Reasonable water use - pre-treatment industry development trend
1 Introduction
China's fresh water resources are in short supply, and per capita fresh water resources are less than 1/4 of the world's per capita, and the distribution is extremely uneven. More than 400 of the country's large and medium-sized cities are water-scarce cities, of which 100 are seriously water-deficient cities. There are a number of processes that consume large amounts of fresh water in the pre-painting process. We have written an article on the water-saving method of pre-painting. After the paper was published, we thought about the following questions.
What are the principles that are followed for water conservation? Or is it reasonable to dispose of the cleaning water beforehand?
Do all painted parts have to be washed with deionized water? Of course, washing with deionized water will have great benefits, but is there any benefit to do? For example, for powder electrostatic spraying, it is certainly beneficial to use all new powder instead of recycled powder. Is it not necessary? The answer is of course no. The cost issue is considered here.
For the cleaning water treated before painting, if the cleanliness is very important, can our current water quality indicators meet the quality requirements (basically taken for granted)? How to prove?
With these considerations in mind, we know that in addition to understanding the importance of the process, we must understand the origins of this process. The more we understand the most essential things, the more we can come up with practical scientific methods to make breakthroughs.
We know that some soluble salts in water have an effect on the corrosion resistance of the product. Insoluble salts have an effect on the adhesion. So what degree does the different concentration affect? What is the reasonable range we are looking for?
As far as powder coating is concerned, we can see changes in the following aspects, which may impose new requirements on previous standards and processes.
Change one: housing conditions. Compared with the bungalows, the relatively dry living space, metal corrosion will be reduced;
Change 2: Gas conditions. Environmental metal corrosion of gas is reduced relative to the coal-fired (SO2) kitchen environment;
Change three: painting conditions. In the past, it was mainly painted with paint. The thickness of the paint was about 20μm. The current powder coating thickness is 60-90μm. Compared with the air and moisture conditions, the corrosion rate of the coated product is different. Not the same.
The performance of the coating is tested from two aspects: the laboratory method and the product market performance method, to determine the rationality of the process and to establish the use of fresh water. The conditions in the laboratory are relatively easy to satisfy, but the most important product market performance is rarely investigated, and the adjustment of the process is lagging.
2. The status quo of pre-treatment freshwater utilization
2.1 The addition of tap water is controlled by the pressure of the water pipe, or by the design of the production line, and few people raise objections;
2.2 The discharge of washing water is judged by the eyes of the employees, and some process regulations must be discharged daily;
2.3 The water reuse rate is low, often direct discharge, and has not formed a good habit of reuse of water.
In fact, water that looks clean is sometimes unsuitable for coating applications (such as water with NaCL dissolved), while water that looks dirty may not be used (such as water containing coated powder coating particles). Control, flow control or eye sensory control have unscientific factors and there is room for improvement. Because the increase in water consumption will increase the cost of the product, it has an economic incentive for the company to control the pre-treatment water control method.
3. Determination of the cleanliness of water
Pre-treatment cleaning water is the most scientific method based on cleanliness. Cleanliness can be detected chemically or physically.
3.1 The pH of water, ie [H+], [OH-], can be detected by pH value;
3.2 The concentration of soluble ions in water, such as [Ca2+], [Mg2+], [SO42-], [CO32-] in tap water, can be detected by the conductivity of water; the water-absorbing ions increase the humidity of the substrate, The conditions for the corrosion reaction are created, and the presence of [CL-] accelerates the depolarization and accelerates the corrosion of the metal under the coating.
3.3 The concentration of insoluble suspended particles in water, such as phosphating slag, dust, etc., can be measured by turbidity measurement, light transmittance or particle count; the presence of phosphating slag between the coating and the phosphate film will reduce both sides. Binding force
3.4 The concentration of soluble organic matter in water can be measured by absorbance or colorimetric analysis. Small molecules that decompose organic matter at high temperatures cause pinholes in the coating.
Different areas and different product processes, the detection of water cleanliness can be carried out by one or several of the above methods. By detecting the waste water standard suitable for coating production and using the instrument to control the water supply, the purpose of water saving can be achieved. , to obtain the expected quality and efficiency, economic benefits and environmental benefits.
