Chemical grade silicon is used in the production of silicones and polysilicon. Among them, organic silicon mainly uses industrial silicon of 421#, 411#, 521# and other brands. Compared with aluminum alloys, the content of aluminum elements is higher, usually less than 0.2%. Silica gel not only requires the content of iron, aluminum, and calcium, but also the content of trace elements such as titanium, phosphorus, and boron; while the production of polysilicon is equivalent to the purification of silicon, and there are no strict requirements for the grade of industrial silicon. Requirements such as boron and carbon that affect photoelectric conversion efficiency. The polysilicon that the market is more interested in is currently mainly 421# industrial silicon, and 521#, 441#, 553#, and 99 silicon with a silicon content of about 99% are also widely used. With the advancement of production technology and the need for cost control, some metallurgical grade industrial silicon has gradually become the mainstream consumer specification in the polysilicon industry.
1. Industrial silicon and recycled silicon
In terms of industrial silicon supply, with the advancement of downstream technology, the market’s requirements for the taste of industrial silicon have gradually decreased, and other industrial silicon other than national standards have emerged, such as 97 silicon, recycled silicon, etc. Among them, as a substitute for the low-end market, 97 silicon, also known as external silicon, is a product made of silica and blue carbon smelted in an electric furnace. Its main component silicon content is lower than metal silicon, about 98%. The remaining impurities include iron, aluminum, calcium and other elements, which are mainly used as additives for non-ferrous alloys and can also be used as a substitute for metallic silicon, such as in the field of recycled aluminum alloys. According to ferroalloy online sample statistics, in March 2022, 40 of the 51 submerged furnaces in 97 silicon factories across the country were operating normally, with a monthly output of approximately 25,650 tons, accounting for approximately 9.22% of the institutional silicon metal output. Its main domestic output in 2021 is as follows:
With the development of the photovoltaic industry and the launch of new polysilicon production capacity, the industrial silicon recycling system has been gradually improved in recent years. As a waste recycling product, the production of recycled silicon is gradually increasing. Recycled silicon is a waste material produced by industrial silicon grinding, polysilicon and silicon wafer slicing. It is obtained after smelting in an intermediate frequency furnace and has a nominal silicon content of approximately 99%. SMM data shows that in 2021, my country’s industrial silicon production will be 3.17 million tons, including 970,000 tons of silicon and 85,000 tons of recycled silicon, accounting for 5.7% and 2.7% respectively.
However, it should be noted that the particle size of 97 silicon is too large during the smelting process, resulting in large amounts of slag, difficulty in coming out of the furnace, reduced silicon recovery rate, increased energy consumption, rising furnace bottom, etc., which affects the quality of 97 silicon products. Production. Recycled silicon accounts for a relatively small proportion of the current industrial silicon supply, mainly from the recycling process of downstream production, and part of the output is also directly included in metallic silicon. At this stage, supply statistics mainly focus on the production of various grades of industrial silicon that meet national standards.
Upstream production of industrial silicon
1) Production process
Industrial silicon is made from silica raw materials that are washed, screened, and dried, and then mixed and smelted in an electric furnace according to the selected reducing agent (petroleum coke, clean coal, charcoal, etc.) and raw materials in different proportions. It is essentially a continuous electrothermal chemical reaction in a submersible furnace. The equation is as follows: after power is applied, the electrodes of the submersible furnace emit sparks, reaching a high temperature of more than 1,800 degrees Celsius, heating the materials in the furnace; in this process, silicon is reduced and in a liquid state. After preliminary smelting, the liquid silicon is refined to reduce the content of aluminum and calcium impurities; the silicon ingots are cooled and demoulded, and then undergo processes such as crushing, grading, packaging, weighing, and storage.
