The Suppression Characteristics of NH₄H₂PO₄/Red Mud Composite Powders on Methane Explosion

Abstract

The composite powders composed of red mud (RM) and NH₄H₂PO₄ (NH₄H₂PO₄/RM) were successfully prepared by the anti-solvent method. The composition and structure of the NH₄H₂PO₄/RM composite powders were characterized by the techniques of X-ray diffraction (XRD), SEM, N₂ adsorption-desorption and Thermogravimetry-Differential scanning calorimetry (TG-DSC). The analysis results indicate that the as-prepared samples are composed with uniform nanoparticles and possess the porous structure. The methane explosion suppression characteristics of the NH₄H₂PO₄/RM composite powders were tested by a 20 L spherical explosion system and a 5 L pipe test system. The results show that the NH₄H₂PO₄/RM composite powders possess considerable suppression properties on methane explosion. When the loading content of NH₄H₂PO₄ reached 30%, the maximum pressure and the maximum pressure rise rate of methane explosion were decreased by 35.1% and 95.8%, respectively. When comparing with no powder addition, the time to reach the pressure peak was extended from 0.07 s to 0.50 s. The NH₄H₂PO₄/RM composite powders presented a synergistic suppression effect between NH₄H₂PO₄ and RM, which made it exhibit considerable suppression property than that of pure NH₄H₂PO₄ or red mud powders.

1. Introduction

Gas explosion is an extraordinary serious coal mine disaster that can cause massive damage and casualties. In order to reduce the impact of coal mine gas explosion, researchers have carried out a lot of research works. On the one hand, the refuge chamber has been widely investigated and used for miners who are unable to escape during an accident, which can provide basic survival conditions [1,2,3]. Zhao et al. simulated and analyzed the structure strength of a coal mine mobile refuge chamber under gas explosion load by a finite element method [4]. Li et al. simulated and analyzed the air pressure distribution law in the mine refuge chamber [5]. Zhang et al. studied the design and optimation of the refuge chambers based on the simulation of the gas explosion pressure fields [6]. Jia et al. studied the relationship between the propagation regulation about shock wave of gas explosion and the laneway cross-sectional area [7]. Zhang et al. established the underground mine tunnel model and the refuge chamber model by using the finite element software ANSYS/LA-DYNA, the propagation of methane/air mixture blast waves, and the fluid-structure interactions were also simulated by the Arbitrary Lagrange-Euler (ALE) method [8].

On the other hand, the suppression of gas explosion in the coal mine is also significant for the safety of mine production. Methane is the major component of mine gas and its explosion limits is 5–15%. In the previous studies, water mists [9,10,11,12], inert gas [13,14,15,16], and chemical powders [17,18,19,20,21] have been used to suppress the methane explosion. Because of its excellent performance on methane explosion suppression, chemical powders are studied extensively. In recent years, the composite powders were appreciated and studied widely for its synergistic effect on the explosion suppression. Harris et al. systematically studied the relationship between the particle size distributions, specific surface area, and the explosion suppression effectiveness of the rock dusts [22]. Ni et al. fabricated a new type fire suppressant with core–shell structure while using zeolites and NaHCO₃ [23]. The composites could extinguish the oil fire with less powder amount and shorter time when comparing with the common BC (…) powders. Bertolino et al. report a novel procedure to fabricate multilayer composite biofilms based on halloysite nanotubes (HNTs) and sustainable polymers [24]. The thermal properties analysis of the nanocomposite indicated that the chitosan/halloysite layer generates a flame retardant action on the biomaterials and the multilayer morphology confers a fire resistance to the nanocomposite. In our previous work, the NaHCO₃/red-mud composite powders were prepared and its suppression effect on the methane explosion was tested in detail [25]. Due to the synergistic suppression effect between NaHCO₃ and red-mud, the NaHCO₃/red-mud composite powders exhibited better suppression properties than that of the pure NaHCO₃ and red mud powders. Thus, the use of composite powders gradually became an important development trend for the gas explosion suppression.

