Impacts of Thermal Activation on Physical Properties of Coal Gangue: Integration of Microstructural and Leaching Data

Abstract

The recycling of coal gangue has considerable potential to produce secondary environmental hazards, which significantly influence the high-end application of coal gangue in practical engineering. The present study investigates the effects of activation treatment on the physical, chemical properties and leaching behavior of coal gangue. The mineral composition, micro-pore structure and element leaching were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Thermogravimetry Analysis (TG), Low-Temperature Nitrogen Adsorption (LTNA) and Inductively Coupled Plasma (ICP). The results indicate that kaolinite and pyrite in coal gangue experienced reconstruction after 600 °C during thermal activation. The density of thermally activated coal gangue is increased with the calcination temperature as well as the alkalinity (from 4.8–7.1) due to the burning of organic and the oxidation of pyrite. The calcination treatment induced the reduction in macropore volume (>50 nm), and enhancement in mesopore (<5 nm) volume. Leachable Ni, Cd, Mn, Cu, Zn and Pd decreased by 99%, 67%, 86%, 40%, 99% and 93% after calcination at 800 °C, respectively. The Si and Al in 800 °C calcined coal gangue exhibited a high leaching ability in alkalinity solution; leachable Al reached 106.4 mg/kg, while leachable Si reached 86.1 mg/kg after 48 h of dynamic leaching.

1. Introduction

Coal gangue is a solid waste produced in the process of coal mining and washing, which accounts for 10–15% of the raw coal production. According to the National Bureau of Statistics, the raw coal production in 2023 was approximately 4.71 billion tons in China, and coal gangue production in the same year was approximately 825 million tons. Meanwhile, the coal gangue stock is growing at an average of 700 million tons per year and now covers an area of 70 km². Some statistical data show that the utilization rate of coal gangue in China is only 60–70% [1]. The disposal of coal gangue has been the main problem for the coal mining industry in China.

The considerable quantity of coal gangue has raised significant environmental concerns; since organic matter and sulfide always exist in coal gangue, NO_X, SO_X and other noxious gases and particulates could be released during spontaneous combustion, which pose a considerable risk to the environment [2]. In addition, the leaching of heavy metals was reported as the main form of pollution by the disposal of coal gangue [2]. It has been demonstrated that coal gangue contains a range of leachable heavy metal elements, with the total amount released exceeding the limits set out in relevant standards. These heavy metals and other deleterious elements will percolate through the surface and infiltrate the groundwater, resulting in significant contamination of the surrounding soil and groundwater resources [2]. These properties inhibit the disposal and utilization of coal gangue [3]. Many studies have been conducted on the application of coal gangue after pre-treatment. For example, crushed coal gangue can be used as an aggregate in the preparation of cementitious composites in backfilling. This approach promotes the in situ utilization of coal gangue without the migration of gangue from the well [4]. For the purpose of ecological system protection in most mining regions, the preparation of artificial soil by coal gangue was proposed [5]. Broken coal gangue was applied as a carrier, combining it with microorganisms or other nutrients to transform it into a mulch layer, with the objective of repairing the vegetation ecological environment of the mining region and promoting the sustainable development of the mining region [6]. Nevertheless, the risk of heavy metal leaching in soil from coal gangue poses a serious problem to the safety of the environment and human beings.

