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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.7 REVERSE OSMOSIS
3.7.3 Applications and Restrictions
Exhibit 3-54 shows an example of applying reverse osmosis as a chemical recovery technology. In this example, nickel salts are recovered from rinse water and returned to the bath. A three stage counterflow rinse system is used in order to concentrate and reduce the flow rate of the feed to the RO unit. The 100 gph feed rate is typical for this application. The feed is pumped (high pressure feed pump) through a cartridge filter (typically 5 micron) and through the RO system. The reject passes through a carbon filter to remove bath impurities and is returned to the plating bath. In this example, the surface evaporation of the bath is sufficient to create the necessary head-room. The permeate is returned to the counterflow rinse system.
The primary plating chemical recovery application for RO is nickel plating. This includes Watts nickel and bright nickel plating (ref. 382). A wide range of other successful applications are identified in the literature (mainly articles written by RO equipment manufacturers). Some of the more frequently discussed applications include: brass cyanide, cadmium cyanide, copper cyanide, non-cyanide alkaline zinc, and zinc cyanide (ref. 156, 157, 263). From the Users Survey, there were only two successful application of RO for chemical recovery. These involved recovery of nickel acetate seal and acid zinc drag-out (PS 010, 230). Another shop successfully operates an RO unit on a cadmium cyanide process, but does not return the concentrate to the bath due to their concern over bath contamination (PS 131). This shop indicated that they operate under stringent aircraft manufacturer's specifications.
Reverse osmosis is applicable to the recycle of effluent from an end-of-pipe treatment system. End-of-pipe systems that employ hydroxide precipitation generate an effluent that is relatively free of toxic metals; however, it contains a high concentration of total dissolved solids (TDS). A typical effluent contains between 500 to 4,000 mg/l of TDS (ref. Memtek file). Although approximately 1.2% of the shops responding to the Users Survey indicated that they directly reuse this effluent (see Section 2.6), most sources indicate that it is of insufficient quality for rinsing, especially in critical rinse situations. Typically, rinse quality criteria for functional and bright plating is in the range of 100 to 700 mg/l and 5 to 40 mg/l TDS, respectively (Exhibit 2-16). Most end-of-pipe RO wastewater recycle applications consists of: preconditioning to prevent the precipitation of salts in the membrane; prefiltration; RO filtration and storage. One survey respondent used RO and other recycling technologies in place of an end-of-pipe treatment system (PS 233). Their system is described in Exhibit 3-55, where RO is incorporated into a complex, zero discharge configuration. In this application, RO is employed to upgrade the quality of the ultrafiltration permeate so that it can be reused as rinse water. Because this plant operates at zero discharge, the concentrated stream from the RO unit is hauled off-site for treatment/disposal; ordinarily, it would be treated on-site.
RO is generally not considered applicable to highly concentrated, oxidative solutions, like chromic acid, nitric acid and peroxy-sulfuric etchant. Process chemicals in these solutions can be recovered using RO; however, membrane life-span is a concern (ref. 157). The membrane life-span for these applications was demonstrated on a bench scale to be only 15 to 35% of that for nickel plating applications (ref. 157). However, one supplier advertises use of their equipment for chromate conversion coating (ref. KRC file). That company agrees that membrane life is shortened by this application, but indicated that RO is still cost effective. For this application, the primary competing technology is vacuum evaporation.
The unsuccessful chemical recovery applications from the Users Survey included: Watts nickel (PS 172), zinc cyanide (PS 008), acid zinc (PS 010), and copper cyanide (PS 089). The reasons for failure of these systems relate mostly to fouling, and are discussed in subsection 3.7.7.