Experimental Investigation on Spontaneous Combustion Characteristics of Sulfide Ores with Different Sulfur Content

Abstract

The spontaneous combustion of sulfide ores (SOSC) is an extremely dangerous mining disaster that directly threatens safety production in mines and causes far-reaching negative impacts on the surrounding ecosystem. In this study, oxidation weight gain experiments, self-heating temperature and ignition temperature tests, and thermogravimetric analysis (TGA) were conducted to detect the spontaneous combustion characteristics of sulfide ores with different sulfur contents (40.29%, 34.56%, 24.81%, and 14.2%). The results show that the sulfur content significantly affects the spontaneous combustion characteristics of sulfide ores. As the sulfur content decreased, the oxidized weight gain rate decreased overall, and the self-heating temperature (135, 152.5, 162.5, and 176.9 °C) and ignition temperature (425.3, 438.6, 455.4, and >500 °C) increased. The three combustion stages of the SOSC were divided based on the TG and DTG curves: low-temperature oxidation stage, combustion decomposition stage, and slow burnout stage. Furthermore, KAS and FWO methods were used to obtain the apparent activation energy in the combustion decomposition stage. The apparent activation energy decreased significantly with the increase in the sulfur content. The results of all experiments and analyses showed that sulfide ores with high sulfur content have a stronger tendency to undergo spontaneous combustion. The research results have important theoretical and practical implications for the prevention of SOSC.

1. Introduction

In China, 20%–30% of iron sulfide deposits and more than 10% of nonferrous metal sulfide deposits are high-sulfur deposits, and the spontaneous combustion of sulfide ores (SCSO) is a serious natural disaster faced in the production of high-sulfur mines [1,2]. SCSO is a complex process that involves chemical, physical, and biological interactions. It occurs when sulfide minerals undergo oxidation reactions, generating heat that accumulates and eventually ignites the ore [3]. This phenomenon is particularly prevalent in metal mines rich in sulfide ores, such as pyrite, chalcopyrite, and sphalerite [4]. SCSO not only results in the loss of mineral resources but also poses severe threats to mine safety and environmental protection [5,6]. In addition, as the depth of mines continues to increase, the risk of spontaneous combustion is further exacerbated by the high ground temperature in deep mining environments [7,8]. China’s Regulations for the Specifications for General Surveys of Hidden Disaster-Causing Factors in Mines (KA/T 22-2024) stipulates that sulfide ores and adjacent surrounding rock with sulfur content greater than 15% should be tested for the propensity and period of spontaneous combustion. Therefore, it is important to investigate the spontaneous combustion characteristics of sulfide ore samples to provide basic data for disaster prevention measures [9].

For decades, many scholars have been carrying out research on the thermal properties of sulfide ores’ spontaneous combustion processes. For example, Kong et al. [10] used thermogravimetric–differential scanning calorimetry (TG-DSC) to evaluate the pyrophoric risk of sulfide minerals during storage and demonstrated that the kinetic approach is an effective method for predicting the risk of sulfide minerals. Li et al. [11] studied the spontaneous combustion mechanism and heat stability of sulfide mineral powder through synchronous thermal analysis, and investigated the effects of atmosphere conditions, heating rates and particle size on the thermal stability of pyrite. In addition, many researchers have investigated the self-heating behavior of sulfide–mineral concentrates using the crossing-point temperature method [12] and TGA and DSC measurements [13,14].

