Development direction of environmental protection type pretreatment technology
Text / Li Weizhan
Abstract: With the country's increasing emphasis on environmental protection, pollution prevention has become an urgent problem to be solved in almost all industries, especially in the industrial field. As a forward-looking technology, the development of environmentally friendly coating pretreatment technology will have far-reaching significance for the coating industry, especially the successful application of nano ceramic zirconium salt treatment and silane treatment technology will become a leap in coating pretreatment technology.
Keywords: environmental protection; energy saving and emission reduction; ceramic membrane technology; magnetic separator; nano ceramic zirconium salt treatment; silane treatment; recycling technology (PT-RO)
1 Introduction
With the widespread application of cathodic electrophoresis technology in the coating industry and the increase in people's purchasing power, the coating industry has developed rapidly. However, due to the limitations of investment scale and coating process technology in the early days, many earlier coating production lines were built There are shortcomings, especially the lack of knowledge about low pollution and low emissions. With the worldwide emphasis on environmental protection, environmental pollution has become one of the most concerned issues in today's world. For their own development, coating enterprises will inevitably choose energy-saving and emission-reduction, low-pollution equipment technology as one of the standards for enterprise transformation and construction of production lines, for the purpose of achieving cleaner production. As a heavy-duty paint pretreatment professional in painting and sewage, it should break through the limitations of traditional processes. With the help of the world's outstanding pharmaceutical companies, based on its own characteristics, improve the process, optimize the process, and vigorously develop environmentally friendly coating pretreatment technology, and gradually achieve low Pollution, low emissions.
This article briefly introduces the development direction of the main pollution sources degreasing and phosphating process in the environmental protection technology and the application of water saving and emission reduction in the pretreatment process of painting.
2 Degreasing process
2.1 Development and utilization of ceramic membrane technology
Inorganic ceramic membrane separation technology is a material separation technology based on the sieving effect of porous ceramic media. It uses a dynamic "cross-flow filtration" method that is completely different from the traditional "dead-end filtration" and "filter cake filtration" methods: namely Driven by pressure, the degreasing fluid flows at a certain flow rate on the surface of the inner layer of the membrane tube. Small molecular substances (that is, liquids) penetrate the microporous membrane in a direction perpendicular thereto, and large molecular substances (or solid particles) are trapped, causing the fluid To achieve the purpose of separation, concentration and purification, as an environmentally friendly technological process, it can not only adapt to the temperature of the degreasing liquid at 50 to 60 ° C for a long time, but also have good continuous operation performance, which can effectively separate the oil and water in the degreasing liquid and delay the degreasing liquid. Aging and discharge due to excessive oil pollution in the degreasing fluid. This emerging green process is bound to be a trend in the development of the degreasing process of the pre-coating degreasing process in the future. At present, there are two automotive coating production lines in China's First Automobile Group Corporation that use ceramic membrane oil-water separation devices.
2.2 Application of magnetic separator
The iron powder and welding slag brought by the workpiece in the stamping and welding process will enter the degreasing liquid with the degreasing cleaning, and the residue of these iron chips will directly affect the quality of the workpiece. According to the research of Japan Paka Company: 87% of the dust causing electrophoretic coating dust defects is metal dust (welding slag, cutting powder). After increasing the degreasing cleaning flow rate and setting up a device dedicated to removing iron powder in the cleaning solution After the measures, the quality of the electrophoresis of the workpiece has been improved by nearly 4 times, which shows that an effective iron powder removal device is indispensable. In the early pretreatment degreasing process, due to the low production cycle, most of the basket and bag filters are used to remove iron powder and other fibrous materials. For the current large-scale continuous production line, the production cycle is generally above 20JPH. Bag filter and suspension separator can not effectively intercept fine iron powder, and secondly increase the filter bag and labor cost. According to the production situation, the pre-processing production line with a single-line production cycle of about 30JPH is equipped with a suspension separator In the case of a bag filter, the filter bag needs to be replaced once in the first hot water cleaning within 4 hours, and the fine iron powder under 50μ has not been effectively intercepted. The application of the magnetic separator has been widely recognized at present, and its more reasonable configuration is basket filter-pump-suspension separator-magnetic separator-bag filter-plate heat exchanger to circulate in the tank. After the degreasing liquid is coarsely filtered by the basket filter, it is rotated at high speed in the suspension separator by a pump to leave the coarse slag at the bottom, and the degreasing liquid carrying fine iron powder enters the magnetic separator, because the magnetic rods arranged in the magnetic separator are Under the action of the electric field, it can produce high-strength adsorption force of 10,000-12,000 gauss, which can effectively capture fine iron powder, and send the iron powder to the paper belt filter during slag cleaning, remove the iron powder through the filter paper, and pass the magnetic The degreased liquid of the separator passes through the bag filter to filter the fiber and other impurities into the plate heat exchanger to fully exchange heat and return to the degreasing tank. The application advantage of adding a magnetic separator is that it can save labor and material costs and classify impurities, improve the quality of the workpiece and extend the service life of the degreasing liquid, and reduce the discharge of the degreasing liquid. At present, many domestic production lines use magnetic separators.
