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Thermal stability and mechanism of decomposition of emulsion explosives in the presence of pyrite
Journal of Hazardous Materials 300 (2015) 702–710
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Thermal stability and mechanism of decomposition of emulsion explosives in the presence of pyrite Zhi-Xiang Xu a , Qian Wang a , Xiao-Qi Fu b,∗ a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University Zhenjiang 212013, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• An exothermic reaction occurs at
•
•
• •
about 200 ◦ C between pyrite and ammonium nitrate (emulsion explosives). The essence of reaction between emulsion explosives and pyrite is reaction between ammonium nitrate and pyrite. The excellent thermal stability of emulsion explosives does not mean it was also showed when pyrite was added. A new overall reaction has been proposed as: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g).
a r t i c l e
i n f o
Article history: Received 3 May 2015 Received in revised form 26 July 2015 Accepted 27 July 2015 Available online 4 August 2015 Keywords: Emulsion explosives Ammonium Nitrate pyrite Thermal decomposition
a b s t r a c t The reaction of emulsion explosives (ammonium nitrate) with pyrite was studied using techniques of TG-DTG-DTA. TG–DSC–MS was also used to analyze samples thermal decomposition process. When a mixture of pyrite and emulsion explosives was heated at a constant heating rate of 10 K/min from room temperature to 350 ◦ C, exothermic reactions occurred at about 200 ◦ C. The essence of reaction between emulsion explosives and pyrite is the reaction between ammonium nitrate and pyrite. Emulsion explosives have excellent thermal stability but it does not mean it showed the same excellent thermal stability when pyrite was added. Package emulsion explosives were more suitable to use in pyrite shale than bulk emulsion explosives. The exothermic reaction was considered to take place between ammonium nitrate and pyrite where NO, NO2 , NH3 , SO2 and N2 O gases were produced. Based on the analysis of the gaseous, a new overall reaction was proposed, which was thermodynamically favorable. The results have significant implication in the understanding of stability of emulsion explosives in reactive mining grounds containing pyrite minerals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Fax: +86 511 88780214. E-mail addresses: [email protected], [email protected] (X.-Q. Fu). http://dx.doi.org/10.1016/j.jhazmat.2015.07.069 0304-3894/© 2015 Elsevier B.V. All rights reserved.
Compared with traditional industrial explosives, emulsion explosives (EE) has become major product of commercial explosives for water- resistant, low cost, lower environmental pollution properties etc. [1,2]. EE is used for blasting operations in the mining industry. It is made by dispersing supersaturated ammonium
Z.-X. Xu et al. / Journal of Hazardous Materials 300 (2015) 702–710
nitrate (AN) droplets in hydrocarbon oil containing a surfactant. They are normally sensitized by adding chemically generated gas bubbles or hollow micro-balloons. There are two types of EE, bulk EE and packaged EE. Bulk EE mixed with gas bubble is directly pumped into blast holes and then detonated. Packaged EE has low water and high oxidizer salt content, which is packaged after sensitizing. When EE are used in mining areas containing sulphides rocks, it can react exothermally with the sulphides, like pyrite (FeS2 ). The reaction at low temperature can cause intense heating, which in turn causes premature detonation and misfire. It has been believed that it would be due to the complex reactions between AN and pyrite or the products from the low temperature oxidation of the shale especially when the shale is exposed to the ambient air and moisture. Therefore, a thorough understanding of the factors contributing to the premature detonation is crucial for operational safety. Numerous researches about the reaction of AN with pyrite have been reported [3–10]. Essentially, the reaction of AN with pyrite is an auto-catalytic process, which decreases AN thermal stability. Many years ago, EE has been used in pyrite reactive shale [4–6]. The structure of the emulsion is an advantage. The water-in-oil structure makes AN encapsulated in oil. And AN is not easily accessible to the outside pyrite directly. Only when all the water is evaporated, the reaction between AN and pyrite could be occurred. This process would generally take many hours to a few days, but it cannot ensure operational safety. If the emulsion completely dries out, then the conditions are analogous to having ANFO in the borehole. So it is necessary to comprehend thermal characteristic of EE with pyrite. Although many researches about EE with buffer agent have been reported [11–13], the mechanism of EE with pyrite decomposition is not well understood. And this is of importance for designing safety blasting. Pyrite was easy to be oxidized at humidity condition, pore water often contain dissolved Fe2+ , Fe3+ ions and sulphuric acid. As we know, acidic catalytic species and Fe3+ decrease the thermal decomposition temperature of AN [14,15]. And also, acidic species would influence on thermal stability of EE was reported in the previous Ref. [16]. However, the process of catalytic species catalyzed EE decomposition is still unknown. So, it needs to understand how pyrite catalyzed EE decomposition. The present study aims to obtain kinetics of EE thermal decomposition in the present of pyrite, and understand thermal decomposition process with pyrite. And another aim is to establish a simplified mechanism of the overall reaction of EE and pyrite shale which is supported with evidence of gaseous. The reaction is investigated using the thermal analysis method, which is an effective method to estimate hazards [17]. This understanding will be very useful for safety blasting practice. Hence, firstly, thermal decomposition properties of samples by non-isothermal experiments were carried out, and also kinetics of thermal decomposition were calculated. Then experiments of isothermal experiments were carried out to investigate thermal decomposition of samples at steady temperature condition. Thirdly, the evolved gases were analyzed to investigate thermal decomposition mechanism. At last a new overall reaction would be proposed according to evolved gas analysis.
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Table 1 The properties of different oils. Item
CP
130
Density(25 ◦ C) (kg/m3 ) Viscosity(25 ◦ C) (mm2 /s) Flash point (◦ C)
1250 4206# >230
824 6.12 140
Note: viscosity of CP was measured by SNB-3 viscometer of Shanghai Nirun Intelligent Technology Co., Ltd. The spindle was selected 2# and speed was 70 RPM at 25 ◦ C.
derivative. The properties of CP and D130 are shown in Table 1. The formula of EE was as follows: (1#) water: AN: urea: PIBSA: oil = 16:75:2:2.5:4.5 (2#) water: AN: urea: PIBSA: oil = 20:71:2:2.5:4.5 water: (3#) AN: urea: PIBSA: (52# 50 wt%+D130#50 wt%) = 20:68:5:2.5:4.5 (4#) water: AN: urea: PIBSA: wax = 16:75:2:2.5:4.5 (5#) AN (6#) 50 wt% AN + 50 wt% pyrite The procedure of bulk EE prepared involves two steps. First, the oxidizer solution was prepared by dissolving the ingredients in a large stainless steel beaker heated to a temperature of approximate 80 ◦ C. The oil phase comprised of oil and surfactant was then poured into the mixer bowl when it was heated to 60 ◦ C and maintained. From this time mixing started with a speed of approx 200 rpm, and the hot oxidizer solution was slowly poured into the bowl. After the pouring, the speed of mixing was accelerated (approximately to 1200 rpm) and continued for a few seconds to achieve the final refinement. The samples (1–3#) are basic formula of bulk EE. And 4# is basic formula of package EE. The 4# sample prepared was different from other samples (1–3#). The difference of procedure is the heating temperatures of water phase and oil phase. The water phase is at about 90 ◦ C, and oil phase is at 90 ◦ C. And other parts were the same. After prepared of emulsion matrix, the pyrite was added into matrix, and the mass ratio was 50%, then the sample with pyrite was obtained. And the samples with pyrite were labeled as 1–1#, 2–1#, 3–1# and 4–1#. The pyrite used in this study was come from Inner Mogolia Sheng An chemical Ltd., Co. It contains 45.27 wt% iron and 34.21 wt% sulphur. These pyrite ore were separated from the original rock using a jaw crusher and then pulverized to below 100 m. 3.1. Thermal experiments Non-isothermal experiments were carried out by TG-DTA- DTG (PerkinElmer, Diamond). The samples were then deposited in an open ceramic crucible heated up from room temperature to 350 ◦ C. The heating rate was 10 K/min. The experiment was performed under a constant argon flow of 40 mL/min to sweep away the offspring of samples thermal decomposition. Isothermal TG (PerkinElmer, Pyris1) experiments were run at 120 ◦ C. The isothermal measurements were started from room temperature and held the sample under the isothermal conditions. The heating rate at initial process was 50 K/min. In all experiments, samples of about 5 mg were heated in open ceramic crucible in a flowing atmosphere of nitrogen flow of 40 mL/min to sweep away the offspring of samples thermal decomposition.
