Strong and Selective Ni(II) Extractants Based on Synergistic Mixtures of Sulfonic Acids and Bidentate N-Heterocycles

ABSTRACT

Bidentate 5,5ʹ-alkyl-3,3ʹ-bi-1H-pyrazole and 2-(5-alkyl-1H-pyrazol-3-yl)pyridine ligands, L⁵ and L⁶, have been shown to be stronger synergists for the solvent extraction of Ni(II) from sulfate solutions by dinonylnaphthalene sulfonic acid (DNNSAH) than the structurally related tridentate ligand 2,6-bis-[5-n-nonylpyrazol-3-yl]pyridine, L¹, previously reported by Zhou and Pesic. The bidentate ligands are highly selective, providing the option of sequential recovery of Ni(II) and Co(II) and rejection of other metals commonly found in the liquors resulting from the acidic sulfate leaching of laterite ores. They were the strongest synergists identified in a screening carried out on 18 types of bidentate and tridentate N-heterocyclic ligands, including the recently reported 2-(2ʹ-pyridyl)imidazoles, L⁹⁻¹¹. X-ray crystal structures of Ni(II) complexes of model ligands for L⁵ and L⁶, having t-butyl rather than long-chain alkyl groups and with 2-naphthalene sulfonate rather than DNNSA⁻ as counteranions, show that the [Ni(L)₃]²⁺ complexes form strong H-bonds from the pyrazolyl NH groups to the oxygen atoms of the sulfonate groups, an arrangement that will stabilize [Ni(L)₃·(DNNSA)₂] assemblies and shield their polar functionalities from diluent molecules of the water-immiscible phase. UV–visible spectra and mass spectrometry provide evidence for the strong synergists displacing all water molecules from the inner coordination sphere of the Ni(II) ions.

KEYWORDS:

• Ni recovery
• laterite processing
• synergistic solvent extraction
• outer-sphere bonding
• supramolecular chemistry

Acknowledgments

We thank Solvay S.A., Anglo American, and University of Edinburgh’s Principal’s Career Development Scholarship scheme for the funding for PhD studentships for JWR, MRH, and EDD, respectively, James Cumby for discussions on the refinement of structure [Ni(L²)₂·(2-naphth-SO₃)₂]·nCHCl₃, and Furahi Achebe and Efren Delos Santos (Solvay) for technical assistance in the synthesis of many of the candidate synergists. Discussions with Violina Griffin (Solvay) are gratefully acknowledged.