GB2125780A

GB2125780A - Process of preparing silica gel having desired physical properties

Info

Publication number: GB2125780A
Application number: GB08318437A
Authority: GB
Inventors: Hiromi Sasaki; Takao Miyoshi; Tadashi Tanaka
Original assignee: Central Glass Co Ltd
Current assignee: Central Glass Co Ltd
Priority date: 1982-07-09
Filing date: 1983-07-07
Publication date: 1984-03-14
Also published as: JPS5913620A; JPS636484B2; GB2125780B; GB8318437D0; DE3324740A1; DE3324740C2

Abstract

In preparing silica gel by reaction between an alkali metal silicate such as sodium silicate and a mineral acid such as sulfuric acid, the reaction is carried out such that the concentration of SiO2 in the reaction becomes not lower than 15% by weight, and preferably 20-30% by weight.

Description

SPECIFICATION Process of preparing silica gel having desired physical properties This invention relates to an improvement in a process of preparing silica gel by reaction between an alkali metal silicate and a mineral acid.

Silica gel is expressed generally by the formula SiO2.xH2O, where x is from 0.1 to 0.3, and belongs to gel-like silica. So-called white carbon has a resembling chemical structure, but white carbon is a sort of colloidal silica in the form of ultrafine particles commercialized mainly for use as a reinforcing filler for synthetic rubbers and commonly accepted as a material different from silica gel.

The most familiar method of preparing silica gel is decomposition of an alkali metal silicate with acid. The decomposition reaction is carried out as a wet process, and the product of the decomposition reaction is initially in the form of a jelly-like silica gel. By aging for several hours the silica sol turns into a hydrogel, but this hydrogel contains considerable amounts of by-produced water-soluble salts and therefore needs to be well washed with water to completely wash off the soluble salts. Usually the content of water in the obtained hydrogel amounts to about 80% by weight, so that it is impossible to completely remove the water by mere filtration. Therefore, a very large amount of heat energy is consumed in drying the hydrogel to completely remove the moisture.

For example, Japanese patent application publication No. 56(1981 )-21 726 describes in its example that 4 kg of water needs to be removed to obtain 1 kg of dry silica gel when a sodium silicate solution in which the concentration of SiO2 is 145.3 g/l is decomposed with sulfuric acid having a specific density of 1.050 such that the concentration of SiO2 in the reaction system at the final stage becomes 50-100 g/l. As another exam ple, Japanese patent application primary pu blication No. 51 (1976)-136841 describes that a wet cake containing 86% of water is obtained when silica gel is prepared by making a sodium silicate solution in which the concentration of SiO2 is 9.52% react with 11.4% sulfuric acid such that the concentration of SiO2 in the reaction system at the final stage becomes 8%. As demonstrated by these examples, in conventional processes the decomposition of an alkali metal silicate with a mineral acid is carried out in a relatively dilute solution. Therefore, it is necessary to use apparatus of large capacity, and a large amount of heat energy is required for separation of the by-produced soluble salts which are present in low concentrations in a large volume of dilute solution.

As another matter of inconvenience in conventional processes for the preparation of silica gel, when the reaction is carried out under an acidic condition the product becomes a sol-gel mixture. It is unfavorable to immediately subject the sol-gel mixture to washing because the sol portion flows away together with the washings. Where the washing is performed by a repulping method, the entire quantity of washing water remains in the gel so that the gel after washing is very high in the content of water. Therefore, it is necessary to continue mixing of the sol-gel mixture for more than two hours at an elevated temperature suitable for aging. Of course such a mixing-aging procedure requires a considerable amount of energy.

Silica gel is used for various purposes, and variously different physical properties are required for the respective purpose. For example, importance is attached to a polishing property and refractive index for use in tooth powder or paste, to a thixotropic property and apparent specific density for use in pigments, to physical strength for use as a filler for rubber or plastics and to specific surface area for use as desiccant.

