This example includes a PDF with in-depth explanations of all the calculations and interpretation of the results. It also includes a calculation file that can be downloaded and run in Thermo-Calc if you have a license for the relevant software and databases.
The ladle furnace fulfils many purposes in the steelmaking process. Desulphurization, which we will focus on in this example, is merely one of them.
Desulphurization is usually performed by transferring sulphur (S) that is dissolved in the liquid metal to a CaO-rich slag phase, as shown in the image:
The principle of desulphurization is to transfer the sulphur dissolved in the liquid steel to a CaO-rich slag phase. This process is typically carried out in a ladle furnace (LF).
For this process to be successful, two conditions need to be fulfilled:
The slag must be fully liquid (liquid fraction > 0.9). This is required for kinetic reasons. The slag phase must be fluid so that it can emulsify with the liquid steel and form a large surface area where the reaction between steel and slag can take place.
The slag must take up a large amount of S from the liquid steel (have a “high sulphur capacity”) so that a significant amount of sulphur will move from the liquid steel to the slag phase.
The example explores these two conditions for an equilibrium between liquid steel and slag in a ladle furnace, trying to find good slag compositions.
Note that desulphurization is not carried out in an earlier stage, for instance during steelmaking in a basic oxygen converter (BOF) or an electric arc furnace (EAF), because conditions are predominantly very oxidizing, so it is difficult to decrease the S content in the liquid steel. Desulphurization is therefore usually performed in a subsequent step, during steel refining in a ladle furnace or a vacuum degasser after the steel has been fully killed by a suitable deoxidizing agent and the oxygen activity is low, as shown in the figure above.
In the example, we first investigate how the composition of the slag phase influences the sulphur content in the steel using a grid calculation. We then explore the common problem during steelmaking of what to do when some of the furnace slag, which is oxidized and rich in FeO, is tapped into the ladle, reducing the desulphurizing ability of the slag in the ladle furnace.
Finally, we look at two ways of recovering the desulphurizing ability of the ladle slag - conditioning the slag with deoxidizing agents and reducing the melting point with fluxing agents - to find the optimum slag composition where the slag is at the same time liquid and the S content in the steel is low.
The Process Metallurgy Module in Thermo-Calc makes it easy to set up these calculations and results are obtained after only four calculations.
Plots showing the fraction of liquid slag as a function of SiO2 and Al2O3 content (red contour lines) and amount of S in the liquid steel phase in equilibrium with the slag (blue contour lines) at 1620°C. The optimum slag composition is where the slag is at the same time liquid and the S content in the steel is low. No optimum is found when the slag is contaminated with FeO (top right), but the slag desulphurizing ability can be recovered by either conditioning it with CaC2 (bottom left) or fluxing it with CaF2 (bottom right).
The Process Metallurgy Module can be used to investigate the entire steelmaking process, from scrap to fully refined steel. The examples below investigate other steps in the steelmaking process: