Keith R. Barnard

Synergistic Solvent Extraction of Nickel and Cobalt: A Review of Recent Developments

Abstract Synergistic solvent extraction of nickel and cobalt has been an important research subject since the 1960s. In recent years, several synergistic systems have been further developed for possible industrial application, including carboxylic acids/pyridinecarboxylates for the separation of nickel and calcium; carboxylic acids/α-hydroxyoxime systems for the separation of cobalt from manganese, magnesium, and calcium, and carboxylic acid/α-hydroxyoxime/organophosphate systems for the separation of nickel and cobalt from manganese, magnesium, and calcium. The separation of nickel and cobalt from iron and aluminum using synergistic systems has also been explored. Industrial application of synergistic solvent extraction systems is expected in the near future.

Crystallographic Determination of Three Ni-α-Hydroxyoxime-Carboxylic Acid Synergist Complexes

X-ray crystal structures of three dicarboxylato-bis-α-hydroxyoximenickel(II) complexes have been obtained. These contain a short chained (C8) analogue of LIX®63 hydroxyoxime, along with either benzoate, isobutyrate or propionate. All have pseudo-octahedral structures with monodentate carboxylate anions located cis to one another and neutral, chelating α-hydroxyoxime ligands. Intra-molecular hydrogen bonding between each anionic acids carboxylate group and an adjacent oxime hydroxyl group is evident. Inter-molecular hydrogen bonding is also observed. These provide the first definitive structural elucidation of the types of nickel complexes that could be formed during synergistic extraction by LIX®63 and carboxylic acids.

The Effect of Hydroxyoxime Isomer Conformation on Metal Extraction in the LIX 63/Versatic 10 Synergistic System

LIX 63 contains an aliphatic α-hydroxyoxime as its active component, with two isomeric forms (anti and syn) being present in a 3:2 ratio in “as supplied” reagent. In the present work, > 90% pure syn hydroxyoxime was isolated from LIX 63. Examination of metal extraction properties in systems containing Versatic 10 with anti and/or syn hydroxyoxime established unequivocally that the syn isomer plays no beneficial role in metal extraction in this synergistic solvent extraction system, thereby excluding the possibility of monodentate coordination of hydroxyoxime. Economic consequences arising from this outcome and the opportunity to maximize reagent utilisation are briefly discussed.

Kinetic Separation of Co from Ni, Mg, Mn, and Ca via Synergistic Solvent Extraction

Abstract The combination of LIX 63 and Versatic 10 acts synergistically for the selective extraction of nickel and cobalt from impurities of manganese, magnesium, and calcium. Nickel extraction kinetics is, however, slow relative to cobalt. The present work exploited this difference to selectively remove Co (1.0 g/L) from a Ni-rich feed solution (20 g/L) containing aforementioned impurities to achieve a raffinate Ni:Co ratio > 667. Batch testing was used to assess the effect of various factors on selective metal extraction. The selective stripping of the resulting loaded organic was also assessed. The metal selectivity properties of the organic solution did not deteriorate over five cycles.

Identification of Four Additional Hydroxyoxime Degradation Products formed in the LIX® 63/Versatic 10 Synergistic System and their Effect on Metal Selectivity

Three products arising from LIX® 63 hydroxyoxime degradation have previously been identified, namely 5,8-diethyl-6,7-dodecanedione (diketone), 5,8-diethyldodecan-6-oxime-7-one (keto-oxime) and 5,8-diethyl-7-hydroxy-6-dodecanone (acyloin). In the present work, four additional LIX® 63 degradation products derived from cleavage of the carbon backbone have been detected and identified, namely 2-ethylhexamide, 2-ethylhexanoic acid, 2-ethylhexanal, and 3-heptanone. The amide was isolated and spectroscopically characterized, whereas the other three products were identified primarily using gas chromatography-mass spectrometry. Besides 2-ethylhexanoic acid, the effect of the degradation products on metal selectivity in the synergistic system were assessed and found to have a small detrimental effect on one or more metal ions.

