Modeling the effects of evaporation on acid rock drainage in mine rock samples

Acid rock drainage (ARD) is an environmental concern of great importance to the mining industry. Humidity cell testing is often used to evaluate the acid producing potential of rock piles. It was desired to develop a comprehensive model of the processes that occur in humidity cell testing. Specifically, it was desired to develop a model that considered both ARD reactions without neglecting temperature effects due to reaction or evaporation. To this end, a model of the humidity cell was developed using Comsol Multiphysics™. ID isothermal and adiabatic models of evaporation in a humidity cell were developed. Analytic solutions were used to validate these models. A ID nonad iabatic evaporative model was designed to simulate the actual conditions experienced during humidity cell testing. This model was verified using experimental data. A homogeneous 2D nonadiabatic model of evaporation in a humidity cell was developed and also compared to experimental data with good results. Heterogeneity was added to the 2D model in the form of an inclined layer with different permeabilities to investigate the effects of an irregular air-flow pattern on evaporation. The results showed some interesting flow fields and temperature distributions and a few numerical anomalies that needed to be addressed. Finally, a ID humidity cell model was developed which considered four reactions in addition to the leaching and evaporative processes involved in humidity cell testing. Comparison of this model with experimental data was also good. These models will be used to understand the process of mine rock piles weathering, particularly for a selected mining site. The results will be used to develop a more comprehensive field model that will help predict the stability and the geochemical changes that will occur in a rock pile into the future. Better environmental management of these rock piles will occur with this increase in understanding of the mechanisms involved.