The utilization of renewable resources is on the rise because of the increasing public awareness regarding sustainable development and environment conservation. In the wake of problems associated with fossil fuel consumption, biomass has been recognized as one of the most important alternatives. Rice husk obtained from the rice industry is one of the most abundant agricultural biomass generated in Korea. Rice husk accounts for about 20% of the total weight of rice and has low protein and available carbohydrates contents, but is rich in silica. Many studies have been carried out to evaluate the composition of rice husk and facilitate its utilization as a useful source for silica, bioenergy, and so on.

In this study, an alkali based biorefinery was used to optimize the operating conditions that allow for maximum silica extraction while retaining most organics in the solid phase. Two different types of biorefinery processes for the rice husk were designed: the digestion process and the low temperature deeping process. The integrated manufacturing methods of the organic and inorganic materials originating from each of the processes were investigated in depth.

The rice husk fibers obtained by the digestion process can be used as an alternative to wood pulp fibers and a high concentration black liquid that yielded silica and lignin-silica hybrid materials. The yield and extraction efficiencies of the rice husk fiber and the high concentration black liquid produced by the digestion process were evaluated. The organic matter content in the high concentration black liquid was 9-15%. The rice husks were delignified, yielding about 1% residual ash. The rice husk fiber, when used in combination with UBKP, served as an alternative pulp with improved bulk properties and dehydration rate; the high concentration black liquid was converted into a lignin-silica hybrid through sol-gelation under acidic conditions. The lignin with low thermal stability was combined with silica to produce lignin with improved thermochemical stability.

The production yield and efficiency of the alkali-treated rice hull and low concentration black liquid obtained through the low temperature deeping process were also evaluated. The concentration of black liquid was found to be 4-7%, with 60-70% of its composition being inorganic material. Considering the yield of rice husk fiber and low concentration black liquor, it was adequately optimized under the conditions of 20% NaOH addition, liquid ratio 5:1, and deeping duration of 18 hours. In addition, chlorine dioxide pre-treatment of the low temperature deeping process improved its extraction efficiency from 35% to 88%.

The liquid extract yielded by the low temperature deeping process showed higher inorganic content with relatively lower impurity. The extract was processed to high purity silica by sequentially applying sol-gelation, aging, washing, and heat treatment.By optimizing the conditions for each of the purification processes, high-quality silica was successfully produced with whiteness 90 and a purity of ≥95%. The black liquid obtained through the low temperature deeping process is considered suitable for producing high purity silica.

The alkali-treated rice husk has been modified to maintain the shape of the husk, resulting in increased water absorption; washing and heat treatment were carried out with the application of sulfuric acid and EDTA to effectively remove the remaining alkali metal salt. Alkali-treated rice husk are considered an easy-to-use soil material because of the similarities in their particle shapes and high moisture characteristics.

The biorefinery process could be a viable method for fractionating the lignocellulosicm biomass into the silica and lignin-silica hybrid. The rice husk pulp and the alkali treated rice husk cab be used for the preparation of high-value products. The major appeal of the proposed process is the integrated utilization of component fractions by alkali treatment, so that the manufactured product can be recovered with a valid overall yield. In particular, the low temperature deeping method is performed utilizing water as a solvent in all the steps and operated at atmospheric pressure. From an industrial scale-up perspective, this could avert the use of expensive and energy consuming machineries and provide a simplified safety management. This study demonstrated a possibility for the commercialization of the organic and inorganic materials yielded from the rice husk biomass to produce high value-added products.