Leaching kinetics of synthetic heazlewoodite
The sources of base metals are mainly in the form of oxides or sulphides, of which the sulphides are predominantly present in South Africa. These metals are intergrown platinum group and base metals in the form of alloys and sulphides. In order to produce high grade saleable metals, it is necessary to effectively separate the base metals from the precious metals. By means of a hydrometallurgical process, that is leaching, metals can selectively be extracted from ores. The mechanism of leaching can be described by oxidation-reduction and acid-base reactions. During this study, the leaching of a synthetically prepared heazlewoodite (Ni3S2) nugget was investigated. The parameters that were studied during the thermal leaching investigation are: -different acids; -different temperatures; -nitrogen and oxygen; -copper(lI) and iron(llI) ions. The influence of these parameters is discussed. It was found that the leaching rate appears to be dependant on the orientation of the crystals. The leaching process is partly an oxidation process, which is enhanced by the addition of strong oxidants. This was seen by the high leaching rates yielded by nitric acid. Oxygen and iron(lll) also accelerated the dissolution process. Leaching rates were typically in the order of 0.87 ± 0.02 mg.m-2.s-1 in 0.5 mol.dm-3 H2S04 under an oxygen atmosphere. This rate increased to 12.4 ± 0.20 mg.m-2.s-1 and 15.8 ± 0.13 mg.m-2.s-1 in the presence of Fe(lll) and Cu(ll) ions under an oxygen atmosphere, respectively. Two activation energies were calculated from the thermal data. These values were found to be 28.2 kJ.mol-1 for the initial leaching rate, and 45.75 kJ.mol-1 for the final leaching rate. These values are indicative of a surface chemical rate determining step. The two electrochemical methods used to investigate the oxidation-reduction reaction were cyclic voltammetry and chronopotentiometry. It was found that the dissolution rate determining processes occurred between 0.25 and 0.55 V (v. SHE). Results showed that irreversible oxidation-reduction processes control the electrochemistry of heazlewoodite. Since the oxidation-reduction processes are not the only reactions occurring, the presence of acid-base reactions complicated the description of the dissolution process. A qualitative description of the voltammograms, as well as an empirical model describing the leaching process, is given. In this model the formation of an inert layer is described, which forms by the oxidation of the nickel sulphide surface. The dissolution of the layer in acid was slower than the dissolution of the Ni3S2 which resulted in a decreased leaching rate.