【How silicon is made】-- Material, Making, Processing & Packing

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 reaction between silica and carbon within an electric arc furnace produces silicon.

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 (24-kg) plastic pails or paper bags, 500-lb (227-kg) steel drums or 3,000-lb (1,361-kg) large sacks or boxes.