Modeling and Predicting of Drainage Quality for Sulfide Mine Tailings Impoundments

摘要

Oxidation of sulfide minerals in the sulfide tailings impoundments generates acidic conditions and high concentrations of dissolved heavy metals, sulfates and arsenic in the pore water. The pore water migrates away from the tailings to pose a potential long-term environmental threat to nearby water bodies, including surface water and groundwater systems, when infiltration of precipitation is unhindered, bottom liners are absent and no drainage collection is installed. So there is a great need to develop reliable modeling techniques for the characterization geochemical interactions taking place within the tailings and prediction of potential future environmental hazard which favor further prevention and remediation of the acidic mine drainage. In this paper, a comprehensive kinetic model for tailings-water interaction was established on the basis of considering the coupling and feedback among mony factors and processes such as sulfide oxidation, gangue dissolution, oxygen diffusion, water flow and mass transport, evolution of porosity and permeability of tailings induced by dissolution and precipitation of minerals. We employed the finite element and the updated Lagrangian approach to solve the dynamic equation sets and compile the modeling code. The Xiangxi gold mine(XXGM) located in Hunan province(a city of central-south university), a larger Au-Sb-W deposit with over 120 years' exploitation and over 3.8 million ton tailings, was taken as an example to illustrate the application of this model in the simulation and prediction of the quality of tailings effluents. Considering the impact of tailings impoundments with or without covering layers, we simulated the evolution of the concentrations of various contents (pH,SO_4~(2-),Pb,Zn, Cu,Cd,Sb and Au) in tailings pore water with time and depth. The numerical modeling results suggested that tailings-water interaction at the early stage lead to the acidification and release of heavy metals which are responsible for the persistent pollution to the environment for 30 years. Tailings pore water can be gradually neutralized and contents of polluted species tend to decline markedly with time. It is also shown that the gangue dissolution and organic matter reaction can stimulate the acid neutralization and decrease the diffusion rate of oxygen in tailings impoundments. The contents of pore-water species in upper tailings are significantly higher than in lower tailings and change severely at 6m depth. Therefore, the hydrological zoning phenomenon in tailings impoundments account for geochemical zonation. The fact that modeling results are in agreement with field data confirmed that this model has the potential to predict drainage quality for sulfide mine tailing impoundments.