Executive Summary

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 Fe_1-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:

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 the Inco Clarabell 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 % S^2-), 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 % S^2-. Only slightly lower loading rates for sulphate and iron were observed in the column containing 2 % S^2- but nickel release rates were higher than those in the 6 % S^2- 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 % S^2- 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.

Last Modified: 2003-11-26		Important Notices

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