2. Experiments
3.2. TG-DSC-MS experiments
2.1. Materials and samples prepared
The TG–DSC–MS test was performed with TG-DSC (NETZSCH STA449C) system and MS (NETZSCHQMS403C) system. NETZSCHQMS403C conditions: ionizing electron energy of 70 eV, quartz capillary gas connector, pressure injection 1000 mbar. Approximately 5 mg of sample was heated from 50 ◦ C to 350 ◦ C. The heating
All the emulsifiers, wax, salts of inorganic and chlorinated paraffin (CP, 52#) were industrial product. The oil was D130, which was come from Exxon Mobil Corporation. The emulsifier was PIBSA
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Fig. 2. DTA curves of samples with pyrite.
Fig. 1. DTA curves of samples without pyrite.
rate was 10 K/min, and the samples were carried in Al2 O3 crucibles. High-purity argon was used as purge gas with a gas flow rate of 20 ml/min. 3.3. Decomposition kinetic analysis Non-isothermal curves were carried out at heating rates 2.5, 5, 10 and 20 K/min. The kinetics of the EE (AN) thermal decomposition with and without presence of pyrite can be determined using the Kissinger method [18], which uses the temperature (Tm) at which the reaction rate is at a maximum as obtained from the DTA curves of samples decomposition in experiments with different heating rates. The equation is as follows:
ln
ˇ 2 Tm
= ln
AR Ea
−
Ea RTm
(1)
Where In(ˇ/Tm) is the heating rate (K/min). The slope and intercept from the line plot of against 1/Tm are used to obtain the activation energy and pre-exponential factor, respectively [18]. 4. Results and discussion Decomposition of EE with and without pyrite by non- isothermal experiments As we know, increased water content and added urea to formation of EE can increased thermal stability of EE [19,20]. The CP also was selected to increase thermal stability of EE as inhibitor. The results of DTA were showed in Fig. 1. From the results, we can know that the thermal stability of EE increased as the inhibitor (CP) added. And also increased water content can increase thermal stability of EE. Peak temperature of samples shifted to high temperature significantly. For in open system, the 1–3# samples were endothermic. However, 4# was exothermic. It means that reaction of 4# was more violent than other samples at high temperature. And the thermal stability of 4# was also better than other samples. According to peak temperature, we can know that the packaged EE (4#) was more suitable than bulk EE to use in pyrite shale. And the Ref. [6] obtained the same results. Moreover, many papers have reported that thermal decomposition of EE was a key factor, which influenced EE in pyrite shale safe blasting [5]. It is as much more safety in the operation as the better thermal stability of EE. In order
Fig. 3. TG curves of samples with pyrite.
to demonstrate the conclusion, the next experiments were carried out. The experiments were carried out when samples with pyrite as weight ratio 50 %. The results were showed in Fig. 2. And the heating rate was 10 K/min. There were little discrepancy in peak temperature of samples, and the results were very strange. The results almost have no difference in 1–1#, 2–1#, 3–1# and 4–1#. The peak temperature of 4–1# shifted to a higher temperature while the other three samples almost were the same. However, the thermal stability of 1–4# samples has significant discrepancy as shown in Fig. 1. We thought the reason was related to decomposition process of EE. In order to analyze the strange phenomenon, The TG curves were also analyzed. The TG curves of samples were shown in Fig. 3. From TG curves, it was obtained that droplet of emulsion breaking occurred firstly, which caused water evaporation. After dehydration, decomposition of AN and oil took place under further heating. So, the essence of EE thermal decomposition was AN’s thermal decomposition. The reference also found the same result [21] . According to Fig. 3, we also found that thermal stability of 4–1# was better than other sam-
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Table 2 Activation energy and peak temperature of samples. No.
Peak temperature (◦ C) without pyrite (10k/min)
Peak temperature (◦ C) with pyrite (10k/min)
Activation energy (kJ/mol)
pre-exponential factor
r2
1# 2# 3# 4# 5# 6#
255.47 264.98 276.23 278.23 278.92
199.47 197.68 199.71 205.18
99.80
2.32 E10
0.9885
83.60
1.7 E10
0.9575
93.30 81.94
3.02E10 1.95E10
0.9996 0.9782
196.73
Note: Activation energy was the samples with pyrite except 6#.
Fig. 4. DSC curves of samples with pyrite.
ples, though the water content was also 16%. For D130 is easy to evaporate but wax is not at elevated temperature process, EE with D130 as an oil phase was easier to break than with wax as oil phase. So, according to TG curves, the important information implied that emulsion breaking was a very important factor to thermal decomposition of EE in pyrite shale. For emulsion breaking occurred firstly, then interaction between hydrocarbon fuel and oxidant was carried out and it influenced thermal stability of EE. For volatilization of D130, the samples of 1–1#, 2–1# and 3–1# were much easier breaking than 4–1#. So, the thermal stability of 4–1# was the best among the samples. However, if the process of emulsion breaking cannot be prevented, the discrepancy of EE thermal decomposition was little. Hence the interaction between hydrocarbon fuel and oxidant was the fundamental reaction. And many papers thought the added inhibitor to inner phase of EE can prolong thermal stability to use in pyrite shale [5,6,11,12]. And the increased water amount in the formulation helps to enhance the thermal stability of EE [19]. According to Figs. 1–3, however, it was showed that added inhibitor to EE was difficult to increase thermal stability of EE when pyrite was mixed with EE. According to Figs. 2–3, we can know clearly that the intrinsic thermal stability of EE is the thermal stability of AN. The DSC experiment result also showed the same conclusion as shown in Fig. 4. The exothermic process was appeared in DSC experiment for in sealed system unlike in DTA experiment in open system. The DSC results were the same as DTA results. In a word, from the DSC and DTA curves, a large exothermic peak is observed at about 200 ◦ C in the presence of pyrite. This is in a reasonable agreement with reference [3,7,9,10]. Hence, according to above mentioned, emulsion breaking was a very important process to thermal decomposition
Fig. 5. Results of samples isothermal decomposition curves at 120 ◦ C.
