Study of spontaneous combustion mechanism and heat stability of sulfide minerals powder based on thermal analysis

    Study of spontaneous combustion mechanism and heat stability of sulfide minerals powder based on thermal analysis X. Li, Y.J. Shang, Z.L. Chen, X.F. Chen, Y. Niu, M. Yang, Y. Zhang PII: DOI: Reference:

S0032-5910(16)30908-1 doi:10.1016/j.powtec.2016.12.040 PTEC 12188

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

1 August 2016 5 December 2016 12 December 2016

Please cite this article as: X. Li, Y.J. Shang, Z.L. Chen, X.F. Chen, Y. Niu, M. Yang, Y. Zhang, Study of spontaneous combustion mechanism and heat stability of sulfide minerals powder based on thermal analysis, Powder Technology (2016), doi:10.1016/j.powtec.2016.12.040

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ACCEPTED MANUSCRIPT Spontaneous combustion & heat stability of pyrite

Study of spontaneous combustion mechanism and heat stability of

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sulfide minerals powder based on thermal analysis

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X. Li; Y. J. Shang; Z. L. Chen; X. F. Chen*; Y. Niu; M. Yang; Y. Zhang

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School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China

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*Corresponding author: [email protected];

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Wuhan University of Technology, School of Resources and Environmental

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Engineering, Wuhan 430070, Hubei, China; +8613627212572.

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Abstract

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The spontaneous combustion mechanism of sulfide minerals powder is studied by simultaneous thermal analysis. The chemical reactions of pyrites in the atmospheres of air

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and nitrogen are deduced and the corresponding reaction products are characterized by the

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Raman spectroscopy. The characteristic temperatures of the pyrite reaction process are discussed in different conditions. The results show that the process of oxidation spontaneous combustion can be divided into three stages. Fe3FeSiO4(OH)5 in the minerals is decomposed at low temperature in the oxidation process. FeS2, the main component of the mineral, is

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mainly oxidized in the stage of oxidation spontaneous combustion. However, FeS2 will be

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decomposed under heating in nitrogen atmosphere. Thermal stability of pyrite in air is lower

Keywords

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particle size.

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than that in nitrogen, and its thermal stability will be improved by increasing heating rate and

Pyrite; Spontaneous combustion mechanism; Thermal analysis; Thermal stability

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1 Introduction

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As a kind of sulfide minerals, pyrite powder mainly consists of FeS2 and trace amounts of metal elements, such as As, Sn, Sb and Cu [1]. In sulfide mining, pyrite powder can react

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with oxygen easily during which a large amount of heat is released. Spontaneous combustion

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of pyrite powder may happen when enough heat is accumulated [2]. Oxidation spontaneous combustion of FeS2 is a severe natural disaster in metal mines, as well as in coal gangue [3] and sulfur-containing oil [4]. Therefore, it is of great significance to study the thermal

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stability characteristics of pyrite powder in different conditions.

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Up to now, many scholars have studied the spontaneous combustion mechanism of different sulfides through thermal analysis [5]. Zhang et al. [6] studied the spontaneous

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combustion process of hydrogen sulfide corrosion products by monitoring the contents of

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sulfur in the products and gas compositions exhausted during the oxidation. Richard et al. [7] explored the thermal stability and decomposition kinetics of ammonium nitrate in the presence of pyrite powder. Cai et al. [8] studied spontaneous combustion tendency of iron sulfide

corrosion

by

X-ray

diffraction

(XRD),

Raman

analysis

and

thermo-gravimetric-differential scanning calorimetric (TG-DSC) analysis. Yang et al. [9] studied the apparent activation energy for spontaneous combustion of three kinds of sulfide concentrates under different heating rates. Zhan et al. [10] analyzed the production and kinetic mechanism by X-ray energy dispersive spectrometry (EDS), scanning electron 3

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microscopy (SEM) and thermo-gravimetric analysis (TGA).

