Sulfide Smelting

Abstract

Platinum, iridium, and mercury can be obtained by thermal decomposition of the respective sulfides. Sulfides of cadmium, cobalt, molybdenum, and zinc can be reduced with metals like calcium, magnesium, manganese, sodium, and iron but economics does not permit large-scale production by metallothermic reduction. Nickel sulfide cast into anode can be directly electrolyzed to deposit nickel on cathode. The process has been successful on commercial scale for nickel but not for copper and lead. Conventional pyrometallurgical route based on the combination of roasting of sulfide concentrate in multiple hearth/flash/fluidized bed roasters and subsequent reduction smelting of oxides in a blast furnace of rectangular cross-section has been most widely adopted for production of zinc and lead. Until the very recent past, controlled roasting, matte smelting, and converting have been the most noteworthy method for production of blister copper from chalcopyrite concentrate. Roasting converts most of iron sulfide into iron oxide and copper sulfide into copper oxide/sulfate. But the roasting step has been eliminated from the flow sheet of copper extraction in recently developed faster smelting and converting processes [1, 2]. Smelting happens to be one of the main steps in extraction of metals from sulfide and oxide minerals via the pyrometallurgical route. It is essentially an operation of melting and slag/metal separation involving chemical reactions. Smelting may be defined in two different ways. In general, any process of metal production where metal is obtained in molten state is given the name “smelting.” Thus, production of aluminum by electrothermic reduction of alumina dissolved in cryolite and those of iron, lead, and zinc by carbothermic reduction of hematite and calcined sinters (in case of lead and zinc) would fall in this category. The next level of definition is the overall process of producing primary metals from sulfide ores/minerals by going through a molten stage. The specific definition is the first step of the two-step oxidation of sulfur and iron from sulfide minerals, mainly for copper and nickel production, that is, matte smelting as against to “coverting” in which the matte is further oxidized to produce molten copper or a higher grade nickel matte. The two stages of the metal production operation, namely matte smelting and converting, are related to oxygen potential in two stages as well as heat generation [3]. Oxygen potential does not only affect the slag chemistry (e.g., magnetite formation) but also changes the behavior of impurities. In case, the sulfide mineral is oxidized all the way to metal in one step many more of the impurities would join the metal, instead of the slag and thus much larger amount of heat will be generated. Hence, in the first stage: the “matte smelting step,” much iron, sulfur, and harmful impurities are removed into the slag formed in that stage. The resultant matte is separated and treated in a subsequent step, usually the converting step. © 2021, The Minerals, Metals & Materials Society.