JPH08169711A - Production of high purity silica - Google Patents

Abstract

PURPOSE: To improve quality stability by preventing the contamination of silica due to alkali metals and to efficiently produce a high purity silica extremely small in impurities. CONSTITUTION: In this method for producing the high purity silica by which a solidified matter formed by bringing aq. alkali metal silicate soln. into contact with a water-miscible org. medium or an acid soln. is treated with an acid- containing liq. to extract impurities, and the obtained amorphous silica is separated from the acid-containing liq., then washed with a pure water in which impurities do not substantially include, the amorphous silica is washed while keeping the system constituted by mixing the amorphous silica and the washing water within the pH of 4.2-2.2 at least at the final washing stage, then the silica is separated from the washing water and heated at 800-1400 deg.C.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing high-purity silica.

[0002]

2. Description of the Related Art Conventionally, various methods have been proposed as a method for producing high-purity silica. When an aqueous solution of an alkali metal silicate is used as a raw material, it is contacted with a water-miscible organic medium or an acid solution. A method is known in which a coagulated product is generated, and then the coagulated product is treated with an acid-containing liquid to obtain amorphous silica, and impurities contained therein are extracted and removed. (For example, JP-A-62-3011 previously proposed by the present inventors,
Methods described in JP-A-62-283809, etc. ) By the way, the surface of the silica particles treated with the acid-containing liquid and separated from the treatment liquid has an acid-containing liquid containing the extracted impurities.
(Acid treatment liquid) is attached. In order to obtain high-purity silica, in the conventional method, the separated silica particles are repeatedly washed with pure water containing substantially no impurities as washing water to replace these deposits (impurities). After removal, high-purity silica is obtained.

[0003]

In the above-described conventional method in which pure water containing substantially no impurities is repeatedly used, contaminated silica having a high impurity concentration is sometimes generated. There is a point. According to the result of the analysis by the present inventors, the pollutant is an alkali metal-
In particular, it is often Na. It is considered that the main cause of the contamination is that the impurities eluted from the equipment material of the manufacturing facility are adsorbed by silica. Contamination of the product intermediate silica with impurities reduces the yield of high-purity silica products and becomes a major impediment to stable operation of production equipment. The problem to be solved by the present invention is to develop an improved method capable of preventing contamination of product silica by impurities when obtaining high-purity silica from an aqueous alkali metal silicate solution.

The object of the present invention is to treat an aqueous solution of an alkali metal silicate with a water-miscible organic medium or an acid solution to treat a coagulated product, which is treated with an acid-containing solution to obtain amorphous silica and extract impurities. In the method of washing with water to obtain high-purity silica, an improved method for producing high-purity silica that contributes to stable operation of production equipment by preventing a decrease in product yield due to contamination of product intermediates with alkali metal impurities is provided. To provide.

[0005]

[Means for Solving the Problems] The present inventors diligently studied a method for preventing contamination of product silica by impurities when obtaining high-purity silica from an alkali metal silicate aqueous solution,
It was clarified that the main cause of contamination of the product silica is that the alkali metal impurities eluted from the equipment material into the washing water are adsorbed by the product intermediate silica. That is, the adsorption of alkali metal impurities in the cleaning water to the product intermediate silica is governed by the pH of the cleaning water, and the pH of the cleaning water rapidly increases as the pH approaches from the acidic region to the neutral region, and When repeatedly washing with water, if the pH of the system consisting of the silica to be washed and the washing water at least at the final stage (hereinafter simply referred to as the washing system) is maintained at 4.2 or less, alkali metal The present invention has been completed by finding that impurities can be prevented from being adsorbed on product intermediate silica.

According to the present invention, the "amorphous silica obtained by contacting an aqueous solution of an alkali metal silicate with a water-miscible organic medium or an acid solution, treating a coagulation product with an acid-containing solution to extract impurities, After separating from the acid-containing liquid, in a method of washing with washing water containing substantially no impurities to obtain high-purity silica, the amorphous silica is obtained by at least the final stage of washing with water. The method for producing high-purity silica is characterized in that the pH of the system prepared by mixing the washing water with the washing water is kept in the range of 4.2 to 2.2, and then the washing water is separated from the washing water and heat-treated. " To do.

