Abstract
Rice husks are an important by-product of the rice milling process and are a major waste product of the agricultural industry. Rice husks contain nearly 20 mass% silica, which is present in hydrated amorphous form. They have now become a great source of raw biomass material for manufacturing value-added silicon composite products, including silicon carbide, silicon nitride, silicon tetrachloride, magnesium silicide, pure silicon, zeolite, fillers of rubber and plastic composites, cement, adsorbent, and support of heterogeneous catalysts. The controlled burning or thermal degradation of the rice husks in air or nitrogen leads to the production of white rice husk ash (WRHA) or black rice husk ash (BRHA), respectively.
The present review is an attempt to consolidate and critically analyze the research work carried out so far on the processing, properties, and application of rice husks and the products of its thermal degradation in various laboratories and also highlight some results on the processing and characterization of rice husk ashes (RHAs) and reactive silica obtained in the author’s laboratory. In this connection, the composition, structure, and morphology of the raw rice husks (RRHs) and the products obtained from its thermal degradation in an oxidative or inert atmosphere are described in detail. The controlled burning or pyrolysis of the RRHs in a fluidized-bed reactor is shown to be the most perspective method. The products obtained might successfully be used as fillers of polypropylene (PP) and tetrafluoroethylene-ethylene copolymer (TFE-E) composites, rubbers, and other polymer composites and to replace the expensive synthetic additive as Aerosil, for instance. The physicochemical and physicomechanical characteristics of the obtained composites are described. The RHA-polymer composites can lead to the futuristic “organic–inorganic hybrid materials” with specific properties. Due to the high pozzolanic activity, the rice husk silica also finds application in high strength concrete as a substitute for silica fume.
The abundance of waste from paddy milling industry as well as its interesting complex of behaviors are prerequisites for success in obtaining cheap and valuable products and stipulate new alternatives for its applications. The production of value added materials from rice husks not only facilitates utilization of an abundantly available agro waste but also reduces the environmental pollution and solves a serious ecological problem.