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  • Q

    Why do aluminum powder and dust explode?

    A

    Aluminum alloys, aluminum profiles and other aluminum products are commonly surface treated to be polished and polished, which will produce a large amount of aluminum powder and dust.


    Why do aluminum powder and dust have aluminum powder explosions and dust explosions?


    What is aluminum powder: also known as silver powder, alias aluminum silver powder, good heat transfer performance. The fire hazard of aluminum powder is that its dust flies and mixes with air, and if it encounters Mars, it will explode and burn. Aluminum powder can also form an explosive mixture by mixing with other oxidants. If it is in contact with an acid or a base, hydrogen gas is generated, and hydrogen gas is easily burned and exploded in the air.


    In 2014, the cause of the explosion of Kunshan Zhongrong Metal Products Co., Ltd. was found to be that a large amount of aluminum powder and dust generated during the polishing of aluminum alloy wheels encountered aluminum powder explosion and dust explosion accidents caused by open flames, causing huge casualties.


    There are many kinds of aluminum alloy polishing. Generally, it is polished with wax and then thrown with cloth. In this case, there is naturally a lot of aluminum dust. However, in general aluminum alloy processing plants, there is a suction exhaust device on the side of the polishing machine. Therefore, aluminum powder and dust do not accumulate so much.


    Aluminum powder is a two-stage water-burning item that reacts with water to generate hydrogen and release heat.


    Sometimes the dry aluminum powder explodes because the extremely small dry aluminum powder can be suspended in the air, increasing the contact surface with the air, increasing its chemical activity, once the dust reaches a certain level in the air. When the quantity is encountered, it encounters a fire source and can quickly deflagrate, and instantaneously generates a large amount of heat and combustion products, causing gas, steam and the like to violently expand, resulting in an explosion.


     


    When this concentration reaches the explosion concentration range, it will explode under the influence of certain ignition energy. If there is too much aluminum powder, it may cause the aluminum powder to rise again and form a secondary explosion called blast.


     


    Aluminum powder can also explode, and many people don't know it. Specifically, the long-time polishing operation makes the air in the workshop diffuse a lot of aluminum powder, and the ignition point of aluminum is quite low. It may explode if it encounters fire source or high temperature and friction.


    Since aluminum reacts with water to form hydrogen, this is why it cannot be extinguished with water. There have been cases in which water has been used to extinguish the explosion of aluminum powder, causing another explosion.


    In the event of a fire caused by an aluminum powder explosion, water cannot be used, but a foam fire extinguisher should be used to extinguish the fire (aluminum profile processing). Cover the fire and isolate the fire from the air. This is because aluminum reacts with water to form hydrogen, which is why it cannot be extinguished with water. There have been cases in which water has been used to extinguish the explosion of aluminum powder, causing another explosion.


    Most people don't know that aluminum powder can also explode. Especially the long-time polishing operation makes the air in the working building diffuse a lot of aluminum powder, and the ignition point of aluminum is quite low. As long as it encounters fire source or high temperature and friction, Both may explode.


    Characteristics of aluminum dust: It can be ignited when it encounters a small source of ignition in the air; aluminum powder with grease in the air, such as long-term accumulation and storage, does not disperse heat, and is also prone to spontaneous combustion or explosion, and aluminum The smaller the particle size of the powder, the greater the risk of explosion. When the concentration in the air reaches 40 g/m3 or more, it can explode when exposed to an open flame.


    The flame temperature is high, the burning speed is fast, the explosion power is large, and the radiation heat is strong. When burning, it usually has a green-blue flame, emitting silver-white glare, and the explosion pressure can reach 6.3 kg/cm2. It has great destructiveness and harmfulness to surrounding buildings and personal safety.


    How to save after an aluminum dust explosion accident?


    Flammable metal powder aluminum powder compounds are prone to form explosive dust. Cover with fire asbestos (or sand) first, then use water to save. Selection of Fire Extinguisher In order to effectively extinguish the aluminum powder fire, the fire extinguishing agent must be properly selected and used.


    First of all, aluminum powder can not be fired with water and foam. This is because the surface of aluminum powder usually has an oxide film that does not react with water. However, the newly formed aluminum powder or the aluminum powder which has been detached from the oxide film and the aluminum powder which is burning or being baked at a high temperature on the fire field will rapidly react chemically, and the hydrogen gas which is in danger of explosion and combustion is mixed with air to form an explosive mixture. Therefore, aluminum powder fires are prohibited from water and foam.


    Secondly, the aluminum powder can be burned with chlorine and bromine at normal temperature, and can also react with the halogenated alkane to form a small amount of aluminum chloride, which often leads to explosive combustion. Therefore, aluminum powder fire can not be saved with carbon tetrachloride and 1211 fire extinguishing agent.


    Aluminium powder has a light specific gravity and a small fineness. Once it encounters wind or air spray, it is easy to fly and form an explosive mixture in the air. Therefore, aluminum powder fires cannot be extinguished with a gas fire extinguisher such as carbon dioxide.


