An experimental evaluation of the use of rock phosphate (apatite) for the amelioration of acid-producing coal mine waste

Abstract

The weathering of coal and coal-associated rocks (AMD) containing iron disulphide minerals produces acid mine drainage. (AMD). This is the most important environmental problem in high-sulphur coal basins. It is generally agreed that the major oxidizing agent of FeS 2 is the ferric ion. Experiments have shown that the average equilibrium voltage for the Fe 3+/FeS 2 system is 0.42 which corresponds to a Fe 3+: Fe 2+ ratio of 10 −6: 1. In an ideal solution, if the Fe 3+: Fe 2+ ratio could be reduced to less than 10 −6: 1, the Fe 3+-oxidation of FeS 2 could not take place. It has been shown that both Fe 2+ and Fe 3+ ions can be precipitated by the phosphate ion under ideal conditions at near-neutral pH-conditions, producing a Fe 3+:Fe 2+ ratio of 10 −11: 1. Experiments utilizing rock phosphate (apatite) as a source of phosphate ion showed that the treatment of coal-associated rock containing iron disulphide minerals not only prevented the production of acid, but also terminated the production of acid from materials already undergoing oxidation. The results of these experiments indicate that rock phosphate (apatite) is an effective AMD ameliorant based upon its ability to reduce the Fe 3+: Fe 2+ ion ratio to below 10 −6: 1 and its ability to reduce the subsequent solution potential to the less than 0.42 V, required to allow the oxidation of FeS 2 by Fe 3+.