*3.2. Effect of Slag Structure on the Viscosity of the CaO–SiO2–FeO–Al2O3–MgO Slag*

Figure 4 shows the effect of Al2O3 on the viscosity of the CaO–SiO2–FeO–Al2O3–MgO system at 1873 K. In the CaO–SiO2–MgO system, the viscosity was slightly decreased with the addition of 10 wt% Al2O3. However, the viscosity was simply increased as a the concentration of Al2O3 increased. In the CaO–SiO2–FeO–MgO systems, it is commonly observed that an increase in Al2O3 causes an increase in viscosity. Compared with previous studies that measured viscosity in the CaO–SiO2–Al2O3–MgO system [27,28] or CaO– SiO2–FeO–Al2O3–MgO system [29], the present system showed lower viscosity. The present experiments were carried out in the composition where MgO was saturated at 1823 K. Compared with other studies, the higher MgO concentration resulted in lower viscosity [28]. According to Mysen et al. [30–32], the anionic structure in the aluminosilicate system does not change upon quenching from the molten state. For this reason, the molten slag structure was investigated by analyzing the quenched glass sample. Using FT-IR and Raman spectroscopy, the changes in the network structure with varying Al2O3 and FeO concentrations were evaluated.

**Figure 4.** Effect of Al2O3 on the viscosity of CaO–SiO2–FeO–Al2O3–MgO slag system at 1873 K with varying FeO concentrations.

Figure 5 shows the FT-IR transmittance spectra of the slag samples. According to previous FT-IR investigations of slag structures [19,20,33–36], bands indicating the distinct structural units related to the silicate and aluminate structures can be found in three regions: 1200–800 cm−1, 750–630 cm−1, and 630–450 cm−1, corresponding to [SiO4] tetrahedral symmetric stretching vibrations, [AlO4] tetrahedral asymmetric stretching vibrations, and Si–O–Al bending vibrations, respectively. In the silicate network structure, tetrahedral [SiO4] units can be classified depending on the number of bridging oxygens (BOs). As different units have different symmetric stretching vibrations, the absorption band present in the FT-IR spectrum corresponds to the characteristic bonding states of the different units. The number of BOs in the [SiO4] unit is expressed by *n* in *Q<sup>n</sup> Si*, where 4, 3, 2, and

1 indicate sheets, chains, dimers, and monomers, respectively. Likewise, the number of BOs in the [AlO4] tetrahedral unit is expressed by *n* in *Q<sup>n</sup> Al*. It is commonly observed that the addition of Al2O3 to the CaO–SiO2–MgO or CaO–SiO2–FeO–MgO system introduces an absorption peak at 750–630 cm−1, indicating the formation of [AlO4] tetrahedral units. When the [AlO4] tetrahedral units form a polymerized network structure or become incorporated into the [SiO4] tetrahedral units, a cation is required for charge balancing [35]. The high affinity between Mg2+ and the [AlO4] tetrahedral unit was reported in our previous study [11]. As the viscosity of the CaO–SiO2–FeO–Al2O3–MgO system was measured in the MgO-saturated composition at 1823 K, sufficient Mg2+ existed in the molten slag for the charge balance of the aluminate and aluminosilicate network structures. For this reason, the addition of Al2O3 causes the formation of a network structure, and the viscosity monotonically increases with increasing Al2O3 concentration at a fixed initial concentration of FeO, as shown in Figure 4.

**Figure 5.** FT-IR transmittance spectra of quenched CaO–SiO2–FeO–Al2O3–MgO slag with varying Al2O3 concentration for FeO contents of (**a**) 0 wt%, (**b**) 10 wt%, and (**c**) 20 wt%.

A decrease in viscosity can be observed in Figure 4 as the FeO concentration increases for a fixed initial Al2O3 concentration. Depending on the number of BOs in the [AlO4] tetrahedral units, two distinct absorption bands can appear in the FT-IR spectra [19]. When the BO number is 3 (*Q*<sup>3</sup> *Al*), an absorption band is observed in the range 690–750 cm−1. Otherwise, the absorption band observed between 640 and 680 cm−<sup>1</sup> is attributed to the *Q*2 *Al* unit, where the BO number is 2. As shown in Figure 5, a decrease in transmittance at 640–680 cm−<sup>1</sup> is observed when the initial concentration of FeO is increased at a fixed initial concentration of Al2O3. The increase in *Q*<sup>2</sup> *Al* units with increasing FeO concentration indicates the depolymerization of the [AlO4] tetrahedral network structure. In the molten oxide system, FeO acts as a network modifier. As Fe2+ ions require charge compensation, non-bridging oxygen is formed, which results in depolymerization by reducing the network connectivity.