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The heat stability of sulfide powder is different under various extrinsic conditions. Many factors can affect heat stability of mine materials, such as water content, heating rates, sulfide

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components. Hu et al. [11] studied the kinetic parameters of mechanically activated pyrite, as

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well as non-activated pyrite, using Friedman method at different heating rates. They found that the metastable pyrite is more susceptible to spontaneous combustion than that of non-activated one. Mao et al. [12] investigated the spontaneous combustion of coal at different heating rates and they found the production and oxygen consumption increase

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correspondingly with the rise of heating rate. Yang et al. [13] found the iron-rich sulfide

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powder concentrate exhibits a stronger tendency to spontaneous combustion than that of

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sulfur-rich one.

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The spontaneous combustion mechanism of pyrite powder is studied in this paper through synchronous thermal analysis. In addition, the effects of atmosphere conditions, heating rates and particle size are also investigated.

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2 Experimental

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2.1 Sample preparation

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The pyrite ore was firstly grinded by ball mill after mechanical crushing, and sieved by

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standard screen equipment. Then the samples of different sizes were obtained. The samples were tested by X-ray fluorescence analysis and XRD analysis [14]. The chemical compositions were shown in Table 1. It could be seen that the sample is mainly composed of FeS2 (81.94%), PbS (2.13%) and Fe3FeSiO4(OH)5 (11.46%). XRD pattern and Raman

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spectrum were presented in Figure 1. The characteristic peaks of FeS2, PbS and

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Fe3FeSiO4(OH)5 can be observed in the XRD pattern. The three main peaks at 347, 382 and

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434 cm-1 in the Raman spectrum are the characteristic active modes for pyrite.

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2.2 Experimental

STA6000 Simultaneous thermal analyzer was applied in the tests to study the spontaneous combustion mechanism of pyrite. The experimental conditions of pyrites were given in Table 2, and the chemical compositions of sample 2 and 7 were given in Table 3.

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The sample 2 and 7 were studied at room temperature by a RENISHAW Raman microscope. The spectra were recorded with a resolution of 1 cm-1 and the laser beam

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diameter was 1 μm.

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Table 1 Chemical compositions of sample

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Table 2 Experimental conditions of pyrite Table 3 Chemical compositions of samples

Table 1 Table 2 Table 3

Figure 1 (a) XRD pattern and (b) Raman spectrum of pyrite

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3 Results and discussion

Figure 1

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3.1Thermal oxidation process of pyrite in air

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TG, DSC and DTG curves of pyrites are presented in Figure 2. It can be seen that the oxidation spontaneous combustion process of pyrite can be divided into three stages: thermal decomposition, oxidation spontaneous combustion and extinguishment.

In the stage of thermal decomposition, the DSC curve increases slowly, and the TG curve shows a weight loss of 0.113 mg. In this stage, Fe3FeSiO4(OH)5 is decomposed according to Eq. (1), which mainly causes the weight loss.

2Fe3 FeSiO 4 (OH )5  6FeO  2SiO2  Fe2O3  5H 2O 6

(1)

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completely and maybe very little sample is still going.

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value (0.113 mg). As a result, it can be deduced that most of Fe3FeSiO4(OH)5 was reacted

In the stage of oxidation spontaneous combustion, three exothermic peaks appear in the

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DSC curve. Meanwhile, total weight loss of 3.113 mg is observed, during which, two weight loss peaks appear in the DTG curve. In this stage, sulfur dioxide gas is produced by the reaction with oxygen, which results in the weight loss directly. When the samples are reacted

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2FeS 2  5O2  2FeO  4SO2

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incompletely, the reaction is as following [15]:

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(2)

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When the samples are reacted completely, the oxidation reaction is as following [16]:

4FeS 2  11O2  2Fe2O3  8SO2

(3)

The samples contain traces of PbS, and its reaction is given as following [17]:

PbS  2O2  PbSO4

(4)

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ACCEPTED MANUSCRIPT According to the law of mass conservation, if the reaction 2 occurred completely, the weight loss will be Wmax = 3.215 mg; if the reaction 3 occurred completely, the weight loss will be

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Wmin = 2.679 mg. The experimental weight loss value is between Wmax and Wmin, indicating that both reaction 2 and 3 occurred during the oxidation. The fact that two weight loss peaks

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in the DTG curve also confirms this conclusion.