Though particular attention is directed to variously different aspects of physical properties depending on the intended uses, specific surface area is generally taken as a typical measure of silica gel as a powdery material. Of course the specific surface area value required to silica gel of each grade or for a particular use is variable over a wide range. Physical properties, including specific surface area, of silica gel depend on various factors in the preparation of silica gel by the above described process, such as the concentration of SiO2 in the reaction liquid, reaction temperature, pH of the reaction system, rate of neutralization, coexistance of any or no electrolyte and, in case of coexistence, the kind and concentration of the electrolyte, intensity of stirring and the temperature at the stage of aging of the obtained sol-gel mixture.

The specific surface area of silica gel prepared by the above described process significantly depends on the reaction temperature and the pH of the reaction system and varies over a very wide range. In general silica gel relatively large in specific surface area is obtained when the reaction is carried out under an acidic condition and silica gel relatively small in specific surface area under an alkaline condition. However, even though the pH of the reaction system in the conventional process is set at a given point the specific surface area of the obtained silica gel still varies considerably with the concentration of SiO2 in the reaction system.

In actual practice, therefore, it is not easy to obtain silica gel of which the specific surface area is exactly as specified.

It is an object of the present invention to eliminate the above described problems or inconveniences in the preparation of silica gel by reaction between an alkali metal silicate and a mineral acid, and to provide an improved process by which silica gel having a desired specific surface area and a desired apparent specific density can easily and economically be prepared.

The present invention provides a process of preparing silica gel by reaction between an alkali metal silicate and a mineral acid, which process is characterised in that the reaction is carried out such that the concentration of SiO2 in the reaction system becomes not lower than 15% by weight.

We have discovered that by raising the concentration of SiO2 in the reaction system to 15% by weight or above it is possible to obtain a silica hydrogel in which the content of water is below about 60% without suffering from formation of a soi-gel mixture, and that when the SiO2 concentration is adjusted to such a high level the specific surface area of the formed silica gel no longer depends on the SiO2 concentration but depends primarily on the pH and temperature of the reaction system.

From an economical point of view, it is an important advantage of the process according to the invention that the product of the reaction can easily be filtered and washed with water without the need of using such a large quantity of water as in the conventional processes. Presumably the reason is that because of a very high concentration of the raw material silica gel crystallizes out without intermediately forming a sol or, even if a sol is formed, the sol-to-gel conversion is completed in a very short time. In other words, the reaction between an alkali metal silicate and a mineral acid takes place almost simultaneously upon contact of the silicate with the acid to form silica which is freed immediately, so that the reaction is completed to result in precipitation of silica gel with scarce participation of water molecules in the reaction.

According to the invention, the reaction can be carried out in a relatively small-sized reactor since the amount of water in the reaction system is greatly decreased. Moreover, the consumption of heat energy for drying of the formed silica gel becomes remarkably smaller than in the conventional processes because of the lowered content of water in the washed silica gel. In the conventional processes in which the concentration of SiO2 in the reaction liquid is always below 15% by weight and usually below about 10%, the total energy required for drying of silica gel is the sum of the energy consumed in aging and the energy consumed in dehydration and drying of silica gel after aging. According to literature, a total quantity of steam required for obtaining 1000 kg of dry silica gel amounts to an enormous value such as about 25 tons.

Though this value should be taken as an example, it is certain that in conventional processes more than 4000 kg of water must be evaporated to obtain 1000 kg of dry silica. In contrast, in the process according to the invention it suffices to evaporate about 1000 kg of water, i.e. only about 1/4 of the quantity in the conventional processes, to obtain 1000 kg of dry silica.

When the process according to the invention is performed by employing a suitable reaction temperature and by suitably adjusting the pH of the reaction system in order to obtain silica gel having a desired specific surface area, the apparent specific density of the obtained silica gel depends on the manner of bringing the reactants into contact with each other. That is, relatively dense silica gel is obtained when alkali metal silicate is introduced into mineral acid, and relatively light silica gel is obtained when mineral acid is introduced into alkali metal silicate. Where alkali metal silicate and mineral acid are simultaneously introduced into a reactor, the apparent specific density of the obtained silica gel becomes relatively high when the reaction is carried out under an acidic condition and relatively low under an alkaline condition.