In recent years, combustion technology has been applied to utilize coal gangue for energy production. A binary specimen geopolymer was obtained by mixing coal gangue with red mud after 800 °C thermal activation, which showed much high strength values (up to 7.3 MPa) [7]. Due to the high alkalinity of the red mud, the addition of other hydroxides to the mixing process is no longer necessary [8]. Moreover, the geopolymer cement composite sphere can effectively remove heavy metal elements from industrial wastewater [9]. Part of coal gangue still contains organic components, which can be reused in the thermal power generation process when mixed with a certain proportion of raw coal [2]. This approach has been widely adopted in numerous coal-fired power plants. However, it is associated with a notable increase in the production of secondary solid wastes, such as fly ash and bottom ash [10]. Due to its stable mineral phase composition (the formation of mullite at high temperatures), ash is generally considered to be a poor candidate for practical applications. The bottom of the furnace ash, similarly, is characterized as a stable mineral phase composition (high-temperature formation of mullite), which also renders it poorly reactive. Consequently, the value of the actual application is low, such as roadbed [11]. Coal gangue contains a considerable quantity of clay minerals, including kaolinite and other clay minerals. When subjected to elevated temperatures, these minerals undergo dewatering and reconstruction, resulting in the formation of an amorphous Si-Al structure at a specific temperature range [12]. As the temperature exceeds 1000 °C, recrystallization occurs, leading to the development of mullite, a chemically stable phase [13]. The application of appropriate heat treatment temperatures (600–900 °C) facilitates the formation of a substantial number of Si-Al glass phase structures, which is of paramount importance for the utilization of coal gangue in cementitious materials [14]. Additionally, thermal treatment technology can be employed for the removal or stabilization of hazardous components from specific solid waste materials. The results show that leachable Cu, Pb, Zn and Cr in solid waste are markedly diminished under the influence of temperature [15]. It was proposed that the heavy metals are vaporized or solidified within the solid waste, forming a glassy state under high temperatures [16]. This consequently enhances the security of hazardous waste utilization and mitigates the risk of environmental hazards. However, research related to the leaching law and mechanism of heavy metals from coal gangue under thermal treatment conditions is still limited. To mitigate the environmental hazards associated with the utilization of heat-activated gangue, further investigation is required to elucidate the leaching mechanisms of deleterious heavy metal ions and activated silica-aluminum within the gangue thermal conditions.

This study investigates the physical and leaching properties of coal gangue after calcination at different temperatures (500 °C, 600 °C, 700 °C, 800 °C and 900 °C). The mineral phases evolution, micro-structure development, heavy metal leaching and reactive Si and Al releasing were characterized. The results will provide a comprehensive understanding of the calcination of coal gangue and support basic data for the application of calcined coal gangue in building materials and soil composites in a sustainable way.

2. Materials and Methods

2.1. Materials

The multiple coal gangue sample was derived from Wuhai, Inner Mongolia. Then, the samples were thoroughly mixed. The chemical composition of coal gangue is shown in

Table 1. Chemical composition of coal gangue from Wuhai.

Compound Formula	SiO2	Al2O3	CaO	Fe2O3	K2O	MgO	TiO2	Na2O	SO3
Concentration Unit (wt.%)	60.10	28.66	3.40	2.25	1.95	1.39	0.74	0.59	0.63

.

The coal gangue fractions (<4 mm) were prepared by using a PE125X1500 crusher. The calcination of coal gangue was achieved by using a rotary kiln (OTF-1200X-5D-R-III-G). The calcination process was set at a heating rate of 8 °C/min, a rotational speed of 2.5 r/min, and an aeration (air) volume of 5 L/min to the specified temperature, and then held for 2 h after natural cooling. Following a period of natural cooling, 300 g of calcined gangue samples was placed in an XY-4500 type powder mill. The samples were removed and sieved through a 120-mesh sieve after grinding for a period of 2 min. The same operational process was employed to prepare samples at temperatures of 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C, respectively, for thermal activation of the coal gangue samples. These samples were coded as G26 (control), G500, G600, G700, G800, and G900. The calcination and cooling processes are illustrated in Figure 1.

2.2. Test Methods

2.2.1. Characterization of Mineral Composition of Thermally Activated Coal Gangue

The Bruker D8 ADVANCE X-ray diffractometer with Cu cobalt was employed to analyze the mineral compositions of the G26, G500, G600, G700, G800 and G900 gangue samples. Scanning was conducted with a voltage of 40 kV, and a current of 40 mA; the scanning range was 5° to 90°, and the scanning speed was 8° per minute.

2.2.2. Leaching Properties of Thermally Activated Coal Gangue

A total of 5 g of coal gangue (G26) and thermally activated coal gangue (G500, G600, G700, G800 and G900) was weighed. Afterward, 40 mL of deionized water was added to reach a solid–liquid ratio of 1:8. Then, the mixture was placed in a shaker and shaken at 150 rpm for 24 hrs. Finally, the leachates were collected, after filtering using a 0.45 μm nylon filter membrane. Subsequently, the filtrates were determined utilizing an inductively coupled plasma mass spectrometer (ICP OES 730).

