JPH054323B2 - - Google Patents

Description

[Detailed description of the invention]

a. Industrial Application Field The present invention relates to a method for producing silica gel, and particularly to a method for producing high-purity silica gel that can be used as a raw material for high-purity SiO ₂ . b. Prior Art Conventionally, methods of reducing silicon dioxide with carbon, methods of reducing chloride gas such as silicon trichloride, hydrogenated silane, etc. are known for producing silicon metal. However, as silicon for semiconductors,
Since highly pure silicon is required, the method described above has been used or the obtained metal silicon has been purified, resulting in poor productivity and high cost. However, the purity of silicon required for silicon for solar cells is about 59 to 79, and silicon of this degree of purity is relatively easily achieved by operations such as purifying silicon dioxide at the raw material stage. It is becoming clear that this is the case.
(Unexamined Japanese Patent Publication No. 136116, No. 56-88819, No. 88819, No. 57)
-209814) The purity of the silicon dioxide raw material used in these methods for producing silicon for solar cells is at least 4 nines or higher, and impurities such as boron, phosphorus, etc.
It is preferred that the concentrations of the transition metal and the transition metal are each 1 ppm or less. Some natural resources such as high-quality crystals exist as raw materials for high-purity silicon dioxide, but these resources are limited and expensive, making them unsuitable for mass production. High purification operations were required. As a method for achieving high purity, a flux is added to a silicon dioxide-containing raw material and melted to form a fibrous silicate glass, and this fibrous glass is leached with acid to form a powder.
A method of obtaining a SiO ₂ porous body is known. (Japanese Patent Application Laid-open No. 56-88819, Japanese Patent Application Laid-Open No. 57-209814) However, the above method requires a large amount of equipment and thermal energy to create silicate glass, and it is difficult to leaching silicate glass. Even if the shape is fibrous, there is a drawback that it takes a considerable amount of time. Furthermore, when large-diameter fibers are mixed in the silicate glass fibers, there is a risk that the silicate glass that has not been leached may be mixed into the obtained SiO ₂ raw material and become an impurity. On the other hand, a method is known in which silica gel is obtained by reacting an alkali silicate (commonly known as water glass) with an acid. Silica gel is made of highly pure SiO ₂ , and it is possible to use such silica gel for the above-mentioned direct reduction, but these silica gels usually have a SiO ₂ purity of around 99.5wt%, and even those with high purity have a purity of 99.95wt%. %, and in that state, it could not be used in the production of silicon for solar cells by the above-mentioned direct reduction. This is because with ordinary silica gel, impurities other than SiO ₂ contained in the silica gel (Na ⁺ , Ca ²⁺ , Mg ²⁺ , etc.) are difficult to escape into the silica gel external cleaning solution.
This was due to the difficulty of achieving high purity. It also has excellent filtration and cleaning properties, and has a water content of 60wt.
A method for producing silica gel that is easy to dry and has a dry content of less than
This method of manufacturing silica gel involves mixing and reacting alkali silicate and mineral acid so that the SiO ₂ concentration is 15 wt% or more, but even with this method, it is difficult to clean the silicon to the purity expected as a silicon raw material for solar cells. It was hot. c. Purpose of the Invention The present invention provides a manufacturing method for obtaining silica gel having a purity that can be used for manufacturing silicon for solar cells, and in particular, provides a manufacturing method that can obtain silica gel with a purity that can be used for manufacturing silicon for solar cells.
The purpose is to provide a method for producing silica gel that provides SiO ₂ purity. d Structure and operation of the invention The present invention provides SiO ₂ to form a reaction solution at a concentration of 15% by weight or more, the reaction solution is maintained at room temperature by cooling, and silica gel in which the organic compound is bonded to silicic acid molecules is precipitated from the reaction solution. This is a method for producing high-purity silica gel, in which the precipitated silica gel is then washed with mineral acid. As the alkali silicate in the present invention, alkali silicates with various SiO ₂ /R ₂ O ratios such as lithium silicate, sodium silicate, and potassium silicate can be used. Usually, sodium silicate aqueous solution JIS No. 3 standard ( _SiO2 28~30wt%, Na2O9 _~
10wt%, specific gravity 1.385 or more) is preferred because it is inexpensive and available. The alkali silicate to be mixed with the mineral acid is preferably an aqueous alkali silicate solution prepared by appropriately diluting the alkali silicate with water. Alkaline silicate aqueous solution is obtained by dissolving a molten product of alkali metal silicate in water under high pressure, or by dissolving amorphous silicate (such as amorphous silica or diatom produced during silicon manufacturing) in alkali hydroxide. Those made by various manufacturing methods can be used. As the mineral acid for the reaction, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and mixtures thereof can be used. Among them, sulfuric acid is
It is effective and preferred for removing impurities such as Ti and Zr. Among them, 3-8N sulfuric acid is preferable. Here, the alkali silicate and mineral acid are mixed to form a reaction solution, and the concentration and amount of the alkali silicate and mineral acid are adjusted so that the SiO ₂ concentration in the reaction solution is 15wt% or more, preferably 20wt% or more. It is desirable to select. When the SiO ₂ concentration in the reaction solution is 15wt% or more, silica gel with excellent filterability and washability and low content can be obtained, as is clear from JP-A-59-13620, and the synergistic effect with the effect of the present invention can be obtained. Desirable for having. Further, it is preferable that the SiO ₂ concentration in the reaction solution is 20 wt% or more because the above effect becomes even more remarkable. The present invention is characterized in that a water-soluble organic compound having at least one hydroxyl group or ketone group is included in the reaction solution used for producing silica gel. By including the above-mentioned organic compound in the above-mentioned reaction solution, this