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Separation techniques with supported liquid membranes
Desalination, 68 (1988) 191-202
191
Elsevier Science Publishers B.V., AmsterdAm -- Printed in The Netherlands
Separation Techniques with Supported Liquid Membranes* G. SCHULZ Fa. Enka AG, Technical Membranes, Ohder Strasse 28, 5600 Wuppertal 2 (F.R.G.)
SUMMARY
Now that the problems of the longevity of liquid membranes have been solved with the introduction of multi-circuit processes, the supported liquid membrane technique has received a fresh impetus. W i t h the commercial availability of appropriate membranes and large modules with membrane areas of up to 10 m 2, the industrial application of the supported liquid membrane technique seems to be near. So far, it has only been tested in the laboratory on a pilot plant scale. Unfortunately, the worldwide downwards trend in the price of raw metals has delayed the introduction of this new separation technique into the market. At the moment, smelting is still by far the most economical method of producing metals. Against this background, the supported liquid membrane must be seen to have been thoroughly discussed and tested and must be seen as providing an economical alternative to competing processes. B u t for the m o m e n t it can be seen as a stand-by technology, waiting for a time when the recovery and recycling of metals, including uranium, becomes a more interesting economical prospect. New applications in the field of medicine and especially for gas separation, widely expected b u t at the m o m e n t still in the development stage, make the outlook promising for this multi-faceted technology. Keywords: supported liquid membranes, circuit processes, microporous membranes, metal ion
separation.
SYMBOLS A* - AM - dK,a-dp - D --
specific interface, m2/m 3 membrane area, m 2 outer hollow fiber diameter, m m a x i m u m pore diameter, m diffusion coefficient, m2/s
*Paper presented at the 5th Symposium on Synthetic Membranes in Science and Industry, Tiibingen, F.R.G., September 2-5, 1986.
0011-9164/88/$03.50
© 1988Elsevier Science Publishers B.V.
192 J K pH r R s T
--------~M ~ (0 ~ ---
diffusion rate, g/s equilibrium constant p H value particle radius, m gas constant, J / m o l ' K membrane thickness, m absolute temperature, K packing density of the module porosity of the membrane marginal angle viscosity, Pa s surface tension, m N / m
INTRODUCTION
Up to now the membrane processes introduced in the medical field and technology have been based on the application of so-called passive membranes. The mass transfer through passive membranes occurs with the active share between the permeating components and the membrane material. Passive membranes are merely barriers, which are passed by different substances at different velocities down a gradient. The liquid membrane technique is the first process on an industrial scale, which consists of an operation with active membranes. Active membranes contain complex building agents with high selectivity for the components to be separated. The main fields of application for the liquid membrane technique are: - - hydrometallurgy; - - treatment of wastewater from the chemical industry; galvanic techniques; gas separation; - - m e d i c a l application; --biotechnology. In this work we describe supported liquid membranes ( S L M ) , starting with the theoretical fundamentals, going on to different process developments and ending with the present engineering possibilities and potential applications. r
porousmembranes
passive membranes -1"-- solubility membranes ion-exchangemembranes active membranes _ . ~ liquid membranes L ~ biological membranes
Fig. 1. Characterisation of membranes.
193
THEORETICAL F U N D A M E N T A L S The aqueous solution,which initiallycontains all the ions which can permeate the liquid membrane, is generally referred to as the feed solution. The aqueous solution present on the opposite side of the membrane, which is initiallyfree from the permeable ions, isgenerally referredto as the stripsolution. In principle,liquidmembrane permeation is a solvent extraction process in which the extraction and stripping operations are performed simultaneously. A liquid phase acts as a selective separation layer between the feed and the strip solution. The carrier dissolved in the liquid phase picks up the ions on the feed side of the liquid membrane and transports them by diffusion to the strip side of the membrane. The driving force is a concentration gradient of the ion-carriercomplex between both sides of the membrane. Fig. 2 shows the example of the separation of metal ions from aqueous solutions,the most comm o n "counter transport" through a membrane. Other, commercially less important mass transfer processes through liquid membranes are described in more detailin a number of papers [1 ]. In the aqueous phase IIIa metal ion diffusesto the membrane surface,reacts there with the carrierX, a liquidion-exchanger, forming a metal-carrier complex which diffuses through the liquid membrane (phase II) driven by the existing concentration gradient. At the membrane surface which is in contact with phase I,the thermodynamic equilibrium is shiftedand the complex is split into the carrier and the metal ion. The carrierpicks up a proton and diffuses back to the feed side of the membrane. The whole process consists of the exchange of metal ions for protons, with the carrier acting as a catalyst.In the case of a steady-state process two diffusion steps have to take place simultaneously: the diffusion of the metal-carrier complex from the feed side of the membrane to the stripside and the back-diffusion of the proton loaded carrier. This metal ion transport works as long as protons migrate to the opposite direction. The driving force for this coupled transport is provided by the different aciditiesor p H values of the feed and strip solutions. In this type of liquid membrane -i
carrier~ $":L-'?"?'~.-'!_organic phase ":Wi~ 2H* ~.. ~
pH~ • Phase 11T
2HX
Phase 17
12H*
pH2 Phase I
Fig. 2. Schematic representation of the mass transfer through a liquid membrane.
194 transport, metal ions can be shifted across the membrane against their concentration gradient. In ideal conditions, the dependence of the metal ion flow across the membrane on the difference in pH value on either side of the membrane can be derived as follows: The equilibrium reaction for a divalent metal ion is Me 2+ + 2 H X .
