I’d like to discuss the wastewater treatment process at the polysilicon plant where I work
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I work at the wastewater treatment plant of a polysilicon factory, and we deal with two types of wastewater. One type is the acidic wastewater generated by washing the off-gases produced during the reduction of trichlorosilane to polysilicon in a water scrubber tower; the other type is the hydrofluoric acid wastewater produced during this process. The treatment method for acidic saline wastewater is as follows: The acidic water is pumped to a pre-aeration and adjustment tank through a scrubbing tower. In this tank, flake sodium hydroxide is added to adjust the pH value to 2. Then, the wastewater is pumped to a neutralization and adjustment tank, where liquid quicklime is added to bring the pH value to neutral (the use of quicklime serves to accelerate precipitation and make the water clearer in color). Coagulants are also added to enhance the effectiveness of the neutralization reaction, with mechanical stirring being used for mixing. Then the water enters a high-efficiency inclined tube sedimentation tank, where the precipitates formed as a result of the neutralization of the wastewater are effectively removed. The supernatant fluid then flows to an air flotation unit, where PAM and PAC are added to further reduce the COD, SS, and color intensity of the water, as well as the silicate polymers suspended on the surface of the water. Air flotation relies on tiny bubbles generated by a dispenser to attach to the impurity particles in the water. Float it to the surface and then remove it with a sludge scraper. The water that rises to the surface enters the intermediate tank; 75% of this water passes through a sand filter before going into the recycled water tank. The water in the recycled water tank can be pumped back to the wash tower tank for use in washing purposes. The remaining water is discharged directly after passing through a sand filter and an activated carbon filter. The sludge from the inclined tube sedimentation tank is pumped using a screw pump to a concentration sedimentation tank for further sedimentation. The excess supernatant liquid is discharged into a pre-aeration adjustment tank, after which the remaining wet sludge is pumped via a screw pump to a plate and frame filter press for dewatering; once dried, it is removed and taken away. It is still in the landfilling stage; the ideal approach is to use it as an additive in brick factories for brick production. Due to time constraints, I’ll mention only one; I’ll talk about the others next time I have time. There are also many factors that prevent the treated water from achieving the desired quality; for example, sometimes the addition of quicklime results in sediment floating to the surface, leading to failure in the treatment process. (It’s not clear whether this is due to poor quality of the lime – the cause has yet to be determined.) If sodium hydroxide is added directly to bring the pH to a neutral level without using quicklime, the color intensity of the water is significantly reduced. I don’t know why, but I was wondering if anyone with relevant experience could help explain it
If you don’t have a treatment process for SIO2 already, it will be quite difficult to handle this substance; it also takes up a lot of space. The use of flocculants does not yield good results, and it is likely that treating it with quicklime will not give you the desired outcomes. When CLS reacts with water, HCL and SIO2 are produced! This is quite a headache in exhaust gas treatment; the other aspects are relatively easier to handle! If SIO2 is handled like you people do, I personally think it won’t work! You didn’t make it clear whether SIO2 was removed or not; this is all I can say!
To be more specific, after the scrubbing tower scubs the exhaust gases, most of the SIO2 forms into flocculent bubbles that float on the surface of the water. It is necessary to remove these SIO2 bubbles from the scrubbing tower every day; they dissolve in water when washed with flowing water. The solution is then pumped to a pre-oxidation tank where alkali is added to adjust the pH to 2, and later lime is added in a neutralization tank to bring the pH to neutral. This wastewater subsequently enters a high-efficiency inclined tube sedimentation tank, where the SIO2, SS, and COD in the wastewater are effectively removed through sedimentation.
How will the final waste be disposed of?
It’s quite different from us! It seems that the sediment in yours isn’t easy to remove from the water, right? Think about how to separate the water from the sediment! I can’t reveal the process issues! The final method of waste disposal definitely varies from company to company!
Removing water is very simple; use a plate and frame filter press. The aqueous sediment is pumped in, and under pressure, the water flows out through the filter cloth while the sediment remains in the frame. This method is quite effective for dehydration, but the downside is that the dry sediment inside the frames needs to be removed manually, which is quite troublesome. To achieve optimal results, the filter cloth and frames must be cleaned frequently, which is time-consuming and labor-intensive. I wonder if there are any better methods available. The precipitate can be added as an additive in brick factories for brick firing. It’s a pity that the friend upstairs said it’s not convenient to reveal the techniques; we really want to learn those advanced treatment methods. But I understand, and thanks anyway! ! !
The wastewater treatment generated during polysilicon production is not particularly complex. Firstly, the SIO2 solids can be removed only through filtration. Secondly, it is the removal of F ions that presents difficulties; this is achieved by adding CaO to neutralize them and form CaF2. However, since their concentration is low, determining how to precipitate them poses a problem. It is a comprehensive approach that constitutes the true secret.
The friend upstairs is probably referring to the methods for treating hydrofluoric acid wastewater generated during the production process in the corrosion workshop. Adding lime to form calcium fluoride precipitate is indeed the right approach; therefore, the wastewater must pass through two sedimentation tanks – a primary sedimentation tank and a secondary high-efficiency inclined-tube sedimentation tank – in order to further remove fluoride ions. There are several additional treatment steps as well. In fact, wastewater treatment doesn’t require much technical expertise. What I mentioned above is the process for treating the silica and hydrochloric acid wastewater generated as a result of washing and adsorbing exhaust gases.
What era are we in still using plate and frame filters? There are membrane filters that allow for continuous operation!
You should choose a chamber diaphragm filter press, and it is advisable to have a filter cloth shaking mechanism and an automatic cleaning device. Furthermore, if your sludge is coagulated with iron salts before filtration, its performance in forming cakes after filtration will be much better.