| The movement of contaminated pore water emanating from mine tailings impoundments depends on physical and geochemical processes, including groundwater infiltration, flow rate, sulfide oxidation and acid neutralization reactions. Therefore, an understanding of these processes is critical in prevention and remediation of acid mine drainage. The focus of this study was acid neutralization reactions and their effect on the release of metals contained in the mine tailings. Acid neutralization reactions were studied in the absence of sulfide oxidation using a fully-saturated laboratory column experiment. The column was packed with fresh unoxidized tailings collected at the Kidd Creek metallurgical plant, near Timmins, ON. Dilute sulfuric acid, representing acidic drainage generated in the unsaturated zone of tailings impoundments, was passed through the column continuously. The pH, Eh, alkalinity, major ions and metal concentrations in the column effluent water were measured as a function of time. The experimental results clearly show pH dependence of the metal release to pore water. The pH of water is buffered by a consistent series of minerals, present in the tailings. For the Kidd Creek tailings the series of minerals consists of ankerite, siderite, gibbsite, and chlorite. Elevated concentrations of metals such as Cd, Co, Cr, Ni, Pb, V and Zn were observed in the effluent water as pH decreased. The results of this study are consistent with the previously proposed conceptual model of acid neutralization reactions that has been developed on the basis of field data.; Natrojarosite residue, which is a waste from the zinc refinery, was co-disposed with mill tailings at Kidd Creek. A separate column experiment, using a mixture of tailings and natrojarosite, was conducted to evaluate the impact of this product on future aqueous discharge to the environment. The results were compared with the column experiment containing unaltered tailings. The results show that higher concentrations of metals such as Pb and Zn persist longer in the effluent water from the column containing natrojarosite. The concentrations of metals such as Al, Cd, Cr, Pb and V increase sooner in the effluent from the column containing natrojarosite due to a sooner decrease in pH of the effluent as a consequence of the dissolution of natrojarosite. The sooner decrease in effluent water pH from the natrojarosite column shows that natrojarosite dissolution is an acid-producing reaction under conditions prevailing in many tailings impoundments.; To quantify and discern the reactions occurring in the column that contained only tailings without natrojarosite, reactive transport modelling was conducted using the multicomponent reactive transport model MIN3P. The results of reactive transport modelling suggest that the pH of the effluent can be explained by a series of dissolution-precipitation reactions, involving ankerite-dolomite, siderite, chlorite and gibbsite. The amounts of minerals used in the simulations are consistent with mineralogical analyses. |
