Tantalum  chloride  is used as a feed material for the plasma reduction using hydrogen and in the chloride bath fused salt electrolysis routes to produce  tantalum. Most of the tantalum,  however, occurs  in nature in the oxide form. The traditional chlorination process employs gaseous chlorine  and carbon  as  reductant  at high temperatures (1073-1273 K). Use of carbon tetrachloride as the  chlor- inating agent is expected to reduce the chlorination tem-peratures  significantly resulting in lower  energy cons-umption and  cost of  capital equipment  for chlorination. The purpose of the present study was to explore the use of carbon tetrachloride as an alternative chlorinating agent

for Ta205. The kinetics of chlorination of pure tantalum pentoxide powders with carbon  tetrachloride in nitrogen atmosphere was investigated in the temperature range 733-853 K and  Pccia  in the  range 20-60 kPa.  The kinetic results were found to conform to a diffusion controlled

reaction model. Equilibrium conversions to chloride and the gas phase composition at different temperatures and partial pressures of CCI4 were calculated using the free energy minimisation technique

Brocchi, E A

Chakraborty, M

Jena, P K

Srikanth, S

English

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Proceedings of the International Conference on "Recent Advances in Metallurgical Processes" Golden Jubilee Celebrations, Department of Metallurgy, Indian Institute of Science, Bangalore Eds. D.H. Sastry, E.S. Dwarakadasa, G.N.K. lyengar and S. Subramanian New Age International Publishers, New Delhi (1997) KINETICS OF CHLORINATION OF TANTALUM PENTOXIDE WITH CARBON TETRACHLORIDE M.CHAKRAVORTHYI, S.SRIKANTHI, E.A.BROCCHI2 and P.K.JENA2 National Metallurgical Laboratory, Jamshedpur 831 007, India 2Dept. of Materials Science & Metallurgy, Catholic University of Rio De Janeiro, Brazil Tantalum chloride is used as a feed material for the plasma reduction using hydrogen and in the chloride bath fused salt electrolysis routes to produce tantalum. Most of the tantalum, however, occurs in nature in the oxide form. The traditional chlorination process employs gaseous chlorine and carbon as reductant at high temperatures (1073-1273 K). Use of carbon tetrachloride as the chlorinating agent is expected to reduce the chlorination temperatures significantly resulting in lower energy consumption and cost of capital equipment for chlorination. The purpose of the present study was to explore the use of carbon tetrachloride as an alternative chlorinating agent for Ta205. The kinetics of chlorination of pure tantalum pentoxide powders with carbon tetrachloride in nitrogen atmosphere was investigated in the temperature range 733-853 K and Pccia in the range 20-60 kPa. The kinetic results were found to conform to a diffusion controlled reaction model. Equilibrium conversions to chloride and the gas phase composition at different temperatures and partial pressures of CCI4 were calculated using the free energy minimisation technique. INTRODUCTION Tantalum is used in capacitors, corrosion resistant components and in high temperature alloys as a carbide former, solid-solution strengthener and for imparting microstructural stability at elevated temperatures. It occurs in nature almost exclusively as oxide in association with niobium. Extraction of Ta involves preconcentration of the ores by wet gravity separation followed by dry magnetic or electrostatic methods. The concentrates are then either chlorinated directly or digested in hydrofluoric acid; Nb and Ta separated by solvent extraction and Ta precipitated as pure Ta205 or K2TaF7. Tantalum can then be extracted by i) carbon reduction of tantalum oxide in vacuum, ii) hydrogen reduction of tantalum chloride in plasma, iii) metallothermal reduction of K2TaF7 by sodium and iv) fused salt electrolysis of either oxide or chloride melts. The processing, properties and applications of Ta has been recently reviewed by Kock and Paschen[1] and Gupta[2]. A knowledge of the thermodynamics and kinetics of the chlorination of Ta and Nb is therefore necessary to optimise the process conditions to enable their selective separation and to achieve high yields. Chlorination of mixed oxides of Nb and Ta by carbon tetrachloride was first reported by Ruff and Thomas[3] who tried to separate Nb and Ta by this method. Similar experiments were repeated by Schafer et al[4]. Chlorination of mixed Nb and Ta concentrates with chlorine in the absence of any reducing agent at 1473 K has been patented