Rice Hulls as a Raw Material for Producing Silicon

Abstract

Rice hulls are a relatively high‐volume, low‐cost by‐product commodity which contains the two basic components needed to produce silicon: silica and carbon. Impurity analyses have indicated that rice hulls from various sources are compositionally similar and that they have low concentrations (10–20 ppmw) of aluminum and iron, the major impurities in conventional raw materials used to prepare metallurgical silicon. The levels of the major impurities (Ca, K, Mg, and Mn) in rice hulls can be reduced by about a factor of 100 to around 20 ppmw by hot hydrochloric acid leaching. The doping impurities boron and phosphorus, important in silicon intended for solar cells, were less affected by acid leaching. Their concentrations were found to be 1 and 40 ppmw, respectively, in leached hulls. Coking of unpurified rice hulls produced a material with a C:SiO2 ratio of about 4:1. An increase in concentrations of some impurities was noted since coking results in about a 67% loss in sample weight. The coked hulls were extruded with sucrose as a binder to produce 5 mm diam pellets with an average bulk density of about 800 g/l. The pellets gave favorable changes in bulk density and compression strength with increasing percentage sucrose. The abrasiveness of silica caused die wear in the extruder, resulting in roughly a doubling of the original concentration of iron in the coked hulls. Coked rice hull pellets showed excellent reactivity for producing silicon. Laboratory tests employing the reaction of silicon monoxide with carbon in the pellet showed very high reactivities for both purified and unpurified materials. A test in a small arc furnace of unpurified material showed energy consumption per unit weight of silicon to be about 15%–30% lower than normal. Unpurified rice hulls were used for these tests. Although the silicon produced in the arc furnace test was contaminated, estimates of silicon purity attainable from coked hulls, unpurified and purified, indicate their potential as raw materials for the production of MG‐Si and solar‐grade silicon.

Process Parameters Optimization in Preparing High-Purity Amorphous Silica Originated from Rice Husks

“…17,18) The acid leaching treatment on rice husks before combustion was useful to obtain high purity silica, and also indicated available analytical methods to evaluate the removal of impurities were introduced by using thermogravimetric analysis and scanning electron microscopy observation on acid-leached rice husks. As results in the previous studies, [19][20][21][22][23] strong acid leaching treatment can remove the metallic ingredients from husks. In particular, the use of H 2 SO 4 solutions with 10-20% concentration causes charring of the organics (cellulose, hemi-cellulose and lignin) parts which separates out from the silica structural network of rice husks.…”

Improved Optical Degradation Characteristics of Eu Complex Encapsulated by High-Pressure Annealing

“…Infrared spectroscopy provided information about the bonds present in the chemical structure of the extracted nSiO 2 and plasma-treated nSiO 2 . The spectra are shown in Fig.…”

“…1,2) Leaching of rice husks using various acid media at temperatures ranging from 100 to 120°C and at various time intervals has been conducted to remove metallic impurities and to produce silica with a large surface area. 1,3) Ultrafine SiO 2 has a host of applications in the electronics, ceramic, and polymer material industries.…”

“…Furthermore, modified nSiO 2 also improves the dispersion of the filler in the matrix, which plays a crucial role in determining the mechanical properties of the composite. Hydrophobicity can be increased on nSiO 2 surfaces by using modifying agents such as silanes [16][17][18] that react with the hydroxyl groups.…”

Solar Grade Silicon from Agricultural By-products

“…[1][2][3][4][5][6][7] The motivation for using RH and in our work, rice hull ash (RHA), comes from the fact that all plants (to some extent) extract SiO 2 from the soil. In particular, grasses including rice, wheat, oats and barley have high concentrations of SiO 2 in their hulls, husks or stalks.…”