*2.2. Experimental Procedure*

Figure 3 illustrates the schematic of the experimental setup. First, 100 g of SAD was accurately weighed and mixed thoroughly with a certain amount of alkali. In order to produce the specific compounds during the roasting process, the formulation of the SAD and alkali was controlled according to the molar ratio of Na2O to Al2O3 (n(N/A)). The mixtures were dry pressed at 30 MPa to produce cylindrical samples with a diameter and length of 20 mm and 40 mm, respectively. The cylindrical samples were placed in a corundum crucible with dimensions of 120 mm × 80 mm × 30 mm and roasted in a muffle furnace under an atmospheric environment at a preset temperature for 1 h, with a constant heating rate of 10 ◦C min<sup>−</sup>1. The crucible was taken out and placed in a desiccator to cool to room temperature. The roasting clinkers were crushed to 200 mesh in a mortar and leached in caustic liquor to obtain NaAlO2 solution. The leaching residue was filtered and rinsed several times with boiling water. According to the industrial dissolution conditions of bauxite clinkers, the caustic concentration of the leaching solution was maintained at 60 g· <sup>L</sup><sup>−</sup>1.

**Figure 3.** Schematic of the experimental setup and procedure.

#### *2.3. Characterization Methods*

The Gibbs free energy of the chemical reactions that can occur during roasting was analyzed using the reaction module of the software package Factsage (ver. 7.0, Thermfact, Montreal, Canada). The contents of Al and Na in the roasting clinkers and leaching residues were determined using X-ray fluorescence (XRF), and the recovery efficiencies of the Al and Na were calculated using Equations (1) and (2), respectively:

$$\mathfrak{m}\_{\rm Al} = \left[ \mathrm{Al}\_1 - \mathrm{Al}\_2 \left( \mathrm{Mg}\_1 / \mathrm{Mg}\_2 \right) / \mathrm{Al}\_1 \right] \tag{1}$$

$$\eta\_{\rm Na} = \left[ \rm Na\_1 - \rm Na\_2 (Mg\_1/Mg\_2)/Na\_1 \right. \tag{2}$$

where Al1, Mg1 and Na1 are the contents of Al, Mg and Na in the roasting clinkers, respectively, wt.%; and Al2, Mg2 and Na2 are the contents of Al, Mg and Na in the leaching residues, respectively, wt.%. The content of soluble chlorate in the SAD samples was determined using a chloride ion activity meter (PCL-202, INESA Scientific Instruments, Shanghai, China). The content of AlN in the SAD samples was determined using the Kjeldahl method [27].

For the mineralogical study, an X-ray diffractometer (PW1710, Philips, Amsterdam, The Netherlands) was used with Cu Kα-radiation at 40.0 kV and 30.0 mA. The XRD tests were conducted within the 2<sup>θ</sup> range of 10–60◦ using a scanning speed of 0.1◦·min−1. Before testing, the samples were crushed to pass through a 200-mesh sieve and dried in an oven at 105 ◦C for 2 h. The diffractograms were identified with the help of the ICDD (International Centre for Diffraction Data) Powder Diffraction File (PDF-2) reference database and the Jade (vs. 6.0). The chemical composition of SAD was determined by XRF (mAX, AXIOS, Alamelo, The Netherlands). Micrographs of the roasting clinkers and leaching residues were observed using a scanning electron microscope (Regulus 8100, Hitachi, Japan) at an accelerating voltage of 15.0 kV. The microscopy samples were crushed to pass through a 200-mesh sieve and dried in an oven at 105 ◦C for 2 h, then sprayed evenly on the conductive adhesive and metal-lized with carbon in a vacuum evaporator (Q150T ES, Quorum, East Grinstead, Britain). An EDS (ul-tra-DLD, Shimadzu, Kyoto, Japan) connected with SEM was used to perform elemental anal-ysis on the sample particles. The elements with the content of 3%~20 wt.% have the accuracy of relative error <10%. And the elements with content greater than 20 wt.% have the higher accuracy of relative error <5%.
