Carbothermic Reduction of Amorphous Silica Refined from Diatomaceous Earth

Abstract

Aimed at developing solar-grade Si (SOG-Si) resources, amorphous silica (AS) refined from diatomaceous earth was reduced carbothermically. The reactivity of quartz—typically crystalline silica—also was investigated for comparison. Preliminary experiments confirmed an intermediate phase of SiC during the carbothermic reaction. SiC was produced more easily by heating AS mixed with graphite within 2 hours at 1773 K in a resistance furnace, whereas quartz remained unreacted under the same condition. The AS mixed with SiC then was heated in an electrode impulse furnace. An Si peak was identified in the X-ray diffraction (XRD) pattern of the sample reacted within 30 seconds at 2273 K. Chemical analysis indicated that the mole ratio of reduced Si to initial SiO₂ increased with a heating time of 15–30 seconds. It almost reached a constant depending on the heating temperature. The initial stage may correspond to a significant reduction from SiO₂ to Si in the solid–solid or solid–gas reaction systems. The next stage probably is a slow vaporization of SiO(g). Once the reduced Si melts with SiO₂ at the high temperature, the melt partially covers the surface of SiO₂ to prevent contact with SiC. A better reactivity for refined AS is observed than for quartz.

Appendices

Appendix A. Thermodynamic Consideration

Figure A1 shows the triangular coordinate for the Si-C-O ternary system. Possible condensed phases are Si, SiO₂, SiC, and C, whereas the gaseous phase is considered to be the mixture of SiO(g) and CO(g) at temperatures greater than 1773 K.

Fig. A1

Triangular coordinate for the Si-C-O ternary system. Zones (a) and (b) correspond to SiO₂-SiC-C and SiO₂-SiC-Si system, respectively

The composition of SiO₂ and C mixed at any ratio is represented as the SiO₂-C line in Figure A1. As the reaction between SiO₂ and C proceeds with evolving gaseous SiO(g) and CO(g) at a high temperature, a point representing condensed-phase mixture composition leaves the SiO₂-C line and moves toward the SiO₂-SiC line or the SiC-C line depending on P _CO/P _SiO, where P _SiO and P _CO are the partial pressures of SiO(g) and CO(g), respectively. When SiO₂ and SiC are mixed, the composition of the mixture is located on the SiO₂-SiC line. The composition of the condensed phase moves toward an Si apex, whereas SiO₂ is reduced with SiC to form Si.

Figure A2 represents the temperature dependence of calculated P _SiO and P _CO for the following two ternary systems: SiO₂-SiC-C and SiO₂-SiC-Si.[18] If the mixture with a given ratio of SiO₂ and C is heated, then the point indicating the condensed phase composition on the SiO₂-C line in Figure A1 moves down the line connecting that point on the SiO₂-C line with the “CO” point on the C-O line toward the SiO₂-SiC or the SiC-C line. This shift occurs because P _CO in the SiO₂-SiC-C system is much higher than P _SiO and because gaseous CO vaporizes more significantly than SiO.

Fig. A2

Temperature dependence of P _CO (upper) and P _SiO (lower) for the (a) SiO₂-SiC-C system and (b) SiO₂-SiC-Si systems

However, P _SiO is higher than P _CO at a lower temperature in the SiO₂-SiC-Si system (Figure A2). In such a case, the mixture of SiO₂ and SiC phase evolves more SiO(g) than CO(g). The vector synthesized between the SiO and the CO removal rates should determine the reaction path. The “mixture composition” point on the SiO₂-SiC line may as well shift back along the line connecting that point with the “SiO” point on the SiO₂-Si line. Thus, it may not enter the SiO₂-SiC-Si triangle. This finding makes it difficult for Si to be formed. Above 2073 K, however, p_CO starts to exceed p_SiO in the SiO₂-SiC-Si system (Figure A3) The mixture of the SiO₂ and the SiC phase evolves more CO, and the “mixture” point on SiO₂-SiC line approaches the Si-SiC line. Thus, it becomes efficient to heat the mixture of SiO₂ and SiC at high temperatures exceeding 2073 K to generate Si in laboratory-scale experiments.

Fig. A3

Temperature dependence of P _SiO/P _CO for the (a) SiO₂-SiC-C and (b) the SiO₂-SiC-Si systems

Appendix B

Such a short-duration phenomenon accompanying a relatively slow transport or diffusion process in melt may be beyond the thermodynamic consideration. Generally speaking, the diffusion rate of chemical species in melt is not so fast that uniform composition is formed within 10 to 100 seconds without convection mixing. It is contrasted with an apparent equilibrium reduction reaction across a long period between SiO₂ and C powder at 1773 K inside a resistance furnace.