Process Parameters Optimization in Preparing High-Purity Amorphous Silica Originated from Rice Husks

Abstract

Rice husk, one of the main agricultural disposals in Japan, China, India, and South-east Asian countries, is employed as useful renewable resources to produce energy and high-purity silica (SiO₂), because it consists of 70% organics and about 20% SiO₂. The latter, SiO₂ is obtained by burning rice husks in air. The purity of rice husk ashes, however, is reduced by the remains of both carbon contents originated in organics and original metallic impurities such as K, Na, Ca, P, Al, etc. In the previous works, the utilization of the leaching methods by strong acids with a high concentration for both the remove the metallic impurities and the fragmentation of silica structural network was suggested. As their results, the thermal resolution mechanism of organics was discussed by using TGA showing the mass loss behavior during heating. The objective in this study is to clarify the mechanism of the removal of organic components when applying a dilute H₂SO₄ leaching treatment with 1∼5% solution concentration. In particular, from a viewpoint of the investigation on molecule structural changes of organics, GCMS (Gas Chromatograph Mass Spectrometer) analysis is carried out on acid-leached rice husks. When using only 1% concentration H₂SO₄ solution leaching, a change of cellulose of polysaccharides to levoglucosan via glucose of monosaccharide by hydrolysis and condensation reactions occurred. This reaction accompanied with dehydration always served en endothermic heat, which was found in DTA profile of acid leached rice husk materials. It was also clarified that such a low concentration H₂SO₄ solution was enough to remove alkali metal elements contained in husks. By optimizing the combustion temperature of rice husks after acid leaching, completely amorphous silica ashes, composing of 99.3% SiO₂ and 0.04% carbon, were obtained.