Process review and electrochemistry of nickel sulphides and nickel mattes in acidic sulphate and chloride media

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 (Ni 3 S 2 ) 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 NiS 2 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 NiS 2 formation.

A review of chloride assisted copper sulfide leaching by oxygenated sulfuric acid and mechanistic considerations

“…However, visual evidence for sulfur formation was found after anodization at a much higher potential of 2.35 V (Hillrichs and Bertram,1983). In contrast, the current-potential curve of NiS in 0.5 mol dm -3 H 2 SO 4 showed a peak at a lower potential of ~1.34 V (Muir and Ho, 2006). Moreover, anodic oxidation of NiS produced S, NiS 2 and S 2 O 3 2-at a potential of ~1 V in mixed sulfate/chloride media.…”

“…The general view is that during atmospheric leaching in the presence of sodium chloride in sulfuric acid, a porous and somewhat crystalline sulfur layer is formed on the sulfide surface, instead of a tight passivating sulfur layer which blocks the surface Lawson, 1991a, Muir andHo, 2006;Carneiro and Leao, 2007). Elemental sulfur formed during the leaching of chalcopyrite with ferric chloride is also more porous than that formed with ferric sulfate, thus the leaching rate is an order of magnitude larger in the former (Majima et al, 1985).…”

“…Recent reviews have highlighted the growing interest and conflicting views amongst researchers to understand the mechanistic details of leaching of chalcopyrite and secondary copper minerals in order to promote hydrometallurgical routes to extract copper (Dreisinger, 2006;Watling, 2006;Nicol, 2007;Klauber, 2008;Cordoba et al, 2008a). Chalcocite and covellite are two of the common copper-bearing minerals also formed as intermediate products during the leaching of chalcopyrite, copper matte/residue and copper-nickel sulfides; as shown by some of the reactions relevant to the present study listed in Table 1 (Peters, 1976a,b;Grizo et al, 1982;McDonalds and Langer, 1983;Cheng and Lawson, 1991a,b;Fisher et al, 1992;Grewal et al, 1992;Rademan et al, 1999;Muir and Ho, 2006;Deng et al, 2001;Provis et al, 2003;Nicol and Lazaro, 2003;Ruiz et al, 1998Ruiz et al, , 2007Dreisinger, 2006;Herreros et al, 2006;Vinals, 2007, Cordoba et al, 2008a,b). The dissolution of nickel sulfides via the displacement reactions such as reactions R6-R8 in…”

“…Rotating ring-disc electrochemical studies with nickel matte (Ho, 1996) and chalcopyrite (Lazaro and Nicol, 2006) have revealed the formation of interim sulfur-oxy species such as thiosulfate. Under some leaching conditions, NiS 2 and FeS 2 produce sulfate ions rather than elemental sulfur, while chloride ions appear to modify the passivating sulfur or inhibit the formation of NiS 2 layers (Muir and Ho, 2006).…”

“…4 Table 1 (Rademan et al, 1999;Muir and Ho, 2006) is thermodynamically more favourable with large values of equilibrium constants, compared to the acid dissolution via reactions R4-R5. Moreover, the copper sulfide formed on the surface of millerite via reaction R6 is oxidized more rapidly than the base mineral (Miki and Nicol, 2008a).…”