JPS62283809A - Production of high-purity silica - Google Patents

Abstract

PURPOSE:To produce high-purity silica by forming an aq. alkali silicate soln. to a fibrous gel in a solidifying agent consisting of an acid soln., immersing the gel into a liquid contg. an acid and washing the same, thereby extracting and removing impurities. CONSTITUTION:The alkali silicate expressed by M2O.nSiO2 (M is an alkali metal, n is number of moles number of SiO2 and 0.5-5) is made into the cobwebbing aq. soln. of 2-200 poise viscosity. The aq. soln. is extruded from a spinning nozzle having <=1mm hole diameter into the acid soln. of <=4N concn. and is solidified to form the fibrous gel. The spinning nozzle made of noble metallic alloys or tetrafluoroethylene resin or the nozzle having the surface coated with a noble metal or tetrafluoroethylene resin is used. A sulfuric acid, nitric acid, etc., are used for the acid soln. Such gel is immersed into the liquid contg. the acid and is then washed, by which the impurities are extracted and removed, and high-purity silica is obtd. A sulfuric acid, nitric acid, etc., are used for the liquid contg. the acid and the acid concn. thereof is specified to about 1-2N, the treatment temp. to about 110-140 deg.C and the treatment time to about 1-10min with a continuous type.

Description

[Detailed description of the invention] 3. Detailed description of the invention [Industrial application field] The present invention relates to a method for producing high-purity silica.

For more information, see Alkaline silicate water soluble? The present invention relates to a method for producing high-purity silica containing extremely low content of radioactive impurities such as uranium in addition to alkali metals and chlorine from fl.

High-purity silica is used as a filler and dispersant, as a raw material for transparent quartz glass, special ceramics, etc., and is also expected to be used as a raw material for fillers in resin compositions for encapsulating electronic components.

Synthetic resin compositions containing inorganic fillers such as silica are used as encapsulating materials for electronic components.
It is considered advantageous to incorporate the filler in as much amount as moldability allows from the viewpoint of physical properties such as expansion coefficient, thermal conductivity, moisture permeability, mechanical properties, and cost, and silica-based fillers are preferred in terms of quantity. has been done. However, with the increasing integration of electronic component elements, the problem of malfunction of the elements has arisen. 1
1) It is said that this is caused by alpha rays emitted from radioactive elements such as uranium and thorium, which are contained in v& amounts in units of b, and it is desirable to further reduce the content of such impurities in silica. ing.

The present invention aims to meet such demands.

[Conventional technology]

Known methods for producing high-purity silica include: 1) A method in which silicon tetrachloride purified by distillation, adsorption, liquid phase extraction, etc. is reacted under an oxyhydrogen flame. 2) Methods for producing high-purity silica using an aqueous alkali silicate solution as a raw material include: 2-1) A method for purifying an aqueous alkali silicate solution by treating it with an ion exchange resin: (JP-A No. 60-42217) , *opening 60-422
No. 18 Nato).

2-2) Method for purifying an aqueous alkali silicate solution by treating it with an acid: (JP-A-59-54632, JP-A-60-19101)
No. 6.

JP-A No. 60-204612, etc.) have been proposed.

[Problem that the invention seeks to solve]

Highly pure silica can be produced by these methods, but in the case of method 1), the obtained silica particles are fine particles with an average particle size of the mμ order and have a large specific surface area, and are not suitable for resin compositions for encapsulating electronic components. It is difficult to use it as a filler.

In addition, in the method 2-1), since the purification operation is performed by diluting the 5iOz 74 degrees of aqueous silicate aqueous solution to about 10% by weight or less, it is difficult to prevent silica from precipitating from the silica sol in terms of equipment efficiency. Since the operating conditions for separating and recovering from the mother liquor are complicated, there is a problem in terms of productivity.

Method 2-2) is a simple method and various attempts have been made, but
Even when an aqueous alkaline silicate solution is treated with a high-4 degree acid, phase separation occurs locally, making it difficult to extract impurities. Alkaline silicate water also trying to avoid local phase separation? Even if liquid 8 is extruded into a dilute acid solution, the extruded alkaline silicate components adhere to each other and form large lumps, which significantly reduces the extraction rate of impurities, which is difficult in terms of practicality.

