Abstract: On the basis of a brief discussion on the treatment method of overspray paint mist in the paint shop, a VOC calculation and statistical model method is proposed, and a practical discussion on the reduction of VOC emissions is carried out.
Key words: paint shop; overspray paint mist treatment; VOC control
0 Preface
The car coating process involves many basic subjects such as organic chemistry, polymer chemistry, physical chemistry, and material science. Among them, aromatic hydrocarbon solvents are the most widely used and used in paint materials, such as xylene and high-boiling aromatic hydrocarbon solvents. In addition, ester solvents (butyl lactate), Alcohols (isopropanol), ketone solvents (cyclohexanone, trimethylcyclohexanone) and other volatile organic solvents have caused varying degrees of pollution to the environment. We often refer to VOC as volatile organic compounds, which refer to solvents and some chemical substances that volatilize into the atmosphere during the use of coatings. They are mixtures of organic substances (hydrocarbons, ethylene glycol, alcohol, etc.), For example, paints, PVC, and cleaning thinners all contain VOCs. Most countries and organizations define VOC as any organic compound with a boiling point lower than or equal to 250 °C, such as Germany's definition of VOC as organic volatiles. They have a vapor pressure of 0.01 kPa or more at a temperature scale of 293.15 K, or generate corresponding moisture under the corresponding conditions of use (German Federal Environmental Protection Regulation No. 31).
Modern car painting workshops currently use reciprocating machine and manipulator electrostatic spraying. The paint coating rate of reciprocating machine electrostatic spraying is generally 60%~70%, and the paint coating rate of manipulator electrostatic spraying is generally 80%~90%. The amount of overspray is often volatilized and scattered in the spray booth during the painting process. There are currently three main treatment methods for the treatment of overspray paint mist: 1) Venturi water adsorption; 2) Limestone dry adsorption; 3) E-scrub water treatment. The following is a brief introduction to these three different processing methods.
1 Treatment method of overspray paint mist in paint shop
1.1 Venturi water adsorption treatment method
Venturi water adsorption treatment is currently mostly used in spraying workshops of solvent-based traditional processes. The oversprayed paint mist falls into the circulating water with the settling wind from top to bottom. After the mist paint droplets enter the circulating water, they are dispersed into small particles by the paint mist coagulant. Under the action of electric charge or adsorption, the paint mist condenses The agent uses the different affinity of the two poles to completely wrap the paint droplets. Through chemical action, the paint mist coagulant penetrates into the paint and destroys the functional groups in the paint particles, effectively eliminating the viscosity, and part of the solvent is also absorbed and wrapped. , the viscosity of the paint particles disappears, and the paint slag is a non-adhesive loose group. Before the damaged paint slag floats or sinks, the polymer flocculant in the paint mist coagulant gathers the damaged paint droplets into Larger slag clusters, at the same time, agglomerate the damaged paint particles and other impurities, make them firm and bond, enhance the efficiency of mechanical dehydration and neutralize the system charge, and maintain the ion balance in the system. The slag cluster in the floating Water Tank is removed by the slag scraper. The VOC content of the paint slag formed by these chemical coagulants is generally about 1.5%.
In some systems with relatively large circulation volume or relatively fast circulation speed, sometimes more foam may be generated, and it is necessary to use a defoamer to suppress excessive foam generation. Bactericides are also used in Venturi water adsorption treatment. The circulating water system is a "dead cycle" without overflow. Under the climate conditions in southern CHINA, it is generally used for 1 month in summer (2 months in winter). Bacteria (mainly anaerobic bacteria) will be produced in the circulating water, which will cause odor and excessive foam in the circulating water. Bacteria will produce hydrogen sulfide gas and ammonia salt in the water, which will accelerate the increase of the conductivity of the circulating water.
1.2 Limestone dry adsorption treatment method
Limestone dry adsorption is a paint mist treatment method newly introduced into CHINA in recent years. It is mainly used in the spraying equipment system manufactured by Duerr in Germany. For limestone treatment and E-scrub water treatment. The process flow of limestone treatment of paint mist is shown in Figure 1.