organic compound moderately reacts with the silanol groups of silicic acid molecules to form bonds such as esters during silica gel production. This makes it possible to prevent the silicic acid molecules from becoming large, thereby creating an appropriately fine-grained silica gel structure with a low water content that is easy to wash and filter in subsequent steps. The organic compound used here is preferably a water-soluble organic compound having a hydroxyl group or a ketone group because it reacts with the silanol group and reduces the condensation reaction. Polyethylene glycol is preferred. Similar effects can be obtained with ketones (eg, methyl isobutyl ketone) and monohydric alcohols (eg, butyl alcohol), but they are highly volatile and require appropriate measures to reduce odor and work environment. Water-insoluble materials are not preferred because they require time and effort to mix and react. Although amines and the like may have a similar effect, they are not preferred because their reaction with silanol groups is too strong and the resulting silica gel particles become too fine, requiring much effort for washing and separation. The amount of organic compound added should be determined based on the concentration of the aqueous solution containing the alkali silicate, the initial concentration of the reaction acid, the temperature of the mixed solution, the acid addition rate, etc. It is desirable that the amount is 1 to 40 parts by weight. How to incorporate organic compounds into the reaction solution:
If the concentration of the alkali silicate, which is added to the mineral acid in advance, is high, when the organic compound is added to the aqueous alkali silicate solution, the alkali silicate and the organic compound will react, forming a rubber-like product. This is time consuming as it has to be crushed before the mineral acid is added. Therefore, it is preferable to add the organic compound to the mineral acid in advance, and then add the aqueous alkali silicate solution thereto. The silica gel thus prepared is then washed with acid and with pure water, resulting in high-purity silica gel. The acid used for cleaning can be hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid, as well as mixtures thereof, similar to the mineral acids used for silica gel production.
12N sulfuric or hydrochloric acid and mixtures thereof are preferred. Among the above, those of 3 to 8 are more desirable. These washing operations are desirably carried out at a temperature of 15 to 100°C, more preferably 60 to 95°C. By performing acid washing, the condensation reaction of the silanol groups in the silica gel progresses, and the silica gel is converted to silica gel with a lower water content, and at the same time, the alkali metal or alkaline earth metal ions contained in the silica gel are removed from the washing acid. is extracted. Next, the present invention will be explained based on examples. e Examples Example 1 Add polyethylene glycol to 9 ml of 36-normal sulfuric acid.
200 was added and mixed well. To this, 100 g of No. 3 sodium silicate (JIS standard) was added in about 3 minutes while stirring. Since intense heat generation occurred in the above operation, the reaction was carried out in an ice bath and the reaction temperature was maintained at room temperature. The generated gel has a rough shape and is highly viscous, so after the addition of sodium silicate, continue stirring for about 1 minute using a powerful stirrer to make this silica gel about
It was ground into particles with a size of about 1000 μm. The pH at the end of the reaction was approximately 3. This gel was placed in 200 ml of 6N sulfuric acid and heated to 90°C and maintained for 6 hours. When the above solution was suction filtered with a water pump using Toyo Filter Paper No. 5A in a Buchner funnel with a diameter of 11 cm, it took about 2 minutes to filter the entire amount. Filtered dehydrated silica gel is dried silica gel 100
A hydrated silica gel containing about 180 parts by weight of water and a water content of about 180% was obtained. The obtained hydrous silica gel was added to 200 ml of 6N hydrochloric acid, heated to 90°C, and maintained for 4 hours. Thereafter, when it was suction-filtered in the same manner, it took about 1 minute and 40 seconds to filter the entire amount, and a hydrous silica gel with a water content of about 140% was obtained. Furthermore, add this hydrated silica gel to 200ml of 6N hydrochloric acid.
After heating to 90°C and holding for 6 hours, filtration was performed in the same manner. Filtration time is approximately 1 minute 20 seconds and the water content is approximately
A 100% hydrous silica gel was obtained. Finally, add this hydrated silica gel to 200ml of 6N hydrochloric acid.
In addition, the silica gel was held at 90°C for 6 hours and filtered after filtration, and the water content of the hydrated silica gel was about 90%. This hydrous silica gel was washed several times with 400 ml of pure water, and the water washing was finished when the pH of the filtrate reached approximately 5.4. The hydrated silica gel thus obtained (water content approximately 90
%) was dried at 150°C for 10 hours. The dry silica gel obtained weighed about 25 g. This dried silica gel was decomposed with HF and impurities were analyzed using a plasma emission spectrometer. The results are shown in Table 1. Table 1 shows, as Comparative Example 2, the results of analyzing impurities in commercially available silica gel for column chromatography (C-100 manufactured by Wako Pure Chemical Industries, Ltd.), which is considered to be a conventionally obtained high-purity silica gel, using the same analysis method as above. It is also shown in Table 1. As can be seen from Table 1, the silica gel produced according to the present invention has a much higher purity than that obtained conventionally. Comparative Example 1 Polyethylene glycol 200 in Example 1
The same procedure was performed, except that .
The dry silica gel obtained weighed about 25 g, and the impurity measurement results are shown in Table 1. Further, the filtration time required for filtration after each acid washing and the water content of the silica gel obtained at that time are shown in Table 2. It can be seen from Table 1 that the silica gel obtained in Example 1 is easily purified compared to that obtained by conventional methods. Further, as can be seen from Table 2, the water content in the Examples was lower than that obtained by the conventional method, which had excellent filtration and cleaning properties, and it was also found that the filtration properties were even better. f. Effects of the Invention According to the present invention, the filtration and cleaning properties are excellent.