The inventors of the present invention have improved these problems in the conventional method and produced high-purity silica with an extremely low content of impurities using an aqueous alkali silicate solution as a raw material more efficiently and economically than before. After extensive research, the present invention was completed.

[Means for solving problems]

That is, in the present invention, an aqueous alkali silicate solution is made into a fine fibrous gel in a coagulant, and the resulting fibrous gel is treated with an acid-containing liquid and then with water to extract and remove impurities. This relates to a method of using an acid solution as a coagulant in producing silica; ~5)
A method for producing high-purity silica, which is characterized by using an aqueous solution of an alkali silicate represented by the formula and combining the following steps.

(1) Alkaline silicate water with viscosity ranging from 2 to 200 poise? 8 liquid is coagulated by extruding it into an acid solution with a concentration of 4N or less through a spinning nozzle with a pore size of 1+nm or less,
Process of forming a fibrous gel.

(2) A step in which the obtained fibrous gel is immersed in a liquid containing an acid, and then washed with water to extract and remove impurities to obtain silica.

The present invention will be explained in detail below. Embodiments of the invention include:
It consists of the following two steps.

- Process-1: (fiberization process).

A highly viscous liquid with stringiness (from an aqueous alkali silicate solution)
(hereinafter referred to as a stock solution) is prepared, and this stock solution is coagulated in an acid solution using a fiberizing device to form a fine fibrous gel.

-Step-2=(Impurity extraction step).

The obtained fibrous gel is treated with a solution containing an acid (hereinafter referred to as "treatment") and then with water to extract and remove impurities.

The features of the present invention will be described below.

(1) An aqueous alkali silicate solution is coagulated in an acid solution using a fiberizing device equipped with a spinning nozzle with a pore diameter of 1 mm or less to form a fine fibrous gel.

The resulting fibrous gel has a fine diameter and a high surface area, increasing the efficiency of extracting impurities.

(2) When forming an aqueous alkali silicate solution into a fine fibrous gel; ■ An aqueous alkali silicate solution coagulated in an acid solution:
A solution having a viscosity of 2 to 200 poise is used, and (1) the acid concentration of the acid solution as a coagulant is 4 normal or less.

According to the method of the present invention, which combines the conditions (1) and (1) above, it is surprisingly possible to obtain a fibrous gel having a hollow structure even when using a normal circular hole nozzle, and the solidified product thereof The part maintains a homogeneous and highly swollen state and is obtained in a structure that allows impurities to be easily extracted by acid and water.This, together with the effect of feature Ib (1) above, significantly improves the extraction efficiency of impurities in silica. Can be done.

In the method of the present invention, the raw alkali silicate aqueous solution includes sodium salts and potassium salts of silicic acid.

Aqueous solutions such as lithium salts can be used.

Hereinafter, the case where a sodium silicate aqueous solution is used as the alkali silicate aqueous solution in the method of the present invention will be described as an example.
The above two steps will be explained in order.

[Step-1 = (Fibre-forming step)] A sodium silicate aqueous solution as a raw material is adjusted to a viscosity range suitable for fiber-forming, and is used as a stock solution.

The viscosity range of the stock solution suitable for the method of the present invention is from 2 to 200 poise, particularly preferably from 10 to 100 poise.

When using a sodium silicate aqueous solution with a high 5iOzfi degree (too high viscosity) as a raw material, dilute it appropriately with water before use.

In the case of a sodium silicate aqueous solution containing 30% SiJ cautery, the viscosity is low under normal conditions and the stringability is insufficient, so sodium silicate is polymerized and used in order to impart stringability to the solution.

As methods for polymerizing sodium silicate, a partial neutralization method using an acidic substance, a dehydration concentration method, a method of adding a polyvalent metal salt, etc. have been proposed. Of these, the dehydration and concentration method is the simplest method, and dehydration of a few percent causes the sodium silicate to polymerize and increase its viscosity.

The prepared stock solution is heated to a temperature suitable for fiberization, e.g.
The mixture is kept at 0°C, passed through an appropriate filtration device, and sent to a fiberization device using a metering pump.

The fiberizing device is not particularly limited, and generally an extruder equipped with a spinning nozzle (hereinafter referred to as nozzle) can be used.

