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 abandoned mines. Previous difficulties in removing zinc from circum-neutral mine waters 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. Geochemical modelling indicates that zinc carbonate (smithsonite. ZnCO3) should precipitate from these mine waters if pH can be raised from the ambient value of 7.5 to 8.2. It was postulated that this could be achieved through closed-system reaction with calcite, in a simple gravity-flow, sealed bed of limestone gravel (an "anoxic limestone drain" (ALD)). Previous applications of ALD technology have been for strongly acidic mine waters, with the intention of achieving wholesale alkalinity generation. In this new application, the ALD is meant to achieve a marginal increase-in pH to encourage removal of zinc from the water by smithsonite precipitation. Testing of this concept by laboratory and field experiments has yielded encouraging results. The laboratory system gave an average of 50% reduction in zinc concentration over a 14 h retention period. The field system presently gives a 22% average reduction in zinc concentration over a similar retention time. With further development, it is hoped that ALDs could become a cheap and attractive unit process for treating drainage from abandoned mines. particularly in situations where conventional treatment is not feasible on economic or practical grounds. (C) 2000 Elsevier Science Ltd. Aii rights reserved.