Membrane recycling in the liquid surfactant membrane process

Znd. Eng. Chem. Res. 1993,32, 1431-1437

1431

Membrane Recycling in the Liquid Surfactant Membrane Process I. Abou-Nemeht and A. P. Van Peteghem' Laboratory For Non-Ferrous Metallurgy, University of Gent, Technologiepark 9, Zwijnaarde B-9052, Belgium

A study has been conducted to investigate the influence of membrane recycling on the kinetics of metal permeation and emulsion stability in the liquid surfactant membrane process. The analogous stages of the industrial process, i.e., emulsification, permeation, settling, and splitting, have been reproduced, however, so far on a laboratory scale. It has been revealed that after two cycles of the membrane circulation, the kinetics of metals extraction and emulsion stability have been drastically affected. Chemical and instrumental analysis of the membrane have shown considerable losses in the surfactant (Span 80) during cobalt extraction, while the carrier bis(2-ethylhexy1)phosphoric acid (DBEHPA) has not been affected. This fact has been mainly attributed to the surfactant chemical instability during exploitation and particularly in acidic media. 1. Introduction In recent years, the continued impact of energy conservation increases has resulted in a trend toward increased attention to economics,safe operations, and environmental consciousness. These factors have driven scientists and engineers toward higher integration of process plants promoted by continuous increasing capitals costs. Liquid surfactant membrane (LSM) process as schematically depicted in Figure 1has been hailed to have the potential to solve a number of challenging separation problems. The past few years have seen a relatively great deal of innovation in applying the LSM process to hydrocarbons separation (Li, 1971; Cahn and Li, 1976); phenol and ammonia separation from wastewaters (Kitagawa and Nishikawa, 1976;Kitagawa et al., 1977),and various amines extraction (Baird et al., 1987). Encouraging attempts have been made in biomedical engineering where LSM has been tested for cholesterol (Yagodin et al., 1988) as well as free phenol (Volkelet al., 1982)and toxin extraction from blood (Halwachs et al., 1980). In biotechnology Scheper et al. (1986)have applied LSM to the enzyme catalyzed L-amino acid preparation. Regardless of the recession of metal prices on the international market, many pilot plants have been erected, e.g., for uranium extraction (Bock et al., 1981); for copper extraction (Ho and Li, 1982; Li et al., 19831, and for zinc extraction (Marr, 1984; Draxler and Marr, 1986;Draxler et al., 1986). In addition to the above studied metals, the literature reports a long list of others, such as lanthanides (Teramoto et al., 1986), cobalt (separation from different nitrate, sulfate, chloride (Strzelbicki and Charewicz, 1978,1980),and acetate solutions) (Abou-Nemeh and Van Peteghem, 1989a), copper (Bart et al., 1987; Bunge et al., 1988), chromium (Hochauser and Cussler, 1975; Fuller and Li, 1984; Weiss and Castaneda Zepeta, 1988),manganese (Abou-Nemeh and Van Peteghem, 1989b), sodium (Lamb et al., 1980; Bartsch et al., 1986;Abou-Nemeh and Van Peteghem, 1992b), nickel (SchQgerlet al., 1985; Abou-Nemeh and Van Peteghem, 1992b3,uranium (Yo0 et al., 19881, and many more. More reviews on the process are available elsewhere (Marr and Kopp, 1980; Way et al., 1982; Noble and Way, 1987; Tavlarides et al., 1987; Noble et al., 1988). Successful attempts have been made by Marr and coworkers (Marr, 1984; Draxler e t al., 1986, 1988) to

* To whom correspondence should be addressed. FAX 645668. t Present address: Christian Doppler

(091)

Laboratory, Chemical

and Environmental Engineering Department, Technical University of Graz, Inffeldgasse 25, A-8010 Graz, Austria.

