WHAT IS SOLVENT EXTRACTION

1.1 WHAT IS SOLVENT EXTRACTION?

The term solvent extraction refers to the distribution of a solute between two immiscible liquid phases in contact with each other, i.e., a two-phase distribution of a solute. It can be described as a technique, resting on a strong scientific foundation. Scientists and engineers are concerned with the extent and dynamics of the distribution of different solutes-organic or inorganic-and its use scientifically and industrially for separation of solute mixtures.

The principle of solvent extraction is illustrated in Fig. 1.1. The vessel (a separatory funnel) contains two layers of liquids, one that is generally water (Saq) and the other generally an organic solvent (Sorg). In the example shown, the organic solvent is lighter (i.e., has a lower density) than water, but the opposite situation is also possible. The solute A, which initially is dissolved in only one of the two liquids, eventually distributes between the two phases. When this distribution reaches equilibrium, the solute is at concentration [A]aq in the aque- ous layer and at concentration [A]org in the organic layer. The distribution ratio of the solute

D = [A]org /[A]aq                   (1.1)

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*Retired.
†The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of the term liquid-liquid distribution. However, more traditionally the term solvent extraction (sometimes abbreviated SX) is used in this book.

Fig. 1.1 A schematic representation of solvent extraction (liquid-liquid distribution). A solute A is distributed between the upper layer, for example an organic solvent, and the lower layer, an aqueous phase.

is defined as the ratio of “the total analytical concentration of the substance in the organic phase to its total analytical concentration in the aqueous phase, usually measured at equilibrium” [1], irrespective of whether the organic phase is the lighter or heavier one. If a second solute B is present, the distribution ratio for the various solutes are indicated by DA, DB, etc. If DB is different from DA, A and B can be separated from each other by (single or multistage) solvent extraction. D is also called the distribution coefficient or distribution factor; we here prefer the expression distribution ratio.

For practical purposes, as in industrial applications, it is often more popu- lar to use the percentage extraction %E (sometimes named the extraction fac- tor), which is given by

%E =100D/(1+D) (1.2)

where D is the distribution ratio of the solute (or desired component). For D = 1, the solute is evenly distributed between the two phases. A requirement for practical use of solvent extraction is that a reasonable fraction (percentage) of the desired component is extracted in a single operation (or stage).

Solvent extraction is used in numerous chemical industries to produce pure chemical compounds ranging from pharmaceuticals and biomedicals to heavy organics and metals, in analytical chemistry and in environmental waste purification. The scientific explanation of the distribution ratios observed is based on the fundamental physical chemistry of solute–solvent interaction, activity factors of the solutes in the pure phases, aqueous complexation, and complex-adduct interactions. Most university training provides only elemen- tary knowledge about these fields, which is unsatisfactory from a funda- mental chemical standpoint, as well as for industrial development and for protection of environmental systems. Solvent extraction uses are important in organic, inorganic, and physical chemistry, and in chemical engineering, theoret- ical as well as practical; in this book we try to cover most of these important fields.

None of the authors of this book is an expert in all the aspects of solvent extraction, nor do we believe that any of our readers will try to become one. This book is, therefore, written by authors from various disciplines of chemistry and by chemical engineers. The “scientific level” of the text only requires basic chemistry training, but not on a Ph.D. level, though the text may be quite useful for extra reading even at that level. The text is divided in two parts. The first part covers the fundamental chemistry of the solvent extraction process and the second part the techniques for its use in industry with a large number of applica- tions. In this introductory chapter we try to put solvent extraction in its chemical context, historical as well as modern. The last two chapters describe the most recent applications and theoretical developments.

REFERENCES

1.      Freiser, H.; and Nancollas, G. H.; Compendium of Analytical Nomenclature. Defini- tive Rules 1987. IUPAC. Blackwell Scientific Publications, Oxford (1987).

2.      Blass, E.; Liebl, T.; Ha ̈berl, M.; Solvent Extraction—A Historical Review, Proc. Int. Solv. Extr. Conf. Melbourne, 1996.

3.      3. Ho ̈gfeldt, E.; Stability Constants of Metal-Ion Complexes. Part A: Inorganic Li- gands. IUPAC Chemical Data Series No. 22, Pergamon Press, New York (1982).

4.      McNaught, A. D.; and Wilkinson, A.; IUPAC Compendium of Chemical Terminol- ogy, Second Edition, Blackwell Science (1997).

5.      IUPAC, Quantities, Units and Symbols in Physical Chemistry, Third Edition, (Ed. Ian Mills), Royal Society of Chemistry, Cambridge 2002.

Soure: Solvent Extraction Principles and Practice, Revised and Expanded edited by Jan Rydberg