EI-NO-- 09905089
Norges teknisk-naturvitenskapelige 1052 universitet
in Reactions Trygve
aluminium of
Eidet
ntio
on
carbon
electrolysis ..
anodes M/W &STI -~,J:
2
4
S99
DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Reactions On Carbon A nodes In A luminium Electrolysis .
By
Trygve Eidet
Thesis submitted in partial fulfillment of the requirements for the degree Doktor Ingeni0r.
The Norwegian University of Technology and Science Department of Electrochemistry October 1997
UST OF SYMBOLS -111-
LIST OF SYMBOLS
SYMBOL DESCRIPTION UNIT
v Stoichiometric number aa Charge transfer coefficient (anodic) ac Charge transfer coefficient (cathodic) t] Overpotential [V] 0 Phase shift [°] cr Warburg coefficient [Q/slc] AG° Standard Gibbs energy change of reaction [kJ/mol] ARd Dust generation [wt%] ARt Total bumoff [wt%] Pa Symmetry factor (anodic) [0.5] Pc Symmetry factor (cathodic) [0.5] 6 Electrode coverage (fraction of 1) a> Angular frequency [2n Hz] a Activity a Regression coefficient a Tafel constant [V] A Apparent surface area [cm2] A Preexponential factor Ad apparent, geometric area [cm2] A, “True” electrode area [cm2] b Tafel slope [V/dec.] BAD Baked apparent density, anode [kg/m3] BC Baked carbon c Concentration [mol/dm 3] ca Surface concentration [mol/cm 2] C(O) Chemisorbed, fixed, oxygen atom C(G)„ Chemisorbed, mobile, oxygen atom C(OJ Chemisorbed, fixed, molecular oxygen C(OJ. Chemisorbed, mobile, molecular oxygen ^-"ads Capacitance due to adsorbed intermediate formation [pF/cm 2] cd Current density [A/cm2] C. Electric double layer capacitance [pF/cm 2] CE Current efficiency [%] Cf Free active site
^impfpilch, calc.) Calculated raw material impurity concentration [wt%] or [ppm] ^ impfpilch, exp.) Measured pitch impurity concentration [wt%] or [ppm]
CMe Metal impurity concentration [ppm] CPE Constant phase element n Frequency dispersion parameter (0 < n < 1) in CPE Sulphur concentration [wt%] Predefined impurity concentration [wt%] or [ppm] Oxygen atom adsorbed on carbon electrode surface
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -IV-
CY Coke yield [%] D Diffusion coefficient [cm2/s] DC Delayed coke E Activation energy [kJ/mol] E Potential [V] et- Anode potential corrected for IR-drop [V] EDX Energy dispersive X-ray microanalysis EIS Electrochemical impedance spectroscopy Em Potential amplitude [V] E° Standard potential [V] ET Reversible potential [V] F Faraday constant [96487 C/mol] f Frequency [Hz] FC Fluid coke G Graphite GAD Green apparent density, anode [kg/m3] i Current density [A/cm2] I Current [A] L Anodic current density [A/cm2]
^calh. Cathodic current density [A/cm2]
»* Double layer current density [A/cm2] h Faradaic current density [A/cm2] h Current amplitude [A] Exchange current density [A/cm2] = V=I j k Potential dependent rate constant (anodic) [s'1] k Potential dependent rate constant (cathodic) [s1] k reaction rate [mol/s] K Standard rate constant (anodic) [cm/s] K Standard rate constant (cathodic) [cm/s] ki Effective rate constant (including concentration term) [mol/cm 2 s] Ki Potential dependent, effective rate constant (including concentration term) [mol/cm 2 s] k° Standard rate constant [cm/s] ^ads Inductance due to adsorbed intermediate formation [H] ^out Outer /system inductance [H] m, Weight of loose dust [mg] m2 Gaseous carbon loss [mg] m= Mass of additive [g]
added Mass of added organic chemical [g] ms Mass of green coke [g] mo Original weight of sample [mg] MO Metal oxide “p Mass of precursor [g] ^precursor Mass of coke precursor [g]
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS UST OF SYMBOLS -V-
MW, impurity Molecular weight, impurity [g/mol]
MW, organic Molecular weight, organic additive [g/mol] n Number of electrons transferred after rds n Number of electrons transferred before rds n Total number of electrons transferred Pco Partial pressure of CO [Pa] Pea, Partial pressure of C02 [Pa] PYG Pyrolytic graphite q Charge required for adsorption of the ad-species to complete coverage [C] Q* Charge in the electric double layer [C] R Gas constant [8.314 J/K mol] r Surface roughness from BET [cm2BE/cm28eo j r Surface roughness from EIS [Cdl / CdL PYG] ro Net rate of production of electrons [mol/s] R, Charge transfer resistance of the first reaction step [Q cm2] r, Net rate of production of adsorbed species I [mol/s] Rz Charge transfer resistance of the second step [O cm2] D ■^a'rfcafc.) Calculated air reactivity [mg/gh] D Experimental air reactivity [mg/gh] D ZVC02fi»Zc.; Calculated C02 reactivity [mg/gh] D ZVC02(cjpJ Experimental C02 reactivity [mg/g h] Charge transfer resistance [£2 cm2] rds Rate determining step & Electrolyte resistance [O] Gas reactivity, core samples [mg/cm2 h] & Gas reactivity, granulate [mg/g h] SEM Scanning electron microscope Dust index [%] SPG Ordinary graphite (spectrally pure) T Absolute temperature [K] t Time [s] VBD Vibrated bulk density [g/cm3] v, Rate of reaction i [mol/s] XRD X-ray diffraction y. Adjustable admittance parameter [n l] Z’ Real part of impedance [O] Z” Imaginary part of impedance [Q] 4 Faradaic impedance [Q] Warburg impedance [Q]
KEACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS ACKNOWLEDGMENTS -Vll-
ACKNOWLEDGMENTS
This thesis summarizes 3.5 years of work at the The Department of Electrochemistry at The Norwegian University of Technology and Science. The project was conducted by The Electrolysis Group , Materials Technology Department, of The Foundation for Scientific and Industrial Research (SINTEF). The experimental work was performed at NTNU and at Elkem ASA Research ’ laboratories in Kristiansand, Norway.
Acknowledgments are due to The Norwegian Research Council and the Norwegian aluminium industry, under the EXPOMAT and PROSMAT programs, for financial support.
I am very grateful to my supervisors Morten S0rlie and Jomar Thonstad for then- guidance and support throughout this work.
I am further indebted to Professor Adolf Kisza from The Institute of Chemistry at The University of Wroclaw, Poland, for our fruitful collaboration. He introduced me to electrochemical impedance spectroscopy and supervised my work in this field.
Elkem ASA Research has contributed generously to this project with equipment and expertise. Thanks are due to Gunnar Halvorsen and Knut Henriksen for their support.
I wish to thank Torild Eggen, Thor Rostpl and 0yvind Larsen for assistance on parts of the experimental work, and the staffs at Elkem Research, The Electrolysis Group and The Department of Electrochemistry for help and support.
Professor Klaus J. Huttinger, Institut fur Chemische Technik der Universitat Karlsruhe, Germany, gave helpful suggestions on the laboratory coker apparatus, and Tarconord, Nyborg, Denmark provided the coke precursor fractions.
Finally I would like to thank my family for their support and encouragement during my education, and specially my wife, Inger-Lise, who has shown immense patience.
Kristiansand, October 1997
Trygve Eidet
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS SUMMARY -IX-
SUMMARY The aim of this work was to investigate the main chemical reactions that consume anode carbon during aluminium electrolysis. These reactions are the electrochemical anode reaction (Part I) and the airbum and carboxy reactions (Part II).
Part I: The Anodic Reaction on Carbon Anodes in the Aluminium Electrolysis Studied by Electrochemical Impedance Spectroscopy . The mechanism and kinetics of the electrochemical anode reaction in the aluminium electrolysis was studied at different carbon anode materials in several electrolytes. New developments in electronics and computerized data acquisition and calculations, have improved the possibilities to analyze the reaction by electrochemical impedance spectroscopy (EIS), and this technique was used in laboratory cell experiments at 1000 °C under argon atmosphere. The reaction was studied in the potential range from 1.2 - 2.4 V versus the aluminium reference electrode which gave current densities up to about 1 A/cm2. The total anode reaction can be described as a two-step two-electron charge transfer reaction with an adsorbed intermediate. A preceding chemical reaction step seems to be present. The measured impedance spectra showed the same features for all the investigated anode materials. At low potentials the reaction seems to be influenced by a Warburg diffusion, but this is probably mainly due to parasitic, parallel faradaic reactions at the electrode and/or the surface roughness of the anode. At higher anode potentials, the reaction was under kinetic control by charge transfer reaction. At low frequencies, the Nyquist curves developed a loop in the fourth quadrant of the complex plane diagram. Such pseudo-inductive loops are diagnostic to electrochemical reactions with adsorbed intermediates. At pyrolytic graphite and ordinary graphite in alumina saturated cryolite melts, the recorded impedance spectra were analyzed by an equivalent circuit approach and by simulations based on kinetic equations. This yielded the double layer capacitance, electrode coverage, effective rate constants, and the charge needed to cover the electrode by a monolayer. The anodic reaction was faster on ordinary graphite anode than on pyrolytic graphite anode, and the first electrochemical step was rate determining on both materials. Measurements of the surface roughness by EIS gave the same ratio between the roughness of graphite and pyrolytic graphite as BET measurements did. High roughness of industrial anode materials at low potentials indicated that the melt penetrated pores. The surface roughness of all the investigated carbon materials decreased with increasing potential, reflecting changes in wetted area. Tafel plots with true electrochemical area can be calculated from apparent geometric area and roughness factors. For industrial anode materials, this altered the Tafel slope drastically. Increase in Tafel slope and Tafel constant were seen when adding CaF2 and A1F3 to the cryolite melt. The same effect was observed in melts with 5 wt% and -10 wt% added. Reduction of the alumina content caused an increase in Tafel slope at pyrolytic graphite. The same effect was observed in the melts with 3 wt% and 5 wt% A1203.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -X-
The following general anodic reaction scheme is in accordance with the findings: <=/ Y&R h
R ** Oads. + K-\ A' + ^ O + Zg-
Based on the reaction above the following reaction mechanism of the anodic reaction on carbon in cryolite melts was suggested: 1. Preceding chemical step which reduces the number of F-ligands around the oxide complexes. This facilitates the complexes to react with the carbon surface: F3+x A1 -O-AlF2+(* +1)" <=>F3Al-0-A1F3~ + 2xF" and/or F2+XA1 < g > AlF22
Part II: A Study of the Am and Carboxy Reactivity of Carbon Anodes in the Aluminium Electrolysis The study focused on the effects of inorganic impurities on the reactivity of carbon anodes and cokes. Impurities that were investigated were sulphur, iron, nickel and vanadium. These are common impurities which have been reported to affect airbum and the carboxy reaction at carbon anodes. The surface studies were done with a scanning electron microscope (SEM) and the phase analysis were performed with energy dispersive X-ray microanalysis (EDX) and X-ray diffraction (XRD).
Labscale anodes Six different labscale anodes were made with delayed coke (low sulphur) or fluid coke (high sulphur) and the same binder pitch. Iron was added to the anodes, and the effect of iron, and sulphur on iron, on carboxy reaction at 960 °C was studied. Iron was in the metallic state after baking except when it had access to released sulphurous compounds which converted it to FeS. FeS was stable through baking. Iron is catalytically active in the C-C02 reaction at 960 °C, and iron present as FeS after baking, can be converted to active catalyst (metallic iron) under these conditions.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS SUMMARY -XI-
Iron compounds in contact with carbon in cracks and pores of the carbon material in C0/C02 atmosphere at 960 °C can be reduced to Fe or FeO. If the iron particles become detached from the carbon, they will be oxidized to Fe304 or Fe203 which are catalytically inactive. Catalytic activity caused by iron on fluid coke (high sulphur) was not observed. This may be due to a "steady state" poisoning of the active catalyst particles by sulphur, or may have carbon structural explanations.
Laboratory coker To be able to produce cokes with predefined impurity profiles and with comparable structural characteristics, a laboratory coker was developed. An aromatic oil from the distillation of coal tar pitch was used as a coke precursor. Green coke was produced in the coker under 15 bar pressure to 525 °C. The green coke was crushed and calcined to 1000 °C in a coke bed under atmospheric pressure. This yielded a low air- and carboxy reactivity coke with a low impurity content and physical properties comparable to commercial anode cokes, and it gave the possibility to add controlled amounts of impurities to the coke precursor, and to tailor any wanted impurity content in the produced cokes.
Sulphur Sulphur alone did not have any significant effect on the air- and carboxy reactivities of cokes for S contents in the range from 0.35 to 1.8 wt%. The C02 reactivity in this concentration range was close to being constant. These findings indicate that the rather contradicting effects of sulphur on reactivity reported in the literature are related to indirect effects of sulphur like structural effects and poisoning of catalytic impurities.
Iron Iron was present in the metallic state after calcining in low sulphur cokes, while in the high sulphur cokes a range from pure FeS to metallic iron was found. Iron catalyzed both the 02- and the C02 gasification of cokes. Sulphur caused a reduction in the effect of iron as a catalyst to the carbon-0 2 reaction. It reduced the air reactivity and the rate of increase in reactivity with iron content. The carboxy reactivity was slightly smaller due to S, but the increase in reactivity with iron content was the same for high- and low sulphur cokes.
Vanadium Vanadium in high- and low sulphur cokes was partly in the metallic state and partly converted to vanadium oxide after calcining. None of the vanadium particles had significant sulphur contents. Vanadium was a strong catalyst to both the airbum and the carboxy reaction. The air reactivity of the produced cokes with vanadium additives was too high to give reliable measurements. Thus, vanadium is an extremely strong catalyst to airbum, and if sulphur inhibited the reaction, the effect was not sufficiently large to lower the reactivity into the measurable region. Sulphur caused a significant reduction in the effect of vanadium as a catalyst to the carboxy reaction. It reduced the C02 reactivity and the rate of increase in reactivity with vanadium content. SEM analysis and EDX did not substantiate the difference in reactivity between high and low sulphur cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -Xll-
Nickel Nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after calcining. It did not have any substantial effect on air and carboxy reactivity in the concentration range from 10 to 650 ppm. Addition of sulphur caused a minor reduction in carboxy reactivity as a function of the nickel content.
G eneral remarks on sulphur , iron , vanadium and nickel impurities The relative catalyst strength of the investigated impurities was vanadium > iron > nickel » sulphur alone. This ranking was valid for both airbum and carboxy reaction. Sulphur and nickel are almost equal because they had no significant catalytic effect, although Ni may have a very small effect on C02 reactivity. From these investigations it seems like iron can take part in the catalytic red-ox-cycle as long as it is in good contact with carbon; vanadium takes part in the catalysis as long as the reaction conditions are maintained, while nickel is inactive. The contact, or wetting, between the catalyst particles and the carbon matrix were best for vanadium, intermediate for iron and poor for nickel. This is probably a main reason for the difference in strength of the investigated catalysts. The inhibition of catalytic activity due to sulphur did probably not have the same mechanism for iron and vanadium. Sulphur may inhibit the vanadium catalysis of the carbon-C0 2 reaction by forming a stable non-mobile complex with vanadium. This would prevent vanadium from catalyzing the reaction at active sites. It is also possible that the sulphur released as the carbon was consumed in the catalyzed reaction, gave a "steady state" poisoning of the active catalyst particles. Sulphur was bonded to iron during coke production, and the iron sulphides were converted to metallic iron during the first stage of reaction with air and C02. The iron particles were probably not catalytically active during this conversion, and sulphur therefore reduced the catalytic effect of iron on the carbon gasification reactions.
Pitch coke The developed laboratory coker could be used for carbonization of anode binder pitches. It gave uniform “anode binder cokes” for laboratory use. The coke yield after pressure carbonization and calcination to 1000 °C was typically 88-90 % and the vibrated bulk density of the coke was comparable to calcined anode grade petroleum coke. Gas reactivity of the pitch cokes could be expressed in terms of the impurity contents of the pitch cokes. With simple empirical expressions C02 and air reactivity was correlated to impurity content with correlation coefficients in the range R = 0.95-0.97. With respect to air reactivity, sodium was the most important impurity in binder pitches. Experimental air reactivity results were found to be proportional to the sodium content alone in the pitch cokes with a correlation coefficient of R = 0.90. Reactivity measurements on baked laboratory anodes, using the same calcined petroleum coke as aggregate, but with different pitch binders, gave a qualitative verification of the pitch coke reactivity results, and the reactivity data from the pitch cokes were in accordance with the previous investigations on cokes with impurity additions.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS CONTENTS -xm-
CONTENTS
1. INTRODUCTION...... 1 1.1. ALUMINIUM PRODUCTION...... 1 1.2. CARBON ANODES...... 3 1.3. ANODE CARBON CONSUMPTION...... 4 1.4. SCOPE AND BACKGROUND FOR THIS WORK...... 5
Part I The Anodic Reaction on Carbon Anodes in the Aluminium Electrolysis Studied by Electrochemical Impedance Spectroscopy
2. INTRODUCTION...... 9
3. THEORY...... 13 3.1. GENERAL...... 13 3.1.1. CURRENT-OVERPOTENTIAL RELATION...... 13 3.1.2. IMPEDANCE MEASUREMENTS...... 15 3.1.3. POTENTIAL AND CURRENT AT THE ELECTRODE...... 16 3.1.4. THE FARADAIC IMPEDANCE...... 17 3.1.5. THE RANDLES CIRCUIT...... 18 3.2. TWO STEP REACTION WITH ADSORBED INTERMEDIATE...... 20 4. EXPERIMENTAL______22 4.1. APPARATUS...... 22 4.2. THE EXPERIMENTAL CELL...... 23 4.2.1. CELL DESIGN...... 23 4.2.2. REFERENCE ELECTRODE...... 24 4.2.3. CHEMICALS...... 25 4.3. CARBON ANODES...... 25 4.4. METHOD...... 26 5. RESULTS AND DISCUSSION...... 28 5.1. GENERAL FEATURES OF THE IMPEDANCE SPECTRA...... 28 5.2. EQUIVALENT CIRCUITS...... 36 5.2.1. THE OUTER INDUCTANCE...... 40 5.2.2. ELECTROLYTE RESISTANCE...... 41 5.2.3. THE IMPEDANCE IN THE LOW POTENTIAL RANGE...... 42 5.3. THE ANODIC REACTION ON GRAPHITE...... 43 5.4. SURFACE ROUGHNESS...... 51 5.4.1. SURFACE ROUGHNESS BY EIS...... 51 5.4.2. SURFACE ROUGHNESS BY BET...... 52 5.4.3. ELECTRODE SURFACE ROUGHNESS FITS AT LOW POTENTIALS...... 53 5.4.4. ELECTRODE SURFACE ROUGHNESS FITS AT HIGH POTENTIALS.....54 5.4.5. ELECTRODE SURFACE ROUGHNESS...... 55 5.4.6. TAFEL SLOPES WITH TRUE CURRENT DENSITY...... 56
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -XIV-
5.5. EFFECT OF BATH COMPOSITION ON THE ANODE REACTION...... 59 5.5.1. EFFECT OF CALCIUM FLUORIDE...... 60 5.5.2. EFFECT OF ALUMINIUM FLUORIDE...... 61 5.5.3. EFFECT OF ALUMINA...... 62 5.5.4. SUMMARY...... 63 5.6. SUGGESTIONS ON THE REACTION MECHANISM ON CARBON ANODES IN ALUMINIUM ELECTROLYSIS...... 63 5.7. FUTURE WORK...... 65 6. CONCLUSIONS...... 66
Part II A Study of the A ir and Carboxy Reactivity of Carbon A nodes in the A luminium Electrolysis
7. INTRODUCTION...... 71
8. THEORY...... 72 8.1. CARBON-GAS REACTIONS...... 72 8.1.1. GENERAL...... 72 8.1.2. CARBOXY REACTIVITY...... 74 8.1.3. AIRBURN...... 75 8.2. CATALYSIS OF CARBON-GAS REACTIONS...... 76 8.2.1. GENERAL...... 76 8.2.2. REACTION MECHANISMS...... 78 8.2.3. TOPOGRAPHICAL EFFECTS OF CATALYZED CARBON-GAS REACTIONS...... 79 8.2.4. INHIBITION OF THE CARBON-GAS REACTION...... 81 8.2.5. IRON...... 81 8.2.6. VANADIUM...... 82 8.2.7. NICKEL...... 82 8.2.8. SULPHUR...... 83 8.3. GAS REACTIVITY OF CARBON MATERIALS...... 83 9. EXPERIMENTAL...... 86 9.1. PREPARATIVE EQUIPMENT...... 86 9.1.1. PASTE MIXER...... 86 9.1.2. BAKING FURNACE...... 87 9.1.3. LABORATORY COKER...... 88 9.2. HYDRO ALUMINIUM AIR/CO, REACTIVITY APPARATUS...... 90 9.3. ANALYTICAL INSTRUMENTS...... 92 9.4. MATERIALS...... 93 9.5. METHODS...... 94 9.5.1. LABSCALE ANODES...... 94 9.5.2. PITCH COKES AND CONTROLLED IMPURITY LEVEL COKES...... 95 9.5.3. REACTIVITY...... 97 9.5.4. SEM ANALYSIS...... 97 9.5.5. XRD ANALYSIS...... 98
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS CONTENTS -XV-
10. RESULTS AND DISCUSSION...... 99 10.1. LABSCALE ANODES...... 99 10.1.1. BAKED ANODE MATERIALS...... 99 10.1.2. CARBOXY REACTION AT ANODES WITH IRON ADDED...... 104 10.1.3. AIRBURN AT ANODES WITH IRON ADDED...... 109 10.1.4. SUMMARY...... 110 10.2. LABORATORY COKER...... 111 10.2.1. SUMMARY...... 114 10.3. EFFECTS OF SULPHUR ON THE AIR AND CARBOXY REACTIVITY OF COKES...... 115 10.3.1. SULPHUR IN CALCINED COKES...... 115 10.3.2. THE EFFECT OF SULPHUR ON AIR REACTIVITY...... 116 10.3.3. THE EFFECT OF SULPHUR ON CARBOXY REACTIVITY...... 117 10.3.4. SUMMARY...... 118 10.4. EFFECTS OF IRON ON THE AIR AND CARBOXY REACTIVITY OF COKES...... 118 10.4.1. IRON IN CALCINED COKES...... 118 10.4.2. THE EFFECT OF IRON ON AIR REACTIVITY...... 119 10.4.3. THE EFFECT OF IRON ON CARBOXY REACTIVITY...... 121 10.4.4. THE EFFECT OF SULPHUR ON IRON IN CALCINED COKES...... 122 10.4.5. THE EFFECT OF SULPHUR ON AIRBURN CATALYZED BY IRON..... 123 10.4.6. THE EFFECT OF SULPHUR ON THE CARBOXY REACTION CATALYZED BY IRON...... 125 10.4.7. SUMMARY...... 126 10.5. EFFECTS OF VANADIUM ON THE AIR AND CARBOXY REACTIVITY OF COKES...... 127 10.5.1. VANADIUM IN CALCINED COKES...... 127 10.5.2. THE EFFECT OF VANADIUM ON AIR REACTIVITY...... 128 10.5.3. THE EFFECT OF VANADIUM ON CARBOXY REACTIVITY...... 129 10.5.4. THE EFFECT OF SULPHUR ON VANADIUM IN CALCINED COKES. 131 10.5.5. THE EFFECT OF SULPHUR ON AIRBURN CATALYZED BY VANADIUM...... 132 10.5.6. THE EFFECT OF SULPHUR ON THE CARBOXY REACTION CATALYZED BY VANADIUM...... 133 10.5.7. SUMMARY...... 134 10.6. EFFECTS OF NICKEL ON THE AIR AND CARBOXY REACTIVITY OF COKES...... 135 10.6.1. NICKEL IN CALCINED COKES...... 135 10.6.2. THE EFFECT OF NICKEL ON AIR REACTIVITY...... 136 10.6.3. THE EFFECT OF NICKEL ON CARBOXY REACTIVITY...... 138 10.6.4. THE EFFECT OF SULPHUR ON NICKEL IN CALCINED COKES...... 139 10.6.5. THE EFFECT ON AIRBURN OF SULPHUR ADDED TOGETHER WITH NICKEL...... 140 10.6.6. THE EFFECT ON THE CARBOXY REACTION OF SULPHUR ADDED TOGETHER WITH NICKEL...... 141 10.6.7. SUMMARY...... 143
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -XVI-
10.7. SUMMARY ON THE EFFECTS OF SULPHUR, IRON, VANADIUM AND NICKEL IMPURITIES...... 143 10.8. REACTIVITY OF PITCH COKES...... 145 10.8.1. PITCHES AND PRODUCED PITCH COKES...... 146 10.8.2. CARBOXY AND AIR REACTIVITY OF PITCH COKES...... 151 10.8.3. EFFECT OF PITCH ON ANODE REACTIVITY...... 156 10.8.4. SUMMARY...... 158 10.9. FUTURE WORK...... 159 11. CONCLUSIONS...... 160
LITERATURE...... 165
APPENDIX 1: ESTIMATION OF ANODE AREA AND CURRENT DENSITY...... 173 APPENDIX 2: EFFECT OF EXPERIMENTAL CELL LINING...... 177
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 1. INTRODUCTION -1-
1. INTRODUCTION
1.1. ALUMINIUM PRODUCTION
Aluminium is produced by the electrolysis of alumina (A1203) dissolved in a bath consisting mainly of cryolite. At present, it is only the Hall-Heroult process which is in commercial use. This process was invented independently by Hall and Heroult in the eighteen eighties. The electrolysis is performed in large cells where the liquid metal serves as the cathode, and with a consumable anode consisting of petrol coke with coal tar pitch as binder. The electrolyte is a fluoride melt in which alumina is dissolved. The process is partly “carbothermal ” as the required electrical energy for the production of aluminium is reduced by the electrochemical consumption of the carbon in the anode, see reaction (1.1).
Two carbon anode technologies are in use. Figure 1.1 shows a sketch of a typical Sdderberg electrolysis cell [1]. This cell has a continuously self-baking anode which is called “Spderberg anode” after its inventor. The S0derberg anode is worked by feeding green anode paste briquettes to the top of the anode. As the carbon is consumed by the electrolytic process at the bottom of the anode, the paste sinks down through the steel casing, and bakes as it reaches the hotter zones of the anode.
STUD EXTENSION
BUS BAR
STEEL SHELL
ANODE PASTE
BAKED CARBON
.ELECTROLYTE ALUMINA
CRUST SHELL
SIDE WALL METAL CARBON 777Tr7*7T7777tt CATHOOEBLOCK' RAMMING .PASTE
COLLECTOR INSULATION 'BAR
Figure 1.1. Aluminium electrolysis cell with "S0derberg" anode [1],
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -2-
The other anode technology is the “prebaked ” anodes, shown in Figure 1.2 The anodes in this type of cells are baked before they are put into the cell, and replaced when they are consumed. This technology About 80 % of the worlds annual aluminium production is produced with this technology.
Figure 1.2. Aluminium electrolysis cell with "prebaked" anodes [1].
Anode carbon is consumed in the aluminium production process, and the main reaction is [2]:
A^O,(dissolved) + 3/2 C(s) -> 2 Al(l) + 3/2 C02(g) (1.1) E° 970 "C=-1.17 V (1.2)
The fact that carbon takes part in the electrochemical reaction to produce aluminium reduces the consumption of electrical energy in the process.
A standard industrial electrolyte has the following composition [2]: 1.5-6 wt% A^O,, 4-8 wt% CaF2, 8-13 wt% A1F3 and the balance cryolite (Na3AlF6). The temperature in the electrolyte is normally between 950 and 980 °C. The calcium fluoride in the bath comes from impurities in the alumina, but some companies add CaF2 because it is believed to have some beneficial effects on the process.
Aluminium fluoride is added to reduce the cryolite ratio, i.e. the molar ratio of NaF and A1F3. This is done to be able to run the process at a lower temperature and obtain a better current efficiency. Lithium fluoride (LiF) can be added to the bath to improve the electrical conductivity and to lower the temperature.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 1. INTRODUCTION -3-
Theoretical energy consumption per kg produced aluminium, including heating of reactants, is 6.4 kWh, but the real consumption is about 13 - 18 kWh/ kg A1 [2]. Most of the energy loss is ohmic heat loss in the anode, electrolyte and cathode lining, together with overvoltage and current efficiency < 100 %. The current efficiency is among other things, reduced by too short interelectrode distance, too high operating temperature and the back reaction, see reaction (1.3). To reduce the ohmic loss in the electrolyte, the interelectrode distance is kept as small as possible. This is limited by the risk of short-circuiting due to magneto-hydrodynamic disturbances of the liquid cathode, and by the possibility of increased contact between C02 and liquid aluminium. The latter increases the rate of the back reaction.
1.2. CARBON ANODES
Carbon anodes represent a major production cost in the primary production of aluminium. Approximately 400-450 kg net anode carbon is consumed per tonne aluminium produced, while the theoretical need in the electrochemical reaction is 334 kg C / tonne Al.
The raw materials from which carbon anodes are produced are petroleum coke and coal tar pitch. The coke is a by-product from the oil refineries. The anode producers want coke to have high electrical conductivity, high mechanical strength, high chemical purity and high homogeneity. Such coke properties together with the right processing conditions shall give anodes with high conductivity, good mechanical properties and thermal shock resistance, low reactivity towards C02 and air and low impurity transfer from anodes to metal.
For both anode technologies an anode paste is made from mixing a coke aggregate with controlled grain size distribution with liquid pitch. The binder pitch is normally a coal tar pitch which is produced from a by-product of coal coking. The mixing recipe, mixing time and the mixing temperature of the paste are very important factors in the production of a dense and homogenous anode.
For S0derberg anodes, the anode paste is briquetted and fed directly to the anode top. The paste melts and is baked by the heat from the electrolysis during its movement down to the anode surface where it is consumed. The maximum baking temperature in this process is between 950 and 980 °C. depending of the operating temperature of the cell. Prebaked anodes are made from anode paste which is vibrated or pressed into moulds. After that, the green anode blocks are baked to around 1200 °C.
The baked carbon anodes consist of coke grains bonded together by pitch coke formed during the baking. Thus, when one looks at the anodes from the grain size scale and down, the anodes are more or less inhomogenous with respect to structure, reactivity, strength and purity.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -4-
1.3. ANODE CARBON CONSUMPTION
Figure 1.3 shows the anode carbon consumption in the aluminium electrolysis without any electrochemical side reactions.
Theoretic consumption 0.334 kg C/kg Al | Excess Electrolytic consumption 0.334/CE i cons.
Net consumption Butts ◄— ------► ------►- Gross consumption
Figure 1.3. Total anode carbon consumption in the aluminium electrolysis.
The theoretic consumption in Figure 1.3 is calculated from Equation. (1.1). The current efficiency (CE) in industrial cells is never 100 %, but somewhere between 82 and 95 % [3], This is mainly due to the back reaction [2]:
Al(l) + 3/2C02(g) -> l/2Al203(dissolved) + 3/2CO (1.3)
The extra carbon consumption due to Equation. (1.3) together with the theoretical consumption, is called the electrolytic consumption, and it is thus normally between 0.35 and 0.41 kg C/ kg Al.
The excess consumption in Figure 1.3 is due to carbon dust formation and to the reactions between the anode and the oxidizing gases C02 and air. The carbon gas reactions are called carboxy reaction and airbum. The carbon consumption by these carbon gas reactions is between 0.02 and 0.15 kg.
In addition to oxidative carbon consumption by the C02 and airbum, the reactions cause selective burning, i.e. the gases attack the different structural elements of the anode at different rates. The binder matrix, which is carbonized coal tar pitch and petroleum coke fines, is consumed faster than the petroleum coke grains. The electrochemical anode reaction is consuming the different structural elements at different rates in the same manner. This leads to detachment of coke grains from the anode surface, and so called dusting occurs. This mechanism is shown in
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 1. INTRODUCTION -5-
Figure 1.4.
