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Earth Materials & Resources 1.Indb A ALUMINUM DEPOSITS Aluminum is a ubiquitous metal that is critical to many industries. Because of the characteristics of aluminum ore, it was not practical to produce until the late nineteenth century. Aluminum production remains power-intensive, so most aluminum is recycled. PRINCIPAL TERMS with bauxite (discovered in 1821) as the primary source of aluminum. Bauxite is a rock composed of carbonate rock: a rock composed mainly of cal- several aluminum-rich minerals, such as gibbsite, cium carbonate boehmite, and diaspore, in a heterogeneous mix- electrolysis: process by which liquid or dissolved ture with various iron oxides and the clay mineral metals are separated by electromagnetic attraction kaolin. Kaolin itself is rich in aluminum, and it has heterogeneous mixture: a nonuniform mixture been considered as a potential source. However, ion: an atom with a net charge due to the addition given the abundance of bauxite, kaolin processing is or loss of electrons not of economic interest. karst: a region formed by the weathering of under- There are two primary forms of bauxite: lateritic lying rock, typically limestone or dolomite and karst. Lateritic bauxite is silicate based, whereas mineral: a naturally occurring solid with a specific karst bauxite tends to be carbonate. Karst bauxite chemical composition forms above carbonate rocks, such as limestone ore: chemical compounds of a desired material and dolomite, in a karst region. A karst region is a that are economically viable to exploit large area of carbonate rock formed during periods pressure vessel: a container designed to hold gases in Earth’s history called marine incursions, when a or liquids at a much higher pressure than the sur- shallow ocean covers a region that was formerly dry rounding pressure land. For the karst region to have formed, the ocean rock: a mixture of minerals must have been shallow. strip mining: a method of mining that occurs on Lateritic bauxites are formed atop silicates. the earth’s surface Laterite weathering is intense tropical weathering that produces a soil called laterite. Weathering, the Aluminum Ore process by which soil is formed, requires the bedrock Aluminum is an important metal because of its to be broken down by the conjunction of natural high strength, low density, and high corrosion resis- forces such as wind, rain, and seasonal temperature tance. It is used in items ranging from cans to air- fluctuations. frames. However, extensive use of aluminum by hu- Equally, chemistry plays a large role in the pro- mans is a new phenomenon. cess known as chemical weathering. Organisms play Historically, aluminum production has required a role as well, with many plants secreting chemicals more energy than could be provided. Because of the to break down rocks. Weathering is distinct from chemical characteristics of aluminum, the metal could erosion, which moves materials; weathering occurs not be produced on a large scale until the develop- in situ. ment of the Hall-Heroult process in 1886, in conjunc- Soil is produced in areas of wet weather and vig- tion with dependable and inexpensive electricity. orous plant life—namely, in the tropics. Tropical soil is rich in many important minerals and, particu- Bauxite Ore larly in the Amazon, is poor in nutrients. This type Aluminum is highly reactive and is seldom found of soil occurs throughout the tropics or areas that in its elemental state. As such it is found as ore, once were tropics. Thus, many tropical countries are 1 Aluminum Deposits Earth Materials and Resources good sources for bauxite mining. Bauxite is often settlement pond fail, the resultant slide of red mud found in large deposits near the surface that covers can destroy towns and pollute the environment. large areas; it also can be found in deposits deeper Once the solution cools, the dissolved aluminum underground. hydroxide precipitates out. When it is heated to 980 In addition to bauxite, other potential sources of degrees Celsius (1,800 degrees Fahrenheit), the alu- aluminum are kaolin clay, oil shale, anorthosite, and minum hydroxide decomposes to alumina, releasing coal. However, these sources are not economically vi- water vapor in the process. This process, invented in able, and given the widespread availability of bauxite, 1887 by Carl Josef Bayer and named for him, turns there is no economic incentive to make use of them. bauxite to aluminum oxide. Bayer had been working on supplying alumina to the textile industry, where Mining the alumina was used to fix colors in the cotton-dying Because most bauxite deposits are on the surface, process. (The role is called mordant.) Several pro- bauxite mining often takes the form of strip mining. cesses had already been invented for turning bauxite First, the land above the deposit is cleared of vegeta- to aluminum oxide, but the Bayer process was more tion. Explosives are then used to