In a preferred embodiment, the washing water at least in the final stage is water containing 30 to 300 ppm of acid, and the acid is carbonic acid or sulfuric acid.

The heat treatment is preferably performed at a temperature of 800 to 1400 ° C.

The present invention will be described in detail below. The high-purity silica obtained by the method of the present invention has an extremely low impurity content, and the content of each alkali metal such as Na and K, the alkaline earth metal such as Mg and Ca, and each element of Fe and Al is 1 ppm or less. Is.

An embodiment of the method of the present invention comprises the following three steps. (1) Amorphous Silica Preparation Step An aqueous solution of an alkali metal silicate is brought into contact with a water-miscible organic medium or an acid solution to produce silica gel. Then
The obtained silica gel is treated with an acid-containing solution to obtain amorphous silica, and impurities are extracted to separate it from the acid-containing solution. (2) Amorphous silica washing step The amorphous silica separated from the acid-containing liquid is washed with pure water. At least at the final stage of this water washing, the washing system is washed while keeping the pH of the washing system in the range of 4.2 to 2.2, and then the amorphous silica and the washing water are separated. (3) Amorphous silica heat treatment step The amorphous silica subjected to the above-mentioned washing treatment is heated to a temperature of 800 to 14
Heat treatment in the range of 00 ℃.

The above steps will be described in detail below. (1) Amorphous silica preparation step The method of the present invention comprises treating an aqueous solution of an alkali metal silicate with a water-miscible organic medium or an acid solution to produce a coagulated product (gel), which is treated with an acid-containing solution. It can be applied to the obtained amorphous silica. The method for producing amorphous silica is not particularly limited as long as it is within the range of the above method, and amorphous silica obtained by any method may be used. As the alkali metal silicate as the raw material, sodium salt, potassium salt, lithium salt, etc. of silicic acid can be used.

The method of producing amorphous silica has been previously proposed by the present inventors, for example, in Japanese Patent Laid-Open No. 62-3011 and Japanese Patent Laid-Open No.
62-3012, JP62-283809 or JP62-2838.
No. 10, according to the method described in each publication, a method of extracting impurities by treating the coagulated product obtained by extruding an aqueous solution of an alkali metal silicate through a pore into a water-miscible organic medium or an acid solution, treating the solution with an acid. Can be used.

In a more specific embodiment, an aqueous solution of an alkali metal silicate having a viscosity in the range of 2 to 500 poise is supplied from a nozzle having a pore size in the range of 0.05 to 1 mm to a water-miscible organic medium or a concentration of 4N. It is extruded into a coagulation bath consisting of the following acid solution to coagulate into a fibrous, columnar or granular form, and the obtained coagulated product is treated with an acid-containing liquid.

Examples of the water-miscible organic medium used in the coagulation bath include alcohols such as methanol, ethanol and n-propanol, esters such as methyl acetate and ethyl acetate, ketones such as acetone and methyl ethyl ketone, and dimethylacetamide. Examples thereof include amides such as dimethylformamide, dimethylsulfoxide and the like.

The acid used in the coagulation bath is an inorganic acid such as sulfuric acid, nitric acid or hydrochloric acid, and it is preferable to use sulfuric acid or nitric acid. As the coagulation bath, an aqueous solution of these acids is practically used. preferable. The acid concentration is in the range of 0.1 to 4 N, preferably 0.5 to 3 N, and more preferably 1 to 2 N. The coagulation bath temperature is preferably kept in the range of 10 to 60 ° C.

Acids used in the acid-containing liquid include sulfuric acid,
Among the inorganic acids such as nitric acid and hydrochloric acid, it is preferable to use sulfuric acid and nitric acid, and as the acid-containing liquid, an aqueous solution of these acids is preferable in practice. The acid concentration is in the range of 0.1 to 8 N, preferably 0.5 to 7 N, and more preferably 3 to 6 N.

The treatment of the coagulation product with the acid-containing liquid may be a batch system in which the coagulation product and the acid-containing liquid are mixed, or a continuous system in which the acid-containing liquid is continuously supplied to the coagulation product for treatment.
Any of them may be used, and it may be selected depending on the allowable level of impurities in the product. Furthermore, as a means for increasing the extraction rate of impurities, it is also possible to perform a repeated treatment using a new acid-containing solution in a batch system.

