# The Thermal Decomposition Behavior of Pyrite-Pyrrhotite Mixtures in Nitrogen Atmosphere

Authors: Changshun Tian; Yunzhang Rao; Gang Su; Tao Huang; Cairong Xiang
Journal: Journal of Chemistry
 (2022)

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## Abstract

To assess the thermal transformation process of common sulfide minerals in a nitrogen atmosphere, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and thermogravimetric mass spectrometry are employed to define the influence of the pyrrhotite content in pyrite-pyrrhotite mixtures (mixed minerals). The results indicate that an increase in pyrrhotite content decreases the temperature of the maximum mass loss rate of mixed minerals and reduces its mass loss. The solid-phase transformation of the thermal decomposition of mixed minerals is accelerated because the apparent activation energy of pyrrhotite is lower than that of pyrite and mixed minerals. However, the pyrrhotite makes the mixed minerals easier to sinter and agglomerate, which reduces the total volatilization amount of the gas product, S2; thus, the rate of mass loss decreases.


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## Body

## 1. Introduction

Pyrite and pyrrhotite are common sulfide minerals in mines [1]. Pyrite (FeS2) is widely used in rubber and textile, pyrrhotite (Fe1-xs) is widely used in heavy-metal pollution control, and both of them are widely used in acid production industry [2–5]. Moreover, pyrite and pyrrhotite, as minerals associated with coal, are among the main sources of sulfur dioxide (SO2) emissions during coal combustion and environmental pollution [6]. Because Fe and S have variable valence properties, pyrite and pyrrhotite exhibit extremely high chemical activities and complex crystal structures. At specific conditions, it will burn or even explode, leading to tragic consequences [7, 8]. Therefore, it is of great significance to describe the combustion and explosion processes of pyrite and pyrrhotite.Several articles have been published on the combustion and explosion of a single sulfide mineral. Researchers generally believe that the combustion and explosion processes undergo two stages: thermal decomposition and oxidation; the thermal decomposition process is consistent with the reaction in an inert atmosphere (N2, He, and Ar) [9–11]. Therefore, studying the thermal decomposition process of sulfide minerals is of great value to reveal the mechanism of combustion and explosion.Considerable research has been carried out on the thermal decomposition behavior of pyrite in an inert atmosphere using a wide range of methods. Many of these results show that the thermal decomposition of pyrite is controlled by a chemical reaction and is a surface first-order reaction [12, 13]. Generally, it is considered that the thermal decomposition process is as follows: pyrite ⟶ pyrrhotite ⟶ troilite (FeS) ⟶ Fe, and this process is controlled by the temperature and total S gas pressure in the system [14]. However, Hong et al. [15] believe that the thermal decomposition of pyrite is divided into two steps. The first step is the decomposition of pyrite into pyrrhotite and liquid S atoms, and the second step is the formation of S2 (g) by the combination of liquid S atoms and evaporation. When S dissipates quickly, the main reaction stage is dominated by Step 1, and the apparent activation energy Ea is 297 kJ/mol; when S dissipates slowly, the main reaction stage is conducted by Step 2, and the Ea is approximately 112 kJ/mol. Furthermore, Hoare et al. [16] have studied the reaction mechanism model of a single pyrite particle in an N2 atmosphere and found the rock core shrinkage phenomenon. The decomposition of pyrite into pyrrhotite follows the unreacted core model. The mass loss process of pyrite is divided into two stages. The first stage forms a porous layer of pyrrhotite around each particle, and the second stage is the further decomposition of the material under the pyrrhotite layer. Shi et al. [17] have studied the solid-phase transformation of pyrite in an N2 atmosphere at a temperature gradient of 100°C and found that pyrite does not change significantly when the temperature is lower than 500°C. The pyrite begins to transform into monoclinic pyrrhotite at 500–600°C, and afterward, hexagonal pyrrhotite is observed at 700–800°C; at 900°C, more stable FeS is formed. Additionally, Wang et al. [18] and Li et al. [19] have applied magnetic technology to identify the thermal decomposition process of pyrite. The study shows that there are two thermal decomposition paths: pyrite ⟶ magnetite ⟶ pyrrhotite and pyrite ⟶ pyrrhotite. Thus, the analysis of the thermal decomposition process of pyrite in an inert atmosphere has not yet been unified.Pyrrhotite is one of the main species of sulfurous Fe ore in nature; however, few studies have analyzed pyrrhotite as a single object [20]. Selivanov et al. [21] have considered 270°C as the limit temperature for Fe7S8 to remain stable. Schwarz, Li, Powell, and so on [22–24] have concluded that pyrrhotite undergoes a monoclinic to hexagonal transformation at 300–320°C. Kennedy et al. [25] believe that pyrrhotite and elemental S react at approximately 500°C to generate pyrite. Furthermore, as the temperature increases, the phase of pyrrhotite changes between S and Fe [26].Although the thermal decomposition of pyrite and pyrrhotite has been studied in detail, it is well known that the thermochemical reactions of mixed-mineral sulfide ores are more complex than those of single-mineral samples. Yang et al. [27] have found that an increase in the quality fraction of FeS in an FeS-FeS2 mixture increases the specific surface area, total pore volume, and adsorption capacity of mixed minerals. Almeida et al. [20] have proved that the combination of pyrite and pyrrhotite is beneficial to the production of S by electrochemical experiments. Further, under open-circuit conditions, when pyrite and pyrrhotite are in contact, galvanic cells are produced between the minerals; this is because the reduction rate of Fe ions on the surface of pyrite is greater than that on the surface of pyrrhotite; thus, the dissolution rate of FeS in the system increases. Moreover, the addition of pyrrhotite increases the concentration of Fe ions, and thereafter, reversible half-reactions occur, leading to the inhibition of FeS formation.The purpose of this study is to determine the influence of pyrrhotite content on the thermal decomposition reaction of pyrite-pyrrhotite mixtures (mixed minerals) as the basis for the next step in revealing the combustion and explosion mechanism of the mixed minerals. Therefore, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric mass spectrometry (TG-MS), and other characterization methods were applied to determine the causes of the influence by analyzing solid and gas products and calculating the apparent activation energy.


