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Mining Waste
The modern metal mining industry is essentially one of the largest waste management industries on the globe, with over 95% of the rock mass moved for the extraction of either base-metal or precious metal ores from either open-pit or underground mining operations being waste to the final metal production process. Thus large waste rock piles or tailings impoundments are produced to be managed, sometimes in perpetuity, by the engineering and environmental professionals associated with mining. Today, social acceptance of mining means comfort with well-managed minimal industrial impacts on ecosystem and human health.
In the ore beneficiation and/or enrichment and refining processes there are also secondary waste streams consisting primarily of fine grained rock and mineral concentrate materials. These fine grained wastes, or process tailings, are commonly enriched in sulphur bearing minerals such as pyrite, pyrrhotite and associated metal-rich minerals. Concentrations from modern Ontario mills such as Strathcona (Xstrata Nickel) and Clarabelle (CVRD Inco) may be at least as high as 0.8% nickel and 0.4% copper, representing a potential near-surface resource for extraction given the appropriate hydrometallurgical technologies. The image illustrates a series of engineered cells in the Strathcona Tailings Area, with the dark pyrrhotite-rich tailings in the centre of the image (source: Xstrata Nickel).
These mill tailings materials, both historical and present, are a potential major environmental legacy and liability to the industry if not properly managed because the sulphur-bearing minerals react with air and water to form acidity, known as acid mine drainage (AMD). The acidity in turn promotes the solubilization of heavy metal contaminants such as copper, zinc, nickel, cobalt and arsenic which commonly appear in non-compliant concentrations in water bodies downstream of both active and closed mining activities unless treatment or recovery plants are installed. The environmental impacts of acid mine drainage can be long-term. Even after a mine property has been abandoned, acid mine drainage flows continue for decades, or even for more than a century.
Canada's mining industry currently spends more than $100 million annually to control AMD. These costs cover a wide range of activities: the collection and treatment of acid drainage, the construction of engineered structures (impoundments) to contain mining by-products, the relocation of mining by-products to contaminant areas, and the rehabilitation of mine, mill and contaminant areas after operations cease. But the downside of these conventional methods is that they are expensive and require a long-term commitment. With an estimated 7 billion tonnes of tailings covering 44,000 hectares (MEND, 1994) the potential environmental liability facing the Canadian Minerals Industry was estimated, in a report to MEND, at between $1.8 and $2.9 billion dollars.
Water treatment for AMD management has always been only a cost item for the mine operator (documented in the many MEND reports). Acidic mine water and other feeds to treatments plants frequently contain significant metal values but these metals are almost always lost when converted to hydroxides and sent to sludge impoundments, with very few opportunities for recovery and recycle. To date there has been minimal attempt to recover the metals in the mine tailings impoundments either prior to, or post, deposition in the tailings management areas. There are more than 6000 abandoned mining or exploration, as well as active, sites in, for example, the province of Ontario. Modern mill technologies are still creating pyrrhotite waste streams containing in excess of 0.8% nickel, probably as either solid solution series or as minor pentlandite inclusions or intergrowths hosted within the iron sulphide mineral. This waste stream represents an easily accessible source of nickel given the appropriate hydrometallurgical extraction and purification processes.
