A Continuous liquid-liquid Extractor Arthur E. Werner and Michael Waldichuk, Fisheries Research Board of Canada, Biological Station, Nanaimo, B. C., Canada
of pollution in coastal sea waters, the need arose for concentrating and isolating trace amounts of certain substances. Difficulties encountered in batch extraction of large volumes of sea water led US to try continuous liquid-liquid extraction. Developments in the field of laboratory liquid-liquid extractions have not been rapid (6, 6, 9), but apparatus for extraction of radioactive solutions has been described (2, 4). Other equipment reported in the literature does not provide for continuous flow of the aqueous phase or may be complicated by provision for two solvents, one of them water soluble (1, 8). The elegant apparatus described by Scheibel unfortunately requires external solvent recovery (3). Unavailability of a suitable commercial extractor for the specific problem in our laboratory compelled us to design and build a unit (Figure 1) based on the principle of contact by countercurrent flow of sea water and solvent with continuous recovery of the latter. The extractor is constructed almost entirely of borosilicate glass. Ball and socket joints, providing additional durability, are used wherever flexibility is required, especially on the dog leg, URING INVESTIGATIONS
creases as the concentration of sea water increases; the solution then sinks to colder regions so that the extracted material is removed from contact with the hot surfaces. Thus, thermally unstable substances are prevented from decomposing by prolonged heating. Solvent vapor rises to a reflux condenser, K , and liquefies. The distillate returns through the tube, T, to the extractor. This tube may carry a calibration as the position of the meniscus is a convenient means for measuring the solvent flow rate. Capacity. It is possible to exhaust many liters of sea water with a small amount of solvent-e.g., 250 m1.unless the solvent itself is too soluble in water. I n cases of high solubility-
e.g., diethyl ether, solubility 75 grams pe liter at 20' C.-it is advisable to saturate the sea water with solvent beforehand, adding it directly to reservoir A . Replenishing the solvent in the evaporator, E, during the course of extraction is therefore not necessary. The degree of exhaustion achieved by a given solvent depends on the ratio of the flow rates of solution and solvent. The flow of solution can be varied by changing the flow-restricting capillary, C, and by adjusting the pressure head. The flow of solvent is controlled by adjustment of the heat input to the evaporator by a variable transformer and can be measured by the position of the meniscus on a calibrated portion of tube, T .
H. EXPERIMENTAL
Operation. The sea water descends from a n elevated reservoir, A , and passes over a weir, L , down a mixing tube, M , where contact with ascending droplets of solvent takes place. The extracted sea water leaves the bottom of the mixing tube and siphons into a storage container, B , a t floor level. Regular flow is maintained by a capillary, C, and a constant pressure head device, P , a simplified form of Mariotte flask ( 7 ) . The solvent passes down a narrow, central tube, T , and is dispersed a t the bottom of the mixing tube, ill, by the sintered glass disperser, D. It rises as a stream of fine droplets through the descending sea water, thus ensuring efficient extraction and forms a layer which continuously overflows into the evaporator, E. The latter consists of a tube packed with Berl saddles, lagged with asbestos, and wired for electrical heating. Heat is supplied so that maximum temperature is a t the top of the tube where the solvent enters. The lowest part of the evaporator is the unheated sump, S, from which concentrated extract can be withdrawn through a stopcock, I . -4s the solute-loaded solvent enters the evaporator, the solvent is vaporized. The density of the remaining liquid in-
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ANALYTICAL CHEMISTRY
B Figure 1. Schematic diagram of a continuous liquidliquid extractor (dimensions given in inches)
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Table 1.
Equilibrium Experiments Solution I I1
Original -0.25 Temp., “C. VI, ml. 25 25 VZ, ml. C,, moles/l. 18.68 X 10-3 18.68 X loe3 C1, moles/l. 7.92 x 10-3 Cz, moles/l. 1.12 X 10-2 I
