A Prussian-blue analogue (PBA) ion-chromatography-based technique for selective separation of Rb⁺ (as RbCl) from brines

A new general method is presented for separating pure RbCl_(s) from solutions rich in Na⁺ and K⁺. The method relies on Rb⁺ adsorption via ion exchange performed by self-synthesized PES coated Zn-Hexa-Cyanoferrate material. The procedure starts by passing the wastewater through an ion exchange column, which is thereafter regenerated with 1 M NH₄Cl. If the Rb⁺ absorbed on the column does not reach a minimal predetermined value (e.g., 8%, eq-based), the ammonia is removed by sublimation and the remaining salts are passed again through a Na⁺-preadsorbed column. Once the adsorbed Rb⁺ is substantial (>8%), a chromatography-based separation between Rb⁺ and Na⁺/K⁺ is performed, using a 2^nd column, fully pre-adsorbed with NH₄⁺. First, 0.05M NH₄⁺-solution is used to extract Na⁺ and K⁺ out of the first column, along with a small Rb⁺ mass, which is thereafter partly re-adsorbed on the second column, while Na⁺/K⁺ ions are not. Once the exiting eluent solution is devoid of the competing ions, 1M NH₄⁺-solution is used to extract all the remaining Rb⁺ into the regeneration solution, which is thereafter subjected to water evaporation followed by NH₃/HCl sublimation to result in pure RbCl_(s) product. We used theoretical simulations corroborated by empirical results to present proof of concept for the suggested approach. A detailed cost analysis (Capex and Opex) reveals that the RbCl_(s) production cost does not exceed ∼25% of the current salt price.