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Pollution Prevention and Control Technologies for Plating Operations

Section 3 - Chemical Recovery


3.4.8 Residuals Generation

The primary residuals from ion exchange recovery processes are the regenerants (eluates) and backwash solutions. The regenerants are concentrated wastes and the backwash is dilute. Both solutions are either caustic or acidic, depending on the resin type and application. High metal bearing regenerates (typically cation resin) are: (1) sometimes reused directly in the bath; (2) further processed to recover the metal (e.g., electrowinning); (3) waste treated; or (4) sent to an off-site recovery facility. Low metal bearing regenerants (typically anion resin) and backwash solutions are typically treated on-site. Waste treatment processes generate sludge that is an EPA listed hazardous waste (F006).

The volume of regenerant produced will depend on the regeneration requirement (e.g., lbs of acid per ft3 of resin) and the concentration of acid used (typically 1 to 5%). The regeneration requirement will depend on the resin type, application (metal or complex being recovered) and the configuration (cocurrent vs counterflow). Typical volumes of regenerant are 20 to 50 gal/ft3 of resin. The volume of regenerant waste is sometimes reduced by reusing the last portion of the regenerant, which will be less contaminated with metal and contains free acid. Backwash volumes depend mostly on the equipment design and the application. Typically, backwashing generates 25 to 75 gal/ft3. The backwash is partly reused by some equipment vendors as make-up water for regenerant, in an effort to reduce the total waste volume generated. Because backwash contains only dilute concentrations of pollutants it is typically not a major concern and is treated on-site and discharged. However, for shops working toward zero discharge, the backwash volume could present a significant problem. Both backwash and regenerant can processed by evaporation to reduce the volume requiring disposal. However, this increases the capital and operating costs of the system. Also, evaporation of hazardous wastes is sometimes regulated as a RCRA technology and may require a permit to operate.

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