For the limestone paint mist collection device (see Figure 2) used in the "2010 Water-based Color Paint New Process", it can be set by replacing the limestone in each collection module of the spray booth (for the filter module of the paint inner cavity spray area and the varnish inner cavity spray area Optimize the number of passing vehicles set by the filter module ), increase the number of passing vehicles, reduce limestone consumption to reduce the amount of limestone input, the frequency of replacement has a direct effect on the control of limestone consumption, and it is necessary to regularly verify the rationality of the setting value of passing vehicles , so as to improve the efficiency of limestone to collect paint mist.
1.3 E-Scrub treatment method
The E-Scrub electrostatic paint mist separation system is located below the grid plane of the spray booth, and the spray booths of the topcoat line BC1+2 and CC1+2 have their own E-Scrub system. As it passes through the painting process, the air in the spray booth is loaded with excess paint particles. The overspray mist is drawn together with the air by the circulating air exhaust fan through the separation system. The overspray paint in the air is separated in the separation module. The exhaust fan draws clean air into the exhaust duct arranged next to it. Most of the purified air is re-distributed to the paint spray booth as recirculated air, a small part of the purified air is led by exhaust air treatment into the waste discharge pipe and replaced by the fresh air supplied.
Figure 3 shows the E-Scrub electrostatic separation device for capturing paint mist. The downwind of the paint spray booth passes through the deflector from top to bottom, engulfs the settled paint mist and enters the separation module for electrostatic separation, and the static electricity is provided by a high-voltage generator. After the settled paint mist is absorbed by the separation module, it flows to the collection pan. The downwind is then re-sent into the spray booth by the circulating fan. The picture shows the external structure of E-Scrub, which is the system terminal of E-Scrub paint treatment. The paint is adsorbed on the separating agent through the separation module, and the mixture of paint and separating agent flows into the main tank; after entering the sedimentation tank, it is centrifugally separated to make the paint solid block Separated from the separating agent, then pump the separating agent to the main tank.
Electrostatic separation is the main feature of E-Scrub work, and its separation process is as follows: 1) Power on the settled paint mist; 2) Charged paint mist moves toward the cathode; 3) The paint mist is taken away from the cathode plate by a separator; 4 ) Centrifugal separation in the separation system of the circulation system; 5) The separation agent after centrifugation is reused and pumped to the main tank.
The difference between E-Scrub and the traditional wet Venturi mainly has the following points: 1) The separation agent separation replaces the wet Venturi water curtain, and the water saving is more obvious; 2) The separation module is the core module of the paint mist adsorption and separation; 3) The new Increase exhaust gas bypass incineration, about 15% of downwind in CC section is incinerated; 4)The circulation-based settling wind is fed into the circulation air supply room by the fan; 5) The fresh air coming in from the artificial area of the spray booth will be used as the E-Scrub air seal.
The flow chart of E-Scrub air supply system is shown in Figure 4.
The operating mode of the separating agent is as follows: 1) The separating agent is pumped through the system tank; 2) The separating agent is overflowed in two stages in the separating tank to ensure the liquid level of the separating agent is balanced; 3) The compressed air expansion tube is used for the separating agent The titration flowed and flowed on the surface of the cathode plate; 4) The separating agent overflowed into the main tank.
