ASH

FIELD OF INVENTION

The; present invention relates- to the field of chemistry and particularly to a method and a system for generation of high perfprmanee precipitated silica from rice husk ash.

BACKGROUND

Precipitated silica is a form of synthetic . amorphous silicon dioxide. Precipitated silica finds application in reinforcement of rubbers and plastics, thickening and thixotropy of coatings and paints, printing inks, plastics and cosmetics, anti- blocking of plastic foils, carrier for pesticides and catalysts. Thus, precipitated silica is an important industrial production target on a commercial scale. There are various methods available in the art for producing precipitated silica. One such method known in the art discloses manufacturing of silica by fusion silica sand and sodium carbonate at high temperatures of around 1400°O ⁱ . The sodium silicate obtained is precipitated using either sulfuric acid or hydrochloric acid. One significant disadvantage of the method is that it is highly energy intensive. The method requires the reactants to be heated to high temperatures of around 1400°C. Another method discloses production of -silica from rice husk ash. A sodium silicate is extracted from rice husk ash. The sodium silicate is precipitated using sulfuric acid to obtain silica. One significant disadvantage of the method is that one of the i byproduct of the reaction, sodium sulfate, being environmentally hazardous chemical needs effective treatment.

The Indian patent No. 216477 teaches a sparger based pre!cipitation method to obtain silica from rice husk ash. A sodium silicate solution is extracted from rice husk- ash. Subsequent to the extraction of the sodium silicate solution, a carbon dioxide gas is injected to the sodium silicate solution to obtain precipitate containing silica. Injection of the carbon dioxide gas is achieved through sparger. Few disadvantages of abovementioned- process are difficulty in insertion of the sparger for each batch of the reaction, chances of leakage of sodium silicate solution during sparger insertion and increasea time taken for the completion of the reaction. Hence the process is cumbersome. Thus, there is a need for obtaining precipitated silica from the rice husk ash which is fast, easy to operate and maintain.

BRIEF DESCRIPTION OF DRAWINGS

So that the manner in which the recited features of the indention earn be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. FIG. 1 is a flow chart of the system for obtaining precipitated silica from rice husk ash, according to an embodiment of the invention.

FIG. 2 is a schematic representation system for obtaining precipitated silica from rice husk ash, according to an embodiment of the invention

FIG. 3 is a schematic representation of an ejector assembly, according to an embodiment of the invention.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method for generation of high performance precipitated silica from rice husk ash, The method includes reacting sodium hydroxide with the rice, husk ash in the ratio of about 1 : 1 weight/weight to about 1 :4 weight /wei'ght to obtain a sodium silicate solution. The sodium silicate solution is co-inject with carbon dioxide gas for obtaining a '.slurry comprising of silica and sodium carbonate solution. The slurry is separated to obtain silica. The sodium hydroxide is regenerated from the sodium carbonate solution.

Another aspect of the invention provides a system for generation of high performance precipitated silica from rice husk ash. The system includes a digestion chamber having a plurality of inlets and one outlet. An ejector is coupled to the outlet of the digestion chamber having at least two inlet ports and an outlet port. A precipitation chamber is connected to the outlet port of the ejector. The precipitation chamber has a first outlet port _' and a second outlet port. A finished product chamber is connected to the second outlet port of the precipitation chamber. A regeneration chamber is connected to the first outlet port of the precipitation chamber. The regeneration chamber is coupled to the reaction chamber.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention provide a method and a system for generation of high performance precipitated silica from rice husk ash. The method includes reacting sodium hydroxide with the rice husk ash in the ratio of about 1 :1 weight /weight to about 1 :4 weight /weight to obtain a sodium silicate solution, co-injecting a carbon dioxide gas with the sodium silicate solution for obtaining a slurry comprising of silica and sodium carbonate solution, separating the slurry to obtain silica and regenerating the sodium hydroxide from the _, sodium carbonate solution.

