I
REDUCTION CHLORINATION REACTIONS OF NIOBIUM AND TANTALUM OXIDE CONTAINING MATERIALS
by
EDUARDO DE ALBUQUERQUE BROCCHI B . Sc., M.Sc.
A thesis submitted for the Degree of Doctor of Philosophy of the University of London and for the 9 Diploma of Membership of the Imperial College
Department of Metallurgy and Materials Science Royal School of Mines Imperial College of Science and Technology University of London March 1983
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There's nothing you can make that isn't made There's nothing you can know that isn't known There's nothing you can see that isn't shown Nothing you can say but learn how to play the game
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% ABSTRACT
The present state of the extractive metallurgy of niobium and tantalum, their main sources, uses and probable future applications are described.
The application of chlorination processes, especially for niobium-containing materials, as well as thermodynamic and kinetic aspects of reduction chlorination reactions are reviewed.
The experimental work was carried out on samples suspended in a vertical furnace in which chlorine was admitted from the top and some of the products were collected on a cold-finger placed at the bottom of the furnace tube.
Most of the tests were carried out with a mixture of pure niobium pentoxide and carbon, as reducing agent, in order to study the effects of individual variables such as temperature, porosity and thickness of the sample, percentage of reducing agent in the initial charge, exposed surface area and partial pressure of chlorine.
Temperatures from 650 to 850°C, porosities from 28 to 83%r percentage of reducing agent (%Ci) from 9 to 60% and partial pressures of chlorine from 0.25 to 1.00 atm were employed.
The results indicated that a balance of factors, principally temperature, %Ci and porosity cause the progress of the reaction to be controlled by different mechanisms. For samples of 28% nominal porosity and 9%Ci reacting for example at 700°C the niobium pentoxide is rapidly converted to the same extent throughout the solid mixture (chemical control) while for samples with 40%Ci reacting at 800°C there is the formation of a sharp reaction front which moves to the bottom of the pellet
i v (diffusion control). In the light of these results a mathematical model was applied to describe and predict the progress of the reaction.
Experiments were also carried out with a tin slag and a concentrate of Brazilian pyrochlore in order to study the behaviour of these materials during chlorination and to assess its application for the production of technical niobium pentoxide.
The analysis of tin slag and pyrochlore residues after chlorination were carried out by X-ray diffraction and X-ray fluorescence. In the case of the niobium pentoxide chlorination experiments a scanning electron microscope (SEM) was also used. 0 CONTENTS Page
Abstract iv Table of Contents V1 List of Figures 1X List of Tables xvi
CHAPTER I. INTRODUCTION 1
CHAPTER II. LITERATURE REVIEW 13 2.1 GENERAL 13 2.2 CHLORINATION OF THE OXIDES 14 2.3 CHLORINATION OF FREE OR LOW-OXYGEN CONTAINING 14 MATERIALS 2.4 CHLORINATION OF SLAGS 18 2.5 CHLORINATION OF ORES AND CONCENTRATES 19
CHAPTER III. THEORETICAL BACKGROUND 25 3.1 GENERAL 25 3.2 THERMODYNAMIC CONSIDERATIONS 26 3.3 KINETIC CONSIDERATIONS 30 3.4 GENERAL STRUCTURAL MODEL 33
CHAPTER IV. EXPERIMENTAL METHOD 45 4.1 GENERAL 45 4.2 APPARATUS 45 4.2.1 The chlorine gas train 49 4.2.2 Chlorination system 49 4.3 MATERIAL USED 51 4.4 PREPARATION OF INITIAL SAMPLES 52 4.5 EXPERIMENTAL PROCEDURES 54
CHAPTER V. REDUCTION CHLORINATION OF NIOBIUM 61
PENTOXIDE (Nb2Q5) - RESULTS AND DISCUSSION 5.1 INTRODUCTION 41 5.2 EXPERIMENTAL RESULTS AND DISCUSSION 64
vi Page 5.2.1 General 64
Kinetic Curves of the Reduction Chlorination 67 of Niobium Pentoxide
Experimental Conditions and Results of the 95 Reduction Chlorination of Niobium Pentoxide
5.2.2 General macro and microscopic 110 observation s 5.2.2.a Unreacted pellets 110 5.2.2„b Reacted pellets 112 5.2.3 Effect of sample thickness 117 5.2.4 Effect of chlorine flow rate 122 5.2.5 Effect of the percentage of carbon in 124 the solid mixture (%Ci) 5.2.6 Effect of temperature 144 5.2.7 Effect of gas/solid contact area 161 5.2.8 Effect of chlorine partial pressure 167 5.2.9 Effect of porosity 176
5.3 GENERAL MECHANISM 182 5.4 MATHEMATICAL MODEL 188 Symbols 19.7 5.5 THE MATERIAL COLLECTED ON THE COLD-FINGER 201 5.6 CONCLUSIONS 205
CHAPTER VI. REDUCTION CHLORINATION OF TIN SLAG 208 AND PYROCHLORE CONCENTRATE
6.1 GENERAL 208 6.2 ANALYTICAL PROCEDURE 209 6.2.1 Chlorination residue 209 6.2.2 Condensed species 209 6.3 REDUCTION CHLORINATION OF TIN SLAG 210 6.3.1 Experimental conditions and results 210 6.3.2 The effect of %Ci 211 6.3.3 The effect of temperature 215 6.4 REDUCTION CHLORINATION OF PYROCHLORE 234 CONCENTRATE
vi i Page
6.4.1 The effect of %Ci 234 6.4.2 The effect of temperature 236 6.5 CONCLUSIONS 241
CHAPTER VII. SUGGESTIONS FOR FUTURE WORK 242 7.1 ACADEMIC PURPOSES 242 7.2 PRACTICAL PURPOSES 243
Acknowledgements 244
APPENDIX I List of experimental results 245
APPENDIX II Calculation of porosity and %Nb205 252 content in different sections of partially reacted pellets APPENDIX III Computer programme 257
References 258
vii i LIST OF FIGURES
No. Title Page
1.1 Production of niobium containing materials 11 and their main applications
1.2 Chlorination route for the extraction of 12 niobium and tantalum
3.1 Standard free energy changes for the sequence 37 of chlorination of niobium and tantalum
3.2 Predominance area diagram for the system 38 Nb-O-Cl at 800 K
3.3 Predominance arec diagram for the system 39 Nb-O-Cl at 100 K
3.4 Predominance area diagram for the system 40 Ta-O-Cl at 800 K
3.5 Predominance arec diagram for the system 41 Ta-O-Cl at 1000 K
3.6 Standard free energy changes for the reduction 42 chlorination reactions of Nb20|j
3.7 Standard free energy changes for the reduction 43 chlorination reactions of
3.8 Free energy diagram for the reduction 44 chlorination of Nb and Ta oxy-chlorides
4.1 Chlorination apparatus (photograph) 46
4.2 Chlorination apparatus (drawing) 47
4.3 Chlorine gas train (photograph) 48
4.4 Chlorination apparatus for the runs where 59 a silica spring was employed (drawing)
4.5 Furnace temperature profile 40
ix No. Title Page
Kinetic curves of the reduction chlorination of niobium pentoxide (%Nb20^ reacted as a function of time)
Samples of 28% nominal porosity:
V.I Effect of temperature and pellet thickness 68 (9%Ci)
V.II Effect of temperature (20%Ci, h=2.9 mm) 69
V.III Effect of temperature and pellet thickness 70 (20%Ci)
V.IV Effect of temperature (40%Ci, h=3.4 mm) 71
V.V Effect cf temperature and pellet thickness 72 (40%Ci)
V.VI Effect of temperature and pellet thickness 73 (60%Ci)
V.VII Effect of temperature and pellet/gas contact 74 area (9%Ci, h=10.0 mm)
V.VIII Effect of temperature and pellet/gas contact 75 area (20%Ci, h=10.0 mm)
V.IX Effect of temperature and pellet/gas contact 76 area (40%Ci, h=10.0 mm)
V.X Effect of chlorine partial pressure and 77 pellet thickness (9%Ci, 700°C)
V.XI Effect of chlorine partial pressure and 78 pellet thickness (9%Ci, 800°C)
V.XII Effect of chlorine partial pressure and 79 pellet thickness (40%Ci, 700°C)
V.XIII Effect of chlorine partial pressure and 80 pellet thickness (40%Ci, 800°C)
Samples of 46% nominal porosity
V.XIV Effect of temperature and pellet thickness 81 (9%Ci)
V.XV Effect of temperature (20%Ci, h=4.0 mm) 82
V.XVI Effect of temperature and pellet 83 thickness (40%Ci)
V.XVII Effect of temperature (60%Ci, h=4.5 mm) 84
x No. Title Page
Samples of 83% nominal porosity 85
V.XVIII Effect of temperature and sample 85 thickness (9%Ci)
V.XIX Effect of temperature (2030, h=5.0 mm) 86
V.XX Effect of temperature and sample thickness 87 (20%Ci)
V.XXI Effect of temperature (40%Ci, h=5.0 mm) 88
V.XXII Effect of temperature and sample thickness 89 (40%Ci)
V.XXIII Effect of temperature (60%Ci, h=5.0 mm) 90
V.XXIV Effect of sample thickness (4%Ci, 800°C 91
V.XXV Effect of chlorine partial pressure and 92 temperature (9%Ci, h=10.0 mm)
V.XXVI Effect of chlorine partial pressure and 93 temperature (40%C.i, h=10.0 mm)
Samples of 72% nominal porosity
V.XXVII Few additional runs 94
5.1 Relationship between total weight loss of 66 the sample (%) and percentage of Nb^O^ reacted for different initial carbon content
5.2 Micro-structure of the upper external surface 111 of unreacted pellets
5.3 Micro-structure of different sections of a 114 partially reacted pellet (run 127)
5.4 Micro-structure of different sections of a 115 partially reacted pellet (run 132)
5.5 Micro-structure of different sections of a 116 partially reacted pellet (run 136)
5.6 Effect of sample thickness on the initial rate 119 of reaction (28% nominal porosity)
5.7 Effect of sample thickness on the initial rate 120 of reaction (46% n.p.)
5.8 Effect of sample thickness on the initial rate 121 of reaction (83% n.p.)
xi No. Title Page
5.9a Effect of chlorine flow rate on %Nb90j- reacted 123 (28% n.p.) L D
5.9b Effect of chlorine flow rate on initial rate 123 of reaction (28% n.p.)
5.10 Effect of %Ci on the initial rate of reaction 128 (28% n.p.)
5.11 Effect of %Ci on the extent of reaction 129 (28% n.p.)
5.12 Percentage of Nb^Oc reacted against pellet 132 depth (28% n.p., h=10.0 mm)
5.13 Percentage of Nb«0c reacted against pellet 133 depth (28% n.p.,Zh=5.0 mm)
5.14 Effect of %Ci on the initial rate of 134 reaction (46% n.p., h=4.0 mm)
5.15 Effect of %Ci on the extent of reaction 135 (46% n.p., h=4.0 mm)
5.16 Effect of %Ci on the initial rate of reaction 136 and on the extent of reaction (46% n.p., h=10.0 mm)
5.17 Effect of %Ci on the initial rate of 137 reaction (83% n.p., 700°C)
5.18 Effect of %Ci on the. initial rate of 138 reaction (83% n.p., 800°C)
5.19 Percentage of Nb«0c reacted against pellet 140 depth (46% n.p., h=10.0 mm)
5.20 Top, intermediate and bottom layers of 152 pellets at different extents of reaction
5.21 Sections of partiolly reacted pellets after 153 ignition of carbon
5.22 Initial rate of reaction as a function of 154 temperature (28% n.p.)
5.23 Micro-structure of the upper external 155 surface of partially reacted pellets
5.24 Initial rate of reaction as a function of 15£ temperature (46% n.p.)
5.25 Initial rate of reaction as a function of 157 temperature (83% n.p.)
5.26 Rate of reaction as a function of %Nb20^ 158 reacted
xii No. Title Page
5.27 Cumulative %Nb«Oc reacted against pellet 159 depth (28% n.pf, h=10.0 mm)
5.28 Cumulative %Nb«Oc reacted against pellet 140 depth (44% n.p7, h=10.0 mm)
5.29 Effect of exposed surface area on the 1£3 initial rate of reaction
5.30 Percentage of Nb«0c reacted against pellet 165 depth - (28% n.p7, h=10.0 mm, lateral surface area exposed)
5.31' Sections of partially reacted pellet after 164 ignition of carbon (28% n.p., h=10.0 mm, lateral surface area exposed)
5.32 Initial rate of reaction as a function of 170 chlorine partial pressure
5.33 Percentage of Nb«05 reacted against pellet 173
depth (28% n.p., h=10.0 mm, pr1 =.425, .250)
5.34 Percentage of Nb20^ reacted against pellet 174
depth (28% n.p., h=5.0 mm, pri =.25) 2 5.35 ^2^5 content in the sample as a function 175 of time
5.34 Effect of porosity on the initial rate of 179 reaction (h=10.0 mm)
5.37 Effect of porosity on the initial rate of 180 reaction (h=5.0 mm)
5.38 Effect of porosity on the apparent value of 181 activation energy
5.39 Progress of the reaction zones through samples 184 showing the changes in the reaction rates of each layer as a function of time.
5.40 General representation of how the reaction 187 proceeds in the body of the chlorination charge (h=10.0 mm) according to the values of the temperature, %Ci and porosity of the solid mixture
5.41 Comparison between observed and predicted 199 extent of reaction against pellet depth
5.42 Comparison between observed and predicted 200
%Nb20^ reacted as a function of time
xi i i No . Title Page
5.43 X-ray diffroctograms of residues and products 203
from chlorination of Mb20^
5.44 Reaction products condensed on the cold- 204 finger (photograph)
6.1 Top and bottom surfaces of a partially 212 reacted pellet (tin slag)
6.2 Effect of %Ci on the extent of reaction 214 (tin slag)
6.3 Effect of temperature on the total change 217 of weight
6.4 Effect of temperature on the percentage 218 of slag reacted
6.5 Effect of temperature on the percentage of 219
Nb20 6.6 Effect of temperature on the percentage of 220 Fe202 content in the slag 6.7 Effect of temperature on the %Nb20^ reacted 221 6.8 Effect of temperature on the %Ta20^ reacted 222 6.9 Effect of temperature on the %Fe202 reacted 223 225 6.10 Relationship between % slag reacted and %Nb205 reacted 6.11 Relationship between % slag reacted and 226 Tao0c reacted 6.12 Relationship between % slag reacted and 227 %Fe202 reacted 228 6.13 Free energy diagram for the reduction chlorination of the main oxides present in the tin slag 6.14 X-ray diffractograms of different materials 230 from chlorination of tin slag 6.15 Kinetic curves of some oxides present in 231 the tin slag 6.16 Effect of %Ci on the extent of reaction 235 (pyrochlore concentrate) XIV No. Title Page 6.17 Effect of temperature on the %Nb20^ reacted 239 6.18 X-ray diffractograms of the material 240 collected on the cold-finger after treating II.1 Calculated porosity against partially 255 reacted pellet depth (28% n.p.) II .2 Calculated porosity against partially 256 reacted pellet depth (46% n.p.) ft ft ft xv LIST OF TABLES No. Title Page 1.1 Proven and probable reserves of niobium and 5 tantalum (1981) 1.2 Production of primary niobium and tantalum 6 (1974-1976 average) 1.3 Production Gf niobium in concentrates by 6 country - 1976 1.4 World niobium production capacity (1979 7 and 1985) Tables presenting experimental conditions and results of the reduction chlorination of niobium pentoxide Samples of 28% nominal porosity V.I Runs with top surface area exposed and prl =1.0 96 V.II Runs with top and lateral surface area exposed 100 and p^ =1.0 V.III Runs with top surface area exposed and 101 different chlorine partial pressures Samples of 46% nominal porosity V.IV Runs with top surface area exposed and 102 1 PCI = -° 2 Samples of 83% nominal porosity V.V Runs with top surface area exposed and 104 =1 Pci2 -° V.VI Runs with top surface area exposed and 108 different chlorine partial pressures Samples of 72% nominal porosity V.VII Few additional runs V.VIII Experiments with two separate layers of 109 Nb20^ and carbon xvi No. Title Page 5.1 Sectional analyses carried out with unreacted 62 pellets 5.2 Factors relating % total weight loss and 64 % Nb205 reacted 5.3 Analysis of sectioned pellets (28% initial 130 porosity) 5.4 Analysis of sectioned pellets (46% initial 139 porosity) 5#5 Analysis of different portions of partially 141 reacted samples (83% NP) Comparison of SEM and mass balance analysis 143 1 of pellet sections 5.7 Activation energies for different reaction 145 conditions (28% NP) 5.8 Activation energies for different reaction 148 conditions (46% NP) 5.9 Activation energies for different reaction 148 conditions (83% NP) 5.10 Analysis of sectioned pellets (28% NP and 164 lateral surface area exposed) 5.11 Experimental conditions showing similar changes 168 in the initial rate due to varying chlorine partial pressures 5.12 Analysis of sectioned pellets (28% MP and 171 different chlorine partial pressures) 5.13 Group of experiments to be compared with 194 predicted values from the structural model (pellet of 28% nominal porosity and 1.00 cm thickness) 5.14 Typical output of the computer programme 196 5.15 X-ray diffraction pattern for Nb205 (ASTM 201 card no. 27-1312) 5.16 X-ray diffraction pattern for Nb?05 (ASTM 201 cards nos. 20-804, 27-1311 and 19-862) 5.17 Mass balance regarding Nb20^ reacted and the 202 material collected on the cold-finger x vii No. Title Page 6.1 Analysis of tin slag (%wt) 210 6.2 Experimental results for runs with pellets 213 with different %Ci (tin slag) 6.3 Experimental results for runs at different 216 temperatu res 6.4 Semi-quantitative analysis of partially 22? reacted slag 6.5 Analysis of the leached tin slag (%wt) 233 6.6 Experimental results for runs with pellets 234 with different %Ci (pyrochlore concentrate) 6.7 Experimental results for runs at different 236 temperatures 6.8 Analysis of the material collected on the 238 cold-finger after treating 11.1 Calculation of porosity in different sections 254 of partially reacted pellets (28% n.p., 9%Ci, 700°C, Run 127) 11.2 Calculation of porosity in different sections 254 of partially reacted pellets (28% n.p., 40%Ci, 700°C, Run 132) 11.3 Calculation of %Nb90c content in different 254 sections of partially reacted pellets (28% n.p., 40%Ci, 700°C, Run 132) 11.4 Calculation of porosity in different sections 254 of partially reacted pellets (46% n.p., 9%Ci, 800°C, Run 136) 0 x vii i CHAPTER I INTRODUCTION Tantalum and Niobium are similar in their chemistry and normally occur together in nature. These facts have provided a constant challenge for scientists since their discovery both as regards their identification and more recently their separation. In 1801 an English chemist, Charles Hatchett, isolated a white oxide of an unknown element which he called columbium. In the following year tantalum was discovered by A.G. Ekeberg, a Swedish chemist, who announced the isolation of its oxide. For some time, as a result of a limited number of qualitative experiments, it was thought that columbium and tantalum were the same element (2) . Between 1840 and 1845, Heinrich Rcse, a German chemist, published a series of papers showing the existence of at least two similar metallic elements, tantalum and another which he considered to be a new element called niobium. The following considerable controversy about the identity of the elements known as columbium, tantalum and niobium was finally resolved by the determination of the vapour densities of niobium and tantalum pentachlorides and niobium oxichloride by H. Deville and L. Troost, in 1845-1847, and by the many analyses of the chlorides and fluorides carried out by C. Bloomstrand and M. Marignac in 1844. Eventually, at the fifteenth International Union of Chemistry Congress held in Amsterdam in 1949, niobium was chosen as the international name for the element with an atomic number of 41. Tantalum metal was produced for the first time in 1824 by J. Berzelius but it was heavily contaminated with oxide. In 1905 tantalum was prepared in a ductile state by W. von Bolton, a Polish scientist, through the thermal reduction of I^TaF^ with sodium. The metal was soon introduced to industry and used as the filaments of incandescent electric lamps until 1911 after which tantalum was replaced by tungsten in this role. The principal applications of tantalum have been in the production of electronic components, mainly capacitors. It is also used in the production of vacuum tubes of electronic circuits and in the manufacture of minicomputers. Other fields of application include metalworking machinery, chemical process equipment, and transportation and nuclear industry. Reviews of the recent developments in the extractive metallurgy and uses of tantalum are given by Jones (£) and Borchers and Korineck (£) . Most of the metallic tantalum produced in the recent years has been by reduction or electrolysis of potassium tantalum fluoride (I^TaF^) or by carbon reduction of the oxide. Improvements in the technology of reducing the potassium tantalum fluoride by sodium have led to the adoption of this process as the standard commercial method for making tantalum metal. Since small amounts of impurities affect the properties of tantalum most of the metal produced must be purified. A melting-remelting sequence is normally used which is accomplished by vacuum or inert atmosphere arc melting or by electron beam melting. Several chemists claimed to have produced niobium metal but it is likely that W. von Bolton, in 1907, was the first to prepare the metal in a relatively pure state by the reduction of niobium oxide (Nb20^) with aluminium. Niobium has been mostly employed as an additive in steelmaking. In this field it has been used mainly in the fabrication of high-strength low alloy steel (HSLA), where its presence is applied to control grain size and improve the mechanical properties, and in the production of stainless steel, where niobium can be added in order to stabilize the carbides and improve corrosion resistance. More recently with the development of the nuclear, spacial, military, transportation and superconductor industries the increasing use of niobium has been significant in materials such as superalloys, niobium- based alloys, niobium metal and niobium carbide. Conversion of niobium raw materials into a final product can be accomplished by several different routes (J_) . The most common practice has been to use niobium concentrates in the manufacture of ferroniobium which is directly used in the steelmaking industry. However, for the production of the more sophisticated niobium containing material it is necessary to use chemically prepared oxide or even niobium metal as the starting material. A chart showing the most common intermediate materials and the wide range of applications of niobium is given in Figure 1.1. Niobium has also been consumed, in relatively minor amounts, in the form of permanent-magnet alloys, electronic components, electrical resistance alloys, flame plated alloy coatings, and metal to glass sealing materials. The principal process applied for production of ferroniobium is the traditional aluminothermic method using a mixed charge of niobium concentrate, iron- containing compound and aluminium powder, as starting materials. The production of niobium oxide is generally accompanied by the coextraction of tantalum. By dissolving the niobium containing material (concentrate or tin slags) in a hydrofluoric acid solution followed by the addition of a potassium-containing reagent (KOH) most of the tantalum present is precipitated as potassium tantalum fluoride (I^TaF^) . The more soluble niobium compound (I^NbOFij) remaining in solution can be converted to purified niobium oxide by a sequence of chemical reactions. This process is known as the Marignac fractional crystallization and was used in the extraction of niobium and tantalum from mineral concentrates prior to the adoption of solvent extraction. In liquid-liquid (solvent) extraction, for which methyl isobutyl ketone (MIBK) is employed, separate niobium and tantalum streams are obtained after the initial dissolution of the concentrate with hydrofluoric acid. As in the processing of niobium concentrate to produce ferroniobium, niobium oxide is aluminothermically reduced to produce high purity master alloys and niobium metal. In making the master alloys elements such as iron, nickel and cobalt are added to the charge as oxide and/or metal. For some special applications the refining of niobium metal is necessary and can be achieved satisfactorily by a melting-remelting sequence in an electron beam furnace. The basic method for production of niobium carbide is the reduction of niobium oxide by carbon at high temperature under vacuum or a protective atmosphere. The resources for production of niobium and tantalum containing materials can be broadened if some improvements in extraction technology are achieved in the manufacture of these materials. In this case other identified world resources might become available commercially which correspond approximately to one third of the present proven reserves. In the table below the figures are given for proven and probable reserves excluding secondary products from recycling (4). TABLE 1.1 PROVEN AND PROBABLE RESERVES OF NIOBIUM AND TANTALUM (1981) RESERVES (METAL CONTENT, t x 106) PROVEN PROBABLE NIOBIUM 10 15 TANTALUM 0.1 0.3 Approximately half of the tantalum produced is derived from concentrates and the other half from tin slags. The average production of niobium and tantalum oxide content in concentrates and slags between the years 1974-1976 are given in Table 1.2 (5). TABLE 1.2 PRODUCTION OF PRIMARY NIOBIUM AND TANTALUM 1974-1974 AVERAGE (METRIC TONNES) TONS Nb205 TONS Tcj205 TONS Ta205 TONS Ta205 IN CONC. IN CONC. IN SLAGS TOTAL 10145 513 521 1034 Niobium availability is highly dependent on concentrates from Brazil as this is shown in Table 1.3 (5). TABLE 1.3 PRODUCTION OF NIOBIUM IN CONCENTRATES BY COUNTRY - 1974 COUNTRY TONS Nb205 Brazil 7322,4 Canada 1175,4 Russia 480, 4 Nigeria 284,8 Thailand 141,1 Others 128,3 Total 9755,0 Brazil leads the world in the production and exports of ferroniobium and has recently added «niobium oxide as an upgraded product. The pattern of world production is expected to develop in a similar manner as can be seen in Table 1.4 (4). TABLE 1.4 WORLD NIOBIUM PRODUCTION CAPACITY (THOUSAND POUNDS NIOBIUM CONTENT) PRODUCTION CAPACITY COUNTRY 1979 1985 Brazil 26000 40000 Canada 4000 5000 USSR 1950 2000 Nigeria 800 800 Thailand 450 600 Malaysia 300 300 Au stralia 70 80 Zaire 60 60 Others 180 180 World Total 33810 49020 Until 1980 CBMM (Companhia Brasileira de Mineracao e Metalurgia) had produced well over 120,000 tonnes of pyrochlore concentrates and over 50,000 tonnes of ferroniobium (7). Although a trend in substitution or conservation of expensive and scarce elements such as tantalum is emerging (8), the demand for niobium and tantalum alloys has been increasing. In the case of niobium the probable demand is expected to increase at a rate of 5.6 percent annually in the United States and 4.6 percent in the rest of the world. This would lead to overall demand of approximately 40,000 tons in the year 2000. This consumption would take the world's probable cumulative demand of niobium for a figure near to 600,000 tons in the year 2000 (9). The growing fields of application of the more sophisticated niobium-containing materials has been promoting an increased production of niobium oxide and niobium metal for use in their manufacture. The development of less expensive extractive processes than these now applied is expected to be given a high priority. Among the possible processes chlorination appears as a very attractive route to the production of either niobium oxide or niobium chlorides which can be used in the manufacture of niobium metal. Also, since niobium and tantalum occur together in many minerals, their separation is important and it is reported to be attainable via the chloride route by fractional distillation of the chlorides (.10., or through the selective reduction of the niobium pentachloride to a condensed niobium trichloride with hydrogen (£2) or by reacting the mixed pentachlorides vapours of niobium and tantalum with an alkali-metal chloride powder to form complex compounds of the type, AMCl^, where A is an alkali metal and M is niobium or tantalum (1£, 14). A mixture of niobium and tantalum chlorides can also be treated by solvent extraction in order to separate the two metals (L5 to 1£) . Niobium-Tantalum separation was also achieved by Henderson and Block (1£) through the halogen exchange reactions of calcium fluoride and the pentachloride of these metals. Also, there is a growing interest in the production of inexpensive base materials coated with niobium or tantalum for special applications,. The production of such materials can be achieved through the evaporation of a volatile metal-containing species, such as the niobium and tantalum chlorides, which is subsequently transported to a substrate where the metal is deposited. Glask (2£) has presented a report on the role of chemical vapour deposition as a manufacturing process and Powell et al. have studied the deposition of tantalum and niobium from their volatilized halides (2_1) and reviewed early work on metal deposition (£2) . Amongst the products of chlorination of niobium and tantalum pentoxides are the oxychlorides of these metals. However, the gaseous mixture from the chlorination stage can be passed with excess chlorine through a second furnace containing a bed of carbon at 500°C causing the conversion of oxychlorides into pentachlorides (£3). If niobium oxide is the desired final product from chlorination it can be readily obtained by the hydrolysis of the chlorides and oxychlorides. Although a chlorination plant demands expensive materials to protect them against corrosion, studies have shown that chlorination is the less expensive process for production of pure niobium (74) and that it is the most suitable method for the production of niobium metal of nuclear purity (25). Chlorination can therefore be suitable for treating raw materials which contain a significant amount of tantalum as well as materials with a low tantalum content such as the Brazilian pyrochlore concentrates which are the main source of niobium. An illustrative flowsheet of the processes for the production of niobium oxide and niobium metal, using a chlorination route, is given in Figure 1.2. In industrial scale operations the regeneration of chlorine is an important requirement in terms of ecological and economical factors. Commercial chlorination plants have been operating successfully and their operational layouts have been improved, such as the use of fluidised beds in order to increase productivity through improved solid-gas contacting. In the extractive metallurgy of titanium, where chlorination is an established process, plants have been developed with a larger throughput such as the unit at Stallingborough (Laporte Industries Ltd. - England) which converts approximately 7000 Kg/h metal oxides to chlorides (2_6) . As a promising route for the extractive metallurgy of niobium and tantalum chlorination has been a subject of several investigations, mainly in terms of estimating the possibility of using this process with different starting materials. However most of the research which has been carried out on the chlorination of niobium and tantalum raw materials have consisted of the analysis of results on their technical and practical aspects. Little has been done on the study of the kinetics of the process and on the analysis of the variables which may contribute significantly to the progress of reaction such as the porosity and size of the sample. The main purpose of the present experimental work has been to obtain more detailed information about the rate controlling factors of the chlorination of niobium oxide as well as to study the behaviour of by-products such as tin slag and Brazilian pyrochlore concentrates during their chlorination. Groups of experiments were designed in order to study the effect of the variables independently and the analyses were carried out in such a way to provide understanding of how the reaction proceeds in the body of the sample. Finally a mathematical model was applied to describe the progress of the reaction. STEEL-MAKING HSLA (.025-.150%Nb) ha FERRONIOBIUM STAINLESS 60-70% Nb (.3-1% Nb) MINING hs OTHER ALLOY STEELS -a MINERAL CONCENTRATE TECHNOLOGY HIGH-PURITY SUPERALLOYS MASTER ALLOYS 1-5% Nb "0 NIOBIUM CONTAINING -0 NIOBIUM JML REFINED OXIDE Nb METAL "0 NIOBIUM METAL SPECIAL Nb-OONTAINING -7 NIOBIUM ftl 10Y5 CARBIDE -8 | ^ j Oil and gas pipelines;'railroad equipment, offshore platforms; bridges; transportation Industry. j 2| Chemical processing plants. | 3 \ Neaf resisting steel (l%Mb): furnace-parts; muffles; tubes; retorts. 