3.1 Mineral phase reconstruction procedure
In the SSS, aluminum exists mainly as corundum (Al2O3) and nepheline (NaAlSiO4) phases. Hence, the detailed reactions between Al-bearing minerals with Na/Ca additives are demonstrated in this study. Eqs. (2)-(13) show the possible reactions during the reconstruction process of Al2O3 and NaAlSiO4, and FactSage 8.0 software (Thermfact/CRCT, Montreal, QC, Canda; GTT-Technologies, Herzogenrath, Germany) was used to calculate the changes in the Gibbs free energy of the reactions, and the results are shown in Fig. 4. According to the thermodynamic calculations, Al2O3 reacts preferentially with Na2O or NaOH, forming than NaAlSiO4. The Gibbs free energy (ΔrGmθ) of Eqs. (2)-(5) is negative, indicating that the reaction can occur during the range of modification temperatures. Compared with Al2O3, the NaAlSiO4 reacts preferentially with CaO or Ca(OH)2, and the ΔrGmθ of Eqs. (8) and (9) are negative. The ΔrGmθ values of Eqs. (10) and (11) decrease as the temperature increases, indicating that the reaction is more favorable at high temperatures. When the temperature is less than 700 ℃, Al2O3 and NaAlSiO4 are difficult to react with Na2CO3 or CaCO3. When the temperature is above 725 ℃, Al2O3 can react with Na2CO3 or CaCO3 (Eqs. (6) and (12)). Further elevating the temperature to 825 ℃, NaAlSiO4 can react with CaCO3 (Eq. (13)). At 1125 ℃, NaAlSiO4 can react with Na2CO3 (Eq. (7)). Considering the separation of Al and Si in the subsequent alkaline leaching process, Na and Ca bearing additives are used together.
Al2O3 + Na2O = 2NaAlO2 (2)
NaAlSiO4 + Na2O = NaAlO2 + Na2SiO3 (3)
Al2O3 + 2NaOH = 2NaAlO2 + H2O (4)
NaAlSiO4 + 2NaOH = NaAlO2 + Na2SiO3 + H2O (5)
Al2O3 + Na2CO3 = 2NaAlO2 + CO2 (6)
NaAlSiO4 + Na2CO3 = NaAlO2 + Na2SiO3 + CO2 (7)
Al2O3 + CaO = CaO·Al2O3 (8)
NaAlSiO4 + 2CaO = NaAlO2 + Ca2SiO4 (9)
Al2O3 + Ca(OH)2=CaO·Al2O3 + H2O (10)
NaAlSiO4 + 2Ca(OH)2=NaAlO2 + Ca2SiO4 + 2H2O (11)
Al2O3 + CaCO3 = CaO·Al2O3 + CO2 (12)
NaAlSiO4 + 2CaCO3 = NaAlO2 + Ca2SiO4 + 2CO2 (13)
As aluminum mainly exists as corundum (Al2O3) and nepheline (NaAlSiO4) phases, it is difficult to extract Al2O3 from the SSS by direct leaching. In this paper, Na and Ca-bearing additives were added to the slag to reconstruct the mineral phases of Al-bearing and Si-bearing minerals and selectively extract Al2O3. The roasted slags prepared at different conditions were leached at 95 ℃ for 2 h in an alkaline solution with a concentration of 2 mol/L and with a liquid-to-solid ratio of 10 mL/g.
The effect of roasting temperature on the leaching ratio of Al2O3 and SiO2 is shown in Fig. 5(a). As shown in Fig. 5(a), the yield of the alkaline leaching residue decreases from 63.10 to 59.12% with a temperature increase from 1150 to 1300 ℃. Meanwhile, the leaching rate of Al2O3 and SiO2 increases from 59.78 and 8.84% to 64.83 and 15.49%, respectively. Figure 5(b) shows that the content of Al2O3 decreases with elevating the temperature from 1150 to 1250℃, indicating that most Al-bearing minerals achieve the mineral phase reconstruction, increasing the Al2O3 leaching rate. As the temperature increases from 1250 to 1300 ℃, the content of SiO2 increases due to the mullite decomposition. Meanwhile, the increase in the SiO2 leaching rate results in poor Al extraction selectivity by alkaline leaching. Therefore, the recommended optimal roasting temperature is 1200 ℃.
Figure 5(c) shows the effect of roasting duration on the Al2O3 extraction. The yield of alkaline leaching residue decreases from 63.65 to 55.81%, prolonging the roasting time from 15 to 60 min. Meanwhile, the leaching rate of Al2O3 increases from 53.96 to 70.66%, as shown in Fig. 5(c). The content of Al2O3 decreases with prolonging the duration from 15 to 60 min, meaning that a long roasting time is beneficial to improve the transformation of corundum (Al2O3) and nepheline (NaAlSiO4) to sodium aluminate (NaAlO2), as shown in Fig. 5(d). Overall, the suggested optimal roasting time is 60 min.
The effect of additives on the Al2O3 extraction is shown in Fig. 5(e). The yield of the alkaline leaching residue decreases from 90.30 to 55.10% with an increase of the NaOH dosage from 0 to 40%. Meanwhile, the leaching rate of Al2O3 and SiO2 increases from 21.34 and 2.65% to 71.75 and 25.8%, respectively. Figure 5(f) shows that the mineral phases of the roasted slag obtained at a 40% Ca(OH)2 dosage are Ca2Al2SiO7, Al2O3, and CaTiO3. Increasing the NaOH dosage to 10%, the diffraction peak intensity of Al2O3 and Ca2Al2SiO7 decreases, while the diffraction peak of Na1.65Si1.65Al0.35O4 appears. By further increasing the NaOH dosage to 20%, the diffraction peak intensity of Ca2Al2SiO7 keeps decreasing, while the diffraction peak of Al2O3 disappears. At 30% NaOH and 10% Ca(OH)2, the mineral phases of the modified slag are Na1.65Si1.65Al0.35O4, Na1.75Si1.75Al0.25O4, and CaTiO3. The diffraction peak of Na1.65Si1.65Al0.35O4 is transformed to Na1.95Si1.95Al0.05O4 with an additive content of 40% NaOH. Increasing the NaOH dosage is beneficial to promoting the reconstruction of Al-bearing minerals. However, when only NaOH is added, the leaching rate of SiO2 is 25.80%, indicating that solely adding NaOH disrupts the selective extraction of aluminum. Therefore, the recommended optimal additive contents are 30%NaOH and 10%Ca(OH)2.
The MPRS was obtained by roasting the SSS at 1200 ℃ for 60 min with 30% NaOH and 10% Ca(OH)2. The mineral phases of the MPRS determined are shown in Fig. 5(f). The main mineral phases are Na1.65Al1.65Si0.35O4, Na1.75Al1.75Si0.25O4, and CaTiO3. The mineral phases of Al2O3 and SiO2 in these phases in the MPRS are listed in Tables 5 and 6, respectively. Compared with the SSS, the soluble Al2O3 increases in the MPRS, and the silicate content changes slightly, indicating that adding Ca2+ and Na+ can promote the transformation of corundum (Al2O3) and nepheline (NaAlSiO4) phases to soluble Al2O3. The detailed mechanism of the modification process is described as follows:
Table 5
Occurrence of alumina in the MPRS (%).
Minerals | Alkaline-Soluble | Insoluble | Total |
Al2O3 content | 31.53 | 4.93 | 36.46 |
Fraction | 86.47 | 13.53 | 100 |
Table 6
Occurrence states of silica in the MPRS (%).
Minerals | Free SiO2 | Silicate | Total |
SiO2 content | 0.13 | 7.16 | 7.29 |
Fraction | 1.83 | 98.17 | 100 |
The functional groups of the SSS and MPRS were examined by FTIR spectroscopy (Fig. 6(a)). The peaks at 3320 and 1668 cm− 1 belong to the bending vibration of the O-H band in hydroxyl groups and water absorbed on the surface of SSS, respectively. The absorption peaks at 950 and 422 cm− 1 are related to the antisymmetric vibration of Si-O and the bending vibration of Si-O-Si, respectively. The vibration peaks at 537 and 653 cm− 1 originate from the vibration of Ti-O-Al and the stretching vibration peaks of Al-O in the aluminum oxide octahedron [AlO6]. In the FTIR spectrum of the MPRS, the absorption peak at 968 cm− 1 belongs to the antisymmetric vibration of Si-O; a small shift exists compared to that of SSS due to the influence of Ca2+ and Na+ from the additives, indicating that the Si-O network is gradually split and depolymerized, forming Si-O-Ca after modifying. The absorption peak at 851 cm− 1 is related to the symmetric stretching vibration of Al-O-Al, indicating that the amount of Al3+ participating in the formation of sodium aluminate is increased, facilitating the dissolution of Al2O3 in the leaching process [25–27].
Raman spectroscopy was applied to get better insight into the functional groups of the two slags, and the results are shown in Fig. 6(b). The peaks at 334 and 389 cm− 1 belong to the stretching vibration of Si-O in the SSS. The peaks at 431 and 481 cm− 1 are related to the Si-O-Si bending vibration, cation participation, and its long-range-ordered framework vibration. The peaks in the range of 524–700 cm− 1 belong to the antisymmetric stretching vibration of Al-O. Compared with the wavenumber shifts of SSS, the wavenumber shifts of Al-O disappear. The width and intensity of Ti-O, Si-O, and Al-O-Si vibration peaks increase with the addition of Ca2+ and Na+, implying that the activities of Al2O3 and SiO2 increase, increasing the leaching rates of Al2O3 and SiO2 [28, 29].
Figures 6(c) and (d) show the MAS NMR spectra of 27Al and 29Si in the two slags. The chemical shifts of 27Al in the SSS are 55.70 ppm and 10.69 ppm, the former assigns to 4-coordinated (tetrahedral) Al, and the latter belongs to 6-coordinated (octahedral) Al species. The chemical shift of 29Si is -91.73ppm, which belongs to the Q3 layer groups. Compared with the SSS, an increase of the 4-coordinated Al content and a corresponding decrease for Al in 6-coordinated appear in the MS, indicating that alumina becomes more active. The chemical shift of 29Si in the MPRS belongs to the Q2 chain-shaped structure. The different chemical shifts represent the silicon Qn environments, where n is the number of bridging oxygen atoms linked to other Si atoms for each Q(SiO4) unit [30, 31]. The difference in the Si structure between SSS and MPRS indicates that the polymerization degree of the MPRS is lower than that of the SSS. Meanwhile, the Ca-O-Si bond is formed. The results agree with the FTIR spectra.
In summary, the addition of Ca2+ and Na+ has a significant influence on the structural change of the SSS and is conducive to the transformation of corundum and nepheline to sodium aluminate and calcium silicate.
3.2 Alkaline leaching of the MPR slag
The effect of leaching temperature is examined in the ranges from 65 to 95℃ at a NaOH concentration of 4 mol/L, a liquid-to-solid ratio of 10 mL/g, and a 75%-particle size less than 200 mesh. The results are shown in Fig. 7. It can be seen that prolonging the leaching time positively impacts the Al2O3 recovery rate. The Al2O3 recovery is 66.50% at a leaching temperature of 65 ℃ for 120 min. Meanwhile, the recovery of Al2O3 is 80.66% at a leaching temperature of 95 ℃, indicating that high temperature is conducive to the extraction of Al2O3.
According to the experimental data from Fig. 7, the plots of 1-(1-x)1/3-t and 1-2x/3-(1-x)2/3-t are depicted in Fig. 8, and the G(a)-t correlation coefficient is shown in Table 7. The plot of 1-2x/3-(1-x)2/3-t exhibits an excellent linear relation, indicating that the leaching process is controlled by internal diffusion.
To calculate the apparent activation energy, the plot of lnk-1/T should be a straight line with a slope of -E/R and an intercept of lnk0. According to the Arrhenius equation and Table 7, the linear fitting between lnk-1/T was calculated and presented in Fig. 9. The apparent activation energy of the leaching process is 16.21kJ/mol, which agrees with the results presented in the reference [32] when the reaction process is controlled by internal diffusion. According to the results in Fig. 7, the following kinetic expression can be derived to describe the leaching process: 1-2x/3-(1-x)2/3=[1.61×10− 2×exp(-1949.72/T)]×t.
Table 7
The G(a)-t correlation coefficient obtained from the linear fit of the Al2O3 leaching rate of the MPRS.
Model | 65℃ | 75℃ | 85℃ | 95℃ |
R2 | k | R2 | k | R2 | k | R2 | k |
1-(1-x)1/3 | 0.9111 | 0.0015 | 0.9312 | 0.0017 | 0.9362 | 0.0019 | 0.8987 | 0.0020 |
1-2x/3-(1-x)2/3 | 0.9510 | 0.0005 | 0.9657 | 0.0006 | 0.9679 | 0.0007 | 0.9395 | 0.0008 |
The leaching process was controlled by the internal diffusion of the liquid reactant through the reaction interface. Some parameters were optimized, such as alkaline concentration, leaching temperature, time, liquid-to-solid ratio, and particle size, to improve the Al2O3 extraction.
