ã€China Aluminum Industry Network】 The common surface treatment of aluminum products such as aluminum alloy and aluminum profiles is polishing and grinding, which will produce a large amount of aluminum powder and dust.
Why does aluminum powder and dust cause explosion of aluminum powder and dust explosion?
What is aluminum powder: Also called silver powder, alias aluminum silver powder, good heat transfer performance. The fire hazard of aluminum powder is that its dust is mixed with air and it will explode and burn in the event of Mars. Aluminum powder mixed with other oxidants can also form explosive mixtures. Hydrogen is generated when it comes into contact with acids and alkalis, and hydrogen gas is easily combusted and explodes in the air.
The reason for the explosion of Kunshan Zhongrong Metal Products Co., Ltd. has been to find out that the aluminum powder and dust generated during the polishing of aluminum alloy wheels encountered explosions of aluminum powder and dust explosions in an open flame, causing huge casualties.
There are many types of aluminum alloy polishing. It is generally marked with polishing wax and then cast with cloth wheels, so naturally there will be a lot of aluminum dust. However, general aluminum alloy processing plants have a suction-drainage dust exhaust device on the edge of the polishing machine. Therefore, aluminum powder and dust do not have so much concentration.
Aluminum powder is a secondary water-fired item that reacts with water energy to produce hydrogen gas that releases heat.
Sometimes the dry aluminum powder explodes. This is because the tiny dry aluminum powder particles can be suspended in the air, increasing the contact surface with the air and increasing its chemical activity. Once the dust reaches a certain level in the air, When it is in quantity, it encounters a source of fire and can quickly deflagrate, instantly producing a large amount of heat and combustion products, causing the gas, steam, etc. to expand dramatically, causing the consequences of the explosion.
When this concentration reaches the explosive concentration range, an explosion will occur due to a certain ignition energy. If the aluminum powder is too much, it may cause the aluminum powder to rise again and form a secondary explosion, called an explosion.
Aluminum powder can also explode, it is estimated that many people do not know. Specifically, the long-term polishing operation caused a lot of aluminum powder in the air of the workshop, and the ignition point of aluminum is quite low. As long as the source of fire or high temperature and friction are encountered, it may explode.
Because aluminum reacts with water to produce hydrogen, this is why water cannot be used to extinguish the fire. There have been instances where water was used to extinguish the explosion of aluminum powder and cause another explosion.
Fire caused by explosion of aluminum powder can not use water, but should use a foam fire extinguisher to fire (aluminum processing). Cover the fire with air by covering it with foam. This is because aluminum reacts with water to produce hydrogen, which cannot be extinguished by water. There have been instances where water was used to extinguish the explosion of aluminum powder and cause another explosion. Most people do not know that aluminum powder can also explode. In particular, long-term polishing operations cause a lot of aluminum powder to accumulate in the air inside the workshop. The ignition point of aluminum is quite low, as long as it encounters fire or high temperature and friction. It can all explode.
Aluminum dust characteristics: in the air encountered a small source of ignition that can cause fire; aluminum powder in the air stained with grease, such as long-term accumulation of storage, heat is not scattered, but also easily lead 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 in the event of an open flame. The flame temperature is high, the burning speed is fast, the explosion is powerful, and the radiant heat is strong. When burning, it is generally a green-blue flame that emits strong silver-white light with an explosion pressure of up to 6.3 kg/cm2. It has greater destructive power and harm to the surrounding buildings and personal safety.
How to save after an aluminum dust explosion?
Combustible Metal Powders Aluminum powder compounds tend to form explosive dust. In case of fire, cover with asbestos blanket (or sand) and save with water. Selection of Fire Extinguishers In order to effectively extinguish the fire of aluminum powder, fire extinguishing agents must be properly selected and used. First of all, aluminum powder can not be fired with water and foam because of the fire. This is because the surface of aluminum powder usually has an oxide film that does not react with water. However, the new aluminum powder or aluminum oxide film that has fallen off and the aluminum powder that is burning or under high temperature baking on the fire field will rapidly react chemically, releasing hydrogen and the air with the risk of explosion burning to form an explosive mixture. Therefore, aluminum powder fires are prohibited from using water and foam.
Second, aluminum powder can react with chlorine and bromine at room temperature, and it can react with halogenated alkanes to produce a small amount of aluminum chloride, which catalyzes and often leads to explosive combustion. Therefore, aluminum powder fires can not be saved with carbon tetrachloride and 1211 fire extinguishing agents.