~ Me X2 + 2 H +
and the equilibrium constant K - [MeX2] [H+]2 [ H X ] 2 [Me 2+ ]
(1)
where [ H + ] and [ Me 2+ ] are concentrations in the aqueous phase and [ Me X2I and [ H X ] concentrations in the organic phase of the membrane. This equilibrium constant is the same on both sides of the membrane. Equilibrium occurs if [Me2+] I og [ Me 2+ ] m = 2 ApH
(2)
where ApH is the difference between the p H values on both sides of the membrane. If, for example, the p H value of the strip solution is 1 and t h a t of the feed solution is 4, that means t h a t the difference is 3, a concentration of 1 : 10 6 can be achieved for divalent metal ions such as copper, zinc, nickel, etc.
Parameters influencing properties o[ liquid membranes The mass transfer of complexed metal ions through liquid membranes can be described approximately by the first Fick's law
j__.D'AM ([MeX,]i_ [ M e X , ] m ) $
(3)
where [ M e X , ] i is the concentration of the metal complex in the liquid membrane on the side towards phase I and [MeXn ] m is the concentration of the metal complex in the liquid membrane towards phase III. Following the molecular kinetic interpretation of Nernst, the diffusion coefficient D can be defined as D-
RT 6n~lr
(4)
where ~/is the viscosity of the liquid m e m b r a n e and r the radius of the diffusing components.
195 The above mentioned equations give some indications as how to increase the effectiveness of a liquid membrane: small membrane thickness for short diffusion length; high concentration of carrier molecules in the organic phase; small molecular diameters of the complexing agent; - - h i g h process temperature; low viscosity of the liquid membrane. It has to be taken into account that these requirements sometimes have opposite, less desirable effects. For example, with an increasing carrier concentration the viscosity of the liquid membrane increases too; an increased temperature promotes the dissolution of the organic phase in aqueous solutions. -
-
LIQUIDMEMBRANES In order to meet the requirements of the liquid membrane technique, it is necessary to develop stable films from the liquid. Fig. 3 shows the possibilities for forming thin liquid films. For industrial applications the multiple emulsion and the supported liquid membrane are suitable. lamella
H
foam ~
,,m|| multiple emulsion iiii~iiiiiiii!~iiiiiiiii :--:---..-.-~-_...----_::-...:-.-.. :-.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
microporous support material ~
Fig. 3. Formationof liquid films. :::::::::::::::::::::/organic phase • ~7~'J~ o~i~ii::::~ and carrier striP j I ~ ' i : : ~ - - - " : ~ l
s°,ut Iiii
t
I ~ ! ~ i ~ water phase ~E-~'E~::::-~:-:-iI with trace elements Fig. 4. Multipleemulsion.
196 feed solution organicphase ~ carrier f
Accurel-
membrane
<~
J strip solution
Fig. 5. Schematic representationof a supported liquidmembrane (SLM).
Multiple emulsions [ 2-5 ] In liquid surfactant membrane processes the membrane system (Fig. 4) is in the form of water-in-oil emulsions that are made by mixing stripping solutions with carrier-containing organic solvents. These emulsions are then dispersed in the aqueous feed solutions from which, for example, the metal ions have to be removed. The corresponding phases are stabilized by surfactants. The specific interface for the strip solution is about 106 m2/m 3 and for the organic phase towards the feed side 3000-4000 m2/m 3. An industrial plant working on the basis of the above described technology, has been running since the spring of 1986 at Chemiefaser Lenzing in Austria. It operates according to the concept of Prof. Mart, Graz (Austria) [ 6 ] and was planned for 75 m3/h wastewater containing zinc.
Supported liquid membrane The supported liquid membrane (SLM) technique uses a microporous hydrophobic polymer structure as support for the organic phase incorporating the carrier.The liquidphase is held in the membrane pores by capillarity.The wetting pressure for a hydrophobic membrane can be calculatedfrom Ap=
4 a cos dp
(5)
In this case a is the interface tension of the aqueous phase versus the organic phase. Amine compounds are able to decrease considerably the wetting pressure. In principle, complex building agents highly influence the interface tension and it is advisable to determine experimentally their behaviour before choosing an agent. A schematic representation of a supported liquid membrane is given in Fig. 5. The requirements for the porous support structure are: - - high porosity; small pore diameters; thin walls; - - hydrophobic material; chemical resistant polymer; -
-
197 TABLE I Microporous Accurel ®-polypropylene hollow fibermembranes by Enka AG, Wuppertal
Inner diameter
(mm)
1.8 1.2 0.6 0.3 0.18
Wall thickness
(~)
400 300 300 150 40
Organic phase in the pores
Effective specific
(dm3/m2)
membrane
(mS/m3)
0.367 0.281 0.337 0.169 0.034
area
577 833 1250 2500 5770
low costs. Porous support membranes are availableas flatsheets or hollow fibers.Hollow fiber membranes are preferred because of their higher packing densities, favourable hydrodynsmics and other advantages. The company Enka A G of Wuppertal (West Germany) offersthe following programme for microporous Accurel ®-polypropylene hollow fibermembranes. Relevant data for the liquid membrane permeation are evaluated as well (Table I). The indicated values referto a packing density of 50% in the module, a free volume in the membrane of 75% and an effectivearea for mass transfer according to the porosity of the membrane and referringto the outer diameter of the hollow fiber.The specificarea for the mass transfer A* can be calculated from A*- 4.~'~M
(6)
dK,a
So today, membrane technology has attained mass transfer areas which are
hollow fiber membrane module solution
~
organic phase and carrier
st rilc~ng solution
solution
ed stripping Sotutior
Fig. 6. Schematic representation of a hollow fiber permeator for the SLM technique. Fig. 7. Three-circuit process with SLM.
