Rice Hulls as a Raw Material for Producing Silicon

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 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.