0 Industrial Use of Solvent Extraction

1.4.3 Industrial Use of Solvent Extraction

A surge in interest in solvent extraction occurred in the decades of the 1940s and 1950s initiated by its application for uranium production and for reprocessing of irradiated nuclear materials in the U.S. Manhattan Project. The first large-scale industrial solvent extraction plant for metals purification was built in 1942 by Mallinckrodt Chemical Co., St. Louis, for the production of ton amounts of uranium by selective extraction of uranyl nitrate by ether from aqueous solutions. The high degree of purity (99.9%) required for use of uranium in nuclear reactors was achieved. An explosion led to the replacement of the ether by other solvents (dibutylmethanol and methylisobutylketone). At the same time new types of more efficient metal extractants were introduced, e.g., tri-n-butylphos- phate in 1945 and trioctylamine in 1948. This activity became a great stimulus to the nonnuclear industry, and solvent extraction was introduced as a separation and purification process in a large number of chemical and metallurgical industries in the 1950s and early 1960s. For example, by leaching copper ore with sulphuric acid followed by extraction of this solution with an organic hydroxyaryloxime dissolved in kerosene, several million tons of copper (30% of world production) is now produced annually. This and many other processes are described in later chapters.

For these applications, the technique of solvent extraction had to be further developed and with this a new terminology was also developed. This can be illustrated by considering a process where a desired component in an aqueous solution is extracted with an organic reagent (extractant) dissolved in another organic liquid; note here that the term “organic solvent” is not used because of possible confusion. The term “solvent” could be used for the whole organic phase or for the organic liquid in which the organic extractant is dissolved. Thus the term generally given to the latter is (organic) diluent.

While in laboratory experiments the extraction vessel may be a test tube, or more conveniently some kind of separation funnel (Fig. 1.1), this is not suited for industrial use. Industry prefers to use continuous processes. The simplest separation unit is then the mixer-settler, or some clever development of the same basic principle, as described in Chapter 9. Figure 1.5 pictures a simple mixersettler unit, here used for the removal of iron from an acid solution also containing nickel and cobalt (the same systems as in Fig. 1.3). The mixer (or contactor) is here simply a vessel with a revolving paddle that produces small droplets of one of the liquid phases in the other. This physical mixture flows into and slowly through the separation vessel, which may be a long tank; through the influence of gravity the two phases separate, so that the upper organic kerosene-octanol-amine phase contains the Fe(III) and the lower aqueous CaCl2 phase contains the Co(II) and Ni(II). Numerous variations of the construction of mixersettlers (or MS-units, as they are abbreviated) exist (Chapter 9), often several joined together into MS-batteries. Many such will be described later on.

A diagram of a full basic process is given in Fig. 1.6 to illustrate the common terminology. The incoming aqueous solution is called the feed. It is contacted with the (recycled) solvent phase in a mixer-settler unit. Here we do not indicate the exact type of unit, but only its function (extraction), as commonly is done. After extraction and separation of the phases, the depleted phase becomes the raffinate and the enriched solvent phase becomes the extract or loaded (or pregnant) solvent. The raffinate may undergo a solvent recovery stage to remove any entrained solvent before exiting the process. The extraction process is rarely specific so that other solutes may be co-extracted with the main component. These impurities may be removed with an aqueous scrub solution in a scrub stage producing a scrub extract and a scrub raffinate containing the impurities. The latter may return to the feed solution to maintain an overall water balance. The scrubbed extract is now contacted with another aqueous solution to strip or back-extract the desired component. The stripped solvent then may undergo some regeneration process to prepare the solvent phase for recycle. The loaded (pregnant) strip solution then is treated to remove the de- sired product and the strip solution is recycled. One of the important aspects of this flowsheet is that, wherever possible, liquid phases are recovered and recycled. This is important from both an economic and an environmental standpoint.

