The method described in this paper can solve this problem, that is still using bipolar membrane electrodialysis separation of acid and alkali method, recovery of hydrofluoric acid and nitric acid. Only in this method, the concentration of alkali used in neutralization is required to exceed 1.0N. As a result, the recovery of the acid becomes easier and the concentration of the recovered hydrofluoric acid is increased. Alkali used in the neutralization, in part from the electrodialysis recovery of the alkali pool, and the other part of the base can not be 100% recovery, reuse need to add new alkali, such as the use of solid base. Since the concentration of the salt formed after the neutralization reaction is increased by increasing the alkali concentration, the recovered acid concentration is also increased. In the recovery of hydrofluoric acid-nitric acid spent acid, the alkali that is often used is potassium hydroxide. When the concentration of potassium hydroxide is less than 1.0 N, hydrofluoric acid can not be recovered, and the separation of the bipolar membrane can not be sufficiently exerted.

First, after acid pickling of stainless steel or the like, the resulting waste acid containing nitric acid-hydrofluoric acid is stored in the spent acid tank 2 and then introduced into the neutralization tank 3 through a pipe line and potassium hydroxide having a concentration of more than 1.0 N is added. Then, a metal acid sludge such as Fe (OH) 3, Cr (OH) 3, Ni (OH) 2, etc. is precipitated and filtered, and then the filtrate containing KF and KNO3 is transferred to a bipolar membrane electrodialysis cell 4, and the ionization is performed. In the bipolar membrane electrodialysis cell, KF is ionized into K + and F-, KNO3 is ionized into K + and NO-3, the role of the electric field, K + through the cation exchange membrane 9 enters the caustic chamber 7 and the F- and NO-3 pass through the anion exchange membrane 10 into the acid chamber 6. The water in the solution passes through the bipolar membrane 8 and is ionized into H + and OH- through the bipolar membrane side of the anion exchange membrane into the alkali chamber, and the alkali room in the formation of alkali KOH, reuse in the neutralization tank; H + through the bipolar membrane side of the cation exchange membrane into acid chamber, and the acid anion F- and NO-3 to form HF, HNO3, reuse back to the pickling tank. The neutral desalted water discharged from the salt chamber 5 is discharged through a line or used as a concentration-adjusting solution.