Capture Concentration Techniques

9.3.3.41 Capture/Concentration with Full Reuse of Drag-Out

The pioneer in simple, low-cost methods of reducing waste in the plating shop was Dr. Joseph B. Kushner.9 In Water and Waste Control for the Plating Shop (1972), he describes a "simple waste recovery system" that captures drag-out in a static tank or tanks for return to the plating bath. The drag-out tanks are followed by a rinse tank that flows to the sewer with only trace amounts of polluting salts and is often in compliance with sewer discharge standards. A simplified diagram of this reuse system is shown in Figure 9.2. It is not difficult to automate the direct drag-out recovery process, and commercial units are available.

Evaporation out, Q2

Evaporation out, Q2

Concentrate out, Q2

Concentrate out, Q2

- Water supply in, Q2

Water supply in, Q1

Flowing rinse tank

Double drag-out tank

Work

Rinsewater out to sewer, Q1

FIGURE 9.2 Kushner method of double drag-out for full reuse. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)

The Kushner concept is easily applicable to hot plating baths where the bath evaporation rate equals or exceeds the pour-back rate, Q2. The drag-out concentration depends on the bath drag-out rate, the number of drag-out tanks, the rinsewater flow rate, Q2, the plating bath evaporation rate, and drag-out return rate. The number of drag-out tanks must be based on the available space. The higher the number of counterflowed drag-out tanks, the smaller will be the return rate necessary to obtain good rinsing. The Kushner multiple drag-outs are not feasible if there is no room for the required drag-out tanks. If there is little or no evaporation from the bath, supplementary evaporation should be considered. Bath contamination must be minimized by using purified (RO) water for Q2.

9.3.3.4.2 Capture/Concentration with Partial Reuse of Drag-Out

By adding a trickling water supply and drain, Q3, to the drag-out tank, the application of Kushner's concept can be extended to other metal finishing processes that may not be amenable to full reuse but can allow partial reuse. Figure 9.3 depicts the partial reuse scheme. The trickle concentrate can also be batch treated in a small volume on-site, recycled at a central facility, or mixed with Qj, for discharge, if the combined metal content is below sewer discharge standards.

9.3.4 Waste Reduction Costs and Benefits

The benefits of waste reduction in the metal finishing shop include the following:

1. Reduced chemical cost

2. Reduced water cost

3. Reduced volume of "hazardous" residuals

4. Reduced pretreatment cost.

The benefits of saving valuable chemicals and water and reducing sludge disposal costs can best be illustrated by an example. An electroplating operation discharges 98,400 L/d of wastewater containing 0.91kg of copper, 1.14kg of nickel, and 0.91kg of cyanide. The shop can reduce its

Evaporation out, Q2

Trickle water supply in, Q3 -1 1- Water supply

Evaporation out, Q2

Concentrate out, Q2

Trickle water supply in, Q3 -1 1- Water supply

Double drag-out tank

Trickle concentrate out, Q3

Concentrate out, Q2

Double drag-out tank

Trickle concentrate out, Q3

Water supply in, Q1

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