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Surface Finishing Tutorial

#233 Saving Water for Washing and Cleaning (Countercurrent Multistage Washing)

Category : Environmental conservation
April 1, 2016

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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