WO1993023150A1

WO1993023150A1 - Liquid extraction of aqueous solutions

Info

Publication number: WO1993023150A1
Application number: PCT/EP1993/001217
Authority: WO
Inventors: Myron J. Coplan; Aharon M. Eyal; Avraham M. Baniel; Eyal Bressler
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem; Cohn, Michael
Priority date: 1992-05-18
Filing date: 1993-05-14
Publication date: 1993-11-25
Also published as: IL101906A0

Abstract

An indirect contact extraction method of the kind in which a solute is extracted from an aqueous feed solution into a receiver solution across a permselective membrane that is permeable to said solute and impermeable to the extraction reagent and its association products. In accordance with the invention an aqueous receiver solution is used for the extraction.

Description

LIQUID EXTRACTION OF AQUEOUS SOLUTIONS

GLOSSARY

In the following description and claims the terms and expressions below should be understood as having the following meanings:

Direct contact extraction: a liquid-liquid extraction process of the kind in which an aqueous solution and an organic extractant are intimately mixed and then allowed to separate whereby solute is transferred from the aqueous to the organic phase.

Indirect contact extraction: an extraction process in which a solute is withdrawn from an aqueous feed solution to a receiver solution across a semi-per¬ meable membrane of anv kind. _ _

Extraction reagent: a compound capable of forming an asso¬ ciation product with a solute and thereby to extract such solute from an aqueous feed into a receiver solution.

Receiver solution: a solution containing an extraction re¬ agent.

Organic receiver solution: a receiver solution in which the solvent is organic.

Aqueous receiver solution: a receiver solution in which the solvent is water.

FIELD AND BACKGROUND OF THE INVENTION

The present invention is in the field of indirect contact extraction of solutes from aqueous solutions.

Extraction of solutes, e.g. electrolytes, from aqueous solutions is required for many technological purposes such as in chemical, biotechno- logical and hydrometallurgical production and processes; for decontamina¬ tion of water by the removal of noxious electrolytes therefrom; for the removal of acids from reaction mixtures in the course of on-going chemical reaction or fermentation processes; and the like. Conventionally, solutes are extracted from aqueous solutions by direct contact extraction techniques using water-immiscible organic receiver solutions. In such extraction the choice of the organic solvent of the receiver solution depends in each individual case on factors such as difference in specific gravities of the phases, viscosity and surface tension of the organic solvent, all of which influence the rate and efficiency of phase separation and have, therefore, to be taken into consideration. Because of such constraints it is not always possible to use a receiver solution which, from the point of view of extraction selectivity and capacity, would be most effective.

The direct extraction method has the further disadvantages of being uneconomic in case of a dilute aqueous feed solution, of leading to extractant losses in consequence of inevitable dissolution, entrainment and frequent emulsification of the extractant which represents heavy expense and additionally cause contamination of the product and pollution of the extraction raffinate. Apart from ecological problems of such pollution and contamination, the entrained organic phase may also interfere with on-going reactions, e.g. by adversely affecting and inhibiting microorganisms participating in biotechnological processes such as fermentation, by denaturating proteins present in the aqueous feed solution, and the like.

For all these reasons it has already been suggested to replace direct contact extraction by various indirect contact extraction technologies still using, however, organic solvents for the receiver solution, as described, for example, in "Membrane Handbook" edited by Ho and Sirkar, Van Nostrand Reinhold Publishers, Library of Congress No. 91-43661. One such proposal is based on the use of liquid membranes, either in the form of double emulsions or in the form of a liquid held in the pores of a porous solid support. However, such liquid membranes proved to lose selectivity due to emulsion breakage in double emulsion liquid membranes and due to leakage in supported liquid membranes. In addition, such liquid membranes also cause a substantial loss of organic receiver solution. Recently a new system called "contained liquid membrane" has become known. In accordance with that system, also described in the above Membrane Handbook, an organic liquid phase, the "contained liquid membrane", is separated from an aqueous feed solution by a first solid membrane film and from an aqueous receiver solution by a second solid me brane film. In this system the feed and receiver solutions frequently flow through the lumens of separate groups of hollow fibers submerged in the contained liquid phase of the first membrane. This method also has the potential of possible contamination of the feed and receiver solutions with organic species as in the other methods described above.

