JPH0468246B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing fine silica powder.
More specifically, the present invention relates to a method for efficiently producing fine powder of high-purity silica, which can be used as a raw material for producing silicon for solar cells, through simple operations, for example. Silicon for solar cells is generally required to be highly pure silicon on the order of 5 nines to seven nines in order to improve the efficiency of converting solar energy into electrical energy. As a method for producing such high-purity silicon, a method is already known that uses high-purity silica as a raw material and reduces it to produce high-purity silicon [for example, U.S. Pat. No. 4,247,528] Calligraphy (= Tokukaisho
55-136116)]. In order to produce high-purity silicon for solar cells using the above reduction method, the silica used as a raw material is generally required to have a purity of 4 nines or more, and in particular, boron, phosphorus and It is desired that the concentration of each impurity such as a transition metal is 1 ppm or less. Natural resources such as high-quality crystal can be used as raw material silica with high purity, but such natural resources are limited and expensive, so industrial mass production is not possible. It is not suitable for Therefore, various methods of producing high-purity silica from easily available and inexpensive silicates have been studied. As an example of such conventional methods, silica-containing materials (e.g. silica sand, feldspar, etc.)
A method has been proposed in which a flux is added and melted to form a fibrous silicate glass, and this fibrous glass is leached with acid to obtain a powdered silica (SiO ₂ ) porous material. (See US Pat. No. 4,294,811, DE-OS3123009). However, in order to obtain high-purity silica by leaching, it is necessary to limit the glass composition to one that facilitates leaching, and as a result, in addition to silicon dioxide, large amounts of aluminum oxide and alkaline earth metal salts are added to the glass components. As a result, it is unreasonable to have to remove all large amounts of components other than silicon dioxide after the glass is made into fibers. Furthermore, when large-diameter fibers are mixed in with the silicate glass fibers, there is a risk that unleached silicate glass will mix into the obtained silica 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 [for example, J.G. Beil (J.
G.Vail), “Soluble Silicates”
Silicates” (ACS monograph series),
(See Reinhold, New York, 1952, Vol. 2, p549, etc.) Silica gel is composed of SiO ₂ with relatively high purity, and it is possible to use such silica gel as a raw material for producing silicon by the reduction method described above, but these silica gels are usually
The purity of SiO ₂ is around 99.5wt%, and even if it is of a grade called high purity, it is around 99.95wt% at most, and it cannot be used as it is as a raw material for manufacturing silicon for solar cells, which requires a purity of 4 nines or more. I can't. Therefore, an object of the present invention is to provide a simple and efficient method for producing high-purity silicon fine powder that can be used as a raw material for producing silicon for solar cells. According to the present invention, (a) a water-miscible monovalent silicate is added to an aqueous solution of an alkali metal silicate having a SiO ₂ concentration of 9 to 20% by weight until solid precipitation occurs and the solution becomes cloudy or just before that. and (b) adding to the resulting mixture at least one water-miscible alcohol selected from dihydric alcohols in an amount of 35 to 80% of the stoichiometric amount for neutralizing the alkali metal silicate. (c) The resulting slurry is then brought into contact with the mineral acid at an acidity of at least 1N or more. provided. It should be noted that it should be understood that the term "silica" used in this specification includes not only anhydrous SiO ₂ but also hydrated silica (SiO ₂ xH ₂ O). The alkali metal silicates used as starting materials in the process of the invention include lithium silicate,
These include sodium silicate, potassium silicate, and the like, with sodium silicate being particularly preferred. The SiO ₂ /M ₂ O ratio in these silicates (where M
represents an alkali metal) is not particularly limited and can be freely selected from a wide range, but generally SiO ₂ /M ₂ is within the range of 2 to 4, preferably 2.5 to 3.5.
