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Waste gas treatment in the production of aluminum surface treatment
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1,Introduction
Aluminum profile organic coating surface treatment process will inevitably appear in the coating quality of substandard products , Such as electrophoresis coating often appear defects are low film, yellowing, particles, bubbles and other defects, Powder electrostatic spraying often appear pinhole, bubbles, particles, string color and other defects, If this scrapped, is bound to affect the yield and timely delivery, resulting in unnecessary losses.
So more and more aluminum enterprises are concerned about how to remove the aluminum on the unqualified coating, recycling substrate for rework. In addition, in order to ensure the conductivity of the workpiece during electrostatic spraying, spray coating with organic coating also carried out stripping treatment.
 
The basic requirements for stripping:
1, No damage to the original aluminum alloy workpiece,s, including the appearance of size, material composition, surface hardness, finish, roughness are not allowed to have the slightest change. That can only take off the film, can’t hurt the body;
2, stripping clean and no prints, aluminum surface can’t have the stars left trace, that is, to clean off.
Aluminum coating organic coating stripping method can be divided into three categories: chemical methods, pyrolysis methods and mechanical methods. Chemical desorption method using the use of corrosives, solvents or both; high temperature decomposition method using high temperature furnace, open flame, heat fluidized bed, molten salt bath and laser for stripping; mechanical stripping method using high pressure water, abrasive media , Brush, mill, scraper, cutting machine, low temperature treatment.
 
2.Chemical stripping
The chemical release agent works by softening or dissolving the coating, splitting the coating between the coating and the substrate, and then releasing the film to be mechanically removed. Chemical release agents can be classified according to the operating temperature (hot or cold) and can also be classified according to their composition, such as etchants (acidic or alkaline), solvent or etchant / solvent combinations. Corrosion agent release agent is generally used heating process, stripping temperature of 80 ~ 100 ℃.  Solvent-based chemical release agent using cold process, the working temperature of room temperature or near room temperature. A solvent-based chemical release agent is usually a liquid obtained by mixing solvents such as chlorinated hydrocarbons, ketones, esters, alcohols and benzene. The use of a solvent has a permeation and swelling effect on the coating layer, and it is possible to effectively remove the coating surface of various aluminum surfaces, to peel the coating film directly or to peel the coating film easily. The release of the release agent is achieved by a series of physical and chemical processes such as dissolution, penetration, swelling, peeling and reaction.  There are many chemical release agents that have been commercially available. The advantages of chemical release agent is the recipe varied, can remove acrylic, epoxy, polyester, polyurethane and other high durability coating, and the impact of aluminum substrate is very small. Disadvantages are a potential hazard to health and the environment, which is related to the use of corrosive materials, chlorine-containing solvents and flammable solvents.  Corrosive release agent is usually acidic, is a strong acid such as concentrated sulfuric acid, hydrochloric acid, phosphoric acid and nitric acid composition of the release agent. Due to concentrated hydrochloric acid, nitric acid volatile produce acid mist, while the aluminum substrate corrosion; concentrated phosphoric acid stripping time is longer, the substrate also corrosive effect, so the use of the three kinds of acid less. Concentrated sulfuric acid and aluminum, iron and other metal passivation reaction, the corrosion of aluminum is very small, while the organic matter with a strong dehydration, carbonation and sulfonation and make it soluble in water, so concentrated sulfuric acid is often used as acid release agent The  Since the solvent-borne release agent is cold, that is, the working temperature is room temperature or near room temperature, so their speed of action than the hot corrosive stripping agent is slower, longer coating time. Therefore, the combined release agent came into being. The combination of release agent formulations using corrosive materials and solvents, both the advantages of both. They can remove the most durable and chemically resistant coating. They can work at room temperature or near room temperature, operating temperature is lower than the boiling point of the solvent, the speed of action between the corrosive release agent and solvent-based release agent. Domestic products have been listed, the use of good results.
 
