DESC:Field of Invention

A process for preparing water glass from rice husk ash is disclosed. Particularly, a process for preparing water glass suitable for production of highly dispersible silica is disclosed.

Background

Silica finds application as a catalyst carrier, absorbent for an active material, absorbent, viscosity, texturizing or anti-caking agent, element for battery separators, toothpaste or paper additive, reinforcing filler in silicone matrices or in compositions based on natural or synthetic polymer(s), in particular on elastomer(s), particularly diene elastomers.

Various methods are known for preparing silica. It is further known that the method used for preparing silica also influences the physical and chemical properties thereof. Typically, silica is prepared by a precipitation reaction between sodium silicate (also known as water glass) and an acidifying agent, followed by a filtration and a washing step and, then optionally a step for disintegrating the obtained filter cake.

Traditionally, silicate required for production of silica was obtained by fusing silica sand with sodium carbonate. Presently, a major quantity of silicate is obtained from rice husk ash. Rice husk is an agricultural residue, available abundantly in rice producing countries. India alone produces approximately 12 million tons of rice husk annually. Silica is the major constituent of rice husk ash, making it economically feasible to extract silica, which has wide market. Using rice husk ash for production of silica, also addresses the issue of appropriate disposal of rice husk ash. To prepare silica, silicate is obtained from rice husk ash by reacting with any alkali preferably sodium hydroxide and sodium carbonate to produce sodium silicates (Na2SiO3). Sodium silicate is then reacted with sulfuric acid to generate silica.

However, the known processes have failed to provide a cost effective and commercially viable silica from rice husk ash. There is therefore a need to devise an improved process for production of water glass from rice husk ash. It is also required that such a process is able to produce water glass suitable for producing silica having desired characteristics.

Summary

A process for preparing water glass having Na2O: SiO2 ranging between 1:2.5- 1:3.7 is disclosed. Said process comprises treating rice husk ash with a caustic soda solution and water at a temperature ranging between 90 to 160°C under 2-3.5 bar pressure with stirring at 600-700 rpm for 1-3 hours, followed by separation of water glass from resultant reaction mixture.

A process for preparing highly dispersible silica from rice husk ash is also disclosed. Said process comprises acidifying water glass obtained by a process as claimed in claim 1, with a mineral acid having a concentration of about 0.1 M to 2 M to cause precipitation of highly dispersible silica; and separating the highly dispersible silica.

Detailed Description

For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the disclosed composition and method, and such further applications of the principles of the disclosure therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof.

Reference throughout this specification to “one embodiment” “an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

In the broadest scope, present disclosure relates to a process for producing water glass from rice husk ash. In particular, the present disclosure relates to the preparation of water glass which is suitable for production of highly dispersible silica. Said process comprises of treating rice husk ash with caustic soda solution and water at a temperature ranging between 90 to 160°C under 3-3.5 bar pressure with stirring at 600-700 rpm for 1-3 hours, followed by separation of water glass from resultant reaction mixture.

The present inventors have found that obtaining water glass having Na2O: SiO2 ranging between 1: 2.5- 1: 3.7 is key to production of highly dispersible silica from rice husk ash. Said ratio of Na2O to SiO2 has been achieved by the present inventors by subjecting rice husk ash to treatment as disclosed above.

In accordance with an aspect, carbon is obtained as a by-product in this process.

In accordance with an aspect, water glass prepared using the aforesaid process has Na2O: SiO2 ratio in a range of 1:2.5 to 1:3.7, and preferably 1:3 to 1:3.7 with Na2O content ranging between 6-7% and SiO2 content ranging between 22-24%.

In accordance with an embodiment, rice husk ash is selected such that it comprises of 50-95 % SiO2. Preferably, rice husk ash comprises of 90% SiO2. The SiO2 content of the rice husk ash may be analyzed using any known technique and preferably using a combination of chemical analysis, gravimetric analysis, ICP, AAS and XRF analysis. In accordance with a related embodiment, rice husk ash is thoroughly washed with hot water or an acid before undergoing aforesaid process. Said acid may be any acid including but not limited to HCl, H2SO4.

In accordance with an embodiment, caustic soda solution comprises of 50% sodium hydroxide (NaOH), and preferably 25% NaOH.

In accordance with an embodiment, rice husk ash, caustic soda solution and water are mixed in a ratio ranging between 1: 0.5: 1 to 1: 5: 5 and preferably 1: 0.91: 1.7.

