April 2016 Archives

#237 Surface Treatment and Environmental Conservation

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Surface treatment and environmental conservation

Background of environmental conservation

In the past, metal surface treatment plants including electroplating factories were exposed to criticism that they were responsible for pollution problems resulting from cyanogen or hexavalent chromium. This had forced the management to make a substantial investment out of their scarce monetary resources in preventing such pollution, which caused them to bear with the high running cost.

The Water Pollution Control Act must be enforced for wet surface treatments as it involves water and its drainage. The Poisonous and Hazardous Substances Control Law applies to processes that involve use of acid/alkali, poisonous, or deleterious substances. In addition, the "effluent standards" must be observed for water discharged into public waters. The Sewerage Act applies to water discharged into sewers. Such drainage must be compliant with the "effluent standards" defined by the Sewerage Code of the corresponding district.

If it involves a process that generates harmful gas or mist, the concentration measurement and voluntary inspection are required for the department in charge of the process according to the Industrial Safety and Health Act. This includes an installation of purification systems or adopting an operation management process.

Boilers used for drying plating solution or products as well as bake finish of paints are regulated by the exhaust emission standards under the Air Pollution Control Act.

Each local government has its own "antipollution ordinance" to control water quality, air, noise, vibration and so on.

More restrictions are added to these laws and regulations year after year. Since the initial enactment, the regulatory values have become stricter than ever.

Aside from the pollution problems we talked about so far, the world is also witnessing a deterioration of global environment in recent years, such as the disappearing ozone layer by CFCs used in refrigerant and spray cans, and global warming associated with the carbon dioxide increase resulting from fuel burning of automobiles and thermal power plants.

Various measures have been adopted for preventing further deterioration. One of them is called VOC regulations (VOC refers to Volatile Organic Compounds including organic solvents). Another example is the CO2 emission reduction policy as part of the energy-saving measures. In addition, more companies are now practicing "green procurement", which is a strategy of preventing environmental pollution by avoiding materials containing harmful substances like cadmium, lead, or mercury in the procurement process. These measures have been adopted not only in Japan but also on a worldwide scale with an exception of some countries.

In the European Union (EU) for example, they have enforced the RoHS Directive regulating the harmful substances contained in electrical and electronic devices together with the WEEE Directive regulating the industrial waste such as electrical products since the year 2007. These directives regulate the amount of organic/inorganic harmful substances in products.

The Soil Contamination Countermeasures Act was enacted in Japan to resolve soil contamination problems that occur frequently with redevelopment of old factory site into a residential area. This regulation not only applies to selling the property that was formerly used as factory site but also applies to cases where the land usage changes even when the owner remains the same. The process requires analysis of the soil to check for harmful substances and its concentration. If the soil is contaminated, the land is subject to soil purification.

Let's look into the details of these regulations from the next lecture.

#236 Saving Water for Washing and Cleaning (by Ion Exchange) -2

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There are various combinations of ion-exchange towers. [Fig.1] illustrates an example of towers assembled for purifying plating wastewater. This is probably a good example of learning the types of ions that can be adsorbed by each type of ion-exchange towers.

Fig.1

Based on these adsorption characteristics, ion-exchange towers are proven to be effective in the following usages:

1. If you circulate water in the final rinse tank used for countercurrent multistage washing and adopt the ion-exchange method, it will eliminate fresh feed-water and drainage from the system.
2. In the similar manner, if you use chelating resin that separates and adsorbs certain metals, the resin collects the certain metals from the eluting solution. (Examples of metals are gold, silver, platinum, nickel, copper, etc.)
3. Use an ion-exchange tower to adsorb and eliminate metal of impurities in the plating solution collected by spray washing. By purifying the plating solution, you can extend the operating life and prevent aging or wasting of the plating solution.

[Fig.2] illustrates an example of the rinsing water circulation system with an ion-exchanger assembled in the final rinse tank. This system uses the countercurrent multistage washing method along with the forced evaporation method for the plating solution.

Fig.2

#235 Saving Water for Washing and Cleaning (by Ion Exchange) -1

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Water purification using ion-exchange resin is an old-established method for pure water production. This method can be adopted for surface treatment processes such as the plating wastewater treatment.
Ion-exchange resin is available in the form of cation or anion exchanger. Each of them can be further divided into strongly acidic, mildly acidic, strongly basic, and weakly basic resins. A tower filled with such ion-exchange resin is called an ion-exchange tower. In this tower, water runs from the top toward the bottom. During this time, the following adsorption reaction occurs in the ion-exchange tower:(R: ion-exchange resin)

For the cation-exchange tower, R-H + K+ => R-K + H+
For the anion-exchange tower, R-OH + A- => R-A + OH-

In the cation-exchange tower, hydrogen ion is generated when hydrogen bonded to cation-exchange resin is replaced with cation K+ (Cu2+ or Ni2+, for example) in the treatment water. In the case of anion-exchange resin, hydroxide ion is generated when the hydroxide reacts with the anion A-. Ultimately, water (H2O) is generated by the reaction of H+ and OH-.
The ion-exchange resin can remove cation and anion impurities dissolved in water by the mechanism described here.

