Claims:1. A process for preparing abrasive grade silica, the process comprising:
preparing a sodium silicate solution comprising silicon dioxide and sodium oxide in a ratio in the range of 3.0:1-3.3:1, and water, wherein the sodium silicate solution has a solid concentration in the range of 29-31% w/w and pH in the range of 12-13;
preparing an electrolyte solution by mixing an electrolyte with water at a temperature in the range of 85-90oC wherein the electrolyte solution has the electrolyte in an amount of 10% w/w of the weight of silicon dioxide, sodium ion concentration in the range of 0.3N-0.42N, and pH in the range of 7-7.5;
adding the sodium silicate solution and water to the electrolyte solution to obtain a mixture having pH in the range of 8.2-8.9;
simultaneously adding the sodium silicate solution, water and a first amount of a mineral acid to the mixture to obtain a slurry having a final sodium ion concentration of 0.83 ± 0.01N and pH of 7.9-8.1;
aging the slurry for 5-15 minutes at a temperature in the range of 85-90°C;
adding a second amount of the mineral acid to the aged slurry to bring down the pH of the aged slurry in the range of 3.5-4.2; and
precipitating silica from the aged slurry followed by washing and drying the silica to obtain the abrasive grade silica.

2. The process as claimed in claim 1, wherein the electrolyte is selected from a group consisting of sodium sulfate and sodium chloride.

3. The process as claimed in claim 1 or 2, wherein the electrolyte solution is prepared by mixing the electrolyte with water by stirring at a rate of 30-100 rpm.

4. The process as claimed in any of claims 1-3, wherein the mineral acid is selected from the group consisting of sulfuric acid and hydrochloric acid.

5. The process as claimed in any of claims 1-4, wherein the mineral acid is sulfuric acid having concentration in the range of 50-98% w/v.

6. The process as claimed in any of claims 1-5, wherein the slurry is stirred continuously during the aging.

7. The process as claimed in any of claims 1-6, wherein the precipitating of silica from the aged slurry comprises subjecting the aged slurry to further aging for a period of 5-15 minutes at a temperature in the range of 85-90°C.

8. The process as claimed in any of claims 1-7, wherein the drying comprises spray drying or flash drying.

9. The process as claimed in any of claims 1-8, comprising comminuting the dried silica to obtain a desired particle size distribution.

10. The process as claimed in any of claims 1-9, wherein concentration of silicon dioxide in the sodium silicate solution is 75 g/l.

11. The process as claimed in any of claim 1-10, wherein the abrasive grade silica has
BET surface area in the range of 25-120 m2/g;
DOA oil absorption value in the range of 70-120 ml/100g;
CDBP coefficient in the range of 0.21-0.29;
Sears value in the range of 17.9-21.9 ml/5g;
particle size distributions having d50 value in the range of 6-15 µm;
bulk density value in the range of 250-450 g/l;
linseed oil absorption in the range of 60-130 ml/100g;
water absorption in the range of 70-130 ml/100g;
Einlehner abrasion value in the range of 1.9-3.5 mg loss per 100000 revolutions;
fluoride compatibility value of = 90%; and
viscosity in the range of 150-250 cPs.

12. An abrasive grade silica having:
BET surface area in the range of 25-120 m2/g;
DOA oil absorption value in the range of 70-120 ml/100g;
CDBP coefficient in the range of 0.21-0.29;
Sears value in the range of 17.9-21.9 ml/5g;
particle size distributions having d50 value in the range of 6-15 µm;
bulk density values in the range of 250-450 g/l;
linseed oil absorption in the range of 60-130 ml/100g;
water absorption in the range of 70-130 ml/100g;
Einlehner abrasion value in the range of 1.9-3.5 mg loss per 100000 revolutions;
fluoride compatibility value of = 90%; and
viscosity in the range of 150-250 cPs.
, Description:TECHNICAL FIELD
The present invention relates to a process for preparing synthetic amorphous silicas. Specifically, the present invention relates to a process for preparing abrasive grade silica.