2.3 Phosphorus-free and nitrogen-free degreasing agent
Condensed phosphate has the effect of softening hard water and promoting the dispersion of dirt particles. It also has a high alkalinity, which can dissolve fat dirt through saponification. In addition, it is combined with sodium dodecylbenzenesulfonate These surfactants have obvious synergistic effect, which can greatly improve the cleaning effect. Therefore, phosphate as a degreasing agent has many advantages, but there are also insurmountable shortcomings, which is the effect of over-fertilization. The discharge of phosphorus-containing wastewater into the closed water system will increase the phosphorus content in the water. Phosphorus is one of the three major nutrients produced by plants, thus promoting the reproduction of algae in water. After the algal blooms die out, the rotten odor releases the lack of oxygen in the water, water pollution, and adverse effects on the growth of aquatic organisms. Therefore, in order to prevent algae pollution in water, most countries in the world have increasingly restricted phosphorus and nitrogen emissions. strict. In Europe and Japan, many relevant research institutions are developing a degreasing agent with a process temperature of about 40 ° C, which does not contain phosphorus and nitrogen, and a non-phenolic, degradable surfactant. In the degreasing agent, the use of a surfactant combination with this property and an alkaline builder is the future development direction.
3 Phosphating process
3.1 Phosphating process without nitrite
In the modern fast and high-quality phosphating process, the accelerator is an indispensable ingredient. Without them, this phosphating will lose its practical significance. Nitrite as an oxidizing accelerator plays an important role in the phosphating of zinc salts, but it is unstable in acidic solutions and will decompose to NO. Nitrite is added in the form of sodium salt, so that Na and NO are accumulated in the decomposition solution. In order to improve the working environment, the nitrite-free phosphating process will have great market prospects. Nitrite acts as an oxidant in an acidic solution to oxidize Fe to produce insoluble iron phosphate (slag), but the pH of the reaction area where the substrate is etched rises. Complexing agent. In this case, the use of new accelerators can achieve higher phosphating efficiency. The performance of the phosphating film obtained by this nitrite-free phosphating process is not worse than that of the phosphating film obtained by the traditional process.
3.2 Nano ceramic zirconium salt treatment on metal surface
As a main metal anti-corrosion technology, for half a century, the phosphating and passivation process before coating has been widely used. However, the waste residue from the phosphate in the phosphating process and the toxicity of hexavalent chromium in the passivation process have been Caused high attention in the coating industry. As an emerging technology, the nano-ceramic zirconium salt treatment on the metal surface can also be called a phosphorus-free nano-film technology. It is a solution based on nano-zirconia solution and fluorozirconic acid. It can condense and precipitate on the metal surface. The product is converted into a protective layer of nano ceramic zirconium film. The result of the reaction of the metal with the solution forms a "metal-oxygen-metal" which constitutes the skeleton of any oxide ceramic structure. The coagulation process continues to increase the metal-oxygen-metal crosslink density until the zirconium film is formed. The settling rate of the zirconium film is very fast, and a complete film can be generated in about 20 seconds. The deposition rate can shorten the length of the production line, thereby saving space and additional manufacturing costs. In addition, since the solution does not contain heavy metals and phosphates, no surface conditioning is required, no waste residue is generated, water is saved and emissions are reduced, and it can be reacted at room temperature, saving the production cost of the current zinc phosphate process. The typical process flow is pre-degreasing cleaning-degreasing cleaning-water washing-pure water washing (or RO water washing)-nano ceramic zirconium salt treatment-pure water washing-electrophoresis. Phosphate and hexavalent chromium are used in the current phosphating and passivation process, and the treatment of wastewater and waste residues requires a higher cost, while the nano-ceramic zirconium salt treatment of metal surfaces is an energy-saving and environmentally friendly process technology in the surface treatment industry. It transforms a layer of nano-ceramic zirconium film on the metal surface, does not produce waste residue, does not require a passivation process, so this technology has potential application value of performance.
