Title of Invention

"A PROCESS FOR PRODUCTION REACTIVE SILICA"

Abstract

A process for production of reactive silica characterized by digesting flyash with alkali solution in the temperature range of 25°-150°C for a time period in the range of 30 minutes to 168 hours, separating off the residue by known methods and precipitating silica from the solution by the addition of dilute mineral acids, keeping the precipitated mass for aging, separating the solid silica mass by conventional methods, washing the silica mass till free from anions and finally drying the washed silica mass to get reactive silica

Full Text

The present .invention relates to a process for production of of reactive silica. Silica is one of the abundantly present mineral over the earth crust. In majority of the situations, it exists as integral constituent of many of the rock types in the form of complex alkali / alkaline earths / transition element minerals. In addition, silica also exist in free form in many of the crustal argillaceous mass. Silica, or more commonly sand, as available along the river banks or sea coast is fairly an inert material and largely used as filler in cement / lime based mortar or concrete matrices and certain ceramic bodies.On physico-chemical cosiderations, the reactivity of silica largely depend on its allotropic form besides particle size. The reacivity of silica in amorphous form has long been recognised due to its discreet particle size and surface properties. Some of the recently recognised applications of amorphous reactive silica are dry binder, hydraulic fibre containing heat resistant composites, coatings for siliceous substrate, as integral constituent of cement / lime based grout mixes for ground improvement, high early strength grout mixes, inert filler for rubber latex in tyres and foot wear soles .Chemically pure high grade amorphous silica is used as free flow and anticaking agent in feed stuff, food ingredients, spices, detergents and cleansers. Reactive amorphous silica has wide range applications due to its inherent reactivity in alkaline medium. Reactive silica undergo irreversible chemical reaction with hydrated lime in vast range of concentrations and hydrothermal conditions. Reactive silica in amorphous .form can be used in combinations with lime, cement, slags alongwith other traditional coarse / fine aggregates with advantage of development of early high mechanical strength with qualities of thermal resistance and durability. In cement matrices, finely distributed amorphous silica particles readily react with lime which is formed due to the hydration of cement phase.Several examples of use of amorphous silica in improving the quality of cement concrete mixes are available besides reduction in bleeding and segregation by vastly reducing the bleed water channel. Further, addition of fine grained reactive amorphous silica in concrete mass reduces interparticle friction and improves workability. Reference may be made to ( Her, Ralph K., The colloid chemistry of silica and silicates, Cornell University Press, Itahca, New York, 1955 ) wherein a method to prepare silica powders with a feel of smoothness through ultra fine mechanical grinding of natural sand have been reported, but such products are not fine enough in the colloidal sense. Reduction in the average size of silica grain even to few micron size can however be achieved through ultra fine mechanical grinding procedures. The resulting silica powders although show enhancement in chemical reactivity but lacks in desirable morphological and surface characteristics. Further, such methods for obtaining reactive and fine grades of silica particles, involve exhorbitent cost and energy inputs. Reference may also be made ( Jhons- Mansville Corpn., The story of diatomite , 1953 ) wherein some of naturally occurring amorphous reactive silica resources (e.g. volcanic ash deposits, diatomaceous earth or diatomite ) have been mentioned. However, these resources exist with limited availability that too in specific regions of the world only in small pockets and therefore can not meet the ever increasing demand of construction sector for long time to come. Keeping in view the increasing demand of reactive amorphous silica for various industrial applications, attempts have been made in the past couple of decades for its commercial production in several developed countries. In this regard reference may be made to the compilation of Her, Ralph K.,( The colloid chemistry of silica and silicates, Cornell University Press, Itahca, New York, 1955) wherein approaches based on chemical techniques for obtaining more finely divided forms of silica have been reported. In these approaches, processes involving depolyemerization of silica in one way or another and repolymerizing to the desired colloidally subdivided form are mentioned. Amorphous reactive silica is also obtained through thermal technique wherein silica is vaporized at high temperature to produce SiO2 vapor or by burning ethyl silicate and to collect the "silica fume". Extraction of silica from suitable source through dissolution and subsequently its conversion into a gelatinous precipitate constitute the vital step in obtaining silica with pre-defined surface and morphological attributes through chemical mode. It may be noted that suitable concentration of colloidal silicic acid particles are essential in order to form stable gel structure, besides other conditions like temperature and specific ionic concentration etc. Reference may also be made to the reported work by Van Buzagh, (Colloidal Systems, The Technical Press Ltd., London, 1937, p 149) wherein it is stated that essentially (1) the charge on the ultimate particles be sufficiently reduced either by lowering the pH below 7 or by having solution electrolytes present if the pH is above 7 and (2) the rate of reaction between silanol groups on the surfaces of the particles in contact be catalyzed by OH~ion ( or, if the pH is low, by F-ions). In order to bring the particles together into more compact aggregates as a precipitate instead of a gel, a flocculating agent must be present. In this connection, it is worthwhile to refer the findings of Verwey and Overbeek ( E.J.W.Verwey and J.T.G.Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier Pub. Co.,New York, 1948 , page 12 ) wherein it is mentioned that since a gel is a loose network of colloidal particles interconnected only at few points, in which the network of the dispersion medium is immobilized, it must be prevented from forces which are often operative to stabilize the sol. Thus, it is evident that those materials which tend to permit the formation of more compact aggregates are likely to appear as a precipitate, without any significant entrapment of dispersion medium. Further , reference may also be made to the findings of Bungenberg de Jong, ( Colloid Science, ed. H.R.Kruyt, Elsevier Pub. Co., New York, 1949), where in it is mentioned that the charge on silica particle is greatly reduced in presence of high electrolyte concentration and lead to coagulation rather than gel formation. Reference may also be made to the work of E.J.W.Verwey and J.T.G.Overbeek, (Theory of the Stability of Lyophobic Colloids, Elsevier Pub. Co.,New York, 1948) wherein it is stated that polyvalent cations are even more effective flocculating agents. This implies that magnesium, barium, calcium or aluminium ion are much more effective than sodium in coagulating the silica. It is also important to note that the aggregation of the ultimate silica particles can be accelerated not only by lowering their charge, but also by increasing the concentration of the catalyst which is required to effect their union. Higher concentrations of alkali or a combination of fluoride and low pH have this effect. Reference may also be made to published information (E.J.W.Verwey and J.T.G.Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier Pub. Co.,New York,1948) wherein it is mentioned that water-soluble organic solvents such as alcohol lower the charge on the particles and accelerate flocculatation in alkaline solution. However, in acid sols, where the particles have but little charge, there is little OH- ion present to act as a catalyst to form the necessary - SiOSi- linkages between the particles. Thus, at pH 1 to 3, neither salts nor organic solvents have much effect on the rate of gelling.There have been a great many process proposed for making fine silica by reacting certain types of silicate minerals with acids. In some of these, the silica actually dissolves and is subesequently precipitated; in other , the silica is obtained as an insoluble residue. Reference may also be made to the work of Tsutomu Kuwada and Yujiro Sugawara covered under Japanese Patent 179, 853 ( 1949). In this process reactive silica is obtained through manipulation of conditions of chemical reaction and method of drying in respect of each mineral. Acid clay is extracted with 50 percent H2S04 by circulating the acid for 5 to 10 hours, washing with water, and drying to give active Si02 containing 1 percent A1203. Another method for modifying the polymerization of silica so that the product is a finely divided precipitate instead of a gelatious mass involves the reaction of sodium silicate with an ammonium salt. Reference may also be made to the work reported by W.C.Arsem and J.G.E. Wright covered under US Patent 1,270,093 (General Electric Co., 1918 ) wherein the end product i.e. finely divided silica is obtained through chemical reaction between sodium silicate and ammonium chloride. Under this process the reaction mixture is boiled to form silicic acid followed by filtering and drying the resulting product. In another reference Heuser , R.V. (US Patent 2,114,123, American Cynamide Co., 1938) has reported the process for obtaining precipitated silica gel having large pores by reacting sodium silicate and an ammonium salt under specified conditions at ordinary temperature. The resulting silica gel particles thus obtained are reported to show spherical morphology and range from 0.5 to 5 microns , with a bulk density of 170 to 250 kg per cu m. Silica may also be obtained as a precipitate instead of a gel by reacting sodium silicate with a sodium salt such as sodium sulfate, then acidifying the gel and washing, is covered under British Patent 203,158 ( J. Crosfield and Sons Ltd., 1922) and the end product i.e. the gel exhibited granular structure with ultramicroscopic pores. The use of sodium bicarbonate for this purpose is covered under another British Patent by T.J.I. Craig and A. Kirkham ( British Patent 299,483, Peter Spence & Sons Ltd., 1927 ). In this context reference may also be made to the findings of F. Gewecke covered under German Patent 731,343 (Deutsche Solyvay-werke , A.G.1937) herein the precipitation of silica in presence of fluoride ion is reported. Reference may also be made to various methods reported for the precipitation of silica from solutions of alkali metal silicate with acid at different concentration, pH