Parametric Study of Zinc, Cadmium, and Nickel Ion Recovery Using D2EHPA in a Semi-Pilot Liquid–Liquid Extraction Plant

Industrial effluents containing zinc, cadmium, and nickel represent both an environmental challenge and a valuable secondary resource. This study investigates the simultaneous recovery of Zn(II), Cd(II), and Ni(II) from nitric acid solutions using di-(2-ethylhexyl) phosphoric acid (D2EHPA) dissolved in commercial diesel fuel in a continuous counter-current semi-pilot mixer–settler extraction system. The effects of key operating parameters, including aqueous-phase pH, initial metal concentration, phase ratio, contact time, stirring speed, and stripping agent, were systematically evaluated. Metal concentrations were determined by atomic absorption spectrometry, while Fourier-transform infrared spectroscopy was employed to investigate the extraction mechanism. The optimum operating conditions were pH 2.45 for Zn(II) and pH 5.38 for Cd(II) and Ni(II), a phase ratio of VA/VO = 1.5/3.5, and a stirring speed of 700 rpm. Under these conditions, high extraction efficiencies were achieved, with Zn(II), Cd(II), and Ni(II) recoveries of 98.52%, 76.86%, and 84.04%, respectively. FTIR characterization, together with slope analysis, suggested a dimeric cation-exchange extraction mechanism involving D2EHPA species in the organic phase. Among the stripping agents evaluated under the present experimental conditions, 0.5 M H2SO4 produced the highest stripping efficiencies for Zn(II), Cd(II), and Ni(II). Although the stripping efficiencies remained moderate, particularly for Cd(II) and Ni(II), these results demonstrate the relative effectiveness of H2SO4 compared with the other stripping agents examined. Further optimization of the stripping conditions is expected to improve metal recovery. The proposed process shows significant potential for industrial wastewater treatment, metal recovery, and resource valorization.