Executive Summary

The highly nonlinear nature of the kinetic equations describing the coupled geochemical and physical processes involved in pyrite oxidation has posed serious questions about the predictability of the environmental impact of acid rock drainage. In response to these questions this report describes the results of a research project which has been initiated with the purpose to provide a quantitative analysis of the interrelated elementary chemical and physical processes which are responsible for pyrite oxidation and acid rock drainage (ARD). This project is part of an effort to design practical indicators which would combine quantitative laboratory data, obtained for small samples of waste rock, with large-scale effects observed in waste rock piles.

The main objective of this project was to determine the simple scaling laws which govern the geochemical and thermodynamic behaviour of pyritic rock in waste rock piles. The scaling indicator * which combines information about processes occurring at different scales, is an innovative feature of this project. Effective kinetic equations for coupled chemical reactions involved in pyrite oxidation have been derived. The concentration of dissolved oxygen has been described by a simple formula which gives reasonable quantitative agreement with experimental data for the whole range of temperatures and oxygen concentrations in the gas phase, as required for ARD analysis. The strong decrease of dissolved oxygen concentration with temperature is included in the model. This property has not been included in previous models - the omittance of which could result in an overestimation of the acid generation potential.

Energy and oxygen transport are described by using a reaction-transport model. Strong nonlinear dependence of the effective reaction rates on the physical, mineralogical and chemical parameters has been described by means of a scaling parameter * which can be used as a practical indicator of ARD for waste rock piles. The dimensionless scaling parameter * combines information about pile porosity, pile size, effective reactive surface area, temperature dependence of the rate of pyrite oxidation, oxygen diffusion in the gas phase, heat of the pyrite oxidation reaction, thermal conductivity of waste rock, and ambient temperature.

The sensitivity analysis provides information about the required accuracy of experimental tests and the relative importance of parameters governing different physical and geochemical processes responsible for ARD. In particular, it is shown that the effectiveness of impermeable covers increases with the pyrite concentration. The scaling analysis indicates that geochemical and transport processes operate at the meso-scale in a way fundamentally different from the full-scale. Critical values of the scaling parameter *, at which bifurcations or thermodynamic catastrophes leading to accelerated acid generation rates, have been determined for different scenarios. The critical dependence of ARD on pile porosity, pile size and reactive surface area is one of the conclusions of the bifurcation analysis. The results of the scaling analysis offer the possibility of a cheap and fast preliminary assessment of the expected environmental impact. All parameters and variables of the present model can be measured in independent experiments. The model produces realistic results for acid generation rates without introducing adjustable fitting parameters used by all (known to us) other waste rock models. Several results of this study are significantly different from conclusions and assumptions of other existing models.

Quantitative results presented in this study should be confronted with field data. Additional thermokinetic tests for waste rock samples are required in order to provide reliable entry data for future predictive waste rock models. The effects of convective oxygen transport and water transport on the critical values of the dimensionless scaling parameter should be analyzed in a future study. Usual acid/base accounting tests do not provide data necessary for a predictive model which should generate quantitative information about the effluent. Additional thermokinetic tests are proposed in order to provide experimental information necessary for a predictive waste rock model. We hope that after further modifications and calibration, the scaling analysis presented here may help to properly design and manage waste rock piles and dumps.

Last Modified: 2003-11-26		Important Notices

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