JP4000205B2 - Method for separating cutting powder from silicon waste liquid - Google Patents

Description

【００２０】
【発明の効果】
本発明方法によれば、金属シリコンの切削粉を含有する切削廃水から該切削粉を比較的破壊されにくい凝集物として効率的に除去することができる。
凝集物除去後の廃水は単にｐＨを整えるだけで、一般廃水として廃棄できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for separating metal silicon cutting powder. More specifically, the present invention relates to a method for separating and processing cutting wastewater containing silicon cutting powder easily and inexpensively.
[0002]
[Prior art]
The most common method for producing high-purity polycrystalline silicon is a method in which a core wire of high-purity silicon is heated with current, and trichlorosilane is reacted with hydrogen on the core wire to deposit rod-shaped polycrystalline silicon. Widely done in general. The silicon core wire may be used by cutting a polycrystalline silicon rod obtained by precipitation into a thin rod suitable for the core wire.
[0003]
When cutting a silicon rod, slurry-like cutting wastewater containing silicon cutting fine powder is generally generated. Since cutting wastewater is turbid due to silicon fine powder, abrasives, etc., this waste liquid cannot be discarded as general wastewater. Must be separated. However, silicon in cutting wastewater has the property of generating hydrogen gas in water, so there is a problem that it re-levitates with the hydrogen gas generated by the agglomerated sediment (floc). The sedimentation technique could not be used.
[0004]
Various attempts have been made so far to solve this problem. For example, Japanese Patent Laid-Open No. 63-49291 proposes a method of adding an inorganic flocculant to alkaline polishing waste water, then adding a polymer flocculant and a flotation aid, and then flotation and separation with pressurized water. Further, as an improvement of this method, Japanese Patent Laid-Open No. 6-134469 discloses that hydrogen is first generated in a storage tank, and then adjusted to pH 7-9, and a flocculant is added without generating hydrogen. A method of coagulating sedimentation has been proposed.
[0005]
However, these methods have a problem that the cohesive force of the metal silicon aggregates is weak, and some weak flocs are broken when passing through the pump in the process of supplying to the filtration system. Moreover, although the quality of the treated water satisfied the release standard of 20 mg / L or less as in the environmental regulations, it could not withstand the requirements of further environmental regulations.
[0006]
[Problems to be solved by the invention]
As a result of researches to solve the above problems, the present inventors have found that the cause of the deterioration of the quality of treated water and the breakage of aggregates are caused by uneven mixing of the flocculant. did. In addition, in order to mix uniformly, how to efficiently spread the flocculant inside the aggregated particles, and how to solve the problem that the particles float and are not stirred due to the generation of hydrogen in the mixing process. It became clear that there are two technical issues.
[0007]
[Means for Solving the Problems]
Therefore, as a result of further research, the present inventors believe that if the acidity of silicon cutting wastewater is maintained, the flocculant is evenly distributed, and until now, silicon fine powder always generates hydrogen in an aqueous solution. However, it has been surprisingly found that little hydrogen is generated under acidic conditions. Utilizing these findings, we developed a technology to form metallic silicon agglomerates in which a flocculant is uniformly mixed, and completed a method for efficiently separating agglomerated solids in silicon cutting wastewater, thereby reaching the present invention. .
[0008]
That is, the present invention adds aluminum chloride and / or iron chloride as a flocculant to cutting wastewater containing metal silicon cutting powder, aggregates the cutting powder at a pH of less than 7, and adjusts the pH to 10 or more. This is a method for separating metal silicon cutting powder, wherein the metal silicon aggregate is suspended.
[0009]
One of the features of the present invention is that when silicon cutting fine powder in silicon cutting wastewater is aggregated, an ideal metal silicon aggregate is formed, so that inorganic aggregation occurs at a pH low enough to prevent hydrogen generation. It is to add and mix the agent. When the pH after adding the flocculant is around 7.0 or more, it is not preferable because it causes non-uniform mixing of the solution and generation of hydrogen gas. In order not to generate hydrogen gas, it is preferable that the pH before adding the flocculant and before adjusting the pH is less than 7.0, further less than 5.0, and even less than 4.0. As the acidity increases, the generation of hydrogen gas is suppressed, and as a result, even silicon fine particles are uniformly mixed with the inorganic flocculant like a normal substance that does not generate hydrogen. This is probably because a strong oxide film is formed on the surface of the silicon particles, and the thickness of the film depends on pH. If the silicon cutting wastewater is neutral or acidic, there is no problem because the pH becomes 7.0 or less when the inorganic flocculant is added. However, if the abrasive is mixed with the cutting wastewater and becomes alkaline, before adding the inorganic flocculant, an appropriate non-aggregating acid is added, and the pH after adding the flocculant in advance is adjusted. It is important to control the pH so that it is less than 7.0.
[0010]
As the inorganic flocculant suitable for the present invention, for example, aluminum chloride and iron chloride are preferably used. These can be used alone or in combination. Aluminum chloride and iron chloride may include polyaluminum chloride and polyiron. As these inorganic flocculants, commercially available ones can be used without any limitation. In particular, the amount of waste generated from the entire process can be reduced by using waste from other steps in the polycrystalline silicon manufacturing process. For example, in the production process of trichlorosilane, a by-product is produced from a reactor that produces trichlorosilane by reacting metal silicon and hydrogen chloride, or a reactor that produces trichlorosilane by reacting metal silicon and silicon tetrachloride. As a result, the residual powder is recovered. Since these powders contain high concentrations of aluminum chloride and iron chloride in addition to silicon, mixing these powders with the above-mentioned cutting wastewater adds unnecessary inorganic compounds into the process. As a result, the amount of waste can be reduced. As a matter of course, not only when the powder is mixed with silicon cutting wastewater, but also when the powder is used alone, that is, when mixed with water instead of the cutting wastewater, the same effect is obtained on the aggregation of the powder itself.