## 2. Materials and Methods

### 2.1. Materials and Characterization

The samples of pyrite and pyrrhotite were natural minerals, and they were purchased from a stone specimen firm in Guangdong Province, China. To reduce the mechanically activated influence on the thermal decomposition of ore dust samples [28], after coarse crushing in a laboratory, the ore dust samples were manually ground using a corundum mortar. The samples were broken into relatively small particles for the thermal decomposition analysis; they were sieved using a 200-mesh (75 μm) standard sieve. Subsequently, the screened pyrite and pyrrhotite were mixed in mass ratios of 1 : 0.1, 1 : 0.25, 1 : 0.5, 1 : 0.75, 1 : 1, 1 : 1.25, 1 : 1.5, 1 : 1.75, and 1 : 2, respectively. A laser diffraction analyzer (2000E, Jinan Winner, China) was employed to analyze the particle sizes of pyrite, pyrrhotite, and pyrite-pyrrhotite mixtures with a mass ratio of 1 : 1 (referred to as mixed minerals (1 : 1)). The results are presented in Table 1 and Figure 1. The particle size distribution of most dust particles was below 45 μm, and the median particle size was below 33 μm. SEM (MLA650 F, FEI, USA) was used to analyze the surface structure of the ore dust. The results are shown in Figure 2. The pyrite and pyrrhotite samples showed inhomogeneous particle sizes and irregular shapes. The particle size of some mineral samples under the 200-mesh standard sieve was larger than 75 μm because of the irregular structure. After comparing the results in Figures 1 and 2, it was observed that the result of the particle size analysis of the ore dust samples was in good agreement with the scanning microscopy results. Additionally, the test samples were not dried. Therefore, after drying the samples in a constant-temperature drying oven at 80°C for 24 h, the moisture contents of the pyrite and pyrrhotite samples were measured. The results showed that the two minerals were almost free of moisture.Table 1
Summary of the laser particle size analysis results of the ore dust samples.
Sample typeD90/μmD50/μmD10/μmS/V (specific surface area)/(cm2 cm−3)<10μm/%10–45μm/%45–100μm/%100–200μm/%Pyrite91.9429.043.5515958.2923.3740.6128.527.31Mixed minerals (1 : 1)79.4724.112.8806854.0127.4846.3823.023.12Pyrrhotite80.3228.043.4126200.9124.8642.7627.325.06Figure 1
Particle size analysis curves of the ore dust samples.Figure 2
Surface structure of the ore dust samples: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)The main mineral components of the three samples were identified by XRD (Empyrean, PANalytical, Holland) at room temperature of 27°C. The results showed that FeS2 was the main component in the pyrite, accompanied by a small amount of silica (SiO2), and Fe7S8 was the main component in the pyrrhotite, accompanied by a small amount of FeS2 and SiO2, as shown in Figure 3. The Fe content in the minerals was determined by titration (implementing standards of GB/T 6730.65–2009 [29]), and the content of S in the minerals was determined by iodometry (implementing standards of YS/T575.17–2007 [30]). The contents of Fe and S in pyrite and pyrrhotite were 45.74% and 58.23% and 53.02% and 38.91%, respectively. The ratios of S to Fe in pyrite and pyrrhotite were 2.0285 and 1.1694, respectively, which were in accordance with the stoichiometric values of pyrite (FeS2) and pyrrhotite (Fe7S8); the test results were consistent with those of the XRD analysis.Figure 3
The XRD analysis results of the solid-phase products at the end temperatures of the different stages and the peak temperature of the DTG of the thermal decomposition of the ore dust samples: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)


### 2.2. Experimental Methods

All experiments were carried out in a 99.99% pure N2 atmosphere, and the procedure is shown in Figure 4. First, the thermal decomposition of the pyrite, pyrrhotite, and mixed minerals (1 : 1) was assessed using a thermal analysis instrument (TG/DTA6300, PE, USA). To ensure accurate results, the experiments were conducted in strict accordance with the instrument operating procedures. For each test, the mass of the mineral dust was 5–10 mg, the nitrogen flow was 200 mL/min, and the temperature range was 30–1100°C. The experiments were conducted at a heating rate of 10 °C/min. Under the same scheme, experiments were carried out on mixed minerals with other mass ratios, and the influence of the pyrrhotite content in the mixed minerals was determined.Figure 4
Experimental instrument and process.To analyze the influence mechanism, a tube furnace (TL1700, Nanjing Huike Electric Furnace Inc., China) was used with a gas flow of 200 mL/min, a heating rate of 10 °C/min, and the temperature selected in the thermal analysis test results. The phase variable of the mixed minerals (1 : 1) was investigated by comparing pyrite with pyrrhotite. In the test, 2.5 g of the three kinds of mineral samples was placed in an 80 × 40 × 17 mm trapezoidal corundum crucible. After reaching the test temperature, the temperature was maintained for 20 min, and the product was continuously ventilated until it was cooled to room temperature. The solid products were characterized by XRD and SEM (MLA650 F, FEI, USA). For the TG-MS (STA449F3-QMS403, NETZSCH, Germany), 10 mg of each of the three ore dust samples was used. The gas flow rate of the TG was 50 mL/min, the gas flow rate of the MS was 20 mL/min, and the heating rates were 10 °C/min, according to the conclusion of literature [17–24]. The changes in S and S2 in the gas-phase products of the thermally decomposed mixed minerals (1 : 1) were investigated in real time, and pyrite and pyrrhotite were compared.The magnetic measurements were carried out using a physical property measurement system (PPMS, Quantum Design Inc., China) within a magnetic field of 1 T at a stable temperature of 27°C.


## 2.1. Materials and Characterization

The samples of pyrite and pyrrhotite were natural minerals, and they were purchased from a stone specimen firm in Guangdong Province, China. To reduce the mechanically activated influence on the thermal decomposition of ore dust samples [28], after coarse crushing in a laboratory, the ore dust samples were manually ground using a corundum mortar. The samples were broken into relatively small particles for the thermal decomposition analysis; they were sieved using a 200-mesh (75 μm) standard sieve. Subsequently, the screened pyrite and pyrrhotite were mixed in mass ratios of 1 : 0.1, 1 : 0.25, 1 : 0.5, 1 : 0.75, 1 : 1, 1 : 1.25, 1 : 1.5, 1 : 1.75, and 1 : 2, respectively. A laser diffraction analyzer (2000E, Jinan Winner, China) was employed to analyze the particle sizes of pyrite, pyrrhotite, and pyrite-pyrrhotite mixtures with a mass ratio of 1 : 1 (referred to as mixed minerals (1 : 1)). The results are presented in Table 1 and Figure 1. The particle size distribution of most dust particles was below 45 μm, and the median particle size was below 33 μm. SEM (MLA650 F, FEI, USA) was used to analyze the surface structure of the ore dust. The results are shown in Figure 2. The pyrite and pyrrhotite samples showed inhomogeneous particle sizes and irregular shapes. The particle size of some mineral samples under the 200-mesh standard sieve was larger than 75 μm because of the irregular structure. After comparing the results in Figures 1 and 2, it was observed that the result of the particle size analysis of the ore dust samples was in good agreement with the scanning microscopy results. Additionally, the test samples were not dried. Therefore, after drying the samples in a constant-temperature drying oven at 80°C for 24 h, the moisture contents of the pyrite and pyrrhotite samples were measured. The results showed that the two minerals were almost free of moisture.Table 1
Summary of the laser particle size analysis results of the ore dust samples.
Sample typeD90/μmD50/μmD10/μmS/V (specific surface area)/(cm2 cm−3)<10μm/%10–45μm/%45–100μm/%100–200μm/%Pyrite91.9429.043.5515958.2923.3740.6128.527.31Mixed minerals (1 : 1)79.4724.112.8806854.0127.4846.3823.023.12Pyrrhotite80.3228.043.4126200.9124.8642.7627.325.06Figure 1
Particle size analysis curves of the ore dust samples.Figure 2
Surface structure of the ore dust samples: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)The main mineral components of the three samples were identified by XRD (Empyrean, PANalytical, Holland) at room temperature of 27°C. The results showed that FeS2 was the main component in the pyrite, accompanied by a small amount of silica (SiO2), and Fe7S8 was the main component in the pyrrhotite, accompanied by a small amount of FeS2 and SiO2, as shown in Figure 3. The Fe content in the minerals was determined by titration (implementing standards of GB/T 6730.65–2009 [29]), and the content of S in the minerals was determined by iodometry (implementing standards of YS/T575.17–2007 [30]). The contents of Fe and S in pyrite and pyrrhotite were 45.74% and 58.23% and 53.02% and 38.91%, respectively. The ratios of S to Fe in pyrite and pyrrhotite were 2.0285 and 1.1694, respectively, which were in accordance with the stoichiometric values of pyrite (FeS2) and pyrrhotite (Fe7S8); the test results were consistent with those of the XRD analysis.Figure 3
The XRD analysis results of the solid-phase products at the end temperatures of the different stages and the peak temperature of the DTG of the thermal decomposition of the ore dust samples: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)