of EE. And the process reflected the essence of reaction between EE and pyrite. All the results of thermal decomposition were listed in Table 2. According to Table 2, it was showed that with water content increased, peak temperature of EE was shifted to a higher temperature. However, when pyrite was added, the peak temperature of samples has little difference. For thermal stability of 3# was the best to bulk EE, we also calculated kinetics of 3–1# and 1–1# as a comparison. And the inhibitor (CP) has little function to thermal stability of EE when pyrite was presented. According to above results, the 4# sample was suitable for using in pyrite shale. 4.1. Kinetics of thermal decomposition According to above results and reference, pyrite significantly catalyzed AN and EE. It is well-known that the added catalyst lowers the activation energy of the reaction. Although, activation energy alone is not enough to understand the thermal decomposition process and property, activation energy can provide reasonable information about the critical energy needed to start the decomposition of a compound. In turn the values will help to understand the thermal stability of the samples under investigation. According to the Refs. [22,23], the overall decomposition reaction of pure AN is described by a first-order reaction kinetics, while the kinetic of EE is complex for mixed system. The activation energy values, computed using Kissinger method, are shown in Table 2. In this paper, we have calculated the average activation energy required for the thermal decomposition of pure AN to be 93.90 kJ/mol, while 81.94 kJ/mol was obtained for pyrite catalyzed decomposition. This significant reduction could be identified as catalytic influence of pyrite on the thermal decomposi-
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Fig. 6. The Mass spectra of the gas products of 1#2◦ .
Fig. 7. The Mass spectra of the gas products of 2#2◦ .
tion reaction of AN. Besides a significant reduction in the activation energy values, the plots of activation energy against the catalyzed decomposition, indicating a change in the decomposition reaction pathway. Instead of the first-order reaction kinetics process, the catalyzed decomposition showed the dependence of activation energy on the reaction proceeds and in turn was a multi-step reaction kinetics process. The activation energy value of AN is in reasonable agreement with Refs. [22–24]. After adding pyrite, the activation energy value of AN mixture was decreased significantly. However, the Ref. [3] found AN’s activation energy discrepancy with and without pyrite was little. We also calculated the activation energy of EE with pyrite required for the thermal decomposition. For thermal stability of 3# is better than 1#, we calculated the activation energy of 3–1# and 1-1# samples. The results were also showed in Table 2. For the mixture system of EE, activation energy of EE has a large difference according to Refs. [25,26]. So, it is difficult to estimate thermal stability for the influence of the formation on thermal stability. Moreover, the thermal stability of 1–1# and 3–1# is also difficult to estimate. However, according to peak temperature we can obtain that pyrite catalyzed decomposition of EE. 4.2. Decomposition of EE with and without pyrite by isothermal experiments The Ref. [8] showed that the induction stage of AN with pyrite is only about 35 min at about 40 ◦ C. And Ref. [27] found the induction stage of AN with pyrite is only about 160 min at about 40 ◦ C. According to above thermal analysis results, the isothermal experiments were carried out at 120 ◦ C about 200 min to investigate the thermal stability of samples. The results of isothermal experiment are showed in Fig. 5. Firstly, it was obtained that the water was evaporated (EE sam-
Fig. 8. The Mass spectra of the gas products of 3#2◦ .
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ples). After dehydration, decomposition of AN and oil took place under further heating. Compared to 4–1#, 1–1# and 3–1# samples were easy to break and thermal decomposition. In other words, package EE was more suitable to use than bulk EE in pyrite shale again. And also the result implied that although little discrepancy at initial stage, the thermal stability of 3–1# was better than that of 1–1# after about 140 min. According to Fig. 5, we can know that the curves of samples almost become a straight line, which implies the reaction is proton reaction of AN at this condition. The extent of conversion of 6# was larger than that of 4–1#. It means that the decomposition rate of 6# was faster than 4–1#. However, according to non-isothermal experiment, the discrepancy was a little. Isothermal experiment, hence is an important complementary to illustrate thermal stability. In a word, according to Fig. 5, we can know that package EE (4–1#) was more suitable usage in pyrite shale than bulk EE and ANFO explosives. 4.3. Evolved gas analysis In order to analyze the decomposition mechanism of EE with pyrite, samples were chosen to perform the evolved gas analysis. And also AN with and without samples were also analyzed. From TG curve, it was obtained that droplet of emulsion breaking occurred firstly, which caused water evaporation. After dehydration, decomposition of AN and oil took place under further heating. So, the essential of EE thermal decomposition was AN’s thermal decomposition. Real-time mass spectral analysis of the evolved gases was also performed. Figs. 6–9, , show the ion current curves for mass-to-charge (m/z) ratios = 15–18, 30–32, 44, 46, 63 and 64. The peaks were assigned to • CH3 (m/z = 15,31), NH3 (m/z = 16, 17), H2 O (m/z = 17, 18), NO (m/z = 30), N2 O (m/z = 44), HNO3 (m/z = 30, 46, 63), SO2 (m/z = 64), NO2 (m/z = 46) CO2 (m/z = 28, 44) and • CH2 -OH (m/z = 31). To EE, the species (m/z = 18) was appeared at very low temperature (about 100 ◦ C), and it was ascribed to water. This indicated that water was evaporated from EE and EE was easy to break. For low molecular weight material of D130, it could also be volatilized and it might be pyrolysised at a relative low temperature. The curve (m/z = 15) change demonstrated the reactive intermediates from D130 was presented in Figs. 6–8. For dissociation reaction of AN presented even low temperature [28], the species (m/z = 17)
Fig. 9. The Mass spectra of the gas products of 6#.
at low temperature appeared, and it was ascribed to NH3 . From Figs. 6–9 the curve change showed the NH3 presented. Then ammonia can further pyrolysis but need high temperature. And the HNO3 (g) (m/z = 63) was also detected, it can demonstrate dissociation reaction of AN presented at very low temperature. The Ref. [29] also detected species (m/z = 15, 16, • NH, • NH2 ) at low temperature (100 ◦ C). However, the reference thought it was presented at very high temperature [30,31]. The reference found that the AN based
Fig. 10. Ion current curve of NO(m/z = 30).
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Fig. 11. Ion current curve of N2 O (m/z = 44).
