Why a stiff slag is not short of lime

When a converter or ladle slag goes stiff, the reaction on the floor is almost always the same: add lime. It is the intuitive move — high-basicity slag is made of lime, so a slag that is not behaving must need more of it. In most cases that is exactly the wrong move, and it makes the next heat worse rather than better.

The reason is that a stiff slag is not short of lime. It is short of dissolved lime. In a high-basicity slag, calcium silicate builds a continuous network through the liquid, and the lime you charged sits inside that network as solid islands instead of going into solution. The slag is holding a large amount of unmelted lime in suspension. Total lime content looks high; the useful, dissolved fraction does not.

Adding more lime does not break that network. It reinforces it. Pushing basicity higher takes the slag further from the liquid region of its own phase diagram, which raises the melting point rather than lowering it. The result is a thicker slag, more undissolved lime and a slower slag-steel reaction — which then gets read as "the slag needs more lime" all over again. That loop costs you lime and heat time on every cycle.

What actually changes the situation is bringing the melting point down instead of pushing basicity up. A Slag Fluidizer works by penetrating the viscous high-lime slag with low-melting-point components, breaking the calcium silicate structure apart and pulling both the melting point and the viscosity down. Lime that was sitting as islands goes into solution, and the slag you have already paid for starts doing the job you bought it for.

The practical way to tell whether it is working is not to watch the lime addition. It is to watch the slag. Take a sample and look at whether it runs off the rod as a thin string or comes up lumpy and pasty. Watch the deslagging: a fluid slag comes off the ladle cleanly, while a stiff one tears and leaves a crust behind. And watch the reaction itself — when the slag is running properly, desulphurisation in the refining ladle is the clearest signal, because it depends directly on how well slag and steel can reach each other. The fluidiser is not doing that work; it is putting the slag in a position to do it.

Addition rate is the part that cannot be copied from a datasheet. It depends on your slag composition, your basicity, your tapping temperature and how much undissolved lime you are already carrying. Used properly, a Slag Fluidizer is added at a fraction of the tonnage of the lime it is helping to dissolve. Send us your slag analysis, basicity, steel grade and current consumption, and we will work out a starting dosage for your line.