tailieunhanh - Encyclopedia of Global Resources part 62

Encyclopedia of Global Resources part 62 provides a wide variety of perspectives on both traditional and more recent views of Earth's resources. It serves as a bridge connecting the domains of resource exploitation, environmentalism, geology, and biology, and it explains their interrelationships in terms that students and other nonspecialists can understand. The articles in this set are extremely diverse, with articles covering soil, fisheries, forests, aluminum, the Industrial Revolution, the . Department of the Interior, the hydrologic cycle, glass, and placer mineral deposits. . | 558 Hall-Heroult process Global Resources process is a heavy consumer of electric power and most economical where power is inexpensive. John R. Phillips See also Aluminum Hall-Heroult process Oxides Oxygen Silicates. Hall-Heroult process Category Obtaining and using resources Aluminum is second only to iron as the most used metal. Although aluminum is the most common metal in the Earth s surface 8 percent by weight it is almost always combined with other elements. Aluminum is difficult to separate from the common ores of aluminum such as oxides and silicates. The Hall-Heroult process separates aluminumfrom bauxite ore. Thepro-cess is the only industrial source of aluminum. Definition The Hall-Heroult process is the process by which aluminum is separated from alumina A12O3 through electrolysis. The alumina is made from bauxite ore. The alumina is dissolved in a carbon-lined bath of molten cryolite Na3AlF6. Aluminum fluoride is added to reduce the melting point of cryolite. During electrolysis liquid aluminum is deposited at the cathode. Overview In 1886 Charles Martin Hall an American and Paul Heroult a Frenchman developed the process separately. The key ideas were that aluminum can be isolated by electrolysis and that a small amount of alumina could be dissolved in molten cryolite at a much lower temperature than the melting point of alumina. The energy saved allowed aluminum to be separated at an economical price. Carbon electrodes are used with a low voltage of 3 to 5 volts and a high amperage of up to 350 000 amps. Oxygen is produced at the anode and reacts with the electrode to produce carbon dioxide. The carbon dioxide is exhausted into the atmosphere after it is cleaned. Some hydrogen fluoride is also produced and is removed in a water bath before the carbon dioxide is exhausted. As the alumina is used up new alumina is added by breaking through the solid crust that develops on the surface. The electrolytic cell is lined with carbon but a layer of cryolite .