The method described in brief herein above shall be described in detail with various embodiments and examples. The method for obtaining precipitated silica from' rice husk ash includes reacting sodium hydroxide with rice husk ash. The sodium hydroxide and rice husk ash are reacted in the ratio of about 1 : 1 weight /weight to about 1 :4 weight /weight. In one example of the. invention, the sodium hydroxide and rice husk ash are reacted in the ratio of - 1 :4. Initially, a measured amount of sodium hydroxide is dissolved in the water to obtain a sodium hydroxide solution. The temperature of the sodium hydroxide solution is maintained at 70°C - 95°C. The rice husk ash is then added to the sodium hydroxide solution and allowed to react for a time period of around 1 hour to 3 hours. Upon the completion of the reaction, a slurry containing sodium silicate solution along with the undigested rice husk ash is obtained. The slurry obtained is filtered to obtain sodium silicate solution' The filtration is carried out by any of the commercially available filtration equipments. In one example of the invention, the filtration equipment employed is a filter press. The Sodium silicate solution and the carbon dioxide gas are co-injected!lin the stoichiometric ratio of about 0.25 to about 8. Co-injection is achieved by injecting the sodium silicate solution and the carbon dioxide gas through an ejector at a pressure varying from vacuum to positive pressure. The co-injection enhances the mixing of sodium silicate solution with the carbon dioxide which inturn increases the reaction rate of the precipitation reaction. The precipitation reaction is carried out for pre-determined time duration of about 5 minutes to 120 minutes. Preferably, the precipitation reaction is carried out for the pre-determined time duration of about 20 minutes to 60 minutes. In one example of the invention, the precipitation reaction is carried out for 9 minutes. The temperature for carrying out precipitation reaction is maintained in the range of 50°C to 100°C.

Upon completion of the precipitation reaction, slurry comprising of precipitated silica and the sodium carbonate solution is obtained. The slurry is separated to obtain precipitated silica. The separation comprises of the steps of separating the slurry through a filtration mechanism including but not limited' to a gravity filtration and a suction filtration. The precipitated silica is dried. The drying is carried out by any of the commercially available drying equipments. In one example of the invention, the drying , equipment employed is a spray drier. The temperature for drying the precipitated silica is maintained in the range of 400°C to 700°C. The yield of the silica obtained is; about 95% of the silica present in the rice husk ash. In one example of the invention, the purity of the silica obtained is about 99.5%. In one embodiment of the invention altering the ratio of sodium hydroxide to rice husk ash alters the rheology of the slurry. In another embodiment of the invention altering the stoichiojmetric ratio of carbon dioxide during co-injection alters the rheology of the slurry. Altering the rheology of the slurry results in alteration of the product grade of silica obtained. In one example of the invention, altering the rheology of the slurry alters the product grade to yield powdery silica. In another example of the invention, altering the rheology of the slurry alters the product grade to yield in. micro pearl silica.

The sodium carbonate solution, obtained during the separation of the slurry, is subjected to regeneration, to recover the sodium hydroxide. During regeneration, the sodium carbonate solution is converted to sodium hydroxide by reacting with calcium hydroxide. More than 95% of sodium hydroxide is regenerated. The regenerated sodium hydroxide is re-used during another cycle for the extraction of the silica from the rice husk ash The other byproducts of the reaction include precipitated calcium carbonate. In one example of the invention, the precipitated calcium carbonate is heated to obtain carbon dioxide and calcium oxide. In an alternate example of the invention, the ^' calcium carbonate is used as filler in paper industry.

Various embodiments of the invention also provide a system for generation of high performance precipitated silica from rice husk ash. The system includes a digestion chamber having a plurality of inlet and one outlet. An ejector is coupled to the outlet.of the digestion chamber having at least two inlet ports and an outlet port ^' . A precipitation chamber is connected to the outlet port of the ejector. The precipitation chamber has a first outlet port and a second outlet port. A finished product chamber is connected to the second outlet port of the precipitation chamber. A regeneration chamber is connected to the first outlet port of the precipitation chamber. The regeneration chamber is coupled to the digestion chamber.

FIG. 1 is a flow chart of the system for obtaining precipitated silica from a rice husk ash, according to an embodiment of the invention. The system includes a digestion chamber 1 , a precipitation chamber 3, a finished product chamber 5 and a regeneration chamber 7. The digestion chamber 1 is. connected to the precipitation chamber 3. The precipitation chamber 3 is further connected to the finished product chamber 5 and the regeneration chamber 7. The regeneration chamber 7 is further connected to the digestion chamber 1.

FIG. 2 is a schematic representation of the system for obtaining precipitated silica from a rice husk ash, according to an embodiment of the invention. The digestion chamber 1 is provided with a plurality of interconnected partitions within the digestion chamber 1 The interconnected partitions include an ash digestion area 2, an ash slurry tank 4, an ash filter press 6 and a sodium silicate tank 8. The ash digestion area 2 is provided with a plurality of inlets. The inlets 10, 12 and 14 of the ash digestion area 2 are used for adding a rice husk ash, demineralised water and a sodium hydroxide respectively to ash digestion area 2. Subsequent to the addition of reactants, the sodium hydroxide and the rice husk ash are reacted in the ratio of about 1 : 1 weight/weight to about 1 :4 weight/weight to obtain a slurry containing sodium silicate solution along with the undigested rice husk ash is obtained! The slurry is stored in an ash slurry tank 4. The slurry is filtered ¹ in an ash filter press 6 to obtain sodium silicate solution. The sodium silicate solution obtained is stored in the sodium silicate tank 8. The digestion chamber 1 is provided with an outlet 16. An ejector 18 is coupled to the outlet 16 of the digestion chamber 1. The ejector 18 is selected from a group comprising single stage ejectors, multi-stage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non-condensing stages. The ejector 18 is having a throat to jet ratio of about 2 to about 8. The ejector 18 is having plurality of inlet !ports and an outlet port 24. The inlet ports 20 and 22 o/ the ejector are internally connected to each other. Due to the ejector action, the sodium silicate solution is pumped at the pressure varying from vacuum to positive pressure through inlet port 20 to the ejector 18. Simultaneously the carbon dioxide gas is !also sucked through the internally connected inlet port 22 to the ejector 18. Upon co-injection, partial precipitation reaction between sodium silicate solution and carbon dioxide gas is carried out in the ejector 18.

