Geochemical trends in evaporative tailings ponds - an experimental study

摘要

Environmental best practices and practical considerations commonly require mines to store process water in large reservoirs such as tailings ponds. In many jurisdictions, mining companies need to demonstrate an understanding of the geochemical evolution and environmental impact of such reservoirs. The development of predictive mine drainage chemistry models is commonly complicated by the highly variable geochemical behaviour of the different dissolved constituents in contact with exposed mine materials. In dry environments, the geochemistry of tailings ponds may be driven by evapo-concentration where certain dissolved species become more strongly enriched than others. This discrepancy is especially evident in neutral environments where the concentrations of many species are solubility-controlled.To better understand the relative attenuation mechanisms occurring in an evaporative tailings pond, a small-scale (30L) laboratory pond was subjected to approximately 96% evaporation where seven water samples were collected and analyzed at different evaporation steps over a time period of 21 days. The experimental pond water was in contact with a layer of non-acid generating tailings to provide nucleation and adsorption sites. Measured and theoretical concentration factors for several species were calculated and compared at each evaporation step to put the brine evolution into context.The pH remained circum-neutral and increased slightly from 7.7 to 8.2 over the course of the experiment. Decreasing redox conditions were indicated by an increasing NO_2/NO_3 ratio. Other geochemical trends with degree of evaporation suggested that Ca and alkalinity were limited by calcite precipitation early on, while Ca and sulphate precipitated as gypsum during slightly more evolved stages of the test. Visible gypsum did in fact form on the pond surface, consistent with the timing of the geochemical signature. Other major cations (K, Na, Mg) behaved conservatively. The geochemical behaviour of dissolved trace metals and metalloids was variable. Base metals Cu, Co, N, and Zn were effectively scavenged by co-precipitation and/or adsorption onto secondary phases, most likely Fe-hydroxides. Metalloids that form oxy-anionic complexes were attenuated (As, Sb) or behaved conservatively (Mo, Se) becoming progressively enriched in the evolving brine. For Mo in particular, the dissolved geochemical load increased as the pond water became more brine-like. This is interpreted to be a result of desorption or other mobilizing processes releasing Mo from the material in response to the slight increase in pH. Comparison of the apparent mass loadings removed from solution with the elemental budget in the secondary precipitate indicates that, especially for strongly solubility-controlled species, ion exchange between tailings and water occurs to attain or approach geochemical equilibrium. These results give new insight into the relative solubilities of various species in neutral evaporative tailings ponds, highlighting the effect of minor changes in the ambient regime (e.g., pH, Eh) on the aqueous geochemical composition.