English

Abstract

Acidic and Metalliferous Drainage (AMD) from sulfidic waste rock piles is a major issue facing the mining industry worldwide. Existing dry cover systems are designed to minimize the acid load discharged from waste rock piles by lowering infiltration rates. The strategic blending of alkaline amendments, such as limestone, within waste rock piles is also used to minimize the acid load discharged. The benefit of this blending is limited in most cases due to armoring (passivation) of limestone by precipitates. Armoring can be overcome by placing limestone on top of the waste rock piles, where it only interacts with rain water. Alkalinity released from the limestone-bearing covers can react with acid and metallic salts along preferential flow pathways within a waste rock pile to create inert, precipitate-coated channels that inhibit further reaction (ie. acid release). Due to the low solubility and slow dissolution rates of limestone, acid generation is not completely prevented and may still occur within the waste rock pile. Nevertheless, an overall reduction in the physical interaction between water and acid producing materials can sometimes be achieved using limestone-bearing covers. This mechanism has proven successful at the Freeport mine in Indonesia, which is characterized by very high rainfall. However, the effectiveness of limestone-bearing covers is limited under most climatic conditions, as a result of the low solubility and slow dissolution rates of limestone in near-neutral rainwater. New magnesium-based materials with superior solubility and dissolution rate characteristics to limestone have been developed for use in waste rock covers. These materials will permit the controlled release of alkalinity to infiltrating rainwater over a wide range of climatic conditions. The required amount of the magnesium-based alkalinity producing material in a waste rock cover will be insignificant relative to the amount of acid producing material in the waste rock. As a result, the magnesium-based cover materials represent an innovative and cost-effective solution to minimization, rather than treatment, of acid drainage from waste rock piles. The new materials being developed are based on calcium-enriched caustic magnesia (MgO) with controlled calcination grades and grain sizes to maximize both solubility and dissolution kinetics. Alkalinity concentrations of up to 510 mg/L CaCO3 equivalent in pure water can be achieved with the new materials (c.f. only 10-15 mg/L for limestone), with a typical saturation pH of 9.0-9.5 (c.f. only 8.0-8.5 for limestone). Compared to limestone it is envisaged that relatively small amounts of these new magnesium- based materials can be deployed within existing and new cover systems to minimize short, medium and long term acid discharges, thereby providing a significant advance in the control of AMD (acid and metalliferous drainage) from waste rock piles.