Pyrite behaviour in a tailings pond

This paper deals with the behaviour in a tailings pond of the residues produced during the flotation of a complex sulphide ore. The composition of the tailing was essentially pyrite (63.4%) with minor amounts of Cu, Pb and Zn sulphides (1.4, 0.5 and 0.8, respectively). In addition, sulphite ion and other thiosalts were present in the liquid phase. These reducing products were added at the flotation plant in order to prevent undesirable oxidations. The presence of these reducing products controlled the redox potential of the system, inhibiting oxidation reactions. A laboratory column was used to simulate the evolution of the tailings in the pond. Distilled water was added at the top of the column; the water flowed across the tailings and finally was collected at the bottom for its chemical and microbiological analysis. The actual situation in the industrial site was reproduced by a model consisting of tow columns: one (column S) was the part of the ore under water, and the other (column H-S) was the part of the ore subject to humidity and dryness cycles. These two tests reproduced the system very well: the submerged pyrite did not produce acidic water but the ore exposed to humidity and dryness cycles generated drainages with pH near to 2. The metal and sulphate concentrations were consistent with this. The influence of two other factors was also studied: bacterial activity and the height of solid residue inside the columns. The tests showed that the presence of active bacteria intensified the weathering of the residue and that, when the height of the ore in the column decreased, solid weathering also intensified. Finally, a third factor taken into account was the presence of products with reducing activity associated with the ore as a consequence of the flotation process. This combined action affected the behaviour of the residue, which was also reproduced successfully with the assayed laboratory columns. (C) 2004 Published by Elsevier B.V.