Why does aluminum powder and dust produced by aluminum alloy surface polishing explode?

The common surface treatment of aluminum products such as aluminum alloy and aluminum profile is polishing and polishing, which will produce a lot of aluminum powder and dust Why do aluminum powder explosion and dust explosion occur? What is aluminum powder? It is also called silver powder, also known as aluminum silver powder It has good heat transfer performance The fire risk of aluminum powder lies in that its dust flies and mixes with air, which will explode and burn in case of Mars Aluminum powder can also form explosive mixture when mixed with other oxidants If it contacts with acid and alkali, hydrogen will be produced, and hydrogen is easy to burn and explode in the air The explosion cause of Kunshan Zhongrong metal products Co., Ltd has been found out that a large amount of aluminum powder and dust generated during the polishing of aluminum alloy wheel hub meet with open fire, resulting in aluminum powder explosion and dust explosion accidents, resulting in huge casualties There are many kinds of aluminum alloy polishing Generally, it is polished with polishing wax and then polished with cloth wheel, so there will be a lot of aluminum dust naturally However, in general, aluminum alloy processing plants, there is a large suction exhaust device beside the polishing machine So aluminum powder and dust will not have so much accumulation Aluminum powder is a second-class combustion object in contact with water, which can react with water, generate hydrogen and release heat   Sometimes the dry aluminum powder will explode, because the dry aluminum powder with very small particles can suspend in the air, increase the contact surface with the air, and increase its chemical activity Once the dust reaches a certain amount in the air, it can explode rapidly when encountering a fire source, and generate a large amount of heat and combustion products in an instant, so that the gas, steam and so on will expand violently The result of the explosion When the concentration reaches the explosion concentration range, the explosion will occur under the influence of certain ignition energy If there is too much aluminum powder, it may cause the aluminum powder to rise again, forming a secondary explosion, which is called martyrdom explosion Aluminum powder can also explode, which many people don't know Specifically, the long-term polishing operation makes the air of the workshop filled with a lot of aluminum powder, and the ignition point of aluminum is quite low, as long as the fire source or high temperature, friction, may explode Aluminum will react with water to produce hydrogen, which is the reason why water cannot be used to extinguish the fire In the past, there has been a fire that uses water to extinguish the explosion of aluminum powder, causing another explosion Fire caused by aluminum powder explosion can not use water, and foam extinguishers should be used for fire fighting (aluminum profile processing) The fire is separated from the air by foam covering That's why aluminum reacts with water to produce hydrogen, which can't be put out by water In the past, there has been a fire that uses water to extinguish the explosion of aluminum powder, causing another explosion Most people don't know that aluminum powder can also explode, especially the long-term polishing operation makes the air in the workshop filled with a lot of aluminum powder, and the ignition point of aluminum is quite low, as long as it meets the fire source or high temperature, friction, it may explode Characteristics of aluminum dust: when encountering a small ignition source in the air, it can ignite and burn; when encountering an open fire, aluminum powder stained with grease in the air, such as long-term stacking and storage, will not dissipate heat, and will easily cause spontaneous combustion or explosion Moreover, the smaller the particle size of aluminum powder is, the greater the risk of explosion When its concentration in the air reaches more than 40 g / m3, it can explode when encountering an open fire The flame temperature is high, the burning speed is fast, the explosion power is great, and the radiation heat is strong When burning, the flame is generally green and blue, giving off strong silver white light, and the explosion pressure can reach 6.3kg/cm2 It has great destructive power and harmfulness to the surrounding buildings and personal safety How to put out the aluminum dust explosion accident? Flammable metal powder aluminum powder compound is easy to form explosive dust Cover the fire with asbestos felt (or sand) before putting out the fire with water In order to put out the aluminum powder fire effectively, the fire extinguisher must be selected and used correctly First, the fire of aluminum powder can not be extinguishing with water and foam This is because the surface of aluminum powder usually has a layer of oxide film which will not react with water However, the newly made aluminum powder or the aluminum powder whose oxide film has fallen off and the aluminum powder which is burning or baking at high temperature on the fire site will react rapidly, and release the hydrogen which is dangerous for explosion and combustion to mix with air to form explosive mixture Therefore, aluminum fire is prohibited from using water and foam to fight Secondly, aluminum powder can react with chlorine and bromine at room temperature, and react with haloalkanes to produce a small amount of aluminum chloride, which often leads to explosion and combustion Therefore, the aluminum powder fire can not be put out with carbon tetrachloride and 1211 fire extinguishing agent Aluminum powder is light in weight and small in fineness Once encountering wind or air jet, it is easy to fly in the air to form explosive mixture Therefore, aluminum powder fire can not be put out with carbon dioxide and other gas fire extinguishers According to the above characteristics, 7150 fire extinguishing agent or chemical dry powder, dry sand, graphite powder, dry magnesium powder, etc shall be selected for fire fighting Because this kind of fire extinguishing agent can cover the surface of burning aluminum powder, isolate it from the air, and effectively prevent the aluminum powder from flying and mixing with the air, so as to achieve the purpose of smothering fire extinguishing What is aluminum dust explosion? It means that when the dust meets the heat source (open fire or temperature) within the explosion limit, the flame spreads in the whole mixed dust space in an instant, the chemical reaction speed is extremely fast, and a large amount of heat is released at the same time, forming a high temperature and a large pressure The energy of the system is converted into mechanical work and radiation of light and heat, which