In the metal pickling process, often discharge containing nitric acid, hydrofluoric acid and other waste acid. Effective removal of metal sludge, so that nitric acid, hydrofluoric acid and other waste acid by separation and concentration of recycling, is currently in the metal processing problems to be solved.

The waste acid is first sent to the neutralization tank through a pipeline and neutralized by adding alkali solution (such as KOH). The metal is precipitated and then separated and filtered. The filtrate is introduced into a membrane composed of bipolar membrane, cation exchange membrane, anion exchange Membrane bipolar membrane electrodialysis cell, the acid, alkali separation, separation of acid back to the pickling tank, alkali back to the neutralization tank for recycling. However, the recovery of the acid concentration of this method and the original acid compared to the concentration of both decreased, especially in the nitric acid containing waste, the recovery of hydrofluoric acid concentration is very low, difficult to reuse. Therefore, the acid must be added in the re-use of new hydrofluoric acid. Therefore, increasing the concentration of hydrofluoric acid recovered, reducing the discharge of desalting solution, and reducing the amount of hydrofluoric acid to be replenished, is a problem to be solved at present.