DESC:FIELD OF INVENTION
The present invention relates to preparation of precipitated silica from rice husk ash by precipitation using carbon dioxide followed by recovery of sodium carbonate. The process has three stages i.e. digestion of rice husk ash, precipitation of silica from soluble silicate and recovery of sodium carbonate.

BACKGROUND OF INVENTION AND PRIOR ART
Silica is widely used in various industries and has multiple applications in the field of rubbers and plastics, paints, healthcare, cosmetics, toothpastes, catalyst, coatings, carrier agent, insulation, stabilizing and desiccants. Rice husk ash which is waste product of the rice industry is rich in silica. There have been continuous efforts to dispose off the rice husk ash as also numerous efforts have been made to utilize the high silica content available in the rice husk ash.

IN216477 discloses a process for the preparation of precipitated silica SiO2 from rice husk ash in three stages, digestion of rice husk ash, precipitation of silica from soluble silicate and regeneration of caustic solution. Though IN216477 refers to the regeneration of caustic solution and its reuse in the digestion step, the regeneration steps involves treating the byproduct sodium carbonate with either calcium oxide or calcium hydroxide. Such step involves more equipment and also involves multi -step operations which effectively increases the cost of the process and makes the process described in IN216477 non-economical.

Thus there remains a need for a simple and economical yet efficient process that provides silica of high purity and yield.

OBJECTS OF INVENTION
It is an object of the present invention to provide an efficient and economical process for the preparation of precipitated silica from rice husk ash involving efficient sodium carbonate recovery in high yield.

It is another object of the present application to provide a process which helps in eliminating the expensive effluent treatment plant and also cutting down the plant operation cost.

SUMMARY OF INVENTION
The present application provides a process for the preparation of precipitated silica from rice husk involving recovery of sodium carbonate comprising the steps of:
a. Reacting rice husk ash with sodium hydroxide to obtain sodium silicate solution;
b. Reacting the sodium silicate solution with carbon dioxide to obtain precipitated silica and sodium carbonate solution;
c. Evaporating the sodium carbonate solution to recover sodium carbonate by removing water.

DESCRIPTION OF INVENTION
Rice husk ash available as a waste from farms has a silica content of around 80 - 90%, around 5-8% metallic impurities and 10-20% unburnt carbon. This silica can be extracted economically by the proposed process, which meets the requirements of the various industries.

This process consists of three steps namely;
1. Digestion of rice husk ash using sodium hydroxide to prepare sodium silicate;
2. Precipitation of Silicate with Carbon dioxide;
3. Recovery of by-product sodium carbonate by simple method.

The general knowledge in the art that reuse of catalysts or reagents are preferred in terms of cost effectiveness. However, the present inventors found that recovering the by-product, instead of regenerating to be used in the step of digestion of rice husk, results in making the process simple and economical. Accordingly, the present process also eliminates expensive effluent treatment plant and also cutting down the plant operation cost. Such recovered sodium carbonate has varied industrial uses such as raw materials in filtration industry and detergent manufacturing respectively.

Unlike prior art, the present process does not involve introduction of another raw material like lime to regenerate sodium hydroxide thereby making the process of the present application more economical. It has been observed that upon using regenerated sodium hydroxide, which is of low strength and poor quality requires an additional step of purification prior to being reutilized in the step of digestion. Such additional steps add to the cost of the process thus rendering it less viable for industries, especially small scale industries.

In the present application, sodium carbonate is evaporated. Sodium carbonate is a commercially useful item, which is useful in various industries. Further
Sodium Hydroxide is also easily available commercially and incurring huge costs to regenerate the same is not economically feasible. Further Sodium carbonate generation is 1.3 times of sodium hydroxide used in reaction thereby recovery of maximum raw material cost.

Recovered sodium hydroxide by using lime process will affect quality of the silica in second step since calcium carbonate will be contaminated with Recovered sodium Hydroxide. It has been observed by the inventors of the present application that only 50% sodium Hydroxide is recovered and the same is of low quality. This shows that 100% recovery of sodium Hydroxide is not possible and again we have to add fresh sodium hydroxide to complete the reaction. All the efforts to recover sodium hydroxide is not fulfilled totally.
In the present invention, the preparation of precipitated silica involving sodium carbonate recovery comprises the following steps.

a. Preparation Sodium Silicate
Molar ratio of silica in rice Husk to Sodium Hydroxide is in the range of 1: 0.5 to 1: 0.75. In this step the rice Husk Ash is reacted with sodium Hydroxide water solution to convert the silica content present in the rice husk to obtain sodium silicate solution which is separated by filtration. The unreacted carbon is washed and dried used as activated carbon.

b. Making Precipitated silica
Sodium silicate solution from step 1 is treated with Carbon dioxide gas to get precipitated silica and sodium carbonate. In this step, carbon dioxide is bubbled in to the sodium silicate solution at an atmospheric pressure of about 0.5-1kg/sq cm into the precipitator reactor at a temperature ranging between about 85°C to about 90°C. Sodium silicate solution absorbs the residual carbon dioxide and gets converted to precipitated silica and sodium carbonate.

