Sulfide Mineral Coating Process to Control Acid Rock Drainage

Uncontrolled oxidation of sulfide minerals in mine wastes causes acid rock drainage (ARD) that contaminates receiving waters with acid and heavy metals. ARD is an enormous environmental problem, and the most significant environmental issue faced by the mining industry. Effectively dealing with ARD has been and continues to be a formidable challenge for which no global solutions exist. Existing technology for controlling ARD at the source by stopping oxidation of sulfide minerals still is rudimentary and aims to stop either chemical oxidation or biological oxidation, but not both. Both of these chemical and biological processes are important in the formation of ARD. Consequently, both processes must be controlled for a comprehensive solution to the problem. The overall goal of this Phase II research project is to develop a comprehensive solution to the ARD problem by rendering sulfide rock nonreactive to chemical and biological oxidation. The approach involves a combined treatment involving chemical coating and the use of thiocyanate, which is highly and selectively inhibitory to acidophilic microorganisms that catalyze ARD formation. Furthermore, the control of microbial activity may prolong the longevity of chemical coatings that are subject to failure under the severe ARD conditions caused by biooxidation. The specific goals of this Phase II project are to: (1) determine quantitatively the effectiveness of the combined treatment in preventing ARD from developing when applied to fresh waste rock or tailings; (2) determine the effectiveness of the combined treatment in controlling an existing ARD situation; (3) estimate the cost for this ARD treatment; (4) develop an engineering plan for application of the treatment at mine sites; (5) obtain preliminary information on the feasibility of producing a combined controlled-release product; and (6) determine if consolidated phosphate clay waste may be used as a source of phosphate rock. Phase II laboratory tests will be conducted with sulfidic rock from collaborating mining companies that are prepared to conduct field trials if the results are promising. Phase I results were encouraging, showing that the combined treatment could be highly effective both in preventing ARD and in controlling existing ARD. Phase I research and development involved laboratory-accelerated weathering tests employing sulfidic rock from a gold mine in Nevada. Preliminary estimates indicated that the combined treatment would be less costly than existing technology for ARD prevention and control. Furthermore, even where complete control was not obtained, the improvement in water quality with the combined treatment would result in a substantial reduction in capital and operating costs for conventional water treatment, and far less sludge would be produced. The combined treatment approach has the potential to greatly reduce the costs and environmental impacts from ARD. It has wide applicability to mining situations, including application to tailings and waste rock to prevent or treat ARD. It could be used by mining companies for ARD control during or following mining. The component chemicals of the combined treatment are readily available and inexpensive. In some cases, the components already are available at mining sites in process waste streams. Nonetheless, there also may be the opportunity to develop a combined controlled-release product.