Steel drum painting and environmental protection (2)
-- Speech at the 5th Pan-Asian Steel Barrel Industry Conference
Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing
2. Exhaust gas
The source of the exhaust gas from the steel drum is mainly the exhaust of the spray booth and the drying tunnel. The exhaust gas from the spray booth is generated during the exhaust process of the spray booth. In order to ensure good working conditions, the spray booth needs to be ventilated within a wind speed range of 0.25 to 1.5 m/s. Therefore, the exhaust gas from the spray booth is characterized by a large amount of air and a very low concentration of organic volatiles (approximately 10-20 ppm). Inside). In addition to the organic volatile gas, the paint chamber exhaust gas contains a part of the paint mist formed by the overspray of the paint. The particle size of these paint mists is about 20 ~ 200 microns.
The exhaust of the drying tunnel mainly contains solvent vapor, thermal decomposition gas during coating film formation, and exhaust gas generated by combustion using gaseous fuel or liquid fuel. The harmful substances in such exhaust gases are mainly benzenes, esters, alcohols, ethers and ketones, as well as some amines and aldehydes.
The spraying chamber and the drying tunnel exhaust gas have direct combustion method, catalytic combustion method, absorption method and activated carbon adsorption method.
a. Direct combustion method
The direct combustion method is a method in which the exhaust gas generated during the painting and drying process is introduced into the combustion chamber, directly burned in contact with the flame, and the combustible components in the exhaust gas are burned and decomposed into odorless and harmless carbon dioxide and water vapor.
In order to prevent carbon monoxide in the exhaust gas from generating carbon monoxide due to incomplete combustion, in addition to supplying sufficient oxygen in the combustion chamber, it is necessary to have a combustion temperature of 650 to 800 ° C and a residence time of exhaust gas of 0.5 to 1.0 second.
The direct exhaust gas combustion system is composed of a burner, a combustion chamber, a preheater and the like. In order to achieve the desired combustion effect, the burner should be able to form a continuous and stable flame of complete combustion with a large combustion area and sufficient contact of the exhaust gas with the flame.
The direct combustion method system design considers the following points:
(1) Since the paint exhaust gas is a mixed gas containing a plurality of solvent vapors, when the concentration is close to the explosion limit value, combustion occurs, and an explosion occurs. Therefore, in order to prevent explosion, it is necessary to dilute to a predetermined degree of safety with air for a high concentration gas close to the lower limit of the explosion limit.
(2) When directly burning 怯 to treat the exhaust gas generated by painting and drying tunnel, in order to avoid the generation of photochemical smog NOx, the combustion temperature should not exceed 800 °C.
(4) Consider the use of waste heat.
Direct combustion method, easy to manage, simple to maintain, and highly reliable. However, the processing temperature required is high and the fuel is consumed.
b. Catalytic combustion
The catalytic combustion method is a method in which the organic solvent vapor in the exhaust gas is subjected to intense oxidative combustion to generate water and carbon dioxide, thereby removing harmful substances in the exhaust gas.
In the process of catalytic combustion of the exhaust gas, the exhaust gas is sent to the heat exchanger by the fan through the pipeline to heat the exhaust gas to the initial temperature required for catalytic combustion. The preheated exhaust gas is burned through the catalyst layer. The high-temperature clean gas generated by the combustion can be reused in the steel drum drying tunnel.
Due to the presence of the catalyst, the initial combustion temperature of the catalytic combustion exhaust gas is about 250 to 500 ° C, which is much lower than the combustion initiation temperature of the direct combustion method of 650 to 800 ° C, so that it is not necessary to supply a large amount of heat energy from the outside, so that The exhaust gas is completely burned. Fuel consumption is much lower than direct combustion.
The catalytic combustion system consists of a catalytic element, a catalytic combustion chamber, a heat exchanger and a safety control device. It is the core component of the catalytic combustion system, the catalytic element, and the outside is a metal frame made of stainless steel. The internal filling surface is coated with a catalyst. Metal carrier. Most of the catalysts are platinum-based precious metals such as palladium, platinum, and the like. The metal carrier is formed into various shapes, such as a mesh, a honeycomb, a sphere, a column, and the like. Most of the materials of the carrier are heat-resistant alloys such as nickel and chromium, and ceramics.