Further, through the outlet port 24 of the ejector 18, the partially reacted sodium silicate solution and carbon dioxide enter into the precipitation chamber 3.

The precipitation chamber 3 is provided with a plurality of interconnected partitions within the precipitation chamber ¾. The interconnected partitions include a precipitator 26, a silica filtration unit 28 and a sodium carbonate solution tank 30, The precipitation reaction between sodium silicate and carbon dioxide gas is further continued in precipitator 26 to form slurry containing precipitated silica and sodium carbonate solution. The slurry is filtered using the silica filtration unit 28 to .obtain precipitated silica. The precipitated silica is sent to the finished product chamber 5 through an outlet 32 from the silica filtration unit 28 of the precipitation chamber 3. In one example of the invention, the outlet 32 is a conveyor. The sodium carbonate solution obtained from the slurry is stored in the sodium carbonate solution tank 30.

The finished product chamber 5 is provided with a plurality of interconnected partitions within the finished product chamber 5. The interconnected partitions include a spray drier 34 and a packing unit 36. The silica is subjected to drying in the spray drier 34. The silica dried is packed in the packing unit 36. In one example of the invention, the milling unit can also be included. The sodium carbonate solution stored in the sodium carb nate solution tank 30 is sent to the regeneration chamber 7 through the outlet port 38 of the precipitation chamber 3.

The regeneration chamber 7 is provided with a plurality of interconnected partitions within the regeneration chamber 7. The interconnected partitions include a regenerator 40, a calcium carbonate slurry tank 42, a calcium carbonate filter press 44, a calcium carbonate drier 46, a calcium carbonate packing unit 48 and a sodium hydroxide recovery tank 50. The sodium carbonate solution reacts with calcium hydroxide at the temperature of about 70 to 100 °C to regenerate the sotiium hydroxide. The slurry containing sodium hydroxide and calcium carbonate is stored in the calcium carbonate slurry tank 42. The slurry is passed through the calcium carbonate filter press 44. The calcium carbonate thus obtained from the calcium carbonate filter press 44 is further dried in the calcium carbonate drier! 46. The dried calcium carbonate is sent to the calcium carbonate packing unit 48. The sodium hydroxide is transferred from calcium carbonate filter press 44 to sodium hydroxide recovery tank 50. The sodium hydroxide concentration is altered to the required level. The sodium hydroxide is sent to the digestion chamber 1 through the outlet 52 of the regeneration chamber 7 for the re-use of sodium hydroxide in the next cycle for obtaining silica from the rice husk ash.

FIG. 3 is a schematic representation of an ejector assembly, according to an embodiment of the invention. The ejector 118 is having plurality of inlet ports and an outlet port. The ejector.1.8 is

! 0 selected from a group comprising single stage ejectors,: multistage non-conditioning ejectors, multi-stage conditioning ejectors and multi-stage with both condensing and non- condensing stages. The ejector 18 is having a throat to jet ratio of about 2 to about 8. The sodium silicate solution and carbon dioxide gas are passed through the inlet ports 20 and 22 respectively, to the ejector outlet port 24. Further, through the outlet port 24 of the ejector 18, the sodium silicate solution and carbon dioxide are passed to precipitator 3.

Example 1 :

In one example of the invention, co-injection of the carbon dioxide gas and the sodium silicate solution is carried out in the stoichiometric ratio of about 8 on weight/weight. Co-injection is achieved through the ejector 18. The size of the throat and jet of the ejector 18 is 1 : 4. The ejector 18 is having a throat to jet ratio of about 4. Precipitation of about 300 liters of silica containing 5 % sjlicate is completed in about 34 minutes of time. The silica obtained is having a BET surface-area of about 87 m ² /g. The Hg Porosimetry surface area of the silica obtained is 256 m ² /g. The silica obtained has a pore volume of 1.92 cc/g and an average pore diameter of 46 nanometers.

