Phase Transformation of Metal Hexacyanoferrates Containing Rubidium–Cesium to Separate Rubidium and Cesium from Each Other

Abstract

Rb⁺ and Cs⁺ lack independent ore resources in nature, and they often coexist with K⁺ and Na⁺. Hexacyanoferrates containing Rb⁺ and Cs⁺ are often calcinated and leaching to release Rb⁺ and Cs⁺ into solution to separate from co-existing Na⁺ and K⁺. In this study, co-precipitate containing Rb⁺ and Cs⁺, RbCs-CuZnHCF, is treated by phase- transformation method using (NH₄)₂S as phase-transformation agent to achieve the separation of Rb⁺ and Cs⁺ from each other. The kinetics and thermodynamics of the phase-transformation are studied. The results indicated that solid RbCs-CuZnHCF is transformed to Rb⁺ and [Fe(CN)₆]^4- by the aid of S^2-, while Cs-CuZnHCF maintains its original solid state. The removal efficiency of Rb⁺ is 64.22%, while the loss of Cs⁺ is 0.86% under the optimal conditions (T = 60℃, S^2-/(Cu²⁺+Zn²⁺) = 0.6, pH = 3-7, t = 60 min), and the separation coefficient of Rb-Cs was up to 207.89. Thermodynamics analysis reveals that the reaction is endothermic. Kinetic analysis reveals a low activation energy (17.99 kJ/mol) and pre-exponential factor of 10.18 L/ (mol·s), indicative of a diffusion-controlled process. By this method, the high-priced hexacyanoferrates can be re-used as co-precipitant agent to separate Rb⁺ and Cs⁺ from co-existing Na⁺ and K⁺.