In the experiment, a different concentration of KOH was used to observe the recovery of hydrofluoric acid. For example, 10 liters of a 4.0 g /L KOH solution was added to a 10 L metal waste washing solution containing 2.0 eq /L of hydrofluoric acid and 2.0 eq /L of nitric acid to carry out a neutralization reaction to precipitate a metal acid sludge. (Eq /L is the amount of sol equivalent in 1 L of solution). After filtration, the filtrate thus obtained had a KF of 1.0 eq /L and a KNO3 of 1.0 eq /L. Subsequently, the filtrate was transferred to a bipolar membrane electrodialysis cell, and 8.31 L of the mixed acid was recovered. Among them, hydrofluoric acid concentration reached 1.59eq /L, nitric acid concentration reached 2.41eq /L, hydrofluoric acid recovery rate reached 66.2%. The concentration of KF in the recovered 13.53 L desalting solution was 0.5 eq /L.

In the same waste acid solution of 10 L, 35 L of 1.14 eq /L KOH solution was added to carry out neutralization reaction to precipitate the metal acid sludge and filtrate. The obtained 45 L filtrate contained 0.44 eq /L of KF and 0.44 eq of KNO3 /L. Then, the filtrate was transferred to a bipolar membrane electrodialysis cell, and 5 L of a mixed acid was recovered in which the concentration of nitric acid reached 4.0 eq /L and the hydrofluoric acid was not recovered, resulting in a concentration of KF of 35 L in the desalting solution 0.57 eq /L.

From the data analysis of the above examples and comparative examples, it was found that the potassium hydroxide solution used must be at least a certain concentration in order to increase the hydrofluoric acid recovery concentration. In addition, it can be seen from the experiments that the KOH solution used in Experiment 5 was only 10 L when the KOH concentration was increased and the volume of the solution at neutralization was reduced, whereas the amount of the base used in Comparative Example 3 was 35 L, reduced by more than 2/3.