Claims:Field of Invention

A process for preparing sodium silicate from rice husk ash is disclosed. Particularly, 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, 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 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 silica, 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 sodium silicate from rice husk ash is disclosed. Said process comprises fusing the rice husk ash with trona at an elevated temperature ranging between 1200-1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate.

A process for preparing silica from rice husk ash is also disclosed. Said process comprises fusing the rice husk ash with trona at an elevated temperature ranging between 1200- 1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate; dissolving the obtained sodium silicate in an aqueous medium to obtain sodium silicate solution at a temperature of 100-160°C; acidifying said sodium silicate solution to cause precipitation of silica.

Brief Description of Drawings

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

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

Figures 3 shows the Transmission Electron Microscopy (TEM) analysis of silica extracted from rice husk ash using the process of the present disclosure;

Figure 4 shows Field emission scanning electron microscopy analysis of silica extracted from rice husk ash using the process of the present disclosure.

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, the present disclosure relates to a process for preparing silicate from rice husk ash. In particular, said process comprises fusing the rice husk ash with trona at an elevated temperature ranging between 1200-1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate.

The present disclosure also relates to a process for preparing silica from rice husk ash. Said process comprises fusing the rice husk ash with trona at an elevated temperature ranging between 1200- 1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate; dissolving the obtained sodium silicate in an aqueous medium to obtain sodium silicate solution at a temperature of 100-160°C; acidifying said sodium silicate solution to cause precipitation of silica.

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 80-350 m2/g;
- a BET surface area in a range of 90-400 m2/g m2g;
- a DBP oil absorption in a range of 70-350 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 0.08-0.5 g/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 (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, trona comprises sodium carbonate and sodium bicarbonate. In accordance with an embodiment, trona comprises sodium carbonate in an amount ranging between 40-50% and preferably 44%. In accordance with an embodiment, trona comprises sodium bicarbonate in an amount ranging between 25-35%. Trona may also comprise 2-3 % Sodium fluoride, about 2% Sodium chloride and about 6%, insoluble impurities. The composition of trona is illustrated in table 1, below. In accordance with an embodiment, trona is processed to reduce the impurities. Any known process may be used to process trona so as to reduce the amount of impurities. The composition of said processed trona is illustrated below, in table 2.

Table 1: Composition of Trona
Trona Composition
Sodium carbonate 40-47%
Sodium bicarbonate 30-37%
Sodium Fluoride 2-3%
Sodium Chloride 1-2%
Insoluble Impurities 5-6%

Table 2: Composition of Processed Trona
Processed Trona Composition
Sodium carbonate 94-96%
Sodium Fluoride 3-4%
Sodium Chloride 0.4-0.8%
Insoluble Impurities 0.5-1%
Organic material 0.2-0.3%

In accordance with an embodiment, the rice husk ash and trona are fused in w/w ratio ranging between 1:0.8 to 1:1.5. Preferably, the rice husk ash and trona are fused in w/w ratio in a range of 1: 1. In accordance with a related embodiment, the rice husk ash and trona are fused under a pressure in a range of 1 to 6 atm. Preferably, the rice husk ash and trona are fused under a pressure in a range of 1 atm.

In accordance with an embodiment, the sodium silicate and the aqueous medium are mixed in w/w ratio in a range of 1:10- 1:20. Preferably, sodium silicate and the aqueous medium are mixed in w/w ratio in a range of 1:10. In accordance with a related embodiment, the aqueous medium is maintained at a temperature in a range of 90-150°C. In accordance with an embodiment, the aqueous medium is water.

In accordance with an embodiment, the acidification is carried out until silica is precipitated completely. 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. 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 0.625 M. In accordance with an alternate embodiment, the acidification is caused by passing carbon dioxide or sulphur dioxide. Such acidification is caused using carbon dioxide and/or sulphur dioxide in an amount ranging between 3.5-5 gm of CO2 per 100 gm of sodium silicate having 5-7% Na2O and 5-8 gm of SO2 per 100 gm of sodium silicate having 5-7% Na2O. Preferably, 4.96 gm CO2 and 7.5 gm SO2 per 100gm of sodium silicate having 5-7% Na2O is used.

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.

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, in environmental remediation, and in 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.

Examples:

Example 1: Precipitation of silica by preparing sodium silicate from RHA using sodium carbonate, Trona and Processed Trona

Three separate processes were carried out by mixing 12.5 grams of powdered RHA and 18.75 gm of powder trona, 12.5 grams of sodium carbonate and 15 grams processed trona in three beakers. The mixture was heated at 1200°C for two hours in a muffle furnace. The glassy solid sodium silicate was taken out and dissolved in hot water after grinding. The solution of sodium silicate was centrifuged to remove the impurities. Thereafter, precipitated silica was obtained by acidifying the above sodium silicate solution by adding 1.25M sulphuric acid. The precipitate was washed twice with distilled water followed by drying.