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Furthermore, in the last phase, the DSC, TG and DTG curves tend to be flat.

Figure 2

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Figure 2 TG, DSC and DTG curves in air

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Raman spectrum of pyrite's oxidation and decomposition products is shown in Figure 3. The Raman spectrum highlights two intensive peaks at 228 cm-1 and 265 cm-1, and three

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weaker ones at 370 cm-1, 580 cm-1 and 1285 cm-1. The peaks at 228 cm-1 and 265 cm-1 are

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close to the characteristic peaks of Fe2O3.The peak at 580 cm-1 may be a characteristic one of FeO. The results indicate that the product is mainly composed of Fe2O3 and FeO, which is consistent with the results of chemical analysis above. The peaks at 370 cm-1 and 1285 cm-1 are characteristic peaks of FeOOH and the peak intensity is weak, which indicates a small amount of FeOOH existing in the products. FeOOH may come from the reactions process of Fe2O3 and the absorbed water or FeO oxidation and water absorption.

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ACCEPTED MANUSCRIPT Figure 3 Raman spectrum of the product in air

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3.2 Thermal decomposition process of pyrite in nitrogen

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Figure 3

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From the TG, DSC and DTG curves (Figure 4) in nitrogen atmosphere, it can be seen that a small endothermic peak appears in the DSC curve. Compared with that in air atmosphere, the

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reaction temperature interval is narrowed down, and the peak temperature is higher. Only one peak appears in the TG curve with a weight loss of 2.317 mg. The reaction may be expressed

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as [18]:

(5)

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2FeS 2  2FeS  S 2

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In the reaction 5, the volatile S2 is escaped by carrier gas immediately. According to the law

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of mass conservation, if the reaction 5 occurred completely, the weight loss will be 2.244 mg. The reaction 1 performs without oxygen, which the weight loss is 0.132 mg in a nitrogen flow. As a result, the weight loss will be 2.376 mg if Fe3FeSiO4(OH)5 and FeS2 are reacted completely, which is just slightly greater than the measured value (2.317 mg), and the difference is no more than 2.48%, indicating that both the reaction 1 and 5 happen in nitrogen atmosphere and the reactions are almost complete.

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ACCEPTED MANUSCRIPT The characteristic parameters of the sample in different atmospheres are presented in Table 4. The characteristic temperature of the sample in air is lower than that in nitrogen, mainly

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due to the intense exothermic reaction with lower thermal stability in air, while FeS2

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Figure 4 TG, DSC and DTG curves in nitrogen

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endothermic decomposition reaction results in higher thermal stability in nitrogen.

Table 4 Characterization parameters of samples at different atmosphere

Figure 4

Table 4

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Figure 5 shows the Raman spectra of the pyrite decomposition product in nitrogen. The

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spectrum is composed of two intense peaks at 228 cm-1 and 277 cm-1, and three weak ones at 386 cm-1, 588 cm-1 and 1287 cm-1. Meanwhile, the peaks at 228 cm-1 and 277 cm-1 are the

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characteristic peaks of Fe2O3. The peak at 588 cm-1 may be the characteristic peak of FeO.

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According to the discussion above, the FeS exists in the products. FeS is reflected by the intensive peak at 282 cm-1 and a weaker one at 208 cm-1, which are in good agreement with the results in the literature. The peak value is close to the one of Fe2O3, therefore, the Raman peaks of Fe2O3 and FeS may overlap each other. The peaks at 386 cm-1 and 1287 cm-1 clearly shows the presence of a small amount of FeOOH.

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ACCEPTED MANUSCRIPT XRD was employed to confirm the resultant products. As shown in Figure 6a, the characteristic peaks of Fe2O3 (JCPDS No. 33-0664), FeO (JCPDS No. 89-2468) and FeOOH

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(JCPDS No. 77-0244) are observed. In the XRD pattern of the products obtained in N2, except for Fe2O3 and FeO, the peaks attributed to FeS (JCPDS No. 65-9124) are presented.