In order to ensure the normal operation of catalytic combustion, the following points should be considered when designing a catalytic combustion system:
(1) The concentration of exhaust gas. If the concentration of exhaust gas is too low, the combustion effect is poor. If the combustion effect is increased, the fuel consumption will be caused. The concentration of the exhaust gas is too high, the heat of combustion is high, and the temperature rises high. Not only will the catalyst burn out, the service life of the catalyst is lowered, but also the treatment is improper. It will cause an explosion. The concentration of organic matter in the exhaust gas is preferably maintained at 10-15 g/m3.
(2) The appropriate preheating temperature is determined according to the composition of different organic substances contained in the exhaust gas. The preheating temperature is too low to perform catalytic combustion, and the preheating temperature is too high, resulting in waste of fuel. Due to the different composition of the exhaust gas, the preheating temperature is also different. For the organic volatiles produced by the amino baking varnish commonly used in our steel drum industry, the preheating temperature is about 250 ~ 300 ° C; and the organic volatilization generated by the epoxy coating The preheating temperature is about 400-500 °C.
(3) The residence time of the exhaust gas in the catalyst layer is an important factor affecting the treatment effect of the exhaust gas. The long residence time will increase the thickness of the catalyst layer, resulting in waste of catalytic materials, short residence time, and unclean exhaust gas treatment. The residence time of the exhaust gas in the catalyst layer is preferably from 0.14 to 0.24 seconds.
(4) Consider adequate oxygen supply conditions. The organic solvent component in the exhaust gas generates a strong oxidation reaction by the catalyst, and converts the substance to generate carbon dioxide and water. Therefore, in order to sufficiently carry out the combustion, a sufficient supply of oxygen must be provided. If the oxygen supply is insufficient, the exhaust gas purification cannot be completed, and other harmful substances such as carbon monoxide or the like are generated, or carbon deposits are caused on the surface of the catalyst, so that the catalyst activity is lowered.
(5) Consider the poisoning and degradation of the catalyst. In theory, in the absence of external pollution, the catalyst can be used continuously, but in fact, pollution of water vapor, heavy metals, dust, paint mist, etc. is unavoidable. These pollutions cause catalyst poisoning, which reduces the activity of the lighter and completely loses the activity. Therefore, when designing, the catalyst contamination should be minimized and the service life of the catalyst should be improved. After the catalyst is poisoned, it can be restored by regeneration treatment.
c. Absorption method
The absorption method generally uses a liquid as an absorbent to cause harmful components of the exhaust gas to be absorbed in the liquid.
The absorption process of the liquid absorbent for gas is a transfer and mass transfer process of a gaseous substance which occurs due to molecular diffusion of gas and through an interphase membrane of a gas phase and a liquid phase. The driving force of diffusion and diffusion, at the gas phase interface film, is the partial pressure of gas and the partial pressure difference of gas at the interface. At the interface of the liquid phase interface, the concentration of the dissolved gaseous substance at the liquidus interface is in the liquid phase. The concentration of dissolved gases is poor. In the stable absorption process, the solute concentration in the liquid at the interface between the two phases and the solute partial pressure in the gas constitute a dynamic equilibrium state.
From the principle and process of carrying out the absorption method, the key to the absorption method is to select an absorbent which can effectively absorb the absorbed material. For the exhaust gas during the coating construction process, it is difficult to use an absorbent because it is a thin mixture of various organic volatiles. If the main component in the exhaust gas is a hydrophilic solvent, water can be used as the absorbent: if the main component in the exhaust gas is a hydrophobic solvent, it is necessary to use an absorbent which has a good absorption effect on the solvent.
