*3.4. Effects of n(Na2O)/n(SiO2) on the Phase*

Setting the reaction system to *n*(SiO2)/*n*(Al2O3) = 2.0; *n*(H2O)/*n*(Na2O) = 60; aging at 50 ◦C for 1 h; crystallization at 94 ◦C for 4 h; and changing *n*(Na2O)/*n*(SiO2) = 1.2, 1.7, 1.9, and 2.1, respectively, after finishing reactions, the XRD analyses of the products are shown in Figure 9.

In Figure 9, the intensity of these spectra is high and their shapes are similar. This is because *n*(Na2O)/*n*(SiO2) works together with *n*(H2O)/*n*(Na2O); they change the alkalinity of the solution and affect the growth rate of zeolite. When the *n*(Na2O)/*n*(SiO2) ratio is larger, the system alkalinity is higher, which promotes the dissolution rate of the materials and the growth rate of NaA zeolite crystal; when *n*(Na2O)/*n*(SiO2) = 2.1, the intensity of diffraction peaks is high and there are no other miscellaneous crystals, so *n*(Na2O)/*n*(SiO2) = 2.1 is selected.

**Figure 9.** Effects of *n*(Na2O)/*n*(SiO2) on the phase.

## *3.5. Effects of n(H2O)/n(Na2O) on the Phase*

Setting the reaction system to *n*(SiO2)/*n*(Al2O3) = 2.0; *n*(Na2O)/*n*(SiO2) = 1.7; the aging temperature as 50 ◦C; aging time as 2 h; crystallization at 94 ◦C for 4 h; and changing *n*(H2O)/*n*(Na2O) = 50, 55, 60, and 65 respectively, after the reactions, the XRD analyses are shown in Figure 10.

**Figure 10.** Effect of *n*(H2O)/*n*(Na2O) on the phase.

We can see from Figure 10, as the value of *n*(H2O)/*n*(Na2O) increases, the diffraction peak intensity decreases gradually. When *n*(H2O)/*n*(Na2O) = 55, the intensity of the diffraction peaks are at maximum, and this is due to the high alkalinity concentration, which lead to large viscosity of the system and is not conducive to the mass transfer process, but is adverse to the dissolution of solid silicon and aluminum components in the system [35]. The reaction system cannot provide sufficient highly active raw material for crystal growth, which causes the intensity of the peaks to decrease, and so *n*(H2O)/*n*(Na2O) = 55 is selected.

#### *3.6. Effect of Aging Temperature on the Phase*

Setting reaction system *n*(SiO2)/*n*(Al2O3) = 2.0; *n*(H2O)/*n*(Na2O) = 55; *n*(Na2O)/*n*(SiO2) = 2.1; the aging time as 2 h; crystallization at 94 ◦C for 4 h; and changing aging temperature to 30 ◦C, 40 ◦C, 50 ◦C, and 60 ◦C respectively, after the crystallization reaction, the XRD analyses of the products are shown in Figure 11.

**Figure 11.** Effect of aging temperature on the phase.

The effect of aging temperature is reflected in the process of transforming the liquid sol into zeolite. It can be seen from Figure 11 that when the aging temperature is 30 ◦C or 40 ◦C, the intensity of diffraction peaks is low; when the aging temperature is 50 ◦C or 60 ◦C, the intensity is high, and this is because the low-temperature aging stage can improve the nucleation rate, reduce the grain size, and increase the number of crystals [36]. When the aging temperature is 50 ◦C or 60 ◦C, the raw materials can be fully dissolved and the nucleation will grow sooner, but if the aging temperature is too high, it will reduce the number of nucleation, so the proper aging temperature is 60 ◦C.

#### *3.7. Effect of Crystallization Time on the Phase and Morphology*

Setting reaction system *n*(SiO2)/*n*(Al2O3) = 2.0; *n*(H2O)/*n*(Na2O) = 65; *n*(Na2O)/*n*(SiO2) = 2.1; aging at 60 ◦C for 2 h; crystallization temperature as 94 ◦C, and changing the crystallization time to 1 h, 2 h, 3 h, and4hrespectively, after the reactions, the XRD patterns of the products are shown in Figure 12, and the SEM images are shown in Figures 13–16.

The crystallization time is particularly important for synthesis zeolites; it mainly affects their crystallinity. We can see from Figure 12, when the crystallization time is 1 h or 2 h, the intensity of the peaks is low; when the crystallization time is 1 h, just a small number of little NaA crystals begin to appear (Figure 13); most of the crystals are about 1 μm in size. With the extension of reaction time to 2–4 h, a large number of crystals begin to appear, the crystals are a regular cubic shape and the particle size is about 2 μm. At the same time, when the crystallization time is 4 h, the intensity of the peaks is relatively higher, which is due to the longer crystallization time that gives zeolite enough time to grow, so the crystallization reaction time is determined to be 4 h.

