Background

Although a number of variations exist, sustainable development is most commonly defined as development that meets the needs of the present without compromising the ability of future generations to meet their own needs². [1] The principles of sustainable development involve integrating economic activity with environmental integrity, social concerns, and effective government systems. [2] These principles have had a growing influence on the development of environmental and social policy in recent decades, and have been adopted and promoted by a number of international organizations, including the United Nations and World Bank. [1]

A number of industry associations, including the International Council on Mining and Metals, and the Mining Association of Canada, have endorsed the principles of sustainable development and have developed their own guidelines to promote sustainable practices among their member organizations.

Environmentally sustainable mining

The continued discoveries of new oil, coal, and mineral reserves, improved recycling of materials, and advances in technology in recent years have largely lessened the fears of running out of non-renewable resources. [3] For instance, the development of technologies including froth floatation for processing certain metal sulphide ores, the Solvent Extraction-Electrowinning process for obtaining copper, and the use of cyanide in gold extraction have all made previously uneconomic grades of ore suitable for mining, thus increasing economically viable mineral reserves. [4]

However, although mining itself may occur on a relatively small land area, the associated infrastructure and pollution from mining activities have the potential to affect the health of ecosystems and reduce their ability to provide the goods and services necessary for human and environmental well-being. [3] These services include the purification of air and water and the decomposition of waste materials, which can be compromised where the ecosystems are overwhelmed by high levels of pollution. The importance of a healthy environment to future generations is recognized as a “pillar” of sustainable development.

In order to be more environmentally sustainable, mining operations are increasingly conducted in a manner that minimizes their impact on the surrounding environment, and leaves mine sites in an acceptable state for re-use by people or ecosystems. A number of management strategies and technologies are being developed and used by the mining industry to reduce the environmental impacts of mining, and are discussed below.

Reducing inputs

Water

Water is used in a number of applications at mine sites. By diverting surface water and pumping groundwater, mining operations can reduce both the quantity and quality of water available downstream for aquatic ecosystems and other industrial and municipal water users, especially in areas with arid climates.

In response to water scarcity in many mining regions, a number of innovative water conservation practices are being developed and implemented to reduce water use. In Canada, mining has one of the highest water recycling rates among the industrial sectors, and between 1996 and 2005 reduced its total water intake by 33%. [5] This drop in water use occurred at the same time as the value of production increased 48%, meaning the water intake per dollar of production (or water-use intensity) has also declined.

Energy

Mining and metal processing can be very energy-intensive processes. For instance, diesel fuel is used by trucks and excavators during mining, electricity is used to grind ore and refine copper and aluminum, and coal is required in order to smelt iron ore and make steel. [1] The extraction of fossil fuels (coal, oil, and gas), and the construction of infrastructure required for energy generation have their own environmental impacts, including the production of greenhouse gases and increased risk of environmental contamination along the energy supply route. Reducing energy consumption at mines can reduce greenhouse gas emissions and extend the life of fossil fuel reserves in addition to reducing operating costs and therefore the cost of the commodity being mined. [1]

Some examples of ways mining companies are reducing their energy consumption include Alcoa’s RopeCon transport system at its Jamalco Operations in Jamaica, which generates electricity while transporting bauxite ore downhill from the mine to the rail station, and Kennecott Utah Copper’s haul truck idle management project, which was recognized by the Utah Pollution Prevention Association with an Outstanding Achievement in Pollution Prevention Award in 2010. Mining companies are also investigating renewable energy sources to reduce costs and reliance on external energy sources including solar power in Chile and wind turbines at the Diavik Diamond Mine in the Northwest Territories.

Land disruption

Mine sites currently disturb a small fraction of the Earth’s total land surface. For instance, less than 0.01% of Canada’s land area has been used for the production of minerals and metals since mining began over 100 years ago. [6] However, mining activities use land at every stage of the mine cycle, including exploration, construction, operation, closure, and post-closure. [7] Vegetation is cleared for the construction of buildings, roads, and powerlines, open pits or tunnels are dug to gain access to the ore, and waste storage facilities such as tailings ponds are expanded over the life of the mine, potentially leading to habitat loss and deforestation. [7]

There are a number of ways to reduce the land-use impacts of mining. [7] These include reducing the overall footprint of the mining area, minimizing the amount of waste produced and stored, maintaining biodiversity by transplanting or culturing any endangered plants found on site, and planning mines around existing infrastructure where possible. [7] Although current technology requires ores to be excavated in order to produce metals, research in areas such as biomining offers the possibility of mining with minimal land disruption in the future.

Reducing outputs

Waste

Mine waste includes solid waste, mine water, and air particles, which can vary significantly in their composition and potential for environmental contamination. In addition to preventing soil, water, and air pollution, waste management plans are required in order to select and design appropriate storage facilities for the large volumes of waste produced at most mine sites.

It is generally recognized that preventing pollution is more economic and effective at reducing environmental impacts than cleaning it up later on (i.e., leaving a legacy of environmental degradation for future generations). [4] Methods for minimizing and eliminating wastes in the production of minerals and metal commodities include [1]:

• Using cleaner production techniques
• Environmental control technologies
• Using waste as raw material, and
• Reducing the amount of waste produced through process re-engineering.

Water management strategies are used to reduce the volume of waste water produced, and if necessary, to treat it to an acceptable quality before it is released. Over the past 30 years, most countries have passed formal environmental legislation describing acceptable standards of human impacts to air, water, and land. As a result, mine waste management plans are increasingly required in order to obtain a mining permit in many parts of the world. [4, 8]

Acid rock drainage

Acid rock drainage (ARD) forms when sulphide minerals in waste rock and ore at a mine site are exposed to air and water. ARD can pollute surface and groundwater with acidity and dissolved metals, which can adversely affect aquatic organisms and water users downstream. A number of strategies are used to predict, prevent, and mitigate ARD at mine sites, and are described in further detail here.