In accordance with an embodiment, rice husk ash is treated with caustic soda solution and water at a temperature ranging between 90 to 160°C, and preferably at around 130 to 250°C. In accordance with an embodiment, said treatment is carried out for 1 to 3 hours, and preferably for 1 to 2 hours.

In accordance with an embodiment, water glass is separated from the resultant mixture by using any known technique and preferably by centrifugation.

In accordance with an embodiment, said process is carried out in a high pressure reactor.

A process of preparing silica is also disclosed. Said process comprises of acidifying water glass obtained using aforesaid process using a mineral acid to obtain a precipitate. The precipitate was washed twice with ample distilled water followed by drying to obtain silica powder.

In accordance with an aspect, the above disclosed process results in obtaining precipitated silica having the following physico-chemical characteristic data:
- a CTAB surface area in a range of 50-350m2/g;
- a BET surface area in a range of 60-400 m2/g m2g;
- a DBP/DOA oil absorption in a range of 60-350 ml/100g;
- a CDBP coefficient (DA) in range of 0.4-0.9; and
- a sears number (V2) in a range of 12-30 ml/ (5g).

In accordance with an embodiment, said precipitated silica alongside the parameters mentioned above has one or more of the following physico-chemical parameters, independently of one another:
- average primary particle size in a range of 10-100 nm;
- average particulate aggregate size (% of particles) in a range of 200-2000 nm;
- a micro-pore area in a range of 9-75 m2/g;
- tapped density in a range of 0.08-0.5g/cc;
- bulk density in a range of 100-300 g/l;
- a micro pore volume ranging from 0.005-0.025 cm³/g;
- a pore diameter ranging from 100-350 Å;
- a moisture loss of 3-7% by weight, on drying for two hours at 105°C;
- a pH value of 5.5-7(5 % in water);
- a Wk coefficient number less than 3.4;
- a SiO2 content of greater than 97 %; and
- soluble salts content of less than 0.5%.

In accordance with an embodiment, acidification is caused using a mineral acid selected from a group consisting of sulphuric acid, hydrochloric acid, nitric acid, organic acids like citric acid, acetic acid, oxalic acid and preferably sulphuric acid. In accordance with a related embodiment, the mineral acid has a molarity in a range of 0.1 M to 2 M, and preferably around 1.25 M. In accordance with yet another embodiment, the water glass and the mineral acid are added in a ratio ranging between 0.5: 1 to 1: 3, and preferably 1: 1.21. The acidification is carried out until silica is precipitated completely.

In accordance with an embodiment, the precipitated silica obtained upon completion of reaction is filtered followed by washing. Washing is done to eliminate the by-products, obtained as a result of reaction. Thus obtained precipitated silica is then subjected to a drying step. The drying step may be carried out by spray drying, spin flash drying, or vacuum tray drying. Alternatively, the wet cake is subjected to short-term drying, followed by addition of a dispersing agent in a suitable solvent. The dispersion may then be dried to obtain precipitated silica. In accordance with an embodiment, the dispersion of silica is prepared using a dispersing agent selected from a group consisting of metal salt of saturated & unsaturated fatty esters with long hydrocarbon chain/ fatty acids in an appropriate solvent selected from a group consisting of butanol, butanone, toluene and acetone.

The silica according to the present disclosure can be used in tyre rubber, rice roller rubber, shoe sole rubber or any other elastomers. Specifically, the silica disclosed herein is suitable for use as filler in vulcanizable or vulcanized elastomer compositions. The vulcanized elastomer composition can be used for the manufacture of tyre and other rubber products. In accordance with an embodiment, said silica may be used as a reinforcing filler in a quantity in a range of 7 to 90 phr. Any conventional process may be used to form vulcanizable or vulcanized elastomer compositions using the above disclosed silica as reinforcing filler.

Specific Embodiments are Described Below

A process for preparing water glass having Na2O: SiO2 ranging between 1:2.5- 1:3.7 the process comprising treating rice husk ash with a caustic soda solution and water at a temperature ranging between 90 to 160°C under 2-3.5 bar pressure with stirring at 600-700 rpm for 1-3 hours, followed by separation of water glass from resultant reaction mixture.

Said process wherein the rice husk ash, caustic soda solution and water are mixed in a ratio ranging between 1: 0.5: 1 to 1: 5: 5.

Said process wherein the rice husk ash, caustic soda solution and water are mixed in a ratio ranging between 1: 0.91: 1.7.