has a strong destructive force The characteristics of aluminum dust explosion (1) multiple explosions are the biggest characteristics of dust explosion; (2) the minimum ignition energy required for dust explosion is higher, generally more than tens of MJ; (3) compared with combustible gas explosion, the pressure of dust explosion rises slowly, the duration of higher pressure is long, the energy released is large, and the destructive force is strong The burning speed of dust is smaller than that of gas Because of its long burning time and large energy, the degree of damage and burning is much more serious This is because the content of carbon and hydrogen in the dust is high, and the content of combustible materials is high If compared with the highest value of energy, dust explosion is several times that of gas explosion, and the temperature can reach more than 2000-3000 ℃, and the maximum explosion pressure is 345-690kpa; (4) dust combustion has to go through complex processes such as heating, melting, dissociation, evaporation, etc., and the time required for dust from contacting the fire source to explosion, that is, the induction period is longer than gas explosion, reaching tens of seconds; (5) powder Dust explosion can cause another explosion of dust in other parts of the building And the pressure of the second explosion is greater than that of the first explosion, which is more destructive How to prevent aluminum dust explosion? First, reduce the concentration of dust in the air The equipment with good airtight performance shall be used to minimize the dust flying out At the same time, effective ventilation and dust removal equipment shall be installed to strengthen the cleaning work The second is to control the indoor temperature The third is to improve the equipment and control the fire source In places with dust explosion risk, explosion-proof motors, explosion-proof electric lamps and explosion-proof switches shall be used Fourth, the scope of explosion should be controlled in advance The fifth is to control the temperature and oxygen content For containers with dust deposition, cooling measures shall be taken, and inert gas can be filled if necessary to dilute the oxygen content Fire fighting measures and emergency plan for grinding and polishing workshop of aluminum alloy, aluminum profile and other aluminum products 1 Establish a strong fire headquarters Once the aluminum powder explodes and burns, it often causes casualties, and the fire scene is more complex, and there is a danger of secondary explosion Therefore, in the process of fire fighting, it is necessary to establish a strong fire field headquarters to consider the whole task of fire fighting The task of the fire field headquarters is to quickly organize forces to detect the fire, formulate correct tactical measures, arrange combat tasks, issue combat orders, mobilize various vehicles to save people, evacuate materials, supply fire extinguishing agent (sand transport), etc 2 pay attention to the correct use of extinguishing agent Both the 7150 fire extinguisher and the chemical dry powder fire extinguisher use compressed nitrogen as the power to spray the agent on the combustibles Therefore, when using these two fire extinguishers, the spraying pressure should not be too large to prevent dust flying and explosion When dry sand and asbestos are used to put out the fire, the method of "one enclosure, two covers and three burials" can be adopted, that is, when seizing the fire, first use dry sand to surround the burning aluminum powder from four sides to a certain extent, and then use asbestos to cover it Finally, use sand to bury it gently (generally the sand thickness is 30-50 cm) In addition, the above fire extinguishing agents can also be used in combination In case of fire in the workshop, the combustion of kerosene pool (tank) and some equipment used for production in the workshop can be extinguished with dry chemical powder or 7150 fire extinguishing agent first, and then, when the temperature in the workshop is relatively weak, the aluminum powder burned by 7150 fire extinguishing agent, asbestos quilt and dry sand can be used for encircling 3 Block first, then encircle If a small amount of aluminum powder does not form a large-scale fire, we can adopt the tactics of quick decision, using asbestos quilt and dry sand to eliminate the fire directly In case of large area combustion caused by explosion, the tactics of first blocking and then extinguishing fire shall be adopted According to the characteristics of aluminum fire, the following problems should be paid attention to in the process of fire fighting 1 Do not open the window at will during the fire fighting, so as to avoid the formation of the wind that makes the aluminum powder fly, causing the explosion to hurt people 2 Fire fighters should pay attention to observe and use terrain and objects as a cover to prevent radiation heat, scorching body, house collapse and explosion causing casualties 3 Make full use of safety equipment and equipment to protect themselves and facilitate the action of team members 4 Close all valves of the ball mill and process system in the workshop in time, shut down the machine, cut off the power and then put out the fire In order to find out the cause of Kunshan aluminum dust explosion accident, we must first find out what is dust explosion Dust explosion is an explosion phenomenon when the combustible dust suspended in the air touches the fire source such as open fire or electric spark Because the surface area of the small particles of the solid material is much larger than that of the same weight of the block material, it is easy to ignite Common combustible dust includes: metal (such as magnesium powder, aluminum powder), coal, grain (such as wheat, starch), feed (such as blood powder, fish powder), agricultural and sideline products (such as cotton, tobacco), forest products (such as paper powder, wood powder), synthetic materials (such as plastic, dye) Combustible dust does not mean that it will explode The results show that there are five conditions for dust explosion: 1 The dust concentration is moderate If the concentration is low, the dust particle spacing is too large, the flame is difficult to spread, and it will not explode; if the concentration is too high, the oxygen will not be enough, and it will not explode (generally 30-40 grams per cubic meter of aluminum magnesium dust will reach the explosion concentration) 2 There should be a certain amount of oxygen This is a necessary condition for dust combustion 3 There should be enough ignition sources The minimum ignition energy of dust explosion is 1-2 orders of magnitude larger than that of gas explosion 4 The dust must be in the state of dust cloud, i.e in suspension 5 The dust cloud should be in a relatively sealed space