BACKGROUND
Synthetically produced abrasive grade amorphous silica is an important ingredient in many of the toothpaste formulations available today. It not only improves the cleaning performance of the formulations but is also relatively safe, non-toxic, and is compatible with other ingredients of the formulations, including glycerine, sorbitol, xylitol, thickening agents, detergents, coloring agents, and fragrance materials.
The low abrasive silicas (Radioactive Dentin Abrasion or RDA:= 70) are used in children toothpaste formulations, medium abrasive silicas (RDA:71-100) are used in regular toothpaste formulations and high abrasive silicas (RDA:101-150) are used in toothpaste formulations, where higher cleaning performances are needed. The critical parameters control for abrasive grade silica synthesis are its specific surface area (25-120 m2/g), bulk density (250-450 g/l), average particle size distribution (6-15 µm), and oil absorption (70-120 ml/100g) values.
The abrasive grade silica is conventionally prepared by admixing dilute alkali silicate solutions with strong aqueous mineral acids under conditions where aggregation to the sol and gel cannot occur, stirring and then filtering out the precipitated silica. The resulting precipitate is then washed, dried, and comminuted to the desired size. Silica obtained by the conventional process is highly abrasive and can cause damage to the dental layer by frequent brushing. It is desirable to have silica with optimum abrasiveness and good cleaning properties.

SUMMARY
The present disclosure relates to a process for preparing abrasive grade silica. The process comprises preparing a sodium silicate solution comprising silicon dioxide and sodium oxide in a ratio in the range of 3.0:1-3.3:1, and water, wherein the sodium silicate solution has a solid concentration in the range of 29-31% w/w and pH in the range of 12-13; preparing an electrolyte solution by mixing an electrolyte with water at a temperature in the range of 85-90oC wherein the electrolyte solution has the electrolyte in an amount of 10% w/w of the weight of silicon dioxide, sodium ion concentration in the range of 0.3N-0.42N, and pH in the range of 7-7.5; adding the sodium silicate solution and water to the electrolyte solution to obtain a mixture having pH in the range of 8.2-8.9; simultaneously adding the sodium silicate solution, water and a first amount of a mineral acid to the mixture to obtain a slurry having a final sodium ion concentration of 0.83 ± 0.01N and pH of 7.9-8.1; aging the slurry for 5-15 minutes at a temperature in the range of 85-90°C; adding a second amount of the mineral acid to the aged slurry to bring down the pH of the aged slurry in the range of 3.5-4.2; and precipitating silica from the aged slurry followed by washing and drying the silica to obtain the abrasive grade silica.
The present disclosure also relates to abrasive grade silica having BET surface area in the ranges of 25-120 m2/g; DOA oil absorption value in the range of 70-120 ml/100g; CDBP coefficient in the range of 0.21-0.29; Sears value in the range of 17.9-21.9 ml/5g; particle size distributions having d50 value in the range of 6-15 µm; bulk density values in the range of 250-450 g/l; linseed oil absorption in the range of 60-130 ml/100g; water absorption in the range of 70-130 ml/100g; Einlehner abrasion value in the range of 1.9-3.5 mg loss per 100000 revolutions; fluoride compatibility value of = 90%; and viscosity in the range of 150-250 cPs.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 depicts the effect of sodium ion concentration on the BET surface area of the abrasive grade silica obtained in accordance with various embodiments of the present disclosure.
FIG. 2 depicts the effect of sodium ion concentration on Sears values of the abrasive grade silica obtained in accordance with various embodiments of the present disclosure.

