Effects of Pyrrhotite on the Combustion Behavior and the Kinetic Mechanism of Pyrite-Pyrrhotite Mixture Powders in the Air

. In this study, we performed a comparative analysis of the combustion behavior of pyrite, pyrrhotite, and pyrite-pyrrhotite mixture (mixed mineral) powders in an air atmosphere. To study the infuence of the pyrrhotite content in mixed mineral powders on the combustion behavior in the air, thermogravimetric mass spectrometry, X-ray difraction analysis, and scanning electron microscopy were employed. Te results indicated that pyrrhotite lead to a weight gain in the mixed minerals during the combustion process. Pyrrhotite particles are more easily adsorbed on the surface of pyrite particles during mixed mineral combustion due to their strong ability to absorb oxygen, which accelerates pyrite combustion. Te weight loss of mixed minerals decreased during the combustion process with increasing pyrrhotite content, resulting from pyrite encapsulation by agglomerated and sintered pyrrhotite during combustion. Te calculated kinetic parameters and phase analysis results suggested that pyrite combustion is consistent with the shrinking core mechanism, and in the combustion process, the irregular pyrite particle shrank into a spherical particle; the combustion products of pyrrhotite grew in a layer-by-layer manner. Pyrrhotite combustion corresponded to the three-dimensional difusion mechanism, and mixed mineral combustion was dominated by the shrinking core mechanism and supplemented by the three-dimensional difusion mechanism. SO 2 , as the main combustion product, was continuously generated and volatilized in the reaction, signifying that the combustion reaction of pyrite is a two-phase reaction involving gas and solid.


Introduction
Iron sulfde minerals play a key role in geochemistry, marine systems, oil storage and transportation, and environmental management [1][2][3][4][5]. As iron sulfde minerals, pyrite (FeS 2 ) is widely applied to sulfuric acid production, metallurgy, and other industries [6,7], whereas pyrrhotite (Fe 1− x S) has practical application values in biomedicine, magnetic material manufacturing, water treatment, and battery processing, among other felds [8,9]. However, large amounts of iron sulfde dust produced during iron sulfde mineral mining, storage, and transportation pose the risk of combustion and even explosion if dust explosion pentagon conditions are met [10,11]. Moreover, pyrite and pyrrhotite, as minerals associated with coal and chalcopyrite, are among the main sources of sulfur dioxide emissions during the combustion of coal and chalcopyrite leaching, which is a signifcant cause of environmental pollution [12][13][14]. Accidents caused by combustion and explosion, as well as pollution in iron sulfde mines, have resulted in signifcant damage and loss of life in several countries in the past decades [15,16]. Terefore, to address the challenges associated with the use of iron sulfde minerals, it is essential to study the phase transition behavior of iron sulfde combustion in an air atmosphere.
Pyrite combustion in the air is controlled by temperature and oxygen concentration, among other factors, and involves the transformation of two diverse forms [17]. Some researchers have proposed that pyrite directly converts to hematite (Fe 2 O 3 ; one-step direct oxidation theory) [18][19][20][21][22][23][24][25], while others have considered that pyrite frst decomposes into porous pyrrhotite and that the porous pyrrhotite further oxidizes to other iron oxides (step-by-step theory) [17,[26][27][28]. Subject to temperature, one-step direct oxidation theory suggests that pyrite directly oxidizes to produce Fe 2 O 3 at temperatures below 530°C, and sulfate formation occurs during the reaction [23]. Te oxidation products of pyrite at 1200 and 1500°C are Fe 2 O 3 and Fe 3 O 4 , respectively [19,20]. However, according to step-by-step theory, pyrite frst decomposes to form magnetic pyrite (pyrrhotite) at 610°C, followed by further oxidation of pyrrhotite, with the end products comprising FeS 2 , Fe 1− x S, and Fe 2 O 3 /Fe 3 O 4 [27]. In addition, some studies have demonstrated that pyrrhotite appears at low temperatures (T < 270°C), but pyrrhotite oxidizes to iron oxides at high temperatures [29]. Infuenced by the concentration of oxygen, the one-step direct oxidation theory considers that Fe 2 O 3 is the stabilized oxide in a hypoxic atmosphere at ∼1327°C [18], and Fe 3 O 4 is formed at high temperatures [19,20]. Fe 3 O 4 is the only stabilized product under high oxygen concentration conditions when the combustion temperature is above 1427°C, while Fe 2 O 3 only exists at temperatures below 1227°C [17,21]. According to the step-by-step theory, only pyrrhotite is formed in atmospheres of 100 ppm to 1009 ppm O 2 gas, in a temperature range of 484-538°C [26]. Tus, there is no consensus regarding the combustion process of pyrite, and this topic requires further research.