The biggest problem when using a nozzle is the occurrence of adhesion problems of the stock solution extruded from the nozzle to the nozzle exit surface.

As is well known, sodium silicate aqueous solution is a viscous liquid that has a high affinity with metals, and has the property of solidifying into plows with a slight decrease in water content, and is also used as an adhesive. As you can see, sodium silicate water? When the stock solution consisting of 8 liquids solidifies while adhering to the nozzle surface, a strong bond is formed between the sodium silicate and the nozzle surface, and it is extremely difficult to separate this bond. When the coagulated material adheres to the nozzle surface, the stock solution extruded from adjacent holes will adhere and coagulate one after another, until it becomes impossible to continue the fiberizing operation.

Such a phenomenon is likely to occur when the nozzle used has a small hole diameter and a large number of holes. The solution to this is:
The goal is to minimize the adhesion between the nozzle surface and the stock solution.

The present inventors conducted various studies on the material of the nozzle to be used, and when converting an aqueous alkali silicate solution into a fine fibrous gel, we decided to use a nozzle made of noble metal alloys such as gold-platinum alloy or tetrafluoroethylene (hereinafter referred to as When using a nozzle made of TFE-based resin, or a nozzle whose nozzle surface is coated with precious metals or TFE-based resin, the ability of gelled alkali silicate to separate from the nozzle is significantly improved, and the alkali silicate is removed from the nozzle surface. It has been found that a remarkable effect can be obtained in preventing the occurrence of adhesion troubles.

The noble metals referred to in the present invention include gold, platinum, gold, palladium, etc., and can be coated on the nozzle surface by a normal coating process.

The TFE resin referred to in the present invention is polytetrafluoroethylene (P
TFE), copolymers of TFE and hexafluoropropylene, copolymers of TFE and perfluoroalkyl vinyl ether, copolymers of ethylene and TFE, copolymers of ethylene and vinyl fluoride, ethylene and vinylidene fluoride, and copolymers of ethylene and chlorotrifluoroethylene.

The nozzle surface is coated with TFE resin according to the usual method.
If necessary, coating may be performed after applying a primer to the outer surface of the nozzle.

In addition to the wet method, various methods can be used for fiberization, such as extruding an aqueous alkali silicate solution into the air through a nozzle and then treating it with an acid solution to coagulate it. From the viewpoint of preventing adhesion to the material, the dry method is more advantageous than the dry method.

In the method of the invention, a stock solution is forced through a nozzle immersed in a coagulation bath. The extruded stock solution coagulates into fibers in a coagulation bath and becomes a gel. This fibrous gel is taken off by rollers or sent to the next step using a suitable transport device such as a belt conveyor.

The hole diameter of the nozzle used in this process is 0.05 to 1.0 m.
The range is preferably o, 1 to 0.3 bars.

As the nozzle, a normal circular hole nozzle is used, but a modified cross-section hole nozzle or a hollow fiber spinning nozzle may also be used.

In the method of the present invention, a hollow fibrous gel can be obtained without particularly using a hollow fiber spinning nozzle, and a good impurity extraction effect can be obtained in step-2.

Mixing fine air bubbles into the fibrous gel is also effective in increasing the impurity extraction efficiency.

Methods for mixing fine air bubbles into the fibrous gel include using a stock solution prepared by stirring so that air is drawn into the liquid, using a chemical blowing agent that decomposes into the stock solution by heating to generate gas, or using a chemical blowing agent that generates gas by heating the stock solution. Add a liquid low boiling point substance at
Various methods can be used, such as a method of fiberizing the stock solution while heating it, or a method of utilizing the cavitation phenomenon of a pump that sends the stock solution to the fiberization device.

As the coagulant used in the coagulation bath of the present invention, an acid solution is used. As the acid, inorganic acids such as sulfuric acid, nitric acid and monohydrochloric acid are used, and sulfuric acid and nitric acid are preferably used. Further, as the acid solution, a water/8 solution of these acids is practically preferable.

The acid concentration of the acid solution as a coagulant is 0.1 normal or more,
It is recommended that the range be 4 or less.