    According to the above characteristics, the fire of aluminum powder should be saved with 7150 fire extinguishing agent or chemical dry powder, dry sand, graphite powder and dry magnesium powder. Because this kind of fire extinguishing agent can cover the surface of the burning aluminum powder, it is isolated from the air, and can effectively prevent the aluminum powder from flying and mixing with the air, thereby achieving the purpose of suffocating fire extinguishing.


    What is an aluminum dust explosion?


    That is, the dust is in the explosion limit range, encountering the heat source (open flame or temperature), the flame instantaneously propagates through the entire mixed dust space, the chemical reaction speed is extremely fast, and at the same time, a large amount of heat is released, forming a high temperature and a large pressure, the system The energy is converted into mechanical work as well as light and heat radiation, which is very destructive.


    Characteristics of aluminum dust explosion


    (1) Multiple explosions are the biggest feature of dust explosions;


    (2) The minimum ignition energy required for dust explosion is higher, generally above tens of millijoules;


    (3) Compared with the explosion of flammable gas, the dust explosion pressure rises slowly, the higher pressure lasts for a long time, the energy released is large, and the destructive power is strong. The burning rate of dust is smaller than that of gas. Because of its long burning time and large energy generation, the degree of damage and burning is much more serious. This is because the carbon and hydrogen content in the dust is high, and the fuel content is high. If the comparison is made according to the highest value of the generated energy, the dust explosion is several times that of the gas explosion, the temperature can reach 2000-3000 °C or above, and the maximum explosion pressure is 345-690 kPa; (4) the dust combustion should be heated, melted, dissociated, evaporated, etc. In complex processes, the time required for the dust to contact the fire source to explode is longer than the gas explosion for tens of seconds; (5) the dust explosion can cause the dust in other parts of the building to explode again. Moreover, the second explosion pressure is greater than the first explosion pressure and is more destructive.


    How to prevent aluminum dust explosion


    One is to reduce the concentration of dust in the air. Use equipment with good sealing performance to minimize dust flying and escaping. At the same time, install effective ventilation and dust removal equipment to enhance cleaning. The second is to control the indoor temperature. The third is to improve equipment and control the source of fire. Explosion-proof motors, explosion-proof lights, and explosion-proof switches are required in places where there is a danger of dust explosion. Fourth, the scope of the explosion should be controlled in advance. The fifth is to control the temperature and oxygen level. For containers with dust deposits, cooling measures should be taken, and if necessary, inert gas can be charged to dilute the oxygen content.


    Fire-fighting measures and emergency plans for grinding and polishing of aluminum products such as aluminum alloy and aluminum profiles


    1. Establish a strong fire command headquarters. Once the aluminum powder explodes and burns, it often causes casualties, and the fire situation is more complicated and there is a danger of secondary explosion. Therefore, a strong fire field command should be established during the fight to ensure that the entire fire fighting mission is considered. The mission of the Fire Command is to quickly organize forces to detect fires, formulate correct tactical measures, arrange combat missions, issue combat orders, and mobilize various vehicles to save people, evacuate materials, and supply fire-extinguishing agents (sand sand).


    2. Take care to use the extinguishing agent properly. Both the 7150 fire extinguisher and the chemical dry powder fire extinguisher use compressed nitrogen as the power to spray the medicament onto the combustion products. Therefore, when using these two fire extinguishers, the injection pressure should not be too large to prevent the dust from flying and exploding. When using dry sand or asbestos to extinguish the fire, the method of "one circumference, two cover, three burial" can be adopted. That is, when siege fire, the dry aluminum powder is first used to surround the fired aluminum powder to a certain extent. Cover it with asbestos and, finally, bury it gently with sand (generally sand thickness is 30-50 cm). In addition, the above fire extinguishing agents can also be used in combination. If there is a fire in the production workshop, use chemical dry powder or 7150 fire extinguishing agent to extinguish the combustion of the kerosene tank (tank) and some equipment used in the workshop. Then, when the temperature in the workshop is relatively weak, use 7150 fire extinguishing agent. , asbestos and dry sand cofferdam burning aluminum powder.


    3. First block, then encircle. If a small amount of aluminum powder is burned without a large-scale fire, a quick-fixing tactic can be adopted, and the asbestos and dry sand can be used to directly eliminate the fire. If an explosion occurs to form a large area of combustion, the tactics of first blocking and then extinguishing the fire should be taken.


    Attention should be paid to the fire of aluminum powder


    According to the characteristics of aluminum powder fire burning, the following problems should be noted in the process of saving.


    1. In the fight, it is not allowed to open the window at will, so as to avoid the ion of a wind that causes the aluminum powder to fly, causing an explosion to injure people.


    2. Firefighters should pay attention to the observation and use of topographical features as a cover to prevent radiant heat, burning bodies, collapse of houses and explosions causing casualties.


    3. Make full use of safety equipment and equipment to protect themselves and facilitate team members' actions.


    4. Close all kinds of valves in the ball mill and process system in the workshop, stop the machine first, then stop the power and then save.