The silica obtained using the three processes was compared and the results have been tabulated in Table 3, below.
Table 3: Results

Sr. no. Composition details % of silica w.r.t RHA
1 RHA : Trona = 1 :1.5 (by weight)
71%
2 RHA : Processed Trona = 1 :1.2 (by weight) 76%
3 RHA : Sodium carbonate = 1 :1 (by weight) 75%

Further, silica extracted after calcination at different temperatures, was observed and tabulated in Table 4, below.

Table 4: Results after calcination

Sample Name % Silica w.r.t. RHA (Heated @ 800°C) % Silicaw.r.t. RHA (Heated @ 900°C)
% Silica w.r.t. RHA (Heated @ 1200°C
RHA: Trona 32 52.7 71
RHA: Processed Trona 39 66.25 76

The properties of the precipitated silica obtained using the processed trona in accordance with the process described above have been tabulated in Table 5, below.
Table 5: Properties of precipitated silica

Sr. No. Properties Method Units Result
1 BET Surface area (N2) ASTM D5604 m2/g 163
2 t-Plot External surface area /CTAB ASTM D5604 m2/g 148
3 t-Plot Micro pore area ASTM D5604 m2/g 15
4 Average primary particle size
(0.1% liquid dispersion) TEM nm 10–30
5 Average aggregate size
(Powder sample) %V Mastersizer 3000 series µm D50 = 11.5
6 DBP absorption value ASTM D6854 ml/100 g SiO2 250
7 Tap density ASTM B527 g/l 290
8 Heat Loss (2 hrs at 105 ?C) ASTM D6738 % 4.8
9 SiO2 content ASTM D297 % > 97
10 pH value (5 % SiO2 in distilled water) ASTM D6739 6
11 Soluble Salts ICP MS % <0.5

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.

Trona is a cheaper source of sodium carbonate and hence reduces the cost of production of silica, as compared to prior known processes. , Description:We Claim:

1. A process for preparing sodium silicate from rice husk ash, the process comprising:
fusing the rice husk ash with trona at an elevated temperature ranging between 1200-1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate.

2. A process as claimed claim 1, wherein the rice husk ash and trona are fused in w/w ratio ranging between 1:1 to 1:1.5.

3. A process as claimed claim 1, wherein trona comprises 40-50 % sodium carbonate, 25-30% sodium bicarbonate, 2-3% sodium fluoride, 2% sodium chloride and 6% insoluble impurities.

4. A process as claimed claim 1, wherein trona is processed prior to fusion with the rice husk ash, to reduce the amount of impurities to about 0.5-1%.

5. A process for preparing silica from rice husk ash, the process comprising:
fusing the rice husk ash with trona at an elevated temperature ranging between 1200- 1500 °C for a time period ranging between 2-4 hours to obtain sodium silicate;
dissolving the obtained sodium silicate in an aqueous medium to obtain sodium silicate solution at a temperature of 100-160°C;
acidifying said sodium silicate solution to cause precipitation of silica.

6. A process as claimed claim 5, wherein the rice husk ash and trona are fused in w/w ratio ranging between 1:1 to 1:1.5.

7. A process as claimed claim 5, wherein trona comprises 40-50 % sodium carbonate, 25-30% sodium bicarbonate, 2-3% sodium fluoride, 2% sodium chloride and 6% insoluble impurities.

8. A process as claimed claim 5, wherein trona is processed prior to fusion with the rice husk ash, to reduce the amount of impurities to about 0.5-1%.

9. A process as claimed in claim 5, wherein sodium silicate and the aqueous medium are mixed in w/w ratio in a range of 1:10 to 1:20.

10. A process as claimed in claim 5, wherein 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 or by passing carbon dioxide or sulphur dioxide.

11. A process as claimed in claim 5, wherein the precipitated silica has:

- a CTAB surface area in a range of 80-350 m2/g;
- a BET surface area in a range of 90-400 m2/g m2g
- a DBP oil absorption in a range of 70-350 ml/100g;
- a CDBP coefficient (DA) in range of 0.4-0.9; and
- a sears number (V2) in a range of 12-30.

Dated this 14th day of March, 2017

Sneha Agarwal
Of Obhan & Associates
Agent for the Applicant
Patent Agent No. 1969