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Bailey B.L.,University of Waterloo | Smith L.J.D.,University of Waterloo | Smith L.J.D.,Rio Tinto Diavik Diamond Mines Inc. | Blowes D.W.,University of Waterloo | And 3 more authors.
Applied Geochemistry | Year: 2013

During mining operations, explosives are used to fragment rock into workable size fractions. Mine-water chemistry can be affected by blasting agent residuals, including NH3, NO2-, NO3-, Cl-, and ClO4-. At the Diavik diamond mine, Northwest Territories, Canada, waste rock generated from open-pit and underground mining is stockpiled on site. Three large-scale test piles measuring 60 by 50m at the base and 15m in height, along with four 2×2m lysimeters each 2m in height, were constructed at Diavik as part of a comprehensive research program to evaluate the quality of water emanating from waste rock stockpiles. Ongoing monitoring of the water chemistry since 2007 shows that blasting residuals comprise a large proportion of the dissolved constituents in the initial pore water and effluent. Leach tests conducted on freshly blasted rock from Diavik indicate the mass of N released corresponds to a 5.4% N loss from the blasting explosives; this mass is in the range for N loss reported for blasting operations at Diavik during the period when the test piles were constructed. The total mass of N released from the lysimeters was also within this range. The three large-scale test piles have only released a small fraction of the N estimated to be contained within them. Blasting of waste rock contributes SO42- to effluent through the oxidation of sulfide minerals in the rock during the blast. During the initial flush of water, the test pile that contained waste rock with the higher S content was observed to release higher concentrations of SO42- than the test pile with lower S content waste rock. Mass-balance calculations based on the ratios of SO42- to total N can be used to estimate the relative contributions of sulfide oxidation within the test piles and SO42- released when S in the host rock is oxidized during blasting. These calculations provide an estimate of S mass released during the first flush of the test piles. © 2012 Elsevier Ltd. Source


Smith L.J.D.,University of Waterloo | Smith L.J.D.,Rio Tinto Diavik Diamond Mines Inc. | Moncur M.C.,University of Waterloo | Moncur M.C.,Alberta Innovates Technology Futures | And 5 more authors.
Applied Geochemistry | Year: 2013

The physicochemical processes that affect acid mine drainage (AMD) in unsaturated waste rock piles and the capabilities of small-scale laboratory experiments to predict AMD from waste rock are not well understood. An integrated laboratory and field study to measure and compare low sulfide waste rock and drainage characteristics at various scales has been initiated. This paper describes the design, construction and instrumentation of three field-scale experimental waste rock piles (test piles), and six active zone lysimeters at the Diavik diamond mine in the Northwest Territories, Canada. The test piles are comprised of granitic and sulfide-bearing metasedimentary waste rock excavated during open pit mining operations. One test pile contains waste rock with a target S content of <0.04wt.% S; the second test pile contains waste rock with a target S content of >0.08wt.% S; and the third test pile contains the higher sulfide waste rock (>0.08wt.% S) and was re-sloped and capped with a low permeability till layer and a low sulfide waste rock cover. The first two test piles are approximately 15m high with bases of 50m by 60m, and the re-sloped test pile has a larger base of 80m by 125m. Instrumentation was selected to measure matrix flow, geochemistry of pore water and drainage, gas-phase O2 concentration, temperature evolution, microbiological populations, waste rock permeability to air, and thermal conductivity, as well as to resolve mass and flow balances. Instrument locations were selected to characterize coupled physicochemical processes at multiple scales and the evolution of those processes over time. Instruments were installed at a density such that the number of instruments that survived construction (40% to >80% by instrument type) was sufficient to allow adequate characterization of the physicochemical processes occurring at various scales in the test piles. © 2012 Elsevier Ltd. Source


Smith L.J.D.,University of Waterloo | Smith L.J.D.,Rio Tinto Diavik Diamond Mines Inc. | Blowes D.W.,University of Waterloo | Jambor J.L.,Leslie Research and Consulting | And 3 more authors.
Applied Geochemistry | Year: 2013

