Acid generation modelling : Equity Silver waste rock dumps Knapp, R. A.; Scharer, Jeno M.; Pettit, C. M.; Patterson, Robert J.; Nicholson, Ronald V.
The Equity Silver mine near Houston B.C. is proposing to cease operations in 1992. The large open pit operation has produced more than 80 million tonnes of waste rock which for the most part has been placed into surface dumps. The waste contains about 4 percent sulphide minerals and is a strong acid generator. Since 1981 Equity Silver Mines Limited (Equity) have been intercepting and treating seepage during which time there has been a steadily increasing load of acidity. Over the past several years Equity have developed their plans for decommissioning. The plan includes the control of acid release rates through the capping of the dumps with a low permeability cover. This plan was implemented over much of the dump in 1990/91. The preliminary indications is that the cover is functioning well. The decommissioning plan for Equity calls for perpetual treatment of seepage and runoff from the dumps. This requires substantial operating costs for labour, power, chemicals and maintenance. In order to provide assurance that these funds would be available over the long-term, Equity have been requested to post a bond. A Technical Committee was formed with representation from Equity and several government agencies to review bonding requirements. The committee met on 6 occasions and presented their best estimate of potential long-term costs and the net present value of a bond for several scenarios. The committees approach included empirical estimates of acid production for a series of assumptions. The concerns of the committee were that they did not have sound basis for determining peak acid production rates, the duration of peak rates and the decline in acid production over time or long-term acid production rates. As a result they applied a broad range to their assumptions. Equity was comfortable with the Technical Committee reports but wanted to confirm the data by having a site specific ARD (Acid Rock Drainage) model developed for their site. The model development was completed in 1991 and calibrated to the historical data. Model runs were completed for till, uncompacted clay and compacted clay covers. Model results confirm the shape of the ARD curve and suggest that the Technical Committee estimates were conservative but not unreasonable. The net present value of the bond based upon the ARD model results overlaps the committees estimates. This paper presents information regarding the general structure of the ARD model, estimates of ARD production by the model for the various cover scenarios, and compares the model results with several empirical estimates.
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