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Polysilicon production industrial wastewater zero discharge technology


Polysilicon Industrial Wastewater Filter Element

At present, polycrystalline silicon mainly adopts multi-wire cutting technology. Therefore, the old Cutting fluid needs to be discharged continuously during the cutting process. During the cutting process, about 50% of the silicon material is mixed into the Cutting fluid composed of polyethylene glycol (PEG) Cutting fluid and silicon carbide powder (SiC) abrasive, and new Cutting fluid is constantly supplemented, which will produce a lot of cutting waste liquid.

The silicon powder in the wastewater of crystal silicon processing is very fine, with particle sizes ranging from 0.1 to 1mm. Most small particles are suspended in water and difficult to deposit. The treatment method for crystal silicon wastewater is to add flocculants such as Aluminium chlorohydrate, polymeric Iron(III) chloride, polyacrylamide, etc. into the water. After flocculation and sedimentation of silicon-containing wastewater, solid-liquid separation is carried out through a sintered filter.

In the polysilicon industry, most of the industrial wastewater currently produced has not achieved zero-discharge technology. This paper compares and analyzes the application of zero-discharge technology in various industries, and analyzes the characteristics of sewage in the polysilicon industry. application feasibility.

The concept of zero discharge of industrial wastewater

Simply put, zero discharge of waste water means that in the production process, the waste water is reused after treatment and not discharged, so as to achieve green environmental protection and sustainable development. The key to zero discharge of industrial wastewater is whether it can effectively reduce the salt content in industrial wastewater.

Comparison of zero-discharge technologies for wastewater

(1) Incineration technology

Incineration technology is generally at a temperature of 800 to 950 °C. Combustible organic matter in industrial wastewater or organic matter added with combustion accelerants reacts with oxygen to generate water, CO2, inorganic ash and thermal energy, and realizes the process of zero emission.

The main steps of the process include feed pretreatment, high temperature incineration, heat recovery and flue gas treatment.

Incineration technology is mainly aimed at wastewater with high organic content. For wastewater with low organic content, it is difficult to burn due to its low calorific value. Usually, it is necessary to concentrate the wastewater, increase the calorific value and then burn it, otherwise the energy consumption will be high and the investment will be large. Incineration technology generally has problems such as equipment coking, secondary pollution of dust, and dioxins in exhaust gas.

(2) Evaporative crystallization technology

Evaporation crystallization technology is a relatively mature chemical unit in industrial production.

It is widely used in metallurgy, chemical industry, sewage treatment, seawater desalination and other processes.

For the high-salt wastewater formed by the in-depth treatment of wastewater, the evaporative crystallization process can generally be used to achieve zero discharge.

At this stage, the main evaporation technologies at home and abroad include multiple effect evaporation (MEE), mechanical vapor recompression evaporation (MVR), horizontal film spray evaporation (MVC) and so on.

The main process of MEE is that multiple evaporators are connected and operated, and the secondary steam produced by the upper-stage evaporator is used as the heat source of the next-stage evaporator to improve the heat utilization rate.

The advantages are that the pretreatment of influent water is simple, the application is flexible, and the system is stable, but the heat source needs to be continuously supplemented. On the basis of multi-effect evaporation, MVR supplements steam at one time, and it will not be supplemented after normal operation. The energy saving effect is remarkable, and the equipment occupies a small area, but the investment is high.

Compared with MVR, MVC has the same requirements for secondary steam utilization, which improves the evaporation efficiency, forms an evaporation film under the action of gravity, and has a shorter heating time and higher evaporation efficiency.

(3) Combined evaporation technology

At present, industrial wastewater is mainly divided into organic wastewater and inorganic wastewater, including low-salt wastewater and high-salt wastewater.

According to the wastewater of different materials, the combined evaporation technology is used to treat different types of wastewater. Selecting the appropriate combination can maximize the realization of zero discharge, and play a better role in saving energy, reducing equipment investment and saving operating costs. .

When the wastewater contains high concentration of organic matter, the evaporative crystallization-incineration technology can be used to achieve zero discharge, and the process is simple and the treatment is thorough; when the amount of wastewater is large and the organic matter and salinity are low, if the wastewater is directly treated by evaporation, equipment investment If it is larger, it is generally pre-concentrated by the membrane method to reduce the amount of water treated by evaporation.

When the water quality of the evaporative condensate is high, the evaporative condensate should be further treated by combining biochemical methods, membrane methods, ion exchange adsorption methods and other methods.

In terms of further reducing energy consumption, domestic and foreign researchers consider applying new energy sources such as solar energy and wind energy to the mechanical vapor recompression process.

Feasibility of zero discharge of wastewater in polysilicon production process

The wastewater in the polysilicon production process is mainly inorganic wastewater, mainly containing silicon, chlorine, metal ions, etc. Generally, the salt concentration is low, the water volume is large, and it does not have the conditions for direct evaporation and crystallization. It can reduce equipment investment, operating costs, and achieve environmental protection and zero-emission requirements.

Through the discussion of the above various technologies, the wastewater discharged from polysilicon production, using the above technologies, has the feasibility of zero discharge.

To sum up, the zero-discharge technology of industrial wastewater has been successfully applied in industries such as coal chemical industry, chlor-alkali, and power plants. With the severe environmental protection trend, zero-discharge of wastewater from polysilicon production will become inevitable.

Through membrane concentration pretreatment, combined with mechanical vapor recompression evaporation, combined with ion exchange adsorption according to water quality requirements, cold discharge can be achieved economically.

At the same time, the development of new combined cold emission technologies such as the utilization of wind energy and solar energy has good prospects.