Other proposals for the performance of indirect contact extraction included the use of highly plasticized polymer membranes, solute-permeable membranes in the form of rather dilute gels formed in the pores of solvent-stable porous membranes, and non-porous polymer membranes which, in operation, are swollen by water and organic solvent thus forming a transition zone which provides for relatively high diffusivity. None of these techniques was found to be quite satisfactory, among others for the reason that they did not overcome the problem of leakage of the organic phase into the aqueous feed solution. It is accordingly the object of the present invention to provide a new method of indirect contact extraction of solute from aqueous solutions, free of any organic phase pollution problems.

SUMMARY OF THE INVENTION The present invention is based on a new concept according to which in the indirect extraction of solute from an aqueous solution across a membrane, the receiver solution is also aqueous and holds an extraction reagent of appropriate water solubility and of a kind which does not permeate from the receiver to the feed solution across the particular membrane used, neither by itself nor in association with the extracted solute. In this way the solvent on both sides of the membrane is water and problems of solvent contamination of the feed solution do not arise. This new concept is fundamentally different from prior art indirect extraction methods of aqueous solutions where the receiver phase solvent is organic. - D -

Thus, in accordance with the present invention there is provided an indirect contact extraction method of the kind in which a solute is extracted from an aqueous feed solution into a receiver solution across a permselective membrane that is permeable to said solute and impermeable to the extraction reagent and its association products, characterized in that an aqueous receiver solution is used for the extraction.

Solutes that can be extracted in accordance with the invention include water-soluble organic or inorganic cations and anions, salts, acids, bases, amino acids and neutral hydrophilic organic compounds with molecular weights of up to 500.

Similar as in the prior art, the extraction reagents used in the performance of the method according to the present invention are selected in accordance with the nature of the solute to be extracted. For example, when cations are to be extracted from an aqueous feed solution the extraction reagent will be selected from compounds such as oximes; crown ethers; acids and their salts, preferably salts that are capable of complex formation such as, for example, citrates and tartrates; ethylenediamine tetracetic acid (EDTA) and its phosphate analogs; water-soluble complexing or chelating polymers, e.g. polyacrylic acid and its salts, polyacrylamide, poly(ethylene imine) and its salts, poly(ethylenesulfonic acid) and its salts, poly(methacrylic acid) and its salts, poly(posphoric acid) and its salts, polycyclic molecules; and many others.

In the case of anion and acid solutes, extraction reagents may be selected from amongst various basic compounds including water soluble organic bases and their salts such as triethylamine, triethanolamine, polyamines, tetramethyl ammonium bromide and the like.

For separating acids or salts of acids of complexing nature from other acids or their salts, transition metals may, for example, be used as extraction reagents. Where salts are to be extracted, amino acids and peptides may be used as extraction reagents.

Depending on the circumstances of the case, the extraction reagent may be so selected as to be specific to a particular solute which is thus selectively extracted from the feed solution.

Aqueous solutions are generally less viscous than organic ones and thus provide for higher diffusion coefficients. Accordingly, by using aqueous receiver solution better mixing and higher mass-transfer rates in membrane boundary layers can be achieved as compared to organic receiver solutions.

Membranes used in the performance of the present invention can be selected from among a large variety of permselective polymeric or inorganic membranes. They may be of the kind used in ultrafiltration where the selective layer of the membrane is porous. Water moves more or less freely through the pores and selective permeation of any solute is determined largely by its size relative to the pore size in the selective layer so that, for example, species of molecular weights greater than, say, 1000 may not pass through the pores while other species of molecular weights less than 1000 pass more or less freely through the permselective layer. Thus, in accordance with the present invention, a relatively high molecular weight extraction reagent in the receiver solution on one side of the membrane would be barred from crossing its permselective layer while a solute in the feed solution on the opposite side of the selective layer is not so barred. The prior art describes methods of metal ion separation from aqueous feed utilizing a combination of high molecular weight water soluble complexants and ultrafiltration membranes, in which the complexant is added to the feed and complexes the metal ion to be separated. The water and non-complexed species are driven through the membrane concentrating the complexed ion in the feed side of the membrane (Spivakov et al., Nature, 315, 313 (1985)). The bulk of the feed solution is transferred through the membrane requiring relatively large membrane area. The present invention is distinguished fundamentally from such methods in that in accordance with the invention the high molecular weight water soluble complexant performs as the extraction reagent in the receiver solution and the metal ion to be transferred is extracted into the latter.

