Water Journal : Water Journal February 2014
WATER FEBRUARY 2014 38 Feature Article Most mining operations require at least a nominal quantity of water with which to perform critical operations such as drilling, dust control and minimal ore processing. Many water uses are insensitive to water quality, merely requiring a nominal volume with which to perform essential operations. Other uses, typically mineral concentration based on otation, might dictate that certain minimum standards of quality be maintained to recover economic percentages of mineral values at suf cient grade to keep the mine pro table. Most mining operations reuse water to the largest extent possible, within constraints imposed by quality requirements, water availability and discharge considerations. Surplus water from precipitation or from the mine is discharged if it is not needed to operate the mine and associated crushing and grinding systems. Transport of mineral products over long distances through conveyance pipelines can cause water resources at the point of origin to become depleted and introduce contaminants into the water during conveyance that make the water undesirable at the nal destination. This can occur with coal, for example, with the leaching of common salts, boron, heavy metals, uoride and other undesirable constituents. Water that accompanies coal through long-haul pipelines is not normally returned to the point of origin to be reused for additional coal shipments because of the cost of constructing a second, parallel pipeline, and because contaminants leaching from the coal would accumulate after many cycles of reuse. Figure 2 shows a typical distribution of water use in mining. SOURCES OF WATER Access to a secure and stable water supply is critical to mining operations; without water a mine cannot operate. Water sources often need to be shared by multiple users, while at the same time leaving enough water for ecosystem functioning. Mines obtain water from a variety of sources, including direct harvesting from the environment (surface water and groundwater), water reused from other sources, on-site recycling and town water supplies, in line with approved water management plans. Mines often use water that is unsuitable for other purposes, such as deep saline groundwater or town sewage ef uent. This lower-quality water can be used directly for purposes such as dust suppression, or it can be treated to a higher quality. Sometimes mines experience a natural and continuous in ow of water, for example in the pit or underground tunnels, and this water needs to be removed (through pumping) so that access to the mine workings stays open. This is known as dewatering. This water is often either released into a receiving water source or used by the mine in the production processes. However, dewatering can lower groundwater tables or deplete surface water. Many mines recycle a signi cant amount of their water for reuse on-site, with some mines recycling up to 80 per cent of all water used. In other cases, mines source water from external ef uent streams, with some mining operations sourcing up to 50 per cent of their water from local ef uent. These practices reduce demand on water drawn from the environment. Most mines penetrate into water-producing formations or fracture systems during exploration or operation. Depending on the nature of the ore and the geochemical conditions of the formation, this groundwater might either be of good quality or be contaminated to the extent that treatment is needed before discharge. Mine water must be removed from operating mines to prevent ooding, the removal rate equalling the in ow rate. Except for cases in which the mine is elevated above the surrounding topography, mine water must be pumped to a treatment system or to a discharge point. Energy consumption can be signi cant, not only because of large volumes, but also because of appreciable lift from deep within the mine to the surface, often several thousand feet. If water is used in mining or in ore processing at a mine site, the mine water can be used for production. Some mines are water de cient, necessitating the import of water from offsite. ENVIRONMENTAL IMPACTS Successfully treating mining ef uents presents major challenges for water treatment companies, which are frequently faced with remote sites and extreme environmental conditions, signi cant uctuations in water quality, and a variety of contaminants. Each mine requires a tailored wastewater treatment system to ensure the treated ef uents (which can also be from tailing processes or mine dewatering) meet site-speci c conditions and the required quality to allow reuse of the water within the mine. Ensuring sustainability of the water supply is an important factor for any mining operation. Wastewater quality uctuates signi cantly from mine to mine. The impact of the mining industry on the environment has been a public concern, with growing appreciation of the natural environment and increasing awareness of the possible harmful effects that the industry's activities can cause. The extractive nature of mining operations creates a variety of impacts on the environment before, during and after mining operations. The extent and nature of impacts can range from minimal to signi cant, depending on a range of factors associated with each mine. These factors include: the characteristic of the ore body; the type of technology and extraction methods used in mining; and the on-site processing of minerals. The environmental impacts of mining include erosion, formation of sinkholes, loss of biodiversity, and contamination of soil, groundwater and surface water by chemicals from mining processes. Mining can deplete surface and groundwater supplies. The lowered pH and increased metal content may damage aquatic animals and vegetation, as well as humans and other organisms that drink from the streams or eat plants and animals that have bioaccumulated hazardous substances from the stream. Groundwater withdrawals may damage or destroy streamside habitat many kilometres from the actual mine site. Groundwater can be contaminated when there is a hydraulic connection between surface and groundwater, when there is mining below the water table, and when waters in ltrate through surface materials (including overlying wastes or other material) into the groundwater. Blasting, underground mine excavation and collapse, and exploration drilling can all create pathways for water seepage through mines into groundwater. Typical Water Use in Mining Processing 65% 1% 3% 9% 22% Dust Supression Mining Camp Use Rehabilitation, Care & Maintenance Exploration Figure. 2: Typical breakdown of water use in mining.
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