Coproduction of silicon nitride & oxynitride whiskers and precipitated silica from industrial rice husk ash

Abstract

Rice industrialization generates large amounts of rice husk, and when this husk is burned, substantial volumes of ash are produced as a result. The lack of a sustainable application for these ashes constitutes a serious environmental liability. Employing rice husk ash as a starting material to manufacture higher-value products fulfills both environmental and economical purposes. Particularly, the presence of silica and carbon in intimate contact makes rice husk ash an attractive precursor for carbothermal reduction and nitridation processes. In the present work, a pre-treatment was applied to a locally sourced industrial rice husk ash, aiming to render not only a more appropriate starting material for carbothermal reduction and nitridation, but also precipitated silica as a by-product. This process involved a simple alkaline digestion step followed by filtering and acid precipitation of the solubilized silica. For carbothermal reduction and nitridation, the treated ash was introduced in graphite crucibles with graphite lids and held at soaking temperatures between 1200 °C and 1400 °C under a nitrogen atmosphere for 3 h. The pretreatment imposed on the ash permitted the adjustment of the carbon-to-silica ratio, producing a substantial improvement in the carbothermal reduction and nitridation results, and enabling the simultaneous obtention of precipitated silica. α- and β- silicon nitride, and silicon oxynitride whiskers were obtained through carbothermal reduction and nitridation of the treated rice husk ash. The whiskers obtained had lengths in the millimeter range with cross-sections ranging from about 100 nm up to 500 nm.