Third, the coating pollution treatment

Contamination in the process of steel drum coating mainly exists in three forms: waste water, waste gas and waste residue.

1, waste water treatment

The source of waste water in the steel drum industry mainly comes from two aspects, namely the waste water produced during the steel drum pre-treatment process and the waste water generated by the paint spray curtain. The waste water generated in the pre-treatment process is the main source of steel tank coating wastewater. The main current wastewater treatment methods are physical treatment, chemical treatment and biological treatment.

Among them, the physical treatment method can be classified into a precipitation method (separation of sediments by gravity or centrifugal force), a filtration method (separation of suspensions from waste water by a filter cloth or filter paper), a combustion method (concentration and drying of waste water after combustion), and an adsorption method. (Adsorption with porous active material), Air floatation (using bubbles and adhesion of particles in waste water to float them up), Membrane separation method (Using ultrafiltration, reverse osmosis to separate waste water).

Chemical methods can be divided into neutralization methods (neutralization with acids and alkalis), oxidation-reduction methods (oxidation reduction of ions with sodium sulfite, air, chlorides, etc.) and coagulation methods (precipitation separation with coagulants). , ion exchange method (ion exchange resin for ion exchange treatment of wastewater containing heavy metal ions).

The biological treatment method includes a biological filter method (biofilm adsorption on the surface of a biological filter to oxidize organic matters) and an activated sludge method (using an activated sludge containing microorganisms to oxidize and decompose organic matters).

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

The pretreatment of most barrel plants in China is mainly oil removal and phosphating, while the oil removal method is currently mainly used for chemical degreasing; the phosphating solution is currently mainly used for iron and manganese phosphating. In addition, most of the oil removal and phosphating solutions used by the barrel plant are supplemental (there is no need to replace the new solution during the application process but only add up), so the pre-treatment wastewater from the steel drum coating is mainly oily and contains surface Active agent, sodium hydroxide, trisodium phosphate, sodium carbonate, sodium silicate, etc., as well as alkaline degreasers for various types of auxiliaries, and contains phosphoric acid, phosphate, nitrate, nitrite, molybdic acid Salt, heavy metal ions, etc., as well as various types of additives such as acidic phosphating liquid shower wastewater. For the domestic steel drum industry under current conditions, the simplest method is to set sedimentation tanks, and add alum or polyaluminum chloride, polyferric sulfate, polyacrylamide and other coagulants to the precipitation tank according to different types of pretreatment liquids. The layers are allowed to precipitate, and then the pH of the supernatant liquid is adjusted to 6-9 for dilution discharge. (In general, the COD and BOD of the steel drum pretreatment liquid after the agglomerating and separation of diluted wastewater, after dilution The indicators can all meet the emission requirements), and the sediment will be handled by the environmental protection department.

The waste water generated during the spraying process is mainly the waste water generated by the water curtain set up to catch the paint mist during the spraying process. The treatment of this type of waste water can be reused only by adding coagulant to the water tank. The separated condensate can be processed by the environmental protection department.

2. Exhaust gas

The source of steel drum coating exhaust gas is mainly the exhaust of painting booths and drying tunnels. Exhaust gas from the spray booth is generated during the exhaust process of the spray booth. In order to ensure good working conditions, the paint booth needs to be ventilated within a wind speed range of 0.25 to 1.5 m/s. Therefore, the exhaust gas from the paint booth is characterized by large air volume and extremely low concentration of organic volatiles (approximately 10 to 20 ppm Inside). In addition to organic vapors, the exhaust gas in the paint booth contains—partly due to overspraying of the paint. The particles of these paint mists are approximately 20 to 200 microns in diameter.

The exhaust of a drying tunnel mainly contains solvent vapors, thermally decomposed gases during film formation, and exhaust gas generated by the combustion of gaseous fuels or liquid fuels. The harmful substances in such exhaust gas are mainly benzenes, esters, alcohols, ethers and ketones as well as some amines and aldehydes.

Spray chamber and drying tunnel exhaust gas treatment methods include direct combustion, catalytic combustion, absorption, and activated carbon adsorption.

a. Direct combustion

The direct combustion method introduces the exhaust gas generated during painting and drying into the combustion chamber and directly contacts the flame to burn, and burns and decomposes combustible components in the exhaust gas to become a method of odorless and harmless carbon dioxide and water vapor.

In order to prevent the generation of carbon monoxide due to incomplete combustion of hydrocarbons in the exhaust gas, in addition to supplying sufficient oxygen in the combustion chamber, it is also necessary to have a combustion temperature of 650 to 800°C and an exhaust gas residence time of 0.5 to 1.0 second.

Exhaust gas direct combustion system is composed of burner, combustion chamber, preheater and other parts. In order to achieve the desired combustion effect, the burner should be able to form a continuous and stable fully-burning flame, the combustion area should be large, and the exhaust gas should be in full contact with the flame.

Direct combustion method system design considers the following points:

(1) Because the paint exhaust gas is a mixed gas containing many kinds of solvent vapors, when it burns when its concentration is close to the explosion limit, it will cause an explosion. Therefore, in order to prevent explosion, high concentration gas near the lower limit of the explosion limit needs to be diluted with air to a predetermined degree of safety.