To evaluate the leaching of Al and Si at different pH values, the solutions were configured with pH 3, 5, 7, 9 and 12. The G600 and G800 gangue samples should be weighed at 5 g each, and added to 40 mL of solutions with different pH values at a solid–liquid ratio of 1:8. The configured gangue mixture was placed in a shaker and shaken at 150 rpm for 4, 8 and 48 h. The supernatant was then taken at the end of the shaking period, and the gangue filtrate was filtered with a 0.45 μm nylon filter membrane in order to obtain the gangue filtrate.

2.2.3. Thermogravimetric Analysis of the Gangue During Thermal Activation

The thermal decomposition of the gangue was characterized using the STA449F5 thermogravimetric analyzer. The test atmospheres were nitrogen (N₂) and air (N₂/O₂), with the rate of temperature increase set at 10 K/min and the temperature range from 25 to 1000 °C.

2.2.4. Characterization of Pore Evolution in Thermally Activated Gangue

To identify the pore structure development of coal gangue during calcination, the gangue samples G26, G500, G600, G700, G800 and G900 were tested by using an ASAP 2460 specific surface area analyzer. The nitrogen adsorption–desorption method was carried out at a temperature of −77 °C. The pore size distribution of the gangue samples was calculated using the Brunauer–Emmett–Teller (BET) method.

2.2.5. Micro-Morphological Testing of Thermally Activated Gangue

The G26, G500, G600, G700, G800 and G900 gangue samples were characterized using a SU8600 scanning electron microscope at 15 kV. Prior to testing, the samples were sprayed with gold.

2.2.6. Methods for Testing the Density of Gangue Powder

The mass of the empty specific gravity bottle (with stopper) (m₁) was weighed. The specific gravity bottle was filled with gangue powder (m₂). The appropriate amount of distilled water was added to the specific gravity bottle, ensuring that the liquid surface was approximately 5 mm from the gangue powder, and it was left overnight (m₃). The specific gravity bottle was cleaned with distilled water. Then, it was filled with distilled water (m₄). Additionally, the temperature of the distilled water utilized was recorded (t). Finally, the calculation of density was carried out as follows:

$$\delta ={\displaystyle \frac{\rho (m_2-m_1)}{\left(m_4-m_3\right)+(m_2-m_1)}}$$

(1)

In step 4, $\rho$ the researcher determined the density of distilled water at a specified temperature (t).

3. Results and Discussion

3.1. Effect of Calcination Temperature on the Composition of Gangue Phases

The XRD spectra of the gangue samples subjected to thermal activation at varying temperatures are illustrated in Figure 2.

The identified mineral compositions of the original coal gangue sample (G26) are kaolinite, dickite, quartz, and pyrite. After thermal activation, the position and intensity of the diffraction peaks in the sample changed, indicating that the mineral composition of the coal gangue was reconstructed due to thermal treatment. For G500, the position of the primary diffraction peaks in the coal gangue remained unaltered. However, the intensity of the diffraction peaks associated with kaolinite and dickite exhibited a notable decline in comparison to G26. This indicated that at 500 °C, the kaolinite and dickite was decreased. After increasing the thermal activation temperature to 600 degrees, the diffraction peaks of kaolinite, dickite and pyrite became undetectable, while the diffraction peaks of hematite and quartz remained unchanged. This observation suggests that kaolinite, dickite and pyrite experienced a complete decomposition and reconstruction process under thermal activation at 600 degrees. As the temperature increased gradually, new mineral phases were observed in the XRD analysis.

