KR0184742B1 - Process for preparing antibiotic and porous silica gel powder - Google Patents

Abstract

The present invention has a specific gravity of 0.02-0.30 g / cc, specific surface area of 200-900 m ² g, prepared by neutralization reaction of an alkali silicate solution with an acid solution containing antibacterial substances (silver, copper, gold, zinc, platinum, etc.). To prepare antimicrobial porous silica gel powder with pore volume of 0.2-3.5cc / g and pore size of 10-500Å and maximum antimicrobial content of 10%, alkaline aqueous silicate solution and acid solution containing antimicrobial material are sprayed. After the reaction, the present invention relates to a method for producing an antimicrobial porous silica gel powder having effective antimicrobial properties by aging, filtration and drying in a short time.

Description

Method for preparing antimicrobial porous silica gel powder

The present invention relates to a method for producing an antimicrobial porous silica gel powder, and more particularly, an aqueous alkali silicate solution as a raw material and an acid solution containing an antimicrobial substance are added and reacted in a spray form, followed by aging, filtration and drying in a short time. The present invention relates to a method for preparing an antimicrobial porous silica gel powder having effective antibacterial properties.

In general, the antimicrobial porous silica powder controls the porosity (pore volume, pore size, specific surface area) and particle size to make various fibers (patient clothes, white clothes, surgical clothes, etc.), films (audio, video, packaging, etc.), home appliances, antimicrobial paints It is used as a filler of toothpaste, toothpaste, and plastics. Recently, the demand for antimicrobial properties of materials and articles is gradually increasing, and thus the field of application of antimicrobial porous silica powder is increasing.

It is a long known fact that precious metals such as silver, copper, gold, zinc or platinum have antimicrobial properties. In fact, silver or copper is a new material that replaces chlorine and other toxic flesh microorganisms. It is becoming. An aqueous solution of silver nitrate (A9AO ₃ ), copper sulfate (CuSO ₄ ), or cupric chloride (CuCl ₂ ) containing silver or copper ions has a very large microbial effect. However, since these aqueous solutions are easily reduced, there is a problem with persistence. Therefore, research and development on inorganic materials (antimicrobial ceramics) having antimicrobial activity has recently begun actively in Japan.

A typical method for producing antimicrobial ceramics is to carry a metal such as silver, copper, gold, zinc, platinum, or the like on a carrier such as amorphous silica, zeolite, calcium phosphate, zirconium phosphate, soluble glass, titania, or the like as a catalyst. Silica fine powder is already used in the industry because of its excellent physicochemical properties and relatively inexpensive manufacturing cost. Anti-sticking agent for film (audio, video, packaging, etc.), matting agent for home appliances and paints, abrasive and thickener for toothpaste, and filler for plastic It is widely used, and the demand for antimicrobial silica having antimicrobial properties while maintaining the above characteristics by the industry's demand for antimicrobial is increasing rapidly. In the case of silica, it is known that the antimicrobial metal can be supported on the silica powder carrier by 2-3%.

As a general method for producing an antimicrobial porous silica carrier, 1) vapor phase decomposition method using silicon tetrachloride (Japanese Patent Application Laid-Open No. 58-, 410313, Japanese Patent Application Laid-Open No. 62-3011), and 2) sol-gel method using silicon alkoxide (Japan) Japanese Patent Application Laid-Open No. 63-166777), and 3) a carrier prepared by neutralization of alkali silicate with acid (US Patent No. 4675122, Japanese Patent Laid-Open No. 3-23487). Gold, zinc, platinum, and the like) are known.

However, the method of 1) has the disadvantages such as toxic and corrosive at the reaction, and the pores are formed only on the particle surface, the method of 2) has the advantage of obtaining a high-purity powder, there is a problem of economic efficiency. In addition, the method of 3) is the most widely used because of the low raw material price and easy handling in the manufacturing method, but the mixing reaction between the raw materials is done by dropping method (Dropping) should be less than 20% concentration of alkali silicate solution as the raw material And, since the contact between the raw material is made locally, there is a disadvantage that the reaction is non-uniform. In addition, it is manufactured by washing with alkaline water for a long time to increase the pore volume (3 ~ 4 days / batch), the uniformity of each product lot is large, the manufacturing cost is high, and due to the long-term aging / washing process There are disadvantages such as outflow of catalyst material. In addition, there is a problem that a complex process, ie, pulverization, crushing, assembly, etc., has to go through a manufacturing process when controlling the particle diameter and shape.