The Effect of LIX 63 Hydroxyoxime Degradation Products on Kinetics-Based Metal Extraction

The potential of the LIX® 63 hydroxyoxime/Versatic 10 combination to kinetically directly separate cobalt (1 g/L) from a nickel rich (20 g/L) synthetic leach solution while rejecting impurities has been highlighted previously. In the present work, the stability of the LIX 63 hydroxyoxime has been assessed under conditions relevant to a commercially prospective kinetics-based operation, and the results compared with those obtained in previously investigated equilibrium-based systems. Importantly, no adverse effect of the four main hydroxyoxime degradation products on metal extraction and stripping kinetics was found, even when these were added at high concentrations.

Difference Hirshfeld fingerprint plots: a tool for studying polymorphs

A new tool has been developed to help elucidate the differences in packing between different polymorphs, especially when the differences of interest are small. The technique builds upon the Hirshfeld fingerprint plot pioneered by Spackman and co-workers by subtracting the value at every point in a fingerprint plot from the value at every point in another. This is found to reveal differences that are not readily apparent to the eye. By summing the absolute values of these differences, a quantitative measure of the difference between two fingerprint plots can be obtained. The technique was applied to Ni and Cu trans-bis(2-hydroxy-5-methylphenonethanoneoximato) complexes determined at two temperatures, with the Ni complex displaying temperature-dependent polymorphism. Difference Hirshfeld fingerprint plots were also generated for calculated structures from DFT simulations that were performed on the experimental structures. These demonstrated that the simulations reproduced the fine detail of the packing.

Synthesis of a t-α-Hydroxy Oxime and its Synergistic Behavior with Versatic 10

The aliphatic α-hydroxyoxime LIX® 63 contains a secondary hydroxyl group that is susceptible to oxidation. In the present work, an alternative oxidation-resistant α-hydroxyoxime containing a tertiary hydroxyl group, namely (1-hydroxycyclohexyl)-phenyl ketone oxime (“phenyl oxime”), has been synthesized and characterized. Combining phenyl oxime with Versatic 10 results in synergistic metal extraction behavior. The ability of this ligand to withstand oxidation is demonstrated. Other benefits are also evident, including its existing wholly in the active anti isomeric form and exhibiting fast nickel kinetics in the synergistic system. Phenyl oxime readily undergoes aqueous acid-catalyzed hydrolysis and so is not commercially practical. Addition of carbon side-chains could help to overcome this.

Modelling Cobalt Solvent Extraction using Aspen Custom Modeler

Abstract The cobalt solvent extraction system using Cyanex 272, a phosphinic acid based extractant, has been modelled using the Aspen Custom Modeler mathematical modelling software. The principle advantage of this method is that the model can easily be imported into Aspen Plus and run as part of an integrated flowsheet containing other unit operations. The cobalt solvent extraction circuit operates on a counter-current basis, with the barren organic entering the final stage and the aqueous feed entering at the first stage. Since the metal extraction efficiencies were dependent on the conditions of the outlet streams, a solver must be selected to simultaneously solve a set of algebraic nonlinear model equations. Initial sensitivity analysis for a single stage Aspen Custom Modeler model has shown that increasing pH or the organic to aqueous (O:A) ratio significantly increases individual metal extraction efficiencies. To achieve the ultimate aim of maximising cobalt extraction while minimising magnesium and nickel co- extraction and reagent consumption, an economic objective function has been formulated within the optimisation problem to solve for the optimum pH setpoint and O:A ratio. The optimised single stage results indicate operating at pH 4.5 and O:A of 0. 78 to achieve 95% cobalt extraction, while limiting nickel extraction to

Development of an integrated model for cobalt solvent extraction using Cyanex 272

A model of metal extraction based on pH isotherms was generated and applied to a cobalt solvent extraction (SX) circuit. Cyanex 272 (bis-(2,4,4-trimethylpentyl) phosphinic acid) was used as the organic extractant due to its selectivity for cobalt over nickel in the extraction process. Experiments were conducted for cobalt, nickel and magnesium extraction, with the latter two representing impurity elements in Co SX. The methods for determining metal extraction incorporated the effects of temperature, solution composition and pH on the equilibrium constant k, and hence on the overall extraction extent. This information was applied to a multi-stage mixer-settler model consisting of integrated extraction units. The initial mathematical model for cobalt, which was built in Matlab can be further developed to include the impurity elements and incorporate the scrubbing and stripping units. Future work will focus on using the model for process optimisation.