## 2.2. Experimental Methods

All experiments were carried out in a 99.99% pure N2 atmosphere, and the procedure is shown in Figure 4. First, the thermal decomposition of the pyrite, pyrrhotite, and mixed minerals (1 : 1) was assessed using a thermal analysis instrument (TG/DTA6300, PE, USA). To ensure accurate results, the experiments were conducted in strict accordance with the instrument operating procedures. For each test, the mass of the mineral dust was 5–10 mg, the nitrogen flow was 200 mL/min, and the temperature range was 30–1100°C. The experiments were conducted at a heating rate of 10 °C/min. Under the same scheme, experiments were carried out on mixed minerals with other mass ratios, and the influence of the pyrrhotite content in the mixed minerals was determined.Figure 4
Experimental instrument and process.To analyze the influence mechanism, a tube furnace (TL1700, Nanjing Huike Electric Furnace Inc., China) was used with a gas flow of 200 mL/min, a heating rate of 10 °C/min, and the temperature selected in the thermal analysis test results. The phase variable of the mixed minerals (1 : 1) was investigated by comparing pyrite with pyrrhotite. In the test, 2.5 g of the three kinds of mineral samples was placed in an 80 × 40 × 17 mm trapezoidal corundum crucible. After reaching the test temperature, the temperature was maintained for 20 min, and the product was continuously ventilated until it was cooled to room temperature. The solid products were characterized by XRD and SEM (MLA650 F, FEI, USA). For the TG-MS (STA449F3-QMS403, NETZSCH, Germany), 10 mg of each of the three ore dust samples was used. The gas flow rate of the TG was 50 mL/min, the gas flow rate of the MS was 20 mL/min, and the heating rates were 10 °C/min, according to the conclusion of literature [17–24]. The changes in S and S2 in the gas-phase products of the thermally decomposed mixed minerals (1 : 1) were investigated in real time, and pyrite and pyrrhotite were compared.The magnetic measurements were carried out using a physical property measurement system (PPMS, Quantum Design Inc., China) within a magnetic field of 1 T at a stable temperature of 27°C.


## 3. Results and Discussion

### 3.1. Thermal Decomposition Behavior of Each Mineral in N2 Atmosphere: Pyrite, Mixed Minerals (1 : 1), and Pyrrhotite

The thermal analysis results for pyrite, mixed minerals (1 : 1), and pyrrhotite are shown in Figure5. The thermal decomposition process of the three ore dust samples was accompanied by mass loss and could be divided into three stages. The total mass loss of pyrite Δm=25.22% was higher than that of the mixed minerals (1 : 1) Δm=16.12% and pyrrhotite Δm=5.81%.Figure 5
Thermal decomposition of the ore dust samples; △m1, △m2, and △m3 are the first, second, and third stages of mass loss, respectively.Only a small amount of mass loss occurred in the first stage: 2.61%, 1.56%, and 0.92%, respectively. The end temperatures were 560, 470, and 500°C, respectively. Because the sample was dried without free water and bound water and the result of S/Fe characterized in Section2.2 was larger than the theoretical value, there was excess S. Therefore, we believe that a small amount of elemental S was volatilized by heating.The highest mass losses occurred in the second stage, and they were 19.35%, 10.72%, and 2.63%, respectively, and the end temperatures were 560, 470, and 500°C, respectively. This trend was attributed to the decomposition of the main minerals in the pyrite dust [31]. The maximum mass loss rates occurred at 602, 607, and 610°C, which were 0.511, 0.209, and 0.0157 mg/min, respectively. Further, the derivative thermogravimetry (DTG) curves showed a peak. Furthermore, it was observed that although the reaction rate of the mixed minerals (1 : 1) did not increase and the heating rate remained the same (10 °C/min), the mixed minerals (1 : 1) attained the maximum mass change rate earlier because of the pyrrhotite.In the third stage, the mass decreased to 3.26%, 3.84%, and 2.26%, respectively. Study shows that the mass loss of pyrite is related to the slow and continuous desulfurization of the intermediate pyrrhotite product, and the final product is troilite, which possesses a stable structure and composition [17]. As demonstrated above, the slow and continuous desulfurization reaction of pyrrhotite and mixed minerals (1 : 1) occurred simultaneously, and this requires further verification.


### 3.2. The Influence of Pyrrhotite Content on the Thermal Decomposition of the Pyrite-Pyrrhotite Mixtures

As shown in Figure6, an increase in the content of pyrrhotite decreases the peak temperature of the thermal decomposition (peak of DTG) of the mixed minerals; this indicates that pyrrhotite accelerates the thermal decomposition of the mixed minerals. Moreover, an increase in the content of pyrrhotite decreased the mass loss of the mixed minerals, which showed that the reaction intensity of pyrrhotite was not as strong as that of pyrite. Further, the reaction of pyrite was more violent, the heat loss was greater, and this has been confirmed in an explosion test in previous literature [32]. Previously, it was considered that the volatilization of volatile gas is the cause of mass loss in the gas-solid reaction of dust thermal decomposition [33]. Therefore, the problem of reaction intensity can be determined from the chemical reaction products, and this phenomenon is discussed in Section 4.1.Figure 6
The influence of pyrrhotite content on the thermal decomposition of the mixed minerals.