ANODE
1. Petrol coke grain 2. Porosity (open and closed) 3. Binder matrix 4. Loose carbon particle
Figure 1.4. Selective burning of anode.
The carbon loss to butts is loss connected to anode failure, regular changing of anodes ("Prebaked" cells) or loss on cell shut down. The net consumption due to butts is reduced by recycling of the usable parts of the butts.
1.4. SCOPE AND BACKGROUND FOR THIS WORK
This study is a part of a partly industrially financed research program on carbon consumption and raw materials properties in the aluminium industry. The program aims to reduce the carbon consumption in the primary production, and to give a better process understanding and control.
Work has been done to reduce the carbon consumption all the time since Hall’s and Heroult’s patents in 1886. In the endeavor to achieve this, one has two main choices. The technology and the carbon materials of today can be improved to reduce the excess carbon consumption, or one can replace carbon with an inert anode material which is not consumed in the process. The last option has, in spite of great research activity, not yet been demonstrated on an industrial scale.
The scope of this work is to investigate the main chemical reactions that consume anode carbon during aluminium electrolysis. These reactions are the electrochemical anode reaction and the airbum and carboxy reactions.
The first part of this thesis is on the kinetics and mechanism of the electrochemical anode reaction in aluminium electrolysis. Despite all the research activity on this subject [10], this reaction is still not fully understood. New developments in electronics and computerized data acquisition and calculations, have improved the possibilities to study the reaction by electrochemical impedance spectroscopy (EIS).
REACTIONS ON CARBON ANODES IN TBE ALUMINIUM ELECTROLYSIS -6-
Professor A. Kisza from The Institute of Chemistry at The University of Wroclaw, Poland, has been a key advisor by his participation in this part of the work.
The second part of this thesis is on air and carboxy reactivity of carbon anodes. The aim of this part was to study the effects of inorganic impurities on the reactivity of carbon anodes in the aluminium industry. Special attention was given to sulphur, because its effect on the carbon gasification is not very well understood. Sulphur is always present in anodes, and it is expected that the sulphur content of available anode cokes will increase in the future. It has also been suggested that sulphur poisons catalyzing impurities in the anodes. Other impurities that were investigated were iron, nickel and vanadium, which are common impurities in anodes which have been reported to catalyze carbon gasification.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS Part I
The A nodic Reaction on Carbon A nodes in the A luminium Electrolysis Studied by Electrochemical Impedance Spectroscopy .
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 2. INTRODUCTION -9-
In this first part of the thesis, the kinetics and the mechanism of the anodic reaction on carbon anodes in aluminium electrolysis are investigated by means of electrochemical impedance spectroscopy.
2. INTRODUCTION
Molten cryolite (Na^AlFJ shows a high solubility of alumina (A1203), ~ 22 mol% at 1010 °C [4], which makes it an ideal solvent for alumina in the electro winning of aluminium. When a cryolite melt with dissolved alumina (bath) is electrolyzed with a carbon anode and an aluminium cathode, two cell reactions are possible;
—Al203 + — C - Al + — CO- (2.1) 2 2 3 4 4 2 or -AZ,0,+-C = AI+-CO (2.2) 2 2 3 2 2
The standard potentials of these two reactions at 1010 °C are -1.16 V and -1.02 V respectively [5], The anode reaction is associated with a high anodic overpotential which is in the order of 0.5 V at 1 A/cm2. It has been shown by gas analysis, that at current densities above 0.1 A/cm2 the primary anode product is C02 [6]-[8]. In carefully performed experiments, only minor amounts (<5%) of CO is found, and this CO is most probably produced by the secondary reaction:
C02 + C = 2CO (2.3) where the carbon reactant is not the polarized electrode surface, but rather loose carbon particles or the interior of the porous anode.
The predominance of C02 is also supported by thermodynamic arguments, first demonstrated by Drossbach [9]. For the reaction:
1 3 — Al~Ov — Al H—O- (2.4) 2 4 the standard potential is -2.190 V at 1010 °C [5]. For reaction (2.1) with an anodic overpotential of 0.5 V, the polarization potential becomes 1.66 V, which according to Equation (1.4) corresponds to an oxygen partial pressure of 4.5 10"9 atm. At this oxygen pressure the equation:
C0+~02 = C02 (2.5) is shifted far to the right at 1010 °C (PCo^ / Pco = 1700), showing that CO is unstable at the polarized anode surface.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -10- PARTI
Experimental studies of the mechanism of reaction (2.1) are difficult to perform for a number of reasons:
a) Commercial carbon materials which consist of two phases, an aggregate coke and a binder (pitch coke), are coarse-grained, inhomogeneous and porous.
b) The consumption of the carbon is not uniform, as the more reactive phase (the binder phase) is consumed preferentially.
c) The wetted surface area may change as the anode is consumed, and it may change with the polarization potential,
d) The vigorous gas evolution at the anode changes the wetted area and causes potential or current oscillations.
For these reasons the available studies on the reaction mechanism of reaction (2.1) show a great scatter and do not allow any clear conclusion to be drawn. Available works up to 1980 have been reviewed by Gijotheim et. al. [10]. It should be mentioned that the discrepancies in the literature partly are due to the fact that the experimental set up (such as carbon material and bath composition) is different in many of the works and thus not directly comparable.
Many workers recorded Tafel slopes, but the numerical values vary widely from 0.09 to 0.55 V/decade. The most common values are between 0.15 and 0.25 . Some workers have found two slopes; in most cases the second being steeper than the first, but also the opposite has been observed [10]. Table 2.1 shows a collection of Tafel slopes from the literature with some exchange current densities and experimental conditions given.
Voltammetric curves show a reversible absorption peak prior to the C02 peak [11], undoubtedly due to chemisorption of oxygen species at carbon. Beyond the COz peak discharge of fluoride species takes place, but these processes will not be treated in the present work. Chronopotentiometric measurements yield similar results [10].
The first impedance measurements of reaction (1.1) were performed by Drossbach et al. [12] and by Drossbach and Hashino [13] with graphite anodes in Li3AlF6 - Na3AlF6 - A1203 melts at 870-900 °C. Somewhat flattened Nyquist plots were observed, which were attributed to reaction control, according to the nomenclature introduced by Vetter [14]. Thonstad [15] carried out similar studies in cryolite-alumina melts at 1000 °C, using a wide range of carbon materials. For graphite and pyrolytic graphite the results were interpreted as reaction controlled kinetics, i.e. involving one or more intermediate slow steps. For commercial carbons, especially at low current densities, a Warburg impedance was observed, giving indications of partly diffusion control, possibly inside the porous material.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 2. INTRODUCTION -11-
Dewing [16] pointed to the fact that the maximum in the imaginary component (capacitive term) was shifted towards higher frequencies with increasing current density. It was suggested that this shift could be due to the fact that the reaction partly occurs in the pores of the material. Vetyukov and Akgva [17] found that the pseudo capacitance of the anode showed sharp maxima and minima with varying potential, which was attributed to chemisorption of oxygen-containing species.
Table 2.1. Tafel slopes, b, and exchange current densities, io, from different current density (cd) ranges at various anode materials and temperatures, published by several authors.
b i„ cd range Anode Temp. Author(s) [V/dec.] [A/cm2] [A/cm2] rci Solli [19] 0.40 10"3 <0.06 G1 970 0.10 10'8 0.06-1.75 0.30 103 <0.06 BC2 970 0.085 10"7 0.06-1.75 Jarek, 0.20-0.44 0.01-0.1 BC2 Thonstad [11] 0.18-0.24 0.1-0.5 1010 Dewing, 0.295 0.0015 0.04-0.2 G1 van der Kouwe [53] <0.1 0.2-0.5 960- 0.295 0.005 0.5-4.0 BC2 980 Thonstad [15] 0.21-0.23 0.006 0.03-0.3 PYG3 1010- 0.20-0.28 0.024 0.03-1 G1, BC2 1020 Dumas, 0.095 several 985 Brenet [54] types Paunovic [55] 0.29 0.08-0.1 G1 1010 0.14 >0.1 Piontelli, Mazza, 0.27-0.33 0.05-0.1 G1 1050 Pedeferri [56] 0.04 0.1-0.5 Thonstad, 0.55 0.04-1.5 G1 1000 Hove [57] 0.45 0.04-0.7 BC2 Richards, 0.32 0.012 0.04-0.5 G1 1010 Welch [58]
1. Ordinary graphite. 2. Baked carbon. 3. Pyrolytic graphite.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -12- PARTI
Picard et. al. [18] studied impedance spectra of reaction (2.1), and gave a qualitative interpretation of impedance spectra obtained by computer assisted impedance measurements in the frequency range 0.1 Hz to 10000 Hz. They proposed the following three-step mechanism at low current densities and low overpotentials:
(2.6)
AlOFl~x + C-> COads + AIF3~X + 2e~ (2.7)
AZOF;-' + C6L, -» CO%,) + A/F,^ + 2«T (2.8)
The reaction steps above are diffusion of the oxyfluoroaluminate species from the bulk of the melt to the surface of the graphite anode (Eq. (2.6)), dissociation of the AlOF/ ' species and adsorption of the oxide anions (O2), followed by their discharge and formation of an adsorbed carbon-oxygen species (Eq. (2.7)), and dissociation of the AlOF/ ' species and adsorption of oxide ions (O2 ), followed by their discharge in the presence of CO^ and the desorption of gaseous C02 (Eq. (2.8)).
Solli [19] concluded that there are two different Tafel regions for the anodic reaction in the electrolysis of alumina dissolved in cryolite melt. The current/overvoltage curve had one slope in the lower current density range (i < 0.06 A/cm2) where electrochemical CO formation was assumed to be dominating, and another slope in the higher current density range (0.06 < i < 1.75 A/cm2) where the following mechanism was suggested:
A1202F42' suface (2.9)
A1202F4 2 +xC + 2F = C,0 (ads) + Al2OF62 +2e" (2.10)
Al.CyV + CxO (ads) + 2F = C02(g) + Al20F62 +2e" (2.11)
The three reaction steps above are: Transport of alumina species by diffusion from the bulk of the bath to the anode surface, charge transfer giving electrosorption of oxygen to the carbon anode, and finally charge transfer giving direct desorption of COr Step 3 is suggested to be rate determining at normal current densities, but at low current densities step 1 may become increasingly important. These results were obtained by computer assisted impedance measurements in the frequency range 1 to 6-104 Hz.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 3. THEORY -13-
3. THEORY
3.1. GENERAL
3.1.1. CURRENT-OVERPOTENTIAL RELATION
To describe a general relation between current and potential in an electrochemical cell the following simple charge-transfer reaction at the working electrode is considered:
O + ne o R (3.1)
When the cell is at equilibrium, no net current flows. This denotes that there are no concentration gradients at the electrode surface, and the reversible potential, E", of this reaction is given by the Nemst equation:
(3.2)
It is common to replace the activities with concentrations assuming that the activity coefficients of the two species are close to equal. “Equilibrium ” does not mean that no reaction is occurring. What it means is that the reaction is equally fast in both directions. The magnitude of the two equal and oppositely directed currents that are flowing at the electrode surface at equilibrium, is known as the exchange current density, i:
(3.3)
At any potential apart from E" the net current density at the electrode, i, is given by the sum of the current density in cathodic direction and the current density in anodic direction:
Lk. + L = ~nFkc% + nFkc'.o (3.4)
The current density is thus determined by the surface concentration (activity) of the reacting species and the rate constants. The rate constants of electrochemical reactions are potential dependent:
and where a and ac are constants known as the charge transfer coefficients of the anodic and cathodic reactions respectively. For a simple electron transfer reaction like (3.1) they are equal to the symmetry factors and /) respectively, multiplied by the number of electrons transferred, n. The Rvalues are between 0 and 1 and the sum (/^ + j8 c) is 1. It is commonly accepted that the symmetry factors usually are close to 0.5.
REACTIONS ON CARBON ANODES IN THE ALVMINIVM ELECTROLYSIS -14- PARTI
The overpotential, r\, at the working electrode is defined as the deviation of the potential from E":
\T]\ IE -ET\ -IR.‘el (3.6) where E is the measured potential, and IRel is the ohmic voltage drop between the reference and the working electrode. The exchange current density, io, is the cathodic- and anodic current density at T] = 0. This, combined with Equation (3.4) and (3.5), gives the Butler-Volmer equation:
(3.7)
This is the fundamental equation of electrode kinetics. It gives the relationship between exchange current density, transfer coefficients, overpotential and current density.
The Butler-Volmer equation has three limiting forms:
At high positive overpotentials the second term may be ignored, and by taking logarithms the anodic current density is then given by:
(3.8)
For high negative overpotentials the first term may be ignored, and the cathodic current density is given by:
log i = log ia + —-—V (39) 2.3 RT
Equations (3.8) and (3.9) are known as Tafel equations which are usually expressed on the form:
q = a + b log i (3.10a)
However, in this thesis Tafel equations are expressed as:
Eco„ = a’ + b log i (3.10b) where Emn . is the electrode potential (corrected for IR drop in the electrolyte), a’ is the sum of E"' and the Tafel constant a: , 23AT . a=-71Tlologi.gz» + E" (3.11) and b is the Tafel slope:
REACTIONS ON CARRON ANODES IN THE ALUMINIUM ElECTROLYSIS 3. THEORY -15-
2.3 RT b = ■ (3.12) aF
For a reaction controlled multistep processes, the Tafel coefficients are determined by the rate-determining step, and the transfer coefficients are given by [20]:
P (n-n-n) + n (1- P )(n-n-n) + h a„ = —------and a„ = -—------(3.13) where v is the stoichiometric number (the number of times a given step must take place for the overall reaction to occur once), n is the total number of electrons transferred and n and n are the numbers of electrons transferred before and after the rate-determining step, respectively, all for one act of the overall reaction.
At very small values of T], where r] « (RT/ct nF) and 17 « (RT/rr nF), the two exponential terms of Equation (3.7) may be expanded to a series, which gives, ignoring quadric and higher terms, this simple equation:
i = i^V (3.14)
3.1.2. IMPEDANCE MEASUREMENTS
The a.c. impedance technique is based on the measurement of the response in current and potential when a sinusoidal potential signal with small amplitude and varying frequency, is superimposed on the electrode potential of an electrode in an electrochemical cell. From such measurements mechanistic and kinetic information about the electrode reactions may be obtained.
The main advantages of the method are: - The system is only brought slightly out of is original state by the small amplitude, sinusoidal potential. The response of the system is therefore representative for the electrode potential at which the measurement takes place. - Because of the small perturbations caused by the measurements, it is possible to record the effect of a high number of cycles, and thereby getting very exact results as mean values over the total measurement time. - Also because of the small perturbations it is possible to treat the response theoretically by linearised (or other simplified) current-potential relations. - Since the potential signal has varying frequency it is possible to measure relaxation phenomena with relaxation times varying by several orders of magnitude. - It is often convenient to compare data from a.c. impedance measurements with electrical equivalent circuits to get a good interpretation of the results.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -16- PARTI
3.1.3. POTENTIAL AND CURRENT AT THE ELECTRODE
The purely sinusoidal potential perturbation superimposed on the electrode potential in a.c. impedance measurements may be expressed as:
E = Em sin cot , co - 2itf (3.15) where co is the angular frequency (2n times the conventional frequency, /, in Hertz) and Em is the amplitude of the sinusoidal potential. The ac current in the cell will be a function of the frequency of the potential signal, and the current is thus given by:
/ = Im sin (cot +
The current density, i, at the electrode under investigation is given by the sum of the faradaic current density, if, and the electric double layer current density, im.
* = :/+V (3.17) where if represents the part of the current that is consumed in the red/ox-reaction, and im. represents the part of the current that is charging the double layer. The faradaic current density is given by the Butler-Volmer equation (3.7), and i# is given by:
z/f) A(C' /frj (3.18) where Qm is the charge in the double layer and Cm is its capacitance.
Impedance measurements in a three electrode system are performed by passing the current between the working electrode and the counter electrode, and measuring the impedance between the working electrode and the reference electrode. The fact that the electrolyte between the reference and the working electrode has a resistance, Rd , together with Equation (3.17) gives an equivalent electric circuit to the electrochemical system between the reference electrode and the working electrode containing a resistance in series with a parallel of a capacitance, Cdl, and a faradaic impedance , Zf, where Zf represents the resistance to charge transfer by the red/ox- reaction at the electrode.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 3. THEORY -17-
3.1.4. THE FARADAIC IMPEDANCE
MacDonald [21] deduced an equation equivalent to (3.7):
I = nFAk°[c0(0, t)e~h(E-E°) - cR(0, f)ea<£-£°)] (3.19) where / is the total current and A is the electrode surface area1. As mentioned above, because of the small perturbations in potential it is possible to treat the response theoretically by linearised current-potential relations. The current is a function of the electrode potential, E, and of the surface concentrations, c(0,f), of the reacting species. Therefore, the change in I due to any variation in potential and surface concentrations is given by the total Laplace differential [21]:
d I f d l' fdl' dl = dE + dc0 + dc D (3.20) d E d c, d c ° 1E.C R J E,
Dividing by dE and noting that the faradaic impedance is given by:
dE (3.21) dl we obtain under steady state and sinusoidal perturbation [21]:
P ! Dp2 ~ 7 IDf Zf=Rc,+ (1 ~j)co-V2 (3.22) nFA-Jl
Where j = V-T and :
{d!Id c0')EiCg (d I/d cR)Eco and Y = (3.23)
Equation (3.22) may be written as:
Zf = Ra + aafm - jam1'2 (3.24) where <7is equal to the quantity in braces from Equation (3.22). Equation (3.24) shows that the faradaic impedance can be represented by the charge transfer resistance, Ra , and a so-called Warburg impedance, Zw , in series. The Warburg impedance, Zw, is given by the two last terms in Equation (3.24), and is an impedance caused by slow mass transfer to the electrode surface.
1. Note that MacDonald uses the opposite sign for anodic and cathodic currents compared to Eq. (3.7)
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -18- PAsrr
3.1.5. THE RANDLES CIRCUIT
From the argument above it can be seen that the impedance of the system between the reference and the working electrode may be represented by the equivalent circuit shown in Figure 3.1. This circuit is called the Randles circuit. The ohmic resistance between the reference electrode and the working electrode is Rd, Cdl is the double layer capacitance, Ra is the charge-transfer resistance and Zw is the Warburg impedance (diffusion).
Reference Electrode „ Udl II------„ Rel T—W------
—/wv'—W— Rc t zw
Figure 3.1. The Randles circuit. Equivalent circuit of a simple charge-transfer reaction at a planar surface.
The total impedance, Z, of the circuit in Figure 3.1 is given by a real, Z’, and imaginary, Z”, parts:
Z = Z’+jZ” (3.25)
T=R I (3.26) (l + <7(0 vlcdl f +(o2Cl{Rct + <7CQ- U2)2
z,. -(w% + Q(0 mY + ao) m(] + aCdl(Q^2) (3.27) (l + oCa(o-y2f + (D2C2(Ra + oo)-V2)2
If Z” is plotted against Z' at each frequency, the resulting plot is called a Nyquist plot.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 3. THEORY -19-
Figure 3.2 shows a Nyquist plot of the impedance of the equivalent circuit in Figure 3.1. It represents a simple charge transfer reaction under semi-infinite diffusion. The figure visualizes how Z’ and Z” change with the frequency of the superimposed sinusoidal potential signal at a constant electrode potential.
Kinetic control Diffuriancoiiml
Rj + R, Z
Figure 3.2. Nyquist plot for a simple charge transfer reaction under semi-infinite diffusion.
The limits of the real and imaginary parts of the impedance are:
Z’ (m->0) = Rd + RC, + aoi10 (3.28)
Z”(m->0) = -2a2 Cm - am"2 (3.29)
This implies that a plot of -Z” against Z’ gives a straight line with slope of 45° as
/L Z (ft) —> =°) = Rel + (3.30)
Z" (® -».,) =----- 6)C^ (3.31) i+to^&:
It can be demonstrated [21] on isolation of to from the above equations, that the high frequency impedance data describes a semi-circle. The radius of the circle is Ra and the high frequency intercept of the circle with the Z’-axis is Rd (see Figure 3.2).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -20- PARTI
3.2. TWO STEP REACTION WITH ADSORBED INTERMEDIATE
If electrochemical impedance spectroscopy (EIS) is used to study an electrode reaction that includes an adsorbed intermediate, a pseudo-inductive behavior of the frequency dependence of the impedance is observed. This pseudo-inductive behavior is seen in the Z (real) vs. -Z" (imaginary) plot as one or two semi-circles in the fourth quadrant of the complex-plane diagram. This is known from the study of the mechanism of electrode reactions like the oxidation of metals [22]-[24] and corrosion processes [25], [26].
In the analysis of a two-step electrochemical reaction with one adsorbed intermediate, two approaches which describe the pseudo-inductive loop, are found in the electrochemical literature. In the first, by Armstrong et. al. [27], three types of possible complex-plane plots, including pseudo-inductive behavior, were calculated by giving possible values of two resistances , R0 and a time constant t, which are mathematically equivalent to the behaviour of elements in an equivalent circuit.
In the second approach, Bai and Conway [28] analyze the impedance of processes with one or two adsorbed intermediates. An approach based on reaction kinetics was preferred to calculations involving an equivalent circuit. The interfacial impedance was expressed in terms of rate constants and coverage by intermediates as in steady- state kinetic analysis. The "equivalent circuit" approach [29]-[31] commonly used in mathematical analysis of impedance data is, according to Bai and Conway, somewhat arbitrary and empirical, as the components of such an equivalent circuit are not always identifiable with the properties of individual reaction steps.
Equation (3.32) describes an electrode reaction with an adsorbed intermediate with Bai and Conway's notation [28]:
(3.32) where Kt are potential dependent, effective rate constants, i.e. they include respective concentration terms:
(3.33)
z 2.303E) K_i = k_ t exp (3.34) x and RT (3.35) are the Tafel slopes (i =1,2).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 3. THEORY -21-
The faradaic reaction rate is expressed in terms of the net rate of production of electrons, r0, and the net rate of production of adsorbed species, rt:
(3.36)
q d6 t at (3.37) where:
vl=Kl{\-Q)-K_ ie (3.38)
v2=K2e-K_2(l-9) (3.39) and I is the current, 6 is the electrode coverage, q is the charge required for adsorption of the ad-species to complete coverage and F is the Faraday constant.
At steady state:
and from Equation (3.37):
(3.41) the electrode coverage 9 is readily obtained:
K1 + K_t + K2 + K_2 (3.42)
For details of the impedance calculation the reader is referred to the original paper of Bai and Conway [28]. It has been assumed in the above derivation that diffusion in reaction (3.32) is unimportant, and that there is no outer inductance and electrolyte resistance.
The equations derived by Bai and Conway allow for the simulation (and fitting) of impedance spectra of a two-step electrochemical reaction with an adsorbed intermediate. This treatment shows the electrochemical nature of the pseudo-inductive behaviour.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -22- PARTI
4. EXPERIMENTAL
4.1. APPARATUS
Figure 4.1 shows schematically the apparatus used in the a.c. impedance measurements (EIS).
Solartron 1250 gpib
Potentiometer
Potentiostat
Figure 4.1. Apparatus for a.c. impedance measurements.
The items in Figure 4.1 are:
Solartron 1250 Frequency Response Analyzer. Frequency range 0 - 64 000 Hz.
High Power Potentiostat Wenking HP 72, connected to Wenking Model SMP 72 Scanning Potentiometer. The maximum current output of the Potentiostat is 10 A.
Closed laboratory furnace with Eurotherm controller and resistance heating. The length of the furnace tube is 70 cm, and its inner diameter is 8 cm.
Experimental cell (inside the furnace, see Chapter 4.2)
Reference resistance, 0.10.
GPIB (General Purpose Integrated Bus) data connection between the Solartron and a PC. The computer program for a.c. impedance measurements is PCSOL.exe.
REACTIONS ON CARBON ANODES IN THE ALUMINUM ELECTROLYSIS 4. EXPERIMENTAL -23-
The potentiostat was controlled by the potentiometer to give the specified anode potential. The generator unit in the Solartron produced an alternating potential signal (+/- 15 mV) of varying frequency, which was superimposed on the cell potential. At every frequency of the potential signal from the generator unit, two quantities were recorded by the Solartron. These quantities were the response in potential between the reference electrode and the anode (recorded on Channel 2), and the response in current through the system. The current response was registered as the potential response over the reference resistance (recorded on Channel 1). From these responses the real and imaginary parts of the impedance were calculated for every a.c. frequency.
4.2. THE EXPERIMENTAL CELL
4.2.1. CELL DESIGN
Figure 4.2 shows the experimental cell used in all the experiments. The cell was devised after several preliminary experiments with different cell designs. A cell design with an aluminium pool at the bottom of the crucible as counter electrode, a cell design with a steel disk at the bottom of the crucible as counter electrode and a cell design with a tungsten rod as counter electrode were tested. The alternative cell designs were excluded because the setup in Figure 4.2 gave least measurement scatter.
The experimental cell was contained in a graphite crucible lined with an alumina tube at the side wall. The alumina linin g was added to help keeping the electrolyte saturated with respect to alumina. Test experiments with and without lining showed that the alumina lining did not influence on the results from the impedance measurements, but the system inductance was slightly altered (see Appendix 2).
The cell had three electrodes. The working electrode / anode at which the reaction under investigation was taking place, was made from four different carbon materials described later. The current was lead to this electrode through the furnace lid by a steel rod shielded by an alumina tube.
The counter electrode / cathode was simply the graphite bottom of the cell. This provided a fairly high cathode to anode area ratio. The ratio was high to make sure that the impedance from the fast, and thus low impedance cathode reaction, had as small an effect on the cell impedance as possible. The current was lead to the cathode through the bottom of the furnace by the steel supporter for the experimental cell. The last electrode, the reference, is described below.
The cell lining was not included in experiments with unsaturated melts. The temperature in the cell was measured with a Pt/Pt 10% Rh thermocouple contained in an alumina tube (BN tube in melts not saturated with alumina).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -24- PARTI
1.
1. Anode contact, steel rod 5. Crucible bottom (counter electrode), covered by alumina tube. 6. Thermocouple in closed alumina tube. 2. Reference electrode, Al. 7. Graphite crucible. 3. Anode, carbon. 8. Alumina saturated cryolite melt 4. Alumina lining, tube. ______
Figure 4.2. Experimental cell.
4.2.2. REFERENCE ELECTRODE
The reference electrode is shown in Figure 4.2. It was made from an alumina tube with one closed end. The tube had two small holes about 2.5 cm above the bottom. Inside, it had a smaller alumina tube with a tungsten wire cemented inside. The W wire created contact to pure aluminium in the bottom of the biger alumina tube, and the aluminium was in contact with the electrolyte through the small holes. This reference electrode is stable, and it is working well from the start of the experiment because the melt is in direct contact with the aluminium. In experiments with unsaturated melts the alumina tube was replaced by a boron nitride tube. Several reference electrodes were tested by putting them into the same melt, and no potential difference between any of the electrodes were found. The measured potential, E, between the working electrode (carbon anode) and the reference electrode is given by Equation (3.6), and it can be written:
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 4. EXPERIMENTAL -25-
E = IZH + n. + IRel (4.1) where E"v is the reversible potential of the cell reaction, % is the anodic overpotential and IRcl is the ohmic potential drop between the reference electrode and the anode which can be measured by EIS.
4.2.3. CHEMICALS
The electrolyte was molten, hand-picked Greenland cryolite crystals (Kryolite- selskabet 0resund, Denmark) with additions:
y-alumina (A12G3, Fluka Chemika, pro analysis). Heat treated to 1100 °C. Added in all experiments. In experiments with alumina saturated melts, A1203 was added in excess. Melts with 3 and 5 wt% alumina were also produced.
Calcium fluoride (CaF2, Merck, Percip. pure). Used without further purification. Added in experiments to investigate the effect of bath composition on the anode reaction. CaF2 concentrations were 0 wt%, 5 wt% and 10 wt%.
Aluminium fluoride (A1F3, Norzink, technical grade). Sublimed under vacuum (p < 2 torr) at 1000 °C. Added in experiments to investigate the effect of bath composition on the anode reaction. A1F3 concentrations were 0 wt%, 5 wt% and 11 wt%.
4.3. CARBON ANODES
Four different anode materials were used:
Pyrolytic graphite (SINTEC, Germany) Ordinary graphite, spectrally pure (ultra “F” purity, Ultra Carbon, USA) Industrial anode, type A (based on Statoil coke) Industrial anode, type B (based on ARCO coke)
Pyrolytic graphite has a well ordered structure, and is thus very dense. Measurements on this material should be little influenced by effects from pores or other surface imperfections.
The ordinary graphite grade was chosen because of its low content of impurities in order to avoid possible impurity effects.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -26- PARTI
The industrial type anodes were made at Elkem Research ’ laboratories specially for these experiments. They were made from the same binder pitch (Tarconord, 120 °C Mettler softening point) but with different cokes. The maximum coke grain size was 2 mm to make sure that any grain at the surface of the small anodes used in the experiments, did not cause an unrepresentative spectrum. Table 4.1 gives some data for the four anode materials.
Table 4.1. Material data of four carbon anode materials.
Density Ash content S content Diameter Material Label [g/cm3] [wt%] [wt%] [mm] Pyrolytic graphite PYG 2.22 max. 0.0100 - 5.00 Ordinary graphite SPG 1.60 < 0.0005 - 6.15 Industrial anode Type A 1.51 0.26 1.18 11.0 Industrial anode TypeB 1.52 0.34 2.43 11.0
4.4. METHOD
The experiments on the different carbon anodes (described above) were performed under the following conditions:
Argon atmosphere 1000 ± 4 °C. Graphite crucible with inner diameter 6.6 cm and height 10.4 cm. Electrolyte (bath): cryolite with A12G3 added. (In some of the experiments A1F3 and/or CaF2 were also added)
Preelectrolysis was run prior to all the experiments at 1.2 V for 45 minutes (/ ~ 0.01 A), and then subsequently at 2.2 V for 15 minutes (/ - 3 A) in order to condition the anode surface and to reduce the amount of impurities in the bath.
Every experiment (same anode, electrolyte and temperature) consisted of two frequency scans at each potential. Measurements were made at every 0.1 V from 1.2 V to 2.3 - 2.5 V against the aluminium reference electrode. At each frequency scan 50 measurements per frequency decade were recorded in the frequency range of 10 Hz - 64 000 Hz. 65 000 Hz is the upper limit of the frequency generator. Thus, every a.c. impedance spectrum had about 200 measurement points. At the end of every frequency scan, the direct current was read off the amperemetre on the potentiostaL
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 4. EXPERIMENTAL -27-
The low limit of the potential range is close to the reversible potential of the overall cell reaction:
AlA(dissolved) + 3/2 C(s) -> 2 Al(l) + 3/2 C02(g) (4.2)
The high limit of the potential range is close to potentials where one can expect influence from the onset of anode effect, i.e. where the surface concentration of alumina at the anode is very low, and the fluorides in the electrolyte starts to decompose to give CF4 and COF2. The gas evolution at these potentials was very intensive, which made measurements at higher potentials rather scattered.
All experiments (same anode, electrolyte and temperature in the whole potential range) were repeated once. If the results of the experiment and the reproduction were close, they were averaged. Otherwise a third experiment was run to get reproducible data. The latter case occurred only for experiments that failed. Experimental failure was due to several problems like cracking of the reference electrode, cell leakage, thermocouple failure, furnace tube cracking, furnace controller break-down and anode breakage.
The geometric surface area of the carbon anode was typically about 7 cm2. The area changed during the experiment due to the electrochemical consumption of carbon. This was accounted for as shown in Appendix 1.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -28- PARTl
5. RESULTS AND DISCUSSION
5.1. GENERAL FEATURES OF THE IMPEDANCE SPECTRA
The recording of the spectra in Figure 5.1 and Figure 5.2 was done in a saturated cryolite-alumina melt at 1000 °C at varying anode potentials as described in Chapter 4. The figures show Nyquist plots of the impedance spectra at several anode potentials (including IR-drop) from an experiment with a pyrolytic graphite anode. Every point at the curves represents the real (Z’) and imaginary (Z”) part of die a.c. impedance of the anode reaction at a specific a.c. frequency. The given anode potentials are the potential drops between the anode and the aluminium reference electrode.
