Separation of Nickel from High-Concentration Cobalt Solutions by Aminophosphonic Acid: Solvent Extraction, Extraction Mechanism and Purification Applications

Abstract

The efficient separation of small amounts of nickel from high-concentration cobalt solutions is of significance for the preparation of high-pure cobalt. Herein, (2-ethylhexylamino) methyl phosphonic acid mono-2-ethylhexyl ester (HEHAMP, denoted as HL) was firstly employed for the selective extraction and separation of Co(II) and Ni(II) from acidic solutions. Due to the relatively weak electronegativity of nitrogen atom bearded in the extractant, HEHAMP preferentially extracts Ni(II) over Co(II) at low acidities, which is contrary to the extraction behavior of Co(II) and Ni(II) with commonly used extractants such as P507 and Cyanex 272. The highest separation factor of 5.94 for Ni/Co could be achieved at a Co/Ni molar ratio of 80/1 in chloride media. The relatively low temperature is beneficial for Ni/Co separation. The extracted species were determined to be CoL2 and NiL2. Microscopic properties of the possible structures were proposed based on the experimental and density functional theory (DFT) calculations. Because of the higher chelation binding energy, Ni(II) exhibits a higher ligand-binding affinity compared to Co(II), which facilitates the formation of more structurally robust hexa-coordinated complexes featuring two five-membered rings. This may be the predominant mechanisms for Ni/Co separation. A cascade extraction process for cobalt purification was developed using HEHAMP as extractant, by which the purity of cobalt increases from 95% to 99.6%, with good chemical stability and reusability. HEHAMP has great potential in the separation of Ni(II) from high-concentration cobalt solutions.

Keywords: nickel, cobalt, Solvent extraction, Separation factor, Aminophosphonic acid