Efficient Enrichment of Rubidium and Cesium from High-Salt Solution by Copper Hexacyanoferrate Co-Precipitation

Cesium (Cs) and rubidium (Rb) are important rare alkali metal elements that are scarce and dispersed in nature. They often coexist with high amounts of Na⁺ and K⁺. In this study, copper hexacyanoferrate (CuHCF) was used as a co-precipitant to enrich Cs⁺ and Rb⁺ in a high-salt solution. The effect of parameters, such as type and dose of divalent metal ions, solution pH, material dropping speed, reaction time, and temperature, was investigated for the recovery rate of Cs⁺ and Rb⁺. Results showed that the recovery rates of Cs⁺ and Rb⁺ were 99.78% and 88.84% at the optimal Cu²⁺/(Cs⁺+Rb⁺) molar ratio of 0.5, respectively. The reaction could be completed within 10 min in pH 3–8. The detailed characterization of the Cs⁺ and Rb⁺ containing co-precipitants, CsRb–CuHCF, confirm a general formula of Cs_1.09Rb_0.81K_0.16Cu_0.97Fe(CN)₆. The (Cs⁺+Rb⁺)/(Na⁺+K⁺) increased significantly from 0.074 to 28.47 before and after precipitation, which realized the effective enrichment of Cs⁺ and Rb⁺. Studies of the precipitation reaction mechanism showed that the precipitant reacted first with Cs⁺ and then with Rb⁺. This process offers a promising avenue for the enrichment and separation of Cs⁺ and Rb⁺ from high-salty solutions.