Restoring environmental function at mine sites

Mining is a relatively temporary activity, and mine sites have finite operating lives which are determined by the size and quality of the ore deposit being mined. Mine site reclamation and closure activities aim to restore land disturbed by mining activities to an acceptable state for re-use by people or ecosystems.

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Mitigating impacts

At many sites, the key reclamation, soil treatment, and water quality concerns owe their origin to the same process — the oxidation of sulfide minerals, especially the iron sulfide, pyrite. Oxidation of sulfide minerals can produce acidic conditions that release metals in both waste materials and water.

Mining in the early days took place at a time when environmental impacts were not as well understood and, most importantly, not a matter of significant concern. As a result, historical mine sites may still have areas that are not reclaimed, remnants of facilities, and untreated water. This inherited legacy of environmental damage from mining is not indicative of the mining cycle today.

Now, mine closure and a number of activities to mitigate the impacts of mining are an integral part of all metal mine planning and mineral development from the discovery phase through to closure:

• Reclamation
• Soil treatment
• Water treatment
• Preventing acid rock drainage
• Controlling gas emissions

Reclamation

Reclamation entails the re-establishing of viable soils and vegetation at a mine site. Although regulatory agencies may require complex reclamation designs, simple approaches can be very effective. One simple approach depends on adding lime or other materials that will neutralize acidity plus a cover of top soil or suitable growth medium to promote vegetation growth. Modifying slopes and other surfaces and planting vegetation as part of the process stabilizes the soil material and prevents erosion and surface water infiltration. Even this simple approach is likely to cost a few thousand dollars per acre to implement. Where soils have a sustained high acidity, the costs of using this approach can increase, sometimes to tens of thousands of dollars per acre. The challenge to find cost-effective reclamation approaches continues.

Promising reclamation options in the future may include using sludge,  “biosolids,” from municipal waste water treatment processes as an organic soil amendment, and growing plant species that are more tolerant of acidic conditions.

Soil Treatment

High levels of metals in soils, not just acidity, can be harmful to plants, animals, and, in some cases, people. A common approach used in dealing with contaminated soil is to move it to specially designed repositories. This approach can be very expensive and controversial, but it is sometimes required. With this approach, the volume and toxicity of the soil is not reduced, the soil is just relocated. Effective soil treatment approaches in the future depend upon better understanding of the risks associated with metals in mine wastes. These “natural” metals in minerals may not be as readily available in the biosphere, and therefore, they may not be as toxic as the metals in processed forms, such as lead in gasoline.

Future approaches may include:

• Using chemical methods to stabilize metals in soils, making them less mobile and biologically available.
• Using bacteriacides that stop the bacterial growth that promotes the oxidation of pyrite and the accompanying formation of sulfuric acid.
• Using bioliners, such as low permeability and compacted manure, as barriers at the base of waste piles.
• Permanently flooding waste materials containing pyrite to cut off the source of oxygen, stop the development of acidic conditions, and prevent mobilization of metals.

Water Treatment

The most common treatment for acidic and metal-bearing waters is the addition of a neutralizing material, such as lime, to reduce the acidity. This “active” treatment process, which causes the dissolved metals to precipitate from the water, usually requires the construction of a treatment facility. The ongoing maintenance that such a plant requires makes this treatment technique very expensive.

Aside from the expense, some active treatment plants generate large amounts of sludge. Disposal of the sludge is a major problem. Because of the cost and the physical challenges of dealing with sludge, alternatives to active treatment facilities are needed. Some possible alternatives include:

• Using “passive” wetland systems to treat metal-bearing water. This approach has been successfully used where the volumes and acidity of the water are not too great. Passive wetland systems have the added advantage of creating desirable wildlife habitat.
• Using in-situ treatment zones where reactive materials or electric currents are placed in the subsurface so that water passing through them would be treated.
• Combining treatment with the recovery of useful materials from contaminated water.

Preventing Acid Rock Drainage

Although the discharge of acidic drainage presents several challenges to protecting water quality, the significance and widespread occurrence of acid rock drainage warrant special efforts to prevent or minimize its occurrence. Prevention must be addressed during exploration activities, before the beginning of newly-planned mining operations. In some cases, it may even be possible to prevent or reduce acid rock drainage in old or abandoned mining areas. Current and potential treatment approaches for acid rock drainage are similar to those already described. Possible measures to prevent or significantly reduce acid rock drainage include:

• Flooding of old underground mine workings to cut off the oxygen supply necessary to the sustained generation of acidic waters.
• Sealing exposed surfaces in underground workings with a coating of material that is non-reactive or impermeable to inhibit the oxidation process.
• Backfilling mine workings with reactive materials that can neutralize and treat waters that pass through them.
• Adding chemicals to the water in flooded surface and underground mine workings that can inhibit acid-generating chemical reactions and precipitate coatings that will seal off groundwater migration routes.
• Isolating contaminated waters at depth by stratification, allowing viable habitat to develop near the surface in the water that fills large open pits.

Controlling Smelter Emissions

Smelter emissions, especially sulfur dioxide and particulate materials, have historically presented significant environmental problems. Modern smelting technology has met this challenge by drastically reducing the amount of emissions. An example is the modernized smelter built by Kennecott Utah Copper that processes ore concentrates from the Bingham Canyon Mine near Salt Lake City. Using technology developed by the Finnish company Outokumpu, this smelter has reduced sulfur dioxide emissions to 95 percent of previous permitted levels. This smelter, which came online in 1995, is the cleanest in the world. It captures 99.9 percent of the emitted sulfur.

Article Source: http://www.americangeosciences.org/