If an acid solution with an acid concentration exceeding 4N is used as a coagulant, the resulting silica fibers will become too dense, making it difficult to extract the impurities contained inside in the next step-2. . Furthermore, if the acid concentration of the acid solution as a coagulant is less than 0.1 normal, the coagulation rate of the alkali silicate is too low and the fibrous gels tend to adhere to each other, which is not practical.

For this reason, the acid concentration of the acid solution as a coagulant is preferably in the range of 0.1 to 4N, preferably 0.1N to 4N.
It is in the range of 5 to 3 normal, more preferably 1 to 2 normal.

When the conditions specified in this specification are combined as the viscosity of the stock solution, the pore size of the nozzle, and the acid concentration of the acid solution used as a coagulant, the coagulation rate of the alkali silicate is moderate and surprisingly it has a hollow structure. Moreover, a highly swollen transparent fibrous gel can be obtained. The obtained gel is subjected to the next step-2
The swollen state is maintained even in this state, and dealkalization proceeds in this state.

The surface of the resulting fibrous gel has numerous scale-like cracks, and the presence of these cracks allows the acid to easily penetrate into the gel, resulting in a fine and hollow structure. Due to the synergistic effect with the high surface area characteristic of fibers,
The extraction efficiency of impurities is significantly improved.

Furthermore, the presence of the crank can significantly reduce the load of the pulverization process when adjusting the particle size of the obtained silica.

In this specification, the highly swollen state of the gel means a state where the gel has a high liquid content, and the degree of swelling can be indicated by the liquid content of the gel.

There is a preferable range for the liquid content of the gel in which impurities are easily extracted. The liquid content is calculated using the following formula: % formula %-1: Approximately Log of the sample is taken, and the weight of the sample is obtained by removing the adhering liquid using a centrifuge at room temperature (processing at 100 OG for 10 minutes), (g).

w2: After the above treatment, the weight of the sample dried at 150'C for 4 hours and left to cool to room temperature in a Tenkater, (g) 1
When calculated using, for example, sodium silicate #3 as a raw material.
The preferable range of liquid content of the fibrous gel when using No. 1 is about 80 to 150%.

If the liquid content is less than 80%, the structure of the formed gel becomes too dense, making it difficult to extract the impurities contained inside in the next step-2.

On the other hand, when the liquid content exceeds 150%, the silica concentration in the gel becomes too low even if the resulting gel is transparent, and the volume shrinkage of the gel increases in step-2, causing impurities inside. The result is devitrified silica that still contains silica. Even if the impurity extraction operation is repeated on silica in such a state, it is difficult to remove the impurities as the object of the present invention.

In addition, even if the fibrous gel has a liquid content within this range, the silica concentration in the gel is uneven, and the gel is partially whitened and devitrified because of impurities, which is the purpose of the present invention. Difficult to remove.

Since the coagulation rate of alkali silicate varies greatly depending on the type of acid used as a coagulant, it is difficult to determine the coagulation bath temperature unambiguously, but a temperature of about 10 to 60°C is usually good.

In addition, if the same type of acid solution used in the next impurity extraction process is used as a coagulant, the steps such as coagulant recovery and waste liquid treatment that are required when using a coagulant such as an organic solvent can be avoided. It is advantageous to be able to omit this.

The fibrous gel can be removed at a rate of 1 to 1 per minute using a roller type.
It is a conveyor type and is normally operated at a speed of about 0.1 to 50 m/min.

[Step-2: (Impurity extraction step)] The fibrous gel obtained in step-1 is treated with a liquid containing an acid in this step. Acids are sulfuric acid and hydrochloric acid.

Inorganic acids such as nitric acid and organic acids such as formic acid.
It is preferable to use sulfuric acid, nitric acid, etc.

Further, as the treatment liquid, an aqueous solution of these acids is practically preferable.

The acid treatment operation in this step can be carried out in one step, but in order to extract and remove the amount of impurities, it is necessary to divide the treatment into at least two steps and use each step separately. It is also possible to perform a multi-step process in which the process liquid is renewed.

A common method for extracting impurities is to use a highly concentrated acid, but in the method of the present invention, in order to maintain as much as possible the gel structure formed in step-1, from which impurities can be easily removed, It is preferable that the acid δ1 degree of the treatment liquid is low.

The acid concentration for one night of treatment is 4N or less, preferably 0.5-3
normal, more preferably in the range of 1 to 2 normal.

