The Diavik waste rock project: Initial geochemical response from a low sulfide waste rock pile |
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| Institution: | 1. Energy, Mining and Environment Research Center, National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC V6T 1W5, Canada;2. Minesite Drainage Assessment Group, Surrey, BC, Canada;3. Equity Silver Mine, Reclamation Operations, Goldcorp Inc., PO Box 1450, Houston, BC V0J 1Z0, Canada |
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| Abstract: | Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053 wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1 year. Sulfate concentrations reached 2000 mg L?1 when ambient temperatures were >10 °C, and decreased as ambient temperatures declined to <0 °C. The pH decreased to <5, concomitant with an alkalinity minimum of <1 mg L?1 (as total CaCO3), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were <0.36 and <0.11 mg L?1, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13 mg L?1), Ni (<7.0 mg L?1), Co (<1.5 mg L?1), Zn (<0.5 mg L?1), Cd (<0.008 mg L?1) and Cu (<0.05 mg L?1) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite Fe1?xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite CuFeS2] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO4, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock. |
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