## 3 Results and Discussion
### Suppression Properties of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM Composite Powders  
The suppression properties of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders with different amount of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) (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 NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/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 NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\), and enhanced gradually with increasing the amount of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\). When the loading amount is more than 20%, the suppression performance of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders on the maximum explosion pressure is even better than the pure NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) powders. The 30%-NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders show a better suppression effect than the pure NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) 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 NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders and the pure NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) and red mud (RM) powders are illustrated in Figure 6B, and the detailed data are shown in Table 1.  
Figure 5: Thermogravimetry-Differential scanning calorimetry (TG-DSC) curves of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders. Figure 6: The suppression effect of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders: the explosion pressure curves (**A**), the maximum pressure, the maximum rate of pressure rise, and the time of pressure peak arriving (**B**). From Table 1, we can see that when adding the 30% NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/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\(\cdot\)s\({}^{-1}\) to 1.72 MPa\(\cdot\)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, NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\), and the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/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 NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) powders was added, the time is 50 ms and 132 ms, respectively. When the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders with the different amount of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) (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 NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders present a much better suppression effect on flame propagation speed than that of pure RM powders. In addition, the 30%-NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders exhibit the best inhibition performance, which is consistent with the results that were tested by the 20 L spherical explosive system.