Diavik Waste Rock Project: Oxygen Transport in Covered and Uncovered Piles

摘要

The rate of sulfide oxidation in waste rock dumps is often controlled by the transport of 0_2 into the piles. At Diavik Diamond Mines, 300 km north of Yellowkife, NWT,Canada, three 15-m high waste rock piles were constructed. Measurements of 0_2concentrations within the piles, and pore-gas pressures in and around one of the test piles,have been made to investigate rates of 0_2consumption and gas transport. The Type I andType III piles are distinguished by their sulfide contents; 0.028 ± 0.020 wt. %S and 0.052± 0.035 wt. %S, respectively. The other pile, referred to as the Covered Test Pile, has acore that is similar to the Type III pile (0.082 wt. %S) but it is covered by 1 m of lowpermeability till, intended to limit 0_2 and water transport, and 3 m of Type I thermalcover, designed to keep the till layer below the active freeze-thaw layer, thus keeping itpermanently frozen. In the Type III pile gas pressures have been measured using acontinuous data logging system over the summer seasons in 2007 and 2008. The systemmeasures pore-gas pressure at 49 locations within the pile and surface air pressure at 14locations around the pile at approximately 1 minute intervals. The measured pressure dataat individual sampling points show a correlation to wind speed and wind direction.Measured wind speed and wind direction are very erratic; however, the frequentmeasurements allow discernable trends in pressure gradients within the pile to beobserved. Observed pressure gradients reveal that complex spatial heterogeneity withinthe waste rock pile controls the magnitude and direction of air flow through the pile. Ineach of the three piles, 0_2concentrations have been measured at weekly to monthlytimescales at up to 500 sampling points within the pile. In the Type I and Type III piles,0_2is at the atmospheric concentration, indicating that gas transport does not limit the rateof oxidation in these test piles. At some locations in the Covered Test Pile, 0_2concentrations show a progressive depletion over the summer months followed by anincrease in CO_2concentrations. Rates of0_2depletion and concentration gradients wereused to calculate sulfide oxidation rates within the pile. The estimated rates range from1.7 x 10~(-11)to 4.1 x 10~(-kg0_2m~(-3)s~(-1),which is low compared other waste rock pileswith higher sulfide content, but is comparable to rates derived from humidity cellexperiments on the Diavik Type III rock.