Why can aluminum powder, dust that aluminium alloy surface polishing produce explode?
Aluminum alloy, aluminum profile and other aluminum products common surface treatment is polishing, grinding, will produce a lot of aluminum powder, dust.
Why do aluminum powder and dust explode and dust explode?
What is aluminum powder: also called silver powder, alias aluminum silver powder, heat transfer performance is good.
The fire danger of aluminum powder is that its dust flies and mixes with the air, causing an explosion and combustion in the event of a spark.
Aluminum powder mixed with other oxidants can also form an explosive mixture.
Contact with acids and bases produces hydrogen gas, which can easily burn and explode in air.
The cause of the explosion of Kunshan Zhongrong Metal Products Co., LTD has been found out, which is caused by a large amount of aluminum powder and dust produced by aluminum alloy wheel hub during polishing.
There are many kinds of aluminum alloy polishing.
It is to hit on polishing wax to use cloth wheel to throw commonly, there is a lot of aluminium dust naturally so.
But the average aluminum alloy processing plant is a polishing machine on the side of a large suction exhaust dust device.
So the aluminum powder and the dust don't accumulate that much.
Aluminum powder is a secondary combustion object with water, which reacts with water to produce hydrogen and heat.
Sometimes dry aluminum powder explosion, this is because the tiny particles drying aluminium powder can be suspended in the air, increased with the air contact surface, increase its chemical activity, once reach a certain amount of dust in the air, the ignition source, can quickly deflagration, instantly create a lot of heat and combustion products, make the gas, steam, such as severe inflation, the explosion consequences.
When this concentration reaches the range of explosion concentration, an explosion will occur under the influence of certain ignition energy. If there is too much aluminum powder, it may cause the aluminum powder to be raised again and form a second explosion, which is called martyrs explosion.
Aluminum powder can also explode, probably many people do not know.
To be specific, the long polishing operation filled the air of the workshop with a lot of aluminum powder, and aluminum ignition point is quite low, any encounter with a fire or high temperature, friction, can explode.
Aluminum reacts with water to produce hydrogen, which is why you can't use water to put out a fire.
There have been previous instances of a fire using water to extinguish aluminum powder and trigger another explosion.
Fire caused by aluminum powder explosion should not be put out by water, but by foam extinguisher (aluminum extrusion processing).
Through the foam cover, the fire and air isolation.
That's why you can't put out a fire with water because aluminum reacts with water to produce hydrogen.
There have been previous instances of a fire using water to extinguish aluminum powder and trigger another explosion.
Most people do not know that aluminum powder can also explode, especially after a long period of polishing work so that the air in the workshop is filled with a lot of aluminum powder, and aluminum ignition point is quite low, as long as the source of fire or high temperature, friction, may explode.
Characteristics of aluminum dust: it can be ignited when it encounters a small ignition source in the air;
The aluminum powder with grease in the air, such as long-term storage, heat collection and retention, is also prone to spontaneous combustion or explosion. Moreover, the smaller the particle size of aluminum powder is, the greater the explosion risk will be. When the concentration of aluminum powder in the air reaches more than 40g/m3, it will explode in the presence of open fire.
The flame has high temperature, fast burning speed, large explosive power and strong radiant heat.
When burning, it usually shows green blue flame and emits silver white strong light. The explosion pressure can reach 6.3 kg/cm 2.
It is destructive and harmful to the surrounding buildings and personal safety.
How to put out after happening aluminium dust explosion accident?
Flammable metal powder aluminum powder compound is easy to form explosive dust.
Cover with asbestos felt (or sand) before extinguishing with water.
Selection of Fire Extinguishers In order to effectively extinguish aluminum powder fires, the correct selection and use of fire extinguishers must be.
First of all, in case of fire, aluminum powder cannot be put out with water or foam. This is because there is usually a layer of oxide film on the surface of aluminum powder, so it will not react with water.
However, the newly made aluminum powder or aluminum powder with peeling oxide film, as well as the aluminum powder burning on the fire site or under high temperature baking will quickly undergo chemical reaction, releasing hydrogen gas which is dangerous for explosive combustion and forming explosive mixture with air.
Therefore, aluminum powder fire is prohibited to use water and foam extinguishing.
Secondly, aluminum powder can react with chlorine and bromine at room temperature, and react with halogenated alkanes to produce a small amount of aluminum chloride, which often leads to explosive combustion.
Therefore, aluminum powder fire can not be put out with carbon tetrachloride, 1211 extinguisher.
Aluminum powder is light in proportion and small in fineness. It is easy to form explosive mixture in the air in case of wind or air spray.
