Thermodynamic Modeling of Salting Effects in Solvent Extraction of Cobalt(II) from Chloride Media by the Basic Extractant Methyltrioctylammonium Chloride

Abstract

The design and optimization of solvent extraction processes for metal separations are challenging tasks due to the large number of adjustable parameters. A quantitative predictive solvent extraction model could help to determine the optimal parameters for solvent extraction flow sheets, but such predictive models are not available yet. The main difficulties for such models are the large deviations from ideal thermodynamic behavior in both the aqueous and organic phases due to high solute concentrations. We constructed a molecular thermodynamic model for the extraction of CoCl 2 from different chloride salts by 0.2 mol L –1 trioctylmethylammonium chloride in toluene using the OLI mixed-solvent electrolyte (OLI-MSE) framework. This was accomplished by analyzing the water and hydrochloric acid content of the organic phase, measuring the water activity of the system, and using metal complex speciation and solvent extraction data. The full extractant concentration range cannot be modeled by the OLI-MSE framework as this framework lacks a description for reversed micelle formation. Nevertheless, salting effects and the behavior of hydrochloric acid can be accurately described with the presented extraction model, without determining specific Co(II)–salt cation interaction parameters. The resulting model shows that the salting effects originate from indirect salt cation–solvent interactions that influence the availability of water in the aqueous and organic phases.

Hard–Soft Interactions in Solvent Extraction with Basic Extractants: Comparing Zinc and Cadmium Halides

“…Extractants consisting of a quaternary ammonium cation with long alkyl chains and a counteranion typically organize into reverse micellar structures due to their surfactant-like properties. − Most of the water in the organic phase is then found inside these reverse micelles. This can also explain the large amount of water that can be dissolved in organic phases with a high quaternary ammonium extractant concentration . It is now possible to come back to the unexpected trend in the distribution of acids to the organic phase (Figure ).…”

“…This can also explain the large amount of water that can be dissolved in organic phases with a high quaternary ammonium extractant concentration. 39 It is now possible to come back to the unexpected trend in the distribution of acids to the organic phase (Figure 3). It was observed that the more hydrophilic HCl is distributed more to the organic phase than the more hydrophobic HBr.…”

“…Consequently, there is less water in the organic phase that can destabilize the extracted metal complexes associated with the extractant, which enhances its extraction. 39 Second, the larger halides will result in larger metal−halide complexes, in which the charge of the metal complex is more delocalized. Moreover, these bonds are less polarized due to a smaller difference in electronegativity between the halide and the metal.…”

“…First, less water is found in the organic phase containing an extractant with a more hydrophobic anion (Figure ). Consequently, there is less water in the organic phase that can destabilize the extracted metal complexes associated with the extractant, which enhances its extraction . Second, the larger halides will result in larger metal–halide complexes, in which the charge of the metal complex is more delocalized.…”

Molecular Forces in Liquid–Liquid Extraction

“…136 Nevertheless, the road is long, and better theories need to be developed for every aspect of colloidal soft-matter approaches, especially if they are to be applied to the nonequilibrium transfer of ions, where the hysteresis of backextraction cannot be obtained. Also, improvements in the modeling in the aqueous phase 137 to decipher phenomena such as the co-ion specificity (e.g., NO 3 − vs Cl − ) or the boost in extraction efficiency when using the background, nonextracted salt 22 are very important. The challenge remains to obtain the activities and speciation of all solutes in supersaturated solutions, with the aim of better modeling of the polar core of the aggregate.…”