3.2.2. Effects of Different Factors on the Mineralogical Phases

The mineralogical analysis of the roasting clinkers and leaching residues at various roasting temperatures is illustrated in Figure 6. The main phases in the roasting clinkers included NaAlO2, MgO, NaFeO2 and NaAlSiO4, which was consistent with the results of the thermodynamic analysis. The diffraction peak intensity of NaAlO2 in the roasting clinkers significantly increased from 1000 ◦C to 1050 ◦C, which was the critical solid phase of the Al recovery. Moreover, SiO2 played a negative role in the recovery of Al because SiO2 was able to react with soluble NaAlO2 to produce insoluble NaAlSiO4.

**Figure 6.** XRD patterns of the roasting clinkers (**a**) and leaching residues (**b**) under different roasting temperatures.

After the leaching process, the NaAlO2 in the roasting clinkers was completely dissolved, and the NaFeO2 was hydrolyzed to generate insoluble Fe(OH)3 and release NaOH. Therefore, the main phases in the leaching residues were MgO, MgAl2O4, Fe(OH)3 and NaAlSiO4. The diffraction peak intensity of MgO in the leaching residues continued to increase from 1000 ◦C to 1150 ◦C, while the diffraction peak intensity of MgAl2O4 significantly decreased. This indicates that an increase in the temperature was beneficial to the reaction (8). A small amount of MgAl2O4 was still observed in the roasting clinkers and leaching residues due to the low amounts of Na2CO3. The phase composition of the roasting temperature above 1150 ◦C remained unchanged, indicating that the mineralogical phase of the roasting clinkers was more effective at 1150 ◦C, which is consistent with the results shown in Figure 5c.

The XRD patterns of the roasting clinkers and leaching residues at different n(N/A) are shown in Figure 7. As the n(N/A) increased, the diffraction peak intensities of NaAlO2 and MgO in the roasting clinkers continued to increase, while the peak intensity of MgAl2O4 continued to decrease. This shows that the reaction (8) was facilitated by increasing the n(N/A). When the n(N/A) exceeded the value of 1.2, the SiO2 disappeared and NaAlSiO4 continued to decrease, while Na1.95Al1.95Si0.05O4 was generated as a result of the reaction (12), and this was easily dissolved in the alkali solution [29].

0.1SiO2 + 1.95Na2CO3 + 1.95Al2O3 = 2Na1.95Al1.95Si0.05O4 + 1.95CO2 (12)

**Figure 7.** XRD patterns of the roasting clinkers (**a**) and leaching residues (**b**) under different Na2CO3/SAD mass ratios.

With the increase in the n(N/A) from 1.0 to 1.3, the MgO in the leaching residues significantly increased, while the MgAl2O4 continued to decrease, which is consistent with the XRD results of the roasting clinkers. The phase composition of the system with an n(N/A) of over 1.3 was essentially the same as that of the system with a lower molar ratio, indicating that the mineralogical phases of the roasting clinkers with the n(N/A) of 1.3 were more effective, which is consistent with the results shown in Figure 5d.

#### *3.3. Effects of Different Factors on the Leaching System*

Since the structure of solid NaAlO2 is different from the structure of the aluminate ions in the solution, the dissolution of NaAlO2 in the roasting clinkers is actually a chemical reaction, as given by the reaction (13) [30].

$$\mathrm{Na\_2O\cdot Al\_2O\_3(s) + 4H\_2O} = \mathrm{Na^+} + 2\mathrm{Al(OH)\_4^-} \tag{13}$$

In this section, the effects of various leaching factors including the leaching temperature, leaching time and liquid-to-solid ratio on the recovery of Al were evaluated. These experiments were conducted at the roasting temperature of 1150 ◦C, n(N/A) of 1.3 and roasting time of 1 h.

## 3.3.1. Effects of the Leaching Temperature

Figure 8a shows the recovery efficiency of the Al element in the roasting clinkers at different leaching temperatures as a function of the leaching time. The reaction was severe within the first 15 min, and the NaAlO2 in the roasting clinkers was continuously dissolved. However, no significant increase in the recovery of the Al was observed with the further extension of the time. When the leaching temperature was increased from 30 ◦C to 90 ◦C, the 15 min Al recovery increased from 68.79% to 88.11%. The leaching temperature had a significant impact on the recovery of the Al, because the increase in the temperature was beneficial in accelerating the mass transfer and diffusion.

**Figure 8.** Recovery of Al in the roasting clinkers under different leaching temperatures (**a**) and L/S ratios (**b**), with fitting experimental data for different reaction orders: *n* =0(**c**), *n* =1(**d**) and *n* =2(**e**), and the half-life of leaching process at *n* = 1 and *n* =2(**f**).
