*5.2. Effect of Basicity on the Occurrence State of Sulfur in Slag*

The micro-area distribution of the sulfur in the S1 and S3 slag samples in Figure 8 were analyzed by scanning electron microscopy (SEM-EDS), as shown. Through the surface scanning analysis data, it was found that the matrix phase around the CaS grains contained dispersed sulfur elements; thus, it can be determined that the slag matrix phase in the solidification process contained unprecipitated amorphous sulfur. The sulfur of the synthetic slag existed in two different states, namely, CaS crystal and amorphous sulfur. The sulfur elements distributed in the aggregated state of the molten slag were CaS crystals, while the sulfur elements distributed in the dispersed state occurred in the matrix phase as an amorphous structure. Comparing the microzone distribution of the sulfur elements in Figure 8a,b, it can be discovered that the amorphous structure density of the sulfur elements in the S3 slag sample was higher than that in the S1 slag sample, and the content of amorphous sulfur in the slag increased with an increase in basicity. The key reason of such a phenomenon was that the crystallization temperature of the CaS in the molten slag decreased with the increase in basicity and the supercooling zone of the CaS crystallization decreased during solidification in the same cooling system (air cooling), which reduced the amount of precipitation of the CaS phase and increased the amorphous sulfur content in matrix phase. Moreover, the thermodynamic calculation of sulfur precipitation during solidification in Figure 2 indicated that the sulfur in the slag was completely precipitated in the form of CaS, but the surface scanning distribution analytical results showed that the amorphous sulfur content of matrix phase increased with the increase in basicity. This was mainly because the kinetic condition of S2<sup>−</sup> diffusion in the matrix phase deteriorated with the decrease in slag temperature during solidification, while certain S2<sup>−</sup> in the matrix phase failed to diffuse to the surface of the CaS crystal nucleus to form CaS crystals and occurred in the form of an amorphous structure in matrix phase. *Minerals* **2021**, *11*, x FOR PEER REVIEW 11 of 12

**Figure 8.** Surface scanning analysis of the sulfur elements in molten slag. (**a**)-S1 with a basicity of 2.5, (**b**)-S3 with a basicity of 3.5. **Figure 8.** Surface scanning analysis of the sulfur elements in molten slag. (**a**) S1 with a basicity of 2.5, (**b**) S3 with a basicity of 3.5.

increase of the slag basicity.

State Key Laboratory of Refractories and Metallurgy.

**Conflicts of Interest:** The authors declare no conflicts of interest.

perature of CaS in slag decreased with the increase of basicity;

of sulfur in matrix phase was mainly homogeneous nucleation;

(1) According to the calculated results of molten slag solidification process based on thermodynamic database FactSage8.1, it was concluded that sulfur in KR desulfurization slag was mainly precipitated in the form of CaS, and the crystallization tem-

(2) CaS grains mainly precipitated along silicate grain boundaries in low-basicity (*R* = 2.5 and 3.0) slags, and the precipitation behavior of CaS was mainly heterogeneous nucleation. There were fewer CaS grains precipitated along silicate grain boundaries in molten slags with high basicity (*R* = 3.5, 4.0 and 4.5), and the precipitation behavior

(3) The number and the size of CaS grains decreased and increased respectively with the

**Author Contributions:** R.Z. drafted the manuscript and conducted the experiments, J.J. helped to conduct the experimental work, and J.L. modified and polished the draft. Y.Y. and H.Z. helped to read the manuscript. All authors have read and agreed to the published version of the manuscript. **Funding:** The work was supported by the National Natural Science Foundation of China (No. 51974210, 52074197), the Hubei Provincial Natural Science Foundation (No. 2019CFB697), and the

**Acknowledgments:** The authors would like to thank the National Natural Science Foundation of

China and the Hubei Provincial Natural Science Foundation for their financial supports.

**6. Conclusions** 

Based on the above, research status of KR desulfurization slag and the experimental results are discussed. At present, the comprehensive utilization process of KR desulfurization slag in China is not considered to remove sulfur from KR desulfurization slag and to reuse it in converter smelting processes. Therefore, the results are beneficial to provide a theoretical basis for the subsequent removal of sulfur from slag through oxidizing atmosphere.

## **6. Conclusions**


**Author Contributions:** R.Z. drafted the manuscript and conducted the experiments, J.J. helped to conduct the experimental work, and J.L. modified and polished the draft. Y.Y. and H.Z. helped to read the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was supported by the National Natural Science Foundation of China (No. 51974210, 52074197), the Hubei Provincial Natural Science Foundation (No. 2019CFB697), and the State Key Laboratory of Refractories and Metallurgy.

**Acknowledgments:** The authors would like to thank the National Natural Science Foundation of China and the Hubei Provincial Natural Science Foundation for their financial supports.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

