A STUDY OF THE INITIAL STAGES OF WEATHERING OF RED DOG MINE WASTE
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A major environmental issue in the mining industry is the acid rock drainage (ARD) produced when sulphidic minerals in the mine wastes are exposed to water and oxygen. The acidity of ARD can solubilize metals and metalloids which negatively impacts surrounding ecosystems, aquatic life, and human health. Pyrite is the most abundant metal sulphide and it is the main mineral that generates ARD from the Red Dog Mine wastes. The Red Dog Mine in Alaska, USA is an open-pit operation that extracts zinc and lead from a high-grade deposit. The high pyrite content of its waste rocks has produced a significant amount of ARD throughout the surrounding area. The main factors in the generation of acid from the Red Dog Mine wastes were investigated. The effect of particle size, wetting and drying cycles, elevated temperature, and type of water were determined using pH, solution potential, and electrical conductivity as initial indicators. It was found that elevated temperature, when combined with continuous shaking, was the most effective in expediting the onset of acid generation. The conditions for acid generation in terms of pH and Eh were determined. Shake flask oxidation tests were conducted to study the dissolution behaviours of lead from galena, zinc from sphalerite, and iron from pyrite during the initial stages of weathering of the Red Dog waste rock. Each metal behaved quite distinctly, with lead dissolving first, followed by zinc and then iron, although there was some overlap in behavior. Lead solubility was initially low and subsequently decreased over time which was attributed to the reduced solubility of lead at high sulphate concentrations. Sphalerite dissolved after galena and the zinc solubility continuously increased with time throughout the test. In the presence of soluble iron, the dissolution rate of sphalerite increased. The zinc concentration in the leachate exhibited a one-to-one correspondence with the sulphate content, indicating that the majority of the sulphate came from sphalerite. Iron was not detected in solution until about 40 days and was present at low concentrations. Experimental conditions and the theoretical model indicated the formation of ferrihydrite Fe(OH)3 at shorter weathering times (and hence low sulphate concentrations) and the formation of jarosite (KFe3+3(OH)6(SO4)2) at longer weathering times (and hence higher sulphate concentrations). The effectiveness of natural phosphate rock (NPR) addition for treatment of the ARD produced from the accelerated oxidation of the Red Dog Mine wastes was investigated using the same shake flask experimental conditions. Although it was found that an application rate of 20% or higher was sufficient to neutralize the ARD, more detailed analysis of the leachates and weathered waste rocks is needed to confirm if there were any reductions in metals leaching and the speciation of any secondary precipitates that may have formed.