Zinc removal from hard, circum-neutral mine waters using a novel closed-bed limestone reactor

Zinc is an ecotoxic metal commonly associated with discharges from abandone d mines. Previous difficulties in removing zinc from circum-neutral mine wa ters with anaerobic wetlands have led to the development of a novel method to remove zinc from some mine waters of the North Pennine Orefield, UK. Geo chemical modelling indicates that zinc carbonate (smithsonite. ZnCO3) shoul d precipitate from these mine waters if pH can be raised from the ambient v alue of 7.5 to 8.2. It was postulated that this could be achieved through c losed-system reaction with calcite, in a simple gravity-flow, sealed bed of limestone gravel (an "anoxic limestone drain" (ALD)). Previous application s of ALD technology have been for strongly acidic mine waters, with the int ention of achieving wholesale alkalinity generation. In this new applicatio n, the ALD is meant to achieve a marginal increase-in pH to encourage remov al of zinc from the water by smithsonite precipitation. Testing of this con cept by laboratory and field experiments has yielded encouraging results. T he laboratory system gave an average of 50% reduction in zinc concentration over a 14 h retention period. The field system presently gives a 22% avera ge reduction in zinc concentration over a similar retention time. With furt her development, it is hoped that ALDs could become a cheap and attractive unit process for treating drainage from abandoned mines. particularly in si tuations where conventional treatment is not feasible on economic or practi cal grounds. (C) 2000 Elsevier Science Ltd. Aii rights reserved.