conditions and temperature to prevent the formation of a gelatious mass and to promote the preceipitation of silica in a finely divided form (Her, Ralph K., The colloid chemistry of silica and silicates, Cornell University Press, Itahca, New York, 1955). In all the aforesaid discussion it is well indicated that amorphous silica is best obtained from its solution with minimal of coagulation wherein charge on silica particle play a vital role. In the process dealt with in the present invention, silica is solubilised from the constitutent glassy matix of flyash in alkaline medium and the resulting system is subjected to gelation process by lowering alkalinity by acidification of the system at low temperatures ( 10 to 20ºC) , which have not been covered in totality in earlier reported findings. The main object of the present invention is to provide process for production of reactive silica. Another object of the present invention is to provide a process for production of reactive silica from flyash which obviates the draw backs of getting suitable grade of natural silica resource. Yet another object of the present invention is to evolve utilization of low value industrial waste from thermal power plants i.e. flyash, which obviates the problem of its disposal. Still another object of the present invention is to evolve methodology for the conversion of waste siliceous mass into value added product which obviates the drawback of limited availability of reactive silica at exhorbitant cost. Still another object of the present invention is to evolve methodology for the conversion of waste siliceous mass into value added product which obviate the adverse impact on natural environment involving air, water and land over the earth crust. Accordingly the present invention provides a process for production of reactive silica characterized by digesting flyash with alkali solution in the temperature range of 25°-150°C for a time period in the range of 30 minutes to 168 hours, separating off the residue by known methods and precipitating silica from the solution by the addition of dilute mineral acids, keeping the precipitated mass for aging, separating the solid silica mass by conventional methods, washing the silica mass till free from anions and finally drying the washed silica mass to get reactive silica. In an embodiment of the present invention the flyash used may be such as pond ash or hopper ash. In another embodiment of the present invention the alkali solution used may be sodium hydroxide, potassium hydroxide, sodium corbonate, potassium carbonate. In yet another embodiment of the present invention the digestion of flyash may be effected both at normal or elevated atmospheric pressure (in the range of 1 to 6 atmosphere) in alkali resistant alloy pressure vessel. In yet another embodiment of the present invention the separation of unreacted solid residue after alkali digestion of flyash may be effected by decantation, filteration centrifugation. In yet another embodiment of the present invention the acidification of solubilised silica mass may be effected by mineral acids such as hydrochloric acid, nitric acid, sulphuric acid solutions. In yet another embodiment of the present invention the acidification of solubilised silica mass may be effected at temperatures in the range of 5° to 40°C. In yet another embodiment the aging of precipitated silica mass may be effected at a temperature in the range of 5° to 40°C for a minimum period of 8 hours. In yet another embodiment the washing of precipitated reactive silica mass may be effected by distilled or deionised water, ethanol and mixture thereof. In yet another embodiment the filteration and washing of precipitated reactive silica mass may be effected through filter press assembly of appropriate capacity. In yet another embodiment of present invention the drying of washed reactive silica mass may be effected at temperature below 70°C both at normal or reduced pressure. The present invention provides a process for utilisation of flyash in manufacture of reactive silica, which comprises selection of silica rich flyash (above 60 percent of silica on dry weight basis and unburnt carbon content below 2 percent) collected from electrostatic precipitator (hopper ash) or ash disposal pond (pond ash) and containing 90 percent of the particles below 20 micron siza, the flyash is subsequently digested in alkali resistant alloy (e.g. stainless steel) pressure vessel to sustain upto 6 kg/cm2 internal pressure, with alkali solution (i.e. sodium hydroxides/ sodium carbonates / potassium hydroxide / potassium carbonate) with concentration ranging between 1 to 8 M for a period ranging between 30 minutes to 168 hours maintaining a liquid : solid ratio between 1 to 5 (on weight basis) in the temperature range of 25° to 150° C or at 10 to 20 ° C above ambient temperature at 1.0 to 2.0 times the atmospheric pressure so as the maximal hydro - thermal decomposition of complex silicate takes place. Resulting solublised silicate in liquid phase is separtated out through seiving on a hardened filter paper sheet with pore size openning equivalent to 0.5 micron at temperature ranging between 10°-20° C involving decantation in the initial stages followed by filteration. The unreacted solid mass is washed off with dilute warm solution of respective alkali hydroxide or alkali carbonate solution (0.1 to 0.8 M concentration). Separtated liquid phase constitute the rich source of silicate ion with impurities of alumina content which