To quantitatively evaluate the silicate structure changes with varying Al2O3 concentration in the present CaO–SiO2–FeO–Al2O3–MgO system, Raman scattering measurements were performed. Figure 6 shows the original Raman spectra and Raman deconvoluted bands within the 400–1100 cm−<sup>1</sup> range. Referring to the appropriate references listed in Table 2 [19–21,30,36–42], the Raman spectra were fitted by a Gaussian function and the corresponding structural units of the slag were identified with the aid of Peakfit 4 (Systat Software, San Jose, CA, United States). The relative fractions of the tetrahedral silicate structure units with varying BO numbers *Q<sup>n</sup> Si* were qualitatively evaluated by integrating the areas of the corresponding Gaussian-deconvoluted peaks. As shown in Figure 7, the number of *Q*<sup>1</sup> *Si* structural units gradually decreased with increasing Al2O3 concentration.

In contrast, the numbers of *Q*<sup>2</sup> *Si* and *<sup>Q</sup>*<sup>3</sup> *Si* structural units increased with increasing concentrations of Al2O3. The increase in the number of silicate structure units with higher BO numbers indicates the polymerization of the silicate network structure. According to Wang et al. [29] who studied the structure of a CaO–SiO2–FeO–Al2O3–MgO slag system using Raman spectroscopy and magic-angle-spinning nuclear magnetic resonance spectroscopy, a more polymerized silicate network structure was observed with higher Al2O3 concentration. The [AlO4] tetrahedral structural unit can be associated with the [SiO4] tetrahedral structural unit, thereby increasing the degree of polymerization. Yao et al. [43] also reported silicate network polymerization by the addition of Al2O3. When Al2O3 functions as a network former for tetrahedral structural units, it can be associated with non-bridging oxygen in the [SiO4] tetrahedral structural units, thus strengthening the silicate network structure. Therefore, the addition of Al2O3 to the CaO–SiO2–FeO–Al2O3– MgO system results in the polymerization of the molten slag system by the formation of an [AlO4] tetrahedral network structure associated with the [SiO4] tetrahedral network structure units.

**Figure 6.** Raman spectra of quenched CaO–SiO2–FeO–Al2O3–MgO slag with 10 wt% initial FeO concentration and varying initial Al2O3 concentrations of (**a**) 10 wt%, (**b**) 20 wt%, and (**c**) 30 wt%.


**Table 2.** Reference Raman peak positions and corresponding assigned aluminate and silicate units.

**Figure 7.** Relationship between relative area fractions of silicate tetrahedral structure (*Q<sup>n</sup> Si*) and initial concentration of Al2O3 in the CaO–SiO2–FeO–Al2O3–MgO slag system at a fixed initial concentration of 10 wt% FeO.

#### **4. Conclusions**

Understanding the thermophysical properties of molten CaO–SiO2–FeO–Al2O3–MgO systems is significant for FeO reduction by Al dross addition in the EAF process. In the present study, the viscosity of a CaO–SiO2–FeO–Al2O3 system with a high concentration of MgO, which reached saturation at 1823 K, was measured by varying the FeO and Al2O3 concentrations at a fixed CaO/SiO2 ratio. Structural changes in the molten slag system with composition variations were investigated using FT-IR and Raman spectroscopy. The following conclusions were drawn from the present study.


3. According to the intermediate-range order structural investigation by Raman spectroscopy, the silicate network structure was polymerized with increasing Al2O3 concentration. Quantitative evaluation of the *Q<sup>n</sup> Si* structural units revealed an increase in *Q*2 *Si* and *<sup>Q</sup>*<sup>3</sup> *Si* units with a decrease in *<sup>Q</sup>*<sup>1</sup> *Si* units with increasing Al2O3 concentration, indicating the polymerization of the silicate structure. The association of the [AlO4] tetrahedral units with the [SiO4] tetrahedral silicate network induced the polymerization of the slag structure and an increase in the viscosity of the molten slag.

**Author Contributions:** Conceptualization, Y.K. and D.-J.M.; methodology, Y.K.; formal analysis, Y.K.; investigation, Y.K.; data curation, Y.K.; writing—original draft preparation, Y.K.; writing—review and editing, D.-J.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministry of Science, ICT, and Future Planning of Korea, grant number GP2020-013.

**Acknowledgments:** Youngjae Kim acknowledges financial support from the Basic Research Project no. GP2020-013 of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT, and Future Planning of Korea.

**Conflicts of Interest:** The authors declare no competing financial interest.
