Ferrosilicon (FeSi)
title: "Ferrosilicon (FeSi)" description: "Ferrosilicon grades FeSi75 and FeSi65, their use as steel deoxidizers and in magnesium production, and how ferrosilicon differs from metallurgical silicon." section: "upstream"
What Is Ferrosilicon?
Ferrosilicon is an iron-silicon alloy produced in electric arc furnaces by reducing silica with coke in the presence of iron scrap or iron ore. Unlike metallurgical silicon (>98% Si), ferrosilicon contains 15–90% silicon with the balance being iron, and is engineered for metallurgical — not chemical — end uses.
The reaction: SiO₂ + C + Fe → FeSi + CO↑ (simplified)
China produces approximately 60–65% of global ferrosilicon output, with significant additional capacity in Russia, Norway, Iceland, and Brazil.
Main Grades
| Grade | Si content (%) | Fe (%) | Typical applications |
|---|---|---|---|
| FeSi75 | 72–80 | balance | Steel deoxidation, inoculation; most traded grade |
| FeSi65 | 63–68 | balance | Steel deoxidation, cast iron inoculant |
| FeSi45 | 41–47 | balance | Steel alloying, inoculant for cast iron |
| FeSi25 | 23–30 | balance | Steelmaking; lower silicon addition |
| Micro-Si / FeSi90 | 88–92 | balance | Magnesium production (Pidgeon process) |
FeSi75 is the global benchmark grade for trading and pricing.
Key Applications
Steel deoxidation is the largest use. Liquid steel in the converter or ladle contains dissolved oxygen that causes porosity and brittleness in the solidified product. Adding ferrosilicon forms SiO₂ inclusions that float to the slag, producing cleaner, stronger steel. Inoculant addition rates are typically 0.5–3 kg FeSi75 per tonne of steel.
Magnesium production (Pidgeon process) consumes FeSi90 as the reducing agent: dolomite (CaMg(CO₃)₂) calcined to MgO + CaO, then reduced by ferrosilicon in a retort at 1200°C under vacuum to yield metallic magnesium vapour. This is how nearly all of China's magnesium metal is produced.
Foundry inoculation — adding FeSi65 to molten cast iron at tapping controls graphite morphology (flake → spheroidal), dramatically improving ductility and tensile strength.
Distinction from Metallurgical Silicon
| Feature | Ferrosilicon (FeSi75) | Metallurgical Silicon (3303) |
|---|---|---|
| Si content | ~75% | ~99% |
| Fe content | ~25% | <0.30% |
| Primary use | Metallurgical (steelmaking, Mg) | Chemical (silicones, polysilicon) |
| Price driver | Steel/metal markets | Silicone/PV demand |
Ferrosilicon and metallurgical silicon share quartz and carbon as inputs but diverge sharply in purity, price dynamics, and downstream industries. They are not interchangeable.