A material having an O ratio (weight) is suitable. In particular, sodium silicate (SiO ₂ : 28-30wt%, Na ₂ O 9-10wt%
%, specific gravity: 1.385 or more) is inexpensive and industrially easy to handle. Furthermore, as alkali metal silicates that can be prepared manually in the form of an aqueous solution, melt-molded products of alkali metal silicate dissolved in water under high pressure, or amorphous silicates (amorphous silica or Diatoms, etc.) dissolved in alkali hydroxide can also be used as starting materials in the method of the invention. According to the method of the present invention, first, an aqueous solution of an alkali metal silicate as described above is prepared. At that time, the concentration of alkali metal silicate in the aqueous solution is
The concentration is such that the SiO ₂ concentration is in the range of 9 to 20% by weight, preferably 11 to 17% by weight, and more preferably 12 to 16% by weight. However, within the above range
If an aqueous solution of alkali metal silicate with a SiO ₂ concentration can be produced industrially, it can be used as it is or further diluted with water, or
If an aqueous solution of an alkali metal silicate having a SiO ₂ concentration can be produced industrially, it can be diluted with water to obtain an aqueous alkali metal silicate solution according to the present invention. For example, an alkali metal silicate aqueous solution that can be suitably used in the present invention can be obtained by diluting the aforementioned JIS No. 3 standard sodium silicate aqueous solution with water to a volume of 1.5 to 5 times. A water-miscible alcohol selected from water-miscible monohydric and dihydric alcohols is first added to the alkali metal silicate aqueous solution prepared in this way. As used herein, the term "water-miscible alcohol" refers to an alcohol having a solubility in water (distilled water) at 25° C. of at least 50 g/or more, preferably 100 g/or more. However, as a water-miscible monohydric alcohol,
For example, methyl alcohol, ethyl alcohol,
Examples include alkanols such as n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, and 2-hexanol. Among these, alkanols having 1 to 4 carbon atoms are preferred, and especially, Number of carbon atoms: 1
~3 alkanols are preferred. Furthermore, the water-miscible dihydric alcohol may be of any type with low molecular weight, such as alkanediols such as ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol; and polyalkylene oxides such as polyethylene oxide having an average molecular weight of 1000 or less, preferably 200 to 600, and polypropylene oxide having an average molecular weight of 900 or less, preferably 200 to 400, among which alkylene diols having 2 to 4 carbon atoms. and polyalkylene oxides having an average molecular weight of 200 to 600 are preferred, particularly polyalkylene oxides having an average molecular weight of 200 to 600.
400 polyethylene oxide is preferred. The water-miscible alcohols mentioned above can be used alone or in combination of two or more. Water-miscible alcohols which can be used particularly advantageously include polyethylene oxides having an average molecular weight of 200 to 400. When the above-mentioned water-miscible alcohol is gradually added to an aqueous alkali metal silicate solution, solid precipitation occurs after a certain amount has been added, making the solution cloudy, and as the addition continues, the precipitation gradually increases. go. One major feature of the present invention is that the addition of a water-miscible alcohol to an aqueous alkali metal silicate solution causes precipitation of solids and the solution begins to become cloudy (this point is defined herein as the "turbidity point"). The goal is to stop at that point. As a result, precipitation of the fine silica powder in the subsequent step (b) and leaching of impurities from the fine silica powder with mineral acid in the step (c) can be carried out smoothly and efficiently. It becomes possible to obtain fine powder of high purity silica. When the water-miscible alcohol is added in an amount far below the "cloud point", gelation occurs when the mineral acid is added in step (b), making it difficult to obtain a fine powder precipitate, and instead forming a paste-like mixture. Temperature: Significantly inferior precipitation tends to occur. On the other hand, if the amount is added significantly above the "turbidity point", it becomes difficult to obtain highly pure silica. The reason for this is probably that due to the dehydration effect of alcohols, leaching with mineral acids is not sufficiently carried out and a large amount of silicate precipitates are formed. Therefore, a suitable amount of water-miscible alcohol to be added is within a range of ±30% or less, more preferably within a range of ±20% or less of the amount of alcohol at the "cloudy point", centered on the "cloudy point", Most preferably, it is within a range of ±10% or less. As used herein, the "turbidity point" can be determined by the method described below for the starting aqueous alkali metal silicate solution to which the method of the present invention is applied. That is, the alkali metal silicate aqueous solution having a SiO ₂ concentration of 9 to 20% as a starting material is used as SiO ₂ and approximately
Take an amount equivalent to 100g, put it in a 2-liter beaker, and stir using a magnetic stirrer (stirring bar length: 40mm) at a speed that allows the vortex generated by the rotation to reach the stirring bar. Add the water-miscible alcohol at a rate of approximately 10 ml/min.
Dilute turbidity may occur in the solution during the process, but by continuing to add water-miscible alcohol and observing with the naked eye, the turbidity (TURVIDITY) TE/F = 100 is equivalent to C2-5 of DIN38404. do
Compare with the STANDARD FORMAZINE SUSPENSION solution and stop adding alcohol when the same turbidity is reached. The amount of water-miscible alcohol added up to this point is defined as the "turbidity point."