## 3.1. Thermal Decomposition Behavior of Each Mineral in N2 Atmosphere: Pyrite, Mixed Minerals (1 : 1), and Pyrrhotite

The thermal analysis results for pyrite, mixed minerals (1 : 1), and pyrrhotite are shown in Figure5. The thermal decomposition process of the three ore dust samples was accompanied by mass loss and could be divided into three stages. The total mass loss of pyrite Δm=25.22% was higher than that of the mixed minerals (1 : 1) Δm=16.12% and pyrrhotite Δm=5.81%.Figure 5
Thermal decomposition of the ore dust samples; △m1, △m2, and △m3 are the first, second, and third stages of mass loss, respectively.Only a small amount of mass loss occurred in the first stage: 2.61%, 1.56%, and 0.92%, respectively. The end temperatures were 560, 470, and 500°C, respectively. Because the sample was dried without free water and bound water and the result of S/Fe characterized in Section2.2 was larger than the theoretical value, there was excess S. Therefore, we believe that a small amount of elemental S was volatilized by heating.The highest mass losses occurred in the second stage, and they were 19.35%, 10.72%, and 2.63%, respectively, and the end temperatures were 560, 470, and 500°C, respectively. This trend was attributed to the decomposition of the main minerals in the pyrite dust [31]. The maximum mass loss rates occurred at 602, 607, and 610°C, which were 0.511, 0.209, and 0.0157 mg/min, respectively. Further, the derivative thermogravimetry (DTG) curves showed a peak. Furthermore, it was observed that although the reaction rate of the mixed minerals (1 : 1) did not increase and the heating rate remained the same (10 °C/min), the mixed minerals (1 : 1) attained the maximum mass change rate earlier because of the pyrrhotite.In the third stage, the mass decreased to 3.26%, 3.84%, and 2.26%, respectively. Study shows that the mass loss of pyrite is related to the slow and continuous desulfurization of the intermediate pyrrhotite product, and the final product is troilite, which possesses a stable structure and composition [17]. As demonstrated above, the slow and continuous desulfurization reaction of pyrrhotite and mixed minerals (1 : 1) occurred simultaneously, and this requires further verification.


## 3.2. The Influence of Pyrrhotite Content on the Thermal Decomposition of the Pyrite-Pyrrhotite Mixtures

As shown in Figure6, an increase in the content of pyrrhotite decreases the peak temperature of the thermal decomposition (peak of DTG) of the mixed minerals; this indicates that pyrrhotite accelerates the thermal decomposition of the mixed minerals. Moreover, an increase in the content of pyrrhotite decreased the mass loss of the mixed minerals, which showed that the reaction intensity of pyrrhotite was not as strong as that of pyrite. Further, the reaction of pyrite was more violent, the heat loss was greater, and this has been confirmed in an explosion test in previous literature [32]. Previously, it was considered that the volatilization of volatile gas is the cause of mass loss in the gas-solid reaction of dust thermal decomposition [33]. Therefore, the problem of reaction intensity can be determined from the chemical reaction products, and this phenomenon is discussed in Section 4.1.Figure 6
The influence of pyrrhotite content on the thermal decomposition of the mixed minerals.


## 4. The Influence of Pyrrhotite on the Thermal Decomposition Mechanism of the Pyrite-Pyrrhotite Mixtures

A comprehensive discussion of the thermal decomposition of pyrite and pyrrhotite in the N2 atmosphere was based on the literature data, the solid phase, gas phase, and changes in the surface structure, providing data support for analyzing the effect of pyrrhotite on the thermal decomposition of mixed minerals.


### 4.1. Pyrite

The changes in the solid phase, gas phase, and surface structure during the thermal decomposition of pyrite at different temperatures are shown in Figures3(a), 7(a), and 8(a), respectively. From 27°C to 390°C, the elemental S gas curve gradually decreased. The solid-phase analysis results at this temperature showed that pyrite did not decompose, and the melting point of S was approximately 112.8°C. This indicated that the mass loss resulted from the volatilization of elemental S gas and confirmed the conjecture of mass loss in the first stage, as presented in Section 3.1. The pyrite particles had a smooth surface at 550°C, which was demarcated with no phase change. At 560°C, a small number of pores appeared on the surface of the pyrite, and the surface of the finer particles became smooth, accompanied by sintering. Additionally, a small amount of S2 gas was emitted from the surface of the pyrite, which showed that the pyrite began to undergo thermal decomposition. Compared with that in literature [17], the temperature range for the conversion of pyrite to pyrrhotite can be compressed to 560–600°C.Figure 7
The real-time gas-phase transformation process of thermal decomposition: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)Figure 8
Changes in the surface structure of the thermal decomposition products: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)Furthermore, at 626°C, the pyrite surface pores gradually increased, showing a honeycomb shape. The small particles were agglomerated into blocks, and only the presence of hexagonal pyrrhotite (Fe1−xS) was detected; there was an endothermic peak in the differential scanning calorimetry (DSC) curve, indicating that the thermal decomposition reaction was the most violent. According to the literature [17], the pyrrhotite produced at 560–626°C completed the transformation from monoclinic to hexagonal, the S2 peak was narrow with high intensity, and this indicated that S2 was the main product of the thermal decomposition of pyrite. At 645°C, the pores on the surface of the pyrite were completely opened, the pores were shallow, and the bottom of the pores was visible, indicating that the reaction only occurred on the surface. Additionally, the characteristic peaks of pyrite at 645°C were significantly lower than those at 626°C, and the increased characteristic peaks of Fe1-xS indicated that pyrite transformed into hexagonal pyrrhotite.At 1100°C, the surface of the pyrite particles was porous, and the particles were approximately spherical; when the particles were further enlarged, a hexagonal columnar shape was observed. The XRD analysis only showed hexagonal troilite (FeS) products, and the thermal decomposition reaction continued. The amount of S2 produced exhibited an upward trend, and the height of the S2 production trend was equal to the peak value, while the amount of produced S changed slightly, indicating that the gas product was still dominated by S2. These results are consistent with those reported in literature [17].In summary, the thermal decomposition of pyrite in the N2 atmosphere was a two-step chemical reaction, which can be described by the following equations:(1)1−xFeS2s=Fe1−xSs+0.5−xS2g,(2)Fe1−xSs=1−xFeSs+0.5xS2g.


