Water

NaOH

Figure IS. Reciprocating-flow ion exchanger. SOURCE. Cialw, <3.0., i«S5 (Bit. 10)

resins are inefficient for treating wastewaters with pH values greater than 7.0. For recovery of metals to be cost-effective, metal wastewaters should be free of complexing agents (e.g., cyanide).

Pretreatment Reouirements--Suspended solids must be removed from the wastewater before passing the stream through ion exchange systems. Filtration systems are placed in-line before the ion exchange system. For low-volume systems, activated-carbon, deep-bed, diatomaceous earth precoat, and resin filters can be used to pretreat the stream. The filters are replaced when head losses result in excessive filter cycle times. Multimedia sand filters, which contain backwashing regeneration systems, are used in large-volume systems.

Certain organics {e.g., aromatics) and complexing agents (e.g., cyanide) become irreversibly sorbed onto the exchange resin and reduce the effectiveness of metal removal. Oxidants, such as chromic or nitric acid, can damage the resin structure. Sodium metabisulfite can be added to reduce resin damage. Using activated carbon for pretreatment can prevent detrimental organics from reaching the ion exchange resin.

Posttreatment Reoui rements--Wastewater from ion exchange contains spent regenerating solution, wash, and filtered solids. Regenerating solutions, especially highly concentrated RF1E solutions, are amenable to metals recovery by methods such as electrowinning. The filtered solids from the pretreatment systems can generally be landfilled provided the wastes intended for disposal meet the land disposal provisions set in 40 CFR Part 266. Electroplating regenerating solutions are typically returned to the plating baths in art effort to economize production. Metal chelates can be processed for metals recovery by electrodialysis or precipitation.

Performance Data-Ion exchange systems have proven to be effective in the removal of barium, cadmium, chromium {VI), copper, lead, mercury, nickel, selenium, silver, uranium, and zinc. Table 45 presents data on the effectiveness of ion exchange in removing soluble metal ions in commercial, pilot-plant, and laboratory operations. In the electroplating industry, ion exchange has achieved 98 percent removal of cadmium, 99+ percent removal of nickel, 99+ percent removal of selenium, and 91,7 percent removal of silver. A 40-gal/min ion exchange/electrolytic recovery system installed at the Tingstol Company in Chicago to recover copper from printed-circuit-board rinse waters yielded effluent concentrations below 1.5 ppm from feed concentrations up to 500 ppm copper.

The electroplating industry is currently using ion exchange for the removal/recovery of cadmium, chromium, copper, nickel, silver, and zinc.

Chelating agents have been shown to achieve 90 percent removal efficiencies of lead (>I percent) in contaminated soil; however, removals fall off dramatically as contaminant concentrations decrease. The performance of chelating agents depends greatly on the specific waste matrix and must be evaluated on an individual basis.27

TABLE 45. ION EXCHANGE REMOVAL EFFECTIVENESS*

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