For our steel drum industry, the absorption method is not very practical. China's steel drum coating is the most environmentally polluting PVF coating, not only its low solid content, but also more than 75% of the organic volatiles in the baking process to the environment, and due to volatilization The material has a large specific gravity and strong adhesion, so that the volatile mist can not be discharged in the vicinity of the drying tunnel for a long time, which has a great influence on the human body and the environment. The mist generated during the baking of PVF paint is in the form of droplet particles of dimethyl phthalate, so it cannot be called smoke (smoke is composed of solid particles). For such a mist, we can actually use water as an absorbent, but here we need to put a quote as an absorbent, because the role of water here is to cool down the dimethyl phthalate vapor.
I have done experiments in the laboratory to condense mist of dimethyl phthalate with water spray. The dimethyl phthalate mist produced by baking the PVF coating in the oven is sucked into the water stream by a water flow negative pressure device (commonly known as water mouse), and no dimethyl phthalate vapor escapes during the baking process. At the end of the experiment, the already liquefied dimethyl phthalate that had settled under the water was found in the water reservoir under the water flow negative.
Therefore, when we are baking PVF steel drums, can we design a shower tower to extract the mist generated in the drying tunnel, so that the drying tunnel maintains a negative pressure, and the negative pressure is not so much that the heat loss of the drying tunnel is excessive, and The resulting mist cannot escape and diverge. The extracted mist is sent to the shower tower from the bottom to the bottom, and the water is sprayed down. In order to bring the two into full contact, a plurality of trays may be provided in the middle or a large number of small porcelain rings may be filled. Thus, during the flow contact between the mist and the water, at the phase film, the mist is condensed by the water and carried downward to allow the gas to be purified. Clean air is discharged from the upper portion of the absorption tower, and water containing the dimethyl phthalate liquid flows into the water tank from the lower portion of the spray tower. Since the specific gravity of dimethyl phthalate reaches 1.19, it is much higher than water, and the liquefied dimethyl phthalate sinks to the bottom of the water, so the upper part of the water tank can be provided with an overflow port, and the overflowed water can be circulated. Use, when you just need to open the bottom valve of the sink to release the recovered dimethyl phthalate and recycle it.
d, activated carbon adsorption method
In fact, in addition to adsorption with activated carbon, active aluminum, silica gel, molecular sieve and other adsorption methods can also be used. Among them, the comprehensive absorption performance of molecular sieves is the best. However, considering the cost and the actual situation of the steel drum industry, it is most practical to use activated carbon to adsorb the exhaust gas of the baking coating.
Activated carbon is hydrophobic and has a high adsorption efficiency for organic solvents. In addition, activated carbon has a specific surface area that is much higher than other adsorbents, and is generally 500 to 1,500 m 2 /g. Therefore, in purifying organic solvent exhaust gas, activated carbon is often used as an adsorbent.
Activated carbon has different specific surface areas and physical properties due to the different raw materials and methods of activation. In general, the use of granular activated carbon of the husk type is better. The particle size of granular activated carbon is generally about 5 mm. The smaller the particle size, the higher the resistance to passage, but the higher the adsorption efficiency.
The exhaust gas from the spray booth and the drying tunnel passes through the dust collector and the cooler to remove the paint mist and dust, and is lowered to a certain temperature, and then sent to the adsorption tower through the fan from the lower portion of the adsorption tower. In the adsorption tower, the harmful components contained in the exhaust gas are adsorbed and concentrated on the large solid phase surface of the activated carbon, thereby achieving the purpose of purifying the exhaust gas. The purified air is discharged into the atmosphere through an induced draft fan.
As the adsorption process progresses, the activated carbon gradually loses its adsorption efficiency and reaches a saturated state. At this time, the steam is introduced into the adsorption tower through a steam pipe connected to the upper portion of the adsorption tower to desorb the activated carbon, and the deactivated activated carbon is desorbed. Can continue to use. The mixed gas of the desorbed water vapor and the organic volatile vapor of the coating enters the condenser, cools it, agglomerates into a liquid, and flows into the separator. In the separator, the coating organic volatile liquid is separated from the water and recovered.