1 ^| Gas turbines; rocket parts; heat resistant and combustion equipment; jet engines-. petroleum, nuclear and auto Industry. | 51 Parts In high temperature vacuum furnaces; aerospace applications (rocket Darts); nuclear reactors. | 6 | Fusion reactors. ( 7 | Nuclear engineering equipment; space and military Industry; superconductors. | 8 1 Machine cutting tools. PRODUCTION OF NIOBIUM CONTAINING MATERIALS AND THEIR MAIN APPLICATIONS FIGURE 1.1 Ta containing Ta Rerkrhbrides Treatment" material (Smrafion rfc/Ta) Nb cortaining Nb material Nb Nb Nb containing alloys Figure 12. CNorincdion route fix the extract-ion of Niobium and Tantalum. CHAPTER III LITERATURE REVIEW 2.1) GENERAL The conversion to chlorides of niobium and tantalum pentoxides has emerged as a suitable route in the extractive metallurgy of these metals and investigations have been carried out with a variety of starting materials and different chlorinating agents. According to Marzys (27) the earliest recorded suggestion of attacking niobium and tantalum containing minerals with a chlorinating agent (sulphur monochloride or a mixture of sulphur dichloride and chlorine) appears to have been made by E.F. Smith in 1898. Other early investigations, such as the attack of pentoxides with SOCI2 or CCI4, have also been reviewed by Fairbrother (1^) . The direct action of chlorine on niobium or tantalum pentoxide can only be effective at high temperatures and Cuvelliez (28) has patented a process for the separation of niobium and tantalum based on this behaviour. It is claimed that at 1050°C the chlorination of N^O^ can be attained (e.g. 75% reacted during 7 hours) while Ta20^ remains unaffected. The present section is organized according to the starting material to be chlorinated, as follows: i) oxides of niobium and tantalum, ii) free or low- oxygen containing materials, iii) slags and iv) ores and concentrates. 2.2) CHLORINATION OF THE OXIDES The action of chlorine on a mixture of carbon and niobium pentoxide at temperatures between 320 and 1000°C was studied by Lind and Ingles (23^) and by Fairbrother et al. (29_) . Both investigations revealed the simultaneous formation of niobium pentachloride and niobium oxychloride. Fairbrother also studied the corresponding tantalum reaction at 400°C. The results showed that tantalum oxychloride may have been formed and decomposed into the pentachloride and the pentoxide which was observed in the sublimate product. He also suggests that oxychlorides of niobium and tantalum are formed as the primary products of chlorination in this method rather than by a reaction between pentoxide and pentachloride. Also, tantalum oxychloride was found to be less stable than the corresponding niobium compound. The kinetics of the chlorination of niobium and tantalum pentoxides with chlorine in the presence of excess of graphite powder have been reported by Mehra et al. (3j3) (3_1) . For both oxides the amount reacted was found to be directly proportional to the time of reaction and surface area of the pellet. The rate of chlorination was independent of the flow rate of the chlorine in the range studied (0.1-0.4 1/min) but proportional to partial pressure of chlorine (prl ) in \ 2 the case of niobium and p^ in the case of tantalum. The initial rates of Nb20^ ^and Ta^^ reacted at various temperatures in the range between 600 and 750°C were determined and the apparent values of the activation energy were found to be 184.1 kJ/mol and 129.7 kJ/mol respectively. It was also suggested that "mobile adsorption" of chlorine molecule in case of niobium pentoxide and that of half chlorine molecule in case of tantalum pentoxide isi the rate-controlling step. 15 These mechanisms were expressed by the following equations respectively: ~ KT -E/RT r = C i. e 9 (2iTmKT r % KT 1 -E/RT r = Cg _ r e h 2 (2iTmKT) 2 where, r = reaction rate (molecules of Nb20,j/cm .sec) Cg = conc. of Cl2 (molecules/cc) K = Boltzman constant (erg/degree) T = temperature (K) E = activation energy (kj/mol) R = gas constant (kJ/degree mol) h = Planck's constant (erg.sec.) m = mass of the molecule (gr.) The predicted and experimental results were compared in terms of the initial rate of reaction at 690°C. Diffusion of the gaseous reactant and the products have not been taken into account because of the hifcjh apparent values of activation energy obtained This may have occurred due to a poor solid-solid contact since the pellet made up of Nb or Ta pentoxide was only covered with graphite powder. Carbon tetrachloride has also been used for the chlorination of both oxides and the results from different investigations showed that Nb^^ can be chlorinated 'at about 200-250°C while Ta202 is only gasified at temperatures above 350°C (32). The kinetics of chlorination of the pentoxides of niobium and tantalum by chlorine and carbon-monoxide were studied by Srinivasan and Jena (3_3) . With both pentoxides the amount reacted per unit time was found to be directly proportional to the surface area of the pellets and the partial pressures of CO and C^. The flow rate of the gas mixture did not influence the rate of chlorination appreciably. The effect of temperature in the range between 525 and 750°C was studied and the apparent values of activation energy for the chlorination of Nb20^ and Ta20^ were found to be 31.8 and 21.3 kJ/mol respectively. It is also suggested that "immobile adsorption" or first order chemical reaction of COCI2 molecule with the surface sites, is the rate determining step for both the oxides. Chlorination of niobium pentoxide with phosgene at 250, 320 and 360°C were performed by Stefanyuk and Morozov (34). Other chlorinating agents which have been reported as reacting with Nb and Ta pentoxides include aluminium-trichloride, sulphur mono- and dichloride and thionyl chloride (1^) . 2.3) CHLORINATION OF FREE OR LOW-OXYGEN CONTAINING MATERIALS Direct chlorination of commercial ferro-niobium with some tantalum content was studied by Mcintosh and Broadley (£2). They used the chlorination route not only as a means of converting the niobium and tantalum to a form suitable for treatment but also to separate these two metals. They presented the work divided in the following stages: (i) Chlorination of ferro-niobium to give a mixture of volatile chlorides of the elements present. (ii) The purification of the mixed chlorides by removing iron and some of the minor constituents by reduction with hydrogen. (iii) Separation of the niobium and tantalum by reduction of the NbCl5 to NbCl3 with hydrogen at 500°C. (iv) Reduction of the niobium trichloride with hydrogen at 400-1000°C to produce niobium metal powder. As an alternative to stages (ii) and (iii) Steele and Geldart (JUL^) investigated the fractional distillation of the mixed chlorides. The results obtained have shown that practically complete removal of ferric chloride can be achieved by this method and that reasonable separation of tantalum and niobium has been obtained considering the experimental conditions (about half of the niobium pentachloride in the starting material was obtained with less than 0.8 percent of tantalum pentachloride). The residue left was analysed and the results indicated that complete removal of niobium and tantalum pentachloride can be achieved by distillation. In order to avoid the presence of oxygen in the starting material for chlorination Kroll (3J5) suggests for the treatment of low grade concentrates: 1) Reduction of the material with carbon at high temperature in an inert atmosphere. 2) Reaction of the product, which contains the carbides of niobium, tantalum and titanium, with sulphuric acid to remove any remaining iron. 3) Chlorination of the final carbides in order to obtain the chlorides of the valuable metals. Economically the process does not appear very attractive. Nieberlein (34) developed a process for deposits containing minor constituents such as niobium. The process consisted of reducing the mineral concentrate with coke at high temperature to form a carbide-suboxide sinter followed by chlorination in the range between 400-500°C and finally fractional distillation of the chlorides. The chlorination stage recovered 97, 83 and 87 percent of the titanium and 82, 50 and 87 percent of niobium from brookite, rutile and ilmenite sinters respectively. The initial niobium and titanium contents in these sinters were 1.5, 1.0, 0.5 and 42.5, 33.9 and 20.3% respectively. May and Engel (^Z^ suggested that the most efficient procedure for niobium and tantalum extraction, particularly from low-grade materials such as tin slags, was that involving the chlorination of ferrotantalum- niobium alloy prepared by carbothermic reduction. 2.4) CHLORINATION OF SLAGS In order to recover niobium and tantalum from slags from tin extraction Block (31$) reacted chlorine with a mixture of slag (containing about 20% of + Nb20^ and 12% of Fe^^) and carbon. Block stated that the presence of iron is doubly inconvenient because it caused contamination of the tantalum and niobium chlorides as well as prejudiced the gasification of these metals since the ferric chloride first produced was reduced to the less volatile ferrous chloride which coated the unreacted slag and tended to prevent chlorination. May and Engel (^7) also studied the reaction of chlorine with a mixture of tin slag (19.3% (Ta + Nb)20^ and 16% FeO contents) and carbon. They also found that the ferrous chloride formed soon coated the solid particles causing the reaction to stop. Removal of iron and other soluble materials through a leaching with HC1 increased the (Ta + Nb)20^ content to 36% with an almost total recovery. Chlorination experiments carried out with the acid-leached slags at 700°C converted about 90% of the tantalum and niobium to a chloride product containing less than 5% of metallic impurities. «No attempt was made to separate the chlorides. Tin slags from Metallurgie Hoboken Overpelt containing about 4% of combined niobium and tantalum pentoxide was mixed with charcoal and then chlorinated by Moguel (3_9) . A small extent of reaction was obtained in the range of temperature between 800 and 1100°C. Samples of slag containing about 0.4 percent niobium were chlorinated by Barr et al. (40). They found that the average content of Nb20^ in the condensed FeClg fraction was 3.15% and that approximately 32% of the niobium was recovered in this fraction. Henderson (41) used the idea reported by himself and Block (1_9) for chlorinating Geomine slags by adding calcium fluoride to the chlorination charge, to achieve higher extraction of tantalum and niobium by selective gasification and condensation. 2.5) CHLORINATION OF ORES AND CONCENTRATES Chlorination has also been applied on several ores and concentrates of niobium and tantalum. Lind and Ingles (^3) stated that the removal of niobium from niobite ore could be achieved under reduction chlorination conditions similar to those employed with Nb20^. The reduction chlorination of niobium ores and concentrates from Canada was investigated by Chakravarty and Prince (42). They found that when chlorine gas is used reaction temperatures of at least 650°C are necessary for maximum gasification of niobium. Henderson et al. (4£) reported the chlorination of euxenite concentrates in the presence of carbon and sodium chloride at 500°C for small and large scale tests. This work indicated that 17 to 28% sodium chloride in a mixture of ore, carbon and NaCl prevents the volatilization of the chlorides of iron and uranium due to the formation of a non-volatile type of double salt. May et al. (16) used a heated rock-salt column (400 C) to remove the iron and uranium chlorides from the volatile mixture of chlorides and oxychlorides produced during the reduction chlorination (500-600°C) of euxenite concentrates. The tantalum, niobium and titanium chlorides passed from the salt column into a heated condenser (150°C) where the tantalum and columbium chlorides were removed from the volatile chloride stream. The low-melting, non-volatile complex of sodium- uranium-ferric chloride contained 66% of the uranium and the major part of the iron. The tantalum-niobium chloride product represented a recovery of 96% of these metals and was processed further by solvent extraction to separate the two metals. Henderson (41) also tested the function of the salt column placed between the reactor and condenser and the results confirmed those given by May. Moreover, he studied the reduction chlorination of euxenite concentrates with the addition of sodium chloride in the charge at 500°C. The experimental results showed that a sodium chloride to ore ratio of 0.2 in a an ore-carbon mixture restricted virtually all uranium and iron to the non-volatile fraction. May and Engel (£7) studied the reduction chlorination of a columbite concentrate containing 66.0% niobium pentoxide and 6.4% tantalum pentoxide in the range of temperature between 400 and 800°C. The results indicated that chlorination of columbite-carbon mixtures for 90 and 180 minutes at 400°C gasified 55.2% and 96.7% of niobium and tantalum. At temperatures of 650°C and higher, more than 99% of the valuable metals was gasified in 90 minutes. The purity of the chloride products obtained ranged from 88 to 95% niobium and tantalum chlorides and no attempt was mode to separate these metals. Whalley et al. (43) studied the preparation of NbClij from concentrates containing 24.7% Nb20^/ 17.6% CaO, 14.1% Si02 and 12.3% Fe203 (magnesium, titanium, aluminium and rare earths oxides were also present) by reduction chlorination. The gaseous products of reaction passed through a vertical tube containing coarse granules of sodium chloride kept at 300°C. The aluminium and ferric chlorides were effectiyely removed from the gas stream due to the formation of a low melting eutectic of NaCl-AlCl^-FeClg. The remaining mixed gases then passed through a bed of coarse carbon, where additional chlorine was supplied, in order to convert any oxy- chlorides present into normal chlorides. Finally selective condensation was used to separate the niobium pentachloride from the other gaseous species. Whalley et al. suggested the following experimental conditions and reagent consumption for the production of niobium pentachloride: Reaction at temperatures between 600°C and 700°C for 30-120 minutes. Consumption of 0.81 gr. of chlorine and 1.0 gr. of carbon per gram of concentrate. Reduction of niobium pentachloride with hydrogen was carried out in two stages as suggested by Mcintosh (£0) and according to the following reactions: 550°C NbCl5 + H2 • NbCl3 + 2HC1 650-825°C NbCl3 + 3/2 H2 • Nb + 3HC1 The niobium produced contained an appreciable amount of hydrogen which was removed by heating in vacuum at temperatures ranging from 25°C to 1000°C for 2 hours. The kinetics of the reduction chlorination of loparite 8.8% (Ta-Nb)205 , pyrochlore 62.5% Nb205 and euxenite 47.0% (Ta-Nb)20^ was studied by Stefanyuk and Morozov (34). Experiments were carried out in the range of temperature between 430 and 780°C and the apparent values of activation energies were found to be 107.9 kJ/mol, 102.9 kJ/mol and 101.3 kJ/mol for the minerals loparite, pyrochlore and euxenite respectively. The effect of the chlorine partial pressure on the rate of chlorination was observed and the apparent orders of reaction with respect to chlorine were found to be 0.49 for loparite (at 800°C) and 0.58 for pyrochlore (at 600°C). Habashi and Malinsky (45) studied the possibility of applying chlorination on pyrochlore concentrates (52.0% Nb20^) from Quebec, Canada in order to produce technical niobium oxide. Pyrochlore concentrates were found to be resistant to the action of gaseous HC1 ct 700°C as well as to the action of chlorine up to 900°C and to the action of (Cl9 + CO) mixture up to 700°C. The results from the experiments with HC1 disagree with those given by Harris and Jackson (46) who found that niobium could be gasified from the Mrima ore in a stream of HC1 gas in the range of temperature between 500 and 700°C. Reaction of the pyrochlore concentrates with (Cl2 + S02) mixture gasified 60% of the initial niobium content in 6 hours at 400°C. The volatilized product analyzed 42.4% Nb and 53.5% Cl which corresponds very nearly to NbOCl^. The action of chlorine on a mixture of pyrochlore concentrate and carbon was also studied. The results showed that when the carbon content in the mixture was 35% all the niobium was gasified in 40 minutes at 700°C. The average Cl2 consumption in nine tests was 22.4 g which corresponds to about 1.4 g per gram of Nb20^ produced. Analysis of the condensed material indicated the presence of NbOCl3. A material balance for a typical reduction chlorination test (reaction of chlorine with a mixture of pyrochlore concentrate and carbon) showed a final product containing 96.4% of niobium pentoxide and an identical niobium recovery. The chlorination of columbite (10.2% Nb and 11.2% Ta) in a fluidized bed reactor was studied by Olsen and Block (47). The reaction rate was found to be independent of chlorine concentration and proportional to 0.13 power of the mineral concentration. This -3 corresponds to a reaction rate .constant of 5.22 x 10 g.mineral/(cc.)(min.) . The conversion (X) was related to the time of reaction (t) according to the equation: = 0092 1 (1-X) or °- More recently, Berjoan and Meubus (48) investigated the direct chlorination of a pyrochlore mineral at 1000, 1400 and 1800°C. Analysis of the condensed vapours from these reactions indicated the presence of different compounds. Berjoan and Meubus thus suggested different mechanisms for the reaction between chlorine and pyrochlore according to the temperature. Meubus also studied the effect of chlorine partial pressure on the rate of reaction during direct chlorination of a pyrochlore at temperatures between 1600 and 1950°C (49), as well as the kinetics of this reaction in the gas-solid phase for temperatures between 1100 and 1300°C (50). In the former case (49) Mewbus observed that the mixture of NaNbO^ and CaNb^^, initially present in the starting material, was gasified in two distinct steps. The chlorination of the NaNbO^ and the subsequent chlorination reaction of the CaNb^^. In the later investigation (50,) Meubus states that kinetic studies of the reaction between chlorine and pyrochlore can be reduced to the study of the CaNb20^ chlorination kinetics. In order to obtain information on the true reaction mechanisms invblved and avoid effects such as difficulty of access of the chlorine throughout the sample, the rate of reaction was determined from decreasing amounts of initial solid samples. The rate of reaction obtained to extrapolation to nil sample value was considered to be the true reaction rate if a single particle were subjected to chlorination under the prevailing conditions. The energy of activation was found to be 129 kJ/mol and the reaction rate nearly first order with respect to the chlorine concentration at the solid-gas interface. The more general use of chlorine in extractive metallurgy has also been reported in the literature (51 to 54). CHAPTER III THEORETICAL BACKGROUND 3.1) GENERAL The control of the quantity of a desired product obtained during a given reaction time, under particular conditions, is the main concern in the design of chemical processers. Thermodynamics can be applied to calculate the composition of a system at equilibrium and to predict the conditions under which the desired product can be obtained. Thermodynamic data alone are not enough to calculate rates of reactions or to determine the mechanism by which the reaction takes place. These factors are established by the study of the chemical reaction kinetics which also provide information about the effect caused by variables on the rate of reaction. However, in order to obtain a mathematical expression which closely predicts and describes the actual kinetics, it may be necessary to take into account not only parameters arising from the chemical reaction step but also from mass transport phenomena where the form in which reactant and products are present may become important factors in determining the overall rate of reaction. There has, therefore, been a growing interest in developing mathematical models in which structural variables of the system are either explicitly included into the reaction rate expressions or are incorporated into parameters which appear in the mathematical equationsoderived to describe the progress of the reaction. 3.2) THERMODYNAMIC CONSIDERATIONS Because most of the present work is devoted to the study of the reduction chlorination of niobium pentoxide, a thermodynamic appraisal of some of the reactions related to the process is relevant. Tantalum is also included as this metal is normally present in niobium-containing materials and a comparison of their behaviour is useful. For this purpose the standard free energies of some reactions were calculated as function of temperature and represented graphically. The calculations were made by means of a computer programme (55) and based on the most recent available thermodynamic data (56). The equations in which the programme was based are the standard thermodynamic relationships: AG° = AHj - TASj r T where AH? = AH? + AC dT 298 P 298 r T AC dT 298 The AC for the reaction is expressed as AC = a + bT + cT -2 + dT 2 P In Figure 3.1 are given the standard free energy changes for the sequence of chlorination reactions of niobium and tantalum. It can be seen that in the range between 800 and 1000 K the stable niobium chlorides are NbCl^ and the lower chlorides NbCl2 and NbCl2 33- In the case of tantalum the intermediate chlorides are unstable with respect to TaCl^ and Ta. By using a diagram such as Figure 3.1 it is possible to predict the most probable chlorides which are produced during the direct chlorination of some common metals as well as to evaluate the possibility of separation of the chlorides by preferential reduction as used by Jeffes (57). When oxy-chlorides are also formed the thermo- dynamic basis of an oxide chlorination reaction is better represented by an equilibrium diagram of the system metal-oxygen-chlorine for a given temperature as used by Knacke (54) to discuss the systems U-rO-rCl and Ti-O-Cl. In the case of niobium and tantalum it is known that the presence of oxygen in the starting material for chlorination results in the formation of oxy- chlorides, especially in the case of niobium. Figures 3.2 to 3.5 are the predominance diagrams for the systems Nb-O-Cl and Ta-O-Cl at 800 and 1000 K. When the data for a particular system are plotted on a log p^ versus log Pq diagram the straight line relationship is thei boundary of thermo- dynamic stability for the compound. According to the phase rule, at a fixed temperature and total pressure of the gaseous phase, a maximum of three condensed phases can coexist in the three component system. In Figures 3.2 and 3.3 it can be seen that lower chlorides (NbCl2"NbCl3 at 800 K and NbCl2 at 1000 K) and oxy-chloride (Nb09Cl at 800 and 1000 K) separate 28 the stability areas of NbCl^ and the oxides. It is interesting to note that by increasing the temperature from 800 to 1000 K the calculated boundary between NbCl5 and NbOCl3 approaches that of NbCl5 and Nb02Cl. This indicates that at higher temperatures the formation of a NbOClg stability area is expected. In the case of tantalum (Figures 3.4 and 3.5) there are no stability areas for oxy-chlorides under the conditions selected in the diagrams. Predominance area diagrams may also be useful in predicting reaction mechanisms. According to Figures 3.2 and 3.3 for instance, the direct chlorination of Nb20^ NbCl^ would take place through the formation of an intermediate compound. On the other hand, TaCl^ could be obtained from direct chlorination of Ta20^ without the formation of intermediate oxy-chlorides (Figures 3.4 and 3.5). These observations are in agreement with reported results on the chlorination of Nb20^ and Ta20^ where the formation of a niobium oxy-chloride is more pronounced than that of tantalum which becomes a significant product only at higher temperatures. So far the direct chlorination of niobium and tantalum oxides has been considered but as well known an efficient chlorination of the oxides of these metals can only be effected in the presence of a reducing agent. This is because niobium and tantalum oxides are more stable than their chlorides so that the oxygen potential in the system must be lowered. Figures 3.6 and 3.7 show the standard free energy changes for the reduction chlorination reactions of 29 Nb20^ and Tc^O^ respectively. The products of the reactions used for calculations are the chlorides of these metals and C02. It can be seen from these figures that in terms of standard free energy changes most of the reactions are possible within a large range of temperatures, the experimental study of the equilibrium for any of these reactions is hindered by the fact that many of the reactions take place simultaneously. Figures 3.4 and 3.7 also show that the production of a solid Nb02Cl or Ta02Cl are thermodynamically the most favourable. However, these oxy-chlorides have not been reported as a product of chlorination in the literature. A mixture of NbCl^ and NbOCl^ have been reported as the main product of the reduction chlorination of niobium oxide (23) (29) and TaCl^ as virtually the only product in the case of tantalum oxide (2J? ) • This is likely to be due to kinetic reasons and suggests that NbOCl^, NbCl^ and TaCl^ may be formed not only as primary products of chlorination but also through the following reactions: 2Me02Cl(s) + C + 2C12 = 2MeOCl3 + CC>2 Me02Cl(s) + C + 2C12 = MeCl5 + C02 Figure 3.8 shows the standard free energy changes for these reactions. It can be seen that NbOCl3, NbCl^ and TaCl^ are thermodynamically the most probable end-products which is in accordance with experimental observations. Also given in Figure 3.4 is the standard free energy change for the reaction C02 + C = 2C0. It may be seen that up to 970 - 980 K (=700°C) the i tendency of the reduction chlorination reactions are to produce C02. Above that temperature all reactions will have more negative AG° values when CO is produced so that gas mixtures with C0/C02 ratios greater than 1.0 will result. This is illustrated by the AG° versus T line for the reaction 2/5 Nb20^ + 2C + 4/5Cl2 = 4/5NbCl2 + 2C0 which is also included in Figure 3.6. From Figure 3.8 it would appear that if NbCl^ and TaCl^ were the desired products of the reaction the reduction chlorination should be carried out at low temperatures (below 500 K). However the chemical reaction is likely to be slow at this range of temperatures. 3.3) KINETIC CONSIDERATIONS When a chemical equation is written for thermodynamic analysis only the overall reaction is considered. The intermediate paths, which may actually be followed, are not taken into account. The kinetic analysis aims to establish the reaction steps and eventually to determine the rate of the overall reaction. The kinetic analysis of chemical reactions has generally to be carried out on the basis of experimental work. The basic kinetic laws normally used to interpret the rate of a homogeneous chemical reaction (measured by the change of the concentration of reactant or product with time) deal with the concentrations of reactants and temperature. Based on experimental observation and later explained by the collision theory it is found that the rate of disappearance of A occurring during a reaction aA + bB. . . pP + qQ ... can often be expressed as r = ^A = kC? C? where n is the order of reaction -ar A B with respect to A and m is the order with respect to B. The dependence of the reaction rate constant, k, on temperature can be related to the energy of activation -E/RT by the Arrhenius equation k = Ae where A is the frequency factor and E is the energy of activation. The order of a reaction may be used as a first criterion to determine the nature of the reaction mechanism. If the overall reaction rate varies with the reactant concentration according to a simple law (such as a first or second order reaction) the mechanism may be a simple one. In this case the reaction order can coincide with the value from the stoichiometric equation for any of the reactants. However, if a no integral value is found to be the reaction order, a complex process consisting of several steps may be necessary to describe the mechanism of the reaction. The order of reaction can be determined through an iterative procedure (integration method) when the change in concentration of a reactant or product is known as a function of time. In this case the order of reaction is determined according to the best fitting of the experimental results compared with predicted values given by the integrated kinetic equation (e.g. kt = In C^ for a first order reaction). Then the reaction rate constant can also be determined. The integration method may produce an incorrect interpretation of the order of a reaction because the decreasing of the reaction rate may not be due only to the decreasing concentration of the reactant but to the formation of intermediates or other factors occurring during the course of the reaction. A second procedure can also be used (differential method) which deals with the initial rate. In this method several runs are carried out, each of them with a different initial concentration of the reactant A. If the reaction rate is expressed by r = k C^ n it can be written after taking logarithms: log r = n log C^ + log k. When the initial rates are determined for different values of C^ the plot of the logarithm of the initial rate against the logarithm of the concentration of A may produce a straight line. In this case the order of the reaction (n) and the reaction rate constant (k) can be readily determined. The differential method will reveal the correct non-integral order if the reaction rate cannot be expressed as a function of the concentration raised to integral powers. The analysis of the effect of temperature on the reaction rate constant is normally accomplished by determining the activation energy of the reaction through an Arrhenius plot. It has been shown that while high values of activation energy (E^ > 40 kJ/mol) are typical of processes controlled by the chemical reaction step lower values of activation energy (E^ < 30 kJ/mol) are characteristic of processes where mass transfer phenomena assumes a more important role in the control of the overall reaction rate (diffusion controlled processes ) . For heterogeneous systems (such as those involving gas-solid reactions) general structural models (taking into account the effects of both chemical reaction and mass transfer on the overall reaction rate) have then been used to determine the rate controlling step as well as to derive mathematical expressions which describe the progress of the reaction. 3.4) GENERAL STRUCTURAL MODEL In a number of mathematical models for gas-solid reactions it has been assumed that the solid is dense and that the reaction proceeds through a sequence of steps such as: (i) Diffusion of the reactant gas through an existing gas film surrounding the solid. (ii) Diffusion of the reactant gas through a solid porous layer formed on the outside of the dense solid. (iii) Chemical reaction of the reactant gas with the dense solid at the reaction interface. (iv) Diffusion of gaseous products through the porous layer. (v) Diffusion of gaseous products through the gas film to the bulk of the fluid. A number of assumptions are made (e.g. the system is isothermal and the solid keeps its shape and size during the reaction) and mathematical expressions can be derived based on the rate controlling step. Most of the mechanisms proposed for gas-solid reactions are described in terms of one of three reaction models: 1) chemical control, 2) diffusion control and 3) mixed chemical and diffusion control. Comparison between the predicted values and the experimental results can be used to determine which model best represents the reaction process. When a dense solid reacts with a gas the reaction interface is well defined and the progress of the reaction may be described in terms of the shrinking-core model. In this model, after some reaction has occurred, the solid phase consists of an unreacted core surrounded by a reacted porous layer. As the reaction proceeds the reacted zone expands progressively and the diffusion of reactants or products through this region may be one of the rate limiting steps. If the solid is assumed to be spherical the mathematical expressions which relate the fractional time for complete reaction (i) and the fractional conversion of the solid (X) can be written as(58): 1) For chemical reaction control i = 1 - (1 - X)1/3 2) For diffusion control 2.1) Gas film diffusion control 2.2) Control by diffusion through the porous layer i = 1-3(1-X)2/3 + 2(1-X) 3) For mixed chemical and diffusion control 35 where r o = radius of solid (dense solid + porous layer) r c = radius of unreacted dense solid . resistance to diffusion k r _ m o _through porous layer and De resistance in the gas film where k m = mass transfer coefficient D = diffusion coefficient e The shrinking-core model can be regarded as an ideal limiting situation since the penetration of the gas into th'e solid may actually occur. In this case the reaction interface has no simple geometry and the shrinking-core model may not represent the progress of the reaction under conditions of chemical control although it may be used to describe the rate of conversion when the chemical reaction is fast and the penetration of the reactant gas is limited to a thin layer. Also, in the shrinking-core model the structural parameters are incorporated implicitly into a reaction rate constant although structural changes may occur to different extents within different parts of the solid. These facts (typical of gas-solid reactions) have been responsible for a growing interest in developing alternative mathematical models which include a quantitative assessment of the role played by structural parameters in the overall rate equation (59 to 72). One of the main difficulties of the mathematical representation of these models is how to include structural changes in a final expression. This fact sometimes has led to a very complex mathematical treatment where the numerical solution of the differential equations, used to describe the models, requires a substantial amount of time and computational work. The models proposed in the literature cover a wide variety of situations depending on several factors (e.g. shape and size of the solid, formation of a solid product, type of bed, etc.) and assumptions (e.g. the system is isothermal, diffusion of the gaseous reactant through the product layer of the individual grains does not affect the rate, etc.). The derived mathematical expressions will depend on these factors and assumptions and therefore will best represent how the reaction proceeds if applied to systems where theoretical and experimental conditions are comparable. A series of models for gas-solid reactions have been presented and briefly discussed by Froment and Bischoff (73 ). In Section 5.4 of this work a mathematical model is used in order to interpret and predict the progress of the reduction chlorination reaction of niobium pentoxide . 300 400 500 600 700 800 900 L_ i 1)00 1100 1200 1300 20 _1_ 83.7 •2TqQ3+Q2=2TQCU(G) 23Nban3+a2=23Nba4 -1255 4TQC125+C12=4TQCIb -167.4 0 r2Nbal+Cl2=2Nbas -AG -AG0 (KGal/mol) (KJ/md) -2092 QffTa+Q2=OflTaa2.5 ziaa4(G)+a2=2TQa5 -2S10 Nb.d2=NbCl2 -2929 reactant product melting point M (0 -3B4.7 booing point B qd subum. point S 0 Figure 3.1. Standard free energy charges for the sequence of chlcrination reactions of niobium and tantalum. 38 Figure 3.2. Predominance area diagramfor th e system Nb-O-CI at 800 K. •fcg Po2 Figure 33. Predomjnanoe area diagramfor th e system Nb-O-Cl d 1000K. Rgtre 34. Predominance area dngamfor th e system ti-O-CI at 800 K. -kgpo2 Figure 35 Precfominance area dfogramfor th e system Ta-O-Cl af 1000 K. 42 BOO 400 30 p ' ' -41.8 -83.7 1 2 -167.4 I -2S10 -3B4.7 —4B5 Figure 3.6. Standard free energy changesfor th e reduction chlorinahcn reactions of Nb2Os 300 400 500 600 700 BOO 900 DOO 1100 Figure 3.7. Standard free energy changes for the reduction Ufciiuliun reactions of Ta20s 44 10—i k K1B \ X •2Ta02Cl(S)4 C*2Cl2* 2Ta0a3 (G) +C02 \ \ Too X® So TO IE tx5o \ T(K) -W \ 2Ta02Cl(S)*02Cl2« \ «2Ta0Cl3(S)+C02 \ —41.8 ' \ \ 1 2Nb02Cl(S)*C*2a2 -2Nb0Q3(G)+C02 —83.7 2 o 53 Nb02a(SUC*2Cl2 >NbQ5+G02 -1255 —167.4 •TaO2 Cl (S) • C+2Cl2 =2TaQ s* GD2 -50- —209.2 -55- 100 200 300 400 5002)0 700800 T(°C) Figure 38. Free energy diagram fix the reduction chbrination of Nb and Ta ay-chlorides. CHAPTER III EXPERIMENTAL METHOD 4.1) GENERAL All the experiments were carried out in basically the same apparatus with some variations in the procedure used. A photograph of the chlorination equipment is shown in Figure 4.1. Its main dimensions together with its internal construction are illustrated by a sectional drawing in Figure 4.2. A gas train supplied!the chlorine at a measured flow rate into the top of the reaction tube. A photograph of the chlorine gas train is shown in Figure 4.3. Some of the chlorination products were collected on a cold finger placed at the bottom of the furnace, its temperature being controlled by a measured internal air flow. After the cold finger the gases were passed through a cooled gas trap before reaching the exit of the equipment where a vacuum pump was used to avoid contamination of the laboratory atmosphere by extracting chlorides and excess chlorine and thereby maintaining a slightly negative pressure in the apparatus. 4.2) APPARATUS The complete experimental system used consisted of two separate parts: the chlorine gas train and the chlorination system itself. Figure 4.1. Chbrindion apparatus. i Tap 1 Tap 2 Figure 4.2. ChlorinaHon apparatus. 48 Figure 4.3. Chlorine gas train. 1. Chlorine gas cylinder. 2. Activated charcoal column. 3. Molecular sie/e column. 4. Glass ball rotameter. 4.2.1) THE CHLORINE GAS TRAIN The chlorine from a cylinder was first treated by passing over activated charcoal kept at about 300°C and then dried by passing through a second column filled with molecular sieve grade AW500 manufactured by Union Carbide. The flow rate of the treated chlorine was controlled by a glass ball rotameter type D manufactured by Glass Precision Eng. placed at the end of the chlorine gas train, before reaching the furnace. The chlorine gas train was completely enclosed in a fume cupboard. 4.2.2) CHLORINATION SYSTEM The chlorination system can be basically divided into three parts: 1) the taps, bulb and glass head located above the furnace, 2) the furnace tube itself and 3) the glass items placed below the furnace. 1) The taps and the glass bulb of the top part were arranged to provide accurately measured flow-rates of chlorine or of chlorine-argon mixtures. The experimental procedures are described in more detail in Section 4.5. A platinum wire was used to suspend the sample from one end. The other end was supported by a hook located on a support inside the glass head or onto the lower end of a silica spring. In the latter case the top end of the silica spring was supported from the glass head. Two different types of glass heads were used at the top of the reaction tube. One of these was used in the runs where a silica spring was employed to follow the total weight loss of the sample. This was arranged in such a way that the silica spring was contained in a separate glass compartment. The dividing wall between the two compartments was pierced by a hole through which the platinum wire passed. By this means condensation of compounds from the chlorination reaction on the spring and on the glass wall around it was avoided so that it was possible to follow the movement of its lower end by a micrometer. The device described above, with details of its dimensions, is shown in Figure 4.4. The second type of glass head was used in most of the experiments described in the present work. It consisted of a cover for the reaction tube fitted with a hook and a side arm through which chlorine and/or argon entered the reaction zone. 2) The furnace was wound with a Kanthal resistance element, positioned vertically and fitted with a removable mullite (A^O^SiC^) tube. The furnace temperature was controlled by a Eurotherm regulator model LP96 DHS/SCR/10A using a Pt-Pt/Rh(13%) thermocouple. A second Pt-Pt/Rh(13%) thermocouple located close to the control thermocouple was connected to a temperature chart recorder model TE220 manufactured by Tekman. Both thermocouples were placed in the space between the furnace tube and the reaction tube. The temperature inside and outside the reaction tube were compared before the experimental work and were found to be the same. 3) The bottom part of the chlorination system consisted of a cold finger followed by two glass traps. After these the gas passed through a water ejector vacuum pump which extracted any volatile chlorides and excess chlorine. The cold finger, with air passing through it, had its temperature controlled by an air flow meter type RS3 (Glass Precision Eng.). The tip of a Pt-Pt/Rh(13%) thermocouple was attached to the inner wall of the cold finger tip and was also connected to the chart recorder. The chlorination system was enclosed in a safety cabinet which had fume extraction facilities in case of leakage. In addition to the chlorination system a glass ball rotameter type B (Glass Precision Eng.) was used for the control of the argon flow rate. The photograph in Figure 4.1 shows the chlorination system as it was used in most of the experiments described in the present work with all the parts connected up as for an experimental run. 4.3) MATERIALS USED High purity argon and technical grade chlorine both supplied by BOC were used in all experiments. The chlorine typical analysis is given below: chlorine 99.8% bromine 1600 ppm nitrogen trichloride 60 ppm moisture 50 ppm non-volatile residues 150 ppm Graphite powder non-pleetable-grade 1 (specpure) manufactured by Johnson Matthey Chemicals Ltd. was used as the reducing agent. (Impurities detected as given by the manufacturer Si = 1 ppm. Ca, Cu, Fe, Mg, Na < 1 ppm). Specpure niobium oxide also manufactured by Johnson Matthey Chemicals Ltd. (impurities detected, Ag = 1 ppm, Si < 1 ppm), tin slag from Metallurgie Hoboken Overpelt, Belgium and a concentrate of pyrochlore supplied by CBMM - Brazil were the starting materials for the experiments. Details of the compositions of these materials are given in the individual sections describing their chlorination. 4.4) PREPARATION OF INITIAL SAMPLES The materials used to prepare a sample were separately weighted out in the proportions required in an aluminous porcelain crucible and thoroughly mixed. A half inch diameter die was then filled with the mixture and a predetermined pressure was applied to it in order to produce a pellet of the desired height and porosity. The pellet height was then measured with a micrometer and it was placed in a weighed recrystal1ized alumina crucible fitted with a platinum wire bridle and the crucible plus the pellet was re-weighed. The weight of the pellet was calculated by difference and used to determine the actual mass of the starting materials and consequently, since the specific mass of these materials are known, the porosity of the pellet. In the experiments where the material was not pelletized the mixture of starting materials was placed directly into the reaction crucible to a level which produced the height and porosity required for the sample. The reaction crucible used during the tests with tin slag and Brazilian pyrochlore consisted of a cylindrical tube fitted with two pieces of platinum wire arranged in a cross which in turn supported a layer of alumina wool onto which the pellet was placed. In most of the experiments with pure niobium oxide a constant area was left exposed to the attack of chlorine in order to study the effect of variables such as height and porosity of the sample under comparable conditions. A type of crucible was therefore used which was closed at the bottom and into which the pellet fitted closely so that only the top area was exposed to reaction with chlorine. When the initial charge height was close to 1 cm it just reached the top of the crucible. Two procedures were used when the shorter samples were reacted. The first was to place the charge on the bottom of the crucible so that a space was left between the top of the charge and upper edge of the crucible. The second was to place the charge on pressed alumina wool layer on the bottom of the crucible. In this case the top of the sample was level with the crucible edge. It was found that when identical experiments were carried out using these two procedures no differences were observed in the results . The observed temperature profile of the furnace was such that its hottest zone was approximately 1 cm long at a constant temperature. The samples were alwaysi placed within the constant temperature zone so that the fact that shorter samples were placed at slightly different positions caused no effect on the results. Figure 4.5 illustrates how various samples were positioned inside the reaction tube and the furnace temperature profile with its constant temperature zone for 700°C and 800°C. 4.5) EXPERIMENTAL PROCEDURES Three different procedures were used in the chlorination experiments, one of them being used mostly in the runs with pure niobium oxide. During a first set of runs, which were of an exploratory nature, in order to produce an adequate range of levels of the variables, a silica spring was used to follow the total weight loss of the sample continuously. The tests were conducted by heating the charge in an argon atmosphere to a given temperature for a certain period of time. At this stage the argon flow was stopped and dry chlorine was introduced at the desired rate instead. The change of weight was measured by observing the movement of the silica spring which supported the charge hung from its end through a travelling micrometer. The spring had been previously calibrated so it was possible to follow the change of weight of the charge during the time when chlorine was passed through the reaction zone of the furnace. Readings were taken at given intervals of time, depending on the reaction rate, in order to plot the graph of change of weight of the charge versus time. At the end of each test argon was readmitted to flush out the chlorine. The temperature was decreased and the residue in the crucible and the material condensed on the cold finger were then collected. The spring length was chosen so that the sample was situated in the hottest zone of the furnace at the start of the experiment (when the spring was at its greatest extension). Each spring therefore limited the experimental work to samples of the same weight. It was also important that the change of weight in the sample should not be very great since in this case the crucible would move up or down to a position outside the constant temperature zone, causing difficulties in the accurate control of the reaction temperature. Because of these limitations due to the use of silica springs a second procedure was adopted. This was the conventional method of carrying out experiments for varying reaction times, all the other variables involved being kept constant. The procedure to control temperature and admit chlorine into the furnace were similar to the previous method. This method required the preparation of groups of identical samples to be chlorinated under certain conditions for varying periods of time. A disadvantage of this method was the long time required to obtain a complete graph of the total weight lost versus time. Since many experiments were expected to be carried out with pure niobium oxide and a relationship was observed between the total weight loss of the sample and the percentage of niobium oxide reacted which was independent of temperature (as described in Sec 5.2.1)/ a third method was devised and used in most of the experimental work. In this method, after the chlorine was admitted into the furnace as previously described, the reaction was interrupted at particular times by switching the gas flow from chlorine to argon. The crucible was then removed from the furnace and the total weight loss of the sample measured. The crucible was then replaced into the furnace, the argon stream continued, and after about five minutes, for the temperature of the charge to regain a constant value, the chlorine was readmitted. This point was considered to be the restarting of the chlorination and after a certain time the procedure of removing the crucible from the furnace was repeated and the total weight loss of the sample could be determined for the cumulative time of chlorination. The use of this method made it possible to carry out more runs within a given period of time and also had the advantage of dealing with the same sample at each experiment avoiding possible differences in the results caused by different initial samples. Chlorination experiments using the methods described were carried out under the same conditions and the results did not show any appreciable differences. However, small changes in the experimental procedure such as the time expended to remove the crucible from the reaction tube - due to the residual chlorine inside the reaction tube which can cause the reaction to continue - and the time used for the temperature of the charge to regain a constant value, may be responsible for some variations in the experimental results. In the experiments where the partial pressure of chlorine was varied from unity, taps 1, 2 and 3 (Fig.4.2) were turned on/off at certain times and the bulb was used as mixing chamber. In these experiments the argon was admitted first into the furnace at a particular flow rate. The chlorine was then also admitted and the flow rate of both gases regulated until they were stabilized at the values which corresponded to the desired ratio. Tap 3 was then closed and the crucible was placed in the furnace. After a short time, for the sample to reach the desired temperature, tap 3 was opened and this was taken as the starting time of the chlorination. The reaction was allowed to take place for a given period of time when tap 3 was closed to interrupt the gas flow and the sample removed from the furnace in order to be weighed. During this time the bulb and reaction tube were flushed with argon. The process was then repeated and the total weight loss of the sample was determined for the cumulative time of reaction. The whole system was normally cleaned between the runs but consecutive identical runs were carried out without cleaning - so that some condensed materials were present on the inside of the apparatus. No difference was observed on the results in terms of initial rate or total weight lost. The type and method of analysis used in this work is given separately in the sections dealing specifically with niobium oxide, tin slag and Brazil pyrochlore. Figure 4.4. Chforination apparatus employing silica spring. 60 Reaction tube Fig. 45. Furnaoe temperature profile. CHAPTER III REDUCTION CHLORI NAT ION OF NIOBIUM PENTOXIDE (Nb2QR) RESULTS AND DISCUSSION 5.1) INTRODUCTION Reduction chlorination experiments of niobium pentoxide were carried out under a variety of conditions. The variables studied were chlorine flow rate and partial pressure, sample porosity, percentage of reducing agent in the solid mixture, sample thickness, temperature and gas/solid interfacial area. Most of the tests were duplicated and all the observed results recorded and the curves of %Nb20^ reacted versus time were plotted. Experiments were repeated a number of times for various different purposes such as: 1) to check the reproducibility of the results, 2) to test the effect of varying the experimental procedures described in Chapter 4, 3) to analyse the sample residues after varying extents of reaction, 4) to compare the residues obtained under different experimental conditions at the same degree of reaction. Two different products could be analysed after each experiment: the reaction residue and the material collected on the cold-finger (mcf). The present work is principally concerned with the reactions occurring in the reaction sample so that most of the discussion will be based on the percentage and quantity of niobium oxide reacted. The data obtained from the mcf are presented and briefly discussed in Section 5.5. Qualitative analyses were carried out by X-ray diffraction in both reaction residues and mcf. Microscopic examination of selected unreacted and partially-reacted sectioned pellets were performed using a "Cambridge model S600" and a "Jeol 35" scanning electron microscopes. Quantitative analysis of niobium oxide was also carried out when the JSM 35 was used. The percentage of niobium pentoxide reacted was calculated after ignition of the graphite present in the residue. In some cases the residue was hydrolised in boiling water or dilute ammonium hydroxide and evaporated to dryness before ignition of the graphite in case some chloride compounds were present in the samples. This treatment was not found to lead to different results so that the former procedure was adopted. Fractional sections of the pellet were analysed by the same method in order to provide a profile of the niobium pentoxide reacted through the sample. The homogeneity of the starting pellets was tested by gravimetric analysis (Table 5.1) and by SEM (Table 5.6) and found to be satisfactory. TABLE 5.1 Sectional analyses carried out with unreacted pellets Initial carbon Section Nb0Oc + C Nbo0c %C in each (*) (mm) (*) (ir •) (§r?) section 4.65 41.1 1.5841 1.4391 9.15 9.00 4.62 40.8 1.4331 1.3053 8.92 2.05 18.1 0.7375 0.6725 8.81 3.27 31.4 1.0382 0.8307 19.99 20.02 3.84 36.8 1.1828 0.9450 20.10 1.31 12.6 0.5850 0.4682 19.. 97 2.00 19.2 0.6402 0.5124 19.97 Because the porosity of the samples affected the reaction kinetics the results are presented and compared in the first instance for each porosity tested. A discussion of the effect of the porosity is then presented. The porosities of the samples were calculated from the specific gravities of niobium pentoxide 3 3 (4.47 g/cm ) and carbon (2.25 g/cm ) (74 ) and the volume of the sample. The density of niobium pentoxide was confirmed using a specific gravity bottle according to the British Standard Methods of Test (£5 ). For the two lower porosities (28 and 46%) the initial samples were compacted pellets and in the other case (83%) they were formed of loose powder. Experiments were also carried out at 72% nominal porosity (Table V.VII) but they were not sufficiently extensive to be included in the. present discussion. Some preliminary experiments were carried out to select the conditions to be used in the reaction studies and these are not described further. Unless otherwise indicated the following results refer to experiments carried out with a 2 sample with an exposed top surface area of 1.29 cm and a chlorine flow rate of 0.1 1/min. 5.2) EXPERIMENTAL RESULTS AND DISCUSSION 5.2.1) GENERAL In Appendix I are listed the results of the chlorination experiments in terms of time of reaction, percentage of the total sample (Nb20^ + C) reacted and percentage of niobium pentoxide reacted. Because of the complexities of the possible reactions taking place due to the formation of CO, C02, NbCl^ and NbOCl^ a group of reaction samples produced under widely varying conditions was examined to determine the correlation between the sample weight loss and the percentage loss of Nb20^. The results are shown in Figure 5.1 and it was found that there was a very close relationship between these variables for various initial carbon contents of the samples, as shown in Table 5.2. TABLE 5.2 Factors relating % total weight loss and X Nb^O,- reacted CORRELATION %Ci FACTOR COEFFICIENT 9 1.010 .9998 20 1.145 .9998 40 1.548 .9983 60 2.295 .9998 The factors in Table 5.2 were used to calculate intermediate values of niobium pentoxide reacted based on the results of the total weight lost, as given in Appendix I. A graph for each experimental set of conditions was then plotted in terms of percentage of Nb20^ reacted versus time. These plots are presented in Figures V.I to V.XXVII showing the effects of temperature and/or sample thickness. The figures in brackets identify the run numbers as given in Appendix I. The experimental conditions are given in Tables V.I to V.VIII. Table V.VIII also shows the experiments in which the niobiumi pentoxide and carbon were present in separate layers. % TOTAL WEIGHT LOSS 4 NOMINAL T(°C) SYMBOL POROSITY (%) Pcl2 28 700 100 A 28 800 100 V 46 700 100 o 46 800 too 83 700 100 • 83 800 too o 83 700 025 0 83 800 025 0 % Figure 5.1. Relationship betweentotal weigh t loss of the sample (%) and percentage of Nb205 reacted for different initial carbon contents. ft ft ft KINETIC CURVES OF THE REDUCTION CHLORINATION OF NIOBIUM PENTOXIDE (%Nb20^ reacted as a function of time) ft 68 (60) ,(53) (119-162) (95) 1(50) •c Mr (94-118-127-160) (164) p 0 30 60 90 120 TIME (MIN.) Figure V.I Effect of temperature and pellet thickness. 