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Thus, the main components for the oxidation products are Fe2O3 and FeO. The main products

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of pyrite under N2 thermal decomposition are FeS, Fe2O3 and FeO.

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Figure 6 XRD patterns of the products in (a) air and (b) nitrogen

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Figure 5 Raman spectrum of the product in nitrogen

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3.3 Thermal oxidation process of pyrite at different heating rate

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The TG, DSC and DTG curves (Figure 7) and characteristic parameters (Table 5) of the

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sample at different heating rates show that the overall trend of reaction is consistent. With increasing heating rate, the peak temperatures in the TG and DSC curves turn to be higher, and the maximum weight loss rate is decreased [19]. Meanwhile, the initial temperatures, peak temperatures, termination temperatures and temperatures corresponding to maximum weight loss becomes higher, indicating that higher heating rate will cause higher reaction temperature for pyrite. With increasing heating rate, the heating effects per unit time is increased to higher peak temperature. As a result, the thermal stability of pyrite will be higher. With increasing heating rate, the temperature difference between inside and outside the 11

ACCEPTED MANUSCRIPT sample becomes greater. Due to the lower temperature inside the sample, the reaction doesn’t occur in time. With the temperature difference becomes more obvious, the reaction is delayed

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and then the reaction becomes slow. As a result, the reaction temperature ranges become

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wider. Meanwhile, the extinguishing temperature tends to be higher.

Figure 7 (a) DSC, (b) TG and (c) DTG curves of sample with different heating

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rate

Table 5

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Table 5 Characterization parameters of samples with different heating rate

Figure 7

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3.4 Thermal oxidation process of pyrite at different particle size

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The TG, DSC and DTG curves and characteristic parameters of the samples with different

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sizes are presented in Figure 8 and Table 6, respectively, showing that the whole trend of the reaction keeps the same. With decreasing the size, the initial temperatures, peak temperatures, termination temperatures and temperature corresponding to maximum weight loss become lower. The reaction temperature interval becomes narrowed. Due to smaller particle size, the area of sample exposed to the air is larger [20]. Therefore, the reaction becomes faster and the reaction reaches equilibrium state more easily. Meanwhile, the decomposition degree becomes greater at a given temperature. As a result, the reaction is more likely to occur in low temperature. Furthermore, the reaction becomes faster and the temperature range 12

ACCEPTED MANUSCRIPT becomes narrower. The smaller size leads to the bigger loading density of the sample. Therefore, the capacity of heat conduction becomes greater and the peak temperature

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decreases. The results show that smaller size pyrite tend to reaction at lower temperature and

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worsens the thermal stability.

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Table 6 Characterization parameters of samples with different diameters

Table 6

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Figure 8 (a) DSC, (b) TG and (c) DTG curves of samples with different sizes

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4 Conclusions

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Spontaneous combustion mechanism and heat stability of sulfide minerals powder were

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studied. The conclusion was given as following:

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(1) By discussing the process of pyrite powder oxidation spontaneous combustion, the process could be divided into three stages, including thermal decomposition, oxidation spontaneous combustion and extinguishment. In addition, Fe3FeSiO4(OH)5 was decomposed at low temperature, followed by FeS2 oxidation and decomposition.

(2) Combined thermal analysis with Raman spectra, pyrite's oxidation and decomposition products are mainly FeO and Fe2O3, while in nitrogen atmosphere, the decomposition products are mainly composed of FeS, FeO and Fe2O3. 13

ACCEPTED MANUSCRIPT (3) Thermal stability of pyrite powder was studied through thermal analysis, and the thermal stability of pyrite in air atmosphere was lower than that in nitrogen atmosphere. The

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atmosphere played an important role on the thermal stability of pyrite. Furthermore, the thermal stability of pyrite was enhanced with increasing the heating rate and decreasing the

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particle size.

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Acknowledgements

This research was financially supported by the Natural Science Foundation of China (Nos.

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51174153 and 51374164) and the National Key Research and Development Program of

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China (No. 2016YFC0802801) and the Hubei Natural Science Foundation (No.

