Characteristics of Raw Materials
The XRD analysis of the raw materials (Figure 3) shows that iron presents as hematite (Fe2O3) and, in a lower amount, as goethite (FeOOH). Titanium appears as anatase (TiO2) and calcium as calcite (CaCO3). Aluminum is found in hydroxides as gibbsite AlO(OH)3, diaspore α-AlO(OH), and boehmite β-AlO(OH), in complex alumino-sodium-silicate phases as Na1.15Al1.15Si0.85O4 and cancrinite (Na6Ca2(AlSiO4)6(CO3)2(H2O)2), and as katoite (Ca3Al2(SiO4)(OH)8) in the BR.
Table II shows the XRF analysis of the raw materials used in the present study. BR contains a significant amount of Fe2O3, 43.59 wt pct, while the alumina content is 22.78 wt pct, indicating levels of alumina losses that occur in the Bayer process.
Characteristics of Slags
The XRD results of the produced slags are illustrated in Figures 4 and 5. It was observed that there are significant differences compared to the mineralogical compositions of the raw materials (Figure 3) in the smelting reduction. Obviously, there are no Fe- and Na-containing phases in the produced slags. According to Figure 4, the main phases in slag S-1 are C2AS and CA. Weak peaks of C12A7 and CT were also detected.
C12A7 is the dominant phase for slag S-2, while larnite, β-C2S, CT, CA, and C2AS were also detected. Slag S-3 contains C12A7, β-C2S, CT, CA, and weaker peaks of C2AS in comparison with slags S-1 and S-2. The carbon detected in all the mentioned slags was due to the contamination from the graphite crucibles and thermowell.
The XRD results of the S-3 with different cooling paths are given in Figure 5. The slag S-3-S contains C12A7, CT, C3A and C2AS. The slag S-3-R contains C2AS, CT, C3A, β-C2S and C12A7. The peaks of C2AS are obviously weaker in the S-3-S slag in comparison with the S-3-R, while C12A7 has a stronger peak in S-3-S than S-3-R.
The other minor phases, such as Ca3Ti2O7, anorthite CA2S, and CaTi2O4, which may form in the slags, could not be identified precisely due to their low intensity peaks in addition to the overlapping of several phases.
According to the XRF analysis in Table III, the slags were composed mostly of CaO, Al2O3, SiO2, and TiO2 oxides. Slightly lower SiO2 content than expected was observed for S-3-S (in comparison with S-3 and S-3-R), which may be attributed to some inhomogeneities in the used raw materials or little fluctuation in temperature at elevated temperatures.
The XRF analysis reports the total iron content as Fe2O3; however, as will be discussed in Section 3, the Fe content of the slags is attributed to metallic entraps, based on the microstructural analysis. To further justify this, magnetic separation of iron by a normal magnet was applied in the S-3-R sample prior to its analysis. As seen in Table III, the Fe content was significantly reduced. Hence, the Fe2O3 content of the XRF has been transformed to metallic Fe in Table III.
As the WDS results are more reliable than those using EDS for the determination of the chemical composition of the phases, we present the WDS results. In Figure 6 and Table IV, the BSE images and WDS analysis of the phases in slags S-1 through S-3 and S-3-R in different locations are presented. We emphasize that we observed even distribution of SiO2 in the main phases (overall distribution), mostly for samples S-2, S-3-S, and S-3-R. However, the compositions of the same phases in different samples were observed to be different regarding the content of the impurities (SiO2 and TiO2) in the calcium aluminate phases, as seen in the WDS analysis results in Table IV.
In slag S-1, three phases were observed and marked as A, B, and C. The continuous phase A that appears as light gray is attributed to the C2AS phase. The dark gray phase B is rich in CaO and Al2O3, and its ratio corresponds to CA2, not CA as detected with the XRD analysis. The bright-phase C has a high concentration of CaO and TiO2, which could be attributed to the CT phase, but it contains 10.8 wt pct Al2O3. However, a phase like this was not observed in the XRD analysis. According to Dunyushkina et al., Al ions could incorporate in the CT structure even if this phase appears as CT in the XRD spectra. In slag S-2, phase A (dark gray) has an acicular shape and the C/A mass ratio is close to the theoretical mass ratio of C12A7; however, it contains some Si and Ti impurities. It is worth noting that based on the recent work by Azof, this phase may be C12A7 with Si ions in the locations of some Al ions and is so called “Si-mayenite.” The solidified structure B seems to be a mixture of small C2S, CT, and possible CA phases distributed nonuniformly. The fine structure phase C may be the C12A7 phase containing Si impurities.
