Separation of copper by liquid surfactant membranes

J, inorg, nacl, Chem. VoL 40, pp. 1415-1421 © Pergamon Press Ltd., 1978. Printed in Great Britain

0022-190217810701-1415/$02,0010

SEPARATION OF COPPER BY LIQUID SURFACTANT MEMBRANES J. STRZELBICKI and W. CHAREW1CZ Technical University of Wroclaw, Institute of Inorganic Chemistry and Metallurgy of Rare Elements, 50-370 Wroclaw, Poland (Received 23 December 1976; received [or publication 14 December 1977)

Abstract--Liquid surfactant membrane separation of cupric cations from acidic aqueous solution with the liquid ion exchanger di(2-ethylhexyl)phosphoric acid: D2EHPA was studied. The effects on the process of the emulsifier concentration, Span 80, over the range from 0.4 to 1.5% (vol.), of the stirring from 300 to 470 rpm, of the membrane viscosity (polybutadiene addition) from 2.0 cP to 4.3 cP, of the KNO3 concentration in the external solution from 0.0 to 1.8 M, of the pH of that solution from 1.5 to 4.5 and of the initial copper concentration over the range from 0.01 g/I to 7.5 g/I have been determined. Results are discussed in terms of membrane stability and thickness. A model for copper separation by liquid surfactant membrane is presented based on the multiple diffusion of ion-exchanger molecule between membrane interfaces. The effectiveness of liquid D2EHPA membranes for copper recovery from acidic aqueous solution is demonstrated with better utilization of D2EHPA over a corresponding liquid extraction process.

INTRODUCTION ' aqueous nitric acid with 10 cc of organic solution for 20 rain at Since N. N. Li published his first papers on the ap- constant rotation of 2000rpm. Then 20cc of resultant emulsion plication of liquid surfactant membranes for separation was stirred with 100cc of aqueous cupric solution. The organic of hydrocarbons[I--4] other applications of this novel phase (liquid membrane) was prepared by dissolution of emulsifier: Span 80 (Int. Enzymes Ltd.), D2EHPA 98%, and separation technique have been studied. Liquid polybutadiene of average molecular weight equal to 6.000 in membranes were shown to be useful for recovery of cyclohexane as the solvent. Concentrations of the liquid phenol, ammonia, organic acids, amines, and hydrogen membranecompounds were varied, excluding that of D2EHPA sulfide from waste water[5,6]. Moreover, certain studies which was maintained at 6.3% (vol.)(I.87× l0 -I M). As external were done on the separation of inorganic ions, e.g. solution, aqueous cupric nitrate and potassium nitrate were used chromate[7-9], usually at respective concentrations of 1.0g/dm3 (I.57 x 10 2 M) The work presented here contains the results of stu- and 0.5 M. These concentrations were varied in some experidies on cupric cation separation from aqueous solution, ments from 0.01g Cull to 7.5g Cull and from 0 to 1.8M for The system applied consisted of two aqueous solu- KNO3. tions separated by an organic liquid membrane. One of the solutions was cupric salt solution as the external Internal Liquid membror~ [ External solution solution l high Cu~'con phase of water-in-oil-in-water type emulsion. The second high acidity a n d l~'-'~ centrot,on at aqueous solution of a concentrated inorganic acid was i.~reo~,n~ Cu~" ~ [ ,. ) ,o, oe,~,,y the internal phase of that emulsion. The liquid membrane concentration (~O2EHpN~../ (organic phase of the emulsion) contained di(2-ethyl'~-~cu~ S,oge~ hexyl)phosphoric acid (D2EHPA) as an ion-exchanger. .~..~. j D2EH E~ha,~o, p A protons (O2EH~~ D2EHPA reacts with cupric cations at the membrane ~ ~"/~f~e~ ,,th eup,,e interface forming Cu(D2EHPh which is soluble in the to,o° organic membrane. The compound diffuses through the cu n liquid membrane toward the internal interface of the (~'~1 ~ I OiffusJonof Cu-D2EHPA emulsion droplet, where it decomposes because of the ~ I eo~p~. rn~r uot Stage 171 high acidity of internal aqueous phase. Released cupric ~ ~7 R h cations remain in this phase while free D2EHPA mole . . . . . ee,e onge o, cupr,c ~'~ eat,on for h d cules diffuse back to the external interface of the .~.. ~~ . ~ y ,ogen membrane (Fig. 1). ~ ,oos EXPERIMENTAL mixing water-in-oil emulsion, where nitric acid solution was the dispersed phase and liquid membrane was the organic phase, with an aqueous solution of cupric salt (Fig. 2). The dispersed phase of water-in-oil emulsion was named as "internal solution" and cupric salt aqueous solution as "'external solution". Organic phase (liquid membrane) contained an emulsifier as well as the liquid ion-exchanger: (D2EHPA). The above water-in-oil emulsion was mixed with external solution of cupric salt by means of a mechanical stirrer. A laboratory-scale preparation procedure of liquid membrane systems is shown in Fig. 3. The water-in-oil emulsion was made by stirring I0 cc of 2 M

