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Sample entry with codes <document> <source><bibcitation><bibcit.fielded><title>How
Products Are <citation><print><volume>7</volume> <publisher>Gale</publisher> <pubdate><year year=2002></pubdate> <source><byline><composed.name>Loretta Hall</composed.name></byline></source> <doc.body> <para>The metallic element aluminum is the third most plentiful element in the earth's crust, comprising 8% of the planet's soil and rocks (oxygen and silicon make up 47% and 28%, respectively). In nature, aluminum is found only in chemical compounds with other elements such as sulphur, silicon, and oxygen. Pure, metallic aluminum can be economically produced only from aluminum oxide ore.</para> <para>Metallic aluminum has many properties that make it useful in a wide range of applications. It is lightweight, strong, nonmagnetic, and nontoxic. It conducts heat and electricity and reflects heat and light. It is strong but easily workable, and it retains its strength under extreme cold without becoming brittle. The surface of aluminum quickly oxidizes to form an invisible barrier to corrosion. Furthermore, aluminum can easily and economically be recycled into new products.</para> <head>Background </head> <para>In 1886, two 22-year-old scientists independently developed a smelting process that made economical mass production of aluminum possible. Known as the Hall-Heroult process after its American and French inventors, the process is still the primary method of aluminum production today. The Bayer process for refining aluminum ore, developed in 1888 by an Austrian chemist, also contributed significantly to the economical mass production of aluminum.</para> <para>In 1884, 125 lb (60 kg) of aluminum was produced in the United States, and it sold for about the same unit price as silver. In 1995, U.S. plants produced 7.8 billion lb (3.6 million metric tons) of aluminum, and the price of silver was seventy-five times as much as the price of aluminum.</para> <head>Raw Materials</head> <para>Cryolite, a chemical compound composed of sodium, aluminum, and fluorine, is used as the electrolyte (current-conducting medium) in the smelting operation. Naturally occurring cryolite was once mined in Greenland, but the compound is now produced synthetically for use in the production of aluminum. Aluminum fluoride is added to lower the melting point of the electrolyte solution.</para> <para>The other major ingredient used in the smelting operation
is carbon. Carbon electrodes transmit the electric current through the
electrolyte. During the smelting operation, some of the carbon is consumed
as it combines with oxygen to form carbon dioxide. In fact, about half
a pound (0.2 kg) of carbon is used for every pound (2.2 kg) of aluminum
produced. Some of the carbon used in aluminum smelting is a byproduct
of oil refining; additional carbon is obtained <para>Because aluminum smelting involves passing an electric current through a molten electrolyte, it requires large amounts of electrical energy. On average, production of 2 lb (1 kg) of aluminum requires 15 kilowatt-hours (kWh) of energy. The cost of electricity represents about one-third of the cost of smelting aluminum.</para> <head n=1>The Manufacturing Process</head> <head n=2>The Bayer process</head> <para><num>2</num>The slurry is pumped into a digester, a tank that functions like a pressure cooker. The slurry is heated to 230-520°F (110-270°C) under a pressure of 50 lb/in<superscript>2</superscript> (340 kPa). These conditions are maintained for a time ranging from half an hour to several hours. Additional caustic soda may be added to ensure that all aluminum-containing compounds are dissolved.</para> <para><num>3</num>The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process.</para> <para><num>4</num>The slurry is pumped into a settling tank. As the slurry rests in this tank, impurities that will not dissolve in the caustic soda settle to the bottom of the vessel. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminum in the settling tank remains dissolved in the caustic soda. The residue (called “red ud”) that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.</para> <para><num>5</num>After the impurities have settled out, the remaining liquid, which looks somewhat like coffee, is pumped through a series of cloth filters. Any fine particles of impurities that remain in the solution are trapped by the filters. This material is washed to recover alumina and caustic soda that can be reused.</para> <para><num>6</num>The filtered liquid is pumped through
a series of six-story-tall precipitation tanks. Seed crystals of alumina
hydrate (alumina bonded to water molecules) are added through the top
of each tank. The seed crystals grow as they settle through the liquid
and dissolved alumina attaches to them.</para> <head n=2>The Hall-Heroult process</head> <para><num>8</num>Within the reduction pot, alumina crystals are dissolved in molten cryolite at a temperature of 1,760-1,780°F (960-970°C) to form an electrolyte solution that will conduct electricity from the carbon rods to the carbon-lined bed of the pot. A direct current (4-6 volts and 100,000-230,000 amperes) is passed through the solution. The resulting reaction breaks the bonds between the aluminum and oxygen atoms in the alumina molecules. The oxygen that is released is attracted to the carbon rods, where it forms carbon dioxide. The freed aluminum atoms settle to the bottom of the pot as molten metal.</para> <para>The smelting process is a continuous one, with more alumina being added to the cryolite solution to replace the decomposed compound. A constant electric current is maintained. Heat generated by the flow of electricity at the bottom electrode keeps the contents of the pot in a liquid state, but a crust tends to form atop the molten electrolyte. Periodically, the crust is broken to allow more alumina to be added for processing. The pure molten aluminum accumulates at the bottom of the pot and is siphoned off. The pots are operated 24 hours a day, seven days a week.</para> <para><num>9</num>A crucible is moved down the potline,
collecting 9,000 lb (4,000 kg) of molten aluminum, which is 99.8% pure.
The metal is transferred to a holding furnace and then cast (poured into
molds) as ingots. One common technique is to pour the molten aluminum
into a <head n=1>Byproducts/Waste</head> <para>Several types of waste products are generated by decomposition of carbon electrodes during the smelting operation. Aluminum plants in the United States create significant amounts of greenhouse gases, generating about 5.5 million tons (5 million metric tons) of carbon dioxide and 3,300 tons (3,000 metric tons) of perfluorocarbons (compounds of carbon and fluorine) each year.</para> <para>Approximately 120,000 tons (110,000 metric tons) of spent potlining (SPL) material is removed from aluminum reduction pots each year. Designated a hazardous material by the Environmental Protection Agency (EPA), SPL has posed a significant disposal problem for the industry. In 1996, the first in a planned series of recycling plants opened; these plants transform SPL into glass frit, an intermediate product from which glass and ceramics can be manufactured. Ultimately, the recycled SPL appears in such products as ceramic tile, glass fibers, and asphalt shingle granules.</para> <head n=1>The Future</head> <doc.foot> <grouptitle level= 2>Books</grouptitle> <bibcitation><bibcit.composed>Russell, Allen S. “Aluminum.”
<title>McGraw-Hill Encyclopedia of Science & Technology.</title>
New York: <grouptitle>Periodicals</grouptitle> <bibcitation><bibcit.composed>Thompson, James V. “Alumina:
Simple <grouptitle>Other</grouptitle> <bibcitation><bibcit.composed>Reynolds Metals Company. <http://www.reynoldswrap.com/gbu/bauxitealumina>
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