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One of the main reasons for pre-coating blistering (low water resistance and wettability) and corrosion of deionized water treatment equipment under the paint film is the presence of water-soluble salts and alkalis on the coating surface or in the coating. This is often caused by unclean washing, poor quality of clean water (or inadequate control) or secondary contamination of the coated surface after surface treatment processes (degreasing, phosphating, electrophoretic coating, pickling, neutralization, etc.). Otherwise, the impurity of washing results in the formation of coating, which brings the treatment fluid of the former process to the next process, and causes the imbalance or transformation of the treatment fluid of the next process, resulting in the waste of the treatment drugs and the quality of the electrophoretic film.
For example, on the premise of ensuring the cleanliness of the coated surface after washing, and on the other hand, coating water treatment equipment can reduce water consumption, sewage discharge and the loss of treatment drugs, which is a question that should be paid special attention to in the technical planning of coating line, coating equipment planning and production management.
In order to obtain excellent coatings, save capital and save energy and reduce emission, it is necessary to grasp the following key technologies in the washing process of glass tank membrane cleaning water equipment.
(1) Strictly control the water quality of washing water for painting
Water capital of painting workshop is usually tap water (sometimes referred to as urban water, industrial water or groundwater, River water). Because of the difference of water source, pollution degree and time, the quality of tap water in different regions is also different, so we should pay attention to the local water quality and its change scale in the technical planning of painting line.
The water quality of tap water suitable for industrial coating should meet the following criteria: pH 5-7; conductivity less than 200 mS/cm; Cl-or SO42-below 20 mg/L (or SO42-+Cl-below 35 mg/L); total hardness (in terms of CaCO3) below 100 mg/L; transpiration residue below 100 mg/L.
Water quality of tap water beyond the above base should be purified on time, reused or water supply washing water should be thoroughly selected pure water and reverse process water replenishment clean (see Figure 1).
The water quality of pure water (also known as DIW) for painting should be controlled under the condition of strict request, whose conductivity is below 1 ugS/cm (25 C). Usually, the conductivity can be controlled under 5 u S/cm in production.
(2) Drawing up a benchmark for washing cleanliness
Washing cleanliness (or cleanliness) is usually expressed by the surface water quality of the coating after washing with water. The essence of the washing process is to use the principle of water dilution and replacement to wash out the treatment solution adhering to the coating; the cleanliness of the coating can be expressed by dilution multiples, such as when the water quality of the washed surface reaches 1/500 of the treatment solution (such as degreasing solution and phosphating solution), it is 500 times of the dilution (1/1000 is 1000 times of the dilution).
In order to ensure the high quality of the coating, it is necessary to choose pure water washing before painting (the final washing after pretreatment) and after electrophoretic painting. The surface water quality of the coating should be close to that of pure water. Usually, the conductivity of the dripping water from the workpiece is controlled to be less than 30 muS/cm.
In addition, before painting, the water droplets and accumulated water on the surface of the coating after washing should be removed. Because the water droplets concentrate 1000 times in the drying process, if the water quality is not up to standard, white saline-alkali spots will occur after drying.
(3) The number of times of washing (also known as the number of times), the time of washing and the number of times of water supply washing: after the treatment of the treatment liquid, it can be cleaned with movable water (that is, water is not recycled), although it can also be cleaned, but it consumes a lot of water. According to the dilution multiple, if the liquid carrying capacity of the clean surface is 100 mL/m2 and the dilution is 500 times, the water consumption will be as high as 50L/m2. In industrial painting production occasions, it is not possible to use mobile water, but to wash repeatedly with circulating water section. Usually, the degree of cleanliness required by the technical requirements can be reached, requiring 2 to 4 times of washing. Sometimes poorly washable medicines (such as caustic sodium, silicate, etc.) need to be washed repeatedly before they can be cleaned.
Washing time: In spray cleaning occasions, the planning time is usually no more than 30 seconds, because each time (section) washing only replaces and dilutes the surface treatment liquid or the front clean water, without chemical or accumulation effect, so there is no strict control request for washing time. In high-speed continuous production occasions, each washing time 10 seconds can be. In the soaking and cleaning occasion, it is easy to be immersed in the cleaner. Adding washing time will not improve the washing effect as adding washing times, but will spoil the kinetic energy and affect the efficacy.
