Aquatech membrane technology for recovery of acid/base values for salt streams

Abstract

Bipolar membrane water-splitting technology, which is presently being commercialized by the Aquatech Systems unit of Allied-Signal, is a cost effective route to recovery and recycling of salt streams. Such streams are routinely generated in processing operations such as metal pickling, rayon manufacture, flue gas scrubbing, fermentation, etc. Recovery and disposal of such salts is proving to be increasingly expensive becuase of the high cost of capital and the stringent environmental regulations. In the Aquatech process, the salt stream is directly converted to its consituent acid and base using a direct current driving force. The process, which is electrodialytic, uses a two- or three-compartment cell incorporating bipolar and monopolar membranes. The first commercial application of this technology is on the recovery and recycling of mixed acids used in stainless steel pickling. The spent acids containing metal fluorides and nitrates, form a hazardous waste that presents a significant disposal problem. In our process the spent acid is neutralized with a soluble potassium base (KOH) to precipitate the metal hydroxides. The filtrate from the recovery step containing the soluble salt KF/KNO 3 is then processed in a three-compartment Aquatech cell to regenerate the mixed acid and base. The acid is then sent back to pickling while the base is recycled to the steel production step. The process thereby eliminates a waste disposal problem while generating valuable acids. The Aquatech technology is applicable to sodium sulfate recovery on an even larger scale. Such streams are generated during neutralization, ion-exchange and rayon manufacturing operations. These streams can be converted directly to caustic soda and sulfuric acid and recycled to the font end of the operating plant. Some of these streams are quite dilute and consequently require preconcentration via conventional electrodialysis. The concentrated salt solution is then processed in the Aquatech cell. Depending on the desired product quality two- or three-compartment arrangement may be required. Details of the process, pretreatment requirements and process economics are discussed.