The method of supporting the antimicrobial substance on the silica carrier prepared by the above method is roughly classified into an impregnation method, an ion exchange method, and a precipitation method. In the method of impregnation, the carrier is brought into contact with a solution containing an antimicrobial substance and supported by spraying, evaporation drying, and adsorption. The spraying method is to spray and support a solution containing an antimicrobial component while the carrier is put in an evaporator and stirred. The evaporation drying method is a method in which a carrier is immersed in a solution containing antimicrobial substances and then the solvent is blown off. The ion exchange method is mainly used to support antimicrobial substances on silica, zeolite, alumina, etc., but it has the advantage that the antimicrobial substances can be uniformly distributed. However, the amount of antimicrobial materials that can be supported is small, and the supporting time is excessive.

As described above, the supporting method of the conventional antimicrobial substance described above has to carry the antimicrobial substance again after preparing the carrier, so that excessive time is used on the supporting material, and the content of the antimicrobial substance is limited, and it must go through a complicated process. As a result, the manufacturing cost also increases.

In addition, in the process of preparing the silica carrier by the method of preparing the silica powder of 3), there is also a method of preparing the antimicrobial silica by adding a raw material containing an antimicrobial material during the initial synthesis of the raw material, but the aging / washing process is excessive. There is a disadvantage that a significant amount of antimicrobial material is lost.

That is, in the conventional method, it is not easy to manufacture a highly porous silica carrier, and there is a limit to the amount of an antibacterial substance that can be loaded thereon, and the manufacturing time is long and the manufacturing cost is high. In addition, in the case of supporting the antimicrobial material on the carrier, it is difficult to carry the homogeneous material on the already prepared carrier, and the manufacturing process is complicated and the economic efficiency is low. In addition, even when the antimicrobial material is added to prepare a carrier, there is a disadvantage in that the antimicrobial material is lost due to a long time aging / washing process.

The present inventors solve the above problems of the prior art to produce an antimicrobial highly porous carrier, as well as to provide a silica gel powder with a high content of antimicrobial material.

Still another object of the present invention is to provide an antimicrobial substance in a homogeneous reaction distribution with silica gel powder, and to provide a manufacturing process simply.

1 is a powder manufacturing process chart according to the present invention.

2 is a reactor according to the present invention.

3 is a detailed view of a raw material input device according to the present invention.

The present invention comprises the steps of (a) adding an acid solution to the alkali silicate aqueous solution to obtain a mixture in the neutralization reaction;

(b) producing a silica hydride from the reaction mixture;

(c) gelling and closing the silica hydrosol to obtain a silica gel slurry;

(d) A method for producing a low density porous silica gel powder by separating, washing, drying and pulverizing the silica gel slurry, wherein the aqueous alkali silicate solution and silver, copper, gold and zinc are used as raw materials in the step (a). Or a method of producing an antimicrobial porous silica gel, characterized in that the reaction mixture is made by neutralizing the inorganic acid aqueous solution containing an antimicrobial substance such as platinum in the form of a spray, and further added sulfuric acid aqueous solution containing the antimicrobial substance to the reaction tank.

Hereinafter, the present invention will be described in more detail.

In the method for producing the antimicrobial porous silica gel powder according to the present invention, first, a partial neutralization reaction is performed by spraying an alkaline acid aqueous solution, which is a raw material, and an inorganic acid solution containing an antimicrobial substance such as silver, copper, gold, zinc or platinum in a spray form into a raw material input device. To prepare a primary mixture.

As the aqueous alkali silicate solution used as a raw material in the present invention, for example, an aqueous solution of sodium silicate or an aqueous solution of potassium silicate can be used, and in particular, it is preferable to use an aqueous solution of sodium silicate because it can be easily supplied in large quantities at low cost.

The silica concentration of the alkali silicate solution used in the present invention is suitably 20 to 26% by weight based on the total aqueous solution. In this case, when the silica concentration is less than 20% by weight, the productivity is lowered. On the contrary, when the concentration is higher than 26% by weight, the viscosity is turned on, which may cause process trouble such as clogging of the transfer pipe or the spray nozzle. Can not do it.

