Phosphating slag removal systems are commonly used in automobile painting lines

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Phosphating deslagging systems are commonly used in automobile painting lines. Author: Wang Haijun, Fourth Design and Research Institute of Machinery Industry. Source: AI “Automobile Manufacturing Industry”. Phosphating the surface of metal materials before coating can significantly improve the adhesion and corrosion resistance of the coating, which is why it is widely used in various industries. Especially in the automobile industry, the phosphated layer is used as a base layer for electrophoretic painting, with almost 100% adoption of this practice. During the phosphating process, the formation of phosphating sludge is inevitable. If excessive phosphating slag is not removed from the bath in a timely manner, it will not only contaminate the phosphating solution and shorten its service life, but also adhere to the surface of the vehicle body, affecting the quality of the phosphating coating as well as the overall painting quality of the vehicle ； In jet systems, an excessive amount of phosphating slag can easily cause clogging of nozzles and pipelines, reducing the heat transfer efficiency of the heat exchange system and preventing the entire production line from operating properly. Therefore, it is necessary to find effective ways to keep the level of phosphating slag in the bath within a certain range; this has led to the development of various phosphating slag removal systems (devices). In recent years, with the rapid development of China’s automobile industry, nearly forty modern car body painting lines of international advanced standards have been built in the country. Even more encouraging is the fact that several of these high-end car painting lines were entirely designed and manufactured by Chinese engineers, indicating that the design level of such painting lines in China has reached a new height. As a professional designer in the field of coating, I have been involved in the design of various types of coating lines on many occasions, and I have deep insights into the progress made in coating technology in China. This article introduces several phosphating slag removal systems that are widely used in automobile coating lines in recent years; it also reflects the current level of equipment configuration for phosphating slag removal systems in such lines. Inclined plate sedimentation tank filtration system: 1. Inclined plate sedimentation tank + paper belt filter (system configuration shown in Figure 1). When the inclined plate sedimentation tank + paper belt filter is in operation, the phosphating residue is initially precipitated and concentrated in the conical tank of the phosphating tank; it is then pumped by pump P1 into the phosphating sedimentation tank, entering the bottom of this tank through specially designed liquid guide pipes. From there, it rises slowly at a very low flow rate, passing through the inclined plate area. Under the combined effect of gravity and the pressure exerted by the inclined plates, the phosphating residue settles rapidly at the bottom of the cone, while the clear liquid formed at the upper part flows back to the phosphating tank via overflow pipes. The slag-rich liquid at the bottom of the sedimentation tank is controlled by an automatic valve to flow onto the chain plate of the tape filter; filter paper is placed on this chain plate. The phosphating solution settles due to gravity, passes through the filter paper, and flows into the liquid storage tank located below the chain plate, from where it is pumped back to the phosphating tank using pump P2 ； The phosphating slag is trapped by the filter paper and gradually thickens on the chain plate. Once it reaches a certain thickness, the liquid level rises, and a sensor sends a signal that commands the pneumatic valve to stop supplying the phosphating slag. The chain plate then starts moving, pushing the filter paper forward; the phosphating slag together with the filter paper falls into the waste bin. A new piece of filter paper is automatically placed on the chain plate, the liquid level drops, and the sensor instructs the chain plate to stop moving, thus starting the next working cycle. The phosphating slag removal system with this configuration is characterized by relatively low equipment investment, simple operation, and small floor space. If multiple phosphating cones are installed at the bottom of the phosphating tank and controlled by automatic valves to open in turn, the slag removal efficiency can be further improved. The disadvantages are high consumption of filter paper, high moisture content in the resulting phosphating slag, and significant loss of phosphating solution. The phosphating slag removal system of this type was previously used in automobile painting lines; since the output of those lines was low in the past, it was sufficient to meet the production requirements. However, with the increase in automation levels and output of automobile painting lines in recent years, this type of slag removal system is no longer in use. 2. Sedimentation tank + plate and frame filter press (system configuration shown in Figure 2). As can be seen from Figure 2, the configuration of this slag removal system is basically the same as that of the “sedimentation tank + paper belt filter” system, with the difference being that the paper belt filter has been replaced by a plate and frame filter press. During operation, pump P2 pumps the phosphating solution containing excess slag into the plate and frame filter press, where filtration is carried out by the filter