### 4.2. Pyrrhotite

Figures3(c), 7(c), and 8(c) show the variations in the solid phase, gas phase, and surface structure during the thermal decomposition of pyrrhotite at different temperatures, respectively. At 323°C, an endothermic peak appeared in the DSC curve of pyrrhotite, the pyrrhotite was transformed from monoclinic to hexagonal, and S2 was still the main gas product. These findings were consistent with those reported in literature [21–24]. At 500°C, the surface of the particles was smooth and did not agglomerate, which was different from that of pyrite. Fine particle agglomeration was more obvious at 550°C, and the surface of large particles was still smooth; this was due to the relatively strong magnetism of pyrrhotite [18]. Additionally, the peak value of FeS2 increased at 500°C and 550°C, indicating that monoclinic pyrrhotite, Fe7S8, decomposed to form a small amount of FeS2. This phenomenon is consistent with previously reported experimental results [24]. The aggregated small particles became loose and porous at 578°C, indicating that most of the gas was generated and volatilized out of the particles through the pore channels. The XRD results showed that Fe1−xS was produced by the thermal decomposition of FeS2 and FeS2 in the original ore sample. This phenomenon was in good agreement with the above-mentioned thermal decomposition of pyrite.Nevertheless, at 670°C, most of the fine pyrrhotite particles agglomerated into spheres because of pyrolysis. The surface of the fine particles was rough and porous, the surface of the large particles remained unchanged, and only Fe1−xS was produced. At 1100°C, particle agglomeration was more obvious. The particle surface was the same as that of pyrite, presenting a hexagonal columnar shape. Only hexagonal Fe1−xS was found in the residue by XRD analysis. Further, at 578–1100°C, the phase of pyrrhotite changed between S and Fe as the temperature increased [26]; at 1100°C, only troilite existed in the thermal decomposition products of pyrite, as presented in Section 4.1. However, it is not shown in the XRD analysis results of the pyrrhotite thermal decomposition. The above phenomenon occurred because the Fe1−xS peak masked a small amount of FeS peak formed by the thermal decomposition of pyrite. References [34, 35] believe that XRD analysis has errors in distinguishing the phases of adjacent objects through diffraction peaks. Therefore, the XRD analysis results in this study were acceptable. Compared with the peaks of S and S2 at 500–1100°C, the peak of S2 was steeper; therefore, S2 remained the main gas product.In summary, the thermal decomposition reaction equations of pyrrhotite in the N2 atmosphere are shown as follows:(3)1−xFe7S8s=7Fe1−xSs+0.5−4xS2g,(4)Fe1−xSs+1−2xSs=1−xFeS2s.


### 4.3. Pyrite-Pyrrhotite Mixtures

In the thermal decomposition process, the solid phase, gas phase, and surface structure changes of the mixed minerals (1 : 1) at different temperatures are shown in Figures3(b), 7(b), and 8(b). At 470°C, the surface of the large particles was smooth without pores, and the particles were distinct with no obvious change. At 550°C, micropores appeared on the surface of the large particles, and finer particles were adsorbed on the surface of the large particles. The results of the XRD analysis at 470°C and 550°C showed that hexagonal Fe1−xS and FeS2 were present. Further, the peak value of pyrite decreased, indicating the transformation of pyrite and the formation of pyrrhotite. However, at 560°C, part of the peak value of single pyrite decreased, and part of the peak value increased; the Fe1−xS phase was not formed, indicating that the addition of pyrrhotite was conducive to the thermal decomposition of pyrite, which can explain the experimental phenomenon presented in Section 3.2. Additionally, the peak of pyrrhotite began to decrease, indicating that the transformation from monoclinic to hexagonal was completed. It was confirmed in a previous report [20] that the combination of pyrite and pyrrhotite is beneficial to the generation of S, and the curve of the gas product, S2, showed an upward trend, further confirming the occurrence of the reaction.Moreover, at 610°C, the pores on the surface of the large particles increased, and the agglomeration of small particles was observed. There was an endothermic peak in the DSC curve, and a large amount of S2 gas was generated near this temperature. At 620°C, the pores on the surface of the large particles increased and were completely opened. Small particles were adsorbed on the surface of the large particles. However, the pores indicated that the reaction occurred not only on the surface of small particles but also on the surface of large particles at the relatively low part of the adsorption layer. The peak value of FeS2 in the mixed minerals (1 : 1) was lower than that in the single pyrite, which indicated that pyrrhotite was beneficial to the thermal decomposition of pyrite at high temperatures. At 1100°C, the surfaces of the particles were porous and honeycomb-like. Occasionally, some particles were not honeycomb-like. The pores of the mixed minerals were less than those of pyrite, which showed that the reaction intensity of the mixed ore was lower than that of pyrite. When a single particle was further enlarged, it was found that its shape was hexagonal columnar, and the XRD results were the same as those of pyrrhotite: only hexagonal Fe1−xS was observed. This may be because the addition of pyrrhotite inhibited the formation of FeS from pyrite [20]; however, a small amount of FeS peaks was masked by Fe1−xS. Additionally, the S2 analysis curve still showed an upward trend, indicating that the mixed minerals underwent decomposition reaction, which was the same for pyrite and pyrrhotite at temperatures higher than 1100°C.In summary, the thermal decomposition reaction equation of the pyrite-pyrrhotite mixtures in the N2 atmosphere is shown in the following equation:(5)1−xFeS2s+1−xFe7S8s=8Fe1−xSs+1−5xS2g.


### 4.4. Verification of the Analysis Results

To verify the accuracy of the thermal decomposition products of pyrite, mixed minerals (1 : 1), and pyrrhotite at 1100°C, the magnetic analysis of the three products was carried out, and the results are shown in Figure9. The magnetic properties of the three products were considerably weak and almost negligible. However, as shown in Figure 9, the thermal decomposition products of the pyrrhotite and mixed minerals (1 : 1) show enhanced antidemagnetization abilities and are equal. In the XRD analysis results, hexagonal pyrrhotite Fe1−xS was observed. The thermal decomposition products of pyrite exhibited a weak antidemagnetization ability. In the XRD analysis results, FeS was observed. The results of the validity test are consistent with the results of literature [22]; therefore, the thermal analysis XRD results are acceptable.Figure 9
Magnetic analysis results of the thermal decomposition products at 1100°C. The magnetic measurements were carried out at a stable temperature of 27°C.Additionally, the above-mentioned TG and TG-MS tests were successively carried out, and the errors of the DTG peak value were 0.97%, 1.15%, and 0.34%, respectively. The errors may be caused by the different ventilation rates of N2 and the different masses of the analysis samples [36], which was considered acceptable because the heating rate was 10°C/min.


