**3. Results and Discussion**

#### *3.1. Thermodynamic Analysis*

The amounts of the reactants and products under different roasting conditions were predicted using thermodynamic analysis. Two alkalis (NaOH and Na2CO3) were selected as additives to perform the thermodynamic analysis. The possible reactions and the changes in Gibbs free energy are shown in Equations (3)–(11) and Figure 4.

$$\text{Al}\_2\text{O}\_3 + 2\text{NaOH} = 2\text{NaAlO}\_2 + \text{H}\_2\text{O} \tag{3}$$

$$\text{MgAl}\_2\text{O}\_4 + 2\text{NaOH} = 2\text{NaAlO}\_2 + \text{MgO} + \text{H}\_2\text{O} \tag{4}$$

$$\text{Fe}\_2\text{O}\_3 + 2\text{NaOH} = 2\text{NaFeO}\_2 + \text{H}\_2\text{O} \tag{5}$$

$$2\text{AlN} + 2\text{NaOH} + 1.5\text{O}\_2 = 2\text{NaAlO}\_2 + \text{H}\_2\text{O} + \text{N}\_2\tag{6}$$

$$\text{Al}\_2\text{O}\_3 + \text{Na}\_2\text{CO}\_3 = 2\text{NaAlO}\_2 + \text{CO}\_2 \tag{7}$$

$$\rm MgAl\_2O\_4 + Na\_2CO\_3 = 2NaAlO\_2 + MgO + CO\_2 \tag{8}$$

$$\text{Fe}\_2\text{O}\_3 + \text{Na}\_2\text{CO}\_3 = 2\text{NaFeO}\_2 + \text{CO}\_2 \tag{9}$$

$$2\text{AlN} + \text{Na}\_2\text{CO}\_3 + 1.5\text{O}\_2 = 2\text{NaAlO}\_2 + \text{CO}\_2 + \text{N}\_2\tag{10}$$

$$\text{NaO}\_2 + \text{NaAlO}\_2 = \text{NaAlSiO}\_4 \tag{11}$$

**Figure 4.** Relationship of the changes in Gibbs free energy with the temperature for the reactions during the roasting process.

From the reaction curves (3)–(6), it can be seen that the Gibbs free energy of the reactions of Al2O3, MgAl2O4, Fe2O3 and AlN with NaOH were negative at room temperature, which indicates that the thermodynamic reaction conditions are sufficient. The Al2O3 of MgAl2O4 phase was combined with NaOH to obtain NaAlO2 and release MgO. Moreover, AlN was oxidized to generate non-toxic N2. The reaction curves (7)–(9) show that the Gibbs free energy values of the reactions were negative only at higher temperatures, which indicates that Na2CO3 was less active compared to NaOH. The reaction curve (10) shows that the thermodynamic conditions for the reaction of AlN and Na2CO3 to produce NaAlO2, CO2 and N2 were sufficient. From the reaction curve (11), it is clear that insoluble NaAlSiO4 was produced by the combination of SiO2 and NaAlO2, resulting in the loss of alumina in the SAD.

These results demonstrate that the reactions of the SAD with the two alkali additives (NaOH and Na2CO3) were feasible. All the Al-containing compounds (Al, Al2O3, AlN and MgAl2O4) in SAD can react with alkali additives to form readily soluble NaAlO2. In addition, the other impurities (Mg, Fe, and Si) generated insoluble species, thus achieving the separation and extraction of valuable aluminum elements from SAD.

#### *3.2. Effects of Different Factors on the Roasting System*

In order to ensure that the desired compounds are produced in the clinkers, the extent of the roasting of the mixed materials should be strictly controlled. Since both the alkali additives can thermodynamically react with SAD to produce the expected compounds, the effects of these alkali additives on the roasting system were examined.

The effects of different factors on the recoveries of Al and Na were studied by conducting a variety of experiments for each additive type, various roasting temperatures and ingredients depending on the n(N/A) (see Table 2 and Figure 5).


**Table 2.** List of roasting parameters studied in the experiments.

**Figure 5.** Recovery of Al and Na in roasting clinkers under different roasting conditions: roasting temperature with NaOH additive (**a**), n(N/A) with NaOH additive (**b**), roasting temperature with Na2CO3 additive (**c**), and n(N/A) with Na2CO3 additive (**d**).

3.2.1. Effects of Different Factors on the Recoveries of Al and Na

For the alkali additives, NaOH and Na2CO3, the recoveries of Al and Na in the clinkers under different temperatures are shown in Figures 5a and 5b, respectively. As is evident from Figure 5a,b, the roasting temperature had a significant effect on the recoveries of Al and Na. With the increase in the roasting temperature, the recoveries of both the Al and Na initially increased and then decreased. When NaOH was used as the additive, the recoveries of Al and Na reached the maximum values of 83.47% and 89.06%, respectively, at the roasting temperature of 950 ◦C. However, when Na2CO3 was used as the additive, the highest recoveries of Al and Na had values of 80.90% and 88.72%, respectively, at the roasting temperature of 1100 ◦C. Additionally, the recoveries of Al and Na were similar in the within ±50 ◦C interval of the optimum temperature, which indicates that the range of the optimum temperature was wide and simple to control.

On the one hand, the mixed materials primarily relied on the solid−solid reaction during the roasting process. The generation of less liquid phase in the clinkers at low roasting temperatures led to slow reaction rates, large porosity and poor dissolution performance. On the other hand, the generation of a large amount of liquid phase in the clinkers at high roasting temperatures resulted in the rapid volatilization of the alkali, low porosity and poor dissolution performance [28]. Therefore, the appropriate liquid phase and porosity of clinkers are available only at a certain roasting temperature, which offers a better reaction rate and dissolution performance.

The n(N/A) is a critical parameter which affects the extraction of the valuable Al element from SAD. The n(N/A) represents the amount of alkali additives which have been added to the system. For the additives NaOH and Na2CO3, the recoveries of Al and Na in the clinkers under different n(N/A) are shown in Figures 5c and 5d, respectively. As shown in Figure 5c, when the n(N/A) was increased from 1.0 to 1.2, the recovery of Al increased from 83.47% to 91.83%, whereas that of Na increased from 89.36% to 93.64%. When Na2CO3 was used as the additive, the n(N/A) of 1.3 was optimal for extracting NaAlO2, while the Al and Na recoveries were 90.79% and 92.03%, respectively. This could be due to the fact that the low quantities of alkali were not sufficient enough to adequately support the reaction of the aluminum-containing compounds (Al, Al2O3, AlN, and MgAl2O4) in SAD with Na2O to produce soluble NaAlO2, thus resulting in a poor dissolution performance of the clinkers. With the increase in the n(N/A), the recoveries of Al and Na gradually declined. This can be attributed to the production of some insoluble substances due to excessive amounts of alkali, which prevented the further dissolution of NaAlO2.

It is worth mentioning that the NaOH additive resulted in a lower roasting temperature and slightly higher Al and Na recoveries compared to the Na2CO3 additive under the optimal roasting conditions. However, due to the NaOH deliquescence, the mixed materials were able to easily absorb the moisture in the air during the experiment, thus causing the hydrolysis of the AlN in SAD and releasing a large amount of toxic ammonia. Additionally, the produced cylindrical samples quickly lost their strengths, forming into a slurry. Therefore, Na2CO3 was selected as the appropriate additive for the subsequent experiments.
