Efficient separation and recovery of strontium and calcium by non-saponification complexing solvent extraction using EHEHPA and ammonium citrate

Due to their highly similar chemical properties, the separation of strontium and calcium has long been a great challenge. Although conventional saponification extraction enables Sr/Ca separation, it mainly suffers from inherent limitations including high-salinity wastewater generation and process complexity. This study developed a novel extraction approach utilizing ammonium citrate as a complexing agent in an EHEHPA/chloride system. Under optimized operational parameters (20 % EHEHPA, 0.02 mol/L AC, 30 min extraction duration, and 0.05 mol/L HCl for stripping), the system demonstrated remarkable separation performance with a separation factor reaching 161.5, which corresponded to a 27 times increase compared to saponification approaches. A four-stage cross-current extraction process yielded a strontium carbonate product with 91.06 % purity. Mechanistic studies indicated that the exceptional performance of this technology originated from the triple synergistic effects of ammonium citrate: robust pH buffering that stabilized the extraction system; selective coordination chemistry that dramatically increased the Sr 2+ /Ca 2+ reactivity differential; and interfacial activity modification that boosted mass transfer at the interface. This environmentally benign method can minimize the generation of ammonia‑nitrogen wastewater while operating under ambient conditions. Not only does it provide an innovative solution for greener recycling of strontium resources, it also offers valuable technical insights into the efficient separation of other similar metals.