198
comparable with those of energy-consuming jet nozzles or venturi scrubbers [71. MEMBRANEMODULESAND PROCESSES Hollow fiber modules are manufactured in such a way that a bundle of parallel fibers are potted with synthetic resin at the front sides of a cartridge. The hollow fibers are open at the front sides (see Fig. 6). In a module the walls of the hollow fiber membranes are impregnated with the organic solvent containing the liquid ion-exchanger, called the carrier. The feed solution flows through the lumen of the hollow fibers and the aqueous strip solution circulates on the shell side of the fibers. If the feed solution is passing only once through the hollow fiber module the amount of metal removed is not enough. The feed solution has to be circulated continuously through the module. This recycling mode operation can be realized in one- or multi-circuit processes. In the beginning the experiments with SLM were done according to the scheme shown in Fig. 6. The main disadvantage of this system is the limited lifetime of the liquid membrane due to the dissolution of the organic solvent in aqueous solutions. The limited lifetime of the liquid membrane was the main reason for the delay in the commercial introduction of this technology, along with the fact that no specific membranes and modules were available. In 1984, however, Kim [8] introduced a new method for the SLM technique with no problems of membrane degradation. The principle of the three-circuit process is shown in Fig. 7. There are two membrane stages: one for extraction and one for stripping. The organic solvent picks up the metal ions in the extraction module out of the feed solution. These ions are then removed from the organic solution in the stripping module, and the regenerated solvent is recycled to the extraction module. Because of the generally counter-current flow of the organic and aqueous phases with extended contact areas, a single membrane permeator acts as a multi-stage device. Considering the kinetics of the extraction and re-extraction step, one can see that the extraction process is 5-6 times higher (depending on the process parameters) than the re-extraction process. As a consequence the re-extraction step needs a correspondingly higher membrane area. One reason for the slow re-extraction could be the strong complexing tendency of the carrier molecules. The laminar flow conditions in the membrane result in a very slow regeneration of the interface between the organic- and the strip-phase. So the dissociation of the ion-carrier complexes is too low. This is the reason why mixer-settler systems should be preferred for the re-extraction step. A schematic view of a combined membrane-mixer settler process is illustrated in Fig. 8. The feed solution flows through the shell side of the hollow fiber permeator. Inside the fibers a suspension out of the organic phase and strip solution circulates. The organic phase makes up about 70% of the emul-
199
stdpping ~ memtxane module •
__._,.Io
/
. organic phase and camer
E
nl o
J
=~ is
conce-~t rate
8 lc
[ "/ -
~
regenerate
./
I Fir 0,163 m 2 ds/di= 0,6/0,3 mm "--Feed= 2000 ppm Cu. pH 3.5 Strlp-15000 ppm Cu, pH O.e 0.2
Fig. 8. Coupled membrane-emulsionprocess.
J
I
I.I.
0,4 0,6 0,8 1 organic part of emulsion
Fig. 9. Copper fluxthrough the S L M versus the part of organic phase in the emulsion.
sion. Operating in this way avoids the need for a settlerfor the extraction loop. Only the product flow has to be separated. Fig. 9 demonstrates the copper flux through a supported liquidmembrane by various organic parts in the emulsion [9]. Experiments with a coupled membrane-emulsion process show that liquid membrane permeation takes place utilizingthe fast extraction effect. The advantages and disadvantages of the S L M versus the multiple emulsions are summarized in Table II [10 ].
EXPERIMENTAL
RESULTS
Various papers are published [11-14 ] about applications of S L M techniques for differentmetal ions and organic components. Particularattention was payed T A B L E II Advantages and disadvantages of the S L M versus the multiple emulsion Advantages
Disadvantages
Very small volume of organic phase and carrier
Costs for the support material Pressure loss for small hollow fiber diameters and high packing densities Relatively clean feed solutions necessary (otherwise prefdtration) Chemical cleaning of the polymer membrane, necessary from time to time The interface tension between the liquid membrane and the aqueous feed solution must be higher than 15 mN/m
Favourable hydrodynamics, intensivemass transfer (without back mixing) Low releaseof organic phase to the aqueous feed solution N o surfactants N o membrane instability Selectivere-extractionpossible Suitable for the separationof gases [15,16]
200 to the mass transfer through an SLM as function of the membrane properties (e.g. support material, pore size, membrane thickness, organic solution, carrier concentration, etc.), varying the physical properties, hydrodynamic conditions and operating parameters (e.g. different pH values on either side of the membrane, feed concentrations, concentration of the strip solutions, etc.) The experiments were made with an SLM system, as illustrated in Fig. 6. From these results the following conclusions can be derived (note that these are not valid for all systems):
Membrane properties - - t h e thinner the membrane, the greater the flux (up to 50/Lm a linear dependency, below this, a disproportional increase); - - t h e lower the viscosity of the liquid membrane, the greater the flux; - - an optimal carrier concentration exists in the organic phase (often between 10-30 vol.% ); - - t h e higher the efficiency, the lower the selectivity of the liquid membrane; - - t h e higher the efficiency, the lower the permeability of the fast permeating component; - - a n increase in temperature increases the solubility of the organic phase in the feed phase.
