New methods to prepare high-purity silica

Abstract

Two methods are described to prepare high-purity silica to be employed as a low-cost starting material for solar-grade silicon production. In the first process, abundantly available quartz sand is purified by fusing it with glass forming oxides to form a melt from which thin glass fibres are drawn. Subsequent treatment of the fibres with hot HCl leads to an ion exchange whereby all non-siliceous oxides are removed from the glass network leaving an insoluble matrix of high-purity silica. The second method employs sodium hexafluosilicate (Na₂SiF₆), prepared from fluosilic acid, a by-product of the fertilizer industry. The Na₂SiF₆ is thermally decomposed to yield gaseous SiF₄ which is subsequently hydrolysed in an ammonia solution. With both methods silica having a purity of >99.99% was obtained.