also get solubilised during the process forming sodium aluminate. Solubilised alumina content can however be eliminated through optimal pH control, while acidifying the liquid phase containing silica as silicate ion so as to obtain silicic acid. Silicic acid thus separated out polymerises with substantial reduction of the electrical charge on discreet silica particle, at critical hydrogen ion concentration. Though, polyvalent cations are more effective as flocculating agents, however, do not yield the desired particle size distribution in the system. Further, silicic acid or silica hydrates as formed during acidification stage of solubilised silica concentrate is allowed to undergo aging for a minimum period of 8 hrs. at temperatures in the range of 10°-20°C and separated out through conventional methods involving sedimentation, sieving through hardened acid resistant filter paper under low pressure. The separated silica mass is thoroughly washed with cold and very dilute nitric acid solution (0.01 to 0.05 N concentration), deionised water and thereafter with (1:1) alcohol water mixture followed by washing with absolute alcohol till the washings are free from anions. Silica mass thus obtained, is dried at low temperatures below 70°C for 6 to 48 hours at normal or reduced atmospheric pressure prior to storage in air sealed containers for subsequent use. The dried silica mass thus obtained shows the predominance of amorphous silica phase through X-ray diffraction analysis and the total silica content was found to vary in the range of 78 to 85 percent with association of other constituents (i.e. oxides of iron / aluminium, alkali / alkaline metal etc.) in low proportions except physically and chemically adsorbed water contents. Further, it is stated that yield of dried reactive silica from flyash mass depends on the conditions of alkali digestion, temperature and H ion concentration during acidification of solubilised silica and aging conditions of precipitated silica mass, besides, on over all proportion of amorphous / glassy constituents associated with flyash. The process for the manufacture of reactive silica from flyash involves (i) Selection of suitable flyash mass, with low unburnt carbon and fine grained texture, as coarse grained flyash with excessive unburnt carbon is liable to cause appreciable loss both in the quality of end product besides increase in the cost of production of amorphous reactive silica (ii) The flyash mass is subsequently digested with caustic soda ( sodium hydroxide comm. grade ) . Although other alkalies / alkaline materials belonging to alkali metal hydroxides or soluble carbonates such as sodium carbonate, potasium hydroxide, potasium carbonte are effective in solubilising silica from amorphous / glassy matrix of flyash at optimal ionic concentrations, the use of sodium hydroxide is preferred because of cost considerations and low solubilising time involved in digestion process (iii) Flyash and sodium hydroxide solution are digested in an alkali resistant pressure vessel at temperatures close to the boiling temperature of the mixture. Although the solubilisation of silica takes place even at ambient temperatures, the time required for solubilisation of silica is usually more and (iv) solubilised silica can subsequently be seperated out in the form of fine grained hydrated silica or silicic acid particles with the addition of dilute and cold mineral acids such as HCL, HN03 or H2S04. These particles are associated with surface charge and exhibit colloidal character. Generalised process flow chart of the process is shown below : (Diagram Removed) A typical chemical mode of seperating silica from flyash as silicic acid is shown below, wherein hydrochloric acid is used in the reaction: (Equation Removed) The formation of silica agglomerates involving gelation and or precipitaion largely depends on the ionic concentration in the reaction medium besides temperature at which the above reaction is allowed to accomplish. In this regard, it is worthwhile to express the polymerisation process of the silicate ion in the following simple manner : (Equation Removed) Assuming the coordination number of silicon with respect to oxygen to be 4, the polymerisation process may be chemically expresses as follows : (Equation Removed) Further, if the coordination number of silica in the soluble silicates is assummed to be 6, the process may be chemically expressed as follows : (Equation Removed) Accordingly, it may therefore be visualized that silica gel is made up of a polymer of Si(OH)4 in which each unit is coordinated with the next in linear polymer. An alternative representation of the above chemical reaction may be expressed as : (Equation Removed) The following examples are given by way of illustration of present invention and therefore should not be construed to limit the scope of present invention. EXAMPLE 1 In this experiment one kg of pond ash from coal based Super Thermal Power Plant, Vijayawada was mixed with 5 lit solution of sodium hydroxide of 8 M concentration in a stainless steel container and treated for one week (168 hours) at ambient temperature with occassional stirring with stainless steel rod. The mix was kept undisturbed for another 2 days to facilitate sedimentation. Solid unreacted residue was subsequently sieved off through filter paper (equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled to temperature of 10°C and dilute hydrochloric acid solution ( 