Therefore, aluminum fire can not use carbon dioxide and other gas extinguishers to put out.
According to the above characteristics, 7150 extinguishing agent, chemical dry powder, dry sand, graphite powder and dry magnesium powder should be used for extinguishing.
Because this kind of extinguishing agent can cover the surface of burning aluminum powder, make it isolated from the air, and effectively prevent aluminum powder flying and mixing with the air, so as to achieve the purpose of smothering the fire.
What is aluminum dust explosion?
Dust is within the scope of the explosion limit, encounter heat source (open flame or temperature), flame instantaneous spread to the whole mixed dust in space, the chemical reaction speed, release a large amount of heat at the same time, the formation of high temperature and a lot of pressure, the energy of the system is converted into mechanical work, as well as light and heat radiation, has a strong destructive power.
Characteristics of aluminum dust explosion
(1) Multiple explosions are the biggest characteristic of dust explosions;
(2) The minimum ignition energy required for dust explosion is relatively high, generally above tens of millijoules;
(3) Compared with the explosion of combustible gas, dust explosion pressure rises slowly, the higher pressure lasts for a long time, the energy released is large, and the destructive force is strong.
Dust combustion rate is lower than gas, because of its long combustion time and large energy generation, so the destruction and burning degree is much more serious.
This is because of the high content of carbon and hydrogen in the dust, that is, more combustible content.
If compared according to the highest value of energy produced, dust explosion is several times that of gas explosion, the temperature can reach above 2000-3000℃, and the maximum explosion pressure is 345-690kPa.
(4) Dust combustion goes through complex processes such as heating, melting, dissociation and evaporation, and the time required for dust from contact with fire source to explosion, i.e. induction period, is longer than that of gas explosion, up to tens of seconds.
(5) Dust explosion can cause dust explosion in other parts of the building.
Moreover, the pressure of the second explosion is higher than that of the first explosion, which is more destructive.
How to prevent aluminum dust explosion
One is to reduce the concentration of dust in the air.
Adopt airtight equipment with good performance to reduce dust flying and escaping as far as possible. Meanwhile, install effective ventilation and dust removal equipment to strengthen cleaning work.
2 it is to want to control indoor temperature.
The third is to improve the equipment, control the fire source.
Explosion-proof motor, explosion-proof lamp and explosion-proof switch should be used in places with dust explosion danger.
Fourth, the scope of the explosion should be controlled beforehand.
Five is to control the temperature and oxygen content.
Where there is dust deposition, there should be cooling measures, and if necessary, inert gas can be filled to dilute the oxygen content.
Fire fighting measures and emergency plan of grinding and polishing workshop for aluminum alloy, aluminum profile and other aluminum products
1.
Establish a strong fire site command.
Once the aluminum powder explodes and burns, it often causes casualties. Moreover, the situation at the fire site is complex, and there is a risk of secondary explosion.
Therefore, in the process of fighting and rescue should be set up a strong fire site headquarters, to the entire fire task for unified consideration.
The main tasks of the fire headquarters are to quickly organize forces to detect the fire, formulate correct tactical measures, arrange combat tasks, issue combat orders, mobilize vehicles to rescue people, evacuate supplies, supply fire extinguishing agents (sand transport) and so on.
2.
Pay attention to the correct use of fire extinguishing agent.
Both 7150 fire extinguishers and chemical dry powder fire extinguishers use compressed nitrogen as the power to spray the agent on the burner. Therefore, the jet pressure should not be too high when using these two fire extinguishers to prevent dust from blowing up.
In the use of dry sand, asbestos is fire, can use a 'surround, two cover, three buried' method, that is, in the siege of the fire, the first dry sand will burn aluminum powder from around, around to a certain extent, then covered with asbestos, finally, gently buried with sand (generally 30 to 50 cm thick sand).
In addition, the above fire extinguishing agent can also be used in combination.
If there is a fire in the workshop, you can first use chemical dry powder or 7150 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, 7150 extinguishing agent, asbestos and dry sand are used to eliminate the burning aluminum powder.
3.
First stop, then kill.
If a small amount of aluminum powder combustion does not form a large range of fire, we can adopt the tactics of quick battle, using asbestos to be, dry sand directly eliminate the fire.
If there is a large area of combustion caused by an explosion, the tactics of first blocking and then extinguishing the fire should be adopted.
Problems to be paid attention to in fire of extinguishing aluminum powder
According to the characteristics of aluminum powder fire, the following problems should be paid attention to in the process of fighting and rescuing.
1.
In the rescue, do not open the window at will, in order to avoid the formation of a draught aluminum powder flying, causing explosions.
