JP2928020B2 - Silicon recovery method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering silicon from an aqueous solution in which fine silicon particles are suspended.

[0002]

2. Description of the Related Art A semiconductor silicon chip is manufactured by cutting and polishing a silicon ingot. The shavings generated in the step of cutting the ingot and the step of polishing the cut wafer are contained in a state where fine silicon particles are suspended in water used in the step of cutting and polishing. This quantity has become enormous with the development of semiconductors.

[0003]

The fine silicon particles generated in the above steps are hard to aggregate and do not settle easily. This is generally thought to be for the following two reasons.
One is that since the surface of the silicon particles is charged, there is an electrical repulsion between the particles. The other is that the surface of the silicon particles is oxidized because the surface is oxidized, and steric hindrance by the hydrated layer inhibits interparticle bonding.

On the other hand, as a method for recovering such silicon particles, a physical method and a chemical method are known.

[0005] As a physical method, there is a method of filtration or centrifugation, but the silicon particles are too practical to be clogged with filtration because the particle size is too small, which is not practical. There is also a centrifugal separation method. However, the concentration of silicon particles is too low with respect to moisture, so that the efficiency of centrifugal separation is low, and equipment and operation are costly.

As a chemical method, there is a coagulation precipitation method using various coagulants. In order to remove the surface charge, there are inorganic acids such as sulfuric acid and hydrochloric acid and inorganic salts such as aluminum sulfate. However, since there is a hydrated layer, the efficiency is low and it is necessary to add a large amount. As a result, the quality of the recovered silicon deteriorates, and a great deal of time is required for cleaning. Also, if these are washed off with water or the like, the surface is charged again, so that they are suspended, which greatly hinders the recovery. Alcohols may be used to remove the hydrated layer, but have a problem in that the efficiency is poor because the effect of eliminating charges is not so high. There is also a polymer flocculant for cross-linking between particles, but it is difficult to remove it from the recovered silicon, and it is difficult to improve the quality of silicon. On the other hand, it has been reported that hydrofluoric acid is added to dissolve an oxide layer on the surface of silicon particles, thereby making it difficult to hydrate and causing coagulation and precipitation (Japanese Patent Laid-Open No. 61-215211).
No.). However, for efficient coagulation and sedimentation, the charge quenching effect, which is another factor, is also necessary.If hydrofluoric acid is added in an amount necessary for this purpose, the silicon is dissolved by the hydrofluoric acid and the silicon is recovered. The rate drops.

The present invention has been made in view of the above circumstances, and eliminates the surface charge of silicon particles suspended in water and, at the same time, does not form a hydrated layer by hydrophobically modifying the surface. And then collect the silicon efficiently and in high yield with a coagulant that can be easily removed after the coagulation and separation have been performed to minimize the amount of dissolved silicon. The purpose is to purify purified silicon.

[0008]

In order to achieve the above object, in the method for recovering silicon according to the present invention, hydrofluoric acid and a water-soluble inorganic electrolyte are added as an aggregating agent to an aqueous solution in which silicon particles are suspended. It aggregates and separates particles.

Further, the silicon particles separated from the aqueous solution are subjected to a water washing treatment to purify the recovered silicon.

[0010]

The suspension of silicon particles in the present invention includes, for example, an aqueous solution containing fine silicon particles generated in a step of cutting a silicon ingot or a step of polishing a silicon wafer.

The average particle size of these actual silicon particles is
Generally, it is about 10 μm to about 0.1 μm. In addition, the present invention can be applied to all aqueous solutions in which silicon particles that are suspended and hardly settle exist wholly or partially.

Next, hydrofluoric acid to be added to this suspension refers to a normal one, and there is no particular limitation. It can be used by diluting it with water.

[0013] The water-soluble inorganic electrolyte refers to an inorganic acid or an inorganic salt which can be dissociated and dissolved in water. For example, examples of the inorganic acid include hydrochloric acid and sulfuric acid. The inorganic salt is a salt of an inorganic acid and an inorganic base, for example, fluorides such as ammonium fluoride, sodium fluoride, and potassium fluoride; And iron salts such as ferrous sulfate and ferric sulfate, and aluminum salts such as ammonium sulfate.

The amount of hydrofluoric acid added to the suspension of the present invention is preferably at least 0.01 mol / l, and
mol / l, more preferably less than 0.2 mol / l. If it is less than 0.01 mol / l, the effect of improving the sedimentation speed of the silicon particles will be small. Also, 0.
If it is 5 mol / l or more, the tendency to dissolve silicon becomes strong, and the recovery rate of silicon decreases. Therefore, it is preferable to minimize the amount of addition.

The amount of the water-soluble inorganic electrolyte to be added together with the hydrofluoric acid is preferably at least 0.01 mol / l, and if it is less than 0.01 mol / l, the effect of improving the sedimentation speed of the silicon particles is reduced. The upper limit of the addition amount is not particularly limited as long as it is an amount capable of efficient coagulation separation,
It is more efficient and less troublesome to remove the recovered silicon by washing it with water if it is minimized.