The present invention has made it possible to industrially prepare silica gel of variously different physical properties according to the intended use of the product by selectively varying or controlling some factors in the reaction between an alkali metal silicate and a mineral acid.

Silica gel prepared by the process of the invention can be pulverized into very fine particles comparable to the particles of white carbon and serves all the purposes for which white carbon or silica gel prepared by conventional processes have been used. For example, this silica gel is useful as adsorbent, desiccant, base of tooth powder or paste, filler for rubbers and plastics, viscosity increasing agent in paint and ink and matting agent for paper.

In the accompanying drawings: Figure 1 is a graph showing the relationship found in examples of the present invention between the pH of the reaction system and the specific surface area of the obtained silica gel; Figure 2 is a graph showing the relationship found in examples of the present invention between the concentration of SiO2 in the reaction system and the specific surface area of the obtained silica gel; Figure 3 is a graph showing the same matter as the graph of Figure 2 with respect to some processes not in accordance with the present invention; and Figure 4 is a graph showing the relationship between the quantity of water used for washing the reaction product and the content of Na2O in the obtained silica gel with respect to examples of the invention and some processes not in accordance with the invention.

Similarly to the conventional processes for the preparation of silica gel, a process according to the invention uses an alkali metal silicate and a mineral acid as the starting materials. Sodium silicate, potassium silicate and lithium silicate can be named as suitable examples of the alkali metal silicate, and it is usual to employ sodium silicate which is low in price. Usualiy the alkali metal silicate is used in the form of aqueous solution. As to the mineral acid, it is possible to use sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid, but it is preferred to use sulfuric acid.

According to the invention, an alkali metal silicate is made to react with a mineral acid such that the concentration of SiO2 in the reaction liquid becomes not lower than 15% by weight, and preferably not lower than 20% by weight. There is no strict upper limit to the concentration of SiO2 in the reaction liquid, but in practice an excessively high concentration of SiO2 offers difficuity in intensely and uniformly stirring the reaction system. Therefore, usually it is suitable to avoid the SiO2 concentration exceeding 30% by weight.

The SlO2 concentration can be adjusted to a desired level by diluting either, or both, of the mineral acid and the alkali metal silicate solution with water to a suitable concentration. Where concentrated sulfuric acid is employed as the mineral acid, it is preferable to dilute the acid before its introduction into the reaction vessel because by doing so the heat of dilution is dissipated outside the reaction system so that the reaction temperature can be controlled without affected by the heat of dilution. The reaction temperature is usually in the range from room temperature to about 100 C.

When an alkali metal silicate solution and a mineral acid are mixed such that the concentration of SiO2 in the mixture becomes not lower than 15% by weight and that the pH of the mixture takes an intended value, which may be from about 0.5 to about 12, the entire mixture assumes the state of a large lump. By stirring the mixture the lump is sheared into sleet-like pieces that adhere to the blade of the stirrer and gradually turn into a tacky pasty state. Further stirring results in that water exudes from the pasty material and that the reaction system becomes a low viscosity slurry. The reaction operation is terminated at this stage, and the slurry is filtered to recover the solid product. Usually the reaction in a process according to the invention is completed in about one hour.

As an advantage of the invention, a silica hydrogel in which the content of water is below 60% can easily be obtained by washing the filtered solid product with water. That is, silica gel as the product of this process can readily be filtered a,nd washed, and the washing can be accomplished by using only a small quantity of water. It is an additional effect of the exudation of water from the silica gel present in the reaction liquid that impurity metal salts such as aluminum and iron salts originated in the alkali metal silicate spontaneously transfer into the motor liquid, so that silica gel of very high purity can be obtained. Therefore, silica gel prepared by a process according to the invention can be used as the material for fine ceramics that are required to be extremely low in the content of impurity.