(2) Amorphous silica washing step By the treatment with an acid-containing liquid (acid treatment), impurities in the coagulated substance are transferred into the acid-containing liquid (acid treatment liquid) as the extractant.
After the acid treatment, the produced amorphous silica and the acid treatment liquid as the extractant are separated by a usual solid-liquid separation operation such as filtration. At this time, on the surface of the wet silica separated from the acid treatment liquid, a part of the acid treatment liquid containing impurities extracted from the solidified substance remains attached.

When an aqueous solution of an alkali metal silicate is used as a raw material, the acid treatment liquid remaining on the surface of the wet silica is a raw material which is the highest concentration of impurities together with impurities in the raw material extracted by the acid treatment. It contains salts produced by neutralization of the alkali metals therein and excess acid content of the acid-containing liquid used. When obtaining high-purity silica, the wet silica is substantially free of impurities in order to reduce the concentration of impurities remaining on the surface of the wet silica separated from the acid treatment liquid. After washing with pure water and solid-liquid separation, these deposits (impurities) are diluted and replaced.

In the washing and solid-liquid separation treatments using pure water, the concentration of impurities attached to or contained in the treated silica is
Repeat as needed until an acceptable level of impurities in the product silica is reached. At this stage, the pH of the cleaning system tends to be in the neutral range. In the conventional method for producing high-purity silica, the washing treatment is completed at this stage. However, as described above, such washing treatment sometimes causes contamination of the washed product intermediate silica with alkali metal impurities, which reduces the yield of high-purity silica products and hinders stable operation of production equipment. There is a problem.

In the method of the present invention, the pH of the washing system at least in the final stage of the washing with water is kept at 4.2 or less, preferably in the range of 4.2 to 2.2, so that the adsorption of alkali metal onto silica is prevented and the intermediate product is removed. Prevents the occurrence of contamination of body silica. As a means for adjusting the pH of the washing system to 4.2 or less, an acid may be added to pure water as washing water used at least at the final stage of washing with water. In the final stage of water washing, the concentration of impurities other than the acid attached to or contained in the treated silica has reached a level that is acceptable for the product silica, and the amount of acid remaining on the treated silica is It is not necessary to add acid to the washing water if the concentration can bring the pH of the washing system below 4.2. As the acid to be added, it is preferable to use a small amount of acid that exhibits the effect of preventing alkali contamination and is easily volatilized from silica by heat treatment. Specifically, inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid and carbonic acid, and organic acids such as formic acid and acetic acid can be used. Practically, carbonic acid and sulfuric acid are preferable.

In the method of the present invention, the amorphous silica is treated with water containing 30 to 300 ppm, preferably 40 to 200 ppm, and more preferably 50 to 100 ppm of acid at least in the final stage of washing with water. Can be washed. Acid concentration is 30 p
If it is less than pm, the effect of preventing the adsorption of alkali metals to silica cannot be expected, while if it exceeds 300 ppm, the acid content may remain in the silica even if heat treatment is applied,
The load for preventing equipment corrosion or environmental pollution due to volatilized acid content increases.

The mode of the washing operation in the method of the present invention is not particularly limited, and the washing operation which is usually performed for the granular material can be selected. As such an operation, for example, a washing water is pressed into a layer of wet silica which is separated from the acid-treated solution by a conventional method such as filtration / centrifugation and in which a part of the acid-treated solution remains. A method of permeating and replacing the remaining acid treatment liquid with washing water (also called rinse washing), or a cake washing method in which wet silica is dispersed in a washing water in a slurry form to remove substances adhering to silica. (Also referred to as repulp washing) can be exemplified. At least at the final stage of washing with water, the washing water is usually used in an amount of 5 to 15 times the weight of the amorphous silica to be washed.

(3) Heat Treatment Step of Amorphous Silica Acid is attached to the surface of the amorphous silica obtained by the method of finishing the washing treatment with a system containing acid. By the way, regarding the high-purity silica to be obtained by the method of the present invention, its application-particularly in the filler of the VLSI encapsulating resin composition, the presence of ionic impurities causes corrosion of VLSI lead wires, lead frames, etc. Therefore, its concentration is strictly regulated. The amount of ionic impurities contained in the high-purity silica can be evaluated by the value of the electric conductivity of the extracted water obtained by boiling and extracting the high-purity silica in pure water. Is 10 μS / cm
The following is desired. In order to meet such a demand for high purification, the acid adhering to the surface of the amorphous silica is removed by the heat treatment in this step.