Figure 1.6 shows a situation where each process-extraction, scrubbing, and stripping-occurs in a single operation or stage. This is generally not efficient because of the finite value of the distribution coefficient. Thus if D has a value of 100 (i.e., E = 99%), then after one extraction, the organic extract phase will contain approximately 99 parts and the aqueous phase 1 part. To achieve a greater extraction, the aqueous raffinate should be contacted with another portion of the solvent after which the new organic phase contains 0.99 parts, and the aqueous raffinate now only 0.01 part. Therefore, two extraction stages will provide 99.99% extraction (with “2 volumes” of the organic phase, but only “1 volume” of the aqueous phase).

Three different ways of connecting such stages are possible: namely, cocurrent, crosscurrent, and countercurrent (see Fig. 1.7). In cocurrent extraction, the two phases flow in the same direction between the various contactors. A simple inspection of the diagram will show that with this configuration no advantage is gained over a single contact because, providing equilibrium is reached in the first contactor, the separated flows are in equilibrium when entering the second contactor so no change in relative concentrations will occur. In the second configuration (b), crosscurrent, the raffinate is contacted with a sample of fresh solvent. This is the classical way of extracting a product in the laboratory when using a separatory funnel and will give an enhanced recovery of the solute. However, on an industrial scale, this is seldom used because it results in the production of a multitude of product phases containing a reducing concentration of the desired solute. These have to be combined before stripping resulting in a much larger volume of loaded solvent to be treated with consequences for plant size and economics. The third configuration (c), countercurrent, is the one generally chosen by industry. The phase volumes remain constant and by feeding the two phases, feed and solvent, at opposite ends of the bank of contactors, the driving force for extraction, i.e., the solute concentration difference between the two phases, is maximized. Chapters in the second part of the book will extend this discussion.

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixersettlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7.

In the early analytical applications of solvent extraction, optimal extraction or separation conditions were obtained empirically. This was unsatisfactory and general mathematical descriptions were developed by a number of researchers in many countries. This was especially important for largescale industrial use and is an activity that continues today almost entirely with computers.

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0 Extractants

1.4.2 Extractants

During the years 1900 to 1940, solvent extraction was mainly used by the organic chemists for separating organic substances. Since in these systems, the solute (or desired component) often exists in only one single molecular form, such systems are referred to as nonreactive extraction systems; here the distribution ratio equals the distribution constant.

However, it was also discovered that many organic substances, mainly weak acids, could complex metals in the aqueous phase to form a complex soluble in organic solvents. A typical reaction can be written

M z + ( a q ) + z H A ( a q  o r  o r g ) →← M A z ( o r g ) + z H + ( a q ) ( 1 . 6 )

which indicates that the organic acid HA may be taken from the aqueous or the organic phase. This is an example of reactive extraction. It became a tool for the analytical chemist, when the extracted metal complex showed a specific color that could be identified spectrometrically. The reagent responsible for forming the extractable complex is termed the extractant.

The industrial use of solvent extraction of inorganic compounds grew out of the analytical work. As both areas, analytical as well as industrial, needed both better extractants and an understanding of the reaction steps in the solutions in order to optimize the applications, theoretical interpretations of the molecular reactions in the solutions became a necessity, as will be described in later chapters.

The increased use of computer graphics for modeling molecular structures and chemical reactions has opened a path for the synthesis of tailormade extractants. Thus the future promises new varieties of extractants with highly selective properties for the desired process.

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0 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.

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0 1.2 SOLVENT EXTRACTION IS A FUNDAMENTAL SEPARATION PROCESS

1.2 SOLVENT EXTRACTION IS A FUNDAMENTAL SEPARATION PROCESS

Under normal conditions, matter can appear in three forms of aggregation: solid, liquid, and gas. These forms or physical states are consequences of various interactions between the atomic or molecular species. The interactions are governed by internal chemical properties (various types of bonding) and external physical properties (temperature and pressure). Most small molecules can be transformed between these states (e.g., H₂O into ice, water, and steam) by a moderate change of temperature and/or pressure. Between these physical states— or phases—there is a sharp boundary (phase boundary), which makes it possi- ble to separate the phases—for example, ice may be removed from water by filtration. The most fundamental of chemical properties is the ability to undergo such phase transformations, the use of which allows the simplest method for isolation of pure compounds from natural materials.