[Fig.1] One cycle of ion-exchange resin

However, a specific limit applies to how much ion-exchange resin can replace. After exceeding a certain amount of ions, the resin is no longer capable of adsorbing more ions. Therefore, it is necessary to perform the "regeneration" task to restore the adsorption ability of the resin. The following reaction occurs in the regeneration process:

For the cation-exchange tower, R-K + H+ => R-H + K+
For the anion-exchange tower, R-A + OH- => R-OH + A-

As shown in [Fig.1], this series of tasks are carried out in the cycle of backwash, regeneration, and cleaning processes. The backwash step loosens up resins in the tower. To regenerate the cation-exchange resin, acids with H+ such as hydrochloric acid and sulfuric acid will be used. To regenerate the anion-exchange resin, alkali with OH- such as NaOH will be used. The cleaning step eliminates residual chemicals from the regeneration process.

[Fig.1] illustrates the system with a spray-cleaning unit adopted prior to the process of countercurrent multistage washing. Spray cleaning is a highly effective method of cleaning products using a small amount of water.

Fig.1

This system has the two characteristics.
One of them is the significant reduction of plating solution brought into the first rinse tank since almost all of the plating solution will be removed by spray cleaning. This can greatly reduce the overall water usage for washing and cleaning.
The second characteristic is that relatively thick plating solution separated by spray cleaning can be restored back to the plating bath as long as the amount is within a range of natural evaporation from the plating bath. The amount of natural evaporation increases when the operating temperature of the plating bath is relatively high. As shown in [Fig.1], the plating solution can be collected easily under this condition.
If it is not possible to restore the entire plating solution collected due to insufficient evaporation volume, collect them by forcibly vaporizing the separated solution. To vaporize plating solution forcibly, adopt the "air evaporation" method used for cooling towers. At this time, the heating unit of the plating bath supplies latent heat of water evaporation.
When the solution has low bath temperature or is likely to dissolve, the batch-type vacuum evaporation can be also adopted.

Evaporation of the plating solution is significantly affected by its temperature and circulating volume, exhaust airflow rate, external temperature and humidity, etc.[Fig.2] shows the conditions of evaporation.

Fig.2

Most of the water used in surface treatment plants will be for washing and cleaning. Starting from the pre-processing, each process including plating, anodic oxide coating, chemical conversion treatments, and a series of post-processing steps involves washing and cleaning by water before moving onto the next process. Our objective here is to save water by reducing the amount of water usage for washing and cleaning.

In general, "water washing" is performed in a rinse tank while running a small amount of water into it. In this process, the plating solution attached to the product will be dispersed into water when you soak the product into a rinse tank.

A rinse tank unit consisting of two to three tiers instead of one tank is used for this water washing process. In this process, the product is supplied from one side while fresh cleaning water flows from the other side. This method is called "countercurrent multistage washing". In this method, the water of the uppermost stream is always clean even when the water on the bottom of the rinse tank is heavily contaminated.

Now, let's calculate the amount of water required for washing and cleaning in the following conditions:

Plating solution concentration: 375 g/L, pumping-out amount of plating solution: 3.8 L/H, and final concentration of the rinse tank: 0.047 g/L
(Note: g/L represents the number of grams in one liter. L/H represents the number of liters to be pumped out per hour.)

[Fig.1] shows the example of using one rinse tank. [Fig.2] is the example of using two tanks. [Fig.3] is the example of using three tanks. The rinse water amount, which was 30,300 L/H for one tank, is reduced to 340 L/H for two-tier washing. If you use three tanks, the water required is only 76 L/H. These examples conclude that multistage washing is effective in reducing the water usage drastically.

[Fig.3] [Fig.2] [Fig.3]

The above numbers were calculated by the following formula:

Quantity of water for countercurrent multistage washing
formula
In this formula, W is the required quantity of water (L/H), D is the quantity of water to be pumped out (L/H), n is the number of rinse tanks, C0 is the concentration of plating solution (g/L), and Cn is the final concentration of the rinse tank (g/L).

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