Rice, which provides a major source of food for billions of people, covers 1% of the surface of the earth. Rice husk (RH), the outer covering of paddy rice, is an important byproduct during the milling process, and residue ash is generated after the burning of RH that is called rice husk ash (RHA), which is a primary waste material in the agricultural industry. The main components in RH are lignin, cellulose, and hemicellulose, which are generally named lignocellulose. RH also contains ca. 15 to 28 wt% of silica. The high content of silica in RH presents opportunities for the preparation of value-added silicon-based materials. Since the 1970s, various silicon-based materials, including silica, silicon carbide, silicon nitride, silicon tetrachloride, zeolite, and silicates, have been successfully synthesized using RH as the silicon source. This field of research has been significantly advanced and expanded in the past decade spurred by the global attention on sustainable and renewable resources.
Top1. Introduction
Rice, which provides a major source of food for billions of people, covers 1% of the surface of the earth. Rice husk (RH), the outer covering of paddy rice, is an important by-product during the milling process and residue ash is generated after the burning of RH and the ash is called rice husk ash (RHA), which is a primary waste material in the agricultural industry (Azat et al., 2019).
The main components in RH are lignin, cellulose, and hemicellulose, which are generally named lignocellulose. RH also contains ca. 15 to 28 wt% of silica (Kerimkulova et al., 2019; Askaruly et al., 2019; Azat et al., 2019). The high content of silica in RH presents opportunities for the preparation of value-added silicon-based materials. Since the 1970s, various silicon based materials, including silica, silicon carbide, silicon nitride, silicon tetrachloride, zeolite, and silicates, have been successfully synthesized using RH as the silicon source (Sunb & Gong, 2001). This field of research has been significantly advanced and expanded in the past decade spurred by the global attention on sustainable and renewable resources.
Extraction of silica from rice husk is an emerging trend in the current research field. Large amount of rice husk (RH) are treated as waste and disposed off at the landfill site. These waste materials can also cause fire, which may lead to severe environmental pollutions. The airborne particles produced from dust may induce respiratory disease to human beings (Bock & Su, 1970). The burning of rice husk results in the formation of Rice Husk Ash (RHA) with major SiO2 content with 10 to 20% of carbon and traces of other organic components depending on the burning conditions, the furnace type, the rice variety, the climate and the geographical area (Brindley & Zussman, 1959).
Moreover, the commonly used silica precursor like tetraethyl orthosilicate (TEOS) is more expensive, and hence rice husk ash (RHA) and other waste sources having silica are used as an alternative (JB d'Espinose de la Caillerie et al., 1995; Mansurov et al., 2013). Acid leaching of the rice husk ash was carried out to remove soluble elemental impurities and hence it increases the purity of the silica content. The organic compounds in Rice Husk and other waste materials can be decomposed under Sintering conditions. The metallic impurities can be transferred to soluble ions by simple acid treatment (Azat et al., 2014; Tauanov et al., 2020). With the aforementioned method, sintering the sources at high temperature requires high energy and the utilization of acid especially the volatile hydrochloric acid not only corrupts the reaction equipment but also brings an extra financial cost and pollutes environment. Hence, sulfuric acid treatment was selected for acid treatment and the purification treatment was done in two steps. The first step is leaching with acid solution to remove the soluble metallic impurities and the second one is sintering the leached sample in order to reduce the total carbon content. The expected results of these two-unit processes are an overall increase in the purity of the silica content.