Synthesis of metallic silicon by reaction of silica in rice hulls with aluminum and its hydrochlorination

Abstract

Silica in rice hulls (rice hull silica) reacted with aluminum to synthesize silicon and alumina at the temperature of 900–1225 °C. Rice hull silica contained about 20 wt% in rice hulls is amorphous and has high surface area with fine particles. Reactivity of rice hull silica added potassium carbonate for aluminum was higher than rice hull silica, silica sand and artificial amorphous silica because the addition of potassium carbonate caused the melt of rice hull silica surface and improved the contact between rice hull silica and aluminum melt.Si in the product of the reaction of rice hull silica with aluminum at 1225 °C completely reacted with hydrogen chloride to form chlorosilanes without catalyst at 300 °C. The silicon and the unreacted aluminum existed as a eutectic alloy, Si7Al3, in the product. Since aluminum in the eutectic alloy first reacted with hydrogen chloride to sublimate as aluminum chloride, hydrogen chloride easily diffused in the eutectic alloy having empty aluminum sites, so that silicon in the eutectic alloy easily reacted with hydrogen chloride.