Alternatively, membranes used in this invention may be of the type used in hyperfiltration, such as in reverse osmosis, where there are no detectable pores in the permselective layer which is believed to be essentially fully dense. Selective permeation of solutes in a homogeneous solution, for example salts dissolved in water, is a function of the specific chemistries of the membrane substance and the permeating solutes. In uncharged membranes such as, for example polyvinylidene fluoride membranes, the solvent of the solution to be separated has a finite solubility in the membrane substance and a practical diffusion rate through it under a chemical or pressure gradient, and they may carry with them one or more solutes while the solutes per se are virtually insoluble in the membrane substance and/or have an imperceptible diffusion rate in it. However, in so- called fully-dense membranes with specific charged substituents, migration of solutes having like or opposite charges across the membranes is facilitated or suppressed, as the case may be, due to the presence of the charged substituents.

In accordance with the present invention, the extraction reagent in the receiver solution is barred from crossing the selective layer of the membrane by charge effects or other specific chemical phenomena while the solute in the feed solution is free to migrate across the selective layer from the feed solution into the extraction solution. In accordance with the invention a cation exchange membrane is used in conjunction with an anionic extraction reagent for the extraction of a cationic solute, and an anion exchange membrane is used in conjunction with a cationic extraction reagent for the extraction of an anionic solute. Cation exchange membranes of excellent chemical stability are available, and are relatively inexpensive. Conventional cation exchange membranes suitable for use include membranes carrying sulfonic groups, such as sulfonated polystyrene, sulfonated polyethylene, sulfonated polysulfone, sulfonic perfluorinated polyolefins, etc. Commercially available anion exchange membranes, suitable for use, usually carry quaternary amino groups. Modified polysulfone and other anion-exchanging polymers that carry amino groups are also suitable.

Organic receiver solutions require solvent-stable membranes, which limit the choice of membranes to be used. It is, for example, difficult to produce membranes that combine high solvent stability with good stability in high acidity or high basicity. This limitation is removed in accordance with the present invention by using an aqueous receiver solution.

Aqueous solutions are generally more polar than organic solutions, enhancing dissociation of salts and acids and stabilizing ionic species such as, for example, protonated low molecular weight amine bases, zwitterions, transition metal cations and anions of complexing chelating carboxylic acids. As a result, higher concentrations of these species, acting as extraction reagents, are obtainable in aqueous receiver solution compared to organic receiver solutions, leading to higher mass transfer across the membrane. These extraction reagents are blocked more efficiently by ion- exchange membranes from permeation to feed and product solutions when in aqueous receiver solution than when in organic solution.

Usually, the extracted solute, whether a valuable product or an undesirable contaminant, should be removed from the aqueous receiver solution in order to liberate the extraction reagent and make the aqueous extractant solution available for further extraction. Such removal can be done in any suitable manner. In accordance with a preferred embodiment of the method according to the invention, the extracted solute is transferred from the aqueous receiver solution to a third aqueous solution referred to herein as "product solution", across a second permselective membrane permeable to the extracted solute and impermeable to the extraction reagent and its association products. For the transport of the extracted solute from the aqueous receiver solution to the aqueous product solution, the association product of the extracted solute and the extraction reagent has to decompose. Such decomposition may be mediated by pH changes, by displacement with another reagent, through oxidation/reduction (as known for example for uranium or iron extraction), etc.

Instead of transferring the extracted solute from the receiver solution to an aqueous product solution, the transfer may also be to a vapor phase, e.g. in case of a volatile specie, or to a solid phase.