In the present invention, the hydrofluoric acid and the water-soluble inorganic electrolyte attached to the silicon separated by coagulation and precipitation can be removed to improve the purity of the silicon by washing with water. Specifically, it can be performed by repeating decantation with water or adding water to the silicon residue during filtration.

As described above, when the combined use of hydrofluoric acid and a water-soluble inorganic electrolyte of the present invention is used as a flocculant, silicon can be recovered efficiently and with little loss. Such an effect combines the hydrophobizing effect of hydrofluoric acid on the surface of silicon particles with the charge elimination effect of the hydrofluoric acid and the water-soluble inorganic electrolyte on the silicon particle surface. It is considered that the use of the combination significantly reduced the amount of the compound. In addition, the recovered silicon can be easily purified to a high degree, and it is difficult to resuspend even after washing, and can be efficiently separated and collected.However, since hydrofluoric acid and water-soluble inorganic electrolyte are soluble in water, these can be easily washed by water. This is probably because the silicon particles can be removed and the surface of the silicon particles is hydrophobically modified, so that they are easily aggregated.

[0018]

Embodiments of the present invention will be described below.

A coagulant consisting of hydrofluoric acid and a water-soluble inorganic electrolyte was added to the aqueous solution in which the silicon particles were suspended to prepare the mixtures shown in Examples 1 to 10. Table 1 shows the average particle size of the silicon particles, the silicon concentration, the type of the added hydrofluoric acid, and the type and amount of the water-soluble inorganic electrolyte for each example.

Next, 80 ml of each sample was stirred for 3 minutes and then left to measure the sedimentation rate of silicon. The sedimentation speed was determined by collecting the supernatant at a fixed time and measuring the turbidity. The method for measuring turbidity is JIS K01
According to 01 9.2, a calibration curve was prepared using a kaolin standard solution. Table 3 shows the measurement results.

The amount of silicon dissolved by the addition of these flocculants can be determined by filtering a sample solution 10 hours after the addition of the flocculant with a 0.1 μm filter paper, and subjecting the filtrate to atomic absorption analysis. I asked. The dissolution rate of silicon was calculated from the dissolution amount, and the results are shown in Table 3.

With respect to the samples of Examples 1 and 6, the precipitated silicon was filtered, and the operation of adding 100 ml of pure water to the silicon residue on the filter paper and suctioning was repeated three times to wash. The washed silicon was transferred to a beaker, and 80 ml of pure water was added. After stirring for 3 minutes, the re-sedimentation property was measured. The measurement of the sedimentation velocity was measured by the same method as described above. Table 3 shows the measurement results as Examples 11 and 12.

For the sample of Example 9, the operation of decanting the sedimented and separated silicon supernatant, adding 100 ml of pure water, stirring and leaving, and decanting was repeated three times. And 80m of pure water
After the addition and stirring, the sedimentation velocity was measured in the same manner as in the previous example. Table 3 shows the measurement results as Example 13.

Further, with respect to Examples 11 to 13, the electric conductivity of the supernatant liquid when the sedimentation property was observed was measured, and the cleaning degree of silicon was examined. Table 5 shows the results.

For comparison, the samples shown in Table 2 to which no coagulant was added, the samples to which hydrofluoric acid or other coagulants were added were designated as Comparative Examples 1 to 10, and
After the suspension was stirred for 3 minutes, the sedimentation rate of silicon and the dissolution rate of silicon were measured in the same manner as in the example. Table 4 shows the results. Then, the electrical conductivity of the silicon suspensions of Comparative Examples 1 and 2 was measured, and the results were compared with Comparative Examples 11 and 12, respectively.
As shown in Table 5.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

As is clear from the comparison between the examples and the comparative examples, when hydrofluoric acid and a water-soluble inorganic electrolyte are added to the suspension of silicon particles as a flocculant,
It has been found that silicon particles can be coagulated and precipitated efficiently and with little loss and can be separated. In addition, it was found that the recovered silicon could be washed with water to be highly purified, and the sedimentation and separability after washing with water was good, and it was difficult to resuspend.

[0032]

As described above, according to the present invention, silicon can be efficiently separated and recovered from a suspension of silicon particles at a high yield. In addition, it is possible to purify the recovered silicon as needed, and as a result, it is possible to effectively recycle them.

Continuation of front page (72) Inventor Minoru Tsurumi 30-2-1-103 Enokaoka, Midori-ku, Yokohama-shi, Kanagawa Prefecture (56) References JP-A-4-358594 (JP, A) JP-A-4-27425 ( JP, A) JP-A-3-84098 (JP, A) JP-A-61-215211 (JP, A) JP-A-48-37958 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , (DB name) B01D 21/01 C02F 1/52 C01B 33/02-33/039

Claims (2)

(57) [Claims] 1. A method for recovering silicon, characterized in that hydrofluoric acid and a water-soluble inorganic electrolyte are added as an aggregating agent to an aqueous solution in which silicon particles are suspended, whereby silicon particles are aggregated and precipitated to separate silicon. 2. The method for recovering silicon according to claim 1, wherein the silicon particles separated from the aqueous solution are subjected to a water washing treatment to purify the recovered silicon.