Pyrrhotite is generally studied as an intermediate product during pyrite combustion, but only few studies have been performed employing pyrrhotite as a reactant [30][31][32][33][34][35]. Ozdeniz and Kelebek [30] found that pyrrhotite combustion occurs through a spontaneous self-heating reaction. Cruz et al. [31] suggested that the reactivity of pyrrhotite is controlled by the formation of an oxidation product layer, which can encapsulate and passivate the surface of pyrrhotite, and that the elemental S layer is dominant. Zhao et al. [32] revealed that O 2 exhibits greater adsorption energy on the pyrrhotite surface than on the pyrite surface. Dunn and Chamberlain [33] showed that the ignition temperature decreases with an increase in the pyrrhotite content. Alksnis et al. [34] prepared iron oxides and sulfur dioxide by roasting pyrrhotite. Te optimum roasting conditions were estimated as follows: the gas fow rate was at least 200 mL/ min, the temperature was 850°C, and the oxygen partial pressure was equal to that in the air. Luo et al. [8] proposed that the pyrrhotite combustion process occurs in four stages: oxidative decomposition of pyrrhotite, formation of ferric sulfate, decomposition of ferric sulfate, and formation of hematite. Moreover, Lv et al. [35] theorized that CO 2 participates in the entire oxidative decomposition process of pyrrhotite. Nevertheless, the studies on pyrrhotite combustion are relatively scarcer than those on pyrite combustion.
Te combustion behavior of mixtures is more complex than that of single compounds, and therefore, associated studies are rather sparse. Te experiments performed by Yang et al. [36] using a FeS-FeS 2 mixture in a nitrogen atmosphere revealed that the strength of surface adsorption and oxygen storage capacity of the FeS-FeS 2 mixture increase with the increasing mass fraction and fractal dimension of FeS, leading to spontaneous combustion of sulfde ore. Li et al. [37] studied the phase transition process of a FeS-FeS 2 mixture in a helium and H 2 S atmosphere at high temperatures and found that the oxidation of sulfur is a prerequisite for the formation of pyrite. However, the combustion process and the mechanism in an air atmosphere remain unclear and necessitate further studies.
To explore the infuence of the pyrrhotite content on the combustion of pyrite-pyrrhotite mixtures (mixed mineral), we used thermogravimetric mass spectrometry (TG-MS), X-ray difraction (XRD) analysis, and scanning electron microscopy (SEM) in this study to determine solid and gaseous products. By calculating apparent activation energy, the associated chemical process and the kinetic mechanism, as well as the role of the pyrrhotite content in pyrite combustion, were determined. Te study is aimed at providing a theoretical basis for the chemical reaction, revealing pyrrhotite participation in pyrite dust explosions, and the knowledge of which may help prevent pyrite mountain fres and explosion accidents in the future.

Materials and Characterization.
Commercial pure pyrite and pyrrhotite (Guangzhou Huadu District Huadong Yeshi Stone Specimen Firm, Guangdong, China) were used to conduct the tests. After grinding, samples were sieved with a 200-mesh standard sieve (pore diameter � 75 μm), and the sieved pyrite and pyrrhotite were, respectively, mixed at 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. Te particle sizes and the surface structure of pyrite, pyrrhotite, and pyrite-pyrrhotite mixtures at a mass ratio of 1 :1 (referred to as mixed minerals (1 :1)) were studied using a laser difraction analyzer (2000E, Jinan Winner, China) and SEM (MLA650F, FEI, USA), and the results of which are shown in Figure 1. Te three minerals exhibit nonuniform particle sizes and irregularly shaped structures, and the majority of the mineral particles were <45 μm in size, with a median particle size of <33 μm. Te results of the particle size analysis were in good agreement with the SEM analysis results. Te moisture contents of ore samples were determined after drying at 80°C for 24 h in a constanttemperature drying oven, and the tested samples were almost free of moisture.
