Year

2003

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Australia is one of the major coal producing countries in the world. Coal mine abandonment, groundwater rebound due to cessation of dewatering and associated pollution problems are a cause of serious concern throughout the world, and in particular, in Australia. Coal mining often leads to major environmental pollution problems when pyritic mineral is associated with the coal. The oxidation of pyrite usually generates acid mine drainage (AMD). The generation of AMD containing Fe++, Fe+++, SO4-- and H+ is a source of great concern and can contaminate water resources.

Prediction of groundwater inflow into a mining excavation is important for the design an effective dewatering system during the feasibility stage of a surface mining operation. A two-dimensional numerical finite element model called SEEPAV has been used for predicting the inflow of water into a surface mine working. The SEEPAV is a 32-bit graphical software that operates on the PC under Microsoft Windows 95, 98, Me, NT, 2000, and XP operating systems. This model has a capability to simulate saturated/unsaturated flow conditions and to calculate the height of the seepage faces the mining excavation taking into account the hydraulic conductivities and the water content as a function of pore water pressure.

The SEEPAV model with some modifications has also been used to predict the groundwater rebound within backfilled open cut mines. This model has the ability to simulate groundwater flow problems in partially saturated porous media. Flexibility in the model is achieved by assigning different boundary conditions to the model. The results of the model of groundwater rebound are presented and compared with those obtained from analytical solutions, using the existing numerical model as well as with the field monitored data observed in the UK . This model calculates realistic results that can be used by mine operators and environmental engineers to control the quality of mine drainage in a backfilled open cut mine operation.

A two-dimensional numerical finite volume model using PHOENICS as a computational fluid dynamic (CFD) package has been developed to simulate the transport of oxygen and long-term oxidation of pyrite as well as transport of oxidation products from a backfilled open cut coal mine. Gaseous diffusion was considered to be the principal mechanism for the transport of oxygen through the spoil. It was assumed that both oxygen and ferric iron produced by bacterially mediated oxidation of ferrous iron participate in the oxidation of pyrite. The pyrite oxidation reaction is based on shrinking-core model. The model takes into account the effects of both surface reaction kinetics and the rate of oxidant diffusion into the particle. The model also assumes the particles have spherical shape and considers the rate of pyrite oxidation to be first with respect to oxidant concentration and pyrite surface area. Complexation of ferric iron is assumed to take place within the spoil solution. The model can take into account the role of sulphate reduction bacteria governed by Monod type kinetics. The model also incorporates acid neutralisation reactions, linear and equilibrium-controlled ion exchange reactions as well as the effects of precipitation of ferric iron. In this present model, chemical and bacterial oxidation of Fe2+, pyrite oxidation by oxygen and ferric iron, oxygen diffusion and bacterial sulphate reduction are relatively slow and assumed to be kinetically controlled but ion exchange and complexation reactions as well as precipitation reactions are fast and these are assumed to be equilibrium controlled reactions. Comparisons were made with published numerical modelling results and close agreement was achieved.

It was found that for the development of inflow and post-mining rebound models, a study of the hydrogeological characteristics of spoil is important. A comprehensive model of pyrite oxidation in the backfilled site of an open cut mine should not ignore the role of iron-oxidising bacteria. In the absence of bacteria, oxygen is the only important oxidiser of pyrite and the oxidation rate is highly dependent on the effective diffusion coefficient.

Although the oxidation of pyrite in the presence of air and bacteria is unavoidable, the numerical model developed here provides useful tool for assessing the effectiveness of a rehabilitation strategy to reduce pollutant production within oxidising pyritic material.

Comments

Accompanying disc can be consulted with the hard copy of the thesis in the Archives Collection, call no. is 622.292/11

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.