Industrial uses of rice husk
ANNUAL production of paddy in India is more than 177 million tonnes, out of which nearly 11 per cent is contributed by Punjab. Paddy has to pass through several milling operations such as dehulling, polishing and grading before obtaining good quality white rice. This job is being done by about 30,000 modern mills and 19,000 shellers in the country, while 2,500 shellers alone in Punjab get engaged in this business.
Shelling of such a large amount of paddy results in the collection of significant quantity of husk, which contributes nearly 20 per cent of the grain. Unless properly disposed of, accumulation of husk poses several problems of quick flaming and spreading while blowing with air. Low bulk density and large volume of husk stalemate its safe storage. Though low nutritive value, abrasive, fragile and porous nature of husk restrict its worthily utilisation, the following are some of the products which could be prepared while using cellulose, lignin and silica of the husk.
Composite boards: Panel boards for exterior or interior uses in the form of wall boards, floor or ceiling can be prepared from rice husk with or without resin. While using synthetic resin at the rate of 8 per cent by weight, the moisture content of the husk is increased to 20 per cent. The ingredients are mixed, pressed at 1300 to 1600 C under a pressure of 80 km/cm2 for 30 minutes and conditioned for 12 hours. Either sulphuric acid, urea or both @ of 3 per cent could be added to make the board water proof.
In the second process without any binder, the husk is ground to a particle size of 40 to 60 mesh, mixed with dilute sulphuric acid (2 per cent) @ 5-8 per cent by weight of the husk and pressed in a hot hydraulic press at 1650 C under a pressure of 60 to 70 kg/cm2 for 20-25 minutes.
Furfural: The husk is treated with sulphuric acid till complete digestion of lignin. The liquefied husk is steam distilled to recover furfural. The yield of furfural varies from 4 to 12 per cent. It is widely used in synthetic rubber and oil refining industries.
Sodium silicate: The husk is burnt to grey ash, which is extracted by cold or hot process using 10 per cent sodium hydroxide solution or a mixture of sodium carbonate and sodium hydroxide in the ratio of 10:1. Sodium hydroxide can also be replaced with calcium hydroxide. Sodium silicate obtained in the dilute form is bleached with activated carbon and then concentrated to 550 Be by heating and evaporation. Sodium silicate is used in the manufacturing of soap, detergents, silica gel and water proof cement. Silica gel is extensively used in sealing and lamination of paper, bleaching and sizing of textiles and paper pulp.
Molecular sieves: The dilute sodium silicate is mixed with sodium aluminate to prepare alumino-silicate gel which is crystallised to produce a desirable size of molecular sieve powder. Being highly absorbent, molecular sieves are extensively used in chemical, petrochemical and gas industry for desiccation, drying, bulk separation and recovery of gases and liquids.
Silicon tetrachloride: The husk is pyrolysed land chlorinated at 10000 C to provide intimate mixture of silica and carbon in the form of silicon tetrachloride, free from most of the other inorganic chlorides. Silicon tetrachloride is used in the manufacture of electronic grade silicon, colloidal silica and silicon polymers.
Activated carbon: The husk is saturated with 35 per cent solution of potassium thiocyanate, potassium carbonate or zinc chloride. Upon drying the treated husk is carbonised at 3000 to 5000 C and activated with steam at 7500 C or with CO2 at 8000 to 9000 C, followed by treatment with caustic soda for silica. The carbon is dried and ground to pass 100 mesh screen.
Electrical porcelain tiles: The white ash of husk is treated with sodium oxide, calcium carbonate, boric acid and alumina. The porcelain material possesses high electrical resistivity, high chemical resistance towards minerals acids and low sintering temperature.
Char: It is prepared by controlled pyrolytic conversion of husk such that the carbon contents of residue are more than 1.5 per cent. Char is used as insulating or anti-slagging material in iron and steel industry, as coagulant and filter aid in treatment of contaminated water, and as anti-skid abrasive in tyres and shoe soles. Char also supports the rhizobium multiplication and survival for more than six months.
Fuel
Husk is one of the most abundant renewable agriculture-based fuel material. The calorific value of husk is 3000 Kcal/kg. One tonne of paddy or 200 kg of husk generates nearly 600,000 Kcal hear per hour. Assuming mechanical conversion efficiency of 30 per cent, the heat is sufficient to undertake parboiling and milling operations. Thus, husk can be successfully used as fuel for boilers, furnaces and even for domestic stoves.
Boiler fuel: The husk fired boiler produce 15 to 18 kg steam/m2 of the surface. A mechanically drafted combustion-tube boiler capable of generating, 1500 kg steam per hour, have a fuel rate of 333 kg/hr of husk with an effective husk and steam ratio of 3.66 kg steam/kg husk.
Furnace fuel: The furnace is designed for a burning capacity of 200 kg husk/hr to generate 500 kg steam. The husk is introduced into the horizontal cyclone furnace along with air to impart whirling motion to husk, or it is introduced into the combustion chamber in the form of a jet. The furnace can be coupled with air or water heat exchanger to utiliser heat in the flue gases. The hot gases in the boiler can also be used for drying of paddy.
Steel/Metal Plants: Rice Husk Ash is used as Tundish/Label covering compound - iOTA MCC - a powdered RHA liquid metal covering compound based on rice husk ash.
Briquettes: The husk is compressed or extruded adopting Hullite machine to prepare briquettes for using as industrial or domestic fuel. Briquettes in the form of bricks or small cubes diminish the problem of density and are easy to transport.
By Tejinder Singh
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