Building and characterizing low sulfide instrumented waste rock piles: Pile design and construction, particle size and sulfur characterization, and initial geochemical response

Abstract

A rigorous laboratory and field study to measure and compare low sulfide waste rock and drainage characteristics at various scales has been designed and implemented. The field study was constructed at the Diavik diamond mine in the Northwest Territories, Canada. Three well-instrumented, 15 m high test piles and three sets of 2 m scale experiments were constructed from run of mine waste rock. Diavik waste rock is comprised of granite and metasedimentary biotite schist country rock. The biotite schist contains the sulfide minerals, principally pyrrhotite. Diavik segregates waste rock based on sulfur content. One test pile contains waste rock with 0.035 wt. % S, within the operational sulfur target of < 0.04 wt. % S for lower sulfur waste rock designation. The second pile contains waste rock with 0.053 wt. % S, lower than the operational sulfur target of > 0.08 wt. % S for the higher sulfur waste rock designation. The third pile contains a core of 0.082 wt. % S waste rock which is within the operational sulfur target of > 0.08 wt. % S for the higher sulfur waste rock. The third pile has been re-contoured and capped by a 1.5 m of till and 3 m of lower sulfide waste rock as per the current reclamation plan for the higher sulfide waste rock pile. The test piles were built using standard end-dumping and push-dumping methods. Instrumentation was installed at the base of each pile and on four angle of repose tip faces, as well as in the covers of the third pile. Instrumentation was selected to measure matrix flow, pore water and bulk pile geochemistry, gas-phase oxygen and carbon dioxide concentrations, temperature evolution, microbiological populations, permeability to air, and thermal conductivity, and to resolve mass and flow balances. Instruments were designed to permit measurements at multiple scales. During pile construction samples of the < 50 mm fraction of waste rock were collected. The samples were analysed for sulfur content and particle size distribution. Particle size distributions for the lower and higher sulfur waste rock are similar but the higher sulfur waste rock has a higher proportion of fines. Particle size distributions for both waste rock types suggest the piles have rock-like characteristics rather than soil-like characteristics. Sulfur concentrations vary with the scale of measurement: concentrations of smaller size fractions are higher than larger size fractions. Acid-base accounting using standard methods and site-specific mineralogical information suggest that the waste rock is acid generating. However, when acid-base accounting is compared to effluent pH and alkalinity, the data suggest these calculations may be conservative. Drainage effluent from the higher sulfide test pile was measured for field parameters (pH, Eh, alkalinity) and dissolved cations, anions and nutrients. The geochemical equilibration model MINTEQA2 was used to interpret potential geochemical controls on solution chemistry. The pH decreases to < 5, concomitant with the minimum alkalinity of < 1 mg L-1 (as total CaCO3), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Sulfate concentrations reach 1995 mg L-1. Calculated saturation indices of Al (oxy)hydroxides and Al hydroxysulfate species, and pH suggest Al oxyhydroxide dissolution is buffering pH at times. Concentrations of Fe (< 0.37 mg L-1), Fe (II) and calculated saturation indices of Fe(III) (oxy)hydroxide species suggests that Fe is predominantly Fe(III) and Fe is being controlled by secondary mineral precipitation. The dissolved trace metals Mn (<19.2 mg L-1), Ni (<10.4 mg L-1), Co (<1.8 mg L-1), Zn (<0.9 mg L-1), Cd (<0.015 mg L-1) and Cu (<0.05 mg L-1) show increasing trends in the effluent water. No dissolved trace metals appear to have secondary mineral controls. Elevated SO4, Al, Fe dissolved metals Ni, Co, Zn, Cd and Cu, and depressed pH values suggest sulfide mineral oxidation is occurring in the test pile containing 0.053 wt. % S.