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Reactions on Carbon Anodes in Aluminium Electrolysis

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Reactions on Carbon Anodes in Aluminium Electrolysis EI-NO--1052 09905089 Trygve Eidet .. -~,J: Reactions on carbon anodes in aluminium electrolysis of ntio M/W 2 4 S99 &STI universitet teknisk-naturvitenskapelige Norges 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
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