In slag S-3, the bright phase A (Figure 6 and Table IV) was rich in TiO2 and CaO, as in S-1, it was high in Al2O3; in the XRD analysis, no Ca-Al-Ti-O phase was observed. The light gray phase B is composed of CaO and Al2O3, with the C/A mass ratio similar to the C12A7. Sulfur has a higher concentration in comparison with the rest of the phases, and based on the literature, the C12A7 phase dissolves S in its structure. Thus, we may claim that this phase is the C12A7 phase, as identified in the XRD analysis. The solidified structure C could be a mixture of C12A7, C2S, and CT, which supports the XRD result. The measured analysis of phase D was difficult to evaluate due to that phase’s size and morphology. However, based on WDS and XRD analysis, it could be a mixture of C2AS, C12A7, and C2S phases, but the identification is not precise in this case. It is observed that metallic iron particles were entrapped in the slag structure, as seen in Figure 6 for slag S-3.
The detected phases in the S-3-S and S-3-R slags were found to be impure, as SiO2 and TiO2 were detected in all the analyzed points. This is possibly due to the increased cooling rate, in comparison with the S-3 slag. In the S-3-R sample, it is found that the rapid cooling rate is accompanied with the formation of porosity (Figure 6). Phase A is likely a mixture of C12A7, CT, and C2AS, but it is evidenced that Si is evenly dispersed. Phase B could be the C2AS phase detected in the XRD analysis but seems not to be stoichiometrically the same. For clarity, the WDS results of slag S-3-S are not included.
Characteristics of Pig Iron
The chemical analysis of the pig iron samples (Table V) shows that the Fe content of all the metals is ~92 wt pct. The Si content is low and varies between 0.22 and 0.02 wt pct, while the titanium concentration varies from 1.4 to 0.38 wt pct. Cu did not appear in the chemical composition of the raw materials (Table II), possibly due to its low concentration, while we see a small amount of it in the metal phase.
Typical examples of the micrographs of the metallic phase are given in Figure 7. The microstructural analysis of the produced pig irons indicates that they have a relatively similar microstructure, where the graphite flakes are distributed in the matrix, as seen in Figure 7.
At the surface of the metallic phase and mostly close to the graphite flakes, complex carbides were formed that primarily consisted of titanium and vanadium as indicated with the mapping. However, Si and Cr elements were found to be evenly distributed in the Fe matrix. In the metallic phase, M-3-R was observed with less carbides formed in comparison with M-3. This is possibly due to less Ti in the M-3-R (Table V) in addition to the applied cooling, as will be discussed subsequently.
Theoretical and Modeling Calculations
Equilibrium study and distribution of elements
The predicted equilibrium compositions of metal and slag phases in the molten state are listed in Table VI. The calculations were carried out at the experimental temperature of 1650 °C, and the metal phase was assumed to be carbon saturated. As seen in Table VI, a relatively good agreement was observed between the calculated and experimental (Table III) contents of Al2O3 and CaO. Besides, a higher but not significant difference was observed for the SiO2 and TiO2. Moreover, FactSage predicts that parts of TiO2 and Cr2O3 will be reduced to Ti2O3 and CrO, respectively.
Scheil–Gulliver cooling calculations
The Scheil–Gulliver cooling calculations are shown in Figure 8. The calculations were done based on the liquid slag compositions previously calculated (Table VI) concentrated in the main components (above 2 pct). The main phases that appear for S-1 in descending order are CA, C12A7, α-C2S, CaTi (ss) that is a Ca3Ti2O7 (s)-Ca3Ti2O6 (s) solid solution, PERO that is a perovskite Ca2Ti2O6 (s)-Ca2Ti2O5 (s) solid solution, and γ-C2S.
For slag S-2, the predicted phases are C12A7, α-C2S, CaTi (ss), C3A, and Ca3Ti2O7, and for slag S-3, they are C12A7, α-C2S, CA, CaTi (ss), C3A, and Ca3Ti2O7.