Analysis and Countermeasures of Accidents in Aluminum Smelting Furnace Explosion

Keywords: aluminum smelting furnace; explosion accident; explosion limit; reason; countermeasure

1 Overview of explosion accidents

A factory gas melting aluminum furnace exploded during the melting of aluminum alloy. In the accident team, the aluminum alloy ingot was cast by the semi-continuous method. After the aluminum liquid in the furnace was cooled, the molten material was continuously smelted without cooling down or cleaning the bottom of the furnace. When charging the furnace, the charging truck first pours a box (1.5 t) of 3A21 (LF21) mixed high-magnesium aluminum alloy waste material into the silo, and then divides it into 5 furnaces, and then adds 5A05 (LF5) to the furnace. The remainder of the press and the 1.48t aluminum wire were divided into four tanks. The first and second tanks were added to the area, and an explosion occurred in the second tank.

The flames burned when the facer, neck, and hands of the Tiancheong who is working on the material are exploded; the 25mm thick organic windshield is broken into three pieces by the shock wave generated by the explosion and the controllers are deformed. Gas furnace furnace arches and vertical flue arches are all collapsed; firebricks on the south side of the furnace walls are separated from the heat-insulating bricks; drums on the southeast side of the furnace shell steel plate are deformed; burners on the burner head are displaced; gas and air ducts are blown at the top. The section of the flange butt joint above the area, the pull-pull on the furnace shell, the accident flap of the gas branch pipe in one area was torn; the window frame and glass of the gas-melting aluminum furnace plant were all damaged, and other adjacent buildings were damaged. Both the property and the factory were damaged in varying degrees.

After the explosion, the on-site conditions are as follows: the two furnace doors are fully open, the flue gate is in the ascending state, and the feeding crane is in front of the furnace door in one zone, and the feed trough is tilted in the furnace. In the vicinity of the furnace door slag pan in one zone, there were 7 pieces of residual remnants of cake and several aluminum wires, but there were no traces of molten aluminum.

2 Cause Analysis

Normally explosions can be divided into physical explosions and chemical explosions. Physical explosions are due to sudden changes in phase and pressure. For example, the explosion of molten aluminum caused by the mold water during the casting of molten aluminum is a chemical explosion caused by the extremely rapid chemical reaction of the material and the explosion caused by the high temperature and high pressure. For example, explosion caused by combustible gas such as gas and natural gas mixed with air. The conditions required for the explosion of a flammable mixture are: (1) Concentration of the flammable mixture is within the explosion concentration limit; (2) There is sufficient ignition energy.

The destruction energy of the blast wave of the accident explosion is expressed in terms of TMT equivalent. Calculate using formula (1)[1]:

U=1000(R/R')3

In the formula: U-- explosion accident shock wave TNT equivalent, kg TNT;

R-distance between the damage and the accident center, m;

R'--1000kg TNT ground explosion experimental data, m (see Table 1).

According to the accident site destruction situation and related accident investigation results, it was found that the “most of the damaged windows were damaged” within about 113.4 m from the explosion center. It can be known that the explosion was overpressure between 784kPa and 980kPa at 113.4m from the explosion center. From this, it was determined that R=113.4m, and R'=166m to 144m determine from this: R=I 13.4 m, R'=l 66 m to 144 m.

According to formula (1) available:

U Chong max=488.37kg TNT

U flush min=318.8 kg TNT

According to 90% of the total explosion energy of the shock wave energy, the explosion energy of this accident is approximately:

U max=U Chong max/0.9=542.6 kg TNT

U min=U Chong min/0.9=354 kg TNT

After the accident, some people thought it was due to moisture in the material or explosion caused by incomplete combustion of the gas. But at that time, the gas furnace instrument records that the gas and air pressure and flow rate had been stable for 1 h before explosion, and there was no obvious fluctuation. The ratio was in line with the process requirements and there could not be excess gas in the furnace, so the conditions for gas explosion were not enough. Accidents due to the moisture in the material and the dampness of the tool during the aluminum smelting process occurred occasionally, but never reached such a large destructive force, and no aluminum was found on the door water jacket, furnace floor, and feed car body on site. The splashing of the liquid does not match the splashing of molten aluminum when the water overheats in the molten aluminum. During the explosion, both the furnace door and the flue gate are open. Therefore, no accidental explosion of steam overpressure occurs in the furnace. This basically negates the possibility of this explosion caused by the moisture in the material.

A further analysis of the accident revealed that the liquid contained in the press pack made by pressing the aluminum scrap by the press packer was not completely water, but was actually the lubricant used in the press packer. The sampling of the remaining debris on site was 2.8%, and if each pack was calculated based on 50 kg, the oil content was 1.4 kg, and the total amount of oil containing 1.4 t was about 39.2 kg.