Red mud is the waste residue of alumina industry, which is mainly composed of Al₂O₃, CaO, SiO₂, and Fe₂O₃, and caused serious environmental pollution. The recycle and utilization of red mud have become an urgent problem. On this condition, many works have been done to find the novel utilization of red mud [26,27,28,29,30]. In our previous research, the red mud was modified by a facile method and presented a considerable suppression property on methane explosion [31]. Producing methane explosion suppression reagents with red mud could both solve the pollution problem and bring considerable economic value. NH₄H₂PO₄ is a kind of common fire-extinguishing agent. Kordylewski and Amrogowicz studied the suppression effect of NH₄H₂PO₄ and NaHCO₃ powders on the methane explosion, and the results indicated that the NH₄H₂PO₄ powders had a better suppression effect on the methane explosion [32]. However, to the best of our knowledge, there is still no research about the preparation and methane explosion suppression application of the NH₄H₂PO₄ and red mud composite powders.

Herein, a new composite (NH₄H₂PO₄/RM) for methane explosion suppression was successfully prepared with NH₄H₂PO₄ and red mud. The composition and structure of the NH₄H₂PO₄/RM composite powders were characterized, and their methane explosion suppression properties were tested in detail. Furthermore, the suppression mechanism of the NH₄H₂PO₄/RM composite powders on methane explosion was also discussed.

2. Materials and Methods

2.1. Materials

NH₄H₂PO₄/RM composite powders were prepared with the modified red mud and NH₄H₂PO₄ particles by an anti-solvent method. The raw red mud was provided by Henan Zhongmei Aluminum Corporation, China (major compositions: SiO₂ 16.94 wt %; Al₂O₃ 23.25 wt %; Fe₂O₃ 15.05 wt %; CaO 16.94 wt %; TiO₂ 4.27 wt %; Na₂O 3.72 wt %; MgO 1.93 wt %). The raw red mud was dried, ground, and sieved to a size of <150 μm. Other chemical reagents were of analytical grade. The modified red mud powders were prepared at first and the method was shown in the literature [31]. The preparation process of NH₄H₂PO₄/RM composite powders with different weight percentage of NH₄H₂PO₄ (5%, 10%, 15%, 10%, 25%, and 30%) was as follows. Firstly, a certain amount of NH₄H₂PO₄ was solved in deionized water to become a saturated solution. Then, 10 g modified red mud powders were dispersed in the anti-solvent ethanol to become a suspension solution. Under magnetic stirring, the prepared suspension solution of modified red mud was added into the NH₄H₂PO₄ saturated solution. The obtained mixture was stirred for 2 h, aged for 4 h, and then the precipitate was separated by filtration and dried in vacuum for 12 h. Finally, NH₄H₂PO₄/RM composite powders with a different weight percentage of NH₄H₂PO₄ were obtained, and denoted as 5%-NH₄H₂PO₄/RM, 10%-NH₄H₂PO₄/RM, 15%-NH₄H₂PO₄/RM, 20%-NH₄H₂PO₄/RM, 25%-NH₄H₂PO₄/RM, and 30%-NH₄H₂PO₄/RM.

2.2. Characterization

X-ray diffraction (XRD) analysis was carried on Bruker-AXS D8 (Bruker, Madison, WI, USA) with CuKα radiation at 40 kV and 25 mA. Field emission scanning electron microscopy (FESEM, Quanta™ 250 FEG) (FEI, Eindhoven, The Netherlands) was used to analyze the morphologies of the prepared samples. N₂ adsorption-desorption isotherms were obtained on a Quantachrome Autosorb-iQ sorption analyzer (Quantachrome, Boynton Beach, FL, USA). Before carrying out the measurement, the samples were degassed at 150 °C for more than 6 h. The specific surface areas (S_BET) of the samples were calculated while following the multi-point BET (Brunauer-Emmett-Teller) procedure. Thermogravimetry-Differential scanning calorimetry (TG-DSC) analysis was completed on a Simultaneous Thermal Analyzer (NETZSCH, Selb, Germany) in a flow of air (20 mL·min⁻¹) at a heating rate of 10 °C·min⁻¹.

2.3. Explosion Experiment Device and Test Process

The effects on the methane-air premixed gas explosion pressure of NH₄H₂PO₄/RM composite powders with different weight percentage of NH₄H₂PO₄ were tested by the 20 L spherical explosion system. The illustration of 20 L spherical explosion system was presented in Figure 1. The system mainly includes the gas distribution system, ignition system, powder injection system, and data acquisition system. Firstly, the 9.5% methane/air was mixed by the partial pressure method. Then, the NH₄H₂PO₄/RM composite powders were put into the powder storage tank and the 2 MPa injection air was introduced into the tank. The high-pressure air and the powders were injected and dispersed into the spherical vessel after pushing the ignition button. The igniter is electrical pulse and the ignition energy is about 100 J. The explosion pressure was collected by the pressure sensor and then saved by the computer at the same time.

The average velocity of flame propagation was measured by a pipe test system with a cross-sectional area of 100 × 100 mm² and a length of 500 mm. The illustration of the pipe test system was shown in our previous published paper [18]. Firstly, the NH₄H₂PO₄/RM composite powders were put into the powder container. Then, the premixed 9.5% methane/air was introduced into the pipe. The powders were injected into the pipe by the high-pressure gas and then the methane/air was ignited by the electrical pulse igniter. Meanwhile, the flame propagation images were captured by a high-speed camera and saved in the computer.

3. Results and Discussion

3.1. Sample Characterization

The X-diffraction patterns of NH₄H₂PO₄/RM composite powders, red mud, and NH₄H₂PO₄ are shown in Figure 2, respectively. From Figure 2, we can see that the NH₄H₂PO₄/RM composite powders and red mud powders have the similar X-ray diffraction patterns. The diffraction patterns of the composite powders also present the characteristic diffraction peaks of NH₄H₂PO₄ (2θ = 19.6°, 27.6°, 36.4°, 42.1°) after loading the NH₄H₂PO₄ particles. The results indicate that the red mud was the main component of the composite powders and the NH₄H₂PO₄ particles were loaded on the red mud successfully.

The SEM images of NH₄H₂PO₄ powders, red mud, and the NH₄H₂PO₄/RM composite powders are shown in Figure 3. As shown in Figure 3A, the sizes of NH₄H₂PO₄ particles are large and not uniform, and the diameters are from 5 to 20 μm. The surface of red mud is rich in pores and the pores are in irregular distribution, as shown in Figure 3B. The surface of the NH₄H₂PO₄/RM is coated with small particles according to Figure 3C, which indicates that NH₄H₂PO₄ are successfully loaded on the surface of red mud, forming the uniform composite powders.

The N₂ adsorption-desorption isotherms of red mud and the NH₄H₂PO₄/RM composite powders are shown in Figure 4. Both of the isotherms are type IV in the IUPAC classification with the H4 hysteresis loop. The results indicate that these two powders possess the mesoporous structure, and capillary condensation occurred during the adsorption-desorption process. After loading the NH₄H₂PO₄ particles, the adsorption capacity of the composite is lower than red mud in the P/P₀ range of 0.5–1.0, indicating the decrease of mesopores in the NH₄H₂PO₄/RM composite powders and the formation of a granulation structure during the loading process. The data of this experiment confirmed the changes of the NH₄H₂PO₄/RM morphology that are presented in SEM images (Figure 3). The specific surface area and pore volume of red mud are 123.16 m²·g^‒1 and 0.158 cm³·g^‒1. After loading the NH₄H₂PO₄ particles, the data of the composite powders are reduced to 75.83 m²·g^‒1 and 0.116 cm³·g^‒1. It means that the NH₄H₂PO₄ particles were loaded on the surface and/or in the pore of red mud.

The TG-DSC curves of the NH₄H₂PO₄/RM composite powders are shown in Figure 5. From the TG curve, there are three weight loss stages in the whole process. The first stage from 120 °C to 220 °C is attributed to the lost and the thermal decomposition of adsorbed and intercalated moisture of the NH₄H₂PO₄/RM composite powders. The second stage from 220 °C to 300 °C could be ascribed to the decomposition of NH₄H₂PO₄. The third stage from 300 °C to 420 °C could be ascribed to the decomposition of the hydroxides (such as: Al(OH)₃ and Fe(OH)₃). After 420 °C, there is no further weight loss, indicating that the composite powder became stable. The total weight loss of the NH₄H₂PO₄/RM composite powders is 93.7%. The DSC curve of the composite powders exhibited three endothermic peaks, which is consistent with the TG curve. According to the DSC curve, the total endothermic quantity of the NH₄H₂PO₄/RM composite powders is 1257.9 J·g⁻¹, indicating that the composite powders presented an excellent heat absorption performance.

