**2. Experimental Process**

The slag system was CaO-SiO2-MgO-Al2O3-Cr2O3-FeO and the component of the slag sample is shown in Table 1. The raw materials included CaO, SiO2, MgO, Al2O3, Cr2O3, and FeC2O4·2H2O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). First, the raw materials were weighed based on Table 1 and then mixed. 0.10 wt% H3BO3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added into the mixtures to prevent the pulverization of the synthetic samples. Second, mixtures weighing 200 g were put into a molybdenum crucible in the carbon-tube furnace. The furnace was warmed to 1600 ◦C at a heating rate of 10 ◦C/min under a nitrogen atmosphere and held for 30 min. Then, the temperature was decreased to 1450 ◦C at a rate of 20 ◦C/min, and the slag samples were taken out from the furnace and quenched with water to obtain synthetic slag samples at different sampling times, as shown in Figure 1. The sampling times were 0, 5, 10, 20, 30, 40, 60, 80, 100, and 120 min. Finally, the slag samples were ground, polished, and sprayed with gold powder.

**Table 1.** The chemical composition of stainless-steel slag, g.

**Figure 1.** The heating procedure adopted in the experiments.

The characterization of the samples was conducted with SEM-EDS and XRD. The size of the spinel crystals was measured with Image-Pro Plus 6.0 (IPP). According to the Scheil-Gulliver equation, FactSage7.1 was applied to simulate the phase transformation and precipitation of the spinel phase during solidification of the molten slag. The specific setting conditions are shown as follows.

Database: FactPS, FToxide, FSstel;

Compound setting: idea gas, pure solid;

Solution phase setting: FToxid-SLAGA, FToxid-SPINA, FToxid-MeO\_A, FToxid-bC2SA, FToxid-aC2SA, FToxid-Mel\_A.

The FToxid-SLAGA was set as the target phase of Scheil-Gulliver cooling. The setting temperature was 2000 ◦C and the solidification step was 10 ◦C. The calculation process was terminated automatically when the target phase completely disappeared. The results were exported as pictures and edited with FactSage7.1.