69 800°C 850°C 750°C (14-33-45) (46) (4-11-12-13-32-48) 700°C h (1-6-7-10- i 20-31-44- 47) 650°C in (17-18-19-34-35) O JD so 90 TIME (MIN) Figure VJ. Effect of temperature 70 800°C-h:2.9 TIME(MIN) Figure V.DI. Effect of temperature and pellet thickness. 71 9 750°C TIME (MIN) 41 Figure V.IV. Effect of temperature. 72 h=5.0-800°C h=50-700°C TIME (MIN) Figure V.V. Effect oftemperature an d pellet thickness. ft 73 h=3.6-800°C h=3.6-730°C Rgure V.VI. Effect of temperature and pellet thickness. TIME (MIN) Figure V.VJD. Effect of temperature and pellet/gas contact area. 700°C, AT TIME (MIN) — Figure V.VJD. Effect of temperature and pellet/gas contact area. KXH TIME(MIN) — Figure V.VJD. Effect of temperature and pellet/gas contact area. hrfi.O-pcl^l.00 TIME (MIN.) Fig V.X. Effect of chlorine partial pressure and pellet thickness. 78 h=10.0 - pa =1.00 (Fig. V.I) TIME (MIN.) Fig. V. XIII. Effect of chlorine partial pressure and pellet thickness. 79 TIME (MINI Fig.V. XIII . Effect of chlorine partial pressure and pellet thickness. 80 Fig. V. XIII. Effect of chlorine partial pressure and pellet thickness. 81 TIME (MIN.) fig. V.XIV. Effect- of temperature and pellet thickness. Figure V.XXIII. Effect of temperature. 83 h=45-800°C TIME (MIN.) Fig. V. XVI. Effect of temperature and pellet thickness. 84 TIMEfMIN.) Rg.V.XVn. Effect of temperature. 85 (97-102-106-209) (59-63-126-213) © (54-65) (92-1CB-TH-159-205) • (49-101-110-158-202) • 90 TIME (MIN) Figure V.XVDI. Effect of temperature and sample thickness. 86 Figure V.XXI. Effect of temperature. 87 TIME (MIN) Figure V.XX. Effect* of temperature and sample thickness. 88 TIME (MIN) Figure V.XXI. Effect of temperature. 89 TIME (MIN.) Fig. V.XXD. Effect oftemperature an d sample thickness. TIME (MIN) Figure V.XXIII. Effect of temperature. 91 TIME (MIN) Figure XXIV. Effect of sample thickness. 92 Fig V. XXV. Effect of chlorine partial pressure and temperature. 93 TIME (MIN.) Fig V. XXVI. Effect of chlorine partial pressure and temperature. 94 TIME (MIN.) Fig V. XXVI Few additional runs. EXPERIMENTAL CONDITIONS AND RESULTS OF THE REDUCTION CHLORINATION OF NIOBIUM PENTOXIDE Unless otherwise indicated chlorine flow rate = 0.1 1/min TABLE V.II Top andlateral surfac e area exposed - Chlorine partial pressure: 1.00 EXPERIMENTAL CONDITIONS (Nominal Por•osit y : 28%) EXPERIMENTAL RESULTS Calc. % Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initia I Initial ( mm) Initial T Time Of Run Weight Rate . Porosity fRafe 3\ Weight Reaction Loss Reacted IV.Nb 0j 103 Nominal Actual Nominal Actual (#C ) 2 iL- (%) (gO (min) c/.) ('/.) \ min / \ min / 28.1 9.31 2.63 0.999 700 120 50 61.2 62.8 0.73 6.61 30.5 9.0 9.23 2.7 2.72 0.999 750 120 53 84.0 85.3 1.63 14.78 30.9 9.23 2.74 0.999 800 90 60 94.0 95.0 2.46 22.31 34.2 9.0 9.10 5.4 5.45 1.899 700 90 164 47.0 48.4 0.53 9.15 33.4 9.15 5.41 1.906 800 40 166 53.0 54.3 1.63 28.23 25.4 9.0 9.01 8.0 7.58 2.994 700 120 90 46.7 47.7 0.41 11.17 24.6 9.15 10.03 3.998 700 60 94 23.3 32.4 9.0 9.05 10.0 10.30 3.687 700 30 118 11.7 0.41 14.42 28.6 9.09 10.58 3.996 700 60 127 23.7 23.8 27.1 8.96 9.82 3.793 700 120 160 49.2 50.5 33.1 9.04 10.45 3.700 800 120 119 95.1 96.5 27.4 9.0 8.95 10.0 9.86 3.794 800 55 162 53.2 54.3 1.14 39.73 24.8 9.14 10.06 4.001 800 120 95 89.2 27.6 20.04 2.86 0.997 650 30 17 10.1 11.2 27.3 20.10 2.85 0.997 650 60 18 26.6 27.4 20.0 20.17 2.9 2.86 0.998 650 45 19 16.7 18.8 0.41 3.27 '27.8 20.21 2.88 1.000 650 120 34 41.6 28.1 20.32 2.89 0.998 650 120 35 44.6 TABLE V.I (Cont'd) EXPE RIMENTAL CONDITIONS (Nominal Por•osit y :28%) EXPERIMENTAL RESULTS Total Nb 0 Initial Initial Calc. % In itial Carbon Pellet Thickness Pellet Overall 2 5 ( mm) Initial T Time Of Run Weight Rate . Porosity Reacted gr .103 Weight Reaction Loss ( V.Nb205) Nominal Actual Nominal Actual C C ) (%) (go (min) c/o CM \ min / V min / 28.2 20.04 2.88 0.996 700 60 1 62.4 71.5 27.3 20.18 2.87 1.003 700 10 6 9.0 11.7 28.5 20.2 6 2.91 1.000 700 30 7 33.6 38.5 28.7 20.02 2.90 0.996 700 30 8* 36.5 41.8 28.9 20.0 20.01 2.9 2.91 0.996 700 30 y ** 36.8 42.1 1.25 9.97 27.1 20.78 2.88 1.005 700 45- 10 47.6 54.5 28.5 20.14 2.90 0.99C 700 15 20 13.5 27.3 20.09 2.85 0.997 700 120 31 81.0 93.4 27.4 20.25 2.87 1.001 700 120 44 84.1 97.3 27.7 20.13 2.87 0.998 700 90 47 75.4 28.5 20.08 2.90 0.998 750 60 4 82.0 94.8 27.9 20.04 2.88 0.999 750 30 11 57.6 65.9 28.4 20.0 20.18 2.9 2.91 1.002 750 15 12 26.8 30.5 2.19 17.49 28.3 20.11 2.90 1.000 750 60 13 81.8 92.9 27.8 20.33 2.89 1.002 750 120 32 84.8 98.1 2 6.7 20.07 2.83 0.998 750 90 48 85.8 98.4 28.5 20.03 2.90 0.998 800 30 14 79.2 90.7 27.5 20.21 2.87 1.000 800 30 15* 77.7 89.0 28.0 20.0 20.15 2.9 2.88 0.998 800 30 16** 77.6 89.3 3.33 26.57 27.8 20.21 2.88 1.000 800 60 33 87.0 99.6 29.0 20.17 2.93 1.001 800 30 45 78.8 27.9 20.0 20.18 2.9 2.88 0.999 850 35 46 88.4 100.0 4.65 37.08 •CHLORINE FLOW =0.2 Jl/min - •*: CHLORINE FLOW = 0.3 K,/min ir * 0 * TABLE V.I (Cont'd) EXPEF IIMENTAL COND.[HON S (Nomi nal Por•osit y : 28%) EXPERIMENTAL RESULTS Calc. % Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initial Initial ( mm) Initial Porosity T Time Of Run Weight Rate . R ate v Weight Reacted 3 (%) Nominal Actual Nominal Actual (*C ) Reaction Loss ( */.Nb2 Of,) gr JO tg n (min) (•/.) (•/.) 26.9 20.03 9.93 3.494 700 110 151* 56.7 27.5 20.04 65.9 10.01 3.495 700 85 153** 43.5 26.2 20.07 50.4 9.84 3.497 700 80 161 41.1 29.9 20.0 20.02 47.9 10.0 10.27 3.467 700 15 192 8.1 31.1 20.01 0.59 16.41 10.44 3.466 700 35 . 193 18.1 30.5 20.04 10.36 3.466 700 60- 194 31.2 30.4 20.04 36.6 10.35 3.468 700 80 196 42.8 27.7 20.02 50.2 10.02 3.489 800 120 152* 76.1 27.5 20.04 87.7 10.02 3.499 800 50 154** 44.0 26.1 20.04 50.7 9.84 3.502 800 90 163 66.2 30.6 20.0 20.00 76.7 10.0 10.36 3.463 800 10 197 8.0 30.6 20.01 1.25 34.76 10.36 3.460 800 20 198 22.9 30.7 20.02 26.8 10.37 3.459 800 15 199 17.7 30.9 20.01 20.2 10.41 3.463 800 30 200 35.1 41.4 29.1 40.0 40.06 3.4 3.41 0.999 700 120 52 63.5 30.7 40.05 97.7 1.70 10.18 40.0 3.48 0.997 750 30 55 49.4 28.1 39.99 3.4 75.1 3.35 0.996 750 60 56 63.8 2.62 15.66 30.4 40.0 40.00 3.4 3.44 0.990 800 60 61 66.1 32.1 40.04 99.8 3.74 22.33 40.0 5.0 5.35 1.502 700 50 165 46.3 71.7 26.5 40.11 1.47 4.93 1.498 700 33 168 31.6 48.8 13.21 31.6 40.0 40.06 5.0 5.30 1.498 800 20 167 34.8 53.3 3.64 32.68 •CHLORINE FL0W= 0.2 fi./min ** CHLORINE FLOW = 0.3 £,/mil * ft ft ft * ft TABLE V.I (Cont'd) EXPEF OMENTAL CONDITIONS (Nominal Poi"Osit y : 28%) EXPERIMENTAL RESULTS Calc. % Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initial Initial ( mm) Initial Porosity T Time Of Run Weight Rate . Reacted 3 Weight Reaction Loss lv.Nb2o5J gr JO (%) Nominal Actual Nominal Actual Cc) (go (min) c/.) \ min / \ min / 30.8 40.03 10.47 2.994 700 60 113 35.0 30.9 40.07 10.50 3.000 700 120 121 49.9 An n xinu n.u 77.1 29.5 40.01 10.28 2.995 700 60 132 34.8 54.4 0.93 16.72 28.3 40.00 10.13 3.003 700 35 138 21.5 32.9 31.5 40.00 10.60 3.001 800 70 114 45.9 70.1 32.5 40.0 40.03 10.0 10.76 3.002 800 120' 120 62.2 95.0 2.25 40.47 29.5 40.05 10.28 2.995 800 38 133 33.4 52.5 25.9 59.99 3.71 0.996 700 90 71 43.3 24.1 60.0 59.98 3.6 3.55 0.976 700 45 72 40.0 2.44 9.63 28.6 60.04 3.86 0.997 800 60 77 44.4 4.67 18.61 24.1 60.38 9.63 2.641 700 30 155 12.0 27.4 0.88 23.9 60.0 60.37 10.0 9.63 2.647 700 60 156 22.5 51.6 9.22 25.4 60.36 9.80 2.641 800 42 157 22.6 51.2 1.64 17.17 TABLE V.II Top and lateral surface area exposed - Chlorine partial pressure: 1.00 EXPEF UMENTAL CONDITIONS (Nominal Porosity : 28%) EXPERIMENTAL RESULTS Calc. % Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initial Initial ( mm) Initial Porosity T Time Of Run Weight Rate . R ^te \ Reacted 3 Nominal Actual Weight Reaction Loss f V.Nb205 gr . I0 (%) Nominal Actual (go ('c) (min) (•/.) \ min / V min / 35.7 8.98 9.0 10.0 11.07 3.768 700 60 220 43.1 44.2 0.87 29.84 35.9 8.98 11.07 3.759 800 30 222 90.5 92.2 3.85 131.73 31.0 20.02 20.0 10.0 10.43 3.463 700 40 195 51.7 59.8 1.45 40.16 31.0 20.02 10.43 3.465 800 13- 201 53.4 61.2 4.70 130.25 29.0 40.01 10.12 2.971 700 40 221 59.8 88.1 28.9 40.01 2.55 45.48 40.0 10.0 10.12 2.975 700 25 226 37.2 57.4 28.8 40.02 10.12 800 15 223 54.2 82.6 2.977 7.33 130.64 29.2 39.96 10.12 2.963 800 8 225 35.3 53.6 28. 2 40.01 40.0 10.0 10.07 2.991 700 120 83 * 62.9 96.9 1.38 24.76 28.0 40.04 10.05 2.992 800 60 84* 66.0 3.56 63.87 •Lateral surface area covered with silica wool 4 ft ft 4 ft ft TABLE V.III Top surface area exposed and different / EXPERIMENTAL CONDITIONS (Nominal Porosity : 28%) EXPERIMENTAL RESULTS Calc. % Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initia I Initial ( mm) Initial T Time Of Run Weight Rate . Rate Porosity f 3\ Weight Reaction Loss Reacted 1 •/.Nb 0 10 (%) Nominal Actual Nominal Actual (go ('c ) (aim) c/.) 2 5 iL- ( min) c/o \ min / V min / 29.2 8.97 10.12 3.795 700 0.250 120 140 19.8 20.1 0.16 5.53 29. 6 8.95 10.19 3.795 700 0. 625 150 182 49.9 51.0 0.34 11.75 29. 2 9 8.93 10.0 10.10 3.791 800 0.250 120 141 43.8 44.3 0.36 2 7.6 8.96 9.88 3.788 800 0.250 45 149 19.5 19.8 12.42 29.9 9.03 10.24 3.803 800 0.625 65 186 49.0 49.9 0.82 28.37 33.9 9.14 5.44 5 0 1.903 700 0.250 180 178 44.0 45.0 0.26 4.50 33.6 9.20 5.43 1.907 800 0.250 60 180 51.3 52.4 - 0.86 14.89 28.1 39.99 10.07 2.992 700 0.250 120 139 25.3 38.8 0.33 5.93 33.0 40.03 10.80 2.992 700 0.625 85 184 33.9 51.8 0. 66 11.84 28.7 39.99 10.0 10.15 2.994 800 0.250 120 142 37.5 56.3 0.67 28.1 40 40.01 10.10 2.995 800 0.250 70 147 26.6 39.9 12.04 28.3 40.04 10.11 2.997 800 0.625 50 187 34.3 52.0 1.49 26.78 31.7 40.02 5 O 5.30 1.497 700 0.250 70 179 30.0 45.9 0. 66 5.93 31.3 40.03 5.27 1.497 800 0.250 35 181 33.3 49.9 1.64 14.72 ft ft % # ft ft TABLE V.IV Top surface area exposed - Chlorine partial pressure: 1.00 EXPERIMENTAL CONDITION? ( Nominal Porosity : 4 6 %) EXPERIMENTAL RESULTS Calc. %Initial Carbon Pellet Thickness Pellet Overall To.tal Nb205 Initial Initial (mm) Initial T Time Of Run Porosity Weight Rate Rate Weight Reacted 5 Reaction Loss f-/.Nb2o5 ] jgr. XT ] (%) Nominal Actual Nominal Actual (go fC) (min) \ min / \min / 47.9 9.01 9.0 4.0 3.61 0. 996 700 120 66 49.9 0.56 5.08 49 .7 8.97 3.74 0.996 800 120 73 94.6 95.4 2.67 24.21 46.4 9.16 11.37 3.221 700 120 91 45.1 46.3 47.8 9.0 9.01 10.0 9.70 2. 680 700 120 124 47.1 40.0 0.43 11.14 47.5 9.22 9.55 2.652 700 60' 174 25.9 26.0 45.5 9.13 10.06 2.901 800 120 96 94.2 46.7 9.14 9.0 10.0 9.58 2.701 800 60 122 76.6 78.1 1.44 3 6.47 45.4 8.98 10.01 2.898 800 35 136 45.7 46.3 48.5 9.26 9.73 2.647 800 18 177 25.5 26.1 47.5 20.06 3.94 0.996 700 120 67 77.2 89.0 1.18 9.40 47.9 20.0 20.09 4.0 3.97 0.995 800 60 74 86.9 99.4 43.9 20.03 3.59 0.969 800 5 80 15.9 17.7 3.43 26.93 48.8 40.05 4.67 0.989 700 120 68 63.3 97.4 44. 6 40.0 40.00 4.5 4.14 0.948 700 10 81 14.2 2.11 12.26 48.3 40.25 4.69 1.001 800 60 76 64.6 5.71 34.15 * ft - t ft ft ft TABLE V.IV (Cont'd) EXPERIMENTAL CONDITION? ( Nominal Porosity : 4 6 %) EXPERIMENTAL RESULTS Calc. %Initial Carbon Pellet Thickness Pellet Overall Total Nb205 Initial Initial (mm) Initial T Time Of Run Weight Rate Rate „ Porosity Reacted (%) Weight Reaction Loss Nb (_gr_. f?\ Nominal Actual Nominal Actual tgd ro (min) (•'• 2°5 ] (•/.) \ min / \min / 48.7 40.06 9.25 1.962 700 93.9 40. 6 40.04 9.33 2.292 700 It 42! 4 40.1 40.14 8.35 2.064 700 35 175 26.5 40.3 1.33 17. 69 43.7 39.76 12.60 2.278 700 60 93 44.8 40.0 10.0 67.8 50.0 40.09 12.05 2.490 700 120 86 62.7 48.4 40.02 9.20 1.9 64 800 40 98 54.1 82.1 43.2 40.15 8.80 2.066 800 15 176 29.5 44.5 3.70 46.66 39.3 40.03 9.10 2.283 800 20 100 30.9 49.0 60.01 4.26 0.786 700 120 69 42.3 97.3 3.89 12.23 60.0 4.5 53.7 59.96 4.75 0.797 800 60 75 44.6 8.57 27.35 TABLE V.II Top andlateral surfac e area exposed - Chlorine partial pressure: 1.00 EXPERIMENTAL CONDITIONS (Nominal Porosity : 83%) EXPERIMENTAL RESULTS Calc. Sample Sample Overall Total Nb 0 Initial Initial %Initial Carbon Run 2 5 Initial Time Of Weight Rate i Rate \ Porosity Thickness Reacted gr_. 101 Nominal Acfual (mm) Weight Reaction c/.) F^OsI (%) (go (min) \ mm ' \min / 83.3 9.20 0.442 700 120 49 24.7 25.2 83.6 9.00 0.432 700 120 101 29.4 83.0 9.0 9.62 5.0 0.448 700 45 110 11.2 10.8 0.28 1.16 82.4 9.18 0.466 700 60 158 15.1 81.5 8.76 0.492 700 45 202 10.5 83.7 9.11 0.432 9.0 5.0 750 120 54 52.5 83.4 8.89 0.439 750 120 65 54.4 55.1 0.99 3.92 83.5 8.84 0.438 800 60 59 68.7 69.2 83.5 8.70 0.438 800 60 63 9.0 5.0 73.3 2.62 83.5 9.11 0.436 800 60 126 74.2 75.0 10.51 82.7 9.74 0.454 800 60 213 80.6 79.5 9.17 1.086 700 120 92 36.0 82.3 9.66 0.933 700 120 103 42.1 42.9 78.4 9.0 9.62 10.0 1.135 700 60 111 25.9 26.0 0.43 3.85 83.1 9.38 0.893 700 120 159 37.4 37.7 83.1 9.32 0.891 700 45 205 15.8 16.0 78.5 9.70 1.134 800 120 97 97.2 82.4 9.32 0.932 800 15 102 36.0 9.0 36.4 2.62 82.5 9.27 10.0 0.926 800 60 106 90.6 91.9 23.18 82.6 9.55 0.916 800 30 209 55.0 56.3 TABLE V.V (Cont'd) EXPERIt CENTAL CONDITIONS (Nominal Porosity : 83%) EXPERIMENTAL RESULTS Calc. Sample Sample %Initial Carbon Overall Total Nb205 Initial Initial Initial T Run Porosity Time Of Weight Rate I Rate » Thickness Reacted 3 Nominal Actual Weight ('C ) Reaction L (V.N^OJ \GR_. 10 (%) (mm) (GO (mm) m CM 78.4 20.0 20.08 3.0 0.312 700 60 23 19.4 20.1 (0.37) (0.92) 83.3 20 .32 20.0 0.401 650 60 26 3.5 83.3 20.38 5.0 0.402 650 120 36 9.2 0:08 0:26 83.5 20.32 10.5 0.396 700 60 27 30.3 83. 6 20.63 34.9 0.392 700 120 ' 29 51.0 83.3 20.39 58.5 0.402 700 120 30 50. 6 83.2 20.0 20.43 57.6 5.0 0.402 700 120 42* 51.3 82.8 20. 60 0. 63 2.04 0.411 700 60 128 36.6 82.1 21.11 41.4 0.426 700 40 173 21.7 82.0 21.10 25.3 0.429 700 30 203 15.1 82.3 20.40 0.425 700 120 216 56.3 83.3 20. 26 64.2 0.401 750 120 37 79.9 83.4 20.0 20.16 91.9 5.0 0.400 750 120 38 81.3 83.4 20.22 93.1 2.55 8.15 0.400 750 120 39 78.6 83.3 20.24 90.9 0.402 800 60 40 86.2 83.5 20.28 98.0 20.0 0.396 800 15 41* 69.9 82.9 20.38 5.0 80.2 0.410 800 20 130 76.3 7.62 24. 65 82.4 21.19 0.420 800 15 207 67.5 75.5 83.4 20 .0 20. 28 5.0 0.400 850 15 43 85.4 11.25 35.87 *Chlorine flow = 0.3 1/min TABLE V.V (Cont'd) EXPERIMENTAL CONDITIONS (Nominal Porosity : 83 %) EXPERIt 1ENTAL RESULTS CaTc. Sample Sample Overall Total Nb205 Initial Initial %Initial Carbon T Run Initial 1 Time Of Weight Rate i Rate . Porosity Thickness Weight ('U Reacted V.NBJOJ 10 Nominal Actual (mm) Reaction CM 1 Jl- J (%) (go (min) \ min ' \min / 82.0 20.27 0.8 64 700 120 129 78.4 90.2 83.1 20.52 0.809 700 60 137 55.1 63.5 82.4 20.0 20.27 10.0 0.846 700 50 172 40.1 46.4 1.00 6.73 82.0 20.98 0.860 700 30 204 22.0 25.0 82.1 20.93 0.856 700 30 206 24.6 28.8 82.2 20.52 0.852 700 35 217 30.8 36.0 82.7 20.34 0.832 800 20 190 40.7 46.3 82.0 20.0 21.18 10.0 0.860 800 20 208 43.2 50. 6 2. 67 17.94 82.1 20.89 0.853 800 30 210 63.5 73.4 78.6 30.0 29.30 5.0 0.457 700 20 171 14.8 18.4 0.92 2.97 83.3 40.12 0.344 700 120 51 5J.. 7 78. 6 83.6 40.79 0.337 700 60 105 38.9 59.8 82.0 40.0 40.00 5.0 0.373 700 40 107 31.3 48.5 1.62 3.38 84.5 40.00 0.320 700 10 109 10.7 15.2 83.6 40.06 0.338 700 20 125 19.4 27.4 82.0 41.02 0.370 700 65 170 42.1 63.1 81.5 42.16 0.376 700 60 214 41.2 62.6 83.5 40 .03 0.341 750 20 57 50.3 76.9 83.5 40.0 39 .98 5.0 0.341 750 60 58 59.8 90.7 6.00 12.26 83. 6 40.00 0.340 750 120 64 63.1 96.4 83.5 39 .91 0.341 800 60 62 66.8 99.3 40.0 5.0 9.33 19.73 82.1 40.76 0 .368 800 10 212 51.0 74.8 o ON TABLE V.V. (Cont'd) EXPERIMENTAL CONDITIONS (Nominal Porosity : 83 %) EXPERIMENTAL RESULTS Cafe. Sample Sample Overall Total Nb 0 Initial Initial % Initial Carbon T Run 2 5 Initial Time Of Weight Rate . Rate i Porosity Thickness Reacted Nominal Actual Weight ru Reaction (V.N^OJ 111- 10) (%) (mm) (go (min) \ min ' \min / 80.5 40.00 0.806 700 30 108 43.0 65.5 82. 6 40.0 40.02 10.0 0.719 700 60 117 59.3 91.3 2.67 11.88 82.6 40.3? 0.717 700 30 16? 40.9 62.9 81.4 42.06 0.756 700 60 215 57.3 91.4 82. 6 40.02 0.718 800 20 • 116 61.8 93.9 82.5 40.0 40.29 10.0 0.720 800 10 191 43.3 66.3 7.42 31.90 82.6 40. 22 0.718 800 15 211 51.9 78.7 83.3 60.06 60 0 5 0 0.302 700 120 70 35.9 81.4 3.2 3.86 83.1 59.65 0.307 800 30 79 43.3 16.0 19.82 82.2 4.30 5.0 0.493 800 60 218 42.9 41.8 1.3 6.13 82.0 4.0 4.45 10.0 0.995 800 60 219 52.1 50.9 1.3 12.35 82.0 4.43 0.993 800 20 224 25.4 24.7 TABLE V.VI Top surface area exposed and different EXPERIMENTAL CONDITIONS (Nominal Porosity : 83%) EXPERIMENTAL RESULTS Calc. Sample Sample Overall Yolnitial Carbon T Total Nb205 Initial Initial Initial Run Porosity Time Of Weight Rate Rate \ Thickness Weight ( C Reacted 3 Nominal Actual (mm) (atm) Reaction Loss •/.Nbp'Os gr . 10j (%) (go (min) c/.) min / 82.6 8. 60 0.928 700 0.250 120 143 21.6 21.6 0. 18 1.53 82.3 8.96 0.908 700 0.625 120 183 35.3 36.3 0. 36 2.98 82.4 9.0 8.82 10.0 0.930 800 0.250 120 145 85.1 85.7 0. 7.29 82.5 8.81 0.929 800 0.250 120 150 80.1 80.3 86 83.5 8.73 0.874 800 0.625 60 188 78.7 80.2 1. 92 15.32 82.6 39.93 0.720 700 0.250 120 144 52.7 80.6 1. 10 4.76 82.5 40.35 0.722 700 0.625 60 185 45.7 69.4 1. 67 7.19 82.4 40.05 0.724 800 0.250 60 146 64.5 97 . 6 40.0 10.0 2. 23 9.75 82.2 39 .90 0.733 800 0.250 45 148 49.8 16.3 82.5 40.41 0.720 800 0. 625 20 189 43.8 66.4 4. 80 20.59 TABLE V.VII Few additional runs EXPERIMENTAL CONDITION S (Nominal Porosity :72%) EXPERIMENTAL RESULTS Sample Overall Total 5 Initial Initial Calc. % Initial Carbon Sample Nb 2 0 Initia I Time Of Weight Rate Rate Thickness T Run Reacted 3 Porosity Weight Reaction Loss (•/.) 1 •/.Nb205) 1 gr_. 10 J Actual (mm) % (%) Nominal (gr) ( C) (min) \ min / \ mi n / 71.5 20.08 5.0 0.685 650 60 22 9.6 10.3 (0.18) (0.99) 70.5 20.0 20.18 3.5 0.497 700 60 2 50.2 57.5 70.5 20.00 3.5 0.497 700 30 3 25.4 29.1 0. 97 3.85 72.9 33 .0 33. 66 0.587 700 60 24 50.5 69.2 5.0 1.15 3. 64 71.9 33.39 0.365 700 30 25 25.1 33.9 73.8 40.-06 1.083 700 60 87 59.5 1.82 11.81 73 .7 40.0 40.31 10 .0 1.087 700 60 89 56.8 73 .8 40.09 1.082 800 30 88 62.5 4.57 29. 62 TABLE V.VIII Experiments with two separate layers of Nb 20tj and carbon EXPERIMENTAL CONDITIONS EXP. RESULTS Overall Total Nb2 O5 Nb 2 O5 Carbon Sample T Time Of Run Weight Thickness Loss Reacted Weight Thickness Porosity Weight Thickness Porosity (mm) Reaction ( C ) (gr) (mm) (•/.) (g n (mm) c/.j (min) c.) 1.386 5.77 58.4 0. 264 5.0 81.8 10.77 30 800 134 1.0 1.068 7.00 73.5 0.180 3.5 82. 2 10.50 35 850 135 1.6 - 5.2.2) GENERAL MACRO AND MICROSCOPIC OBSERVATIONS 5.2.2a) UNREACTED PELLETS S.E.M. photographs showing the surface of unreacted pellets with 9 and 20% carbon contents are given in Figures 5.2a and 5.2b. In both cases the carbon is present in agglomerates in a number of areas of various sizes which may or may not touch each other. The interfacial area of contact between the niobium pentoxide and carbon (Figure 5.2c) is seen to be a function of the relative amount of these substances present in the sample. In Figures 5.2c and 5.2d are shown the typical internal structures of the niobium pentoxide zone in pellets of two different porosities (28 and 46%). It can be seen that while the pores between the particles were within a narrow size range in the 28% porosity pellet (Figures 5.2c/d) a large number of macro pores appear for the 46% porosity pellets (Figures 5.2e/f ). The size and distribution of the niobium pentoxide particles were uniform throughout the sample except for occasional larger crystals. No significant variation was detected in the internal structure when a sectioned unreacted pellet was analysed from top to bottom in the JSM 35. 111 1* k FIGURE 5.2 MICRO-STRUCTURE OF THE UPPER EXTERNAL SURFACE OF UNREACTED PELLETS ft ft (a) (b) Pellet nominal porosity: 28% Pellet nominal porosity: 28% Carbon content: 9% Carbon content: 20% Magnification: 20X Magnification: 20X (c) (d) Pellet nominal porosity: 28% Nb20^ zone Carbon content: 9% Pellet nominal porosity: 28% Maanification: 2K Carbon content: 9% Magnification: 5K (e) (f) Pellet nominal porosity: 42% Nb20^ zone Carbon content: 9% Pellet nominal porosity: 42% Magnification: 2K Carbon content: 9% Magnification: 5K 5.2.2b) REACTED PELLETS Measurement of the pellet dimensions using a micrometer did not indicate any alteration in the pellet size during the reaction when an excess carbon was used. Even when the percentage of initial carbon was 9% (approximately the stoichiometric value as shown in Figure 5.1) the pellet kept its size and shape up to a considerable degree of reaction. A non-uniform change in the porosity of the reacted pellets was noted since the top surface of the pellets were normally soft and very porous whereas the bottom layers appeared less affected by the reaction or even unreacted. The high porosity of the top surface of the pellet extended to a variable extent into each pellet according to experimental conditions. The diffractograms of residues from a number of runs showed only the patterns of niobium pentoxide (Figure 5.43). This suggests that the reaction is unlikely to proceed through an intermediate stage of reduction of the Nb20^ by carbon (see Section 5.2.9.) As a part of the investigation into the internal structure of partially reacted pellets, polished sections were examined and analysed by SEM (Jeol 35). Since the top parts of these pellets were very porous and unsuitable for mounting the analyses were carried out on the lower parts of the residual pellets. Photographs were taken at three approximately equidistant points and are presented in Figures 5.3 to 5.5. These showed: 1) The absence of any sharply defined reaction f ront. 113 2) That the top layers of the samples were more porous than the lower ones. 3) That there were no variations in structure across the section layers except for occasional macropores. 4) That the internal structure of the residue with 40% initial carbon content was more porous than that with 9% initial carbon content. 5) That the overall quantity of carbon left in each layer of the sample was significantly greater in the top layers of the residue with 40% Ci than with 9% Ci. 6) That the main features observed when the initial pellet had a higher porosity (46%) were similar to those for pellets with 28% nominal porosity. The information obtained from the SEM will be compared with the gravimetric analyses of sectioned pellets and used for the characterization of the reaction behaviour. FIGURE 5.3 MICRO-STRUCTURE OF DIFFERENT SECTIONS OF A PARTIALLY REACTED PELLET - Initial nominal porosity: 28% - Initial carbon content: 9% - Reaction at 700°C - Run: 127 SECTION OF TOP LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification: 2000X (JSM Analysis: 33.6% Nb) SECTION OF MIDDLE LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification 2000X (JSM Analysis: 41.9% Nb) SECTION OF BOTTOM LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification: 2000X (JSM Analysis: 43.3% Nb) FIGURE 5.3 MICRO-STRUCTURE OF DIFFERENT SECTIONS OF A PARTIALLY REACTED PELLET - Initial nominal porosity: 28% - Initial carbon content: 40% - Reaction at 700°C - Run: 132 SECTION OF TOP LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification: 2000X (J SM Analysis: 12.8% Nb) SECTION OF MIDDLE LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification: 2000X (JSM Analysis: 18.0% Nb) SECTION OF BOTTOM LAYERS OF THE SAMPLE (a) Magnification: 100X (b) Magnification: 2000X (JSM Analysis: 28.5% Nb) FIGURE 5.3 MICRO-STRUCTURE OF DIFFERENT SECTIONS OF A PARTIALLY REACTED PELLET - Initial nominal porosity: 46% - Initial carbon content: 9% - Reaction at 800°C - Run: 136 * 117 5.2.3) EFFECT OF SAMPLE THICKNESS It was noted that in experiments using the same initial pellet weights but with different percentages of carbon the results indicated that the thickness of the pellet could be a significant parameter in determining the overall reaction rate. Experiments were thus carried out with samples of different initial weights but of constant volume. The results are shown in Figures V.I, III, V,.VI, X, XI, XII, XIII, XIV, XVI, XVIII, XX, XXII and V.XXIV. The initial reaction rates represented both as percentage and weight of niobium pentoxide reacted per minute are given in Tables V.I to V.VII. Figure 5.6 shows the effect of varying pellet thickness under a number of experimental conditions for pellets with 28% initial porosity. It can be seen that although the initial rate in terms of percentage of niobium pentoxide reacted decreases for thicker pellets the absolute quantity reacted tends to increase . The results for1 pellets of 46% nominal porosity closely resemble those for 28% porosity (Figure 5.7). This general trend is an indication that the chlorine penetrates to varying extends inside the pellet from the start of the reaction. It is thus possible that the initial rate was affected by the pellet thickness due to the resistance of the pellet to chlorine penetration which, in turn, is a function of the reactivity of the system in the top layers of the pellet. Therefore it is thought that the chlorine 118 penetration was less effective when the initial carbon content was 60% since the experiments carried out under this condition did not show an increase in the reaction rate (gr. Nb20^/min) for thicker pellets. However this trend (greater reaction rate (gr./min) for thicker pellets) also suggests that although the wall effect (penetration of chlorine in the space between the pellet and crucible) had not been apparent £ during the analysis from pellets sectioning (as illustrated in Sections 5.2.5 and 5.2.6) it may have contributed to this trend (Section 5.2.7.). It was then concluded that the analysis of the effect of the experiment variables on the reaction ^ rates and structural mechanism could be better interpreted if runs were performed on pellets of equal dimensions. At 83% nominal porosity an anomalous effect was observed at 700°C in that increasing the sample thickness caused the reaction to be faster both in terms of percentage and weight of niobium pentoxide reacted (Figure 5.8a). This effect was not observed at 800°C where the results (Figure 5.8b) resembled that observed in the denser reaction samples. (See Section 5.2.9.). # 4 4.8 60%Cj,800°C V, 4.4- 4.0- 3.6- a%Ci,800°C c 3.2- E 28- 20%C|,800°C 2.4- 60%Ci,7DOPCA 2.0- £ 16- 1.2- 20%Ci<700°CD 20%Q,7XPC * 0.8- 0.4- i—r 29 3.6 10.0 PELLET THICKNESS (mm) 4.4 44 40%Cjl800°C© 4.0- 3.6- ./9%cj,800°c c 3.2- POROSITY: 28% E A0%Ci,800oC 28- 24- in o N 20- vO 1.6- 1.2- 0.8- 04- t r TT 2.7 3.4 5.0 54 8.0 10.0 PELLET THICKNESS (mm) Fig.5.6. Effect of pellet thickness on the initial rate. 120 m x ci cm JD PELLET THEKNESS(mm) t r 1 r 3.7 4.7 9.0 10.0 PELLET THICKNESS(mm) Fig 5.7. Effect of pellet thickness on the initial rate. dT JDcm SAMPLE THICKNESS (mm) £ 5.0 10.0 11.0 SAMPLE THICKNESS (mm) Fig. 5.8. Effect of sample thickness on the initial rate. 5.2.4) EFFECT OF CHLORINE FLOW RATE The effect of chlorine flow rate was studied at two stages. Initially a group of experiments were performed to determine ifs effect on the extent of reaction. Later the results were analysed in terms of initial rate of reaction. Figure 5.9a shows the percentage of niobium oxide reacted for different chlorine flow rates at 700 and 800°C after 30 minutes. It can be seen that the chlorine flow rate had no effect on the extent of reaction after a given time. This fact was the first evidence that the rate at which the chlorine is transferred from the bulk of the gas to the surface of the pellet was not a significant factor in the overall reaction mechanism. This conclusion was confirmed when another group of experiments using a thicker pellet was carried out. Figure 5.9b shows that a change in the chlorine flow rate did not cause the initial rate of gasification of niobium pentoxide to be significantly altered. Similar results were observed when chlorine flow rates of 0.1 and 0.3 1/min were used on reaction samples of 83% nominal porosity as can be seen in Table V.V and Appendix I. 123 100 90 TT" o 800°C (14) (15) (16) 80 POROSITY: 28% 70 20% Cj 3 h=2.9mm oc 60 in 1=30 min. ^ 50 (7) (8) (9• ) vO _0_ 40 TT 730°C 30 20 -r -r~ 0.1 0.2 0.3 CHUHNE FUWR4TE(l/min.) Fig.5.9a. Effect of chlorine flew rate on %Nb205 reacted. | 1.50- (163) (152) (154) s 1.25- o_ —o- —o- 800°C e in O too-I 0.25 0 0.1 0.2 0.3 CHLORINE FLOW RATE(l/min.) Fig 5.9b. Effect of chlorine flow rate on initial rate. 124 5.2.5) EFFECT OF THE PERCENTAGE OF CARBON IN THE SOLID MIXTURE (%Ci) The effect of the percentage of carbon in the initial reaction sample was studied in the range between 9 and 60%. The lower limit was very close to the stoichiometric amount according to experimental results 0 (Figure 5 . 