Analyses of several nickel mattes from around the world show a
significant variation in their copper content. Typically, matte contains an
alloy, heazlewoodite and a copper sulphide phase. The various
commercial methods of processing these mattes are reviewed and some of
the fundamental factors which determine the choice of leaching system
are discussed. Electrochemical studies on pure heazlewoodite and
millerite 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. Differences in the electrochemical
behaviour of nickel sulphides in chloride media are attributed to greater
crystallinity of sulphur and less formation of'NiS2. The electrochemistry
of mattes differs because of the presence of alloy and copper. Copper in
the matte modifies the potential for alloy dissolution and the rate of
anodic dissolution of nickel. 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 matte
in chloride media using Cu(II)/Clz as the oxidant.