Fig. 12. Ion current curve of species (m/z = 32).
explosives decomposition is the pyrolysis of oil to form reactive intermediates, and then they are oxidized by decomposition productions of AN [32]. So the main component of the species (m/z = 15, • CH ) maybe was • CH for D130 pyrolysis. The Ref. [32] thought 3 3 • CH was oxidized to • CH -OH at first step. Hence, the ion cur3 2 rent of species (m/z = 31) can affirm the hypothesis. And the species (m/z = 31) was further oxidized to carbon dioxide (m/z = 44) by AN. In reaction process, NO2 (m/z = 46) also was detected. When pyrite was added, the important offspring was SO2 (m/z = 64). It also has been detected in Figs. 6–9. The species (m/z = 44) maybe was CO2 or N2 O. To 6#, species (m/z = 44) is only N2 O. To EE with pyrite, species (m/z = 44) is a mixture of N2 O and CO2 . Based on above analysis of the results from the present study, it is proposed that the overall exothermic reaction at about 200 ◦ C is a reaction between melted AN,surfactant, oil phase and pyrite. The essence of the reaction between EE and pyrite is a reaction between
AN and pyrite. The ion current change of species suggested that the reaction between AN and pyrite is rapid. 4.4. A new overall reaction equation between AN (EE) and pyrite Many papers researched reaction between AN and pyrite, and the mechanisms were proposed [7,9,10]. None of the sensible reactions proposed by former researchers can satisfactorily explain the generation of NO, N2 O, NH3 and SO2 gases at the specific peaks. Therefore, a new overall reaction at the first exothermic peak is proposed as follows: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g)
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According to TG curves, we can know that the essence of EE’s thermal decomposition is thermal decomposition reaction between AN and additives. Hence when pyrite was added to EE, thermal decomposition was the reaction among AN, pyrite, oil and surfactant. Certainly, thermal decomposition of AN was the fundamental. The thermal decomposition of AN has been quite extensively studied for many years. Obviously, no single mechanism can explain all of the aspects of its decomposition characteristics. It is generally accepted that thermal decomposition is initiated by an endothermic proton transfer reaction as shown in Reaction (1). The proton transfer reaction occurs at low temperature and has relatively low speed. And NH3 and HNO3 were all detected as shown in Reaction (1). NH4 NO3 ↔ NH3 + HNO3
(1)
R-CH3 → CH3 • + R •
(2)
CH3 • + HNO3 → – CH2 –OH → – HC = O → CO → CO2
(3)
The reference found that the AN based explosives decomposition is the pyrolysis of oil to form reactive intermediates, and then they are oxidized by decomposition productions of AN [32]. The pyrolysis production was formed as shown in Reaction (2) and oxidized by HNO3 to form CO2 in Reaction (3). The Refs. [33–35] also found HNO3 can directly oxidize active materials. The mass spectra results have demonstrated species (m/z = 31,44) presented. As we know, Fe2+ can be easily oxidized by HNO3 at very low temperature. From Figs. 10 and 11 and , it was found that NO and N2 O were all detected at very low temperature. Importantly, the decomposed temperature for forming N2 O is lower than that of NO. It’s implied that N2 O was first formed at very low temperature (∼140 ◦ C). However, the Ref. [8] thought at low temperature NO was formed firstly. To EE with pyrite, the species was mixture of CO2 and N2 O, but 6# was only N2 O. And it also implied that reaction between EE with pyrite was very violent for onset temperature was about 120 ◦ C. The Ref. [7] thought NO was formed by NO3 − and Fe2 + . And it did not consider N2 O presented in overall reaction. And N2 O was not detected at high temperature at their experiment condition. However, according to Fig. 11, it was showed that N2 O was presented obviously. The reference thought that NH3 was first oxidized to form N2 O [36]. The Ref. [37] also found N2 O presented at AN thermal decomposition process. Hence, we thought that N2 O presented in reaction between EE (AN) with pyrite. And also Ref. [38] thought sulphur released from the reaction between HNO3 and FeS2 . In other words, they thought gas sulphur presented in reaction between AN and pyrite. At low temperature, the species (m/z = 32) which listed in Fig. 12, maybe was oxygen. And as temperature elevated, the concentration of oxygen was decreased. Hence the ion current of species also decreased. The change of curves at about 210 ◦ C maybe was caused by gas sulphur appeared. However, according to Figs. 6–9, SO2 (m/z = 64) appeared only at about 150 ◦ C. Hence, we thought that NO, N2 O and SO2 may be formed by the reduction of HNO3 from AN as follows: FeS2 + H+ + NO3 − → Fe3+ + SO2 ↑ + NO ↑ + N2 O ↑ + H2 O
(4)
Another important question was about ferric ion or ferrous ion. As we know, conversion of ferrous to ferric iron is very easy. However, an important factor was that at present study the oxygen balance was negative, like 6# is −0.5333 g/g. So it was not provided additional oxygen to oxidize ferrous ion. Hence, we thought ferric ion and ferrous ion were all presented in the present study. And also Ref. [7] found Fe2 O3 presented in sample. For NO2 presented as reactive intermediate, NO2 was not steady in present condition. Hence, NO2 was detected but it was not listed in reaction equation, also like HNO3 and CH2 OH. Furthermore, the CO2 presented in the gases when EE thermal decomposition with FeS2 .
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The last important question was about the ration of gas produced by reaction. The Ref. [7] found molar ration of NO to SO2 is approximately 2 at exothermic peak. And we also found the same result. In present paper, the molar ration of N2 O to SO2 is approximately one. Hence, according to above analysis and mass spectra results, a new overall reaction at the first exothermic peak is proposed as follows: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g)
(5)
8. Conclusion The paper describes the investigations of pyrite influenced the thermal stability of emulsion explosives (EE) and ammonium nitrate (AN). The thermal decomposition of AN with EE was carried out with and without pyrite. The presence of pyrite reduces the decomposition temperature and accelerates the rate of decomposition of AN and EE. The activation energy and the pre-exponential factor were also calculated. An exothermic reaction occurs at about 200 ◦ C between pyrite and AN (emulsion explosives). The essence of reaction between EE and pyrite is reaction between AN and pyrite. The excellent thermal stability of EE does not mean it was also showed same result when pyrite was added. Package EE were more suitable in usage in pyrite shale than bulk EE. A new overall reaction has been proposed about AN with pyrite shale according to mass spectra analysis. 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g).
Acknowledgement This paper was supported by Inner Mogolia Sheng An chemical Ltd., Co. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.07. 069 References [1] E.G. Mahadevan, Ammonium nitrate explosives for civil applications-slurries, emulsions and ammonium nitrate fuel oils, Wiley-VCH, Weinheim, 2013. [2] Xuguang Wang, Emulsion explosives, Metallurgical Industry Press, Beijing, 2008. [3] R. Gunawan, D. Zhang, Thermal stability and kinetics of decomposition of ammonium nitrate in the presence of pyrite, J. Hazard. Mater. 165 (2009) 751–758. [4] P. Bellairs, The development of an inhibited explosives for black pyrite reactive shale, in: Proceedings of the 22th annual symposium on explosives and blasting technique, Orlando, Orlando, Florida, USA, 1996, pp. 361–375. [5] Tetsuya Sawada, Koichi Kurokawa, Fumihiko Sumiya, et al., Development of heat resistant emulsion explosives, Proceedings of the 8th annual symposium on explosives and blasting technique, Orlando, Florida, USA (1992) 73–80. [6] R.P. Proulx, D.S. Scovira, Drilling and blasting in hot and reactive ground conditions at barrick goldstrike’s meikle mine, Proceedings of the 26th annual symposium on explosives and blasting technique, Salt Lake City, Utah (2000) 319–331. [7] R. Gunawan, S. Freij, D. Zhang, et al., A mechanistic study into the reactions of ammonium nitrate with pyrite, Chem. Eng. Sci. 61 (2006) 5781–5790. [8] P. Priyananda, A.M. Djerdjev, J. Gore, et al., Premature detonation of an NH4 NO3 emulsion in reactive ground, J. Hazard. Mater. 283 (2015) 314–320. [9] J.A. Rumball, The interaction of partially weathered sulphides in the Mt. McRae shale formation with ammonium nitrate, in: Ph.D. Thesis, Murdoch University, Perth, Australia, 1991.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Thermal stability and mechanism of decomposition of emulsion explosives in the presence of pyrite Zhi-Xiang Xu a , Qian Wang a , Xiao-Qi Fu b,∗ a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University Zhenjiang 212013, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• An exothermic reaction occurs at
•
•
• •
about 200 ◦ C between pyrite and ammonium nitrate (emulsion explosives). The essence of reaction between emulsion explosives and pyrite is reaction between ammonium nitrate and pyrite. The excellent thermal stability of emulsion explosives does not mean it was also showed when pyrite was added. A new overall reaction has been proposed as: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g).
