Spent Acid Recovery Using Diffusion Dialysis

Each year, million gallons of acid solutions are used by the Department of Defense DoD and its support contractors in various metal finishing operations such as stripping, etching, activation, passivation and pickling. Over time, these acids become contaminated with metals and are discarded at considerable expense as hazardous waste. Often the acid content of the discarded acid is comparable to the beginning solution. Diffusion dialysis is a recently commercialized membrane separation technology that can be used for the recovery of a wide variety of acids from high strength, metal contaminated solutions such as produced in metal finishing operations. It is extremely easy to install and operate, requiring only a source of deionized water and electricity, and it produces an acid stream that has an acid strength comparable to that of the contaminated acid, making the acid potentially suitable for replacing and maintaining metal-finishing bath contents without further processing. In this Environmental Security Technology Certification Program ESTCP project, commercially available diffusion dialysis systems were deployed and operated at two DoD metal finishing facilities to verify performance and reuse potential of the recovered acid. The first unit was deployed in stand-alone, batch-processing mode at Tobyhanna Army Depot, Tobyhanna, PA to recover nitric and sulfuric acids from spent copper bright dip CBD, and nitric acid and ammonium bifluoride from spent magnesium bright dip MBD. Spent acid accumulation from the CBD and MBD processes at Tobyhanna is approximately 800 gallons per year. The second unit was deployed at the Rock Island Arsenal, Rock Island, IL in dedicated, continuous-processing mode to purify and return hydrochloric acid to a 4000-gallon chrome-stripping bath. The studies demonstrated that diffusion dialysis is a highly reliable and a viable acid recovery technology for the recovery of nitric, sulfuric, hydrofluoric, and hydrochloric acids.