Te compositions of the tested samples and combustion products were analyzed using XRD (Empyrean, PANalytical, Holland) at 27°C. Te contents of Fe and S in the ore samples were determined using titration (implementing standards of GB/T 6730.65-2009 [38]) and the direct combustion-iodometric method (implementing standards of YS/T575. 17-2007 [39]), respectively. Te results showed that FeS 2 and monoclinic Fe 7 S 8 are the main components in pyrite and pyrrhotite, respectively. Te contents of Fe and S in pyrite were 45.74% and 58.23%, respectively, and those in pyrrhotite were 53.02% and 38.91%, respectively. Te ratios of S to Fe in pyrite and pyrrhotite were 2.0285 and 1.1694, respectively, which were in good agreement with their stoichiometric values, and the measured results were consistent with the results of the XRD analysis.

Experimental Methods.
Te experimental procedure is shown in Figure 2. TG-MS (TA449F3-QMS403, NETZSCH, Germany) was used to estimate the weight loss and real-time gas products of pyrite, pyrrhotite, and mixed minerals (1 : 1). Te airfow rate for the TG analysis was 50 mL/min, the nitrogen fow rate of MS was 20 mL/min, and the heating rate was 10°C/min. To study the infuence of the pyrrhotite content on mixed mineral combustion, measurements were conducted using TG (Hitachi, STA7200, Japan) at the same airfow rate and heating rate as in TG-MS. Te mass of each of the aforementioned test samples was 10 ± 0.5 mg.
Based on the results of the TG-MS test, the combustion test was performed at the diferential scanning calorimetry (DSC) peak temperature in an electric box furnace (KSL-1200X-M, HF-Kejing, China) to study solid products. First, 2.5 g each of pyrite, pyrrhotite, and mixed minerals (1 : 1) was placed in a trapezoidal corundum crucible with dimensions of 80 × 40 × 17 mm, where the airfow rate was set at 200 mL/min and the heating rate was 10°C/min. After achieving the test temperature and maintaining it at a constant value for 20 min, the samples were allowed to ventilate until the combustion products were cooled to room temperature in the box, and the products were then characterized using XRD and SEM.

Combustion Behavior of Iron Sulfde Minerals in an Air
Atmosphere. Te weight loss and real-time variation of oxygenated sulfde gas products of the three ore dust samples during combustion in the air are shown in Figure 3. Te total weight loss of pyrite, mixed minerals (1 : 1), and pyrrhotite was 34.11%, 25.20%, and 18.38%, respectively.
Compared with the combustion of pyrrhotite and mixed minerals (1 : 1), pyrite combustion only involved four stages, and the second stage of the weight gain was not observed, as presented in Figure 3 and Table 1. In the frst stage, pyrite, mixed minerals, and pyrrhotite exhibited lower weight losses than they did in the other stages, and short exothermic peaks on the DSC curve appeared at 157, 150, and 171°C. Considering the elemental analysis results in Section 2.1, namely, the presence of excess S monomers, and S melts at ∼112.8°C, we assume that the peak is generated due to the reaction of S with O 2 to form SO 2 . Hence, an increase in the SO 2 curve can also confrm this result.
In the second stage, pyrite underwent a weight loss of 15.58%. Elemental sulfur on the surface of pyrite particles volatilized [40] and reacted with O 2 to form SO 2 , with SO 2 production being maximum at ∼476°C. Notably, small amounts of SO, SO 3 , and COS were simultaneously observed, which is consistent with the results of the studies by Jorgensen and Moyle [23] and Lv et al. [35], wherein SO and COS were detected, respectively. However, the weights of pyrrhotite and mixed minerals increased by 3.24% and 0.79%, respectively, which is associated with the formation of FeSO 4 and Fe 2 O 3 from oxidative decomposition of pyrrhotite [8,41]. Additionally, a large amount of gaseous SO 3 was formed.
In the third stage, the weight loss of pyrite was 16.69%. An endothermic peak was observed at 659.3°C on the DSC curves. Hu et al. [17] reported that sulfate would be formed at approximately 600-650°C, and the endothermic peak was a result of sulfate decomposition, which was verifed by analyzing solid-phase products. Pyrrhotite showed a weight loss of 4.36%, which is related to the emission of S due to pyrrhotite decomposition at ∼550°C [8,42]. S, released during combustion, reacted with O 2 to form SO, SO 2 , SO 3 , and COS, and the amount of SO 2 produced was maximum at ∼512°C. Te weight loss of mixed minerals was 11.33%, which may be due to the gasifcation of S originating from original pyrrhotite [41] and pyrrhotite formed through pyrite pyrolysis [40].
