Tailored synergistic hydrophobic eutectic mixtures for ultra‐selective recovery of cobalt from calcium‐rich solutions

The selective recovery of cobalt (Co) from industrial wastewater is increasingly important for sustainable resource utilization but remains challenging in calcium‐rich media. In this work, a hydrophobic eutectic mixture was tailored by pairing tri‐ n ‐octylamine as the hydrogen bond acceptor with bis ‐ (2,4,4 ‐ trimethylpentyl) ‐ phosphinic acid as the hydrogen bond donor. This system integrates highly selective Co extraction with saponification‐free and diluent‐free operation, achieving a separation factor (SF Co/Ca ) exceeding 3389.4. Mechanistic analysis suggested that Co was extracted in the form of chloride‐rich anionic complexes. Using a counter‐current extraction flowsheet, effective separation of trace Co (0.50 g/L) from calcium‐rich saponification wastewater (Ca >40 g/L) was achieved. The Co concentration in the stripping solution reached 19.3 g/L, enabling subsequent precipitation of high‐purity CoCO 3 and resulting in an overall Co recovery of 96.2%. This work provides a feasible strategy for the selective recovery and enrichment of Co from industrial wastewater.