(2) Direct combustion enthalpy When the exhaust gas produced by painting and drying tunnels is used, the combustion temperature should not exceed 800°C in order to avoid the generation of photochemical smog NOx.

(4) Consider the use of waste heat.

The direct combustion method is easy to manage, simple to maintain, and highly reliable. However, it requires high processing temperatures and consumes more fuel.

b. Catalytic combustion

Catalytic combustion is a method in which a catalyst catalyzes the oxidative combustion of organic solvent vapors in exhaust gas to produce water and carbon dioxide, thereby eliminating harmful substances in the exhaust gas.

During the catalytic combustion of the exhaust gas, the exhaust gas is sent to the heat exchanger through a duct through a fan, and the exhaust gas is heated to an initial temperature required for catalytic combustion. The preheated exhaust gas is burned through the catalyst layer. The high-temperature clean gas generated by combustion can be reused in the drying tunnel.

Because of the presence of the catalyst, the initial temperature of the combustion of the exhaust gas by the catalytic combustion method is about 250-500°C, which is much lower than the combustion starting temperature of the direct combustion method of 650-800°C. Therefore, it is not necessary to supply a large amount of heat energy from the outside and it is possible to make The exhaust gas is completely burned. The consumption of fuel is much lower than that of the direct combustion method.

The catalytic combustion system consists of catalytic elements, catalytic combustion chambers, heat exchangers, and safety control devices. It serves as the catalytic element of the catalytic combustion system's core component. 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 and platinum. The metal carrier is made into various shapes such as mesh, honeycomb, sphere, column, and the like. The carrier materials are mostly nickel, chromium and other heat-resistant alloys and ceramics.

In order to ensure the normal operation of catalytic combustion, the following points need to be considered when designing a catalytic combustion system:

(1) Exhaust gas concentration. If the exhaust gas concentration is too low, the combustion effect is poor. If the combustion effect is increased, it will inevitably cause fuel consumption. Excessive exhaust gas concentration, high combustion heat, and high temperature rise will not only burn out the catalyst, reduce the service life of the catalyst, but also handle it improperly. It will cause an explosion. The concentration of organics in the exhaust gas is preferably maintained at 10-15 g/m3.

(2) Determine the appropriate preheating temperature based on the composition of the different organics contained in the exhaust gas. If the preheating temperature is too low, catalytic combustion cannot be performed, and if the preheating temperature is too high, fuel waste results. Because the exhaust gas contains different components, the preheating temperature is also different. For the organic volatiles produced by the amino baking varnish commonly used in the steel drum industry, the preheating temperature is about 250 ~ 300 °C; and the organic volatiles produced by the epoxy coating are 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. A long residence time will increase the thickness of the catalyst layer, resulting in waste of catalytic materials, short residence time, and unsatisfactory exhaust gas treatment. The residence time of the exhaust gas in the catalyst layer is preferably 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 through the catalyst and converts the substance to produce carbon dioxide and water. Therefore, sufficient combustion must be performed to provide sufficient oxygen supply. 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 deposition is caused on the surface of the catalyst, so that the activity of the catalyst is decreased.

(5) Consider the problem of catalyst poisoning and activity degradation. Theoretically, in the absence of external pollution, the catalyst can be used continuously. In fact, the pollution of water vapor, heavy metals, dust, paint mist, etc. is unavoidable. These pollutions cause catalyst poisoning, lighter activity is reduced, and heavy ones completely lose activity. Therefore, during the design, catalyst poisoning should be minimized to increase the service life of the catalyst. After the poisoning of the catalyst, its performance can be restored by regeneration treatment.

c. Absorption method

Absorption is generally a method in which a liquid is used as an absorbent to allow harmful components of the exhaust gas to be absorbed in the liquid.

The absorption process of a gas liquid absorbent is a transfer and mass transfer process of a gas substance that occurs due to molecular diffusion of gas through an interphase film of a gas phase and a liquid phase. The driving force of diffusion transfer is the difference between gas partial pressure and gas partial pressure at the interface of the gas phase interface membrane. At the liquid phase interface membrane, the concentration of dissolved gas substances at the liquid phase interface is in the liquid phase. The dissolved gas material concentration is poor. In the stable absorption process, the solute concentration in the liquid at the interface of the two phases and the partial pressure of the solute in the gas constitute a dynamic equilibrium state.

From the point of view of the principle and process of the absorption method, the key to the absorption method is to select an absorbent that can effectively absorb the absorbed substance. For the exhaust gas during the coating construction process, it is more difficult to use absorbents because of the lean mixture of organic volatiles. If the main component of the exhaust gas is a hydrophilic solvent, water can be used as an absorbent: If the main component of the exhaust gas is a hydrophobic solvent, it is necessary to use a good absorption effect for this solvent.