It has been confirmed [17] that kaolinite experienced a thermal decomposition under the medium temperatures (450–700 °C), as shown in Equation (1):

Al₂Si O₂₅(OH)₄ (s)→Al₂O₃ − 2SiO₂ (s) + 2H₂O (g)

(2)

Furthermore, dikalite is well known to be decomposed in the temperature range of 450–700 °C [18]; similar residues were observed to those in kaolinite (as shown in Equation (2)):

Al₂Si₂O₅(OH)₄ (s)→Al₂O₃ − 2SiO₂ (s) + 2H₂O (g)

(3)

Meanwhile, pyrite in coal gangue was also reported to react with oxygen in air when the temperature reached 500~700 °C. Consequently, the process of oxidation occurs directly at the surface of pyrite, and ferrite oxide and sulfur dioxide can be produced (as shown in Equation (3)):

2FeS₂ (s) + 5.5O₂ (g)→Fe₂O₃ (s) + 4SO₂ (g)

(4)

G500 shows that the intensity of the characteristic peak of kaolinite (d = 0.67424 nm) is weakened and the other diffraction peaks do not change much compared to G26, indicating that there is no significant change in mineral compositions. After the calcination temperature reached 600 °C, the peak of kaolinite (d = 0.67424 nm) that was invisible disappeared, which is due to the fact that kaolinite removes a large number of hydroxyl groups and its crystal structure is disrupted, changing from kaolinite to metakaolinite [19]. It can be seen that after 600 °C, a further increase in calcination temperature (900 °C) shows no significant influence on the mineral phase of the coal gangue. This is in agreement with the study of Wang et al. [20].

In addition, the appearance of hematite in the calcined coal gangue samples after 600 °C resulted in a color change in the treated samples. During calcination, pyrite (FeS) reacted with oxygen and produced SO₂ and hematite (Fe₂O₃), which is also consistent with the color of the calcined gangue samples (Figure 3). There is no significant change in the diffraction peaks of G600, G700, G800 and G900, indicating that the structure of the mineral enters a relatively stable temperature range. This is also in agreement with the results of Zhang et al. [18].

To further analyze the decomposition or reaction of the mineral phase in gangue during the heat treatment process, the original gangue samples were thermogravimetrically analyzed under nitrogen and air atmospheres and the test results are shown in Figure 4.

Under the air atmosphere, from 400 °C, the mass loss of the coal gangue exhibited a rapid increase, which persisted until approximately 700 °C, resulting in a distinct peak in the DTG curve. This phenomenon is primarily attributable to the dehydroxylation of kaolinite and dikalite, as well as the phase transition to amorphous biotite [21] and amorphous quasi-dikalite [22], and the simultaneous direct oxidation of pyrite to hematite [18], at which point the mass loss is 11.81%.

The peaks of the DTG curves under air and nitrogen atmospheres are basically the same, but the two peaks of kaolinite dehydroxylation at 447 °C and 527 °C in the DTG curves under the air atmosphere are shifted to the low-temperature region by 40 °C. And the mass loss before 600 °C under the air atmosphere is more obvious than under the nitrogen atmosphere, which indicates that a large number of organic combustible components present in the gangue under the action of O₂ are burned and decomposed; so, the gangue combustion exotherm plays the role of internal heat source and accelerates the thermal decomposition of kaolinite [23]. The weight loss of the sample under the nitrogen atmosphere at temperatures between 26 °C and 1000 °C (15.77%) is less than that of the sample under the air atmosphere at the same temperatures (26.44%). The discrepancy between the two can be attributed to the free carbon content of the gangue [24].

It can be observed that thermal treatment has the potential to alter the mineral phases present, resulting in the formation of more reactive metakaolinite and quasi-dickite. Consequently, this may exert an influence on the ion leaching process from the heat-activated gangue itself.

3.2. Effect of Calcination Temperature on the Physical Properties of Coal Gangue

3.2.1. Effect of Calcination Temperature on Density of Coal Gangue

The density evolution of coal gangue after different thermal activation is shown in Figure 5.

As illustrated in Figure 5, the density of coal gangue increases with the increase in calcination temperature. During the process of thermal activation, the gradual increase in temperature leads to the combustion and transformation of combustible components in coal gangue into carbon dioxide. Therefore, the solid volume of organic components was decreased. Furthermore, the transformation of pyrite into hematite also contributed to the increase in density. It is known that the density of pyrite is approximately 4.6–5.2 g/cm³ [25], while the density of hematite is approximately 5.00 g/cm³ [26]. In addition, the minerals present in gangue, such as kaolinite and dickite, undergo a transformation into denser minerals, accompanied by the loss of the water phase [27]. At elevated temperatures, for instance, 900 °C, clay minerals undergo recrystallization and sintering, resulting in the formation of more compact structures.

