*Article* **Preparation of NaA Zeolite from High Iron and Quartz Contents Coal Gangue by Acid Leaching—Alkali Melting Activation and Hydrothermal Synthesis**

**Deshun Kong 1,2 and Rongli Jiang 1,\***


**Abstract:** In this study, NaA zeolite was successfully synthesized from coal gangue with high contents of iron and quartz as the main raw material. The results show that most iron ions can be removed from coal gangue after calcination at 700 ◦C for 2 h, leaching in hydrochloric acid with a mass fraction of 20% for 7 h and a liquid-solid ratio of 3.5:1. When *m* (acid leached residue of calcined gangue):*m* (Na2CO3) = 1.1 and melting at 750 ◦C for 2 h, the quartz and other aluminosilicates turn into nepheline, which dissolve in water. Finally, the optimum conditions of synthesis NaA zeolite are as follows: *n*(SiO2)/*n*(Al2O3) = 2.0, *n*(Na2O)/*n*(SiO2) = 2.1, *n*(H2O)/*n*(Na2O) = 55, aging at 60 ◦C for 2 h, and crystallization at 94 ◦C for 4 h. This shows that the high iron and quartz contents coal gangue can be used for the synthesis of NaA zeolite.

**Keywords:** high iron and quartz contents coal gangue; acid leaching; alkali melting; hydrothermal reaction; NaA zeolite

## **1. Introduction**

Coal gangue is the main discharged waste of the coal industry [1,2]. The arbitrary stacking of it seriously influences the safety of the ecological environment [3,4]. Recently, the development and utilization of coal gangue as a kind of new resource has gained interest [5–7]. Many products have been prepared from coal gangue, but the common utilization methods of coal gangue are as power plant fuel [8] and to prepare building materials [9], including brick [10], cement clinker products [11] and other products. The utilization technology level is lower and the industrial added value is not high. On the other hand, zeolite synthesis often uses cheap minerals or waste, which can save the cost of raw materials. Coal gangue can be used as raw material for preparing zeolites [12,13]. In recent years, researchers have noticed the great potential of coal gangue in zeolite synthesis [14–16].

Zeolites are micropore and mesoporous hydrated aluminosilicates containing alkali elements, alkaline earth metals, or other cations, whose structure is built up with a framework of tetrahedral molecules, which are linked with shared oxygen atoms [17]. Due to their unique properties, zeolites have been used in many fields, such as in agriculture [18], chemical technology [19], oil refining [20], and others for their porous characteristics, ionexchange properties, and catalytic performance [21]. Many studies have succeeded in the conversion of low iron-bearing coal gangue into synthetic zeolites [22–24]. However, coal gangue with high iron content is a kind of material with poor quality. It is difficult to synthesize high quality zeolite because iron ions will affect the whiteness and performance of the products, therefore, there are only a few studies concerning what happens in the transformation process of the whole NaA zeolite preparation from high iron content coal gangue.

**Citation:** Kong, D.; Jiang, R. Preparation of NaA Zeolite from High Iron and Quartz Contents Coal Gangue by Acid Leaching—Alkali Melting Activation and Hydrothermal Synthesis. *Crystals* **2021**, *11*, 1198. https://doi.org/ 10.3390/cryst11101198

Academic Editor: Sergio Brutti

Received: 12 September 2021 Accepted: 30 September 2021 Published: 3 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In this work, coal gangue is firstly calcined and then leached by acid to remove most of the iron ions; at the same time, there is very low chemical activity of kaolinite and quartz in the coal gangue, which are necessary to be activated before synthesizing zeolite. However, the quartz in gangue is difficult to activate by heating, which will directly enter the zeolite products [25]. In zeolite synthesis, the alkali melting method, as an activator for the formation of soluble aluminate and silicate [26–28], is adopted to activate the materials richly with inert silica or alumina in the presence of alkali. In the process, the acid-leached residue of coal gangue is activated absolutely by Na2CO3 powder at 750 ◦C for 2 h after calcination of the coal gangue powder at 700 ◦C for 2 h, and leaching in 20% hydrochloric acid at 90 ◦C for 7 h under stirring. The NaA zeolite is obtained following aging and hydrothermal synthesis. For further discussion of the mechanism of phase transformations, the gangue, acid-leached residue, alkali-melted intermediate products, and the NaA zeolite, which are prepared under different conditions, are detected by XRD; the phase transformation laws are obtained; and the suitable technological conditions for preparing NaA zeolite from high iron and quartz contents coal gangue are determined.

## **2. Experimental**

#### *2.1. Materials*

The coal gangue comes from Wangjiazhai Coal Mine in Liupanshui, Guizhou Province, China, which is made into powder of less than 200 mesh. NaAiO2 and Na2CO3 are analytically pure (AR), Tianjin city Beichen Founder Reagent Factory; Tianjin, China; NaOH is AR, Tianjin Zhiyuan Chemical Reagents Co., Ltd. Tianjin, China; and hydrochloric acid is AR, Chongqing Chuandong Chemical Co., Ltd. Chongqing, China.

#### *2.2. Technological Process*

The coal gangue is calcined at 700 ◦C for 2 h and then leached by 20% (in mass) hydrochloric acid; the liquid to solid ratio is 3.5:1 (volume/mL:mass/g) at 90 ◦C for 7 h; and the acid leached residue is obtained after filtering and drying, and then the residue is mixed with sodium carbonate at the ratio of *m* (acid leaching residue):*m* (Na2CO3) = 1.1, melting at 750 ◦C for 2 h in the muffle, by adjusting the reacting proportion and aging; the hydrothermal crystallization is completed under the stirring condition of 94 ◦C for several hours, and NaA powder zeolite is obtained after washing and drying.

The preparation process of NaA zeolites from high-iron content coal gangue is shown in Figure 1.

#### *2.3. Characterization*

TD-2500 type X-ray diffraction instrument (XRD, China) was used for CuK<sup>α</sup> (λ for K<sup>α</sup> = 1.54059 Å), 2θ = 3◦ (min)–65◦ (max), with a step width of 0.04◦. The major chemical elemental compositions were detected by Thermo Electron ARL9900XP+ type X-ray fluorescence spectrometer (XRF, Massachusetts, USA). The morphology of the products was detected by Zeiss evo18 type scanning electron microscope (SEM, Jena, Germany).

**Figure 1.** Process flow chart of preparing NaA zeolite from high iron coal gangue.

#### **3. Results and Discussions**

#### *3.1. Major Chemical Elements Analysis of Coal Gangue and the Acid Leached Residue*

The raw coal gangue and the acid leached residue are dried at 105 ◦C for 12 h [29], the main compositions are shown in Table 1.


**Table 1.** Main compositions of coal gangue and the acid-leaching residue/*wt* %.

We can see from Table 1 that the main compositions of the gangue are silicon, aluminum, and iron; obviously the iron element content is too high, which will affect the properties of the NaA product, so it must be removed. After acid leaching by hydrochloric acid, the major chemical elements of the residue are silicon and aluminum; the contents of other elements are low, *n*(SiO2)/*n*(Al2O3) = 13.46, compared with NaA zeolite (*n*(SiO2)/*n*(Al2O3) = 2.0), so it is necessary for NaAlO2 to adjust the *n*(SiO2)/*n*(Al2O3) ratio of the system.

The morphology analyses of the gangue powder, calcined gangue, and residue are shown in Figures 2–5.

**Figure 2.** SEM image of raw gangue powder.

**Figure 3.** SEM image of 700 ◦C calcined gangue.

**Figure 4.** SEM image of acid-leached residue.

**Figure 5.** The SEM image of acid-leached residue (taken from Figure 4).

As seen, the particles of raw gangue powder are relatively uniform at 15–20 μm or larger in Figure 2; after calcination, the particle size decreases and fragments appear. After acid leaching, the number of flaky debris increases significantly. Most particle sizes are less than 5 μm, which is due to the overflow of water in coal gangue after calcination. Moreover, kaolinite decomposes into amorphous Al2O3 and SiO2, at 700 ◦C; most of the iron content and part of Al2O3 are dissolved in the acid solution.

In order to observe the particle morphology of acid-leached residue more clearly, we enlarged a region in Figure 4 and marked it red, then got Figure 5. Figure 5 shows that many small particles become smoother, this is because the edges and corners of some particles are worn off by stirring, which makes the raw material more fragmented, so it is conducive to the alkali-melting reaction in the next step.

#### *3.2. Phase Analysis of the Residue and Activation Product*

Figure 6 shows that the crystal substances in the coal gangue mainly contain kaolinite and quartz, as well as a small amount of pyrite and siderite. Table 1 shows that the iron element content in the coal gangue is very high, but the diffraction peaks of iron-containing materials are very weak in Figure 6, which indicates that the iron materials are amorphous. After calcination, kaolinite turns into metakaolin, its crystal structure is destroyed, and the carbon in coal gangue is removed.

