Pyrite and pyrrhotite are the most abundant sulphides in mine wastes worldwide. While
there is a large body of information related to the weathering of pyrite and the effects of
this process on water quality, there is a significant deficiency of information on the
weathering reactions and the controls on pyrrhotite reaction rates. Unlike pyrite,
pyrrhotite represents a range of chemical composition as indicated by the formula
Fe1-xS in which x can vary from 0 to 0.125. This also implies that there is an inherent
deficiency of iron in the crystal structure, possibly representing less structural stability
than that of pyrite. Several crystallographic forms of pyrrhotite are known.
The objectives of this investigation were:
- to assess the kinetic controls on pyrrhotite oxidation;
- investigate the effects of crystal structure, metal impurities, surface area,
and bacterial catalysis on oxidation reaction rates; - assess the dynamics and effects on water quality of pyrrhotite oxidation
in tailings column studies; and - develop a modelling approach consistent with the mechanisms and
controls on pyrrhotite oxidation reactions.
This study was conducted in three distinct phases. The first phase involved twelve
distinct pyrrhotite samples for a detailed study of fundamental chemical kinetics of
oxidation by oxygen and ferric iron. The second phase comprised characterization and
kinetic studies on actual pyrrhotite concentrate obtained from Inco’s Clarabelle Mill
(Sudbury). The pyrrhotite concentrate material was studied to assess the oxidation
kinetics for different conditions of temperature, pH and bacterial catalysis. The third
phase of the investigation involved testing of pyrrhotite tailings material in columns
designed to assess the effects of sulphur content (2 % to 6 % S2-), bacterial inoculation
(Thiobacillus ferrooxidans), the presence of calcite (1 % CaCO3) and enstalite (5 %
MgSiO3) as neutralizing solids, and the presence of fine-grained pyrrhotite material (<
45 m m) in the tailings.
The specific surface areas (area/mass) of selected particle size-fractions varied
significantly among pyrrhotite specimens and exhibited values that were a factor of 2 to
10 times greater than those of similar size pyrite particles and 6 to 40 times greater
than calculated theoretical surface areas assuming spherical smooth geometry. The
rates of abiotic oxidation by oxygen as exhibited by iron production were, on average,
ten times greater than rates for pyrite oxidation under similar conditions. Oxidation
rates by ferric iron, however, were about one-fourth of those for pyrite oxidation under
similar conditions. The effect of temperature was similar to that observed for pyrite
oxidation with activation energy values in the range of 50 to 60 kJ mol-1.
Crystallographic structure and trace metal content showed no consistently significant
effects.
The effect of bacterial inoculation differed with pH and temperature. The maximum
biologic rate of sulphate production was observed at pH = 4 with rates that were
approximately 10 times those in non-biologic tests. Non-biologic rates and biologic
rates approached similar values at pH values of 2 and 6. The column studies
confirmed the effects of bacterial activity on oxidation rates with similar behaviour in a
column that had been inoculated and one that had not been inoculated. Oxidation rates
and loading rates for sulphate, iron and nickel were similar in both columns containing
6 % S2-. Only slightly lower loading rates for sulphate and iron were observed in the
column containing 2 % S2- but nickel release rates were higher than those in the 6 %
S2- column. The oxygen consumption rates reflected the loading rates of pyrrhotite
oxidation products. In general, the oxygen consumption rates were a factor of 3 to 10
higher than the stoichiometrically equivalent sulphate production rates initially but
oxygen consumption and sulphate release rates converged with time. The average
molar ratio of Fe : SO4 was about 0.9 to 0.96 suggesting that, on average, pyrrhotite
oxidation produced Fe2+ and SO4 2- stoichiometrically.
The presence of calcite in the 6 % S2- tailings resulted in similar oxygen consumption
and sulphate release rates to those observed for the non-carbonate tailings. However,
the calcite maintained a near neutral pH condition in the pore water with the result that
Fe and Ni release rates were significantly lower. The presence of enstatite,
representing a silicate buffer, resulted in porewater with pH = 4 to 5 and some
additional ferric hydroxide precipitation with subsequent decreases in nickel release
rates compared to the control. The difference between the carbonate and silicate
buffered tailings was the pH of the porewater with the carbonate column maintaining
near neutral pH and causing almost complete precipitation of the iron.
The results indicate that characteristics of pyrrhotite such as metal impurities and
crystallographic form do not affect oxidation rates significantly. The surface area of the
pyrrhotite particles is by far, the most significant parameter required to assess
oxidation rates for pyrrhotite in tailings. The results of the column tests indicate that
oxygen transport is the most important phenomenon and small uncertainties in reaction
kinetics will not significantly affect long-term predictions for tailings oxidation when both
kinetics and oxygen mass transport are considered in geochemical modelling.