In accordance with the invention it is possible to effect a so- called "uphill pumping" of a solute from a dilute feed solution via a receiver solution into a concentrated product solution. With high solvation capacity and high diffusivity, aqueous receiver solutions provide for the application of the co- and counter transport of ions as driving forces for concentration of the transferred solute in the receiver and/or the product solution. Thus, high concentrations of NaCl in an aqueous solution, from which Fe³⁺ should be removed, enhance co-transport of Fe³⁺ and Cl^" as FeCl₄ ^" using anion exchange membranes and cationic extraction reagent. Similarly, high concentrations of Na₂S0₄ in the product solution lead to nearly complete transport of Cu²⁺ from a sulfate containing aqueous feed solution, using cation exchange membranes and anionic extracting reagents. The counter transport of Na⁺ ions provide for concentration of the Cu²⁺ ions in the product solution. Uphill pumping of the transferred solute requires high feed to product solution flow rates, easily achieved according to the method of the invention. Electrolytes containing aqueous solutions have generally much higher conductivity than organic phases. Hence, aqueous receiver solution provides for application of an electric field as a driving force for acceleration of mass-transport and for uphill pumping concentration of the transferred solute. This applies to a receiver solution as well as to a product solution.

In most cases water may be allowed to transfer freely between the two or three aqueous solutions, as the case may be. Where, however, this is undesired, e.g. in order to avoid undue dilution of the feed and/or product solution by water transfer, either of these solutions may be put under suitably adjusted physical pressure or high ionic strength by adding some available solutes such as, for example, sodium chloride, whereby water permeation is practically eliminated.

In the method according to the invention selectivity of transfer from the feed into the receiver solution is provided by the extraction reagent while the permselective membrane that partitions between the aqueous solutions serves merely for avoiding the escape of the extraction reagent from the receiver to either of the feed and product solutions. As the method according to the invention is free of any emulsification, entrainment, specific gravity and surface tension problems, the extraction reagent can be selected exclusively on the basis of its degree of water solubility and selective affinity to a desired solute and in this way the extraction can be optimized. The method according to the invention is clearly distinguished from ultrafiltration methods where large amounts of water or other solvents are transferred across an ultrafiltration membrane while the solutes are retained on the membrane. For the performance of the method according to the present invention a variety of commercially available membrane systems and modules may be used such as a simple three-compartment cell in which the medium compartment holds the receiver solution, one of the flanking compartments holds the feed solution and the other the product solution. For continuous operation it is possible to use a multi- compartment apparatus in which a plurality of receiver solution compart¬ ments alternate with feed solution and product solution compartments through which the respective solutions circulate co-currently or counter- currently, as may be desired; or also a spiral-type module such as described, for example, by M. Teramoto et al, Proc. ISEC (Int. Solv. Extr. Conf.) 1988, II, 110.

In accordance with another embodiment of this invention there is provided an arrangement of first and second groups of hollow fiber membranes. The fibers of one group may have selective layers which are the same or different from those of the other group. The two groups are disposed within a common containment vessel or tank (hereinafter "receiver solution vessel") filled with a receiver solution. A feed solution carrying one or more solutes is pumped through the lumens of a first group of hollow fibers, and a product solution is pumped through the lumens of the second group of hollow fibers.

The permselective layer of each group of fibers is such that the extraction reagent is barred from permeating across the membrane into the lumens. However, water from any of the solutions may permeate without detriment the permselective layer of each group of fibers.

By the judicious selection of permselective layer properties and the extraction reagent, migration of solute from the feed solution flowing in the lumens of the first group of fibers into the product solution flowing in the lumens of the second group of fibers is accomplished by several processes going on more or less simultaneously.

Migration of solute from the feed solution flowing in the lumens of the first group of hollow fibers across the permselective layers thereof into the receiver solution is facilitated by specific chemical interaction of solute and extraction reagent. The solute/extraction reagent association product diffuses or convects a very short distance to the selective layer of the second group of hollow fibers through whose lumens the product solution flows. The solute/extraction reagent association product dissociates at the solution interface with the permselective membrane layers so that solute is liberated from the solute/extractant interaction product and is free to migrate across the fibers' walls into product solution flowing through the lumens thereof. Similar transport processes take place in other . kinds of membrane setups. In this embodiment of the invention the hydraulic pressures of the feed solution, receiver solution and product solution are each indepen¬ dently controllable. The effect of this is to permit the permeation of a controlled amount of water across the respective membrane selective layers in one direction or the other. Thus, whatever promotes the desired ultimate transfer of solute between the feed and product solutions may be enhanced by the simultaneous transfer of some water across either the permselective layers of either or both groups of hollow fibers.

