Unit

Figure 21. Closed-loop evaporative recovery system. Source: G. C. Cushnie Jr., 1985 (Ref. 10)

industry, the use of double-effect units is advantageous because two different plating baths can be recovered simultaneously, A possibility of cross-contamination exists, however. Because of the low rinse ratio, three or more rinse tanks are needed to make a closed-loop system economical. Open-loop systems should be used whenever rinsing rates cannot be reduced (see Figure 22). Further treatment may be necessary to recover the metals in the concentrate, depending on the concentrate's composition.

The most effective rinsing technique for increasing metal recovery is counterflow rinsing. The same rinse water can be used twice and through two tanks. Recovery rates depend on the wastewater flow rate, and they increase with the number of counterflowing rinses (as shown in Table 47). Recovery is often set at 99 percent for evaporation systems in the plating industry.

TABLE 47. RECOVERY PERCENTAGE AS A FUNCTION OF NUMBER OF RINSES'

Evaporation Number of counterflowing rinses, % rate _

TABLE 47. RECOVERY PERCENTAGE AS A FUNCTION OF NUMBER OF RINSES'

Evaporation Number of counterflowing rinses, % rate _

1i ters/hour

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