Fixing the liquid-to-solid ratio at 10 mL/g and a 75% particle size less than 200 mesh, the MPR slag was leached at 95 ℃ for 120 min. The effect of the alkaline concentration on the recovery of Al2O3 is illustrated in Fig. 10(a). The yield of leaching residue decreases from 59.49 to 47.70% with an increase in the alkaline concentration from 2 to 4 mol/L. Meanwhile, the leaching rates of Al2O3 and SiO2 increase from 70.66 and 8.93% to 80.66 and 10.29%, respectively. The Al2O3 content in leaching residue decreases from 17.57 to 14.78%. However, the SiO2 content increases from 10.90 to 13.71%. Further increasing the alkaline concentration has a slight impact on the indexes. The dissolution rate of sodium aluminosilicate increases with the alkaline concentration. However, increasing the alkaline concentration further has little effect on the dissolution rate of sodium aluminosilicate, although it raises the operation cost. Therefore, the recommended optimal alkaline concentration is 4 mol/L.
As shown in Fig. 10(b), the leaching temperature is 65 ℃, and the leaching residue yield is 64.12%. The corresponding leaching rates of Al2O3 and SiO2 are 66.50 and 8.97%, respectively, and the grade of Al2O3 and SiO2 is 19.05 and 10.35%, respectively. By increasing the temperature to 95 ℃, the yield decreases to 47.70%. The leaching rates of Al2O3 and SiO2 increase to 80.66 and 10.29%. Meanwhile, the grade of Al2O3 and SiO2 is 14.78 and 13.71%, respectively. The leaching rate of sodium aluminosilicate increases with the leaching temperature, enhancing the leaching rate of Al2O3. Therefore, the proposed optimal leaching temperature is 95℃.
The effect of leaching time on the recovery of Al2O3 is illustrated in Fig. 10(c). The yield decreases from 61.59 to 47.70% with an increase in the leaching time from 60 to 120 min. The leaching rates of Al2O3 and SiO2 increase from 71.82 and 9.26% to 80.66 and 10.29%, respectively. Prolonging the leaching time to 150 min, these indexes change slightly. Therefore, the proposed optimal leaching time is 120 min.
As shown in Fig. 10(d), the leaching rate of Al2O3 increases rapidly as the liquid-to-solid ratio raises from 3 to 10 mL/g. Moreover, as the liquid-to-solid ratio increases, the content of NaOH increases, and the particles possess a higher contact area to react with NaOH. Therefore, the optimal liquid-to-solid ratio is 10 mL/g.
It can be seen from Fig. 10(e) that the leaching rates of Al2O3 and SiO2 increase from 80.66 and 10.29% to 84.91 and 13.40%, respectively, with the particle size increasing from 75 to 90%, passing through a 200 mesh. According to the dynamic model, the leaching process is controlled by internal diffusion. When the particle size is refined, the diffusion rate increases, and the energy barrier required for the reaction decreases. Therefore, the optimal particle size is set at 90%, passing through a 200 mesh.
The main chemical composition of various leaching residues is presented in Table 8. The content of TiO2 and CaO of the alkaline leaching residue is 17.97 and 16.81%, respectively, exhibiting high utilization values. However, there is no effective measure to recover calcium and titanium from RM. In addition, it also contains 12.71% Al2O3, 14.58% SiO2, and 14.54% Na2O, respectively. According to the acid solubility difference between perovskite and gangue, the subsequent acid leaching can further remove Al2O3 and SiO2 to enrich TiO2. Keeping the liquid-to-solid ratio at 10mL/g, the leaching is performed at 30 ℃ for 30 min using 20 wt.% HCl to remove Al2O3 and SiO2, while CaO and TiO2 remain in the residue. The acid-leaching residue contains 46.53% TiO2 and 37.21% CaO. The main impurities in TiO2 were 6.04% Fe, 4.8% Al2O3, and 0.81% SiO2. To compare the mineral phase composition before and after acid leaching, XRD patterns are presented in Fig. 11. Before acid leaching, the main mineral phase is perovskite. In addition, there is a small amount of NaAlSi3O8. However, the diffraction peaks of NaAlSi3O8 disappear after acid leaching, indicating that SiO2 is removed. Additionally, the diffraction peaks of perovskite exhibit increased intensities, indicating that perovskite is purified by removing Al2O3, SiO2, and Na2O.
Table 8
The main chemical composition of various leaching residues (%).
Sample | Fe | Al2O3 | SiO2 | CaO | MgO | K2O | Na2O | TiO2 |
Alkaline leaching | 4.70 | 12.71 | 14.58 | 16.81 | 0.72 | 0.079 | 14.54 | 17.97 |
Acid leaching | 6.04 | 4.80 | 0.81 | 37.21 | 0.80 | 0.012 | 1.15 | 46.53 |