5.5 Mineral resources: formation, mining, environmental impact  (Page 3/43)

Mining and processing ore

There are two kinds of mineral mines, surface mines and underground mines . The kind of mine used depends on the quality of the ore, i.e., concentration of mineral and its distance from the surface. Surface mines include open-pit mines , which commonly involve large holes that extract relatively low-grade metallic ore (see Figure Open Pit Mine ), strip mines , which extract horizontal layers of ore or rock, and placer mines , where gold or diamonds are extracted from river and beach sediment by scooping up (dredging) the sediment and then separating the ore by density. Large, open-pit mines can create huge piles of rock (called overburden) that was removed to expose the ore as well as huge piles of ore for processing. Underground mines, which are used when relatively high-grade ore is too deep for surface mining, involve a network of tunnels to access and extract the ore. Processing metallic ore (e.g., gold, silver, iron, copper, zinc, nickel, and lead) can involve numerous steps including crushing, grinding with water, physically separating the ore minerals from non-ore minerals often by density, and chemically separating the metal from the ore minerals using methods such as smelting    (heating the ore minerals with different chemicals to extract the metal) and leaching    (using chemicals to dissolve the metal from a large volume of crushed rock). The fine-grained waste produced from processing ore is called tailings    . Slag is the glassy unwanted by-product of smelting ore. Many of the nonmetallic minerals and rocks do not require chemical separation techniques.

Mineral resources and sustainability issues

Our heavy dependence on mineral resources presents humanity with some difficult challenges related to sustainability, including how to cope with finite supplies and how to mitigate the enormous environmental impacts of mining and processing ore. As global population growth continues—and perhaps more importantly, as standards of living rise around the world—demand for products made from minerals will increase. In particular, the economies of China, India, Brazil, and a few other countries are growing very quickly, and their demand for critical mineral resources also is accelerating. That means we are depleting our known mineral deposits at an increasing rate, requiring that new deposits be found and put into production. Figure Demand for Nonfuel Minerals Materials shows the large increase in US mineral consumption between 1900 and 2006. Considering that mineral resources are nonrenewable, it is reasonable to ask how long they will last. The Table Strategic Minerals gives a greatly approximated answer to that question for a variety of important and strategic minerals based on the current production and the estimated mineral reserves    . Based on this simplified analysis, the estimated life of these important mineral reserves varies from more than 800 to 20 years. It is important to realize that we will not completely run out of any of these minerals but rather the economically viable mineral deposits will be used up. Additional complications arise if only a few countries produce the mineral and they decide not to export it. This situation is looming for rare earth elements, which currently are produced mainly by China, which is threatening to limit exports of these strategic minerals.