After the turbidity point is exceeded, the turbidity changes rapidly, so it can be determined without using a meter. The addition of water-miscible alcohol to aqueous alkali metal silicate solutions is generally carried out at 15-50°C, preferably at 20°C.
It is desirable to carry out the reaction at a temperature of -40°C with uniform stirring to prevent localized precipitation as much as possible. Further, in order to achieve uniform stirring and mixing, the water-miscible alcohol may be added after being diluted with water in advance in some cases. The water-miscible alcohol can be added continuously or intermittently, and the rate of addition is not critical and depends on the type or concentration of the alkali metal silicate, the type of alcohol, the degree of stirring, etc. It is desirable to avoid the formation of localized precipitates. Alternatively, they may be added while being mixed using a static mixer or the like. To the aqueous alkali metal silicate solution to which the water-miscible alcohol has been added and mixed as described above, mineral acid is then added to precipitate silica as a fine powder. Examples of mineral acids that can be used for this precipitation include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, or two of these.
Among these, sulfuric acid and nitric acid, especially sulfuric acid, are preferred, and these acids usually have a concentration of 2 to 18N, more preferably 3 to 12N.
Can be used at specified concentrations. The amount of these mineral acids added should be within the range of 35 to 80% of the stoichiometric amount required to neutralize the alkali metal silicate, depending on the type of mineral acid used, the alkali metal silicate, and the water miscibility. Although it can be changed depending on the type and concentration of alcohol, preferably the stoichiometric amount is
It can be within the range of 50-70%. When the amount of mineral acid added is less than 35% of the stoichiometric amount, the amount of silica precipitated is small and the yield of the desired fine powder silica is reduced. It is thought that the unprecipitated silica is colloidal or sol-like silica, and cannot be captured by over-operation, resulting in a decrease in the yield of fine powder silica. On the other hand, when the amount of mineral acid added exceeds 80% of the stoichiometric amount, the pH of the reaction solution approaches neutrality and the gelation rate increases, resulting in formation of gelled silica in addition to granular precipitation. In addition to impairing properties, it becomes difficult to obtain high purity silica. Addition of mineral acid is controlled while stirring to suppress gel formation as much as possible (taking care not to substantially increase the viscosity of the mixture) and to ensure that silica precipitates as a solid fine powder. However, it is desirable to do this gradually over a certain amount of time. In particular, it is desirable to slowly add mineral acid in an amount up to 35% of the stoichiometric amount required to neutralize the alkali metal silicate, and the rate is Although it depends on the concentration of the alkali metal silicate in the aqueous solution and the concentration of the mineral acid added, it is generally convenient to conduct the reaction for at least 5 minutes, preferably for 10 minutes or more. More specifically, the SiO ₂ concentration in the alkali metal silicate aqueous solution used as the starting material was 12
~16% by weight, usually for at least 10 minutes
It is optimal to add over 15 to 60 minutes, and it is desirable to add over a longer time as the SiO ₂ concentration in the aqueous alkali metal silicate solution increases. Furthermore, as mentioned above, it is preferable to add the mineral acid while stirring strongly to prevent gel formation, even if it is only locally. It is desirable to perform this continuously or intermittently, monitoring with a PH meter as necessary to ensure that the pH does not fall below 10.5. The temperature of the mixed solution during this addition is generally maintained at a relatively low temperature within the range of 15 to 50°C, more preferably 20 to 40°C. As described above, by adding mineral acid to the mixture of aqueous alkali metal silicate solution and water-miscible alcohol, silica is gradually precipitated in the form of fine powder as the mineral acid is added, forming a slurry. arise.
The slurry after addition of mineral acid may be transferred to the next step immediately, but in some cases, it may be transferred for a certain period of time.