a r t i c l e
i n f o
Article history: Received 3 May 2015 Received in revised form 26 July 2015 Accepted 27 July 2015 Available online 4 August 2015 Keywords: Emulsion explosives Ammonium Nitrate pyrite Thermal decomposition
a b s t r a c t The reaction of emulsion explosives (ammonium nitrate) with pyrite was studied using techniques of TG-DTG-DTA. TG–DSC–MS was also used to analyze samples thermal decomposition process. When a mixture of pyrite and emulsion explosives was heated at a constant heating rate of 10 K/min from room temperature to 350 ◦ C, exothermic reactions occurred at about 200 ◦ C. The essence of reaction between emulsion explosives and pyrite is the reaction between ammonium nitrate and pyrite. Emulsion explosives have excellent thermal stability but it does not mean it showed the same excellent thermal stability when pyrite was added. Package emulsion explosives were more suitable to use in pyrite shale than bulk emulsion explosives. The exothermic reaction was considered to take place between ammonium nitrate and pyrite where NO, NO2 , NH3 , SO2 and N2 O gases were produced. Based on the analysis of the gaseous, a new overall reaction was proposed, which was thermodynamically favorable. The results have significant implication in the understanding of stability of emulsion explosives in reactive mining grounds containing pyrite minerals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Fax: +86 511 88780214. E-mail addresses: [email protected], [email protected] (X.-Q. Fu). http://dx.doi.org/10.1016/j.jhazmat.2015.07.069 0304-3894/© 2015 Elsevier B.V. All rights reserved.
Compared with traditional industrial explosives, emulsion explosives (EE) has become major product of commercial explosives for water- resistant, low cost, lower environmental pollution properties etc. [1,2]. EE is used for blasting operations in the mining industry. It is made by dispersing supersaturated ammonium
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nitrate (AN) droplets in hydrocarbon oil containing a surfactant. They are normally sensitized by adding chemically generated gas bubbles or hollow micro-balloons. There are two types of EE, bulk EE and packaged EE. Bulk EE mixed with gas bubble is directly pumped into blast holes and then detonated. Packaged EE has low water and high oxidizer salt content, which is packaged after sensitizing. When EE are used in mining areas containing sulphides rocks, it can react exothermally with the sulphides, like pyrite (FeS2 ). The reaction at low temperature can cause intense heating, which in turn causes premature detonation and misfire. It has been believed that it would be due to the complex reactions between AN and pyrite or the products from the low temperature oxidation of the shale especially when the shale is exposed to the ambient air and moisture. Therefore, a thorough understanding of the factors contributing to the premature detonation is crucial for operational safety. Numerous researches about the reaction of AN with pyrite have been reported [3–10]. Essentially, the reaction of AN with pyrite is an auto-catalytic process, which decreases AN thermal stability. Many years ago, EE has been used in pyrite reactive shale [4–6]. The structure of the emulsion is an advantage. The water-in-oil structure makes AN encapsulated in oil. And AN is not easily accessible to the outside pyrite directly. Only when all the water is evaporated, the reaction between AN and pyrite could be occurred. This process would generally take many hours to a few days, but it cannot ensure operational safety. If the emulsion completely dries out, then the conditions are analogous to having ANFO in the borehole. So it is necessary to comprehend thermal characteristic of EE with pyrite. Although many researches about EE with buffer agent have been reported [11–13], the mechanism of EE with pyrite decomposition is not well understood. And this is of importance for designing safety blasting. Pyrite was easy to be oxidized at humidity condition, pore water often contain dissolved Fe2+ , Fe3+ ions and sulphuric acid. As we know, acidic catalytic species and Fe3+ decrease the thermal decomposition temperature of AN [14,15]. And also, acidic species would influence on thermal stability of EE was reported in the previous Ref. [16]. However, the process of catalytic species catalyzed EE decomposition is still unknown. So, it needs to understand how pyrite catalyzed EE decomposition. The present study aims to obtain kinetics of EE thermal decomposition in the present of pyrite, and understand thermal decomposition process with pyrite. And another aim is to establish a simplified mechanism of the overall reaction of EE and pyrite shale which is supported with evidence of gaseous. The reaction is investigated using the thermal analysis method, which is an effective method to estimate hazards [17]. This understanding will be very useful for safety blasting practice. Hence, firstly, thermal decomposition properties of samples by non-isothermal experiments were carried out, and also kinetics of thermal decomposition were calculated. Then experiments of isothermal experiments were carried out to investigate thermal decomposition of samples at steady temperature condition. Thirdly, the evolved gases were analyzed to investigate thermal decomposition mechanism. At last a new overall reaction would be proposed according to evolved gas analysis.
703
Table 1 The properties of different oils. Item
CP
130
Density(25 ◦ C) (kg/m3 ) Viscosity(25 ◦ C) (mm2 /s) Flash point (◦ C)
1250 4206# >230
824 6.12 140
Note: viscosity of CP was measured by SNB-3 viscometer of Shanghai Nirun Intelligent Technology Co., Ltd. The spindle was selected 2# and speed was 70 RPM at 25 ◦ C.
derivative. The properties of CP and D130 are shown in Table 1. The formula of EE was as follows: (1#) water: AN: urea: PIBSA: oil = 16:75:2:2.5:4.5 (2#) water: AN: urea: PIBSA: oil = 20:71:2:2.5:4.5 water: (3#) AN: urea: PIBSA: (52# 50 wt%+D130#50 wt%) = 20:68:5:2.5:4.5 (4#) water: AN: urea: PIBSA: wax = 16:75:2:2.5:4.5 (5#) AN (6#) 50 wt% AN + 50 wt% pyrite The procedure of bulk EE prepared involves two steps. First, the oxidizer solution was prepared by dissolving the ingredients in a large stainless steel beaker heated to a temperature of approximate 80 ◦ C. The oil phase comprised of oil and surfactant was then poured into the mixer bowl when it was heated to 60 ◦ C and maintained. From this time mixing started with a speed of approx 200 rpm, and the hot oxidizer solution was slowly poured into the bowl. After the pouring, the speed of mixing was accelerated (approximately to 1200 rpm) and continued for a few seconds to achieve the final refinement. The samples (1–3#) are basic formula of bulk EE. And 4# is basic formula of package EE. The 4# sample prepared was different from other samples (1–3#). The difference of procedure is the heating temperatures of water phase and oil phase. The water phase is at about 90 ◦ C, and oil phase is at 90 ◦ C. And other parts were the same. After prepared of emulsion matrix, the pyrite was added into matrix, and the mass ratio was 50%, then the sample with pyrite was obtained. And the samples with pyrite were labeled as 1–1#, 2–1#, 3–1# and 4–1#. The pyrite used in this study was come from Inner Mogolia Sheng An chemical Ltd., Co. It contains 45.27 wt% iron and 34.21 wt% sulphur. These pyrite ore were separated from the original rock using a jaw crusher and then pulverized to below 100 m. 3.1. Thermal experiments Non-isothermal experiments were carried out by TG-DTA- DTG (PerkinElmer, Diamond). The samples were then deposited in an open ceramic crucible heated up from room temperature to 350 ◦ C. The heating rate was 10 K/min. The experiment was performed under a constant argon flow of 40 mL/min to sweep away the offspring of samples thermal decomposition. Isothermal TG (PerkinElmer, Pyris1) experiments were run at 120 ◦ C. The isothermal measurements were started from room temperature and held the sample under the isothermal conditions. The heating rate at initial process was 50 K/min. In all experiments, samples of about 5 mg were heated in open ceramic crucible in a flowing atmosphere of nitrogen flow of 40 mL/min to sweep away the offspring of samples thermal decomposition.