3.2. Suppression Properties of the NH₄H₂PO₄/RM Composite Powders

The suppression properties of the NH₄H₂PO₄/RM composite powders with different amount of NH₄H₂PO₄ (weight percentage of 5%, 10%, 15%, 20%, 25%, and 30%) were tested and the results are presented in Figure 6. Figure 6A shows the 9.5% premixed methane-air explosion pressure curves with no powders, pure red mud powders, and the NH₄H₂PO₄/RM composite powders. It can be seen from Figure 6A that the maximum explosion pressure is decreased and the time of pressure peak arriving is delayed by adding suppression powders. The explosion suppression effect of red mud was improved after loading NH₄H₂PO₄, and enhanced gradually with increasing the amount of NH₄H₂PO₄. When the loading amount is more than 20%, the suppression performance of the NH₄H₂PO₄/RM composite powders on the maximum explosion pressure is even better than the pure NH₄H₂PO₄ powders. The 30%-NH₄H₂PO₄/RM composite powders show a better suppression effect than the pure NH₄H₂PO₄ powders both in the maximum explosion pressure and the time of pressure peak arriving. The maximum rate of pressure rise and the time of pressure peak arriving of the NH₄H₂PO₄/RM composite powders and the pure NH₄H₂PO₄ and red mud (RM) powders are illustrated in Figure 6B, and the detailed data are shown in

Table 1. The explosion parameters of methane-air premixed gas with different powders.

Samples	Max Pressure (MPa)	The Rate of Max-Pressure Rise (MPa·s−1)	Time of Pressure Peak Arriving (s)	Decline Rate of Max Pressure (%)	Decline Rate of Max Pressure Rise (%)	Delay Time of Pressure Peak Arriving (s)
No powders	0.749	41.16	0.07	0	0	0
RM	0.651	21.48	0.11	13.1	47.8	0.04
5%-NH4H2PO4/RM	0.607	8.79	0.23	19.9	78.6	0.16
10%-NH4H2PO4/RM	0.571	6.16	0.26	23.8	85.1	0.19
15%-NH4H2PO4/RM	0.562	5.17	0.29	24.9	87.4	0.22
20%-NH4H2PO4/RM	0.519	4.32	0.36	30.7	89.5	0.29
25%-NH4H2PO4/RM	0.497	2.54	0.40	33.6	93.8	0.33
30%-NH4H2PO4/RM	0.486	1.72	0.50	35.1	95.8	0.43
NH4H2PO4	0.545	2.99	0.45	27.2	92.8	0.38

.

From Table 1, we can see that when adding the 30% NH₄H₂PO₄/RM composite powders as the suppression agent, the maximum explosion pressure is decreased from 0.749 MPa (no powders) to 0.486 MPa, which reduce 35.1%. Accordingly, the maximum rate of pressure rise is decreased from 41.16 MPa·s⁻¹ to 1.72 MPa·s⁻¹, which reduce 95.8%, and the delay time of pressure peak arriving is increased from 0.07 s to 0.50 s, which delay 0.43 s.

The flame propagation images of the 9.5% premixed methane-air explosion with the red mud, NH₄H₂PO₄, and the NH₄H₂PO₄/RM composite powders are shown in Figure 7. As shown in this figure, the time of explosion flame propagating to the top of the pipe is 26 ms with no powders addition. When the pure red mud or the NH₄H₂PO₄ powders was added, the time is 50 ms and 132 ms, respectively. When the NH₄H₂PO₄/RM composite powders with the different amount of NH₄H₂PO₄ (5%, 10%, 15%, 20%, 25%, and 30%) were added, the time of explosion flame reached the top are 73 ms, 85 ms, 97 ms, 110 ms, 130 ms, and 160 ms, respectively. The results indicate that the NH₄H₂PO₄/RM composite powders present a much better suppression effect on flame propagation speed than that of pure RM powders. In addition, the 30%-NH₄H₂PO₄/RM composite powders exhibit the best inhibition performance, which is consistent with the results that were tested by the 20 L spherical explosive system.

