How Is Silicon Metal Made?

Industrial silicon is obtained by smelting in an electric arc furnace with silica (quartz sand) as raw material and carbonaceous material as reducing agent. Commonly used reducing agents are mainly charcoal, petroleum coke, coal, etc., which can reduce metal silicon from silica. Silica and reducing agent undergo reduction chemical reaction under high temperature environment in the furnace.

(1) Silica requirements

Industrial silicon production requires refined materials to be fed into the furnace. The component requirements for silica are: SiO2 ≥ 99%, Fe2O3 ≤ 0.15%, Al2O3 < 0.2%, CaO < 0.1%, and the total impurities are preferably < 0.6%.

In addition, silica is usually in the form of quartz or pebbles, and should have high heat resistance. When added to the furnace, it should rarely crack after the phase change caused by heat, and the starting temperature of strong damage should be as high as possible. The most important characteristic of silica is the reducibility of SiO2.

Theoretically, the reducing agents that can be used for industrial silicon smelting include charcoal, petroleum coke, semi-coke, asphalt coke and bituminous coal. However, considering the relationship between subsequent impurity removal, there are strict requirements for reducing agents. Generally, the reducing agent is required to have high fixed carbon, low ash content, moderate volatile matter, low moisture content, high resistivity, strong reaction activity and certain mechanical strength.

After washing, screening and drying the raw silica, the raw materials are mixed in different proportions according to the type of reducing agent used. The proportion of each material is controlled by a computer program, mixed and put into the electric furnace; the highly automated and large-scale feeding process is carried out continuously. Electric current is passed through the electrode to heat the material in the brain. Its liquidus line is above 1410℃. The refining needs to reach a high temperature of more than 1800 degrees Celsius. The silicon is reduced in the brain and is in liquid state. It is released through the silicon outlet and cast into silicon ingots. After crushing and packaging, it is sold as industrial silicon powder.

The gas overflowing from the electric furnace is dusted, and the silicon powder is recovered to obtain microsilica powder products; microsilica is mixed with concrete to improve the various properties of concrete; it can also be used as a raw material for returning to the electric furnace, a filler for rubber, an anti-caking agent for fertilizers, a raw material for silicate bricks and refractory materials, a catalyst for production and other uses.

SiO2+2C═Si+2CO (T=2073k)

SiO2+C═SiO+CO (side reaction)

The purity of industrial silicon produced is between 98% and 99%, and the impurities come from silica and carbon materials (including petroleum coke, clean coal carbon electrodes, etc.). The temperature in the furnace is an important factor affecting the smelting of industrial silicon. The temperature distribution not only affects the quality of the chemical reaction in the furnace, but also has different degrees of damage to the arc furnace equipment, such as electrode ablation and furnace lining damage. Most of the metallic silicon produced by the above is in block form. If silicon blocks are used as raw materials for crushing and refining, ball milling, roller milling, and whirling methods are generally used to produce industrial silicon powder. The finished product particle size can be controlled within the range of 30-425 meshes through process adjustment.

The preparation technologies of solar polysilicon mainly include chemical method and physical method. At present, the mainstream process for preparing solar-grade polysilicon is chemical method, which accounts for about 80% of the global output. The physical method is also called the metallurgical method, which is similar to the metal smelting and purification process. At present, the chemical method for purifying industrial silicon has a higher purity, but the metallurgical method has a short process, is environmentally friendly, low cost, and has more development prospects. The metallurgical method refers to a method of preparing solar-grade polysilicon using industrial silicon as raw material, using a combination of hydrometallurgy, vacuum melting, oxidation refining, directional solidification, special field melting and other technologies.

The characteristic of the metallurgical method is that silicon does not participate in any chemical reaction during the purification process. Relying on the difference in physical properties between silicon and impurities, impurities are removed by metallurgical smelting to obtain polysilicon that meets the performance requirements of solar cells. It has the advantages of low cost, safety and environmental protection, and has become a research hotspot in this field.