Field of Invention A process for preparing silica from rice husk ash is disclosed. Background

Precipitated 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 elastomerfs), 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 a silicate, in particular an alkali metal silicate, 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 si lica, silicate is obtained from rice husk ash by fusing rice husk ash with sodium hydroxide. However, sodium hydroxide being expensive, increases the overall cost of the production of silica.

There is therefore a need to devise an improved process which could reduce the overall cost of production of silica. It is also required that the such a process is able to produce silica having desired characteristics. Summary

A process for preparing silica from rice husk ash is disclosed. Said process comprises fusing the rice husk ash with an alkali metal sulphate at an elevated temperature ranging between 1200 to 1500 °C for a time period ranging between 2-4 hours to obtain an alkali metal silicate; dissolving the obtained alkali metal silicate in an aqueous medium to obtain an alkali metal silicate solution and a first by-product comprising carbon dioxide and sulphur dioxide; acidifying the alkali metal silicate solution by addition of the first byproduct, to cause precipitation of silica and release of a second by-product comprising said alkali metal sulphate.

Brief Description of Drawings

Figure 1 illustrates the process for preparing silica in accordance with an embodiment of the present disclosure;

Figure 2 shows the photograph of the silica obtained using the process of the present disclosure; Figure 3 shows the X-ray Diffraction measurements of silica extracted from rice husk ash using the process of the present disclosure;

Figures 4 shows the Transmission Electron Microscopy (TEM) analysis of silica extracted from rice husk ash using the process of the present disclosure;

Figure 5 shows Field emission scanning electron microscopy analysis of silica extracted from rice husk ash using the process of the present disclosure.

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 preparing precipitated silica from rice husk ash. In particular, the present disclosure relates to a process for preparing silica by fusing rice husk ash with an alkali metal sulphate at an elevated temperature ranging between 1200 to 1500 °C for a time period ranging between 2-4 hours to obtain an alkali metal silicate; dissolving the obtained alkali metal silicate in an aqueous medium to obtain an alkali metal silicate solution and a first by-product comprising carbon dioxide and sulphur dioxide; acidifying the alkali metal silicate solution by addition of the first by-product, to cause precipitation of silica and release of a second by-product comprising said alkali metal sulphate.

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 80-300 m ² /g;

a BET surface area in a range of 90-350 m ² /g m ² g;

a DBP oil absorption in a range of 70-350 ml/lOOg;

a CDBP coefficient (DA) in range of 0.4- 0.9; and a sears number (V ₂ ) 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 m ² /g;

- tapped density in a range of 0.08-0.5 g/cc;

- bulk density in a range of 100-300 g/1;

- a micro pore volume ranging from 0.005-0.025 cm ³ /g;

- a pore diameter ranging from 100-350 A;

- a moisture loss of 3-7% by weight, on drying for two hours at 105°C;

- a pH value of 5.5-7.5 (5 % in water);

- a Wk coefficient number less than 3.4;

- a S1O2 content of greater than 97 %; and

- soluble salts content of less than 0.5%. In accordance with an embodiment, the rice husk ash and the alkali metal sulphate are fused in w/w ratio in a range of 1 :0.7 to 1 :2.4. Preferably, the rice husk ash and the alkali metal sulphate are fused in w/w ratio of 1 : 1. In accordance with a related embodiment, the rice husk ash and the alkali metal sulphate are fused under a pressure in a range of 1 atm. In accordance with an embodiment, the alkali metal silicate and the aqueous medium are mixed in w/w ratio in a range of 1 : 10 to 1 :20. Preferably, the alkali metal silicate and the aqueous medium are mixed in w/w ratio in a range of 1 : 10.

In accordance with an embodiment, the aqueous medium is maintained at a temperature in a range of 100-160 °C. In accordance with an embodiment, the aqueous medium is water. In accordance with an embodiment, the alkali metal sulphate is selected from a group consisting of sodium sulphate, potassium sulphate and magnesium sulphate,, and preferably the alkali metal sulphate is sodium sulphate. In accordance with an embodiment, the acidification of the alkali metal silicate solution is carried out in a batch manner such that the ratio of the alkali metal silicate solution to the first by-product is in a range of 10: 1— 20: 1 and is preferably 11 : 1. In accordance with an embodiment, such acidification is caused using carbon dioxide or sulphur dioxide in an amount ranging between 3.5-5 gm of C0 ₂ per 100 gm of sodium silicate having 5-7% Na ₂ 0 and 5-8 gm of S0 ₂ per 100 gm of sodium silicate having 5-7% Na ₂ 0. Preferably, 4.96 gm C0 ₂ and 7.5 gm S0 ₂ per lOOgm of sodium silicate having 5-7% Na ₂ 0 is used. In accordance with a related embodiment, 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 and unsaturated fatty esters with long hydrocarbon chain/ fatty acids in an appropriate solvent selected from a group consisting of butanol, butanone, toluene and acetone.