In the fourth stage, the weight loss of pyrite was only 0.90%, and it continued above 800°C. Furthermore, the weight losses of pyrrhotite and mixed minerals were 12.70% and 11.16%, respectively, resulting from further pyrolysis of FeSO 4 formed via pyrrhotite decomposition to release SO 2 in the temperature range of 600-900°C [8]. Moreover, an endothermic peak was detected at 663.4°C, due to the decomposition of the molten material [Fe 2 (SO 4 ) 3 ] 2 ·Fe 2 O 3 [43].
In the ffth stage, the weight losses of pyrrhotite and mixed minerals had low values of 0.62% and 2.48%, respectively, and they continued at 800°C. Furthermore, the slope of the TG curve was very small, indicating that the combustion reaction was essentially completed at that time.

Infuence of the Pyrrhotite Content on the Combustion of Pyrite-Pyrrhotite Mixtures.
Te efect of the pyrrhotite content on the combustion of pyrite-pyrrhotite mixtures in an air atmosphere is shown in Figure 4. Te frst peak temperature on the DTG curve generally increased with an increase in the pyrrhotite content (except for a few points, e.g., the mass ratio 1 : 0.1), which may be due to the fact that pyrrhotite contains more amount of sulfur and takes longer to completely combust than pyrite does. Te second peak temperature on the DTG curve decreased with an increase in the pyrrhotite content because pyrrhotite is easier to oxidize [44]. With an increase in the pyrrhotite content, the amounts of FeSO 4 and Fe 2 O 3 formed via decomposition increased and the reaction rate increased. Although the two peak temperatures fuctuated, there was no variation in the third and fourth peak temperatures on the DTG curve with an increase in the pyrrhotite content, which indicated that pyrrhotite mainly infuenced the initial and weight gain stages of combustion of the pyrite-pyrrhotite mixture.
Moreover, the weight loss rate of mixed ore evidently decreased with an increase in the pyrrhotite content, which indicated that the combustion of pyrite is more violent than that of pyrrhotite in the air, and pyrite combusted with more gas volatilized, as shown in Figure 4. Tis phenomenon has also been observed in explosion experiments [45].

Characteristics of Solid Products from Iron Sulfde Minerals Combustion.
Te combustion products of the three samples in the air were characterized using XRD at room temperature (27°C), the peak temperature on the DSC curve, and the fnal test temperature (800°C), and the results are International Journal of Chemical Engineering 5 The first peak temperature The second peak temperature The third peak temperature The fourth peak temperature Weight loss rate     International Journal of Chemical Engineering shown in Figure 5. Surface morphology changes in combustion products are shown in Figure 6. As shown in Figures 5(a) and 6(a), at 157°C, dispersed pyrite particles with smaller sizes were adsorbed on the surface of large particles, which are dense and nonporous. No phase change was observed at that time, indicating that the weight loss is caused by S volatilization. Te phase change in pyrite occurred at 467°C, and hematite was formed, which is consistent with the experimental results reported by Aracena et al. [22], Jorgensen and Moyal [23], and Schorr and Everhart [25]. At 467°C, as small particles were sintered, some cracks developed on the surface of large particles, and the gas volatilized from cracks. At 530°C, the surface of large particles became smoother and cracks became larger, and all pyrite was consumed and converted to Fe 2 O 3 . At 659°C, the particles gradually shrank and tended to be ellipsoidal in shape. Compared with the particles at 530°C, the cracking degree on the surface of single particles increased, and fne particles were more agglomerated. However, intermediate sulfate was not detected at this time. Jorgensen and Moyal [46] suspected that the XRD analysis conducted for distinguishing adjacent phases through diffraction peaks had errors. At 800°C, both sintered particles and large particles possessed an ellipsoidal structure and the combustion products were Fe 2 O 3 , which is the same as at 530°C. Pyrrhotite, the intermediate product described in [26,27,47], was not found in the results of the XRD analysis. FeO, the fnal product described in [48], was also not detected. Combined with the analysis results of the gas-phase products in Section 3.1, the chemical reaction equation of pyrite combustion in the air is shown in the equation as follows: At 423°C, the phase of pyrrhotite began to change, and as the temperature increased from 423°C to 800°C, the crystalline phase of Fe 2 O 3 gradually stabilized, as shown in Figure 5(c). Additionally, FeSO 4 was detected at 423°C and 470°C, confrming