6N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0, so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH) 4 was seperated from the liquid phase through filteration over acid resistant Grade Whatman filter paper 542. Care is taken to thorough wash the silica gel free from chloride ion, initially washing with deionised water followed by 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 65°C and stored in air tight plastic containers. The dried material thus obtained weighed 284g. EXAMPLE 2 In this experiment one kg of hopper ash from Super thermal Power Plant , Kaniha (Orissa) was mixed with 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 16 hours at 60 + 2°C with occassional stirring with glass or stainless steel rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 541) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH) ^ was seperated from the liquid phase through filteration over acid resistant Whatman filter paper grade 542. Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. The dried material weighed 346g. EXAMPLE 3 In this experiment one kg of hopper ash from Super thermal Power Plant , Kaniha was mixed with 2 lit solution of sodium carbonate of 6 M concentration in a stainless steel container and digested for 24 hours at 60 + 2°C with occassional stirring with glass or stainless steel rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH) 4 was seperated from the liquid phase through filteration over acid resistant Whatman filter paper grade 542 . Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. The dried material weighed 298g. EXAMPLE 4 In this experiment one kg of hopper ash from Super thermal Power Plant , Kaniha was mixed with 2 lit solution of potassium hydroxide of 4 M concentration in a stainless steel container and digested for 16 hours at 60 + 2°C with occassional stirring with glass or stainless steel rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liquid phase through filteration over acid resistant Whatman filter paper grade 542. Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. The dried material weighed 356g. EXAMPLE 5. In another experiment one kg of flyash from NTPC Thermal Power Station, Talcher (Orissa) was treated 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 16 hours at 60 + 2°C with occassional stirring with glass rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C) . The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liqiud phase through filteration over acid resistant Whatman filter paper grade 542. Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. EXAMPLE 6 In another experiment one kg of flyash from Punjab State Electricity Board Thermal Power Station, Ropar (Punjab) was treated 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 16 hours at 60 + 2°C with occassional stirring with glass rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper (equivalent to Grade Whatman 541) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liqiud phase through filteration over acid resistant Whatman filter paper grade 542. Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. EXAMPLE 7 In another experiment one kg of flyash from MTPC Thermal Power Station, Badarpur (Delhi) was treated 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 16 hours at 60 + 2°C with occassional stirring with glass rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C) . The seperated liquid phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 4N cone) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liqiud phase through filteration over acid resistant Whatman filter paper grade 542 . Care is taken to thoroughly wash the precipitated silica mass free from chloride ion. Initial washings are done with cold and very dilute nitric acid (0.05 N conc.) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70°C and stored in air tight plastic containers. EXAMPLE 8 In another experiment one kg of flyash from same source as given under example 1 was mixed with 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 6 hours over steam bath at 80 to 85 deg centigrade with occassional stirring with glass rod. The mix was kept undisturbed for another 6 hours to facilitate sedimentation. The mixture was subsequently sieved off through filter paper ( equivalent to Grade Whatman 42 ) and the contents on the filter paper were washed to recover all adhered silcate ion with hot distilled/ deionised water( 60°C ). The seperated liquid, phase was cooled down to 10°C in a ice bath and dilute hydrochloric acid solution( 8N cone) was added dropwise with constant stirring till the pH of the reaction mixture is brought down to 3.0 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liqiud phase through filteration over alkali resistant Grade Whatman filter paper 542. Care is taken to thoroughly wash the silica gel free from chloride ion. Initial washings are done with warm and very dilute nitric acid ( 0.05 N concentration ), deionised water followed by washing with 50 percent alcohol-water mixture and finally 2-3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 65°C and stored in air tight plastic containers. The dried silica mass thus obtained weighed 342g. EXAMPLE 9 In this experiment one kg of hopper ash from the same source as given under example 2 was mixed with 2 lit solution of sodium hydroxide of 4 M concentration in a stainless steel container and digested for 30 minutes in a pressure vessel at 2 atm steam pressure. The digested mix was kept undisturbed for about 2 hours so as to bring down the pressure and temperature to ambient