DEMONSTRATION STUDY: TREATMENT OF HEAP LEACH PAD DRAIN DOWN SOLUTION: PROOF-OF-PRINCIPLE BENCH STUDY 1

A 20-week bench-scale study was conducted by Golder Associates to determine whether passive treatment could effectively remove weak and dissociable (WAD) cyanide and NO3 from the drain down solution emanating from a decommissioned gold heap leach pad. An anaerobic treatment scenario was chosen and three different substrates were chosen. Limestone at 10% by weight was in all three reactors, two of the reactors had between 80 and 90% hay/straw, and a third reactor had 45% hay/straw and 45 % potato mash. These are local agricultural products and by-products. Because the historic concentration of nitrate-N ranged from 142 to 297 mg/L, the flow rate into the reactors was based on the hydraulic retention time needed to remove nitrate-N down to 10 mg/L, and this was determined to be 20 days. All three substrates were effective at removing WAD cyanide and nitrate-N to below 0.04 mg/L and 1.0 mg/L, respectively. In the effluents, other constituents that could be the products of cyanide and nitrate-N degradation were analyzed and only ammonia- N was detected at concentrations from 5 to 10 mg/L. The most recent regulatory reference value has been set at 10 mg/L N for the total N in all nitrogen species. Under this monitoring requirement, the sum of nitrate-N and ammonia-N would be below the 10 mg/L N reference value.