Converter slopping during a low-carbon blow
A 250 t top-blown converter is midway through a low-carbon heat with a 0.05% C target. The charge is 78% hot metal (4.2% C, 0.35% Mn, 0.06% P) and 22% scrap. Lime and dolomite are charged to target slag basicity CaO/SiO2 = 3.0. At 55% of the blow, oxygen flow is 52,000 Nm3/h and lance height is 1.45 m. The off-gas CO2/CO ratio has fallen and the lance is vibrating. A slag sample assays 24% FeO, 9% MgO, and basicity 2.2; bath temperature is 1610 C against a target turndown temperature of 1665 C. Slopping begins, and the operator proposes to lower the lance further and add 2 t of lime immediately. The vessel freeboard is limited and the gas-cleaning system is near its differential-pressure limit. The heat must meet 0.030% max P and 0.025% max S at turndown.
1. Diagnose the most likely slopping mechanism and assess whether lowering the lance and adding 2 t of lime is an appropriate immediate response. Give the sequence you would follow in the next few minutes.
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The combination of basicity 2.2, 24% FeO, a falling CO2/CO ratio and lance vibration points to a foamy, gas-rich slag: decarburization is producing CO faster than the slag can drain, and the lime is not fully dissolved. Lowering the lance concentrates oxygen on a small bath area, raises local carbon oxidation and CO generation, and normally makes foaming worse rather than better. Adding 2 t of lime in one lot can chill and thicken the slag and trigger a transient eruption, especially with the gas-cleaning system already near its pressure limit.
Immediate sequence: raise the lance by 200–300 mm, reduce oxygen flow by 10–15%, and increase bottom stirring if the vessel has it. Add lime or dolomite in 300–500 kg increments rather than one large lot, with a short mix before the next addition. Take a bath temperature and a slag sample, check off-gas trends and lance vibration, and do not attempt to recover by over-blowing. Resume the blow only when foam height is falling and the off-gas ratio is stable. The heat still needs a dephosphorization plan, but preventing a slop comes first.
2. The heat must still meet 0.030% P at turndown. Explain how the current slag and temperature affect dephosphorization and what you would do to recover the heat without causing another slop.
Read the worked answer
Dephosphorization needs a basic, oxidizing slag and a relatively low temperature. At basicity 2.2 the slag is too low to give a good phosphorus partition; 24% FeO is adequate, but bath temperature is already 1610 C and will rise further, which favors phosphorus returning to the metal. Simply blowing harder to raise FeO would worsen foaming and temperature.
Recovery should aim to raise basicity without a large temperature penalty. Add lime in controlled increments, target about 3.0–3.2 at turndown, and use scrap or other coolant if the heat is running hot enough to threaten the turndown temperature. Keep the lance high enough to avoid a second foam surge, and use bottom stirring to improve slag-metal mixing. Check phosphorus on a sublance or sample before turndown; if P is still above 0.030%, a short reblow with lime and coolant is preferable to tapping off-spec. Sulfur removal also needs basicity, but it prefers lower FeO, so target a balance rather than maximum FeO.
3. Recommend two changes for the next heat and state what data you would collect to verify that the changes work.
Read the worked answer
First, revise the flux charge using a lime mass balance rather than a fixed addition: account for scrap dilution, hot-metal silicon, and target basicity 3.2, and use smaller lime additions early so the slag can dissolve them. Second, change the lance schedule to a higher early lance position and a lower oxygen flow until foam is established, then step down gradually; this reduces the peak CO generation rate that drives slopping.
Verification data should include lance height and oxygen flow against time, off-gas CO and CO2, lance vibration, bath temperature at 50–60% of blow, slag samples for CaO, SiO2, FeO and MgO, and turndown P and S. Track slopping incidents and gas-cleaning differential pressure per heat. If foam still builds, check lime quality and sizing, because coarse or poorly calcined lime dissolves slowly and leaves effective basicity below the target even when the charged ratio looks correct.