Process parameters If the process parameters change, SLM reacts as follows: - - t h e concentration factor (101-104) of the fast permeating component increases linearly with the difference in pH value over the membrane; --the membrane permeability increases with the pH value of the feed solution up to a maximum and then decreases; - - t h e velocity of the strip solution has no influence on the flux through the membrane; --the concentration of the strip solution has almost no influence on membrane permeability; - - t h e higher the flow on the feed side, the higher the concentration at the exit of the module and the lower the recovery of components out of the feed solution; - - l o w concentrations in the feed solution result in drastically reduced permeation rates (for copper, lower than 100 p p m ) . All experiments have shown that the flux rates through a liquid membrane depend on the operating parameters and process designs [ 8,9,11 ]. An interesting result relating to the field of application of SLM is illustrated in Fig. 10. The flux through the membrane decreases dramatically if the content of copper in the feed solution is less than 100 ppm, making the process uneconomical at low concentrations. Economic calculations show that the SLM process is not a suitale technology for the removal of trace elements from wastewater, i.e. water treatment, but for specific separations it may be preferable
201
f
(.,1
5
0
PP-I~bers 0,3/0.6 mm F,,-lulsmnwith 70% (Kerosin + 10% UX 6 3 / 7 0 Fir 0,163 m 2
I
5OO
I
I
IOO0
[CulKl.. 1 b:)Pm]
Fig. 10. S L M permeabilityversus feed concentration [9].
to conventional processes, for example, for the separation of metal ions from extraction solutions in the fieldof ore leaching. COMPETING
TECHNOLOGIES
The SLM technique was originally developed for hydrometallurgical applications. Competing technologies in this field of application are solvent extrac-
FEEO*
..AL
0
9 9 . 9 + % PURE
J
Fig. 11. Hydrometallurgical circuitprocesses. T A B L E III Advantages and disadvantages of the S L M versus solventextraction Advantages
Disadvantages
Ratio organic phase: feed phase ~< 1
Sensitiveto tensides
Very expensive carrierscan be used
Costs for the support material
Low dissolutionof organic phase in the aqueous feed phase
Relatively sensitive to particles in the feed solution (clogging of the membrane module; polluting of the membrane surface)
Strong separationbetween organic and feed phase; no mutual pollutionof the streams High selectivity
202
tion, ion-exchanger and chemical precipitation. Chemical precipitation is primarily appropriate for high metal ion concentrations, but does not allow the metal to be directly recovered in elementary form. Ion-exchangers have a lower selectivityfor metal ions with the same charge and are mainly used when there are very low concentrations of metallic ions in the feed solution. Consequently, conventional solvent extraction is SLM's main competitor according to the closed loop processes of Fig. 11. This figure is generally applicable to the new concepts of the S L M processes too. The only difference between the two technologiesliesin the layout of the extractionequipment (either mixer-settler or membrane modules). A comparison between the S L M technique and solvent extraction characterises S L M as shown in Table III.
REFERENCES 1 R. Marr and A. Kopp, Fliissigmembran-Technik-Ubersicht iiber Ph~nomene, Transportmechanismen und Modellbildungen, Chem.-Ing.-Tech., 52 (5) (1980) 399-410. 2 N.N. Li, AIChE J., 17 (1971) 459. 3 R.Pa Cahn and N. Li, Separation of organic compounds by liquid membrane processes, J. Membr. Sci.,1 (1976) 129-142. 4 J. Draxler and R. Marr, Multiple Emulsion - lhr Einsatz bei der Fliissig-Membran-Permeation zur selektiven Abtrennung von Metallen aus w~issrigenLSsungen. Chem. Ind., November (1983) 698-702. 5 T.A. Hatton and D.S. Wardius, Analysis of staged liquid surfactant membrane operations, AIChE J., 30 (6) (1984) 934-944. 6 Fa. Chemiefaser Lenzing AG, Austrian Patent Application No. 373 859, 15-07-i983. 7 S. Ripperger, Die blasenfreieBe- und Entgasung von Fliissigkeitenmit mikroporSsen Membranen, Chem.-Ing.-Tech., 58 (4) (1986) 322-383. 8 B.M. Kim, Membrane-based solvent extraction for selectiveremoval and recovery of metals, J. Membr. Sci.,21 (1984) 5-19. 9 K. Schneider and Th. Rintelen, Riickgewinnung von Metallen mit Hilfe tr~gergestiitzter Fliissigmembranen, Chem.-Ing.-Tech., 58 (10) (1986) 800-802. 10 K.-H. Lee, D.F. Evans and E.L. Cussler, Selective copper recovery with two types of liquid membranes, AIChE J., 24 (5) (1978) 860-868. 11 M. Taramoto and H. Tanimoto, Mechanism of copper permeation through hollow fiberliquid membranes, Sep. Sci. Technol., 18 (10) (1983) 871-892. 12 P.R. Danesi, A simplifiedmodel for the coupled transport of metal ions through hollow fiber supported liquidmembranes, J. Membr. Sci.,20 (1984) 231-248. 13 G.A. Kordosky, The chemistry of metals recovery using Lix reagents, Company information by Fa. Henkel, 1844 West Grand Road, Suite 104, Tucson, Arizona 85745-1273. 14 D. Rohde, Uran aim Phosphorsiiure: L~gerfristig Chancen, Chem. Ind., February (1985) 76-79. 15 R.W. Baker, Liquid membranes for the production of oxygen-enriched air,Paper presented at the 9th Energy Technology Conference, Washington D.C., Febr. 17, 1982. 16 W.J. Ward Ill, A membrane system for carbon dioxide control in lifesupported systems, Portable Life supported Systems, Conference Papers NASA-SP-234 (1969).