3 High temperature decomposition stripping
The high temperature decomposition and stripping equipment, including open flame, high temperature furnace, fluidized bed and molten salt bath. At high temperatures, most organic coatings can be pyrolyzed in a relatively short period of time. The advantage is that the stripping speed is fast and thorough, the disadvantage is that energy consumption and damage to some of the substrate, and aluminum and other large aluminum products are not applicable, can only be used for small aluminum parts.  High temperature furnace stripping is in the high temperature and low oxygen atmosphere on the organic coating part of the pyrolysis. The first stage will produce volatile organic compounds, leaving carbon and inorganic compounds; the second stage will remain on the aluminum in the high temperature and excessive oxygen combustion, the formation of carbon dioxide. The inert pigments and fillers remaining on the substrate thereafter are mechanically removed. In order to comply with the air quality standard, a post-burner in the high-temperature furnace exhaust device burns the residual organic volatile compounds to form carbon dioxide and water. In a more efficient system, the heat exchanger can circulate energy. The hot fluidized bed stripping is the use of a pyrolysis process in a fluidized bed, and the fluid in the fluidized bed is hot sand. Fluidized gases can be hypoxic or oxygen-rich, depending on the process requirements. The closed loop system has a dust collector, a rear burner and a heat exchanger. The organic volatile compounds are converted into carbon dioxide and water by the burned burner and discharged. Heat exchangers improve thermal efficiency.  The molten salt bath is stripped using a patented technology for melting, oxidizing and inorganic salt salt bath processes. The coated workpieces are immersed in a salt bath for 5 to 10 minutes, which is related to salt formulation and coating composition. The gas discharged from the circulatory system is treated to meet air quality standards. Some patented molten salt baths have been developed that can be operated at lower temperatures to subject some aluminum products to release.  Laser stripping is a high-tech method that utilizes the energy of the laser beam to heat the organic coating. The laser beam is automatically moved along the workpiece, and the coating is decomposed as it passes through the coating. The process is very slow, in the formation of the substrate on the best off the role.
 
4. Mechanical stripping
Mechanical stripping includes many conventional methods such as hand and electric tools for grinding, scraping and cutting, shot peening, and the like. Mechanical stripping also includes high pressure water stripping and low temperature stripping. Mechanical stripping is usually supplemented by other methods to completely remove loose coating residues.  Manual grinding, scraping and cutting are still used in actual work. The method used in this method has a grinding pad, sandpaper wire brush, scraper, chisel hammer, rotary wire brush and laminated polishing wheel, the advantage is the stripping speed, the disadvantage is the labor intensity.  Sandblasting stripping uses a variety of grinding media, grinding media through compressed air or low pressure water generated by the centrifugal force on the workpiece. The centrifugal sandblasting machine utilizes motor-driven wheels to throw the media onto the surface of the coating. In air sandblasting, the medium is thrown onto the surface of the workpiece with the aid of a compressed air flow. These physical processes are carried out in a closed container, and therefore only for small aluminum products.  The sanding medium is sprayed in low pressure water. The advantage of sandblasting is that it can quickly remove the coating from most substrates and meet environmental requirements. The disadvantage is that the workpiece is subject to the impact of the medium 2 twisting and wear. At present, sandblasting stripping media are sand, steel shot, plastic, ice crystal and carbon dioxide beads.  Traditional sand and steel balls and other grinding media has been used for sandblasting. Because of their high hardness, it is also the most abrasive. They have the advantage that they may be masked by the twisting and wear of the substrate.  High pressure water stripping takes advantage of the pressure of 15000 ~ 50000Pai of the impact of the workpiece surface and remove the coating, and the use of specially designed nozzle, by changing the water pressure, impact angle and residence time and other parameters, or even get rid of the most durable coating membrane.
 
5. Concluding remarks
There is no "best" method for removing organic coatings. All of the methods mentioned above are effective for stripping. Only after careful consideration of the size of the stripped aluminum profile, coating properties, operating costs, environmental impact and safety factors such as workers can make an accurate choice.

Updated:2017-12-13 | Return
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