There are many factors that can affect SCSO, such as mineral composition, heating rates, mechanical activation, and pre-oxidation. Some scholars have studied the role of mineralogical content on the self-heating behavior of sulfide ores. They found that the higher the pyrrhotite, pentlandite, and chalcopyrite contents, the higher the self-heating propensity of the sulfide ore [15]. Yang et al. studied the roasting thermodynamic process of African cobalt-rich copper sulfide ore using the TG-DTG method at different heating rates. The results show that the heating rate has a significant influence on the ore phase transition during the oxidation process [16]. Many researchers have considered the effect of mechanical activation on the oxidation and combustion characteristics of sulfide minerals. They found that the chemical reaction activity of sulfide minerals increased with mechanical activation [17,18,19]. Zhao et al. [20] analyzed the effects of water immersion on the thermodynamic properties of sulfide ores through nitrogen adsorption, X-ray diffraction and TGA experiments. Sulfide ores were also more susceptible to spontaneous combustion after the pre-oxidation [21]. In addition, prediction and detection [7,22,23,24], prevention and control technologies [6,25,26,27] and risk assessment for SCSO [28,29] have been studied experimentally and theoretically for decades.

Numerous researchers have contributed toward SCSO. However, there has still been no systematic analysis of the effect of the primary combustion element sulfur of SCSO. In order to further explore the mechanism of SCSO, four sulfide ore samples from Huzhaoshan mine in the Qixiashan lead–zinc mining area with different sulfur contents were selected to further investigate the characteristics of SOSC. The variation rules of parameters such as oxidized weight gain rate, self-heating temperature and ignition temperature and the apparent activation energy of sulfide ores were analyzed. The effects of sulfur content on spontaneous combustion characteristics were qualitatively discussed. The research results aim to provide some support for the prevention and control of SCSO during mining.

2. Experimental Samples and Method

2.1. Sulfide Ore Sample Preparation

The sulfide ores were obtained from the Huzhaoshan mine in the Qixiashan lead–zinc mining area of Jiangsu Province. Before the experiment, large ore samples were packaged in sealed bags and transported to the laboratory. The experimental samples were prepared by removing the surface oxide layer (approximately 5 mm) by hammering. The ore samples were then crushed to 2 cm with a low drop of a heavy hammer. Finally, a vibrating mill was used to grind the crushed samples to 80 mesh in small quantities. The experimental samples were protected by nitrogen filling at room temperature and placed in wide-mouthed sand flasks. Prepared experimental samples with a particle size of less than 20 μm made up approximately 75% or more. The particle size distribution of the experimental samples is shown in Figure 1.

Sulfur content is an important factor in the spontaneous combustion of sulfide ores. In this experiment, the main mineral composition of the samples was investigated using the X-ray diffraction (XRD) technique, as shown in Figure 2. The composition and content of the chemical elements in the sulfide ores were measured using X-ray fluorescence (XRF). The chemical compositions of the samples are presented in

Table 1. Chemical composition of the experimental samples.

Sample Name	Mass Fraction (%)
	S	Fe	Pb	Zn	Si	Mn	Al	Ca	Mg	Cu
YM-01	40.29	10.61	0.998	0.459	4.54	7.17	3.24	2.22	0.581	0.012
YM-02	34.56	36.59	0.579	1.27	1.68	2.01	1.14	1.37	0.461	0.01
YM-03	24.81	25.72	1.81	3.2	2.07	1.47	0.526	4.78	1.47	-
YM-04	14.2	22.25	6.2	16.28	3.55	3.34	3.07	10.15	2.93	0.015

.

The results of the elemental analysis of the samples showed that Fe and S accounted for the highest proportions in the ore samples. In addition, the samples contained Fe, Pb, Pb Zn, and other metallic elements. The XRD results showed that the main components of the sulfide ores were FeS₂, CuFeS₂, ZnS, and PbS, and the main impurity was SiO₂. The main compounds in the samples are listed in

Table 2. Main compound contents of the experimental samples.

Compounds	YM-01	YM-02	YM-03	YM-04
FeS2	36.63	32.59	24.81	18.25
ZnS	6.29	12.51	14.98	17.15
PbS	3.13	2.21	3.87	4.09
CuFeS2	0.56	0.32	-	0.40
Fe2O3	5.26	11.63	9.68	7.81
SiO2	9.71	3.58	4.44	7.58

.

2.2. Experimental Approach

2.2.1. Oxidation Weight Gain Test

The higher the oxidizing activity of the ore sample, the higher the oxygen uptake and the greater the mass increase in the sample. Therefore, the oxidation rate can be determined by periodically measuring the rate of increase in the oxidized mass of the ore sample to determine its oxidation rate and thus characterize the oxidation reaction activity of the ore [30]. The weighed ore sample was evenly spread out on the Petri dish to increase the contact surface of the ore sample with air, and then this Petri dish containing the ore sample was placed in a constant temperature and humidity chamber. To approximate the underground mining environment, the temperature was 40 ± 1 °C and the relative humidity was 60%. Every other day, the weight of the mineral sample was compared to the weight change in each specimen, which reacted to the speed of oxidation of each mineral sample. To facilitate the analysis and comparison, the measured weight of each specimen was converted to a weight-gain rate. The weight gain rate is measured as a period of time, the unit initial weight specimen weight gain percentage, and can be expressed as follows

$$P={\displaystyle \frac{\Delta W}{W_0}}={\displaystyle \frac{W-W_0}{W_0}}\times 100\%$$

(1)

where P is the weight gain rate (%), ΔW is the weight gain of the samples after a period of oxidation (g), W₀ is the initial weight of the samples (g), and W is the weight of the samples after a period of time (g).

2.2.2. Self-Heating Temperature and Ignition Temperature Test

The self-heating phenomenon is the core process and necessary stage leading to SCSO [31,32]. The self-heating temperature test employed the equilibrium tracking method, as shown in Figure 3. This method involves the use of a constant-temperature oil bath to artificially heat the ore samples, typically setting the initial temperature between 30 and 40 °C and supplying an appropriate amount of oxygen. The procedure continued until the temperature of the ore samples equaled the ambient temperature, at which point a constant temperature was maintained for approximately 30 min. If the temperature of the ore samples does not exceed the ambient temperature, this indicates that the ore samples do not exhibit significant self-heating. Subsequently, the ambient temperature is increased, with the temperature increment being approximately 10 °C.

The ignition temperature experiment for the samples used the GB/T 16430-2018 method [33], which is a minimum ignition temperature test for sulfide ore dust layers. The experimental equipment is illustrated in Figure 4. During the test, the hot plate furnace was first heated to a constant temperature, and then the sample to be tested was placed on the hot plate, and a dust layer of a specified 5 mm thickness was formed. The temperature of the hot plate was rapidly increased to a constant temperature before placing the sample, and the dust layer was observed to determine whether it ignited. During the course of the experiment, the sulfide ore dust layer underwent oxidative self-heating as the temperature increased, and ignition occurred when a certain temperature was attained.

2.2.3. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis can measure the change in sample mass with temperature under programmed temperature control to obtain the TG curve [34]. This test technique is an effective method for investigating the thermal behavior of minerals [35]. The Mettler Toledo TG/DSC STARe System was used for the experiments. The experiments were performed using approximately 20 mg of each sample. The atmosphere was air at a gas flow rate of 40 mL/min. The raw samples were heated from room temperature to 850 °C at heating rates of 5, 10, 15, and 25 °C min⁻¹.

Combustion kinetics methods based on thermogravimetric analysis can be mainly divided into the model-fitting method and model-free method. The model-fitting method may lead to incorrect results owing to the kinetic compensation effect arising from the selection of the reaction mechanism function. The model-free method, also known as the iso-conversion rate method, can obtain more reliable apparent activation energy values without assuming the mechanism function and is recommended by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC). It is widely used in thermodynamic analyses, and the commonly used model-free methods are the Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) models [36]. The expression of the FWO and KAS methods is based on the following equations.

FWO method:

$$\ln \beta =\ln \left({\displaystyle \frac{AE}{RG\left(\alpha \right)}}\right)-5.331-1.052{\displaystyle \frac{E}{RT}}$$

(2)

KAS method:

$$\ln \left({\displaystyle \frac{\beta }{T^2}}\right)=\ln \left({\displaystyle \frac{AR}{EG\left(\alpha \right)}}\right)-{\displaystyle \frac{E}{RT}}$$

(3)

where E is the apparent activation energy (J/mol), A is the pre-exponential factor (1/s), R is the gas constant (J/mol·K), β is the heating rate (K/min), T is the temperature corresponding to different heating rates β at a given value of conversion α (K), and G(α) represents the integral form of the mechanism function.

The apparent activation energy E was calculated from the slope of the regression lines obtained by plotting ln(β) (FWO method) or ln(β/T2) (KAS method) against 1/T for different conversion values (α). In addition, the pre-exponential factor A and other thermodynamic parameters such as enthalpy ΔH, Gibbs free energy ΔG, and entropy ΔS were calculated with the following equations [37].

$$A=\beta \exp \left({\displaystyle \frac{E}{R{T}_m}}\right)/\left(R{T}_m^2\right)$$

(4)

$$\Delta H=E-RT$$

(5)

$$\Delta G=E+R{T}_m\ln {\displaystyle \frac{K_{B}T}{hA}}$$

(6)

$$\Delta S={\displaystyle \frac{\Delta H-\Delta G}{T_m}}$$

(7)

where T_m is the average DTG peak temperature (K), K_B is the Boltzmann constant (1.38 × 10⁻²³ J/K), and h is Planck’s constant (6.626 × 10⁻³⁴ J/s).

3. Results and Discussion

3.1. Weight Gain of Sulfide Ore Samples

The oxidation weight gain test lasted 30 days, and the oxidation process of the ore samples under constant temperature and humidity was observed every 3 days. The sample appearances before and after the oxidation test are given in Figure 5. It can be seen that the samples were mostly earthy gray with smooth surfaces before oxidation. After oxidation under constant temperature and humidity for 1 month, the samples gradually turned dark brown. In addition, after oxidation, the samples showed different degrees of agglomeration: samples YM-01 and YM-02 showed strong agglomeration, YM-03 showed weak agglomeration, and YM-04 showed slight agglomeration.

Figure 6 illustrates the oxidation weight gain rate curves for each ore sample over time. It is evident that as the oxidation duration increases, the oxidation weight gain rate for each ore sample increases consistently. Notably, there were variations in the magnitude of increase among the different ore samples. By the conclusion of the experiment, the YM-01 ore sample exhibited the highest oxidation weight gain rate of 1.227%, whereas the YM-04 sample demonstrated the lowest rate of 0.088%. The average oxidation weight gain rate across the ore samples was 0.764%. Based on the oxidation caking characteristics and the weight gain rate of each ore sample, the oxidation performance was assessed as follows: YM-01 = YM-02 > YM-03 > YM-04. This closely aligned with the sulfur content. This suggests a positive correlation between the oxidation performance of the samples and their sulfur contents.

3.2. Self-Heating Temperature and Ignition Temperature

Figure 7 shows the self-heating and ignition temperatures of various sulfide ore samples. It can be seen from Figure 7 that the self-heating temperatures of the samples were 135 °C, 152.5 °C, 162.5 °C, and 176.9 °C, respectively. It is evident that as the sulfur content of the samples increases, there is an obvious decrease in the self-heating temperature, which shows a negative correlation with the sulfur content. The ignition temperatures of the ore samples were 425.3 °C, 438.6 °C, 455.4 °C, and >500 °C (the YM-04 sample did not catch fire in the experimental temperature range), respectively. It can be found that with the increase in sulfur content, significantly less energy is required for the combustion of samples and a higher risk of SOSC.

3.3. Kinetic Analysis of the Oxidation Reaction

3.3.1. Combustion Stage Division

Figure 8 presents the TG and DTG curves for the sulfide ore samples at different heating rates (5 °C/min, 10 °C/min, 15 °C/min, and 20 °C/min). Although the sulfur content of the samples varied, the TG and DTG curves showed the same trends. The mass of sulfide ores samples heated from room temperature to 850 °C under an air atmosphere generally demonstrated three processes of slow increase, drastic decrease, and gradual stabilization. According to previous research [11,38], based on the mass loss stages, the sample combustion process can be subdivided into three stages.

Stage I is the low-temperature oxidation stage (room temperature—T₁). This stage occurred at a temperature lower than approximately 400 °C, and the TG curve exhibited a weak mass gain or mass loss phenomenon, with a quality loss of approximately −0.8% to 1.24%. The mass gain phenomenon was mainly due to the oxygen adsorption of the samples, and the mass loss phenomenon was caused by trace mineral decomposition in the sulfide ore samples.

Stage II is the combustion decomposition stage (T₁–T₂). This stage mainly involves the decomposition and depolymerization of sulfide ore, which continuously generates gas that escapes from the sample, and the mass of the sulfide ore samples gradually decreases. The mass loss was approximately 14.3%–27.3% relative to the initial stage, with an average of 21%. Furthermore, according to the TG and DTG curves under different heating rates in this stage, it can be observed that the curves shifted to the high-temperature direction as the heating rate increased. This phenomenon was mainly due to the fact that the temperature gradient between the crucible and the experimental sample increases as the heating rate increases. To achieve the same thermal equilibrium, a higher temperature is required, which leads to a shift in the TG curve towards higher temperatures. In addition, the residues after the TGA were analyzed using the XRD (Figure 9). The XRD pattern of the residues showed that the new products after TGA were Fe₂O₃, FeSO₄, ZnSO₄, and PbSO₄. In this stage, the main component may undergo chemical reactions [35]:

$${4\mathrm{FeS}}_2{+11\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{O}}_3+8{\mathrm{SO}}_2$$

(8)

$${\mathrm{FeS}}_2{+3\mathrm{O}}_2{=2\mathrm{FeSO}}_4+{\mathrm{SO}}_2$$

(9)

$${\mathrm{ZnS}+2\mathrm{O}}_2{=\mathrm{ZnSO}}_4$$

(10)

$${\mathrm{PbS}+2\mathrm{O}}_2{=\mathrm{PbSO}}_4$$

(11)

Stage Ⅲ is the slow burnout stage (T₂—850 °C). At this stage, the mass loss of the ore sample began to slow down and gradually tended to level off. The majority of the macromolecular compounds decomposed into gas molecules in Stage II, and only non-decomposable or non-reacting substances remained until the sample was burned out and its weight no longer changed. The maximum mass loss of the sample at this stage was approximately 1.6% to 5.6%, with an average of 3.6%.

3.3.2. Effects of Sulfur Content on Thermogravimetric Properties

According to the TG-DTG curve,

Table 3. Temperature range and quality loss rate in stage II for four sulfide ore samples.

Sample Name	Initial Temperature T1/°C	Final Temperature T2/°C	Mass Loss in Stage II/%
			5 K/min	10 K/min	15 K/min	20 K/min	Average Value
YM-01	402.5	610.3	27.3	26.1	23.6	20.8	24.5
YM-02	402.7	591.7	27.0	25.2	23.2	21.5	24.2
YM-03	401.6	570.4	20.2	21.6	20.4	19.0	20.3
YM-04	401.9	558.2	16.7	15.2	14.8	13.4	15.0

lists the temperature range and quality loss rate in stage II for the four sulfide ore samples at four heating rates.

As shown in Table 3, the initial temperatures were similar for each sample (402.2 ± 0.5 °C), but the final temperatures varied greatly in stage II, and the final temperature gradually decreased with an increase in the sulfur content of the ore samples. With an increase in the sulfur content, the final temperature gradually decreased (610.3 °C, 591.7 °C, 570.4 °C, and 558.2 °C, respectively), and the enlargement in the temperature range of stage II became increasingly obvious. In addition, through the mass loss of different samples at this stage, it can be observed that the mass loss decreased as the sulfur content decreased (the average values obtained for different heating rates were 24.5%, 24.2%, 20.3%, and 15.0%, respectively). The results indicate that SOSC primarily involves a reaction between elemental sulfur and oxygen.

