Similarly, a liquid–liquid reversible equilibrium system can govern a solvent extraction process, which is intended to separate, concentrate, or purify a particular component from a mixed solution, such as a fermentation broth, a leaching solution from a mineral acid reaction, or a waste stream from other operations.
One can refer to the basic reference books15–18 and note that the “official” denomination is “liquid–liquid extraction,” but most people in the field keep calling them “solvent extraction processes.”
Generally, one is considering two liquid phases, but there also exists invariant systems with three liquid phases at equilibrium, according to Gibbs’ “phases law.” At least one of these systems was used in the IMI “cleaning” process for separation of clean phosphoric acid from wet phosphoric acid (see Chapter 4, Section 4.4.3, Reference 26).
Again, in almost all processes of practical importance, there are many components in each phase. One has to define all these components and their ranges of concentration and to choose, on one hand, the variable of major interest and on the other hand, the ranges of parameters (such as the con- centrations or ratios of the other constituents and the physical conditions) that can be covered in a reasonable experimental program. If one is not careful in his choice, the number of tests required can easily shoot up exponentially. The “distribution coefficient” of this major variable can be correlated and used for multiple stage process calculations in the defined ranges of parameters. The major difference from the vapor–liquid physical equilibrium systems is that in most liquid–liquid extraction processes, the major variable of interest in any particular process can be either an ionic or a molecular entity, according to the chemical extraction mechanism.
Once this procedure is well understood, the bench-scale experimental program for the development of a separation process based on solvent extrac- tion can be relatively straightforward. The technique of the so-called “separa- tion funnels” tests is based on equilibration in defined conditions, sampling and analyses, and it can be carried out routinely by laboratory technicians. This fact of life was probably one of the main reasons for the successful development of many dozens of new solvent extraction processes in the years 1960 through 1980 in various countries. Very promising R&D programs in this field are continuing nowadays inside some of the large industrial corporations, although not much is published about that at international conferences.
In this connection, it is important to stress the experimental technique called “limiting conditions,” which makes it easier to study the effect of one variable at a time. If, for example, 50 ml of a starting aqueous solution are mixed with 50 ml of a solvent phase, the concentrations of the component of interest after equilibration will probably change significantly in both phases. If, for example, a series of tests are done at different temperatures, the quantitative results can be “all over the place” and a lot of tests will be required to find a working hypothesis to explain the results. But if 100 ml of the starting aqueous solution are mixed with 1 ml of solvent, the chosen composition of the aqueous phase will change very slightly, while that of the small organic phase can change very significantly. Thus, three to four tests at different temperatures should give a clear indication of the effect of that variable for the particular chosen aqueous composition.
The experimental procedure is also simpler for processes in which the solvent added is composed of a single component, such as butanol, pentanol, methyl iso butyl ketone (MIBK), and so on. But, for other processes, the composition of the solvent phase added can be quite complex by itself and may present a large number of additional components and parameters, such as the nature of the extractant (i.e., one particular tertiary amine from the dozens of tertiary amines commercially available), of the modifiers (i.e., one of many long-chained alcohols available for such duty), and of the diluent (i.e., a light, saturated hydrocarbon), and the relative weight ratios of these three classes of components.
In the typical practice of solvent extraction process development, one would generally start with a screening procedure. (Even my granddaughter knows by now that, in real life, the Princess would have to kiss many Frogs before she would find, maybe, her Prince.) This “screening” procedure is generally started with one effective composition “formula” found in previous publi- cations, or in the suppliers’ recommendations. This formula may not neces- sarily be the optimal composition for the specific case studied, so that some “exploration tests” with moderate changes outside this range should be done preferably before any specific commitment. But the prevailing attitude has often been: “Let’s start with the composition that works, and we will opti- mize later.” But as often happens, everybody is too busy “later” to look back at this exact composition. This is a well-known pitfall.
It has also been observed that the chemical behavior of some extractants (in particular tertiary amines) does change as the “new” reagent (straight from the bottle) is “aged” after a few dozens cycles of loading/regeneration. This change, which may include a significant shift in the equilibrium curve, can be due most probably to the elimination of some traces of impurities which remained in the “new” reagent from its synthesis, and possibly also to the oxidation of unsaturated bonds in the experimental manipulations. Since the plant will be working eventually with an aged extractant, the testing conditions and results should reflect that change from the beginning. An additional form of aging occurs in functioning plants due to accu- mulation of certain impurities in the solvent cycle. Although a continuous purification procedure is generally used on a side stream, there is an eco- nomic limit to such purification and any plant has to live with a certain level of impurities in the solvent. This effect is difficult to reproduce from these early tests, but has to be accounted for in the design safety factors.