Thermodynamic Investigation of High Temperature Ta2O5 Chlorination with C2Cl4

Navarro, R. C. S., Zocatelli, T. F., Brocchi, E. A.

Chlorination has been applied as an industrial process for recovery of metals from many resources – slags, tailings, mineral concentrates. Under the usual conditions, chlorine (Cl2) reacts at high temperature (973 – 1273 K) with the metal containing material, usually of oxide nature, under the presence of a reducing agent, for example, a carbon bearing material such as graphite, in order to promote both the driving force and kinetics. In recent years, considerable research has been stimulated regarding the search of new chlorinating agents, which could preserve both the thermodynamic viability and reaction kinetics, but also with an associated reduced environmental impact. In this context organic chlorinated molecules, such as CCl4 and C2Cl4, appear to be excellent candidates, on one side, understood by their gaseous nature, which favor the contact with the oxide particles, and on the other, by their reducing (presence of carbon) and chlorinating (presence of chlorine) power. However, the inherent stability of these last chlorinating agents imposes some restrictions regarding the temperature and agent partial pressure admitted into the reaction chamber. Therefore, the present work focuses on a thermodynamic study of the possible chlorination of pure Ta2O5 with C2Cl4, thereby exploring the chlorination potential of the mentioned agent in comparison with the usual route (Cl2 with graphite). It is understood that the formation of graphite through thermal decomposition of C2Cl4 should be avoided. So, some simulations have been performed considering the incorporation of oxygen in the reactor’s atmosphere. It is demonstrated that C2Cl4 can indeed be used in replacement to Cl2 for the reaction with Ta2O5 with temperature varying between 873 and 1173 K, thereby resulting in gaseous TaCl5 and TaOCl3 as the major chlorinated products under continuous flow. However, the results suggest that the presence of oxygen can be important for oxidizing the non-reacted C2Cl4 to Cl2 and CO or CO2, specially when the atmosphere becomes progressively concentrated in C2Cl4. Finally, experimental results on high purity Ta2O5 samples at 1173 K revealed that diluted C2Cl4 (Rc = 0.03) can indeed result in gaseous products, preferentially TaCl5, according to the simulations. Fine solid particles were identified in the absorption KOH solutions, which is explained by the decomposition of part of the injected C2Cl4 as it travels between reactor entrance and sample