Published on 09/02/2014
First Update 16/03/2017
Second Update 23/05/2018
Rice Husk and Silica
Rice husk is one of the highest produced agricultural process residues. Almost 1.2×108 tons of rice husk is generated across the globe. This huge amount of waste is an environmental nuisance. Rice husk is traditionally utilized as a fertilizer additive, as fuel and in landfill or paving applications.
India produces about 25 million tonnes of rice husk each year. When burnt, this rise husk generates ash which is a great environmental problem damaging our surroundings and land.
The rice plant absorbs silica in the form of silicic acid from the soil during its life cycle. This silica accumulates round the cellulose micro compartments and naturally exists in the form of nano particles, particles which do not permit microorganisms to enter the seed but which allow air and other gases to go in and come out. Rice husk is a natural reservoir for nano structured silica and its derivatives. These silica nano particles can also be processed directly from rice husks. Silica obtained in this way is relatively pure and accounts for as much as 20% of the dry weight of the Rice husk. Rice husk ash also contains 60% to 80% amorphous silica which has wide spread applications in various industry. Carbon and sodium carbonate may also be gainfully extracted.
Extraction plants for batch processing of Silica are indigenously available in India. It is estimated that one tonne of paddy milled, generates 200 kg. of husk, 200 kg husk gives 50 kg Ash and the ash contains 60 to 80% silica. This would mean that about 40 kg of silica could be extracted per tonne of paddy. There are other estimates that suggest that for every 1000 Kg. of paddy milled, about 250 kg (28%) of husk is produced and when this husk is burnt in the boilers, about 70 kg (25%) of rice husk ash is generated that contains about 60 Kg silica (85-90%).
Ash generated from the combustion of rice husk for cogeneration can be reused to extract silica. Being of biological origin, this silica is non toxic and can be used in the rubber re-enforcement (Tyre Industry) , Plastic re-enforcement, agriculture (reportedly in animal food also), food, health care and cosmetics, catalysts , in pulp and paper processing and ,as a cleansing agent in tooth pastes and as an anticaking agent in the Food industry. Elemental silica has a wide range of traditional applications in metallurgy, synthesis of silicone and in the semi conductor industry. Nanostructure silicon, because of its unique properties and small size finds wide application in nano electronics, photonics, biotechnology, energy harvesting and energy storage. Silicon has a large theoretical specific charge capacity (4200 mA gm./l), almost 10 times more than the standard graphite electrodes. Silicon anodes are however highly prone to fractures. Reducing the size of silicon to the nano scale can prevent fractures. This makes nano silicon a very interesting anode material for the next generation of Li-ion batteries for electric vehicles and portable electronics.
A process for silica precipitation developed at the CGPL, Indian Institute of Science Bangalore involves digestion of the rice husk with caustic soda. This produces a solution of sodium silicate which is filtered and the filtrate taken for precipitation.
Precipitation involves passing carbon dioxide through the silicate solution and filtering the precipitated silica. The filtrate containing sodium carbonate is taken for regeneration where it is reacted with Calcium Hydroxide in the regenerator. The calcium carbonate thus formed is again recovered as solid and the sodium hydroxide used in the precipitation.
The precipitated silica is a white amorphous powder, fluffy with a purity of >98%, surface area of 50 to 300 m2/gm, bulk density of 120-400 g/litre ,and loss on ignition 3.0 to 6.0%, pH of 5% slurry, 6.3+-0.5 and heat loss of 4.0-7.0%. The technology at an estimated ash requirement of 1.6 tons/ton of silica and an estimated plant cost of Rs. 25 million/ton/day was estimated to produce silica at a cost of Rs. 30/kg as against a selling cost of Rs. 50/Kg.
Another technology employs specially developed thermal reactors under controlled temperatures. This can give amorphous silica from 90 to 98% concentration. The process does not use any chemicals. The traditional Carbothermic reduction of silicon dioxide requires a very high temperature of 20000C for reduction. This is higher than the melting point (14100C) of silicon.
Rice husk can also be directly converted into Si Nano particles which have demonstrated a high degree of performance as Li-ion battery anodes. Raw rice husk is converted first to nano SiO2 by thermally decomposing the organic matter. This is followed by a magnesio thermic reduction to produce nano Silicon. The essence of the process lies in managing the strongly exothermic reactions in such a way that the nano structure property of silicon in rice husk is preserved.Rice husk is burnt in air (1100oC) to give bulk SiO2. HCL leaching to remove metal ions followed by burning at moderate temperatures (7000C) produces amorphous SiO2 nano particles with an average diameter of 80 nm. These nano particles are loosely interconnected and macroscopically preserved in the shape of the Rice Husk. This nano SiO2 is then reduced to give nano Si. The use of magnesium as a reducing agent allows the reduction at a temperature of 6500C, facilitating the possibility of nano structure preservation. The obtained Si nano particles have sizes around 22 nm., a surface area of about 117m2/gm. and are porous. This gives them a superior performance as a Li-ion battery anode.
High performance nano silicon anodes are currently manufactured by high temperature, high energy pyrolysis of silane/poliysilane/halosilane precursors or laser ablation of bulk silicon. These are expensive processing routes. In order to be comparable to the cost of graphite electrodes, less expensive methods need to be evolved. The extracted pure silicon from rice husk can be coated in carbon and used in anodes in lithium coin cells. These anodes have been observed to be more efficient from anodes made with silicon alloys. The interconnected porous structure of the silicon, enabled the formation of solid electrolyte interfaces which resulted in the anodes made from rice husk silicon to have a high coulomb efficiency and excellent discharge capacity retention. Anodes made from silicon alloys suffer from capacity fading because of their high volume change which can reach 300%, causes the alloys to fracture and unstable SEIS to form.
Extraction of amorphous silica from Corn Cob Ash has also been attempted. Silica gel has been produced by dissolving Corn Cob Ash with Alkali Solution to form sodium silicate solution. The pH was lowered to 7.0 by adding HCL to form silica aquagel followed by drying to form silica Xerogel. The silica yield was 52.32% while the moisture content was 2.89%.
Cement has been successfully produced production of cement from rice husk from the agricultural waste and rice husk ash. For this production, rice husk was pre-carbonized in a pilot-plant and was further decarbonized in an electric furnace to produce rice husk ash. 24.5% rice husk ash was mixed with other raw materials (sourced locally) for producing white portland cement and the cement produced was used to make a concrete slab. The formulated cement slab and commercial cement slab were tested for their physical characteristics and chemical composition. The results of tests confirmed that produced cement was of similar standard to commercial cement. Based on the results, the has been recommended for developing countries since it would help reduce problems of rice husks as farm wastes.