## 4.1. Pyrite

The changes in the solid phase, gas phase, and surface structure during the thermal decomposition of pyrite at different temperatures are shown in Figures3(a), 7(a), and 8(a), respectively. From 27°C to 390°C, the elemental S gas curve gradually decreased. The solid-phase analysis results at this temperature showed that pyrite did not decompose, and the melting point of S was approximately 112.8°C. This indicated that the mass loss resulted from the volatilization of elemental S gas and confirmed the conjecture of mass loss in the first stage, as presented in Section 3.1. The pyrite particles had a smooth surface at 550°C, which was demarcated with no phase change. At 560°C, a small number of pores appeared on the surface of the pyrite, and the surface of the finer particles became smooth, accompanied by sintering. Additionally, a small amount of S2 gas was emitted from the surface of the pyrite, which showed that the pyrite began to undergo thermal decomposition. Compared with that in literature [17], the temperature range for the conversion of pyrite to pyrrhotite can be compressed to 560–600°C.Figure 7
The real-time gas-phase transformation process of thermal decomposition: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)Figure 8
Changes in the surface structure of the thermal decomposition products: (a) pyrite, (b) mixed minerals (1 : 1), and (c) pyrrhotite.
(a)(b)(c)Furthermore, at 626°C, the pyrite surface pores gradually increased, showing a honeycomb shape. The small particles were agglomerated into blocks, and only the presence of hexagonal pyrrhotite (Fe1−xS) was detected; there was an endothermic peak in the differential scanning calorimetry (DSC) curve, indicating that the thermal decomposition reaction was the most violent. According to the literature [17], the pyrrhotite produced at 560–626°C completed the transformation from monoclinic to hexagonal, the S2 peak was narrow with high intensity, and this indicated that S2 was the main product of the thermal decomposition of pyrite. At 645°C, the pores on the surface of the pyrite were completely opened, the pores were shallow, and the bottom of the pores was visible, indicating that the reaction only occurred on the surface. Additionally, the characteristic peaks of pyrite at 645°C were significantly lower than those at 626°C, and the increased characteristic peaks of Fe1-xS indicated that pyrite transformed into hexagonal pyrrhotite.At 1100°C, the surface of the pyrite particles was porous, and the particles were approximately spherical; when the particles were further enlarged, a hexagonal columnar shape was observed. The XRD analysis only showed hexagonal troilite (FeS) products, and the thermal decomposition reaction continued. The amount of S2 produced exhibited an upward trend, and the height of the S2 production trend was equal to the peak value, while the amount of produced S changed slightly, indicating that the gas product was still dominated by S2. These results are consistent with those reported in literature [17].In summary, the thermal decomposition of pyrite in the N2 atmosphere was a two-step chemical reaction, which can be described by the following equations:(1)1−xFeS2s=Fe1−xSs+0.5−xS2g,(2)Fe1−xSs=1−xFeSs+0.5xS2g.


## 4.2. Pyrrhotite

Figures3(c), 7(c), and 8(c) show the variations in the solid phase, gas phase, and surface structure during the thermal decomposition of pyrrhotite at different temperatures, respectively. At 323°C, an endothermic peak appeared in the DSC curve of pyrrhotite, the pyrrhotite was transformed from monoclinic to hexagonal, and S2 was still the main gas product. These findings were consistent with those reported in literature [21–24]. At 500°C, the surface of the particles was smooth and did not agglomerate, which was different from that of pyrite. Fine particle agglomeration was more obvious at 550°C, and the surface of large particles was still smooth; this was due to the relatively strong magnetism of pyrrhotite [18]. Additionally, the peak value of FeS2 increased at 500°C and 550°C, indicating that monoclinic pyrrhotite, Fe7S8, decomposed to form a small amount of FeS2. This phenomenon is consistent with previously reported experimental results [24]. The aggregated small particles became loose and porous at 578°C, indicating that most of the gas was generated and volatilized out of the particles through the pore channels. The XRD results showed that Fe1−xS was produced by the thermal decomposition of FeS2 and FeS2 in the original ore sample. This phenomenon was in good agreement with the above-mentioned thermal decomposition of pyrite.Nevertheless, at 670°C, most of the fine pyrrhotite particles agglomerated into spheres because of pyrolysis. The surface of the fine particles was rough and porous, the surface of the large particles remained unchanged, and only Fe1−xS was produced. At 1100°C, particle agglomeration was more obvious. The particle surface was the same as that of pyrite, presenting a hexagonal columnar shape. Only hexagonal Fe1−xS was found in the residue by XRD analysis. Further, at 578–1100°C, the phase of pyrrhotite changed between S and Fe as the temperature increased [26]; at 1100°C, only troilite existed in the thermal decomposition products of pyrite, as presented in Section 4.1. However, it is not shown in the XRD analysis results of the pyrrhotite thermal decomposition. The above phenomenon occurred because the Fe1−xS peak masked a small amount of FeS peak formed by the thermal decomposition of pyrite. References [34, 35] believe that XRD analysis has errors in distinguishing the phases of adjacent objects through diffraction peaks. Therefore, the XRD analysis results in this study were acceptable. Compared with the peaks of S and S2 at 500–1100°C, the peak of S2 was steeper; therefore, S2 remained the main gas product.In summary, the thermal decomposition reaction equations of pyrrhotite in the N2 atmosphere are shown as follows:(3)1−xFe7S8s=7Fe1−xSs+0.5−4xS2g,(4)Fe1−xSs+1−2xSs=1−xFeS2s.


## 4.3. Pyrite-Pyrrhotite Mixtures

In the thermal decomposition process, the solid phase, gas phase, and surface structure changes of the mixed minerals (1 : 1) at different temperatures are shown in Figures3(b), 7(b), and 8(b). At 470°C, the surface of the large particles was smooth without pores, and the particles were distinct with no obvious change. At 550°C, micropores appeared on the surface of the large particles, and finer particles were adsorbed on the surface of the large particles. The results of the XRD analysis at 470°C and 550°C showed that hexagonal Fe1−xS and FeS2 were present. Further, the peak value of pyrite decreased, indicating the transformation of pyrite and the formation of pyrrhotite. However, at 560°C, part of the peak value of single pyrite decreased, and part of the peak value increased; the Fe1−xS phase was not formed, indicating that the addition of pyrrhotite was conducive to the thermal decomposition of pyrite, which can explain the experimental phenomenon presented in Section 3.2. Additionally, the peak of pyrrhotite began to decrease, indicating that the transformation from monoclinic to hexagonal was completed. It was confirmed in a previous report [20] that the combination of pyrite and pyrrhotite is beneficial to the generation of S, and the curve of the gas product, S2, showed an upward trend, further confirming the occurrence of the reaction.Moreover, at 610°C, the pores on the surface of the large particles increased, and the agglomeration of small particles was observed. There was an endothermic peak in the DSC curve, and a large amount of S2 gas was generated near this temperature. At 620°C, the pores on the surface of the large particles increased and were completely opened. Small particles were adsorbed on the surface of the large particles. However, the pores indicated that the reaction occurred not only on the surface of small particles but also on the surface of large particles at the relatively low part of the adsorption layer. The peak value of FeS2 in the mixed minerals (1 : 1) was lower than that in the single pyrite, which indicated that pyrrhotite was beneficial to the thermal decomposition of pyrite at high temperatures. At 1100°C, the surfaces of the particles were porous and honeycomb-like. Occasionally, some particles were not honeycomb-like. The pores of the mixed minerals were less than those of pyrite, which showed that the reaction intensity of the mixed ore was lower than that of pyrite. When a single particle was further enlarged, it was found that its shape was hexagonal columnar, and the XRD results were the same as those of pyrrhotite: only hexagonal Fe1−xS was observed. This may be because the addition of pyrrhotite inhibited the formation of FeS from pyrite [20]; however, a small amount of FeS peaks was masked by Fe1−xS. Additionally, the S2 analysis curve still showed an upward trend, indicating that the mixed minerals underwent decomposition reaction, which was the same for pyrite and pyrrhotite at temperatures higher than 1100°C.In summary, the thermal decomposition reaction equation of the pyrite-pyrrhotite mixtures in the N2 atmosphere is shown in the following equation:(5)1−xFeS2s+1−xFe7S8s=8Fe1−xSs+1−5xS2g.


