A THREE DIMENSIONAL FINITE VOLUME MODEL FOR ACID MINE DRAINAGE SIMULATION IN OPENCAST MINE BACKFILL

摘要

The backfill of opencast mines often contains pyrite which may be oxidised and produce acid mine drainage (AMD) if it is exposed to the atmosphere. AMD is often characterised by high concentrations of iron, high sulphate and low pH and it is Considered to be a major cause of long-term poor water quality and is the source of many environmental problems. This paper presents a three dimensional numerical model that incorporates oxygen balance, pyrite oxidation, groundwater flow and transport of the oxidation products in order to describe the long-term pyrite oxidation and pollutant leaching within the backfill of an opencast mine. A diffusion process governs the oxygen transport within the pore space of the backfill. A shrinking-core concept describes the oxidation of pyrite particles. The transport model incorporates physical processes (advection, diffusion) and chemical processes. A multi-purpose CFD (computational fluid dynamics) software called PHOENICS version 3.6.1 incorporating a numerical finite volume discretisation scheme has been modified by creating a PHOENICS pre-processing input file in the PHOENICS input language (Q1_AMD_3D) and developing a subroutine called GROUND_ AMD3D in the FORTRAN 99 language. One and two dimensional modelling was performed to calibrate the three dimensional model. It was found that the oxidation of only a small fraction of pyrite is enough to cause long-term environmental problem due to acid mine drainage generation. The lowering of pH in the range between 2.5 to 3.5 results in the bacterial oxidation of pyrite being enhanced. Subsequently the bacterial action produces more Fe~(3+), SO_4~(2-) , H~+ and Fe~(2+) . The results obtained from three-dimensional modelling can be used for designing an effective site remediation program in order to minimise environmental effects arising from abandoned backfilled opencast mines.