Environmentally Benign Reuse of Agricultural Wastes to Prepare High-Purity Silica from Rice Husks

Rice husk, one of main agricultural wastes in South-east Asia countries, is well known as renewable resource to produce both energy and high-purity silica (SiO2), because it consists of 70% organics and 20% amorphous SiO2. The conventional process of recovering silica originating from rice husks requires a leaching treatment by strong acid solutions, such as sulfuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3), to remove metallic impurities from the rice husks before combustion. The use of these acid solutions, however, is hazardous to the environment and human life, and difficult for process-cost-saving in mass-production due to expensive and complicated disposal systems. In this study, an environmentally benign, harmless to human as well as economical process to produce high-purity amorphous SiO2 powder originating from rice husks has been established without using strong acids. From the viewpoint of a harmless influence on human body, carboxylic acids such as a citric acid or oxalic acid are employed in the acid leaching treatment of rice husks. In this alkali impurities are effectively removed from husks via a chelate reaction between carboxyl (-COOH) and alkali elements. In the experiment, first of all, it was shown that the carboxylic acid leaching was effective in promoting the hydrolysis of organics by DTA and GCMS. From experimental results on the optimization of concentration and temperature of the carboxylic acid solution by DTA, XRF and ICP analysis, it was found that a concentration of 5% or more was enough to both increase SiO2 purity over 99% and remarkably reduce alkali metal impurities in the rice husk ashes. Amorphous SiO2 powder originating from rice husks has a porous structure, so that it was also easy to control its particle size by dry milling or a grinding process.