Iron losses during desulphurisation of hot metal

Sammanfattning: After injection of calcium carbide and magnesium during desulphurisation of hot metal, the slag is normally solid and contains large amounts of iron. Besides the enclosed iron droplets in the slag, drawn-off hot metal during slag skimming also accounts for iron losses during desulphurisation of hot metal. Iron losses during hot metal desulphurisation using both calcium carbide (mono-injection), and calcium carbide and magnesium (co-injection), have been studied by large-scale investigations of slag from the slag pit as well as slag sampling during the desulphurisation process at SSAB EMEA in Luleå.An alkali-containing mineral, nepheline syenite, was mixed together with thecalcium carbide to facilitate slag formation during desulphurisation and to enhance the separation of iron from the slag. Even though the addition of nepheline syenite resulted in a more fine-grained slag, no reduction in the iron content in the slag before slag skimming was observed. The addition of 5 wt-% of nepheline syenite to the calcium carbide during mono-injection decreased the magnetic fraction of the slag from the slag pit from 2.5 wt-% to 1.9 wt-%. The decrease in the larger magnetic fractions combined with the improved iron yield may arise from a more effective slag skimming due to the change in slag consistency.During co-injection with magnesium and calcium carbide, the magnetic fraction of the slag from the slag pit decreased from 3.2 wt-% to 3.1 wt-% with the addition of 10 wt-% of nepheline syenite. Even though nepheline syenite has a positive effect on the slag skimming, due to a decrease in larger magnetic fractions, the total magnetic iron losses have not decreased significantly. A plausible explanation for the lack of improved iron yield during co-injection in the present study could be that the increased turbulence during injection of magnesium counteracts any positive effect from addition of nepheline syenite on the slag properties. Finally, no negative effect on the reagent efficiency was observed during the trials with addition of nepheline syenite.