191
Elsevier Science Publishers B.V., AmsterdAm -- Printed in The Netherlands
Separation Techniques with Supported Liquid Membranes* G. SCHULZ Fa. Enka AG, Technical Membranes, Ohder Strasse 28, 5600 Wuppertal 2 (F.R.G.)
SUMMARY
Now that the problems of the longevity of liquid membranes have been solved with the introduction of multi-circuit processes, the supported liquid membrane technique has received a fresh impetus. W i t h the commercial availability of appropriate membranes and large modules with membrane areas of up to 10 m 2, the industrial application of the supported liquid membrane technique seems to be near. So far, it has only been tested in the laboratory on a pilot plant scale. Unfortunately, the worldwide downwards trend in the price of raw metals has delayed the introduction of this new separation technique into the market. At the moment, smelting is still by far the most economical method of producing metals. Against this background, the supported liquid membrane must be seen to have been thoroughly discussed and tested and must be seen as providing an economical alternative to competing processes. B u t for the m o m e n t it can be seen as a stand-by technology, waiting for a time when the recovery and recycling of metals, including uranium, becomes a more interesting economical prospect. New applications in the field of medicine and especially for gas separation, widely expected b u t at the m o m e n t still in the development stage, make the outlook promising for this multi-faceted technology. Keywords: supported liquid membranes, circuit processes, microporous membranes, metal ion
separation.
SYMBOLS A* - AM - dK,a-dp - D --
specific interface, m2/m 3 membrane area, m 2 outer hollow fiber diameter, m m a x i m u m pore diameter, m diffusion coefficient, m2/s
*Paper presented at the 5th Symposium on Synthetic Membranes in Science and Industry, Tiibingen, F.R.G., September 2-5, 1986.
0011-9164/88/$03.50
© 1988Elsevier Science Publishers B.V.
192 J K pH r R s T
--------~M ~ (0 ~ ---
diffusion rate, g/s equilibrium constant p H value particle radius, m gas constant, J / m o l ' K membrane thickness, m absolute temperature, K packing density of the module porosity of the membrane marginal angle viscosity, Pa s surface tension, m N / m
INTRODUCTION
Up to now the membrane processes introduced in the medical field and technology have been based on the application of so-called passive membranes. The mass transfer through passive membranes occurs with the active share between the permeating components and the membrane material. Passive membranes are merely barriers, which are passed by different substances at different velocities down a gradient. The liquid membrane technique is the first process on an industrial scale, which consists of an operation with active membranes. Active membranes contain complex building agents with high selectivity for the components to be separated. The main fields of application for the liquid membrane technique are: - - hydrometallurgy; - - treatment of wastewater from the chemical industry; galvanic techniques; gas separation; - - m e d i c a l application; --biotechnology. In this work we describe supported liquid membranes ( S L M ) , starting with the theoretical fundamentals, going on to different process developments and ending with the present engineering possibilities and potential applications. r
porousmembranes
passive membranes -1"-- solubility membranes ion-exchangemembranes active membranes _ . ~ liquid membranes L ~ biological membranes
Fig. 1. Characterisation of membranes.
193
THEORETICAL F U N D A M E N T A L S The aqueous solution,which initiallycontains all the ions which can permeate the liquid membrane, is generally referred to as the feed solution. The aqueous solution present on the opposite side of the membrane, which is initiallyfree from the permeable ions, isgenerally referredto as the stripsolution. In principle,liquidmembrane permeation is a solvent extraction process in which the extraction and stripping operations are performed simultaneously. A liquid phase acts as a selective separation layer between the feed and the strip solution. The carrier dissolved in the liquid phase picks up the ions on the feed side of the liquid membrane and transports them by diffusion to the strip side of the membrane. The driving force is a concentration gradient of the ion-carriercomplex between both sides of the membrane. Fig. 2 shows the example of the separation of metal ions from aqueous solutions,the most comm o n "counter transport" through a membrane. Other, commercially less important mass transfer processes through liquid membranes are described in more detailin a number of papers [1 ]. In the aqueous phase IIIa metal ion diffusesto the membrane surface,reacts there with the carrierX, a liquidion-exchanger, forming a metal-carrier complex which diffuses through the liquid membrane (phase II) driven by the existing concentration gradient. At the membrane surface which is in contact with phase I,the thermodynamic equilibrium is shiftedand the complex is split into the carrier and the metal ion. The carrierpicks up a proton and diffuses back to the feed side of the membrane. The whole process consists of the exchange of metal ions for protons, with the carrier acting as a catalyst.In the case of a steady-state process two diffusion steps have to take place simultaneously: the diffusion of the metal-carrier complex from the feed side of the membrane to the stripside and the back-diffusion of the proton loaded carrier. This metal ion transport works as long as protons migrate to the opposite direction. The driving force for this coupled transport is provided by the different aciditiesor p H values of the feed and strip solutions. In this type of liquid membrane -i
carrier~ $":L-'?"?'~.-'!_organic phase ":Wi~ 2H* ~.. ~
pH~ • Phase 11T
2HX
Phase 17
12H*
pH2 Phase I
Fig. 2. Schematic representation of the mass transfer through a liquid membrane.
194 transport, metal ions can be shifted across the membrane against their concentration gradient. In ideal conditions, the dependence of the metal ion flow across the membrane on the difference in pH value on either side of the membrane can be derived as follows: The equilibrium reaction for a divalent metal ion is Me 2+ + 2 H X .