The design of the activated carbon adsorption tower should pay attention to:
(1) A pretreatment device should be provided to remove the paint mist contained in the exhaust gas before entering the adsorption tower and reduce it to a suitable temperature to increase the adsorption efficiency of the activated carbon. Otherwise, the adsorption rate of the activated carbon is lowered due to the surface of the activated carbon being covered by the coating particles and the temperature of the exhaust gas being high.
(2) Explosion prevention and prevention of spontaneous combustion of the activated carbon layer should be considered.
(3) The regeneration and replenishment of inert charcoal should be considered to ensure the continuous progress of the adsorption process.
(4) The uniformity of the gas should be considered to prevent the occurrence of phenomena such as channel flow, bias current and short circuit during the adsorption process, and to ensure a certain adsorption efficiency.
3, the treatment of spray paint and drying tunnel waste paint residue
There is currently no ideal treatment for spray paint and drying tunnel waste paint residues. In the past, more methods were buried and self-burning. However, due to pollution of the soil and pollution of crops after infiltration, the landfill was banned by many countries; it is not advisable to burn these waste paint residues by themselves. Without special combustion equipment, it is impossible to fully burn the waste paint residue, but it will produce a lot of carbon monoxide and other toxic substances to pollute the environment.
However, many local environmental protection departments have already built centralized waste incineration plants. These incineration plants will classify garbage into different categories, burn the combustible garbage, and turn waste into treasure to become a valuable energy source. After contacting the local environmental protection department, many of these incineration plants will collect the garbage that needs to be incinerated at regular intervals.
Fourth, ways to improve coating pollution
Now the benefits of the steel drum industry are not very good, but most of the barrel factories spend their energy on management, steel, paint, energy and other aspects of reducing consumption, if we can reduce the pollution of coatings, we can also find ways to reduce consumption and increase efficiency. This is not a good thing for the benefit of the country and for the benefit of the people.
1, pre-treatment shower water exchange
As mentioned above, most of the degreasing liquids currently used in China's steel drum industry are chemical degreasers containing surfactants, sodium hydroxide, trisodium phosphate, sodium carbonate, sodium silicate and various additives. It is alkaline, so the time of rinsing after de-oiling is long, and it becomes alkaline wastewater. At this time, it is necessary to replace the shower water.
Most phosphating solutions are acidic liquids containing phosphoric acid, phosphate, nitrate, molybdate and various additives. The leaching water after phosphating has become acidic for a long time. Waste water, then do not change the water, it will affect the paint coating.
At this time, if the two showers are replaced with each other, unexpected results will occur.
The surface of the steel drum after degreasing is also alkaline due to the relationship between the oil and the oil. If it is washed with phosphating wastewater containing a small amount of phosphating solution, it is not only partially neutralized. The surface of the barrel is alkaline, and it is also advantageous for the subsequent phosphating process.
On the surface of the phosphating steel barrel, the surface of the steel barrel is acidic due to the phosphating solution. If the alkaline water is used to remove the oil, the surface of the phosphating layer of the acidic steel barrel is partially neutralized. However, there is a problem to be paid attention to here. In addition to being alkaline, the draining wastewater of the oil also contains grease that is cleaned from the surface of the steel drum. Therefore, if the oil content is high, the phosphating surface of the steel drum will be contaminated. That is, the amount of oil in the oil-washing wastewater is very low, and it is necessary to re-evaluate the clear water to make sure that the surface of the phosphating layer of the steel drum is clean.
The reuse of such interchangeable shower water wastewater not only reduces sewage discharge but also saves water.
2, change the paint variety and spraying method
In addition to pre-treatment, there are two main ways to improve the pollution of steel drum coating, which is to change the coating variety and construction method, and to eliminate or reduce the production of harmful objects in the steel drum from the root wood. If the solvent-based paint that causes serious pollution is changed into water-based paint, powder paint or high-solids paint to eliminate or reduce the pollution of the organic volatiles of the paint to the atmosphere, adopt high-efficiency coating method to improve the utilization rate of the paint and reduce The scattering of paint, etc.