Three large-scale instrumented waste rock piles were constructed at the Diavik Diamond Mine in the Northwest Territories, Canada. These experimental waste rock piles (test piles) are 15. m high and are part of an integrated field and laboratory research program to characterize and compare low-sulfide waste rock and drainage at various scales. During test pile construction, samples of the <50. mm fraction of waste rock were collected from two types of waste rock that are segregated during mining operations based on S content. The samples were analyzed for S content and particle size distribution. One test pile contained waste rock with an average of 0.035. wt.% S in the <50. mm fraction, within the operational S target of <0.04. wt.% S for the lower S waste rock type. The second test pile contained waste rock with an average of 0.053. wt.% S in the <50. mm fraction, lower than the operational S target of >0.08. wt.% S for the higher S waste rock type. The third test pile has a low permeability till layer and a low sulfide waste rock thermal layer covering a core of waste rock with average 0.082. wt.% S in the <50. mm fraction, which is within the operational S target of >0.08. wt.% S for the higher S waste rock. Particle size distributions for the lower and higher S waste rock are similar, but the higher S waste rock has a higher proportion of fine-grained particles. Sulfur determinations for discrete particle sizes of the <50. mm fraction illustrate higher S concentrations in smaller particles for both the lower S waste rock and the higher S waste rock. Similarly, S concentrations calculated for the >10. m scale, from composite blast hole cuttings, are lower than those calculated for the <50. mm scale. Acid-base accounting using standard methods and site-specific mineralogical information was used to calculate the ratio of neutralization potential to acid generating potential. A comparison of calculation approaches to pH and alkalinity data from humidity cell and test pile effluent suggest that ratios are very sensitive to the calculation method. The preferred calculation method was selected by comparing calculation results to pH and alkalinity data from humidity cell effluent collected over 95. weeks and test pile effluent collected over five field seasons. The preferred acid-base accounting values were obtained by calculating the average neutralization potential divided by the average acid potential of a sample set. This approach indicates that waste rock with >0.05. wt.% S is of uncertain acid-generating potential and effluent data indicate this waste rock generates acidic effluent; whereas lower S waste rock does not produce acidic effluent, consistent with the acid-base accounting predictions. © 2013 Elsevier Ltd. Source


Neuner M.,University of British Columbia | Neuner M.,Golder Associates | Smith L.,University of British Columbia | Blowes D.W.,University of Waterloo | And 5 more authors.
Applied Geochemistry | Year: 2013

A field experiment is being carried out at the Diavik diamond mine in northern Canada to investigate the influence of unsaturated flow behavior on the quality of drainage from mine waste rock piles in a region of continuous permafrost. This paper is part of a series describing processes affecting the weathering of waste rock and transport of reaction products at this site; here the focus is on unsaturated water flow and its role in mass loading. Two 15m-high instrumented test piles have been built on 60m by 50m collection systems, each consisting of lysimeters and a large impermeable high-density polyethylene (HDPE) liner. Collection lysimeters are installed nearby to investigate infiltration in the upper 2m of the waste rock. Porosity, water retention curves, and hydraulic conductivity functions are estimated from field measurements and for samples ranging in size from 200cm3 to 16m3. Net infiltration in 2007 is estimated to have been 37% of the rainfall for mean annual rainfall conditions. Early-season infiltration freezes and is remobilized as the waste rock thaws. Wetting fronts migrate at rates of 0.2-0.4md-1 in response to common rainfall events and up to 5md-1 in response to intense rainfall. Pore water and non-reactive solutes travel at rates of <10-2 to 3×10-2md-1 in response to common rainfall events and up to 0.7md-1 in response to intense rainfall. Time-varying SO4 mass loading from the base of the test piles is dictated primarily by the flow behavior, rather than by changes in solute concentrations. © 2012 Elsevier Ltd. Source


Langman J.B.,University of Idaho | Langman J.B.,University of Waterloo | Blowes D.W.,University of Waterloo | Sinclair S.A.,University of Waterloo | And 9 more authors.
Journal of Geochemical Exploration | Year: 2015

Leachate from a humidity cell experiment provided a geochemical framework to evaluate the early evolution of weathering of the same waste rock in an Arctic environment. Comparison of laboratory and field results indicates the hydrogeochemical system within the higher sulfide (0.01-0.27wt.%S) waste rock pile has attained a peak weathering state, indicated by a shift to acid neutralization by weathering products of Al-bearing minerals, stabilization of the pH near 4.5, mobility of metals from sulfide and Al-bearing minerals, and a highly correlated relation between SO4 release and outflow from the waste rock pile. Further weathering of the waste rock should be driven by external environmental factors of temperature and precipitation/infiltration instead of internal factors of wetting and transient acid-neutralization processes. An evaluation of sulfide depletion indicates that 4% of the sulfide in the <6.3-mm fraction of this waste rock pile, corresponding to 2% of the sulfide in the <50-mm fraction, has been removed through oxidation and leaching as SO4. Weathering is strongly seasonal because of the Arctic climate, which produced a daily sulfide-depletion rate corresponding 0-0.02% and a peak annual depletion of 100kg or 1.5% of the remaining sulfide content in the <6.3-mm fraction. Peak sulfide weathering is expected to continue until about 15% of the available sulfide is depleted, similar to an observed decrease in sulfide weathering in the laboratory. Estimates of the reactive surface area of the sulfide minerals and a peak rate constant were used to evaluate the sulfide percent in the fine fraction of rock in the pile undergoing weathering during the annual freeze-thaw cycle, which can be used to estimate a climate rate factor to adjust the weathering mass through the seasonal changes. For modeling of future leachate, laboratory results indicate that an exponential rate limiting factor is necessary to account for slowing of sulfide oxidation after peak weathering because of the formation of secondary minerals that inhibit element release. © 2015 Elsevier B.V. Source

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