EARLY STEPS TOWARD THE USE OF SOLVENT EXTRACTION

1.3 EARLY STEPS TOWARD THE USE OF SOLVENT EXTRACTION

Around 500 B.C.E. the Greek philosophers recognized four elements: earth, water, phlogiston (~air), and fire. This view harmonizes with the present concept of three physical states of aggregation (solid, liquid, gas) and heat. Aristotle (~350 B.C.E.) emphasized that these “elements” were not eternal, but could be changed into each other. Five thousand years earlier, scientists already had found that when certain green minerals were heated in a coal fire, metallic copper was obtained. In Aristotle’s time it was known how to produce metals such as copper, gold, tin, lead, silver, iron, mercury, and arsenic. Even earlier, by transmuting certain “earths” with fire, ceramics and glasses had been produced. Many of these arts were probably developed by the Egyptians, the first true chemists. The word “alchemy” is derived from Arabic and Greek and is supposed to mean “art of transmutations as practiced by the Egyptians.” Fruit juices were fermented, oils and fats were squeezed out of vegetables and animal parts, and purified by digestion with earths, bones, etc. Crucibles, retorts, and even distillation equipment seem to have been in use; see Fig. 1.4. We must think of these early alchemists as endlessly mixing, heating, boiling, digesting, cooling, etc., everything they could collect from nature. The purpose of these transmutations varied: for lamps, weapons, pigments, perfumes, and poisons; for cosmetics and medicines to prevent aging and to prolong life (elixir vitae); for tanning chemicals, soap, anesthetics, and also for making gold. In fact, they did succeed in producing gold-like metals (e.g., brass). For example, it is known that they heated odorous leaves in alkaline water with fats and oils, so that ointments and perfumes could be enriched in the cooled solidified fat, and that these products were extensively used in the ancient courts, and perhaps even among the general population. If this is considered to be solvent extraction, it is truly one of the oldest chemical techniques. It is also likely that the Egyptians knew how to distill alcohol, long before it is described by the Arab Kautilya and the Greek Aristotle about 300 B.C.E. (See also Ref. [2].)

This experimentation more or less came to a halt during the Greek civilization. The Greeks were philosophers, and not so much experimentalists; Aristotle was a philosopher and a systematizer (systems technician, in modern language), not an experimentalist. The Greeks were followed by the Romans who were administrators, and by the Christians who considered alchemy to be ungodly. Although alchemy was practiced during subsequent centuries, particuarly in the Arabian world, it became suspect and was banned by many rulers (though encouraged in secrecy by others). About 500 years ago, alchemy was rather openly revived in Europe, particularly at local courts, and progressed within a few centuries into modern science.

Fig. 1.4 (a) Equipment used by alchemists, according to an Alexandrian manuscript (about 300 B.C.E. to A.D. 300). (b) Apparatus for fractionated distillation. Front page of Philosophi ac Alchimistae Maximi by Johannes Greininger, Strasbourg, 1531. The original work is ascribed to the great eighth-century Arab alchemist, Abu Musa Jabir. 

Digestion of various earths (or digested earths) with alcohol produces many organic solvents (ether, acetone, etc.). These solvents could be obtained in pure form through distillation. Such organic solvents could have been produced many thousands years ago, because of the obvious knowledge of distilla- tion [2]. However, 200 years ago only a few pure solvents seem to be known: besides the natural water and oils (and kerosene) only alcohol, ether, and “etheric oils” were acknowledged. It is difficult to trace organic solvents far back in history. The reason may simply be that organic compounds obtained by distillation of mixtures of natural products were found to be rather uninteresting (except for alcohol), because at that time they seemed to have very little practical value, and they certainly could not be used to produce gold. Because solvent extraction requires pure organic solvents of limited aqueous miscibility, it is then understandable why solvent extraction historically is considered (perhaps falsely) to be a newcomer among chemical separation methods.

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. 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