Explosion protection and environmental protection in the production of metal containers
In the manufacture of metal containers, it is inseparable from the drying of the paint. Before the production of tinplate cans, it is necessary to carry out the printing of iron. Even if pre-coated steel plates are used, pre-painting of steel plates is also required. When cold-rolled plates are used to produce metal containers, they must be printed. Therefore, the printing of iron and coating is an important process for the production of metal containers, but also the main way to cause environmental pollution, but also an important part of safe production. Over the years, explosions caused by painting have been blazing new trails. Environmental pollution caused by painting has become a target of public criticism.
In recent years, more and more attention has been paid to the explosion-proof and anti-pollution treatment of paint drying ovens for metal containers at home and abroad. There are many new technologies and new equipments, but for most companies, the most important issue is still Know and value. Here, we combine the latest achievements at home and abroad to talk about the explosion-proof and anti-pollution problems in the production of metal container printing and coating.
Explosion proof coating drying furnace
In the production of metal containers, when the coating film is dried in a curing oven, most of the organic solvent present in the coating film will gradually volatilize. In an environment where the temperature of the furnace is 150°C to 350°C or higher, the solvent may volatilize when mixing with air in the paint, which is extremely dangerous.
There are usually two ways to prevent explosions:
One is to control the organic solvent content in the furnace atmosphere. Use necessary monitoring means to place it at 1/4, 1/6 or even 1/8 of the explosion limit
the following. Once the organic solvent content reaches the limiting concentration, the coating equipment will automatically interrupt the coating. In addition, the amount of ventilation and exhaust capacity can be designed based on the coating used and the production speed to ensure that the amount of volatile organic solvent does not exceed the control standard.
The second is to control the oxygen content in the furnace atmosphere, that is, using the so-called inert gas method, the furnace gas is divided into two pipelines, all the way to the burner to burn again, and the other way to mix with the high temperature hot air burner output, and then enter the furnace This reduces the oxygen content in the furnace atmosphere.
When the content of organic solvent is higher than the explosion limit, the increase of oxygen content tends to make the atmosphere tend to be explosive. If the pressure in the furnace is reduced, the introduction of air will increase the risk of explosion. Therefore, it is required that the smaller the furnace mouth, the better. However, the furnace mouth is too small, which is inconvenient for the production operation. The extremely narrow furnace mouth does not work in practice.
Japan Ocean Co., Ltd. adopts the method of reducing the oxygen content in the furnace to control it at 3% to 5%, and solves the explosion problem of the heating furnace. This method can allow the volatile content of solvent in the furnace gas to be relatively high and can still be recycled. However, to ensure safety, equipment and control system requirements are very high.
Hot air heating exhaust gas circulation and secondary incineration
The hot air in the drying oven still has a relatively high temperature at discharge, usually 400° C. or even higher, and the hot air contains a large amount of organic solvent volatiles in the coating. Therefore, it cannot be directly discharged into the atmosphere to avoid air pollution. All countries and countries in the world have strict regulations on environmental protection and require that the total VOC emissions be reduced year by year. If they are burned, the method is simple, but it wastes a lot of heat, making the heat efficiency of the furnace only 10%. % to 15%.
Nowadays, there are many large-scale drying furnaces in the world that adopt furnace gas circulation and secondary incineration methods to create a special secondary incineration system. Using the heat energy in the exhaust gas, the contained solvent is also converted into heat energy for use. This not only avoids the environmental pollution of the solvent, but also greatly improves the thermal efficiency of the curing oven. The thermal efficiency of the furnace for furnace gas circulation and secondary incineration is close to 80%.
Non-hot air heated exhaust gas treatment
When far-infrared heating or induction heating is used for curing, the amount of exhaust gas is small, and the concentration of organic solvents is high. Cooling or catalytic oxidation is usually used to treat the exhaust gas.
1. Condensation method: Liquid nitrogen gasification cooling is used to recover organic solvents. The exhaust gas containing the solvent is usually cooled by a heat exchanger, and the solvent is collected as a liquid, so that a highly pure and reusable solvent can be obtained, and the recovery rate of the solvent is also 90% to 95%. The vaporized nitrogen can also be returned to the furnace to maintain the inert atmosphere in the furnace.
2. Catalytic Oxidation: Catalytic oxidation of organic solvents in exhaust gas using heavy metal catalysts. The disadvantage of this method is that the exhaust gas needs to be preheated to about 400°C to perform the oxidation reaction, which consumes a certain amount of heat. In addition, the catalyst is prone to poisoning and requires frequent replacement.
Wastewater treatment
The waste water produced in the production of metal containers is mainly due to the dripping of waste water before the product is painted or the waste liquid that has been replaced. Including degreased alkaline wastewater, zinc-containing, fluoride-containing phosphating wastewater, chromium-containing passivation and activated wastewater. Pollutants are mainly hexavalent and trivalent chromium ions, zinc ions and fluoride ions. The purpose of wastewater treatment is to reduce their content in the drainage and avoid their environmental pollution. The general method of wastewater treatment is to chemically treat the wastewater and react these ions with added chemicals to produce difficult-to-dissolve precipitates, which are then separated from the water and processed separately.
The specific approach is to first reduce Cr6+ wastewater to make it a Cr3+ wastewater, and then adjust the pH
As a result, Cr3+ is precipitated as Cr(OH) and separated from the solution. At the same time, the acidic waste water containing defatted alkali and phosphatized waste liquid were mixed, and the pH was adjusted with alkali to precipitate heavy metals therein. The part of the water separated from the precipitate is mixed with the chromium-containing waste water treated as mentioned above. Through the precipitation process, various precipitates are separated out, and finally the pH is adjusted again.
After the value is discharged, the precipitate is filtered out, dried, and treated separately.
Waste residue treatment
The waste generated during the coating process is generated from waste paint, sludge formed by overspray, and pretreatment sludge.
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