导流板是沉降风经过的区域，带有油漆，长期不清洁，导流板漆皮褶起，掉落入分离模块，造成高压报警。分离剂滴定槽是容易积漆的地方， 积漆过多将导致沉降风不能透过分离模块，不能起到静电分离。收集盘积漆过厚过多，带入主槽的积漆也多，会造成主槽出口的絮凝泵、储槽泵的堵塞、损坏阳极板锯齿释电极容易积漆，这是不可避免的，如果不能清洁到位，则影响电场效应。阴极板上附着油漆过厚，将导致阳极与阴极之间极距过小，引起漏电流，系统要求关闭。
漆雾处理的 3 种方式各有特点，可以视各个车间的实际情况选择，但保护环境是发展的最终方向。
表 1是 3 种处理方式费用的比较。
2 VOC 计算统计模式
VOC 的计算目前采用 1 a 计算统计 1 次， 在 1 a的时间内计算所使用的溶剂量， 即全年有机溶剂投入量减去有机溶剂量或者有机黏合剂的处理量 （由于这些溶剂化学或物理性能， 所以需要通过燃烧处理进行销毁的），再减去收集的废料中所包含的有机溶剂的量（对废气、废水的回收处理进行销毁的），然后According to油漆涂装车间生产的车型面积换算从而得出 VOC 的排放（g/m2）。 由于涂装车间的整体设计布置不同， 设备能力、功效有异，所以不同车间相同工序中产生的 VOC挥发量也有差别， 鉴于计算 VOC 涉及的材料数据较多，所以车间需要建立一个 VOC 控制小组，每月统计材料的消耗， According to每月各个颜色的消耗来计算每月VOC 的产生量。 与此相对应的还需要收集统计每月废稀料、 废渣的回收以及废稀料、 废渣中溶剂的质量分数；每月多少 VOC 挥发通过焚烧炉燃烧排放。 至于如点修补处的环己酮、 喷蜡处的 VOC 则需要专业的仪器测量得到数据。 油漆车间的 VOC 排放主要来自于溶剂型色漆及中涂漆， 采用水性漆可大幅降低 VOC排放，VOC 排放仅为传统溶剂型油漆工艺的 1/3 左右，达到欧盟 VOC 的排放标准。 表 2 为溶剂型色漆和水性色漆的 VOC 含量对比。
表 2 溶剂型漆和水性漆的 VOC 含量对比
采用无中涂的“2010 水性漆工艺”的 VOC 排放将进一步降低，VOC 排放仅为 24 g/m2。
目前涂装车间既有传统溶剂型工艺又有“2010新工艺”。传统工艺的回收主要包括废稀料及废渣的回收，与此对应的“2010新工艺”主要回收废稀料和沾有油漆颗粒的石灰石。而在生产过程中喷蜡的飞雾以及点修补的环己酮飞扬物则无法回收统计。所以提高废弃物的回收是控制VOC的有效手段。油漆管路的清洗以及机械手喷涂时的换色清洗消耗的溶剂都需要详细记录和汇总。其中如何多次使用（减少投入）清洗溶剂也是一个可优化的课题。
3降低VOC排放实践探讨
VOC的产生贯穿了油漆车间整个工艺过程，从电泳涂装的电泳槽、UBS喷涂、中涂底漆喷涂、色漆、清漆喷涂、整理线的点修补、空腔注蜡、车身门板喷蜡以及各个烘房都会产生不同数量的VOC挥发排放。油漆车间有些区域的VOC挥发目前能够计算但无法实行控制，比如生产过程中脱脂槽、电泳槽、UBS喷房、喷漆后的静止室中VOC的挥发；有些区域产生的VOC可以通过管理方式来进行优化控制。
降低VOC排放的方式很多，以下从油漆材料、油漆工艺、涂装设备及过程控制等方面进行探讨。
1）对于溶剂型油漆，丙烯酸树脂大都用二甲苯作稀料调制黏度，溶剂型油漆中稀料的含量视油漆的特性和用途大都保持在55%左右，如底漆中稀料的含量一般在45%、色漆的稀料含量在50%~72%（不同颜色含量不同）；而水性色漆的溶剂含量明显降低，只有14%~18%，所以采用水性色漆替代溶剂型色漆是汽车行业的不二选择。
2）多采用单色漆。单色漆因材料本身原因需要添加的稀料较金属漆要少。车身众多的油漆颜色对客户而言是美不胜收的目视体验，而对生产企业和社会环境却是无奈的选择。生产线每增加一个颜色就多了切换的可能，每切换一次颜色，视金属漆或单色漆不同会增加1.35~2.00kg的稀料消耗，这其中一部分色漆稀料会被回收进入调漆间，另有一部分稀料会进入水幕文丘里，将来从漆渣中回收，也有小部分会挥发至循环风中排放进入大气层，对环境造成一定的影响。所以选择单色漆或精减油漆颜色无疑可以减少VOC排放。
3）提高设备涂装的有效利用率。目前油漆涂装车间使用溶剂型油漆的喷涂设备主要有往复机和机器人两类。喷涂油漆工艺According to油漆材料（金属漆、单色漆）的不同而有差异。金属漆采用ESTA和Spraymate空气喷涂相结合的方式，单色漆采用100%的ESTA喷涂。一般常规换色1次，消耗油漆0.5kg，稀料在2kg左右。目前德国Duerr公司和瑞士ABB公司均有改进喷涂设备换色阀的新举措：增加Pushout功能，通过换色阀的时间控制减少油漆及稀料的损耗。
4）减少空气喷枪的喷涂，有效降低单耗。举例：反射银传统喷涂一般采用ESTA和Spraymate空气喷涂相结合的方式，近来PPG公司开发新产品取消空气喷枪喷涂，有效地降低了单耗。
5）提高同色连喷率（同色连台数）。所谓同色连台数就是喷涂设备喷车的次数/颜色改变次数之比。同色连台数越大，换色次数越小，换色损耗越小。所以油漆车间降低油漆材料消耗可从车身车间开始。首先车身车间尽量做到同种颜色排序上线，保证从车身车间进入油漆车间的车身尽可能同种颜色相连。企业对低比例颜色（俗称小颜色）集中安排生产，即生产部门将收到的订单颜色产量集中在几天内全部喷涂完成，减少油漆颜色频繁切换而带来的油漆、稀料损耗。所以油漆车间提高同色连台数，就减少了色漆及稀料的投入。
