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.