Absorption is not practical for our steel drum industry. China's steel barrel coating is the PVF coating that has the greatest environmental pollution. Not only does it have a low solids content, but organic volatiles that account for more than 75% of the coating amount in the baking process have to go to the environment. 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 impact on the human body and the environment. The fog generated during the baking of PVF paint is in the form of droplets of dimethyl phthalate and cannot be called smoke (the smoke is composed of solid particles). For this kind of fog, we can actually use water as an absorbent, but here as an absorbent we need to make a quotation mark, because the effect of water here is only to condense the dimethyl phthalate vapor cooling down.

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 was pumped into the water stream by a negative pressure flow device (commonly known as water rat). No dimethyl phthalate vapor escaped during the baking process. At the end of the experiment, the liquefied dimethyl phthalate that had submerged in the bottom of the water was found in the water reservoir under the water under pressure.

Therefore, when baking PVF steel drums, we can design a shower tower to extract the fog generated in the tunnel, so that the drying tunnel maintains a negative pressure, so that the size of the negative pressure will not cause excessive loss of the thermal energy of the tunnel, and The resulting fog cannot escape the divergence. The extracted mist is sent to the shower tower, from the bottom to the bottom of the tower, and the water is sprayed from above. In order to make full contact between the two, a plurality of trays or a large number of small ceramic rings may be arranged in the middle. In this way, during the flow contact between the mist and the water, the mist is condensed by the water at the interphase film and carried downwards, so that the gas can be purified. Clean air is discharged from the upper part of the absorption tower, and water containing dimethyl phthalate liquid flows from the lower part of the spray tower into the water tank. Since the proportion of dimethyl phthalate reaches 1.19, which is much higher than that of water, the liquefied dimethyl phthalate sinks to the bottom of the water. Therefore, an overflow port may be provided in the upper part of the water tank, and overflowing water may be recycled. Use, only need to open the bottom valve of the sink to release dimethyl phthalate.

d, activated carbon adsorption

In fact, in addition to the use of activated carbon adsorption, active aluminum, silica gel, molecular sieve 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 conditions of the steel drum industry, the use of activated carbon to adsorb the exhaust gas of baked paint is most practical.

Activated carbon is hydrophobic and has a high adsorption efficiency for organic solvents. In addition, activated carbon has a much higher specific surface area than other adsorbents, typically 500 to 1500 m 2 /g. Therefore, in purifying organic solvent exhaust gas, activated carbon is often used as an adsorbent.

Activated carbon has different specific surface area and physical properties due to the different materials and methods of activation. Under normal circumstances, the use of granular granular activated carbon is better. The particle size of granular activated carbon is generally about 5 mm. The smaller the particle size, the higher the resistance to passing, but the higher the adsorption efficiency.

The exhaust gas from the painting room and the drying tunnel passes through the dust collector and the cooler to remove the paint mist and dust, and after lowering to a certain temperature, it is sent to the adsorption tower through the fan from the lower part of the adsorption tower. In the adsorption tower, harmful components contained in the exhaust gas are adsorbed and concentrated on the large solid surface of the activated carbon, thereby achieving the purpose of purifying the exhaust gas. The purified air is discharged into the atmosphere through a draft fan.

With the progress of the adsorption process, the activated carbon gradually loses its adsorption efficiency and reaches its saturation state. At this time, the steam is connected to the upper part of the adsorption tower to pass the steam into the adsorption tower, so that the activated carbon is desorption, and the activated carbon is desorption and regeneration. Can continue to use. The mixed gas of desorbed water vapor and coating organic volatiles vapor enters the condenser, cools it, and is condensed into a liquid and flows into the separator. In the separator, the paint organic volatile liquid is separated from the water and recovered.

The design of activated carbon adsorption tower should pay attention to:

(1) A pretreatment device should be installed to remove exhaust gas from the exhaust gas before entering the adsorption tower and reduce it to a suitable temperature to increase the adsorption efficiency of activated carbon. Otherwise, the adsorption rate of the activated carbon decreases due to the surface of the activated carbon covered by the paint particles and the exhaust gas temperature is high.

(2) Explosion protection and prevention of spontaneous combustion of the activated carbon layer should be considered.

(3) The regeneration and replenishment of inert carbon should be considered to ensure continuous adsorption.

(4) The homogeneity of the gas distribution should be considered to prevent the occurrence of channeling, biased flow, and short circuit during the adsorption process and ensure a certain adsorption efficiency.

3, spray paint room and drying tunnel waste residue treatment

Paint spray booths and drying tunnels are currently not ideally disposed of. In the past, the practice was to bury it and burn it by itself. However, because of the pollution to the soil and the contamination of the water source after the infiltration, the burial was prohibited by many countries; it is also not desirable to burn these waste paint residues by themselves. Without dedicated combustion equipment, it is impossible to fully burn the spent paint residue, but it will produce a large amount of carbon monoxide and other toxic substances to pollute the environment.

However, many local environmental protection agencies have already built centralized waste incineration plants. These incinerators will classify wastes, concentrate combustible waste, and turn waste into valuable energy. After contact with the local environmental protection department, many of these incinerators regularly collect garbage that needs to be burned.

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