2. Experiments
3.2. TG-DSC-MS experiments
2.1. Materials and samples prepared
The TG–DSC–MS test was performed with TG-DSC (NETZSCH STA449C) system and MS (NETZSCHQMS403C) system. NETZSCHQMS403C conditions: ionizing electron energy of 70 eV, quartz capillary gas connector, pressure injection 1000 mbar. Approximately 5 mg of sample was heated from 50 ◦ C to 350 ◦ C. The heating
All the emulsifiers, wax, salts of inorganic and chlorinated paraffin (CP, 52#) were industrial product. The oil was D130, which was come from Exxon Mobil Corporation. The emulsifier was PIBSA
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Fig. 2. DTA curves of samples with pyrite.
Fig. 1. DTA curves of samples without pyrite.
rate was 10 K/min, and the samples were carried in Al2 O3 crucibles. High-purity argon was used as purge gas with a gas flow rate of 20 ml/min. 3.3. Decomposition kinetic analysis Non-isothermal curves were carried out at heating rates 2.5, 5, 10 and 20 K/min. The kinetics of the EE (AN) thermal decomposition with and without presence of pyrite can be determined using the Kissinger method [18], which uses the temperature (Tm) at which the reaction rate is at a maximum as obtained from the DTA curves of samples decomposition in experiments with different heating rates. The equation is as follows:
ln
ˇ 2 Tm
= ln
AR Ea
−
Ea RTm
(1)
Where In(ˇ/Tm) is the heating rate (K/min). The slope and intercept from the line plot of against 1/Tm are used to obtain the activation energy and pre-exponential factor, respectively [18]. 4. Results and discussion Decomposition of EE with and without pyrite by non- isothermal experiments As we know, increased water content and added urea to formation of EE can increased thermal stability of EE [19,20]. The CP also was selected to increase thermal stability of EE as inhibitor. The results of DTA were showed in Fig. 1. From the results, we can know that the thermal stability of EE increased as the inhibitor (CP) added. And also increased water content can increase thermal stability of EE. Peak temperature of samples shifted to high temperature significantly. For in open system, the 1–3# samples were endothermic. However, 4# was exothermic. It means that reaction of 4# was more violent than other samples at high temperature. And the thermal stability of 4# was also better than other samples. According to peak temperature, we can know that the packaged EE (4#) was more suitable than bulk EE to use in pyrite shale. And the Ref. [6] obtained the same results. Moreover, many papers have reported that thermal decomposition of EE was a key factor, which influenced EE in pyrite shale safe blasting [5]. It is as much more safety in the operation as the better thermal stability of EE. In order
Fig. 3. TG curves of samples with pyrite.
to demonstrate the conclusion, the next experiments were carried out. The experiments were carried out when samples with pyrite as weight ratio 50 %. The results were showed in Fig. 2. And the heating rate was 10 K/min. There were little discrepancy in peak temperature of samples, and the results were very strange. The results almost have no difference in 1–1#, 2–1#, 3–1# and 4–1#. The peak temperature of 4–1# shifted to a higher temperature while the other three samples almost were the same. However, the thermal stability of 1–4# samples has significant discrepancy as shown in Fig. 1. We thought the reason was related to decomposition process of EE. In order to analyze the strange phenomenon, The TG curves were also analyzed. The TG curves of samples were shown in Fig. 3. From TG curves, it was obtained that droplet of emulsion breaking occurred firstly, which caused water evaporation. After dehydration, decomposition of AN and oil took place under further heating. So, the essence of EE thermal decomposition was AN’s thermal decomposition. The reference also found the same result [21] . According to Fig. 3, we also found that thermal stability of 4–1# was better than other sam-
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705
Table 2 Activation energy and peak temperature of samples. No.
Peak temperature (◦ C) without pyrite (10k/min)
Peak temperature (◦ C) with pyrite (10k/min)
Activation energy (kJ/mol)
pre-exponential factor
r2
1# 2# 3# 4# 5# 6#
255.47 264.98 276.23 278.23 278.92
199.47 197.68 199.71 205.18
99.80
2.32 E10
0.9885
83.60
1.7 E10
0.9575
93.30 81.94
3.02E10 1.95E10
0.9996 0.9782
196.73
Note: Activation energy was the samples with pyrite except 6#.
Fig. 4. DSC curves of samples with pyrite.
ples, though the water content was also 16%. For D130 is easy to evaporate but wax is not at elevated temperature process, EE with D130 as an oil phase was easier to break than with wax as oil phase. So, according to TG curves, the important information implied that emulsion breaking was a very important factor to thermal decomposition of EE in pyrite shale. For emulsion breaking occurred firstly, then interaction between hydrocarbon fuel and oxidant was carried out and it influenced thermal stability of EE. For volatilization of D130, the samples of 1–1#, 2–1# and 3–1# were much easier breaking than 4–1#. So, the thermal stability of 4–1# was the best among the samples. However, if the process of emulsion breaking cannot be prevented, the discrepancy of EE thermal decomposition was little. Hence the interaction between hydrocarbon fuel and oxidant was the fundamental reaction. And many papers thought the added inhibitor to inner phase of EE can prolong thermal stability to use in pyrite shale [5,6,11,12]. And the increased water amount in the formulation helps to enhance the thermal stability of EE [19]. According to Figs. 1–3, however, it was showed that added inhibitor to EE was difficult to increase thermal stability of EE when pyrite was mixed with EE. According to Figs. 2–3, we can know clearly that the intrinsic thermal stability of EE is the thermal stability of AN. The DSC experiment result also showed the same conclusion as shown in Fig. 4. The exothermic process was appeared in DSC experiment for in sealed system unlike in DTA experiment in open system. The DSC results were the same as DTA results. In a word, from the DSC and DTA curves, a large exothermic peak is observed at about 200 ◦ C in the presence of pyrite. This is in a reasonable agreement with reference [3,7,9,10]. Hence, according to above mentioned, emulsion breaking was a very important process to thermal decomposition
Fig. 5. Results of samples isothermal decomposition curves at 120 ◦ C.
