One. Introduction to Industrial Silicon
Silicon is one of the most widely distributed elements in nature, with an earth crust content of about 26.3%, second only to oxygen, and is a semi-metal between metals and non-metals. The crystal structure of silicon determines the physical and chemical properties of silicon. Silicon has a regular tetrahedral stable structure similar to diamond, and silicon atoms are connected by covalent bonds, so silicon has properties such as high hardness and high melting and boiling points.
In the photovoltaic industry, industrial silicon is the main raw material for manufacturing materials such as polysilicon; industrial silicon is also the most important raw material for organosilicon, and is used to produce products such as silane, silicone, and silicone oil; in the alloy industry, industrial silicon is mainly used as an additive for non-ferrous alloys. When silicon is added to certain non-ferrous metals, it can improve the strength, hardness and wear resistance of the base metal, and sometimes it can also enhance the casting and welding properties of the base metal; industrial silicon can remove dissolved oxygen in molten metal as a deoxidizer; silicon alone can improve the strength and corrosion resistance of steel and certain alloys as an alloying agent. Adding an appropriate amount of silicon to cast iron to make high-silicon ferroalloy can significantly enhance the corrosion resistance of cast iron to a variety of chemical reagents.
Industrial silicon, also known as metallic silicon or crystalline silicon, is an important industrial material and is widely used in metallurgy, chemical industry, machinery, electrical appliances, aviation, shipbuilding, energy and other fields. Metallic silicon is a product smelted from silica and carbonaceous reducing agent in a submerged arc furnace. The content of the main component silicon element is about 98% (in recent years, silicon with a Si content of 99.99% is also listed in metallic silicon), and the remaining impurities are iron, aluminum, calcium, etc. Due to its different uses, it is divided into various specifications. According to the content of iron, aluminum and calcium in metallic silicon, metallic silicon can be divided into different grades such as 553 and 441. Industrial silicon is mainly used as an alloying agent, deoxidizer, reducing agent and chemical raw material for metallurgical products. The downstream of industrial silicon is mainly polysilicon, aluminum alloy and organosilicon, accounting for 30.28%, 26.82% and 38.03% respectively.
Two. Raw materials for production
Industrial silicon is obtained by smelting in an electric arc furnace using silica (quartz sand) as raw material and carbonaceous materials as reducing agents. Commonly used reducing agents include charcoal, petroleum coke, coal, etc., which can reduce metallic silicon from silica. Silica and reducing agents undergo a reduction chemical reaction under high temperature in the furnace.
1. Silica requirements
Industrial silicon production requires refined materials to be put 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.
2. Requirements and types of reducing agents for industrial silicon smelting
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 impurities 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.
3. Production Process
After washing, screening and drying, the raw silica is 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. 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.
In the production of industrial silicon, the reduction formula of silicon is:
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 mesh through process adjustment.
Preparation of solar polysilicon
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 production. The physical method is also called metallurgical method, which is similar to the metal smelting and purification process. At present, the purity of industrial silicon purified by chemical method is higher, but the metallurgical method has a short process, is environmentally friendly, low cost and has more development prospects. The metallurgical method refers to the 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.
