Silicon

Silicon

Background

Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates. Silicon's compounds are also found in water, in the atmosphere, in many plants, and even in certain animals.

Silicon is the fourteenth element of the periodic table and is a Group IVA element, along with carbon germanium, tin and lead. Pure silicon is a dark gray solid with the same crystalline structure as diamond. Its chemical and physical properties are similar to this material. Silicon has a melting point of 2570° F (1410° C), a boiling point of 4271° F (2355° C), and a density of 2.33 g/cm3.

When silicon is heated it reacts with the halogens (fluorine, chlorine, bromine, and iodine) to form halides. It reacts with certain metals to form silicides and when heated in an electric furnace with carbon, a wear resistant ceramic called silicon carbide is produced. Hydrofluoric acid is the only acid that affects silicon. At higher temperatures, silicon is attacked by water vapor or by oxygen to form a surface layer of silicon dioxide.

When silicon is purified and doped with such elements as boron, phosphorus and arsenic, it is used as a semiconductor in various applications. For maximum purity, a chemical process is used that reduces silicon tetrachloride or trichlorosilane to silicon. Single crystals are grown by slowly drawing seed crystals from molten silicon.

Silicon of lower purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, alumiinum, and bronze. When small amounts of silicon are added to aluminum, aluminum becomes easier to cast and also has improved strength, hardness, and other properties. In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents.

In 1999, world production was around 640,000 metric tons (excluding China), with Brazil, France, Norway and the United States major producers. This is a continued decline compared to the last several years (653,000 tons in 1998 and 664,000 in 1997). Though data is not available, China is believed to be the largest producer, followed by the United States. One estimate puts China's production capacity as high as 400,000 metric tons per year, with over 400 producers. Exports from this country have increased in recent years.

Consumption of silicon metal in the United States was roughly 262,000 metric tons, at a cost of 57 cents per pound. The annual growth rate during 1980-1995 was about 3.5% for silicon demand by the aluminum industry and about 8% by the chemical industry. Demand by the chemical industry (mainly silicones) was affected by the Asian economic crisis of the late 1990s.

History

Silicon was first isolated and described as an element in 1824 by a Swedish chemist, Jons Jacob Berzelius. An impure form was obtained in 1811. Crystalline silicon was first produced in 1854 using electrolysis.

The type of furnace now used to make silicon, the electric arc furnace, was first invented in 1899 by French inventor Paul Louis Toussaint Heroult to make steel. The first electric arc furnace in the United States was installed in Syracuse, New York in 1905. In recent years, furnace technology, including the electrodes used for heating elements, has improved.

Raw Materials

Silicon metal is made from the reaction of silica (silicon dioxide, SiO2) and carbon materials like coke, coal and wood chips. Silica is typically received in the form of metallurgical grade gravel. This gravel is 99.5% silica, and is 3 x 1 or 6 x 1 in (8 x 3 cm or 15 x 3 cm) in size. The coal is usually of low ash content (1-3% to minimize calcium, aluminum, and iron impurities), contains around 60% carbon, and is sized to match that of the gravel. Wood chips are usually hardwood of 1/2 x 1/8 inch size (1 x. 3 cm size). All materials are received as specified by the manufacturer.

The Manufacturing Process

The basic process heats silica and coke in a submerged electric arc furnace to high temperatures. High temperatures are required to produce a reaction where the oxygen is removed, leaving behind silicon. This is known as a reduction process. In this process, metal carbides usually form first at the lower temperatures. As silicon is formed, it displaces the carbon. Refining processes are used to improve purity.

The Reduction Process

• 1 The raw materials are weighed and then placed into the furnace through the top using the fume hood, buckets, or cars. A typical batch contains 1000 lb (453 kg) each of gravel and chips, and 550 lb (250 kg) of coal. The lid of the furnace, which contains electrodes, is placed into position. Electric current is passed through the electrodes to form an arc. The heat generated by this arc (a temperature of 4000° F or 2350 ° C) melts the material and results in the reaction of sand with carbon to form silicon and carbon monoxide. This process takes about six to eight hours. The furnace is continuously charged with the batches of raw materials.
• 2 While the metal is in the molten state, it is treated with oxygen and air to reduce the amount of calcium and aluminum impurities. Depending on the grade, silicon metal contains 98.5-99.99% silicon with trace amounts of iron, calcium and aluminum.

Cooling/Crushing

• 3 Oxidized material, called slag, is poured off into pots and cooled. The silicon metal is cooled in large cast iron trays about 8 ft (2.4 m) across and 8 in (20 cm) deep. After cooling, the metal is dumped from the mold into a truck, weighed and then dumped in the storage pile. Dumping the metal from the mold to the truck breaks it up sufficiently for storage. Before shipping, the metal is sized according to customer specifications, which may require a crushing process using jaw or cone crushers.

Packaging

• 4 Silicon metal is usually packaged in large sacks or wooden boxes weighing up to 3,000 lb (1,361 kg). In powder form, silicon is packaged in 50-lb (23-kg) plastic pails or paper bags, 500-lb (227-kg) steel drums or 3,000-lb (1,361-kg) large sacks or boxes.

Quality Control

Statistical process control is used to ensure quality. Computer-controlled systems are used to manage the overall process and evaluate statistical data. The two major process parameters that must be controlled are amounts of raw materials used and furnace temperatures. Laboratory testing is used to monitor the chemical composition of the final product and to research methods to improve the composition by adjusting the manufacturing process. Quality audits and regular assessments of suppliers also ensure that quality is maintained from extraction of raw materials through shipping of the final product.

Byproducts/Waste

With statistical process control, waste is kept to a minimum. A byproduct of the process, silica fume, is sold to the refractory and cement industries to improve strength of their products. Silica fume also is used for heat insulation, filler for rubber, polymers, grouts and other applications. The cooled slag is broken down into smaller pieces and sold to other companies for further processing. Some companies crush it into sandblasting material. Because electric arc furnaces emit particulate emissions, manufacturers must also comply with the Environmental Protection Agency's (EPA) regulations.

The Future

Though industry analysts predicted demand for chemical-grade silicon by Western countries would increase at an annual average rate of about 7% until 2003, this growth may be slower due to recent economic declines in Asia and Japan. If supplies continue to outpace demand, prices may continue to drop. The outlook for the automotive market is positive, as more car makers switch to an aluminum-silicon alloy for various components.

Other methods for making silicon are being investigated, including supercooling liquid to form bulk amorphous silicon and a hydrothermal method for making porous silicon powder for optical applications.

Where to Learn More

Books

Kirk-Othmer. Encyclopedia of Chemical Technology. New York: John Wiley & Sons, Inc. 1985.

Periodicals

Bendix, Jeffrey. "The Heart of Globe is in Cleveland." Cleveland Enterprise (Fall 1991).

Ward, Patti. "Heroult Electric Arc Furnace Stands the Test of Time." Iron and Steelmaker 26, no. 11 (November 1999). http://www.issource.org/magazine/Web/9911/Ward-9911.htm.

Other

Annual Minerals Review: Silicon. U.S. Geological Survey, 1998.

Mineral Commodity Summaries: Silicon. U.S. Geological Survey, February 2000.

Mineral Industry Surveys: Silicon in February 2000. U.S. Geological Survey, May 2000.

—Laurel M. Sheppard