The main source of pollution in steel drum coating is the coating itself, and the most important pollution of the coating to the environment is organic volatiles. However, organic volatiles are an important component of solvent-based coatings, so water-based paints and powder coatings are worthy of advocacy from an environmental point of view and development direction. In fact, some barrel mills have already used water-based paints and powder coatings in part, and even some barrel mills have used powder coatings. Of course, from the current production needs of the steel drum industry, it is impossible to completely dispose of solvent-based coatings. What we can do is to use high-solids solvent-based coatings as much as possible. Direct, most economical, most effective, and the most immediate way. Moreover, this is not only conducive to environmental protection, but also an effective means of reducing consumption and increasing efficiency.
Changing the coating method and improving the spraying device are also effective means to improve coating coating efficiency and reduce paint mist scattering. We can analyze the comparison of the three most commonly used spray methods in our steel drum industry:
The air spray takes the paint out by means of the negative pressure caused by the compressed air flow and is dispersed into a mist by the spray gun to be sprayed on the surface of the steel drum. The air sprayed paint mist is carried by the airflow. The paint mist is mixed with a large amount of air, which causes the paint to spread with the airflow. Moreover, the organic volatiles in the paint accelerate the volatilization rate due to the diffusion of the airflow. . Therefore, air spraying is the most environmentally harmful method in several methods of coating, and the waste of coating is also the largest.
High-pressure airless spraying is to apply high pressure to the paint by the high-pressure piston in the closed container, and the high-pressure paint is passed through the high-pressure hose and the spray gun, and the pressure energy is converted into the velocity energy through the olive-shaped nozzle hole, so that the paint is atomized into the particles to be sprayed. Steel drum surface. Compared with air spraying, since the paint mist does not contain a large amount of air, the coating does not splash with the airflow, the utilization rate of the paint is remarkably improved, and environmental pollution is greatly reduced. And because high-pressure airless spray can spray a coating with a much higher viscosity than air spray, the thinner used in the spray can be reduced, which also reduces environmental pollution, and of course reduces the coating cost.
Compared to the above two spraying methods, electrostatic spraying has minimal environmental pollution. Electrostatic spraying is the application of a negative high-voltage electrical connection generated by a high-voltage electrostatic generator to a metal sharp-edged or tip-discharged spray gun electrode, while the steel drum is grounded through a suspension chain or other transfer device to form a gap between the negative electrode and the steel drum. Uneven electrostatic field. By corona phenomenon, firstly, a large amount of electrons are excited and emitted near the negative electrode (the spray cup). After the paint mist enters the electric field, it combines with the electrons to form negatively charged particles. Under the action of the electric field force, it is directed to fly to the positively charged steel drum. surface. Electrostatic spraying does not or rarely scatters the paint mist into the air, which is the main cause of minimal environmental pollution in several spray methods. If a negative-voltage DC high-voltage power grid with an enameled wire is installed behind and on both sides of the steel drum in the spray booth, the inner wall of the spray booth will not be contaminated with paint mist due to the repelling effect of the paint mist which is also negatively charged. .
There are many ways to reduce the environmental pollution of steel drum coating. For example, the coatings currently used in our steel drum industry, except for nitrocellulose lacquer and X-891 coating at room temperature and fast drying, can reduce the viscosity by heating ( For example, X-622 epoxy steel barrel special inner coating, the temperature is increased from 25 ° C to 40 ° C, the viscosity can be reduced by about 20%), so that the paint can be reduced or even not used to spray the paint; in spraying, especially The use of a linear nozzle with a small spray angle in air spray can reduce the splash of paint mist; the use of high-quality spray equipment can also make the paint mist more refined, less scattered, and better coating effect.
As long as we pay attention to it, the pollution of the steel barrel coating will definitely be minimized, the coating material consumption of the coating will be reduced to a minimum, and the coating quality of the steel drum will be better.
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