Evaluation of factors affecting pyrite oxidation and subsequent pollutant generation in backfilled open cut coal mines
The oxidation of iron sulphide minerals, especially pyrite, by oxygen and water in the presence of certain bacteria causes acid mine drainage (AMD), with high concentrations of iron and sulphate and low pH. The results of a numerical finite volume simulation of AMD generation at abandoned backfilled open cut mines are presented. The governing equations of pyrite oxidation, oxygen diffusion and solute transport have been solved by modifying a commercial computational fluid dynamics package called PHOENICS. The pyrite oxidation rate is described by a shrinking-core model for spherical pyritic grains. Oxygen transport within the backfill material is assumed to be by diffusion. A sensitivity analysis of the major parameters that influence the rate of the pyrite oxidation and subsequent pollutant ·generation has been carri€1d out. The parameters considered were the effective diffusion coefficient for oxygen transpol1, the surface recharge rate, the radius of spoil particles containing pyrite, the pyrite content, and the role of bacteria on the pyrite oxidation rate. It was found that the pyrite oxidation rate and .the subsequent pollutant loading are most sensitive to the effective diffusion coefficient in the preSence of iron-oxidising bacteria, although the effect of the particle radius should not be ignored. An increasing· diffusion coefficient for oxygen transport increased the rate of pyrite oxidation and . subsequently increased the ferrous iron and sulphate concentrations and decreased the solution pH. ... Iron-oxidising bacteria increased the ferric/ferrous ratio. The bacterially mediated oxidation of pyrite is . highly dependent on the pl-f. When the pH is maintained in the range between 2.5 to 3.5, the bacterial oxidation of pyrite is enhanced. Subsequently, the bacterial action .produces more Fe3 +, SO/', W, and . : Fif+. The results of such. sensitivity analyses are important in assessing and evaluating the physical .. and chemical processes involved in the generation of pollutants and the transport of dissolved from an open cut mine into an aquifer.