that the weight gain stage was caused by the production of Fe 2 O 3 and FeSO 4 , which was consistent with the result reported in [8]. Meanwhile, the yield of SO, SO 2 , and SO 3 increased, especially that of SO 3 , which was maximum at 435.1°C, as shown in Equation (2). Notably, the phase of pyrrhotite changed from monoclinic Fe 7 S 8 to hexagonal Fe 1− x S at 423°C and 470°C, which was consistent with the pyrolysis phenomenon in a N 2 atmosphere [49]. Te reaction is shown in Equation (3). Small particles were adsorbed on large particles with a smooth surface and underwent sintering and cracking and formation of a dense sintered body at 171°C, 423°C, and 470°C, respectively, as shown in Figure 5(c). In addition, Fe 3 O 4 and [Fe 2 (SO 4 ) 3 ] 2 ·Fe 2 O 3 originating from the oxidation of pyrrhotite at 425-520°C and 663.4°C, respectively, were not found [43]. Moreover, Fe 1− x S and FeSO 4 were converted to Fe 2 O 3 , resulting in the formation of a large amount of SO 2 and SO 3 at 470-525°C, which occurred before obtaining the reaction temperature in [43]. Te reaction equations are shown in Equations (4) and (5). Te occurrence of diferent phases may be related to the heating rate [50,51]. Te cracking phenomenon was more apparent at 525°C than at 470°C, indicating that the degree of reaction at 525°C was higher than that at 470°C and that the maximum amount of gaseous products was produced at that temperature. It further demonstrates that the amount of gas products generated is related to the cracking degree and that the output of gas products increases with an increase in the extent of cracking. At 663°C, a new material with a thin crystal structure was produced. Considering the XRD analysis results, it is conjectured that the new material is sulfate formed via the thermal decomposition of the molten material [Fe 2 (SO 4 ) 3 ] 2 ·Fe 2 O 3 . Te surface structure of pyrrhotite at 800°C was almost the same as that at 663°C. Compared with the case of pyrite, no cracking phenomenon was observed, indicating that the oxidation combustion mechanisms of pyrrhotite and pyrite are diferent. In summary, from the analysis results of gas-phase products, the chemical reaction equation of pyrrhotite combustion in the air can be simplifed to Equation (6) without considering the intermediate process: However, the phase transformation of mixed minerals occurred at 452°C. At 452°C and 474°C, the phase transformation of mixed minerals was similar to that of pyrrhotite, and the combustion products contained Fe 1− x S, FeS 2 , Fe 2 O 3 , and FeSO 4 , which were responsible for line patterns, as shown in Figure 3(b), similar to those in Figure 3(c). Moreover, mixed minerals gained weight at 410°C, while pyrrhotite gained weight at 480°C, as shown in 8 International Journal of Chemical Engineering

Figures 3(b) and 3(c)
. It was confrmed that the weight gain is caused by the formation of Fe 2 O 3 and FeSO 4 , indicating that the addition of pyrrhotite promoted the formation of Fe 2 O 3 and FeSO 4 in the pyrite-pyrrhotite mixture, which accelerated the reaction process in the weight gain stage. When the temperature exceeded 525°C, all the mixed minerals reacted to form Fe 2 O 3 , indicating that the conversion of both pyrrhotite and pyrite was complete. Kennedy and Sturman [43] found that pyrite FeS 2 formed in the oxidation of pyrrhotite and theorized that pyrite FeS 2 did not participate in the oxidation of Fe 1-x S. Terefore, considering the formation of gas-phase products, we theorized that the combustion of the pyrite and pyrrhotite mixture in the air occurs independently, with the chemical reaction equations shown in Equations (1) and (6). Upon analyzing the particle surface structure of the combustion products of mixed minerals (1 : 1), the products were similar to pyrite combustion products, as shown in Figure 6(b). Terefore, it may be considered that the interaction between minerals may be signifcantly infuenced by the pyrite composition.

Analysis of the Combustion Kinetic Mechanism of Iron
Sulfde Minerals. Te apparent activation energy (Ea) can be used to describe the thermodynamic mechanism of nonisothermal and heterogeneous reaction systems [52].
Using the Coats-Redfern method, Ea of pyrite, mixed minerals (1 : 1), and pyrrhotite during the combustion process was calculated as follows: Herein, 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 − 1 ·mol − 1 ), T is the absolute temperature (K), g(a) is the integral function of the reaction model, and a is the decomposition conversion rate of three kinds of ore samples (%). a � (m 0 − m t )/(m 0 − m ∞ ), where m 0 is the initial mass of the samples, m t is the mass of the samples at time t, and m ∞ is the fnal mass of the samples.