In the present invention, any inorganic acid such as hydrochloric acid, nitric acid, acetic acid, and sulfuric acid may be used as the inorganic acid, but sulfuric acid may be advantageous in consideration of reactivity with the raw material, impurities removal effect, manufacturing cost, and the like. The concentration of acid in the inorganic acid solution is preferably 6 to 15% by weight based on the total solution. If the acid concentration is less than 6%, there is an advantage that can uniformly induce a uniform reaction of silica and inorganic acid and removal of the alkali component, but there is a disadvantage to increase the size of the reaction tank, there is a disadvantage to increase the manufacturing cost, On the contrary, when the acid concentration exceeds 15% by weight, sufficient mixing with the alkali silicate aqueous solution is difficult, so that a homogeneous reaction cannot be expected. The value of the product will fall.

As an antimicrobial material used in the present invention, silver (AgNo ₃ , Ag ₃ O ₄ P), copper (CuN ₂ O ₆ , CuCl ₂ ), gold (AuCl), zinc (ZnCl ₂ , ZnN ₂ O ₆ ), pulse gold ( An aqueous solution such as H ₂ PtCl ₆ ) may be used, and the amount of the antimicrobial substance may be added in advance to the inorganic acid aqueous solution up to 10% of the antimicrobial substance used for the dry silica used as a raw material in the alkali silicate aqueous solution.

Then, according to the present invention, it is preferable to maintain the pH of the primary mixture of the aqueous alkali solution of silicate and the inorganic acid solution containing the antimicrobial substance in the raw material input device, for example, 11.6 or more. If the pH is kept lower than 11.6, the gelation phenomenon occurs momentarily in the process, such as clogging of the inlet.

As described above, the primary mixture by the partial neutralization reaction of the aqueous alkali silicate solution obtained from the raw material input device and the inorganic acid solution containing the antimicrobial substance is introduced into a reaction tank having a stirrer, and then forms an antimicrobial silica gel hydrosol and gelates in a few minutes to prevent antimicrobial activity. Converted to silica gel hydrogel. The converted antimicrobial silica gel hydrogel is disintegrated by an agitator before it is hardened to produce an antimicrobial silica gel slurry.

In particular, according to the present invention, when the primary mixture is added to the reaction tank, it is possible to add an inorganic acid solution containing an antimicrobial agent, which is to lower the pH of the mixture, for example, the pH of the mixture, for example, 10.8 to 11.4 The inorganic acid solution containing the antimicrobial substance is added continuously until maintained. If the pH of the mixture in the reaction tank is less than 10.8, a sudden gel occurs. On the contrary, if the pH exceeds 11.4, the gelation time is long and the process time is lengthened.

On the other hand, since the disintegration time of the obtained antimicrobial silica hydrogel is related to the size of the gel particle finally prepared, it may be selectively changed depending on the use, but generally 20 minutes to 30 minutes is appropriate.

As described above, the antimicrobial silica gel slurry containing microgels prepared by the partial neutralization and disintegration process is pure silica gel slurry by complete neutralization by adding an acid solution to completely remove the alkali component remaining by partial neutralization. It becomes At this time, the addition amount of the acid solution is preferably added until the pH of the silica gel slurry solution is 2.5, for example.

The antimicrobial silica gel slurry obtained above is aged under an alkaline atmosphere in order to increase the pore size between particles to control the pore size and pore volume.

NaOH, NH ₄ OH, or NH ₃ gas may be used as the alkali added for the alkaline atmosphere. Preferably, ammonia without incorporation of impurities is preferable, and the most preferable form is ammonia water (NH ₄ OH). good.

Alkali dosing method is preferably added to the silica gel slurry in the form of a spray through a nozzle.

On the other hand, the aging conditions in an alkaline atmosphere is preferably carried out for 30 minutes to 3 hours at a temperature of 60 to 100 ℃ and pH 7.5 to 10.5, it can be prepared a gel suitable for the application by changing the conditions according to the desired properties. . In particular, the higher the aging temperature and pH, the longer the aging time, the larger the size of the pores.

Specifically, to prepare antimicrobial silica gel, specific gravity 0.02-0.30 g / cc, specific surface area 200-900 m ² / g, pore volume 0.2- when aging 40 to 80 minutes at a temperature of 85 to 95 ° C., pH 8.5 to 9.0 An antimicrobial porous silica gel powder of 3.5 cc / g and a pore size of 10-500 kPa and a maximum antimicrobial content of 10% can be prepared.