The water insoluble precipitated silica, is separated by filtration and is washed free from other metallic impurities, dried and used as final product.
Purity of silica is about 99% and above and soluble salt is less than 1%. All other parameters as per IS, particularly IS-12076 -1986.

The drying or water separation from silica is done by centrifuging or press filtering and using convenient drying system (spin flash drying or Spray drying or Rotary drying) resulting in effective drying of the product.

c. Sodium carbonate recovery.
Sodium carbonate solution obtained in step (b) is evaporated to obtain free sodium carbonate. In this step, evaporation is done in multiple effect evaporator by applying thermo compression ejector which consume about 25% steam requirement per kg of water evaporation. Pure water is recycled to process and achieving zero effluent discharges and less water consumption only.

Typically, the temperature of evaporation varies from 60 °C to 100°C and involves four stage multiple evaporation, which is operated under vacuum. It is continuous evaporation and capacity of evaporator is designed based on generation of sodium carbonate solution.

EXAMPLES
Example 1

Good quality Rice Husk Ask, i.e. well burnt (700-750?) boiler outlet RHA is reacted with 1.5N sodium hydroxide solution at temp 90? to a period of 3 hours. After reaction the sodium silicate solution is separated using plate and frame filter press or notch filter or Automated vertical pressure filter.

Making Precipitated silica
Diluted solution i.e.5-6% silica content and 1:3 ratio sodium silicate solution and Carbon dioxide is bubbled in to the sodium silicate solution at an atmospheric pressure of about 0.5-1 kg/sq. cm into the precipitator reactor at a temperature ranging between about 85°C to about 90°C for about an hour to yield precipitated silica and sodium carbonate.

The water insoluble precipitated silica, is separated by filtration and is washed free from other metallic impurities, dried and used as final product. Washed and neutralized silica cake is further dried using spray dryer or spin flash dryer or Rotary vacuum paddle dryer.

Sodium carbonate recovery.
Sodium carbonate solution obtained is evaporated to obtain free sodium carbonate. A multiple evaporator system to evaporate water present in sodium carbonate solution. This system is also called as zero Liquid discharge plant. Multiple evaporator system will concentrate the solution up to 4% to 50% of concentration. Further concentrate using crystalliser to produce sodium carbonate powder.

Pilot plant test Result:
Sl. No Parameters Units Result Testing methods

1

Silica as SiO2 on ignition basis
%
99.21

IS-12076 -1986

2

Loss on Ignition
%
6.06
(Moisture loss of 10.15)
IS-12076 -1986

3

Moisture Content
%
4.09
IS-12076 -1986

4
pH of Aqueous suspension 5%
-
6.89

IS-12076 -1986

5
BET Single point surface area

M2/gm
153.06-405.7
ASTM 5604
6
Soluble salt

%
0.70

FC-126

,CLAIMS:1. A process of preparing precipitated silica from rice husk comprising the steps of:
a. reacting rice husk ash with sodium hydroxide to obtain sodium silicate solution;
b. reacting the sodium silicate solution with carbon dioxide to obtain precipitated silica and sodium carbonate solution; and
c. evaporating the sodium carbonate solution to recover sodium carbonate by removing water.

2. The process as claimed in claim 1, wherein the molar ratio of silica in rice husk to sodium hydroxide is in the range of 1:0.5 to 1: 0.75.

3. The process as claimed in claim 1, wherein carbon dioxide is bubbled into the sodium silicate solution at a pressure of about 0.5- 1 kg/cm2.

4. The process as claimed in claim 3, wherein carbon dioxide is bubbled into the sodium silicate solution at a temperature between about 85°C to about 90°C.

5. The process as claimed in claim 1, wherein the silica obtained in step (b) is subjected to further steps of purification and drying.

6. The process as claimed in claim 1, wherein in step (c) the evapouration involves 4 stages.

7. The process as claimed in claim 6, wherein the temperature varies from 60°C to 100°C.

8. Silica substantially free from any metallic impurities as obtainable by the process of claim 1.