The anodic current densities were in the range 0.005 A/cm2 to 0.20 A/cm2 in Figure 5.1 and Figure 5.2.
iii ig anod e potei tials ----1.5 V
----1.8 V
7 0.10 ----1.9 V
Z VO Figure 5.1. a.c. Impedance spectra at several anode potentials (including IR- drop) measured at pyrolytic graphite.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -29-
a -0.02
. -0.03 1.9 V ---- 2.0 V
Figure 5.2. a.c. impedance spectra at three anode potentials (including IR-drop) measured at pyrolytic graphite.
The sequence of a.c. impedance spectra shown in Figure 5.1 and Figure 5.2 demonstrates how the impedance of the anodic reaction changes with the anode potential (including IR-drop). Figure 5.3 provides better information on how the shape of the plots change in the whole measured potential range.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -30- PARTI
10 kHz I t '
Z.reel, [t*ml ->
100 Hz
Z.re* tomi
Figure 5.3. a.c. impedance spectra at 12 anode potentials (including IR-drop) measured at pyrolytic graphite (continued on next pages).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -31-
Figure 5.3. (Continued) a.c. impedance spectra at 12 anode potentials (including IR-drop) measured at pyrolytic graphite (continued on next pages).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -32- PARTl
10 kHz---
1.7 V
36 kHz
Zxed. lotml
1.8 V
40 kHz
Z.reaf, townl (xlCM)
^“1 kHz
1.9 V
40 kHz
Z.ted. lohml 0
Figure 5.3. (Continued) a.c. impedance spectra at 12 anode potentials (including IR-drop) measured at pyrolytic graphite (continued on next pages).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -33-
2.0 V
381Hz
2. real, lohml 0t1O'-3)
10 kHz
2.1 V
Z.real, fohnil (xlO"-3)
Figure 5.3. (Continued) a.c. impedance spectra at 12 anode potentials (including IR-drop) measured at pyrolytic graphite (continued on next page).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROL YSIS -34- PARTI
10 Hz
2.2 V
36 kHz
Zreel, fehml 6ri0^3)
100 Hz
2.3 V
32 kHz
Zreel, lohml 000^3)
Figure 5.3. (Continued) a.c. impedance spectra at 12 anode potentials (including IR-drop) measured at pyrolytic graphite.
The figure gives qualitative information about the mechanism of the electrode reaction which is under investigation:
At low potentials (1.2 - 1.4 V) the Z'/Z"-curve appears to be influenced by a Warburg diffusion (see Figure 3.2). Other authors have also observed this [18], [19] and they have indeed interpreted it as an effect from the diffusion of dissolved alumina to the anode surface. However, since the current is very small and the cryolite is saturated with respect to alumina, diffusion control of the reaction under investigation is not expected at low potentials. Most of the current running at these potentials is probably mainly due to additional, parasitic faradaic reactions at the electrode. This must be due to impurities present in the melt. The impurities are present in small concentrations, and could thus be the origin of the Warburg impedance seen in the spectra. Another reason for an apparent diffusion control may be the surface roughness of the anode due
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS J. RESULTS AND DISCUSSION -35-
to its porosity. It has been shown by de Levie [32] that some types of porosity can give rise to an apparent Warburg impedance.
At higher anode potentials the complex plane plots are more and more curved. This suggests kinetic control by charge transfer reaction. The curves also develop a loop at low frequencies in the fourth quadrant of the complex plane diagram. Such pseudo- inductive loops are diagnostic to electrochemical reactions with adsorbed intermediates [18], [28]. Thus, the studied reaction is an electrochemical reaction with an electroactive, adsorbed intermediate. As the potential increases, the curves are shifted more and more into the fourth quadrant of the complex plane diagram.
The features commented on here are common to all the investigated anode materials and electrolytes. The main difference is that the spectra measured on pyrolytic graphite are the smoothest. This is probably because pyrolytic graphite has the most ordered structure, and therefore any effects from pores and uneven reactivity of the surface are minimized. The smooth, well defined surface of this material is also likely to have a more constant coverage by gas bubbles than the ordinary graphite and industrial anodes which have coarser surfaces that make it easier for the bubbles to stay attached to the anode.
In the past several other authors have suggested reaction sequences involving either four one-electron steps or two two-electron steps with the formation of intermediate adsorbed species. Most treatments indicate that a two step mechanism gives an adequate description of the process, and this approach is supported by the present impedance measurements, and it will be adopted here (see Chapter 2.). The general reaction mechanism can then be written:
02'(diss.) + xC_ -> CP*. + 2e (5.1)
02(diss.) + Cp_ -> C02(g) + 2e (5.2) where O2 represent one or more oxygen-containing complex ions, and Cx O denote the carbon-oxygen surface compound which is the adsorbed intermediate.
The features of the spectra presented here coincides well with the spectra recorded by Picard et. al. [18], and by Solli [19].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -36- PARTI
5.2. EQUIVALENT CIRCUITS
The "equivalent circuit" approach [29]-[31] commonly used in mathematical analysis of impedance data is, according to Bai and Conway [28], somewhat arbitrary and empirical, as the components of an equivalent circuit are not always identifiable with the properties of individual reaction steps.
As shown in Chapter 3.2, it is possible to analyze impedance spectra with the aid of kinetic equations. Unfortunately, there are certain disadvantages to this approach. For one adsorbed intermediate the theory involves 9 parameters: 4 rate constants (K), KA, K2 , KJ, 2 Tafel slopes (b l , b 2), the electrode potential (E), the double layer capacitance (CJ and the charge (q) required for adsorption of the ad-species to complete coverage. The number of parameters to be fitted is thus fairly large. Furthermore, the analyzed spectrum should not contain any contribution from outer inductance, electrolyte resistance and diffusion impedance, but that is the case for all recorded impedance spectra. If these contributions are significant, the preparation of a "purely kinetic " impedance spectrum is impossible without the use of some type of equivalent circuit. The circuit is necessary for the subtraction of the outer inductance, electrolyte resistance and diffusion impedance from the spectra, and therefore an equivalent circuit that describes our system in the whole potential range is needed.
Figure 5.4 a) shows an equivalent circuit of the a.c. impedance of a two step faradaic electrode reaction with adsorbed intermediate proposed by Armstrong and Henderson [33]. This circuit works well for the spectra recorded at lower potentials where no pseudo-inductive loop exists. Harrington and Conway [34] proposed the equivalent circuit shown in Figure 5.4 b) for the same reaction scheme. This circuit only works for the spectra in the high potential range with the inductive loop.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -37-
Figure 5.4. Equivalent circuits of the a.c. impedance of faradaic reactions with two steps and an adsorbed intermediate: a) low potential range, b) high potential range.
On the basis of the circuits presented in Figure 5.4, Kisza [35] presented the equivalent circuit in Figure 5.5 a). It consists of the double layer capacitance, Cffl, in parallel with the charge transfer resistance of the first reaction step, R, , and the impedance due to the adsorbed intermediate in series. The impedance due to the adsorbed intermediate is described by , Lads, with the charge transfer resistance of the second step, R,, in parallel.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -38- PARTI
a)
c.
b)
Figure 5.5. Equivalent circuit for the anodic reaction on carbon anodes in alumina saturated cryolite melts involving two charge transfer reactions with intermediate adsorption, a) general case proposed by Kisza [34], b) circuit including outer inductance and electrolyte resistance. If electrolyte resistance, Rd, and outer inductance, Lmt, are included in the circuit as shown in Figure 5.5 b), an equivalent circuit that covers the whole potential range of the anodic reaction on carbon anodes in alumina saturated cryolite melts is obtained. Figure 5.6 shows impedance spectra and fit by the circuit in Figure 5.5 b) at several anode potentials. The low potential range (1.2 - 1.4 V) was not fitted because the spectra at these potentials were probably dominated by other faradaic reactions and/or the porosity of the anode material.
When the equivalent circuit is established, the circuit components can be calculated to fit the measured data by Non Linear Least Squares fit (NLLS). The simulations were performed by “Equivalent Circuit”, a computer program designed for this purpose by Boukamp [30]. Figure 5.6 shows the measured impedance spectra with the simulations superimposed, for anode potentials from 1.5 V to 2.4 V. The spectra in the figure were obtained from measurements on pyrolytic graphite. The spectra are displayed as Bode
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -39- plots because this gives a good impression of how well the fits match the measurements. In Bode plots the real and imaginary parts of the impedance are plotted separately versus the frequency of the a.c. signal.
1.5 V 1.6 V
Frequency, [Hz] -> Frequency, [Hz} -> 1.7 V 1.8 V
2.1V 2.2 V
23 V 2.4 V
I
Figure 5.6. Bode plots of impedance spectra (markers) with simulations by NLLS-fit superimposed (lines). Anode potentials from 1.5 V to 2.4 V.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -40- PARTI
Figure 5.6 illustrates that the established equivalent circuit provides impedance spectra that are very similar to the measured spectra at all anode potentials. This indicates that the proposed equivalent circuit describes the actual anode reaction mechanism well.
The fits in Figure 5.6 were made in the frequency range from 50 - 45000 Hz. The lower limit was elected to avoid influence from the bubble release frequency. The evolution and release of gas bubbles at the electrode causes the current to oscillate, and this may interfere with the impedance spectra. The high frequency limit of the used data was decided by noise in the system probably caused by the potendostat.
In the present study the equivalent circuit, shown in Figure 5.5 b), will be used for the following purposes:
The fitting of the recorded impedance spectra with the equivalent circuit, allows for the evaluation of the outer inductance and electrolyte resistance. These elements can be subtracted from the data, and spectra suited for the treatment by Bai and Conways theory are produced. In addition, the electrolyte resistance can be used to determine the anode potential corrected for the ohmic drop in the electrolyte, EMir:
= E - IRel (5.3)
Ecor[ is the sum of Ercv and r\a and can thus be plotted against the logarithm of the current density to yield the Tafel slope of the investigated electrode reaction.
The data from the fit of the equivalent circuit (Cd), Eron ) give a good starting vector to the program which evaluates the rate constants of the two steps and all the other parameters used in the kinetic theory (see below).
Due to severe experimental conditions (1000 °C and gas evolution) the impedance spectra of the anodic reaction on carbon electrodes in alumina saturated cryolite melts are usually rather scattered. These problems can also be solved with the help of the equivalent circuit. It can be used for the smoothing of the recorded impedance spectra, since the application of Bai and Conway ’s theory requires smooth impedance spectra.
5.2.1. THE OUTER INDUCTANCE
The outer inductance or system inductance, Lom, represents the outer inductance of the system caused by the inductance of the cables, connections and apparatus. This inductance was minimized by using shielded leads, still it is clearly seen in Figure 5.3 that the impedance at high frequencies is dominated by the outer inductance. By fitting all the impedance spectra of 4 experiments in alumina saturated cryolite melts in alumina lined cells (see Figure 4.2) the outer inductance of that system was evaluated to be:
Lout = (5.7 ±0.3). 107 H
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -41-
This value is a system constant, and was used in all the fits in this experimental system. Thus, one less variable has to be fitted, and better accuracy is obtained in “noisy experiments ”.
In the experiments in cells without alumina lining, the outer inductance is altered due to the fact that horizontal currents were introduced in the cell. The outer inductance for these systems was found to be:
Lmt = (2.2 ±0.2). 10' H
5.2.2. ELECTROLYTE RESISTANCE
The electrolyte resistance, Re], is the ohmic resistance of the electrolyte between the reference electrode and the anode, including connections.
As mentioned above, it is necessary to know the ohmic resistance between the reference- and the working electrode to be able to calculate the IR-drop (see Eq. (4.1)). As shown in Figure 3.2, the Z’/Z”-plot for the Randles circuit intersects the Z’-axis at Rd. As Figure 5.1 and 5.2 show, the intercept with the Z’-axis is shifted to higher resistance as the anode potential is increased.
This is not surprising since both the gas bubbles formed during the electrolysis and the reduced anode area due to electrolytic carbon consumption, should cause increased resistance. However, in this system, kinetic parameters will inf luence on the impedance at all measurement frequencies, and therefore the “intercept value” is not determined by the electrolyte resistance alone.
The intercepts in Figure 5.1 and 5.2 are also shifted to lower measurement frequency as the potential rises due to the fact that the pseudo-inductive nature of the system is increasing with potential. This means that kinetic effects will influence more on the “intercept values” found at higher potentials.
To get an Rel value as exact as possible, the whole measured impedance spectrum should be fitted to a good model, and from that Rd can be derived. Figure 5.7 shows Tafel plots with the anode potential corrected for the IR-drop in different ways. The current density, i, is calculated from the geometric anode area (see Appendix 1), and the results were obtained on a pyrolytic graphite anode.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -42- PARTI
2.6 T
2.4 -- Measured E Ecorr. with Rel. from equivalent cirquit 2.2 -- Ecorr. with Rel. from intersect value
1.8 -
1.6 -
1.4 -
1.2 --
log ( i )
Figure 5.7. Tafel plots with the anode potential corrected for the IR-drop in different ways.
The figure shows that the Rcl obtained from the fit of the equivalent circuit gives a more correct Erorr value compared to the “intercept values”.
5.2.3. THE IMPEDANCE IN THE LOW POTENTIAL RANGE.
These spectra will be discussed in greater detail later (see Chapter 5.4.2.), but as mentioned in Chapter 5.1, the shape of the impedance spectra at low potentials (1.2 - 1.4 V versus the aluminium reference electrode) are most likely dominated by the fact that most of the current flowing at these potentials is due to additional, parasitic faradaic reactions at the electrode. This must be due to impurities present in the melt, and one of several possible candidates is the ferrous-ferric oxidation-reduction couple.
When the equivalent circuit that describes rough surfaces at low potentials (see Chapter 5.4.), is fitted to the spectra at these low potentials, a good fit is obtained, see Figure 5.15. The parameters of the fit supports the assumption of an impurity dominated current at low potentials because the value of the calculated n in the fit is very close to 0.5 (Warburg). The value of a (Eq. (5.6), below) was - 10 Q s'in , and that allows for a rough estimate of the concentration of the impurity. The calculated value was 0.0025 mol/1 which is reasonable for the concentration of iron impurities in the type of cryolite used (assuming the diffusion coefficient to be D=1 Iff5 cms1).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -43-
5.3. THE ANODIC REACTION ON GRAPHITE
In this section the impedance spectra recorded at regular graphite (SPG) and pyrolytic graphite (PYG) anodes in alumina saturated cryolite melts at 1000 °C are presented. The experimental procedure is described in 4. EXPERIMENTAL. The results have partly been published elsewhere [36]. Although it is basically possible to obtain the current density from the impedance data only, in this thesis the current reading from the meter on the potentiostat were preferred for the evaluation of the Tafel curves.
The analysis of the recorded impedance spectra in the potential range from 1.5 V to 2.2 V versus the aluminium reference electrode is illustrated by the impedance spectrum of a pyrolytic graphite anode at 1.890 V shown in Figure 5.8 a). The impedance spectrum was fitted with the equivalent circuit presented in Figure 5.4 b). The outer inductance and electrolyte resistance derived from the fit, were subtracted from the impedance spectrum using the DATA CRUNCHER in Boukamp ’s program [30]. The resulting impedance spectrum is shown in Figure 5.8 b). This spectrum was then fitted by the equivalent circuit presented in Figure 5.4 a). The fit is shown as the line in the Bode plot in Figure 5.8 c).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -44- PARTI
a) b)
c)
mn iiiiiin tttwuii[uin imiH+
Frequency, [Hz]
Figure 5.8. Impedance spectrum of a PYG anode in an alumina saturated cryolite melt, 1.890 V versus the aluminium electrode at 1000 °C: a) Experimental impedance spectrum (Nyquist plot). b ) The spectrum after subtraction of outer inductance and electrolyte resistance (Nyquist plot). c) The NLLS-fit by the equivalent circuit presented in Fig. 4.4. b) (line) superimposed on the spectrum in b) (Bode plot).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -45-
The parameters of the equivalent circuit are shown in Table 5.2 for graphite, and in Table 5.3 for pyrolytic graphite. The potentials, Earr, are corrected for the potential drop due to the electrolyte resistance. The outer inductance was constant and equal to (5.7 ± 0.3) • 10 7 H, and the current density, i, was calculated as shown in Appendix 1.
Table 5.2. Values of the equivalent circuit parameters from fitting of the impedance spectra recorded on graphite anode (geometric area ~ 5.08 cm2) at several potentials.
Ecorr Cd, Ri R2 i rvi mi [pF cm2] mi mi [A/cm2] 1.476 0.211 28 0.284 0.0731 0.038 1.510 0.211 43 0.125 0.0270 0.087 1.521 0.210 43 0.075 0.0193 0.153 1.558 0.210 41 0.052 0.0156 0.217 1.571 0.213 50 0.040 0.0136 0.255 1.588 0.216 62 0.028 0.0108 0.359 1.605 0.220 61 0.022 0.0094 0.442 1.617 0.224 61 0.018 0.0083 0.523 1.622 0.230 65 0.015 0.0075 0.629 1.656 0.236 62 0.013 0.0067 0.715 1.665 0.245 72 0.010 0.0061 0.825
Table 5.3. Values of the equivalent circuit parameters from fitting of the impedance spectra recorded on pyrolytic graphite anode (geometric area ~ 9.5 cm2) at several potentials.
Ec„rr Re, Ca, Ri «2 i rvi mi rpF cm2] mi mi [A/cm2] 1.455 0.184 45 0.257 0.252 0.014 1.540 0.181 25 0.153 0.058 0.039 1.586 0.178 14 0.105 0.036 0.066 1.622 0.178 14 0.067 0.027 0.103 1.637 0.181 19 0.044 0.022 0.145 1.657 0.180 19 0.036 0.017 0.192 1.687 0.182 19 0.031 0.014 0.238 1.697 0.186 21 0.025 0.012 0.286 1.735 0.184 17 0.027 0.011 0.328 1.734 0.195 24 0.019 0.009 0.369 1.757 0.198 24 0.018 0.009 0.408
The data presented in Tables 5.1 and 5.2 were used for Tafel plots, and are shown for two experiments in Figure 5.9.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -46- PARTI
2.000
1.750
1.000
log (i)
Figure 5.9. Tafel plots for pyrolytic graphite (PYG) and ordinary graphite(SPG) anodes.
Linear regression of the data in Figure 5.9 gave these equations:
SPG: Erorr = 1.66 + 0.14 log i (R2 == 0.995) (5.4)
PYG: E_ = 1.80 + 0.18 log i (R2:= 0.954) (5-5)
The curves were reproduced (SPG x 2, PYG x 4), and the discrepancy between measurements was less than 0.04 for both Tafel coefficients. The Tafel slopes is higher than the slopes found by Solli [19], but they are in the same region as the values found by several other workers (see Table 2.1).
Eq. (5.4) and (5.5) show that the ordinary graphite was more reactive, i.e. gave lower overpotential, than the pyrolytic graphite at all potentials. This was expected because the pyrolytic graphite has a well ordered structure with few active sites compared to the fine grained ordinary graphite.
If the cell reaction is given by Eq. (1.1), the reversible potential of the anode reaction in saturated cryolite melts at 1000 °C is 1.16 V [5], Thus, the exchange current densities of the two anodes, io , can be calculated from Eq. (3.8):
SPG: io = 3-10" A/cm2
PYG: io = 3 • 10" A/cm2
These values are much higher than the values presented by Solli [19], but they are close to the values found by other authors (see Table 2.1).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -47-
If the first step in the proposed mechanism (Eq. (5.1) and (5.2)) is the rate determining step (rds), the theoretical value of the Tafel slope would be 0.252 V/dec. (assuming the symmetry factor, /3, to be 0.5 in Eq. (3.13), gives a= 1). On the other hand, if the second step is assumed to be the rds (Eq. (3.13), gives aa= 3), the value of the Tafel slope should be 0.084 V/decade. Only one Tafel slope is observed in the studied potential range as seen in Figure 5.9, its value being intermediate between the two limits calculated above.
The smoothed impedance spectra (see Figure 5.8 c)) was fitted by the kinetic theory using a simple NLLS-fit FORTRAN program [37]. Figure 5.10 shows the impedance spectrum from Figure 5.8 c) (line), fitted with Bai and Conway's theory (markers).
0 kHz
■ 0.01 100 kHz
10 Hz
100 Hz
Figure 5.10. Impedance spectrum from Fig. 4.7 b) (line), fitted with Bai and Conway's theory (markers).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -48- PARTI
The fits yielded the values of two Tafel slopes (b,, b 2), the forward reaction rate constants (Kp K^) of the two reaction steps (see Equations (3.33) and (3.34)), the charge (q) needed for coverage of the electrode surface by a monolayer, the electrode coverage (6) and the double layer capacitance (Cdl).
The two Tafel slopes, b, and b 2, for the SPG and PYG anodes obtained from the fit of the smoothed impedance spectra to the kinetic equations were:
SPG: b, = 0.13 V/decade, b 2 = 0.21 V/decade, PYG: b, = 0.23 V/decade, b 2 = 0.39 V/decade.
The b,-values are in fair agreement with the value from the equivalent circuit approach (Eq. (5.4) and (5.5)). The Tafel slopes were calculated by comparing Equation (3.33) with the exponential fits from the values of the rate constants in Figure 5.13 and Figure 5.14 (see below). The scatter of the fitted points show that these values are interesting only as a check for the approximate magnitude of the data.
The other parameters from the fits are potential dependent, and they are therefore presented in graphs. Figure 5.11 shows the potential dependence of the double layer capacitance for the two types of anodes studied.
k 40.0
Figure 5.11. Double layer capacitance for the SPG and PYG anodes vs. potential.
The double layer capacitance of SPG was around 60 pF/cm 2 which is in fair agreement with the previous measurements by Thonstad [38] and Picard [18]. It is also well recognized that the value of the double layer capacitance is a good measure of the electroactive electrode area. As expected, the double layer capacitance of the almost monocrystalline PYG surface is much lower, around 20 pF/cm 2.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -49-
The values of q, which is the charge required for adsorption of the intermediate species to complete coverage, was of the order of 5-10'3 C/cm2. This is reasonable taking into account that geometric area was used for the calculation of the electrode area of these anodes [28].
Figure 5.12 presents the potential dependence of the electrode surface coverage by the adsorbed intermediates for the two types of anodes studied. The coverage at a given potential depends mainly upon the value of the forward effective rate constant of the second step, K,, which for both anodes is greater than the rate constant of the first step (see Eq. (3.11)). The surface coverage of SPG is greater than of PYG, but it remains small in both cases -10%.
® o.io
Figure 5.12. Surface coverage of SPG and PYG anodes vs. potential.
The rate of the anodic reaction in terms of the kinetic theory, is best presented by the values of the potential dependent, effective forward rate constants, Kt and Kj, which include the concentration terms. Their values as calculated by Eq. (3.33), are presented as a function of potential in Figure 5.13 and Figure 5.14. The solid line represents the NLLS-Fit by the exponential Eq. (3.33).
It is seen from the figures that in the investigated potential range, both steps are faster at the SPG anode, which was expected from the fact that the Tafel curves showed that the ordinary graphite was more reactive than the pyrolytic graphite. It is also seen that the second step is faster than the first one at both types of anodes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -50- PARTI
5.00E-05 4.50E-05 4.00E-05 a SPG 3.50E-05 • PYG
2.50E-05 2.00E-05 1.50E-05 1.00E-05 5.00E-06
ECorr. [V]
Figure 5.13. The forward rate constant for the first step, K,, at SPG and PYG anodes vs. potential (K, equations from fitting the data to Equation (3.33)).
8.00E-04 a SPG • PYG 6.00E-04
5.00E-04
4.00E-04
3.00E-04
2.00E-04
1.00E-04
O.OOEfOO
Ecoit . [V]
Figure 5.14. The forward rate constant for the second step, K2, at SPG and PYG anodes vs. potential (K2 equations from fitting the data to Equation (3.33)).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -51-
5.4. SURFACE ROUGHNESS
In this section the impedance spectra recorded at graphite anodes, pyrolytic graphite anodes and two types of industrial anode materials in alumina saturated cryolite melts are presented. The experimental procedure is described in 4. EXPERIMENTAL.
5.4.1. SURFACE ROUGHNESS BY EIS
Carbon anode materials are more or less porous, and the porosity leads to a rough electrode surface. The dimensions of the pores of the industrial anode materials are large enough to describe the materials as being macroporous. The a.c. impedance of a simple, faradaic electrode reaction on a rough electrode surface can be described by the equivalent circuit presented in Figure 5.15 [39]. The circuit consists of the outer inductance, Lml, and electrolyte resistance, Rd, in series with a so-called constant phase element, CPE, which is in parallel with the charge transfer resistance, Ra.
L» R, -W^Wv- R„ ------WV------1
Figure 5.15. Equivalent circuit of the a.c. impedance of a fradaic electrode reaction taking place on a rough electrode surface.
The presence of a CPE is known from several systems [40]-[43]. The impedance of the CPE is defined according to the equation [30]:
1 ^CPE (5.6) )" where co is the angular frequency of the a.c. signal, Ya is the adjustable admittance parameter related to the double layer capacitance, j = V-T and n is the frequency dispersion parameter (0 < n < 1). The CPE is a very general element. In fact, for n = 0
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -52- PARTI it represents a resistance, for n = 1 it represents a capacitance, for n = 0.5 it represents a Warburg and for n= -1 it represents an inductance. When the CPE acts as a Warburg impedance, the Warburg coefficient, a, is determined by [44]:
RT 1 (5.7) n2F2A-JlD C; where Cg°° is the bulk concentration of the diffusing species.
When the inhomogeneity of the electrode surface is mainly of geometric origin, the double layer capacitance can be calculated from the components in Figure 5.14 (Eq. (5.6)) by the following equation [43]:
1 1 R, + R,. (5.8)
The double layer capacitance evaluated from this equation, is related to the true, electroactive surface. Above, the double layer capacitance of the smooth surface pyrolytic graphite was found to be 20 pF/cm 2 at high potentials. If this value is accepted as the double layer capacitance of a carbon anode with no effect from roughness (r = 1), the roughness of any other carbon electrode surface is given by:
r = (5.9) 20-HT6 where Cdl is the true double layer capacitance found by Equation (5.8). Used in this sense, the term roughness stands for the ratio between true electroactive surface area and the geometric surface area.
5.4.2. SURFACE ROUGHNESS BY BET
As described above, the roughness of an electrode surface can be determined from electroche mical impedance measurements. To compare the values obtained in this way, the BET surface of two equally shaped cylinders of the graphite (SPG) and the pyrolytic graphite (PYG) were measured:
BET of SPG = 0.46 m2/g
BET of PYG = 0.12 m2/g
These measurements were not very accurate, but they gave an indication of the surface area, and the relative difference in surface area between the two materials. The industrial anodes could not be machined to small enough dimensions to be measured
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -53- in the BET apparatus. From the BET data and the geometry of the samples, the surface roughness, r [cm2BET/cm2geoJ, could be estimated:
SPG: r ~ 600
PYG: r ~ 200
These values express the ratio between the surface area of the open porosity accessible for the nitrogen gas and the geometric, apparent surface area. The surface accessible for the nitrogen gas is larger than the surface accessible to a cryolite melt because the electrolyte is unable to penetrate as small pores as the nitrogen gas due to wetting properties.
5.4.3. ELECTRODE SURFACE ROUGHNESS FITS AT LOW POTENTIALS
As described above (4.2.3.), the impedance spectra at low potentials (1.2 - 1.4 V) are most likely dominated by the fact that most of the current flowing at these potentials is mainly due to additional, parallel faradaic reactions at the electrode. Whether the current is due to one or more faradaic reactions on the electrode surface will not affect the values obtained from the equivalent circuit in Figure 5.15.
When the equivalent circuit that describes impedance spectra for a rough electrode surface (Figure 5.15), is fitted to the spectra at these low potentials, a good fit is obtained. Figure 5.16 shows the fit (line) to an impedance spectrum (markers) recorded at 1.3 V versus the aluminium reference electrode for the industrial anode material.
- ANK04 100rrt- +DateZ-real
10rrt- 310m a
3 1m
1k Frequency, [Hz]
Figure 5.16. The fit (line) to an impedance spectrum (markers) recorded at 1.3 V versus the aluminium reference electrode measured at industrial anode material.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -54- PARTI
5.4.4. ELECTRODE SURFACE ROUGHNESS FITS AT HIGH POTENTIALS
The application of the equivalent circuit in Figure 5.15 is limited to the low potential range where the proper anodic reaction contributes little to the measured impedance spectra. At higher potentials (1.4 V < E < 2.1 V versus the aluminium reference electrode) the equivalent circuit presented in Figure 5.17 must be used. The only change compared to the circuit in Figure 4.5 b) is that the double layer capacitance has been replaced by a CPE to account for the effect of surface roughness.
Figure 5.17. Equivalent circuit of the a.c. impedance of a faradaic electrode reaction on a rough electrode surface with a two step reaction with adsorbed intermediate.
Figure 5.18 shows the fit (line) to an impedance spectrum (markers) recorded at 1.5 V versus the aluminium reference electrode.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -55-
ANK14 100m 4-Data Z-reai XData Z-imag
10rrr
100u 100u~
Frequency, [Hz] -->
Figure 5.18. The fit (line) to an impedance spectrum (markers) recorded at 1.5 V versus the aluminium reference electrode measured at industrial anode material.
Equation (5.8) is still applicable for the evaluation of the true Cm, but Rtt has to be replaced by (R, + R%).
5.4.5. ELECTRODE SURFACE ROUGHNESS
Table 5.4 shows the surface roughness calculated from the impedance spectra at different anode potentials for four anode materials.
At 1.2 V, the roughness of SPG is approximately three times the roughness of PYG. This is in accordance with the BET measurements referred above. The high roughness of the industrial anode materials at low potentials indicates that the melt penetrates pores and crevices in the electrode surface, but as the potential increases the pores are filled with gas, and the roughness of the materials is drastically reduced to quite low values. This indicates that even at the porous industrial anodes, only the electrode surface is wetted by the electrolyte.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -56- PARTl
Table 5.4. Surface roughness calculated from the impedance spectra at different anode potentials for four anode materials.
Pyrolytic Graphite Industrial anode Industrial anode graphite (PYG) (SPG) Type A TypeB Ew r Em„. r Eox, r E__ r [V] [V] [V] [V] 1.200 10.9 1.195 28.3 1.196 2490 1.191 881 1.310 6.0 1.297 16.6 1.279 1610 1.270 514 1.390 3.3 1.398 6.6 1.346 1310 1.344 319 1.455 2.4 1.476 1.8 1.398 423 1.394 145 1.540 1.4 1.510 2.1 1.440 111 1.431 53.9 1.586 0.8 1.521 2.3 1.473 32.9 1.471 17.0 1.622 0.9 1.558 2.4 1.504 15.1 1.487 9.7 1.637 0.9 1.571 2.4 1.523 6.5 1.507 6.7 1.657 1.0 1.588 3.0 1.550 6.7 1.531 4.0 1.687 0.9 1.605 2.9 1.579 5.1 1.551 3.5 1.697 1.1 1.617 2.9 1.603 3.8 1.576 2.2
1.735 1.0 1.622 2.9 - - 1.604 1.6 1.734 1.0 1.656 2.5 - - 1.634 1.5
5.4.6. TAFEL SLOPES WITH TRUE CURRENT DENSITY
When the surface roughness was obtained, the “true” current density could be calculated from the electroactive area of the electrode, At, which is given by:
A, = rAs (5.10) where Ag is the apparent, geometric area which is the area used as the electrode area in the Tafel plots presented previously (see Appendix 1). Table 5.5 presents the Tafel equations based on the apparent, geometric area, and the Tafel equations based on the electroactive area.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -57-
Table 5.5. Tafel equations measured on four different anode materials based on apparent, geometric area and on electroactive area. IS a>
Geometric area Electroactive area -C
Pyrolytic graphite 0.01-0.9 E^ = 1.80+ 0.18 log i E^ = 1.80 + 0.18 log i
Graphite 0.01-0.8 Ew. = 166 + 0.14 log i Ero„ = 1.73 + 0.16 log i Ind. anode, type A 0.07-0.7 Emrr = 1.68 + 0.26 log i E = 1 65+ 0.08 log i
Ind. anode, type B 0.06-0.5 Ew. = 1-69 + 0.25 log i E„ = 1.65 + 0.08 log i
At the PYG and SPG anodes the roughness factor is fairly small, and therefore no significant difference is observed between the Tafel equations based on the two types of electrode areas. At the industrial anode materials, which are more porous, the situation is totally different. When the surface roughness is accounted for, the Tafel slopes decrease from approximately 0.25 V/dec. to 0.08 V/dec. A Tafel slope of 0.252 V/dec. corresponds to the theoretical value of the Tafel slope when the first reaction step is rate determining, and a Tafel slope of 0.084 corresponds to the theoretical value of the Tafel slope when the second reaction step is rate determining (see Chapter 5.3. and Chapter 3.1.1.). Thus, surface roughness should be known when Tafel slopes are calculated in this system.