    To find out the cause of the Kunshan aluminum dust explosion accident, you must first find out what is a dust explosion.


    Dust explosion is an explosion phenomenon when combustible dust suspended in the air touches an ignition source such as an open flame or an electric spark. Because the tiny particles of solid matter have a much larger surface area than the bulk material of the same weight, it is very It is easy to catch fire. Common flammable dust includes: metal (such as magnesium powder, aluminum powder), coal, grain (such as wheat, starch), feed (such as blood meal, fish meal), agricultural and sideline products (such as cotton, tobacco), forest products (such as paper powder, Wood powder), synthetic materials (such as plastics, dyes).


    Dust is flammable and does not mean that it will explode. Research results show that dust explosions generally have five conditions:


    The dust concentration is moderate. The concentration is low, the distance between the dust particles is too large, the flame is difficult to spread, and it will not explode; if the concentration is too high, the oxygen will be insufficient and will not explode (generally 30 to 40 grams per cubic meter of aluminum-magnesium dust reaches the explosive concentration) )


    2. Have a certain amount of oxygen. This is a necessary condition for dust burning.


    3. There must be enough ignition source. The minimum ignition energy required for a dust explosion is 1-2 orders of magnitude larger than a gas explosion.


    4. The dust must be in a dust cloud state, that is, in a suspended state.


    5. The dust cloud should be in a relatively sealed space so that the temperature and pressure can rise sharply.


    It is generally believed that as long as the explosive dust meets the conditions 1 and 2, it means that there is a possibility that an accident may occur. Therefore, some places with poor air circulation, such as coal mines, flour mills, sugar mills, textile mills, sulfur plants, feed, plastics, metal processing plants and grain depots, are prone to dust explosions.


    Dust explosions often occur more than once, and it is highly probable that a second explosion will occur. The first explosion blows the dust deposited on the ground and on the equipment. In a short time after the explosion, the explosion center will form a negative pressure, sucking in the surrounding fresh air and mixing it with the raised dust, causing a secondary explosion. . In the second explosion, the concentration of dust is generally higher, so the power will be greater.


    Which link is the most problematic?


    According to official inion, the explosion was caused by an open flame that detonated the dust. That is to say, there were at least two safety hazards in the workshop: there was an open flame and dust reaching the explosion concentration. According to the regulations, the safety requirements of such workshops are extremely high, and there are strict regulations on dust concentration, ventilation and open flame.


    The employees of the nearby company said that the company went to work at 7:30 in the morning. The time of the accident was during the shift between the morning shift and the evening shift. The personnel were confused, the accident occurred or related to it.


    Xiao Wang, an employee of the accident company, said that he had seen a pipe that sucked dust, which was filled with ash, but did not see anyone cleaned up.


    Zhong Shengjun, an associate professor at the Institute of Industrial Explosion and Protection of Northeastern University, said in an interview with the Express reporter that although the cause of the accident is still under investigation, the polishing process is similar, mainly consisting of a polishing machine and a dust removal system. Among them, the dust removal system is the most likely to cause Dust gathers and causes an explosion.


    According to the statistics of Dongda Explosion Protection Institute, from 2010 to May 2011, there were 8 metal polishing dust explosion accidents in China, which led to the tragedy of 11 deaths and 53 injuries, accounting for about 30% of the dust explosion accidents in the same period.


    Zhong Shengjun said that the incident factory polished the alloy wheels and may produce magnesium alloy powder. This composition is similar to the mixture of magnesium, aluminum and iron powder in fireworks. Electrostatics, mechanical friction, collision, electrical failure and spontaneous combustion are all possible sources of ignition. However, for news reports on metal polishing dust explosions, the ignition is usually attributed to Electrical equipment failure, or the cause of the accident is being perfunctory in the investigation, and the underlying reasons for poor supervision have never been mentioned.


    Judging from the news reports, the above accidents have one thing in common: an explosion occurred in the dust removal system, causing casualties, and a second explosion occurred in the workshop. Zhong Shengjun analyzed that in the past, the polishing process unit was small in scale, and the dust removal system was separately set, and the danger was relatively dispersed. Now, the centralized dust removal system connects multiple polishing process units together, and in the event of an accident, the degree of damage is greatly increased.


    Zhong Shengjun said that the current dust removal system distributes two types of bag filters and water filters. Because of the low maintenance cost, the former is favored by enterprises, accounting for more than 90% of all dust removal systems, but there is often a problem: the ventilation and dust removal system is not strictly in accordance with the specifications, resulting in dust accumulation. For example, the control wind speed does not meet the requirements, or the ventilation dust removal system is not set according to the requirements of explosion protection, including the setting of the pipeline, the setting of series and parallel connection, whether the selection of the dust collector is explosion-proof, whether there is Mars detection and extinction measures.


    Yin Chengliang, deputy dean of the Automotive Engineering Research Institute of Shanghai Jiaotong University, also believes that the accident was caused by the company neglecting the dust collection equipment and dust warning device for cost saving.


     


    Source: China Aluminum Network


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