Suppression of aluminum dust explosions by expandable graphite

Research on the Inhibition Effect of NaCl on the Explosion of Mg-Al Alloy Powder

A study was conducted on the explosion overpressure and flame propagation law of magnesium-aluminum (Mg-Al) alloy powder, and the suppression mechanism of sodium chloride (NaCl) on the explosion of magnesium-aluminum alloy powder was explored. Adding NaCl powder can effectively reduce the explosion pressure, flame front position, and flame propagation speed. The higher the amount of NaCl powder added, the lower the explosion pressure of magnesium-aluminum alloy powder, the slower the flame propagation speed, and the lower the flame brightness. NaCl adsorbed on Mg-Al alloy powder isolated heat transfer and played a cooling role. The Cl- produced by NaCl decomposition will react with the free radicals H+ and OH- in the reaction system, which will reduce the concentration of H+ and OH- in the combustion process and hinder the propagation and expansion of the flame. The research results provide theoretical guidance for the explosion prevention of Mg-Al alloy powder and the preparation of a physical-chemical compound explosion suppressor in the later stage.

Inhibition characteristics research of aluminum alloy polishing dust explosion through addition of ultrafine Al(OH)3 inerting agent

Investigations into the deactivation of explosion sensitivity and reduction of flame propagation for aluminium alloy polishing wastes were carried out by the addition of ultrafine Al(OH)3 inerting agent. Meanwhile, high-purity aluminium powders with similar mean diameters were also used as a comparative study. The explosion propagation characteristics of high-purity aluminium dust and aluminium alloy polishing waste dust under different inerting ratios (ε ) were tested and investigated using a standardised Hartmann tester and a developed experimental platform. The results show that the minimum ignition energy of high-purity aluminium powder is between 40 and 45 mJ, and the minimum ignition energy of aluminium alloy polishing waste is between 500 and 550 mJ, which is one order of magnitude higher than that of high-purity aluminium powder. The lower explosion limit concentration of aluminium alloy polishing waste dust is 150 g/m3, which is 53.33% of that of high-purity aluminium powder. According to the analysis of the SEM image, the main reason is that the spherical particles of high-purity aluminium dust have a folded surface and good dispersion. Compared with the smooth fibre surface of aluminium alloy polishing waste dust, the former is easier to contact with air and the contact area is larger. Therefore, in engineering practice, it is not appropriate to use high-purity aluminium dust-related explosion parameters as the basis for the risk assessment of combustion and explosion at aluminium alloy polishing work sites. In addition, as the dust concentration decreases, the combustion intensity of high-purity aluminium dust and aluminium alloy polishing waste dust also decreases, and the flame propagation appears to be a discontinuous phenomenon. The peak flame propagation velocity of aluminium alloy polishing waste is 7.368 m/s at a concentration of 300 g/m3, which is 56.85% of that of high-purity aluminium powder. As the inerting ratio increases, the propagation velocity of the explosion flame slows down. When the inerting ratio reaches 30%, the minimum ignition energy of aluminium alloy polishing waste is inerted to 1 J, and self-sustained flame propagation cannot be formed. The results show that the ultra-fine Al(OH)3 powder has a significant inerting effect and is a realistic possibility in the production of aluminium alloy polishing.

Effects of dust dispersibility on the suppressant enhanced explosion parameter (SEEP) in flame propagation of Al dust clouds

This work presents an overview about the suppressant enhanced explosion parameter (SEEP) phenomenon in aluminum dust explosion moderation. The SEEP phenomenon can be attributed to either the flammable gas produced by decomposition of insufficient chemical suppressant so as to form an explosible hybrid mixture, or to the improvement in dust dispersibility caused by small amounts of thermal inhibitor so as to form better dispersed dust clouds. Aluminum (Al) and four particle sizes of alumina (Al₂O₃) were used to confirm a physically caused SEEP phenomenon by performing flame propagation experiments. Higher flame spread velocities (FSVs) in Al dust clouds were found in the presence of 5 or 10% <150 and <45-µm Al₂O₃ powder. Adding micro-sized Al₂O₃ disrupted inter-particle contact in combustible dusts, decreased inter-particle forces, and formed dust clouds with better dispersibility, thereby decreasing the effective particle size distribution (PSD) of Al dust. A strong thermal effect brought about by 2.5 µm Al₂O₃ overcame the negative effect of improved dispersion, preventing SEEP from occurring. The addition of 50 nm Al₂O₃ increased cohesion of powder mixtures, and decreased dust dispersibility. With benefits from both dispersion suppression and the thermal effect, Al flame propagation was well quenched.