by Cuvelliez[5,6]. Chlorination of mixed oxides using gaseous chlorine at lower temperatures (813-873 K) in presence of graphite has been studied by Hussain and Jena[7]. Subsequently, Mehra and co-workers[8,9] carried out a detailed analysis on the kinetics of the chlorination of pure niobium and tantalum pentoxides in the temperature range 873-1023 K by chlorine in 178 the presence of graphite. They observed that the chlorination of Nb2O5 has a higher activation energy (184 kJ/mol) than Ta205 (130 kJ/mol). The rate of reaction was found to be proportional to the partial pressure of chlorine in case of Nb2O5 and the square root of the partial pressure of chlorine for Ta205. In a subsequent work, Srinivasan and Jena[10] investigated the kinetics of chlorination of Nb2O5 and Ta205 by chlorine and carbon monoxide in the temperature range 798-1023 K. Activation energies for the chlorination of Nb2O5 and Ta205 in the presence of CO were determined to be 31.8 and 21.3 kJ/mol respectively indicating that the rate of chlorination with CO and C12 is faster than with C and C12. They also found that the rates of chlorination of Nb2O5 and Ta205 vary linearly with pco and Paz in the range of partial pressures and temperature adopted in their study. It can therefore be expected that the use of carbon tetrachloride as a chlorinating cum reducing agent will accelerate the chlorination process enabling use of lower temperatures resulting in lower energy consumption and cost of capital equipment. Use of carbon tetrachloride for reduction chlorination has been reported earlier for TiO2 (anatase, ilmenite) by Jena et 411] in the range 523-673 K and by Lavacchia et al[12] in the range 893-973 K, and for ZrO2 by Jena et al[13] in the range 700-750K. Kinetics of carbo-chlorination of refractory metal oxides including Nb2O5 and Ta205 have also been studied recently by Allain et al[14] in the temperature range 653-1273 K using the gas mixture consisting of C12+CO+N2. They found that for chlorination at lower temperatures (<923 K), the activation energies for the chlorination of Nb2O5 and Ta205 were 75 and 100 kJ/mol respectively. The activation energies reported by Allain et al are much higher than that reported by Srinivasan and Jena[] 0] for similar studies and further the activation energy for the chlorination of Ta205 was higher than that of Nb2O5 in contradiction to the earlier results of Jena et al[8-10]. No systematic study is reported in the literature on the kinetics of the chlorination of pure tantalum pentoxide. The purpose of the present work was to investigate the kinetics of chlorination of pure tantalum pentoxide using carbon tetrachloride in the presence of nitrogen. EXPERIMENTAL Specpure Ta205 obtained from Atomic Energy Laboratory, Brazil, chemically pure CC14 and high purity dry N2 gas were used in this study. The experimental apparatus used to • conduct the chlorination experiments was identical to that used earlier by Jena and co-workers[13]. The set up consists of a transparent silica tube horizontally placed inside an electric resistance furnace. The temperature of the furnace was controlled to within ±2°C. The nitrogen from the cylinder was bubbled at the desired rate monitored by a flowmeter through the liquid carbon tetrachloride, kept in a flask and connected to the chlorinator. The flask has three passages, one for the introduction of N2 into the liquid CCI4, the second to pass the nitrogen gas into the chlorinator and the third for the inlet of the N2+CC14 mixture into the chlorinator. The flask was heated by a water bath to the desired temperature maintained within ±2°C by a controller. The pressure inside the flask was measured by a manometer. The outgoing product gases were cooled in a condenser and liquid TaC15 was collected in a flask. The other off gases containing an excess of CC14, C12, COCl2, CO, CO2 and N2 were bubbled through caustic soda solution and released through the exhaust. The experimental procedure usually folldWed was to preheat the reactor to the desired temperature, then introduce the pre-weighed charge held in an alumina boat into the reactor and continue heating till the temperature stabilizes at the desired value. The reactor was thoroughly flushed with pure dry N2 prior to introducing the sample into the reactor to eliminate the presence of moisture and air in the reactor. When the experimental temperature, as measured by a chromel-alumel thermocouple placed inside the reactor and in contact with the alumina boat stabilised for 