Silicon Purification: Purity, Energy and Environmental Trade-Offs Across Five Technological Routes

Global polycrystalline silicon output exceeds one million tonnes per year and is expanding at roughly 14% annually. Meeting demand across purity grades – from metallurgical (MG-Si, 2N–3N) to electronic (EG-Si, 11N) – requires a coordinated portfolio of purification technologies with distinct selectivity. This review systematically surveys seventeen industrial and emerging purification techniques organized into five methodological classes: metallurgical (carbothermic reduction, slag refining, gas blowing, acid leaching, vacuum evaporation, directional solidification), chemical (Siemens chlorosilane, Union Carbide silane, Ethyl fluorosilane), physical (zone refining, electron beam melting, plasma refining), electrochemical (molten salt electrolysis, FFC Cambridge, molten oxide electrolysis, three-layer electrorefining), and crystallization (Czochralski). Each method is assessed in terms of physico-chemical mechanism, achievable purity, specific energy demand, and carbon footprint, with emphasis on 2023–2025 literature. The Siemens process retains 80–85% of the polysilicon market but carries an energy demand of 65–160 kWh/kg and a carbon footprint reaching 57 t CO2 per t Si. Silane-based fluidized-bed reactors offer a credible near-term alternative at 30–50 kWh/kg, while electrochemical routes demonstrate laboratory-scale demands of 2–10 kWh/kg with near-zero carbon footprint. A hybrid metallurgical-electrochemical roadmap emerges as the most promising trajectory for sustainable solar- and electronic-grade silicon production.

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