1 ) . Figure 5.10 shows the initial rates at which niobium pentoxide reacted at different %Ci for given experimental conditions. From this figure it appears that the initial rates both as percentage and as weight of Nb20^ reacted tend to increase with increasing %Ci ¥ to a maximum at 40% Ci and to fall thereafter. This behaviour can also be seen in Figure 5.11 where is shown the effect of the %Ci on the extent of reaction of niobium pentoxide at 700 and 800°C for different times of chlorination. These observations suggest the existence of an optimum contact area between the solid reactants per unit volume (see Section 5.2.2a) which leads to an optimum ratio of niobium oxide and carbon. This behaviour may also be attributed to the fact that an increased %Ci causes the reaction to be faster in the top layers of the pellet. Therefore for longer pellets the overall %Nb20^ reacted may decrease at high %Ci because little reaction takes place in the lower parts of the pellet. On the other hand, for shorter pellets an increase in the initial % Nb20^ reacted can be expected at high %Ci (see Figure 5.6) since the reaction, from its start, takes place in most of the pellet layers. The effect of the %Ci on the reaction of niobium pentcxide in various layers of pellets of 28% nominal porosity can be seen in Figures 5.12 and 5.13. The plots presented in these figures show percentage of niobium pentoxide reacted against pellet depth and were drawn based on the data given in Table 5.3 and according to the procedure described in Section 5.1. From these plots it can be seen that for the same overall extent of reaction (about 50%) an increase in the %Ci causes an increase in the percentage of niobium oxide reacted in the top layers of the pellet. When pellets at different extents of reaction were analysed it was observed that the reacticn proceeds through a reaction front moving to the bottom of the pellet. It was also noted that some niobium pentoxide was generally removed from the lower sections of the pellet. From Figures 5.12 and 5.13 it can be seen that the shape of the reaction profile is steeper for lower %Ci and at lower temperatures and that these two effects can balance each other (e.g. reactions at 700°C with 40% Ci and 800°C with 9% Ci). At 46 and 83% nominal porosity the effect of increasing the %Ci is to increase the initial rate in terms of % reacted (Figures 5.14 to 5.18) without the peaks observed at 28% porosity at about 40% Ci. In terms of grammes of Nb20^ reacted no general trend was observed such as the falling off at higher carbon contents encountered in the 28% porosity samples with 10.0 mm thickness. These results indicate that by increasing the initial sample porosity the reaction takes place in a greater portion of the pellet. Even at higher %Ci the gasification of Nb20^ in the start of reaction is not taking place only in the top layers of the pellet. This can be explained by the fact that an increased porosity was responsible for a less effective contact between the particles of niobium pentoxide and carbon causing a slower chemical reaction and a deeper chlorine penetration . This trend (increase in the initial % Nb20^ reacted/ min at high %Ci) is similar to those of shorter pellets of 28% nominal porosity. The changes in the initial rate of Nb20^ reacted (gr./min) may be attributed to the presence of different quantities of niobium pentoxide in the initial reaction samples. The effect of the %Ci on the reaction of Nb20^ in various layers of pellets of 46% nominal porosity can be seen in Figure 5.19. The plots presented in this figure were drawn based on the data given in Table 5.4. It can be seen that although a slightly greater percentage of Nb20^ reacted occurred in the bottom layers of the pellets of 46% nominal porosity the shape of the curves are similar to those for pellets of 28% porosity (Figure 5.12). Besides the increased percentage of Nb20^ reacted in the top layers of the pellet at higher %Ci it is interesting to note that at 700°C and 9% Ci the reaction is taking place almost uniformly throughout the pellet. A completely opposite mechanism occurs at 800 C and 40% Ci. These facts seem to indicate that the resistance to the chlorine penetration into deeper sections of the pellet at 800°C and 40% Ci can be attributed in a greater extent to chemical reasons (chlorine is consumed in the top layers due to a fast chemical reaction) rather than physical reasons (difficult chlorine access to the bottom parts of the pellet due to low porosity) independently of the initial pellet porosity. This conclusion was supported by the analysis carried out in different portions of the residue when the initial sample was of 83% nominal porosity as can be seen in Table 5.5. It shows that while the reaction at 700°C appears to take place evenly in the whole sample independently of the %Ci used at 800°C there is a greater amount of niobium pentoxide reacted from the top sections of the sample. At the other extreme (700°C and 9% Ci) a slow chemical reaction would allow the chlorine to react rapidly in the bottom layers of the sample even if it was of low porosity. Therefore a balance between %Ci, porosity and temperature would determine the degree of the chlorine penetration into the pellet as well as the overall progress of the reaction. As part of the experimental work quantitative analyses in the JSM-35 were carried out on a sectioned unreacted pellet and on the lower parts of partially reacted pellets in order to compare the results with those taken from a mass balance based on Figures 5.1, 5.12 and 5.19. The quantitative analyses in the JSM-35 were carried out in terms of percentage of elemental niobium present in particular sections of the samples. The comparison was performed with the desired values taken from mass balance after making suitable corrections for the pellet changes during reaction. The assumptions made in calculating these values are presented in Appendix II (Tables II.1 to II.4). When the unreacted pellet was subjected to analysis the local porosity was assumed to be equal to the calculated overall porosity of the pellet. The results, as given in Table 5.6., can be considered to be in good agreement. Also, it shows the uniform distribution of Nb20^ throughout the unreacted pellet and supports the results obtained through the analysis of sectioned fractions of reacted pellets. # 0 INITIAL RATE (% Nb A REACTED I min. Nb^ CONTENT IN THE PELLET (gr.) U1 o .0 .0 rsj •P> o §0 0 1*0 o OH 8 8 S 8- 0s r—i 8- —1 1 1 1 1 1 1 r~ 3 INITIAL RATE (gr. Nb205/min.x10 ) 60-i 55- 50- 45- POR3STTY:28% h=100mm 40- 700°C 35- 30- <3 25- Z s 0 20- 15- 10- 5- 0 - T T T 0 10 20 30 40 50 60 %Ci 55- 50- 45- 40- a 35- 20' 30- Ln s 25- 20- POROSITY: 28% 15- h=10.0mm 10- 800°C 5 - 0 T T T T 0 10 20 30 40 50 60 %G Fig. 5.11. Effect of %Cj on the extent of reaction. T(°C) Tabic 5.3. Analysis of Sectioned Pellets. POROSITY:2B% - h«10.0mm - pd2 «1.00 atm. Overall Time % %TbkiNbft % Ci T(°C) RUN SECTTN(%) %REACTED .(Min.) (ENTRBUTI> REACTED 700 1(24.5) 49.6 51.1 (23.8) 60 127 B(75.5) 15.4 48.9 T(29.5) 57.7 33.7 9.0 700 120 160 1(44.0) 55.1 48.0 50.5 B(26.5) 34.8 18.3 T(29.4) 93.2 50.5 800 55 162 1(42.7) 53.0 41.7 54.3 B(27.9) 15.2 7.8 T(35.4) 99.7 53.5 700 110 151 1(28.3) 67.2 28.9 65.9 8(36.3) 31.9 17.6 700 85 153 TJ58.1) 70.3 81.0 50.4 8(41.9) 22.6 19.0 TJ25.4) 76.2 40.4 700 80 161 1(23.4) 61.8 30.2 47.9 B(51.2) 27.6 29.4 T(48.6) 63.0 83.6 700 60 194 I(29.0) 20.7 16.4 36.6 B(22.4) T(48.3) 59.0 56.8 20.0 700 80 196 1(26.8) 48.9 26.1 50.2 B(24.9) 34.5 17.1 T(54.9) 100.0 62.5 800 120 152 K17.7) 99.7 20.1 87.7 B(27.4) 55.5 17.4 T(34.4) 96.2 69.4 800 50 154 1(37.4) 34.1 25.2 50.7 B(26.0) 10.5 5.4 1(57.4) 99.5 74.5 800 90 163 K12.1) 81.0 12.8 76.7 B(30.5) 32.1 12.7 1(13.7) 92.6 48.0 800 20 198 11(32.9) 36.4 45.4 26.8 12(31.5) 5.6 6.6 B(21.9) 1(51.1) 39.8 100.0 800 15 199 1(29.2) 20.2 B(19.7) 1J17.9) 99.6 43.1 J 800 30 200 1(51.8) 39.9 49.9 41.4 B(30.3) 5.6 7.0 T(°C) » We 5.3 (oont.) POROSITY:28% - h« 10.0 mm - pd2«1.00otm. Overall Time % Nb£ % C, T(°C) RUN SECTION (%) % REACTED (Min.) aONTRBUTI> REACTED 700 120 121 T(52.4) 99.2 47.R 77.1 B(47.4) 52.1 32.2 T(24.3) 98.1 43.8 700 40 132 1(10.3) 78.5 14.9 (54.4) B(45.4) 34.3 41.3 T(44.9) 55.4 74.1 35 138 32.9 40.0 700 B(55.1) 14.3 23.9 T(47.2) 99.9 47.3 70.1 800 70 114 B(52.8) 43.4 32.7 100.0 80.9 800 120 120 T(74.8) 95.0 8(23.2) 78.7 19.1 T(34.0) 99.2 44.2 800 38 133 (52.5) 8(44.0) 28.5 35.8 T(20.8) 51.4 ' 39,1 700 30 155 1(33.2) 32.0 38.8 27.4 B(44.0) 13.1 22.1 T(19.0) 92.8 34.2 40.0 700 40 154 11(29.1) 48.0 38.3 51.4 12(24.9) 33.1 14.0 # B(27.0) 22.1 11.5 T(30.4) 99.1 59.3 800 42 157 1(30.4) 49.0 29.3 51.2 B(3R.8) 15.1 11.4 PORQSIT Y : 28% - h« 5.0mm - pd2 * 1.00 atm. Overall Time % %T)tdiN)A % C, T(°C) RUN SECTION (%) % REACTED (Min) OONTRffiUTI* REACTED 48.4 700 90 144 T(52.7) 52.7 4R.4 9.0 B(47.3) 43.4 47.3 800 144 T(50.0) 49.4 44.1 40 B(50.0) 39.0 35.9 54.3 83.3 700 50 145 T(34.9) 42.8 71.7 B(43.1) 45.0 57.2 40.0 T(44.1) 700 33 148 58.4 55.3 48. R B(53.9) 40.5 44.7 T(45.9) 79.8 800 20 147 48.7 53.3 B(54.1) 30.9 31.3 * PELLET DEPTH PELLET DEPTH PELLET DEPTH PELLET DEPTH ft \ $ •N tf 5 •8 ^ A 5* \ 8 h^S \ \ 3? PELLET DEPTH PELLET DEPTH PELLET DEPTH PELLET DEPTH uu r-o 0 0 PELLET DEPTH PELLET DEPTH NO <5? PELLET DEPTH PELLET DEPTH W 5? un UJ CT UJ kj SP r-o Lea 0 UJ 8 UJ T(°C) 1.0- % i t 0.9- POROSITY: 46% h=4.0 mm ^CTI 0.8- |0.7 - 0.6- ¥ Ii—i 0.5- i 0.4- Ln 0.3- 2 0.2- » 0.1- 0- TIME (MIN.) O X d "E or 20 30 40 50 % INITIAL CARBON CONTENT Fig5.14. Effect of on ttie initialrate(Nomhal porosity:46% , h=4.0mm) T(°C) 100 90 H 80-J TO - 60- 50- df _CQM 40- NO 30- POROSITY: 46% h=4.0mm 20- 700°C 10- 0 T T T T T T 0 10 20 30 40 50 60 % INITIAL CARBON CONTENT 100 90- POROSITY: 46% h=4.0mm 80- 800°C 70- 60- cr 50- in O CM 40- 30- 20- 10-i 0 T 1 T 0 10 20 30 40 50 60 % INITIAL CARBON CONTENT Figure 5.15. Effect of %C,- on the extent of reaction. T(°C) 4.0 500 C 'E $2 x 37.5 | £ IS 20 n 25.0 fc £ £ 10 A M2.5 0 T 0 0 20 40 %C. 100 90- POROSITY: 46% h=10.0mm 80- 70- • 60- 50- JD vO 30- /v> 20- 10- 0 - T T 0 20 AO %C, Fig. 5.16. Effect of %Cj on the initial rate and on the extent of reaction. T(°C) C E §JD i p 12 c Q ht) 1 rrT" P X 2H h 8 i POROSITY: 83% h=10.0mm "6 ? 700°C £ £ cc 1 - k i Fig. 5.17. Effect of the %Cj on the initial rate. 3' LTI INITIAL RATE (% ^ REACTED /min.) - INITIAL RATE (% Nb20^ REACTED I min.) as rn NJ 4> O I j l x • rp ciH 3 if BJ 5: ? BH sH B4 sg n sH uT C* "5" U1 3 INITIAL RATEfgr.NbA Imin.xlO ) INITIAL RATE(gr.NbA/min.x103) uu GD T(°C) ft* 5A. Ardyss of Sectioned Pallet-. * POROSITY: 46% - h« 10.0mm - p^slOatm. PELLET % Total Overall Time % NbA % of Portion RUN section NbA %C T(°C) REACTED ENTRHJIDN (Min.) (%) REACTED T(28.6) 30.6 33.7 700 60 174 11(20.2) 30.5 23.7 26.0 12(21.7) 23.6 19.7 B(29.5) 20.2 22.9 T(20.6) 58.0 25.8 700 120 91 1(36.4) 48.3 38.0 46.3 8(43.0) 39.1 36.2 T(23.2) 56.5 27.3 9.0 700 120 124 1(26.8) 48.9 27.3 (48.0) 8(50.0) 43.6 45.4 T(58.3) 70.6 800 60 122 94.6 78.1 R(41;7) 55.0 29.4 T(48.5) 73.4 78.0 800 35 136 8(51.5) 20.2 22.0 (46.3) 1(25.5) 50.2 49.0 800 18 177 1(48.6) 22.4 41.7 26.1 8(25.9) 9.4 9.9 1(74.5) 700 60 93 76.0 83.5 B(25.5) 43.8 16.5 67.8 T(34.6) 58.1 49.8 40.0 700 35 175 1(36.7) 39.3 35.7 40.3 B(28.7) 20.4 14.5 T(36.6) 94.8 77.9 800 15 176 1(30.9) 20.6 14.3 44.5 B(32.5) 10.7 7.8 i25" Ln vD PELLET DEPTH PELLET DEPTH I I ro J? a D £ -s fo \N0O n0Ps fVN PELLET DEPTH PELLET DEPTH £ ie ii o 3 \ 3 \ UJ \ s\ V 3? \ 3 '' Table 5.5. Analysis of different portions of partially reacted samples. POROSITY: 03% - h =10.0 mm Overall % NbjOs %C PORTIONS time of %Ci T(°C) RUN Pcl2 reaction REACTED IN RESIDUE % wt. %c (min) T(36.6) 10.0 9 700 45 205 16.0 9.5 B(63.4) 9.3 T(32.7) 12.8 800 30 209 56.3 12.2 B (67.3) 11.9 T(25.4) 28.7 700 50 172 46.4 28.7 B(74.6) 28.7 T(17.0) 25.0 700 30 206 28.8 25.3 1(24.2) 26.4 B(58.8) 25.0 T(29.3) 25.7 20 700 35 217 36.0 26.5 B(70.7) 26.8 100% T(26.9) 34.3 800 20 190 46.3 27.9 B(73.1) 25.5 T(23.3) 37.7 BOO 20 208 50.6 31.5 B(76.7) 29.6 T(36.5) 55.8 800 30 210 73.4 42.4 B(63.5) 34.7 1(18.5) 62.4 700 30 169 62.9 62.5 B(81.5) 62.6 40 T(21.2) 90.5 800 10 191 66.3 64.5 B(78.8) 57.5 T(16.2) 90.6 800 15 211 78.7 73.6 B(83.8) 70.3 T(42.2) 10.9 9 700 120 183 36.3 10.4 8(57.8) 10.0 T(30.6) 14.9 800 60 180 80.2 14.2 B(69.4) 13.9 62.5% 1(37.4) 63.8 700 60 185 69.4 66.4 B(62.6) 67.9 40 T(10.3) 89.7 800 20 189 66.4 64.4 1(17.3) 84.1 B(72.4) 56.1 T(28.3) 8.6 25.0% 9 700 120 143 21.6 8.7 8(71.7) 8.7 Table 5.5. (oont.) POROSITY :83% - h = 5.0 mm Overall % Nb 0 %C IN PORTION %Ci T(°C) Time of RUN 2 5 Pcl2 Raaction. REACTED RESIDUE %wt %c 700 40 173 25.3 24.7 T(38.6) 24.5 B(61.4) 24.9 T(56.6) 33.0 20 700 120 216 64.2 34.3 B(43.4) 35.9 T(62.3) 44.1 100% 800 15 207 75.5 40.5 B(37.7) 34.6 T(66.4) 6206 700 60 214 62.6 63.2 B(33.6) 64.3 40 T(39„7) 72.0 800 10 212 74.8 69.5 B(60.3) 67.8 Table 5.6. Comparison of SEM and Mass Babnoe Analyses of Relief Sections. SAMPLE SECTION ESTIMATED LOCAL Kb(%) CALCULATE FROM HEIGHT ROOT OF SEM ANALYSIS (APPENDft D) FROM MASS RUN (mm) FROM BOTTOM (mm) %NbA POROSITY BALANCE TOP 27.9 (3.67) 60.0 30.0 29.4 11 32.3 (2.93) 60.0 30.0 29.4 STARTING 4.40 12 27.8 (2.20) 60.0 30.0 29.4 MATERIAL 13 27.9 (1.46) 60.0 30.0 29.4 29.4 W>%Cj) BOTTOM 31.5 (0.73) 60.0 30.0 30%KHN1Y TOP 33.6 (6.32) 91.0 50.3 31.7 11 41.9 (4.74) 91.0 34.8 41.5 127 7.30 12 45.2 (3.16) 91.0 29.2 45.1 BOTTOM 43.3 (1.58) 91.0 28.6 45.5 TOP 12.8 (5.36) 42.7 49.3 15.2 11 18.0 (4.02) 49.2 43.5 19.5 132 6.70 12 27.2 (2.68) 53.7 38.3 23.2 BOTTOM 28.5 (1.34) 56.9 34.2 26.2 TOP 30.1 (3.75) 91.0 57.7 26.9 136 5.00 I 34.9 (2.50) 91.0 54.0 29.3 BOTTOM 34.9 (1.25) 91.0 51.7 30.8 5.2.6) EFFECT OF TEMPERATURE The effect of temperature on pellets of 28% nominal porosity was macroscopically observed as illustrated in Figure 5.20, which shows the upper surface of pellets at different degrees of reaction for experiments carried out at 700 and 800°C and also three lower sections of a partially reacted pellet. It can be noted that for the same approximate extent of reaction the amount of niobium pentoxide lost from the top layers of the pellet is greater at 800°C. Although a comparison of the bottom layers does not show such clear differences it can be seen that there is a more pronounced gradient of niobium oxide concentration in the body of the residue from the reaction at 800°C. This fact and the variation of surface porosity between top and bottom of the pellet are consistent with the plots given in Figures 5.12 and 5.13 which show that at 800°C there was a smaller amount of chlorine penetration than at 700°C independently of the carbon content and pellet thickness. These facts are illustrated in Figure 5.21 which shows different porosities (resulting from conversion of niobium pentoxide) according to the position of the layer in the residual pellet after ignition of the unreacted carbon. The effect of temperature was also studied using the Arrhenius relationship for pellets of different thickness and %Ci. The initial reaction rate values were determined from the plots in Figures V.I to V.VI and which are given in Table V.I. These plots are shown in Figures 5.22(a) and 5.22(b). Examination of the results for pellets of 2.9 mm thickness and 20% Ci shows two separate temperature regions. Up to about 700°C the reaction rate is more sensitive to temperature suggesting a chemical reaction controlling region. At higher temperatures the value of the activation energy seems to be equal to those of pellets with 40% Ci. An increase in the activation energy for pellets with 9% Ci can also be observed in the same range of temperature. The same plot but for pellets of 10.0 mm thickness and different carbon contents is given in Figure 5.22(b). It can be seen that the same values of activation energies are found for 20 and 40% Ci. However, there were small positive and negative variations for pellets with 9 and 60% Ci respectively. The calculated values for the activation energies obtained through the slopes of the straight lines are given in Table 5.7. TABLE 5.7 ACTIVATION ENERGIES FOR DIFFERENT REACTION CONDITIONS (SAMPLE NOMINAL POROSITY 28%) PELLET ENERGY %Ci THICKNESS(mm) (kJ/mol) 2.7 105.4 9.0 10.0 88.7 2.9 76.1 20.0 10.0 76.6 3.4 76.1 40.0 5.0 76.1 10.0 76.6 3.6 54.0 60.0 10.0 56.5 The fact that the values of the activation energies do not change by increasing the initial carbon content from 20 to 40% indicates that there was a similar resistance to the chlorine penetration into the pellet in this range of %Ci. The smaller activation energies found for higher %Ci suggests that at this level there was a greater resistance to the chlorine penetration into the pellet compared with the previous case. The greater sensibility of the rate to increasing temperatures observed for pellets with 9% indicates an increased dependence of the overall initial rate on the chemical reaction at this level of %Ci. The drop in the reaction rate at 650°C may be due to a greater difficulty of gasification of niobium pentoxide under these experimental conditions (20%Ci and 650°C) as illustrated in Figure 5.23. This figure shows the top surfaces of three pellets from which different percentages of 1^20^ had reacted as photographed in the SEM (Cambridge model). It can be seen that as the reaction proceeds a greater amount of a condensed phase was formed on the sample surface which may have led to a decrease in the overall reaction rate. The temperature coefficients of the reactions of 46 and 83% nominal porosity samples were also observed (Figures 5.24 and 5.25) and the calculated values are presented in Tables 5.8 and 5.9. Although higher values of activation energies were found for higher nominal porosities the general trend was similar to that for samples of 28% porosity: The greater and smaller values of activation energies were found for reaction samples with lower and higher %Ci respectively. This behaviour indicates that an increase in %Ci is accompanied by a smaller contribution of the chemical step for the overall reaction rate. T(°C) Varying the sample thickness did not alter the apparent values of activation energies for 28 and 46% nominal porosity (except for samples with 46% nominal porosity and 9% Ci) but at 83% porosity an increase in sample thickness appeared significantly to lower the values of the activation energies (see Section 5.2.9.). The values of activation energies found for samples of 83% porosity with 10.0 mm thickness and for samples of 46% porosity show the same trend observed for the denser reaction samples. For shorter samples of 83% nominal porosity it was observed that considerably high apparent values of activation energies were found at all values of %Ci. This indicates that under these experimental conditions there was a chemical reaction controlling mechanism within this range of %Ci. Although the calculation of the apparent activation energies based on the initial rates can be used to indicate the nature of the mechanism which controls the overall reaction rate this may be altered in the course of reaction due to structural changes in the body of the sample. As can be seen in Figures 5.12, 5.13 and 5.19 the upper parts of a residual pellet from ar experiment performed at 800°C and 40% Ci consists of excess carbon when the overall Nb20^ reacted is about 50%. This may lead to a decrease in the overall reaction rate as the reaction proceeds as can be seen in Figures 5.26 which shows the approximate overall reaction rate as a function of the percentage of Nb20^ reacted for pellets of 28 and 46% nominal porosity reacted at different experimental conditions (the number in brackets are reaction times). TABLE 5.8 ACTIVATION ENERGIES FOR DIFFERENT REACTION CONDITIONS (SAMPLE NOMINAL POROSITY: 46%) PELLET ENERGY %Ci THICKNESS(mm) (kJ/mol) 4.0 135.6 9.0 10.0 104.6 20.0 4.0 89.5 4.5 89.5 40.0 10.0 89.5 60.0 4.5 68. 6 TABLE 5.9 ACTIVATION ENERGIES FOR DIFFERENT REACTION CONDITIONS (SAMPLE NOMINAL POROSITY: 83%) PELLET ENERGY %Ci THICKNESS(mm) (kJ/mol) 5.0 199.6 9.0 10.0 156.5 5.0 199.6 20.0 10.0 85.4 5.0 152.3 40.0 10.0 85.4 60.0 5.0 139.7 It is thus concluded that the residue of a fast reaction occurring in the upper sections of the pellet (e.g. the reactions wi th pellets with 40% Ci at 800 C) may cause a drop in the overall rate of reaction due to an increasing difficulty for the chlorine to react in the lower sections. In conditions where the chlorine has easy access to the deeper sections of the sample the overall rate of reaction would not be altered (Figure 5.26(b)) up to a certain critical value when the increasing porosity would cause a drop in the overall rate due to a poor solid-solid contact (e.g. Figure V.XIV for h = 10.0 - 800°C). In order to investigate the combined effect of temperature and percentage of carbon in the initial pellet a different type of analysis can be carried out if the cumulative percentage of niobium pentoxide is plotted against pellet depth. The calculation is based on the percent contribution of each analysed section to the total quantity of Nb20,- reacted. The data, extracted from analysis of the sectioned pellets, are presented in Table 5.3 (for pellets of 28% nominal porosity) and Table 5.4 (for pellets of 46% nominal porosity). The plots are illustrated in Figures 5.27 and 5.28. In these figures the broken line represents the plot expected if the reaction had taken place throughout the pellet to the same extent. In this case there would1have been no resistance to the penetration of chlorine in the body of the sample. The velocity of chlorine diffusion would then be high and the chemical reaction step would become rate controlling. Under these conditions the percent of niobium oxide reacted would be independent of the sample dimensions since the reaction would occur evenly throughout the sample. Conversely, if the rate of the chemical reaction presents no resistance to the progress of the reaction, a straight line would be expected from the top of the pellet to a point at one fourth, half or three fourths of the pellet depth depending if the total niobium oxide reacted was 25, 50, or 75%. From Figure 5.27(a) it can be seen that the reactions with pellets containing initially 9 or 20% of carbon at 700°C would most probably show a chemical reaction control. On the other hand for 800°C and carbon contents of 20, 40 or 60% the curve shows a greater deviation from the broken line (Figure 5.27(b)). It also shows that the niobium pentoxide is removed more completely from layers closer to the top surface of the pellets. As was expected for these conditions, the temperature plays an important role in increasing the extent of reaction in the upper layers of the pellet. However, at a given temperature, the percentage cf carbon only effects the degree of conversion in the top layers up to a certain value. From these plots it appears that all the experiments analysed can be represented by three curves. This suggests that although the reaction rate may vary for different reaction conditions the overall structural mechanism may be the same. It is in accordance with the previously observed fact that a decrease in temperature can be compensated by an increase in the carbon content of the initial pellet, the overall reaction exhibiting the same structural transformations. Figure 5.28 for high porosity experiments shows the same trend observed in Figure 5.27 although each of the experiment conditions studied are represented by different curves. Assuming that at 700°C and 9% Ci the chlorine can diffuse readily through mcst of the pellet and that the overall reaction is chemically controlled it can be seen from Figures 5.27 and 5.28 that an increasing in the temperature and/or in the %Ci may lead the overall reaction to a diffusion regime which is reached at 800°C and high %Ci. T(°C) Fig. 5.20. Top, intermediate and bottom layers of pellets at different extents of reaction. Unreacted pellet 800°C 700°C RUN %Nb205 TIME TIME %Nb205 RUN REACTED (MIN.) (MIN.) REACTED 197 9.2 10 15 9.3 192 199 20.2 15 35 20.7 193 198 25.8 20 60 35.6 194 & 200 41.4 30 70 50.2 196 REACTION CDNDIT1DNS: 28% initial porosity 20% Cj h=10.0 mm Pd2=1.00 atm. T(°C) * + FIGURE 5.21 SECTIONS OF PARTIALLY REACTED PELLETS AFTER IGNITION OF CARBON RUN 141 194 198 / Top of \ / Top of \ / Top of \ / Section 11. \ / Section T. \ / Section 11. \ IRef. Table J I Ref. Table J IRef. Table I \5.12 y \5.3 y \5.3 y =m205 70.0 75.0 80.0 Reacted Ref: Fig. 5.34 Ref: Fig. 5.12 Ref: Fig. 5.12 / Top of /Top of \ /Top of \ 1 Section I2.\ I Section I. \ I Section 12. \ I Ref: Table J 1 Ref: Table J I Ref: Table J V 5.12 J \ 5.3 y v 5.3 y =Nb205 35.0 40.0 10.0 Reacted Ref: Fig. 5.34 Ref: Fig. 5.12 Ref: Fig. 5.12 / Bottom of \ I Bottom of \ / Bottom of \ 1 Section B I I Section B ) 1 Section B 1 \ Ref: Table / V Ref: Table j \ Ref: Table / \ 5.12 7 \ 5.3 y \ 5.3 y =Nb205 - - - Reacted T(°C) 800 750 700 650 10 8 6 5 20%Cilh=2.9ov^ 4 © c 3 e 5 2 9%Ci,h=2.7\A 40%Cj,h=5.0| 1 \ 3 .8 \ As .6 \ .5' \ V .4 \ .3- POROSITY: 28% \ .2- 1/1123 MXJB 1/1023 1/973 1/923 1/T(K) T(°C) 850 800 73) 700 650 10 8 6 <60%Cilh=3.6 5 4 c e 3 Q 2 1 H o POROSITY: 28% £ *4 h=10.0mm •3H 2H 1/113 1/1073 1/103 1/973 1/93 1/TIK) Fig5.22. Initial rate as a function of temperature (Nominal porosity: 28%). T(°C) FIGURE 5.23 MICRO-STRUCTURE OF THE UPPER EXTERNAL SURFACE OF PARTIALLY REACTED PELLETS - Initial nominal porosity: 28% - Initial carbon content: 20% - Reaction at 650°C Overall time of reaction: 30 min Total weight lost: 10.1% Run: 17 (a) Magnification: IK (b) Magnification: 5K Overall time of reaction: 60 min Total weight lost: 26.6% Run: 18 (a) Magnification: IK (b) Magnification: 5K Overall time of reaction: 120 min Total weight lost: 44.6% Run: 35 (a) Magnification: IK (b) Magnification: 5K I 156 10 8 6 A0%Cj, h=4.5 5 40%C„ h=10.0 4 20%Cj, h=4.0 £ 3 s 9%Cil h=4.0 2 9%Cj, h=10.0 ct: 1 .8 £ .6 .5 U .3 2 t- 1/1073 1/1023 1/973 1/T(K) Fig. 5.24. Initial rate as a function oftemperature (Nomina l Rxosify: 46%). T(°C) 157 850 800 750 650 30 20 © 10 C 'e 8 6 5 4 3 2 £ POROSITY: 8B% < .6 h=5.0mm 3 H T r 1/1123 1/10/3 1/1023 1/973 1/923 1/T(K) T(°C) 850 800 750 700 650 10 _j i • 8 6 5 c 4 E 3 - 2 - dT 1 ^ -8 POROSITY: 83% I! h=10.0mm ~ .2 1/1123 1/1073 VVB 1/973 W3 1/T(K) Fig. 5.25. Initial rate as a function oftemperature (Nomina l porosity:83%). E N? 0s ^oNb^ REACTED c E %Nbft -REACTED Fig 5.26. Rite ofreaction a s a function of Nb^ reacted. 