6) Reduce rework rate. Reducing the rework rate is the annual control goal of each paint shop, but in the past it was always from the perspective of improving JPH and reducing material costs. If the rework rate is reduced by 1%, 0.1 kg of thinner can be reduced in the consumption of a single paint material, and the reduction of solvent input is an important factor in reducing VOC emissions.
7) Process optimization. Optimize cleaning procedures and reduce the content of cleaning solvents. During the spraying process, when the paint color is switched, it is necessary to use a cleaning agent to clean the spray cup and nozzle, so as to avoid color difference caused by the residual color in the spray cup. For each color change cleaning, a mixture of water-based paint cleaning agent and deionized water with a mass ratio of 3:7 is required. The amount of cleaning agent depends on the ratio of cleaning agent in the mixture (initial planning is set to 2:8). The number of color changes per day depends on the continuous spray rate of the same color of the paint color during spraying. If the continuous spray rate of the same color is high and the number of color changes is small, the amount of cleaning agent used throughout the day will be less. On the contrary, the amount of cleaning agent used will be more. The amount of agent used depends on the number of color changes and the ratio of the mixture. Therefore, it is recommended that the paint shop should adopt the combination mode of "cleaning agent + deionized water" for short cleaning: that is, 2 times of 10 vehicles can be cleaned with cleaning agent, and 8 times can be cleaned with deionized water, which can effectively save the investment of cleaning agent .
4 Epilogue
With the widespread adoption of water-based paint coating technology and Volkswagen's "2010 New Technology", the traditional way of Venturi water adsorption of paint mist is being replaced by limestone dry adsorption treatment and E-scrub water treatment. The new paint mist treatment method has great advantages in saving water and electricity, especially in reducing VOC emissions.
Using water-based paint is a good way to reduce VOC emissions. "2010 Process" is based on the use of water-based paint instead of solvent-based paint, and cancels the intermediate coating process, which is more environmentally friendly. The VOC emission is less than 25g/m2, reaching the international professional level. In addition to using water-based paint, VOC emissions can also be reduced by optimizing control from the perspectives of paint technology, coating equipment, and process control.