Example 2:

In one example of the invention, co-injection of the carbon dioxide gas and the sodium silicate solution is carried out in the stoichiometric ratio of about 7.5 on weight/weight. Co-injection is achieved through an ejector 18. The size of the throat and jet of the ejector 18 is 1 :4 the ejector 18 is having a throat to jet ratio of about 4. Precipitation of about 900 liters of silica containing 5 % silicate is completed in about 52 minutes of time. The silica obtained is having a BET surface area of about 251 m /g. . The 5 Hg ]Porosimetry surface area of the silica obtained is 185m ² /g, has! a pore volume of 1.83cc/g and an average pore diameter of 62.81 nanometers.

Example 3:

In one example of the invention, co-injection of the carbon

! 0 dioxide, gas and the sodium silicate solution is carried out in the

i

stoichiometric ratio of about 23 on weight/weight. Co-injection is achieved through an ejector 18. The size of the throat and jet of the ^: ejector 18 are increased proportionately by two times compared to size of the throat and jet of the example 2 while

! 5 maintaining the throat to jet ratio of about 4. Due to increase in the size of the throat and jet, precipitation of about 900 liters of silic!a containing 5 % silicate is completed in about 9 minutes of time. The silica obtained is having a BET surface area of„about ■ 403m ² /g. The Hg Porosimetry surface area of the silica obtained

20 is 268m ² /g. Further, the silica obtained has a pore volume of

1.31 cc/g and average pore diameter of 36.81 nanometers.

Example 4:

In one example of the invention, co-injection of the carbon dioxide gas and the sodium silicate solution is carried out in the 25 stoichiometric ratio of about 1 : 10 on weight/weight. Co-injection is achieved through an ejector 18. The size of the throat and jet of the ejector 18 is 1 :4. The ejector 18 is having a throat to jet ratio of about 4. Precipitation of about 900 liters of silica

I 2 containing 5 % silicate is completed in about 33 minutes of time. The silica obtained is having a BET surface area of about 239.m ² /g. The Hg Porosimetry surface area of the silica obtained is 205m ² /g. The silica obtained has a pore volume of 2.49cc/g and has an average pore diameter of 105 nanometers.

Example 5:

In one example of the invention, co-injection of the carbon dioxide gas and the sodium silicate solution is carried out in the stoichiometric ratio of about 5 on wt/wt. The co-injeeti'on is achieved through an ejector 18. The size of the throat and jet of the .ejector 18 is 1 :4. The ejector 18 is having a throat to jet ratio of about 4. Precipitation of about 900 liters of silica containing 5 % silicate is completed in about 53 minutes of time. The silica obtained is having a BET surface area of about 226m ² /g-. The Hg Porosimetry surface area of the silica obtained is 174'm ² /g. The^ silica obtained has a ^' pore volume of 3.45cc/g and has an average pore diameter of 304 nanometers.

Example 6:

In one example of the invention, co-injection of the carbon dioxide gas and the sodium silicate solution is carried out in the stoichiometric ratio of about 2.5 on wt/wt. . Co-injection is achieved through an ejector 18. The size of the throat and jet of the ejector 18 is 1 :4. The ejector 18 is having a throat to jet ratio of about 4. Precipitation of about 900 liters of silica containing 5 % silicate is completed in about 53 minutes of time. The silica obtained is having a BET surface area of about 206m /g The Hg Porosimetry surface area of the silica obtained is 137m ² /g.

I 3 Further, the pore volume of the silica is 3.94cc/g and has an average pore diameter of 606 nanometers.

The present invention provides a faster method of obtaining silica from rice husk ash. One of the advantages of the method is that the entire reaction is completed within time duration of two hours. The silica obtained from the method has a yield of about 75%. Further, the method enables regeneration of sodium hydroxide.

Another advantage of the method is that the desired specification of the silica pore range including but not limited to meso pore range, micro pore range, and macro pore range can be obtained by altering the sodium silica solution to carbon dioxide flow rate ratio in the ejector, the throat to jet ratio of the ejector, throat length of the ejector.

Silica obtained by the method as described herein above is physically characterized to determine surface area, density, re volume, pore diameter. In one example, the surface area of silica calculated by BET method is in the range of 150 m ² /g to 400m ² /g. In another example of the invention, the Hg pore volume ranges from 1.2cc/g to1.5cc/g.

The invention provides a method for obtaining precipitated silica. One: advantage of the method is that about 95% of the silica present in the rice husk ash is obtained. Further, the purity of the silica obtained is about 99.5%. The product grade of the _. silica obtained can be altered by altering the ratio of the various reactants, thereby providing two distinct forms of silica, in a single method. The foregoing description of the invention has been given merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.