**Figure 12.** Effect of crystallization time on the phase.

**Figure 13.** The crystallization time is 1 h.

**Figure 14.** The crystallization time is 2 h.

**Figure 15.** The crystallization time is 3 h.

**Figure 16.** The crystallization time is 4 h.

#### *3.8. XRD and SEM Analyses of the Product under Optimized Conditions*

The product prepared under the above optimized conditions was detected by XRD and SEM; the results are shown in Figures 17 and 18.

After searching, the d values and 2 theta values are in good agreement with PDF card: 39-0223, so the product can be confirmed as NaA zeolite. As shown in Figure 17, the product is pure NaA zeolite, the diffraction peaks are sharp, and the intensity of the diffraction peaks are high, which indicates that the crystallinity of the product is high. Figure 18 shows that NaA zeolites prepared under the optimized conditions have regular cubic shapes and a uniform particle size of about 2.5 μm.

**Figure 17.** XRD spectra of the product under optimized conditions.

**Figure 18.** SEM photo of the product under optimized conditions.

## *3.9. Preparation Mechanism*

The NaA zeolite is from high iron content coal gangue; many chemical reactions occurred in the process, in addition to the reactions mentioned above. Nepheline is produced by high temperature reaction and can be hydrolyzed under alkaline conditions. The equations are as follows:

$$2\text{NaAlSiO}\_4 + 4\text{NaOH} = 2\text{NaAlO}\_2 + 2\text{Na}\_2\text{SiO}\_3 + 2\text{H}\_2\text{O} \tag{5}$$

$$\text{SiO}\_2 + 2\text{NaOH} = \text{Na}\_2\text{SiO}\_3 + \text{H}\_2\text{O} \tag{6}$$

$$\text{Al}\_2\text{O}\_3 + 2\text{NaOH} = 2\text{NaAlO}\_2 + \text{H}\_2\text{O} \tag{7}$$

The NaAlSiO4 dissolves in NaOH solution to form NaAlO2 and Na2SiO3, the amorphous SiO2 in the residue reacts with NaOH solution to form Na2SiO3, and amorphous

Al2O3 in the residue reacts with NaOH solution to form NaAlO2 [37–39]. In the silicate ions and aluminate ions system, the primary structural unit of silica–alumina zeolite skeleton is a silica–oxygen tetrahedron and alumina–oxygen tetrahedron, which are both called TO4, as shown in Figure 19.

**Figure 19.** The formation process of NaA zeolite.

As the hydrothermal reaction continues, the TO4 in the reaction system is connected through oxygen atoms, and the tetrahedrons formed are connected through an oxygen bridge to form a ring; four tetrahedrons make up a four-membered ring and six tetrahedrons make up a six-membered ring (the oxygen atoms are omitted, Figure 19) [40]. These rings are reduced to quadrangles and hexagons, and each of them have a Si or Al ion at the tip of the corner. Under the action bridging of cations, silicon and aluminum ions in the rings are further condensed around cations and continue to connect in three-dimensional space to form β cages (Figure 19). The β cage is a chamfered octahedron containing 6 fourmembered rings; 8 six-membered rings; and 24 angular apex, β-cages arranged in the form of body-centered cubes, which are connected by a double quaternion ring, resulting in an α cage (Figure 19) and a three-dimensional skeleton structure at the center of the cell.

When the cages are formed, they will continue to form the zeolite cages; the zeolite cages will continue to expand in a three-dimensional direction according to the bodycentered cubic structure. When there has been a certain amount of expansion, they will generate a certain geometric shape of the grain. The little grain seeds continue to grow in the hydrothermal system and finally form NaA zeolite crystals.

#### **4. Conclusions**

Through this study, these conclusions are drawn as follows:


Compared with [41,42], the preparation conditions obtained in this study are more mild, the purity of the product is higher, and there are no impurity crystals in the product.

**Author Contributions:** D.K.: conceptualization, writing—original draft, investigation, methodology, data curation. R.J.: methodology, writing-reviewing, editing and supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study is financially supported by Guizhou Provincial Education Department's Scientific and Technological Innovation Team Project (NO. [2017] 054), Guizhou Science and Technology Foundation Project (NO. [2018] 1142), and Liupanshui City Science and Technology Foundation (NO. 52020-2019-05-17).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