159 fc H i— K 25 50 75 100 CUMULATIVE %Nb2Os REACTED-— fc LU Q tf 75 100 CUMULATIVE %Nb205 REACTED— OBS: TOTAL PERCENTAGE OF Nb20^ REACTED IS ABOUT 50% IN ALL RUNS. Fig 5.27. Cumulative %Nb20^ reacted against pellet depth (Nominal porosity;23%, h=100 mm.). POROSITY: 46% h=10.0mm T %NbA %Ci Run Syntol PC) tended 9.0 700 26.0 174 E 9.0 800 25.1 m ® 90 700 46.1 91 • 90 700 48.0 124 • fc a 90 800 46.3 136 O 40.0 700 40.3 175 A 40.0 800 445 176 V a 50 75 100 CUMULATIVE %Nb£ REACTED Figure 5.28. Cumulative %Nb2^ reacted against pellet depth. On O 5.2.7) EFFECT OF GAS/SOLID CONTACT AREA The effect of the exposed external surface area was studied by placing the pellet in a larger crucible (19 mm ID) which allowed the chlorine free access to the sides of the sample surface. Experiments were carried out with pellets of 28% porosity (10.0 mm thickness) containing 9, 20 and 40% of carbon at 700 and 800°C. TableV.II shows the data for these experiments and Figures V.VII to V.IX illustrate the results obtained. As was expected the results show that the reaction rate was dependent on the exposed surface area according to the reaction conditions as illustrated in Figure 5.29. From this figure it can be seen that the reaction rate is more affected by changes in the external surface area at 800°C being most significantly altered when the initial carbon content in the pellet was 40%. Under conditions where a rapid chemical reaction occurs, it would be expected that increasing the exposed surface area of the pellet would lead to an increase in the initial reaction rate which was approximately proportional to the total exposed surface area. In the case of pellets with 40% Ci reacted at 800°C, an increase in the initial reaction rate of 3.3 times was observed, the ratio of exposed surfaces being 4.1. This is considered to support the conclusion that under these conditions, the chemical reaction is rapid. The much smaller effect with the pellet containing 9% Ci at 700°C for example, indicated that under these conditions the chemical reaction was slow and that diffusion within the pellet was not so much affected by the change in exposed area. Between these 162 extremes of behaviour a mixed control of the reaction would be expected. These results agree with the conclusions drawn from Figure 5.27. In the course of the experimental work two runs were carried out in which the lateral surfaces of the pellets were covered with silica wool. The results of these runs are also given in Table V.II and plotted ip Figure 5.29. The rates of reaction observed were intermediate between those with only the top of the pellet exposed and with the sides reacting, but closer to the former case. In order to observe the extent of reaction throughout the samples, sectioned fractions of partially reacted pellets were analysed. The results are listed in Table 5.10 and the plots are shown in Figure 5.30. This figure indicates that the reaction proceeds in a similar manner from top to bottom layers of the pellet independently of temperature and percentage of carbon contents. An illustration of this can be seen in Figure 5.31 which shows, for several extents of reaction, how the niobium pentoxide remaining in agglomerated form was located within horizontal sections of the pellets. These residues were obtained after ignition of the unreacted carbon. An increased porosity can also be seen from Figure 5.31 (due to conversion of ^20^) in the body of the residues. This indicates that there was a penetration of the chlorine into the pellet. If analysis were carried out in a vertically sectioned residual pellet a plot such as illustrated below would be observed. %NbA REACTED 0 At AP AT EXPOSED SURFACE AREA STMB0L TOP SURFACE OF PELLET At TOP AND LATERAL SURFACE AT TOP AN: LATERAL SURFACE Ar GMRED WTTH 3LEA W30L Fbrosity:28%f h=1Q.0mm. %C( T(°C) SYMBOL 9 730 • 9 800 o » 20 730 A 20 800 V 40 700 X 40 800 X 40 TOO + 40 800 + * Fig.5.29. Effect of exposed surfboe area on the initial note. 164 Table 5.10. Analysis of Sectioned Pellets. Lateral surface area of pellet POROSITY: 28% - h=t).0mm - pa =1.00 %Ub O % Relief 2 s % Total T(°C) Time of RUN Contri- % Cj Section Reacted bution. T(36.0) 42.1 700 60 220 1(29.7) 43.5 44.2 9.0 B(34.3) 46.9 800 30 222 - - - 92.2 20.0 700 40 195 T(49.9) 62.6 59.8 B(50.1) 57.1 800 13 201 T(44.3) 58.9 61.2 B(55.7) 63.1 T(48.3) 84.7 700 40 221 88.1 B(51.7) 91.3 T(34.7) 58.7 40.0 700 25 226 I(28.9) 56.6 57.4 B(36.4) 56.7 T(54.7) 80.8 15 223 82.6 800 B(45.3) 84.7 T(66.2) 55.5 8 225 53.6 800 B(33.8) 49.9 165 %Nb20s REACTED - %Nb205 REACTED 0 25 50 75 25 50 75 44.2%, Of £ %NbA REACTED %Nb2Os REACTED 25 50 75 tX) 0 25 50 75 100 61.2' 13' 23%Ci %Nb20^ REACTED %NbA REACTED 0 25 50 75 100 L_ L_ L_ 57.4% j 88.1% | 25' ! 40' ! 40%c, a 700 °C 800°C Rg.5.30: Rrantoge cf Nb^ reacted against pellet depth (lateral surface of pellet exposed, nominal porosity:28%, h=10.0mm.). FIGURE 5.31 SECTIONS OF PARTIALLY REACTED PELLETS AFTER IGNITION OF CARBON Initial nominal porosity: 28% h = 10.0 mm Lateral surface area exposed Overall % Total Top Bottom %Ci T time of Run Nb^ Section Section reaction (°C) (min.) reacted o o 9.0 - - - - o o 9.0 700 60 44.2 220 * o o 40.0 700 25 57.4 226 # o o 20.0 800 13 61.2 201 m o o 40.0 800 15 82.6 223 o o 40.0 700 40 88.1 221 5.2.8) EFFECT OF CHLORINE PARTIAL PRESSURE The effect of chlorine partial pressure was studied in the range from 25 to 100% under various experimental conditions. The data from these runs are presented in Table V.III(for samples of 28% nominal porosity) and Table V.VI (for samples of 83% nominal porosity). The results are illustrated in Figures V.X to V.XIII and in Figures V.XXV and V.XXVI. The order of the reaction with respect to chlorine was obtained by plotting the chlorine partial pressure against the initial rate of reaction on a log-log scale as shown in Figures 5.32 and 5.33. As can be seen in these figures the effect of varying the chlorine partial pressure in the reacting gas for samples of varying porosity were very similar. From these figures the slopes of the straight lines appear to be similar although the range of values of the inferred reaction orders lies between 0.68 and 0.86 with respect to chlorine. The value of the slope of these lines which best satisfies the four groups of points is 0.77 for 28% nominal porosity compared with 0.73 for 83% nominal porosity. However, the diffusional resistance in the reaction of porous solids may lead to the existence of a gradient in the concentration of the reactant gas within the sample. In this case the reaction order may change from a low value at the outside surface to a value approaching unity in the interior of the sample . This behaviour makes it difficult to interpret the effect / of the gaseous concentration on the reaction rate through an equation related with reaction order 1 /n (e.g. r a C^ , n > 1, where r = reaction rate and C^ = gaseous reactant concentration on the reaction surface) when both diffusional and chemical kinetics are important aspects in the reaction mechanism ( 76 )• 168 A comparison of Figures 5.32a and 5.32b supports the previous conclusion that a balance between %Ci, temperature and porosity determines the overall reaction mechanism. The existence of four groups of lines within the same region of these figures for example show that the initial reaction rate (%Nb20^/min) may have similar changes in their values under certain experimental conditions, as illustrated in Table 5.11. TABLE 5.11 EXPERIMENTAL CONDITIONS SHOWING SIMILAR CHANGES IN THE INITIAL RATE DUE TO VARYING CHLORINE PARTIAL PRESSURES POROSITY %Ci T(°C) GROUP 28 9 700 A 83 9 700 28 9 800 B 28 40 700 28 40 800 83 9 800 C 83 40 700 83 40 800 D In order to investigate the effect of chlorine partial pressure on the extent of reaction throughout partially reacted pellets analysis of sectioned fractions were carried out. The results are listed in Table 5.12 and the plots are presented in Figures 5 . 3-3 and 5.34. m The profiles of the niobium pentoxide reacted are very similar to those in Figures 5.12 and 5.13. From this, it appears that although the reaction rate decreases for lower chlorine concentrations, the transformation within the pellet proceeds in a similar manner. Unlike the effect of temperature and %Ci, an increase in the chlorine partial pressure does not cause a greater conversion of niobium pentoxide of the top layers of the pellet. This was also noted from Figure 5.27 where the results for 62,5% chlorine partial pressures were included. The experiments with samples of 28% nominal porosity also suggest that the drop in the overall rate of reaction in the case of pellets with 40% Ci (Figure 5.35) may be related to the formation of a gradient of the chlorine concentration within the pellet due to the increasing difficulty for the gaseous products to diffuse through the layer formed on the upper sections. POROSITY: ©% h=10.0 mm 40%cj(axrc 40%C p 9%Ci(700 C l—i—i—I i I Q2 03 OA OS 0j6 Q7 OB 0.910 CHLOT£ FfcRTIAL PRESSURE Fig5.32. Initial rate as a function of chlorine partial pressure. TABLE 5.12. Analysis of Sectioned Pellets. POROSITY: 28% - h« 10.0 mm - Pd?« 0.625 O U PELLET VETQ Time % NbA % % %Ci T(°C) of Rarttn RUN SECTION ODNTRI- TOTAL fbfis REACTED (Min.) (%) BUHON REACTED 1(22.2) 40.9 38.5 700 150 182 1(41.8) 48.5 39.7 51.0 B(24.0) 42.7 21.8 9.0 T(32.5) 84.4 54.4 FIOO 45 184 1(41.5) 41.4 34.4 49.9 B(24.0) 17.7 9.2 1(24.7) 95.8 49.4 40.0 700 85 184 11(17.8) 47.3 23.1 51.8 12(25.0) 37.3 18.0 B(30.5) 14.0 9.5 PCIROSITY : 2B°/ 'O - h=10.0 mm PCI2=0.625 Overall Time PELLET % Nb205 % % %Ci T(°C) of Reaction RUN SECTION CDNTO- TOTAL NbjOj REACTED (Min.) (%) BUTDN REACTED T(29.4) 99.9 54.5 40.0 800 50 187 11(21.4) 73.4 30.3 52.0 12(22.8) 18.8 8.2 8(24.4) 9.9 5.0 PC ROSITY:28% - h=10.0 mm - Pd7 = 0.250 Overall Time' PELLET %Nb205 % % %Cj T(°C) of Reaction RUN SfclllUN CDNTTC- TUIAL to20s REACTED (Min.) (%) BUTDN REACTED T(48.5) 37.2 90.0 700 120 140 1(31.7) 4.4 10.0 20.1 B(19.8) T(34.5) 81.4 47.0 9.0 800 120 141 11(17.3) 44.8 18.3 44.3 12(22.4) 22.3 11.4 B(23.4) 4.3 3.3 T(48.4) 35.9 88.0 800 45 149 19.8 B(51.4) 4.5 12.0 T(49.5) 41.4 7R.3 700 120 139 38.8 B(50.5) 14.7 21.7 T(39.2) 98.8 48.7 40.0 800 120 142 I 12.7) 72.9 14.4 54.3 R/Zft 17 A 1A 9 1(19.4) 99.7 48.9 ROO 70 147 1(29.4) 53.4 39.5 39.9 B(51.0) 9.1 11.4 0 0 0 0 9 0 Table 5.12.(cant.) POROSITY: 28% - h = 5.00 mm - pri?= 0.250 Overall Time PELLET %Nb205 GONTRI% - % Cj T(°C) of Reaction RUN SECTION TOTAL Nt^Os % REACTED (Min.) (%) BLTnON REACTED T(48.6) 45.8 49.4 700 180 178 45.0 9.0 B(51.4) 44.3 50.6 T(49.5) 69.7 65.8 800 60 180 52.4 B(50.5) 35.5 34.2 T(41.8) 56.8 51.7 700 70 179 45.9 B(58.2) 38.1 48.3 40.0 T(54.5) 76.0 82.9 800 35 181 49.9 B(45.5) 18.8 17.1 tvD PELLET DEPTH PELLET DEPTH PELLET DEPTH PELLET DEPTH ii $ d rn o PELLET DEPTH PELLET DEPTH PELLET DEPTH PELLET DEPTH •o £ ii i o \ V bi \u3 £ \jj \ A& > -or 8 174 0 POROSITY:28%, h=5.0mfn, pa,=0.25 %NbA REACTED %Nbft REACTED 9%Ci fc 700°C a a 800°C i— I— a a ni REACTED Q. %Nb205 REACTED 75 40%Ci Fig 5.34. Percentage of N^Os reacted againsf pellet depth. * 0 PCRHTY:2B%, h=t).0mm, 800°C 9%Q Pd2=0.25 i 1 1 30 60 90 123 TIME TIME ui Fig.5.35. Nb205 content in the sample as a function of time. 5.2.9) EFFECT OF POROSITY The effects of varying the sample porosities of pellets nominally 5.0 and 10.0 mm thick at 700 and 800°C are shown in Figures 5.36 and 5.37. In most cases it will be seen that an increase in porosity results in an increase of the initial reaction rates. This is probable due to the greater ease of penetration of chlorine into the bulk of the pellet at higher porosities, resulting in a higher overall rate of the %Nb205 reacted. The behaviour of the shorter sample reacted at 700°C (fall of the initial reaction rate for the most porous samples) is thought to be related to the mechanism whereby oxygen is transferred from combination with niobium to form a carbon containing compound,- , At the very beginning of the process, reaction between Nb20^ and C can only occur at the points of contact between the solid phases and the gaseous products of this reaction include CO, C02, NbOClg and carbon-chlorine compounds such as phosgene. Under the conditions of slow chemical reaction at 700°C it is possible that the accumulation of these intermediates in the gas phase plays a controlling role in determining the rate of reaction. A balance is then set up in which the degree of solid-solid contact (which is diminished by higher porosity) is countered by the accumulation of gaseous intermediates which facilitate the transfer of oxygen. In the thinner sample (where the concentration of the chlorine is basically uniform throughout the bed from the start of reaction) the concentration of these gaseous intermediates may be small causing a slower reaction to take place. This mechanism would also explain the results found for experiments carried out at 700°C and different sample thicknesses. In these experiments an increased initial rate was observed for thicker samples ( Section 5.2.3). This behaviour seems supported by some mechanism proposed in the literature (23)(30)(34)( 77J . It is suggested (23^) (30^) that the primary product of chlorination is NbOCl^ which then comes into contact with carbon and chlorine when it is partially converted into NbCl^. As the conversion of Nb20^ to NbOCl^ is unlikely to take place in one step (see Section 3.2) it is thought that an unstable intermediate oxychloride of niobium is formed by the reaction of carbon and chlorine with Nb20^ which by reacting with a gaseous molecule of chlorine forms the volatile primary product (NbOClg). It is also suggested ( 3_0) that the rate determining step is the reaction between the intermediate oxychloride of niobium . formed on the active centre with a molecule of chlorine to produce NbOCl^ Other observations ( 34) ( 77) suggest that phosgene is the most probable intermediate in reduction chlorination In this case the reaction proceeds by the formation of radicals of the C0C1 type which react with the oxide to form chlorides and carbon dioxide. The latter then partially reacts with carbon and regenerates the active reaction centres. The existence of an intermediate formed by the reaction of carbon and chlorine with Nb20^ agrees with the experiments carried out with two separate layers of Nb20^ and carbon (Table V.VIII). These show the importance of mixing the two solid reactants for the reaction to proceed, since this provides a much greater and well distributed number of active reaction centres. Samples of increased porosities generally show a greater sensitivity to temperature as can be observed from Figure 5.38. These results indicate that in 5.0 mm samples an increase in porosity increases the importance of the chemical step in controlling the reaction rate. With 10.0 mm samples this is the case with stoichiometric carbon contents, but when an excess of carbon is present, the mechanism controlling the reaction of these samples appears to be less sensitive to porosity. These may also be related to the balance between temperature, %Ci, porosity and sample thickness which according to their values will cause the reaction to be controlled by different mechanisms. An example of how temperature and the size of the sample may determine the reaction mechanism is illustrated on the figure below ( 7.6) . TEMPERATURE 80 100 INITIAL POROSITY (%)—— 80 800PC, h=10.0mm. a 6.0 H so 4 I M- INITIAL POROSITY (%) Fig5.36. Effect- cf porosity cn the Htd reaction rate. 180 20 40 60 80 100 INITIAL POROSITY (%) Rg.5.37. Effect of porosity on the initial reaction rate(h=50mm.). 181 50 209.2 8 40- h* 10.0 mm. -167.4 H25.5 20 and 40%G g 20- h83.7 ~ 10- kl.8 0 T T T rO 20 AO 60 80 100 INITIAL POROSITY (%) •209.2 40- h=5.0mm. -167.4 A0%Cj M25.5 o E 5 20H k3.7 2 < 10- kl.8 INITIAL POROSITY (%) fig 5.38. Effect of porosity on the apparent \due of activation energy. 5.3) GENERAL MECHANISM When the amount of reacted in the top sections of the sample is greater than in the bottom the analysis carried out suggests that after the admission of chlorine there is a rapid formation of a partially reacted layer which acts as a reaction front. The rate of conversion of Nb20^ in the top of this layer decreases (due to a decreased extent of con tact between niobium oxide and carbon - observed by an increased porosity) while in the bottom layer the rate of niobium oxide conversion increases (due to an increased ease for the chlorine to react at this level) up to a certain critical value when the increasing porosity becomes more significant despite the higher concentration of chlorine. At this point the reaction front has already moved to a deeper portion of the pellet. In this way the reaction front moves continuously to the bottom of the pellet and may reach it before all the niobium oxide present in the top of the pellet has been reacted, (this can be seen in some plots of Figures 5.12, 5.13 and 5.19). The depth of the initial partially reacted layer is a function of the chlorine penetration through the sample which in turn is a function of the chemical reaction rate. When the velocity of diffusion is smaller than that of chemical reaction the chlorine reacts in a thin layer and the effect of the pellet thickness is small or non-existent. Also in this case the overall reaction rate decreases continuously due to changes in the structure of the top layers of the pellet, as illustrated in Figure 5.26. When the depth of the initial partially reacted layer is greater the velocity of diffusion may be high compared with the rate of chemical reaction and the chemical regimes may predominate (which is the case of experiments at 700°C and 9% carbon content). It should also be noted that the overall rate of niobium oxide volatilization may be constant during a certain time of reaction (e.g. experiments at 700°C with h = 10.0 mm - Figures V.^V.Il^V.V and v.vi ) although the contribution of each layer of the pellet for a given value of total reacted is varying continuously. This behaviour suggests that in this case the maximum rate of niobium pentoxide transformed attained in each pellet layer decreases for deeper layers. This fact can be explained by the existence of a chlorine concentration profile through the pellet since the reaction rate in each layer may be proportional to the corresponding chlorine concentration. The existence of the chlorine concentration profile may be attributed to the fact that chlorine has to pass through a partially reacted layer and/or excess carbon (when the initial carbon is greater than 9%) in order to reach the bottom parts of the pellet. If the material left on the top of the pellet does not offer any resistance to the passage of the chlorine the rate of transformation at each of the deeper layers would then be determined by the flow of chlorine that crosses each layer. When the reaction takes place evenly in the whole sample (e.g. 700°C - 83% nominal porosity or 700°C - 9%Ci - 46% nominal porosity) the effect of the chlorine diffusion is not important in determining the overall reaction rate. A uniform concentration of the chlorine throughout the sample can be rapidly attained and the rate of 1^20^ gasified from each layer is basically the same. This rate will decrease with time when the increasing porosity reaches a critical value which eventually causes the collapse of the charge. Figure 5.39 illustrates graphically the mechanism discussed above showing the zones formed in the body of the sample and the proposed changes in the reaction rate for each section of the sample in the course of reaction. The progress of the reaction may therefore take place through a uniform conversion of niobium pentoxide throughout the chlorination charge (favoured by low temperature, low %Ci and high porosity) or through a sharp reaction front moving to the interior of the solid mixture (favoured by high temperature, high %Ci and low porosity). Intermediate conditions result in a reaction pattern lying between these extremes. Since the reaction behaviour is determined by a balance between the most Important variables of the system the observations of the combined effect of temperature, %Ci and porosity of the solid mixture on the progress of the reaction can be analysed in order to provide a general representation of how the reaction proceeds in the body of the chlorination charge. i) Under conditions of high temperature and %Ci the chemical reaction takes place rapidly resulting in poor chlorine penetration into the solid mixture even when it is formed of loose powder (points (1) on Figure 5.40). ii) At low temperatures and %Ci the chemical step is slow and the reaction may take place throughout the pellet to the same extent independently of the porosity of the solid mixture (points (2) on Figure 5 .40). iii) Under intermediate conditions, increasing porosity will increase the chlorine penetration into the solid charge due to a poor Nb20^-carbon contact which causes the chemical reaction to be slow so that access of the chlorine into deeper sections occurs (points (3)/(4) ar.d (5) on Figure 5.40). iv) The importance of %Ci and porosity of the solid mixture in determining the overall behaviour of the system decreases for increasing temperature. It follows that for increasing temperature lower values of %Ci (at any porosity) or higher values of the charge porosity (at any %Ci) may be sufficient to maintain the overall reaction behaviour under the same mechanism. These considerations are illustrated in a general representation of the behaviour of the system (Figure 5.40) showing how the balance between temperature, %Ci and porosity may determine the progress of the reaction throughout the chlorination charge. 186 tate of reaction fcite c^ neqption Fig. 5.39. Progress of ttiereaction zone s through samples showing the changes in the reaction rates of each layer as a function of time. xJly X * 7yX« wXwX (C) Deep initial chlorine penetration - X5?x* x/ ^XX v.. • • Reaction takes plaoe evenly throu^i the whole sample (Chemical Con- trol). x (M) Intermediate initial chlorine penet- *x£*?x x*x\**S xxx x ration - Reaction front moves to pxxx XXx*3C** XX **x iveMMSS the bottom of the sample (Mixed .-.v.*. :«.•:•: ».•••. (Z1 Central). (D) Root initial chlorine penetration - x Formation of a sharp reaction £xx xx x x x xxxXX X Xx X Xx *X*X* xX X xxx *x xXXx*x x front (Diffusion Control). : XX x x Xx xx x X X xX XX|. x* XxX x x xx j»xx;x x* x X b Jx x J*JxxX*X* X REACTION ZONES x x x x x x (1) Well reacted zone. (Rate in each xxx5xx xxx xx xx *x x Sxx x « x xxV layer decreases due to increasing xxxx*,£xxxxxxxxx porosity) ^X^XjxXtxKix^X*xwxxxx*j(i * (2) Reacting zone. (3) Uhreacted zore. xX XxX X xX (4) Completely reacted zone. ^ x x xV 2 xxxxxXxJx*ScXxxJx x xx x xxx xjxx xxxxxxx£xx xx;xxxxxx x x «X*XX X Ax *5xx£ \ a > faai* ti h t3 l t5 t6 <1 t2 tj ti. H <6 TIME TIME 187 • Fig. 5.40. General representation cf how the reaction proceeds in the body of the dtrhation charge (h=T).Omm) according to the vetoes of the temperature, %Cj and porosity of the solid mixture. Area C: Uniform reaction ttrouc^iut the charge(Chemical Control). Area D: Formation of a sharp reaction front (Diffusion Control). Area M: Intermediate behaviour (Mixed Control). 5.4) MATHEMATICAL MODEL Because structural parameters affect the rate of reaction and may vary in its course the analytical treatment of heterogeneous reactions, such as the reduction chlorination of niobium pentoxide, normally requires a set of simultaneous differential equations which have to be solvednumerically. It follows that a number of the reported mathematical models for gas-solid reactions are not immediately accessible for the interpretation of experimental data or for prediction of the behaviour of a given system. Szekely et al. showed that the solution of approximate analytical expressions may provide a convenient means to resolve this problem since there is a reasonable agreement between the exact numerical solution and the predictions based on the approximate equations in a large number of cases ( 66) ( 67) ( 69)(70). In this model Szekely et al. proposed a dimensionless representation which leads to an approximate dX relationship between (where X is the fractional solid conversion and t* is the dimensionless time) and the other system parameters. Such treatment provides a convenient way of prediction and inter- pretation of experimental results. Also, the definition of a gas-solid reaction modulus (a) allows the assessment of the relative importance of chemical reaction and diffusion. For the case of a reactant gas entering a reactor bed the procedure presented in references (76) and (78) will be followed in developing the mathematical equations for the general structural model. In order to derive a mathematical expression which describes the progress of the reaction a number of simplifying assumptions are necessary such as: the rate of reaction of a solid may be related to the rate of change of gas composition with distance from the surface. the system displays isothermal behaviour and equimolar counterdiffusion. the effective diffusities of gaseous reactant and product are the same throughout the bed the solid structure is uniform and keeps its shape and size during the reaction, the porous solid is formed of an aggregate of fine grains which have the same shape as the solid structure. diffusion of the gaseous reactant through the product layer of the individual grains does not affect the rate. Under these conditions the system may be described by establishing a mass balance on the reactant gas and on the reactant solid. If the rate at which the reaction front moves is small with respect to the rate of transport of the gaseous reactant (a fact normally observed in gas-solid reactions), the conservation of the gaseous reactant for an irreversible reaction may be expressed as:* u k a C C y 8y = - m ( G - GS) The mass balance on the reactant solid may be expressed by the equation: (CG"CGS) = (Ji£) For most practical purposes Eq.(2) may be expressed in terms of the fraction of the solid reacted as function of time using the relationship (76). *Definitions of the symbols used in this section are given on page 197. 190 (1-e) 3CS (1-£)(1-£V) „ dX (3m) TT = 5 S 3T Then by applying the dimensionless concentration of gaseous reactant (V = C^/C^q) on Eq. (1) it can be wri t ten as : dv (1-e)(1-ev) dX ~uy pG 37 - 5 PSc 3-ItT (4) Rate of change Rate of reaction of gas compo- of solid sition with distance from the surface The rate of reaction of a solid particle with gas depends on the conversion of the solid reactant due to changing surface area available for reaction. Thus for a given gas composition in an infinitesimally thin section of the bed and introducing the dimensionless variables bk A / Crn - CDn\ *• -si ^r *?)* -irPTrs"-«><'-vX1 + rr>» «> y g g G t K F V and N* = pE -p (7) e p equation (4) for an irreversible reaction may be written as: - "If. = (8) where 8' = (?) g-g(X) + a [^(X) + 2,] 191 The quantity a is the dimensionless gas-solid reaction modulus which incorporates both structural and kinetic parameters and is very useful in characterizing the behaviour of the system. In physical terms this modulus represents the ratio Rate of chemical reaction Rate of diffusion It is defined as follows: F V f(l-e.,)kA , HP1 V-irH1 <'•*-> (,0) d • e g e When (5. approaches zero the overall rate is controlled by chemical kinetics, the reactant concentration tends to be uniform within the pellet and equal to that in the bulk of the gas stream. The solid conversion X is described by the relationship: t* = gF (X) (11) g where gp is the conversion function defined as follows: g 1/F gF = 1 " 0-X) g (12) g When a approaches infinity the overall rate is controlled by diffusion of the gaseous reactant within the pellet and the solid conversion X is given by 2 f = cj pF (X) (13) P where pF is defined as follows: P PF (X) = X + (l-X)ln(l-X) (14) P 192 When approximate solutions derived from asymptotic regimes (a 0 or a were compared with the exact numerical solution it was found that (70)(76)(78): 2 1) When a < 0.3 (a £ 0.1) the a = 0 asymptote is approached. Thus when this criterion is met the system is chemically controlled. 2) The a oo asymptote is approached when 2 a > 3.0 (or a > 10). In this case the is under a diffusion controlled regime. A general expression was found to be satisfactory when the time dependence of solid conversion is given by: 2 t* = gF (X) + a [p (X) + (15) g L p J It can be seen that Eq.(15) will be reduced to Eq.(11) or Eq.(13) when the system is under chemical or diffusion controlled regime respectively. Equation (15) shows the relative importance of chemical reaction and diffusion through the parameter a which provides the numerical criterion for determining the controlling regime. Eq.(l5) also shows that the time required to attain a given conversion is the sum of the times to reach the same conversion under the control of steps such as chemical reaction and mass transfer. The extent of reaction at any infinitesimally thin section of the bed is a function of the depth of this section and varies with time. In order to obtain the expression which gives the profile of solid reacted throughout the pellet the following initial and boundary conditions are applied. 193 X = 0, t* = 0 (the extent of reaction is zero at the start of reaction) y = y* = o, t*>0 (gas composition is maintained constant at the surface of the bed) Eq.(5) is then integrated and the following expression has been derived (70)(76). dX P'dy Ht* B ' exp (U) Finally the desired relationship between X, y and t* can be obtained by integrating equation (16). r t* - r X = B ' exp - B' dy dt (17) » o where 8' ] (18) - 1(1-X)_1/2 -a2 ln(l-X) + Then by estimating values for a and N* it is possible to obtain the profile of the extent of reaction (X) throughout the pellet depth (y*) at different reaction times (t*). Since an iterative method has to be used to solve equation (17) a computer programme was written and is presented in Appendix III. The programme also calculates the overall percentage of niobium pentoxide reacted based on the calculated extent of reaction (X) at different depths of the pellet (y*). When extreme values of o were used (a = 0.1 and a = 10.0) it was found that values of N* in the range -3 2 10 to 10 provided predicted profiles comparable with the experimental results given in Figures 5.12 and 5.19. 194 Profiles characteristic of chemical reaction control (a < 0.3) were found for 10~3 < N* < 10"2 while profiles expected from diffusion control (a 3.0) were found for values of N* greater than about 10. Since a = 0.3 and a = 3.0 are characteristic values of chemical kinetic and diffusion controlled mechanisms respectively and in the light of the experimental results presented in the previous section (e.g. see Figure 5.27 and Table 5.11) the data fed into the computer programme were divided into three groups according to Table 5.13. ' TABLE 5.13 GROUPS OF EXPERIMENTS TO BE COMPARED WITH PREDICTED VALUES FROM THE STRUCTURAL MODEL (PELLET OF 28% NOMINAL POROSITY AND 1.00 cm THICKNESS). GROUP REGIME (CONTROL) EXPERIMENTS Chemical Reaction 1 a = 0 .3 700°C-9%Ci N *=.01 Mixed 700°C-40%Ci 2 a = l. 0 N*=l.0 800°C-9%Ci Dif fu sion 3 a = 3 .0 800°C-40%Ci N*=10.0 A typical output of the programme is given in Table 5.14 corresponding to the chemical reaction controlled regime. The plots corresponding to these data together with those corresponding to mixed and diffusion controlled regimes are illustrated in Figure 5.41. The adequacy of the model was tested by also plotting in this figure the observed results for pellets reacted to an extent of about 50% overall niobium pentoxide. The structural model appears to represent the general behaviour of the reaction front within the pellet reasonably well. Closer fits could possibly be obtained if different values of a and N* were tested. The predicted overall extent of reaction as a function of time could also be exactly determined if Eq.