[1]
Pyrite (FeS2) is by far the most naturally abundant of
the metal sulfide minerals and is the most significant
contaminant of coal, causing sulfurous emissions on combustion. The major technological objective of this work is an investigation of the feasibility of novel electrochemical coal desulfurisation processes based on direct or indirect electrochemical decomposition of the associated pyrite. However, the fundamental information derived will also have relevance to:
a) improving grades and recoveries in sulfide mineral froth flotation, the principal industrial process by which minerals are concentrated from their ores;
b) defining the conditions under which pyrite could be stabilised to prevent acid mine drainage, in which pyrite oxidises spontaneously in oxygen-containing environments, with net production of protons;
c) its possible use as a material for solar energy harvesting.
The reaction kinetics and mechanisms of oxidation and reduction of well characterised natural and synthetic pyrite in aqueous solutions are being investigated by cyclic voltammetry, chronoamperometry, X-ray photoelectron spectroscopy (XPS) and in-situ Fourier transform infrared spectroscopy (FTIR).
The feasibility has been established of removing sulfur from pyrite by the reduction reaction:
[1]
though unfortunately, this occurs in parallel with the evolution of hydrogen. Sulfur could be recovered by subsequent oxidation of the hydrogen sulfide and the hydrogen used as a fuel. This should be less energy intensive than utilising the oxidation reactions, which are also being studied:
[2]
and at higher electrode potentials and pH > = 2:
[3a]
[3b]
The reason for the severe kinetic limitation to the oxidative decomposition rate of pyrite in the potential range
-0.4 to + 0.5 V vs. S.C.E. has yet to be established. The
electrochemical, XPS and FTIR results all indicated that when pyrite was oxidised in 1 kmol HCl m-3 at potentials
< 0.5 vs. S.C.E., neither elemental sulfur, nor adsorbed Fe(III) species were formed, implying that reaction [2] occurred and the pyrite electrode surface composition remained essentially unchanged.