Preferential extraction of Ni(II) over Co(II) by arylsulphonic acid in the presence of pyridinecarboxylate ester: Experimental and DFT calculations

Selective extraction of Ni(II) from base metals is a critical step in the direct nickel synergistic extraction process. Significant synergistic effect and preferential extraction of Ni(II) over Co(II) were observed by using the synergistic extractant containing dinonylnaphthalene sulfonic acid (HDNNS) and 2-ethylhexyl 4-pyridinecarboxylate ester (ligand L) in Escaid 110. The selectivity sequence followed the order: Ni(II) > Co(II) > Al(III) > Mg(II) > Fe (III). In order to elucidate the separation difference between Ni(II) and Co(II) which had similar chemical properties, statistical thermodynamics analysis and relativistic density functional theory (DFT) calculations were applied to provide the selective mechanism from both macroscopic and microscopic perspectives. The synergistic extraction diagrams showed that the optimum molar ratio of HDNNS and the ligand L was at 1:2 for Ni(II) and Co(II) extraction, and statistical thermodynamics analysis indicated that the overall extraction process was endothermic. According to DFT calculation results, the optimized geometries of the Ni(II) and Co(II) complexes were in accordance with the crystallographical data. The thermodynamical examination for the Ni and Co extraction was further conducted using a thermodynamical cycle. The difference in the change of Gibbs free energies for the formation of the Ni(II) complex was more negative than that of the Co(II) complex, which was in agreement with the experimental data. It could be concluded that the synergistic extractant had a higher affinity for Ni(II) than Co(II), which might account for the preferential extraction of Ni(II) over Co(II). The results established a computational model capable of predicting the selectivity order of metal ions, which would provide new insights into the separation mechanism of Ni(II) and Co(II) with synergistic extractants. (C) 2019 Elsevier B.V. All rights reserved.