In accordance with an embodiment, the process results in obtaining a yield of 25-50% w.r.t. weight of rice husk ash and preferably 30 % w.r.t. weight of rice husk ash. In accordance with an embodiment, after precipitation of silica, alkali metal sulphate is obtained as a by-product which is then regenerated and recycled for fusion with fresh rice husk ash. Thus, negligible/ no alkali metal sulphate is lost during the process. The silica according to the present disclosure can be used in tyre rubber, rice roller rubber, shoe sole rubber or any other elastomers, cosmetic especially dental application, environmental remediation and paints. 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 silica from rice husk ash, the process comprising fusing the rice husk ash with an alkali metal sulphate at an elevated temperature rangingbetween 1200 to 1500 °C for a time period ranging between 2-4hours to obtain an alkali metal silicate; dissolving the obtained alkali metal silicate in an aqueous medium to obtain an alkali metal silicate solution and a first by-product comprising carbon dioxide and sulphur dioxide; acidifying the alkali metal silicate solution by addition of the first by-product, to cause precipitation of silica and release of a second by-product comprising said alkali metal sulphate.

Such a process wherein the rice husk ash and the alkali metal sulphate are fused in w/w ratio in a range of 1 :0.8 to 1 :2.4. Such a process wherein the alkali metal silicate and the aqueous medium are mixed in w/w ratio in a range of 1 : 10 to 1 :20.

Such a process wherein the aqueous medium is maintained at a temperature in a range of 100 - 160°C.

Such a process wherein the acidification of the alkali metal silicate solution is carried out in batch manner such that the ratio of the alkali metal silicate solution to the first byproduct is in a range of 10: 1 - 20: 1. Such a process wherein the acidification is carried out until silica is precipitated completely.

Such a process wherein the alkali metal sulphate is selected from a group consisting of sodium sulphate, potassium sulphate and magnesium sulphate.

Such a process wherein the precipitated silica has: a CTAB surface area in a range of 80-350 m ² /g;

- a BET surface area in a range of 90-400 m ² /g m ² g

a DBP oil absorption in a range of 70-350 ml/100 g;

a CDBP coefficient (DA) in range of 0.4-0.7; and

a sears number (V ₂ ) in a range of 12-30 ml/5 g. Examples

Example 1: Preparation of Silica using Rice Husk Ash and Sodium sulphate

12.5 grams of powdered RHA and 10.0 gm of powder sodium sulphate were mixed properly. The mixture was heated at 1400°C for two hours in a muffle furnace. The solid sodium silicate was taken out and dissolved in hot water after grinding. The sodium silicate solution was centrifuged to remove the impurities. Precipitated silica was obtained by acidifying the above sodium silicate solution by adding 4.96 gm C0 ₂ and 7.5 gm SO2 per lOOgm of sodium silicate (5-7% Na20). The precipitate was washed twice with distilled water followed by drying. The analysis of synthesized precipitated silica was done and its properties were tabulated in the below table, table 1.

Table 1 : Properties of precipitated silica

Industrial Applicability

The process disclosed here in an inexpensive and easy process of preparing silica. The process enables using rice husk ash for production of commercially viable silica, which has otherwise not been obtained in any prior known processes. The silica according to the present disclosure can be used in tyre rubber, rice roller rubber, shoe sole rubber or any other elastomers, cosmetic especially dental application, environmental remediation and paints.

The process utilizes the by-products viz. carbon dioxide and sulphur dioxide by re-cycling the by-products for acidification of metal silicate. Hence, the process reduces waste. This further reduces the costs towards waste disposal. Using the by-products for acidification also eliminates the requirement of mineral acids for acidification of metal silicate. This further eliminates the necessity of down steam processing, required in prior known processes. Additionally, as after precipitation of silica, alkali metal sulphate is obtained as a byproduct, negligible/ no alkali metal sulphate is lost during the process.