materialize the process on an industrial scale for zinc separation and recovery. One of the major advantages of the process is the recycling of the encapsulated solute (zinc sulfate) directly to the main technological stream after presaturation. The other positive aspect of the process is the recycling of the membrane after the emulsion being split in an electric field. However, the recycling of the used membrane is likelyto be possible after its regeneration (make-up), Le., a supplemental addition of ingredients to reproduce its previous chemical and physical composition. Thus, the emulsion from the recycled membrane should maintain the appropriate carrier, surfactant concentrations, internal droplet size distribution, and viscosity. Over the years, membrane recycling and regeneration have not been studied thoroughly and systematically and the literature seems to have little to offer on the subject, even though it is very sketchy and controversial. Hsu and Li (1985) have investigated membrane recovery from the emulsion by applying an electric field and aimed at studying the influence of the applied voltage and insulation material of the electrode on the rate of emulsion splitting. Kreichbaumer and Marr (1985)have reported the influence of an electric field, phase ratio, surfactant concentration, viscosity, and many others on the rate of splitting. However, no data have been reported on the membrane chemical stability, degradation, etc. Gutknecht et al. (1986) in an extensive study on multicomponent permeation have investigated the effect of the number of membrane cycles on the rate of emulsion breakdown, and an almost stable rate of splitting was found. No significant effect of the electric field on the membrane has been reported. Furthermore, on recycling both the membrane and the stripping phase (IP) a slightly higher rate of splitting of the emulsion of the recycled membrane was in a similar attempt observed. Abou-Nemeh et al. (1992~) on recycling the membrane, however, through the emulsification-aplitting loop have found that the membrane gradually deteriorated after each cycle. The phenomenon was justified by the surfactant chemical decomposition during the course of emulsification. Schlljsser and KOssaczky (1988), in an attempt to standardize the permeation tests, have reported the influence of the number of membrane regenerations on ammonia perstraction and noted a significant drop in the kinetics after one cycle. This finding, in fact, is in contradiction to the previous results reported by Gutknecht et al. (19861, who declared an increase in the kinetics of metals extraction. However, it should be noted at this juncture that, despite the chemical difference of the separated solutes (the former are metals while the latter is ammonia), the chemical

1993 American Chemical Society 0888-5885/93/2632-1431~0~.~/~

1432 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993

1. ER: Emulsification reactor 2. EES: Emulsion electrostatical splitter

3. ET: Emulsion tank

4.ETT:Feedtank 5. H.T.G.: High tension generator

6.IPT:Internal phase reagent tank 7.

Membraue tank

8. M.P Microprocasor

9.PFT Purified feed tank 10. PR Pmeation reactor 11. pH: pH indicator 12. RIPST: R e w v d intemal phase solute tank 13. RPMC Mixiug speed control

I

14. ST: Settler 15. TC:Temperatun control 16. TI: Temperatun indicator Figure 1. Schematic diagram of the liquid Surfactant membrane process.

composition of both emulsions (acidic reagents of the inner phase, Span 80 as a surfactant and D2EHPA and LIX 64N as a carrier and modifier, respectively) remains essentially the same. From the preceding review it can be seen that there is an urgent need to investigate the recovery and recycling of the membrane phase of the emulsion used in the LSM process. Therefore, the objectives of this work are to obtain quantitative and qualitative information on membrane recycling and to identify the precursors of its decay during the emulsification-permeationaplittingtechnologicalloop. 2. Experimental Section 2.1. Emulsification Setup, Materials, and Procedure. The emulsion was prepared by adding the membrane (organic phase) dropwise to the internal aqueous phase containing 2 M sulfuric acid as a stripping agent. The membrane consists of 3 vol % sorbitan monooleate (Span 80) as a surfactant (Lot No. V-3106, kindly supplied by ICI-Germany), 5.5 vol % bis(2-ethylhexy1)phosphoric acid (DBEHPA) of 98% purity (Lot No. 31291608, purchased from Johnson Matthey Ltd.), and kerosene (Shellsol T) purchased from Belgium-Shell as a matrix of the membrane. All chemicals wre used without purification or further treatment. The emulsification was performed in an ultra-high-speed Ultra-Turrax T-45 laboratory-type homogenizer for 10min total mixing time and at an impellar speed of 12 000 min-'. Cooling of the mixture was maintained during emulsification to avoid heating up of the emulsion caused by the high shearing stress. The final temperature noted was 32 "C. 2.2. Permeation Setup. The permeation experiments were conducted in a cylindrical bench-scale reactor of an active volume of 0.7 L. The internal construction of the reactor resembles one compartment of an Old-ShueRushton column. The dimensions of the baffles, impeller blades, height, length, and depth of the reactor were designed to avoid "creaming" during mixing and enhance turbulence. Further details on the setup construction can be found elsewhere (Abou-Nemeh and Van Peteghem,