3.2.2. Effect of Calcination Temperature on the Pore Structure of Coal Gangue

Figure 6 illustrates the quantity of nitrogen adsorption and desorption on the surface of coal gangue particles and SEM image after calcination at different temperatures. The adsorption isotherm of G26 and G500 exhibited minimal discernible characteristics. Afterward, as the calcination temperature increased, the adsorption isotherm of the calcined coal gangue samples after thermal activation at 600 °C, 700 °C, 800 °C, and 900 °C exhibited a gradual convergence towards the isotherms of Type II, with a bias towards the axis of relative pressure at relative pressure p/p₀ values between 0 and 0.4. This indicates that the force between the sample and the nitrogen is weak, which suggests that the sample contains almost no micropores. When the relative pressure p/p₀ was greater than 0.5, the isotherm displayed a pronounced increase and a concave downward shape, indicative of the emergence of the phenomenon of adsorption and capillary coalescence of the multimolecular layer. This observation substantiates the hypothesis that the sample contains a substantial number of mesopores and macropores. At a relative pressure of p/p₀ = 1, the adsorption curve displayed an infinite ascending trajectory, indicating that the adsorption process had not yet reached saturation and that the sample contained a substantial number of macropores.

Furthermore, the desorption branch is located above the adsorption branch, thereby forming a hysteresis loop. The shape of the hysteresis loop is indicative of the presence of a pore structure within the gangue samples. An examination of the hysteresis loops reveals that they are all of the H3-type [28], which suggests that the six gangue samples contain slit-like mesopores and macropores. This corroborates the conclusion of the XRD analysis that, with the increase in temperature of the heat treatment, the coal and other organic matter interspersed in the gangue underwent combustion, resulting in the formation of a loose and porous structure within the gangue. Furthermore, after the thermal activation temperature reached 600 °C, kaolinite began to remove hydroxyl groups, resulting in the destruction of the crystal structure and the formation of a loose and porous structure [29]. This resulted in the formation of additional mesopores. As the phase transformation process of kaolinite was finished, the number of mesopores increased gradually. This resulted in the increase in the width of the hysteresis ring. The thermogravimetric analysis curve indicates that when the temperature reached 700 °C, the phase transformation of the minerals in the coal gangue was complete. Consequently, the nitrogen adsorption and desorption curves of G700, G800 and G900 exhibit similar characteristics. However, after calcination at 800 °C, most of the holes disappeared, which was due to the phenomenon of sintering and melting of some minerals at this temperature, filling most of the holes.

3.2.3. Effect of Calcination Temperature on the Pore Size Distribution of Coal Gangue

The pore size distribution of coal gangue before and after calcination is shown in Figure 7.

The International Union of Pure and Applied Chemistry (IUPAC) classifies the pores in powder materials according to their size as micropores: pore size <2 nm; mesopores: pore size 2–50 nm; macropores: pore size >50 nm [30]. The pore size distributions of the gangue samples are analyzed in Figure 7.

The pore size of G26 is primarily distributed within the ranges of 2.06–2.66 nm, 21.68–27.28 nm, and 44.63–88.53 nm. The number of mesopores is minimal, while the number of macropores is considerable. When the thermal activation temperature reached 500 °C, the organic matter present, such as coal, was unable to undergo complete combustion. Additionally, no phase transformation occurred in kaolinite at 500 °C. Consequently, the pore size distributions of G500 and G26 remained largely identical. The mesopores’ sizes of G600 are distributed between 2.37 and 2.71 nm and 21.86 and 27.53 nm. The distribution is analogous to that observed in G26 and G500. The extensive pore size distribution is attributed to the complete combustion of coal gangue, which resulted in the formation of a multitude of macropores. As the thermal activation temperature was increased, the peak of G700 and G800 disappeared at 61.69–152.65 nm, and a peak appeared at 21.73 nm. This indicates that, from 700 °C, the macropores disappeared and new mesopores emerged. This is due to the decomposition and transformation of kaolinite into metakaolin at temperatures between 550 °C and 850 °C, resulting in an increase in the volume of the latter. During the metakaolin transformation process, it can fill part of the macropores. Consequently, a number of macropores transform into mesopores. As a thermal activation temperature of 900 °C, the main pores in G900 are mesopores, and the volume of pores smaller than 5 nm decreases markedly. This is due to the fact that, at 900 °C, coal gangue undergoes sintering and melting.