For example, the slurry may be aged for 5 to 30 minutes, and the aging can be carried out by maintaining the resulting slurry at room temperature while stirring. The slurry thus produced is then further contacted with a mineral acid to leach impurities from the precipitated silica powder in the slurry. The mineral acid used for this leaching can be the same as that used to generate the precipitate of fine silica powder, and it does not necessarily have to be the same type of acid used to generate the precipitate of fine silica powder. However, acids of the same type are generally preferred, and among these, sulfuric acid and nitric acid, especially the former sulfuric acid, are preferred. The above-mentioned slurry and mineral acid can be brought into contact by adding the mineral acid to the slurry, or by adding the slurry to the mineral acid and thoroughly stirring and mixing the mixture. The rate of addition of the mineral acid or slurry in this case is not critical and is arbitrary, but preferably at a rate sufficient to allow intimate contact between the silica powder in the slurry and the mineral acid. The amount of mineral acid used is such that the acidity of the slurry after mixing is at least 1N or more, preferably 1.5 to 4N,
More preferably, the amount is within the range of 2 to 3 normal. If the acidity is lower than 1N, leaching will be insufficient and high purity silica will be difficult to obtain, and it is economically undesirable to maintain the acidity high. It is desirable to contact the fine silica powder in the slurry with the mineral acid at a relatively high temperature of 70°C or higher to promote leaching, especially
Temperatures within the range of 90-100°C are preferred. Also,
The duration of said contact is usually 10 minutes or more, more preferably
It is within the range of 30 minutes to 3 hours. The slurry from which impurities have been extracted by leaching as described above is then filtered to recover fine silica powder. The percake is further washed with mineral acid and/or water if necessary. Hydrochloric acid or nitric acid is suitable as the mineral acid used for this cleaning, and its concentration can generally be 1 to 12N, preferably 2 to 8N. By this acid washing, highly hydrolyzable impurities such as aluminum, titanium, and zirconium that may be present in the overcake can be further removed, thereby making it possible to obtain silica of higher purity. The above-mentioned washing with mineral acid can be carried out at a temperature of usually 60 to 100°C, and the washing with water can be carried out one or more times at a temperature of usually 15 to 50°C. The final silica gel fine powder produced is e.g.
It is dried at a temperature of 100-150℃. This generally results in a finely divided silica powder of high purity with a particle size in the range of 5 to 500 microns. According to the method of the present invention described above, in the silica precipitation step, silica is precipitated in a state where it is easily leached in the acid leaching step that follows the step, so that the silica fine powder obtained by the present invention can be obtained as described below. As is clear from the examples, it has extremely high purity, that is, 4 nines or more, and can be used satisfactorily as a raw material for producing silicon for solar cells. Moreover, the slurry of fine silica powder obtained by the method of the present invention is extremely hypersensitive, and the method of the present invention has very good operability. Next, the present invention will be explained in more detail with reference to Examples. Example 1 Mix 375 g of water glass (SiO ₂ concentration approximately 30 wt%) with a SiO ₂ /Na ₂ O molar ratio of 3.2 and 450 ml of water to obtain 13.6 wt%
A raw material solution containing SiO ₂ was prepared. While stirring this raw material solution, add an average molecular weight of 200 to this solution.
of polyethylene oxide was slowly added at a rate of about 10 ml/min. When 73 ml of polyethylene oxide was added, the solution started to become cloudy, so the addition of polyethylene oxide was stopped. Then, while stirring the solution, add sulfuric acid aqueous solution (25wt%) to this solution.
Slowly pour 80ml (at a speed of about 10ml/min)
Added. After the addition of sulfuric acid was completed, stirring was continued for an additional 5 minutes to prevent non-uniform pH. Addition of sulfuric acid produced a precipitate in the solution, and the amount of precipitate increased as the sulfuric acid was added, but no noticeable change in solution viscosity was observed. The particle size of the generated precipitate was approximately 1000 μm or less. All operations up to this point were performed at room temperature without any special operations such as heating. The addition of 80 ml of 25 wt% sulfuric acid corresponded to 50% of the amount of acid required to neutralize 375 g of water glass, and the pH of the mixture was about 11. Thereafter, 750 ml of sulfuric acid (25 wt%) was added to the reaction solution in which a precipitate was formed by the above operation for the purpose of washing impurities, and the mixture was heated to 90° C. and maintained for 2 hours. The acid concentration of the mixed solution at this time corresponds to about 24 normal. Since most of the precipitate in the mixed solution had settled, the mixture was stirred for about 5 seconds every 30 minutes before and during heating. The solution was then filtered under suction to separate and remove the precipitate. This precipitate contained approximately 200 parts by weight of water per 100 parts by weight of SiO ₂ (water content approximately 200%).