3.3.3. Apparent Activation Energy

Model-free methods such as FWO and KWS are effective in comparing the apparent activation energies, although there are limitations such as the inability to resolve intermediates and the difficulty in distinguishing the dominant steps in complex solid-phase reactions. The conversion rate was from 0.1 to 0.9 with an interval of 0.1, and the fitting results of the combustion decomposition stage (T₁–T₂) at four heating rates based on the FWO and KAS methods described in Section 2.2.3 are shown in Figure 10. The activation energies obtained from the corresponding slopes are listed in

Table 4. Apparent activation energy of sulfide ore samples obtained by FWO and KAS methods.

Conversion α	Apparent Activation Energy E (kJ/mol)
	FWO Method				KAS Method
	YM-01	YM-02	YM-03	YM-04	YM-01	YM-02	YM-03	YM-04
0.1	161.0	190.1	217.9	201.4	157.4	188.0	217.4	199.9
0.2	123.6	200.9	202.5	210.6	117.9	199.0	200.9	209.3
0.3	97.0	124.5	181.8	223.9	89.6	118.3	178.9	223.2
0.4	125.7	115.9	178.9	224.1	119.4	109.1	175.6	223.2
0.5	135.1	139.5	206.5	231.3	129.0	133.7	204.3	230.6
0.6	125.5	159.0	278.5	289.5	118.7	154.1	279.9	291.7
0.7	146.0	239.7	299.1	334.2	140.0	238.7	301.4	338.5
0.8	149.8	255.4	287.8	335.3	143.8	255.0	289.3	339.4
0.9	147.7	247.6	309.3	539.3	141.2	246.6	311.8	553.9
Average	134.6	185.8	240.3	287.7	128.6	182.5	240.0	290.0

.

The apparent activation energy is the minimum energy required for the combustion reaction, and a higher apparent activation energy means it is more difficult for SCSO to occur. Figure 10 shows a good linear fitting result, indicating that the apparent activation energy obtained has a high degree of reliability. As can be seen from Table 4, the apparent activation energy values of the samples calculated using FWO and KAS were not significantly different, indicating that the calculated apparent activation energy is reliable.

As Table 4 indicates, the average apparent activation energies of YM-01 calculated by FWO and KAS models were 134.6 kJ/mol and 128.6 kJ/mol, with 185.8 kJ/mol and 182.5 kJ/mol for YM-02, 240.3 kJ/mol and 240.0 kJ/mol for YM-03, and 287.7 kJ/mol and 290.0 kJ/mol for YM-03, respectively. Figure 11 shows the relationship between the sulfur content and the apparent activation energy of the sulfide ores. It can be observed that with an increase in the sulfur content, the apparent activation energy for the combustion decomposition stage obtained by the two kinetic analysis methods decreased significantly. The main reason for this result could be explained as follows: the main process of SCSO is the oxidation reaction of sulfide (mainly FeS₂) with oxygen and water; the higher the sulfur content, the higher the total amount of sulfide involved in these exothermic oxidation reactions, which means a higher oxidation rate and reaction heat. This indicates that the higher the sulfur content, the greater the tendency toward SCSO. In addition, according to the research by Yang et al. [39] on the classification criteria of spontaneous combustion tendencies due to the apparent activation energy for sulfide ores, when the apparent activation energy value is less than 180 kJ/mol, the spontaneous combustion hazard level is high. Combined with the results shown in Figure 11, it can be inferred that the sulfide ore with a sulfur content of approximately 30% or more has a greater propensity for spontaneous combustion, and therefore prevention and control measures are indispensable for those ore.