1989a; Abou-Nemeh, 1991). A temperature sensor (TI) and pH electrode (PHI) were inserted through the tightfitting lid of the reactor, and both of them were interfaced to a microprocessor (MP) WTW 3000 pH meter for continuous measuring of the temperature and pH of the raffinate. Additionally, a sampling port was installed inside the reactor for the raffinate and emulsion sampling. An emulsion conduit pipe was also installed just above the tip of the impeller to ensure a good breakup of the emulsion droplets. Mixing of emulsion and treated feed was performed by an Ika-Ruhr-Werke RW-20 stirrer at a fixed speed of 250 min-1 and was controlled by a Digitial RW-20 DZMl speed counter. To maintain a constant temperature of the reacting mixture, the reactor was thermostated (TC) at 20 "C and regulated to within kO.1 "C. 2.2.1. Permeation Materials and Trials. For the subject of investigations Co2+ was chosen as a model metallic cation in this work. In fact the choice of Co2+ permeation from acetate solution was a simulation of a real industrial effluent (Abou-Nemeh and Van Peteghem, 1992b). In this manner, Co(CH&00)~4H20of reagent grade quality purchased from Union Chimique Belge (UCB) was dissolved in distilled water. The pH of the feed (pH = 3.0) was adjusted by adding acetic acid of 99.4% purity and its concentration was equal to 2.2 M. A typical permeation experiment was carried out as follows: The feed was pumped to the reactor, and its temperature reached the required reaction temperature (20 "C). Then the emulsion was pumped through an emulsion pipe via a three-way valve connected to a calibrated tube so that the flow rate could be monitored. The flow rate of the emulsion was set to 10 mL/min, and consequently, after 15 min a steady state mode of the reactor was attained. 2.3. Splitting Setup. The experimental unit used for emulsion breaking is a glass laboratory-scale electrostatical splitter. Its constructional design resembles a shell-andtube heat exchanger. The tube in fact is a stainless steel bar (diameter = 9.4 mm) and is the high-tension electrode, while the shell is the second electrode. For the jacket to

Ind. Eng. Chem. Res., Vol. 32, No. 7,1993 1433 Table I. Experimental Conditions for Membrane Recycling Emulsification Stage 25-32 "C temperature 12 OOO min-l stirring speed (emulsion preparation) 10 min total stirring time phase ratio (membrane1 211 internal aqueous phase) membrane composition 5.5 vol % D2EHPA 3.0 vol % span 80 92.5 vol % Shellsol T internal aqueous phase composition 2M sulfuric acid Permeation Stage temperature mixing speed maximum mixing time treat ratio (emulsionlfeed) COa+ concentration of the feed initial pH of the feed

20 "C 250 min-1 70 min 701350 1 g/L 3.0

Splitting Stage temperature ac electric field strength frequency emulsion volume

20 "C 1248 V/cm 1OOO Hz 80 mL

be conductive, it was filled with 4 M NaOH solution. The stainless steel bar was exactly mounted in the center of the tube ensuring an equidistance (d = 10 mm) from the bar to the glass wall of the shell. The splitter was maintained with two inlets and three outlets: for electrolyte filling and draining off, emulsion input, internal aqueous-phase output, and membrane output (see AbouNemeh (1991) and Abou-Nemeh et al. (1992~)for further details). Apart from the electrostatic splitter a key element of the installation is the high-tension generator (HTG) with a capability to generate a 3000 V/cm maximum ac field. Three additional elements were interfaced to the HTG apparatus: an MX 579 ITT digital voltmeter to monitor the tension of the applied field, a GX 239 IT" signal generator to set and control the frequency, and, finally, an OX 170C ITT oscilloscope to control the input sinusoidal signal. 2.3.1. Splitting Trials. After settling the feedemulsion system, the purified feed was separated and the emulsion rich in metal was pumped to the splitter. The splitting experiments were carried out in a continuous mode; Le., the emulsion was pumped to the splitting zone and the internal aqueous phase was continuously drained away from this zone. Only the residue of the unsplit emulsion and the membrane was left in the splitting zone, which were then centrifuged for 45 s for clear separation of the phases. This has been done to correlate the residual volume of the aqueous phase-if any was left-with the final volume obtained to verify the mass balance of the emulsion. To start up the experiment, the high-tension generator, the digital voltmeter, and the signal generator were set to the desired voltage and frequency, respectively. After all the required parameters were set (see Table I) the HTG was put in a "stand-by" position. Meanwhile, the emulsion was fed to the splitting zone. As soon as all the emulsion was present in the splitting zone, the HTG was turned on and the time for splitting was recorded. 2.4. Analytical Section. 2.4.1. Raffinate Analysis. During the course of metal permeation, aliquots were withdrawn from the reactor to trace the evolution of the raffinate concentration profile versus time. Early samples were diluted by means of a Hamilton MicrG Lab. 2000

a

4,

Figure 2. Schematic diagram of the emulsification-permeationsettling-splitting loop of the membrane recycling study.

