Electroreduction, or electrodeoxidation, of pelleted SiO2 powder (left image) or its mixture with other metal oxide powders in molten CaCl2 produces pure Si powder (right image), or the respective silicon alloy powder. Being advantageous by simplicity and resulting in less CO2 emission, the electrochemical approach has an energy consumption that is below 13 kWh (kg of Si)−1.
The electrochemical reduction of solid SiO2 (quartz) to Si is studied in molten CaCl2 at 1173 K. Experimental observations are compared and agree well with a novel penetration model in relation with electrochemistry at the dynamic conductor|insulator|electrolyte three-phase interlines. The findings show that the reduction of a cylindrical quartz pellet at certain potentials is mainly determined by the diffusion of the O(2-) ions and also the ohmic polarisation in the reduction-generated porous silicon layer. The reduction rate increases with the overpotential to a maximum after which the process is retarded, most likely due to precipitation of CaO in the reaction region (cathodic passivation). Data are reported on the reduction rate, current efficiency and energy consumption during the electroreduction of quartz under potentiostatic conditions. These theoretical and experimental findings form the basis for an in-depth discussion on the optimisation of the electroreduction method for the production of silicon.
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