Feed and product solutions are fed into the lumen of each hollow fiber of one group at one end thereof and are withdrawn at the opposite end. By one mode of operation the receiver solution is kept stagnant within the receiver solution vessel, while in accordance with another mode the receiver solution flows across the vessel continuously or intermittentlv. By one mode, hollow fibers of the two groups extend to a large part of their length in more or less parallel arrays, with the flow of the feed and product solutions being either co- or counter-current. The receiver solution may be essentially a stagnant fluid environment bathing the external surfaces of the fibers, or it may be moved along the fiber surfaces by pumping or agitation.

By another mode the first and second groups of hollow fiber membranes are contained in two different vessels and the receiver solution is circulated between them. In a process optimized for solute transfer, the solvent volume balance may be such that, unless some corrective measures are taken, there will occur a gradual volume change of the receiver solution with concurrent dilution or concentration of the extraction reagent. To compensate for this in a steady-state operation, receiver solution may be continuously recircu- lated through a secondary membrane module where, by hydraulic or osmotic pressure effects, its volume and the concentration of receiver solution are adjusted before it is returned to the primary containment vessel where it performs its primary facilitated transport function.

The hollow fibers may be of the so-called "asymmetric membrane" type wherein the permselective layer is a skin integrally formed from the same polymer on one of the surfaces of the hollow fiber wall. Alternatively, the hollow fibers may be of the so-called "composite membrane" type where a thin, permselective polymer layer is deposited on the surface of a porous hollow fiber made of another polymer, which latter has little or no permselectivity of its own. Permselective layers in either membrane type may be formed or deposited on the inside or the outside of the hollow fiber wall. Optimum operation of the overall process may depend on location of the permselective layer. U.S.5,169,529 to Carroll et al. describes a hollow fiber module system employing two sets of fibers immersed in a common fluid and in that respect resembles the hollow fiber embodiment of the instant invention. However, '529 is directed to mediated transport of a gas species from one group of fibers, the feed fibers, through a contained liquid membrane to a second set of fibers, the strip fibers. The fibers of '529 have no selectivity function, one set being only the conduit means for the feed gas and the other set being only the conduit means for the gas species which permeates through the liquid. Continuous removal of products from fermentation liquor has been recognized for a long time as a means of improving productivity of fermentation. Many studies were made of. continuous extraction of product during fermentation. Industrial application is, however, hampered by organic extractant related difficulties. Efficient extractants, for example strong organic bases for the extraction of fermentation produced carboxylic acids, are toxic to the micro-organism. Avoidance of organic receiver solution and efficient blocking of extraction reagent in the indirect contact extraction according to this invention removes these difficulties and provides for continuous removal of fermentation products. Thus, in accordance with one particular application of the invention a fermentation product, e.g. a carboxylic acid produced by fermentation, is withdrawn continuously from the fermentation liquor by indirect contact extraction in the course of fermentation. To this end the fermentation liquor can be circulated through a suitably designed membrane module located outside the fermentor vessel and comprising at least one fermentation liquor feed solution compartment and one aqueous receiver solution compartment- If desired, the fermentation liquor may be filtered before its introduction into the membrane module. Alternatively, a membrane module with wide enouεh channels is used. By a different mode of this particular application, the aqueous receiver solution flows through the fermentor, for example in the lumens of hollow fibers submerged therein.

Further areas of technology to which the invention is applicable include selective removal of cadmium from wet process phosphoric acid; removal of heavy metal impurities and polar-organic compounds from industrial waste prior to disposal; purification of proteins; and many more.

DESCRIPTION OF THE DRAWINGS

Some apparatus for carrying out the method according to the invention are shown, by way of example only, in the accompanying drawings in which:

Fig. 1 shows schematically a three-compartment cell for batchwise operation;

Fig. 2 shows schematically a multi-compartment cell for continuous operation;

Fig. 3 shows schematically a spiral module for continuous operation; Fig. 4 shows a hollow fiber module with stagnant receiver solution; and

Fig. 5 shows a hollow fiber module with circulating receiver solution.