(2) were solved. However this equation is inadequate for a reaction involving two solid components, such as Nb20^ and carbon, because the values of the variables related to the solid reactant could not be determined. Therefore in order to obtain predicted values of the overall extent of reaction as a function of time the relationship between t and t* was determined by the ratio of their values corresponding to an overall extent of reaction of 50%. In Figure 5.42 the comparison between predicted and experimental results is presented. Good agreement was found between predicted and observed results which indicate that the procedure used to relate the real time with the dimensionless time leads to an adequate representation of the experimental data. 196 Table 5.14. Typical output of the computer programme. No. of iterations tJ Xi X y Tol. 2.50000 .50000 .14405 0 . 35595 0 . 00 098 Area 2. 500 00 Overall . 14405 .14307 2 .5 00 00 o/ .14405 .13904 5000 0 .00501 Under 2.50000 /o . 14405 .13512 0000 0 . 00 893 the 5000 0 .01274 2.500 00 Reacted .14405 .13130 Curve 2 2 . 5 00 CO .13130 .13129 1.5000 0 . 00 00 2 1 2 .5 00 00 .13130 .12758 2. 0000 0 . 00 372 1 2.50000 .13130 . 12398 2.5000 0 . 00 732 1 2. 5 00 00 .13130 .12049 3. 0000 0 .01082 2 2.50000 .12049 .12047 3. 0000 0 .00001 1 2.50000 . 120 49 .11708 3.5000 0 . 00341 1 2.50000 . 12049 .11378 4. 0000 0 . 00 671 1 2.50000 . 120 49 .11057 4.50000 . 00992 1 2.50000 . 12049 .10745 5.00000 .01304 2 2.50000 . 107 45 . 10744 5.00000 . 00 001 .64495 .12897 1 5 . 0 00 00 .10745 .28602 0 . 17858 2 5.00000 .28602 . 28672 0 . 00 070 1 5.00000 .28602 .27862 .50000 . 00741 1 5.00000 .28602 .27074 1.0000 0 . 01528 2 5.00000 . 270 7-4 .27068 1. 00 00 0 . 00 006 1 5 . 000 00 . 27074 .26303 1*5000 0 . 00 771 1 5. 000 00 . 27074 .25560 2.O000 0 . 01514 2 5.00000 .25560 .25554 2.00000 . 00 005 1 5 . 000 00 .25560 .24832 2.5000 0 . 00727 1 5.00000 .25560 .24131 3.00000 . 01429 2 5 .00000 .24131 .24126 3.00000 . 00 00 4 1 5.00000 .24131 .23445 3.50000 . 00686 1 5 • 0 CO 00 .24131 .22783 4 . 0000 0 .0134 8 7 5.00000 .22783 .22779 4.00000 . 00 00 3 1 5.00000 . 227 83 .22136 4.50 OB 0 . 00647 .21511 5.0000 0 .01272 1 5.00000 .22783 . 00 003 2 5.00000 .21511 .21508 5. 0000 0 1.26377 .25275 1 7.50000 .21511 .42962 0 .21451 2 7.50000 . 42962 .43132 0 .00 170 1 7.50000 . 42962 .41909 .50000 .01052 2 7.50000 .4190 9 .41899 .5000 0 . 00 010 1 7 .500 00 .41909 .40712 1.00000 . 01198 2 7. 5 00 00 .40712 .40701 1 . 0000 0 . 00 01 1 1 7 .5 0000 .40712 .39548 1.5000 0 .01164 2 7.50000 .39548 .39539 1.50000 . 00 009 1 7.50000 . 3554 8 .38419 2.0000 0 .01129 2 7.50000 . 3641 9 .35411 2. 0000 0 . 00 008 1 7.50000 .38419 .37323 2.5000 0 .01096 2 7. 5 0000 .37323 .37316 2.50000 . 00007 1 7.50000 .37323 .36259 3.0000 0 .01064 ? 7.50000 . 36259 .36253 3.0000 0 . 00 007 1 7.50000 . 36259 .35227 3.5 000 0 . 01033 2 7.50000 . 35227 .35221 3.50000 . 00 006 1 7.50000 .35227 .34224 4. 0000 0 .01002 2 7 .50000 . 34224 .34219 4.0 000 0 . 00 005 1 7.50000 .34224 .33251 4.50000 . 00 973 1 7 .500 00 . 342 2 4 .32310 5. 00000 . 01914 2 7.50000 . 32310 .32302 5.00000 . 00 008 1.87740 .37548 1 10.00000 .32310 .57381 0 .25071 2 10.00000 . 573 81 .57767 0 . 00386 1 10.00000 .57381 .56123 • 5000 0 . 01258 2 10.00000 . 56123 .56096 .50000 .00027 1 10.00000 .56123 .54500 1. 0000 0 . 01623 2 10.00000 . 545 0 0 .54469 1.00000 . 00031 1 10.00000 . 545 0 0 .52920 1.5000 0 .01580 2 10.00000 .52920 .52894 1.50000 . 00026 1 10.00000 .52920 .51390 2.00000 . 01530 2 10.00000 .51390 .51367 2m00000 . 00023 1 10.00000 .51390 .49908 2*50000 . 01462 2 10. 000 00 .49908 .49888 2.50000 • 00 020 1 10.00000 . 4990 8 .48472 3. 0000 0 .01436 2 10.00000 .46472 .48455 3.00000 . 00 017 1 10.00000 .48472 .47079 3.50000 .01392- 2 10.00000 .47079 .47064 3.5000 0 . 00015 1 10.00000 . 47079 .45729 4.00000 . 01350 4.00000 . 00 013 2 10.00000 . 457 2 9 .45716 .01310 1 10.00000 .45729 .44419 4.5 000 0 4.5000 0 . 00 01 1 2 10.00000 .44419 .44406 .01270 1 10.00000 .4 4419 .43149 5.00000 . 00 010 2 10.00 000 . 431 49 .43139 5.o o no o 2.50402 .50080 SYMBOLS Surface area/unit volume of bed Ap External surface area of grain and pellet respectively Stoichiometric coefficient Reactant gas concentration in the bulk of the gas stream Reactant gas concentration within the pellet (moles of gas reactant/unit interstitial volume) Reactant gas concentration at the surface of the particle Concentration of the gaseous products in the bulk of the gas stream Concentration of the solid reactant (moles of solid reactant/unit volume of porous solid) Effective diffusivity in pellet Molecular diffusivity Fp Shape factor for grain and pellet, respectively (=2 for cylinders) (X) Conversion function defined by Eq. ( 12 ) Reaction rate constant Equilibrium constant Mass-transfer coefficient Modified Sherwood number defined by Eq.(7) (X) Conversion function defined by Eq.( 14) Time Dimensionless time defined by Eq.(5) D Reactant gas velocity or -y Vp Volume of solid grain and pellet, respectively Fractional conversion of solid reactant Depth of the pellet Dimensionless depth of the pellet defined by Eq.(6) Particle porosity (%) Pore volume in pellet {%) Molar density of reactant gas Molar density of reactant solid Generalized gas-solid reaction modulus defined by Eq.(?) Dimensionless concentration of gas reactant «VCGO> 199 0 75 25.3%,t"=50 /SlS%,t=12(fl 50.1%, fitX) (a) |Chemically Controlled Regime. CXXK-9%C,) X Fig 5.41.tolperison betwee n observed ——( ) and predicted ( ) extent of reaction against pellet depth. !T r so 1IJB 22.2 88B 116.4 TIME (MIN.)—— Fig. 5.42. Comparison between observed and predicted % Nb^ reacted as a function of time. 5.5) THE MATERIAL COLLECTED ON THE COLD-FINGER The material collected on the cold-finger (MCF) from a selected number of experiments were subjected to X-ray diffraction analysis. The diffractograms of the MCF (Figure 5.43) did not indicate the presence of niobium chlorides or oxychloride. The diffraction lines coincide with those given by the ASTM card no. 27-1312 (Table 5.15) which corresponds to niobium pentoxide. Therefore, the niobium compounds produced by chlorination (probably a mixture of NbClg and NbOC^) should have undergone decomposition into niobium pentoxide while being prepared for analysis. TABLE 5.15 : X-RAY DIFFRACTION PATTERN FOR Nb^ (ASTM CARD No. 27-1312) dI/I o 1 97 3.95 3 .14p 3.115 2.463 ' 3 1.802 2.442 1.812 X X-rcy diffraction analysis of the condensed species after hydrolysis and/or calcination were similar with those of the unreacted niobium pentoxide (see Figure 5.43) and coincide with data given by the ASTM cards numbers 20-804, 27-1311 and 19-862 as shown in Table 5.16. TABLE 5.16 X-RAY DIFFRACTION PATTERNS FOR NbpO ASTM dI/I CARD O 20-804 3.56x 3.789 3.599 2 .78, 4.79^ 4.75,5 3.74^ 2.84,5 27-1311 3.77x 2.78x 3.59g 3 .56g 2.73, 4.754 3.574 3.184 19-862 3.75x 3.598 1.918 1 .49, 2.98< 2.79 2.54^ 2 32 i - 4 The temperature of the tip of the cold-finger was altered in a number of runs and it was found that a temperature of about 100°C was necessary in order to provide a reasonable amount of condensed material on the cold-finger and also to diminish condensation on the inner wall of the reaction tube. In order to evaluate the efficiency of the recovery of niobium pentoxide which could be obtained as condensed material a group of experiments were carried out where the temperature of the tip of the cold-finger was kept at about 100°C/ The experiments for this investigation were carried out without removing the cold-finger from its place in the middle of the runs. A photograph of the condensed material on the cold-finger is shown in Figure 5.44 and Table 5.17 presents the material balance obtained for these experiments (runs 218 to 226). TABLE 5.17 MASS BALANCE REGARDING Nb205 REACTED AND THE MATERIAL COLLECTED ON THE COLD-FINGER (IN GRAMMES) Nb205 MCF MCF after being reacted After runs after calcinated few weeks 10.62 14.01 12.40 8.80 If all compound obtained after calcination was niobium pentoxide (as indicated by the X-ray diffraction analysis) the recovery of Nb20^ as a condensed product was 8 2.9%. These results indicate that reduction chlorination may be an efficient method of producing technical niobium pentoxide especially when applied on rich starting materials. 203 FIGURE 5.43 X-RAY DIFFRACTOGRAHS OF RESIDUES AND PRODUCTS FROM CHLORINATION OF Nb 2o5 1,111 11,1111111 I I III I II I I ,, II I I, I I I , , I I I Ill I I II II I !II I I I'I I I I I I I I I II II " II II II II I II II I I I I I I 1111111111 I, I d~CX) 40 20 10 5 4 3 2.5 2 1.5 1 : Residue from chlorination (Run:204) (25.0% Nb2o5 reacted) 2: Residue from chlorination (Pun: 206) (28.8% Nb2o5 reacted) 3: Residue from chlorination (Run:208) (50.6% Nb 2o5 reacted) 4: Residue from chlorination (Run:210) (73.4% Nb 2o5 reacted) 5: ~A aterial collected on the cold-finger 6: Material collected on the cold-finger after a few days 7: Material collected on the cold-finger after being calcinated 8: Specpure Nb 2o5 manufactured by Johnson Matthey Chemicals Ltd. Figure 5.44. Reaction products condensed on the oold-finger. 5.6) CONCLUSIONS The investigation on the reduction chlorination of niobium pentoxide led to a number of conclusions which are summarized below. 1. It was found that the ratio of niobium pentoxide to carbon reacting was approximately constant within the conditions studied and equal to 10.1 (Figure 5.1). 2. The initial depth of the chlorine penetration into the sample depends on the experimental conditions. For fast chemical reaction (e.g. 800°C/ 40% Ci) the depth of this zone is small and the thickness of the sample did not affect the initial rate of reaction (gr. Nb^^/min). For a deep penetration of the chlorine the thickness of the sample did affect the rate of reaction . 3. The chlorine flow rate in the range between 0.1 and 0.3 l/min did not affect the overall reaction rate. 4. It was found that for pellets of 28% nominal porosity there is an optimum contact area between the solid reactants per unit volume which corresponds to a ratio of Nb2C>5 and C (^40% Ci) . At 46 and 83% nominal porosity the effect of increasing the %Ci was to increase the initial rate in terms of %Nb20^ reacted . 5. The lack of chlorine penetration into deeper sections of samples with 40%Ci during the initial periods of reactions at 800°C can be attributed to a greater extent to chemical reasons (chlorine is consumed in the top layers of the sample) rather than physical reasons (difficult chlorine penetration due to low porosity of the sample). 6. At 700°C and 9%Ci the chemical reaction is slow the chlorine can penetrate rapidly into the bottom layers of the sample even if it is of low porosity. 7. In conditions of fast chemical reaction the unreacted material left in the top layers of the pellet causes a drop in the rate of reaction due to an increasing difficulty for the chlorine to react in the bottom layers. When the chemical reaction is slow there is no formation of such a layer in the top sections of the pellet. The rate of reaction is not altered up to a certain extent of reaction when the increasing porosity of the sample causes a drop in the rate due to poor solid-solid contact. In the intermediate conditions the changes in the rates of reaction are due to one or both of these factors according to the initial sample porosity and percentage of carbon content. 8. The initial reaction rate is more affected by changes in the exposed external surface area at 800°C ai 40%Ci. A much smaller effect occurs during reaction at 700 C and with pellets containing 9%Ci. 9. Although the reaction rate decreases at lower chlorine partial pressures the changes within the pellet take place in a similar manner. Unlike the effect of temperature and %Ci, an increase in the chlorine partial pressure does not change the degree of conversion of niobium pentoxide of the top layers of the pellet compared with that in the pellet as a whole. 10. The chemical reaction steps are thought to take place through a mechanism whereby oxygen is transferred from combination with niobium to form a carbon containing compound. This intermediate gaseous species (e.g. radicals of the C0C1 type formed at the active reaction centres) could then react with Nb20^ to form an oxide of carbon and niobium oxychloride. While the latter would be partially converted to niobium pentachloride the former regenerates the active reaction centres. 11. According to the values of temperature, %Ci, porosity and thickness of the sample the progress of the reaction is controlled by different mechanisms. For pellets of 28% nominal porosity and 10.0 mm thickness for example an increase in the temperature and in the %Ci can lead the overall reaction from a chemically controlled mechanism (700°C - 9%Ci) to a diffusion controlled regime (800°C - 20 to 60%Ci). Between these conditions a mixed control takes place (Fig.5.40). 12. In the light of these results a mathematical model was applied. It was found that the extent of reaction throughout the pellet could be reasonably described by equations (17 ) and (18 ) and according to Table 5.13. 13. By relating real time and dimensionless time according to their values corresponding to 50% Nb20^ reacted good agreement was found between predicted and observed results in terms of %Nb20^ reacted as a function of time . 14. The large amount of Nb20^ recovered on the cold- finger indicates that reduction chlorination can be used to produce technical niobium pentoxide especially if applied on rich starting material. CHAPTER VI REDUCTION CHLORINATION OF TIN SLAG AND PYROCHLORE CONCENTRATE 6.1) GENERAL Reduction chlorination tests were carried out on tin slag from Metallurgie Hoboken Overpelt and on pyrochlore concentrate from CBMM-Brazil in order principally to study the reactivity of niobium, tantalum and iron oxides with chlorine in the presence of carbon. For this purpose the effects of temperature and of the amount of reducing agents in the mixture were studied in order to obtain basic information about the conditions under which the chlorination reaction of the valuable metals would occur at useful extents and rates. Preliminary experiments were carried out using a silica spring where the total change of weight of the sample with time was observed by a travelling micrometer as described in Section 2.2. Experiments were then carried out at varying temperatures and times. The starting material consisted of a mixture (-1.0 gr.) of tin slag or pyrochlore concentrate (both ground to minus 200 mesh) and specpure graphite, which were pelleted without using any binder. The chlorine flow rate used was 0.1 1/min. The pellets were very similar although small variations were found in the thickness of the pellets when different percentage of carbon were used. In all experiments the internal diameter of the crucible was greater than the diameter of the pellet. The temperature of the tip of the cold-finger was maintained at about 150°C during the tin slag experiments and at about 110°C during the runs with pyrochlore concentrate. 6.2) ANALYTICAL PROCEDURE The residues and the material collected on the cold-finger were analysed by the following procedures: 6.2.1) CHLORINATION RESIDUE The residue was hydrolyzed either in boiling water or with dilute ammonium hydroxide and after evaporating to dryness and ignition at 900°C the percentage of charge reacted was calculated. Qualitative analyses were carried out by X-ray diffraction. Quantitative analyses for niobium, tantalum and iron oxide were carried out by X-ray fluorescence (79. ) an<^ the amounts of these oxides gasified determined. 6.2.2) CONDENSED SPECIES Some of the material collected on the cold-finger was hygroscopic. It was hydrolysed in boiling water and evaporated to dryness. The material from some tests were also calcined in air at 1200°C so that oxides could be identified. All of these materials have been analysed by X-ray diffraction and by X-ray fluorescence as described above. 6.3) REDUCTION CHLORINATION OF TIN SLAG 6.3.1) EXPERIMENTAL CONDITIONS AND RESULTS Reduction chlorination experiments on tin slag were carried out with samples of 1.0 g weight and approximately 3.0 mm thickness. The specific gravity of the slag was determined according to the procedure described in reference ( 75) and found to be 3.75. This led to a pellet of 26.2% and 24.3% calculated porosity when the %Ci was 5.1 and 20.2% respectively. The approximate analysis of the slag was stated by the supplier and is given in Table 6.1. TABLE 6.1 ANALYSIS OF TIN SLAG (% wt.) Ta205 4.04 Nb205 3.29 wo3 2.32 Ti02 4.00 ai2o3 6.55 MgO 10.20 CaO 13.00 Si02 17.50 Fe 22.65 Sn 1 .17 Pb 0.31 Cu 0.13 The results obtained by X-ray fluorescence were 3.26% Nb205, 4.14% Ta205 and 32.62% Fe203 which are in agreement with the values given. Electron micro-probe studies showed that niobium and tantalum may be associated with calcium and titanium in regions of smaller than average iron content. It was also observed that the valuable metals segregated in particular areas, while the iron appeared scattered through most of the sample ( 39) . In Tables 6.2 and 6.3 are given the experimental conditions and results. The results are presented in terms of % total weight lost (TWL), % weight lost of the pellet (WLP), % slag reacted and the % of Nb205, Ta205 and Pe202 remaining in the partially reacted slag. The difference between TWL and WLP is due to the formation of a hygroscopic deposit during the reaction which was normally found in the bottom of the pellet, as illustrated in Figure 6.1. This portion of the specimen was removed before further treatment of the residue of chlorination. 6.3.2) THE EFFECT OF %Ci The effect of %Ci was studied in the range between zero and 20%. For this purpose all the experiments were carried out at 850°C for 45 minutes. an< reac e The percentages of Nb20^, ^ l d were then calculated. These results are presented in Table 6.2 and plotted on Figure 6.2. Under these experimental conditions, the percentage of Nb20^ and Tg20^ reacted do not increase significantly for %Ci greater than about 14%. The effect of temperature was then studied by carrying out experiments with samples containing 14% of carbon. 212 Cm 0 0 m Figure 6.1. Top and bottom surfaces of a partially reacted pellet. (Bottom picture shews hygroscopic deposit) TABLE 6.2 EXPERIMENTAL RESULTS FOR RUNS WITH PELLETS WITH DIFFERENT %Ci T = 850°C - t = 45 min. %wt. IN THE UNREACTED % % % SLAG % REACTED* TWL WLP SLAG RUN %Ci REACTED Nb Ta205 Fe203 Nb205 Ta205 Fe203 2°5 1 0 +5.0 - 20.0 3.82 4.78 27.80 6.3 7.6 31.8 2 5.1 13.7 13.7 41.4 4. 62 5.45 16.10 17.0 22.9 71.1 3 10.3 18.3 19.8 59.9 3 .74 4.83 4.25 54.0 53. 2 94.8 4 11.3 19.4 25.7 70.0 1.21 2.06 1.82 88.9 85.1 98.3 5 11.4 22.5 26.8 64.8 2. 61 3.74 2.53 71.8 68. 2 97.3 6 12.1 24.8 28.7 65.1 2.78 3.81 1.98 70.2 67.9 97.9 7 14.0 25.7 28.8 68.9 0.86 1.45 1.26 91.8 89.1 98.8 8 20.2 31.8 36.9 74.1 0.17 0.38 0.39 98.7 97.6 99.7 (*)Calculated for the following initial compositions: Nb20^= 3.26, Ta20^ = 4.14, Fe^^ = 32.62 ro UJ Fig.6.2. Effect of %Cj on the extent of reaction. 215 6.3.3) THE EFFECT OF TEMPERATURE The effect of temperature was studied between 800 and 900°C. Experiments were carried out at different times of reaction and the treated residues analysed. The experimental results are presented in Table 6.3 and illustrated on Figures 6.3 to 6.9. From these figures it appears that in the first minutes of reaction the Nb and Ta pentoxides are not gasified. Although the reaction at 800c'C for about 55 minutes could produce a residue with an approximately unaltered weight percentage of Nb20£; and Ta20«j the proportion reacted of these oxides is between 65 and 70%. At 850 and 900°C basically all the niobium and tantalum pentoxides are gasified in 60 and 30 minutes respectively. The gasification of iron is very rapid up to 70% or 85% conversion of FFe 2C>3 at 800 or 900°C respectively (Figure 6.9). 9 9 216 * TABLE 6.3 EXPERIMENTAL RESULTS FOR RUNS AT DIFFERENT TEMPERATURES (14% Ci) T=R00°C %wt. IN THE UNREACTED % REACTED % % % SLAG TWL WLP SLAG RUN TIME REACTED Nb 0 Nb 0 o205 Fe 0 (min) 2 5 To2°5 F<-2°3 2 5 T 2 3 9 10 5.3 9.2 34.0 4.25 5.30 23.75 14.0 15.5 51.9 10 15 7.5 10.4 43.5 4.51 5.65 17.50 21.8 22.9 69.7 11 25 12.3 18.3 51.3 4.37 5.42 10.75 34.7 36.2 84.0 12 35 16.1 20.1 55.4 4.16 5.25 6.25 43.1 43.4 91.5 13 45 18.4 24.2 61.5 3.60 4.92 3.75 57.5 54.3 95. 6 14A 60 23.1 27.3 63.1 2.81 4.06 1.51 68. 2 63.8 98.3 14B 60 19.9 25.7 64.8 2.58 3.81 2.34 72.1 67.6 97.5 T = 850 C % wt. IN THE UNREACTED % REACTED % % % SLAG WLP SLAG RUN TIME TWL REACTED O205 Nb 0 o205 (min ) Nb205 T Fe2°3 2 5 T Fe2°3 15 7Jj 4.3 8.5 36.2 4 . 28 5.40 22.36 16.2 16.8 56.3 16A 15 15.1 20.4 55.4 3 . 99 5.12 7.75 45.4 44.8 89.4 16B 15 14 . 2 19.7 53.7 4 . 20 5.36 8.60 40.4 40.1 87.8 17 25 20. 6 24.1 63.2 3.15 4.25 2.10 64 .4 62 . 2 97 . 6 18A 30 22.9 26.2 64.5 2.75 3.R1 2.02 70.1 67. 3 97.8 188 30 22.6 25.1 64.9 2.34 3.73 1.51 74.8 68 .4 98.4 7 45 25.7 28.8 68.9 0.86 1 .45 1.26 31.8 89.1 98.8 19 60 29.3 35.1 72.9 0.25 0.43 0.75 97.9 97.2 99.4 900 C % wt. IN THE UNREACTED % REACTED % % % SLAG TWL WLP SLAG RUN TIME READIED Nb 0 TO 0 Fe 0 (min) Nb205 TO205 Fe2°3 2 5 2 5 2 3 20 5 4.2 7.8 37.5 4.38 5.44 21.43 16.0 17.9 58.9 21 10 15.7 19.6 56.5 4.16 5.40 6.25 44.5 43.3 91.7 22 15 23.1 27.3 64.5 2.80 4.42 2.05 69.5 62.1 97.8 23A 20 25.4 28.2 68 .o 1 .65 2.78 0.86 83.3 78.5 99.2 23B 20 24 .8 28.4 67.3 1 .84 3 .04 1.32 81.5 76.0 98.7 24 30 27.9 33.1 72.2 0.35 0.55 0.25 97.0 96.3 99.8 25 60 37.1 45.6 84 .8 0.20 0.24 99 .1 99.1 100.0 217 218 TIME(MIN.) —• Rg. 6.4. Effect oftemperature o n the percentage of slag needed. 219 Fig. 6.5. Effect of temperature on the percentage of Nb205 and Ta205 contents in the slag. 220 40 n 30- vO c£ 20 -| UJ z CD AO 50 60 TIME (MINI Fig. 6.6. Effect oftemperature o n the percentage of Fe2OB content in the slag. 221 TIME (MIN.) Fig. 6.7. Effect of temperature on the percentage of Nb205 reacted. 222 TIME (MIN.) Fig. 6.8. Effect of temperature on the percentage of Ta205 reacted. 223 TIME (MIN.) Fig 6.9. Effect of temperature on the percentage of F^C^ reacted. A relationship between % slag reacted and the anc' reacted are presented in Figures 6.10 to 6.12 respectively. An increase in the percentage of slag reacted is accompanied by an increase in the percentage of the oxide reacted and that such relationship is independent of the reaction temperature. While the percentage of the iron oxide reacted appears to show a linear relationship with the percentage of slag reacted from the start of reaction, the percentage reacted of Nb and Ta pentoxides increase more significantly between 50 and 70% of slag reacted. These results indicate that unless a very well controlled condenser system or an apparatus device (such as the presence of a column filled with granules of sodium chloride to remove iron chloride from the gas stream) is used, the iron chlorides will be recovered along with the chlorides and oxy-chlorides of niobium and tantalum. In the case of other oxides present in the slag it can be seen from the Gibbs free energy changes (Figure 6.13) that the reduction chlcrination of all these oxides is feasible. Therefore the products of the reaction depend also on kinetic factors. It is interesting to note that the alkali earth metal chlorides are low in volatility and might remain in the solid or liquid state while the more volatile compounds are transported. This was confirmed by the diffraction patterns of the specimen collected on the bottom of the partially reacted pellet which matched those for CaCl2.2H20 as can be seen in Figure 6.14. Since the diffractogram did not show any line corresponding to MgCl2 it is thought that the chlorination of the magnesium oxide is not as fast as the chlorination of the calcium oxide. 225 « 100-, f J 90 J I I •P 80 J 1 70 A ? 4 <04 A 90 A df £ A* v0 40 H / / 30-1 14%Cj L 0OO°C / / o 8SPC / • 900PC 20 A / / 10 H 0 -T—I 1 1 r 1 1 1 ' 1 1 0 t) 20 30 40 90 60 70 80 % SLAG REACTED Fig 6.10. Rsbhonship between %slog reacted and %NbA reacted O' /I I I I? 1° / d &/ / A / / / 0/ A/ / / K%C, / / A 800°C / A o 880PC / • 800PC • I i l 1—I 1 1—i 1 1 1 1-1 1 I 1020BOA0 50 60 70 80 % SLAG REACTED Fig 6.11. febticrafrp between % slog reacted and %Ta& reacted. 227 100-1 90- 80 H 70 H (OA a 50 H c5° if * aoH 14% c, A 800°C 30 H o 8B0PC • 900PC 20 H 10 H 1 1 oH i -j 1 1—i 1 1 1 1 i 1 I 10 20 30 40 SO 60 70 80 % SLAG REACTED Fig. 6.12. Relationship between % slog reacted end % F^ reacted. 10 2GW0, •C+2CU=28Wa6 • COj— 20 r25^A+C+2a2=45TQCl5+CD2 2OTQA^C^2a2=4OTdXl3+CD2-1 2/3 AI2O3 •C+2C12 =2/3Al2CU • OO2 • 30 n 40 S02^2a2=Sial,|5)4(D2-1 90 60 L2GMjA^C+2a2=4aNb0a3 70 + ^2/5NbA L*2a2s4/5NbQs+0D2 TA^C+2a2=Tia1,(G)4C02 80 Sn02*(>2a2=SnCVC02 90 - 2fieO+C+3Cl2= 2FeQ3+CD2T 100 - no - 120- 2FeO+C+3Cl2=Fe2Cl6 (G) +GD2 130- 140- 190- 160- • 2GaO +C • 2Cl2=2GaQ2(S) • CD2 170- 180- n 1 1 1 1 1 1 T D0200300400900600700800 (°C) — Fig. 6.13. Free energy diagramfor tti ereduction cHorinatc n of the main aides present in the tin slag Diffractograms of some treated chlorination residues are also presented on Figure 6.14. Although a search through the powder diffraction files index had been carried out, a satisfactory identification of the compounds was not possible due to a large number of similar diffraction lines for different compounds. However it can be seen that despite some variations in the intensity of the lines, the compounds present in all residues are basically the same. This suggests that the bulk of the residue should be stable at the conditions studied and only chlorination at temperatures higher than 900°C could gasify these compounds. Semi-quantitative analysis of the partially reacted slag were also carried out by X-ray fluorescence. Although subject to an uncertainty of +50% these results could provide an idea of the kinetics of gasification of the oxides analysed when compared with the curve % slag reacted versus time. In Table 6.4 is given the results obtained and in Figure 6.15 are presented the kinetic curves for reaction at 350°C of some oxides present in the slag. TABLE 6.4 SEMI-QUANTITATIVE ANALYSIS OF PARTIALLY REACTED SLAG RUN % SLAG Nb205 Ta205 Fe203 Ti02 CaO Si02 SnO reacted 10 43.5 4.7 5.6 15.0 1.7 12.0 1.0 28.0 0.5 12 55.4 4.3 5.2 - 0.8 9.4 0.7 31.4 0.2 23B 67.3 1.9 2.8 - - 4.1 0.5 40.6 - 19 72.9 0.1 0.4 - - 0.4 0.8 56.8 - FIGURE 6.14 X-~AY DIFFRACTOGRAMS OF DIFFERENT MATERI ALS FROM 230 CHLORINATION OF TIN SLAG \,1111111111111 I I L\, II I I Ill II I, I I I I, II I Ill I I I, II Iill ,, II I I I I I I I I IIIII Ill II ,, II I I, II I I I I I I 1,11 dI I I II I I co 40 20 10 5 4 3 2.5 2 1.5 l. ' Material collected ir. the bottom of partially reacted pellet (Run:9) 2. II II II II II II II II II (Run: 10) 3. II II II II II II II II II (Run:ll) 4. II II II II II II II II II (Run: 12) 5. Analar calcium chloride-dihydrate (manufactued by BDH Chemicals Ltd.) 6. Partially reacted slag after treating (Run:24) 7. 1•·, II· II II II (Run:l9) 8. II II II II II (Run:23A) 9. II II II II II (Run:l8B) 10. Material collected on the cold-finger after treating (Run:lO) 11. II II II II II II II (Run:ll) 12. II II II II II II II (Run:l7) 13. II II II II II II II (Run:22) 14. Specpure Nb o (manufactured by Johnson Matthey Chemicals Ltd.) 2 5 II II II II II II 15. Specpure Ta2o5 ( ) 60-i TIME (MIN.) Fig. 6.15. Kinetic curves for some oxides present in the tin slag. While calcium oxide reacts rapidly, the silica reaction is much slower. In the case of Ti02 it appears that its chlorination only takes place sometime after the start of reaction (when most of the iron was gasified). By also anc plotting the kinetic curves for Nb20^, Ta2^5 ' on Figure 6.15 it can be seen that under identical conditions the chlorination of the oxides takes place to different extents and in the following sequence: Ca -»• Fe Nb = Ta -»• Ti + Si Referring to the material collected on the cold- finger the diffractograms obtained could not provide a satisfactory identification of the compounds present due to a large number of similar lines corresponding to different compounds (Figure 6.14). X-ray fluorescence analyses were carried out on the material collected on the cold-finger after hydrolysis and/or calcination. Although a compound with 30% of tantalum oxide had been obtained the results are far from being considered satisfactory in terms of recovery of either niobium or tantalum. It has also been observed that the percentages of tantalum pentoxide present and recovered in all samples were higher than those for niobium pentoxide. Therefore it appears that the deposition of tantalum compounds on the cold-finger occurs more readily than of volatile niobium compound s. In order to eliminate iron in the starting material a hot leach with HC1 was carried out on the tin slag prior to chlorination. A dissolution of 57% of the slag in 6 hours of acid attack suggests that a more suitable starting material for chlorination was obtained. The acid leaching 233 dissolved most of the iron (93.5% in terms of Fe2^3) and other acid-soluble materials and in so doing increased the niobium and tantalum pentoxide content to 8.43 and 7.17% with a recovery of 93.9 and 95.5% respectively. Analyses by X-ray fluorescence was also carried out on the residue of the leaching and are presented in Table 6.5 TABLE 6.5 ANALYSIS OF THE LEACHED TIN SLAG (% wt.) SEMI-QUANTITATIVE Nb205 Ta205 Fe203 W03 Ti02 CaO Si02 SnO 7.17 8.43 5.08 5.5 14.2 3.2. 