In a process according to the invention wherein the concentration of SiO2 in the reaction liquid becomes at least 15% by weight, it is very important to intensely and uniformly stir the reaction system because silica formed by the reaction in this process does not remain, or remains only for an extremely short time, in the sol state and solidifies to hydrogel practically instantaneously. Therefore, it is difficult to achieve thorough mixing of the reactants and the reaction product and obtain silica gel of intended properties when the stirring is as mild and slow as in the conventional processes. To obtain good results by utilizing the present invention, it is necessary to perform intense and vigorous stirring so as to apply a strong shearing force to the reaction system to thereby quickly crush the intermediate product and complete the reaction in a short time.Accordingly it is recommended to use a reactor that has strong stirring ability such as a kneader, doubie-roll screw mixer or high-speed mixer.

In a process according to the invention, the specific surface area of the obtained silica gel scarcely depends on the concentration of SiO2 in the reaction liquid and can easily be controlled by adjusting the reaction temperature and/or pH of the reaction system, though somewhat affected also by the manner of bringing the reactants into contact with each other. For example, if it is wished to obtain silica gel of which the specific surface area is larger than 500 m2/g, the wish can be met by carrying out the reaction at a temperature below about 60"C and rendering the pH of the reaction system at the end of the reaction below about 3.5.

There are three ways of bringing the reactants into contact with each other: addition of the alkali metal silicate to the mineral acid, addition of the mineral acid to the alkali metal silicate, and simultaneous feed of the alkali metal silicate and the mineral acid into the reactor. In the present invention any one of these three ways may arbitrarily be employed though there are minute differences in the mode of reaction among these three ways. When the alkali metal silicate is added to the mineral acid the reaction liquid becomes very high in viscosity during an initial stage of the reaction, but the viscosity lowers when kneading is performed for about 10 min after arrival of the pH value at an intended point.When the acid is added to the silicate, the pH of the reaction system becomes as intended soon after the start of the reaction, and little change occurs in the viscosity of the reaction liquid even when kneading is performed for about 10 min. Where the silicate and the acid are simultaneously fed into the reactor the viscosity of the reaction liquid depends greatly on the pH of the reaction system, and in general the viscosity becomes higher when the pH is above 7 than in the cases of the pH being below 7.

As mentioned hereinbefore, even though the pH of the reaction liquid is controlled to a determined value the apparent specific density of the obtained silica gel differs considerably depending on whether the alkali metal silicate is added to the mineral acid or the acid is added to the silicate. For example, when a sodium silicate solution (in which the concentration of SiO2 was 29% and the concentration of Na2O was 9%) was added to 98% sulfuric acid such that the pH of the reaction system at the end of the reaction became 3.5, silica gel obtained by filtering, washing and dyring the solid product of the reaction and pulverizing the dried product into fine particles having a mean particle size of 11 iim was a dense silica gel of which the apparent specific density was 0.60.However, silica gel prepared by adding 98% sulfuric acid to the same sodium silicate solution so as to render the pH of the reaction system at the end of the reaction 3.5 and treated in the aforementioned manner was a light silica gel having an apparent specific density of 0.15. Where the alkali metal silicate and the mineral acid are simultaneously fed into the reactor, a relatively dense silica gel is obtained when the pH of the reaction system is below 7 and a relatively light silica gel when the pH is above 7.

The invention will further be illustrated by the following nonlimitative examples.

EXAMPLES 1A-1C In Example 1A, 300 g of 98% sulfuric acid was charged in a 2-liter kneader having Z-shaped blades and, continuing stirring, an aqueous solution of sodium silicate (in which the mole ratio SiO2/Na2O was 3.1 and the concentration of SiO2 was 29% by weight) was added to the sulfuric acid at a rate of 100 g/min. The reaction temperature was kept at 35"C. The addition of the sodium silicate solution was terminated when the concentration of SiO2 in the reaction system reached 24-26% by weight and the pH of the reaction system became close to a predetermined value, which was varied over the range from 1 to 10 as shown in the following Table 1. After that stirring of the reaction system was continued for 10 min, and then the pH of the reaction system was measured again.The reaction product was taken out of the kneader and subjected to centrifugal separation. The recovered solid phase in the form of wet cake was washed with water and dried at 105 Cto obtain dry silica gel.

