Extraction of Cs+and Sr2+from HNO3Solution Using Macrocyclic Polyethers

Abstract

A series of crown compounds has been studied in connection with the development of a solvent extraction process for removing Cs⁺ and Srs²⁺ from acidic high activity nuclear waste. Crown compounds were investigated because of their ability to form organic soluble complexes with the alkali metals and the alkaline earth metals. The solvent (tributyl phosphate and kerosene) was chosen because of its compatibility with the Purex process currently used at the Savannah River Plant for plutonium and uranium purification. The crown compounds were found not to be sufficiently strong complexing agents to extract these metals from an aqueous phase with an inorganic anion such as nitrate or chloride. However, the use of large organic soluble anions which also functioned as liquid ion exchangers made it possible to extract Cs⁺ from 3 M HNO₃ while leaving Sr²⁺ and La³⁺ behind. The use of .02 M bis-(4,4′(5′)-[q-hydroxyheptyl]-benzo)-18-crown-6 in 0.076 M (5 vol-%) didodecylnaphthalene sulfonic acid (DNS)-27 vol-% TBP-68 vol-% kerosene gave the most favorable results for cesium extraction. The two phases separated cleanly and rapidly, yielding a distribution coefficient of 2.0 org/aq from 3 M HNQ₃. Strontium and lanthanum did not extract significantly at this acid concentration. The distribution data in the low acid region (10⁻⁴ M) are best explained by steric hindrance effects. The lanthanum did not extract at any acid concentration while the strontium extracted very well by a liquid ion-exchange mechanism but the addition of the crown compound did not improve the distribution. The addition of the crown compound improved the Cs⁺ distribution markedly in the low acid region even though the Cs⁺ diameter is larger than the 18-crown-6-crown cavity while Sr²⁺ should fit very well.