Utilization of silica in rice hulls as raw materials for silicon semiconductors

Abstract

Highly pure silicon and silicon compounds are required in high technology products such as semiconductors and solar cells. Rice hulls consist of 71 to 87wt% organic components such as cellulose and 13 to 29wt% inorganic components. In rice hulls, 87 to 97wt% of the inorganic components are silica (SiO₂). Rice plants absorb water-soluble siliceous ions via the roots. These ions are transported to stems, leaves and rice hulls by sap flow. In rice hulls, siliceous ions accumulate at the cuticle outside of the epidermis. The production of pure silicon compound (SiCl₄) and of silicon metal as the starting material for purification from rice hull ash is proposed to utilize rice hull SiO₂ in high technology industries. In one process, SiO₂ in rice hull ash reacts with Cl₂ on the presence of C and is converted to SiCl₄. SiO₂ + 2Cl₂ + 2C → SiCl₄ + 2CO The SiCl₄ is purified by distillation. The production of SiCl₄ from rice hull ash is much more efficient than that from mineral SiO₂. In another process, SiO₂ in rice hull ash reacts with aluminum (Al) to synthesize Si and Al₂O₃.3SiO₂ + 4Al → 3Si + 2Al₂O₃ The reactivity of rice hull SiO₂ is higher than that of mineral SiO₂. Although rice hulls are now utilized for agricultural materials and fuel, rice hull SiO₂ also has high potential for use as industrial raw materials.