(609e) Solvent Extraction and Separation of Nickel and Vanadium From Sulfate Leach Liquor of Power Plant Fly Ash Using D2EHPA and Cyanex 272

Fly ash generated by power plants is categorized as a special waste by US EPA.  These residues from fossil fuel combustion are captured by pollution control equipment and disposed in special landfills.  In landfills, leaching of heavy metals such as nickel and vanadium into ground water, soil, and/or surface water is the main potential environmental concern.  Furthermore, due to the limited storage capacity of landfills, safe disposal becomes more and more costly.  Therefore, it is imperative to develop means of both diverting these types of wastes from the solid waste stream, and also recovering nickel and vanadium as valuable metals from fly ashes from environmental and economical viewpoints.  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 contains about 2.6% nickel and 3.3% vanadium and according to the leach liquor after the sulfuric acid leaching, synthetic sulfate solutions of nickel and vanadium were prepared in 2gr/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 1.0–5.0 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 95% and 80% for nickel and vanadium, respectively. However, the co-extraction of nickel and vanadium by D2EHPA can be increased with increasing equilibrium pH and temperature.  It was shown that using sole D2EHPA, pH₅₀ (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, 40, and 60°C and two different diluents including kerosene and carbon tetrachloride were studied. As a result, optimum separation of vanadium over nickel was achieved with a Cyanex 272 to D2EHPA ratio of 3. Based on the optimum results, pH₅₀ values for nickel and vanadium were changed from 3.5 to 4.5 and from 2 to 1.75, respectively.