Said process wherein the caustic soda solution comprises of up to 50 % sodium hydroxide (NaOH).

Said process wherein the caustic soda solution comprises of 25 % sodium hydroxide (NaOH).

Said process wherein the obtained water glass has a Na2O content ranging between 6-7% and a SiO2 content ranging between 22-24%.

Said process wherein the rice husk ash is subjected to an washing step to remove undesired metal and metal oxides, before treatment with caustic soda and water.

A process for preparing highly dispersible silica from rice husk ash, the process comprising:
- acidifying water glass obtained by a process as claimed in claim 1, with a mineral acid having a concentration of about 0.1 M to 2 M to cause precipitation of highly dispersible silica; and
- separating the highly dispersible silica.

Said process wherein the water glass and the mineral acid are added in a ratio ranging between 0.5: 1 to 1: 3.

Said process wherein the acidification is caused by a mineral acid selected from a group consisting of sulphuric acid, hydrochloric acid, nitric acid, organic acids like citric acid, acetic acid, and oxalic acid, or by passing carbon dioxide or sulphur dioxide.

Said process wherein the precipitated silica has:
- a CTAB surface area in a range of 50-350 m2/g;
- a BET surface area in a range of 60-400 m2/g m2g;
- a DBP/DOA oil absorption in a range of 60-350 ml/100g;
- a CDBP coefficient (DA) in range of 0.4-0.9; and
- a sears number (V2) in a range of 12-30 ml/ (5g).

Examples

Example 1: Preparation of sodium silicate (water glass)

Composition of rice husk ash:
Composition of rice husk ash was analyzed by employing inductively coupled plasma atomic emission spectroscopy (ICP-AES) and titration methods for ash content estimations. The composition of rice husk ash is illustrated in Table 1, below.

Table 1: Composition of Rice Husk Ash
Component Amount (in %)
SiO2 88 – 92
Carbon (Ash) 4 – 6
K2O, Na2O, CaO, MgO, MnO, Fe2O3
and other impurities 4 – 6

Composition of caustic soda solution
Composition of caustic soda solution was analyzed by acid-base titration method where the percentage of NaOH in caustic soda solution was determined as 25% by weight.

Step 1: Pretreatment of rice husk ash
Rice husk ash was soaked in water and boiled with HCl (6N) at 90°C for 1 hour to remove metallic and metal oxide contaminants. This pre-treated rice husk ash was washed with copious amount of water to adjust pH to 6-7. If needed, NaOH solution can be used to adjust pH. Rice husk ash was then dried at 90°C to obtain a moisture level of 4-5 %.

Step 2: Treatment of rice husk ash
500 grams of milled rice husk ash was mixed in 350ml caustic soda solution (25% NaOH) and 850 mL of distilled water. The resultant mixture was transferred into a high pressure reactor of 2 liter capacity having maximum working pressure 150 kg/m2 and maximum working temperature of 400°C. The reactor was closed properly and the temperature of the reactor was set at 150°C with stirring at 600-700 rpm. The reactor was switched and the temperature was allowed to rise up to 150°C. The reaction was allowed to continue for 3 hours. During reaction, inbuilt pressure reaches 3-3.5 bar. After 1-3 hour of reaction, heating was stopped and the temperature of the reactor was allowed to reach to room temperature.

Step 3: Separation of water glass
The reaction slurry was taken out and centrifuged at 4000 rpm for 20 minutes. The obtained filtrate is slurry of sodium silicate, which was further, processed using a vacuum filtration device to remove any suspended carbon particles.

Observation: The ratio of Na2O and SiO2 of the above obtained water glass was evaluated by titration method and was found to be in the range of 1:3 to 1: 3.3. The properties of the obtained water glass have been listed in table 2, below.

Table 2: Properties of water glass from rice husk ash
Sr. No Characteristics Units Sodium silicate
1 Appearance Viscous liquid
2 Specific Gravity g/cc 1.25
3 SiO2 content % 22.9
4 Na2O content % 6.8
5 Na2O:SiO2 - 1:3.3
6 Total solid content % 29.5
7 Iron ppm 40
8 Phosphorous ppm 262
9 Aluminium ppm 258
10 Manganese ppm 6
11 Magnesium ppm 42
12 Potassium ppm 3006
13 Calcium ppm 364

Example 2: Preparation of silica using water glass obtained in Example 1
Above water glass was used for precipitation of silica, which was obtained by acidifying using 1.25 M sulfuric acid. The precipitate was washed twice with ample distilled water followed by drying to powder. The properties of the obtained water glass have been listed in table 3, below.