DESCRIPTION OF THE APPARATUS

The apparatus 1 schematically shown in Fig. 1 is a three- compartment cell having a first compartment 2 fitted with a magnetic stirring bar 3 and holding a feed solution; a second, median compartment 4 fitted with a stirring bar 5 and holding an aqueous receiver solution; and a third compartment 6 fitted with a stirring bar 7 and holding a product solution. The compartments are separated from each other by permselective membranes 8 and 9.

In operation a desired solute permeates from the aqueous feed solution in compartment 2 into the aqueous receiver solution in compartment 4 and from there into the aqueous product solution in compartment 6.

The apparatus 10 shown schematically in Fig. 2 comprises a plurality of receiver solution compartments 11, feed solution compartments

12 and product solution compartments 13. The feed solution compartments

12 are associated with a manifold 14 and the product solution compartments with a manifold 15.

The compartments are separated from each other by a plurality of permselective membranes 16.

In operation, feed solution is circulated through compartment 12 and product solution through compartments 13 in a counter-current fashion and in this way the desired solute is transferred from the feed solution across two membranes 16 and via a body of receiver solution into the product solution. The receiver solution inside compartments 11 may also circulate or else be static, with agitation if desired.

Fig. 3 shows schematically a spiral-type module 18 having a spiral receiver solution compartment 19 flanked by two spiral compartments 20 and 21 serving for circulation of feed solution and product solution, respectively. The compartments are separated from each other by a spiral permselective membrane 22. In operation the feed and product solutions are circulated through the respective compartments 20 and 21 either co- currently or counter-currently. The receiver solution in compartment 19 may either be static or in circulation.

Fig. 4 shows schematically a hollow fiber module wherein first and second groups of hollow fibers 23 and 24 pass through a receiver solution vessel 25. Feed solution flows through, say fibers 23, while product solution flows through fibers 24 and vessel 25 is filled with receiver solution. In this schematic representation the receiver solution is stagnant and in operation solute from the feed solution permeates across the perm¬ selective membranes of fibers 23 into the body of the receiver solution inside vessel 25 and from there across the membranes of fibers 24 into the product solution.

In Fig. 5 the hollow fiber module is the same as in Fig. 4 and the same numerals are used for similar parts. As shown, the module comprises first and second groups of hollow of fibers 23 and 24 for conducting, respectively, feed and product solutions across the receiver solution vessel 25 filled with receiver solution. The latter is circulated by means of pump 26 through a secondary membrane station 27 having a concentrate return duct 28 and a permeate outlet duct 29. The pressure on the secondary membrane is controlled by a valve 30 whereby the flow rates on the first and second groups of hollow fibers 23 and 24 is adjusted and the volume and concentration of the receiver solution are maintained at desired levels.

For better understanding the invention will now be illustrated in the following laboratory scale working examples to which it is not limited.

Example 1

A two compartment cell was used, with a permselective membrane having an active area of 2cm² separating between the feed and the receiver solutions. The membrane was a Neosepta CM- 1™ (Tokuyama Soda) cation exchange membrane (exchange capacity of 2-2.5) pre-treated in a CuSO₄ solution. 20 ml of an aqueous feed solution containing 1M CuSO₄ and 1M Na₂S0₄ was introduced to the feed solution compartment and 20 ml of an aqueous receiver solution containing 0.03M sodium citrate was introduced into the receiver solution compartment. The cell was mechani¬ cally shaken for 20 hours and thereafter analysis of the aqueous solution in the receiver compartment showed a Cu concentration of 0.013M.

This example shows the selectivity of the transport. Cu²⁺ ions from the feed solution compartment transfer into the receiver solution compartment of the cell while Na⁺ ions transfer in the opposite direction.

Cu²⁺ to Na⁺ equivalent ratio in the receiver solution was much higher than the ratio in the feed solution.

Example 2

The experiment in Example 1 was repeated with only one change: the receiver solution, however, contains 0.03M sodium sulfate instead of sodium citrate. After 20 hours shaking the receiver solution contained less than 0.001M Cu²⁺ which shows that the enhanced transport and the selectivity in Example 1 was due to the reagent used there.

Example 3

In a cell similar to that of Example 1 and with the same cation exchange membrane, IM aqueous CuSO₄ was used as feed solution. The receiver solution contained IM sodium lactate. For comparison a similar experiment was conducted with pure water in the receiver solution compartment. After 20 hours shaking, Cu²⁺ concentrations in the receiver solution compartment in these two experiments were 0.21M and less than 0.001M, respectively.