4l'. 2 1.2 Chlorination of the leached slag-carbon mixture ^ at 800°C showed a faster volatilization of Ta and Nb pentoxides in comparison with the untreated slag. Although a material collected on the cold-finger (after being calcinated) was obtained containing about 40 and 20% of tantalum and niobium pentoxide respectively, the recovery of these oxides were only 54.2 and 30.8%. ¥ The acid-leached slags appear to be amenable to reduction chlorination but a more detailed investigation of the process is necessary in order better to determine conditions for separation and recovery of the valuable metals . 234 6.4) REDUCTION CHLORINATION OF PYROCHLORE CONCENTRATE Reduction chlorination experiments on pyrochlore concentrate were carried out using the same procedure as those for experiments with tin slag. The specific gravity of the pyrochlore concentrate was found to be 4.08. The calculated porosity of the pellets was about 29.5% (+2.0%). Quantitative analysis by X-ray fluorescence confirmed that the percentage of Nb20^ in this material was about 60% (80). Tantalum pentoxide was detected in very small amounts (<1%) and the residue and material collected on the cold-finger were analysed only in terms of Nb20^. 6.4.1) THE EFFECT OF %Ci The effect of %Ci was studied in the range between zero and 22%. For this purpose the experiments were carried out at 750°C for 60 minutes. The results are presented in Table 6.6 and plotted on Figure 6.16. TABLE 6.6 EXPERIMENTAL RESULTS FOR RUNS WITH PELLETS WITH DIFFERENT %Ci T = 750°C - t = 60 min %Nb«0c % % PYROCHLORE IN THE %Nb90,- TWL RUN %Ci REACTED RESIDUE REACTED 1 0 3.2 9.1 62.4 5.5 2 5.5 39.0 62.4 67.5 57.7 3 10.8 51.9 77.3 50.5 80.9 4 15.9 58.1 90.0 23.6 96.1 5 21.3 54.2 90.5 20.8 96.7 235 %Cj Fig. 6.16. Effect of %Cj on ttie extent of reaction. When the carbon content in the pellet is about 15%, most of the niobium pentoxide was gasified in 60 minutes at 750°C. Although this level of %Ci does not necessarily represent the optimum condition for reactions at temperatures lower than 750°C, it was chosen in order to study the effect of temperature. 6.4.2) THE EFFECT OF TEMPERATURE The effect of temperature was studied in the range between 650 and 750°C. The results are presented in Table 6.7 and the plot %NboOc reacted versus time is given in Figure 6.17. TABLE 6.7 EXPERIMENTAL RESULTS FOR RUNS AT DIFFERENT TEMPERATURES (INITIAL Nb205 IN PYROCHLORE: 64.04%) (15% Ci) 0/ Time of 'o %Nb205 %Nb205 RUN T reaction Pyrochlore in the % reacted (°C) (min.) TWL reacted residue 6 650 15 20.6 36.4 64.62 35.8 7 650 30 31.0 50.3 61.66 52.1 8 700 4 17.9 30.5 65.15 29.3 ? 700 7 25.7 43.8 64.27 43.6 10 700 15 35.4 62.2 65.92 60.2 11 700 20 39.7 63.9 61.76 65.2 12 700 30 45.6 71.5 52.98 76.4 13 750 2-i 13.4 29.0 63.90 29.2 14A 750 5 27.2 50.8 65.92 49.4 14B 750 5 29.5 54.1 68.70 50.8 15 750 10 36.3 63.2 65.98 62.1 16A 750 15 40.1 66.9 63.40 67.3 16B 750 15 43.1 73.2 59.98 74.9 17 750 20 43.6 74.1 61.57 75.1 18A 750 30 50.9 80.1 49.64 84.6 18B 750 30 55.1 88.0 41.04 92.3 From Table 6.7 it can be seen that the values of %Nb20^ reacted are similar to those of the percentage of pyrochlore reacted. This suggests that, within the range of temperature tested, Nb/^ is gasified continuously from the start of reaction without interference from other oxides present in the concent rate. Figure 6.17 shows that reduction chlorination of concentrate of pyrochlore is affected by temperature within the range studied. It appears that for practical purposes the reaction should be carried out at temperatures higher than 700 C. At 750UC, for example, 50% of Nb20^ is transformed into volatile chlorides and oxy-chlorides in 5 minutes while at 650°C it is necessary to have a reaction time of approximately 30 minutes. The apparent value of the activation energy was found to be 84.8 kJ/mol which is close to the apparent activation energy of the chlorination of pure niobium pentoxide with pellets of 28% nominal porosity, 20% Ci and 2.9 mm thickness (Ea = 76.1 kJ/mol - Table 5.7). It can also be noted that the rate of %Nb20^ reacted decreases during the course of reaction. This behaviour may be similar to those observed in the experiments with pure niobium pentoxide in which a fast chemical reaction in the top layers of the pellet and/or an ever increasing porosity of the pellet led to a decrease of the overall reaction rate (Chapter 5). Because the reaction may be taking place to the same extent throughout the pellet (pellet thickness is small (-3.0 mm) and the sides as well as the top surfcce of the pellet are exposed to chlorine) the former case is unlikely to occur. Therefore the decrease in the rate of Nb2C>2 reacted may be related to the decreasing degree of contact between carbon and concentrate. This appears to be supported by the fact that the critical degree of solid-solid contact, which is attained when the rate of reaction starts dropping, increases with increasing temperature (Figure 4.17). The material collected on the cold-finger was also examined. Initially a group of treated samples were analysed by X-ray diffraction. The diffraction lines of all compounds analysed matched those of pure niobium pentoxide (Figure 6.18) indicating that Nb20^ was being obtained. The recovery of Nb20^ was then calculated from X-ray fluorescence analyses. The results obtained are given in Table 6. <8. TABLE 6.8 ANALYSIS OF THE MATERIAL COLLECTED ON THE COLD-FINGER AFTER TREATING %Nb205 in %Nb205 RUN the sample recovered 2 97.51 60.4 3 98. 99 68.6 4 90.72 61.6 8 84.33 60.8 9 93. 67 79.5 11 92. 20 81.7 12 88.41 72.6 14A 98. 21 61.6 18 87.48 76.1 Although no special device had been used to collect the volatile products of chlorination efficiently a recovery between 60 and 80% of the gasified Nb20^ was attained. Also, the high percentage of Nb20^ content in this material confirms the conclusion drawn in Section 5.5 that reduction chlorination can be used as a process for production of niobium pentoxide when applied on rich starting material - such as the pyrochlore concentrate . 239 TIME (MIN.) Fig. 6.17. Effect of temperature on the %Nb2C^ reacted. 240 F 'I GU RE 6 . 18 X-RAY DIFFRACTOGRAMS OF THE MATERIAL COLLECTED ON THE COLD-FINGER AFTER TREATING. 1. Run 3 2. Run 4 3. Run 5 4. Run 12 5. Specpure Nb (manufactured by Johnson Matthey 2o5 Chemicals Ltd.) 6.5) CONCLUSIONS The reduction chlorination experiments on tin slag and pyrochlore concentrate led to the following conclusions : - 1) In order to obtain most of the niobium and tantalum pentcxide gasified within a practical reaction time the reduction chlorination of tin slag requires a high tempe ratu re . 2) The reaction of the various oxides present in the slag occurs to different extents in the following sequence: Ca Fe Nb E Ta Ti -»• Si. 3) Most of the niobium and tantalum pentoxides are gasified in the range between 50 and 80% of slag reacted. 4) An acid-leaching of the tin slag can dissolve most of the iron and in so doing increase the niobium and tantalum pentoxide content to 8.43 and 7.17% re spectively. 5) The leached tin slag can be chlorinated at lower temperatures (in comparison with the untreated slag) in order to produce a condensed specimen rich in niobium and tantalum. 6) Reduction chlorination of concentrate of pyrochlore can be a potentially useful method for production of technical niobium pentoxide. CHAPTER VII SUGGESTIONS FOR FUTURE WORK 7.1) ACADEMIC PURPOSES Further experiments with niobium pentoxide should be carried out under conditions of chemical reaction control in order to study the chemical reaction mechanism in more detail as well as to determine the chemical kinetic parameters. Experiments should also be conducted under conditions of diffusion control in order to determine the pore diffusion parameters. Procedures for such measurements are described in references (7j$) and (78). The knowledge of the kinetic and structural parameters may greatly increase the understanding of the reduction chlorination of rich niobium pentoxide concentrates and thus provide a more scientific basis for the quantitative analysis of the progress of the reaction. A scientific and practical approach to an eventual design of a reactor unit should also be attained. The effect of varying particle and pore sizes, and the degree of mixing of the ingredients on the reaction rate as well as the changes of these variables in the course of reaction could be studied in order to determine the mechanism which controls the progress of reaction throughout the solid particles that make up the solid agglomerate. Investigations on reduction chlorination of niobium pentoxide mixed with other oxides could be carried out in order to study the effect of the presence of these oxides on the gasification of Nb20^. Since a mixture of oxides do not represent the actual phases which may exist in natural raw materials experiments could also be carried out on synthetic compounds where the concentration of the original oxides is known. The use of fluidized-bed should be tested in the reduction chlorination experiments since it provides a potentially attractive alternative to fixed-bed for carrying out gas-solid reactions. 7.2) PRACTICAL PURPOSES Further experiments with the leached tin-slag could be performed in order to study the possibility of producing a condensed material rich in niobium and tantalum. Further investigations on the kinetic of the reduction chlorination of pyrochlore concentrates should be carried out using an experimental apparatus where the recovery of the gasified niobium pentoxide was the main concern. Thus the whole process of production technical Nb20^ through reduction chlorination could be analysed. If it were successful, experiments should then be carried out under conditions which would provide practical information for an essential development of the process on a larger scale. ACKNOWLEDGEMENTS I am greatly indebted to my supervisor Professor J.H.E. Jeffes, for his guidance and support. I gratefully acknowledge the provision of a scholarship by the Brazilian Research Council (CNPq) which made this work possible. To all members of the Metallurgy Department - I.C. who were involved in this work I would like to express my appreciation of the co-operation received and in particular I wish to thank Mr. R. Rudkin for his technical assistance, Mr. R. Sweeney for his help during the X-ray fluorescence analysis and Dr. H.M. Flower for the quantitative analysis on the SEM (Jeol 35). I am also grateful to Dr. V. Rajakumar (now at Div. of Fossils Fuels, C.S.I.R.O. Australia) for many helpful discussions and to Dr. E. Vargas (now at Civil Engineering Dept. - P.U.C./R.J. - Brazil) for the advice on the computer programme. Thanks are also extended to Mrs. Sylvia Greenwood for her efficient typing, to Mr. Haroldo Cantanhede for his excellent work in drawing the figures of this thesis and to Valeska for her cheerful encouragement and help in constructing the tables. I also wish to express my gratitude to all members of the Metallurgy Department -P.U.C./R.J. - Brazil for encouraging me to carry out this course and in particular I would like to mention J.B. Bruno for his initial incentive and L.A.C. Teixeira - and his wife Rosaly - for all the support they gave me when I first arrived in England. Financial support for material was provided by Metallurgie Hoboken Overpelt - Belgium and by the Brazilian Research Council (CNPq). 245 APPENDIX I LIST OF EXPERIMENTAL RESULTS (Run, time of reaction, total weight loss of the sample and %Nb«Oc reacted) Run 1 2 3 4 6 7 9 9 10 11 12 13 14 15 16 17 18 Time 60 60 30 60 10 30 30 30 45 30 15 60 (min) 30 30 30 30 60 T.W.L c/.) 62.4 50.2 25.4 82.0 9.0 33.6 36.5 36.8 47.6 57.6 26.8 81.8 79.2 77.7 77.6 10.1 26.6 % Nb205 71.5 57.5 29 .1 94.8 11.7 38.5 41.8 42.1 54.5 65.9 30.5 92.9 90.7 89.0 89.3 Reached 11.2 Run 19 20 22 23 24 25 26 27 29 30 31 32 33 Time 45 15 60 60 60 30 60 60 30 60 90 120 120 120 75 120 (mm) 20 30 40 60 T.W.L 16.7 13.5 9.6 19.4 50.5 25.1 3.5 CM 30.3 14.6 31.7 42.7 51.0 50.6 81.0 81.6 B4.8 57.5 79.6 35.4 87.0 % Nb205 18.8 10.3 20.1 69.2 33.9 34.9 58.5 57.6 93.4 98.1 99 . 6 Reached Run 34 35 36 37 38 39 Time 20 40 60 30 100 120 30 60 120 60 120 30 (min) 60 90 120 20 40 60 90 120 10 60 120 T.W.L. (•/.) 7.5 15.5 23.8 30.£ 36.6 41.6 12.C 24.7 44.6 4.2 9.2 52.3 70.5 77.f 79 .9 41.6 62.8 73.( 78.8 81.3 26. t70. 4 78.6 %Nb205j 10.5 91.9 93.1 90.9 Rearhed Run 40 41 42 43 44 45 46 Time 10 20 30 45 60 5 15 60 90 120 5 10 15 30 60 90 120 L0 20 10 (min) 30 5 15 20 25 35 T.W.L. 53.7 75.8 81.7 84.5 86.2 34.6 69.9 31.9 42.6 51.3 45.7 78.0 85.4 CM 33.9 64.3 80.2 84.1 29.0 57.3 78.8 20.8 40.4 57.1 73.7 B5. 6 88.4 % Nb205 98.( 80.2 97.3 100.0 Reached Run 47 48 49 50 51 Time 15 30 45 60 90 15 30 45 60 75 90 30 60 90 120 30 60 90 90 120 (min) 120 30 60 T.W.L. 15.i (•/.) 32.6 47.8 60.6 75.4 28.9 58.5 78.0 83.4 85.1 85.8 9.3 16.2 21.2 24.7 21.6 40.0 52.5 61.2 26.9 40.7 47.4 51.7 % Nb 2 0 5 Reached 98.4 25.2 62.8 78.6 Run 52 53 54 55 Time (min) 15 31 45 60 75 120 15 30 45 60 80 100 120 15 30 45 60 80 100 120 10 20 30 T.W.L. 16.3 33.6 48.2 58. 6 61.8 63.5 23.8 43.5 58.5 68.9 77.1 81.2 84.0 15.6 26.7 35.5 42.7 45.2 49.4 52.5 14.8 33.5 49 .4 % Nb205 Reacted 97.7 85.3 75.1 Run 56 57 58 59 60 61 Time (min) 10 30 40 60 15 20 5 10 L5 30 60 20 40 60 10 20 30 40 50 60 90 10 20 30 40 60 T.W.L. 16.5 46.8 56.4 63.8 44.3 50.3 19.5 32.7 40.3 53.2 59 .8 41.7 61.1 68.7 24.2 48.5 69.4 80.4 86.5 90.0 c/.) • 94.0 22.6 42.5 58.3 65.4 66.1 % Nb205 Reacted 76.9 90.7 69.2 95.0 99.8 Run 62 63 64 65 66 Time (min) 5 10 15 20 30 60 10 20 30 45 60 20 40 60 120 30 60 120 30 60 90 L 20 T.W.L. 30.4 50.6 60.3 62.9 65.5 66.8 27.4 45.7 56.4 67.4 73.3 48.8 56.2 59.4 63.1 24.3 41.9 54.4 16.0 30.6 41.7 49.9 % Nb205 Reacted 99.3 96.4 55.1 Run 67 68 69 70 71 72 Time (min) 20 10 60 90 120 20 40 60 90 120 15 30 45 60 120 15 30 60 120 15 30 60 90 15 45 T.W.L. 19.9 38.5 53.2 69.7 77.2 27.2 49.6 60.2 62.6 63.3 24.1 38.6 41.0 41.5 42.3 19.0 26.5 32.0 35.9 16.1 29.3 42.3 43.3 16.1 40.0 CM %Nb205 Reacted 89.0 97.4 97.3 81.4 Run 73 74 75 Time (min) 10 20 30 45 60 90 120 5 11 16 21 30 45 60 5 10 15 20 40 60 T.W.L. 24.4 46.6 63.2 77.6 85.8 92.5 94.6 15.4 32.8 49.0 63.1 80.8 36.2 06.9 18.9 35.1 42.2 44.2 44.6 44. 6 %Nb205 Reacted 95.4 99.4 100. Run 76 77 79 80 81 Time 5 10 15 20 25 30 45 60 10 20 30 40 60 5 10 15 30 (min) 5 10 T.W.L. (•/.) 18.1 33.2 46.5 57.2 62.8 64.6 65.0 65.0 19.9 32.7 41.5 44.2 44.4 31.2 38.7 41.0 43.3 15.9 14.2 % Nb205 Reacted 17.7 Run 83 84 86 87 88 Time (mm) 15 30 60 90 120 10 20 30 40 60 10 20 30 40 60 90 120 30 60 5 10 15 20 30 T.W.L. 13.2 26.7 51.9 60.5 62.9 23.4 43.1 55.6 63.0 66.0 8.6 17.0 25.7 33.1 45.8 57.5 62.7 33.9 59.5 14.8 28.8 40. 6 50.3 62.5 % Nb205 Reacted 96.9 Run 89 90 91 92 93 9 k 95 96 Time (min) 20 40 60 30 60 90 120 30 60 90 120 60 L 20 10 30 60 30 60 10 30 120 10 30 60 120 T.W.L. 22.1 42.0 56.8 10.3 22.8 35.2 46.7 12.1 24.3 35.3 45.1 24.5 36.0 7.9 23.9 44.8 f/.) 10.8 23.3 10.5 31.4 89.2 14.1 36.0 66.0 94.2 % Nb205 47.7 46.3 67.8 Reacted Run 97 98 99 100 101 102 103 Time (mm) 25 60 90 120 5 10 20 40 10 35 5 10 20 15 30 60 120 15 15 30 60 120 T.W.L 54.6 86.7 96.2 97.2 12.1 22.4 35.7 54.1 7.8 27.7 9.8 18.3 )0.9 3.5 7.0 14.2 29.4 36.0 6.8 13.9 26.3 42.1 % Nb205 82.1 42.4 36.4 42.9 Reacted Run 105 106 107 108 109 11 0 111 113 1U Time (min) 10 20 30 60 10 30 60 15 50 10 30 10 15 45 15 30 60 20 40 63 10 20 45 70 T.W. L. 10.9 20.4 27.3 38.9 26.2 63.1 ?0.6 14.5 31.3 17.4 43.0 10.7 2.8 11.2 6.6 14.2 25.9 12.5 24.8 35.0 13.1 22.1 36.6 45.9 VoUb2C>5 59.8 91.9 48.5 65.5 15.2 10.8 26.0 70.1 Reacted ro 4> CD Run 116 117 110 119 120 121 122 Time (min) 5 10 15 20 10 20 30 45 60 30 10 31 45 70 120 10 30 60 120 30 60 120 10 30 60 T.W.L (•/.) 24.3 39.9 53.4 61.8 15.9 30.1 42.9 53.7 59.3 11.7 11.7 35.4 47.7 68.7 95.1 13.6 29.9 44.1 62.2 16.8 30.7 49.9 15.3 43.5 76.6 % Nb20s 93.9 Reacted 91.3 96.i 95.0 77.1 78 .1 Run 123 124 12 5 12 6 127 128 129 130 132 133 Time 20 40 80 30 75 120 10 20 10 20 40 60 30 60 30 (min) 60 20 60 120 5 20 30 60 10 38 T.W.L. 20.2 38.8 61.2 12.9 31.8 47.1 11.1 19.4 29.2 45.7 63.4 74.2 11.3 23.7 16.3 36.6 19.£ 52.3 78.4 32.2 76.3 17.4 34.8 13.0 33.4 % Nb20s 93.9 (48.0 27.4 75.0 (23.8 41.4 90.2 (54.4 Reacted (52.5 Run 134 135 136 137 138 139 140 141 Time (min) 30 35 10 35 20 40 60 12 35 20 40 60 90 120 30 60 90 120 20 40 60 90 120 T.W.L. 1.0 1.6 L5.0 45.7 17.4 35.7 55.1 7.4 11.5 4.4 9.2 13.0 19.4 25.3 5.0 >.6 14.8 19.8 6.9 14.1 21.7 32.9 43.£ % Nb20s (46.3 63.5 32.9 38.8 20.1 44.: Reacted Run 142 143 144 145 146 147 Time 15 45 75 120 30 60 90 120 20 40 60 90 120 15 30 45 60 90 120 8 (min) 15 30 60 15 40 70 T.W.L. 5.8 19.4 27.6 37.5 5.4 10.7 15.6 21.6 13.8 24.0 33.5 45.8 52.7 15.7 27.2 38.7 49.5 69.9 85.1 11.4 19.2 39.3 64.5 7.2 17.1 26.6 %Nb205 56.3 21.6 80.6 85.7 97.6 39.9 Reacted Run 148 149 150 151 152 153 Time 10 20 30 45 30 45 10 30 60 90 120 20 35 55 85 110 10 15 25 (min) 35 45 80 120 25 55 85 T.W.L. 12.7 24.2 35.3 49.8 13.5 19.5 7.7 23.8 47.9 67.2 80.1 9.6 18.4 28.4 44.0 56.7 10.1 17.9 25.8 37.1 (6.2 62.9 76.1 14.3 29.0 43.5 %Nb205 76.3 19.8 80.3 Reacted 65.9 87.7 50.4 # * « • * • Run 154 155 156 157 158 159 160 161 Time (min) 30 40 50 30 15 30 45 60 10 20 30 42 15 30 60 15 30 60 90 120 60 120 40 80 T.W. L. 32.0 38.5 44.0 12.0 6.0 11.8 17.4 22.5 7.1 13.2 17.9 22.6 3.8 7.6 15.1 5.5 11.2 20.8 29.9 37.4 (V.) 23.4 49.2 20.3 41.1 % Nb205 Reacted 50.7 27.4 51.6 51.2 37.7 50.5 47.9 Run 162 163 164 165 166 167 168 169 170 171 Time 50 40 (mm) 20 55 20 40 55 90 20 50 90 20 10 10 16 20 20 33 5 30 10 35 65 10 20 T.W. L. 18.8 53.2 20.2 37.1 46. £ 66.2 9.9 28.1 47.0 19.7 46.3 15.7 53.0 20. J 29.7 34.8 18.( 31.6 9.5 40.9 10.1 27.6 42.1 7.7 14.8 r/.) % Nb2o5 54.3 76.7 48.4 71.7 54.3 53.3 48.8 62.9 63.1 18.4 Reacted Run 172 173 174 175 176 177 178 179 180 181 Time (mm) 20 40 50 20 40 30 60 15 35 15 18 30 60 120 180 20 50 70 30 60 15 35 T.W.L. 15.4 40.1 14.1 25.9 26.5 29.5 15.4 31.4 CM 29.8 21.7 11.6 13.5 25.5 7.5 44.0 8.3 22.3 30.0 25.8 51.3 15.6 33.3 % Nb2 05 . 46.4 25.2 26.0 40.3 44.5 26.1 45.0 45.9 52.4 49.9 Reacted Run 182 183 184 185 186 187 188 Time (min) 30 60 120 150 30 60 120 15 30 60 85 15 30 45 60 20 40 65 15 30 50 15 30 45 60 T.W.L. (•/.) 7.3 19.6 40.9 49.9 10.8 19.7 35.3 5.2 11.1 24.9 33.9 16.0 30.6 38.7 45.7 16.3 31.8 49.0 14.0 24.6 34.3 27.5 51.0 67.5 '8.7 % Nb2 O5 51.0 36.3 51.8 69.4 49.9 52.0 30.2 Reacted Run 189 190 191 192 193 194 195 196 197 198 199 200 201 Time 10 15 20 10 (mm) 20 10 15 35 60 20 40 35 80 10 20 15 30 5 10 13 T.W.L. r/.) 27.1 36.8 43.8 24.3 40.7 43.3 8.1 18.1 31.2 23.7 51.7 18.3 42.8 8.0 22.9 17.7 35.1 20.4 41.7 53.4 % Nb205 Reacted 66.4 46.3 66.3 36.6 59.8 50.2 26.8 20.2 41.4 61.2 U1 o o C M Run 202 205 204 5 206 207 208 209 210 211 212 213 2K Tims 15 45 15 30 15 30 15 45 (mini 30 15 5 20 10 30 10 20 30 15 10 30 60 30 60 T.W.L. 3.8 10.5 8.5 15.1 11.0 22.C 5.3 15.8 24.6 c/.) 67.5 11.0 43.J 19.4 55.0 23.2 44.3 63.5 51.9 51.0 57.9 80.6 27.4 41.2 %Nb205 25.( Reacted 16.0 28.8 75.5 50.< 56.3 73.' 78.7 74.8 52.6 Run 215 216 2 17 218 219 220 221 222 27 3 12k 225 22 6 Tims 25 60 30 60 (min) 120 5 35 30 60 30 60 30 60 20 40 15 30 7 15 20 8 25 T.W.L. 35.2 57.3 c/.j 20.: 33.4 56.3 4.4 30.8 35.5 42.9 37.: 52.1 25.0 43.i 31.7 59.8 53.7 90.5 30.4 54.2 25.4 35.3 37.2 % Nb205 91.4 Reacted 64.2 36.0 41.8 50.9 44.2 88.1 92.2 82.6 24.7 53.6 57.4 APPENDIX II CALCULATION OF POROSITY AND %Nt>205 CONTENT IN DIFFERENT SECTIONS OF PARTIALLY REACTED PELLETS In order to compare the quantitative results obtained from the SEM (Jeol 45) with those from gravimetric analysis the values of both porosity and percentage of Nb^^ content in the area analysed by the SEM had to be evaluated (Section 5.2.5). It was presumed that by drawing the curve of calculated porosity versus pellet depth the porosity of the analysed area could be determined since the approximate point of analysis was known. The calculation of the %Nb20^ content was based on the relationship presented in Table 5.1 and on the %Nb20 Because the partially reacted pellet kept its initial shape and size it was considered as a certain number of identical slices in which the initial Nb20^ and C contents were known. The unreacted amount of niobium pentoxide and carbon was then calculated and the porosity of the slices determined in order to obtain the desired profile of the porosity through the pellet depth. In the case of run 132 a second sectional division of the pellet had to be applied because the %Nb20^ content in each section could not be assumed constant (%Ci ^ 9%). Part of the pellet was considered as four slices of equal thickness in such a way that the approximate point of analysis was located in their middles (Figure 11.1(b)). Calculation was then performed and the %Nb20^ in each slice was assumed to be the %Nb20^ in the area analysed by the SEM (Table II.3). Tables II.1 to II.4 and Figures II.1 and II.2 illustrate the procedure described in this section and present the results obtained. TABLE 1.1. RUN \fol. of each section (cm3) Initial Nb^ n each sechn(gn Initial C in each sectionlqr) 127 0.1516 0.4037 0.0404 SECTIONS OF T1H E PELLET 1 2 3 4 5 6 7 8 9 50.0 48.0 45.0 35.0 15.0 5.0 Unreocted Nb«0^ (gr.) .2019 .2099 .2220 .2624 .3431 .3835 .4037 .4037 .4037 Ifrireocted C (pr.) .0202 .0210 .0222 .0263 .0343 .0384 .0404 .0404 .0404 Calc. porosity {%) '64.3 6219 60.7 55.3 39.3 32.1 28.6 28.6 28.6 TABLE H.2. RUN VfoL of each section (cm3) Initicil M)A h each sacticn(crJ Initicil C in each section(qr.) 132 0.1473 0.1996 0.1331 SECTI0 NS OF 71H E PELLET 1 2 3 4 5 6 7 8 9 X NbjO, reocted 100.0 92.5 82.5 70.0 55.0 40.0 30.0 17.5 7.5 Unreocted Nb.Oj (gr.) .0150 .0349 .0600 .0898 .1198 .1397 .1647 .1*46 Unreocted C (gr.) .1178 .1189 .1205 .1223 .1247 .1269 .1285 .1304 .1320 Cole, porosity (%) 64.5 61.8 58.3 54.0 4P.7 43.5 40.0 35.6 32.1 TABLE JL3. RUN Initial Nb205 in each section (gr.) Initial C in each section (gr.) 132 0.2200 0.1467 SECTIONS OF THE PE LLET 1 2 3 4 SNbjO^ reacted 53.3 38.3 25.0 13.3 Unreocted Nb70,(gr.) 0.1027 0.1357 0.1650 0.1907 Unreacted NbM.-Cfgr. 0.2404 0.2760 0.3075 0.3352 3 X wt. Nb2C.« ' 42.7 49.2 53.7 56.9 TABLE 1.4. RUN VbL of each section (cm3) Initial NbA in each section^: Initial C h each section (gr.) 136 0.1435 0.293 0.029 SECTIONS OF T HE PEL JET 1 2 3 4 5 6 7 8 9 10 *Nb«0, reacted 87.1 81.1 72.7 60.6 43.9 29.5 21.2 15.9 12.1 10.6 Unreacted HbjO- (gr.) .0378 .0554 .0800 .1154 .1644 .2066 .2312 .2464 .2575 .2619 Unreocted (T(gr.) .0037 .0055 .0079 .0115 .0162 .0204 .0225 .0244 .0255 .0260 Calc. porosity (X) 93.0 89.7 85.1 78.4 69.4 61.5 57.0 54.0 52.0 51.1 255 POROSITY (%) —— OE Golculated Fbrosity of the section. *=Approximate Point of Analysis. | = Fbrosity of the analysed area. POROSITY (%) Fig. D.1. Calculated Porosity against partially reacted pellet depth. POROSITY (%) 4Ei. 4 10C 0 51.7 540 57.7 PELLE 1 r RUN: 136 -— | / INITIAL POROSITY: 45.4% j 9%C, SAMPL E MOLDE D o = Calculated Porosity of the section. * = Approximate Point cf Analysis. | e Porosity of the analysed area. Fig. D.2. Galculated Porosity against partially reacted pellet depth. 257 * APPENDIX ffl Gxrputer Programme. 1. 0 00 00 0B PROGRAM FIS(IN PUT,OUTPUT,TAPE5=INPUT,TAPE6=0UTPUT) 2. 002131B 01 MENS I ON XI( 100) ,YI (100) ,TI (10) 3. 002131B OATA TOL,AT/.01,5.7 <4. 002131B PEAD(5,100 0)N,SIG,AN 5. 0 02L63B 1000 FORMAT(110 .3F10.0) 6. 002L6 3B REA0(5,100 2)DY I, XIP 7. 002L70B 1002 FORMAT(2F1 0.0) 8. 002L70B RE AO (5 ,1003)NN , (TI( I) , 1=1,71 0025006 1003 F CRMAT(110 .7F10.0) i?: 00250 OB WRITE(6,20 10) N,SIG,AN,OYI,XIP 11. 0025106 2010 FORMAT(IX./• I 10,LF15.5,//) 12. 0 025106 WRITEI6t 20 20) NN, (TI( I) ,1 = 1,7) 13. 00252 OB 2020 FORMAT (IX,/^I10, TF15.5,//) 1L. 002520B 00 300 K=1 15. 0025226 YI P= 0. 0025228 M= C 1716. 002523B 15 C (NTINUE 18. 0025256 M=M • 1 19. 00252 66 OC 100 1=1 ,N 20. 0025306 Al=-.5*(1. 21. 0025366 P=1.-XIP /(1.-XIP)**.5) 22. 0025L0B A3=-SIG**2 •ALOG(P) 23. 0025L5B A5 = S IG** 2*2./A N 2L. 0025L6B BETA=1./(A KA3*A5) 25. 0025518 AL F=-BETA* YIP 26. 0025536 X=TI(K)*BE 27. 0025606 OIF=X-XIP TA*EXP(ALF) 26. 0025626 DIF=ABS(ni 29. 0025638 WRITE(6,20 F) 30. 0025768 FORMAT (IX, UUfl.fll.tfSf110,5F15,5,) 31. 0025766 2000 IF (OIF.LE,TOL)G0 0)I ,TI(K).XIPO TO 25 , X,YIP,OIF 32. 0026006 XIP=X 33. 0026006 IF(XIP.GT.1.)XIP=.9 9 3%. 00260LB IF(I.EO.N) XIP=,99 35. 0026106 100 CONTINUE 36. 0 026128 25 CONTINUE 37. 0026136 XI(M»«XIP 38. 0026136 YI(Ml*YIP 39. 0Q2615B YIP=VIP*0Y <•0. 002617B IFCYIP.LE.5.)G O TO 15 LI. 0026216 A P=0. L2. 0026216 00 200 J= 2 L3. 0 0262 LB OIFX=XI(J- l)-XI(J) LL. 0026256 AR= AR» ( (YI L5. 0026326 CONTINUE L6, 002633B 200 AR =AR«-5# *X (J)*YI(J-l))»0 IPX)/2. L7 • 0026356 PERC= A R/AT L6. 002636B WRITE(6,20 I(M01) )AR,°ER C L9. 0 D26LLB FCRMAT(1X, 2F15.5) 50. 0026LLB 2003010 CONTINUE 51. 0026L6B STOP 52. 0026L78 ENO REFERENCES MILLER, G.L. "Tantalum and Niobium". Butterworths Scientific Publications - 1959. FAIRBROTHER, F. "The chemistry of niobium and tantalum". A collection of monographs edited by P.L.Robinson. Elsevier Publishing Co. - 1947. JONES, T.S. "Tantalum" Mineral facts and problems - 1980 Edition. Bureau of Mines, Bulletin 471 - pp.911-924. BORCHERS, P. and KORINEK, G.J. "Extractive Metallurgy of Tantalum" in "Extractive Metallurgy of Refractory Metals". Edited by SOHN, H.Y., NORMAN CARLSON, 0. and THOMAS SMITH, J. The Metallurgical Society of AIME - 1981 - pp.95-104. WUTHRICH, H.R. "Non-ferrous metals in our industrial world" Trans. Sec. C - IMM - vol.90 - Dec/1981 - pp.CI21-C124. OTTLEY, D.J. "Technical, economic and other factors in the gravity concentration of tin, tungsten, columbium and tantalum ores" Minerals Sci. Engng - vol.11, no.2 - April/1979, pp.99-121. JONES, T.S. "Columbium" Mineral facts and problems - 1980 Edition. Bureau of Mines, Bulletin 471 - pp.215-224. STUART, H., PARAISO, 0. de S. and FUCCIO, R. "Ferrocolumbium production at CBMM, Brasil". Iron and Steelmaker - vol.7, no.5 - May/1980 - pp .11-18 . 259 8. "Trends in special duty materials technology" Metal Progress - vol.119, no.l - Jan/1981 - pp.51-52. 9. STIPP, H.E. "Columbium" Mineral Commodity Profiles, MCP-10 - Jan/1978. U.S. Dept. of the Interior - Bureau of Mines. 10. McINTOSH, A.B. "The development of niobium" J. Inst. Metals - vol.85 - 1956/57 - pp.367-372. 11. STEELE, B. and GELDART, D. "Distillation of volatile chlorides as a means of obtaining pure niobium". Paper no.16 in "Extraction and refining of the rare metals". The Institution of Mining and Metallurgy - 1957 - pp.287-294. 12. McINTOSH, A.B. and BROADLEY, J.S. "The extraction of pure niobium by a chlorination process". Paper no.15 in "Extraction and refining of the rare metals". The Institution of Mining and Metallurgy - 1957 - pp.272-286. 13. SADOWAY, D.R. and FLENGAS, S.N. "A new process for the separation of tantalum from niobium". Metallurgical Transactions B - AIME - vol.llB - March/1980 - pp.57-62. 14. SADOWAY, D.R. and FLENGAS, S.N. "The thermodynamics of tantalum/niobium separation by gas-solid reaction in chloride systems" in "Extractive Metallurgy of Refractory Metals" Edited by SOHN, H.Y., NORMAN CARLSON, 0. and THOMAS SMITH, J. The Metallurgical Society of AIME - 1981 - pp.107-126. 260 15. WERNING, J.R. and HIGBIE, K.B. "Tantalum and Niobium - Separation by Liquid-Liquid Extraction: Industrial and Engineering Chemistry - vol.46, no.12 - Dec./1954 - pp.2431-2494. 16. MAY, S.L., TEWS, J.L., HENDERSON, A.W. and GRUZENSKY,W.G. "Extractive metallurgy of euxenite" U.S.B.M. - R.I. no.5531 - 1959. 17. HUNTER, W.L. and HIGBIE, K.B. ' "Separating tantalum and columbium by solvent extraction: HF-HCl-Diethy1 Ketone System" U.S.B.M. - R.I. no.5918 - 1961. 18. MERRIL, C.C. and COUCH, D.E. "Separation of columbium, tantalum, titanium and zirconium from titanium chlorination residues" U.S.B.M. - R.I. no.7671 - 1972. 19. HENDERSON, A.W. and BLOCK, F.E. "Calcium fluoride additions to chloriration reactions" U.S.B.M. - R.I. no.5919 - 1960. 20. GLASKI, F.A. "A report on the current role of chemical vapour deposition as a manufacturing process" in "Chemical vapour deposition of refractory metals, alloys and compounds" Edited by Schaffhauser, A.C. American Nuclear Society - 1967. 21. POWELL, C.F., CAMPBELL, I.E. and GONSER, B.W. "The deposition of tantalum and columbium from their volatilized halides" J. Electrochem. Soc. - vol.93 - 1949 - pp. 258-265. 22. POWELL, C.F. 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