In order to remove the acid adhering to the surface of the amorphous silica, it is desirable to perform heat treatment at a temperature of 800 ° C. or higher. The treatment conditions can be selected according to the desired quality and physical properties of the desired product silica. Considering the physical properties of product silica, the temperature of heat treatment is 900 ~
1400 ° C, preferably 1050 to 1300 ° C, more preferably 1100
The range of ~ 1250 ° C is preferable. When the heat treatment temperature is lower than 800 ° C., acid remains in the obtained amorphous silica, and the high-purity silica intended by the method of the present invention cannot be obtained. The heat treatment time depends on the heat treatment method and device, and is, for example, in the range of 30 minutes to 50 hours in the stationary system and in the range of 1 second to 10 hours in the fluidized bed or moving bed system.

When sulfuric acid is used as the acid by such heat treatment, the concentration of residual sulfate ion is 1 ppm or less, and the electric conductivity of the extracted water obtained by boiling and extracting in pure water is measured. It is possible to obtain amorphous silica having a value of 1 μS / cm or less.

The apparatus used for the heat treatment is
It is sufficient that the raw material silica can be maintained at a predetermined temperature, and a tubular furnace, a box furnace, a tunnel furnace, or the like can be used. Further, the heating source is optional, and electric heat or combustion gas is an economical heat source. In addition, plasma heating and an image furnace can be used.

[0028]

The method of the present invention will be specifically described below with reference to examples and comparative examples. The present invention is not limited to the examples below. Example 1 JIS No. 3 water glass was heated and concentrated to obtain a viscosity at 20 ° C.
It was set to 300 cps. About 8 liters of this water glass was pressurized with a pump, passed through a filter (opening 70 μm), and then a nozzle (pore diameter: 0.
2mm, 50 holes) 8% by weight kept at 50 ℃
0.4m / s into a coagulation bath containing 300 liters of sulfuric acid
Extruded at the speed of. The fibrous silica was immersed in 10 times the amount of newly prepared 8 wt% sulfuric acid aqueous solution as an acid-containing solution and stirred at a temperature of about 95 ° C for about 1 hour to extract impurities. The amorphous silica was separated from the acid-containing liquid. Then, it was washed four times with pure water in an amount 10 times that of the silica to be washed. The sulfuric acid concentration in the fourth washing liquid was about 8 ppm.

The final washing was the fifth washing, and washing water having a sulfuric acid concentration of 50 ppm prepared by adding sulfuric acid to pure water was washed with 10 times the amount of silica to be washed. Then, the wet silica obtained by performing solid-liquid separation by filtration was dried at a temperature of 120 ° C. for 8 hours by a hot air dryer, and the water content (weight reduction rate based on heat-treated silica) was 7% by weight. 3.7 kg of some amorphous dry silica was obtained. The sulfuric acid concentration of this amorphous dry silica was about 50 ppm.

A quartz beaker was filled with 50 g of the amorphous dry silica and heat-treated at a temperature of 1250 ° C. for 1 hour using an electric furnace. The impurity concentration of the silica thus obtained was Na: 0.3 ppm, K: 0.1 ppm or less, Mg: 0.1 ppm or less, Ca: 0.1 ppm, Fe: 0.5 ppm, SO ₄ ^2- : 0.1 ppm. .

The amorphous dry silica and the heat-treated silica are dispersed in pure water to prepare a slurry having a concentration of 10% by weight, which is obtained by boiling and extracting for 8 hours under conditions without volatilization and concentration. The electrical conductivity of the obtained supernatant (extracted water) at 25 ° C was measured. The value of electric conductivity was 40 μS / cm for dry silica and 1 μS / cm for heat-treated silica.

