|dc.description.abstract||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:
-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.||