Painting and environmental protection of the hotte

Coating and environmental protection of steel drums (II)

III. treatment of coating pollution

pollutants in the coating process of steel drums mainly exist in three forms, namely waste water, waste gas and waste residue

1, waste water treatment

the sources of waste water in the steel barrel industry mainly come from two aspects, namely, the waste water produced during the pretreatment of steel barrels and the waste water produced by the water curtain of the paint booth. The wastewater generated in the pretreatment process is the main source of steel barrel coating wastewater. At present, the main wastewater treatment methods include physical treatment, chemical treatment and biological treatment

among them, physical treatment methods can be divided into precipitation method (using gravity or centrifugal force to separate the sediment), filtration method (using filter cloth or filter paper to separate the suspended solids from the wastewater), combustion method (concentrating and drying the wastewater and burning it), adsorption method (using porous active substances to adsorb), air flotation method (using bubbles and particles in the wastewater to adhere to make it float and separate), membrane separation method (using ultrafiltration and reverse osmosis to separate the wastewater)

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

biological treatment methods include biofilter method (using biofilm on the surface of biofilter to adsorb and oxidize organic matter) and activated sludge method (using activated sludge containing microorganisms to oxidize and decompose organic matter)

typical and practical wastewater treatment processes are as follows: (omitted)

the pretreatment of most barrel factories in China is mainly degreasing and phosphating, and the degreasing method is mainly chemical degreasing at present; At present, the phosphating solution mainly uses iron manganese phosphating. In addition, the degreasing liquid and phosphating liquid used in most barrel factories are of the additive type (the new liquid is not replaced in the process of use, but only added). Therefore, the pretreatment wastewater of steel barrel coating is mainly alkaline degreasing liquid flushing wastewater containing oil stains and surfactants, sodium hydroxide, trisodium phosphate, sodium carbonate, sodium silicate, etc., as well as various additives, as well as phosphoric acid, phosphate, nitrate, nitrite, molybdate Heavy metal ions and all kinds of additives, etc. acidic phosphating solution rinsing wastewater. For the domestic steel drum industry under the current situation, the simplest way is to set up a sedimentation tank, and add alum or polyaluminum chloride, polyferric sulfate, polyacrylamide and other coagulants into the sedimentation tank according to different types of pretreatment liquid to make it precipitate in layers, Then adjust the pH value of the supernatant to 6~9, and then dilute and discharge (generally, after coagulation, separation and dilution, the COD and BOD indexes of the rinsing wastewater of the pretreatment liquid of the steel barrel can meet the discharge requirements), and hand over the sediment to the environmental protection department for treatment

the wastewater generated in the spraying process is mainly the wastewater generated by the water curtain set up to capture the paint mist during the spraying process. The treatment of this kind of wastewater only needs to add coagulant in the pool to make the water recycled. The separated condensate can be treated by the environmental protection department

2. Waste gas

the source of waste gas from steel barrel coating is mainly the exhaust of paint spraying room and drying channel. The exhaust gas of the paint booth is generated during the exhaust process of the paint booth. In order to ensure good working conditions, the spray booth needs to be ventilated within the wind speed range of 0.25 ~ 1.5 m/s. therefore, the exhaust gas discharged from the spray booth is characterized by large air volume and extremely low concentration of organic volatiles (about 10~20ppm). In addition to the organic volatile gas, the exhaust gas from the paint booth also contains some paint mist formed by overspray of paint. The particle diameter of these paint mist is about 20 ~ 200 microns

the exhaust gas of the drying channel mainly contains solvent vapor, thermal decomposition gas in the process of coating film formation, and exhaust gas generated by the combustion of gaseous fuel or liquid fuel. The harmful substances in this kind of waste gas are mainly benzene, esters, alcohols, ethers and ketones, as well as some amines and aldehydes

there are direct combustion, catalytic combustion, absorption and activated carbon adsorption methods for the treatment of exhaust gas from paint booth and drying channel

a. direct combustion method

direct combustion method is a method that introduces the waste gas generated in the process of painting and drying into the combustion chamber, directly contacts and burns with the flame, burns and decomposes the combustible components in the waste gas, and turns it into odorless and harmless carbon dioxide and water vapor

in order to prevent the hydrocarbon in the exhaust gas from generating carbon monoxide due to incomplete combustion, in addition to supplying sufficient oxygen, there must be a combustion temperature of 650 ~ 800 ℃ and a retention time of 0.5 ~ 1.0 seconds in the combustion chamber

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

the current problem of direct combustion lithium-ion batteries is that the anode and cathode materials are not satisfactory. The following points should be considered in the design:

(1) because the paint waste gas is a mixture of gas containing a variety of solvent vapors, when its concentration is close to the explosion limit, it will explode. Therefore, in order to prevent explosion, high concentration gas close to the lower limit of explosion limit value needs to be diluted with air to a predetermined degree of safety

(2) when direct combustion is used to treat the exhaust gas produced by painting and drying channel, in order to avoid the production of photochemical smog substance NOx, the combustion temperature should not exceed 800 ℃

