Traceability of Silicon

 

Silica needs to be purified before it can be used in industrial production. The current mainstream purification method is to add silica to carbon and throw it into a high-temperature resistance furnace for melting. At high temperature, SiO2 will react with C to form SiC and CO, and then SiC will further reduce SiO2 to Si and CO, translated in Chinese, means that silica, the main component of quartz, will react with the carbon in the coke to form silicon carbide and carbon monoxide, and the silicon carbide will react with the remaining silica to form a higher purity Silicon and carbon monoxide. These higher-purity silicon are denser, so they are deposited under the resistance furnace, and after they are extracted, they are metal silicon powder that can be used in industrial production.

 

Metal silicon powder is also called silicon micro-powder, which should be distinguished from micro-silicon powder. Micro-silica fume, also known as silica fume, is the smoke and dust generated during the firing of metallic silicon. The main component is also SiO2, so it can be collected and returned to the furnace, or sold as an additive for cement, building materials, etc. The silicon powder we usually talk about refers to metal silicon powder, not micro silicon powder.

 

The fired metal silicon powder still contains impurities. The three most common impurities are iron, aluminum and calcium, so the content of the three impurities is usually used to distinguish the grade of metal silicon powder in the market. For example, 553 silicon is 0.5% iron impurities, 0.5% aluminum, and 0.3% calcium. Another example is 2202 silicon, which is 0.2% iron, 0.2% aluminum, and 0.02% calcium. At present, the mainstream classification in the market is to divide metal silicon powder into metallurgical grade silicon and chemical grade silicon. %, the product basically starts with 2 characters.

 

As the name suggests, metallurgical grade silicon is generally used in alloy smelting, and the annual consumption of metallurgical grade silicon in the smelting industry is as high as 60%. Another use of metallurgical grade silicon is the production of high-purity silicon, which is discussed in more detail below. Chemical grade silicon is mainly used for the production of organic silicon. Of course, chemical grade silicon can also be used to prepare high-purity silicon, but it is more wasteful.

 

Metallurgical grade silicon is not pure enough and needs further purification before it can be used in solar cell wafers and silicon for electronics. After metallurgical grade silicon is purified, it is called high-purity silicon. The grade is calculated by N. For example, the silicon content of 99.99% is 4N, and 99.9999% is 6N. The purity of solar grade silicon should be above 4N, generally 4-6N polycrystalline silicon or monocrystalline silicon; while the requirements for electronic grade silicon are higher, usually 6-11N monocrystalline silicon.

 

There are many purification methods for metallurgical grade silicon. Currently, there are two large-scale purification methods used by listed companies, namely the improved Siemens method (represented by Tongwei) and the fluidized bed method (represented by GCL-Poly New Energy). represent). The starting point of the two methods is the same. The metal silicon powder cannot be directly purified. It needs to be converted into an intermediate substance, and then the intermediate substance is fractionated, deposited and purified, and finally the silicon is reduced.

 

The intermediate substance selected by the modified Siemens method is trichlorosilane. Low-purity silicon powder first reacts with hydrogen chloride to generate trichlorosilane containing impurities (impurities include silicon chloride, dichlorosilane, etc.). Trichlorosilane is liquid at room temperature, so it can be purified by fractional distillation. The silicon in the high-purity trichlorosilane is replaced with hydrogen. The high-purity silicon prepared by the modified Siemens method is rod-shaped silicon. The disadvantage is that the power consumption is high, the production waste is large, and the cost is difficult to reduce.

 

The fluidized bed method selects silane as the intermediate material. The preparation of silane is relatively simple, that is, continuously injecting hydrogen into trichlorosilane containing impurities. Both trichlorosilane and impurities can react with hydrogen to generate silane SiH4 and hydrochloric acid HCl. The boiling points of these mixed solutions are different. Therefore, it can be separated by distillation, and finally high-purity silane gas can be obtained. At the same time as the preparation of silane, high-purity silicon seed crystals need to be added to the melting furnace, the silicon seed crystals are melted into a fluidized bed, and then the silane is fed into the furnace. Silane decomposes into silicon and hydrogen at high temperature, silicon will be deposited on the surface of silicon seed crystal, and hydrogen will be discharged and recycled. After such a process, more and more silicon is deposited on the surface of the high-purity silicon seed crystal and gradually grows up, so some people call this vapor deposition method "silicon growth".

 

The advantages of the fluidized bed method are less power consumption, less waste and waste gas, and continuous discharge, which can greatly improve the efficiency of crystal pulling. The formation of bubbles leads to "hydrogen hopping" during crystal pulling, so how to dehydrogenate high-purity silicon is a technical difficulty at present. In addition, the fluidized bed method has high requirements on the purity of the silicon seed crystal powder, but in order to burn it into a fluidized bed and reduce air bubbles in the fluidized bed, the silicon seed crystal usually needs to be mechanically ground, and the silicon seed crystal will collide with the equipment during grinding. , it is likely to be mixed with other impurities, resulting in a decrease in the purity of the seed crystal. The high-purity silicon prepared by the fluidized bed method is granular silicon powder, which is different from the rod-shaped silicon block of the Siemens method.

 

Judging from the current situation, the fluidized bed method has no way to replace the Siemens method, but to a certain extent, it can squeeze the market of the Siemens method, especially in the solar-grade silicon market. After investigation, the most mature technology of using granular silicon to pull monocrystalline silicon among listed companies should be LONGi. According to the company, the grade of monocrystalline silicon pulled by granular silicon is now comparable to that of rod-shaped monocrystalline silicon. 's tertiary product. With the improvement of technology, the fluidized bed method and the improved Siemens method will form a situation of two parts, just like the ternary lithium battery and lithium iron phosphate battery of lithium battery.

 

In general, the silicon industry, from silicon ore to downstream organic silicon products, polysilicon and monocrystalline silicon, has four development trends. The first is high purification, the second is energy saving and emission reduction, and the third is To reduce costs, the fourth is industrial integration. Needless to say about high purity, the growth rate of demand for high-purity silicon products is much higher than that of low-purity silicon products, and the price gap of silicon with different purities is widening wildly. In the end, the purest must win the world. The focus of energy saving, emission reduction and cost reduction lies in the utilization of electricity and by-products, and enterprises that use the fluidized bed method in these two points may have greater advantages. The last point is the most important, and it is also related to whether a company can seize the highest point in the silicon industry

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