loosen the soil be- efficient. fore heavy equipment digs up the bauxite-rich soils. Alumina has been used in many industries. This method often makes use of bucket-wheel exca- Because it is chemically inert, it is often used as filler vators and chain-bucket excavators. in plastics and sunscreen. It also can be used as an Bucket-wheel and chain-bucket excavators are abrasive as a substitute for diamonds. Alumina also some of the largest machines made by humans. is used in many kinds of sandpaper. Additionally, While the specific mechanisms are different, each alumina can be used as a catalyst to help convert has a series of excavating buckets on a loop that pick hydrogen sulfide into sulfur. This helps clean emis- up the dirt and place it on a belt that leads to a cen- sions, including the emissions of factories. tral hub. Once there, the soil is placed into trucks or onto a belt that takes the bauxite to a refinery. These Hall-Heroult Processing machines are effective but can be expensive to repair. Alumina must be processed to make aluminum; Often, bulldozers and backhoes are used to the same this is done through the Hall-Heroult process, effect. Although efficient, such mining practices whereby aluminum is formed through electrolysis. disrupt the environment, particularly in tropical re- While most such processes can use water, the high gions. In the process, land must be cleared over the reactivity of aluminum makes the use of water im- active mine area. The mined area must then be re- possible. Thus, alumina must be dissolved in cryo- stored, or the land will face erosion. Even with soil lite (sodium hexafluoroaluminate), a light-colored restoration, the environment is damaged. rare mineral in which alumina is soluble. An interesting property of cryolite is that its refrac- Bayer Process tive index is close to that of water. This means it bends Bauxite is only 30 to 54 percent alumina. Once light the same way water does and thus would be near mined, bauxite ore is heated to about 150 to 200 de- invisible if put in a bowl of water. Cryolite is used as grees Celsius (302 to 392 degrees Fahrenheit) in a flux. A flux is a substance used to preserve purity and pressure vessel with a solution of sodium hydroxide. to clean and improve flow in the metal being worked The sodium hydroxide solution converts the bauxite with. When smelting, fluxes remove impurities. This into aluminum hydroxide, which then dissolves in is especially important with aluminum because of its the hydroxide solution. The rest of the bauxite does high reactivity. not dissolve, forming a waste product called red mud. Both alumina and cryolite have high melting points It is formed mostly of oxidized iron. of more than 1,000 degrees Celsius (1,832 degrees Little can be done with red mud, as it cannot be Fahrenheit). The resultant mixture is then electrolyzed, built upon and is highly basic. It must be dried in with the liquid aluminum being precipitated on the settlement ponds and stored in landfills. One of the cathode. A voltage of between 3 and 5 volts is required, main problems associated with bauxite refining is the with the rate of precipitation proportional to the cur- management of this red mud. Additionally, should a rent. The liquid aluminum is then siphoned off. 2 Earth Materials and Resources Aluminum Deposits The process releases carbon dioxide and hydrogen Places fluoride, which are vented. Hydrogen fluoride is neu- Geologists locate bauxite by prospecting for it. tralized to sodium fluoride. The process requires They take core samples of soils around the world in much electricity, both for deposition and for heating regions suitable for bauxite and examine them for purposes. As a result, most aluminum smelters are evidence of the ore. Through study of the samples, located near sources of relatively inexpensive elec- they can learn the quality and quantity of the bauxite tricity, such as hydroelectric stations. Because of the at the site. high energy requirements for producing aluminum, Global resources are estimated to be between 55 the metal was treated like gold before the develop- and 75 billion tons, sufficient for current demand ment of the Hall-Heroult process. for several centuries. Of these resources, 33 per- The Hall-Heroult process was discovered nearly cent is in Africa, 24 percent in Oceania, 22 percent simultaneously by two chemists, leading to the pro- in South America and the Caribbean, and 15 per- cess’s name. One chemist, Charles Martin Hall, in- cent in Asia. Jamaica has a particularly high con- vented the process in 1886. Heroult came up with centration, and reports suggest that there could the idea a few months later. Hall, however, patented be large deposits in Vietnam (of about 11,000 the process in 1889 and then founded the Pittsburgh megatons), which would makes these sites the Reduction Company, later to become the Aluminum largest bauxite reserves in the world.
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