Neutralisation of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests

This study constitutes a first stage of the elaboration of a concept for setting up capillary barriers composed of reactive tailings and alkaline industrial wastes. The aim of this barrier is (1) short-term mitigation by neutralising acid produced by Fe sulfides and (2) long-term mitigation by stabilisation of secondary products. The alkaline industrial wastes, cement kiln dust (CKD) and red mud bauxite (RMB), are used to produce neutral conditions. A series of time-controlled static leaching experiments were performed. Samples of reactive tailings were prepared containing 0, 2, 5, and 10% alkaline material. The water:solid ratios were also varied (3:1, 6:1, and 20:1) in order to characterise the geochemical balance between liquid and solid phases. The data are used to discern whether mineral solubility controls exist for particular ions. The pH results show that the use of 5% CKD, 10% CKD and 10% (CKD+RMB) allows neutral pH conditions to be maintained in the reactive layer. At these percentages, the concentrations of Al, Fe, Cu, Zn and SO4 in solution are significantly reduced compared with those obtained with the reactive tailings only. Aluminum concentrations are principally controlled by secondary phases like boehmite and gibbsite; Fe, by goethite and ferrihydrite; Cu and Zn, by hydroxides. Calcium and SO4 concentrations are controlled by precipitation of gypsum. The concentrations of Na and K are found not to be controlled by mineral solubility.