Is crystalline silicon bad for the environment?
(1) The largest by-product of the production of crystalline silicon is highly polluting to the environment
In people’s impressions, new energy sources often have a “clean and green” image. In the world, photovoltaic power generation is also known as the most promising green energy technology in the future, but why a “green” company will become a neighboring residents report What about the heavily polluted objects? In fact, the main raw material of photovoltaic cells-polysilicon, has a long history of high energy consumption and high pollution in the manufacturing process.
The preparation process of polysilicon is the purification process of silicon raw materials. At present, the mainstream polysilicon production method in the world is the modified Siemens method. The polysilicon produced by this method accounts for about 85% of the total global output of polysilicon. In the process of manufacturing polysilicon using the modified Siemens method, toxic raw materials such as hydrogen chloride are used, and by-products such as silicon tetrachloride and triclosan are generated at the same time. Silicon tetrachloride is the largest by-product in the production of polysilicon. Statistics show that for every 1 ton of polysilicon purified, 10 to 15 tons of silicon tetrachloride will be produced. Silicon tetrachloride that has not been recycled is a toxic and harmful liquid with strong corrosiveness. When exposed to humid air, silicon tetrachloride decomposes into silicic acid and the highly toxic gas hydrogen chloride, which is a strong irritation to human eyes, skin, and respiratory tract. sex.
(2) The by-products in the polysilicon production process are not conducive to recycling. A complete modified Siemens production model should include a closed loop process of silicon tetrachloride. The silicon tetrachloride generated during the reaction can be hydrogenated to generate trichlorohydrogen. Silicon is recycled and recovered. Relying on this method, the conversion rate of silicon tetrachloride in international photovoltaic companies can reach 34%, but in China, due to the low level of technology, it is often less than 20%. The silicon tetrachloride management loopholes have had a bad impact on the environment.
When the modified Siemens method is used to produce polysilicon, in addition to improving product quality and reducing production energy consumption, it is also necessary to treat a large amount of waste gas, waste water, waste residue and other pollutants generated during the production process, otherwise it will easily cause adverse effects on the environment. However, in order to make profits, some companies disregard the consequences and discharge untreated silicon tetrachloride waste liquid at will, causing unrecoverable damage to the ecological environment. As shown in the picture, vegetables withered due to silicon tetrachloride pollution.
According to statistics, 90% of China’s solar cells are used for export. When people bear the environmental and energy costs of solar cells, they hardly enjoy the green and convenient solar power generation. Internationally, there is a strict management and recycling system for the production process of polysilicon, but in China, neither the production technology nor the management standards as a whole cannot be in line with international standards.
①Chinese polysilicon manufacturers often neglect the disposal of hazardous substances for profit
It is estimated that the production costs of polysilicon companies that add hydrogenation workshops will increase by about 30%. In order to maintain the competitiveness of product prices in the market, most of Chinese polysilicon companies have not added hydrogenation workshops for the harmless treatment of silicon tetrachloride, but generally adopt the following two methods: One is to set up a dedicated storage tank area in the plant area. Store silicon tetrachloride; the second is to bury it deep and fall into chaos。
②Improper storage of polysilicon production waste can easily pollute the environment. If storage methods are used, this means that polysilicon production enterprises must have sufficient sites and containers to store the increasing amount of silicon tetrachloride. Judging from the current situation, in the absence of these conditions and effective supervision in some enterprises, the stored waste will inevitably cause leakage. The polysilicon factory reported by the villagers earlier was due to improper stacking of waste materials, which caused part of the sewage to enter the rainwater pipeline. Compared with container storage, deep burial is a lower cost storage method. However, due to the strong corrosiveness and toxicity of silicon tetrachloride, it will inevitably cause devastating pollution of the surrounding soil and groundwater system.
③Environmental pollution by dumping of waste has occurred many times
Another method of stealing silicon tetrachloride is more direct and harsh. Since 2011, pollution incidents involving the dumping of silicon tetrachloride across the land have occurred in Jiangsu, Shandong, and Liaoning. In April 2011, about 600 tons of silicon tetrachloride waste liquid entered the urban sewage treatment plant in large quantities through a certain urban drainage pipe network, resulting in the death of all biological strains in the plant and severe damage to the equipment. As a result, the treatment plant was paralyzed for more than two months. After the sewage treatment plant was shut down, the production and domestic sewage in the urban area poured back into the moat, causing the river to become black and smelly. These silicon tetrachloride waste liquid eventually entered the largest state-owned fishery in a certain province, and about 400 tons of fish and 500 million artificially hatched fish eggs died in one month.