## 4.4. Verification of the Analysis Results

To verify the accuracy of the thermal decomposition products of pyrite, mixed minerals (1 : 1), and pyrrhotite at 1100°C, the magnetic analysis of the three products was carried out, and the results are shown in Figure9. The magnetic properties of the three products were considerably weak and almost negligible. However, as shown in Figure 9, the thermal decomposition products of the pyrrhotite and mixed minerals (1 : 1) show enhanced antidemagnetization abilities and are equal. In the XRD analysis results, hexagonal pyrrhotite Fe1−xS was observed. The thermal decomposition products of pyrite exhibited a weak antidemagnetization ability. In the XRD analysis results, FeS was observed. The results of the validity test are consistent with the results of literature [22]; therefore, the thermal analysis XRD results are acceptable.Figure 9
Magnetic analysis results of the thermal decomposition products at 1100°C. The magnetic measurements were carried out at a stable temperature of 27°C.Additionally, the above-mentioned TG and TG-MS tests were successively carried out, and the errors of the DTG peak value were 0.97%, 1.15%, and 0.34%, respectively. The errors may be caused by the different ventilation rates of N2 and the different masses of the analysis samples [36], which was considered acceptable because the heating rate was 10°C/min.


## 5. Kinetic Mechanism and Model

### 5.1. Kinetic Mechanism Analysis

The apparent activation energyEa, as a research object, can significantly describe the thermodynamic mechanism of nonisothermal and heterogeneous reaction systems [37]. Hoare et al. [16] and Lv et al. [38] believed that the thermal decomposition process of pyrite could be described by the shrinking sphere model and three-dimensional diffusion model; the decomposition process of pyrrhotite corresponds with the three-dimensional diffusion model. Combined with the analysis of the chemical mechanism presented in Section 4, the Coats–Redfern method was used to calculate the Ea of pyrite, mixed minerals (1 : 1), and pyrrhotite at 560–630°C, as shown in the following equation:(6)lngaT2=lnARβEa−EaRT,where β is the heating rate (°C/min), A is the preexponential factor (min−1), Ea is the apparent activation energy of the reaction (kJ/mol), R is the universal gas constant (8.314 J/K·mol−1), T is the absolute temperature (K), gα is the integral function of the reaction model, and a is the decomposition conversion rate of the three samples (%), a=m0−mt/m0−m∞, m0 is the initial mass of the sample, mt is the mass of the sample at time t, and mt is the final mass of the sample.The solution process was to draw thega/T2−1/T curve according to the integral function and 39 common ga listed in reference [37]. Ea was obtained according to the slope of the straight line, and A was obtained according to the intercept. After calculating Ea of the three ore samples, the results are presented in Table 2. The table lists models a, b, and e described in literature [16, 38] and lists models a, c, and d of 39 common functions with the highest correlation coefficient, whichever is higher. After the comparison, the Ea values of pyrite, mixed minerals (1 : 1), and pyrrhotite were calculated to be 147.18, 87.20, and 27.21 kJ/mol, respectively. The results showed that pyrrhotite contained more active reactants and required less energy for thermal decomposition reactions, and chemical reactions were more likely to occur; these findings are consistent with the experimental results.Table 2
Calculation results of the apparent activation energy of the three ore dust samples.
Sample typeFunctionMechanismgαR2Ea/kJ·mol−1A/s−1ModelPyriteInverse Jander equationThree-dimensional diffusion, 3D1+a1/3−120.9810147.181.49 × 105aPyriteShrinkage spherical (volume)Shrinkage spherical31−1−a1/30.955889.045.6 × 101bMixed minerals (1 : 1)Mampel power rulePhase boundary reaction (one-dimensional,n = 2)a20.975487.202.95 × 101cPyrrhotiteThird orderMultistage reaction1−a−20.991427.213.70 × 105dPyrrhotiteJander equationThree-dimensional diffusion, spherical symmetry, 3D1−1−a1/320.965741.592.01 × 103e


### 5.2. Kinetic Process Model and Influence Mechanism

Studies have shown that the surface reaction of pyrite and pyrrhotite dust in an N2 atmosphere is the main reaction [12, 13]. Although the pyrite and pyrrhotite used in this study contained a small amount of SiO2, it has been proven that SiO2 is an inert material [39] and does not participate in the reaction. From the XRD and TG-MS analysis results, it was observed that the thermal decomposition reaction of pyrite, pyrrhotite, and mixed minerals was a gas-solid two-phase reaction, and the pyrite and pyrrhotite decomposition processes were included in the thermal decomposition reaction of mixed minerals. Using the SEM analysis results, the thermal decomposition kinetic process of mixed minerals in the N2 atmosphere was analyzed and presented in this section, as shown in Figure 10.Figure 10
Thermal decomposition reaction model of the mixed minerals in N2 atmosphere.The thermal decomposition process of the mixed minerals consisted of three stages. In the first stage, when pyrite was heated, elemental S volatilized from the cracks of pyrrhotite fine particles and the surface where large pyrite particles adhered. Additionally, some pyrrhotite fine particles were adsorbed on the surface of pyrite, and some agglomerated together; splitting occurred on the surface of pyrite, and a small amount of S2 volatilized. In the second stage, most of the pores appeared on the surface of pyrite, and the thermal decomposition of the pyrite particles and pyrrhotite fine particles occurred when heated simultaneously, producing gas S2 and volatilizing. The fine particles adsorbed on the surface of pyrite and the agglomerated fine particles were further agglomerated by heat. Pyrite underwent a nuclear shrinkage reaction [16], resulting in a reduced particle volume. In the third stage, the amount of gas, S2, gradually reduced. The large particles adsorbed with fine particles and the aggregated small particles were agglomerated and compact, showing hexagonal columns, and all of them reacted to form hexagonal pyrrhotite, Fe1−xS.There are three reasons why pyrrhotite accelerated the thermal decomposition of mixed minerals. First, the adsorption of small particles increased the specific surface area of large particles, enhanced the thermal decomposition reaction activity, and accelerated thermal decomposition. This phenomenon can be confirmed from the results presented in Tables1 and 2. Second, fine particles were adsorbed on the surface of the large pyrite particles because of the positive magnetic force of the Fe element in pyrrhotite. The precipitation of the negatively charged S (S−1) in pyrite was accelerated, resulting in the early appearance of pores on the surface of pyrite, which can be confirmed in Figure 8(b). Third, fine particles were adsorbed on the surface of large pyrite particles because the reaction temperature of the fine particles was relatively low, and the gas product, S2, was volatilized earlier. The volatilization of the gas products produced by the fine particles increased the surface tension of the large pyrite particles, accelerated the surface splitting of the large pyrite particles, and reduced the thermal decomposition reaction temperature of pyrite, as shown in Figures 7(b) and 8(b).