**Figure 6.** XRD patterns of coal gangue powder at 750 ◦C, calcined powder, and acid-leached residue.

The quartz is inert and it is difficult for it to participate in the crystallization reaction directly, so it must to be activated before the crystallization reaction. Therefore, according to the mass ratio of *m* (acid leached residue):*m* (sodium carbonate) = 1:1.1, after mixing the sodium carbonate evenly with the residue, the mixture is melted at 750 ◦C for 2 h, then the melted powder is analyzed by XRD and added to the water to be fully grinded then filtered; the XRD analyses are shown in Figure 7.

**Figure 7.** XRD spectra of alkali-melted product and its water washing filter residue.

Figure 7 shows that the alkali melted product is mainly nepheline (nepheline, PDF card number: 76-1733), its chemical formula is NaAlSiO4, and the main reaction equations of acid-leached filter residue with sodium carbonate at 750 ◦C are as follows:

$$\rm Na\_2CO\_3 + SiO\_2 = Na\_2SiO\_3 + CO\_2\uparrow \tag{1}$$

$$\text{Al}\_2\text{O}\_3 \cdot 2\text{SiO}\_2 + \text{Na}\_2\text{CO}\_3 = 2\text{NaAlSiO}\_4 + \text{CO}\_2\uparrow\tag{2}$$

The metakaolin and quartz, which contain silicon and aluminum substances, can react with Na2CO3 at 750 ◦C, and nepheline can dissolve in water, so it can be used as silicon and aluminum sources to synthesize NaA zeolite. Therefore, this method can completely activate the quartz in the acid-leached residue; it not only takes full advantage of the silicon and aluminum sources in the residue but also avoids the quartz entering NaA zeolite products, which increases the quality of NaA products, so the soluble intermediate product NaAlSiO4 with higher chemical activity is beneficial to the hydrothermal crystallization reaction.

The rest filter residue contains amorphous SiO2 and Al2O3, they both have high chemical activity and can react with sodium aqueous solution in the hydrothermal synthesis period; the reactions are as follows:

$$\text{SiO}\_2 + 2\text{NaOH} = \text{Na}\_2\text{SiO}\_3 + \text{H}\_2\text{O} \tag{3}$$

$$\text{Al}\_2\text{O}\_3 + 2\text{NaOH} = 2\text{NaAlO}\_2 + \text{H}\_2\text{O} \tag{4}$$

Na2SiO3 and NaAlO2 can be used as the active silicon source and aluminum sources to synthesis NaA zeolite. Because the synthesis of NaA zeolite is closely related to *n*(SiO2)/*n*(Al2O3), *n*(Na2O)/*n*(SiO2), *n*(H2O)/*n*(Na2O), aging temperature, and crystallization time, so the experiments focused on these five factors.

#### *3.3. Effects of n(SiO2)/n(Al2O3) on the Product Phase*

Setting the reaction system to *n*(H2O)/*n*(Na2O) = 50; *n*(Na2O)/*n*(SiO2) = 1.2; aging at 50 ◦C for 1 h; crystallization at 94 ◦C for 4 h,; and changing *n*(SiO2)/*n*(Al2O3) = 1.0, 1.5, 2.0, and 2.5 respectively, after hydrothermal crystallization reaction, the XRD analyses are shown in Figure 8.

**Figure 8.** Effect of n(SiO2)/n(Al2O3) on the phase.

Figure 8 shows when *n*(SiO2)/*n*(Al2O3) = 1.0, the diffraction peak intensity of the product is very low, this is because the crystallinity and the zeolite type highly depend on the *n*(SiO2)/*n*(Al2O3) ratios [30–34]; when *n*(SiO2)/*n*(Al2O3) = 1.0 or 1.5, they deviate from the target NaA zeolite product *n*(SiO2)/*n*(Al2O3) = 2.0, which is not conducive to the nucleation and growth of zeolite; when *n*(SiO2)/*n*(Al2O3) = 2.0, the diffraction intensity increases. The search result shows that the final product is NaA zeolite (PDF card number: 39-0223), there is a further increase of *n*(SiO2)/*n*(Al2O3) = 2.5, and the diffraction intensity also increases. If the *n*(SiO2)/*n*(Al2O3) ratio is too high, the higher *n*(SiO2)/*n*(Al2O3) ratio products such as NaX and NaP zeolites may appear; in order to take full use of the silicon and aluminum components, *n*(SiO2)/*n*(Al2O3) = 2.0 is selected.