In Examples 1 B and 1 C, the above described process was performed by varying the reaction temperature to 60"C and to 100"C, respectively.

For every run of Examples lA, 1 B and 1 C, Table 1 shows the reaction conditions, moisture content in the obtained wet cake, and the specific surface area and apparent specific density of the dry silica gel. The apparent specific density was measured on a sample prepared by pulverizing the dry silica gel to fine particles having a mean particle size of 11 ,am (not only in Examples 1A-1 C but also in the subsequent examples and experiments for reference).

TABLE 1 Reaction Physical Properties Conditions of Silica Gel Temp. pH Conc. Water Specific Apparent ( C) of SiO2 Content in Surface Specific (%) Wet Cake Area Density (%) (m2lg) 35 1 24 40 625 0.60 35 2 24 40 580 0.60 35 4 25 35 500 0.60 Example 1A 35 6 25 45 400 0.52 35 8 26 52 260 0.50 35 10 26 55 210 0.45 60 1 24 40 600 0.55 60 2 24 50 560 0.50 Example 1 B 60 4 25 50 480 0.55 60 8 26 45 300 0.45 100 1 24 50 400 0.50 100 2 24 40 380 0.55 100 4 25 45 325 0.45 Example 1C 100 6 25 55 260 0.35 100 8 26 45 185 0.35 100 10 26 50 125 0.30 EXAMPLE 2 In this example, silica gel was prepared by adding sulfuric acid to sodium silicate.

In the kneader used in Example 1, 1500 g of the sodium silicate solution described in Example 1 was charged and, continuing stirring, 98% sulfuric acid was added to the sodium silicate solution at a rate of 15 g/min. The reaction temperature was kept at 60"C. The addition of sulfuric acid was terminated when the concentratin of SiO2 in the reaction system reached 24-26% by weight and the pH of the reaction system became closed to a predetermined value, which was varired over the range from 1 to 10 as shown in the following Table 2. After that stirring of the reaction system was continued for 10 min. The product of the reaction was treated in the same manner as in Example 1.Table 2 shows variations in the physical properties of the product with the variation in the pH of the reaction system.

TABLE 2 (Example 2) Reaction Physical Properties Conditions of Silica Gel Temp. pH Conc. Water Specific Apparent ( C) of SiO2 Content in Surface Specific (%) Wet Cake Area Density (%) (m2lg) 60 1 24 55 550 0.20 60 2 24 55 530 0.25 60 4 25 50 495 0.25 60 6 25 50 430 0.20 60 8 26 55 325 0.15 60 9 26 50 250 0.15 60 10 26 55 160 0.15 EXAMPLES 3A and 3B In Example 3A, silica gel was prepared generally in the same way as in Example 1A but by using 35% hydrochloric acid in place of the sulfuric acid used in Example 1A. That is, the sodium silicate solution described in Example 1A was added to 35% hydrochloric acid initially charded in the kneader.

In Example 3B, silica gel was prepared by adding 35% hydrochloric acid to the aforementioned sodium silicate solution. In other respects the reaction was carried out in the same way as in Example 3A.

Table 3 shows the reaction conditions and the results obtained in Examples 3A and 3B.

TABLE 3 Reaction Physical Properties Conditions of silica Gel Temp. pH Conc. Water Specific Apparent ( C) of SiO2 Content in Surface Specific (%) Wet Cake Area Density (%) (m2/G) Example 3A 35 2 19 50 580 0.55 Example 35 8 19 55 200 0.16 With respect to Examples 1 to 3, Figure 1 shows the relationship between the pH of the reaction system and the specific surface area of the obtained silica gel.