Synthetic silica gel slurry is aged as described above can be separated by a solid-liquid separation device. For example, a centrifuge, a filter press, a decantation, or a filtration method may be used as the solid-liquid separation device. However, in the case of industrialization, a centrifuge may be easily used for mass production and easy to wash.

The solid-liquid separated silica gel particles may be washed to remove the forget-me-not and unreacted acid solution contained in the gel particles. As water used for washing, for example, general industrial water or pure water can be used, and industrial water and pure water can be selectively used depending on the use and grade.

In particular, the higher the temperature of the washing water, the higher the cleaning effect. Preferably, a small amount of ammonium carbonate or the like, which is effective in removing the ammonia salt, may be used to increase the washing effect.

Finally, the wet silica gel washed as above is dried.

Even if the synthesis of silica gel is well performed until the drying process, if the drying method and conditions are not appropriate, it is difficult to prepare the targeted antimicrobial low density porous silica gel by pore shrinkage of the silica gel.

In order to improve the fluidity of the powder, if the powder is desired, the antimicrobial hydrogel, which has been aged / washed, is slurried by high-speed agitation, and then dried using a spray dryer. If the shape of the powder is not a problem, the hydrogel is dried using an oven dryer, a vacuum dryer, a fluidized bed dryer, or the like without a separate slurrying process. Drying methods and means vary, but if the drying time of the silica gel is prolonged, the pore size and pore volume of the silica gel are greatly changed by shrinkage of the gel body due to surface tension during drying.

Therefore, in order to manufacture an antimicrobial low density silica dry gel body, shorter drying time is preferable. As a dryer which can be dried in a short time, a vacuum dryer, a spray dryer, a flash grinding dryer, or the like can be used, and a grinding dryer suitable for mass production and a continuous process is suitable for drying in a few seconds.

The dried gel body thus prepared is pulverized according to the particle size suitable for the purpose. Ball mills, hammer mills, grind mills, jet mills, etc. may be used as the grinder, but jet mills may be used to prepare fine porous silica powder having an average particle diameter of 4 μm or less.

The antimicrobial porous silica gel powder obtained according to the method of the present invention has a specific gravity of 0.02-0.30 g / cc, specific surface area of 200-900 m ² / g, pore volume of 0.2-3.5cc / g and pore size of 10-500Å, maximum antimicrobial content 10 It has the properties of antimicrobial porous silica gel powder in% (based on silica dry weight).

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 is a manufacturing process chart of the antimicrobial porous silica gel used in the present invention.

The manufacturing process shown in FIG. 1 is similar to the process shown in Patent Application No. 94-26821 filed by the applicant. More detailed description of this process is as follows.

In FIG. 1, the aqueous alkali silicate solution stored in the raw material container 1 is first sprayed into the raw material input device by the nozzle 4 through the valve 2 and the valve 3. At the same time, the sulfuric acid aqueous solution containing the antimicrobial substance such as silver, copper, gold, zinc or platinum stored in the raw material container passes through the valve 7 and the valve 8 and is discharged by the raw material input device by the nozzle 9. 5) is sprayed into. The raw materials introduced into the raw material input device 5 form a primary mixture by partial neutralization.

In the raw material input device (%) according to the present invention, as shown in an enlarged view in FIG. 2A, nozzles 4 and 9 are respectively provided to supply raw materials in a spray form from both sides to the upper side, and the spray surface of the nozzle is shown in FIG. As shown in 2b, the nozzle hole is formed radially.

On the other hand, it is preferable to set the raw material input pressure of the nozzle 4 to be about 3 atmospheres or more, and it is preferable to set the raw material input pressure of the nozzle 9 to be about 2 atmospheres or more.

If the raw material input pressure is at or below the above range, the spray discharge amount to the raw material input device 5 will not be sufficient, and the spray angle will be 55 ° or less, so that uniform mixing will not occur.

The material of the nozzles 4 and 9 used in the present invention is preferably made of ceramics having excellent acid resistance and abrasion resistance, and the raw material input device 5 checks the degree of mixing to adjust the raw material input pressure by the nozzles 4 and 9. It is desirable to make it transparent so that it can be adjusted.