5 100 T tCls,.. PO RN 23 min- " 42-53 	 13-70 	 70400 	 104-Ik? 	 107-2011 PARTICLE EtrE,M• 179 min, the mixture of CC14+N2 was introduced into the reactor for specified periods. At the end of the specified period, the sample was removed from the reactor in hot condition, cooled to room temperature and then weighed. The effects of chlorination time, temperature and partial pressure of carbon tetrachloride were studied. The latter was varied by changing the temperature of the bath while maintaining a constant flow of nitrogen. The partial pressure of CC14 was calculated from the change in volume of the liquid in the flask during a particular period of chlorination, the density of gaseous CC14 and the total quantity of nitrogen bubbled through the flask over the same period. Temperature was varied in the range 733-853 K and times up to 90 min were adopted. The chlorination behaviour as a function of time was studied at every 5 to 10 minute interval. RESULTS AND DISCUSSION The effect of particle size of Ta205 powders on its chlorination behaviour in the range 43 to 208pm (T=833 K, pcc14 0 kPa and time=75 min) is shown in Fig. 1. It is seen that within the limits of experimental uncertainty, the % conversions to TaC15 is independent of particle size or surface area. The variation of percent conversion to TaC15 with time as a function of temperature in the range 733-853 K is shown in Fig. 2 at pcc,4=60 Fig.! Effect of particle size on 	 Fig.2 Effect of Temperature on chlorination chlorination of Ta205 	 of Ta205 The plots appear to show a parabolic behaviour. It was observed that an increase in temperature causes an increase in percentage of tantalum recovered as chloride. The extent of chlorination increased with time up to 70 minutes, after which, near steady state conditions were achieved. Nearly 90% of the tantalum pentoxide was converted to chloride in 35 minutes at 853 K and pca4=60 kPa. The effect of partial pressures of carbon tetrachloride on the conversions of Ta205 to TaC15 is shown in Fig. 3 at 833 K. Within the range of measurements (pca4=20-60 kPa), the amount reacted increases with rise in pca4• The various kinetic models reported in the literature were examined to assess the reaction mechanism in the chlorination of pure Ta205 by gaseous carbon tetrachloride. The absence of surface area dependence of the chlorination of tantalum pentoxide indicates that the overall process is diffusion controlled. The results were found to fit best to Jander's equation: (1-(1-X)"31=kt 	 I 0.2 180 0-2 rc 1-2 	 1-3 VT n103 11'4 NE Iwiel Fig.3. Effect of pccu on chlorination of 	 Fig.4. Plot of (1-(1-X)15)2 against time at Ta205 	 different temperatures which is valid for a diffusion controlled gas-solid reaction where the particles are spherical. The term X is the fraction reacted, t is the time and k is the kinetic rate constant. A plot of the function (1-(1-X)18)2 as a function of time of reaction is shown in Fig 4 for all temperatures of study. The linear relationship of this function with time suggests that the chlorination of pure Ta205 is a diffusion controlled reaction in the range 733-853K. This plot was used to calculate the kinetic rate constant k for different T and the Arrhenius plot is constructed in Fig. 5. The activation energy for the chlorination of pure Ta205 by CC14 computed from this plot was found to be 24 kJ/mol. The value of activation energy obtained in this study for the chlorination of Ta205 using CC14 in the presence of N2 is closer to that obtained with use of C12 as chlorinating agent in the presence of CO+N2 (32 kJ/mol)[10] than that obtained with gaseous C12 in the presence of graphite (130 kJ/mol)[9]. This would imply that the CC14 would decompose to C12 and C at the temperatures of the experiment and react with Ta205. Fig.5. Arrhenius plot for chlorination 	 Fig.6. Plot of (1-(1-X)13)2 against time at of Ta205 	 different pca4 Further, it can be inferred from the lower activation energies that the carbon obtained from the decomposition of CC14 is far more reactive than elemental graphite or some CO 181 forms which reacts with Ta205. Jena et al[l 3] have also proposed that the CCI4 decomposes to Cl2 and C before chlorination. The variation of the function (141- X/1")2 with Pccia is shown in Fig. 6. It is evident that this function is directly proportional to pca4 implying a first order dependance of the rate constant (k) on pca4• To determine the equilibrium condensed and gas phase composition at the temperatures and partial pressures of