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liquid surfactant membrane. Moreover the low surface concentration of the emulsifier causes membrane thin(~ [ ~ ,~~"(~ (CC ÷ ions) ning which improves the diffusion process. Incomplete d ~ ~ saturation of the oil-water interface with the low membrane thickness results in membrane instability and Internal solution an internal solution permeation, which appears as the ~ ~L2/\ r,"~t-~ J (aqueous.HNO~) lowering of the copper flux through the membrane. At ~ s ~ the emulsifier concentration of 0.6% (vol.) the emulsion em stability increases together with the increase of the copliquid surfactant membrane ~ ~ ,~...k~ per flux to the internal solution. The copper recovery also :ontinuousemulsion phase) ~ increases with the emulsion stability and reaches 36 per cent and 94 per cent for Span 80 concentrations equal to 0.4 and 0.6% (voi.), respectively. Further increase of the Fig. 2. Liquidsurfactant membranesystemfor copper separation, emulsifier concentration had no significant effect on the copper recovery while maximum recovery time increased. One possible explanation of this phenomena is the I I water-in-oil inhibition of ion-exchange reaction rate at the interface ~ ~ ~ emulsi°n ;~...~/-~1 saturated with emulsifier m°lecules" An°ther explanati°n z~-'t ,, is given by the increasing diffusion distance due to membrane thickening. The kinetic curves (Fig. 4) go through a characteristic t ~ minimum. This describes the situation when copper flux water-in-oil-in-waterI ~c=,~:~ I to the internal solution reaches the counter flux caused emulsion by decomposition of the emulsion. Thereafter the copper concentration in the external solution increases. Separation of the copper at various membrane [Mixing] [SettlncjJ viscosities was studied in the same system with an Fig. 3. Preparation ofwater-in-oil-in-wateremulsion, emulsifier concentration constant at 1.0% (vol.). For viscosities between 2.0 cP and 4.3 cP it was found that emulsion stability increased with the organic phase The acidity of the external solution was controlled by a pH- membrane viscosity. The highest copper separation was meter and maintained constant by addition of 2M aqueous potassium hydroxide. For most of the experiments it was kept at achieved for membrane viscosity of 3.1 cP (Fig. 6). At 2.25. which is pHo.5 for copper liquid extraction with lower viscosities the copper recovery decreases while at D2EHPA[12].Someexperiments were done at pH 1.5 to 4.5. The higher time of maximum copper recovery is substantially copper concentration was determined by means of a radiotracer increases. This is probably caused by variable membrane technique (~Cu) using a multichannel gamma radiation analyzer thickness, which decreases for low viscosity and results ND-1200with a semiconductor(Ge/Li)probe. in the emulsion instability. At high viscosity of the membrane its thickness grows and the emulsion is very stable but the diffusion distance increases. RESULTS AND DISCUSSION Based on the above results the following composition The composition of liquid membrane seems to have an of the organic phase was chosen for further studies: important influence on the separation process. The cyclohexane solution ofpolybutadiene(viscosity3.1cP), emulsifier, which stabilizes the emulsion forms an ad- D2EHPA 6.3% (vol.), and Span 80 1.0% (vol.). sorptive layer at the membrane interface and governs the The stirring effect was studied in the range 310 to diffusion rate. The effect of the emulsifier concentration 470 rpm, with external solution pH equal to 2.25 containof the cupric ion separation was studied at pH of ex- ing 1.0gCu/1 and0.5 M K N O 3 . C o p p e r recovery (Fig. 7) as ternal phase 2.25, containing copper nitrate and potas- well as emulsion stability (Fig. 8) decreased with stirring. slum nitrate at concentrations of 1.0g/l and 0.5 M, The reverse effective was observed for the time of respectively. The viscosity of the organic phase (liquid maximum recovery. The liquid membrane separation membrane) was maintained equal to 3.1 cP with the process depends on the stirring where at high stirring takes polybutadiene. The internal phase was 2M aqueous nitric the interracial area increases with membrane thinning. It acid. Copper recovery and separation rate (Fig. 4) and the promotes diffusion but decreases the emulsion stability water-in-oil emulsion stability (Fig. 5) were found to and thus recovery. depend significantly on the emulsifier concentration. The It follows from the results of previous experiments on volume of resultant internal solution is a measure of the liquid extraction of copper with the D2EHPA that the emulsion stability and while it is smaller than the the pH of the aqueous solution substantially governs the initial volume (10cc) the emulsion is unstable and de- process of copper separation (12). As the liquid composes during the experiment. At high emulsifier membrane separation is based on phenomena similar to concentrations the final volume was higher than the those governing the liquid extraction, strong dependence intial.At the concentration of Span 80 equal to 0.4% (vol.), of liquid membrane copper separation on pH of the water-in-oil emulsion is fairly unstable and up to 80 per external solution should be expected. The pH effect was cent decomposition occurs. The surface concentration of studies in the range 1.5 to 4.5 (Fig. 9) and at the constant the emulsifier is in that case low and D2EHPA molecules stirring speed of 300 rpm. The increase of pH leads to have much more freedom of movement at the interface, higher copper recovery in shorter times. For pH of the thus binding the cupric cations from the external solu- external solution of 1.5, the copper recovery is about tion. It appears as an increase of copper flux through the 10% while for pH of 4.5 it reaches 97% in 25 rain of

