                      !                      !! "#  $%  & ’ (()* "$  $  %!#   !! !  !! "#  & +, - ../0.)( & 1 " In this investigation, recovery and separation of nickel and vanadium from leach liquors of power plant fly ash were investigated by solvent extraction using D2EHPA, Cyanex 272, and their mixtures in various proportions. The mentioned fly ash mainly contains porous unburned carbon, sulfates, oxides of vanadium, nickel and iron. For this reason, synthetic sulfate solutions of nickel and vanadium were prepared in 2 gr/L of each metal. After dilution of the organic solvents in kerosene with the ratio of 1 to 4, experiments were carried out in the pH range of 1.0–7.0 in steps of 0.5 at ambient temperature. The investigation of solvent extraction of nickel and vanadium by sole D2EHPA was performed and the extraction percentages determined to be 90% and 80% for nickel and vanadium, respectively. However, the co1extraction of nickel and vanadium by D2EHPA can be increased with increasing equilibrium pH and temperature. It was shown that using sole D2EHPA, pH 50 (the pH at 50% metal extraction) values for nickel and vanadium were 3.5 and 2, respectively; which is not appropriate for the efficient separation of nickel and vanadium simultaneously. Adding Cyanex 272 to D2EHPA in the organic phase, leads to a right shifting of extraction isotherm of nickel and a slight left shifting of the extraction isotherm of vanadium and finally improves the separation of nickel over vanadium. To optimize the recovery and separation process of nickel and vanadium from the sulfate leach liquor, the influence of different D2EHPA to Cyanex 272 ratios, various temperatures such as 25, 35, 45, and 55°C were studied. As a result, optimum separation of vanadium over nickel was achieved with a Cyanex 272 to D2EHPA ratio of 0.35 M: 0.25 M. Based on the optimum results, pH 50 values for nickel and vanadium were changed from 3.5 to 4.75 and from 2 to 1.75, respectively. Keywords: Nickel, Vanadium, Solvent extraction, D2EHPA, Cyanex 272. # Due to the wide applications of vanadium and nickel in the steel industry as alloying elements, ceaseless extraction of mineral resources of these two metals are becoming more and more insufficient to answer the rapid growing demands especially in industrialized countries. In recent years, mainly because of depleting in main sources of nickel and vanadium, researchers have been trying to discover and extract these metals from secondary sources such as fly ashes, crude oil, power plant residues, and spent catalysts [1, 2].