Electric Submerged Arc Furnaces

Submerged Arc Furnaces (SAFs) are high-power electric furnaces where graphite electrodes are submerged in a conductive slag/charge, creating intense arcs for chemical reduction of ores at extreme temperatures. Used primarily for producing:

• Metallurgical-grade silicon (~96%+) for PV solar cells and electronics.
• Ferroalloys — ferrosilicon, ferromanganese, ferrochrome, ferroaluminum.
• Phosphorus (using the Wöhler process).

Process Flow

1. Charge preparation: Ores, carbon reductants (coke, coal, charcoal), and fluxes (e.g., silica, limestone) are crushed, mixed, and continuously fed.
2. Heating: Submerged electric arcs between electrodes heat the slag to 2,000–2,200°C, driving carbothermal reduction.
   • For silicon: SiO2 + C -> Si + CO (off-gases may oxidize producing SO2 and SiO2 particulates)
   • For phosphorus: 15 C + 9 SiO2 + apatite -> P2 + Ca-silicate slag + CO
3. Continuous tapping: Molten metal (Si, ferroalloy, P) and slag are tapped off separately.

Emissions & Efficiency Impact

• Direct CO2 from reductants: ~4.7–5 t CO2 per ton of silicon produced.
• Total life-cycle emissions: ~13–14 t CO2e/t Si including extraction and processing.
• Byproduct gases (CO/SiO) require off-gas management due to particulate emissions and NO from combustion.

Supply Chain & Deployment Challenges

• Heavy electrical infrastructure: Requires tens to hundreds of MVA capacity plus capable grid or dedicated power plants.
• Electrode & refractory supply: Graphite electrodes, silica/magnesia refractories must sustain extreme thermal, chemical, and electrical stress.
• Raw material prep: Grinding, flux mixing, and continuous feed systems are capital-intensive.
• Environmental controls: Capture technology for SiO/CO and particulate emissions is essential.