cloth; the clear liquid then flows out through hidden pipes and returns to the phosphating tank. The phosphating slag remains trapped within the plate and frame structure, accumulating over time. When it reaches a certain level, the resistance increases and the pressure rises. The pressure sensor PG then triggers the pump P2 to stop supplying fluid; simultaneously, valve V1 is manually closed and valve V2 is opened to introduce compressed air for dehydration and drying, thereby reducing the amount of phosphating solution in the filter cake. After drying, close valve V2. Manually loosen the clamping bolts of the plate frames, remove the phosphating slag from within them, wash the plate frames, then reassemble and tighten them to prepare for the next working cycle. Compared with belt filter presses, plate and frame filter presses have the advantages of a larger filtration area and lower moisture content in the phosphating slag produced, thereby avoiding unnecessary waste of phosphating solution. At present, most domestically produced plate and frame filter presses are operated manually, resulting in a low level of automation. Although there are automatic versions of such filter presses that feature automatic pressing and slag discharge, their reliability is low, which limits the widespread use of this slag removal system. Due to the low level of automation, its processing capacity is greatly limited; as a result, the sedimentation tank + plate and frame filter press slag removal system is no longer used in large-scale, continuous automotive painting lines, and is generally only employed in small-batch, batch-type automotive painting lines. 3. Sedimentation tank + FK phosphating filter press (system configuration shown in Figure 3). As can be seen from Figure 3, this slag removal system also represents a variation of the slag removal machines compared to the first two types; the basic working process of the FK series of filter presses is shown in Figure 4. The basic operating process of the FK series filter presses: the slag removal system of “settling tank + FK phosphating filter press” is generally of the inlet type. The FK series of filter presses are fully automatic devices; all processes such as pressure filtration, aeration for dehydration, and the removal of the filter cake are controlled automatically by PLCs, resulting in a filter cake with a very low water content. The filtration process is regulated both by the filtration time and by pressure limits. This entire system features a high degree of automation, and it works well when used in conjunction with phosphating tanks. It is a type of slag removal system that has been widely used in automotive painting lines in recent years, and it is employed in both commercial vehicle and car painting lines. However, this system also has certain drawbacks: the filter paper used in the FK series of slag removers cannot be automatically cleaned and reused; instead, once a roll of filter paper is used up, it must be cleaned manually before it can be used again ； After the system has been in operation for a long time, some phosphating residues will adhere to parts such as pipes and filtration chambers. The system cannot undergo automatic acid cleaning as a whole; manual disassembly and cleaning are required, which is rather troublesome ； Additionally, due to the limitations in the processing capacity of the slag remover, it is generally used only in automobile painting lines with low production volumes (annual output of less than 100,000 units). 4. Double sedimentation tanks + concentration and settling tank + FK filter press (system configuration shown in Figure 5). As can be seen from Figure 5, this configuration of the phosphating slag removal system represents an improvement on the “sedimentation tank + FK phosphating filter press” setup; its purpose is to increase the capacity for handling phosphating slag. The two inclined-plate sedimentation tanks are controlled by automatic valves to alternate in discharging slag, thereby increasing the sedimentation time and raising the concentration of the phosphating slag. This slag is then sent to the concentration and settling tank for further sedimentation and concentration, while the clear liquid overflows to a secondary tank and is pumped back to the phosphating tank using a recovery pump. The phosphating filter press is fed with liquid from a concentration and settling tank; the high concentration improves the slag removal efficiency of the filter press. The entire system boasts a high slag removal efficiency, and it is generally used in car painting lines with large production volumes (150,000 to 200,000 vehicles per year). Full-filter type slag removal system 1. ALSI full-filter press (system configuration shown in Figure 6). The ALSI full-filter press is manufactured by Air and Liquid Systems Inc. of the United States. Its basic principle of slag removal is the same as that of the FK series of slag removers, but its performance is much better than that of the FK series: it has a high processing capacity, with the processing capacity of the various models ranging from 20 to 1500 GPM, allowing users to choose a model with the appropriate processing capacity based on their needs ； The requirements of the process regarding slag removal can be met without the need for a phosphating sedimentation tank; the entire phosphating solution is filtered, allowing for thorough slag removal and maintaining a low level of slag in the phosphating tank ； The program allows the filter paper to automatically rewind itself after the residue has been removed, enabling it to be reused multiple times automatically ； The phosphating slag adhered to the pipes and filtration chambers can be easily removed through pickling, thereby reducing the workload for workers. Due to the import of the entire equipment, the investment is high; it is generally used in car painting lines with large production volumes (with an annual output of over 200,000 vehicles). 