The raw material primarily mixed in the raw material input device 5 passes through the valve 11 and is introduced into the reaction tank 13 by the nozzle 12. At the same time, in order to lower the pH of the primary raw material mixture, an acid solution remaining in the raw material container 6 passes through the valve 7 and the valve, and an appropriate amount is introduced into the reactor 13 by the nozzle 12.

At this time, the pH of the raw material mixture in the reaction tank is measured by the pH meter (15).

As shown in an enlarged view in FIG. 3, the reaction vessel 13 may increase the degree of mixing during the reaction of the raw material mixture and may dissolve when a gel is generated during the reaction. And a stirrer 19 provided with three types of agitating vanes composed of a turbine-type impeller 18.

In addition, the reactor 13 surrounds the outside of the heater 20 in order to maintain a constant temperature during the reaction. As shown in FIG. 1, the heater 20 serves to heat the fruit from the fruit device 21 to the outside of the reaction vessel 13.

And, the side of the reaction tank 13 is attached to the pH measuring device 15 that can measure the pH of the reactant in the reaction tank 13 and the temperature sensor 22 that can measure the temperature of the reaction tank (1) have.

If necessary, a baffle plate 23 may be attached to the inner side of the reaction vessel 13 to further enhance the mixing of the reactants as shown in FIG. 3.

On the other hand, the material of the reaction tank 13 is acid-resistant, it is necessary to be a heat-resistant material that withstands heat, specifically, for example, stainless 316L is preferable.

The reaction mixture introduced into the reaction tank 13 forms a silica gel hydrosol and forms a silica gel hydrogel in a few minutes.

The resulting silica hydrogel is disintegrated by vigorous stirring of the stirrer 19 as shown in FIG. 3 above before the gel solidifies into a silica gel slurry.

In order to remove the alkali component remaining by partial neutralization in the silica gel slurry containing the fine gel formed in the reactor 13, the acid solution stored in the raw material container 24 is passed through the valve 25 and the valve 26. Further addition to the reactor 13 neutralizes the slurry completely to give a pure silica gel slurry. Thus prepared silica gel slurry is aged in a basic atmosphere to control the pores, the ammonia water stored in the raw material container 27 to the reaction tank 13 by the nozzle 12 through the valve 28 and the valve 14. Add.

The silica gel matured in the reactor 1 is applied to the solid-liquid separator 30 through the draw valve 29, separated and washed, and then sent to the dryer 31 for drying.

The dry gel body thus prepared is controlled to have a low-density porous silica gel powder by controlling the particle size by air grinding method (jet mill) according to the use.

On the other hand, reference numerals 32 and 33 are valves for adjusting the fruit supplied from the fruit device 21.

Hereinafter, the present invention will be described in detail with reference to Examples.

Example 1

40 kg of sodium silicate No. 3 (silica concentration 28%) solution was diluted with 8 kg of water to prepare an aqueous solution containing about 23% silica concentration. In addition, diluted dilute sulfuric acid (98%) 1.5L, silver nitrate aqueous solution (10%) 8.96L, and water 26.04L to prepare a dilute sulfuric acid aqueous solution. 48 kg of a sodium silicate aqueous solution and 30.5 L of an aqueous solution of silver nitrate were added through a raw material injection nozzle over 6 minutes. After the addition, the remaining silver nitrate-containing aqueous sulfuric acid solution 5L was added through a spray nozzle attached to the reactor to produce a silver-containing silica hydrosol.

At this time, the pH of the reaction mixture was maintained at 11.06. The resulting hydrosol was converted to silver containing silica gel in water. At this time, the gel was disintegrated with strong stirring to prepare a slurry including the microgel. Disintegration was performed for 20 minutes. Next, in order to remove the unreacted alkali (soda component) contained in the silver-containing silica gel, a sulfuric acid dilute solution containing 3 l of sulfuric acid (98%) and 30 l of water was added over 15 minutes. At this time, the pH of the slurry was maintained at 2.0.

Subsequently, 110 L (28%) of ammonia water was added to increase the pore volume and maintained at pH 9.8, and aged for 50 minutes while maintaining the temperature at 80 ° C. Thereafter, filtration and washing were performed to prepare a silver cake containing gel.