DETAILED DESCRIPTION
The present disclosure relates to a process for preparing abrasive grade silica. The process comprises preparing a sodium silicate solution comprising silicon dioxide and sodium oxide in a ratio in the range of 3.0:1-3.3:1, and water, wherein the sodium silicate solution has a solid concentration in the range of 29-31% w/w and pH in the range of 12-13; preparing an electrolyte solution by mixing an electrolyte with water at a temperature in the range of 85-90oC wherein the electrolyte solution has the electrolyte in an amount of 10% w/w of the weight of silicon dioxide, sodium ion concentration in the range of 0.3N-0.42N, and pH in the range of 7-7.5; adding the sodium silicate solution and water to the electrolyte solution to obtain a mixture having pH in the range of 8.2-8.9; simultaneously adding the sodium silicate solution, water and a first amount of a mineral acid to the mixture to obtain a slurry having a final sodium ion concentration of 0.83 ± 0.01N and pH of 7.9-8.1; aging the slurry for 5-15 minutes at a temperature in the range of 85-90°C; adding a second amount of the mineral acid to the aged slurry to bring down the pH of the aged slurry in the range of 3.5-4.2; and precipitating silica from the aged slurry followed by washing and drying the silica to obtain the abrasive grade silica.
In accordance with an embodiment, the concentration of silicon dioxide in the sodium silicate solution is 75 g/l.
In accordance with an embodiment, the electrolyte is selected from sodium sulfate and sodium chloride. In a specific embodiment, the electrolyte is sodium chloride.
In accordance with an embodiment, the electrolyte solution is prepared by mixing the electrolyte with water by stirring at a rate of 30-100 rpm.
Any suitable mineral acid may be used in the process. Suitable examples of the mineral acid include sulfuric acid and hydrochloric acid. In accordance with an embodiment, the mineral acid has a concentration in the range of 50-98% w/v. In accordance with an embodiment, the mineral acid is sulfuric acid.
In accordance with an embodiment, the slurry is stirred continuously during the aging. In an embodiment, the aged slurry is subjected to further aging for a period of 5-15 minutes at a temperature in the range of 85-90°C.
The silica obtained from the process may be dried by any suitable method such as spray drying or flash drying.
In an embodiment, the dried silica is comminuted to obtain a desired particle size distribution.
The present disclosure also relates to abrasive grade silica. The abrasive grade silica has BET surface area in the range of 25-120 m2/g; DOA oil absorption value in the range of 70-120 ml/100g; CDBP coefficient in the range of 0.21-0.29; Sears value in the range of 17.9-21.9 ml/5g; particle size distributions having d50 value in the range of 6-15 µm; bulk density values in the range of 250-450 g/l; linseed oil absorption in the range of 60-130 ml/100g; water absorption in the range of 70-130 ml/100g; Einlehner abrasion value in the range of 1.9-3.5 mg loss per 100000 revolutions; fluoride compatibility value of = 90%; and viscosity in the range of 150-250 cPs.

EXAMPLES
Example 1: Preparation of Low Abrasive Grade Silica: An aqueous solution of sodium silicate having a solid content of approximately 30% by weight (Na2O to SiO2 ratio of 1:3.2, silica by weight of 23%, Na2O by weight of 7%) was prepared in water. The pH of the solution was maintained at 12.5 ±0.5.
50% sulfuric acid solution (w/v) was prepared by slowly adding 500 ml of 98% concentrated sulfuric acid (specific gravity ~1.84 g/cc) to 500 mL of distilled water.
To synthesize precipitated silica, 2.45 liters of distilled water along with 52.5 g sodium sulfate salt (10% with respect to the. total SiO2 dry powder) was taken in a properly cleaned 10 liter beaker on a hot plate stirrer. The temperature was set at 85°C with a stirring rate of 300 rpm. 300 ml of sulfuric acid, 1.84 liters of sodium silicate solution, and 2.44 liters of distilled water were taken in three separate beakers. Three metering pumps were calibrated to pump each of the above three feed solutions. Sulfuric acid pumping rate was set at 6-7 ml/min., sodium silicate solution pumping rate was set at 40.9 ml/min. and distilled water pumping rate was set at 54 ml/min. Once the temperature of the reaction vessel attained 85°C, the addition of sodium silicate solution and water was initiated simultaneously for the first 30 seconds to achieve a pH value of 8.2-8.9.
In phase I of the reaction, the addition of sulfuric acid was initiated in addition to the flow of sodium silicate solution and water. For the next 45 minutes of the reaction, the pH of the solution mixture was maintained at 8.0 ± 0.1, by controlling the flow of sulfuric acid. Post 45 minutes of reaction, the flow of all the three feed solutions (sodium silicate, sulfuric acid, and water) was stopped. The solution mixture was aged for 5 min with the stirrer on while maintaining the temperature at 85°C.
In the second phase of the reaction, only sulfuric acid was added to the reaction vessel at a flow rate of 50 ml/min. The pH of the reaction slurry was brought down rapidly from 8.0 ± 0.1 to 3.5-4.0. After pH adjustment, the addition of sulfuric acid was stopped. The reaction mixture was allowed to age for another 10 minutes at 85°C while stirring. The precipitated slurry was then collected from the reactor vessel.
The precipitate was either centrifuged at 4000 rpm for 5 minutes or passed through a filter press followed by washing of soluble salt like sodium sulfate. The washing was continued until the wash water TDS value reached 1000 ppm or less. The solid content of the washed wet cake thus obtained was 20-30% by weight. For flash drying, the washed silica cake was introduced to a flash dryer without any further changes. For spray drying, the silica cake was homogenized in the form of slurry with a total silica content of 8 to 20%. The pH of the slurry was maintained at 5 to 6 by either addition of sulfuric acid or ammonia. The resultant slurry was then spray dried to get fine dried silica powder.