) was made of silica. Add 750ml of hydrochloric acid (20wt%) to the obtained silica.
was added, heated to 90°C and held for 1 hour, and the solution was filtered under suction to separate the silica again. Thereafter, the same hydrochloric acid washing was repeated twice, and the washing with purified water and filtration were repeated until the pH of the solution was 4 or higher. The obtained silica was dried at 150°C for 10 hours and weighed to give about 100g of silica. The impurity concentration in the obtained silica was measured by plasma emission spectrometry and mass spectrometry. The results are shown in Table 1 below. In addition, the results of similarly measuring the amount of impurities in commercially available silica gel for chromatographic analysis (Wako Pure Chemical Industries, Ltd. C-100) and industrial silica gel, which are said to be of high purity, are shown in Comparative Example 1 and Comparative Example 2 in Section 1 below. Also shown in the table. As is clear from Table 1, the silica produced by the production method of the present invention has a much higher purity than conventional commercially available silica. Example 2 The amount of polyethylene oxide (average molecular weight 200) added was 75 ml (the turbidity point was 73 ml, so the turbidity point was
About 105 g of silica was obtained by carrying out the same operation as in Example 1, except that the amount of sulfuric acid (25 wt%) added was 112 ml. (The amount of sulfuric acid added (112 ml) is 70% of the amount required for neutralization.) The amount of impurities in the obtained silica is shown in Table 1 below. As in Example 1, the purity was high. Example 3 A silica precipitate was prepared in the same manner as in Example 1, except that 90 ml of methyl alcohol, which is the "turbidity point", was added instead of polyethylene oxide. Silica with a water content of 245% was obtained after leaching with sulfuric acid similar to that in Example 1. The obtained silica was washed with 750 ml of hydrochloric acid and kept at 90° C. for 1 hour, which was repeated twice and then washed with purified water until the pH of the washing solution reached 4 as in Example 1. The weight of the obtained silica when dried at 150° C. for 10 hours was 101 g. The impurity content is shown in Table 1 below. Here, silica that can be used as a raw material for manufacturing silicon for solar cells is required to have high purity and low manufacturing cost. In other words, high productivity is required. In order to produce high-purity fine powder silica, it is necessary to repeatedly separate the silica and the cleaning liquid in order to reliably remove impurities, and usually several over-operations are required.
It goes without saying that the shorter the elapsed time at this time, the better the productivity. The present invention is characterized in that the finely divided silica has high purity and that a precipitate with good permeability, that is, with high productivity, can be obtained. Therefore, the following examples were used to investigate the transient nature of silica produced by the production method of the present invention. _Examples 4 to ₁₅ Water glass (SiO ₂
SiO ₂ (concentration approximately 30wt%) mixed with 375g and 450ml of water
Twelve raw material solutions with a concentration of 13.6 wt% were prepared, and each of the various water-miscible alcohols shown in Table 2 was added to each in an amount corresponding to 0.7 to 1.3 times the "turbidity point". Then, 80 ml of sulfuric acid (25 wt%) was added over about 15 minutes to precipitate silica, and 210 ml of sulfuric acid (25 wt%) was added to adjust the pH of the mixture to 1.
After keeping it at 90°C for 1 hour, it was suctioned off. Using a Buchner type dowel with an inner diameter of 11 cm and chemical analysis paper of the same diameter, the liquid was suctioned with a water aspirator and the time until the liquid ran out was measured. Table 2 shows the amounts of various water-miscible alcohols added and the time required. A short elapsed time is a great advantage in industrial production, and it can be seen that the elapsed time is short when water-miscible alcohol is added in an amount of ±30% of the "turbidity point." Comparative Example 3 Silica was prepared in the same manner as in Example 1, except that the amount of polyethylene oxide with an average molecular weight of 200 was changed to 38 ml, which is about 50% of the "turbidity point". When 80 ml of sulfuric acid (25 wt%) had been added, the silica precipitate was not powdery but pasty. More specifically, it was a mixture of powdery precipitate and pasty gel. Subsequently, 750 ml of sulfuric acid (25 wt%) was added to prepare Example 1.
After the same heat treatment at 90°C for 2 hours, suction filtration was performed. However, this solution contained a paste-like precipitate and therefore could not be absorbed. In addition, the amount of sulfuric acid added in the first stage reaction was 56 ml and 94 ml.
However, both were mixed with paste-like precipitates and could not be absorbed. Comparative Example 4 112 ml of polyethylene oxide with an average molecular weight of 200 was added, which corresponds to approximately 150% of the "turbidity point".
