Production and characterization of silica materials from rice husk ash by different combustion processes

Rice husk is a type of waste widely generated in Brazil and worldwide. It is often applied as biomass for energy generation. Rice husk ash (RHA), a solid waste generated in the combustion process for energy generation, is comprised primarily of silica and carbon. In this study, two types of RHAs, obtained by different industrial combustion processes (moving grate furnace and fluidized bed), were subjected to chemical or thermal treatments in different atmospheric flows (Ar/H2, N2, O2 gases, compressed air, and vacuum). The particle size, specific weight, surface area, electrical properties, crystalline structure, and chemical composition were analyzed by different characterization techniques. The results indicated that thermal treatments generate powders with a high silica content; amorphous structures were found in some silica materials (those obtained by chemical treatment and thermal treatments in Ar/H2 flow, compressed air, and vacuum), and others were crystalline in the cristobalite phase (mainly samples treated in the O2 and N2 atmospheric flows). The yield of each process varied, obtaining silica concentrations above 99.3%. Samples with significant analytical carbon content showed higher values of electrical conductivity; this result is probably due to the incorporation of carbon atoms into the cristobalite lattice. Samples treated with O2 atmospheric flows showed resistivity values of 1008 Ω.cm with very low carbon levels. This work concluded that carbonaceous silica materials (which may be new materials in the Si-C-O system) may be obtained during the thermal treatments of rice husk ash.