After being mixed by the mixer, the silica gel is sent to the filter press for solid-liquid separation operation through the slurry pump, and the filter residue is transported to the slag yard for unified stacking. The separated silica gel is scraped out by the scraper and sent to the sludge collection tank, and then the submersible sludge pump is used to drive the silica gel into the mixing tank, and after stirring, it is sent to the filter press for solid-liquid separation.

The industrial wastewater that removes silica gel is salt wastewater, which is generally acidic and has a pH value of about 2. The wastewater is neutralized by acid and alkali, and then evaporated, crystallized, and filtered to extract the salts. The treated water is returned to production for recycling. At present, many domestic enterprises do not consider recycling salt, but stack it centrally.

1. The working principle of the evaporator

The evaporation process refers to the heating of a solution containing non-volatile solvents, so that the water evaporates and the remaining solvent concentration increases continuously. An evaporator is used in this evaporation process. During the evaporation process, a large amount of secondary steam will be generated, and the secondary steam will be continuously moved out to make the solution and steam in dynamic equilibrium, and the evaporation process cannot be carried out in this equilibrium state. If the secondary steam is moved out of the direct condensation, this operation is called single-effect evaporation. If the secondary steam is directly directed to the inlet of the evaporator of the second stage, and the secondary steam is used as the heating source of the second stage evaporator, this method of effective use of latent heat of secondary steam is called multi-effect evaporation.

The evaporation process has the following characteristics:

(1) Heat transfer properties

Phase change process occurs on both sides of the heat transfer wall, one side is solution boiling, and the other side is steam condensation, which is a constant temperature heat transfer process.

(2) Change of boiling point of solution

In general, the saturated vapor pressure of a solution containing non-evaporative solutes is lower than that of pure water at high temperatures. That is, at the same pressure, the boiling point of pure water is lower than that of the solution. Therefore, when the amount of steam is heated, the temperature difference between evaporating pure water and evaporating solution is larger. Usually, the higher the solute composition, the more obvious this phenomenon is.

(3) Solution properties

The viscosity of the solution will increase with the evaporation process, and the corrosive properties of the corrosive solute will also increase. Crystals precipitate during evaporation and are easy to polymerize or decompose at high temperatures. Sometimes foam or scale can form as the process progresses.

(4) Foam entrainment

Due to the entrained liquid foam, it is easy to lose material and contaminate the condensing equipment. Therefore, during the second steam condensation, the entrained liquid foam must be removed.

(5) Energy utilization In the evaporation process, secondary steam is usually generated, and its latent heat is huge, so its utilization value needs to be effectively exerted.

2. Various evaporators used to treat salt wastewater in the production process of polysilicon

At present, in the domestic polysilicon production process, thin film evaporators, three-effect evaporators and four-effect evaporators and other equipment can be used to treat salt wastewater. They all use the principle of evaporation and concentration to evaporate the water in the wastewater, and NaCl condenses and crystallizes to achieve effective separation of wastewater.

2.1 Working principle of thin film evaporator

The film evaporator is a modern, high-efficiency evaporator that requires vacuum operation, forced film formation through a rotating film scraper, and flow at high flow rates. This method has a short residence time and high heat transfer efficiency. In this process, the continuous feed is continuously scraped into a liquid film of uniform thickness by the scraper on the heating surface, and then the liquid film moves downward, and the residue is discharged from the bottom in the process of downward flow, and the components with low boiling point will be evaporated and enter the atmosphere.

2.2 Working principle of three-effect evaporator

The three-effect evaporator adopts the method of series connection. The secondary steam generated by the first evaporator is to use its latent heat of evaporation to introduce heat to the inlet of the second evaporator. The secondary steam condensation of the first evaporator is the inlet heating of the second evaporator. This operation can be achieved as long as the pressure distribution of each evaporator is controlled. The first evaporator that passes through the heating steam is the first effect, the first evaporator that passes through the secondary steam is the second effect, and the evaporator that passes through the next secondary steam is the third effect.

2.3 Working principle of four-effect evaporator

Four-effect evaporator refers to the process of connecting four evaporators in series, and the secondary steam generated by each stage evaporator enters the next stage evaporator in turn as heating steam. When evaporating in the four-effect evaporator, the material is evenly distributed in each evaporation tube through the distribution tray on the top of the evaporator. Under the action of gravity, the material flows out from top to bottom through the evaporation tube, forming a film on the inner wall of the tube, and the film on the inner wall exchanges heat with the heating steam on the outside, and the film liquid evaporates. Since the flow direction of secondary steam is consistent in each evaporator, it is easy to distribute the material liquid into a thin film state along the inner wall of the evaporation tube, which improves the heat transfer efficiency.

3. Conclusion

The multi-effect evaporator improves the evaporation efficiency, that is, the utilization rate of secondary steam. From the above analysis, it is more energy-efficient to use a multi-effect evaporator to evaporate the same amount of water, and the amount of steam and circulating water that needs to be consumed is less. Therefore, in industrial production, multi-effect evaporators such as four-effect evaporators are generally used, which have higher evaporation efficiency and are more widely used.