electronic diluter. The concentration of the metal was determined spectrophotometrically by using a PerkinElmer and/or Unicam-Pye atomic absorption spectrophotometer. Each run was reproduced at least in triplicate, and the data thus presented are accurate within k3.4 % . 2.4.2. Emulsion Analysis. In the kinetic study of the surfactant decompositon during metal permeation, samples of the emulsion were drawn versus time and split electrostatically. The split emulsion thus obtained was then centrifuged for 45 s and the organic phase (membrane) was decanted and analyzed for D2EHPA and oleic acid content. Also, the membrane was analyzed before and after the emulsification to determine the initial conditions of the system. For this purpose the potentiometric technique was extensively applied. Freshly prepared sodium ethoxide of 0.03-0.08M was used as a titrant. The potentiogram showed two well-defined peaks of D2EHPA and oleic acid, and thus their concentrations were calculated from the mass balance. Further details on the technique and method can be found elsewhere (AbouNemeh and Van Peteghem, 1992d). The chemical composition of Span 80 and the molecular mass of the reactive components (esters) were equally determined by gel permeation (Abou-Nemeh and Van Peteghem, 1990) and gas (Abou-Nemeh, 1991) chromatography. 3. Results and Discussion 3.1. Effect of Membrane Recycling on Co2+Permeation. Four sets of experimetnal runs were conducted to study the effect of membrane recycling on the kinetics of cobalt permeation and emulsion stability. In Table I, the experimental operating conditions investigated are summarized. The main principle of these trials relies on extracting the metal in question from the contaminated feed, settling the system for phase separation, and emulsion electrostatic splitting into the membrane (organic phase) and internal aqueous phase rich in solute. A schematic diagram of the emulsification-permeation-splittingtechnological loop is shown in Figure 2. After each cycle a fresh internal phase reagent (2 M sulfuric acid) was added to the recycledmembrane in the emulsification stage. Also, fresh feed (1 g/L Co2+) was treated with the emulsion prepared from the recycled membrane. From this scheme can be seen that the membrane phase was recycled without regeneration until its exhaustion. In Figure 3, the dimensionless ratio of the raffinate concentration to the feed initial one is presented as a function of the reaction time. As is evident from Figure 3,after one cycle of the membrane phase, the kinetics of metal permeation was not signifi-

1434 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993

g 8

SP

'A

0.8

.___._ -~ I

0

+ 0

A

FRESH EMULSION CYCLE1 CYCLE2 CYCLE3

FRESHEMULSION

~

+

I

0

A

CYCLE1 CYCLE2 CYCLE3

1

1 I

0.60.4

..- d

TIME (min)

10

20

30

40

TIME (min)

Figure 3. Effect of membrane recycling on the kinetics of cobalt permeation.

Figure 4. Effect of membrane recycling on the pH of the raffinate during cobalt permeation.

cantly affected and the increase in the concentration ratio from 0.18 to 0.23 after 15 min residence time is rather small. However, on further membrane recycling a remarkable drop in the kinetics was observed. The concentration ratio increased from 0.23 to 0.42 and 0.56 after two and three cycles, respectively. This successive drop in the permeation efficiency by factor of 4 is rather significant and vital for the whole process of cobalt separation. At an earlier stage, it was thought that the accumulated unstripped carrier-metal complex formed in the membrane phase ultimately caused the retardation in kinetics. However, this hypothesis was soon rejected because of the high carrier surface concentration at the membrane/external phase interface in comparison with the solute concentration and relatively fast stripping reaction of Co2+at the membrane/internal phase interface. Therefore, the justification of such behavior lies somewhere else. 3.2. Effect of Membrane Recycling on the pH of the Raffinate. Extraction of divalent metals (M2+)by dimmerized organophosphorous acidic extractants (HX)2 can be represented by the following reaction proposed by Kolarik (1971) as follows:

respectively. Despite the fact of metal uptake inhibition, proton transport from the internal phase to the feed continued unabated. Simultaneously, the raffiiate profiie has shown a linear steady increase in cobalt concentration versus time. Both phenomena unequivocally indicate a significant leakage of the internal aqueous phase reagent (2 M H2S04)which resulted in a significant pH drop and undesired contamination of the raffinate. Summarizing previous thoughts, it can be concluded that the higher the number of membrane cycles, the higher the pH drop of the raffinate which is due to the deterioration of emulsion stability, and consequently, the permeation efficiency was lowered. The item of emulsion stability and related phenomena will be discussed later. 3.3. Emulsion Splitting for Membrane Recovery. The splitting stage deals with breaking the emulsion to recover the membrane and the concentrated solute. In this manner, the emulsion rich in solute used earlier was split in an electric field. The course of the splitting experiment was evaluated by measuring the internal aqueous phase volume drained off from the splitting zone against time. The emulsion splitting efficiency is defined as the ratio of the measured volume of the internal aqueous phase a t time t to the sum of the initial volume of the inner phase and that of water transported during permeation. This can be represented by

As the process of metal extraction proceeds forward, the pH of the raffinate, i.e., the purified feed, drops as a result of the ion-exchange mechanism. During the course of permeation the pH of the raffinate was continuously measured. The results of pH measurements versus time have been displayed in Figure 4. As can be seen from this figure, the higher the number of the membrane cycles the lower the pH; such a drop in the pH of the raffinate cannot be justified by the coupled mechanism transport. Moreover, it has been observed that for times in excess of about 15 min residence time, the pH differences and cobalt concentration differences do not indicate the attainment of state of equilibrium. For the fresh membrane, the pH drop increments for every 5 rnin for times in excess of 15 rnin were 0.08, 0.05, 0.07, 0.09,0.06 and 0.02 (pH units) which corresponds to 6,11, 19, 7, 9, and 5 ppm of metal uptake, respectively. This fact clearly indicates that metal extraction proceeded until the end. However,the situation looks extremely different for the recycled membrane, where the metal uptake was already retarded after 20, 15, and 10 min for the first, second, and third cycle, respectively. The pH of the raffinate at which the retardation in metal extraction took place was 2.20,2.27, and 2.40 for the recycled membrane,

sp= (Vi$/ Vi,T) x 100

(2)

where (3)

and V~,H*O is calculated from the emulsion mass balance as follows: (4)

Therefore, knowing the initial internal aqueous phase volume, the volume of the transferred water after permeation, and that measured during splitting, the splitting efficiency can be estimated. The emulsion obtained after metal permeation was subjected to an ac electric field. The operating conditions are summarized in Table I. From Figure 5, the splitting efficienciesof the emulsion for four cycles of the membrane were 18,62,83, and 91?6 for fresh, first, second, and third cycle of the membrane, respectively for 240 s splitting time. The higher the number of cycles, the higher the splitting efficiency. This fact can be attributed to the change in the membrane composition, and consequently, any change will dramatically affect the emulsion stability.

Ind. Eng. Chem. Res., Vol. 32, No. 7,1993 1435

v

0.013 ,

0.00 I

0 . 9 TIME (sec)

Figure 5. Effect of membrane recycling on the kinetics of emulsion splitting in an electric field.

This is due to alteration in the viscosity and mainly the surfactant concentration (Abou-Nemehand Van Peteghem, 1992d). Kreichbaumer and Marr (1985)have reported analogous results on the influence of the surfactant (Span 80) concentration on splitting efficiency. The higher the concentration, the lower the splitting efficiency. Therefore, it is most likely that the surfactant has been significantly affected. Consequently, the higher the number of membrane cycles, the easier the emulsion breakdown. This fact implies that a higher rate of enhanced coalescence of the internal phase droplets took place. This enhancement of the emulsion demulsification can only be attributed to a significant irreversible qualitative and quantitative change of the surfactant in the membrane phase as will be seen in the next section. 3.4. Membrane Analysis during Metal Permeation. An analogous series of permeation experiments was carried out and aimed at the elucidation of the precursors of the membrane decay during cobalt permeation. The main principle of this kinetic study relies on sampling the emulsion during Co2+ permeation, splitting it electrostatically, and then analyzing the chemical composition with particular emphasis on the carrier and surfactant used. 3.4.1. The Surfactant. The surfactant (Span 80) is a mixture of different esters (sorbitan mono-, di-, tri-, and tetraoleate (RCOOR’)) and starting materials (sorbitol (R’OH) and oleic acid (RCOOH))and water (Abou-Nemeh and Van Peteghem, 1990; Abou-Nemeh, 1991). In the presence of water and traces of acids, the reactive compounds (esters) undergo the reaction of hydrolysis and can be represented by H+