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exceeded 90%. At a concentration of 7.5 g/I and 80 rain of stirring, however, the recovery was only 27%. SUMMARY AND CONCLUSIONS It has been shown that cupric ions can be effectively recovered from an aqueous solution by the liquid surfactant membrane separation process. The main advantage of this process over liquid extraction is the combination of extraction and stripping stages in one operation. The copper recovery reaches 95% with ionexchanger (D2EHPA) amounts of about two-fold less than for the stoichiometric extraction. For an initial copper concentration in the external solution of 7.5 g

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J. STRZELBICKI and W. CHAREWICZ

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Fig. 6. Kinetics of copper separationat various viscosity of the liquid membrane.(Membrane: 6.3% vol. of D2EHPA, and 1.0%vol. of Spansolution in cyclohexane. Stirring 310rpm.) Viscosity of membrane:D, 2.0 cP; A, 3.1 cP; x, 4.3 cP.

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Fig. 11. Kinetics of copper separation at various initial copper concentration. (co, initial copper concentration, c, actual copper concentration.) co equal to: x, 0.01 g Cull; A, 0.1 g Cull; $ , 1.0 g Cull; rq, 7.5 g Cull.

Cull the amount of ion-exchanger is four-fold less. It reinforces the model of liquid surfactant membrane separation based on the multiple diffusion of ion-exchanger molecules between membrane interfaces. The maximum recovery time for the system studied varied from 25 to 100 minutes and is relatively high in comparison to the adequate extraction process where it is about 15 men[13,14]. It can be postulated that copper permeability through the liquid membrane depends on three mechanisms: Chemical reaction of the separated ion with the ionexchanger at the external interference,

Diffusion of the resultant compound through the membrane, Chemical reaction of that compound at the internal interface. 'The first stage depends mainly on the pH of the external solution, where the decrease of its acidity causes an increase of D2EHPA protons substitution rate. The ldnetics of this reaction is also influenced by the concentration of electrolyte in the external solution. Permeation of the copper-D'2EHPA complex through the liquid surfactant membrane depends on the membrane thickness. The rate of copper re-exchange at

Separation of copper by liquid surfactant membranes the internal interface of liquid membrane is probably limited by surface concentration of the Cu/D2EHP/2 complex and thus by its surface concentration of the Cu/D2EHP/2 complex and thus by its surface activity. Further studies on inorganic ions separation by liquid surfactant membranes should include the determination of the composition and structure of adsorptive layers developed at membrane interfaces. REFERENCES 1. N. N. Li, U.S. Pat. 3A10,794 (Nov. 12, 1968). 2. N. N. Li, Ind. Eng. Chem. Process Des. Develop. 10, 215 (1971). 3. hi. N. Li, AIChEJ 17,459 (1971). 4. N. N. Li, R. P. Cahn and A. L. Shrier, U.S. Pat. 3,617,546 (Nov. 2, 1971).

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5. N. N. Li and A. L. Shrier, Recent Developments in Separation Science, Vol. 1, p. 163. Chemical Rubber Co., Cleveland, Ohio (1972). 6. R. P. Cahn and N. N. Li, Separation Sci. 9, 505 (1974). 7. D. K. Schiffer, A. Hochhauser, D. F. Evans and E. L. Cussler, Nature 250, 484 (1974). 8. E. L. Cussler and D. F. Evans, Separation and Purilication Methods,3, 399 (1974). 9. A. M. Hochhauser and E. L. Cussler, AIChE Symposium Series, 71 (152), 136 (1976). 10. W.CharewiczandW. Walkowiak, Physiochemicalproblemsof Mineral Processing. 8, 191 Wroclaw (1974). 11. J. Strzelbicki and W. Charewicz, Wind. Chem. 31,639 (1977). 12. R. GrimmandZ. Kola~ik, J. lnorg. Nucl. Chem. 36,189(1974). 13. J. E. Barnes, J. H. Setchfield and G. O. R. Williams,J. Inorg. Nucl. Chem. 38, 1065 (1976). 14. hi. L. Brisk and W. J. McManamey, J. Appl. Chem. 19, 103 (1969).