CCI4 used in the experiments, free energy minimisation calculations were carried out using the SOLGASMIX programme[15]. All phases in the Ta-O-C1-C-N system reported in the literature and whose thermodynamic data is available were included in the analysis. For simplicity, a single step minimisation was done. Thermodynamic data were taken from JANAF thennochemical tables[16]. Species whose data were not available in JANAF, were taken from the recent compilation of Kubaschewski et al[17]. The results obtained for a typical case at 853 K is given in Table I. It was seen that for all the temperatures and partial pressures of N2 and CC14 adopted in this study, no tantalum carbides or nitrides form. However, substantial amounts of solid carbon is left behind. The equilibrium gas consists mostly of C12, N2, TaC15, CO, CO2, COC12 and some unreacted CCI4. The other species of tantalum chlorides and oxychlorides are found to be negligible. Jena et alp 0,11] and Allain et al[14] propose that considerable amounts of COC12 forms during the chlorination reaction in presence of C or CO which dissociate at temperatures above 873 K. The free energy minimisation calculations substantiates their observation. Table I. Equilibrium Condensed and Gas Phase Compositions at 853 K. Specie Moles Input Moles Output Partial Pre- ssure, atm Specie Mole Input Moles Output Partial Pre- ssure, atm Invariant Condensed Phases Gas Phase C 0.0 0.5026 CO2 0.0 8.34e-4 5.93e-4 Ta 0.0 0.0 COCI 0.0 9.76e-8 6.93e-8 Ta205 0.00226 0.0 TaCls 0.0 4.53e-3 3.21e-3 TaC 0.0 0.0 CO 0.0 7.74e-3 5.5e-3 Ta2C 0.0 0.0 CCI2 0.0 4.0e-14 2.82e-14 TaN 0.0 0.0 C120 0.0 8.0e-23 5.67e-23 Ta2N 0.0 0.0 CCI 0.0 1.0e-25 7.18e-26 TaCls 0.0 0.0 C102 0.0 1.2e-37 8.31e-38 TaC14 0.0 0.0 TaCI 0.0 3.3e-50 2.3e-50 TaC13 0.0 0.0 TaC12 0.0 2.2e-34 1.59e-34 Ta0C13 0.0 0.0 TaC13 0.0 3.8e-22 2.72e-22 Gas Phase TaC14 0.0 8.0e-11 5.72e-11 N2 0.3470 0.3470 0.2465 02 0.0 4.9e-28 3.46e-28 C12 0.0 1.0130 0.7195 TaO 0.0 1.1e-60 8.08e-61 CC14 0.546 0.0329 0.0234 TaO2 0.0 1.3e-53 9.03e-54 COC12 0.0 1.9e-3 1.35e-3 Ta0C13 0.0 CONCLUSIONS Carbon tetrachloride was found to be a very efficient chlorinating agent for tantalum pentoxide even at low temperatures (733-853 K). Particle size appears to have negligible effect on the chlorination of Ta205. The amount of tantalum chlorinated was found to increase with temperature in the range 733-853 K. Nearly 90% of the tantalum oxide could be 182 converted to its chloride at 853 K in 35 minutes at a carbon tetrachforide pressure of 60 kPa. The kinetic results were found to conform to a diffusion controlled reaction model proposed by Jander. From the Arrhenius plot, the activation energy of the process was estimated to be 24 kJ/mol. The reaction rate constant was also found to be directly proportional to the partial pressure of carbon tetrachloride in the range 20-60 kPa. REFERENCES 1. Kock, W. and P.Paschen, J. Metals, (1989), 33 2. Gupta, C.K., Int. Met. Rev., 29(6) (1984) 405 3. Ruff, 0. and F.Thomas, Z. Anorg. Allg. Chem., 156 (1926) 213 4. Schafer, H., L.Bayer and C.Pietruck, Z. Anorg. Allg. Chem., 266 (1951) 140 5. Cuvelliez, F., British Patent No.507, 124 (1939) 6. Cuvelliez, F , British Patent No. 644, 454 (1950) 7. Hussain Zahed and P.K.Jena, Trans. Ind. Inst. Met., 15 (1962) 220 8. Mehra, O.K., Zahed Hussain and P.K.Jena, Trans. Ind. Inst. Met., 19 (1966) 53 9. Mehra, O.K. and P.K.Jena, Trans. Ind. Inst. Met., 20 (1967) 209 10. Srinivasan, K.R. and P.K.Jena, Trans. Ind. Inst. Met., 21 (1968) 35 11. Jena, P.K., D.H.Gameiro and E.A.Brocchi, Trans. Inst. Min. Met. ,100 (1991) C65 12. Lavacchia R., L.Piga, F.Ponchetti and L.Chacon, Trans.Inst.Min.Met., 102(1993) C174 13. Jena, P.K., E.A.Brocchi and T.F.Villela, Metall. Trans. B (in press) 14. Allain, E., H.Djona and G.H.Gabel, in Conference on Light Metals, The Minerals, Metals and Materials Society AIME, 420 Commonwealth Dr., Warrendale, USA, 1995 15. Flyn, H., A.E.Morris, and D.Carter, University of Missouri-Rolla, Copyright 1986 16. Stull, D.R. and H.Prophet, (Eds) JANAF Thermochemical Tables, 2nd ed., US Dept. of Commerce, Washington 1971 (Suppl. 1975) 17. Kubaschewski, 0., O.Knacke and K.Hesselman, Thermochemical Properties of Inorganic Substances, 2nd ed., Springer Verlag, Dusseldorf, Germany, 1991 

Kinetics of Chlorination of Tantalum Pentoxide with Carbon Tetrachloride

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Kinetics of Chlorination of Tantalum Pentoxide with Carbon Tetrachloride

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