After benefi ciation, concentrated ore is processed to produce pure elemental metals and alloys. A variety of metallurgical processes specific to an element are involved, so their environmental profiles vary as well. Iron ore and aluminium ore (known as bauxite) are oxides, and a large energy input is required to recover the pure metal.
In the case of iron, ore is first converted in blast furnaces to pig iron, then into finished steel. The most typical reducer in blast furnaces is coke from coal. Although new pollution control and energy efficiency investments have improved the environmental performance of the coking process, coke production is the dirtiest process in the ferrous metal sector (Ayres 1997), at least in
When exposed to moisture and oxygen, tailings, ore, and wastes that contain sulfur or sulfide can generate acid through bacterial oxidation. This acidity is not only hazardous to fauna and flora but may also cause heavy metal contamination. The formation of acid drainage and resultant contaminants have been described as the largest environmental problem caused by the mining industry in the United States (USEPA 2000).
some countries. The iron and steel sector is one of the major industrial emitters of CO2, and carbon accounting in coke ovens and blast furnaces is important in greenhouse gas emission inventories. The recovery of by-products in steelmaking, such as blast furnace slag, minimizes solid waste generation efficiently; slag can then be used in cement production.
In the case of aluminium, bauxite is fi rst converted into dehydrated alumina (Al2O3). This process generates large amounts of caustic waste called "red mud." Next, alumina is reduced to pure aluminium metal through electrolysis (Ayres 1997). This smelting process requires huge inputs of electricity. Different sources of electricity are used depending on the region. In Latin America, hydroelectricity is the dominant source, whereas coal-fired electricity is used predominantly in Australia. Therefore, embodied CO2 emissions in primary aluminium differ considerably from one region to another.
The content of metals other than iron and aluminium in their ores are much smaller than iron ore and bauxite, even after beneficiation. Prior to the actual smelting stage, most sulfi de ore concentrates are first converted to oxides to drive off the sulfur as SO2. Historically, this proved to be one of the dirtiest of all industrial processes (Ayres 1997), but most facilities now capture the SO2 and reuse it as sulfuric acid.
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