4, spray pretreatment water saving method
4.1 Process
Taking cold-rolled sheets as an example, the ideal spray-type pre-treatment performs the following processes:
Pre-rinsing→pre-degumming→degreasing→water washing→table adjustment→phosphating→water washing→mixing water washing→deionized water washing
According to the concept of water saving design, the above process is divided into two water saving units, namely a degreasing water saving unit and a phosphating water saving unit.
4.2 Degreasing unit of water
Including the increased pre-flushing tank, degreasing tank 1, degreasing tank 2 and water washing tank, in addition to the connecting pipe 1 between the original degreasing tanks, the connecting pipe 2 is added between the pre-flushing and the washing tank, and the liquid level of the washing tank is maintained. Higher than the level of the pre-flush tank. Add a pump to the washing tank to ensure that a small amount of sediment can be withdrawn. The bottom of the washing tank has an inclination angle of 15° or more to ensure a small amount of sediment is collected at the pump pipe end. Only use tap water and water pipes in the washing tank to remove the tap water filling port on other tanks.
The water pumped out of the pump can be replenished with the degreasing tank 2, or the degreasing tank 1 can be replenished via the degreasing tank 2 (reduced by high temperature evaporation and carrying), and the degreasing tank solution can be prepared to rinse the degreasing tanks 1 and 2. The pre-flushing tank is supplemented by the difference in water level and is discharged through the pre-flushing tank to complete the multiple recycling of water. The water quality of the washing tank is measured and controlled by the water hygienic meter.
In terms of process, the water washing tank is not discharged, and the degreasing bath and the washing tank are used to wash the water in the tank.
The pre-flushing tank is used to rinse the re-sprayed workpiece, the newly removed coated hanger (to ensure good electrical conductivity), and the long-term dust-filled workpiece to reduce the pollution level of the degreasing tank, and Prevent fouling of the nozzles of the spray pipe. The pre-flushing pump can be stopped during normal production to save power and reduce noise. The water level of the pre-flushing tank is kept extremely low, and the circulation of the water can be maintained. The workpiece that is not washed down by the suspension can be picked up from the pre-flushing tank in time.
4.3 water-saving phosphating unit
The phosphating tank does not need to be separately added with tap water, but is supplemented by a pump that is added by the phosphating water washing tank. The bottom of the tank is designed as a bevel, the angle is greater than 15°, and the tap water is added by a connecting pipe to mix the washing tank (tap water + go) Ionized water), the mixed water washing tank is supplemented by tap water and deionized water; a water cleansing meter is added, and the tap water is automatically controlled and supplemented according to the water cleanliness data. The washing tank and the mixed washing tank are not discharged. The water pumped out of the pump is also used to flush the phosphating tank. Water that does not meet the required water quality is exported from the abandoned water pipe for wastewater treatment.
4.5 meaning
4.5.1 Digitally check the cleanliness of the cleaning water before painting, formulate a reasonable water quality range, and adapt it to the coating production of different workpieces;
4.5.2 Automatically add fresh tap water according to the water quality;
4.5.3 Increase the pipe connection and pump extraction, increase the reuse rate of water, and no longer simply specify the timing discharge;
4.5.4 with a sufficiently inclined groove bottom surface to ensure that the sediment in the tank is pumped out without changing the water in the tank;
4.5.5 Save water while ensuring the quality of the coating.
6, tank dip pretreatment water saving design
Some complex shapes (with cavity structure) or intermittent production, slot immersion pre-treatment has certain advantages, the standard connection pipe interface for cleaning water is reserved when designing the trough, and the standard cleaning water discharge nozzle, Combine tap water into the tube to form a similar water saving unit. Additional monitoring of the wash water and automatic addition of controls can be added if necessary. Practice has proved that this method has significant water saving effect.
The bottom angle of the tank is about 15°, which is convenient for the sediment to be cleaned.
6 Conclusion
Cleaning is an important process before painting, and it is also the key process of fresh water consumption. The reasonable use of cleaning water determines the dual meaning of ensuring quality and saving resources. The reasonable process scope is realized according to the monitoring results. Means of the above objectives. It has a groundbreaking significance in the domestic coating field and has broad prospects for promotion.
Round Shape Tempered Glass Cover,Rectangle Tempered Glass Lid,Square Tempered Glass Lid,G Type Glass Lid
Hebei Bozheng Glasswork Co.,Ltd , https://www.bozhengglass.com