Example 2: Preparation of the Medium Abrasive Grade Silica: An aqueous solution of sodium silicate having a solid content of approximately 30% by weight (Na2O to SiO2 ratio of 1:3.2, silica by weight of 23%, Na2O by weight of 7%) was prepared in water. The pH of the solution was maintained at 12.5 ±0.5.
50% sulfuric acid solution (w/v) was prepared by slowly adding 500 ml of 98% concentrated sulfuric acid (specific gravity ~1.84 g/cc) to 500 mL of distilled water.
To synthesize precipitated silica, 2.09 liters of distilled water along with 52.5 g sodium sulfate salt (10% with respect to the. total SiO2 dry powder) was taken in a properly cleaned 10 liter beaker on a hot plate stirrer. The temperature was set at 85°C with a stirring rate of 300 rpm. 300 ml of sulfuric acid, 1.84 liters of sodium silicate solution, and 2.79 liters of distilled water were taken in three separate beakers. Three metering pumps were calibrated to pump each of the above three feed solutions. Sulfuric acid pumping rate was set at 6-7 ml/min., sodium silicate solution pumping rate was set at 40.9 ml/min. and distilled water pumping rate was set at 62 ml/min. Once the temperature of the reaction vessel attained 85°C, the addition of sodium silicate solution and water was initiated simultaneously for the first 30 seconds to achieve a pH value of 8.2-8.9.
In phase I of the reaction, the addition of sulfuric acid was initiated in addition to the flow of sodium silicate solution and water. For the next 45 minutes of the reaction, the pH of the solution mixture was maintained at 8.0 ± 0.1, by controlling the flow of sulfuric acid. Post 45 minutes of reaction, the flow of all the three feed solutions (sodium silicate, sulfuric acid, and water) was stopped. The solution mixture was aged for 5 min with the stirrer on while maintaining the temperature at 85°C.
In the second phase of the reaction, only sulfuric acid was added to the reaction vessel at a flow rate of 50 ml/min. The pH of the reaction slurry was brought down rapidly from 8.0 ± 0.1 to 3.5-4.0. After pH adjustment, the addition of sulfuric acid was stopped. The reaction mixture was allowed to age for another 10 minutes at 85°C while stirring. The precipitated slurry was then collected from the reactor vessel.
The precipitate was either centrifuged at 4000 rpm for 5 minutes or passed through a filter press followed by washing of soluble salt like sodium sulfate. The washing was continued until the wash water TDS value reached 1000 ppm or less. The solid content of the washed wet cake thus obtained was 20-30% by weight. For flash drying, the washed silica cake was introduced to a flash dryer without any further changes. For spray drying, the silica cake was homogenized in the form of slurry with a total silica content of 8 to 20%. The pH of the slurry was maintained at 5 to 6 by either addition of sulfuric acid or ammonia. The resultant slurry was then spray dried to get fine dried silica powder.
Example 3: Preparation of the High Abrasive Grade Silica: An aqueous solution of sodium silicate having a solid content of approximately 30% by weight (Na2O to SiO2 ratio of 1:3.2, silica by weight of 23%, Na2O by weight of 7%) was prepared in water. The pH of the solution was maintained at 12.5 ±0.5.
50% sulfuric acid solution (w/v) was prepared by slowly adding 500 ml of 98% concentrated sulfuric acid (specific gravity ~1.84 g/cc) to 500 ml of distilled water.
To synthesize precipitated silica, 1.75 liters of distilled water along with 52.5 g sodium sulfate salt (10% with respect to the. total SiO2 dry powder) was taken in a properly cleaned 10 liter beaker on a hot plate stirrer. The temperature was set at 85°C with a stirring rate of 300 rpm. 300 ml of sulfuric acid, 1.84 liters of sodium silicate solution, and 3.34 liters of distilled water were taken in three separate beakers. Three metering pumps were calibrated to pump each of the above three feed solutions. Sulfuric acid pumping rate was set at 6-7 ml/min., sodium silicate solution pumping rate was set at 40.9 ml/min. and distilled water pumping rate was set at 76.5 ml/min. Once the temperature of the reaction vessel attained 85°C, the addition of sodium silicate solution and water was initiated simultaneously for the first 30 seconds to achieve a pH value of 8.2-8.9.
In phase I of the reaction, the addition of sulfuric acid was initiated in addition to the flow of sodium silicate solution and water. For the next 45 minutes of the reaction, the pH of the solution mixture was maintained at 8.0 ± 0.1, by controlling the flow of sulfuric acid. Post 45 minutes of reaction, the flow of all the three feed solutions (sodium silicate, sulfuric acid, and water) was stopped. The solution mixture was aged for 5 min with the stirrer on while maintaining the temperature at 85°C.
In the second phase of the reaction, only sulfuric acid was added to the reaction vessel at a flow rate of 50 ml/min. The pH of the reaction slurry was brought down rapidly from 8.0 ± 0.1 to 3.5-4.0. After pH adjustment, the addition of sulfuric acid was stopped. The reaction mixture was allowed to age for another 10 minutes at 85°C while stirring. The precipitated slurry was then collected from the reactor vessel.
The precipitate was either centrifuged at 4000 rpm for 5 minutes or passed through a filter press followed by washing of soluble salt like sodium sulfate. The washing was continued until the wash water TDS value reached 1000 ppm or less. The solid content of the washed wet cake thus obtained was 20-30% by weight. For flash drying, the washed silica cake was introduced to a flash dryer without any further changes. For spray drying, the silica cake was homogenized in the form of slurry with a total silica content of 8 to 20%. The pH of the slurry was maintained at 5 to 6 by either addition of sulfuric acid or ammonia. The resultant slurry was then spray dried to get fine dried silica powder.
Table 1: Reaction Parameters and Properties of the Synthesized Abrasive Grade Silica
Properties Low abrasive Medium abrasive High abrasive Comments
Reaction Parameters of synthesis of abrasive precipitated silica
Temperature (°C) 85 85 85 The temperature of all the reactions was kept constant.
pH during reaction 7.9-8.1
7.9-8.1
7.9-8.1
For all the reactions, pH was maintained at 7.9-8.1
Final pH of the reaction slurry 3.5-4.0 3.5-4.0 3.5-4.0 For all the reactions, final pH was maintained at 3.5-4.0
Initial Na+ ion concentration 0.30N 0.35N 0.42N Na+ ion concentration gradually increased from low to high, which in turn also gradually increased abrasiveness.
Na+ ion concentration after complete reaction 0.83N 0.83N 0.83N After complete reaction, Na+ ion concentration of all the reactions was kept at 0.83N.
Physical properties of abrasive silica
pH of 5% aqueous silica dispersion 6.56 6.65 6.49 pH of 5% water dispersion of all samples was 6.3-6.6
BET surface area by titration method (m2/g) 60-120 40-60 25-40 BET surface area decreased with increase in Na+ concentration. (FIG. 1)
Bulk density (g/l) 250-300 280-320 300-450 Bulk density increased with decrease in surface area
Moisture (%) 4.5 4.8 4.6 Moisture of all the samples was in the range of 4.3-4.6
DOA (ml/100g) 110-120 80-100 70-100 DOA of all the samples was in the range of 70-120 ml/100g
Sears Number (ml/5g) 20-22 17.6-20.5 17.1-17.6 With the increase in the surface area, Sears value increased because of more surface silanol groups.