of EE. And the process reflected the essence of reaction between EE and pyrite. All the results of thermal decomposition were listed in Table 2. According to Table 2, it was showed that with water content increased, peak temperature of EE was shifted to a higher temperature. However, when pyrite was added, the peak temperature of samples has little difference. For thermal stability of 3# was the best to bulk EE, we also calculated kinetics of 3–1# and 1–1# as a comparison. And the inhibitor (CP) has little function to thermal stability of EE when pyrite was presented. According to above results, the 4# sample was suitable for using in pyrite shale. 4.1. Kinetics of thermal decomposition According to above results and reference, pyrite significantly catalyzed AN and EE. It is well-known that the added catalyst lowers the activation energy of the reaction. Although, activation energy alone is not enough to understand the thermal decomposition process and property, activation energy can provide reasonable information about the critical energy needed to start the decomposition of a compound. In turn the values will help to understand the thermal stability of the samples under investigation. According to the Refs. [22,23], the overall decomposition reaction of pure AN is described by a first-order reaction kinetics, while the kinetic of EE is complex for mixed system. The activation energy values, computed using Kissinger method, are shown in Table 2. In this paper, we have calculated the average activation energy required for the thermal decomposition of pure AN to be 93.90 kJ/mol, while 81.94 kJ/mol was obtained for pyrite catalyzed decomposition. This significant reduction could be identified as catalytic influence of pyrite on the thermal decomposi-
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Fig. 6. The Mass spectra of the gas products of 1#2◦ .
Fig. 7. The Mass spectra of the gas products of 2#2◦ .
tion reaction of AN. Besides a significant reduction in the activation energy values, the plots of activation energy against the catalyzed decomposition, indicating a change in the decomposition reaction pathway. Instead of the first-order reaction kinetics process, the catalyzed decomposition showed the dependence of activation energy on the reaction proceeds and in turn was a multi-step reaction kinetics process. The activation energy value of AN is in reasonable agreement with Refs. [22–24]. After adding pyrite, the activation energy value of AN mixture was decreased significantly. However, the Ref. [3] found AN’s activation energy discrepancy with and without pyrite was little. We also calculated the activation energy of EE with pyrite required for the thermal decomposition. For thermal stability of 3# is better than 1#, we calculated the activation energy of 3–1# and 1-1# samples. The results were also showed in Table 2. For the mixture system of EE, activation energy of EE has a large difference according to Refs. [25,26]. So, it is difficult to estimate thermal stability for the influence of the formation on thermal stability. Moreover, the thermal stability of 1–1# and 3–1# is also difficult to estimate. However, according to peak temperature we can obtain that pyrite catalyzed decomposition of EE. 4.2. Decomposition of EE with and without pyrite by isothermal experiments The Ref. [8] showed that the induction stage of AN with pyrite is only about 35 min at about 40 ◦ C. And Ref. [27] found the induction stage of AN with pyrite is only about 160 min at about 40 ◦ C. According to above thermal analysis results, the isothermal experiments were carried out at 120 ◦ C about 200 min to investigate the thermal stability of samples. The results of isothermal experiment are showed in Fig. 5. Firstly, it was obtained that the water was evaporated (EE sam-
Fig. 8. The Mass spectra of the gas products of 3#2◦ .
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707
ples). After dehydration, decomposition of AN and oil took place under further heating. Compared to 4–1#, 1–1# and 3–1# samples were easy to break and thermal decomposition. In other words, package EE was more suitable to use than bulk EE in pyrite shale again. And also the result implied that although little discrepancy at initial stage, the thermal stability of 3–1# was better than that of 1–1# after about 140 min. According to Fig. 5, we can know that the curves of samples almost become a straight line, which implies the reaction is proton reaction of AN at this condition. The extent of conversion of 6# was larger than that of 4–1#. It means that the decomposition rate of 6# was faster than 4–1#. However, according to non-isothermal experiment, the discrepancy was a little. Isothermal experiment, hence is an important complementary to illustrate thermal stability. In a word, according to Fig. 5, we can know that package EE (4–1#) was more suitable usage in pyrite shale than bulk EE and ANFO explosives. 4.3. Evolved gas analysis In order to analyze the decomposition mechanism of EE with pyrite, samples were chosen to perform the evolved gas analysis. And also AN with and without samples were also analyzed. From TG curve, it was obtained that droplet of emulsion breaking occurred firstly, which caused water evaporation. After dehydration, decomposition of AN and oil took place under further heating. So, the essential of EE thermal decomposition was AN’s thermal decomposition. Real-time mass spectral analysis of the evolved gases was also performed. Figs. 6–9, , show the ion current curves for mass-to-charge (m/z) ratios = 15–18, 30–32, 44, 46, 63 and 64. The peaks were assigned to • CH3 (m/z = 15,31), NH3 (m/z = 16, 17), H2 O (m/z = 17, 18), NO (m/z = 30), N2 O (m/z = 44), HNO3 (m/z = 30, 46, 63), SO2 (m/z = 64), NO2 (m/z = 46) CO2 (m/z = 28, 44) and • CH2 -OH (m/z = 31). To EE, the species (m/z = 18) was appeared at very low temperature (about 100 ◦ C), and it was ascribed to water. This indicated that water was evaporated from EE and EE was easy to break. For low molecular weight material of D130, it could also be volatilized and it might be pyrolysised at a relative low temperature. The curve (m/z = 15) change demonstrated the reactive intermediates from D130 was presented in Figs. 6–8. For dissociation reaction of AN presented even low temperature [28], the species (m/z = 17)
Fig. 9. The Mass spectra of the gas products of 6#.
at low temperature appeared, and it was ascribed to NH3 . From Figs. 6–9 the curve change showed the NH3 presented. Then ammonia can further pyrolysis but need high temperature. And the HNO3 (g) (m/z = 63) was also detected, it can demonstrate dissociation reaction of AN presented at very low temperature. The Ref. [29] also detected species (m/z = 15, 16, • NH, • NH2 ) at low temperature (100 ◦ C). However, the reference thought it was presented at very high temperature [30,31]. The reference found that the AN based
Fig. 10. Ion current curve of NO(m/z = 30).
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Fig. 11. Ion current curve of N2 O (m/z = 44).
Fig. 12. Ion current curve of species (m/z = 32).
