Ingeniero

5
Solvent Extraction Kinetics
PIER ROBERTO DANESI
Vienna, Austria

5.1

International Atomic Energy Agency,

GENERAL PRINCIPLES

The kinetics of solvent extraction is a function of both the various chemical
reactions occurring in the system and the rates of diffusion of the various species
that control the chemistry of the extraction process.

5.1.1

Rate-Controlling Role
of theChemical Reactions

The dependence of the kinetics on the chemical reactions is easily understood
by considering that the final products of any extraction process are usually in a
chemical state different from the initial unreacted species. This is true even for
the simple partition of neutral molecules between two immiscible liquid phases,
where the chemical change is in the solvationenvironment of the extracted
species. More drastic chemical changes take place in the extraction of a metal
cation from an aqueous solution by a chelating extractant dissolved in an organic
diluent. In the extraction some of the solvation water molecules can be removed
from the metal ion, and a new coordination compound, soluble in the organic
phase, is formed with the chelating group of theextractant. In addition, the
extracting reagent can undergo an acid dissociation reaction and, together with
the extractant-metal complex, can undergo changes in aggregation in the organic
phase. Consequently, whenever at least one of the chemical steps of the overall
reaction mechanism is slow enough, compared with the diffusion rate, the kinetics of extraction would depend on the rate of theslow chemical reactions.

Copyright © 2004 by Taylor & Francis Group, LLC

For solvent extraction systems, we have two additional complications.
First, the chemical reactions can take place, at least in principle, in two bulk
phases, since we are dealing with two immiscible liquid layers. Second, the
chemical reactions can occur in the two-dimensional region called the liquid–
liquidinterface, that separates the two immiscible liquids, or in a thin volume
region very close to it. When interfacial chemical reactions are important, the
situation is analogous to that describing the kinetics of the chemical reactions
encountered in heterogeneous catalysis and in some electrode processes. Although a relatively large number of sophisticated techniques are available for
studyingchemical reactions at solid–fluid interfaces, very few tools have been
developed to investigate chemical changes occurring at liquid–liquid interfaces.
Our knowledge of such interfacial reactions, therefore, is still limited and is
based largely on indirect evidence and speculations.

5.1.2

Rate-Controlling Role
of the Diffusional Processes

To understand the dependence of the extractionkinetics on the rate of diffusion
of the various chemical species that participate in the extraction reaction, we
first have to distinguish between diffusion in the bulk phases and diffusion
through the thin layers adjacent to the interface. In most solvent extraction processes of practical interest, both the aqueous and the organic phase are efficiently stirred. It follows that transport ofmaterial from the bulk of the phases
up to a region very close to the interface can be considered instantaneous and
that diffusion in the bulk of the phases can be neglected. Nevertheless, diffusional processes can still have an appreciable influence on the solvent extraction
kinetics. For example, even when the two phases are vigorously stirred, it is
possible to describe interfacial diffusion byassuming the existence of two stagnant thin layers of finite thickness located on the aqueous and organic side of
the interface. This model of the interface, often referred to as the two-film theory [1,2], is extremely useful for describing extraction kinetics that are controlled by diffusion occurring in proximity to the interface. The two-film model
is used throughout our treatment. Other...