conditions. The treated/ digested mix at ambient temperature was taken out of pressure vessel and kept aside undisturbed prior to filteration through filter paper (equivalent to Grade Whatman 541 ) and the contents on the filter paper were washed to recover all adhered silcate ion with warm distilled / deionised water (60°C). The seperated liquid phase was cooled down to 10 °C in a ice bath and dilute hydrochloric acid solution( 8N conc) was added dropwise with constant stirring till the pH of the reaction mixture is brought down between 3 - 4 so as to avoid contamination of aluminium hydroxide. The gel mass of silicic acid i.e. Si (OH)4 was seperated from the liqiud phase through filteration over alkali resistant Grade Whatman filter paper 542. Care is taken to thorough wash the silica gel free from chloride ion. Initial washing are done with cold and very dilute nitric acid ( 0.05 N concentration ) followed by washing with deionised water, 50 percent alcohol-water mixture and finally 2-3 times with absolute alcohol. The silica gel is taken off from the filter paper assembly and air dried for 48 hours in the laboratory. The air dried mass is subsequently dried in electric oven for 2 hours at 70 °C and stored in air tight plastic containers. The dried silica mass thus obtained weighed 364g. The experiments covered under examples 1 to 9 clearly highlight the application of the methodology for the manufacture of reactive / amorphous silica from flyash for use in different industrial applications. The invention make use of the amorphous/ glassy constitutent of the flyash mass to yield fine grained amorphous silica through its degradation / dissolution under alkaline medium. As per the known practices alkali metal silicates librates silica particles when their pH is lowered to acidic range. The alkali metal silicate in the system are formed when amorphous / glassy constitutents of flyash mass are digested with sodium hydroxide solutions of moderate concentration. The yield of the end product was found to increase significantly when the temperature and pressure conditions are slightly elevated during digestion process and acidification, filteration and washings of silica particles is done at temperatures below 15°C. The main advantages of the present invention are: 1. Amorphous silica is obtained from low value siliceous waste material i.e. flyash involving solublisation and seperation at preset ionic equlibrium 2. Amorphous silica with vast utilisation potential is obtained through chemical process wherein the surface and morphological features can be controlled within the desired range. 3. The present invention is environment friendly and helps in improving the overall quality of life of habitat residing in the vicinity of flyash disposal sites 4. The left over material residue after extraction of amorphous silica could also be used for seperation of other elements such as titanium, aluminium etc. 5. The invention offer wide scope for employment generation in rural areas for setting up of small units for the production of amorphous silica to cater the needs of small scale industrial units manufacturing shoe soles, rubber tyre, electric cables etc., besides construction industry. 6. The present invention provides enough opportunities for the manufacture of value added amorphous silica from low val*ue industrial by-product which is otherwise a waste causing serious concern amongst environmentalists, planners and engineers for its effecient and cost effective management and disposal. We Claim: 1. A process for production of reactive silica characterized by digesting flyash with alkali solution in the temperature range of 25°-150°C for a time period in the range of 30 minutes to 168 hours, separating off the residue by known methods and precipitating silica from the solution by the addition of dilute mineral acids, keeping the precipitated mass for aging, separating the solid silica mass by conventional methods, washing the silica mass till free from anions and finally drying the washed silica mass to get reactive silica. 2. A process as claimed in claim 1 wherein flyash used is taken from pond ash or hopper ash. 3. A process as claimed in claims 1 and 2 wherein alkali solution for digestion of flyash is sodium hydroide / potassium hydroxide / sodium carbonate / potassium carbonate. 4. A process as claimed in claims 1-3 wherein the digestion of flyash is effected at normal and elevated pressure in the range of 1 to 6 atmosphere in alkali resistant alloy pressure vessel. 5. A process as claimed in claims 1-4 wherein the mineral acid used is such as hydrochloric acid, nitric acid, sulphuric acid. 6. A process as claimed in claims 1-5 wherein aging of precipitated silica mass is effected at a temperature in the range of 10° to 20°C for a minimum period of 8 hours. 7. A process as claimed in claims 1-6 wherein the washing of precipitated reactive silica mass is effected by distilled / deionised water, ethanol and mixture there of. 8. A process as claimed in claims 1-7 wherein the drying of washed reactive silica mass is done at a temperature below 70°C at atmospheric pressure or reduced atmospheric pressure. 9. A process for production of reactive silica, substantially as herien described with reference to the examples.

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35-del-2000-correspondence-others.pdf

35-del-2000-correspondence-po.pdf

35-del-2000-description (complete).pdf

35-del-2000-form-1.pdf

35-del-2000-form-19.pdf

35-del-2000-form-2.pdf