The silver-containing gel cake thus prepared was filled in a reactor with a stirrer, followed by stirring for 20 minutes. Gel cakes with a water content of 75% were then slurried. The prepared silver-containing gel slurry was spray dried using a parting dryer (NIRO). Spraying conditions were the disk rotation speed of 1200 times per minute, the inlet and outlet temperatures of 250 ℃, 105 ℃, respectively, the binder was added 0.1% vinyl alcohol polymer.

Example 2

The antimicrobial silica gel powder was prepared in the same manner as in Example 1 except that the silver-containing gel cake was dried in a flash dryer in Example 1.

The operating conditions of the flash dryer are 350 ° C inlet temperature, 180 ° C outlet temperature and Rotor rpm 600.

Example 3

In preparing the aqueous solution containing sulfuric acid in Example 2, an antimicrobial silica powder was prepared in the same manner as in Example 2 except that 2.24 ml of silver nitrate solution (10%) was added and 35 L of water was added.

Comparative Example 1

Antimicrobial amorphous silica was prepared in the same manner as in Example 2, except that an aqueous metal ion solution was not added.

The measurement results of the physical properties of Examples 1 to 3 and Comparative Example 1 are shown in Table 1 below.

[Test Example]

Examples 1 to 3 were tested for the antimicrobial activity of the prepared silver-containing antimicrobial porous silica.

For the antimicrobial test, bacteria used Pseudomonas and Escherichia coli, which are common cells. 30 g of silver-containing silica was added to 100 ml of the solution adjusted to Pseudomonas to 10 ² cells / ml, and 100 ml of the solution to be adjusted to E. coli 4 × 10 ⁶ cells / ml. . After stirring for 1 minute, 3 minutes, and 10 minutes (antibacterial treatment time) while maintaining the temperature at 35 ° C, the silica dispersion was vacuum filtered. The viable cell count of the filtrate was measured with an Automated Colony Counter. The measurement results are shown in Table 2 below.

By using the spray raw material introduction device introduced in the present invention, the antimicrobial substance was added at the same time as the raw material reaction, specific gravity 0.02-0.30g / cc, specific surface area 200-900 m ² g, pore volume 0.2-3.5cc / g and pore size 10 -500 kPa, antimicrobial porous silica gel powder with a maximum antimicrobial content of 10% was able to be produced in a short time. Antimicrobial silica loaded with antimicrobial materials such as silver, copper, gold, zinc, and platinum is used for conventional silica, namely, anti-sticking agents for films (audio, video, packaging, etc.), home appliances, paint matting agents, and toothpaste abrasives. And can be used as a thickener, a plastic filler, but may have the effect of adding antimicrobial properties in the application field.

Claims (6)

The present invention comprises the steps of (a) adding an acid solution to the aqueous alkali silicate solution to obtain a mixture in the neutralization reaction; (b) producing a silica sol from the reaction mixture; (c) gelling and pulverizing the silica car sol to obtain a silica gel slurry; (d) a method for preparing a low density porous silica gel powder by separating, washing, drying and pulverizing the silica gel slurries, wherein the aqueous alkaline silicate solution and silver, copper, gold, zinc or A method of producing an antimicrobial porous silica gel, characterized in that a reaction mixture is prepared by neutralization of an inorganic acid aqueous solution containing an antimicrobial substance such as platinum, and an aqueous sulfuric acid solution containing the antimicrobial substance is added to the reactor. The antimicrobial porous silica gel powder prepared according to claim 1 has a specific gravity of 0.02-0.30 g / cc, specific surface area of 200-900 ml / g, pore volume of 0.2-3.5cc / g and pore size of 10-500 kPa, and maximum antimicrobial content. Method for producing an antimicrobial porous silica gel, characterized in that to be 10%. The method for preparing an antimicrobial porous silica gel according to claim 1, wherein the silica concentration of the alkali silicate solution is 20 to 26 wt% based on the total aqueous solution. The method of claim 1, wherein the acid concentration of the inorganic acid solution is 6 to 15 relative to the total solution. Method for producing an antimicrobial porous silica gel, characterized in that the weight%. The method of claim 1, wherein the amount of the antimicrobial substance to be added is used by adding up to 10% of the antimicrobial substance to the aqueous inorganic acid solution in advance with respect to the dry silica commonly used as an ingredient in the alkali silicate aqueous solution. . The method of claim 1, wherein in the drying / milling step (d), the spherical silica is spray dried by slurrying the gel cake, and the non-spherical silica is a fluidized bed dryer.