[0011]
By sufficiently stirring the acidic solution to which the inorganic flocculant is added, the flocculant can be uniformly dispersed around each silicon particle. Since it is before the start of aggregation, there is no problem with vigorous stirring. By this operation, the strength of the finally formed metal silicon aggregate can be increased.
[0012]
By adding an alkaline compound to an acidic cutting wastewater slurry to which an inorganic flocculant has been added and is well stirred, a floating substance of metal silicon agglomerates can be formed. The type of the alkaline compound is not particularly limited, but an aqueous sodium hydroxide solution can be particularly preferably used because of the high mixing speed. The pH of the solution after adding the alkaline compound is preferably 10 or more, more preferably 11 or more. When the pH of the solution is 9.0 or lower, firm aggregates are not formed, and thus aggregates are often broken in the solution transfer process. Moreover, some ss components remain on the treated water side, and the color of the liquid becomes yellowish brown. On the other hand, when the pH of the solution is 10 or more, solid metal silicon aggregates are formed. Therefore, the aggregates are hardly destroyed even in the solution transfer step. Further, since the aggregate is strong, there is almost no ss component on the treated water side, and the color of the liquid is colorless and transparent. In the above method, a commonly used polymer flocculant or the like can also be used as an auxiliary means for aggregation.
[0013]
Since the metal silicon aggregate obtained in the present invention is strong, the aggregate can be easily recovered. Although the means is not particularly limited, for example, it can be filtered as it is. The means for filtration is not particularly limited, but if the ultrafiltration method is used, the discharged treated water becomes very clear. The agglomerates formed in the present invention are strong enough to hardly collapse even if supplied to the ultrafiltration step. As the ultrafiltration membrane suitable for the present invention, those having a molecular weight cut-off of 1000 or more, further 10,000 or more, further 100,000 or more are suitably employed.
[0014]
In addition, a flotation separation method can also be suitably used as a method other than filtration. When the pH of the liquid containing metal silicon agglomerates is 10 or more, even if a strong oxide film is formed around the silicon fine powder, the film is almost completely decomposed and hydrogen is generated after a short time. Begin to. Metal silicon agglomerates containing hydrogen bubbles begin to float, but the agglomerates are strong, so they rise fairly together. For example, the size can be 5 to 50 mm. Utilizing this property, the metal silicon aggregates can be recovered by a flotation separation method. In addition to using hydrogen gas bubbles as the floating means, pressurized water or a floating aid can be used as an auxiliary means. When the treated water after separation of ss by the above-mentioned method is discharged as general waste water, it can be performed without any problem just by neutralizing with hydrochloric acid or the like.
[0015]
【Example】
Each 200 kg of waste liquid obtained when the polycrystalline silicon rod was cut was collected, and ss in the waste liquid was collected under various conditions. The ss concentration in the cutting wastewater was 0.1%.
[0016]
Example 1
In the production process of trichlorosilane, 500 g of particles having an average particle diameter of 2 μm discharged from a fluidized bed reactor for reacting silicon and silicon tetrachloride are dispersed in 5 liters of water, and then mixed with 200 liters of cutting waste water. Then, 0.1 g of a polymer flocculant was added and stirred vigorously. The pH of the solution at this time was 4.0. Next, 10% aqueous sodium hydroxide solution was added to adjust the pH to 11.0, and the mixture was left after stirring was stopped. After the aggregates were lifted, the treated water collected at the bottom was collected and analyzed. The analysis value of this treated water is shown in Table 1.
[0017]
Example 2
16 ml of a commercially available polyaluminum chloride aqueous solution was mixed with 200 liters of cutting waste water, 0.1 g of a polymer flocculant was added, and the mixture was vigorously stirred. The pH of the solution at this time was 4.0. Next, a 10% aqueous sodium hydroxide solution was added so that the pH was 11.0, and stirring was stopped and the mixture was allowed to stand. The aggregates floated and settled repeatedly after floating. The liquid in which the aggregates float was separated using an ultrafiltration membrane having a fractional molecular weight of 100,000, and treated water was collected and analyzed. The analysis value of the treated water at this time is shown in Table 1.
[0018]
Comparative Example 1
Sodium hydroxide was added to 200 liters of cutting wastewater to adjust the pH to 11.5. Next, 16 ml of a commercially available polyaluminum chloride aqueous solution was added, and 0.1 g of a polymer flocculant was further added, stirred and allowed to stand. The pH at this time was 7.0. At this time, almost all of the aggregate was settled and separated. The analysis value of the supernatant treated water obtained at this time is shown in Table 1.
[0019]
[Table 1]

[0020]
【The invention's effect】
According to the method of the present invention, the cutting powder can be efficiently removed from the cutting waste water containing the cutting powder of metal silicon as an aggregate that is relatively difficult to break.
The waste water after removing the aggregates can be discarded as general waste water simply by adjusting the pH.

Claims (4)

Aluminum chloride and / or iron chloride is added as a flocculant to cutting waste water containing metal silicon cutting powder, the cutting powder is aggregated at a pH of less than 7, and the pH is adjusted to 10 or more to obtain a metal silicon aggregate. A method for separating metal silicon cutting powder, which is characterized by floating.   A powder containing silicon, aluminum chloride, and iron chloride, recovered as a by-product of a reaction in which metal silicon and silicon tetrachloride or metal silicon and hydrogen chloride are reacted to produce trichlorosilane, is used as a flocculant. 2. The separation method according to 1.   The separation method according to claim 1, wherein the floating metal silicon agglomerates are further recovered by a floating separation method.   The separation method according to claim 1, wherein the floating metal silicon aggregate is further recovered by an ultrafiltration method.