Phase transformations in quartz used in silicon and ferrosilicon

Abstract

Different quartz types are used as raw material for silicon and ferrosilicon production. Upon heating, the quartz will experience phase transformations to other silica polymorphs. In this study the phase transformation from quartz to cristobalite, via an amorphous intermediate state, have been investigated in four different quartz types. Samples of approximately 10g and 100g were heated to 1600 °C and 1700 °C. After cooling, the amount of quartz, cristobalite and amorphous phase were measured using X-ray diffraction. Type F showed the strongest ability to transform from quartz to cristobalite, while type A had the slowest transformation. A model of the transformations was made. The rate constant, k1, for the transformation from quartz to amorphous phase varied between 0.002h-1 and 0.184h-1 at 1600 °C. The activation energy for the transformations were different for the various quartz types. Type A had an activation energy of 600kJ/mol, type F had 500kJ/mol and type D and G had around and below 200kJ/mol. The reasons for the different abilities to transform are discussed, and it is suspected that properties decreasing the crystallinity will increase the ability to transform.