In a gas mixture such as the earth’s atmosphere, the ratio of oxygen to nitrogen decreases slightly with atmospheric height because of the greater gravitational attraction of oxygen. However, the gravitational field of the earth is not enough for efficient separation of these gases, which, however, can be separated by ultracentrifugation and by diffusion techniques. In crushed iron ore it is pos- sible to separate the magnetite crystals Fe₃O₄ from the silicate gangue material by physical selection under a microscope or by a magnetic field. In chemical engineering such separation techniques are referred to as nonequilibrium processes. Other common nonequilibrium processes are electrolysis, electrophore- sis, and filtration.

In contrast to these we have the equilibrium processes of sublimation, absorption, dissolution, precipitation, evaporation, and condensation, through which the physical states of solid, liquid, and gas are connected. For example, the common crystallization of salts from sea water involves all three phases. Distillation, which is essential for producing organic solvents, is a two-step evaporation (liquid ⇒ gas) condensation (gas ⇒ liquid) process.

In Fig. 1.2, phase transformations are put into their context of physical processes used for separation of mixtures of chemical compounds. However, the figure has been drawn asymmetrically in that two liquids (I and II) are indicated. Most people are familiar with several organic liquids, like kerosene, ether, benzene, etc., that are only partially miscible with water. This lack of miscibility allows an equilibrium between two liquids that are separated from each other by a common phase boundary. Thus the conventional physical system of three phases (gas, liquid, and solid, counting all solid phases as one), which ordinarily are available to all chemists, is expanded to four phases when two immiscible liquids are involved. This can be of great advantage, as will be seen when reading this book.

Fig. 1.2 Model of a four-phase system consisting of two liquid phases (e.g., an aqueous and an organic phase) in equilibrium with a gas phase and a solid phase.

Solutes have differing solubilities in different liquids due to variations in the strength of the interaction of solute molecules with those of the solvent. Thus, in a system of two immiscible or only partially miscible solvents, different solutes become unevenly distributed between the two solvent phases, and as noted earlier, this is the basis for the solvent extraction technique. In this con- text, “solvent” almost invariably means “organic solvent.” This uneven distribution is illustrated in Fig. 1.3, which shows the extractability into a kerosene solution of the different metals that appear when stainless steel is dissolved in aqueous acid chloride solution. The metals Mo, Zn, and Fe(III) are easily extracted into the organic solvent mixture at low chloride ion concentration, and Cu, Co, Fe(II), and Mn at intermediate concentration, while even at the highest chloride concentration in the system, Ni and Cr are poorly extracted. This is used industrially for separating the metals in super-alloy scrap in order to recover the most valuable ones.

Fig. 1.3 Percentage of extraction of various metals from a solution of dissolved stainless steel scrap, at 40oC. The organic phase is 25% tertiary amine (Alamine 336), 15% dodecanol (Loral C12) and 60% kerosene (Nysolvin 75A). The aqueous phase is a CaCl2 solution at pH 2.

The three main separation processes between solid, gas, and liquid have long been known, while solvent extraction is a relatively new separation technique, as is described in the brief historical review in next two sections. Nevertheless, because all solutes (organic as well as inorganic) can be made more or less soluble in aqueous and organic phases, the number of applications of solvent extraction is almost limitless. Since large-scale industrial solvent extraction is a continuous process (in contrast to laboratory batch processes) and can be made more selective than the conventional gas–liquid–solid separation techniques, it offers numerous industrial possibilities to achieve desired separation efficiently and economically.

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. 

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0 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)

-----------------------

*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.

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