Effect of Different SiO2 Polymorphs on the Reaction Between SiO2 and SiC in Si Production

Marit Buhaug Folstad,Halvard Tveit,Eli Ringdalen,Merete Tangstad

doi:10.1007/s11663-020-02053-x

Abstract

This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate.

Highlights

METALLURGICAL silicon is produced industrially by carbothermic reduction of SiO2 in a submerged arc furnace
As the rate of phase transformation is slow and increases with increasing temperature, it may be that the time interval at the highest temperatures is not great enough to give significantly different SiO2 phase compositions
The experiments conducted in this study showed no significant difference in the reaction rate between SiO2 and Silicon carbide (SiC) in Reaction [5] regarding the effect of quartz, amorphous silica or cristobalite

Summary

INTRODUCTION

METALLURGICAL silicon is produced industrially by carbothermic reduction of SiO2 in a submerged arc furnace. One of the main reactions of the SiO2 in the furnace is with SiC, and several researchers have studied the reaction rate of Reaction [5] at different temperature intervals.[2,3,4,5,6] In addition, the effects of the gas composition and stoichiometry of the reaction are studied, and reaction mechanisms have been proposed. Pure silica melts at around 1720 °C, but for industrial quartz sources exposed to conditions as in industrial furnaces, found softening and melting temperatures are between 1600 °C and 1800 °C.[8] The reaction between SiO2 and SiC may occur with various polymorphs and different solid, softened and melted stages. Different heating rates gives different time intervals at the transformation temperatures and were expected to give significantly different amounts of the different SiO2 polymorphs Amorphous silica forms both from quartz and softened/melted b-cristobalite. Tridymite was not found in any of the samples analyzed in this study, and it is believed that tridymite needs specific

XRD Results

Effect of Different Heating Rates on Phase Transformations in SiO2

Reduction Reaction Between SiO2 and SiC

Effect of SiO2 Polymorphs

Effect of Temperature

CONCLUSION