EXAMPLE 4 Using the same materials and under the same reaction conditions as in Example 1 B, sulfuric acid and sodium silicate solution were simultaneously and continuously introduced into a jacketed kneader of continuous type. The product of the reaction was treated in the same manner as in Example 1 B. Table 4 shows the reaction conditions and the results obtained in Example 4.

TABLE 4 (Example 4) Reaction Physical Properties Conditions of Silica Gel Temp. pH Conc. Water Specific Apparent ( C) of SiO2 Content in Surface Specific (%) Wet Cake Area Density (%) (m2lg) 60 1 25 35 660 0.60 60 2 25 35 625 0.55 60 4 25 40 540 0.45 60 6 25 50 440 0.35 60 8 25 55 310 0.20 60 10 25 55 110 0.15 EXAMPLES 5A-5C In Examples 5A and 5B, silica gel was prepared generally in the same manner as in Examples 1A and 1 B with the exception that the concentrated sulfuric acid was diluted in advance to five different concentrations such that the concentration of SiO2 in the reaction system at the end of the addition of the sodium silicate solution became 15%, 18%, 20% and 23%, respectively.

In Example 5C silica gel was prepared generally in the same manner as in Example 2, by adding sulfuric acid to the sodium silicate solution, with the exception that sulfuric acid was diluted in advance to the above described concentrations.

Table 5 shows the reaction conditions and the results obtained in Examples 5A-5C, and Figure 2 shows the relationship between the concentration of SiO2 in each reaction system of Examples 5A-5C and the specific surface area of the obtained silica gel.

TABLE 5 Reaction Physical properties Conditions - of Silica Gel Temp. pH Conc. Water Specific ( C) of SiO2 Content in Surface (%) Wet Cake Area (%) (m2lg) 35 1.5 15 57 630 35 1.5 18 50 615 Example 5A 35 1.5 20 47 625 35 1.5 23 40 630 60 4.0 15 55 425 60 4.0 18 55 440 Example 5B 60 4.0 20 47 430 60 4.0 23 45 425 60 4.0 15 50 410 60 4.0 18 47 460 60 4.0 20 47 415 60 4.0 23 50 450 Example 5C 60 8.0 15 55 175 60 8.0 18 55 190 60 8.0 20 50 200 60 8.0 23 55 200 Reference 1 In this experiment 1000 g of 30% sulfuric acid was put into a 5-liter beaker and, continuing stirring, a sodium silicate solution (in which the mole ratio SiO2/Na2O was 3.1, and the concentration of SiO2 was 14% by weight) was added to the sulfuric acid at a rate of 30 g/m in. The reaction temperature was kept at 60"C.

The addition of the sodium silicate solution was continued until the pH of the reaction system became 1.0. At this stage the concentration of SiO2 in the reaction system was 8.0% by weight. After that the reaction product was maintained at 70"C for 12 hr for the purpose of aging. A hard gel obtained in this way was crushed so as to pass through a sieve having 2 mm openings, and the crushed gel was filtered, washed, dried and further pulverized to obtain silica gel in the form of fine powder. The content of water in the wet cake was 85%, and the specific surface area of the powdery silica gel was 600 m2/g.

Reference 2 The process of Reference 1 was repeated generally similarly with the exception that the reaction was continued until the pH of the reaction system became 1.5 or 4.0 and that in either case the concentrations of sulfuric acid and sodium silicate solution were varied such that the concentration of SiO2 in the reaction system at the final stage became 1%, 5%, 10% and 13% by weight, respectively. In some runs sodium hydroxide was used for the purpose of adjusting the pH to the intended value, Table 6 shows the reaction conditions and the results obtained in Reference 2.

Reference 3 In this experiment, 3000 g of a sodium silicate solution (in which the mole ratio SiO2/Na2O was 3.1, and the concentration of SiO2 was 14% by weight) was put into a 5-liter beaker and, continuing stirring, sulfuric acid was added to the sodium silicate solution at a rate of 10 g/min. The addition of sulfuric acid was continued until the pH of the reaction system became 4 or 8, and in either case the concentration of sulfuric acid was varied such that the concentration of SiO2 in the reaction system at the final stage became 1%, 5%, 10% and 13%, respectively. In every run the reaction temperature was kept at 60"C.