From this we see that one of the reasons of the variations in the Tafel constants in the literature may be variations in surface roughness of the materials which have been investigated. It also tells us that conditioning of the anode surface prior to the experiment is important to get reproducible results.
The Tafel slopes from the industrial anodes based on electroactive area is corresponding very well with the slopes found by Soil! [19], and the Tafel equations of the industrial anode materials also show that they were more reactive, i.e. gave lower overpotential, than the graphites at all potentials. This was expected because the industrial anode materials have a much more amorphous structure than the graphites.
If the cell reaction is given by Eq. (1.1), the reversible potential of the anode reaction in saturated cryolite melts at 1000 °C is 1.16 V [5]. Thus, the exchange current densities of the two industrial anodes, io , can be calculated from Eq. (3.8):
Ind. anode, type A: io = 7 • 107 A/cm2
Ind. anode, type B: io = 7 • 107 A/cm2
These values are much lower than the ones found for graphites (see Chapter 5.3.). This is due to the change in tafel slopes, and reflects the apparent change from the first to the second reaction step as the rate determining step.
The derived (,-values also correspond well with the values presented by Solli [19], but they are much lower than values found by other authors (see Table 2.1).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -58- PARTI
Previously it was shown that the first reaction step is rate determining at graphite, while the second reaction step seems to be rate determining at industrial anodes. The electrolyte was the same in all the experiments. Thus, the difference has to be due to the anode materials. The porosity and the homogeneity are two main differences between the graphite materials and the industrial anodes. The graphites are partly crystalline, macroscopically homogenous materials. The PYG was very dense (2.22 g/cm3), and the SPG had fine grain size and small pores. The industrial anode materials are amorphous, composites of calcined petrol coke and baked binder pitch coke. This makes the materials inhomogenous at a macroscopic level. The binder pitch coke is preferably consumed by the anode reaction during the preelectrolysis, leaving the anode surface very rough as the coke grains protrude from the surface (see Figure 1.4). The industrial materials are also quite porous. Thus, the difference in porosity and homogeneity have the same consequence; a very rough electrode surface of the industrial anode materials and a fairly smooth surface of the graphites.
Due to their disordered structure, the industrial anode materials are much more reactive than the graphites. This may favor the first step more than the second step, and the second reaction step may become rate determining because it is slower relative to the first step. It is also possible that the C02 pressure may be higher in the pores and crevices than it may become at the smooth graphite surface, and thus shifting the second reaction step to the left.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -59-
5.5. EFFECT OF BATH COMPOSITION ON THE ANODE REACTION
This section is dealing with the impedance spectra measured on graphite anodes in cryolite melts at 1000 °C with different alumina concentrations and added fluorides. Table 5.6 shows the investigated bath compositions. The experimental procedure is described in 4. EXPERIMENTAL.
Table 5.6. Bath compositions. Pyrolytic graphite anode was used in electrolytes 1-3, and ordinary graphite was used in electrolytes 4-8.
ai 2o3 AIF, CaF 2 Electrolyte nr. [wt%] [wt%] [wt%]
1 saturated1 2 5 3 3 4 saturated1 5 5 saturated1 11 6 saturated1 5 7 saturated1 10
1. The alumina saturation concentration was 11.9 wt% in pure cryolite at 1000 °C, 11.5 wt% in cryolite with 5 wt% A1F3 added, 10.8 wt% in cryolite with 10 wt% A1F, added and 10.9 wt% in cryolite with 5 wt% CaF, added (calculated by [45] based on [46]-[49]; 10 wt% CaF2 in cryolite was outside the limit of the bath compositions with known alumina solubility).
The results were worked out in the same manner as the previous section; the equivalent circuit approach was used to obtain the electrolyte resistance from the impedance spectra, and the values of the electrolyte resistance were used to determine E^ for the Tafel plots.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -60- PARTI
5.5.1. EFFECT OF CALCIUM FLUORIDE
Figure 5.19 presents the linear regression Tafel curves from the measurements with different CaF2 concentrations.
2.000
CaH 10 wt% CaF2 5 wt% Cryolite
-1.50
Figure 5.19. Tafel curves from the measurements at ordinary graphite anodes with different CaF 2 concentrations.
The addition of CaF2 caused an increase in Tafel slope from 0.14 V/dec. to 0.25 V/dec. The same effect was observed in the melts with 5 wt% and 10 wt% CaF2. The sum of the reversible potential and the Tafel constant (a’ in Eq. (3.10b)) was also shifted from 1.67 V to 1.85 V on the addition of CaF2.
Fellner et al. [85] found that CaF2 decreases the wetting of carbon by the electrolyte, which should give a lowered wetted surface area. Thus, the real current density would be higher than the apparent current density, yielding a higher apparent tafel slope. This is in accordance with the above findings.
Solli [19] did not observe any change in the Tafel slope due to CaF2, and she reported reduced Tafel constants as a function of CaF2-content, but it is not straight forward to compare these results because Solli used a totally different cell arrangement, and the intersect value to estimate the LR-drop (see Chapter 5.2.2.).
Welch and Richards [86] did not observe any effect of CaF2 on anodic overvoltage.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -61-
5.5.2. EFFECT OF ALUMINIUM FLUORIDE
Figure 5.20 presents the linear regression Tafel curves from the measurements with different A1F3 concentrations.
2.000
1.900
1.800
> 1.700 i M 1.600
1.500 ----- A®, 11 wt% 5 9 fcwt 1.400
1.300 -1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 logi
Figure 5.20. Tafel curves from the measurements at ordinary graphite anodes with different A1F3 concentrations.
The addition of A1F3 caused an increase in Tafel slope from 0.14 V/dec. to 0.22 V/dec. The same effect was observed in the melts with 5 wt% and 11 wt% A1F3. The sum of the reversible potential and the Tafel constant (a’ in Eq. (3.10)) was also shifted from 1.67 V to 1.83 V on the addition of A1F3.
Solli [19] did not observe any change in the Tafel slope due to A1F3, but she reported increased Tafel constants as a function of AlF3-content.
Welch and Richards [86] did not observe any effect of A1F3 either on anodic overvoltage.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -62- PARTI
5.5.3. EFFECT OF ALUMINA
To be able to control the alumina concentration in the melt, the experimental cell had to be modified. The alumina lining was removed, and the reference electrode container had to be made from boron nitride instead of alumina. This lead to the introduction of more vertical currents in the experimental cell because the crucible wall acted as a cathode. The change in vertical current flow slightly altered the outer inductance. All other results were constant (see Appendix 2).
Figure 5.21 presents the linear regression Tafel curves from the measurements with different A1203 concentrations.
2.000
1.900
1.800
1.700
M 1.600
1.500 ---- 3 wt% -----5 wt% 1.400 —Saturated
Figure 5.21. Tafel curves from the measurements at pyrolytic graphite anodes with different A1203 concentrations.
The reduction of alumina content caused an increase in Tafel slope from 0.18 V/dec. to 0.25 V/dec. The same effect was observed in the melts with 3 wt% and 5 wt% A1203. The sum of the reversible potential and the Tafel constant (a’ in Eq. (3.10b)) was also slightly shifted from 1.79 V to 1.84 V on the change in alumina content which was expected since the reversible potential increases when the alumina concentration is reduced.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -63-
5.5.4. SUMMARY
The effects of varying electrolyte composition on the anodic reaction need to be studied in greater detail before any clear conclusions can be drawn.
The obtained Tafel curves in the melts with compositions other than pure cryolite saturated with alumina were very similar. Their average Tafel slope was (0.24 ± 0.02) V/dec., and their average a’ (sum of the reversible potential and the Tafel constant a) was (1.84 ± 0.01) V. This indicates that all the investigated changes in electrolyte composition have similar effects on the anode reaction. Since the same results were obtained on two different carbon anodes, these effects should be caused by changes in the electrolyte structure.
The Tafel slopes close to 0.25 V/dec. indicate that the first reaction step in the proposed mechanism (Eq. (5.1) and (5.2)) is rate determining.
5.6. SUGGESTIONS ON THE REACTION MECHANISM ON CARBON ANODES IN ALUMINIUM ELECTROLYSIS
In Chapter 5.1. it was concluded that the reaction was a two step two electron transfer reaction. Above, it is seen that this reaction has a preceding chemical reaction step that seems to affect the first electrochemical step more than the second. This may indicate that two different oxide complexes is discharged in the two electrochemical steps. Thus, the anodic reaction scheme may generally be described by:
y<=># (5.11) h
(5.12) K~i
t o+2f- (5.13)
Where R and R ’ may be different oxide containing complexes.
Sterten [50] claimed, based on structural model calculations, that the oxide containing complexes in cryolite melts could be described by:
F3A1 - O - A1F32- + 2xF F3+x A1 - O - A1F^X X+1) " (5.14)
F2A1 < q > A1F22- + 2xF" <=> F2+x A1 < ° > A1F2£+1)- (5.15)
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -64- PARTI
The anode reaction consumes oxide anions from the melt, and these have to be extracted from the complexes above. With respect to steric effects, it is most likely that the complexes with the least F-ligands react with the carbon surface. Thus, reaction (5.14) and (5.15) (from right to left) may be described as preceding chemical reaction steps to the electrochemical anode reaction.
In Chapter 5.3. the first reaction step in the proposed electrochemical reaction mechanism (Eq. (5.1) and (5.2)) was found to be rate determining. This conclusion was not obvious. From a mechanistic point of view, the rate of the second step might have been expected to be slower than the first step. This may indicate that different oxide containing complexes take part in the two electrochemical steps, and that a preceding chemical step should be included in the mechanism of this electrode reaction. This finding is supported by the higher value of the second Tafel slope obtained by the fit. It is known from other electrode systems, that a preceding chemical reaction, possibly influenced by the high electric field in the electric double layer, causes an apparent increase of the Tafel slope of the second step [51].
Addition of CaF2 to the melt is, according to Gijotheim and Kvande [1], expected to give free Ca2+ ions. This would add F ions to the melt and thus shift the proposed preceding chemical steps to the right (Eq. (5.14) and (5.15)).
Sterten [50] reports that A1F3 forms a number of complex anions, but all contain additional F ions, and the addition of A1F3 should thus shift the proposed preceding chemical steps to the left (Eq. (5.14) and (5.15)).
Since the addition of CaF2 and A1F3 probably have opposite effects on the preceding chemical reaction step, the additions may affect the reaction in different ways. It was mentioned above that the effect of CaF, may be due to the influence of CaF2 on the wetting properties.
A1F3 (as A1F4 in the melt) may take part in the first electrochemical reaction step as a reaction product (see Eq. (5.18) in the suggested mechanism below), and the addition of A1F3 would thus slow down this reaction step.
Dissolution of alumina to form the complexes in Equations (5.14) and (5.15), consumes ionic species formed from A1F3. This means that the more dissolved alumina, the less “free” A1F3 is left in the melt Addition of alumina would thus shift a first electrochemical reaction step with A1F3 as a reaction product (see Eq. (5.18) in the suggested mechanism below) to the right and the reduction of alumina content would thus slow this reaction step down.
At industrial anodes the second step was found to be rate determining. The industrial anode material is much more reactive than the graphites, and the two electrochemical steps may therefore not be depending on the preceding chemical reaction in the same manner as the graphites.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 5. RESULTS AND DISCUSSION -65-
Thus, from the above findings the anodic reaction on carbon anodes in alumina containing cryolite melts at 1000 °C may be described by two two-electron charge transfer steps with a preceding chemical reaction step and an adsorbed intermediate:
1. Preceding chemical step which reduces the number of F-ligands around the oxide complexes. This facilitates the complexes to react with the carbon surface:
F^Al - O - A1F2+(XX+1)~ <=> F3A1 - O-A1F2" + 2xF~ (5.16) and/or F2+x Al AlF2r>- » F2 A1 < ° > A1F2- + 2xF" (5.17)
2. The first electrochemical step. A two electron transfer step which forms an adsorbed intermediate.
F3AI-O-Allf + xC surface +F-« Cx O^ + 2A1F; + 2e- (5.18)
3. The second electrochemical step. A two electron transfer step which forms C02.
F2A1 A1F2" + Cx O^ + 2F" <=> C02 + F3A1 - O- A1F2" + 2e" (5.19)
This reaction mechanism is a tentative scheme. Other mechanisms may fit the current findings as well. It is possible that the same oxygen containing complexes take part in both electrochemical reaction steps, but that the rate of the steps is dominated by different species.
5.7. FUTURE WORK
There are several possibilities for continuation of this work. A model for evaluation of the kinetic parameters of the preceding chemical step could be developed both as an equivalent circuit and in the form of kinetic equations. This might give a powerful tool to investigate the true effect of different additives to the electrolyte, and it could also give some indications about the structure of the electrolyte.
It would also be interesting to investigate additions to the electrolyte in combinations, especially baths with close to “industrial ” composition. Variation in the cryolite ratio would perhaps give some more answers to the different variation of the electrochemical steps because the stability of the oxide containing complexes is a function of acidity.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -66- PARTI
6. CONCLUSIONS
The anodic reaction on carbon anodes in the aluminium electrolysis was studied by electrochemical impedance spectroscopy (EIS) in a laboratory cell at 1000 °C under argon atmosphere. The reaction was studied in the potential range from 1.2 - 2.4 V versus the aluminium reference electrode which gave current densities up to about 1 A/cm2.
The total anode reaction can be described as a two-step two-electron charge transfer reaction with an adsorbed intermediate. A preceding chemical reaction step seems to be present.
Nyquist plots of the measured impedance spectra showed the same features at all the investigated anode materials. At low potentials (1.2 - 1.4 V versus the aluminium reference electrode) the Z’/Z”-curve seems to be influenced by a Warburg diffusion, but this is probably mainly due to parasitic, parallel faradaic reactions at the electrode and/or the surface roughness of the anode. At higher anode potentials the complex plane plots are more and more curved. This suggests kinetic control by charge transfer reaction. At low frequencies the curves also develop a loop in the fourth quadrant of the complex plane diagram. Such pseudo-inductive loops are diagnostic to electrochemical reactions with adsorbed intermediates.
Electrolyte resistance obtained from the fit of the equivalent circuit gave good values for the evaluation of the IR-drop in the electrolyte.
At pyrolytic graphite and ordinary graphite in alumina saturated cryolite melts, the recorded impedance spectra were analyzed by an equivalent circuit approach and by simulations based on kinetic equations. This yielded the double layer capacitance, electrode coverage, effective rate constants, and the charge needed to cover the electrode by a monolayer. The anodic reaction was faster on ordinary graphite anodes than on pyrolytic graphite anodes, and the first electrochemical step was rate determining on both materials.
Measurements of the surface roughness by EIS gave the same ratio between the roughness of graphite and pyrolytic graphite as BET measurements did. The surface roughness of all the investigated carbon materials decreased with increasing potential reflecting changes in the wetted area. At 1.2 V versus the aluminium reference electrode, the roughness of graphite is approximately three times the roughness of pyrolytic graphite.
The high roughness of the industrial anode materials at low potentials indicates that the melt penetrates pores, but as the potential increases the pores are filled with gas, and the roughness of the materials is drastically reduced.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 6. CONCLUSIONS -67-
At the PYG and SPG anodes the roughness factor is fairly small, and therefore no significant difference is observed between the Tafel equations based on apparent, geometric surface area and area corrected for surface roughness. For the industrial anode materials, which are more porous, the Tafel slopes (b) decrease from approximately 0.25 V/dec. to 0.08 V/dec. when the surface roughness is accounted for. A Tafel slope of 0.252 V/dec. corresponds to the theoretical value when the first reaction step is rate determining, and a Tafel slope of 0.084 corresponds to the theoretical value when the second reaction step is rate determining. Thus, surface roughness should be known when Tafel slopes are calculated in this system.
Addition of CaF2 to the bath caused an increase in Tafel slope from 0.14 V/dec. to 0.25 V/dec. at ordinary graphite. The same effect was observed in the melts with 5 wt% and 10 wt% CaF2. The sum of the reversible potential and the Tafel constant (a) was also shifted from 1.67 V to 1.85 V on the addition of CaF2.
Addition of A1F3 caused an increase in Tafel slope from 0.14 V/dec. to 0.22 V/dec. at ordinary graphite. The same effect was observed in the melts with 5 wt% and 11 wt% A1F3. The sum of the reversible potential and the Tafel constant was also shifted from 1.67 V to 1.83 V on the addition of A1F3.
Reduction of the alumina content caused an increase in Tafel slope from 0.18 V/dec. to 0.25 V/dec. at pyrolytic graphite. The same effect was observed in the melts with 3 wt% and 5 wt% A1203. The sum of the reversible potential and the Tafel constant was also slightly shifted from 1.79 V to 1.84 V on the change in alumina content which was expected since the reversible potential increases when the alumina concentration is reduced.
The obtained Tafel curves in the melts with compositions other then pure cryolite saturated with alumina were very similar. Their average Tafel slope was (0.24 ± 0.02) V/dec., and their average Tafel constant was (1.84 ± 0.01) V.
The following general anodic reaction scheme is in accordance with the above findings:
*/ h
a<=>0_ + 2e~
Kz R' + O, ,<=> 0 + 2e~ K-z
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -68- PARTl
Based on the reaction above the following reaction mechanism of the anodic reaction on carbon in cryolite melts is suggested:
1. Preceding chemical step which reduces the number of F-ligands around the oxide complexes. This facilitates the complexes to react with the carbon surface:
F3+x A1 -O- A1E^+1)- o F3A1-0-A1F2" + 2xF" and/or F2+,A1 < ° > AlF2
2. The first electrochemical step. A two electron transfer step which forms an adsorbed intermediate.
F3A1 — O — Allf + xC sulfacc +F“ <=> Cx O^ + 2A1F- + 2e'
3. The second electrochemical step. A two electron transfer step which forms C02.
F2A1 < g > A1F2- + + 2F" o C02 + F3A1 - O - A1F2" + 2e"
This reaction mechanism is a tentative scheme. Other mechanisms may fit the current findings as well. It is possible that the same oxygen containing complexes take part in both electrochemical reaction steps, but that the rate of the steps is dominated by different species.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS Part II
A Study of the Am and Carboxy Reactivity of Carbon A nodes in the A luminium Electrolysis . REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 7. INTRODUCTION -71-
7. INTRODUCTION
In this second part of the thesis, the air and carboxy reactivity of carbon anodes in the aluminium electrolysis are investigated. The study focuses on the effects of inorganic impurities on the reactivity of carbon anodes and cokes. Special attention was given to sulphur, because its effect on the carbon gasification is not very well understood. Sulphur is always present in anodes, and it is expected that the sulphur content of available anode cokes will increase in the future. It has also been suggested that sulphur poisons catalyzing impurities. Other impurities that were investigated were iron, nickel and vanadium, which are common impurities in anodes which have been reported to catalyze carbon gasification.
One of the factors which leads to excess carbon consumption (see Figure 1.3) is the oxidation of the anodes by air and C02 under evolution of C02 and CO. These reactions are catalyzed by several different inorganic impurities present in the carbon anode materials.
Houston and 0ye [59] give in a literature review references to Si, Fe, V, Ni, Na, S, Ca, Pb, Cu, Zn, Cr, Ti and A1 as catalysts to the reaction between carbon and 02. Sulphur has also been reported as being inactive to this reaction, and A1F3, B and P have been found to be inhibitors. For the carboxy reaction, Fe, V, Ni, Na, Ca, Pb, Cu and A1 are reported as catalysts, while S, A1F3, B and P are inhibitors. Sulphur has also been found to be inactive to this reaction.
Walker et al. [60] examined iron as a catalyst to the carbon-C0 2 reaction. Iron mixed with pure graphite was reacted, and the iron phases were determined indirectly by measuring magnetic susceptibility. Fe-particles containing some Fe095 O (wiistite) were assumed to be catalytically active, while Fe3Q4 (magnetite) was assumed to be catalytically inactive.
Sprite et al. [61] reported that the C02 reactivity of anode materials is reduced by increasing the sulphur content when the S content is between 1 and 3 %. The air reactivity shows a minimum in the same sulphur concentration range.
Bartholomew et al. [62] states that sulphur acts as catalyst poison to iron based catalysts because it forms stable surface sulfides which inhibit adsorption to the surface, and because it can change the electron configuration of the surface. Thus, sulphur poisoning of a catalyst can lead to changes in the selectivity and rate of the catalyzed reaction.
Vanadium is known as one of the most potent catalysts to airbum. Its effect on carboxy reactivity is not that well recognized, and it is generally viewed as a moderate or weak catalyst to the so-called Boudouard reaction [59].
Nickel content in anode cokes often correlate with the vanadium content, and it has therefore been difficult to isolate the effect of nickel on coke gasification. Still several authors have reported nickel as a catalyst to both airbum and the carboxy reaction [59], [63].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -72- PARTll
8. THEORY
8.1. CARBON-GAS REACTIONS
8.1.1. GENERAL
It has proven difficult to obtain a complete understanding of the carbon-gas reactions on carbon anodes. This has several reasons which are discussed below.
The reactions are heterogeneous surface reactions on a surface which is continuously consumed and thereby changed. It is thus hard to obtain a well defined system in which the reactions can be studied [60].
Different types of carbons have unlike structure. They consist of graphitic, molecular lamellae which are built from hexagonal, aromatic rings. The lamellae have varying size, amount of dislocations, planarity, amount of heteroatoms (H, O, N, S, P), and they are stacked in different ways, in different carbons. In addition, the amounts of disordered material and porosity change from one carbon material to another [64]. Thus, it is difficult to obtain comparable results for different types of carbon.
Carbon materials are divided into anisotropic and isotropic materials. Anisotropic materials have a well ordered, graphite like structure, while isotropic carbon materials have a disordered structure which has no properties that are dependent on direction [64].
Because of variations in porosity and structure, different carbons have different numbers of active sites for gas reactions per unit area. Possible active sites are vacancies and dislocations in the graphite structure, and numerous surface structures in the more disordered parts of the material.
The number of active sites is also affected by the amount of inorganic impurities that is present in the material, which may also catalyze the carbon-gas reactions. Catalytically active impurities increase the reactivity of the carbon material simply by introducing more active sites, in addition to the chemical catalysis they perform [64], see Chapter 8.2.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -73-
Marsh [64] summarizes the factors which affect the gas reactivity of carbon materials:
1. Precursor. The substance the material is developed from (e.g. pitch) has a crucial effect on the structure of the final carbon material. It causes a more or less defect lamellae structure.
2. Heat treatment temperature. Increasing heat treatment temperature gives lower density of lattice defects and lower reactivity.
3. Surface area accessible to gas.
4. Catalysis by inorganic impurities.
The gas reactions do not attack evenly the topography of the surface. Selective attack occurs at some places while others remain unreacted. This is due to the fact that the active sites for gas attack mainly are at the edges of the carbon lamellae and at defects in the basal planes of the carbon (vacancies, dislocations, inorganic impurities) [65].
There are thus two types of selective bum at anode materials; (1) the coke and binder are unevenly consumed, as shown in Figure 1.3, mostly because of structural differences, and (2) each coke grain is subject to selective reaction which leads to delamination of the coke lamellae. This is, as mentioned above, due to lattice defects.
Another important factor which has a great influence on the carbon-gas reactions, is the temperature, which affects the mass transfer to and from the surface, the diffusion in pores and the rate of reaction. Walker et al. [66] divided the temperature scale into three ideal zones where the logarithm of the reaction rate is linearly dependent on the reciprocal of the temperature :
Zone 1. At low temperatures the reaction rate is low, and the reaction is chemically controlled all over the accessible surface.
Zone 2. In the medium temperature zone the reaction rate is partly controlled by diffusion in the pores. This gives a concentration gradient of gas reactants which falls to zero inwards in the pore system.
Zone 3. At high temperatures is the reaction rate controlled only by diffusion of gas to the carbon surface, and a diffusion layer is formed.
Figure 8.1 shows the reaction rate versus 1/T as presented by Walker et al. [66].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -74- PART II
ZONEHI
ZONED
ZONE I
Figure 8.1. The change in reaction rate of the carbon gasification with temperature [66].
In practice one observes transition ranges between the zones described above where the reaction rate does not follow an Arrhenius plot for the chemical reactions.
8.1.2. CARBOXY REACTIVITY
Carboxy reaction, also named C02 bum, or the Boudouard reaction, is given by [59]:
C(anode) + C02(g) —> 2CO(g) (8.1)
It has been shown that this reaction can not occur at the polarized anode surface in contact with the electrolyte (see Chapter 2), but it proceeds inside the pores of the carbon anodes. There have been observations indicating Boudouard reaction as far as 10 cm into carbon anodes. C02 bum can also take place at the sides of the anode above the electrolyte, but the reaction is very temperature dependent. It is therefore fastest near the melt where the temperature is highest. At 930 °C, the equilibrium is almost completely shifted to the right, and the reaction rate is nearly doubled from 960 to 1000 °C [59]. The standard Gibbs energy change of reaction (8.1) is shown in Figure 8.2 [69].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -75-
1000 1100 1200 1300 1400 Temperature [K]
Figure 8.2. Standard Gibbs energy change (AG°) of the carboxy reaction as a function of temperature [69].
There is some disagreement about the mechanism of reaction (8.1), but, according to Marsh [64], the most accepted mechanism can be expressed as:
Cf + COz(g) <=> C(O) + CO(g) (8.2)
C(O) -> CO(g) + Cf (8.3) where Cf is a free active site, and C(O) is an active site with an adsorbed oxygen atom.
The reaction mechanism indicates that CO inhibits the reaction because the gas may react backwards with C(O) in Eq. (8.2). This leads to a reduced concentration of C(O), and less CO can be formed by Eq. (8.3).
8.1.3. AIRBURN
The reaction equations of airbum (C-0% reactions) is given by [59]:
C(anode) + 02(g) C02(g), (8.4) 2C(anode) + 02(g) -> 2CO(g), (8.5)
Airbum occurs at the top ("prebaked" cells) and at the sides of the anode. The ratio of the primary products from these reactions, CO/ C02, increases strongly with the temperature. According to Houston and 0ye [59], it can be calculated that the ratio increases from 0.2 to above unity when the temperature is raised from 400 to 550 °C. This means that reaction (8.4) dominates at low- and reaction (8.5) dominates at higher temperatures.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -76- PARTU
The airbum problem in prebaked cells has been reduced by covering the anode top with a mixture of alumina and crushed bath. This inhibits mass transfer of oxygen to the anodes, and thereby decreases the airbum.
The mechanisms of reactions (8.4) and (8.5) are still not well understood. Marsh [64] suggests the following reaction steps to explain the formation of CO and COz:
c, + 0,(8) -> C(OJ or CfOJ. (8.6) -> C(O) + C(0)m and/or (8.7) -» C(Q)_ + C(0)m and/or (8.8) -» C(0) + C(O) (8.9)
C(O) -» CO(g) (8.10) C(O). -» CO(g) (8.11)
C(0).+ C(Q). -> C, + co,(g) (8.12) C(0)m+ C(O) -> C, + co 2(g) (8.13) CO(g)+ C(O) -> C, + co 2(g) (8.14) CO(g)+ C(0)m —> Cf + co 2(g) (8.15) 02(g) + 2C(0) -> 2C02 (8.16) where Cf is a free active site, C(02) is a chemisorbed, fixed, molecular oxygen, C(Oz)m is a chemisorbed, mobile, molecular oxygen, C(0) is chemisorbed, fixed, oxygen atom and C(0)m is chemisorbed, mobile, oxygen atom.
8.2. CATALYSIS OF CARBON-GAS REACTIONS
8.2.1. GENERAL
Inorganic impurities in carbon materials usually affect the rate of the carbon-gas reactions. Some reduce the reaction rate, but very many act as catalysts. Houston and 0ye [59] give in a literature review references to Si, Fe, V, Ni, Na, S, Ca, Pb, Cu, Zn, Cr, Ti and A1 as catalysts to the reaction between carbon and 02. Sulphur has also been reported as inactive to this reaction, and A1F3, B and P have been found to be inhibitors. For the carbon-C0 2 reaction, Fe, V, Ni, Na, Ca, Pb, Cu and A1 are reported as catalysts, while S, A1F3, B and P are inhibitors. Sulphur has also been found to be inactive to this reaction.
It is important to note that catalysis of carbon gasification only causes the reaction to go faster in the temperature range where chemical control is significant (Zone I and II in Figure 8.1). At high temperatures (Zone III) it is mass transfer, not the gasification reaction, that controls the gasification rate.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -77-
In aluminium electrolysis cells, the carboxy reaction becomes important near pot operation temperatures, at lower temperatures the equilibrium is shifted towards the left hand side. Since C02 is the major gaseous species formed at the anode working face, it may also enter the anode open porosity and gasify carbon within the anode body [84]. The airbum reaction has a large negative free energy at all temperatures of interest in aluminium electrolysis but is, due to air access restrictions, mainly of concern at exposed parts of prebaked anodes (top and tapping positions) and under the gas skirts of Spderberg anodes. The top surface temperature of prebaked anodes is often within the range where the chemical reaction is the rate determining step, while temperatures under the gas skirts in Spderberg anodes are so high that transport reactions, i.e. the rate of air leaking in and products removed from the reaction zone, is the rate determining step.
Since the carboxy reaction in prebaked and Spderberg cells and the airbum reaction in prebaked cell operation both are within a temperature range where the chemical gasification reactions may be rate determining, the rate of the carbon consumption may hence be influenced by catalysts and/or some carbon materials properties.
According to Marsh [64], the following factors are decisive to how much the reaction rate of the carbon-gas reaction is increased by a particular metal:
1. The metal. 2. The reacting gas, the type of carbon material and the temperature. 3. The size and dispersion of the catalyst particles in the carbon material. 4. What phase the metal is in. 5. Relative amount of catalyst.
The main reason for the great discrepancies among earlier investigations, may be that many of these did not pay attention to all of the factors above.
Catalysts normally cause the reaction route to be changed, and the new route involves less activation energy, E, for the rate determining step [67]. This makes the reaction go faster.
The change in activation energy is often accompanied by a change in the preexponential factor in the rate expression of the reaction. This is called the compensation effect, and the reaction rate, k, can often be expressed by the Arrhenius equation:
k = Ae~E,RT (8.17) where A is the preexponential factor which is dependent on the density of active sites at the surface.
Figure 8.3 [64] shows how the compensation effect affects the reaction rates of catalyzed- and non-catalyzed reactions. The two rate curves intersect at the isokinetic point where the reactions are equally fast.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROL YSIS -78- PARTII
\ s s
in
Figure 8.3. Arrhenius plots of the rates of catalyzed- and non-catalyzed carbon- gas reactions with isokinetic point [64].
8.2.2. REACTION MECHANISMS
There are two proposed reaction mechanisms for the catalyzed carbon-gas reaction, the oxygen-transfer mechanism and the electron-transfer mechanism [60], [64], [67].
In the oxygen-transfer mechanism the catalyst is viewed as an oxygen carrier which goes through oxidation-reduction cycles [64]:
MO + C02 -> M0.C02 (8.18) M0.C02 +C -> MO + 2CO (8.19) where MO is a metal oxide. The catalytic phase can be oxides and pure metals.
