Method for preparing electronic grade inorganic acids from waste acid solution

A method for preparing electronic grade inorganic acids from a waste acid solution containing hydrofluoric acid, nitric acid and water, comprising the steps of:

• (A) introducing alkali metal salts containing an alkali metal fluoride salt and an alkali metal nitrate salt into the waste acid solution to break azeotrope of hydrofluoric acid and water, the alkali metal salts being present in a molar amount ranging from 20 mol % to 40 mol % based on a total molar amount of the alkali metal salts, hydrofluoric acid, nitric acid and water of the waste acid solution, and then performing a first distillation treatment in a vacuum environment to obtain a hydrogen fluoride vapor that is entrained with mist droplets containing impurities, and a first distillation residue mixture containing nitric acid, water, the alkali metal fluoride salt and the alkali metal nitrate salt;
• (B) removing the mist droplets containing impurities entrained in the hydrogen fluoride vapor so as to obtain a purified hydrogen fluoride vapor;
• (C) subjecting the purified hydrogen fluoride vapor to a first condensation treatment, followed by concentration adjustment using ultrapure water so as to obtain an electronic grade hydrofluoric acid having a purity of 49±0.5%;
• (D) subjecting the first distillation residue mixture obtained in step (A) to a first evaporation treatment in a vacuum environment, such that partial water and nitric acid are evaporated therefrom to form a nitric acid/water mixture, and such that the alkali metal fluoride salt, the alkali metal nitrate salt, and yet-to-be evaporated water are concentrated to form a concentrated solution of the alkali metal salts which is then subjected back to the first distillation treatment of step (A) so as to be recycled;
• (E) introducing an alkali earth metal nitrate salt into the nitric acid/water mixture obtained in step (D) so as to break azeotrope of nitric acid and water, and then performing a second distillation treatment in a vacuum environment to obtain a nitric acid vapor that is entrained with mist droplets containing impurities, and a second distillation residue that is an alkali earth metal nitrate solution containing the alkali earth metal nitrate salt and water;
• (F) removing the mist droplets containing impurities entrained in the nitric acid vapor so as to obtain a purified nitric acid vapor;
• (G) subjecting the purified nitric acid vapor to a second condensation treatment, followed by concentration adjustment using ultrapure water so as to obtain an electronic grade nitric acid having a purity of 70±0.5%;
• and
• (H) subjecting the alkali earth metal nitrate solution obtained in step (E) to a second evaporation treatment, such that partial water is evaporated therefrom to form a concentrated alkali earth metal nitrate solution which is then subjected back to the second distillation treatment of step (E) so as to be recycled,
• wherein in step (A),
• the alkali metal fluoride salt is selected from the group consisting of lithium fluoride, potassium fluoride, cesium fluoride, and combinations thereof,
• the alkali metal nitrate salt is selected from the group consisting of lithium nitrate, potassium nitrate, cesium nitrate, and combinations thereof,
• a molar ratio of the alkali metal fluoride salt to the alkali metal nitrate salt ranges from 0.05:1 to 10:1,
• the vacuum environment has a pressure ranging from 100 mmHg to 500 mmHg, and
• the first distillation treatment is performed in a first distillation tower that includes a polytetrafluoroethylene lining, a bottom portion set at a temperature ranging from 80° C. to 130° C., and a top portion opposite to the bottom portion and set at a temperature ranging from 50° C. to 100° C.