(4) consider the utilization of waste heat

direct combustion method has the advantages of easy management, simple maintenance and high reliability. However, the treatment temperature required is high and fuel consumption is high

b. catalytic combustion method

catalytic combustion method is a method that uses catalysts to make the organic solvent vapor in the exhaust gas oxidize and burn violently, generating water and carbon dioxide, so as to remove the harmful substances in the exhaust gas

in the process of catalytic combustion of exhaust gas, the exhaust gas is sent to the heat exchanger by the fan through the pipeline, and the exhaust gas is heated to the starting 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 channel of the steel barrel

due to the existence of catalyst, the starting temperature of exhaust gas combustion in catalytic combustion method is about 250 ~ 500 ℃, which is much lower than the starting temperature of 650 ~ 800 ℃ in direct combustion method. Therefore, it is not necessary to supply a large amount of heat energy from the outside, so the exhaust gas can be completely burned. The consumption of fuel is much lower than that of direct combustion. These enterprises basically obtain the market by reducing profits

the catalytic combustion system is composed of catalytic elements, catalytic combustion chambers, heat exchangers and safety control devices. As the core component of the catalytic combustion system, the catalytic elements are metal frames made of stainless steel on the outside and metal carriers coated with catalysts on the inside. Most catalysts are platinum based precious metals, such as palladium, platinum, etc. Metal carriers are made into various shapes, such as, honeycomb, spherical, columnar, etc. The carrier materials are mainly nickel, chromium and other heat-resistant alloys and ceramics

in order to ensure the normal operation of catalytic combustion, the following points should be considered when designing the catalytic combustion system

(1) the concentration of exhaust gas. The concentration of exhaust gas is too low, and the combustion effect is poor. If the combustion effect is improved, it is bound to cause fuel consumption. If the concentration of exhaust gas is too high, the combustion heat is large, and the temperature rise is high, it will not only burn out the catalyst, reduce the service life of the catalyst, but also cause explosion accidents if it is not handled properly. The concentration of organic matter in the exhaust gas is best maintained at 10-15 g/m3

(2) the appropriate preheating temperature should be determined according to the composition of different organics contained in the exhaust gas. If the preheating temperature is too low, catalytic combustion cannot be carried out. If the preheating temperature is too high, fuel will be wasted. Due to the different components contained in the waste gas, the preheating temperature is also different. For the organic volatiles produced by the amino baking paint commonly used in our steel barrel industry, the preheating temperature is about 250 ~ 300 ℃; For the organic volatiles produced by epoxy coating, the preheating temperature is about 400 ~ 500 ℃

(3) the residence time of waste gas in the catalyst layer is an important factor affecting the effect of waste gas treatment. A long residence time will increase the thickness of the catalyst layer, resulting in a waste of catalytic materials, short residence time, and unclean waste gas treatment. The retention time of exhaust gas in the catalyst layer is preferably 0.14~0.24 seconds

(4) sufficient oxygen supply conditions should be considered. The organic solvent component in the exhaust gas generates carbon dioxide and water through strong oxidation reaction and material conversion through catalyst. Therefore, to make the combustion fully carried out, there must be sufficient oxygen supply. If the oxygen supply is insufficient, the exhaust gas purification cannot be completed, and other harmful substances, such as carbon monoxide, are generated, or carbon deposits are caused on the surface of the catalyst, reducing the activity of the catalyst

(5) the poisoning and activity decline of catalyst should be considered. Theoretically, the catalyst can be used continuously without external pollution, but in fact, the pollution of water vapor, heavy metals, dust, paint mist and so on is inevitable. These pollutants cause catalyst poisoning, which reduces the activity of light cases and completely loses the activity of heavy cases. Therefore, during the design, the pollution and poisoning of the catalyst should be minimized to improve the service life of the catalyst. After poisoning, the catalyst can be regenerated to restore its performance

c. absorption method

absorption method generally uses liquid as absorbent to make the harmful components of exhaust gas be absorbed in liquid

the absorption of gas with liquid is exactly the absorption process of green and environmental protection defining agent. It is the transfer and mass transfer process of gas substances due to the molecular diffusion of gas and the interphase membrane of gas and liquid. The driving force of diffusion and transfer is the difference between the gas partial pressure and the gas partial pressure at the interface at the gas phase interface facial mask, and the difference between the concentration of dissolved gas substances at the liquid phase interface and the concentration of dissolved gas substances in the liquid phase at the liquid phase interface facial mask. In the process of stable absorption, the solute concentration in the liquid and the solute partial pressure in the gas at the two-phase interface constitute a dynamic equilibrium state

from the perspective 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 material. For the waste gas in the process of coating construction, since it is a thin mixture of various organic volatiles, absorbent is selected; It is more difficult. If the main component in the exhaust gas is hydrophilic solvent, water can be used as absorbent; if the main component in the exhaust gas is hydrophobic solvent, it is necessary to choose absorbent with good absorption effect of this solvent

for our steel barrel industry, the absorption method is not very practical. PVF coating is the most polluting coating on the environment in China. Not only its solid content is low, and more than 75% of the organic volatiles in the coating consumption will run into the environment during the baking process, but also because of the large proportion and strong adhesion of the volatiles, the volatile fog can not be discharged near the baking path for a long time, which has a great impact on human body and the environment. The fog generated in the baking process of PVF coating is in the form of droplet particles of dimethyl phthalate, so it cannot be called smoke (smoke is composed of solid particles). For such fog, we can actually use water as absorbent, but here as absorbent