~ Me X2 + 2 H +
and the equilibrium constant K - [MeX2] [H+]2 [ H X ] 2 [Me 2+ ]
(1)
where [ H + ] and [ Me 2+ ] are concentrations in the aqueous phase and [ Me X2I and [ H X ] concentrations in the organic phase of the membrane. This equilibrium constant is the same on both sides of the membrane. Equilibrium occurs if [Me2+] I og [ Me 2+ ] m = 2 ApH
(2)
where ApH is the difference between the p H values on both sides of the membrane. If, for example, the p H value of the strip solution is 1 and t h a t of the feed solution is 4, that means t h a t the difference is 3, a concentration of 1 : 10 6 can be achieved for divalent metal ions such as copper, zinc, nickel, etc.
Parameters influencing properties o[ liquid membranes The mass transfer of complexed metal ions through liquid membranes can be described approximately by the first Fick's law
j__.D'AM ([MeX,]i_ [ M e X , ] m ) $
(3)
where [ M e X , ] i is the concentration of the metal complex in the liquid membrane on the side towards phase I and [MeXn ] m is the concentration of the metal complex in the liquid membrane towards phase III. Following the molecular kinetic interpretation of Nernst, the diffusion coefficient D can be defined as D-
RT 6n~lr
(4)
where ~/is the viscosity of the liquid m e m b r a n e and r the radius of the diffusing components.
195 The above mentioned equations give some indications as how to increase the effectiveness of a liquid membrane: small membrane thickness for short diffusion length; high concentration of carrier molecules in the organic phase; small molecular diameters of the complexing agent; - - h i g h process temperature; low viscosity of the liquid membrane. It has to be taken into account that these requirements sometimes have opposite, less desirable effects. For example, with an increasing carrier concentration the viscosity of the liquid membrane increases too; an increased temperature promotes the dissolution of the organic phase in aqueous solutions. -
-
LIQUIDMEMBRANES In order to meet the requirements of the liquid membrane technique, it is necessary to develop stable films from the liquid. Fig. 3 shows the possibilities for forming thin liquid films. For industrial applications the multiple emulsion and the supported liquid membrane are suitable. lamella
H
foam ~
,,m|| multiple emulsion iiii~iiiiiiii!~iiiiiiiii :--:---..-.-~-_...----_::-...:-.-.. :-.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
microporous support material ~
Fig. 3. Formationof liquid films. :::::::::::::::::::::/organic phase • ~7~'J~ o~i~ii::::~ and carrier striP j I ~ ' i : : ~ - - - " : ~ l
s°,ut Iiii
t
I ~ ! ~ i ~ water phase ~E-~'E~::::-~:-:-iI with trace elements Fig. 4. Multipleemulsion.
196 feed solution organicphase ~ carrier f
Accurel-
membrane
<~
J strip solution
Fig. 5. Schematic representationof a supported liquidmembrane (SLM).
Multiple emulsions [ 2-5 ] In liquid surfactant membrane processes the membrane system (Fig. 4) is in the form of water-in-oil emulsions that are made by mixing stripping solutions with carrier-containing organic solvents. These emulsions are then dispersed in the aqueous feed solutions from which, for example, the metal ions have to be removed. The corresponding phases are stabilized by surfactants. The specific interface for the strip solution is about 106 m2/m 3 and for the organic phase towards the feed side 3000-4000 m2/m 3. An industrial plant working on the basis of the above described technology, has been running since the spring of 1986 at Chemiefaser Lenzing in Austria. It operates according to the concept of Prof. Mart, Graz (Austria) [ 6 ] and was planned for 75 m3/h wastewater containing zinc.
Supported liquid membrane The supported liquid membrane (SLM) technique uses a microporous hydrophobic polymer structure as support for the organic phase incorporating the carrier.The liquidphase is held in the membrane pores by capillarity.The wetting pressure for a hydrophobic membrane can be calculatedfrom Ap=
4 a cos dp
(5)
In this case a is the interface tension of the aqueous phase versus the organic phase. Amine compounds are able to decrease considerably the wetting pressure. In principle, complex building agents highly influence the interface tension and it is advisable to determine experimentally their behaviour before choosing an agent. A schematic representation of a supported liquid membrane is given in Fig. 5. The requirements for the porous support structure are: - - high porosity; small pore diameters; thin walls; - - hydrophobic material; chemical resistant polymer; -
-
197 TABLE I Microporous Accurel ®-polypropylene hollow fibermembranes by Enka AG, Wuppertal
Inner diameter
(mm)
1.8 1.2 0.6 0.3 0.18
Wall thickness
(~)
400 300 300 150 40
Organic phase in the pores
Effective specific
(dm3/m2)
membrane
(mS/m3)
0.367 0.281 0.337 0.169 0.034
area
577 833 1250 2500 5770
low costs. Porous support membranes are availableas flatsheets or hollow fibers.Hollow fiber membranes are preferred because of their higher packing densities, favourable hydrodynsmics and other advantages. The company Enka A G of Wuppertal (West Germany) offersthe following programme for microporous Accurel ®-polypropylene hollow fibermembranes. Relevant data for the liquid membrane permeation are evaluated as well (Table I). The indicated values referto a packing density of 50% in the module, a free volume in the membrane of 75% and an effectivearea for mass transfer according to the porosity of the membrane and referringto the outer diameter of the hollow fiber.The specificarea for the mass transfer A* can be calculated from A*- 4.~'~M
(6)
dK,a
So today, membrane technology has attained mass transfer areas which are
hollow fiber membrane module solution
~
organic phase and carrier
st rilc~ng solution
solution
ed stripping Sotutior
Fig. 6. Schematic representation of a hollow fiber permeator for the SLM technique. Fig. 7. Three-circuit process with SLM.
