Stainless Steel Desulphurisation
Stainless steel production involves careful control of the chemical reactions in each furnace, with specific conditions tailored to the removal of phosphorus (P), sulphur (S), and carbon (C), while also ensuring the recovery of chromium (Cr) and avoiding chromium losses.
Phosphorus and Sulphur require different conditions for removal: phosphorus needs oxidizing conditions, whereas sulphur needs reducing conditions.
In the Electric Arc Furnace (EAF), oxidizing conditions make phosphorus removal rare due to chromium oxidation risks, so low-phosphorus scrap is used, with any reduction done early before Cr addition. Sulphur removal is inefficient, though CaO-rich slag allows partial desulphurisation.
The Argon Oxygen Decarburization (AOD) Converter uses initial oxidizing conditions for carbon removal (risking Cr2O3 formation), then reducing agents (Si or C) recover chromium and enable desulphurisation to <0.01% sulphur with high-basicity slag and gas stirring.
The Ladle Furnace (LF) provides further reducing conditions for effective sulphur removal, composition fine-tuning, and temperature control.
- Dephosphorization: 4P (metal) + 5O2 + 6CaO (slag) → 2Ca3(PO4)2 (slag)
- Desulphurization: S (metal) + CaO (slag) → CaS (slag) + O (metal)
- Decarburisation: C + O → CO (gas)
- Chromium Recovery: Cr2O3 + 3Si → 2Cr + 3SiO2
Cr2O3 + 3C → 2Cr + 3CO
| Process | Conditions | P Removal | S Removal | Cr Control |
|---|---|---|---|---|
| EAF | Oxidizing | Rare | Partial | Risk of Cr Loss/oxidation |
| AOD | Initially Oxidizing (for Carbon removal) then Reducing |
- | Highly Effective | Cr Recovered (with Si/C) |
| LF | Reducing | - | Effective | Low Cr Oxidation Risk |