SILICA FROM RICE HUSK ASH

Silica (SiO2) is one of the valuable inorganic multipurpose chemical compounds. It can exist in gel, crystalline and amorphous forms. It is the most abandon material on the earth?s crust. However, manufacture of pure silica is energy intensive. A variety of industrial process, involving conventional raw materials require high furnace temperatures (more than 700 degree C). In our Project, a simple chemical process is described which uses a non-conventional raw material rice husk ash for extraction of silica. Rice Husk ash is one of the most silica rich raw materials containing about 90-98% silica (after complete combustion) among the family of other agro-wastes. Rice husk is a popular boiler fuel and the ash generated usually creates disposal problems. The chemical process discussed not only provides a solution for waste disposal but also recovers a valuable silica product, together with certain useful associate recoveries. The other associate recovery is sodium sulphate. Effluent wash water obtained after washing precipitated silica (wet impure silica) contains sodium sulphate. By evaporation of water in multiple effect evaporators, followed by crystallization, filtration and drying, crystals of sodium sulphate are obtained. The residue ash in sodium silica production can be utilized for making good quality bricks. Retained sodium silicate in residue ash acts as a binder and with incorporation of suitable ingredients high quality bricks can be manufactured. From rough estimation the production of paddy in the country about twenty five million tons of rice husk is obtained from rice mills. This is usually burnt resulting in mounts of ash in and around the mills, causing major health problem and pollution. Using rice husk as the raw material two million tons of pure high grade silica can be produced to meet the high demand of various industries that are using it. The main states with large number of rice mills are Andhra Pradesh, Tamil Nadu, Karnataka, and Kerala etc.
The conventional process of manufacturing silica is the reaction of sand with soda ash at about 1500ºC. In our project, a new method of manufacturing silica by reacting rice husk ash with caustic soda is developed.

In Advantage of using rice husk as raw material for precipitated silica are superior and cost effective compared to present technology of producing Silica from quartz. Starts from a raw material of little or no cost and value, which otherwise would cause environmental pollution. Processis energy efficient and also consumes much lower energy compared to an alternative process involving fusion of selected quality of sand.

Rice husk or paddy husk – an agricultural residue is available abundantly in rice producing countries. India alone produces approximately 12 million tons of rice husk annually. Rice husk is generally not advocated as cattle feed because of low cellulose and other sugar contents in it. Furfural and rice bran oil are extracted from rice husk. Rice husk is used by industries as fuel in boilers and for power generation. Rick husk has a high ash content varying from 18-20%. Silica is the major constituent of rice husk ash. High silica (SiO2) content in rice husk ash is economically feasible to extract silica, which has wide market.
Rice is the seed of the monocot plants Oryza sativa (Asian rice) or Oryza glaberrima (African rice). It is normally grown as an annual plant, although in tropical areas it can survive as a perennial and can produce aratoon crop for up to 30 years. Since a large portion of maize crops are grown for purposes other than human consumption, rice is the most important grain with regard to human nutrition and caloric intake, providing more than one fifth of the calories consumed worldwide by the human species. The rice plant can grow to 1–1.8 m (3.3–5.9 ft) tall, occasionally more depending on the variety and soil fertility. It has long, slender leaves 50–100 cm (20–39 in) long and 2–2.5 cm (0.79–0.98 in) broad. The small wind-pollinated flowers are produced in a branched arching to pendulous inflorescence 30–50 cm (12–20 in) long. The edible seed is a grain (caryopsis) 5–12 mm (0.20–0.47 in) long and 2–3 mm (0.079–0.12 in) thick. Rice is the staple food of over half the world’s population. It is the predominant dietary energy source for 17 countries in Asia and the Pacific, 9 countries in North and South America and 8 countries in Africa. Rice provides 20% of the world’s dietary energy supply, while wheat supplies 19% and maize 5%.

Rice Husk

Rice husks are the hard protecting covering of grains of rice. Rice hulls are the coating for the seeds, or grains, of the rice plant. To protect the seed during the growing season, the hull forms from hard materials, including opaline silica and lignin.

One practice, started in the seventeenth century, to separate the rice from hulls, it to put the whole rice into a pan and throw it into the air while the wind blows. The hulls are blown away while the rice fell back into the pan. This happens because the hull isn’t nearly as dense as the rice. These steps are known as winnowing. Later pestles and a simple machine called a rice pounder were developed to remove hulls. In 1885 the modern rice hulling machine was invented in Brazil. During the milling processes, the hulls are removed from the raw grain to reveal whole brown rice, which may then sometimes be milled further to remove the bran layer, resulting in white rice. Rice husk is a by-product of the rice milling industry. It is a unique crop residue with uniform size and high content of ash (14–25%). The silica content of the rice husk ash (RHA) can be as high as 90–98% . This husk can be used as a fertilizer in agriculture or as an additive for cement and concrete fabrication. Due to its high silicon content, rice husk has become a source for preparation of elementary silicon and a number of silicon compounds especially silica silicon carbide and silicon nitride . India is a major producer of rice and finding ways to put the husk to use is imminent. The high silica content in the rice husk ash has attracted interest in discovering ways to use it commercially. Although silica occurs as a component of cells or cell walls in virtually all arial parts of the rice plant, it is most abundant in the husk. Owing to their small diameter, many technological applications, such as thermal insulators, composite fillers, etc., use ultrafine silica powders .We have investigated the possibility of producing high purity silicon from rice husk by purifying the rice husk silica followed by pelletizing and reduction in a modified electric arc furnace. The pelletizing was carried using carbon black as a reductant and sucrose as a binder.