198
comparable with those of energy-consuming jet nozzles or venturi scrubbers [71. MEMBRANEMODULESAND PROCESSES Hollow fiber modules are manufactured in such a way that a bundle of parallel fibers are potted with synthetic resin at the front sides of a cartridge. The hollow fibers are open at the front sides (see Fig. 6). In a module the walls of the hollow fiber membranes are impregnated with the organic solvent containing the liquid ion-exchanger, called the carrier. The feed solution flows through the lumen of the hollow fibers and the aqueous strip solution circulates on the shell side of the fibers. If the feed solution is passing only once through the hollow fiber module the amount of metal removed is not enough. The feed solution has to be circulated continuously through the module. This recycling mode operation can be realized in one- or multi-circuit processes. In the beginning the experiments with SLM were done according to the scheme shown in Fig. 6. The main disadvantage of this system is the limited lifetime of the liquid membrane due to the dissolution of the organic solvent in aqueous solutions. The limited lifetime of the liquid membrane was the main reason for the delay in the commercial introduction of this technology, along with the fact that no specific membranes and modules were available. In 1984, however, Kim [8] introduced a new method for the SLM technique with no problems of membrane degradation. The principle of the three-circuit process is shown in Fig. 7. There are two membrane stages: one for extraction and one for stripping. The organic solvent picks up the metal ions in the extraction module out of the feed solution. These ions are then removed from the organic solution in the stripping module, and the regenerated solvent is recycled to the extraction module. Because of the generally counter-current flow of the organic and aqueous phases with extended contact areas, a single membrane permeator acts as a multi-stage device. Considering the kinetics of the extraction and re-extraction step, one can see that the extraction process is 5-6 times higher (depending on the process parameters) than the re-extraction process. As a consequence the re-extraction step needs a correspondingly higher membrane area. One reason for the slow re-extraction could be the strong complexing tendency of the carrier molecules. The laminar flow conditions in the membrane result in a very slow regeneration of the interface between the organic- and the strip-phase. So the dissociation of the ion-carrier complexes is too low. This is the reason why mixer-settler systems should be preferred for the re-extraction step. A schematic view of a combined membrane-mixer settler process is illustrated in Fig. 8. The feed solution flows through the shell side of the hollow fiber permeator. Inside the fibers a suspension out of the organic phase and strip solution circulates. The organic phase makes up about 70% of the emul-
199
stdpping ~ memtxane module •
__._,.Io
/
. organic phase and camer
E
nl o
J
=~ is
conce-~t rate
8 lc
[ "/ -
~
regenerate
./
I Fir 0,163 m 2 ds/di= 0,6/0,3 mm "--Feed= 2000 ppm Cu. pH 3.5 Strlp-15000 ppm Cu, pH O.e 0.2
Fig. 8. Coupled membrane-emulsionprocess.
J
I
I.I.
0,4 0,6 0,8 1 organic part of emulsion
Fig. 9. Copper fluxthrough the S L M versus the part of organic phase in the emulsion.
sion. Operating in this way avoids the need for a settlerfor the extraction loop. Only the product flow has to be separated. Fig. 9 demonstrates the copper flux through a supported liquidmembrane by various organic parts in the emulsion [9]. Experiments with a coupled membrane-emulsion process show that liquid membrane permeation takes place utilizingthe fast extraction effect. The advantages and disadvantages of the S L M versus the multiple emulsions are summarized in Table II [10 ].
EXPERIMENTAL
RESULTS
Various papers are published [11-14 ] about applications of S L M techniques for differentmetal ions and organic components. Particularattention was payed T A B L E II Advantages and disadvantages of the S L M versus the multiple emulsion Advantages
Disadvantages
Very small volume of organic phase and carrier
Costs for the support material Pressure loss for small hollow fiber diameters and high packing densities Relatively clean feed solutions necessary (otherwise prefdtration) Chemical cleaning of the polymer membrane, necessary from time to time The interface tension between the liquid membrane and the aqueous feed solution must be higher than 15 mN/m
Favourable hydrodynamics, intensivemass transfer (without back mixing) Low releaseof organic phase to the aqueous feed solution N o surfactants N o membrane instability Selectivere-extractionpossible Suitable for the separationof gases [15,16]
200 to the mass transfer through an SLM as function of the membrane properties (e.g. support material, pore size, membrane thickness, organic solution, carrier concentration, etc.), varying the physical properties, hydrodynamic conditions and operating parameters (e.g. different pH values on either side of the membrane, feed concentrations, concentration of the strip solutions, etc.) The experiments were made with an SLM system, as illustrated in Fig. 6. From these results the following conclusions can be derived (note that these are not valid for all systems):
Membrane properties - - t h e thinner the membrane, the greater the flux (up to 50/Lm a linear dependency, below this, a disproportional increase); - - t h e lower the viscosity of the liquid membrane, the greater the flux; - - an optimal carrier concentration exists in the organic phase (often between 10-30 vol.% ); - - t h e higher the efficiency, the lower the selectivity of the liquid membrane; - - t h e higher the efficiency, the lower the permeability of the fast permeating component; - - a n increase in temperature increases the solubility of the organic phase in the feed phase.