In the electron-transfer mechanism the electron configuration of the catalyst is considered. Many of the known catalysts have either unfilled electron shells and can thus accept electrons from the carbon matrix, or they have free electron pairs which can be donated to the carbon matrix. The electron transfers leads to a rearrangement of the Jt-electrons in the carbon, and this weakens the C-C bonds at the edges of the carbon lattice. The bond weakening makes it easier for the edge carbon atoms to create bonds with adsorbed oxygen atoms, and to detach the CO molecule formed [64]:
CO/ + 2C{ -> 3CO + 2e (8.20) 2M+ + C02 + 2e -> MzO + CO (8.21) M20 + C02 —> M2C03 (8.22) where C, is a free active site and e is an electron.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -79-
The oxygen-transfer mechanism is at present the most recognized mechanism [64]. The reasons for this is that it has been shown that the catalyzed reaction only occurs in areas where carbon, catalyst and gas are in physical contact. This would not have been the case under the electron-transfer mechanism which only is dependent on contact between catalyst and carbon to give catalyzed reaction at the edges of the carbon lamellae. The activation energy of the catalyzed reaction is independent of the concentration of catalyst. That would not have been the case if the electron-transfer mechanism was the most correct one.
8.2.3. TOPOGRAPHICAL EFFECTS OF CATALYZED CARBON-GAS REACTIONS
Most investigations of topographical effects of catalyzed carbon-gas reactions have been performed with graphite as the carbon material. The carbon-gas reaction will have a higher rate close to catalyst particles at the carbon material than elsewhere. The carbon surface is thus consumed faster close to the catalyst particles than it is elsewhere. The topographical effects of this phenomenon is separated into three groups [64]:
1. Pitting. Investigations have shown that catalysts placed in vacancies in the basal planes of the graphite structure form pits in the surface, perpendicular to the basal plane, during the reaction. This is shown in Figure 8.4 [64].
Figure 8.4. Pit formation in the graphite structure caused by a catalyst particle (seen from side) [64].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -80- PART II
2. Edge recession. The catalyst spreads out at the edge of a layer in the graphite structure, and the layer is consumed from the side. This is shown in Figure 8.5 [64].
Catalyst film Graphite layer
Edge recession
Figure 8.5. Edge Recession of a basal plane in the graphite structure caused by a catalyst film (seen from side) [64].
3. Channeling. Catalyst particles form channels in the basal plane of the graphite structure. That is, channeling occurs perpendicular to the pitting direction (parallel to the basal planes), and the catalyst particles move at the surface. The particles which form channels sometimes change direction. This is shown in Figure 8.6 [64].
Figure 8.6. Channeling in the basal plane of the graphite structure caused by a catalyst particle (seen from above) [64].
Baker [68] claims that the three different modes of attack are due to the fact that the catalysts wet the carbon surface differently. Good wetting of the carbon surface leads to spreading of the catalyst, and edge recession occurs. Catalysts which wet the surface, but not well enough for spreading to occur, cause channeling. The catalyst is inactive if it does not wet the surface at all.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -81-
8.2.4. INHIBITION OF THE CARBON-GAS REACTION.
There are, according to Marsh [64], five possible ways of reducing the reactivity of carbon materials. The two first ones are connected to the carbon structure:
1. Reduce the open porosity, and thereby the accessible surface area for the gas.
2. Increase the crystalline order of the carbon matrix, and thereby reduce the number of active sites for gas reaction.
The other three possibilities are:
3. Remove the catalytically active phases in the carbon material.
4. Add phases which reduce the rate of the carbon-gas reaction.
5. Form a glassy layer on the carbon surface which creates a barrier to the gas.
Only number 4 represents inhibition of the carbon-gas reaction, if inhibition is interpreted as blocking of active sites at the carbon surface.
McKee [67] states that there are few known inhibitors to oxidation of carbon. The most common are compounds containing phosphorus or halogens, and to some extent boric acid and its derivatives. Some inhibitors reduce the effect of catalytically active phases by reacting with them, and form stable compounds, e.g. phosphates.
8.2.5. IRON
Walker et al. [60] investigated iron as a catalyst for the carboxy reaction. Iron mixed with very pure graphite was reacted with C02, and the iron containing phases were determined indirectly by measuring magnetic susceptibility. Fe-particles containing some Fe095 O (wustite) was assumed to be catalytically active, and Fe304 (magnetite) was assumed to be catalytically inactive. Their conclusion was that iron is gradually deactivated as it is oxidized from metallic iron, via wustite, to magnetite, but that it can be reactivated by reduction with CO. Others have also shown that iron catalyzes the carbon-C0 2 reaction [59].
The catalytic effect of iron on the carbon-O z reaction has been reported to be either moderate or weak [66], [67]. McKee [67] states that iron has a considerable effect on the graphite-0 2 reaction, and that the metal causes both pitting and channeling in the basal plane of the graphite structure. Baker [68] investigated the ability of iron to wet the graphite surface, and found that the metal did not wet it well. It was also stated that Fe did not maintain its catalytic effect with time.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -82- PARTII
8.2.6. VANADIUM
Vanadium is known as one of the most potent catalysts to airbum [59], [70]. Hume proposed the following reaction scheme [71]:
1. When exposed to air at elevated temperatures the vanadium impurity is oxidized:
(8.23)
2. The carbon surface is reducing:
c + v2o 5 O v2o 3 + co 2 (8.24)
The overall reaction is the gasification of carbon by oxygen:
c + o 2 co 2 (8.25)
Baker at al. [72] studied the catalytic effect on air oxidation of graphite by vanadium with the CAEM (Controlled Atmosphere Electron Microscopy) technique. Both vanadium and V2Os were extremely active catalysts for the oxidation of single-crystal graphite around 550 °C. The catalyst was observed as liquid droplets (V2Os melts at 690 °C) which formed channels in the graphite surface, and penetrated between the graphite layers. After reaction, the intermediate oxide V6013 was identified at the graphite surface. The strong catalytic effect by vanadium was ascribed to the ability of the catalyst to wet and spread over the graphite surface.
Vanadium ’s effect on carboxy reactivity is not that well recognized, and it is generally viewed as a moderate or weak catalyst to the Boudouard reaction [59].
Hume [71] states that vanadium is not considered to be a significant catalyst to the carboxy reaction.
8.2.7. NICKEL
The Nickel content in anode cokes often correlates with the vanadium content, and it has therefore been difficult to isolate the effect of nickel on coke gasification. Still several authors have reported nickel as a catalyst to both airbum and the carboxy reaction [59], [63]. McKee [67] refers to several workers who has observed nickel as being a catalyst to carbon gasification for various types of carbon materials.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS *. THEORY -83-
8.2.8. SULPHUR
The effect of sulphur on the carbon-gas reactions has not been very clear. The main reason for this may be that sulphur often occurs in carbon materials together with other impurities, and it has therefore been difficult to isolate the effects of the other impurities from that of sulphur.
Houston and 0ye [59] state in a literature review that sulphur has been reported to be either inactive or weakly catalyzing to the C-02 reaction, and to be inactive to the carbon-C0 2 reaction.
Sprlie et al. [61] reported that the C02 reactivity of anode materials is reduced by increasing the sulphur content for sulphur contents between 1 and 3 %. The air reactivity shows a minimum in the same sulphur content region. It is proposed that sulphur may form sulphidic compounds with the catalyzing impurities in carbon, and thereby act as a catalyst poison.
Bartholomew et al. [62] state that sulphur acts as catalyst poison to iron based catalysts because it forms stable surface sulfides which inhibit adsorption to the surface, and because it can change the electron configuration of the surface. Thus, sulphur poisoning of a catalyst can lead to changes in the selectivity and the rate of the catalyzed reaction.
They also state that metal catalysts are poisoned by sulphur compounds at concentrations well below those necessary for bulk metal sulphide formation. This suggests that two-dimensional surface sulfides are considerably more stable than bulk sulphides. Investigations with iron showed that the surface sulphide has a standard Gibbs energy of formation of -99.2 kJ/mol while bulk iron sulphide has a AG°-value of -55.2 kJ/mol at 850 °C [62]. The measurements were performed under H/H2S- atmosphere.
8.3. GAS REACTIVITY OF CARBON MATERIALS
One of the standard tests to investigate the gas reactivity of carbon materials is the “Hydro Aluminium air/C02 reactivity apparatus ” which was used in all reactivity tests in the present work. The standard reaction temperature for this equipment is 525 °C in air, and 960 °C in C02. The gasification reactions studied are:
C(anode) + C02(g) -> 2CO(g) (8.1) or C(anode) + 02(g) -> C02(g), (8.4) and 2C(anode) + 02(g) -> 2CO(g), (8j)
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -84- PARTII
As previously described (see Chapter 1.3), loose petroleum coke particles may form at the carbon surface due to selective oxidation of the binder matrix. This effect (dusting) gives an additional carbon loss.
The apparatus consists of five furnaces over which thermobalances are mounted. From the thermobalances carbon material samples are suspended inside the furnaces. The carbon samples are heated to the reaction temperature in inert gas (N2). When the predetermined temperature is reached, the gas flow is switched to air or C02. The weight loss due to gasification is continuously recorded at constant temperature and with excess reaction gas. Thermocouples controlling the temperature are mounted close to the surface inside the carbon samples to avoid a run-away exothermic reaction (ignition) in air oxidation. Standard total reaction time is 190 min, but only the last 30 minutes are used to calculate the gas reactivity of the sample, see Figure 8.7. In this (normally) linear weight loss period a regression analysis is made, and the weight loss rate, r ([mg/h]), is calculated.
When baked carbons are tested, a core sample is mounted on a special sample holder. A normal carbon material core sample is a 5 cm high cylinder with 2 cm outer diameter (see Figure 9.6). For coke reactivity testing a granulate in the size range 1-2 mm is screened out, and samples of about 10 g are placed in cylindrical platinum mesh baskets with the same dimensions as the core samples.
After the reaction period, the gas flow is switched back to inert gas during cooling. Loose dust at the core sample surface is carefully brushed off and weighed.
From these measurements the following parameters are normally calculated:
Gas reactivity, core samples [mg/cm2 h]: (8.26)
Gas reactivity, granulate [mg/g h]: (8.27)
Dust index, core samples [%]: s. = 100- ,mi — (8.28) m, +zn 2
Total bumoff [wt%]: AR,=100^+^ (8.29) mo |j$ II Dust generation, core samples [wt%]: 8 (8.30) £ where A is apparent surface area [cm2], m0 is the original weight of sample [mg], the weight of loose dust [mg], and m2 is the gaseous carbon loss [mg].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 8. THEORY -85-
5000
4500
4000
3500 3 3000 2. t 2500 X I 2000 B
Figure 8.7. Typical experimental weight loss curve in C02 at 960 C. The weight loss during the last 30 min is used to calculate the reactivity.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -86- PART II
9. EXPERIMENTAL
9.1. PREPARATIVE EQUIPMENT
9.1.1. PASTEMIXER
Figure 9.1 shows a sketch of the X-blender which was used to mix and knead the anode pastes. It was a WERNER & PFLEIDERER, type LUK 1.0K.
A)
1. Current input with safety switch 6. Power take-off. connected to lid (not shown). 7. Engine. 2. Mixing chamber, width 10 cm, 8. Current input. length 15 cm. 9. Intersection of mixing chamber normal to the 3. Mixing blade: a. 37 o/min, b. 66 o/min. rotation axis of the mixing blades: 4. The shaft of the mixing blade. h = 13.5 cm, d = 7 cm 5. Drive gear.
Figure 9.1. E-blender. A) From above. B) Mixing chamber from side.
The total volume of the mixing chamber was 1.5 1, maximum amount of paste was 1,0 1. The rotation speed of the two mixing blades was unequal. The rotation speed of blade 3a in the figure was 37 rpm, and it was 66 rpm for 3b. The mixing temperature in the E-blender can be preset, but it was still necessary to measure the temperature of
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -87- the pastes because they wet the walls of the mixing chamber unequally. The heat transport from the blender to each paste type is therefore paste dependent. The paste temperatures were measured with a digital thermometer (NORONIX NTD 24 C).
Coke and pitch were preheated before they were added to the E-blender. The preheating was done in a HERAEUS UT 5042 E oven.
9.1.2. BAKING FURNACE
Figure 9.2. shows a sketch of the baking furnace.
A)
1. Weight, 15 kg. 2. Steel tube. 3. Thermocouple, furnace control. 4. Furnace lid, steel lined with refractory bricks. 5. Furnace casing, steel lined with refractory bricks. 6. Pressure rod, stainless steel. 7. Heating element, 10 kW. 8. Coke. 9. Connection for ventilation. 10. Thermocouple. 11. Piston, stainless steel. 12. Casing, steel, height 13 cm, inner diameter 6.0 cm. 13. Base, steel.
Figure 9.2. Baking furnace for baking of carbon materials. A) From front B) From side.
The furnace was built in a steel case lined with refractory bricks. The heating element (10 kW) was flush mounted in the walls, and it was controlled by a Camille Bauer 49- 1P5 programgiver. The furnace lid was of steel lined with refractory brick. It had connection for ventilation and holes for thermocouples and pressure rods. The weights were 15 kg, and the pressure they cause was transferred to the sample by pressure rods and pistons. The casings in which the samples were held during baking, were placed in a coke bed to prevent airbum.
A diamond blade circular saw was used to cut the samples, and polishing was performed on PLANOPOL polishing disks from STRUERS.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -88- P.ART II
9.1.3. LABORATORY COKER
To be able to have full control with the inorganic impurities in the laboratory prepared cokes, a laboratory coker was build. With such an apparatus one can design cokes with known impurity profiles. This can be done by using a pure precursor for the coke, and adding controlled amounts of inorganic impurities. The apparatus can also be used for the preparation of pitch cokes. When coked at atmospheric pressure most binder pitches are turned into foam-like carbons with low bulk density and with C02 and air reactivity properties that are difficult to reproduce. Binder pitch coke yield from a 120°C (Mettler) softening point anode pitch is typically 55-60 wt% under these conditions. In the pressurized laboratory coker the foaming during carbonization is avoided, and the final pitch coke is denser.
The pressure, inert gas flow rate and temperature/time relationship during coking can be set. Figure 9.3 shows a sketch of the laboratory coker which was used to provide pitch cokes and cokes with different impurity levels.
1. Argon cylinder. 2. Manometer. 3. Pressure reduction valve. 4. Flowmeter. 5. Relief valve, cracking pressure 18 Bar. 6. Stainless steel tube, 8 mm outer diameter. 7. Stainless steel tube, 16 mm outer diameter. 8. Furnace. 9. Coke reactor, stainless steel (see fig. 3.2.). 10. Quick joints. 11. Cold trap/pressure stabilizing chamber (identical to 9.). 12. Flow control valve. 13. Flowmeter.
Figure 9.3. Sketch of laboratory coker.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -89-
The argon cylinder provided the desired inert gas flow and pressure. A reduction valve controlled the pressure in the system, while the flow rate of inert gas was read on the flowmeter. Relief valves were placed both up- and downstream from the coke reactor to ensure that there could be no pressure buildup due to blocking by solid deposition in the out- or inlet of the reactor. The furnace had three heating elements which were adjusted to minim um gradient (±3 °C), and it was controlled by an EUROTHERM 904P Controller/ Programmer. The gas flow from the system was controlled by a needle valve, and it was monitored on a flowmeter.
The reactor and cold trap/pressure stabilizing chamber were interchangeable. The reactor is shown in Figure 9.4.
1. Tube for inert gas inlet, 8 mm outer diameter. 2. Tube for gas outlet, 16 mm outer diameter. 3. Top lid with tubes welded on. 4. Copper gasket. 5. Casing, height 15 mm, inner diameter 70 mm. 6. Weld between bolts and casing. 7. Bolt, threaded in both ends. 8. Bottom lid. 9. Nut
Figure 9.4. Cross-section of coke reactor. All parts are in stainless steel except copper gaskets.
The temperature in the reactor was monitored at three locations; on the top lid (between the tubes), on the weld at the middle of the casing and on the bottom lid. The temperatures were registered by weld on, K-type thermocouples. In the critical heat treatment temperature range the maximum temperature gradient over the reactor was 3 °C. The sample holder for the coke precursor was an aluminium soft drink can that facilitated easy removal of the produced green coke.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -90- PART II
9.2. HYDRO ALUMINIUM AIR/CO, REACTIVITY APPARATUS
Figure 9.6 shows the gold mirror lined furnace in which the carbon samples were reacted with air or C02. The furnace was a part of the “Hydro Aluminium air/C02 reactivity apparatus ”. The apparatus had five gold lined furnaces which could be run simultaneously. It was possible to choose the oxidizing gas (C02 or dry air) individually for each furnace, but they had to be run for equally long reaction times. Furnaces which were run at the same time and with the same oxidizing gas, also had to be run at the same temperature. Nitrogen was used as inert gas during heating and cooling.
The thermocouple inside the carbon sample was connected to a Eurotherm controller which regulated the heating element of the furnace. The temperature signal was also registered by a PC which at the same time read the weight/weight loss of the sample. The PC controlled heating time, reaction time, cooling time and the valves which let the three different gases into the furnaces (Dry air, C02 or N2). The gas flow to each furnace was manually adjusted by a valve and a flowmeter.
The sample holder arrangement which was used when polished samples for SEM studies were reacted, is shown in Figure 9.5. The original sample holders were made for standard samples for reactivity measurements which were cylinders with height 5.0 cm and diameter 2.0 cm, as shown in Figure 9.6. To be able to attach the polished samples (chip, ca. 2 x 2 x 0.4 cm) to the sample holder, the bottom 2 cm of a standard sample was cut off. The top piece could then be used to hold the sample. Fiberftax insulation was put above, behind and below the samples to impede oxidation at these surfaces, and to make the samples stick better. This sample holder arrangement gave the opportunity to record the temperature as usual, and thereby reacting the polished samples under exactly the same conditions as the standard samples.
A) B)
1. Alumina suspension rod, diameter 0.6 cm. 2. Sample holder, stainless steel. 3. Thermocouple, Pt/Pt, 10 % Rh, alumina, diameter 0.3 cm. 4. Fiberfrax insulation. 5. Part of standard sample for reactivity measurements. 6. POLISHED SAMPLE, ca. 2 x 2 x 0.4 cm. 7. End nut, stainless steel.
Figure 9.5. Sample holder arrangement for polished samples. A) From front B) From side.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -91-
1. Hook attached to scales, METTLER MP 460 DR.
2. Plug to PID controller.
3. Gas outlet.
4. Furnace lid, alumina.
5. Silica tube, inner diameter 3.7 cm.
6. Alumina hanger, diameter 0.6 cm.
7. Thermocouple, PtZ Pt, 10% Rh, alumina, diameter 0.3 cm.
8. Silica tube lined with gold mirror, height 50 cm.
9. Sample holder, stainless steel.
10. Standard carbon sample, height 5.0 cm, diameter 2.0 cm.
11. Insulation, Fiberfrax.
12. Heating element.
13. Radiation shields, alumina.
14. Ceramic end plate.
15 Gas inlet, C02, dry air and N2.
Figure 9.6. Furnace (gold lined) for reactivity measurements.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -92- PARTII
When coke granulate was tested in the reactivity apparatus the sample holder in Figure 9.5 was replaced by a platinum mesh basket with the same dimensions as a standard core sample. This secured the same temperature control because the thermocouple was placed inside the granulate in the same manner as it was placed inside the core samples.
Carbon samples were weighed on a METTLER AK 160 scales.
9.3. ANALYTICAL INSTRUMENTS
Ash content was determined after combustion of the carbon materials at 700 °C in platinum crucibles in a HERAEUS Laboratory Muffle Furnace MHO.
A PERKIN ELMER 603 Atomic Absorption Spectrophotometer, type "double beam", was used for trace metal analysis of the carbon materials. A mixture of the ash and LiB02 is melted, and dissolved in diluted HNOg (cons. HNG3: distilled water =1:5), before analysis. The apparatus had deuterium background correction, hollow-cathode lamps and acetylene flame. The temperature in the flame was between 2000 and 3000 °C.
Sulphur analysis was performed on a LECO CS-444 Carbon/ Sulphur-Analyzer. This apparatus used oxygen as a carrier gas, and had a "Solid state infrared absorption" detector. The accuracy of the analysis was between 0.1 and 10 ppm, and the range is 0-50 % sulphur. The analyzed samples weighed about 0.02 g.
The scanning electron microscope (SEM) which was used for backscatter- and secondary mode micrographs, was a PHILIPS 515 SEM with LaB6-filament. The SEM had an analyzing system for X-ray micro analysis (XRMA). It was an ED AX PV9900 system which utilized energy dispersive analysis (EDS). The detector was not covered by a window during analysis, and the ZAP factors (correction factors for differences between sample and standard) were calculated by the computer program ED AX. The analytical results were semi quantitative, i.e. the SEM operator identified the peaks representing each element present. The computer then calculated the concentrations assuming that no other than the identified elements were present, and that there was a certain amount of background noise. Neglected peaks were not included in the calculations. Carbon has a too low atomic weight to be evaluated.
The X-ray diffraction analyses were performed with a PHILIPS APD 1700 Automated Powder Diffraction system controlled by a Micro-VAX computer. The XRD instrument was equipped with monochromator, divergence slit (fixed, 1 degree) and X-ray tube with Cu anode. The Micro-Vax computer could identify phases by comparing the measured X-ray spectrum with the spectra it had in its database.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -93-
9.4. MATERIALS
The cokes used for the labscale anodes were:
DO, ordinary petrol coke produced by Statoil in a delayed coker at Mongstad, Norway. FO, fluid coke from Kaiser Aluminium & Chemical Corp., Germany, which is produced in a fluidized bed reactor.
The precursor for the controlled impurity level cokes, AO, was an aromatic oil which was a lighter fraction from the distillation of coal tar pitch supplied by Tarconord, Nyborg, Denmark. Several pitches were used as precursors for pitch cokes, see Chapter 10.8.1.
The additives used in the labscale anodes and controlled impurity level cokes are listed below:
Iron(III)oxide (Fe203), 99%, Sigma-Aldrich , Steinheim, Germany. Iron(II)sulphide (FeS), techn., Sigma-Aldrich , Steinheim, Germany. Iron(m)acetylacetonate (C15H21Fe06), for synthesis, Merck-Schuchardt, Hohnenbrunn bei Munchen, Germany. Nickel(II)acetylacetonate (C10H14NiO4), for synthesis, Merck-Schuchardt, Hohnenbrunn bei Munchen, Germany. Vanadium(III)acetylacetonate (C15H21VOe), 97%, Sigma-Aldrich , Steinheim, Germany. Dibenzothiophene (C12HgS), 98%, Sigma-Aldrich , Steinheim, Germany
Figure 9.7 shows the molecular structure of iron(III)acetylacetonate, nickel(II)- acetylacetonate, vanadium(III)acetylacetonate and dibenzothiophene. These aromatic organic molecules were chosen because they dissolve in molten pitch and in the liquid precursor for the controlled impurity cokes. This gives a uniform impurity distribution in the carbonized materials and minimizes the baking loss of additives.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -94- PARTII
•=o
Figure 9.7. Molecular structure of: a) iron(HI)acetylacetonate, b) nickel(H)- acetylacetonate, c) vanadium(UI)acetylacetonate and d) dibenzothiophene.
9.5. METHODS
9.5.1. LABSCALE ANODES
Six anode materials with different compositions were prepared. They were based on two different types of petroleum coke, delayed coke (DC) and fluid coke (FC). One DC anode was prepared with no iron additive, while Fe203, FeS or iron(III)acetylacetonate were added to three different DC anodes. Fe203 or CI5H21Fe06 were added to two different FC anodes. The coke granulate had a maximum grain size of 2 mm to ensure that all parts of the material were close to the iron additives.
The same binder pitch was used in all the anodes. The recipes of the six anodes are given in Table 9.1. The high pitch content was due to the large amount of fines in the anodes.
Table 9.1. The recipes of the six anodes, AO- AS.
Anode no AO A1 A2 A3 A4 AS Pitch [wt%l 41 41 41 41 37 41 Coke [wt%] 59 59 59 59 63 59 Coke type DC DC DC DC FC FC Iron compound added - FeA FeS C,Ji,,Fe(X Fe„0, C,A,FeOfi
Addition in coke or pitch - Coke Coke Pitch Coke Pitch
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -95-
The preheated coke and pitch for the lab scale anodes were mixed in the mixer shown in Figure 9.1. The anode pastes were mixed for 25 minutes at 200 °C, and the green anodes were baked under pressure in a coke bed to 1000 °C, see Figure 9.2.
Samples of the baked anodes were incinerated, and the ash residue was analyzed with atomic absorption to determine their impurity content.
Other samples of the baked anodes were polished before reactivity testing. The reactions were run in air at 500 °C for 45 minutes and in C02 at 960 °C for 30 minutes. The polished samples were investigated by scanning electron microscopy and x-ray diffraction before and after reaction with air and C02.
9.5.2. PITCH COKES AND CONTROLLED IMPURITY LEVEL COKES
Before the precursors were placed in the coker, additives were dissolved in the aromatic oil by stirring. Due to the green coke yield of 86%, the additions were made according to:
1MYiW,organic m... = 0.86 • c (9.1) LV±MW, impurity where m ... is the mass of added organic chemical, m __ is the mass of coke precursor, Mw „gank/Mw !mpuiIi is grams or of additive per gram elemental impurity (Fe, Ni, S, V) in organic additive, 0.86 is the coke yield and c is the predefined impurity level (if the predefined impurity level is 1 wt%, the c is 1/100). Table 9.2 shows the wanted composition of the produced cokes.
Table 9.2. Wanted composition of produced cokes with controlled impurity levels.
Additive Additive Coke Fe [ppm] S [wt%] Coke Ni/V [ppm] S [wt%]
REF - - Nil 100 - Fel 100 - Ni2 400 -
Fe2 400 - Ni3 1000 - Fe3 1000 - Ni-Sl 100 1.0 Fe-Sl 100 1.0 Ni-S2 400 1.0 Fe-S2 400 1.0 Ni-S3 1000 1.0 Fe-S3 1000 1.0 VI 100 - SI - 0.01 V2 400 - S2 - 0.04 V3 1000 - S3 - 0.1 V-Sl 100 1.0 S4/REF - 1.0 V-S2 400 1.0 S5 - 3.0 V-S3 1000 1.0
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -96- PARTIl
Crushed pitch granulate was placed directly in the coker in the sample container. As mentioned above the sample container was an aluminium soft drink can.
Each batch consisted of about 240 g precursor. The temperature was raised from ambient to 400°C at 30°C/h, from 400°C to 450°C at 10°C/h, from 450°C to 525°C at 3°C/h, and the green coke soaked at 525°C for 4 hours. A pressure of 15 bar was maintained during the coking cycle with an inert gas (Ar) flow rate of about 100 Nml/h. The continuous inert gas flow was more of a safety provision to avoid excessive pressure build-up in the system than for sweeping cracking gases out of the reactor.
After stripping the aluminium can crucible from the green coke body, the coke was crushed to -10 mm and calcined in a graphite crucible with lid. The graphite crucible was then embedded in crushed petroleum coke which again was contained in a stainless steel box with lid. The entire assembly was heated in a furnace equipped with a temperature programmer for precise control of temperature, ramp and soaking time.
The green coke was heated from ambient to 800°C at 100°C/h, from 800°C to 1010°C at 67°C/h and soaked at 1010°C for 2 hours. The reason behind the relatively low maximum heat treatment temperature was to make the coke somewhat more reactive than it would have become at normal prebaked anode baking temperatures and make it more like binder coke in Spderberg anodes where selective reactivity and dusting is a considerably larger problem than in prebaked anodes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 9. EXPERIMENTAL -97-
9.5.3. REACTIVITY
The labscale anode samples were reacted in the reactivity apparatus as described below, except that the measurements in air were at 500 °C for 45 minutes and in C02 at 960 °C for 30 minutes. Thus, the lab scale anode investigations did not give any reactivity data, but were meant for surface studies only.
All the carbon sample types were heated to the reaction temperature for 65 minutes in inert gas (N2, ~ 1.6 Nl/min.). When the predetermined temperature was reached, the gas flow was switched to air (~ 3.6 Nl/min) or C02 (~ 1.8 Nl/min.). The weight loss due to gasification was continuously recorded at constant temperature and with excess reaction gas. Thermocouples controlling the temperature were mounted close to the surface inside the carbon samples to avoid a run-away exothermic reaction (ignition) in air oxidation. The standard total reaction time of 190 min was used, and the last 30 minutes were used to calculate the gas reactivity of the sample. In this (normally) linear weight loss period a regression analysis was made, and the weight loss rate, r [mg/h], was calculated.
For coke reactivity testing a granulate in the size range 1-2 mm was screened out, and samples of about 10 g were placed in cylindrical platinum mesh baskets with the thermocouple mounted inside the granulate.
All carboxy reactivity tests were run at 960 °C, while the airbum measurements were run at 475, 500 or 525 °C depending on the reactivity of the sample. The samples were cooled in inert gas after the test.
9.5.4. SEMANALYSIS
The investigated samples were fastened to the sample holders by carbon tape. In all the scanning electron microscope (SEM) analyses the following were done:
Surface inspection at 25.4x magnification in backscatter (atom number contrast) and secondary mode (topography). Surface inspection at 503x magnification in backscatter and secondary mode. About 15 x-ray microanalyses (XRMA) at different heavy-phase particles at the surface.
In addition, investigations at different large magnifications were made to describe characteristic phenomena, and additional XRMA were performed when needed.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -98- PART II
9.5.5. XRD ANALYSIS
X-ray diffraction measurements were only possible at the labscale anode samples due to their smooth surface. The XRD analyses were performed with the following settings:
Scan area: 10 - 80 ° Step size: 0.040° Measurement time per step: 2s Generator settings: 20 kV, 40 mA
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -99-
10. RESULTS AND DISCUSSION
10.1. LABSCALE ANODES
This work has partly been presented elsewhere [73].
10.1.1. BAKED ANODE MATERIALS
For a surface study of catalytic iron-containing phases at more realistic anode materials with high- and low sulphur contents, six anode materials with different compositions were prepared. They were based on two different types of petroleum coke, delayed coke (DC) and fluid coke (PC). The sulphur content was low in the DC and high in the PC. The coal tar pitch used as a binder had a low sulphur content. One DC anode was prepared with no iron additive, while iron oxide (Pe203), iron sulphide (FeS) or iron(III)acetylacetonate (C^H^FeOJ were added to three different DC anodes. Pe203 or ClsH2]Fe06 were added to two different PC anodes.
This was meant to provide anode materials with high iron contents with and without high sulphur contents. The aim was to try to isolate the effects of iron alone and iron together with sulphur on the air and C02 oxidation of anode materials. An anode with no iron additive was prepared as a reference for comparison.
The same binder pitch was used in all the anodes. The recipes of the six anodes are given in Table 9.1. The anode pastes were mixed for 25 minutes at 200 °C, and the green anodes were baked under pressure in a coke bed to 1000 °C. Samples of the baked anodes were incinerated, and the ash residue was analyzed with atomic absorption to determine their impurity content. The analyses are shown in Table 10.1.
Polished samples of the six anode materials were reacted with air at 500 °C for 45 minutes and with C02 at 960 °C for 30 minutes in a Hydro Aluminium reactivity apparatus, see Chapter 9.2. Heating and cooling were performed in N2 atmosphere.
Unreacted samples, samples reacted with air and samples reacted with C02 were inspected with scanning electron microscope (SEM) and characteristic features were photographed. Several energy dispersive X-ray microanalysis (EDX) were performed at iron phases at the surface of each sample. X-ray diffraction analysis (XRD) was done at the surfaces of the samples for the materials that had iron additives.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -100- PART II
Table 10.1. Iron additive, iron content, sulphur content, ash content, and other impurity levels in the baked anode materials.
