The Problem of Acid Mine Drainage, with an Illustrative Case History

Abstract

Acid mine drainage refers to the oxidation of sulphide minerals, notably pyrite, which occurs when they are exposed to air and water in a mine or are present in spoil or mineral stockpiles. The primary oxidation products of pyrite are ferric and ferrous sulphates, and sulphuric acid. Nonetheless certain conditions have to be fulfilled if acid mine drainage is to develop. If acid mine drainage is to be controlled, then its accurate prediction is necessary. As far as control is concerned, this can be brought about by preventing or inhibiting acid generation, by control of acid migration or by treatment of acid mine drainage. The case history provided deals with the problem of acid mine drainage at a tin mine in the Transvaal, South Africa. As the mine was due to close, the problem of possible ground-water pollution due to acid mine drainage had to be investigated as part of an environmental management program. Waste dumps occur around the tin mine and it was suspected that water draining from these dumps might be a source of pollution. Samples were taken from the dumps for analysis and ground-water samples were collected from boreholes, trenches and pits. Surface water samples were taken from reservoirs and streams. Soil samples were taken from trenches and a stream bed. Analysis of the surface water showed that some samples had low pH values and high concentrations of sulphate. High concentrations of sulphate were also found in ground-water samples. These high concentrations were primarily associated with acid mine drainage. The principal source of acid mine drainage proved to be the pyrite dumps. Water samples taken from the mine workings were not acidic. This was attributed to the presence of an-kerite and calcite in the host rocks which would tend to neutralize acidic waters. However, this water did contain high sulphate concentrations, high electrical conductivity and total dissolved solids. Fortunately ground-water gradients in the area around the dumps are low. This together with the compartmentalization of ground water by dykes suggests that it is unlikely that ground water will be affected beyond the mine lease area.