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. 

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