Gasoline, diesel, kerosene, and various lubricants are all petroleum products. They all have the property of evaporation. In contrast, the flash point of lubricants is higher. Some people mistakenly believe that the ignition of lubricating oil is not easy to burn, it can not be exploded. In fact, lubricating oil is a flammable liquid, which has the danger of burning and explosion in the presence of fire and oxidizer [2]. The lubricating oil in the baled material is vaporized into lubricating oil vapor by the high temperature effect in the gas fused aluminum furnace, and part of it is directly combusted with the oxygen in the furnace. The other part is composed of lubricating oil molecules with long carbon chain molecules in a high temperature environment of nearly 1000°C. Under the crack is short carbon chain of methane, ethane, ethylene, propane and hydrogen and other flammable gases.

The following is an example process of thermal cracking, including initiation, growth, and termination.

Initiation phase: C10H22→·C8H17+·C2H5

Growth stage: C2H5+C10H22→C2H6+C10H21

CH3(CH2)6 CH2CH2CH2·→CH3(CH2)6CH2·+CH2=CH2

CH3(CH2)4CH2CH2·CH2·→CH3(CH2)4CH2+CH2=CH2

CH3(CH2)2CH2CH2CH2·→CH3(CH2)2CH2·+CH2=CH2 etc.

The suspension phase is: 2H·→H2

2CH3→C2H6

CH3·+C2H5·C3H6 or CH4 to C2H4

Experiments show that the higher the temperature, the faster the cracking rate.

Table 2 shows the cracking data of diesel and butane at high temperature [3].

The flammable gas and flammable oil vapor produced by pyrolysis are partly burned off by the oxygen in the excess air in the furnace; the other part is discharged from the flue gas to the outside of the furnace; and some parts are accumulated in the furnace due to complicated flow factors such as eddy currents. An explosion occurs when there is air reaching a certain concentration, ie the explosion limit. See Table 3.

The explosive limit of fuel can be calculated using the following formula [5]:

l/L=0.137×Nc+0.04343

1/L on=0.137×Nc+0.05151

Where: Number of carbon atoms in Nc--hydrocarbons:

L--explosion limit of fuel

Take the average molecular formula of lubricating oil (30# oil) as C20H40. According to the formula above, the explosion limit of oil can be calculated as:

L upper = 3.135% = 365.8 g/m3

L lower = 0.3653% = 42.6 g/m3

Under the explosive limit state, due to the influence of factors such as disturbance and micro-negative pressure in the feeding, the outside air is mixed in, and an explosion occurs under the action of the gas fire source.

Based on the above analysis, the mechanical lubricating oil added to the furnace in the furnace is only a part of it participating in the explosion reaction. Since the explosive limits of materials and explosive power are not a fixed value, they are affected by some conditions and factors. Experiments have shown that the explosion intensity of CH4 in the flowing state is 9 times that of static. In addition, the high temperature of combustible gas, the large surface area, the fast flow velocity on the surface of the liquid, and the high detonation energy all make the explosion limit range wider and the explosion intensity increase sharply. At that time, the gas melting aluminum furnace fully possessed these special conditions.

After the above analysis, it can be inferred that the 9# gas melting aluminum furnace has the following three explosion conditions:

Combustible material: It is derived from the 30# oil brought in by the pressure-packed material and evaporates and cracks in a high-temperature environment to produce flammable gas.

Oxygen: derived from the excess air in the furnace at that time (the ratio of air to gas was 1.23:1 at that time, the ratio of 1.17:1 as stipulated in the gas safety technical regulations, there was surplus air in the furnace); the other source of oxygen was the time of feeding. Into the air.

Detonation energy: There is a large flow of gas burning in the furnace, there is an open flame and high temperatures.

The above three conditions led to the explosion.

3 Suggestions and Countermeasures

People have known about the explosion of wet aluminum containing materials used in aluminum melting furnaces. However, they do not have enough knowledge of oil and gas explosions, and they do not realize that under certain conditions, oils will produce explosive power. Measures should be taken from the following aspects to avoid the repetition of similar accidents.

(1) Training and education of oil knowledge to relevant management personnel and positions.

(2) The baler is strictly prohibited to work under the condition of oil leakage. Once the oil spills, it must be reported level by level, stop the operation immediately, repair and clean the site in time.

(3) The materials with serious oil pollution should be stored separately, and smelting can only be carried out after the oil removal measures such as washing with hot water and drying are adopted. The random sampling and crushing inspection shall be performed on the materials of the cost entity and the management of the receipt inspection shall be strengthened.

(4) Improve the control of the furnace instrumentation system and the requirements for gas quality. The ratio of gas and air flow must be adjusted to 1:1.17 and must not exceed the requirements specified in the safety technical regulations.