Hot Metal Pretreatment: Desulfurization Know-How

Blast furnace hot metal carries 0.03%~0.08% sulfur, while many grades require below 0.01% or even 0.003%. Sulfur causes hot shortness and hurts toughness and weldability. So desulfurize before steelmaking — that's hot metal pretreatment.

Why Not in the Steelmaking Furnace?

Converters/EAFs operate under oxidizing conditions, but desulfurization is efficient under reducing conditions. Pretreatment in the torpedo ladle saves furnace time and uses the right atmosphere.

Main Processes

1. Injection: desulfurizer (mainly CaO lime, sometimes fluorspar/aluminum powder) blown deep into hot metal via lance; CaO reacts with S to form CaS in slag.

2. KR agitation: impeller inserted and spun at high speed, creating a vortex that mixes desulfurizer deep into the melt. Very efficient — sulfur down to 0.001~0.002%, standard for automotive/薄板 grades.

3. Combined: inject then agitate.

Temperature Management

Desulfurizer absorbs heat — hot metal drops 30~50°C. Too cold slows reactions and causes skulling. Measure before (determines dosage) and after (determines furnace readiness).

Judging Results

Sample for sulfur after treatment. White fluffy slag = good; black sticky slag = poor utilization.

Summary

Pretreatment desulfurization blocks sulfur harm before steelmaking. Injection is simple; KR is deep and efficient. Either way, temperature control is critical — the home turf of temperature measurement.