Table 3: Properties of Silica
S. No. Properties Silica Method of Analysis
1 Nature of particles Amorphous Powder XRD analysis
2 Nitrogen surface area (m2/g)-BET 221.92 Multipoint-BET
3 External surface area (m2/g)- N2 214.79 Multipoint-BET
4 Pore surface area (m2/g)-N2 7.127 Multipoint-BET
5 t-Plot micro pore volume (cm³/g) 0.001730 Multipoint-BET
6 Total pore volume 1.547372 Multipoint-BET
7 Total Hg intrusion volume(mL/g) Hg Porosimetry
8 Average pore diameter (nm) 28.05 Hg Porosimetry
9 CTAB surface area (Lab method)-m2/g 210 Standard Test
10 BET:CTAB 1.057 Standard Test
11 DOA /DBP oil absorption (ml/100g) 177.9 Brabender oil absorptometer
12 Bulk Density (g/l) 228 Standard Test
13 Surface functionalization Yes
14 Particle size distribution (d10,50,90 in micron) by % Volume D10=2.09
D50=11.4
D90=37.7 Laser diffraction, Aero
15 MEDIAN PARTICLE SIZE D50=11.4 Laser diffraction, Aero
16 pH (5 % in water) 6.03 Standard Test
17 SiO2 content (%)-(2 hrs. @1000 ?C) = 97 Standard Test
18 Heat Loss (2 hrs. @105 ?C) 5.17 Standard Test
19 Electrical conductivity (µs/cm) = 1200 Standard Test
20 Sears number (ml/5g) 25.88 Standard Test
21 CDBP value 0.389 Standard Test
22 Vicinal/Isolated Silanol group density 24.2% Silica Cross Polarized Magic Angle Spinning (CPMAS)
And Silica Magic Angle Apinning (MAS)
23 Siloxane group density 71.08% Si CPMAS and Si MAS
24 Geminal silanol group density 4.74% Si CPMAS and Si MAS ,CLAIMS:1. A process for preparing water glass having Na2O: SiO2 ranging between 1:2.5- 1:3.7 the process comprising treating rice husk ash with a caustic soda solution and water at a temperature ranging between 90 to 160°C under 2-3.5 bar pressure with stirring at 600-700 rpm for 1-3 hours, followed by separation of water glass from resultant reaction mixture.

2. The process as claimed in claim 1, wherein the rice husk ash, caustic soda solution and water are mixed in a ratio ranging between 1: 0.5: 1 to 1: 5: 5.

3. The process as claimed in claim 2, wherein the rice husk ash, caustic soda solution and water are mixed in a ratio ranging between 1: 0.91: 1.7.

4. The process as claimed in any of the preceding claims, wherein the caustic soda solution comprises of up to 50 % sodium hydroxide (NaOH).

5. The process as claimed in claim 4, wherein the caustic soda solution comprises of 25 % sodium hydroxide (NaOH).

6. The process as claimed in claim 1, wherein the obtained water glass has a Na2O content ranging between 6-7% and a SiO2 content ranging between 22-24%.

7. The process as claimed in claim 1, wherein the rice husk ash is subjected to an washing step to remove undesired metal and metal oxides, before treatment with caustic soda and water.

8. A process for preparing highly dispersible silica from rice husk ash, the process comprising:
- acidifying water glass obtained by a process as claimed in claim 1, with a mineral acid having a concentration of about 0.1 M to 2 M to cause precipitation of highly dispersible silica; and
- separating the highly dispersible silica.
9. The process as claimed in claim 8, wherein the water glass and the mineral acid are added in a ratio ranging between 0.5: 1 to 1: 3.

10. The process as claimed in claim 8, wherein the acidification is caused by a mineral acid selected from a group consisting of sulphuric acid, hydrochloric acid, nitric acid, organic acids like citric acid, acetic acid, and oxalic acid, or by passing carbon dioxide or sulphur dioxide.

11. The process as claimed in claim 8, wherein the precipitated silica has:
- a CTAB surface area in a range of 50-350 m2/g;
- a BET surface area in a range of 60-400 m2/g m2g;
- a DBP/DOA oil absorption in a range of 60-350 ml/100g;
- a CDBP coefficient (DA) in range of 0.4-0.9; and
- a sears number (V2) in a range of 12-30 ml/ (5g).