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2014CFB879).

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ACCEPTED MANUSCRIPT References

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[1] A. Karabulut, G. Budak, R. Polat, et al, EDXRF Analysis of Murgul Pyrite Ore Concentrates, Journal of Quantitative Spectroscopy & Radiative Transfer, vol. 72, pp.

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741–746, 2002.

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[2] F.Q. Yang and C. Wu, Mechanism of Mechanical Activation for Spontaneous Combustion of Sulfide Minerals, Transactions of Nonferrous Metals Society of China, vol. 23, pp.

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276-282, 2013.

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[3] W. Huang, Spontaneous Combustion Mechanism of Gangue in Jingang Coal Mine, Fuel

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and Energy Abstracts, vol. 43, pp. 279, 2002.

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[4] A. Amrani and Z. Aizenshtat, Mechanisms of Sulfur Introduction Chemically Controlled: δ34S imprint, Organic Geochemistry, vol. 35, pp. 1319-1336, 2004.

[5] Q.S. Wang, J.H. Sun, X.F. Chen, et al, Effects of Solvents and Salt on the Thermal Stability of Charged LiCoO2, Materials Research Bulletin, vol. 44, pp. 543-548, 2009.

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ACCEPTED MANUSCRIPT [6] Z.H. Zhang, S.L. Zhao, P. Li, et al, Spontaneous Combustion Process of Hydrogen Sulfide Corrosion Products Formed at Room Temperature, Acta Petrolei Sinica, vol. 28, pp.

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122-126, 2012.

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[7] G. Richard, D.K. Zhang, Thermal Stability and Kinetics of Decomposition of Ammonium Nitrate in the Presence of Pyrite, Journal of Hazardous Materials, vol. 165, pp. 751–758,

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2009.

[8] X. Cai, X.E. Zhao, H.Q. Yao, Spontaneous Combustion Tendency of Iron Sulfide

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Corrosion: Oxidation Characterization and Thermostability, Procedia Engineering, vol. 88, pp.

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356-362, 2014.

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[9] F.Q. Yang, C. Wu, Y. Cui, et al, Apparent Activation Energy for Spontaneous Combustion

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of Sulfide Concentrates in Storage Yard, Trans. Nonferrous Met. Soc. China, vol. 21, pp. 395-401, 2011.

[10] Z. Dou, J.C. Jiang, Z.R. Wang, et al, Kinetic Analysis for Spontaneous Combustion of Sulfurized Rust in Oil Tanks, Journal of Loss Prevention in the Process Industries, vol. 32, pp. 387-392, 2014.

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ACCEPTED MANUSCRIPT [11] H.P. Hu, Q.Y. Chen, Z.L. Yin, et al, Study on the Kinetics of Thermal Decomposition

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of Mechanically Activated Pyrites, Thermochimica Acta, vol. 389, pp. 79-83, 2002.

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of Coal Spontaneous Combustion, Procedia Engineering, vol. 62, pp. 1081-1086, 2013.

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[13] F.Q. Yang, C. Wu, Z.J. Li. Investigation of the Propensity of Sulfide Concentrates to Spontaneous Combustion in Storage, Journal of Loss Prevention in the Process Industries, vol.

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24, pp. 131-137, 2011.

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[14] Z.H. Wang, X.H. Xie, S.M. Xiao, et al, Adsorption Behavior of Glucose on Pyrite Surface Investigated by TG, FTIR and XRD Analyses, Hydrometallurgy, vol. 102, pp. 87-90,

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2010.

[15] S.H. Yin, A.X. Wu, J.Z. Liu, et al, An Experimental Study of Pyrite Bio-leaching as a Way to Control Spontaneous Combustion, Mining Science and Technology, vol. 21, pp. 513–517, 2011.

[16] F.Q. Yang, C. Wu, H. Liu, et al, Thermal Analysis Kinetics of Sulfide Ores for Spontaneous Combustion, Journal of Central South University (Science and Technology), vol. 42, pp. 2469-2474, 2011. 17

ACCEPTED MANUSCRIPT [17] S.I. Sadovnikov, N.S. Kozhevnikova, and A.A. Rempel, Oxidation of Nanocrystalline

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Lead Sulfide in Air , Russian Journal of Inorganic Chemistry, vol. 56, pp. 1864–1869, 2011.

