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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.8 Residuals Generation
The primary residuals generated by the use of membrane electrolysis are spent catholyte and associated sludge. During the process, cations are electrically transported from the anolyte (process bath) through the ion specific membrane into the catholyte. The nature of the residual generated during the process depends mostly on the selection of the catholyte, either acid or caustic. With an acidic electrolyte (typically sulfuric acid), the electroplatable cations deposit on the cathode and the non-electroplatable cations stay in that solution as salts. Periodically, the cathode is cleaned of the sludge-type deposit and the catholyte solution is discarded and replaced (ref. 370). The smallest GOEMA unit has a catholyte volume of 10 gal (20% sulfuric acid). It is typically replaced every 4 months (30 gal/yr) (ref. 384). With a caustic catholyte (a mixture of alkalis used in a patented approach by Ionsep), the high pH converts multi-valent metal cations that enter the catholyte solution into insoluble hydroxides. The hydroxyl ions needed to react with the metal cations that enter are formed at the cathode of the cell. The precipitated metals buildup in the catholyte. With smaller systems, it is usually preferable to replace the catholyte (approximately every one to four weeks). With larger systems, the precipitated metals are separated using a filter press and the catholyte is returned to service (this practice greatly reduces the volume of waste generated by the plating/purification process). Alternatively, a conical bottom catholyte tank can be employed and the solids (with some catholyte) can be drained from the cone of the tank and disposed of as a slurry. In some cases, the metal hydroxides are reusable for other purposes (e.g., ferrous hydroxide, which can be used for making pigments, is produced during applications with steel pickling solutions). The Ionsep catholyte tank typically has a volume of 55 gal (plastic drum), although large installations may have tanks with volumes up to 300 gal.
Two shops operating the Ionsep system provided data on residuals production. PS 006 operated the process on a 2,800 gal hard chromium bath (3 plating tanks). They generated 55 gal/wk (2,860 gal/yr) of spent catholyte. PS 052 operated the process with 12,700 gal of hard chromium solution (7 plating tanks) and generated 55 gal/month (660 gal/yr) of spent catholyte.
If a plating shop using membrane electrolysis has a conventional wastewater treatment system in place, the spent catholyte could be bled to this system or batch treated and then discharged to the general treatment system. Treatment of the catholyte will generate a sludge that under most conditions will be considered a RCRA F006 sludge. The sludge volume will depend mainly on the metal content of the catholyte and the dryness (percent solids) of the resultant sludge.
Spent catholyte may meet the characteristics of a hazardous waste due to its pH and or toxic metals content (particularly chromium). Treatment of the catholyte may be interpreted as a hazardous waste treatment process (RCRA controlled) rather than wastewater treatment (Clean Water Act controlled), especially for shops that have achieved zero discharge. Before developing a treatment and disposal scheme, the plater must investigate applicable RCRA and Clean Water Act rules, including state and local regulations, relative to the treatment process.