Abstract
Analysis of several nickel mattes from around the world shows a significant variation in their nickel and copper content. Typically, matte contains an alloy, heazlewoodite (Ni3S2) and a copper sulphide phase. The various commercial methods of processing these mattes are reviewed and some of the fundamental factors that determine the choice of leaching system are discussed. Electrochemical studies on pure heazlewoodite and millerite (NiS) in sulphate and chloride media are compared which show that nickel sulphides react via a series of nickel deficient sulphides and exhibit passivation as nickel is removed. The electrochemistry of mattes differs owing to the presence of alloy and copper. Copper in the matte modifies the potential for alloy dissolution and the rate of anodic dissolution of nickel. Differences in the electrochemical behaviour of matte in chloride media relative to sulphate media are attributed to greater crystallinity of sulphur and less formation of intermediate NiS2 which oxidises to form thiosulphate and sulphate in solution. At elevated temperatures, Cu(II) in solution exchanges with NiS and completes the leaching of nickel at relatively low potentials without significant oxidation of sulphur to sulphate. It is concluded that there are fundamental advantages to leaching nickel sulphides and nickel matte in the presence of Cu(II) and chloride ion to minimise the production of sulphate ion via NiS2 formation.
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