Carbochlorination kinetics of titanium dioxide with carbon and carbon monoxide as reductant
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INTRODUCTION
IT is well known that natural reserves of certain ores are limited and become gradually depleted. Alternative sources may be low-grade minerals or industrial wastes. For extraction of oxides of transition group metals, a chlorination process can be used. It provides good yield and purity in metal recovery at reasonable cost and, therefore, is being used more frequently in the field of metal extraction from ores and minerals. Most transition group metals exist in ores and minerals in the form of metal oxides. Carbochlorination of the metal oxides can be achieved by using a chlorinating agent (e.g., Cl2, HCl, CCl4, COCl2, etc.) and a reductant (e.g., CO, C) as oxygen sink. For example, when using Cl2 and CO, the following general reaction applies: MaOb 1 bCO 1 bCl2 5 aMCl(2b/a) 1 bCO2 Here, M represents the transition group metal in question. The volatile chlorides of transition metals can be condensed from the effluent gas stream and separated by distillation or sublimation to high-purity products. There is a voluminous literature on carbochlorination. Articles and patents have been published concerning the chlorination of alumina,[1–4] silicon dioxide,[5] Nb2O5,[6] coal flyash,[7] MoO3,[8,9] V2O5,[10] and other minerals like bauxite[11,12] and iron-bearing titaniferous materials.[13,14,15] Investigations of kinetic aspects and the effect of some factors on the recovery of a given metal were covered. Brain and Schuler[16] studied the chlorination of pure titanium dioxide by putting tablets of TiO2 and carbon side by side into a reactor. They concluded that the chlorination rate of TiO2 was 40 to 50 times faster with TiO2-C contact than in the absence of carbon and the acceleration of the chlorination rate decreases with increasing TiO2-C separation. Dunn[17] carried out the chlorination of TiO2 bearing minerals (ilmenite) using CO 1 Cl2 or COCl2 as chlorinating agent and reductant. He claimed that the effect of con-
FENGLIN YANG, Research Assistant, and VLADIMIR HLAVACEK, Professor, are with the Department of Chemical Engineering, SUNY at Buffalo, Buffalo, NY 14260. Manuscript submitted January 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
centration of both CO and Cl2 to the reaction rate is linear and that phosgene tends to decompose at high temperatures and approach the CO 1 Cl2 behavior. Youn and Park[18] and Zhou and Sohn[19] developed mathematical modeling of fluidized-bed chlorination of rutile. However, systematic investigation of kinetic data of carbochlorination of titanium dioxide is not readily available in the literature. In our study, chlorination of two kinds of oxides, TiO2(R)(rutile, powder, ,5 mm, 99.9 pct) and TiO2(A) (anatase, powder, 2325 mesh, 99 pct), was carried out. Temperatures of 800 7C and 1000 7C were chosen as reaction temperatures. The total pressure of 102.1 kPa inside the reactor was kept. The combinations of chlorine with solid carbon or chlorine with carbon monoxide were used for chlorination. The major objective of this study is to understand better the
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