Effect of waste rock particle size on acid mine drainage generation: Practical implications for reactive transport modeling

Mine waste rock poses significant environmental challenges. Evaluating management and reclamation options is particularly complex because of the wide particle size distribution, the non-uniform distribution of acid-generating and buffering minerals, and the variable contribution of the different particle size fractions to acid mine drainage (AMD) generation. Reactive transport simulations can be useful to complement and overcome the limitations of laboratory and field experiments. However, predicting field-scale and long-term geochemical behavior of waste rock requires a better understanding of numerical parameters scale-up. In this study, three waste rocks, with different mineral composition and particle size distribution, were separated into different fractions and tested in the laboratory. Kinetic tests were used to calibrate numerical models and adjust minerals' effective kinetic rate constants to match measured pH and metal concentrations. Calibrated reactive transport simulations were able to reproduce accurately the effect of particle size on pH and sulfate and calcium production rates. Experimental and numerical results confirmed that waste rock oxidation and neutralization rates tended to decrease with increasing particle sizes. Several models were tested and the weighted geometric mean of the effective kinetic rate constants as a function of the proportion of each fraction provided the most accurate estimation of the whole specimen kinetic rate constants. A novel approach to predict waste rock geochemical behavior from a single laboratory test also showed promising results. Overall, these results should contribute to improving the extrapolation of laboratory kinetic test results to field predictions.