Recovery of zinc and iron from hot-dip galvanizing spent pickle liquor using solvent extraction

The disposal of spent pickle liquor from the hot-dip galvanizing process is of serious environmental concern that demands development of safe and economical methods. The present work attempts to extract zinc and iron utilizing a mixture of trialkyl-phosphine oxide (Cyanex 923) as the extractant. Isooctanol and sulfonated kerosene was mixed with the extractant to improve the mass transfer as well as to suppress emulsification. The effect of the extractant concentration, organic phase composition, contact duration, temperature, phase ratio (O/A) were assessed to identify the optimal process conditions. A thorough analysis of the zinc extraction mechanism was carried out utilizing FT-IR, Raman spectroscopy, and ESI-MS analysis. The structures of extractant and extraction complex were simulated using density functional theory (DFT), the overall situation and the local activity index were calculated. The extraction mechanism is proposed based on theoretical simulation and characterization, supported by the experimental results. The extraction process was identified to be exothermic in nature with the extraction efficiency of zinc as high as 95.57% under the optimal conditions. The extraction efficiency of 99.99% for zinc and 1.90% for iron could be achieved in a three-stage countercurrent extraction process. The single stage stripping efficiency of zinc was 95.25% with 0.8 mol/L HNO₃. The extraction efficiency of zinc was 92.50% after five extraction and stripping cycles.