2. Reverse (PS) filtration system (system configuration shown in Figure 7). The reverse (PS) filtration system is a filtration system developed by the Japanese company Pakka Design Engineering. The main components of this system include reverse (PS) filters, concentration tanks, filter presses, etc. The phosphating solution containing slag is pumped in by pump P1 from the bottom of the PS filter. The filter contains 70 filter bags, which are installed in the opposite manner to the traditional approach – they are placed outside the metal support mesh. The clarified phosphating solution passes through the filter bags and returns to the phosphating tank, while the phosphating slag remains attached to the outer surface of the filter bags. After 2 hours of operation, the accumulation of slag causes the pressure in the filter to rise, and the amount of filtrate flowing out begins to decrease; at this point, the backwashing process starts automatically. The filtrate outlet valve closes, and the backwash compressed air valve opens automatically. After 10 seconds, the phosphating slag attached to the surface of the filter bag is blown off by the compressed air and discharged into the concentration tank. After approximately 3 to 6 minutes, the process automatically returns to the filtering mode. The slurry in the concentration tank is pumped by pump P2 to the filter press for pressure filtration, and then compressed air is used for dewatering. The clarified phosphating solution returns to the phosphating tank through filter paper, while the phosphating residue is compressed into solid sludge cakes and removed. The device is automatically controlled by a computer, and the control program can be adjusted as needed; it is capable of effectively removing impurities even when the amount of residue in the phosphating solution is between 100 and 200 ppm. It represents a full-filter type phosphating residue removal system. Due to its one-way flow characteristic, it is very simple to operate and has a high processing capacity of up to 70 m3/h; when used in conjunction with a filter press, it can produce sludge cakes with a very low water content. The drawback is that the PS filter bag is difficult to remove; it is mainly used in painting lines for large cars, and is widely applied in painting lines designed and built by Paccar in China. 3. Full filtration system for plate and frame filter presses (system configuration shown in Figure 8). As can be seen from Figure 8, the equipment configuration for this slag removal system is relatively simple; the key device is the plate and frame filter press (as shown in Figure 9), and imported equipment is generally used. During operation, the automatic valves AV2 and AV3 are opened while AV1 is closed. The phosphating solution containing slag is pumped by pump P1 into the plate and frame filter press for pressure filtration; the clear liquid passes through the filter cloth and returns to the phosphating tank ； After working for a while, phosphating slag continues to accumulate in the filtration chamber, resulting in increased resistance and higher pressure in the pipes. When the set pressure is reached, the pressure sensor PG sends a signal, causing AV1 to turn on while AV2 turns off ； Then the automatic valve AV4 opens, and the process of dehydrating using compressed air begins automatically. After a certain period of time has passed during dehydration, valve AV3 closes; subsequently, AV4 closes while AV5 opens, allowing pressure to be released inside the filter press ； Then the automatic slag discharge process begins; the fully automatic cake discharge device comes into action, ensuring that the phosphating slag cakes can be removed smoothly from the plate and frame structure and fall into the slag collection tray. After that, the plate and frame structure is automatically compressed to start the next working cycle. Due to the high level of automation, the equipment can operate automatically without human intervention once it is started up. In addition, the filter press can be equipped with an automatic filter cloth cleaning system, which reduces the workload for workers. Due to the generally outdated technology, low level of automation, limited processing capacity, and poor reliability of domestically produced plate and frame filter presses, all full-filter presses used for phosphating slag removal are essentially imported products. These devices require significant investment, and they are typically used in car painting lines where high standards for equipment configuration are required. Figure 9 Summary of plate and frame filter presses. Comparing the relatively large-scale automotive painting lines built in China in recent years, the slag removal systems are basically of the types mentioned above; they can be categorized into two types: “inclined plate sedimentation tanks + filter presses” and “fully filtered slag removal systems”. Generally, project owners choose the appropriate slag removal system based on factors such as financial conditions, equipment capacity, and the level of equipment specification. With technological advancements, more advanced and effective slag removal systems may continue to emerge, and as coating professionals, it is essential to stay informed about these developments at all times. (end)