Anode no A1 A2 A3 A4 AS Fe added FeA FeS C,5H2,Fe06 FeA C.A.FeO,
Fe [wt%] 1.73 2.29 0.66 1.30 0.57 S [wt%] 1.13 2.40 1.20 2.57 5.02 Ash [wt%] 2.58 3.72 1.19 2.43 1.12 Na [ppm] 150 200 170 240 290 Mg [ppm] 17 20 13 18 13 A1 [ppm] 150 140 120 100 110 Si [ppm] 280 860 720 850 690 K [ppm] 21 48 23 31 20 Ca [ppm] 160 150 100 110 120 V [ppm] 52 11 46 83 290 Ni [ppm] 90 82 32 120 130 Zn [ppm] 44 97 69 110 120 Pb [ppm] 80 93 79 85 95
As shown in Table 10.1, the iron content of the baked anode materials with iron additives varied between 0.57 and 2.29 wt%. The sulphur content was high in anodes with FC and in the DC anode with FeS added (between 2.4 and 5.0 wt%). In the other anodes (DC with no iron additive, Fe2G3 or C15H21FeOe), the sulphur content was low (about 1.1 wt%). The level of other impurities was normal.
Figure 10.1 and Figure 10.2 show the difference in the contents of heavy phase with and without addition of iron compounds to the anode (heavy phases, like iron compounds, have light (white) colors in SEM images).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -101-
Figure 10.1. Surface of an anode based on DC coke with no iron additive (AO, low sulphur), SEM image (backscatter mode).
Figure 10.2. Surface of an anode based on DC coke with Fe203 added (Al, low sulphur), SEM image (backscatter mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -102- PARTII
The SEM images show that the materials with iron additives had a much higher surface concentration of heavy phases than the material without iron additive. The iron particles were well dispersed in the binder matrix of the anodes. The iron particle size varied with the type of additive. FeS gave the largest particles, and C15H21Fe06 gave the smallest. All the materials were very porous.
The X-ray micro-analysis (EDX) and the XRD analysis showed that iron was in the metallic state after baking, except for FeS which was stable. If the iron impurities had access to sulphurous compounds released during baking, they were converted to FeS. This is illustrated by the XRMA and XRD spectra in Figure 10.3, Figure 10.4 and Figure 10.5.
AT X ELEM 0 K IS.76 S K FE K 160.1282 81 .88
TOTAL
23-OCT-93 1 = 07 : 48 SURER QUANT ? PIRORS 8RLSFQ OFF
,S K*
4.00 1700 8.00 2.40KEU 20eU/ch Pi EDAX
Figure 10.3. XRMA of an impurity particle at the surface of an anode based on DC coke with Fe2Q3 added (Al, low sulphur). Fe2G3 has been converted to metallic iron during baking of the anode.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -103-
ELEM 0 K 1.7827 S K FE K 198.8875
TOTAL 100.00
OCT SUPER RATE = TJMF = 85LSEC PRST OFF
IS K«
C K« 0 K«
28SCNT 20eV/ch A EO AX
Figure 10.4. XRMA of an impurity particle at the surface of an anode based on FC coke with Fe203 added (A4, high sulphur). Fe203 has partly been converted to FeS during baking of the anode.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -104- PARTII
3.50
3.15
2.45 ‘
1.40 ‘
0.35
40.0
24 - 68
Figure 10.5. XKD spectrum and identified phases at the surface of an anode based on DC coke with FeS added (A2). FeS is stable through baking of the anode. The broad big peaks are due to carbon.
10.1.2. CARBOXY REACTION AT ANODES WITH IRON ADDED
After reaction with C02, selective consumption was observed, i.e. C02 consumed the binder matrix faster than the petroleum coke grains. This made the coke grains protrude from the anode surface. This can be seen in Figure 10.6 which shows the anode material with no iron additive after reaction with co2.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -105-
Figure 10.6. Surface of an anode based on DC coke with no iron additive (AO, low sulphur) after reaction with CO,. The surface has been roughened because of selective reaction. (SEM image, secondary mode)
At the surface of the DC anode samples with no iron compounds added, there was no sign of catalytic reaction involving metals.
Surface textural changes caused by the catalytic C-CO, reaction, were observed on DC grains (low sulphur) from samples which had Fe203, FeS or C15H21Fe06 added, as shown in Figure 10.7 and Figure 10.8. The iron particles were associated with pits developed in the layers of the coke, and most of them were attached to the edges of the layers.
This showed that iron was catalytically active in the carbon-C0 2 reaction at 960 °C, and it indicated that iron present as FeS after baking, could be converted to active catalyst (metallic iron) under these conditions. 0degard et al. [75] showed that considerably higher amounts of sulphurous compounds escaped from the anode in the first period after start of electrolysis than would be for uniform consumption of the anode sulphur. They also found that COS is the primary sulphur containing gas. Thus, a possible reaction is: FeS + CO Fe + COS, AG° = -182 kJ/mol (10.1)
The estimated standard Gibbs energy change of this reaction is approximately constant in the temperature range from 500 to 1000 °C due to zero entropy change.
These observations were confirmed by the EDX and XRD analyses. Thus, the fact that iron was present as FeS after baking could not explain the reduced C02 reactivity of high sulphur anodes found before [61].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -106- PARTII
Figure 10.7. Surface textural changes caused by the catalytic C-C02 reaction on a DC anode with Fe2G3 added (Al, low sulphur). (SEM image, secondary mode).
Figure 10.8. Surface textural changes caused by the catalytic C-C02 reaction on a DC anode with FeS added (A2, low sulphur coke). This means that iron initially present as FeS is converted to a catalytically active phase in C02 atmosphere under these conditions. (SEM image, secondary mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -107-
The EDX and XRD analyses showed that under C02 atmosphere at 960 °C, iron compounds in contact with carbon in cracks and pores of the carbon material will be reduced to Fe or FeO. If the iron particles become detached from the carbon, they will be oxidized by C02 to Fe304 or Fe203 which are not catalydcally active phases.
Surface textural changes caused by a catalytic C-C02 reaction were not observed at FC grains (high sulphur) from samples which had Fe203 or C15H21Fe06 added, see Figure 10.9.
Figure 10.9. Surface texture of a FC anode with Fe203 added (A4, high sulphur). There is no sign of catalytic activity in connection with the iron particles. (SEM image, secondary mode).
The lack of catalytic activity on FC may be due to the fact that it had a high sulphur content, and that the sulphur was released as the carbon was consumed in the reaction, giving a "steady state" poisoning of the active catalyst particles. This means that every time a sulphur atom was removed from the surface of a catalyst particle, another sulphur atom from the carbon was available to poison it again.
The EDX and XRD analyses did not show any significant differences between the iron phases on the FC grains and the iron phases on the DC grains. However, these analytical methods would not be able to detect sulphur atoms poisoning the surface of the catalyst particles.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -108- PART II
There may also be structural reasons for the differences in the observed catalytic activity on FC and DC. The polished surfaces of FC grains which contained no visible iron particles, were much less reacted than the same type of DC grains after reaction with C02. This indicates that FC was less reactive than DC in the C-C02 reaction.
Increasing amounts of iron particles did accumulate at the surfaces of the samples as the carbon was consumed, but since the iron was converted to an inactive phase when it lost contact with the carbon, this was not expected to affect the reaction. The accumulation is clearly seen when comparing Figure 10.10. with Figure 10.2.
Figure 10.10. Surface of an anode based on DC coke with Fe203 added (Al, low sulphur) after reaction with C02. The accumulation of iron particles at the sample surface is clearly seen when comparing with Figure 10.2 (SEM image, backscatter mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -109-
10.1.3. AIRBURN AT ANODES WITH IRON ADDED
The reactivity of the anode materials was much higher in C02 at 960 °C than in air at 500 °C. The carbon consuming reaction had therefore not uncovered many iron impurities after 45 minutes, and the airbum study of the labscale anodes were for this reason not very thorough. The reaction time was not increased because it was decided to investigate the problem with a different approach, see Chapter 10.8.
Selective oxidation was not observed on the samples reacted in air, probably due to the fact that the C-02 reaction was less selective, and that too little carbon had been consumed after 45 minutes to show selective airbum.
Preferred attack by the C-02 reaction near the iron particles was observed at the binder matrix of all the samples except on those with iron added as FeS, see Figure 10.11. It could not be concluded from these few observations that this meant that iron is an active catalyst for this reaction, but it was rather ascribed to the fact that the reaction had not proceeded far enough for the iron to be oxidized to Fe304. (Iron catalyzed airbum was seen later, see Chapter 10.4.2.) The FeS phase in the anode materials was stable in air at 500 °C, and therefore the iron bound in this compound had no effect on the oxidation of the anodes.
Figure 10.11. Surface of an anode based on DC coke with Fe203 added (Al, low sulphur) after reaction with air. Preferred attack by the C-02 reaction is seen at the binder matrix near the iron particles. (SEM image, secondary mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -110- PART II
10.1.4. SUMMARY
- Iron was in the metallic state after baking except when it had access to released sulphurous compounds that converted it to FeS. FeS was stable through baking.
- Iron is catalytically active in the C-COz reaction at 960 °C, and iron present as FeS after baking, can be converted to active catalyst (metallic iron) under these conditions.
- Iron compounds in contact with carbon in cracks and pores of the carbon material in C0/C02 atmosphere at 960 °C, can be reduced to Fe or FeO. If the iron particles become detached from the carbon, they will be oxidized to Fe304 or Fe203 which are not catalytically active.
- Catalytic activity caused by iron on FC (high sulphur) was not observed. This may be due to a "steady state" poisoning of the active catalyst particles by sulphur, or have carbon structural explanations.
- These investigations did not give very conclusive results on airbum because the reaction time in air was too short.
This study revealed a need to find a method for investigation of effects of impurities without the uncertainties introduced by using cokes with very different fabrication process, structures and impurity profiles.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -111-
10.2. LABORATORY COKER
Based on a need to be able to predefine the impurity content in cokes and to produce cokes with similar structures but with different impurity contents, a method for production of laboratory cokes was developed, as described in Chapter 9.1.3. This also gave the opportunity to let the investigated cokes have impurity contents in the same concentration range as commercial anode cokes. The apparatus is usable for production of coke from a wide range of organic precursors, distillation fractions and pitches, see Chapter 10.8.
All the equipment presented in Chapter 9 is well tested except the laboratory coker which was developed for this investigation (Chapter 9.1.3.). To test the coker, it was run several times with different coking parameters, and the resulting green cokes were inspected. Table 10.2 presents the different coking parameters of the test runs, and Table 10.3 presents the measured green coke properties. A green coke from Statoil, Mongstad, is included for comparison.
Table 10.2. Coking parameters: mp is the mass of precursor, ma is the mass of additive, Fe-Ac is iron(m)acetylacetonate and DBT is dibenzothiophene. N is the naphthalene distillate fraction and AO is an aromatic oil distillate fraction from distillation of coal tar pitch.
Precursor Additive Heat treatment program Coking Run m. (temp, ramp, temp, level, dwell time) pressure no. Type [g] Type [gl rc/h], [°C], [h] [bar] i. Pitch 154.1 - - (500,495,0),(15,500,14.7), end 15 2. Pitch 220.0 - - (500,100,0),(40,500,6), end 15 3. Pitch 220.0 - - (500,150,0), (10,500,3), end 15 4. Pitch 220.0 - - (500,400,0),(10,450,0),(3,525,4), end 15 5. Pitch 220.0 - - (500,350,0),(10,400,0),(3,525,4), end 15 6. Pitch 50.0 - - (500,400,0),(10,450,0),(3,525,4), end 15 7. Pitch 100.0 - - (500,400,0),(10,450,0),(3,525,4), end 15 8. Pitch 200.0 Fe-Ac 2.0 (500,500,16), end 15 9. Pitch 220.0 Fe-Ac 2.2 (500,500,16), end 15 DBT 1.15 10 Pitch 220.0 DBT 1.15 (500,500,16), end 15 11. N 128.0 - - (30,400,0),(10,450,0),(3,525,4), end 15 12. AO 204.0 - - (30,400,0),(10,450,0),(3,525,4), end 15 13. AO 228.2 - - (30,400,0),(10,450,0),(3,525,4), end 15 14. Mongstad coke - - - - The pitch was a vacuum distilled coal tar pitch from Tarconord, Denmark, with a nominal softening point of 120 °C Mettler. It is used in Elkem’s anode plants.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -112- PART II
Table 10.3. Resulting green coke properties. mg is the mass of green coke, (Green) Coke Yield = 100 m/m,.
Run no. m, Coke Yield Geometric Density Vibrated Bulk Density [gl [%] [g/cm3] [g/cm3]
1. 143.3 92.9 0.57 - 2. 203.9 92.6 0.59 - 3. 205.3 93.3 0.58 - 4. 203.3 92.4 0.59 0.67 5. 203.1 92.3 0.59 0.69 6. 41.4 82.8 0.50 - 7. 81.9 81.9 0.51 - 8. 186.6 93.3 0.56 - 9. 205.2 93.3 0.57 - 10. 204.9 93.1 0.60 - 11. 14.8 11.6 - - 12. 175.4 86.0 0.90 0.70 13. 196.8 86.2 0.90 - 14. - - - 0.73
The general impression of the pitch based green cokes from the laboratory coker was that they were very porous, and that the pores were large (mm range). Still, when they were crushed and the grains in the size range 1-2 mm were sieved out, to perform a standard VBD (vibrated bulk density) measurement, a normal VBD was found. 0.67 and 0.69 g/cm3 compared to 0.73 g/cm3 for the Mongstad coke. The geometric density seems to be independent of the coking parameters, in the parameter range tested. Based on visual inspection and VBD, it was concluded that the coker produces green coke which is comparable to commercial green coke.
Some of the lighter fractions from the vacuum distillation of coal tar pitch were tested to see if they were usable as precursors to coke. The two most important factors were coke yield and purity. Table 10.4. presents the coke yield and the ash content of the precursors and of the resulting green cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -113-
Table 10.4. Coke yield (CY) and ash content of the coke precursors and coke.
Ash content CY Ash content in coke Precursor [wt%] [wt%] [wt%]
Pitch 0.226 92 - Naphthalene fraction 0.000 11.6 - Creosote fraction 0.000 - - AO fraction 0.000 86.0 0.06
The table shows that the pitch was not very pure, and it was therefore not well suited as a precursor for the present purpose. The naphthalene- and creosote fractions had too low CY to be considered as coke precursors.
The AO fraction had a high CY, the geometric density was higher than the coke produced from pitch (see Table 10.3), it had no ash content and the produced green coke only contained 0.06 wt% ash. It is hard to explain the ash content of the coke, but it may be from some stage of the production, either the laboratory coker or the grinding and sieving. Visual inspection revealed that the pores in the green coke looked very similar to the ones in the green coke from Statoil, Mongstad. From these measurements it was decided that the AO fraction was suited as a coke precursor for these investigations.
The green coke from the laboratory coker was crushed in a jaw crusher to - 10 mm grain size after coking, and then calcined to 1000 °C.
As a reference material, a coke with no additives (REF) was prepared from the aromatic oil. Table 10.5 gives the impurity contents of the coke and of a commercial anode coke (CAC) for comparison. The CAC was a low sulphur, low impurity coke which was “cleaner ” than most anode grade cokes it may be compared to.
Table 10.5. Sulphur-, ash- and trace metal contents of the reference material (REF) and of a commercial anode coke (CAC).
Coke S Ash A1 Ca Fe K Mg Na Ni Pb Si V Zn type [%] [%] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm]
REF 0.35 0.049 22 5 28 3 24 45 12 <4 107 <4 3 CAC 1.09 0.053 42 11 112 3 2 21 25 <5 59 60 2
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -114- PAMT II
The impurities present in the reference material coke was assumed to be the background impurities in all the materials produced. Table 10.6 shows reactivity data of the reference- and commercial carbon materials.
Table 10.6. Air- and carboxy reactivity of the reference material (REF) and of a commercial anode coke (CAC).
C02 reactivity Air reactivity Coke type [mg/gh] [mg/gh]
REF 6.3 13.7 CAC 11.3 15.4
The reference has lower reactivities, probably due to the lower impurity content. This shows that the laboratory coker yielded cokes with comparable properties to commercial anode cokes, but with lower impurity content which is beneficial when investigating the effect of added impurities.
10.2.1. SUMMARY
A laboratory coker was developed. An aromatic oil from the distillation of coal tar pitch was used as a coke precursor in the laboratory coker. This yielded a coke with low air- and carboxy reactivity having a low impurity content and properties comparable to commercial petroleum cokes. This gave the possibility to add controlled amounts of impurities to the coke precursor, and to tailor any wanted impurity content in the produced cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -115-
10.3. EFFECTS OF SULPHUR ON THE AIR AND CARBOXY REACTIVITY OF COKES.
This work has partly been presented elsewhere [74].
10.3.1. SULPHUR IN CALCINED COKES
In addition to the reference material, five cokes with increasing sulphur contents were produced (0.01, 0.04, 0.1, 1.0 and 3.0 wt% added). Figure 10.12 shows the actual sulphur content of the calcined cokes versus the wanted composition of produced cokes, see Table 9.1. Sulphur was added to the coke precursors as dibenzothiophene, see Figure 9.7 d).
y = 0.5 x + 0.3
S added [wt%]
Figure 10.12. Sulphur added to coke precursor vs. the sulphur content in calcined coke.
As the figure shows, approximately half of the sulphur was fixed in the coke during the coking and calcining process. The sulphur content in cokes without dibenzothiophene additions were -0.35 wt%. Sulphur does not form separate phases in coke, but it is incorporated in the carbon matrix, and it could therefore not be studied alone in the SEM.
Atmospheric carbonization and calcination of DBT-added pitches (anodes) gave less than 20 % S yield [61].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -116- PART II
10.3.2. THE EFFECT OF SULPHUR ON AIR REACTIVITY
The air reactivities of the cokes with 0.01 and 0.04 wt% S added were lower than the reference material, and the coke with 0.1 wt% was similar to the reference. The cokes which had 1.0 and 3.0 wt% were slightly less reactive, as seen in Figure 10.13. The error bars are the standard deviations of the individual measurements.
y = 10.9
Figure 10.13. Air reactivity of coke aggregate at 525 °C as a function of the sulphur content
Apparently, sulphur alone did not have any large effect on air reactivity in the concentration range between 0.35 and 1.8 wt%. This may indicate that the rather contradicting effects of sulphur reported in the literature [59] are related to indirect effects of sulphur like structural effects or poisoning of catalytic impurities.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -117-
10.3.3. THE EFFECT OF SULPHUR ON CARBOXY REACTIVITY
The carboxy reactivities of the cokes with sulphur additions showed the same pattern as the air reactivity, but in this case the difference in reactivity between the cokes was less than the standard deviations in each measurement. This means that the C02 reactivity was close to being constant in the whole concentration range, see Figure 10.14.
20 - y = 5.5 io -
S [wt%]
Figure 10.14. Carboxy reactivity of coke aggregate at 960 "Casa function of the sulphur content
Similar to air reactivity, sulphur alone did not have any significant effect on C02 reactivity in the concentration range between 0.35 and 1.8 wt%. This too confirms that the discrepancies among the reported effects of sulphur reported in the literature [59] are related to indirect effects of sulphur like structural effects or poisoning of catalytic impurities.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -118- PARTll
10.3.4. SUMMARY
Addition of dibenzothiophene to the coke precursor gave a 50 % sulphur yield in the resulting cokes.
Sulphur did not have a significant effect on the air- and carboxy reactivities of cokes for S contents in the range from 0.35 to 1.8 wt%. The C02 reactivity in this concentration range was close to being constant
These findings indicate that the rather contradicting effects of sulphur on reactivity reported in the literature are related to indirect effects of sulphur like structural effects and poisoning of catalytic impurities.
10.4. EFFECTS OF IRON ON THE AIR AND CARBOXY REACTIVITY OF COKES.
This work has partly been presented elsewhere [74].
10.4.1. IRON IN CALCINED COKES
In addition to the reference material, three cokes with increasing iron contents were produced (100, 400 and 1000 ppm added). Figure 10.15 shows the iron content of the calcined cokes as a function of the amount of iron added to the coke precursor as iron(III)acetylacetonate.
y = 0.9 x + 8.9
Fe added [ppm]
Figure 10.15. Iron added to coke precursor vs. the iron content in calcined coke.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -119-
The figure shows that most of the iron was fixed in the coke during the coking process. Energy dispersive X-ray microanalysis (EDX) showed that iron was predominantly in the metallic state after calcining, but a few of the analyzed particles contained sulphur. Figure 10.16 is a micrograph of a cluster of iron particles in the surface of a calcined coke grain.
Figure 10.16. Surface of calcined coke grain with cluster of iron particles, SEM image (secondary mode).
10.4.2. THE EFFECT OF IRON ON AIR REACTIVITY
The air reactivity of the cokes increased with increasing iron content, as seen in Figure 10.17. The error bars are the standard deviations of the individual measurements. (Note that the concentrations in the figure are ppm, not wt% as above.) The figure shows that iron impurities increase the rate of the airbum of the produced cokes at 525 °C.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -120- PARTII
y = 0.02x + 13.6
Fe [ppm]
Figure 10.17. Air reactivity of coke aggregate at 525 “Casa function of the iron content
Figure 10.18 is a SEM micrograph of iron particles in the surface of a coke grain taken after an air reactivity measurement. A pit has been formed by the cluster of iron particles, and small pits are observed around each particle. The surface texture changes caused by the iron phases indicate that iron catalyzed the C-02 reaction at 525 °C.
Figure 10.18. Surface textural changes caused by the iron catalyzed carbon-O z reaction on a coke grain from an air reactivity measurement, SEM image (secondary mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -121-
The EDX of the iron phases showed that iron was in the metallic state where the particles still were in contact with the carbon matrix, while non catalyzing iron oxide (Fe203/Fe304) was the predominant iron phase among the particles which had been separated from the matrix. This separation effect was caused by the total consumption of the carbon around them. None of the particles had sulphur contents above 5 at%. These findings contradicts the earlier suggestions on the effect of iron on airbum, see Chapter 10.1.3.
10.4.3. THE EFFECT OF IRON ON CARBOXYREACTIVITY
The carboxy reactivity of the cokes increased with increasing iron content, as seen in Figure 10.19. The figure shows that iron impurities increase the rate of the carboxy reaction for the produced cokes at 960 °C.
y = 0.01x4-6.2
Fe [ppm]
Figure 10.19. Carboxy reactivity of coke aggregate at 960 "Casa function of the iron content.
Figure 10.20 is a SEM micrograph of iron particles in the surface of a coke grain from the carboxy reactivity measurements. A pit has been formed by the iron particles, and they are observed sitting at the edges of the carbon lamellae eating their way through them. The surface texture is affected by the presence of the iron, and this indicates that iron catalyzes the carbon-C0 2 reaction at 960 °C.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -122- PART II
Figure 10.20. Surface textural changes caused by the iron catalyzed carbon-C0 2 reaction on a coke grain from a carboxy reactivity measurement, SEM image (secondary mode).
The EDX of the iron phases showed that iron was in the metallic state. No iron oxides were found, which would have indicated that iron is oxidized to a non catalyzing phase as seen for the carbon-0 2 reaction. This effect was observed in a previous study (see Chapter 10.1.2), and it is probably not seen here because it is necessary for the iron to be completely detached from the carbon matrix to become oxidized. None of the analyzed particles had sulphur contents above 1 at%.
10.4.4. THE EFFECT OF SULPHUR ON IRON IN CALCINED COKES
To investigate the effect of sulphur on iron catalysis, a set of cokes corresponding to the ones prepared to study the effect of iron alone was produced. In addition to iron (100, 400 and 1000 ppm), 1 wt% of sulphur was added to the cokes. A reference material with 1 wt% S was also produced. The iron contents of the calcined high sulphur cokes were close to the ones shown in Figure 10.17. The sulphur content was -0.35 wt% in the low sulphur cokes, and -0.85 wt% in the high sulphur cokes.
The EDX showed that the iron particles contained much more sulphur after calcining than in the materials which were described above. A range of sulphur concentrations in iron were found, from pure FeS to metallic iron.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -123-
10.4.5. THE EFFECT OF SULPHUR ON AIRBURN CATALYZED BY IRON
The air reactivity of the cokes with iron and sulphur added did also increase with increasing iron content, but the rate of increase was less than for the low sulphur cokes, and the reactivities of these materials were lower than the corresponding low sulphur materials, as shown in Figure 10.21. This indicates that sulphur reduces the effect of iron as a catalyst to airbum.
y = 0.02x + 13.6
y = 0.01x4-11.6 Fe + 1 wt% S
Fe [ppm]
Figure 10.21. Air reactivity of coke aggregate at 525 'Casa function of the iron content; cokes with and without 1 wt% sulphur added.
Figure 10.22 is a SEM micrograph of iron particles in the surface of a coke grain from the air reactivity measurements. As for the iron particles in low sulphur coke, a pit has been formed by the cluster of iron particles, and small pits are observed around each particle. The surface texture is changed by the iron phases, and this indicates that also iron from the high sulphur coke catalyzed the carbon-0 2 reaction at 525 °C.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -124- PART II
Figure 10.22. Surface textural changes caused by the iron catalyzed carbon-0 2 reaction on a high sulphur coke grain from an air reactivity measurement, SEM image (secondary mode).
The EDX of the iron phases were very similar to the analyses of the particles from the low sulphur coke reacted with air. Iron was in the metallic state where the particles were in contact with the carbon matrix, while non catalyzing iron oxide (Fe203/Fe304) was the dominating iron phase among the particles which had lost contact with the matrix. Some of the particles which were in good contact with the carbon matrix contained sulphur (below 15 at%), but not as much as the particles on the surface of the unreacted coke. This suggests that the particles were converted from a partly sulphidic phase to the metallic state during the initial reaction stage. The conversion process may be the reason why the reactivity was reduced despite the fact that the iron particles were catalytically active in both the low- and the high sulphur cokes. This coincides with the findings of 0degard et al. [75] who showed that considerably higher amounts of sulphurous compounds escaped from the anode in the first period after start of electrolysis than expected for uniform consumption of the anode sulphur.
The carbon matrix did probably not contain enough sulphur to maintain a continuos surface poisoning of the surfaces of the catalyst particles as sulphur was released during the gasification of the carbon.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -125-
10.4.6. THE EFFECT OF SULPHUR ON THE CARBOXYREACTION CATALYZED BY IRON
The carboxy reactivity of the high sulphur cokes increased with increasing iron content, as seen in Figure 10.23, but the difference in reactivity between the high- and low sulphur cokes was very small. For most of the cokes it was smaller than the standard deviation of the individual measurements. The measurements indicate that sulphur reduced the C02 reactivity slightly, but the rate of increase in reactivity with iron content was the same for high- and low sulphur cokes.
& Fe + 1 wt% S
y = O.Olx + 6.2
X 4- 5.
Fe [ppm]
Figure 10.23. Carboxy reactivity of coke aggregate at 960 "Casa function of the iron content; cokes with and without 1 wt% sulphur added.
The surface textural effects from iron particles seen on high sulphur coke were the same as seen on low sulphur coke after carboxy reaction. The micrograph in Figure 10.24 was included to show an iron particle “eating ” its way through a carbon lamella. The observed surface texture changes caused by the iron particles indicate that also iron from the high sulphur coke catalyzed the carbon-C0 2 reaction at 960 °C.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -126- PARTII
Figure 10.24. Surface textural changes caused by the iron catalyzed carbon-C0 2 reaction on a high sulphur coke grain from a carboxy reactivity measurement, SEM image (secondary mode).
The EDX analyses of the iron phases were very similar to the analysis of the particles from the low sulphur coke reacted with C02. None of the analyzed particles had sulphur contents above 1 at%. This too suggests that the particles are converted from a partly sulphidic phase to the metallic state during the initial reaction stage. The decrease in reactivity was smaller for the C02 reaction since this occurred at a higher temperature than the air measurements, and one would therefore expect the conversion from sulphide to metal to proceed at a higher rate.
10.4.7. SUMMARY
Iron from addition of iron(III)acetylacetonate to the coke precursor was almost completely fixed in the coke.
Iron was present in the metallic state after calcining of low sulphur cokes, while in the high sulphur cokes a range from pure FeS to metallic iron was found.
Iron catalyzed both the 02- and the C02 gasification of carbon.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -127-
Sulphur caused a reduction in the effect of iron as a catalyst to the carbon-0 2 reaction. It reduced the air reactivity and the rate of increase in reactivity with iron content The carboxy reactivity was lowered slightly due to sulphur, but the rate of increase in reactivity with iron content was the same for high- and low sulphur cokes.
Surface studies and energy dispersive X-ray micro analysis suggested that sulphur was bonded to iron during coke production, and that the iron sulphides were converted to metallic iron during the first stage of reaction with air and C02. The iron particles were not catalydcally active during this conversion, and sulphur therefore reduced the catalytic effect of iron on the carbon gasification reaction.
10.5. EFFECTS OF VANADIUM ON THE AIR AND CARBOXY REACTIVITY OF COKES.
This work has partly been presented elsewhere [76].
10.5.1. VANADIUM IN CALCINED COKES
In addition to the reference material, three cokes with increasing vanadium contents were produced (100, 400 and 1000 ppm added). Figure 10.25 shows the vanadium content of the calcined cokes as a function of the amount of vanadium added to the coke precursor as vanadium(m)acetylacetonate.
y = 0.3x + 63
V added [ppm]
Figure 10.25. Vanadium added to coke precursor vs. the vanadium content in calcined coke.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -128- PARTII
The figure shows that most of the vanadium was lost during coking. EDX showed that vanadium was predominantly in the metallic state after calcining, while some of it was converted to vanadium oxide (V2O3/V205/V6O13 could not be distinguished by EDX). None of the particles had significant sulphur contents.
10.5.2. THE EFFECT OF VANADIUM ON AIR REACTIVITY
The air reactivity of the produced cokes with vanadium additives was too high to give reliable measurements with the reactivity apparatus used, even at the lowest possible reaction temperature (475 °C). Thus, vanadium is an extremely strong catalyst to airbum.
The air reactivity of the cokes increased enormously with the vanadium content. Figure 10.26 is a SEM micrograph of vanadium particles in the surface of a coke grain taken after an air reactivity measurement. The particles are spread over an area of the surface, and a “crater” has formed in association with the vanadium. The surface texture changes caused by the vanadium phases indicate that vanadium catalyzed the C-02 reaction at 475 °C.
Figure 10.26. Surface textural changes caused by the vanadium catalyzed carbon-0 2 reaction on a coke grain from an air reactivity measurement, SEM image (secondary mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -129-
The EDX of the vanadium phases showed that some vanadium particles were in the metallic state, while others were vanadium oxide. None of the particles had significant sulphur contents.
10.5.3. THE EFFECT OF VANADIUM ON CARBOXYREACTIVITY
The carboxy reactivity of the cokes increased with increasing vanadium content, as seen in Figure 10.27. The figure shows that vanadium impurities increase the rate of the carboxy reaction of the produced cokes at 960 °C.
y = 0.09x + 6
V [ppm]
Figure 10.27. Carboxy reactivity of coke aggregate at 960 "Casa function of the vanadium content
Figure 10.28 is a SEM micrograph of discrete vanadium particles in the surface of a coke grain from the carboxy reactivity measurements. Pits have been formed by the vanadium particles, and they are observed sitting at the edges of the carbon lamellae eating their way through them.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -130- PARTII
Figure 10.28. Surface textural changes caused by the vanadium catalyzed carbon-CO, reaction on a coke grain from a carboxy reactivity measurement, SEM image (secondary mode).
Figure 10.29 is a SEM micrograph of vanadium particles spread over the surface of a coke grain from the carboxy reactivity measurements. The carbon is consumed close to the vanadium. The surface texture is affected by the presence of the vanadium. This indicates that vanadium catalyzed the carbon-C0 2 reaction at 960 °C.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -131-
Figure 10.29. Surface textural changes caused by the vanadium catalyzed carbon-C0 2 reaction on a coke grain from a carboxy reactivity measurement, SEM image (secondary mode).
The EDX of the vanadium phases showed that some vanadium particles were in the metallic state, while most of them were vanadium oxide. None of the particles had significant sulphur contents.
10.5.4. THE EFFECT OF SULPHUR ON VANADIUM IN CALCINED COKES
To investigate the effect of sulphur on vanadium catalysis, a set of cokes corresponding to the ones prepared to study the effect of vanadium alone was produced. In addition to vanadium (100, 400 and 1000 ppm), 1 wt% of sulphur was added to the cokes. A reference material with 1 wt% S was also produced. Figure 10.30 shows the vanadium content of the calcined high sulphur cokes as a function of the amount of vanadium added to the coke precursor as vanadium(IIQacetylacetonate. The sulphur content was -0.35 wt% in the low sulphur cokes, and -0.85 wt% in the high sulphur cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -132- PASTII
Figure 10.30. Vanadium added to coke precursor vs. the vanadium content in calcined high sulphur coke.
