The leaching characteristics of Ni–Cu matte in the acid–oxygen pressure leach process at Impala Platinum

Abstract

A study has been carried out to elucidate the leaching mechanism of Ni–Cu matte in an acid–oxygen (CuSO 4–H 2SO 4–O 2) pressure leach process at Impala Platinum Refineries. This process is based on the initial Sherritt Gordon process at a temperature of 140°C and a pressure of 550 kPa. The complex interaction of the various synthetic minerals with one another in the leaching process has been clarified and indications found that certain crystallographic rules exist for the various synthetic mineral species to leach. Nickel and copper are leached from the sulphide lattice to form nickel and copper sulphides with decreasing Ni to S and Cu to S ratios, i.e., Ni 3S 2–Ni 7S 6–NiS–Ni 3S 4 and Cu 2S–Cu 31S 16–Cu 1.8S–CuS, respectively. The kinetics for the cementation of copper from the leach liquor in the initial stages are very fast, as are the kinetics for the leaching of Ni alloy from the matte. The leaching of Ni alloy creates a porous structure in the matte particle to improve the leaching efficiency of the nickel and copper sulphides. H 2S was detected during the experiment (even with an O 2 partial pressure of 180 kPa) which inhibited the leaching kinetics and led to the formation of a quasi-intermediate nickel sulphide (Ni 7S 6) and copper sulphide (Cu 31S 16) product. The key to the selective leaching of the nickel from the Ni–Cu matte is that while leaching Ni 3S 2, copper ions in solution are continuously precipitated as Cu 2S in a substitution reaction liberating nickel ions into solution. Therefore, nickel is initially leached selectively from the Ni–Cu matte due to the presence of Ni 3S 2 in the solids. The presence of heazlewoodite (Ni 3S 2) is the factor controlling the leaching of copper simultaneously with nickel instead of the effect of the oxidising leach and followed by the non-oxidising leach as it is currently postulated in the literature.