We plotted the curve of g(a)/T 2 − (1/T) based on the integral function and calculated Ea and A from the slope of the curve and intercept, respectively. Ea of the three ore samples in diferent reaction stages are listed in Table 2. In the rapid reaction stage, the Ea values of pyrite, mixed minerals (1 : 1), and pyrrhotite were 194.81, 105.79, and 109.68 kJ/mol, respectively. Te Ea value of mixed minerals (1 : 1) was the smallest, indicating that the addition of  International Journal of Chemical Engineering pyrrhotite facilitated the oxidation combustion reaction of pyrite, which is in agreement with the test results. Additionally, the mechanism analysis demonstrated that the presence of pyrrhotite, which led to a lower reaction order and lower reaction intensity of mixed minerals than those of pyrite, is the reason for the lower weight loss of mixed minerals than that of pyrite.
Based on the previously discussed kinetic mechanism analysis and phase analysis results of combustion products, the combustion process of the pyrite-pyrrhotite mixture in the air was dominated by pyrite and accompanied by the surface heterogeneous combustion of pyrrhotite [53,54], as shown in Figure 7. Pyrrhotite particles were more easily adsorbed on the surface of pyrite particles during mixed mineral combustion due to their strong ability to absorb oxygen [44], which accelerated pyrite combustion. Te reaction mechanism in the main combustion stage of mixed minerals was the same as that of pyrite. However, the weight loss rate of mixed minerals was higher than that of pyrrhotite due to the infuence of pyrite composition. In the initial stage of combustion, S adsorbed in mixed minerals volatilized with the increasing temperature and reacted with O 2 to form SO 2 . When the temperature increased to 410°C, cracks appeared on the surface of pyrite and pyrrhotite sintered to form Fe 2 O 3 and FeSO 4 . At 410-548°C, the cracks on the surface of pyrite became wider and FeSO 4 reacted with O 2 to form Fe 2 O 3 and gaseous SO 3 . When the temperature exceeded 548°C, only one kind of product was obtained, namely, Fe 2 O 3 . During this process, the irregular pyrite particle shrank into a spherical particle through the kinetic mechanism of random nucleation, which indicated that pyrite combustion conforms to the shrinking nucleation model [52]. Furthermore, the combustion products of pyrrhotite grew in a layer-by-layer manner, which indicated that the pyrrhotite combustion conforms to the three-dimensional difusion model. During the entire reaction process, an amount of SO 2 and traces of other gases, such as SO, SO 3 , and COS, were constantly generated and consumed, signifying that the combustion reaction of pyrite was a two-phase reaction involving gas and solid.

Conclusion
In this study, the combustion process and the kinetic mechanism of two typical iron sulfde minerals in the air were analyzed and the infuence of the pyrrhotite content on the combustion of the pyrite-pyrrhotite mixture was investigated. Te conclusions drawn were as follows: (1) With the addition of pyrrhotite, the combustion process of the mixed ore became more complex, and mixed minerals and pyrrhotite underwent a weight gain stage during the combustion process compared with pyrite, due to the formation of Fe 2 O 3 and sulfate FeSO 4 . In the weight gain stage, the weight of pyrrhotite and mixed minerals increased by 3.24% and 0.79%, respectively. (2) Pyrrhotite mainly afected the weight gain stage of mixed minerals. During that stage, the peak temperature of the DTG curve decreased with an increase in the pyrrhotite content, from 480°C to 425°C, which promoted the formation of FeSO 4 . Owing to agglomeration and sintering of pyrrhotite, the weight loss rate of mixed minerals decreased with an increase in the pyrrhotite content. Te weight loss of pyrrhotite was 18.73% less than that of pyrite. (3) Combustion of iron sulfde minerals was a surface heterogeneous reaction involving gas and solid phases, which was controlled by the kinetic model. During the main combustion stage, the kinetic mechanism of pyrite was random nucleation, which accorded with the shrinking nucleation model, whereas that of pyrrhotite accorded with the threedimensional difusion model. Te combustion process of mixed minerals was signifcantly infuenced by pyrite, and the main reaction stage was the same as that of pyrite, which accorded with the random nucleation mechanism. At that time, pyrrhotite showed a three-dimensional difusion trend. (4) Although the chemical reaction equations and conversion kinetics of powder combustion of iron sulfde minerals were established, the analysis of the chemical reaction process was greatly infuenced by the characterization methods used. Tus, there might be diferences between the theoretical results and the actual continuous reaction process. To address this, the testing methods could be modifed. A series of experiments in new in-situ devices with thermocouples and sensors could be used to study the intermediate reaction process.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.