Sulfide ore is a complex composition of various sulfides and oxides. The mineral compositions and particle size are the important factors affecting the characteristic gas concentration of sulfide ore. In future studies, it will be necessary to analyze factors such as particle size and humidity, and ores should be selected from different mines to further verify the viewpoint proposed in this paper. In addition, a systematic study of oxidative thermal changes and gas products is a direction for subsequent research.

3.3.4. Thermodynamic Parameters

Figure 12 shows the thermodynamic parameters of the samples calculated using the FWO and KAS methods. All parameters showed variations with conversion α. This situation indicates that the sample composition is complex and the reactions occurring during the process are also very complicated [40].

Enthalpy change ΔH represents the energy difference between the reagents and the activated complex. The smaller the ΔH value, the more favorable the formation of the production reactants because the potential energy barrier is low. The average ΔH results in order were 128.03 and 121.98 kJ/mol, 179.32 and 175.98 kJ/mol, 233.85 and 233.54 kJ/mol, and 281.34 and 283.57 kJ/mol, calculated by FWO and KAS models for YM-01, YM-02, YM-03, and YM-04, respectively. For all samples, enthalpy change followed the trend in the apparent activation energy.

Gibbs free energy change ΔG indicates the change in the energy of the system towards the equilibrium of reagents and the formation of activated complexes. The ΔG values were 252.46 and 246.41 kJ/mol, 244.24 and 240.9 kJ/mol, 241.65 and 241.34 kJ/mol, and 238.8 and 241.03 kJ/mol for each sample, calculated by FWO and KAS, respectively. For all samples, ΔG values were very close.

Entropy ΔS reflects how near the system is to its own thermodynamic equilibrium. High ΔS values indicate that the sample is far from thermodynamic equilibrium, while a low ΔS value suggests that the sample is approaching a new state close to thermodynamic equilibrium. The ΔS of each sample was negative when the α was low, and it became positive and increased with α. A negative value of ΔS indicates that the complexity of bond dissociation energy of the products is lower than that of the initial reactant. The value of ΔS increased, indicating that the reaction conforms to the principle of entropy increase.

4. Conclusions

In this study, the spontaneous combustion characteristics of sulfide ores with different sulfur contents from the Huzhaoshan mine were studied using oxidation weight gain experiments, self-heating temperature and ignition temperature tests, and thermogravimetric analysis. The main conclusions from the experiment and analysis are as follows.

(1)

The sulfide ore samples exhibited varying degrees of weight gain and agglomeration after oxidation under constant temperature and humidity for one month. The ranking of the oxidation weight gain rate was consistent with the ranking of the sulfur content for each sample. The self-heating temperatures were 135, 152.5, 162.5, and 176.9 °C, and the ignition temperatures were 425.3, 438.6, 455.4, and >500 °C for each sample, respectively. As the sulfur content of the samples increased, the self-heating and ignition temperatures decreased.

(2)

The thermogravimetric analysis of SCSO showed that the combustion process can be subdivided into three stages. Stage I is the low-temperature oxidation stage, during which trace mineral decomposition and oxygen absorption occurred. Stage II is the combustion decomposition stage, during which obvious mass loss occurred due to the decomposition and depolymerization reactions of the sulfide ore. Stage Ⅲ is the slow burnout stage, during which the non-decomposable or non-reacting substances remained until burnout, and the sample weight no longer changed. Otherwise, with an increase in the sulfur content, the enlargement in the temperature range became increasingly obvious, and the mass loss increased in stage II.

(3)

For the combustion decomposition stage, the average activation energies of samples YM-01, YM-02, YM-03, and YM-04 calculated using the FWO method were 134.6, 185.8, 240.3, and 287.7 kJ/mol, respectively, and those calculated using the KAS method were 128.6, 182.5, 240.0, and 290.0 kJ/mol, respectively. Apparent activation energy decreased significantly with the increase in the sulfur content. This indicates that sulfide ores with high sulfur content show a stronger tendency for spontaneous combustion and are indispensable for the development of prevention and control measures.