High-purification of amorphous silica originated from rice husks by combination of polysaccharide hydrolysis and metallic impurities removal

Rice husks and straw, containing 70–75 mass% organics and 15–20 mass% amorphous silica (SiO2), are representative non-eatable biomasses. After the use of the organic elements as fuels, the leavings including large amounts of silica can be used as industrial materials when their purity is 99 mass% or more. Hence, the reuse of high-purity silica will be significantly effective in reducing the total cost of biomass energy in using rice husks and straws. To attain high-purity silica from the remains of the rice husks after air combustion, the optimization of the process conditions of the citric acid leaching treatment and water rinsing process of rice husks were conducted to remove the metallic impurities such as Na, K, Ca, Mg, Fe, Cu, etc. from husks and promote the hydrolysis reaction of polysaccharides. When the citric acid solution with a concentration of 1 mass% or more was used, alkali metal oxides of Na2O and K2O were completely removed. GC–MS analysis showed the progress of the hydrolysis reaction of their hemi-cellulose of rice husks during the leaching process. Carbon content of ashes was drastically reduced to 0.02–0.04 mass% after combustion at 1073–1273 K, and high-purity amorphous silica with 99.5–99.77 mass% were produced from rice husks.

Research highlights▶ To attain high-purity silica from the remains of the rice husks after air combustion, the optimization of the process conditions of the citric acid leaching treatment and water rinsing process of rice husks were conducted to remove the metallic impurities such as Na, K, Ca, Mg, Fe, Cu, etc. from husks and promote the hydrolysis reaction of polysaccharides. ▶ In the use of the citric acid solution with a concentration of 1 mass% or more, the alkali metal oxides of Na2O and K2O were completely removed. This is because of the progress in the hydrolysis reaction of their hemi-cellulose of rice husks during the leaching process shown by GC–MS analysis. ▶ Carbon content of ashes was drastically reduced to 0.02–0.04 mass% after combustion at 1073–1273 K, and high-purity amorphous silica with 99.5–99.77 mass% were produced from rice husks.