It was operated in the same manner as in Example 1. The amount of impurities in the obtained silica is shown in Table 1 below. In this comparative example, the amount of polyethylene oxide added was too large, resulting in poor purity. Comparative Examples 5 to 10 Same as Example 4 except that various water-miscible alcohols shown in Table 2 were added in amounts shown in Table 2 corresponding to outside the range of ±30% of each "turbidity point". The elapsed time of the silica-containing solution after being treated at 90° C. for 1 hour was measured. The type of water-miscible alcohol, the "turbidity point", the amount added and the elapsed time of each are shown in Table 2 below. From the above results, it can be seen that a well-transparent precipitate cannot be obtained outside ±30% of the "turbidity point". Also,
It can be seen that even if 300 ml or more of glycerin and sorbitol are added, the mixture does not become cloudy and has no effect. Comparative Example 11 190 g of the same water glass as in Example 1 was diluted to 54°C (130°F) with 800ml of warm water. 800ml of this solution
of methyl alcohol and 100 ml of water with vigorous stirring. Fine grains of silicate formed and the solution became very cloudy. Bubble carbon dioxide gas into this suspension from the bottom of the suspension for about 2 minutes.
I understood. The pH of the solution is approximately 9 after 40 minutes, and the pH after 50 minutes.
After about 8.60 minutes, the pH became about 7. US
In the example of PATENT2386337, the process ends here and silica is obtained, but in this comparative example, 336ml of sulfuric acid (25%) is added to this solution to increase the purity of the silica.
A further washing operation was carried out by adding and keeping at 90°C for 2 hours. After filtering the same solution, add hydrochloric acid (20%) to 380
ml, washing at 90°C for 1 hour was repeated twice. The amounts of sulfuric acid and hydrochloric acid added were determined so that the concentrations matched those of Example 3. The obtained silica was washed with purified water until the pH of the washing solution reached 4, and dried at 150°C for 10 hours. The impurity content analysis results are shown in Table 1 below. In this comparative example, high purity silica was not obtained because methyl alcohol was used in a large amount, about 5 times the turbidity point. Comparative Example 12 3g of sodium laurylpenzenesulfonate, a surfactant, instead of polyethylene oxide
The procedure was the same as in Example 3, except that the solution was dissolved in water and added. The analysis results of the obtained silica are shown in Section 1 below.
Shown in the table. This comparative example shows that high purity silica cannot be obtained even if a surfactant is added.

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Claims (1)

[Scope of Claims] 1 (a) In an aqueous solution of an alkali metal silicate having a SiO ₂ concentration of 9 to 20% by weight, solid precipitation occurs and the solution becomes turbid or just before that. (b) adding to the resulting mixture 35 to 80% of the stoichiometric amount for neutralizing the alkali metal silicate; A method for producing silica in fine powder form, characterized in that the silica is precipitated as a fine powder by adding an amount of mineral acid, and (c) the resulting slurry is then brought into contact with a mineral acid at an acidity of at least 1 normal or more. . 2. The method according to claim 1, wherein the alkali metal silicate is sodium silicate. 3. The method according to claim 1, wherein the alkali metal silicate aqueous solution has a SiO ₂ concentration of 11 to 17% by weight. 4. The method according to claim 1, wherein the water-miscible alcohol is added in an amount within ±30% of its turbidity point. 5 The water-miscible monohydric alcohol has 1 to 4 carbon atoms.
The method according to claim 1, wherein the alkanol is 6 The water-miscible dihydric alcohol has 2 to 4 carbon atoms.
alkylene diol with an average molecular weight of 200~
600 polyalkylene oxides. 7. Claim 1, wherein the water-miscible alcohol is polyethylene oxide with an average molecular weight of 200 to 400.
The method described in section. 8. The method according to claim 1, wherein the mineral acid is sulfuric acid or nitric acid. 9. The method according to claim 1, wherein in step (b), a mineral acid is added in an amount of 50 to 70% of the stoichiometric amount for neutralizing the alkali metal silicate. 10 Claim 1 in which the mineral acid is added slowly in step (b) while suppressing gel formation.
The method described in section. 11. In step (b), addition of mineral acid in an amount up to 35% of the stoichiometric amount for neutralizing the alkali metal silicate is carried out over at least 5 minutes. the method of. 12. The method according to claim 10, wherein the addition of the mineral acid in step (b) is carried out with stirring so that the pH of the slurry produced does not fall below 10. 13. The method according to claim 1, wherein step (b) is carried out at a temperature within the range of 15 to 50°C. 14. The method according to claim 1, wherein the acidity in step (c) is within the range of 1.5 to 4 normality. 15. The method according to claim 1, wherein step (c) is carried out at a temperature of 70°C or higher. 16. Process according to claim 1, in which, subsequent to step (c), the resulting slurry is filtered and the percake is washed with mineral acid and/or water. 17. The method according to claim 16, wherein the mineral acid is hydrochloric acid or nitric acid.