The figure reveals that more vanadium was fixed in the calcined high sulphur coke than in the low sulphur coke, see Figure 10.25, but EDX results was very similar to the low sulphur coke. Some vanadium particles were in the metallic state, while most of them were converted to vanadium oxide. None of the particles had significant sulphur contents.
10.5.5. THE EFFECT OF SULPHUR ON AIRBURN CATALYZED BY VANADIUM
As mentioned above (Chapter 1.3.2), the air reactivity of the produced cokes with vanadium additives was too high to give reliable measurements with the used reactivity apparatus even at the lowest possible reaction temperature (475 °C). Thus, vanadium is an extremely strong catalyst to airbum, and if sulphur inhibited the reaction, the effect was not sufficiently large to lower the reactivity into the measurable region.
The EDX results of the vanadium phases were very similar to the analyses of the particles from the low sulphur cokes reacted with air. EDX of the vanadium phases showed that some vanadium particles were in the metallic state, while others were vanadium oxide. None of the particles had significant sulphur contents; the highest observed sulphur content being 1.6 at%.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -133-
10.5.6. THE EFFECT OF SULPHUR ON THE CARBOXYREACTION CATALYZED BY VANADIUM
The carboxy reactivity of the high sulphur cokes increased with increasing vanadium content, as seen in Figure 10.31, but the difference in reactivity between the high- and low sulphur cokes was significant. Both the absolute reactivity value and the slope were lowered by sulphur addition, showing that sulphur reduces the effect of vanadium catalysis of the carboxy reaction.
•V CO, i-V CO,+ l wt%S y = 0.09x + 6
y = 0.04x + 6
V [ppm]
Figure 10.31. Carboxy reactivity of coke aggregate at 960 "Casa function of the vanadium content; cokes with and without 1 wt% sulphur added.
The SEM study showed the same surface textural effects as seen for the low sulphur cokes, see Figure 10.28 and Figure 10.29. The EDX was also very similar, showing that some vanadium particles were in the metallic state, while most of them were vanadium oxide. None of the particles had significant sulphur contents; the highest observed S content was 1.4 at%.
Hume [71] found that sulphur inhibits the sodium catalysis of the carbon-CO z reaction by forming a stable non-mobile complex with sodium. This prevents sodium from diffusing into the coke and from catalyzing the reaction at active sites. It may be that sulphur has a similar effect on vanadium. It is also possible that the sulphur was released as the carbon was consumed in the catalyzed reaction, giving a "steady state" poisoning of the active catalyst particles. This means that every time a sulphur atom was removed from the surface of a catalyst particle, another sulphur atom from the carbon was available to poison it again.
Since vanadium was a very active catalyst it would meet more sulphur during reaction than iron, and this might explain why sulphur had a larger inhibition effect on vanadium than on iron, see Chapter 10.4.3.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -134- PARTII
10.5.7. SUMMARY
Only 1/3 of the vanadium from addition of vanadium(HI)acetylacetonate to the pure coke precursor was fixed in the calcined coke, while 70% was fixed when vanadium was added together with 1 wt% sulphur (dibenzothiophene).
Vanadium in high- and low sulphur cokes was partly in the metallic state and partly converted to vanadium oxide after calcining (V20/V205/V6013 could not be distinguished by EDX). None of the vanadium particles had significant sulphur contents.
Vanadium catalyzed both the airbum and the carboxy reaction.
The air reactivity of the produced cokes with vanadium additives was too high to give reliable measurements with the used reactivity apparatus even at the lowest possible reaction temperature (475 °C). Thus, vanadium is an extremely strong catalyst to airbum, and if sulphur inhibited the reaction, the effect was not sufficiently large to lower the reactivity into the measurable region.
Sulphur caused a significant reduction in the effect of vanadium as a catalyst to the carboxy reaction. It reduced the C02 reactivity and the rate of increase in reactivity with vanadium content.
SEM analysis and EDX did not indicate the difference in reactivity between high and low sulphur cokes. It was suggested that sulphur inhibits the vanadium catalysis of the carbon-C0 2 reaction by forming a stable non-mobile complex with vanadium. This would prevent vanadium from catalyzing the reaction at active sites. It is also possible that the sulphur released as the carbon was consumed in the catalyzed reaction, gave a "steady state" poisoning of the active catalyst particles.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -135-
10.6. EFFECTS OF NICKEL ON THE AIR AND CARBOXY REACTIVITY OF COKES.
This work has partly been presented elsewhere [76].
10.6.1. NICKEL IN CALCINED COKES
In addition to the reference material, three cokes with increasing nickel contents were produced (100, 400 and 1000 ppm added). Figure 10.32 shows the nickel content of the calcined cokes as a function of the amount of nickel added to the coke precursor as nickel(II)acetylacetonate.
y = 0.6x + 67
Ni added [ppm]
Figure 10.32. Nickel added to coke precursor vs. the nickel content in calcined coke.
The figure shows that -60% of the nickel was fixed in the coke during coking. EDX showed that nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after calcining. Maximum sulphur content found in a Ni-S-O phase was 30 at %.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -136- PART II
10.6.2. THE EFFECT OF NICKEL ON AIR REACTIVITY
The air reactivity of the cokes is shown in Figure 10.33. It seems like the air reactivity as a function of nickel content went through a minimum. However, it is more likely that the minimum in reactivity was an effect caused by the organic part of the added nickel compound, and that nickel did not have any substantial effect on airbum. This was seen when the air reactivity of the cokes with sulphur and nickel additives was plotted versus amount of substance added to the coke precursor, Figure 10.34.
Ni [ppm]
Figure 10.33. Air reactivity of coke aggregate at 525 "Casa function of the nickel content
Ni Air S Air
Ni or S added [ppm]
Figure 10.34. Air reactivity of coke aggregate at 525 “Casa function of the amount of additives.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -137-
The two curves above superimpose, which indicates that the additions had a common effect. It is more likely that this effect was originated by the organic part of the dibenzothiophene- and nickel(II)acetylacetonate molecules, than that sulphur and nickel are causing the minimum. The aromatic compounds may have a diluting effect on the coke precursor that could change its viscosity, and thus slightly change the carbonization conditions. This may lead to a somewhat smaller reactivity due to a more ordered coke structure. The effect from the organic part of the additives was not seen on the addition of iron(m)acetylacetonate and vanadium(III)acetylacetonate, probably due to the fact that the catalysis by iron and vanadium was overshadowing the effect
Figure 10.35 is a SEM micrograph of nickel particles at the surface of a coke grain taken after an air reactivity measurement. The particles are spherical, and they are not sitting at areas where the surface had been preferably attacked by the reaction. Thus, also the surface texture indicates that nickel did not catalyze the C-02 reaction at 525 °C for the produced cokes.
Figure 10.35. Surface texture of a coke grain with nickel particles from an air reactivity measurement, SEM image (secondary mode).
EDX showed that nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after reaction with air, but the sulphur content was not as high as it was in the calcined cokes (maximum 15 at% against 30 at% in calcined coke).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -138- PARTII
10.6.3. THE EFFECT OF NICKEL ON CARBOXYREACTIVITY
Nickel did not have any great effect on carboxy reactivity. A small increase in reactivity with increasing nickel content is observed, as shown in Figure 10.36.
30 - 25 - y = 0.005x + 6
io -
Ni [ppm]
Figure 10.36. Carboxy reactivity of coke aggregate at 960 "Casa function of the nickel content.
Figure 10.37 is a SEM micrograph of nickel particles in the surface of a coke grain from the carboxy reactivity measurements. The particles are spherical, and they are not sitting at areas where the surface has been preferably attacked by the reaction. Thus, also the surface texture indicates that nickel did not catalyze the carboxy reaction at 960 °C for the produced cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -139-
Figure 10.37. Surface texture of a coke grain with nickel particles from a carboxy reactivity measurement, SEM image (secondary mode).
EDX showed that nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after reaction with C02. The maximum sulphur content was equal to the calcined cokes (maximum 30 at%).
10.6.4. THE EFFECT OF SULPHUR ON NICKEL IN CALCINED COKES
To investigate the effect of sulphur on nickel catalysis, a set of cokes corresponding to the ones prepared to study the effect of nickel alone was produced. In addition to nickel (100, 400 and 1000 ppm), 1 wt% of sulphur was added to the cokes. A reference material with 1 wt% S was also produced. The nickel contents of the calcined high sulphur cokes were close to the ones shown in Figure 10.32. The sulphur content was -0.35 wt% in the low sulphur cokes, and -0.85 wt% in the high sulphur cokes.
EDX was very similar to the low sulphur coke. Nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after calcining. Maximum sulphur content found in a Ni-S-O phase was 30 at %.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -140- PART II
10.6.5. THE EFFECT ONAIRBURN OF SULPHUR ADDED TOGETHER WITH NICKEL
As expected, addition of sulphur did not significantly change the air reactivity of the cokes with nickel additives. The air reactivity of the cokes is shown in Figure 10.38.
NiAir Ni Air + 1 wt% S
■==1—1
Ni [ppm]
Figure 10.38. Air reactivity of coke aggregate at 525 "Casa function of the nickel content; cokes with and without 1 wt% sulphur added.
Figure 10.39 is a SEM micrograph of particles in the surface of a high sulphur coke grain taken after an air reactivity measurement. The small particles sitting at the edge of the pits (top, and left side, of picture) was not nickel, but sodium; showing that there were other catalytic impurities in the produced cokes. The large, bright, spherical particles were nickel phases. The surface was unaffected close to the nickel particles. Thus, also here the surface texture indicates that nickel did not catalyze the C-02 reaction at 525 °C at the produced cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -141-
Figure 10.39. Surface texture of a coke grain with nickel particles from an air reactivity measurement, SEM image (secondary mode).
EDX showed that nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after reaction with air, but the sulphur content was not as high as it was in the calcined cokes (maximum 10 at% against 30 at% in calcined coke).
10.6.6. THE EFFECT ON THE CARBOXY REACTION OF SULPHUR ADDED TOGETHER WITH NICKEL
Addition of sulphur to the coke precursors caused a minor reduction in reactivity, but for most cokes the reduction was smaller than the standard deviation of the individual measurements, see Figure 10.40.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -142- PART II
NiCO; J3 30 —-N1CO2+ 1 wt%S
y = 0.005x + 6
y = 0.003x + 6
Ni [ppm]
Figure 10.40. Carboxy reactivity of coke aggregate at 960 "Casa function of the nickel content; cokes with and without 1 wt% sulphur added.
Figure 10.41 is a SEM micrograph of nickel particles at the surface of a high sulphur coke grain from the carboxy reactivity measurements. The particles are spherical, and they are not sitting in areas where the surface has been preferably attacked by the reaction. Thus, also here the surface texture indicates that nickel did not catalyze the carboxy reaction at 960 °C for the produced cokes.
Figure 10.41. Surface texture of a high sulphur coke grain with nickel particles from a carboxy reactivity measurement, SEM image (secondary mode).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -143-
EDX showed that nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after reaction with COr The maximum sulphur content was equal to the calcined cokes (maximum 30 at%).
10.6.7. SUMMARY
About 60 % of the nickel added as nickel(II)acetylacetonate to the coke precursor was fixed in the calcined coke.
Nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after calcining.
Nickel did not have any substantial effect on air and carboxy reactivity. Addition of sulphur caused a minor reduction in carboxy reactivity as a function of nickel content.
These conclusions are in accordance with a parallel work by Casada et al. [77] who found that nickel in calcined cokes does not have any significant effect on anode performance over the range studied (160 - 370 ppm nickel). They found that air reactivity and C02 reactivity dusting were not statistically different for the anodes tested, and that C02 reactivity of anodes with and without butts were statistically different. The latter differences were not associated with changes in nickel level.
10.7. SUMMARY ON THE EFFECTS OF SULPHUR, IRON, VANADIUM AND NICKEL IMPURITIES
The relative catalyst strength of the investigated impurities was vanadium » iron > nickel = sulphur alone. This ranking was valid for both airbum and carboxy reaction. Sulphur and nickel are equal because they had no significant catalytic effect, although Ni may have a very small effect on C02 reactivity.
Iron, vanadium and nickel were all found partly in the metallic state after calcining. Thus, they should all have the possibility to be involved in oxidation reduction cycles as shown in Chapter 8.2.2. The ability to undergo such cycles depends on the stability of the oxide(s) involved and the number of possible oxidation states which the metal can have under the given conditions. V and Fe have several possible oxidation states, while Ni only has one in addition to elemental Ni.
From these investigations it seems like iron can take part in the red-ox-cycle as long as it is in good contact with carbon, vanadium takes part in the catalysis as long as the reaction conditions are maintained, while nickel never starts to catalyze the reaction.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -144- PARTII
Comparison of Figure 10.20, Figure 10.28 and Figure 10.37 shows that the contact, or wetting, between the catalyst particles and the carbon matrix is best for vanadium, intermediate for iron and poor for nickel. Vanadium is wetting and spreading on the carbon surface, iron is wetting the surface while nickel is non wetting to the carbon surface. This is probably a main reason for the difference in strength of the investigated catalysts.
The inhibition of catalytic activity by sulphur did probably not have the same mechanism for iron and vanadium.
Sulphur may inhibit the vanadium catalysis of the carbon-C0 2 reaction by forming a stable immobile complex with vanadium. This would prevent vanadium from catalyzing the reaction at active sites. It is also possible that the sulphur released as the carbon was consumed in the catalyzed reaction, gave a "steady state" poisoning of the active catalyst particles.
Sulphur was bonded to iron during coke production, and the iron sulphides were converted to metallic iron during the first stage of reaction with air and C02. The iron particles were probably not catalytically active during this conversion, and sulphur therefore reduced the catalytic effect of iron on the carbon gasification reactions.
As mentioned above, the wetting properties of the catalyst particle on the carbon substrate is very important [64]. It is a possibility that sulphur inhibits the catalytic reactions by changing the wetting characteristics of the carbon materials.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -145-
10.8. REACTIVITY OF PITCH COKES
This work has partly been presented elsewhere [78].
Anode carbon “dust” consists normally of petroleum coke particles that loose their anchoring to the anode body when the binder coke reacts preferentially with C02 and/or air [79]. Selective anode reactivity is most often caused by the binder coke being consumed faster than the petroleum coke aggregate. This is the main cause for carbon loss due to dusting, and may cause considerable operational problems (see Chapter 1.3). Excess carbon reactivity will increase the total anode consumption and may, in extreme cases, lead to loss of anodes and out-of-tum anode changes. Important parameters are catalytic impurities [59] and heat treatment temperature differences between petroleum coke and binder coke [80].
Important processing and materials properties that may influence the rate of excess carbon gasification is the maximum heat treatment temperature of anode petroleum coke aggregate and the final baking level of the anode [81], i.e. the binder coke heat treatment temperature and soaking time (at, or near, the maximum baking temperature). In prebaked anode fabrication this baking level can be tailored to give a minimum reactivity difference in binder coke and aggregate coke. In Spderberg cells one do not have this degree of freedom; the anode baking level is restricted by the cell operation temperature, which is approximately 960 °C, and the “soaking time” the anode is subject to given by submersion and anode consumption rate.
The baked anode may, depending on whether it is a prebaked or Spderberg anode, consist of about 10-15 % pitch binder coke, the balance being the calcined petroleum coke aggregate. During laboratory reactivity testing of baked anodes the contribution from the minority pitch binder coke to the overall reactivity of the anode and to other anode properties are hence often overshadowed by the properties of the major petroleum coke component.
In order to evaluate the specific reactive properties of binder cokes in polygranular composite anodes a number of industrially used and prospective anode binder pitches were coked alone in the laboratory coker as described in Chapters 9.1.3 and 9.5.2, and tested for C02 and air reactivity.
When coked at atmospheric pressure most binder pitches are turned into foam-like carbons with low bulk densities and with C02 and air reactivity properties that are difficult to reproduce. Binder pitch coke yield from a 120°C (Mettler) softening point anode pitch is typically 55-60 wt% under these conditions. In order to avoid the foaming during carbonization and make the final pitch coke denser, the laboratory coker was used to make pitch cokes.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -146- PARTII
10.8.1. PITCHES AND PRODUCED PITCH COKES
The pitches that were carbonized, calcined and tested were, for the most part, vacuum anode pitches 1 for anode fabrication with softening points (Mettler) in the range of about 116-123°C (Table 10.7). All these pitches were taken from shipments to aluminium plants (Elkem Aluminium ASA, Norway). In addition there was one thermal pitch 2, some specialty pitches 3 sent to Elkem for testing and a few petroleum pitches 4. All those are also listed in Table 10.7. The four different pitch producers are identified by the letters A-D.
Table 10.7. Description and some characteristic analysis of the pitches.
Softening Coke Pitch point, Mettler yield Remarks rci [wt%] A1 122.4 61.9 Vacuum anode pitch for smelter A2 116.6 60.2 Vacuum anode pitch for smelter1 B1 118.6 59.3 Vacuum anode pitch for smelter1 B2 119.7 60.4 Vacuum anode pitch for smelter1 B3 119.0 59.5 Vacuum anode pitch for smelter1 B4 120.8 60.2 Vacuum anode pitch for smelter1 B5 122.4 60.8 Vacuum anode pitch for smelter1 B6 121.6 61.0 Vacuum anode pitch for smelter1 B7 120.7 59.2 Vacuum anode pitch (batch distilled) to lab 3 B8 131.8 62.0 Vacuum anode pitch (batch distilled) to lab 3 B9 142.5 64.9 Vacuum anode pitch (batch distilled) to lab 3 BIO 112.3 48.1 Petroleum pitch to lab 4 Bll 120.3 50.4 Petroleum pitch to lab 4 Cl 118.4 59.7 Vacuum anode pitch for smelter1 C2 123.0 61.9 Vacuum anode pitch for smelter1 C3 120.0 61.9 Vacuum anode pitch for smelter1 C4 119.8 60.6 Vacuum anode pitch for smelter1 C5 109.2 46.9 Petroleum pitch to lab 4 D1 110.3 58.0 Thermal pitch 2
1. Vacuum anode pitch for smelter: pitch produced by a continuous vacuum distillation process with coal tar (from coal coking) as precursor. 2. Thermal pitch: heat soaked anode pitch for smelter. 3. Vacuum anode pitch (batch distilled) to lab: pitch produced by a batch vacuum distillation process with coal tar (from coal coking) as precursor. 4. Petroleum pitch to lab: pitch produced from petroleum precursor.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION - 147-
Table 10.8 shows the results from the impurity analysis (atomic absorption) of the pitches. Compared to normal anode grade petroleum cokes the pitches were high in Na, Zn and Pb and low in V and Ni. The petroleum pitches BIO and Bll had ash contents so low that it was difficult to obtain enough material for analysis.
Table 10.8. Impurity analysis of pitches (as received).
Pitch Ash s Fe Si V Ni Ca Na K A1 Zn Pb Mg [wt%] [wt%] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] A1 0.26 0.58 170 243 <5 <5 85 203 26 185 205 178 12 A2 0.25 0.58 165 245 <5 <5 81 200 26 195 210 190 12 B1 0.24 0.59 186 187 <5 6 149 195 26 110 274 228 3 B2 0.20 0.47 150 120 <5 6 82 140 13 74 255 290 8 B3 0.17 0.47 170 101 <5 5 71 135 12 71 220 230 8 B4 0.20 0.50 128 135 <5 <5 63 150 15 70 288 275 12 B5 0.22 0.49 120 125 <5 <5 76 215 26 76 323 238 12 B6 0.18 0.50 145 123 <5 <5 61 140 14 77 230 223 8 B7 0.12 0.51 160 56 <5 <5 24 13 10 45 180 175 3 B8 0.22 0.50 235 140 <5 <5 85 155 16 74 265 240 9 B9 0.26 0.51 375 180 <5 <5 100 155 16 88 270 280 10 B10 0.86 Bll 0.54 Cl 0.30 0.61 133 99 <5 <5 80 170 13 62 747 457 6 C2 0.16 0.47 183 157 <5 <5 56 140 15 101 190 193 10 C3 0.15 0.49 91 120 <5 <5 36 110 9 87 160 120 9 C4 0.21 0.49 160 235 <5 <5 59 210 15 113 180 118 15 C5 0.07 0.08 330 60 <5 <5 26 37 5 12 10 12 6 D1 0.15 0.52 190 99 <5 <5 44 52 13 71 205 215 15
The vacuum distilled anode pitches shown in Table 10.7 showed a coke yield, analyzed according to standard procedures under atmospheric pressure [82], in the range of about 59-62 wt%. The calcined pitch cokes from the coke reactor did, however, obtain coke yields in the range of 88-90 wt% (Table 10.9). A typical anode pitch contains about 93 wt% C, 4.5 wt% H, 1 wt% N, 1 wt% O and 0.5 wt% S. If one assumes that most of the hydrogen and oxygen was removed during calcining the actual coke yield was very close to the theoretical maximum. The real coke yield in the anode is higher than what the standard laboratory analysis show but probably less than what the high pressure coke reactor gives.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -148- PARTII
The reactor-coked and calcined petroleum pitches (Table 10.9) gave a considerably lower carbon yield than the coal tar based anode pitches. This was according to expectations since they were less aromatic and their C/H ratio was lower than for coal tar pitches with comparable softening points.
Table 10.9. Gas reactivity values and some other characteristic properties measured on the calcined pitch cokes produced in the laboratory coker.
Pitch VBD Coke yield C02 Reactivity Air Reactivity -2+1 mm 960 °C 475 °C (coke) |kg/m3l fwt%] [mg/g h] [mg/g h] A1 89.2 45.6 13.5 A2 89.3 44.4 16.3 B1 850 88.2 52.9 19.2 B2 842 88.5 52.1 19.6 B3 872 88.2 47.1 19.0 B4 88.3 59.5 22.1 B5 88.5 65.3 23.3 B6 863 88.9 54.1 16.7 B7 846 89.4 14.3 1.9 B8 88.7 77.8 24.3 B9 89.0 81.0 22.6 BIO 865 80.6 7.9 1.4 Bll 854 81.2 5.8 2.4 Cl 892 88.4 67.1 23.4 C2 88.8 48.9 16.9 C3 89.7 33.3 12.4 C4 848 88.8 46.0 19.7 C5 76.0 53.8 6.6 D1 922 88.9 26.8 5.7
Vibrated bulk density (VBD) measurements on the calcined cokes from the reactor (-2 +1 mm fraction) gave results in the same range as measured on calcined anode grade petroleum cokes. Calcined cokes made from vacuum distilled anode pitches (Table 10.9) showed VBD values (-2 +1 mm fraction) in the range of 842-892 kg/m3. For comparison, one of Elkem’s petroleum coke suppliers delivered anode coke through a 5 year period with annual average VBD values in the range of 830-855 kg/m3.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -149-
The impurity analyses of the calcined pitch cokes are shown in Table 10.10. By using the calcined coke analysis data from Table 10.10 together with the coke yield data given in Table 10.9, it was possible to directly compare the impurity concentration, [ppm] or [wt%], in the calcined coke with those of the raw materials:
CY ^imp{pitch, calc.) ” ^impicalcined coke, exp.) ^ qq (10.2)
Table 10.10. Impurity analysis of carbonized and calcined pitch cokes.
Pitch Ash s Fe Si V Ni Ca Na K Al Zn Pb Mg [wt%] [wt%] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] Al 0.30 0.37 240 300 <5 21 100 355 38 190 190 110 26 A2 0.29 0.39 205 300 <5 7 100 305 36 190 180 100 28 B1 0.28 0.32 255 210 <5 8 91 305 33 130 270 140 37 B2 0.22 0.31 180 100 <5 8 54 260 20 67 240 160 30 B3 0.19 0.30 200 95 <5 8 47 230 15 70 205 140 24 B4 0.24 0.34 160 130 <5 5 73 290 20 78 255 160 46 B5 0.26 0.31 120 138 <5 7 84 310 32 90 260 163 41 B6 0.23 0.27 110 150 <5 6 65 260 19 100 205 110 39 B7 0.10 0.36 115 62 <5 4 23 40 9 37 155 87 10 B8 0.28 0.33 255 130 <5 7 72 280 18 69 255 155 45 B9 0.31 0.37 505 125 <5 6 81 315 23 75 260 185 50 B10 0.02 0.77 19 14 5 5 40 2 9 4 5 16 Bll 0.03 0.41 16 13 5 5 51 3 15 3 4 21 Cl 0.31 0.42 155 90 <5 6 75 305 17 78 720 300 33 C2 0.21 0.32 225 150 <5 4 57 215 19 92 170 94 40 C3 0.20 0.34 110 195 <5 6 46 195 18 103 130 35 34 C4 0.25 0.32 190 215 <5 8 64 300 22 130 165 65 55 C5 0.08 0.07 95 40 <5 7 24 102 7 14 26 <5 22 D1 0.18 0.36 165 155 <5 6 47 110 14 78 190 120 35
By plotting the calculated raw material impurity concentration, cldc), against the measured pitch impurity concentration, ev), it was possible to see if impurities were added or removed in the coking and calcining steps compared to the standard pitch analysis method. Figure 10.42 and Figure 10.43 show examples of such correlations for Al, Na, Pb, Zn, K and Mg. The ideal 1:1 correlation curve is drawn as a straight line in each figure.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -150- PAST II
If a given pitch impurity was retained in the calcined pitch coke the plot will be on the 1:1 correlation line. If, for some reason, some impurity concentrations were lower or higher in the calcined coke than they should be based on raw material analysis and coke yield, the data should fall below or above the 1:1 curve, respectively.
o Al ■ Pb • Na ------1:1 corr.
C imp (pitch, exp.) (ppm) Figure 10.42. Correlation between Al, Pb and Na impurity concentrations obtained from raw material (pitch) analysis and those calculated from Eq. (10.2).
• Mg
O K
1:1 corr.
C imp (pitch, exp.) (ppm)
Figure 10.43. Correlation between Mg and K impurity concentrations obtained from raw material (pitch) analysis and those calculated from Eq. (10.2).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -151-
The Al analysis data, shown in Figure 10.42, closely follows the 1:1 line. Although an aluminium can was used as a sample holder for the coke precursor, the cokes did not pick up substantial amounts of Al. On the other hand, the Mg concentration in the calcined pitch cokes was significantly higher than expected (Figure 10.43). It turns out that the Al alloy the can is made of contains about 1.5 wt% Mg, and a small amount of this dissolved and contaminated the coke during the carbonization step. Another minor impurity, K, does not deviate significantly from the 1:1 line (Figure 10.43).
Na and Pb are also plotted in Figure 10.42. There was less Pb found in the pitch cokes than expected from the pitch analysis. The reason was probably some volatilization of Pb (or Pb compounds) during the calcining step to 1000 °C. The calcination temperature was considerably higher than the maximum heat treatment temperature of 700 °C in the carbon (pitch) oxidation step prior to ash analysis. On the other hand, more Na was retained in the calcined pitch coke than predicted from the raw material analysis. The reason for this is not known.
Of the other major impurities listed in Table 10.8 and Table 10.10, Fe, Si, and the total ash content seem to follow the 1:1 correlation. Ca partially follows the correlation but with an overweight of data on the low side. Zn and S are significantly on the low side of the 1:1 correlation, i.e. some of these impurities were volatilized during calcination.
V and Ni concentrations were generally too low to be measured or too close to the quantification limit of the AAS apparatus to give meaningful results. V and Ni are important contaminants in petroleum coke but are almost non-existent in coal-tar pitches. As seen above (Chapter 10.5), especially V is a very strong air oxidation catalyst.
10.8.2. CARBOXYAND AIR REACTIVITY OF PITCH COKES
Experimental C02 and air reactivity values, obtained at 960 °C and 475 °C, respectively, are listed in Table 10.9. The cokes made from the batch distilled coal tar pitch, B7, the petroleum pitches, B10 and Bll, and thermal pitch, Dl, show all remarkably low gas reactivity values. The coke made from petroleum pitch C5 has a low air reactivity but its C02 reactivity is high and comparable to the coal tar pitch cokes.
A correlation matrix for all the trace metal elements, sulphur and reactivity parameters is shown in Table 10.11. The statistical analyses were calculated by a standard computer program [83]. All reactivity-impurity correlations with R > 0.50 are shown in boldface numbers. All impurity-impurity correlations with R > 0.75 are shown in bold italics.
As expected there was a strong correlation between C02 and air reactivity values (R = 0.89). Several of these coke impurities are known to catalyze both the carboxy and airbum reactions [59].
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -152- PART II
There also seemed to be a correlation (R > 0.50) between the reactivity parameters and the concentration of Fe, Ca, Na, K, Zn, Pb and Mg. All of these impurities except Zn are known oxidation catalysts. The reason why Zn came up with medium strong correlation coefficients to both C02 (R = 0.59) and air reaction (R = 0.66) was its covariance with the strong oxidation catalyst Pb (R = 0.93), i.e. when the Pb concentration in the pitch coke (and pitch) varied the Zn concentration varied accordingly.
Table 10.11. Calcined coke reactivity-impurity correlation matrix.
Fe Si Ni Ca Na K Al Zn Pb Mg S CO] Air
Fe - Si 0.42 - Ni 0.23 0.57 - Ca 0.64 0.81 0.47 - Na 0.62 0.69 0.46 0.94 - K 0.53 0.88 0.57 0.94 0.86 - Al 0.41 0.98 0.59 0.84 0.75 0.91 - Zn 0.33 0.16 0.03 0.55 0.58 0.34 0.25 - Pb 0.51 0.16 0.06 0.63 0.65 0.42 0.25 0.93 - Mg 0.55 0.39 -0.04 0.58 0.68 0.43 0.36 0.34 0.40 - S -0.19 -0.16 -0.09 -0.22 -0.24 -0.21 -0.12 -0.05 -0.07 -0.25 - COj 0.67 0.28 0.14 0.71 0.80 0.52 0.30 059 0.69 0.69 -0.46 - Air 0.59 0.39 0.12 0.77 0.90 0.62 0.44 0.66 0.75 0.75 -0.28 0.89 -
Other significant impurity-impurity correlations were within the group of elements Si, Ca, Na, K and Al. Much of this covariance could be attributed to mineral contamination since these elements are among the most commonly present in rock minerals (coal contaminants) and refractory linings in coal tar and coal tar pitch processing equipment. Na is added as soda during pitch distillation in order to neutralize small amounts of HC1 in the tar, and thus prevent corrosion.
Sulphur showed a negative correlation towards both C02 and air reactivity (Table 10.11). The numerical value of the correlation coefficient was very low for air reactivity but close to 0.5 for C02 reactivity. S is a known metal catalyst poison and has been shown to retard C02 and air reactivity of petroleum cokes, see Chapter 10.5 and Chapter 10.6.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -153-
The correlation matrix verifies the previous findings; iron is a catalyst to both air and C02 reactivity, nickel does not have a significant effect and sulphur inhibits the reactions. Vanadium was present in too small amounts to be included in the correlation.
Since the rate of coke gasification at these temperatures (960 °C with C02 and 475 °C with air) is mainly determined by the chemical reactions, the concentration of active catalyst sites at the coke surface may be rate determining. If the total catalytic activity is the sum of the catalytic activity of all the different impurity metals a simple multiregression analysis of reactivity against coke impurity concentrations can be performed:
Rfcalc.) — Og + OjCMefi) + U2CMe(2) + . . . + OnCMefn^ + OgCg (10.3) where a is regression coefficients, cMe is metal impurity concentrations, cs is sulphur concentration. By only including the impurity content of the most significant elements according to the correlation matrix in Table 10.11 (Fe, Ca, Na, K, Pb, Mg and S) in the multiregression analysis, the correlation between calculated COz reactivity
(RCo2tcaic.) and experimental C02 reactivity (RC02,evJ) obtained a correlation coefficient of R = 0.94. If the K and Mg terms were removed from the regression analysis (small absolute values and high relative uncertainty) the correlation coefficient fell to R = 0.91. Any further removal of terms resulted in a significantly poorer correlation. The S coefficient came out negative in all calculations, indicating an inhibiting effect on the C02 gasification.