【table】

[Table] Excellent silica gel was obtained, and silica gel with a low concentration of metal impurities such as Al and Fe caused by the alkali silicate raw material (for example, silica gel with a purity of 99.99 wt% or more as is clear from the examples) was obtained. It is being This is a mixture of an alkali silicate and a mineral acid that contains a water-soluble organic compound having at least a hydroxyl group or a ketone group, and is cooled so that the silica gel production reaction proceeds at room temperature. This is thought to be because the rapid condensation reaction of the silicate silanol groups is inhibited. The silica gel produced according to the present invention is useful as a raw material for high-purity glass or a raw material for Si for solar cells. When the present invention is used to produce silicon dioxide, which is a raw material for Si for solar cells, in addition to mixing the silica gel obtained by the present invention with carbon and reducing it, finely powdered carbon is It is also possible to uniformly disperse carbon in a mixed solution for producing silica gel and use the silica gel in which carbon is dispersed for reduction. Here, incorporating finely divided carbon into the silica gel has the advantage that the silica gel and carbon can be mixed more uniformly, and that the silicon dioxide raw material and the carbon raw material can be simultaneously subjected to high purification operations such as acid washing.

Claims (1)

[Claims] 1. A mineral acid pre-contained with a water-soluble organic compound having at least one hydroxyl group or a ketone group and an alkali silicate are mixed together to reduce the SiO ₂ concentration. The reaction solution is mixed to contain 15% by weight or more, the reaction solution is maintained at room temperature by cooling, and silica gel in which the organic compound is bonded to silicic acid molecules is precipitated from the reaction solution, and then the precipitate is formed. A method for producing high-purity silica gel, in which the produced silica gel is washed with mineral acid. 2. The method for producing high-purity silica gel according to claim 1, wherein the water-soluble organic compound is polyethylene glycol or methyl isobutyl ketone. 3. The method for producing high-purity silica gel according to claims 1 to 2, wherein the amount of the water-soluble organic compound added is 1 to 40 parts by weight per 100 parts by weight of the alkali silicate. 4 The amount of the organic compound added is 100% of the alkali silicate.
The method for producing high-purity silica gel according to claims 1 to 3, wherein the amount is 1 to 40 parts by weight.