The Kinetics of Metal Solvent Extraction

The kinetics of solvent extraction of metal species by organic solution of acidic, basic, and neutral reagents has been reviewed by considering the contribution of both chemical reactions and transport processes on the rate of extraction. The relative predominance of slow chemical reactions and diffusion processes has been shown to depend, in a critical way, both on the hydrodynamic conditions present in the extracting apparatus and on the specific nature and composition of the chemical system involved in the extraction. The general criteria that can help to distinguish between extraction kinetics controlled either by mass transport or chemical reactions have been reviewed. These criteria involve the experimental determination of the influence of a rather large number of physical and chemical variables on the rate of extraction and do not always lead to unambiguous conclusions. The reviewed literature has allowed to draw the following conclusions: (1) because of a lack of complete characterization (especially in terms of hydrodynamics, interfacial pressure, and sufficiently wide concentration ranges), much of the early work on solvent extraction kinetics has rarely led to the definite identification of the rate determining processes; as a consequence, correct extrapolation to the different experimental conditions that are met in practical analytical or technological separations has been often impossible; and (2) more recently a certain number of extraction kinetics studies have appeared which have been carried out in hydrodynamically controlled conditions, exploring a large range of compositions of the species and with knowledge of the interfacial properties of the biphasic system; such studies have contributed to the emergence of a general pattern for the solvent extraction kinetics which is summarized.