Example 2 and Comparative Example 1 Example 1 was repeated except that the fifth washing was performed last time and the washing water having various sulfuric acid concentrations shown in Table 1 prepared by adding sulfuric acid to pure water was used. Was treated according to the method of 1. to obtain amorphous dry silica and heat-treated silica. The sulfuric acid concentration of the washing water used in the fifth washing and the pH of the washing system, the SO ₄ ^2- concentration of the obtained dried silica and heat-treated silica, and the electric conductivity of the boiling extract water, and the heat-treated silica Display Na concentration
-1 shown. In addition, the concentration of impurities other than Na in the obtained silica is K: 0.1 ppm or less, Mg: 0.1 ppm or less, C
a: 0.1 ppm, Fe: 0.5 ppm.

[0033]

[Table 1]

As shown in Table 1, when treated under the conditions specified in the present invention, it was possible to obtain high-purity silica containing very few impurities, which is the object of the present invention (Examples 1 and 2).
. On the other hand, in the fifth (final) washing, when the washing treatment is performed using the washing water that deviates from the conditions specified in the present invention, if the lower limit of the specified conditions is not reached, the alkali metal When the Na concentration of the silica obtained by the contamination caused by the above exceeds 1 ppm (Comparative Examples 1-1 to -2) and, on the other hand, exceeds the upper limit of the specified conditions, the volatilization of the acid content also occurs in the heat-treated silica. Is not enough, SO ₄ ^2- concentration is 4
It is 0 ppm, and the electric conductivity of boiling water is 32.
It was μS / cm (Comparative Example 1-3), but in any case, the high-purity silica aimed at by the present invention could not be obtained.

[0035]

【The invention's effect】

According to the method of the present invention, it is possible to prevent the contamination of silica with alkali metal, improve the stability of the quality of the intended product silica, and efficiently produce high-purity silica containing very few impurities. . The high-purity silica obtained by the method of the present invention is suitable for use as various fillers and dispersants, as a raw material for transparent quartz glass and special ceramics, and as a filler raw material for VLSI encapsulating resin compositions. To be

Claims (9)

[Claims] 1. An amorphous silica obtained by acid-containing an amorphous silica obtained by treating a solidified product produced by contacting an aqueous solution of an alkali metal silicate with a water-miscible organic medium or an acid solution with an acid-containing solution to extract impurities. After separating from the liquid, in a method of obtaining high-purity silica by washing with washing water substantially free of impurities, the amorphous silica is washed with the amorphous silica at least at the final stage of the water washing. A method for producing high-purity silica, which comprises washing the system by mixing it with water while maintaining the pH in the range of 4.2 to 2.2, then separating the system from the washing water and performing heat treatment. 2. The method for producing high-purity silica according to claim 1, wherein the alkali metal silicate is at least one selected from the group consisting of sodium silicate, potassium silicate and lithium silicate. 3. The water-miscible organic medium comprises alcohols such as methanol, ethanol and n-propanol, esters such as methyl acetate and ethyl acetate, ketones such as acetone and methyl ethyl ketone, amides such as dimethylacetamide and dimethylformamide. The method for producing high-purity silica according to claim 1, wherein the high-purity silica is at least one selected from the group consisting of compounds and dimethyl sulfoxide. 4. The high-purity silica according to claim 1, wherein the acid solution is an aqueous solution of at least one acid selected from the group consisting of sulfuric acid, nitric acid and hydrochloric acid, and the acid concentration is in the range of 0.1 to 4 N. Manufacturing method. 5. The high purity according to claim 1, wherein the acid-containing liquid is an aqueous solution of at least one acid selected from the group consisting of sulfuric acid, nitric acid and hydrochloric acid, and the acid concentration is in the range of 0.1 to 8 N. Method for producing silica. 6. The high-purity silica according to claim 1, wherein the acid is at least one acid selected from the group consisting of inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid and carbonic acid, and organic acids such as formic acid and acetic acid. Production method. 7. The method for producing high-purity silica according to claim 1, wherein the washing water in at least the final stage of the washing with water is water containing 30 to 300 ppm of sulfuric acid. 8. The method for producing high-purity silica according to claim 1, wherein the amorphous silica separated from the washing water after the final washing step contains 30 to 300 ppm of sulfuric acid. 9. The method for producing high-purity silica according to claim 1, wherein the heat treatment is performed at a temperature of 800 to 1400 ° C.