In conclusion, the number of coal gangue mesopores increases with an increasing heat treatment temperature, which also confirms the conclusion of the previous section that “the steepening of the hysteresis loop with increasing heat treatment temperature is due to the increase in mesopores”.

3.2.4. Effect of Calcination Temperature on the Specific Surface Area of Coal Gangue

As can be seen from Figure 8, the specific surface area of G500 exhibits a reduction compared to G26; afterward, the specific surface area gradually increases in the range of 500–800 °C. Then, the surface area shows a reduction when the temperature reached 900 °C.

The surface area was calculated according to the BET method. The pore structure was the key factor to determine the calculated surface area of thermally activated coal gangue. As discussed in the above sections, at 500 °C, the number of macropores in the coal gangue increases, resulting in a reduction in the specific surface area. However, as the thermal activation temperature is increased, the macropores undergo a gradual transformation into mesopores, leading to an increase in the specific surface area. At a thermal activation temperature of 900 °C, sintering and melting occur, resulting in a decline in the specific surface area.

3.2.5. Effect of Calcination Temperature on pH of Calcined Coal Gangue

The pH of calcined coal gangue after different thermal activation is shown in Figure 9.

It can be seen that the filtrate of G26 has a pH of 4.7. After calcination, the pH of calcined coal gangue gradually increases to 7.2 (G900). This indicates that the pH of calcined coal gangue could be influenced by the calcination temperature. The reduction in pH with the calcination can be induced by the reaction of pyrite (FeS₂) in coal gangue, which forms an acid under the action of oxidation and water. The specific procedure is as follows [31]:

$$\begin{gathered} 2\mathrm{F}\mathrm{e}{\mathrm{S}}_2+7{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{F}\mathrm{e}\mathrm{S}{\mathrm{O}}_4+2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4 \\ 4\mathrm{F}\mathrm{e}\mathrm{S}{\mathrm{O}}_4+2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4+{\mathrm{O}}_2\to 2\mathrm{F}{\mathrm{e}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+2{\mathrm{H}}_2\mathrm{O} \\ 2\mathrm{F}{\mathrm{e}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+6{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3+3{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4 \end{gathered}$$

As can be seen from XRD Figure 2, with the continuous increase in the heat treatment temperature, the amount of pyrite (FeS₂) gradually decreases and is finally oxidized to hematite (Fe₂O₃), and then a pH increase is observed. Furthermore, pH could strongly influence the leaching behavior of calcined coal gangue.

3.3. Effect of Calcination Temperature on the Leaching of Elements from Gangue

3.3.1. Leaching Evaluation of Calcined Coal Gangue

From the leaching results of calcined coal gangue in Figure 10, it can be seen that the original coal gangue (G26) can release a number of heavy metal ions, such as Cd, Cu, Mn, Ni, Pb and Zn. In general, the heavy metals that can be leached out of coal gangue often exist in sulfide-bound state, carbonate-bound state and silicate-bound state in sulfide minerals, carbonate minerals and silicate minerals [32].

As discussed in the above sections, the mineral phases in the coal gangue experienced a reconstruction during the thermal activation process. Therefore, the mineral phase variation resulted in a notable decrease in heavy metal ion leaching after thermal activation. After calcination at 500 °C, the leaching of Ni presented the highest reduction rate, with 96%, compared to other heavy metals. A slight decrease in Cd and Mn dosage was observed, while the leaching of Cu, Zn, and Pd was enhanced. These observations suggest the potential existence of Cu, Zn, and Pd in the combustible components of the gangue, with the three heavy metal ions undergoing further release following complete combustion.

After the calcination temperature was further increased, the leaching of all heavy metal ions was found to be significantly decreased. At 600 degrees, the leaching of Ni, Cd, Mn, Cu, Zn and Pd decreased by 96%, 84%, 64%, 81%, 98%, and 88%, respectively. At 800 degrees, the leaching of Ni, Cd, Mn, Cu, Zn and Pd decreased by 99%, 67%, 86%, 40%, 99% and 93%, respectively. These observations demonstrate that thermal activation can effectively reduce heavy metal leaching from calcined coal gangue.