Aluminum powder has a light weight and a small fineness, and can easily form an explosive mixture in the air when it encounters wind or air spray. Therefore, aluminum powder fires cannot be saved with gas extinguishers such as carbon dioxide.
According to the above characteristics, the use of 7150 fire extinguishing agent or chemical dry powder, dry sand, graphite powder, dry magnesium powder, etc. shall be used for extinguishing the fire of aluminum powder. Because this type of fire extinguishing agent can cover the surface of the combustion aluminum powder, make it isolated from the air, and can effectively prevent the aluminum powder from being mixed with the air, so as to achieve the purpose of suffocating the fire.
What is an aluminum dust explosion?
That is, within the limits of explosion, the dust meets the heat source (open flame or temperature), and the flame spreads instantaneously throughout the mixed dust space. The chemical reaction speed is extremely fast. At the same time, a large amount of heat is released to form a very high temperature and a large pressure. The energy is transformed into mechanical work as well as light and heat radiation, which has a strong destructive power.
Aluminum dust explosion characteristics
(1) Multiple explosions are the major features of dust explosions;
(2) The smaller ignition energy required for dust explosion is generally higher than tens of millijoules;
(3) Compared with the flammable gas explosion, the dust explosion pressure rises slowly, the high pressure lasts for a long time, the released energy is large, and the destructive power 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 severe. This is because the content of carbon and hydrogen in dust is high, that is, the content of fuel is large. If the comparison is made according to the higher value of the generated energy, the dust explosion is several times larger than the gas explosion. The temperature can reach 2000-3000°C and the larger explosion pressure is 345-690kpa. (4) The dust combustion must be heated and melted, dissociated, and Evaporation and other complex processes, the time from the exposure of the dust to the explosion of the time required for the explosion of the dust to be longer than the gas explosion, up to tens of seconds; (5) Dust explosion can cause dust in other parts of the building to re-explosive. And the second explosion pressure is greater than the pressure of the previous explosion and is more destructive.
How to prevent aluminum dust explosion
The first is to reduce the concentration of dust in the air. Use well-sealed equipment to minimize dust emission, and 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. Where there is a danger of dust explosion, explosion-proof motors, explosion-proof lights and explosion-proof switches shall be used. Fourth, the scope of the explosion should be controlled in advance. The fifth is to control the temperature and oxygen level. Where there is a container for dust deposition, there must be cooling measures, and if necessary, inert gas can be filled to dilute the oxygen content.
Fire-fighting measures and emergency plans for grinding and polishing workshops of aluminum products such as aluminum alloys and aluminum profiles
1. Build a strong fire headquarters. Once the aluminum powder explodes and burns, it often causes casualties, and the situation of the fire field is more complex and there is a risk of secondary explosion. Therefore, a strong fire headquarters should be set up during the rescue process to take a unified look at the entire fire fighting mission. The main task of the fire command headquarters 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, evacuates materials, and supplies fire extinguishing agents (sand-blasting sand).
2. Be careful to use extinguishing agents correctly. The 7150 fire extinguishers and chemical dry powder fire extinguishers are powered by compressed nitrogen and spray the chemicals on the combustion. Therefore, when using these two types of fire extinguishers, the injection pressure should not be too large to prevent the explosion of dust. When dry sand and asbestos are used for extinguishing fires, the “one, two, and three buried†methods can be used. That is, when the fire is siege, the burning aluminum powder is firstly dried around with a dry sand to a certain extent. It is covered with asbestos, and later it is buried gently with sand (usually 30-50 cm thick). In addition, the fire extinguishing agents described above can also be used in combination. If it is a fire in the production workshop, use chemical dry powder or 7150 fire extinguishing agent to extinguish the combustion of the kerosene pool (tank) and some equipment used in the production of the workshop. Then, in the case of relative temperature reduction in the workshop, use 7150 fire extinguishing agent. , asbestos quilt and dry sand cofferdam burning aluminum powder.
3. Block first, then encirclement. If a small amount of aluminum powder is burned without forming a large-scale fire, quick tactics may be adopted to use the asbestos quilt and dry sand to directly eliminate the fire. If an explosion occurs to form a large area of ​​combustion, the tactics of blocking fire first and then extinguishing it should be adopted.
The problems that should be paid attention to when the aluminum powder fire is extinguished
According to the characteristics of burning aluminum powder fire, the following issues should be noted during the rescue process.
1. In the fight, the window must not be opened at will to prevent the formation of a flying hall, so that aluminum powder will fly, causing explosions and wounding.