explosives decomposition is the pyrolysis of oil to form reactive intermediates, and then they are oxidized by decomposition productions of AN [32]. So the main component of the species (m/z = 15, • CH ) maybe was • CH for D130 pyrolysis. The Ref. [32] thought 3 3 • CH was oxidized to • CH -OH at first step. Hence, the ion cur3 2 rent of species (m/z = 31) can affirm the hypothesis. And the species (m/z = 31) was further oxidized to carbon dioxide (m/z = 44) by AN. In reaction process, NO2 (m/z = 46) also was detected. When pyrite was added, the important offspring was SO2 (m/z = 64). It also has been detected in Figs. 6–9. The species (m/z = 44) maybe was CO2 or N2 O. To 6#, species (m/z = 44) is only N2 O. To EE with pyrite, species (m/z = 44) is a mixture of N2 O and CO2 . Based on above analysis of the results from the present study, it is proposed that the overall exothermic reaction at about 200 ◦ C is a reaction between melted AN,surfactant, oil phase and pyrite. The essence of the reaction between EE and pyrite is a reaction between
AN and pyrite. The ion current change of species suggested that the reaction between AN and pyrite is rapid. 4.4. A new overall reaction equation between AN (EE) and pyrite Many papers researched reaction between AN and pyrite, and the mechanisms were proposed [7,9,10]. None of the sensible reactions proposed by former researchers can satisfactorily explain the generation of NO, N2 O, NH3 and SO2 gases at the specific peaks. Therefore, a new overall reaction at the first exothermic peak is proposed as follows: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g)
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According to TG curves, we can know that the essence of EE’s thermal decomposition is thermal decomposition reaction between AN and additives. Hence when pyrite was added to EE, thermal decomposition was the reaction among AN, pyrite, oil and surfactant. Certainly, thermal decomposition of AN was the fundamental. The thermal decomposition of AN has been quite extensively studied for many years. Obviously, no single mechanism can explain all of the aspects of its decomposition characteristics. It is generally accepted that thermal decomposition is initiated by an endothermic proton transfer reaction as shown in Reaction (1). The proton transfer reaction occurs at low temperature and has relatively low speed. And NH3 and HNO3 were all detected as shown in Reaction (1). NH4 NO3 ↔ NH3 + HNO3
(1)
R-CH3 → CH3 • + R •
(2)
CH3 • + HNO3 → – CH2 –OH → – HC = O → CO → CO2
(3)
The reference found that the AN based explosives decomposition is the pyrolysis of oil to form reactive intermediates, and then they are oxidized by decomposition productions of AN [32]. The pyrolysis production was formed as shown in Reaction (2) and oxidized by HNO3 to form CO2 in Reaction (3). The Refs. [33–35] also found HNO3 can directly oxidize active materials. The mass spectra results have demonstrated species (m/z = 31,44) presented. As we know, Fe2+ can be easily oxidized by HNO3 at very low temperature. From Figs. 10 and 11 and , it was found that NO and N2 O were all detected at very low temperature. Importantly, the decomposed temperature for forming N2 O is lower than that of NO. It’s implied that N2 O was first formed at very low temperature (∼140 ◦ C). However, the Ref. [8] thought at low temperature NO was formed firstly. To EE with pyrite, the species was mixture of CO2 and N2 O, but 6# was only N2 O. And it also implied that reaction between EE with pyrite was very violent for onset temperature was about 120 ◦ C. The Ref. [7] thought NO was formed by NO3 − and Fe2 + . And it did not consider N2 O presented in overall reaction. And N2 O was not detected at high temperature at their experiment condition. However, according to Fig. 11, it was showed that N2 O was presented obviously. The reference thought that NH3 was first oxidized to form N2 O [36]. The Ref. [37] also found N2 O presented at AN thermal decomposition process. Hence, we thought that N2 O presented in reaction between EE (AN) with pyrite. And also Ref. [38] thought sulphur released from the reaction between HNO3 and FeS2 . In other words, they thought gas sulphur presented in reaction between AN and pyrite. At low temperature, the species (m/z = 32) which listed in Fig. 12, maybe was oxygen. And as temperature elevated, the concentration of oxygen was decreased. Hence the ion current of species also decreased. The change of curves at about 210 ◦ C maybe was caused by gas sulphur appeared. However, according to Figs. 6–9, SO2 (m/z = 64) appeared only at about 150 ◦ C. Hence, we thought that NO, N2 O and SO2 may be formed by the reduction of HNO3 from AN as follows: FeS2 + H+ + NO3 − → Fe3+ + SO2 ↑ + NO ↑ + N2 O ↑ + H2 O
(4)
Another important question was about ferric ion or ferrous ion. As we know, conversion of ferrous to ferric iron is very easy. However, an important factor was that at present study the oxygen balance was negative, like 6# is −0.5333 g/g. So it was not provided additional oxygen to oxidize ferrous ion. Hence, we thought ferric ion and ferrous ion were all presented in the present study. And also Ref. [7] found Fe2 O3 presented in sample. For NO2 presented as reactive intermediate, NO2 was not steady in present condition. Hence, NO2 was detected but it was not listed in reaction equation, also like HNO3 and CH2 OH. Furthermore, the CO2 presented in the gases when EE thermal decomposition with FeS2 .
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The last important question was about the ration of gas produced by reaction. The Ref. [7] found molar ration of NO to SO2 is approximately 2 at exothermic peak. And we also found the same result. In present paper, the molar ration of N2 O to SO2 is approximately one. Hence, according to above analysis and mass spectra results, a new overall reaction at the first exothermic peak is proposed as follows: 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g)
(5)
8. Conclusion The paper describes the investigations of pyrite influenced the thermal stability of emulsion explosives (EE) and ammonium nitrate (AN). The thermal decomposition of AN with EE was carried out with and without pyrite. The presence of pyrite reduces the decomposition temperature and accelerates the rate of decomposition of AN and EE. The activation energy and the pre-exponential factor were also calculated. An exothermic reaction occurs at about 200 ◦ C between pyrite and AN (emulsion explosives). The essence of reaction between EE and pyrite is reaction between AN and pyrite. The excellent thermal stability of EE does not mean it was also showed same result when pyrite was added. Package EE were more suitable in usage in pyrite shale than bulk EE. A new overall reaction has been proposed about AN with pyrite shale according to mass spectra analysis. 14FeS2 (s) + 91NH4 NO3 (s) → 52NO(g) + 26SO2 (g) + 6Fe2 O3 (s) + 78NH3 (g) + 26N2 O(g) + 2FeSO4 (s) + 65H2 O(g).
Acknowledgement This paper was supported by Inner Mogolia Sheng An chemical Ltd., Co. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.07. 069 References [1] E.G. Mahadevan, Ammonium nitrate explosives for civil applications-slurries, emulsions and ammonium nitrate fuel oils, Wiley-VCH, Weinheim, 2013. [2] Xuguang Wang, Emulsion explosives, Metallurgical Industry Press, Beijing, 2008. [3] R. Gunawan, D. Zhang, Thermal stability and kinetics of decomposition of ammonium nitrate in the presence of pyrite, J. Hazard. Mater. 165 (2009) 751–758. [4] P. Bellairs, The development of an inhibited explosives for black pyrite reactive shale, in: Proceedings of the 22th annual symposium on explosives and blasting technique, Orlando, Orlando, Florida, USA, 1996, pp. 361–375. [5] Tetsuya Sawada, Koichi Kurokawa, Fumihiko Sumiya, et al., Development of heat resistant emulsion explosives, Proceedings of the 8th annual symposium on explosives and blasting technique, Orlando, Florida, USA (1992) 73–80. [6] R.P. Proulx, D.S. Scovira, Drilling and blasting in hot and reactive ground conditions at barrick goldstrike’s meikle mine, Proceedings of the 26th annual symposium on explosives and blasting technique, Salt Lake City, Utah (2000) 319–331. [7] R. Gunawan, S. Freij, D. Zhang, et al., A mechanistic study into the reactions of ammonium nitrate with pyrite, Chem. Eng. Sci. 61 (2006) 5781–5790. [8] P. Priyananda, A.M. Djerdjev, J. Gore, et al., Premature detonation of an NH4 NO3 emulsion in reactive ground, J. Hazard. Mater. 283 (2015) 314–320. [9] J.A. Rumball, The interaction of partially weathered sulphides in the Mt. McRae shale formation with ammonium nitrate, in: Ph.D. Thesis, Murdoch University, Perth, Australia, 1991.
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