Oxidation of cobaltite: Part I. process mineralogy
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I.
INTRODUCTION
C O B A L T is a critical metal for high-temperature alloys. 01 Cobaltite, one of the major sources of cobalt, is a sulfuroarsenide with the formula CoAsS, with some iron or nickel substitution. The largest known domestic reserve of cobalt, located in Idaho, contains 0.65 pet Co, with nearly all of the cobalt found in cobaltite and a minor amount associated with chalcopyrite. Cobalt-containing concentrates generally require a combination of pretreatment by pyrometallurgy and final refining and metal production by hydrometallurgy. [21For cobaltite, the three major processing options, smelting, roast/leaching, and direct leach, are potentially available. t31 With smelting, however, when the arsenic concentration reaches a high level, it begins to contaminate the matte phase and, subsequently, the blister product. This element in blister hinders subsequent electrorefining. [41 Furthermore, since arsenic is easily volatilized, and since arsenic and many of its compounds are toxic, smelting may be detrimental environmentally. Direct-leach methods have been developed using high temperature and high ambient pressure. 13,5[However, for high-grade concentrates of cobaltite, considering the lower capital and operating costs, extraction by a relatively simple roast/leaching technique becomes more attractive, tt] Sulfur and arsenic are removed through roasting, which allows the transformation of cobalt from a difficult-to-dissolve sulfide compound into soluble forms, mainly oxides. Further, hydrometallurgical processing of the matte can be used for cobalt extraction. A roasting/leaching treatment of cobalt-arsenic ores has been practiced for the extraction of cobalt industrially. i7,8,91Direct oxidation of cobaltite and subsequent reduction to minimize the arsenic content have been studied, t~~ These studies have shown that direct oxidation of cobaltite at 873 to 973 K resulted in removal of the majority of sulfur but only partial removal of arsenic from the ores. A more complete study by G.X. WANG, Research Assistant, D. CHANDRA, Professor, and M.C. FUERSTENAU, Echo Bay Mines Distinguished Professor, are with the Department of Chemical and Metallurgical Engineering, University of Nevada-Reno, Reno, NV 89557. Manuscript submitted June 6, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
Holmstrom t~21 of roasting concentrate containing scutterudite (CoAs3) and arsenopyrite showed very poor arsenic removal from these concentrates at all roasting temperatures. Mechanisms of oxidation were not reported. Insight into the oxidation mechanisms for cobaltite is important to ensure not only the efficient extraction of cobalt but also environmentally safe handling of arsenic. The present investigation involved a study of the process mineralogy and the kinetics of oxidation of cobaltite under various conditions through thermogravimetdc analysis (TGA). X-ray diffraction (XRD) analysis and scanning electron microscopic examination of partially roasted eobaltite were conducted to provide information on the chemical compos
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