Heavy metal elements can be classified into three categories: highly volatile elements, semi-volatile elements, and non-volatile elements [16]. In this study, Ni, Cd, Cu, Zn and Pd are related to the highly volatile and semi-volatile categories, which can be removed from solid samples and transformed into gas phase under a tested calcination temperature. Similar results were also collected in the study of heat treatment of MSWI bottom ash [15]. On the other hand, the reduction in pyrite with the increase in calcination temperature (as shown in XRD analysis) also contributed to the decline in heavy metal leaching. Zhou et al. demonstrated [33] that Cu, Zn, Cd, Ni, Cd, and As in coal gangue have a strong correlation with sulfide minerals. These heavy metal elements are primarily associated with sulfide minerals, particularly pyrite (FeS₂) and sphalerite (ZnS). Ide minerals, such as pyrite and sphalerite (ZnS), and Mn are primarily accompanied by silicate and carbonate minerals. Li et al. demonstrated [34] that the Cd element is predominantly associated with sulfide minerals, while the Pb element is primarily accompanied by carbonate minerals and sulfide minerals. Meanwhile, heavy metals always prefer to dissolve in acidic environments; the removal of FeS₂ during calcination promotes the pH of calcined coal gangue. Therefore, all tested heavy metals exhibited a significant decline in the leaching process. This indicated that thermal activation of coal gangue is effective in lowering the risk of environmental pollution.

3.3.2. Alkali Metal Ions

The leaching results of Ca, K, Mg and Na in coal gangue under different temperatures are shown in Figure 11. The leaching of Mg, K and Na from calcined samples gradually decreased with the increased calcination temperature. In a previous study, sodium, potassium, and magnesium elements were predominantly associated with clay minerals in kaolinite [35]. During calcination, kaolinite was restricted and the sodium, potassium, and magnesium elements were volatilized; Ca is primarily associated with carbonates in coal gangue, which have a stable crystal structure and a higher decomposition temperature. Consequently, Ca is less prone to volatilization, exhibiting minimal fluctuations in its leaching amount.

Zhou et al. demonstrated that [35] sodium, potassium, and magnesium elements are predominantly associated with clay minerals in kaolinite. The primary component of coal gangue is the clay mineral kaolinite, which begins to decompose at approximately 550 °C, undergoing a phase transformation into metakaolinite. During this process, the sodium, potassium, and magnesium elements are volatilized. The filtration of Na, K, and Mg is observed to decline when the heat treatment temperature of the gangue reaches 600 °C. Ca is primarily associated with carbonate minerals, which have a stable crystal structure and a higher decomposition temperature. Consequently, Ca is less prone to volatilization, exhibiting minimal fluctuations in its filtration amount.

3.3.3. Reactive Si and Al Leaching Behavior

The formation of reactive Si and Al is the key to the reactivity of kaolinite after calcination. It has been confirmed that the optimal calcination temperature ranges from 600 to 800 °C [23]. These properties give calcined kaolinite a potential utilization in cementitious materials. To confirm the reactive Si and Al formation, calcined coal gangues at 600 °C and 800 °C were selected for test. In Figure 12, the leaching of Al and Si from G600 and G800 is presented.

It is clear that G600 leached out higher amounts of Si and Al compared to G800 under pH of 7. The leaching amount of Al decreased from 1.98 mg/kg (G600) to 1.63 mg/kg (G800); this can be attributed to the elevated calcination temperature, which resulted in a gradual reduction in the content of active alumina in calcined kaolinite. The leaching amount of the Si decreased from 52.46 mg/kg to 18.62 mg/kg. This is due to the fact that a lower calcination temperature will result in an incomplete release of reactive silica minerals, whereas a higher calcination temperature will produce a square quartz phase in place of reactive silica [35]. The reduction in the leaching of aluminum and silicon may also be attributed to the presence of substantial quantities of dissolved organic matter, including fulvic acid, tyrosine [36], and humic acid, within the gangue [37].