3.3. Suppression Mechanism of the NH₄H₂PO₄/RM Composite Powders

According to the results of the explosion experiments, the NH₄H₂PO₄/RM composite powders present the considerable suppression properties on the methane explosion. The schematic diagram of the NH₄H₂PO₄/RM composite powders in the explosion process is illustrated in Figure 8. The suppression mechanism of the NH₄H₂PO₄/RM composite powders could be analyzed from two aspects, as follows.

Physical inhibition effect: The NH₄H₂PO₄/RM composite powders have a good heat absorption performance, as shown in the Figure 5. When the composite powders were added into the explosion vessel, the NH₄H₂PO₄/RM composite powders were separated into the NH₄H₂PO₄ particles and red mud under the high pressure and temperature during the 9.5% premixed methane-air explosion. Then, the NH₄H₂PO₄ particles and the hydroxides (Al(OH)₃, Fe(OH)₃) in the red mud decomposed with absorbing a lot of heat, leading to a good endothermic effect for explosion suppression.

Chemical inhibition effect: The mechanism of methane explosion is shown as the following chain reactions [33].

$${\mathrm{CH}}_4{+\mathrm{O}}_2\to {\mathrm{CH}}_3{+\mathrm{HO}}_2$$

$${\mathrm{CH}}_3{+\mathrm{O}}_2\to {\mathrm{CH}}_2\mathrm{O}+\mathrm{HO}$$

$${\mathrm{HO}+\mathrm{CH}}_4\to {\mathrm{CH}}_3{+\mathrm{H}}_2\mathrm{O}$$

$${\mathrm{CH}}_2\mathrm{O}+\mathrm{HO}\to {\mathrm{CH}}_3{+\mathrm{H}}_2\mathrm{O}$$

$${\mathrm{CH}}_2{\mathrm{O}+\mathrm{O}}_2\to {\mathrm{HO}}_2+\mathrm{CHO}$$

$${\mathrm{CHO}+\mathrm{O}}_2\to {\mathrm{CO}+\mathrm{HO}}_2$$

$${\mathrm{HO}}_2{+\mathrm{CH}}_4\to {\mathrm{H}}_2{\mathrm{O}}_2{+\mathrm{CH}}_3$$

$${\mathrm{HO}}_2{+\mathrm{CH}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{CHO}$$

During the methane explosion process, the NH₄H₂PO₄/RM composite powders decomposed and they generated a lot of gaseous free radicals, which can combine with the generated radicals by methane explosion. In addition, the red mud powders expose more pores on their surface due to the separation of the loaded NH₄H₂PO₄ particles, which can adsorb and capture more free radicals that are generated by methane explosion. Both two reasons simultaneously contribute to the chemical inhibition effect of the NH₄H₂PO₄/RM composite powders.

4. Conclusions

In this work, the NH₄H₂PO₄/RM composite powders with porous structure and high surface area are successfully prepared by using the industrial waste red mud as the base material. The resulting NH₄H₂PO₄/RM composite powders are found to possess considerable suppression properties on methane explosion, and the explosion suppression properties enhanced with increasing the amount of NH₄H₂PO₄ in the composites. When the loading content of NH₄H₂PO₄ reached 30%, the maximum pressure and the maximum pressure rise rate of the 9.5% premixed methane-air explosion were decreased by 35.1% and 95.8%, and the time reached the pressure peak was extended from 0.07 s to 0.50 s. The explosion suppression properties of NH₄H₂PO₄/RM composite powders are caused by the physical and chemical inhibition effect. Due to the unique properties and high-value utilization of the waste from the aluminum industry, the NH₄H₂PO₄/RM composite powders could be a desirable and economical inhibitor for the methane explosion suppression application.