Water supply: It depends on the number of washing times (sections), the ultimate dilution multiple, the amount of liquid taken out by cleaning materials and the method of water supply.
The amount of water (liquid) carried by the coating after each process is related to the structure, shape, mounting method, surface treatment and liquid temperature of the coating (see Fig. 6). Usually the average is 100 mL/m2. In the multi-stage water washing occasion, there are two water supply methods: side-by-side water supply and series water supply. The parallel water supply method is to make up water and drain water in each tank; the series water supply method is to make up water in reverse process, only in washing process. In China, many pretreatment lines are still side-by-side clean water supply or one-time washing, resulting in water capital not being fully utilized and water consumption increased by tens of times, that is, W2 < 1/10 ~1/100] 3W1 (see Fig. 7).
In the case of clean water supply in series, the water supply for washing can be calculated according to the following Kushner approximation formula:
Ultimate dilution factor=(Xn+1-1)/(X-1)
In the formula: n is the number of times of washing; X is the amount of water supply / water taken out.
Detailed examples: In the case of 200 m2/h of clean area with 20 L/h of liquid taken out (i.e. 100 mL/m2), the water supply required for one, two and three washing times is shown in Table 1.
Note: Water supply refers to the amount of water supply (L/h) needed to reach the dilution multiple of 200 m2/h area by washing, and water consumption refers to the amount of water consumption (L/m2) consumed by the dilution multiple of technical request by washing 1 m2 area.
From Table 1, it can be seen that the water supply (water consumption per square metre) of the 2-3 times washing technology (series water supply) using reverse process water supplement is greatly reduced compared with the water supply of the 1 time washing, which greatly improves the water utilization power. In the case of 2-3 washes, the actual water supply is 1.5-2.0L/m2.
(4) Washing method
Three methods of spraying, impregnation or spraying are usually used. If the shape of the cleaned object is simple (no gap in the inner chamber and the appearance that can not be sprayed), spray washing is better, because the mechanical force of spray washing is strong, the amount of circulating water can be less. If the contour structure is complex (with inner cavity, cracks, etc.) of the cleaned objects (such as car body, etc.), the spray-immersion method should be chosen. In the case of spray-immersion connection washing, it is usually repeated spraying plus 1-2 times of soaking, and it is better to spray first and then soak, that is, the soaking process is the final one in the repeated washing process (see figs. 3 and 5).
(5) Washing temperature
Higher washing temperature can improve the solubility of water-soluble saline-alkali. Hot water was originally used for cleaning. Nowadays, room temperature (RT) or normal temperature (i.e. no heating) is used for washing. On the one hand, energy saving, on the other hand, the possibility of secondary rust occurring in the washing process is reduced. Other surface adjustment processes before phosphating require that the temperature of the coated object be controlled is low. After degreasing with hot degreasing solution at 35 C, it has to be cooled by water washing.
In order to make the coatings with large heat capacity dry quickly after washing (easy to blow dry with hot air), hot water is still used to wash after oil removal.
~ 300s; 3. UF spray washing 0.6 ~ 1.0L/m3, 1~2 bar; fresh UF liquid spray 0.3m3/h (automatic control); UF UF liquid pre washing 3 to 4L/m3, 1 bar (Section 1.1) (this process is applied at intermittent time > 1 occasions); F F liquid spray cleaning 50m3/h, 1 bar (area 2); F F liquid spray wash < 2L/m3, 1~2 bar (2.1 2.1); F liquid spray washing (Section 3); F liquid spray washing (Section 3); Fresh F liquid (or pure water from process 11 purified water < 2L/m3, 1-2bar, Section 3.1); UF liquid (or pure water) soaking N02T = 32-45 (Section 4) (Section 4); UF liquid (or pure water) spray washing 8-10m3/h, 1.5 bar (Section 4.1); regenerated pure water spray washing < 2L/m3, l-2bar (Section 4.1); regenerated pure water spray washing < 2L/m3, L-2 bar (Section 4.1); _fresh water sprm3/h (area) Section 5; inclined leaching (Section 5); _leaching > 12 rain, 60-80 C; drying; strong cold cooling (workpiece < 45 C)
Author: Hangzhou Water Shield Technology Co., Ltd.
Website: http://www.zjhzsd.com/
Key words: Deionized water treatment equipment, coating water treatment equipment, glass tank membrane cleaning water equipment
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