Liquid-liquid extraction - Physico-chemical bases of processes

In the previous article to which the reader is invited to refer:

• the principle of the method ;

• a number of related definitions;

• a classification of extraction systems ;

• the main constraints imposed by the industrial use of this technique.

This document focuses on describing the physico-chemical phenomena that need to be understood and mastered in order to achieve optimum separations by liquid-liquid extraction. Indeed, although liquid-liquid extraction may appear at first glance to be a relatively simple technique, the separations it enables to be achieved, particularly on an industrial scale, are in reality the result of the conjunction of a large number of physico-chemical phenomena. The aim of this section is not to give a detailed description of all these phenomena, but to make the reader aware of their existence and to show how they can be used to optimize an industrial process.

Today's process designers have a wide choice of extractants at their disposal . More often than not, its role will be to exploit the properties of these compounds in the most judicious way, without worrying about their physico-chemical origin. Nevertheless, it's worth recalling how an extraction reagent is designed. Generally speaking, to achieve separation under optimum conditions, it is first necessary to select a functional chemical group which satisfies the desired selectivity conditions, i.e. which has strong interactions with certain constituents of the mixture to be separated, and weak interactions with others. Secondly, a hydrocarbon skeleton must be added to the previous functional group so that the whole becomes an extractant molecule.

When it comes to selecting a functional group that meets the required selectivity criteria, various approaches are possible. In the early years of liquid-liquid extraction, extractants were often developed from specific reagents well known in analysis. For example, the active part of the KELEX® 100 molecule is none other than 8-hydroxyquinoline. LIX® 54-100 is based on acetylacetone. More recently, the search for specific functional groups has been aided by the use of molecular modeling techniques [27][30] . Ionophores such as calix[4]arene-crown-6 and calix[4]arene-bis-crown-6, which display remarkable selectivity in favor of cesium ions over sodium ions, are typically the result of research aided by molecular modeling [29] .

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