Process parameters If the process parameters change, SLM reacts as follows: - - t h e concentration factor (101-104) of the fast permeating component increases linearly with the difference in pH value over the membrane; --the membrane permeability increases with the pH value of the feed solution up to a maximum and then decreases; - - t h e velocity of the strip solution has no influence on the flux through the membrane; --the concentration of the strip solution has almost no influence on membrane permeability; - - t h e higher the flow on the feed side, the higher the concentration at the exit of the module and the lower the recovery of components out of the feed solution; - - l o w concentrations in the feed solution result in drastically reduced permeation rates (for copper, lower than 100 p p m ) . All experiments have shown that the flux rates through a liquid membrane depend on the operating parameters and process designs [ 8,9,11 ]. An interesting result relating to the field of application of SLM is illustrated in Fig. 10. The flux through the membrane decreases dramatically if the content of copper in the feed solution is less than 100 ppm, making the process uneconomical at low concentrations. Economic calculations show that the SLM process is not a suitale technology for the removal of trace elements from wastewater, i.e. water treatment, but for specific separations it may be preferable
201
f
(.,1
5
0
PP-I~bers 0,3/0.6 mm F,,-lulsmnwith 70% (Kerosin + 10% UX 6 3 / 7 0 Fir 0,163 m 2
I
5OO
I
I
IOO0
[CulKl.. 1 b:)Pm]
Fig. 10. S L M permeabilityversus feed concentration [9].
to conventional processes, for example, for the separation of metal ions from extraction solutions in the fieldof ore leaching. COMPETING
TECHNOLOGIES
The SLM technique was originally developed for hydrometallurgical applications. Competing technologies in this field of application are solvent extrac-
FEEO*
..AL
0
9 9 . 9 + % PURE
J
Fig. 11. Hydrometallurgical circuitprocesses. T A B L E III Advantages and disadvantages of the S L M versus solventextraction Advantages
Disadvantages
Ratio organic phase: feed phase ~< 1
Sensitiveto tensides
Very expensive carrierscan be used
Costs for the support material
Low dissolutionof organic phase in the aqueous feed phase
Relatively sensitive to particles in the feed solution (clogging of the membrane module; polluting of the membrane surface)
Strong separationbetween organic and feed phase; no mutual pollutionof the streams High selectivity
202
tion, ion-exchanger and chemical precipitation. Chemical precipitation is primarily appropriate for high metal ion concentrations, but does not allow the metal to be directly recovered in elementary form. Ion-exchangers have a lower selectivityfor metal ions with the same charge and are mainly used when there are very low concentrations of metallic ions in the feed solution. Consequently, conventional solvent extraction is SLM's main competitor according to the closed loop processes of Fig. 11. This figure is generally applicable to the new concepts of the S L M processes too. The only difference between the two technologiesliesin the layout of the extractionequipment (either mixer-settler or membrane modules). A comparison between the S L M technique and solvent extraction characterises S L M as shown in Table III.
REFERENCES 1 R. Marr and A. Kopp, Fliissigmembran-Technik-Ubersicht iiber Ph~nomene, Transportmechanismen und Modellbildungen, Chem.-Ing.-Tech., 52 (5) (1980) 399-410. 2 N.N. Li, AIChE J., 17 (1971) 459. 3 R.Pa Cahn and N. Li, Separation of organic compounds by liquid membrane processes, J. Membr. Sci.,1 (1976) 129-142. 4 J. Draxler and R. Marr, Multiple Emulsion - lhr Einsatz bei der Fliissig-Membran-Permeation zur selektiven Abtrennung von Metallen aus w~issrigenLSsungen. Chem. Ind., November (1983) 698-702. 5 T.A. Hatton and D.S. Wardius, Analysis of staged liquid surfactant membrane operations, AIChE J., 30 (6) (1984) 934-944. 6 Fa. Chemiefaser Lenzing AG, Austrian Patent Application No. 373 859, 15-07-i983. 7 S. Ripperger, Die blasenfreieBe- und Entgasung von Fliissigkeitenmit mikroporSsen Membranen, Chem.-Ing.-Tech., 58 (4) (1986) 322-383. 8 B.M. Kim, Membrane-based solvent extraction for selectiveremoval and recovery of metals, J. Membr. Sci.,21 (1984) 5-19. 9 K. Schneider and Th. Rintelen, Riickgewinnung von Metallen mit Hilfe tr~gergestiitzter Fliissigmembranen, Chem.-Ing.-Tech., 58 (10) (1986) 800-802. 10 K.-H. Lee, D.F. Evans and E.L. Cussler, Selective copper recovery with two types of liquid membranes, AIChE J., 24 (5) (1978) 860-868. 11 M. Taramoto and H. Tanimoto, Mechanism of copper permeation through hollow fiberliquid membranes, Sep. Sci. Technol., 18 (10) (1983) 871-892. 12 P.R. Danesi, A simplifiedmodel for the coupled transport of metal ions through hollow fiber supported liquidmembranes, J. Membr. Sci.,20 (1984) 231-248. 13 G.A. Kordosky, The chemistry of metals recovery using Lix reagents, Company information by Fa. Henkel, 1844 West Grand Road, Suite 104, Tucson, Arizona 85745-1273. 14 D. Rohde, Uran aim Phosphorsiiure: L~gerfristig Chancen, Chem. Ind., February (1985) 76-79. 15 R.W. Baker, Liquid membranes for the production of oxygen-enriched air,Paper presented at the 9th Energy Technology Conference, Washington D.C., Febr. 17, 1982. 16 W.J. Ward Ill, A membrane system for carbon dioxide control in lifesupported systems, Portable Life supported Systems, Conference Papers NASA-SP-234 (1969).