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

A study has been carried out to elucidate the leaching mechanism of Ni-Cu m atte in an acid-oxygen (CuSO4-H2SO4-O-2) pressure leach process at Impala P latinum Refineries. This process is based on the initial Sherritt Gordon pr ocess at a temperature of 140 degrees C and a pressure of 550 kPa. The comp lex interaction of the various synthetic minerals with one another in the l eaching process has been clarified and indications found that certain cryst allographic 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., Ni3S2-N i7S6-NiS-Ni3S4 and Cu2S-Cu31S16-Cu1.8S-CuS, respectively. The kinetics for the cementation of copper from the leach liquor in the initial stages are v ery 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 t o improve the leaching efficiency of the nickel and copper sulphides. H2S w as 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 q uasi-intermediate nickel sulphide (Ni7S6) and copper sulphide (Cu31S16) pro duct. The key to the selective leaching of the nickel from the Ni-Cu matte is that while leaching Ni3S2, copper ions in solution are continuously prec ipitated as Cu2S in a substitution reaction liberating nicker ions into sol ution. Therefore, nickel is initially leached selectively from the Ni-Cu ma tte due to the presence of Ni3S2 in the solids. The presence of heazlewoodi te (Ni3S2) is the factor controlling the leaching of copper simultaneously with nickel instead of the effect of the oxidising leach and followed by th e non-oxidising leach as it is currently postulated in the literature. (C) 1999 Elsevier Science B.V. All rights reserved.