Another way to formulate an empirical relationship between reactivity data and coke impurity levels, if one assumes that sulphur is inhibiting the gasification, is to put the S concentration in the denominator:
a\CMe(V) ...... ■Jl~anCMe(n) '■(cak.) = an + (10.4)
By putting the same coke impurity analysis data into Eq. (10.4) as previously done for Eq. (10.3) the RC02(adc2 to RC02fap) relationship improved slightly to R = 0.95. The correlation is shown graphically in Figure 10.44. If also the K and Mg terms were removed from this expression, the correlation is only marginally reduced to R = 0.94. Further removal of impurity terms will also here result in a significantly poorer fit Of these two empirical expressions of reactivity vs. impurity content, Eq. (10.4) seem to fit the C02 reactivity data better.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -154- PARTII
O Eqn.(9), R=0.95
------R=1.00
CO2 reactivity, exp. (mg/g h) Figure 10.44. Correlation between experimental C02 reactivity measurements of calcined “binder” cokes and best fit for C02 reactivity according to Eg. (10.4).
If Eg. (10.3) was used to fit the air reactivity data to the full set of calcined coke impurities (Fe, Ca, Na, K, Pb, Mg, S) the correlation coefficient of the expression was R = 0.97. The RaMcalc) to Rairlevl correlation is shown graphically in Figure 10.45. Removal of the least significant terms from the expression only marginally affected the fit. With only the Na term left (also the constant term ao removed) the fit still had a correlation coefficient as high as R = 0.90:
R^caic., = 0.0673c Na (10.5)
Other investigators [87], [88] have reported that the sodium concentration in anodes must be above a threshold level before sodium has any influence on air reactivity. Such a threshold level was not observed in the Na concentration ranges investigated here (40-355 ppm Na in calcined pitch cokes).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -155-
• Eqn.(8), R=0.97
------R=1.00
Air reactivity, exp. (mg/g h)
Figure 10.45. Correlation between experimental air reactivity measurements of calcined “binder” cokes and best fit for air reactivity according to Eq. (10.3).
A regression fit of air reactivity to Eq. (10.4) gave a poorer fit than to Eq. (10.3). With all impurity terms present the correlation coefficient was R = 0.90. This was not surprising since the calcined coke sulphur content showed a very poor correlation to air reactivity numbers (Table 10.11). Removal of impurity terms up to the point that only Ca, Na, Pb and S were left (ao removed) also resulted in R = 0.90. Attempts to further simplify the expression resulted in a significantly reduced correlation coefficient.
These empirical relationships in general, and air reactivity in particular, cannot be extended to anode cokes and baked anodes since the impurity make-up is different there. Anode grade petroleum cokes contain normally very little Pb (and Zn) while they have a significant concentration of V and Ni. Especially V is a very potent air oxidation catalyst.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -156- PARTII
10.8.3. EFFECT OF PITCH ON ANODE REACTIVITY
As previously mentioned, marginal differences in pitch impurity analysis may have an impact on industrial anode performance but may be masked by the experimental uncertainty in laboratory baked anode reactivity tests. However, if the reactive properties of the binder pitch cokes differed enough, the effects on anodes made with these pitches should be detectable also on laboratory specimens. To verify the findings from pitch cokes against baked anode properties, two laboratory anodes, made with binder pitches that showed considerably different reactive properties, were made and tested. Two anode pitches that gave significant different calcined “binder ” coke reactivity results were chosen as binder pitches for the laboratory made anodes. The choices were pitches B5 and C3. With respect to gas reactivity measured on calcined coke made from these pitches, the gasification rate of B5 was almost twice as high as that measured on C3 in both C02 and air (Table 10.9).
The same anode coke was used with both pitches. A low sulphur (1.15 wt%), low metal impurity anode coke of North Sea origin (Statoil, Mongstad) was chosen. A graph comparing the impurity levels in the two pitches with that of the coke is shown in Figure 10.46.
Element
Figure 10.46. Impurity analysis of the anode pitches B5, C3 and the petroleum coke (PC) are compared.
Since Spderberg anodes have higher binder pitch (coke) content and lower baking temperature than prebaked anodes, a Spderberg anode recipe and Spderberg anode baking simulation (980 °C) was chosen. The anode pastes were targeted to constant viscosity which resulted in a marginally different pitching level. Some green and baked properties are listed in Table 10.12.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -157-
Table 10.12. Some green and baked properties measured on laboratory anodes.
Pitch Open Permea Electrical Ash Anode content GAD BAD porosity bility resistivity content [wt%] [kg/m3] [kg/m3l [vol%] [nPm] [ptim] [wt%]
B5 anode 28.1 1662 1500 23.7 5.45 70.6 0.13
C3 anode 29.0 1669 1500 23.5 4.72 71.9 0.12
The content of calcined petroleum coke aggregate in the baked anodes was close to 90 %. This resulted in a large “dilution ” effect on the binder coke impurity content, as shown in Figure 10.47. However, the content of Fe increases significantly in the baked anode relative to what was expected from raw materials analysis. This is due to Fe pick-up from ball mill wear. What was supposed to be the most potent air oxidation catalyst of all impurities present, V, had the same concentration in both anodes.
300
_ 250 J BB5 anode ------g- ~ 200 □ 03 anode I g 150 o f 100 ft “ 50 ■ _ Ijll,B1 1 0 i J
Element
Figure 10.47. Metal impurity analysis of baked anodes made with B5 and C3 pitches.
Anode reactivity data are presented in Table 10.13 and Table 10.14 for C02 at 960 °C and air at 525 °C, respectively. In accordance with the higher content of elements supposed to be strong carbon oxidation catalysts, notably Ca, Na and Pb, anode B5 showed considerably higher reactivity values for all four parameters, in both C02 and air, than anode C3. Since only the binder coke was different in the two anodes, it was not surprising that it was the dust generation numbers that showed the largest difference, in relative terms.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -158- PART II
Table 10.13. C02 reactivity parameters measured on baked laboratory anodes.
CO, reactivity Dust Dust Total Anode 960 °C index generation bumoff [mg/cm2 h] fwt%l |wt%l fwt%l B5 anode 22.3 19.5 2.2 11.4
C3 anode 18.4 14.5 1.3 9.0
Table 10.14. Air reactivity parameters measured on baked laboratory anodes.
Air reactivity Dust Dust Total Anode 525 °C index generation bumoff [mg/cm2 h] [wt%l [wt%l fwt%l
B5 anode 48.1 28.0 6.2 22.7
C3 anode 34.6 14.3 2.0 13.9
10.8.4. SUMMARY
The developed laboratory coker could be used for carbonization of anode binder pitches at up to 15 bar pressure and under controlled thermal conditions. It gave uniform “anode binder cokes” for laboratory use. The coke yield after pressure carbonization and calcination to 1000 °C was typically 88-90 % and the vibrated bulk density of the coke was comparable to calcined anode grade petroleum coke.
Airbum and carboxy reaction studies showed that gas reactivity can be expressed in terms of the impurity contents of the pitch cokes. With simple empirical expressions one obtains fits of C02 and air reactivity to impurity content with correlation coefficients in the range R = 0.95-0.97.
With respect to air reactivity the sodium content is the most important impurity in binder pitches. Experimental air reactivity results are found to be proportional to the sodium content alone in the coked and calcined pitches with a correlation coefficient of R = 0.90.
Within the investigated Na concentration range (40-355 ppm Na in calcined pitch cokes), there seem not to be a minimum sodium level that does not influence air reactivity.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 10. RESULTS AND DISCUSSION -159-
Reactivity measurements on baked laboratory anodes, using the same calcined petroleum coke as aggregate, but with different pitch binders, gave a qualitative verification of the pitch coke reactivity results.
The reactivity data from pitch cokes were in accordance with the previous findings; iron is a catalyst to both air and C02 reactivity, nickel does not have a significant effect and sulphur inhibits the reactions. Vanadium was present in too small amounts in the pitch cokes to be evaluated.
10.9. FUTURE WORK
Very few basic studies of catalytic impurities in real anode materials have been done. Most workers have studied catalyzed carbon gas reactions on highly pure graphite, often single crystals. It is therefore necessary to verify the findings from ideal systems on real baked carbons. This will include metals, cokes, pitches and baking conditions. Possible synergetic effects of metal impurities should be examined. Anion effects, like differences between NaCl and Na^CO, as a catalyst, is also interesting. The influence from impurities on electrolytic consumption would also need to be examined in more detail.
In this thesis the effect of sulphur on iron, vanadium and nickel has been studied. To get a better understanding of the role of sulphur as an inhibitor in the catalyzed carbon gas reactions, other impurities must be investigated.
All surface investigations in this thesis have been performed ex situ, and the same phases are not necessarily stable at room temperature and at the reaction temperatures. It would of course be very enlightening if in situ techniques could be used. Baker [68] has studied carbon-gas reactions in situ with the CAEM technique (Controlled Atmosphere Electron Microscope) and this seems to be an interesting approach.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -160- PARTII
11. CONCLUSIONS
The air and carboxy reactivity of carbon anodes in the aluminium electrolysis were investigated. The study focused on the effects of inorganic impurities on the reactivity of carbon anodes and cokes. Impurities that were investigated were sulphur, iron, nickel and vanadium, which are common impurities in anodes and have been reported to affect airbum and carboxy reaction at carbon anodes. The surface studies were done with a scanning electron microscope (SEM) and the phase analysis were performed with energy dispersive x-ray microanalysis (EDX) and x-ray diffraction (XRD, at labscale anodes).
LABSCALE ANODES
Six different labscale anodes were made with delayed coke (low sulphur) or fluid coke (high sulphur) and the same binder pitch. Iron were added to the anodes as iron(HI)- acetylacetonate, Fe203 or FeS, and the effect of iron, and sulphur on iron, on airbum at 525 °C and carboxy reaction at 960 °C was studied. From the labscale anode investigations the following conclusions were drawn:
Iron was in the metallic state after baking except when it had access to released sulphurous compounds that converted it to FeS. FeS was stable through baking.
Iron is catalytically active in the C-C02 reaction at 960 °C, and iron present as FeS after baking, can be converted to active catalyst (metallic iron) under these conditions.
Iron compounds in contact with carbon in cracks and pores of the carbon material in C0/C02 atmosphere at 960 °C, can be reduced to Fe or FeO. If the iron particles become detached from the carbon, they will be oxidized to Fe,04 or Fe203 which are not catalytically active.
Catalytic activity caused by iron on FC (high sulphur) was not observed. This may be due to a "steady state" poisoning of the active catalyst particles by sulphur, or have carbon structural explanations.
These investigations did not give very conclusive results on airbum because the reaction time in air was too short, see IRON below.
The labscale anode study revealed a need to find a method for investigation of effects of impurities without the uncertainties introduced by using cokes with very different structures and impurity profiles.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 11. CONCLUSIONS -161-
LABORATORY COKER
To be able to produce cokes with predefined impurity profiles with similar structure, a laboratory coker was developed. An aromatic oil from the distillation of coal tar pitch was used as a coke precursor in the laboratory coker. Green coke was produced in the coker under 15 bar pressure to 525 °C. The green coke was crushed and calcined to 1000 °C in a coke bed under atmospheric pressure. This yielded a coke with low air- and carboxy reactivity having a low impurity content and properties comparable to commercial anode cokes. This gave the possibility to add controlled amounts of impurities to the coke precursor, and to tailor any wanted impurity content in the produced cokes.
SULPHUR
Addition of dibenzothiophene to the coke precursor gave a 50 % sulphur yield in the resulting cokes.
Sulphur did not have a significant effect on the air- and carboxy reactivities of cokes for S contents in the range from 0.35 to 1.8 wt%. The C02 reactivity in this concentration range was close to being constant.
These findings indicate that the rather contradicting effects of sulphur on reactivity reported in the literature are related to indirect effects of sulphur like structural effects and poisoning of catalytic impurities.
IRON
Iron from addition of iron(HI)acetylacetonate to the coke precursor was almost completely fixed in the coke.
Iron was present in the metallic state after calcining of low sulphur cokes, while in the high sulphur cokes a range from pure FeS to metallic iron was found.
Iron catalyzed both the 02- and the C02 gasification of carbon.
Sulphur caused a reduction in the effect of iron as a catalyst to the carbon-0 2 reaction. It reduced the air reactivity and the rate of increase in reactivity with iron content. The carboxy reactivity was lowered slightly due to S, but the rate of increase in reactivity with iron content was the same for high- and low sulphur cokes.
Surface studies and energy dispersive x-ray micro analysis suggested that sulphur was bonded to iron during coke production, and that the iron sulphides were converted to metallic iron during the first stage of reaction with air and C02. The iron particles
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -162- PARTU were not catalytically active during this conversion, and sulphur therefore reduced the catalytic effect of iron on the carbon gasification reaction.
VANADIUM
Only 1/3 of the vanadium from addition of vanadium(HI)acetylacetonate to the pure coke precursor was fixed in the calcined coke, while 70% was fixed when vanadium was added together with 1 wt% sulphur (dibenzothiophene).
After calcining, vanadium in high- and low sulphur cokes was partly in the metallic state and partly converted to vanadium oxide (V203, V2Os and V6013 could not be distinguished by EDX). None of the vanadium particles had significant sulphur contents.
Vanadium catalyzed both the airbum and the carboxy reaction.
The air reactivity of the produced cokes with vanadium additives was too high to give reliable measurements with the used reactivity apparatus even at the lowest possible reaction temperature (475 °C). Thus, vanadium is an extremely strong catalyst to airbum, and if sulphur inhibited the reaction, the effect was not sufficiently large to lower the reactivity into the measurable region.
Sulphur caused a significant reduction in the effect of vanadium as a catalyst to the carboxy reaction. It reduced the C02 reactivity and the rate of increase in reactivity with vanadium content.
SEM analysis and EDX did not indicate the difference in reactivity between high and low sulphur cokes. It was suggested that sulphur inhibits the vanadium catalysis of the carbon-C0 2 reaction by forming a stable non-mobile complex with vanadium. This would prevent vanadium from catalyzing the reaction at active sites. It is also possible that the sulphur released as the carbon was consumed in the catalyzed reaction, gave a "steady state" poisoning of the active catalyst particles.
NICKEL
About 60 % of the nickel added as nickel(II)acetylacetonate to the coke precursor was fixed in the calcined coke.
Nickel was found in the metallic state and as a phase consisting of nickel, sulphur and oxygen after calcining.
Nickel did not have any substantial effect on air and carboxy reactivity. Addition of sulphur caused a minor reduction in carboxy reactivity as a function of nickel content.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS 11. CONCLUSIONS -163-
These conclusions are in accordance with a parallel work who found that nickel in calcined cokes does not have any significant effect on anode performance in the range studied (160 - 370 ppm nickel).
GENERAL REMARKS ON SULPHUR, IRON, VANADIUM AND NICKEL IMPURITIES
The relative catalyst strength of the investigated impurities was vanadium > iron > nickel = sulphur alone. This ranking was valid for both airbum and carboxy reaction. Sulphur and nickel are almost equal because they had no significant catalytic effect, although Ni may have a very small effect on C02 reactivity.
From these investigations it seems like iron can take part in the red-ox-cycle as long as it has good contact with carbon, vanadium takes part in the catalysis as long as the reaction conditions are maintained, while nickel is inactive.
The contact, or wetting, between the catalyst particles and the carbon matrix is best for vanadium, intermediate for iron and poor for nickel. This is probably a main reason for the difference in strength of the investigated catalysts.
The inhibition of catalytic activity by sulphur was probably not due to the same mechanism for iron and vanadium.
Sulphur may inhibit the vanadium catalysis of the carbon-C0 2 reaction by forming a immobile complex with vanadium. This would prevent vanadium from catalyzing the reaction at active sites. It is also possible that the sulphur released as the carbon was consumed in the catalyzed reaction, gave a "steady state" poisoning of the active catalyst particles.
Sulphur was bonded to iron during coke production, and the iron sulphides were converted to metallic iron during the first stage of reaction with air and C02. The iron particles were probably not catalytically active during this conversion, and sulphur therefore reduced the catalytic effect of iron on the carbon gasification reactions.
PITCH COKE
The developed laboratory coker could be used for carbonization of anode binder pitches at up to 15 bar pressure and under controlled thermal conditions. It gave uniform “anode binder cokes” for laboratory use. The coke yield after pressure carbonization and calcination to 1000 °C was typically 88-90 % and the vibrated bulk density of the coke was comparable to calcined anode grade petroleum coke.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -164- PARTII
Airbum and carboxy reaction studies showed that gas reactivity can be expressed in terms of the impurity contents of the pitch cokes. With simple empirical expressions fits of COz and air reactivity to impurity content was obtained. The correlation coefficients were in the range R = 0.95-0.97.
With respect to air reactivity the sodium content is the most important impurity in industrial binder pitches. Experimental air reactivity results are found to be proportional to the sodium content alone in the coked and calcined pitches with a correlation coefficient of R = 0.90.
Reactivity measurements on baked laboratory anodes, using die same calcined petroleum coke as aggregate, but with different pitch binders, gave a qualitative verification of the pitch coke reactivity results, and the reactivity data from the pitch cokes were in accordance with the previous investigations on cokes with impurity additions.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS LITERATURE -165-
LITERATURE
[1] Gijotheim, K. and Kvande, H.: "Introduction to aluminium electrolysis", Aluminium-V erlag, Dusseldorf, 1993.
[2] Thonstad, J.: "Elektrolyseprosesser", Department of Electrochemistry, Norwegian Institute of Technology (NTH), University of Trondheim, Norway, 1992.
[3] Fisher, W. K., Keller, F., Perruchoud, R. C., Light Metals, 1991, p. 681.
[4] Solheim, A., Rolseth, S., Skybakmoen, E., Stpen, L., Sterten, A., Stpre, T., Light metals, 1995, p. 451.
[5] JANAF Thermochemical Tables, 2nd. ed., Nat. Stand. Ref. Data Service, Nat. Bur. Stand. (US), Washington, 1971.
[6] Thonstad, J., J. Electrochem. Soc., Ill, 959 (1964).
[7] Ginsberg, H„ Wrigge, H. C„ Metall, 26, 997 (1972).
[8] Silny A., Utigard, T. A., Light Metals, 1995, p. 205.
[9] Drossbach, P., Z Elektrochemie, 34, 205 (1928), J. Electrochem. Soc., 42, 65 (1936).
[10] Gijotheim, K., Krohn, C., Malinovsky, M., Matiasovsky, K., Thonstad, J.: ’’Aluminium Electrolysis, Fundamentals of the Hall-Heroult Process’’, 2nd ed., Aluminium Verlag, Dusseldorf, 1982.
[11] Jarek, S., Thonstad, J., J. Electrochem. Soc., 134, 856 (1987).
[12] Drossbach, P., Hashino, T., Krahl, P., Pfeifer, W., Chemie Ing. Techn., 33, 84 (1961).
[13] Drossbach, P., Hashino, T., J. Electrochem. Soc. Japan, 33, 229 (1965).
[14] Vetter, K., “Electrochemical Kinetics“, Academic Press, New York, 1967.
[15] Thonstad, J., Electrochim. Acta, 15,1581 (1970).
[16] Dewing, E. W., Can. Met. Quart., 13, 607 (1974).
[17] Vetyukov, M. M„ Akgva, F., Sov. Elektrochem., 6,1806 (1970).
[18] Picard, G. S., Prat, E. C., Bertrand, Y. J., Leroy, M. J., Light metals, 1987, p. 507.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -166-
[19] Solli, L. N., “Carbon anodes in aluminium electrolysis cells: factors affecting anode potential and carbon consumption”, Ph D. Thesis, Norwegian Institute of Technology, Department of Electrochemistry, 1994.
[20] Gileadi, E., Kirowa-Eisner, E., Penciner, J.: “Interfacial Electrochemistry”, Addison-Wesley Publishing Company, Reading, Massachusetts, 1975, p. 54.
[21] MacDonald, D. D., ”Application of Electrochemical Impedance Spectroscopy in Electrochemistry and Corrosion Science”, in “Techniques for Characterization of Electrodes and Electrochemical Processes”, ed’s R. Varma and J. R. Selman, 1991, John Wiley & Sons, Inc., p. 515.
[22] Armstrong, R. D., Edmondson, K., Electrochim. Acta, 18, 937 (1973).
[23] Armstrong, R. D., Bell, M. F., Metcalfe, A. A., “A Specialist Periodical report - Electrochemistry”, Vol. 6, (Edited by H. R. Thrisk), The Alden Press, Oxford, 1978.
[24] MacDonald, D. D„ Real, S., Smedley, S. I., Urquidi-MacDonald, M., J. Electrochem. Soc., 135,2410 (1988).
[25] Keddam, M., Mattos, O. R., Takenoutik, H., J. Electrochem. Soc., 128, 257 (1981).
[26] Epelboin, I, Keddam, M., Mattos, O. R., Takenoutik, H., Proc. 7th International Congress on Metallic Corrosion, 1978.
[27] Armstrong, R. D., Henderson, M., J. Electroanal. Chem., 39, 81 (1972).
[28] Bai, L., Conway, B. E., Electrochim. Acta, 38, 1803 (1993).
[29] MacDonald, J. R., J. Electroanal. Chem. 223,25 (1987).
[30] Boukamp, B., Equivalent circuit, computer program and manual, University of Twente, Holland, 1988/89.
[31] Kisza, A., Polish J. Chem., 69, 819 (1995).
[32] de Levie, R., J. Electroanal. Chem.., 261, 1 (1989).
[33] Armstrong, R. D., Henderson, M., J. Electroanal. Chem., 39, 81 (1972).
[34] Harrington, D. A., Conway, B. E., Electrochim. Acta, 32, 1703 (1987).
[35] Kisza, A., Polish J. Chem., 69, 819 (1995)
[36] Kisza, A., Thonstad, J., Eidet, T., J. Electrochem. Soc., 143, 1840 (1996).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS LITERATURE -167-
[37] NLLS-fit FORTRAN program based on the equations in [28] provided by Professor A. Kisza, Faculty of Chemistry, University of Wroclaw, 50383 Wroclaw, Poland, 1996.
[38] Thonstad, J., J. Appl. Electrochem., 2, 315 (1973).
[39] MacDonald, J. R., “Impedance Spectroscopy”, John Willey & Sons, New York, 1987.
[40] J. R. MacDonald, J. Schoonman, A. P. Lehner, J. Electroanal. Chem., 131,77 (1982).
[41] T. Pajkossy, J Electroanal. Chem., 300, 1 (1990).
[42] P. Los, A. Lasia, J Electroanal. Chem., 333, 115 (1992).
[43] G. J. Brag, A. L. G. van den Eeden, M. Sluyters-Rehbach, J. H. Sluyters, J Electroanal. Chem., 176,275 (1984).
[44] Djokic, S. S., Conway, B. E., “Electroanalytical Methods for Determination of Al203 in Molten Cryolite”, in “Modem Aspects of Electrochemistry”, No.26, (Edited by B. E. Conway, J. O’M. Bockris and R. E. White), Plenum Press, New York, 1994.
[45] Gudbrandsen , H„ Liquidus v 2.0, Computer program, Copyright 1993-1995 SINTEF Materials Technology.
[46] A. Solheim, S. Rolseth, E. Skybakmoen, L. St0en, A. Sterten and T. St0re, Light Metals, 1995, p. 451.
[47] E. W. Dewing, J. Electrochem. Soc., 117,780 (1970).
[48] G. L. Bullard, D. D. Przybycien, Light Metals, 1986, p. 437.
[49] R. D. Peterson, A. T. Tabereaux, Light Metals, 1987, p. 383.
[50] Sterten, A., Electrochim. Acta, 25,1673 (1980).
[51] Galus, Z.: “Fundamentals of Electrochemical Analysis”, Polish Scientific Publishers, Warsaw, 1976.
[52] Kvande, H., Light Metals, 1986, p. 451.
[53] Dewing, E. W., van der Kouwe, E. Th., J. Electrochem. Soc., 120, 358 (1975).
[54] Dumas, D., Brenet, J., Rev. Roum. Chim., 14,1339 (1969).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -168-
[55] Paunovic, M., Electrochim. Metal, 3, 373 (1968).
[56] Piontelli, R., Mazza, B., Pedeferri, P., Electrochim. Acta, 10,1117 (1965).
[57] Thonstad, J., Hove, E., Can. J. of Chem., 42, 1542 (1964).
[58] Richards, N. E„ Welch, B. J., Proc. of the 1st Australian Conference, Sidney and Hobart, 1963, p. 901.
[59] Houston, G. J. and 0ye, H. A., ALUMINIUM 61, p. 250-254, 346-349, 426-428, (1985).
[60] Walker, P. L., Shelef, M and Anderson, R. A.: "Catalysis of carbon gasification”, in "Chemistry and physics of carbon" (Walker, P. L. ed.), Vol. 4, p. 287-383, Marcel Dekker, New York, 1968.
[61] Sprite, M., Kuang, Z. and Thonstad, 1, Light Metals, 1994, p. 659.
[62] Bartholomew, C. H., Agrawal, P. K. and Katzer, J. R„ Adv. Catal. 31,135 (1982).
[63] Rhedey, P. J., Nadkami, S. K., Light Metals, 1986, p. 569.
[64] Marsh, H. Ed.: "Introduction to carbon science", Butterworths, Cornwall, England, 1989.
[65] Thomas, J. M.: "Microscopic studies of graphite oxidation", in "Chemistry and physics of carbon" (Walker, P. L. Ed.), Vol. 1, p. 121-202, Marcel Dekker, New York, 1965.
[66] Walker, P.L., Rusinko, F. and Austin, L. G., Adv. Catal, 11,133 (1959).
[67] McKee, D. W„: "The catalyzed gasification reactions of carbon", in "Chemistry and physics of carbon" (Walker, P. L. Ed.), Vol. 16, p. 1-111, Marcel Dekker, New York, 1981.
[68] Baker, R. T. K„ Carbon, 24, 715 (1986).
[69] Gijotheim, K., Welch, B. J.: “Aluminium Smelter Technology”, Aluminium- Verlag, Dusseldorf, 1988, p. 89.
[70] Rhedey, P. J., Light Metals, 1982, p. 713.
[71] Hume, S. M.,: “Influence of Raw Material Properties on the Reactivity of Carbon Anodes”, Ph.D. Thesis, School of Engineering, University of Auckland, New Zealand, 1994, p. 231.
[72] Baker, R. T. K., Thomas, R. B., Wells, M., Carbon, 13, p. 141 (1975).
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS LITERATURE -169-
[73] Bidet, T., Thonstad, J., S0rlie, M., Conference Proceedings of the 22nd Biennial Conference on Carbon, University of California, San Diego, 1995, p. 638.
[74] Bidet, T., Sprlie, M., Thonstad, J., Light Metals, 1997, p. 511.
[75] 0degard, R., Rpnning, S., Sterten, A., Thonstad, J., Light Metals, 1985, p. 661.
[76] Bidet, T., Sprite, M., Thonstad, J., Conference Proceedings of the 23rd Biennial Conference on Carbon, Penn State University, State College, 1997, p. 436.
[77] Casada, M. R., Rolle, A. J., Chirinos, M., Perez, F„ Romero, S., Eppig, C., Force, G., Paspek, S., Hester, D., DeMori, Z., Turpial, J., Mora, J., Epstein, E., Pirolo, D., Velasco, J.: “Quantification of the Influence of Nickel on Reduction Cell Anodes ”, Light Metals, 1997, p. 489.
[78] Sprite, M., Bidet, T., Conference Proceedings of the 23rd Biennial Conference on Carbon, Penn State University, State College, 1997, p. 430.
[79] Bowitz, O., Bpckman, O. C„ Jahr, J., Sandberg, O., Proc. 1st Ind. Carbon Graphite Conf, 1957, p. 373.
[80] Kuang, Z., Thonstad, J., Sprlie, M., Light Metals, 1994, p. 667.
[81] Foosnaes, T„ Kulseth, N., Linga, H., Naeumann, G. R., Werge-Olsen, A., Light Metals, 1995, p. 649.
[82] ISO 6998-1984 (E).
[83] Cricket Graph version 1.3.2., computer program by J. Rafferty and R. Norling, © 1986 - 1989 Cricket Software, Malvern, PA, USA.
[84] Sadler, B. A., Algie, S. H., Light Metals, 1991, p. 687.
[85] Fellner, P., Lubyova, Z., Chem. Papers, 45, 201 (1991).
[86] Welch, B. J., Richards, N. E., in “Extractive Metallurgy of Aluminium”, Interscience, New York, 1963, p. 15.
[87] Peterson, K. B., Light Metals, 1981, p. 421.
[88] Turner, N. R„ Conference Proceedings of the 4th International Carbon Conference, Baden-Baden, 1986, p. 594.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS A ppendix
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS APPENDIX 1 -173-
APPENDIX 1:
ESTIMATION OF ANODE AREA AND CURRENT DENSITY
The anode area was estimated as shown by this example:
ANODE LENGTH: Before electrolysis: 8.23 cm After electrolysis: 8.21 cm
IMMERSION DEPTH: Before electrolysis, , calculated: 4.18 cm After electrolysis, Ha, measured: 4.16 cm
(The immersion depth before electrolysis was calculated from the change in anode length and the measured immersion depth after electrolysis)
Fig. Al.l. shows a sketch of the anode after electrolysis.
Figure Al.l. Anode after electrolysis.
Measured: d0: 0.615 cm = 2rb (Before electrolysis)
d,: 0.52 cm d^- 0.50 cm dg: 0.47 cm
Calculated: date= (d,+d2+d3)/3 = 2r = 0.497 cm
Calculated areas: Abrfm,= ”rb2 + 2itrbHb = 8.373 cm2 A,=„= nr,2 + 27trHn = 6.685 cm2 AA = - A«fler = 1.688 cm2
REACTIONS ON CARBON ANODES IN THE AWMINIUM ELECTROLYSIS Estimation of anode area at each measurement potential, E:
A(E) = Abrfo „-AAf(E) (Al.l) f(E) in Equation (Al.l) was estimated from four experiments where the measurements were timed, and the carbon consumption at each potential could be calculated from:
(A 1.2) where mc is the mass of the consumed carbon, I is the direct current through the cell, t is time, F is the Faraday constant and Mwc is the molar weight of carbon. It was assumed that the electrode area change was proportional to the consumed mass of carbon. In Fig. A1.2 is the fraction of the consumed carbon plotted vs. the anode potential. This curve was used for estimation of the fraction of the area change as a function of potential.
o.o —t— ------T 1.200 1.400 1.600 1.800 2.000 2.200 2.400 Potential [V]
Figure A1.2. Curve for estimation of the fraction of the area change as a function of potential.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS APPENDIX 1 - 175-
Tab. Al.l. Shows the areas estimated for the sample experiment.
Table Al.l. Areas estimated for the sample experiment
Potential f(E) Area [E] [cm2] 1.200 0.00 8.37 1.300 0.00 8.37 1.400 0.00 8.37 1.500 0.02 8.35 1.600 0.05 8.29 1.700 0.10 8.20 1.800 0.19 8.06 1.900 0.30 7.87 2.000 0.44 7.63 2.100 0.61 7.34 2.200 0.80 7.02 2.300 1.01 6.67
The current density, i [A/cm2], at each potential could then be calculated from the measured current, I [A] and the estimated area, A [cm2]:
i = I/A (A1.2)
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -177-
APPENDIX 2:
EFFECT OF EXPERIMENTAL CELL LINING
Fig. A2.1 shows the linear regression Tafel curves from the measurements with- and without alumina lining in the experimental cell. Both lines are the average of two experiments.
2.000
----- PYG (lined) — 'PYG (unlined)
Figure A2.1. Tafel curves from the measurements at pyrolytic graphite anodes with- and without experimental cell lining.
The Tafel slopes of the curves in Fig. A2.1 were equal (b = 0.18 V/dec.), and the Tafel constant was 1.80 V and 1.79 V for the experimental cell with- and without cell lining, respectively.
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS -178- APPENDIX 2
REACTIONS ON CARBON ANODES IN THE ALUMINIUM ELECTROLYSIS