2.
Fire fighters should observe and use terrain and ground objects as cover to prevent casualties caused by radiant heat, hot bodies, house collapses and explosions.
3.
Make full use of safety equipment and equipment to protect themselves, so as to facilitate the movement of team members.
4.
Timely shut down the ball mill in the workshop and various valves of the process system, first stop, power off and then put out.
If we want to understand the cause of kunshan aluminum dust explosion accident, we must first understand what is dust explosion.
Dust explosion is an explosion phenomenon that occurs when combustible dust suspended in the air touches an open flame or an electric spark. Because tiny particles of solid matter have a much larger surface area than lumps of the same weight, they are easy to catch fire.
Common inflammable dust includes: metals (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 meal, wood meal), and synthetic materials (such as plastics and dyes).
Just because dust is combustible does not mean it will explode.
Research results show that dust explosions generally have five conditions:
1. Moderate dust concentration.
Low concentration, dust particles spacing is too large, flame is difficult to spread, will not explode;
If the concentration is too high, there will be insufficient oxygen and no explosion (generally 30 to 40 grams of aluminum and magnesium dust per cubic meter will reach the explosion concentration).
2. Have enough oxygen.
This is a necessary condition for dust combustion.
3. Have enough ignition sources.
The minimum ignition energy required for a dust explosion is 1-2 orders of magnitude greater than that required for a gas explosion.
4. Dust must be in the dust cloud state, that is, in suspension 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 conditions 1 and 2, it means that there is a potential for accidents.
Therefore, coal mines, flour mills, sugar mills, textile mills, sulfur mills, feed, plastics, metal processing plants and grain warehouses and other places with poor air circulation are prone to dust explosions.
Dust explosion is often more than one, it is very possible to have a second explosion.
The dust deposited on the ground and equipment is blown up by the first explosion, and within a short time after the explosion, negative pressure will form in the explosion center, drawing in the surrounding fresh air and mixing with the dust raised, thus causing the second explosion.
The second explosion, dust concentration is generally higher, so the power will be greater.
Which link is most likely to go wrong
According to the official information, the explosion was caused by an open fire which ignited the dust.
That is to say, there were at least two safety hazards in the workshop: open fire and dust reached explosive concentration.
According to the regulations, the safety requirements of this kind of workshop are extremely high, whether dust concentration, ventilation or open flame has strict rules.
Employees at nearby businesses said the accident occurred or was related to the 7:30 a.m. shift at the business, which occurred during the transition between morning and night shifts.
Xiao Wang, an employee of the company in question, said he had seen a dust-sucking pipe with ashes piled inside, but had not seen anyone clean it.
Zhong Shengjun, associate professor at the Institute of Industrial Explosion and Protection at 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 and mainly consists of polishing machines and dust removal systems, among which dust removal systems are the most likely to cause dust accumulation and explosion.
According to the statistics of dongda Explosion protection Bureau, from 2010 to May 2011, there were 8 metal polishing dust explosion accidents in China, resulting in 11 deaths and 53 injuries, accounting for about 30% of the dust explosion accidents in the same period.
The plant polished the alloy wheels, which could produce magnesium and aluminum powder, Mr. Chung said.
The composition and the fireworks in the magnesium, aluminum, iron powder mixture, electrostatic, mechanical friction, collision, electrical fault and spontaneous combustion is possible ignition source, but for the metal polishing dust explosion of news reports, tend to attribute the ignition reason for electrical equipment failure, or the cause of the accident is under investigation perfunctory, never mention the deep reasons of the poor regulation.
According to news reports, the above accidents have one thing in common: there was an explosion in the dust removal system, and the casualties were all caused by a secondary explosion in the workshop.
Zhong Shengjun analyzed that in the past, the polishing process unit was small in scale, and the dust removal system was set up separately, so the risks were relatively dispersed.
But now the centralized dust removal system connects multiple polishing process units together, once the accident occurs, the harm degree greatly increases.
Zhong shengjun said that the current dust collection system distribution bag dust collector and water dust collector two.
Due to low maintenance cost, the former is favored by enterprises, accounting for more than 90% of all dust removal systems. However, a problem often occurs: the ventilation dust removal system does not strictly follow the specifications, resulting in 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 pipelines, the setting of series and parallel, the selection of dust remover whether explosion protection, Mars detection and extinguishing measures.
Yin Chengliang, deputy director of the Automotive Engineering Research Institute at Shanghai Jiaotong University, also believes that the accident may have been caused by the company's neglect of the exhaust dust collecting equipment and dust warning device in order to save costs.