Although the processing temperature is not particularly limited, it is preferable to carry out the extraction operation at a temperature of 50'C or higher.

When the treatment is performed under pressure at a temperature higher than the boiling point of the treatment liquid at normal pressure, the time required for extracting impurities can be shortened. The higher the temperature during pressure extraction, the better, but considering the corrosion of the equipment due to acid and the energy cost, the temperature should be 100 to 1
50'C1, preferably in the range of 110 to 140°C is practical.

It is desirable that this step be performed while stirring. When the fibrous gel is stirred in the treatment solution, it easily becomes 2 to 5 times long.
It becomes a short fibrous shape of about +n+o.

When the gel is made into short fibers, the dispersibility of the gel by stirring in the treatment liquid becomes extremely good. The short fibrous gel is dispersed in a slurry form in the treatment solution, which facilitates the impurity extraction operation, improves the uniformity of the impurity extraction effect, and significantly reduces impurity extraction results. In addition, since the short fibrous gel has bulkiness which is a characteristic of fibrous materials, liquid separation is extremely easy during washing and filtration operations after impurity extraction treatment.

The acid treatment time is about 30 minutes to 5 hours in the case of a batch method, and about 30 seconds to 30 minutes in the case of a continuous method, preferably about 1 to 10 minutes.

The silica fibers obtained by the acid treatment are then washed with water at a desired temperature, and are subjected to deoxidation and dehydration treatment in combination with a filtration operation if necessary.

The acid used in the present invention is a highly purified product called electronic grade, and the water used for diluting the raw material and the acid used, or for t*Km of silica, etc., is pure water with few impurities. It is preferable.

By this treatment, the content of impurities including radioactive elements in silica is extremely reduced. The impurity content in silica after acid treatment is: alkali metal: approximately 10 ppm or less, chlorine: 3 ppm or less, and uranium: approximately 3 ppm.
b or less.

〔Effect of the invention〕

According to the method of the present invention, high purity silica having an extremely low content of impurities including radioactive elements such as uranium can be obtained using an aqueous alkali silicate solution as a raw material. The resulting silica has a higher purity than that obtained using conventional techniques, so it is used not only as fillers and dispersants, but also as transparent quartz glass, etc.
In addition to being used as a raw material for special ceramics, it is also expected to be used as a raw material for fillers in resin compositions for encapsulating electronic components.

Furthermore, the method of the present invention also has the advantage that manufacturing costs can be reduced compared to conventional methods.

〔Example〕

The method of the present invention will be specifically explained below using Examples and Comparative Examples.

Example-1 Sodium silicate #3 (equivalent to JIS K1408.3, the same applies hereinafter) (SiOz: 28χ+ N a Z O:
9χ, [1:36ppb) was heated to 50°C under reduced pressure and dehydrated and concentrated to obtain a 51 of 432% stock solution for fiberization. The viscosity of this stock solution is approximately 10 at 30°C.
It had 0 poise and good stringability.

After filtering this stock solution, use an extruder to
Coagulation bath maintained at 30°C through a gold-platinum alloy nozzle with 200 holes at a speed of 6 m/min (1N sulfuric acid water)
20p of liquid was pushed out.

The extruded stock solution was solidified by neutralization of Na2O, and turned into a transparent fibrous gel. This fibrous gel had a hollow structure and a twisted shape in which thick and thin parts appeared alternately, and numerous scale-like cracks were formed on the surface.

The fibrous gel was taken out from the coagulation bath using a belt conveyor at a conveyor speed of 1 m/min, and the immersion time of the fibrous gel in the coagulation bath was about 1 minute.

40g of the obtained fibrous gel was added to a treatment solution - 1N aqueous solution of sulfuric acid? &: Immersed in 500CC and heated to 100'C without stirring.
It was treated for 3 hours. The fibrous gel is cleaved finely and has a length of 2
It became a short fiber of ~5 mm.

Next, the obtained short fibrous silica was placed in 500 cc of water, stirred for 10 minutes, and then dehydrated using a Nusoche. After repeated washing with water five times, the concentration of sulfate groups in the silica was less than 1 ppm.

The obtained silica was dried at 150'C overnight, and then ground in an agate grinder to make the particle size distribution uniform, to obtain silica particles. Table 1 shows the impurity content in the obtained silica.

