With the development of the solar energy industry, the demand for polysilicon for batteries grows at a rate of 30% per year. It is expected that by 2011, the global polysilicon demand will reach over 80,000 tons, while China’s current (up to March 2009) polysilicon production capacity Up to 20,000 tons/year, the polysilicon production capacity will reach 100,000 tons/year by 2011. Whether it is a solar cell or a silicon chip, it has a certain service life. When these products reach their life expectancy, how will so much silicon be handled? Recycling is a more feasible technical solution. However, the price of polysilicon has changed a lot. In 2005, the price of polysilicon was only US$30/kg, and it soared to nearly US$500/kg by the end of 2008. However, in the next few years, the price may drop because the production capacity is greater than the demand. Below US$100/kg, it is close to the production cost of domestic companies (70 to 100 US$/kg), while the production cost of large foreign companies is only US$30/kg. If the cost of silicon recycling exceeds this price, companies will have a hard time generating enthusiasm for using recycled silicon. To promote the recycling of silicon, in addition to the legal constraints of environmental protection, reducing the cost of recycling is also an important means, which requires scientists to develop cheaper, cleaner silicon recycling technology.
Silicon is an important raw material for manufacturing solar photovoltaic cells. The purity level of chip-grade polysilicon is required to reach 9 "9" or more (9N ~ 12N), while the purity level of polysilicon required for photovoltaic cells is 6 "9" (6N). How can the recovered silicon be processed to become the raw material for these high purity products? In addition, a large amount of by-products are also produced in the polysilicon production process. Can they also be used?
Photovoltaic battery production and packaging process will have a certain rate of waste, usually 2 to 4%, with the explosive development of the solar energy industry, the total loss of silicon in the production process will gradually increase. At the same time, photovoltaic cells also have a certain service life, and after they have fulfilled their mission, they also need to be recycled so as not to cause a heavy environmental burden.
Photovoltaic cell fragments produced in the production process are first recovered from silver components, processed and remelted, and can also be used as raw materials for photovoltaic cells. Discarded photovoltaic cells need to be disassembled and then recycled. In addition to being used repeatedly in the photovoltaic cell industry, these silicon can also be made into electrical and ceramic materials such as silicon carbide and silicon nitride. At present, many companies such as Solarworld have been engaged in the recycling of spent photovoltaic cells.
The production process of polysilicon mainly includes modified Siemens method, silane method and physical purification method, among which the most widely used method is modified Siemens method. The method first uses silicon powder to react with hydrogen chloride to generate trichlorosilane (or trichlorosilane). After trichlorosilane is subjected to fractionation and distillation, high-purity trichlorosilane is obtained and then reduced by hydrogen gas. The initial high purity polysilicon rods deposit high purity silicon. On the basis of this process, the tail gas dry recovery process is used, and the recycled tetrachlorosilane is used to hydrogenate and regenerate trichlorosilane, which can save a large amount of raw materials. However, due to the low primary conversion rate of this method, the global polysilicon industry still produces a large amount of tetrachlorosilane every year. The current practice is the use of tetrachlorosilane for the production of fumed silica (silicon dioxide), a high value-added product that is widely used as a filler for rubber, plastics and advanced coatings, and as a byproduct hydrogen chloride produced. After recovery, it can also be used to produce trichlorosilane. Both WACKER of Germany and Deshan of Japan currently use this process to treat tetrachlorosilane.