Concentration of nitric acid using the salt effect on vapor-liquid equilibrium of nitric acid-water systems with low concentrations of nitric

When involatile salts are added to an azeotropic mixture, the so-called salt effect changes the vapor-liquid equilibrium, and has a big influence on relative volatility. Aqueous nitric acid is an azeotropic mixture. and cannot be concentrated by ordinary distillation. In this study, we measured the vapor-liquid equilibrium of nitric acid-water-nitrate salt systems with azeotropic composition (x=0-40 mol%) for recycling of nitric acid in solutions from semiconductor manufacturing and metal-surface treatment processes. Aluminum nitrate, magnesium nitrate, calcium nitrate, lithium nitrate and sodium nitrate were used as nitrate salts. For each salt tested the salting-out effect was confirmed and the azeotropic point was eliminated completely by adding salts at concentration of 30-50wt%. Consequently, nitric acid could be separated by distillation from waste solution with a low concentration (under azeotropic composition) of nitric acid. Separation and concentration of nitric acid from dilute aqueous nitric acid was tested by simple distillation using 50wt% lithium nitrate. As a result, nitric acid aqueous solution was concentrated from 5.0 to more than 14.0 mol/dm(3) (yield, 72.2%).