*3.5. Thermal Analysis and Product Characterization*

In order to further evaluate the phase transition of the roasting process, the thermogravimetric and differential scanning calorimetry (TG-DSC) curves of the mixed materials of SAD and Na2CO3 (n(N/A) of 1.3) at a heating rate of 10 ◦C·min−<sup>1</sup> are shown in Figure 10.

**Figure 10.** TG-DSC curves of SAD–Na2CO3 mixtures for the n(N/A) of 1.3.

The weight loss of the mixed materials can be divided into three stages: (1) from room temperature to 550 ◦C, the weight loss was caused by the evaporation of the attached water, and the weight was reduced by 1.24%; (2) from 550 ◦C to 1050 ◦C, CO2 was released due to the reaction of Na2CO3 with the components in the SAD, resulting in a weight loss

of 8.78 wt.%, which is also supported by the fact that Na2CO3 cannot undergo thermal decomposition within the temperature range of 550–1050 ◦C; (3) from 1050 ◦C to 1200 ◦C, the weight of the sample remained basically unchanged, indicating that the main reaction was completed.

The DSC curve showed three obvious endothermic peaks at 564 ◦C, 686 ◦C and 820 ◦C in the heating process of the mixed materials. Additionally, two obvious exothermic peaks at 904 ◦C and 1052 ◦C were observed. The enthalpy changes in the main reactions during the roasting process (see Section 3.1) are presented in Table 3, which indicate that the reactions (7)–(9) were endothermic, while the reactions (10) and (11) were exothermic. Previous studies have shown that Al2O3 and Na2CO3 react mainly at 500–700 ◦C, whereas Fe2O3 reacts with Na2CO3 at about 850 ◦C. Combining the thermodynamics calculations, XRD results and the TG-DSC analysis, the endothermic peaks at 564 ◦C, 686 ◦C and 820 ◦C should be attributed to reactions (7), (8) and (9), respectively. In addition, the exothermic peaks at 904 ◦C and 1052 ◦C corresponded to reactions (11) and (10), respectively, which were determined using the changes in enthalpy. The main chemical reactions of the roasting process were completed at around 1150 ◦C, which was consistent with the optimal roasting temperature derived in Section 3.2.

**Table 3.** Enthalpy changes in the reactions during the roasting process.


The micromorphology of the roasting clinkers and leaching residues with the n(N/A) of 1.3 were observed using a scanning electron microscope (SEM). The energy dispersive spectroscopy (EDS) images are shown in Figure 11e–h. Figure 11a,b shows that the particles in the roasting clinkers were clustered and had a particle size of about 10–30 μm. The shape of the NaAlO2 particles was smooth and irregular. The morphology of the MgO particles was agglomerated and spherical, with a particle size of 2–4 μm. Figure 11c shows that the NaAlSiO4 in the leaching residues had a smooth block-shaped appearance, while MgAl2O4 was octahedral, as shown in Figure 11d.

The chemical compositions of the roasting clinkers and leaching residues under optimal conditions (roasting temperature of 1150 ◦C, Na2CO3/Al2O3 molar ratio of 1.3, roasting time of 1 h, leaching temperature of 90 ◦C, L/S ratio of 10 mL·g−<sup>1</sup> and leaching time of 30 min) are presented in Table 4. The contents of N and Cl in the roasting clinkers reduced to 0.072 wt.% and 0.12 wt.%, respectively. Moreover, the AlN was oxidized to produce harmless N2, and the chloride was evaporated to the gaseous phase, which was recovered after the condensation. The removals of N and Cl reached 98.93% and 97.14%, respectively. Therefore, the green toxification of SAD can be achieved in the roasting process without any pretreatment.

**Table 4.** Compositions of the roasting clinkers and the leaching residues under optimal conditions (%).


**Figure 11.** SEM micrographs of the roasting clinkers (**a**,**b**), leaching residues (**c**,**d**) and EDS (**e**–**h**).