Example 4

The experiments in Example 3 were repeated with IM NiSO₄ in the feed solution compartment, the membrane having been preconditioned in NiSO₄. After 20 hours of shaking the Ni²⁺ concentration in the receiver solution compartment were 0.025 M and less than 0.001M, respectively.

Examples 3 and 4 show the enhancing effect of sodium lactate in the receiver solution on the transport of Cu²⁺ and on Ni2+. They also show that Cu²⁺ transport is more intense than that of Ni²⁺.

Example 5

The receiver solution of experiment 3 containing the copper lactate was diluted to 0.08 M of Cu and introduced into the receiver solution compartment of a cell similar to that of Example 1 and having a similar membrane which are similarly preconditioned. 3M H₂SO₄ was introduced into the receiver solution compartment. After 20 hours of shaking more than 80% of the Cu transferred into the acid containing compartment.

This example shows the ability of recovering the separated solutes from its association with the extraction reagent and of regeneration of the latter.

Claims

CLAIMSt

1. An indirect contact extraction method of the kind in which a solute is extracted from an aqueous feed solution into a receiver solution across a permselective membrane that is permeable to said solute and impermeable to the extraction reagent and its association products, characterized in that an aqueous receiver solution is used for the extraction.

2. A method according to claim 1, characterized in that the extracted solute is re-extracted from the receiver solution into an aqueous product solution by indirect contact extraction across a permselective membrane that is permeable to said solute and impermeable to said extraction reagent and association products thereof.

3. A method according to claim 1 or 2, characterized in that an auxiliary solute is added to the receiver solution.

4. A method according to claim 3, characterized in that an auxiliary solute is added to or is withdrawn from the receiver solution in the course of solute transfer from the feed solution to the product solution.

5. A method according to claim 4, characterized in that the hydraulic pressures of the feed, receiver and product solutions are indepen¬ dently set to control the permeation of water across each permselective membrane layer.

6. A method according to any one of claims 1 to 5, characterized in that said permselective membrane is selected from the group of porous ultrafiltration membranes and fully dense hyperfiltration membranes and the extraction reagent has. a molecular weight of at least 1000.

7. A method according to any one of claims 1 to 5, characterized in that said permselective membrane is a cation exchanger, the extracted solute is cationic and the extraction reagent is anionic.

8. A method according to any one of claims 1 to 5, characterized in that said permselective membrane is an anion exchanger, the extracted solute is anionic and/or acidic and the extraction reagent is cationic.

9. A method according to claim 8, characterized in that an acid is separated from cationic or non-electrolytic impurities by extraction of an aqueous acidic feed solution across an anion exchange membrane into a receiver solution containing a cationic extraction reagent.

10. A method according to claim 9, characterized in that said acid is a fermentation product.

11. A method according to claim 9 or 10, characterized in that said aqueous acidic feed solution comprises a fermentation liquor.

12. A method according to any one of claims 2 to 11, character¬ ized in that each of the feed, receiver and product solutions flow through separate compartments or groups of compartments.

13. A method according to any of the claims 2 to 12, characterized in that the permselective membranes are in the form of first and second groups of hollow fiber membranes having permselective layers which retain the extraction reagent and are permeable to the solute, which first and second groups of hollow fiber membranes are submerged in a body of receiver solution, feed solution being passed through the lumens of said first _

group of hollow fiber membranes and product solution being passed through the lumens of said second group of hollow fiber membranes.

14. A method according to claim 13, characterized in that the receiver solution is circulated between said body of receiver solution and a secondary membrane station whereby the concentration of extraction reagent in the receiver solution is adjusted.

15. A method according to any of the claims 2 to 12, characterized in that the permselective membranes are in the form of first and second groups of hollow fiber membranes having permselective layers which retain the extraction reagent and are permeable to the solute, which first and second groups of hollow fiber membranes are contained in two different vessels and the receiver solution is circulated between them.

16. A method according to any of the claims 1 to 15, characterized in that the selectivity of the membrane separating feed the solution from the receiver solution is different from the selectivity of the membrane separating receiver solution from the product solution.