Preferential recovery and separation of rhodium in the concentrated hydrochloric acid using thin-layer oil membrane extraction based on laminar flow

Abstract

The unique characteristics of the interface environment that distinguish bulk phases can promote or inhibit certain reactions and significantly change chemical reactivity. In this work, the separation of Rh (III) and Fe (III) on the surface of concentrated hydrochloric acid was studied by a thin-layer oil membrane (TOM) extraction based on laminar flow with 18-Crown-6-ether (18C6) as the extractant. A new phenomenon different from conventional extraction was found. The TOM extraction prefers to identify hydrated Rh (III) anions rather than H3O+ ions at the interface due to a lower concentration of free hydrated hydrogen ions on the surface of concentrated hydrochloric acid. However, during the conventional extraction, the H3O+ ions in the bulk phase are preferentially extracted by the 18C6, and then the hydrated rhodium (III) anions could be extracted by the protonated crown ether through the ion association mechanism. Besides, it was found that the TOM extraction could enhance extraction of hydrated rhodium (III) anions by taking advantage of the uniqueness of rhodium species on the interface, and by adjusting the thickness of the oil membrane and the flow rate of the aqueous phase. The decrease in the oil membrane thickness and the increase in the flow rate of the aqueous phase are conducive to the complexation of 18C6 with the hydrated rhodium (III) at the interface, which can better reflect the targeting and the controllability of extracting single species of hydrated rhodium (III) ions at the interface. Considering the applications of selective recovery of Rh (III) from concentrated hydrochloric acid leaching solution of waste automobile catalyst, the selective separability of Rh (III) to other coexisting metal impurity ions such as Fe (III), Cu (II), Co (III) and Ni (II), was also evaluated. This work provides a new strategy for preferential recovery and separation of rhodium in concentrated hydrochloric acid leach solutions containing other base metal ions for possible future's application.