[18] G. Richard, S. Freij, D.K. Zhang, et al, A Mechanistic Study into the Reactions of

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Ammonium Nitrate with Pyrite, Chemical Engineering Science, vol. 61, pp. 5781–5790,

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2006.

[19] M.V. Kök, Non-isothermal DSC and TG/DTG Analysis of the Combustion of Slop

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Asphaltites’, Journal of Thermal Analysis and Calorimetry, vol. 88, pp. 663–668, 2007.

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[20] J.P. Sanders, P.K. Gallagher, Kinetic Analyses Using Simultaneous TG/DSC Measurements part II: Decomposition of Calcium Carbonate Having Different Particle Sizes,

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Journal of Thermal Analysis and Calorimetry, vol. 82, pp. 659–664, 2005.

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Figure 1 (a) XRD pattern and (b) Raman spectrum of pyrite

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Figure 2 TG, DSC and DTG curves in air

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Figure 3 Raman spectrum of the product in air

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Figure 4 TG, DSC and DTG curves in nitrogen

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Figure 5 Raman spectrum of the product in nitrogen

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Figure 6 XRD patterns of the products in (a) air and (b) nitrogen

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Figure 7 (a) DSC, (b) TG and (c) DTG curves of sample with different heating rate

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Figure 8 (a) DSC, (b) TG and (c) DTG curves of samples with different sizes

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Mass fraction/% Mg

Al

Si

S

Ca

Ti

Fe

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O

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Table 1 Chemical compositions of sample

Zn

As

Pb

Ir

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6.63 0.13 0.52 0.80 43.99 0.47 0.39 44.16 0.58 0.13 1.84 0.35

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Table 2 Experimental conditions of pyrite Diameter/mesh gas

/mg

rate/(℃·min-1)

1

10.255

5

180~200

2

9.808

10

180~200

3

10.216

15

180~200

4

9.870

10

5

10.081

6 7

Gas

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Heating

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Weight

Temp.

Flow/(ml·min-1)

range/℃

20ml/min

50~800

air

20ml/min

50~800

air

20ml/min

50~800

100~180

air

20ml/min

50~800

10

200~325

air

20ml/min

50~800

10.177

10

325~400

air

20ml/min

50~800

10.269

10

nitrogen 20ml/min

50~800

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air

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180~200

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NO.

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FeS2

PbS

8.037 0.208

7

8.414 0.219

Fe3FeSiO4(OH)5 others 1.123 1.177

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2

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Weight /mg

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Sample NO.

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Table 3 Chemical compositions of samples

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0.459

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Table 4 Characterization parameters of samples at different atmosphere

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Initial

TG/%

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DSC /℃ Peak

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gas

Termination

point

value

point

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Sample

DTG/℃

Weight Maximum

loss

air

439

534

603

33.93

518

7

nitrogen 591

628

638

22.87

522

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2

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Table 5 Characterization parameters of samples with different heating rate

NO.

(℃·min-1)

Initial

Peak

Termination

point

value

point

5

428

529

2

10

439

534

3

15

444

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604

31

Weight Maximum

loss

542

34.99

515

603

33.93

518

659

33.33

529

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Heating rate

DTG/℃

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Sample

TG/%

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DSC /℃

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Table 6 Characterization parameters of samples with different diameters

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Peak

point

value

100-180

441

578

2

180-200

439

534

5

200-325

436

6

325-400

418

Termination point

DTG/℃

Weight Maximum loss

608

31.2

530

603

33.93

518

604

590

31.68

515

530

585

30.6

514

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Diameter/mesh Initial

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Sample

TG/%

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DSC /℃

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

Thermal decomposition of pyrite under air and nitrogen is investigated.

2.

Thermal stability of pyrite in air is lower than that in nitrogen.

3.

Thermal stability of pyrite is influenced by heating rate and particle size.

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1.

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