Kinetics of chlorination and oxychlorination of chromium (III) oxide
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INTRODUCTION AND LITERATURE REVIEW
REACTIVITY of chlorine toward metals and metal compounds is interesting for the recovery of a valuable element from their ores and from industrial wastes. The chlorides and oxychlorides of transition metals have, generally, a lower boiling point than their oxides. This may allow selective separation of one or several elements, from the gas phase, by controlled cooling of reaction products. Recently, chlorination was used for the recovery of Ta and Nb from their ores[1] and tin slag[2] as well as for the extraction of Mo and V from spent hydrorefining catalysts.[3,4] Chlorination is also investigated for the beneficiation of chromite concentrates and/or selective separation of chromium compounds.[5,6] This article is devoted to the study of the chlorination and the oxychlorination kinetics of chromium (III) oxide using Cl2 1 N2 and Cl2 1 O2 gas mixtures. This investigation is a part of a larger program concerning the chlorination of refractory metals such as MoO3,[7] V2O5,[8] Nb2O5, and Ta2O5.[9] Moreover, the generated data could be used for the beneficiation of chromite ores or concentrates and for the recovery of chromium compounds from industrial wastes. Chlorination of Mo, V, Nb, and Ta oxides, without a reducing agent, generates oxychlorides of these elements. The most common oxychlorides are those described by Eqs. [1] through [4]. The metal oxychloride formation from its oxide does not imply a change of the metal valency and consequently the reaction can be performed in absence of gaseous oxygen. The hexavalent oxychloride of chromium CrO2Cl2(g) is the most stable oxychloride of chromium.[10] For this reason, the formation of CrO2Cl2(g) through the
chlorination of Cr2O3 required extra oxygen, as shown by Eq. [5]. The literature is relatively poor concerning the Cr-O-Cl system. The reaction products of chromium (III) oxide with chlorine could be CrCl2, CrCl3, CrCl4, and CrO2Cl2 depending on the temperature and Cl2 and O2 partial pressures of the system. Morozov and Fefelova[11] studied the equilibrium of chlorination of Cr2O3 with Cl2 between 700 7C and 1000 7C. They mentioned that the chlorination of chromium (III) oxide by chlorine at high temperature could lead to the formation of CrCl2, CrCl3, and CrCl4. These authors suggested that the formation of chromium oxychloride, in the investigated temperature range, was improbable because of its instability at temperatures higher than 450 7C. Sano and Belton[12] used a thermodynamic approach for the formation of CrO2Cl2(g) via the transpiration method. Their experiments consisted of the measurement of the volatilization of chromic oxide in a Cl2 1 O2 1 Ar atmosphere. According to these authors, the chromium predominant product of the chlorination of Cr2O3, with Cl2 1 O2 between 627 7C to 977 7C, is CrO2Cl2(g), as described by Eq. [5]. Reinhold and Hauffe[13] studied the chlorination kinetics of chromium metal and Cr2O3 pellets between 650 7C and 800 7C by thermogravimetric analysis (TGA). They concluded that the rate cons
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