Solvent extraction and liquid membrane separation of rhodium

The aim of this work was to develop a viable solvent-extraction based system for the separation of rhodium (Rh) from aqueous chloride solutions. Ultimately, two different systems were developed. Kelex 100, a commercially available derivative of 8-hydroxyquinoline, was used as the extractant reagent in both of these systems. One of the systems involved the supported liquid membrane (SLM) extraction of Rh. In this system a very thin microporous "Gore-Tex" polymer sheet, impregnated with an organic solution of Kelex 100, served as the SLM. The other system involved the conversion of the chlorocomplexes of Rh to bromocomplexes prior to their solvent extraction with Kelex 100.The results of the lab-scale experiments using a SLM of Kelex 100 having a surface area of 44 cm2 are reported. The optimum conditions for Rh permeation were found as a feed solution of 2.5 M HCl and a strip solution of 0.1 M HCl. The SLM was quite stable at the optimum conditions with no sign of organic loss or membrane deterioration after 72 hours of operation. It was determined that the HCl activity gradient across the membrane acts as the driving force that "pumps" the non-aquated Rh chlorocomplexes against their concentration gradient. The mechanism of Rh permeation was the ion-pair formation between the protonated Kelex 100 and RhCl6 3- complexes. The rate of Rh permeation was in the order of 10-6 mol.m-2.s-1. The mechanism of HCl and H2O permeation, which were co-extracted along with Rh chlorocomplexes, were found to be the hydration of protons at the low feed acid region and the formation of microemulsions at the high feed acid region. The permeated acid and water were separated from the SLM receiving phase by contacting the latter phase with an organic solution of trioctylamine (TOA). The chlorocomplexes of Rh(III) and acid are readily extracted to the TOA organic phase and subsequently subjected to differential stripping with a concentrated solution of Cl- and a mild NaOH solution, respectively. By interfacing the TOA solvent extraction with the SLM of Kelex 100 highly concentrated solutions of Rh (at least 10 times the initial concentration) and raffinates essentially free of rhodium were produced.The UV-Visible investigations revealed that the bromocomplexes of Rh undergo aquation to a much lesser extent than that of the chlorocomplexes. The chlorocomplexes of Rh were converted to bromocomplexes by precipitating first the Na(NH4)2Rh(NO2)6 salt and subsequently dissolving that in an HBr solution. The newly formed bromocomplexes of Rh(III) responded very favorably to extraction with Kelex 100. Relatively high distribution coefficients, about 20, and very steep extraction isotherms were generated. The freshly loaded Kelex 100 organic was efficiently stripped upon contact with a strip solution of 6--8 M HCl and a contact time of 10--12 hours. The developed system shows high promise from a practical implementation point of view