3.3 Silane treatment on metal surface
Silane is a type of silicon-based organic / inorganic hybrid. Before it was found to have anti-corrosion properties, silane was widely used as an adhesive in glass or ceramic reinforced polymer composites. The silane treatment of the metal surface is derived from the basic principle of the interface between the metal and the paint. The role of the interface between the coating and the metal depends on the adhesion. It can be an intermolecular force, an electrostatic attraction, or a chemical bond. When the distance between the two solids is less than 0.4nm, even if no adhesive is used, the two can be bonded. After the silane hydrolysis, the silane molecules form a hydrogen bond with the MeOH group on the metal surface and quickly adsorb to the metal surface. In the subsequent drying process, the SiOH group and the MeOH group further condense and form on the interface Si-O-Me covalently bonds to form a silane film. On the other hand, the remaining silane molecules form a silane film with a Si-O-Si three-dimensional network structure on the metal surface through the condensation reaction between SiOH groups. The advantages are similar to the nano ceramic zirconium salt treatment, the reaction time is short, generally completed within 5 to 30 seconds, the solution does not contain heavy metals and phosphates, no surface conditioning treatment is required, no waste residue is generated, water and emissions are reduced, and it can be used in Reaction at room temperature, shorten the length of the production line, save space, manufacturing costs and production costs; the difference is that the bath is weakly alkaline. In addition, the utilization rate of the silane treatment bath liquid is high, which can reach 250m-400m / Kg, which is incomparable by the current zinc phosphate process. The typical process flow is pre-degreasing cleaning-degreasing cleaning-water washing-pure water washing (or RO water washing)-silane treatment-pure water washing-blowing water (or moisture drying)-electrophoresis. As a kind of "green anti-corrosion technology", silane treatment technology has attracted more and more attention as people's awareness of energy saving, emission reduction and environmental protection has increased. It breaks through the framework of traditional phosphating film formation before painting and is a surface treatment technology belt. Here comes a new concept. Of course, the silane treatment also has shortcomings. The silane film is formed during the dehydration process rather than in water. This will increase the length of the sealed chamber after pretreatment. In the future, the development of underwater film formation technology will surely be Membrane technology opens a new chapter.
4 Water saving and emission reduction
4.1 High salt water recycling
As we all know, the paint shop is a large water-consuming household, and the water consumption of the pre-treatment production line accounts for more than 60% of the entire workshop. Water saving and recycling have become a new direction for the development of pre-treatment equipment technology, and the pure water preparation process The introduction of the discharged high salt water into the pre-treatment production line as spraying and replenishing water after degreasing is a new idea of ​​water saving and emission reduction, and waste water utilization. Generally, there are two fresh water points in the pretreatment line, one is the second washing after degreasing, and the other is the last washing with pure water. Pure water is usually sprayed with pure water. The purpose is not only to maintain the cleanliness of the water washing tank after phosphating, but also to reduce the drip conductivity of the workpiece to less than 30μs / cm to avoid contamination of electrophoresis. Therefore, there is a high requirement for the conductivity of the supplemented water; the second washing after degreasing is usually sprayed and supplemented with industrial water. Its purpose is to rinse the residue on the workpiece and maintain the cleanliness of the washing tank. No special requirements, so high salt water can fully meet its technological requirements. In the paint shop, pure water is mainly used for auxiliary processes such as pretreatment, electrophoresis, air conditioning humidification, and inspection and polishing. According to its flow rate, the amount of high brine produced during the preparation of pure water is slightly less than the supplement of pretreatment industrial water. A storage tank can be installed in the pure water room to collect high salt water, which is sent to the pre-treatment line through a water pump for utilization. At the same time, a float valve or solenoid valve is installed on the storage tank to connect the industrial water pipeline to prevent accidental interruption of high salt water. flow.
4.2 Flushing water circulation regeneration technology (PT-RO)
The pretreatment production line not only accounts for more than 60% of the entire paint shop's water consumption, but also the waste water discharge accounts for more than 60% of the entire paint shop. Therefore, to reduce the amount of sewage in the paint shop, it is necessary to use the pretreatment production process Proceed. The rapid development of RO reverse osmosis technology, especially the successful use of ultrafiltration devices in electrophoresis production lines in the past one or two decades, provides ideas for the pretreatment process in the flushing water recycling technology. The basic process is:
After phosphating, the cleaning water can not only reduce the discharge of cleaning water by a large amount after the regeneration of PT-RO, but also the purified water can be used as a supplementary water instead of fresh pure water to spray pure water, which not only achieves the recycling of recycled water, but also reduces The discharge of wastewater. For now, after phosphating, the flush water recycling technology (PTRO) is not mature because of its immature technology, especially in the PH adjustment and the filtration and interception of phosphating slag before reverse osmosis, so this technology is not extensive. Application, there are not many production lines at home and abroad applying this technology. However, with the development of science and technology, especially the development of ceramic membrane and stainless steel membrane technology, the environmentally friendly technology of PT-RO will surely bring completely closed cleaning to the pre-coating process, achieving nearly "zero" "Emission target.
5 Conclusion
At the same time of economic development, China and the world have put energy and environmental development in the first place. The idea of ​​"development first, governance later" has been abandoned, and all walks of life should fully implement national energy conservation and reduction. Emissions, the concept of environmental protection, truly make good use of energy and protect the environment. As one of the processes with high energy consumption and more waste in the automobile manufacturing process, the pre-painting production line should actively understand the adoption of advanced technologies and equipment at home and abroad, develop high technology content, low cost of process equipment, energy saving and emission reduction, Low-pollution and environmentally friendly technology, truly realize clean production in enterprises.
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