Slag Chemistry & Metallurgy

Every furnace and ladle in a steelworks contains two liquids, not one. Floating on the steel is a layer of molten oxides — slag — typically 100–300 mm deep in a BOF and 50–150 mm in a ladle. It is tempting to think of slag as waste, and eventually most of it is sold or discarded. But while the heat is live, the slag is a working chemical reagent: it is the only place phosphorus and sulphur can go, the absorber that collects the oxide inclusions floating out of the steel, the thermal blanket that slows heat loss, the foam that shields an EAF arc, and the lubricant that keeps a continuous caster mould from sticking.

Almost every steel quality problem traceable to chemistry — high phosphorus, missed sulphur spec, oxide inclusions, refractory erosion, mould powder entrapment — is at root a slag problem. The steelmaker cannot refine what the slag will not hold. That is why melt shop metallurgists speak of "slag practice" as a discipline in itself: the deliberate engineering of slag composition, mass, temperature, and oxidation state at each process stage.

A working definition: slag is a molten solution of metal oxides — predominantly CaO, SiO₂, FeO, MgO, Al₂O₃, MnO, and P₂O₅ — formed from oxidised impurities, deliberate flux additions (burnt lime, dolomitic lime, fluorspar, alumina), eroded refractory, and ore or scale. Its chemistry is described not element by element, as steel is, but by the proportions and interactions of these oxides.