Title of Invention

A PROCESS FOR THE PREPARATION OF FLUFFY VARIETY OF PYROGENIC SILICA

Abstract

The invention relates to a process for preparation of a fluffy variety of pyrogenic silica. The process has been developed for production of fluffy variety of pyrogenic silica at a relatively lower temperature (around 450°C), using a waste material generated by aluminium industries (the spent pot liners) and a commercial grade silicone-oil. The product silica has an average particle size of 4 micrometer and specific surface area of 1.1575 sq. meter/cc. The particle size indicates its potential in commercial use in making a number of industrial products. The process is simple and does not require any special instrument to make the micro-amorphous silica.

Full Text

The present invention relates to a process for preparation of fluffy variety of pyrogenic silica. The present invention particularly relates to a process for the preparation of fluffy variety of pyrogenic silica, at a relatively lower temperature. Pyrogenic silica is termed as micro-amorphous silica, although it is not truly amorphous but consists of regions of local atomic order or crystals of extremely small size, which produces a broad band spectra under ordinary diffraction studies. Common form of micro-amorphous silica have ultimate spherical silica particles of less than 1000 A° in diameter, the surface of which consists of anhydrous SiO2 or SiOH groups. The powders generally occur in coherent condition with a threedimensional aggregate formed by siloxane bonding. These clusters are desegregated in solution phase (generally by alkalies like sodium-hydroxide, ammonia solution etc.) to form sols. Absorption of alkalies on silica surface reduces rate of dissolution & thus growth of clusters retarded. Sols thus formed has great commercial value & a number of these sols (as indicated in Table-1 below) for silica particle sizes from 4 nm to 36 nm finds multi-various uses like binders and stiffners, for modifying frictional properties of waxes and fibers, modifying adhesion between surfaces, reinforcing polymers & rubber, as polishing agent, as viscosity modifier, frosting incandescent buib glass, catalyst support etc. Table-1 : Size of silica particles in commercial silica sols listed by manufacturers. Manufacturers E.I. du Port dc Nemous Wilmington, Del (USA) Nalco Chemical Co., Chicago, II (USA) Nyacol Inc., Ashland, Mass (USA) Nissan Chemical Industries, Tokyo (Japan) Monsanto Ltd., London (England) Commercial name of the sol Ludox Nalcoag Nyacol Snowtex Syton SiO2 (%) 30-40 15-30 30-50 20-30 15-30 Particle dia (nm) 12-21 4-15 8-20 10-20 36-18 Specific surface area (m2/gm) 230-130 750-210 - - 75-150 However, dispersion of pyrogenic silica to a sol is difficult because the particles coalesce to a varying degree and in many cases the surface is partly anhydrous with only a few hydrophilic silanol groups. For this reason, the properties of sols prepared from pyrogenic silicas are generally different from those made in aqueous solution. They do not form strong gels and are of little use as an inorganic binder. In general, fumed silica obtained by flame hydrolysis, do not yield silica that is dispersable in water to produce sols of discrete particles with low viscosity at high concentrations which is a characteristic of sols made by aqueous polymerization process. Nevertheless, with enough processing pyrogenic silicas of ultimate particle sizes 10- 25 nm can be desegregated and dispersed to aqueous sols containing upto 40% silica with suitable mechanical treatment and dispersing agents. Thus colloid milling of pyrogenic silica in water in the presence of boric-acid or alkali-borate is dispersed to produce a 30% sol (Clapsdale L.G and Syracuse M.G. U.S. Pat 2,630,410(1953), Union Carbide and Chemical Co., USA.). Other patented processes for such dispersion of pyrogenic silica involves sodium-silicate, sodium-hydroxide, hydrazine, hydroxylamine, or mixtures with pyrogenic metal-oxides. Amorphous silicas with very small particle size are being prepared by aqueous polymerization process, which involves hydrolysis of gases like SiCl4 and simultaneous polymerization of silicic-acid in water. Stobber & Fink (Stober W, Fink A, and Bohn E. J. Colloid Interface Sc. 26, 62 (1968).) prepared very uniform spherical silica (from few nm to 900 nm) by hydrolyzing a lower alkayl-silicate in an alcohol. Pyrogenic silicas on the other hand, can be prepared by vaporizing silica at very high temperature (around 2500°C). If a reducing agent is present simultaneously so as to form SiO, the sublimation temperature can be brought down to 1700°C (The Chemistry of Silica - R.K. Her, John Wiley & Sons, N.Y (1979), pp 336.). As the monoxide evaporates in an oxidizing atmosphere, the dioxide is condensed in an extremely finely divided form. A remarkable form of silica 'fluffy' form having very low density and appears to flow as water does, was prepared by Jacobson (Jacobson C.A, J. Phys Chem 48, 413 (1936).) from SiF4. The present invention relates to the development of a process for producing such 'fluffy' variety of pyrogenic silica at a much lower temperature (around 450°C) described herein. The main objective of this invention has been to prepare a novel fluffy variety of pyrogenic silica from organic silicon compound such as commercially available silicon-oils together with a waste product (spent pot liner) generated by aluminium industries. Another objective of the invention is to provide a process for preparation of pyrogenic silica at lower temperature. The present invention provides to a process for the preparation of a novel (fluffy variety) of pyrogenic silica from spent pot liner generated by aluminium industries & silicone-oil. Accordingly, the present invention relates to a process for preparation of a fluffy variety of pyrogenic silica comprising the steps of: a) grinding spent pot liners to % inch down to dust size, b). characterized in that reacting the above