Silica gel is an important adsorbent use as desiccant in our daily life. The term ‘gel’ merely indicates the condition of the material it is a hard glassy substance as it is manufactured from raw material. It has different characterization parameters It has porous structure characterised by chemistry of the arrangement of the pores and their porosity size. The fine porous chemistry of the product accounts for its high capacity to absorb water and other moisture contents. The pore area of silica gel varies with the method of its production and different affecting parameters. It is produced by the reaction of dil. sodium silicate and sulphuric acid under suitable condition pH and specific gravity of solution. Few acids cannot be use like HCL because it creates corrosion problems during the process. The mixture is coagulated into a hydrogel which is then thoroghly washed to remove the sodium sulphate formed in the reaction. It is then carefully dried to produce the hard, glassy silica gel of commerce. The time of setting of silicic acid gel is an important consideration in the formation of a spherical silica gel. It is dependent on the cone of 8102, the temperature, the acid used, the pH and the cone of other material present. Silica gel is being produced in different forms 6.3. granualar, spherical and silica gel for chromatography. Silica gel is being produced in different forms e.g. granular, spherical and silica gel for chromatography. For each form to be produced, the method of manufacture is different. Spherical silica gel can be prepared by adjusting the pH of the silicate, or the gel point, in such a way that it set very soon in the form of spheres when we add it drop wise into an organic element, such as an oil, which is being agitated continuously. The spheres are formed because of high interfacial tension between the oil and the sol. They are kept in the oil for such a time that the gel point is reached. The balls solidify and are removed from the solvent. The spherical balls then, are washed in the conventional manner to remove the sodium sulphate formed during the reaction. Silica gel is extensively used as a dehumidifying and dehydration agent. Due to its high adsorptive power, it protects packaged products from damage due to rusting or corrosion. It is also used in pharmaceuticals, pesticides, air conditioning, drying of compressed air and other gases and liquids such as refrigents and oils containing water in suspension.
Silica gel was first known in the hydrosol fora. lts discovery is often ascribed to Sir Thomas Graham, who prepared it in 1881 dil. silica sols obtained by mixing solutions of sodium silicates and hydrochloric said. However the material was known to earlier chemists, who in 1040 reported that amorphous silica in contact with an excess of alkali became ‘liquid’ and that addition of acid quantitatively precipitates the silica from solution. Graham’s contributions seem to have been primarily the development of methods of making of silica sols and quit incidentally the discovery that the hydro gel may be formed when a hydrosol is Eagle Company makes granular silica. Hon-ante Chemical Company makes a small particle, low density. Silica aerosol by replacing‘ the hydro gel water prior to the drying atop and Socony-hobil Oil Company makes a silica gel bead mlichtly aodified by the incorporation of about 3% alumina. Colloidal silica were commercialised about 1940 by national Aluminate Company and are now produced by 3.1.
The main raw materials for the production of spherical silica 5 gel are:
- sodium silicate
- sulphuric acid
- kerosine oil
Sodium silicate always will be the cheapest source of relatively pure and sulphuric acid from which silica gel can be made. However certain natural colloidal alumina-silicates including one clay can be attacked by acid K leaving a residue of porous hydrated $102 which in some instances can be formed into a gel. Also some ortho-silicate minerals easily attacked by acid may prove advantageous in making silica Self Sodium silicate required to the production of silica gel should be of high quality and very transparent in nature. It should contain the followings specifications:
For the production of sols and gels sodium silicate of ratio about 3.3 is usua1y used rather than lower ratios, since less acid is required or neutralisation of alkali per unit of silica and this ratio is available at low cost because such large volumes are produced. However, the 3.8 ratio may be used to some further advantage 1 acid consumption is an important consideration.
NOTE: The silicate of the specifications given above is available from Captain Chemical Industries (pvt), Faisalabad, Pakistan. AKNDS: Acids used most common1y in commercial manufacture are H250?, HCl and 002.
POSITION OP RAW MATERIALS IN PAKISTAN:
All the raw materials mentioned above are indigenously produced and are rather surplus in the country. So, ready availability of the raw materials low capital investments and high rates of return offer a distinct advantage to the small scale manufacturers to venture into this field.
THE CHEMISTRY OF GEL FORMATION:
It is generally agreed that the gelation of silica hydrosol is an outward manifestation of polymerization of the silica units. When the gelation is carried out at an acid pH the silica micelles are extremely small. The three dimensional cross linking of those fine micelles by condensation polymerization which continues during the washing and drying steps leads to “haystack” structure of extremely fine pores.
Si-OH + HO-Si —–+ -O-Si + H20
The small size of silica leads to product with a surface area of more than 800 ma/gm. When the silica hydro gel is washed or aged at a high and temperature, silica transport occurs, leading to growth of the silica beads. As a result the average pore diameter is larger; pore volume increases and surface area decreases.