Distribution of hydrochloric acid in evaporation of nitric acid solutions and rectification distillation of nitric acid

Abstract

In reprocessing spent nuclear fuel, nitric acid process solutions are tbrmed with different levels of radioactivity (raffinates, re-extracLs, etc.). Usually they arc evaporated down in order to reduce their volume, and the liquor vapors are subjected to rectification distillation in order to regenerate the nitric acid and to obtain a condensate suitable for disposal and recycling. Nitric acid process solutions (systems of the nitric acid-water-metal nitrates type) can contain hydrochloric acid impurity, a component which is a corrosion hazard. Therefore data on the distribution of hydrochloric acid between the evaporation and rectification products are of practical interest. Let us present the results of investigations of the distribution under conditions simulating evaporation and rectification distillation. The investigations were carried out using the circulation method in [ l] and reproduction of the continuous evaporation process in [2]. The composition of the salt solutions was specified, the composition of the salt-free solutions was determined: nitric acid was determined by alkalimetric titration, hydrochloric acid was determined by nephelometric titration (with silver nitrate). Data are available on the equilibrium between the liquid and the vapor in the nitric acid-water-hydrochloric acid system at atmospheric pressure and hydrochloric acid concentration greater than 10 g/liter [3]. We have obtained data for salt systems under vacuum and at atmospheric pressure for hydrochloric acid concentration down to 5 g/liter. From analysis of Figs. 1-3, it follows that for the studied concentration, the distribution coefficient for hydrochloric acid (the ratio of the concentration in the condensate of the vapor and the liquid -- the bottoms) may be either less than or greater than I. Therefore for continuous evaporation, the concentration in the bottoms will be higher than in the original solution if K l; and in the condensate it will approach the original value as the degree of evaporation increases. Hydrochloric acid will be absorbed in the reflux on the scrubber trays usually installed to remove radionuclides from the vapor. Accumulation of hydrochloric acid in the bottoms and in the reflux will be greater for a lower nitric acid concentration. The accumulation is promoted by iron nitrate (probably as a result of formation of complexes of iron and chloride ions), and conversely the accumulation is decreased (distillation is promoted) by sodium, aluminum, and uranyl nitrates and phosphoric acid (probably as a result of their salting out effect, causing an increase in the effective concentration of nitric acid [4] and an elevation of the boiling point). We also experimentally established that distillation of hydrochloric acid is also promoted by other components of the raffinates such as manganese, chromium, calcium, lead, cerium, and lanthanum nitrates. The effect of the salts increases as their concentration increases. The liquor vapors will contain the major amount of.nitric and hydrochloric acids. During their rectification, weakly acidic (pH > 1) condensate and acid (usually 10-13 moles/liter) are obtained. The distribution of hydrochloric acid in the upper part of the rectifying column is characterized by the data in Fig. 1 ; the distribution in the lower part and in the still of the column is characterized by the data in Table 1. Since the concentration of nitric acid in the reflux increases downward along the column, the distribution coefficient sharply decreases from the bottom (from the still) to the top plate of the column (from - 170 to - 10-3). As a result, almost all the impurity is distilled off with the vapor; regenerated nitric acid may be obtained with a hydrochloric acid content no