Distillation Separation of Hydrofluoric Acid and Nitric Acid from Acid Waste Using the Salt Effect on Vapor–Liquid Equilibrium

Abstract

This study presents the distillation separation of hydrofluoric acid with use of the salt effect on the vapor–liquid equilibrium for acid aqueous solutions and acid mixtures. The vapor–liquid equilibrium of hydrofluoric acid + salt systems (fluorite, potassium nitrate, cesium nitrate) was measured using an apparatus made of perfluoro alkylvinylether. Cesium nitrate showed a salting-out effect on the vapor–liquid equilibrium of the hydrofluoric acid–water system. Fluorite and potassium nitrate showed a salting-in effect on the hydrofluoric acid–water system. Separation of hydrofluoric acid from an acid mixture containing nitric acid and hydrofluoric acid was tested by the simple distillation treatment using the salt effect of cesium nitrate (45 mass%). An acid mixture of nitric acid (5.0 mol · dm−3) and hydrofluoric acid (5.0 mol · dm−3) was prepared as a sample solution for distillation tests. The concentration of nitric acid in the first distillate decreased from 5.0 mol · dm−3 to 1.13 mol · dm−3, and the concentration of hydrofluoric acid increased to 5.41 mol · dm−3. This first distillate was further distilled without the addition of salt. The concentrations of hydrofluoric acid and nitric acid in the second distillate were 7.21 mol · dm−3 and 0.46 mol · dm−3, respectively. It was thus found that the salt effect on vapor–liquid equilibrium of acid mixtures was effective for the recycling of acids from acid mixture wastes.