Pentlandite (Fe4.5Ni4.5S8) and violarite (FeNi2S4) were
synthesized by dry in vacuo techniques. The products were
analysed by reflected light microscopy, powder X-ray
diffraction and electron microprobe analysis. The synthetic
pentlandite was found to have an average stoichiometry of
Fe4.35Ni4.65S8. A partial phase segregation of pentlandite
into heazlewoodite and pyrrhotite was observed. The
synthetic violarite grains showed a zonal separation into a
Fe1.2Ni1.8S4 core, and a Fe0.5Ni2.5S4 rim. Trace amounts of
pyrite and millerite were also detected.
From a critical review of the thermodynamic data in the
literature, several Eh-pH diagrams were constructed for the
Fe-Ni-S aqueous system. These were compared with
mineralogical evidence obtained from naturally occuring
mineral assemblages.
A study of the oxidative dissolution of pentlandite by
electrochemical techniques was made to clarify the mechanism
by which pentlandite is leached in acid FeCl3 solution. The
techniques used included: potentiometry, linear sweep cyclic
voltammetry, intermittent galvanostatic polarization,
chronopotentiometry and chronoamperometry. The products were
analysed using scanning electron microscopy, powder X-ray
diffraction, electron microprobe analysis, atomic absorption
spectroscopy and gravimetric analysis. The fitting of
experimental results to a simple electron transfer model via
the Sand equation was tested and found to be inappropriate.
A mechanism for the oxidative dissolution of pentlandite
is postulated. In acid solution, pentlandite decomposes
spontaneously, liberating aqueous metal ions and H2S. Under
potentiostatic conditions akin to FeC13 leaching,
pentlandite is oxidized directly to elemental sulphur,
without the formation of any intermediate phases. The lack
of formation of violarite indicates that the system is
substantially perturbed from equilibrium due to slow solid
state diffusion of metal atoms within the sulphur sublattice. The formation of metastable amorphous sulphur as
the alternative product is further evidence of this
perturbation. The physical properties of the sulphur product
layer cause an impediment to mass transport between the bulk
aqueous solution and the mineral surface. However, the
oxidation involves an intrinsically slow 'electron transfer
for the So, Fe2+, Ni2+ / Fe4.5Ni4.5S8 couple which, within the potential range relevant to FeCl3 leaching, is rate
determining for an appreciable part of the reaction. The
implication for extractive hydrometallurgy is discussed.
The use of a convolution transform of voltammetric
currents with a (πt)-1/2 function as applied to simple
electron transfer is described. In addition, the derivation
of a functional form for the treatment of chronoamperometric
data is given. These models were applied in the
determination of the heterogeneous electrochemical
parameters and diffusion coefficients for the FeC13/FeC12
couple in 1M HC1 solution on platinum at 293K, using
computer controlled chronoampermetric techniques. The
results show quasi-reversible behaviour (at 293K), which
implies that electron transfer for this couple would not be
rate determining in the leaching of pentlandite.