PVF coating coating exhaust gas treatment technology
Text / Yang Wenliang
As a steel drum coating, PVF coating has been in China for 30 years, but the characteristics of PVF coating and the treatment of exhaust gas have always been an area that the steel barrel industry does not understand. In recent years, with the strict implementation of the national environmental protection policy, the problem of coating exhaust gas treatment of PVF coatings has inevitably been placed in front of the barrel making enterprises.
Due to the lack of understanding of PVF coatings, the composition and properties of the exhaust gases generated during the coating and drying process are also poorly understood, and the exhaust gas treatment methods painted with ordinary paints do not work, so many companies face PVF coatings. Painting the exhaust gas, nothing can be done.
First, the composition of PVF paint coating exhaust gas
PVF coatings are fluoride coatings. The volatile gas components produced by coating are mainly HF and SiF4, which are one of the main gaseous pollutants in the atmosphere. Its pollution to the atmosphere is much more serious than the pollution of ordinary paint.
HF is a colorless, highly irritating odor and highly corrosive toxic gas. It is highly soluble in water and forms highly corrosive hydrofluoric acid. SiF4 is a colorless asphyxiating gas that is highly soluble in water to form fluorosilicic acid. Fluoride exhaust gas can invade the human body through the respiratory tract, gastrointestinal tract or skin, causing serious harm to human health.
Second, the purification principle of PVF paint coating exhaust gas
At present, advanced PVF waste gas treatment methods at home and abroad are based on the characteristics that both HF and SiF4 are soluble in water, and in most cases, water absorption is used.
The water absorption method is also called wet treatment. The technology uses water, alkaline solution or some salt solution to absorb PVF waste gas, thereby achieving the purpose of purification and recovery. The advantage of this technology is that the purification equipment is small in size and easy to implement, and the treatment process can continuously operate and recover various fluorides, and the treatment efficiency is high and the efficiency is good.
Hydrogen fluoride (HF) is dissolved in water to form hydrofluoric acid, and silicon tetrafluoride (SiF4) is absorbed into water to a certain concentration of fluorosilicic acid (H2SiF6) and silica gel (H2SiO3). After H2SiF6 is reacted with industrial salt, it can produce fluorosilicic acid by-product. Sodium, the main chemical equation is as follows:
3SiF4+3H2O→2H2SiF6+H2SiO3↓
2HF+SiF4→H2SiF6
H2SiF6+2NaCl→Na2SiF6+2HCl
Third, PVF paint coating waste gas treatment process
The defluorination process commonly used in foreign countries is mostly secondary absorption. According to the different equipment used, it is divided into one chamber, one chamber, one chamber, one chamber, one chamber, one chamber, one tower, three towers, and the like. The "chamber" here refers to the water tank absorption chamber, which is generally used as the first-stage absorption. The advantage is that it is not easy to block, and the cleaning is convenient, but the defluorination efficiency is not high. "Device" refers to the venturi absorber, "tower" refers to the turret tower, spray tower, spray cup tower, swirling tower, etc., the defluorination efficiency can reach more than 98%.
In the PVF coating production of steel drums, according to the actual situation of PVF coating in the barrel industry, if the output is not large, the treatment process of the fluorine-containing waste gas can adopt the one-room one-rotation defluorination process, which is divided into three absorptions. The segments are each cycled and thus a higher concentration of fluorosilicic acid by-products can also be obtained. Figure 1 is a flow chart of the treatment process of one chamber and one rotating gas.
1-absorption chamber; 2-swirl plate tower; 3, 4-pump; 5-sump
Figure 1 One-room one-turn PVF waste gas treatment process
The hydrofluoric acid and fluorosilicic acid solution obtained by absorbing HF and SiF4 with water can be used to prepare by-products such as cryolite, fluorosilicone urea and sodium fluorosilicate.
At present, most of the steel drum enterprises in China produce PVF batches, which can be processed by the above process. If the PVF inner coating barrel is continuously produced in large quantities for many years, it is recommended to use the three-tower method for better treatment. The three-column absorption system is treated by spray tower, spray cup tower and swirling plate tower. The tail gas treatment can achieve the discharge of the standard, and the by-product sodium fluorosilicate can also be obtained, which achieves better environmental and economic benefits. Figure 2 shows the three-tower PVF coating waste gas treatment process.
Figure 2 Three-tower PVF coating coating waste gas treatment process
The exhaust gas from the steel drum PVF paint coating production system is sequentially absorbed by the spray tower, the spray cup tower and the spray rotary tray water, and the absorption efficiency of the exhaust gas through the first absorption tower can reach 90%, and the second tower is The absorption efficiency can reach 70%, the absorption efficiency of the third tower is 85%, the total absorption efficiency can reach 99.6%, and the clean water is added from the third tower. The absorption liquid realizes closed cycle without sewage discharge, and the fluorine content emitted from the chimney is less than 0.05G/Nm3, which meets the requirements of industrial pollutant discharge standard (GB 4917).
Figure 3 is a physical diagram of the three-tower PVF coating exhaust gas treatment equipment.
Figure 3 Three-tower PVF coating exhaust gas treatment equipment
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