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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.4 ION EXCHANGE
3.4.1 Overview
Ion exchange is a technology that is very familiar to the plating industry. Of the 318 plating shops responding to the Users Survey, at least 80 or (25%) have used ion exchange, including 34 applications for chemical recovery (the total number of ion exchange users is not known since not types of all applications were covered by the Users Survey). The five most common applications of this technology by the plating industry are: (1) treatment of raw water (e.g., city water) to produce high quality rinse water (includes both softening and deionization); (2) chemical recovery from rinse water; (3) treatment of plating baths to remove contaminants (e.g., tramp metals); (4) as a primary end-of-pipe treatment process; and (5) as a polishing end-of-pipe treatment process to comply with stringent effluent limitations. Chemical recovery applications of ion exchange are covered in this section. Bath maintenance and end-of-pipe treatment applications are covered in Sections 4 and 6, respectively. Raw water treatment is not covered by this project, although some respondents indicated in their survey responses that they use ion exchange for raw water treatment (e.g., PS 127, PS 196, PS 203).
Ion exchange is a chemical reaction wherein an ion from solution is exchanged for a similarly charged ion attached to an immobile solid particle (i.e., ion exchange resin). Ion exchange reactions are stoichiometric (i.e., predictable based on chemical relationships) and reversible. The resins are normally contained in vessels referred to as columns. Solutions are passed through the columns and the exchange occurs. Subsequently, when the capacity of the resins is reached, the ions of interest, which are attached to the resin, are removed during a regeneration step where a strong solution containing the ions originally attached to the resin is passed over the bed.
The strategy employed in using this technology is to exchange somewhat harmless ions (e.g., hydrogen and hydroxyl ions), located on the resin, for ions of interest in the solution (e.g., plating chemicals). In the most basic sense, ion exchange materials are classified as either cationic or anionic. Cation resins exchange hydrogen ions for positively charged ions such as nickel, copper and sodium. Anion resins exchange hydroxyl ions for negatively charged ions such as chromates, sulfates and cyanide.
Ion exchange resins are usually contained in vessels referred to as columns. The basic column consists of a resin bed which is retained in the column with inlet and outlet screens, and service and regeneration flow distributors. Piping and valves are required to direct flow and instrumentation is required to control regeneration timing. The systems are typically operated in cycles consisting of the following steps (ref. 39, 348):
- Service (exhaustion) - Water solution containing ions is passed through the ion exchange column or bed until the exchange sites are exhausted.
- Backwash - The bed is washed (generally with water) in the reverse direction of the service cycle in order to expand and resettle the resin bed.
- Regeneration - The exchanger is regenerated by passing a concentrated solution of the ion originally associated with it through the resin bed; usually a strong mineral acid or base.
- Rinse - Excess regenerant is removed from the exchanger; usually by passing water through it.