Changes in Gold Heap Leach Fluid Chemistry During Mine Closure
AuthorDover, Iain James
AdvisorMiller, Glenn C.
Natural Resources and Environmental Science
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Abstract:Since the 1800’s mining for gold and silver has been a major industry in the western United States. During this period, the mining industry has found that utilization of cyanide solutions is the most efficient method for selectively removing these metals from ore. In the last 40 years or so, a cost-effective method for processing low-grade gold and silver ore is through a heap leaching process, using cyanide applications to large piles of ore. Following months to years of leaching, the heaps and associated ponds that collect the water are reclaimed. However, it is not well understood how various chemical constituents observed in the heap effluent change as closure is completed. Even though the flow rates decrease over time, the contaminant load in the released water can be high, and will depend on the extent of rinsing; either by recirculation or by the flow of meteoric water (rain and snow). Over time, as meteoric water rinses the heap, the concentrations of the easily soluble contaminants will decrease. As closure continues, specific constituents, including cyanide and mercury, will also decrease. Nitrate concentrations will initially increase and then ultimately rinse out. Alternatively, the more tightly held constituents, particularly arsenic, will take longer to elute, and potentially can increase in concentration, depending on changes in pH. Understanding how the contaminant load changes over time will assist in proper management of these facilities. Using 26 years of data collected by the Nevada Division of Environmental Protection, changes in the drainage of a specific closed heap at the Toiyabe Mine in Nevada can be understood by recognizing the physical and chemical characteristics of this precious metals heap.