To lower the iron losses of the hot metal desulphurisation (HMD) process, slag modifiers can be added to the slag. Slag modifiers decrease the apparent viscosity of the HMD slag. Most common slag modifiers in industry contain fluoride as a fluidiser. However, fluoride leads to a higher magnesium consumption and has health, safety and environment issues. Fluoride-free alternatives like nepheline syenite (NS) and fly ash (or pulverised fuel ash, PFA) can decrease the slag’s apparent viscosity. Experiments with HMD slags containing CaF₂, NS and PFA and without slag modifier were performed for slags with a high and an average basicity. The melting points of the slags and their viscosities 1250–1600°C were measured. The experimental results are compared with FactSage calculations. PFA and NS are viable alternatives in the industrial HMD process, as reasonable amounts are sufficient to reach the same lower apparent viscosities and melting points as with CaF₂.

At the hot metal desulphurisation (HMD) stations of Tata Steel’s BOS plant in Port Talbot (UK), KAlF4 is injected together with magnesium in order to increase the fluidity of the slag and thus to decrease the iron loss. Although KAlF4 is successfully contributing to a lower iron loss during HMD, it also has a negative effect on the desulphurisation efficiency of magnesium. Analysis of slag samples from the plant and FactSage calculations showed that the fluorine in KAlF4 reacts with the magnesium, thus preventing the magnesium from reacting with the sulphur.