About Silicon Metal


History and Properties

Silicon is a chemical element with symbol Si and atomic number 14. It is a tetravalent  metalloid.  Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is the second most abundant element in the Earth’s crust after oxygen, naturally occurring in various forms of silicon dioxide or silicates and very rarely in its pure form in volcanic exhalations. Si 2

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. Silicon Metal is known as a semi-metallic or metalloid, having several of the characteristics of metals. This is just a fancy term to describe certain elements that don’t behave exactly like metals or exactly like non-metals. Silicon can have properties like a metal or a non-metal depending on what other elements it combines with. Glass is a very common silicon compound that has non-metal qualities, while the silicon used in electronics can be made to act very much like a metal

 Metallurgical Grade 

Elemental Silicon not alloyed with significant quantities of other elements, and usually > 95%, is often referred to loosely as silicon metal. It makes up about 20% of the world total elemental silicon production, with less than 1 to 2% of total elemental silicon (5–10% of metallurgical grade silicon) ever purified to higher grades for use in electronics. Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal in an electric arc furnace using carbon electrodes. At temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and the silicon undergo the chemical reaction:

SiO2 + 2 C → Si + 2 CO

Liquid silicon collects in the bottom of the furnace, which is then drained and cooled. The silicon produced in this manner is called metallurgical grade silicon and is at least 98% pure. Using this method, silicon carbide (SiC) may also form from an excess of carbon in one or both of the following ways:

SiO2 + C → SiO + CO

SiO + 2 C → SiC + CO

However, provided the concentration of SiO2 is kept high, the silicon carbide can be eliminated by the chemical reaction:

2 SiC + SiO2 → 3 Si + 2 CO

As noted above, metallurgical grade silicon “metal” has its primary use in the aluminium casting industry to make aluminium-silicon alloy parts. The remainder (about 45%) is used by the chemical industry, where it is primarily employed to make fumed silica, with the rest used in production of other fine chemicals such as silanes and some types of silicones.Si 3

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, aluminium, 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

  •  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.
  •  While the metal is in the molten state, it is treated with oxygen and air to reduce the amount of calcium and aluminium impurities. Depending on the grade, silicon metal contains 98.5-99.99% silicon with trace amounts of iron, calcium and aluminium.Si 4

Cooling/Crushing

  • 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 mould into a truck, weighed and then dumped in the storage pile. Dumping the metal from the mould 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. Before shipping, the metal is sized according to customer specifications, which may require a crushing process using jaw or cone crushers.

  Application Of Silicon Metals

  • Silicon metal is used in the primary and secondary aluminium industry to improve the strength of Aluminium. This ability of silicon is of importance to the automotive industry where for example components made from Aluminium Alloys provide a strong and lightweight replacement for heavier cast iron engine blocks and steel wheels. By reducing the weight in automobiles, fuel consumption decreases. As a result green house gas emissions are reduced which is a major contribution to conservation of the environment.
  • Due to its non harmful properties, polymers using Silicon as the base product, known as silicones, provide an alternative to hydrocarbon based products used in the chemical industry. Industrial items such as lubricants, greases and resins, antifoam agents, fabric softeners and everyday consumer products such as skin and hair care products are all made using Silicon as the base product.
  • Silicon is also utilized in the fast growing industries of high speed communication and solar energy. The manufacturing of semiconductor chips, optical fiber and liquid crystal displays is made possible due to the conducting properties of Silicon. Ultra high purity Silicon is being used in the manufacturing of solar panels, making it possible to use the sun’s rays to generate electricity to provide power in a large number of industrial and domestic uses, such as remote telecommunication stations, weather and irrigation projects and street lighting.