Sears value decreased with increase in Na+ concentration. (FIG. 2)
CDBP coefficient/value 0.26-0.29 0.24-0.27 0.21-0.23 CDBP value indicates the low structure of silica. CDBP value was in range 0.21-0.29
Particle size distributions (d50) (µm) 06-15 06-15 06-15 Particle size distribution of all grades was in the range 06-15 µm
Linseed oil absorption (ml/100g) 100-130 90-120 60-90 Linseed oil absorption indicates the low structure of silica. As surface area increased, Linseed oil absorption value increased.
Water absorption (ml/100g) 100-130 90-120 70-90 Water absorption indicates the low structure of silica. As surface area increased, water absorption value increased.
Viscosity (cPs) 200-250 160-210 100-160 This is an important test of silica for toothpaste applications. With increase in surface area, viscosity increased.
Key performance test
Einlehner abrasion value (mg loss per 100000 revolutions) 1.5-2.5 2.5-3.0 3.3-11 Einlehner abrasion value indicates the abrasive properties of silica. With increase in surface area, Einlehner abrasion value increased.
Fluoride compatibility value (%) = 90 = 90 = 90 This is important test of silica for toothpaste applications.

INDUSTRIAL APPLICABILITY

The disclosed process allows preparation of high-quality abrasive grade silica with a wide range of surface area by varying the concentration of sodium ion in the presence of sodium sulfate during the precipitation of silica while maintaining the other reaction parameters such as reaction temperature, solution pH, reaction time, total solid concentration.