Steel drum painting and environmental protection (2)

Steel drum painting and environmental protection (2)

Third, the treatment of coating pollution

The pollutants in the barrel coating process mainly exist in three forms, namely waste water, waste gas and waste residue.

1, wastewater treatment

The source of wastewater in the steel drum industry mainly comes from two aspects, namely the wastewater generated during the pre-treatment of the steel drum and the wastewater generated by the curtain of the spray booth. The wastewater generated in the pretreatment process is the main source of steel drum coating wastewater. At present, the main wastewater treatment methods are physical treatment, chemical treatment and biological treatment.

The physical treatment method can be divided into a sedimentation method (separation of sediment by gravity or centrifugal force), a filtration method (separating suspended solids from waste water by filter cloth or filter paper), a combustion method (concentrating and drying waste water after combustion), and adsorption method. (Adsorption by porous active material), air floatation method (using bubbles and adhesion of particles in waste water to cause floating separation), membrane separation method (separation of wastewater by ultrafiltration and reverse osmosis).

Chemical methods can be divided into neutralization method (neutral treatment with acid and alkali), redox method (oxidation and reduction of ions with sodium sulfite, air, chloride, etc.), coagulation method (separation of suspended solids by coagulant) Ion exchange method (ion exchange using ion exchange resin to treat wastewater containing heavy metal ions).

The biological treatment method includes a biofilter method (using a biofilm on the surface of a biofilter to adsorb an oxidized organic substance) and an activated sludge method (using an activated sludge containing a microorganism to oxidize and decompose an organic substance).

The typical and practical wastewater treatment process is as follows: (omitted)

The pre-treatment of most barrel mills in China is mainly de-oiling and phosphating, while the de-oiling method is mainly used for chemical de-oiling; the phosphating solution is mainly used for ferro-manganese phosphating. And most of the barrels used in the degreasing and phosphating solutions are supplemental (there is no replacement of the new liquid during use, but only added), so the pretreatment wastewater of the steel drum coating mainly contains oil and contains surface. Active agent, sodium hydroxide, trisodium phosphate, sodium carbonate, sodium silicate, etc., as well as alkaline effluent drainage wastewater of various additives, and containing phosphoric acid, phosphate, nitrate, nitrite, molybdic acid Salt, heavy metal ions, and other types of additives such as acidic phosphating solution shower wastewater. For the domestic steel drum industry under the current situation, the simplest method is to set up the sedimentation tank, and add alum or polyaluminum chloride, polyferric sulfate, polyacrylamide and other coagulants to the sedimentation tank according to different types of pretreatment liquid. The layer is precipitated, and then the pH of the supernatant is adjusted to 6~9 and then diluted and discharged (generally, the COD and BOD of the molten wastewater of the steel drum pretreatment liquid after coagulation separation and dilution) The indicators can meet the emission requirements), and the sediments are handed over to the environmental protection department.

The wastewater generated during the spraying process is mainly the wastewater generated by the water curtain provided to capture the paint mist during the spraying process. The treatment of this type of wastewater requires only the addition of a coagulant to the pool to recycle the water. The separated aggregates can be disposed of by the environmental protection department.

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.