Reaction Mechanism and Kinetics of Enargite Oxidation at Roasting Temperatures
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a pyrometallurgical process used in the metallurgical industry to transform sulfide minerals into oxides, sulfates, or other compounds more amenable for subsequent treatment toward the production of metals. In the case of copper concentrates, roasting is commonly used for the elimination of toxic arsenic and other deleterious elements through the gas phase, mainly because copper concentrates containing high amounts of arsenic are not apt for conventional smelting-converting technology because of the high risk of ambient contamination with arsenic compounds and the risk of producing final copper with unacceptable high levels of arsenic. Arsenic in copper concentrates is present primarily as the enargite mineral (Cu3AsS4), and when these copper concentrates contain appreciable amounts of enargite, they must be pretreated to eliminate the arsenic before smelting. Roasting is the preferred pretreatment process,[1–3] where most of the deleterious arsenic is eliminated through the gas phase inasmuch as arsenic compounds (including oxides and sulfides) have high vapor pressures at relatively low temperatures. Thus, the volatilization of arsenic at roasting temperatures should occur rather easily, but in practice, appreciable amounts of arsenic remain in the calcine as nonequilibrium compounds depending on the processing circumstances.[3,4] Although, the arsenic elimination from enargite by thermal decomposition in neutral atmospheres is RAFAEL PADILLA and MARIA C. RUIZ, Professors, and ALVARO ARACENA, Graduate Student, are with the Department of Metallurgical Engineering, University of Concepcion, Edmundo Larenas 285, Concepcion, Chile. Contact e-mail: [email protected] Manuscript submitted October 4, 2011. Article published online May 15, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
relatively well understood, the literature survey did not show conclusive rate data or an indication of the mechanism of arsenic elimination from enargite in oxygen containing atmospheres. On this matter, Kusick and Nadkarni[5] reported some data on the roasting of enargite concentrates in nitrogen and carbon dioxide atmospheres. These investigators focused mainly on the production of arsenic sulfide in the roasting as a final product and discussed the results of some data obtained in rotary and electric arc reactors. Smith and Paredes[6] reported some data on the decomposition of enargite and volatilization of arsenic as sulfide in nitrogen atmosphere. The following chemical reaction was proposed with the formation of two copper sulfides: 2Cu3 AsS4 ðsÞ ¼ Cu2 SðsÞ þ 4CuSðsÞ þ As2 S3 ðgÞ
½1
A somewhat different enargite decomposition reaction in nitrogen atmosphere was reported by Yoshimura[1] as given by Reaction [2]. 2Cu3 AsS4 ðsÞ ¼ 3Cu2 SðsÞ þ As2 S3 ðgÞ þ S2 ðgÞ
½2
Seco et al.[7] studied the decomposition of enargite in nitrogen and air, and they reported some inconclusive results obtained from differential thermal analysis measurements and experiments carried out in a horizontal tube furnace setup. Their data showed a very similar behavio
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