2. Firefighters should pay attention to observing and using topographical features as cover to prevent radiant heat, hot bodies, collapsed houses, and explosions causing casualties.
3. Make full use of safety equipment and equipment to protect yourself and facilitate team members' actions.
4. Timely shut down the various valves of the ball mill and the process system in the workshop. Stop, power off, and then perform the rescue.
In order to understand the causes of the aluminum dust explosion in Kunshan, first of all, it is necessary to clarify what is a dust explosion.
A dust explosion is an explosion that occurs when combustible dust suspended in the air touches an open flame or spark or other source of ignition. Because of the tiny particles of solid matter, it has a much larger surface area than the same mass of massive material, so it is very It is easy to catch fire. Common flammable dusts include: metals (such as magnesium powder, aluminum powder), coal, food (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 flour), synthetic materials (such as plastics, dyes).
Dust is flammable and does not necessarily mean that it will explode. Research results show that dust explosions generally have five conditions:
1. The dust concentration is moderate. When the concentration is low, the distance between dust particles is too large, the flame is difficult to spread and will not explode; if the concentration is too high, the oxygen will be insufficient and it will not explode (generally, the explosive concentration is reached when the dust is between 30 and 40 grams per cubic meter of aluminum and magnesium. a)
2. Have a certain amount of oxygen. This is a necessary condition for dust burning.
3. Have enough ignition source. The smaller 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 must 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 Condition 1 and Condition 2, it means that there is a sign that an accident may occur. Therefore, coal dust, flour mills, sugar mills, textile mills, sulfur plants, feed, plastics, metal processing plants, and grain warehouses are all prone to dust explosions in places with poor air circulation.
Dust explosions often occur more than once, and secondary explosions are most likely to occur. The dust that was deposited on the ground and the equipment was blown by the previous explosion, and in the short time after the explosion, the explosion center formed a negative pressure, sucked in the surrounding fresh air, and mixed with the raised dust, causing secondary explosions. . In the second explosion, the dust concentration is generally higher, so the power will be greater.
Which part is more prone to problems
According to official disclosures, the explosion was due to the open flame detonating dust. In other words, there were at least two safety hazards in the workshop at the time: there was an open fire and dust reaching explosive concentrations. 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.
Employees in nearby companies said that the company went to work at 7:30 in the morning and the time of the accident was during the handover between the morning and evening shifts. The staff was confusing, accidents occurred or related to this.
The company’s employee, Xiao Wang, said that he had seen a dust-collecting pipeline with ash inside, but he did not see anyone cleaning it.
Zhong Shengjun, associate professor of the Institute of Industrial Explosion and Protection at Northeastern University, said in an interview with Express reporter that although the cause of the accident is still under investigation, the polishing process is very similar. It is mainly composed of a polishing machine and dust removal system, among which the dust removal system is more likely to cause Dust builds up and causes an explosion.
According to the statistical data of the Dongda Explosion Protection Institute, from 2010 to May 2011, there were 8 metal polishing dust explosions in China and 11 deaths and 53 injuries were caused, which accounted for about 30% of dust explosions 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 powders in fireworks. Electrostatic, mechanical friction, collisions, electrical faults, and spontaneous combustion are all possible sources of ignition. However, for news reports on metal-polishing dust explosions, the cause of ignition is often attributed to The failure of electrical equipment, or the use of accidents, is being perfunctory in investigations and has never mentioned the underlying causes of poor supervision.
According to news reports, the above accidents have one thing in common: An explosion occurred in the dust removal system, causing casualties, and both occurred in the workshop. Zhong Shengjun analyzed that in the past, the size of the polishing process unit was small, and the dust removal system was installed separately. The risk was relatively dispersed. Now, the centralized dust removal system connects several polishing process units together. Once an accident occurs, the degree of damage is greatly increased.
Zhong Shengjun said that the current dust collection system has two types of bag filter and water filter. Because the maintenance cost is low, the former is favored by enterprises and accounts for more than 90% of all dust removal systems. However, there is often a problem that the ventilation and dust removal systems do not strictly comply with the regulations and cause dust accumulation. For example, the control of wind speed does not meet the requirements, or the ventilation and dust removal system is not set in accordance with the requirements of explosion protection, including the setting of the pipeline, the series connection in parallel, whether the selection of the dust collector is explosion-proof, whether there is Mars detection and extinguishing measures.
Yin Chengliang, deputy director of the Automotive Engineering Research Institute of Shanghai Jiaotong University, also believes that this accident or due to the company's neglect of the exhaust dust collection equipment, dust warning devices to save costs.
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