The mode of action of iron(III) uptake from sulfate solutions by the commercial extractant CYANEX (R) 272, bis(2,4,4-trimethylpentyl)phosphinic acid, has been studied using conventional solvent extraction methods, together with P-31{(1) H} NMR spectroscopy, electrospray ionisation mass spectrometry, and DFT calculations that have not previously been used to analyse the content of the ISOPAR M water-immiscible phase. The maximum Fe-loading recorded was 177% of the theoretical maximum, based on the formation of an Fe(III) complex [Fe(L)(3)] with a 1:3 molar ratio of Fe to phosphinate (L). Inductively coupled plasma optical emission spectroscopy (ICP-OES) indicates that sulfate or hydrogen sulfate ions are co-extracted in a 1:2 molar ratio with iron across the whole of the Fe-loading range, which helps to account for the loadings being greater than 100% of the theoretical value. P-31{(1) H} NMR spectroscopy indicates that the limiting factor in Fe-loading is the availability of uncomplexed CYANEX (R) 272 (sulfate is present in large excess). In contrast to the behaviour of Co(II) and Zn(II) extraction by CYANEX (R) 272, there is no evidence for the formation of polymeric Fe(III) complexes and highly viscous solutions at high metal loadings. The identity of the extracted species is likely to be a polynuclear Fe(III) sulfate complex. [GRAPHICS] .

A combination of mass spectrometry, DFT calculations and 31 P{ 1 H} NMR spectroscopy has been used to define the mode of action of the commercial cobalt extractant, bis(2,4,4‐trimethylpentyl)phosphinic acid (CYANEX®272, L 1 H) in Co recovery. The nature of the Co II complexes formed in the water‐immiscible phase is determined largely by the propensity of phosphinates to form strong interligand H‐bonds in the outer coordination sphere and also to form stable µ 2 ‐Co–O–P–O–Co bridges. At low Co loading levels, the predominant species is the 4:1 complex, [Co( L 1 ·L 1 H) 2 ], in which coordinated neutral phosphinic acid ligands form strong H‐bonds to adjacent anionic phosphinates. At higher Co loading, oligomers such as [( L 1 ·L 1 H)Co( L 1 2 Co) n ( L 1 ·L 1 H)] are formed with µ 2 ‐phosphinate bridging, resulting in a substantial increase in the viscosity of the water‐immiscible phase. The presence of tris(2,4,4‐trimethylpentyl)phosphine oxide ( L 2 ) in the commercial formulation reduces the viscosity because its incorporation into oligomeric complexes such as [( L 2 )Co{ L 1 3 Co L 1 Co L 1 3 Co} m L 1 ] can terminate chains, resulting in a lower average molecular weight. The uptake of Zn by L 1 H shows a very similar dependence of viscosity on loading, and DOSY spectra and mass spectrometry demonstrate that higher molecular weight species are present at high Zn loading.

The configuration of a CYANEX 272 solvent-extraction plant can vary significantly depending on the composition of the pregnant leach solution. Parameters such as the organic-to-aqueous flowrate ratio, the extractant concentration, the pH profile, and the number of stages must be optimized for the given feed conditions. MINCHEM is a modeling program that was specifically designed by Cytec Industries Inc. to allow a rapid assessment of these parameters. The software was used in the present study to determine the optimum pH profile and stage requirements for the extraction and scrubbing circuits of a hypothetical cobalt/nickel plant using CYANEX 272 extractant. The simulations were completed to maximize cobalt recovery from the feed and minimize both impurity transfer in the loaded organic phase and the need for cobalt reprocessing from the scrub stage.