Unit 10: Energy Challenges // Section 13: Material Resources: Metals
Fuels are not the only resources that we mine from the earth. Modern society could not exist without numerous materials such as metals, rocks, and minerals that serve as raw ingredients for buildings, technologies, and industrial processes. In large part, industrialization is a process of refining and employing these materials in ever more sophisticated ways and of substituting machine power (which relies on metals and other advanced components) for human labor.
Metals are used widely in construction, manufacture, and consumer goods and are important material resources for industrial societies. Steel, which is a blend of iron and other compounds, is king of the metals, accounting for more than 90 per cent of global metal production. Many other metals, such as chromium and nickel, are mined primarily for blending with iron to form steels with desirable characteristics, such as corrosion resistance.
Metals are extracted from ores—naturally-occurring rocks and minerals that hold large deposits of high-quality, readily harvested metals. Ores form through many geological processes, including volcanism, underground water flow, and rainfall that alters soils. For example, iron ore deposits generally form when water mobilizes reduced iron (Fe2+) and the iron oxidizes, forming Fe3+, which is insoluble in water and precipitates. This process deposited vast quantities of iron ore on ancient seafloors, yielding what are called banded iron formations. Iron ores also form when underground water percolates through porous rocks such as sandstones, forming ironstones.
The first step in making steel is to extract iron from ore in a process called smelting, a chemical reduction process whose basic steps are several thousand years old. Pulverized ore (taconite) is heated by burning coke (a coal derivative) together with limestone to remove waste materials (Fig. 22). The process yields a mixture of iron and carbon, a residue called "slag," and some emissions. Steel, which is stronger than iron and has other superior qualities such as hardness, corrosion resistance, and electrical conductivity, is made by processing the molten iron-carbon mixture to remove excess carbon while adding other elements (for example, nickel, vanadium, or molybdenum) to obtain the desired properties.
Figure 22. Iron smelting, Carrie furnaces, Rankin, Pennsylvania, 1952
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Source: © Collection of William J. Gaughan, ais 94:3, Archives Service Center, Unitersity of Pittsburgh, Pittsburgh.
Perhaps the second most important metal today is aluminum, which is light, tough, and corrosion-resistant and has high electrical conductivity. Aluminum metal is used today in many manufactured goods, including cars and planes as well as smaller consumer goods.
The primary ore of aluminum is bauxite, which forms when high volumes of rain water move through soils. Typically the water dissolves and removes elements such as sodium, potassium, and calcium, leaving altered soils called laterites that contain significant amounts of highly insoluble metals such as aluminum. Laterites are widespread in tropical environments. To mine aluminum, developers strip off the topsoil and overburden to extract ore, which can require drilling and blasting. Much like coal strip mining, aluminum mining uproots vegetation, displaces wildlife, and pollutes area lakes and rivers.
Aluminum ore is smelted through a complicated process that involves extracting aluminum oxide, then passing high-voltage electricity through it to free the aluminum metal. The process is very energy-intensive: aluminum manufacturers are some of the largest industrial consumers of electricity worldwide and many are located in regions like the Pacific Northwest, where regional electricity prices are relatively low. Aluminum production also generates large quantities of greenhouse gases, although many major companies have formed partnerships with government to reduce these emissions.
Many other metals are important for specialized industrial and manufacturing purposes. For example, copper is used primarily as a conductor of electricity, while titanium is used as a lightweight metal alloy and for white paint pigments. Mining and smelting operations for many metals are similar to the processes involved in making iron. In some ores, such as copper, the metal is bound with sulfur, so mining and smelting these metals produces sulfuric acid and environmental impacts similar to those of acid rain (for more details, see Unit 11, "Atmospheric Pollution").
Sulfide mines and smelting operations often leave major environmental scars. For example, the Berkeley Pit, a former open-pit copper mine in Butte, Montana, is one of the most polluted water bodies in the United States (Fig. 23). The pit contains over 30 million gallons of water with a pH value of 2.5 (highly acidic) that has drained from mine tunnels and shafts feeding into it, and is laced with arsenic and heavy metals including aluminum, cadmium, copper, iron, lead, zinc, and sulfate. In 1995, 342 migrating snow geese died when they mistook the pit water for a safe resting place. Six companies have agreed to pay for cleaning up the pit at an estimated cost of $110 million (footnote 8).
Figure 23. Berkeley Pit, Montana
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Source: © Steve Peters.