said ground material with chromic acid for 3-30 minutes, c). washing the said mixture obtained in step b) with distilled water until neutral & drying in an oven at 80-100°C for 2 hours, d). subjecting the said dry powder to thermal shock for 2-3 minutes in a preheated oven at 950°C, to generate a free flowing graphitic powder, e). mixing the said graphitic powder with equal volume of silicone-oil and heating to 450-500°C to sublime a fluffy pyrogenic variety of silica with average particle size of about 4 micro-meter and surface area of about 1.1575 sq.m/cc. In an embodiment of the present invention wherein the treatment with chronic acid is carried out at a temperature below 140°C. The said spent pot liner (waste cathode blocks) collected from aluminium industries, was first crushed to 1/2 inch down to dust size and then taken into a 500 ml flask provided with a stirrer and thermometer. The assembly was placed inside a small fume-cupboard exhaust of which bubbles through a dilute sodium-hydroxide bottle for absorbing emanated toxic gases. The task was kept submerged in a tray with running tap water for cooling as & when require. Concentrated chromic-acid solution was then added to the crushed material drop by drop with stirring. The reaction is vigorous and exothermic. Care is taken not to allow temperature of reaction mixture exceed 140°C by controlling flow of cold water outside the flask. The reaction is over in about 20 minutes, after which the contents are thoroughly washed with distilled water until neutral. The washed product then dried in an oven at about 80~100°C for 2 hours. The dried sample then thermally shock treated in a preheated oven at about 950°C for 2-3 minutes. The resultant product is a fine graphitic free flowing powder with average particle size of about 20 micro-meter as determined by MALVERN particle size analyzer (Figure-1). The said graphite powder was then placed in another 500 ml round bottom flask & mixed with approximately equal volume of commercial grade silicone-oil. The flask was joined (not air-tight) to another collecting 500 ml flask. The mixture was heated to a temperature of about 450-500°C when fluffy variety of fumed silica deposits inside the collecting flask. Fluffy silica thus produced is extremely light (density about 0.025 gm/cc), white in appearance, and stable for a very long period in ambient conditions. The product dissolves instantaneously in acids like hydrofluoric acid and gets dispersed in water easily by addition of little alkali. Microscopic examination indicates silica particles in a three-dimensional network giving it a fluffy cotton like appearance. MALVERN particle size analysis (Figure-2) shows average particle size of the silica particles about 4 urn and a surface area of 1.1575 m2 per c.c. (495 m2/gm by BET). The following examples are given by way of illustration and therefore should not be construed to limit the scope of present invention. Example 1: Particle size of spent pot liner !/2 inch Treatment time with chromic acid 30 minutes Temp(°C) 140 Average Particle size of graphite 19 micro-meter Ash% 15% Example 2: Particle size of spent pot liner -100BS Treatment time with chromic acid 3 minutes Temp( °C) 140 Average Particle size of graphite 19.04 micro-meter Ash% 14% Example 3: Particle size of spent pot liner '/2 inch to dust(with-100 BS size 40%) Treatment time with chromic acid 25 minutes Temp( °C) 140 Average Particle size of graphite 19.04 micro-meter Ash% 14% EXAMPLE 4: Viscosity of Silicon-oil 300 cP (Brookfield) Volumetric ratio graphite : silicon oil 1:1 Sublimation Temperature 460°C Average particle size of silica 4 micro-meter EXAMPLE 5: Viscosity of Silicon-oil 250 cP (Brookfield) Volumetric ratio graphite : silicon oil 1:1 Sublimation Temperature 455°C Average particle size of silica 4 micro-meter The main advantages of the present invention are : 1. The process generates pyrogenic silica with average particle size 4 micrometer at a relatively low temperature (450-500°C). 2. The process utilizes a waste (spent pot liner) generated by aluminium industries and thereby helps abating pollution generated by the said waste at plant site. 3. The process is simple and easy to operate. It does not require any special apparatus. 4. Sizes of the silica particles produced by invention has a number of commercial applicable & thus has good commercial value. claim :- 1. A process for preparation of a fluffy variety of pyrogenic silica comprising the steps of: a), grinding spent pot liners to 1/2 inch down to dust size, b). characterized in that reacting the above said ground material with chromic acid for 3-30 minutes, c). washing the said mixture obtained in step b) with distilled water until neutral & drying in an oven at 80-100°C for 2 hours, d). subjecting the said dry powder to thermal shock for 2-3 minutes in a preheated oven at 950°C, to generate a free flowing graphitic powder, e). mixing the said graphitic powder with equal volume of silicone-oil and heating to 450-500°C to sublime a fluffy pyrogenic variety of silica with average particle size of about 4 micro-meter and surface area of about 1.1575 sq.m/cc. 2. A process as claimed in claims 1 wherein the treatment with chromic acid is carried out at a temperature below 140°C. 3. A process for preparation of a fluffy variety of pyrogenic silica substantially as herein described with reference to examples accompanying this specification.

Documents:

276-DEL-2003-Abstract-(03-03-2009).pdf

276-del-2003-abstract.pdf

276-DEL-2003-Claims-(03-03-2009).pdf

276-del-2003-claims.pdf

276-DEL-2003-Correspondence-Others-(03-03-2009).pdf

276-del-2003-correspondence-others.pdf

276-del-2003-correspondence-po.pdf

276-DEL-2003-Description (Complete)-(03-03-2009).pdf

276-del-2003-description (complete).pdf

276-DEL-2003-Form-1-(19-05-2009).pdf

276-del-2003-form-1.pdf

276-del-2003-form-13-(06-05-2009).pdf

276-del-2003-form-2.pdf

276-DEL-2003-Form-3-(03-03-2009).pdf

276-del-2003-form-3.pdf