When the pH was 4, the reaction product was treated in the same manner as in Reference 2, but when the pH was 8 it was possible to omit the gelation by aging. The reaction conditions and the results obtained in Reference 3 are contained in Table 6.

TABLE 6 Reaction Physical Properties Conditions of Silica Gel Temp. pH Conc. Water Specific ( C) of SiO2 Contents in Surface (%) Wet Cake Area (%) 35 1.5 1 97 695 35 1.5 5 95 625 35 1.5 10 90 560 35 1.5 13 80 500 Reference 2 60 4 1 95 550 60 4 5 92 525 60 4 10 90 470 60 4 13 85 420 60 4 1 93 395 60 4 5 90 320 60 4 10 85 245 60 4 13 80 225 Reference 3 60 8 1 90 330 60 8 5 90 295 60 8 10 85 200 60 8 13 80 200 With respect to the experiments in References 2 and 3, Figure 3 shows the relationship between the concentration of SiO2 in the reaction system at the final stage and the specific surface area of the obtained silica gel.

Washing test The products of the following runs in Example 5 and References 2 and 3 were subjected to a washing test.

The test method was as described below.

Run No.4 (pH 1.5, SiO2 23%) of Example 5A Run No.3 (pH 4.0, SiO2 20%) of Example 5B Run No. 2 (pH 8.0, SiO2 18%) of Example 5C Run No. 1 (pH 1.5, SiO2 1%) of Reference 2 Run No. 6 (pH 4.0, SiO2 5%) of Reference 2 Run No. 8 (pH 8.0, SiO2 13%) of Reference 3 A predetermined quantity of each sample silica gel was put into a Buchner funnel in which a qualitative filter paper was placed. A variably determined quantity of washing water was added to the sample in the funnel, and the wet sample was subjected to vacuum filtration. After that the silica gel was dried at 105"C and subjected to analysis of the content of Na in order to find the relationship between the quantity of washing water and the content of Na2O in the washed silica gel.

The results of this test are shown in Figure 4, in which the quantity of the washing water is indicated in terms of the weight ratio of the washing water to the dried silica gel.

Claims

1. A process of preparing silica gel by reaction between an alkali metal silicate and a mineral acid, characterized in that the reaction is carried out such that the concentration of SiO2 in the reaction system becomes not lower than 15% by weight.

2. A process according to Claim 1, wherein the concentration of SiO2 in the reaction system becomes at least 20% by weight.

3. A process according to Claim 2, wherein the concentration of SiO2 in the reaction system does not exceed 30% by weight.

4. A process according to any one of the preceding claims, wherein the reaction is carried out by continuing intense stirring of the reaction system.

5. A process according to any one of the preceding claims, wherein said alkali metal silicate is sodium silicate and said mineral acid is sulfuric acid.

6. A process according to any one of the preceding claims, wherein said alkali metal silicate is in the form of aqueous solution.

7. A process according to any one of the preceding claims, wherein the reaction is carried out by adding said alkali metal silicate to said mineral acid.

8. A process according to any one of Claims 1 to 6, wherein the reaction is carried out by adding said mineral acid to said alkali metal silicate.

9. A process according to any one of Claims 1 to 6, wherein the reaction is carried out by simultaneously introducing said alkali metal silicate and said mineral acid into a reactor.

10. A process according to any one of the preceding claims, wherein the reaction temperature is controlled in the range from room temperature to about 100"C and the pH of the reaction system at the final stage of the reaction is controlled to a value in the range from 0.5 to 12 according to a desired value of the specific surface area of the silica gel.

11. A process of preparing silica gel, substantially as herein described in any one of Examples 1A to IC, 2, 3A, 3B,4and 5A to 5C.