CI, II and Th were analyzed by activation analysis.

In the following examples, a special grade reagent manufactured by Hani Chemical was used as the acid, and ion-exchanged water with an electrical conductivity of 1.0 μs/cm (25° C.) or less was used as the water.

Table 1. Impurity content in silica.

Example-2 and Comparative Example-1 Sodium silicate No. 13 (same lot as Example-1) 50o
Dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a Nigu oz.
．． 8X to obtain a clear stock solution. The viscosity of the stock solution is 30゛c
It was 5° poise.

This stock solution was passed from an extruder through a 5O5-316 nozzle coated with putty flonpa, each having a hole diameter of 0.2 mmφ and 50 holes, to give 201 mm each of sulfuric acid aqueous solutions of various concentrations. , to obtain a fibrous gel. All fibrous gels were transparent.

These fibrous gels were treated in the same manner as in Example 1, and the results shown in Table 2 were obtained.

Table 2, percentage of impurities in silica: 5O5-316 without Teflon coating
When a manufactured nozzle was used, the stock solution easily adhered to the nozzle and only a lump-like gel was obtained, and it was difficult to obtain a fibrous gel.

Example-3 and Comparative Example-2゜The stock solution prepared in Example-2 was extracted from an extruder with a pore diameter of 0.2, 0.5, 1.0, and 3.0 for comparison.
5US coated with Teflon with 50 holes each.
316 nozzle or 5O5-316 with gold coating
The gel was extruded into a 2N sulfuric acid aqueous solution M2Off through a manufactured nozzle to obtain a fibrous gel.

The obtained fibrous gel was treated in the same manner as in Example 1, and the impurity extraction results are shown in Table 3 as representative of the Na content in the obtained silica.

Table 3: Impurity content in silica: Among the fibrous gels obtained, all of the gels shown in Example 3 were transparent, but the gel obtained in Comparative Example 2-1 was devitrified. It was white in color, and the silica content was uneven and varied locally.

Example-4 and Comparative Example-3 Sodium silicate #3 (same lot as Example-1) 600
0 g was dehydrated and concentrated under reduced pressure using a vacuum pump while stirring in a Neighborhood maintained at 70°C to obtain raw materials of various viscosities.

These stock solutions were made into fibers according to the procedure of Example-1.

Table 4 shows each condition.

Table 4. Condition during fiberization and impurity content in silica: *Value at 30°C.

In addition, the obtained fibrous gel was treated in the same manner as Jll-1, and the impurity extraction results are shown in Table 4 as representative of the Na content in the obtained silica.

Example 5 The stock solution prepared in Examples 1 to 1 was extruded into 20ffi of a nitric acid aqueous solution having a concentration of 1 according to the procedure of Example 1 to obtain a fibrous gel.

40 g of the obtained fibrous gel was dissolved in 500 g of nitric acid normal aqueous solution.
When placed in a CC and treated at 100'C for 3 hours without stirring, short fiber-like warp strength of 2 to 5 mm in length was obtained. For the subsequent treatments, silica particles were obtained by the same method as in Example-1.

The impurity content in silica is; ・Na: 1. Oppm.

・U: 2.1ppb.

Met.

Patent applicant: Nitto Chemical Industry Co., Ltd. (1 other person)

Claims (1)

[Claims] 1) General formula: M_2O・nSiO_2 (where M is an alkali metal element and n is the number of moles of SiO_2 from 0.5 to 5.
using an aqueous solution of an alkali silicate represented by
A method for producing high-purity silica, characterized by combining the following steps. (1) A process in which an aqueous alkali silicate solution having a viscosity in the range of 2 to 200 poise is extruded into an acid solution with a concentration of 4 normal or less through a spinning nozzle with a pore diameter of 1 mm or less, and coagulated to form a fibrous gel. (2) A step in which the obtained fibrous gel is immersed in a liquid containing an acid, and then washed with water to extract and remove impurities to obtain silica. 2) The method according to claim 1, wherein the spinning nozzle is made of a noble metal alloy or a tetrafluoroethylene (hereinafter referred to as TFE) based resin, or a nozzle whose nozzle surface is coated with a noble metal or a TFE based resin. .