Some studies have also demonstrated that clay minerals [38] and quartz [39] are more readily decomposed in the presence of organic acids. Following an 800 °C heat treatment of the gangue organic matter, complete combustion was observed. However, at 600 °C, gangue may still contain organic matter and carbonaceous minerals [29]. Consequently, during the preparation of the filtrate, the dissolution of organic acids in water facilitates the leaching of the Al and Si elements.

3.3.4. Leaching Rate of Reactive Silica-Aluminum Ions

To investigate the leaching behavior of Al and Si in G800 at different leaching environments, the pH of the leaching process was controlled at 3, 5, 7, 9 and 12. The leaching results are shown in Figure 13.

It is clear to see that the leaching of Al and Si in calcined coal gangue (G800) was controlled by pH and duration. There is no significant change in Al leaching before the pH value reached 12; the increase in leaching duration showed a limited effect on the enhancement in Al leaching. After the pH was increased to 12, the leaching of aluminum exhibited a considerable increase over time; the highest leaching of Al was observed at a shaking time of 48 h. For Si leaching of calcined coal gangue (G800), similar results were recorded. This indicates that in an alkaline environment, reactive Al and Si in calcined coal gangue can be effectively extracted. In other words, these can be used as potential ingredients in cementitious binders of building materials production. A similar observation was also reported in previous studies. Xie et al. demonstrated [40] that the active silica fraction released from coal gangue following calcination and phase transformation is readily dissolved in alkaline solution. The study by He et al. indicated [41] that the Al-O bond length in kaolinite is longer than the Si-O bond length. Additionally, the strength of the Al-O bond is weaker than that of the Si-O bond. Furthermore, the activation energy of the Al-O bond is approximately 76–125 kJ/mol, while that of the Si-O bond is approximately 460 kJ/mol. Therefore, the rate of Al-O bond cleavage is faster, which explains why the leaching of Al is higher than Si.

4. Conclusions

To understand the evolution of coal gangue during the thermal activation process, this study evaluates the development of mineral compositions, micro-structure, and leaching properties of calcined coal gangue at different calcination temperatures. An overall evaluation and analysis are presented. The results provide the essential data for the application of coal gangue with low environmental risk. Some conclusions can be addressed as follows:

• Kaolinite, geodes, pyrite, and quartz are the main mineral components in coal gangue from Wuhai. After calcination, the density of thermally activated coal gangue was increased with the increase in calcination temperature because of the burning of organic and reconstruction of pyrite and kaolinite. Furthermore, the pH was gradually increased from 4.8 (G26) to 7.1 (G900).
• The mineral phase transformation of coal gangue during calcination strongly influenced the pore structure. The calcination process decreased the macropore volume (>50 nm), while the mesopore (<5 nm) volume was increased.
• The leaching results indicate that thermally activated coal gangue was helpful to reduce the leachable heavy metals, including Mn, Zn, Cd, Ni, Cu, and P. At 800 degrees, the leaching of Ni, Cd, Mn, Cu, Zn and Pd decreased by 99%, 67%, 86%, 40%, 99% and 93%, respectively. The reduction in heavy metal in calcined coal gangue can be attributed to the transformation of pyrite and the volatility at high temperatures. Some heavy metal can be solidified in amorphous phases of calcined kaolinite.
• The formation of reactive Si and Al in calcined coal gangue (G800 °C) was confirmed. The high amount of leachable Si and Al was detected in the alkaline environment (pH = 12). After 48 h, leachable Al reached 106.4 mg/kg, while leachable Si reached 86.1 mg/kg. Calcined coal gangue has the potential to be used in the preparation of cementitious material.
• In general, thermal activation is a crucial step in the process of reutilization of coal gangue as it can not only improve the activity of coal gangue but also significantly reduce the amount of heavy metal leaching. Under a reasonable heat treatment temperature, the low environmental risk of treated coal gangue is promising. In addition, it has been mentioned that some organic matters exist in coal gangue, which can be fully utilized during the calcination process, which can effectively reduce energy consumption. In addition, many policies strongly support the recycling and utilization of solid wastes, which promotes the development of new treatment technologies. Therefore, thermally activated coal gangue in building materials and other applications could be potential sustainable materials.