## 5.1. Kinetic Mechanism Analysis

The apparent activation energyEa, as a research object, can significantly describe the thermodynamic mechanism of nonisothermal and heterogeneous reaction systems [37]. Hoare et al. [16] and Lv et al. [38] believed that the thermal decomposition process of pyrite could be described by the shrinking sphere model and three-dimensional diffusion model; the decomposition process of pyrrhotite corresponds with the three-dimensional diffusion model. Combined with the analysis of the chemical mechanism presented in Section 4, the Coats–Redfern method was used to calculate the Ea of pyrite, mixed minerals (1 : 1), and pyrrhotite at 560–630°C, as shown in the following equation:(6)lngaT2=lnARβEa−EaRT,where β is the heating rate (°C/min), A is the preexponential factor (min−1), Ea is the apparent activation energy of the reaction (kJ/mol), R is the universal gas constant (8.314 J/K·mol−1), T is the absolute temperature (K), gα is the integral function of the reaction model, and a is the decomposition conversion rate of the three samples (%), a=m0−mt/m0−m∞, m0 is the initial mass of the sample, mt is the mass of the sample at time t, and mt is the final mass of the sample.The solution process was to draw thega/T2−1/T curve according to the integral function and 39 common ga listed in reference [37]. Ea was obtained according to the slope of the straight line, and A was obtained according to the intercept. After calculating Ea of the three ore samples, the results are presented in Table 2. The table lists models a, b, and e described in literature [16, 38] and lists models a, c, and d of 39 common functions with the highest correlation coefficient, whichever is higher. After the comparison, the Ea values of pyrite, mixed minerals (1 : 1), and pyrrhotite were calculated to be 147.18, 87.20, and 27.21 kJ/mol, respectively. The results showed that pyrrhotite contained more active reactants and required less energy for thermal decomposition reactions, and chemical reactions were more likely to occur; these findings are consistent with the experimental results.Table 2
Calculation results of the apparent activation energy of the three ore dust samples.
Sample typeFunctionMechanismgαR2Ea/kJ·mol−1A/s−1ModelPyriteInverse Jander equationThree-dimensional diffusion, 3D1+a1/3−120.9810147.181.49 × 105aPyriteShrinkage spherical (volume)Shrinkage spherical31−1−a1/30.955889.045.6 × 101bMixed minerals (1 : 1)Mampel power rulePhase boundary reaction (one-dimensional,n = 2)a20.975487.202.95 × 101cPyrrhotiteThird orderMultistage reaction1−a−20.991427.213.70 × 105dPyrrhotiteJander equationThree-dimensional diffusion, spherical symmetry, 3D1−1−a1/320.965741.592.01 × 103e


## 5.2. Kinetic Process Model and Influence Mechanism

Studies have shown that the surface reaction of pyrite and pyrrhotite dust in an N2 atmosphere is the main reaction [12, 13]. Although the pyrite and pyrrhotite used in this study contained a small amount of SiO2, it has been proven that SiO2 is an inert material [39] and does not participate in the reaction. From the XRD and TG-MS analysis results, it was observed that the thermal decomposition reaction of pyrite, pyrrhotite, and mixed minerals was a gas-solid two-phase reaction, and the pyrite and pyrrhotite decomposition processes were included in the thermal decomposition reaction of mixed minerals. Using the SEM analysis results, the thermal decomposition kinetic process of mixed minerals in the N2 atmosphere was analyzed and presented in this section, as shown in Figure 10.Figure 10
Thermal decomposition reaction model of the mixed minerals in N2 atmosphere.The thermal decomposition process of the mixed minerals consisted of three stages. In the first stage, when pyrite was heated, elemental S volatilized from the cracks of pyrrhotite fine particles and the surface where large pyrite particles adhered. Additionally, some pyrrhotite fine particles were adsorbed on the surface of pyrite, and some agglomerated together; splitting occurred on the surface of pyrite, and a small amount of S2 volatilized. In the second stage, most of the pores appeared on the surface of pyrite, and the thermal decomposition of the pyrite particles and pyrrhotite fine particles occurred when heated simultaneously, producing gas S2 and volatilizing. The fine particles adsorbed on the surface of pyrite and the agglomerated fine particles were further agglomerated by heat. Pyrite underwent a nuclear shrinkage reaction [16], resulting in a reduced particle volume. In the third stage, the amount of gas, S2, gradually reduced. The large particles adsorbed with fine particles and the aggregated small particles were agglomerated and compact, showing hexagonal columns, and all of them reacted to form hexagonal pyrrhotite, Fe1−xS.There are three reasons why pyrrhotite accelerated the thermal decomposition of mixed minerals. First, the adsorption of small particles increased the specific surface area of large particles, enhanced the thermal decomposition reaction activity, and accelerated thermal decomposition. This phenomenon can be confirmed from the results presented in Tables1 and 2. Second, fine particles were adsorbed on the surface of the large pyrite particles because of the positive magnetic force of the Fe element in pyrrhotite. The precipitation of the negatively charged S (S−1) in pyrite was accelerated, resulting in the early appearance of pores on the surface of pyrite, which can be confirmed in Figure 8(b). Third, fine particles were adsorbed on the surface of large pyrite particles because the reaction temperature of the fine particles was relatively low, and the gas product, S2, was volatilized earlier. The volatilization of the gas products produced by the fine particles increased the surface tension of the large pyrite particles, accelerated the surface splitting of the large pyrite particles, and reduced the thermal decomposition reaction temperature of pyrite, as shown in Figures 7(b) and 8(b).


## 6. Conclusions

The effect of pyrrhotite content on the thermal decomposition of the pyrite-pyrrhotite mixture in the N2 atmosphere was studied.(1)
The apparent activation energy of pyrrhotite was lower than that of the pyrite and pyrite-pyrrhotite mixture. With an increase in the pyrrhotite content, the specific surface area of the pyrite-pyrrhotite mixtures increased, resulting in a decrease in the reaction peak temperature at the thermal decomposition rate.(2)
Pyrrhotite agglomerated more when pyrrhotite was decomposed. The fine pyrrhotite particles adsorbed on the surface of pyrite blocked the pores of pyrite, resulting in a decrease in the total amount of gas product (S2) and the total mass loss of the pyrite-pyrrhotite mixtures.(3)
The thermal decomposition products of monoclinic pyrrhotite and pyrite-pyrrhotite mixtures at 1100°C were mainly hexagonal pyrrhotite (Fe1-xS), while the thermal decomposition product of pyrite was troilite (FeS).



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*Source: 8160007-2022-01-31.xml*