RCOOR’ + H 2 0 e R’OH

+ RCOOH

(5)

In an earlier paper Abou-Nemeh and Van Peteghem (1992d) have studied the kinetics of the surfactant (Span 80) decomposition during aging of the membrane, and the following kinetic rate equation

-rmm

= rACm= d[RCOOHl/dt = kIRCOOR’1 [H201

(6) was found to fit the experimental data adequately. The rate of Span 80 hydrolysis in the organic phase (membrane) is very slow, and thus, the rate constant was estimated to be (6.93 f 0.2) X lV L/(mol.s) and a conversion of 4% was obtained after 114 h. However, the experimental conditions are extremely different during metal permeation, where a strong acid (2 M sulfuric acid) is involved in the internal aqueous phase, the treated feed has a

6

60

TIME (min) Figure 6. Kinetics of the surfactant (Span80) decomposition and oleic acid production during cobalt permeation.

moderately low pH (pH = 3.0), and strongly adsorbed surfactant molecules a t the membrane/internal and membrane/external aqueous phase interfaces deeply penetrate the aqueous phase with their hydrophilic parts and are exposed to the catalyst (protons). Moreover, the high solute concentration (10 g/L) within several minutes has a far-reaching catalytic impact on the kinetics of Span 80 hydrolysis. It has been reported by Burgess (1978) that the presence of metallic species could enhance the kinetics of hydrolysis of esters even 106 times. Indeed, and as evident from Figure 6, the concentration of oleic acid has increased by a factor of 4within 20 min, which corresponds to more than 10% of Span 80 conversion. Moreover, it can be seen that oleic acid concentration has almost reached an asymptotic value, which suggests that the reaction has attained the state of equilibrium. Furthermore, the higher the order of the ester (secondary, tertiary, etc.) the more difficult the ester hydrolysis (Ingold, 1969; Bamford and Tipper, 1972). Thus, the higher esters of Span 80 which are in appreciable concentrations (dioleate 32.3%, trioleate 19.3%, and tetraoleate 3.5%) (AbouNemeh, 1991) are most likely to undergo the reaction of hydrolysis with difficulty due to steric hindrance. The results depicted in Figure 6 represent the decay of the membrane as a result of the surfactant decomposition for the first cycle, i.e., fresh emulsion. Therefore, the outcome of the membrane analysis for the second and third cycles is an obviousanalogy. Consequently, the chemicalkinetics of reaction 3 will proceed forward with changing the catalyst concentration (fresh internal phase reagent and fresh feed), and as a result the membrane becomes poorer and poorer with the surfactant content and, hence, lower emulsion stability and which, indeed, was observed (see Figure 5). 3.4.2. The Surfactant Decay and Emulsion Stability. From the previous sections it has been shown that membrane recycling without regeneration results in a significant pH drop of the raffinate due to the leakage of the internal aqueous phase reagent and the encapsulated metallic species. This undesired phenomenon takes place due to emulsion instability which is mainly caused by the surfactant decomposition. Rosen (1978)has distinguished six major factors affecting emulsion stability: physical nature of the interfacial film, existence of an electric or steric barrier on the droplets, viscosity of the membrane, size distribution of the droplets, phase volume ratio, and temperature. The reaction of hydrolysis of the surfactant takes place at the interface due to the highly developed interfacial properties of the system. In other words, the surfactant molecules adsorbed at the interface will undergo the

1436 Ind. Eng. Chem. Res., Vol. 32,No. 7, 1993

undergoes the reaction of hydrolysis according to RCOOH + NH,

a

0

d

I

0.15 STw.005

0

. 90

1

s

40 sb TIME (min) Figure 7. Concentration profile of the carrier (DPEHPA) during cobalt permeation.

0

reaction of decomposition at the first place. Consequently, this will affect the mechanical strength of the interfacial film and, thus, the emulsion stability. The interfacial film which acts as a barrier against the internal phase droplets coalescence will be no longer immune against droplet collisions,and hence, the rate of coalescencewill be higher. If this process continues, the dispersed phase will separate from the emulsion and break down. In the present study, the profiles of the pH and metal concentrations of the raffinate are both live evidence of emulsion instability. 3.4.3. The Carrier. Apart from oleic acid, the carrier (D2EHPA)was equally analyzed to trace its profile during metal permeation. The results are displayed in Figure 7. As can be seen, the concentration of D2EHPA was almost constant during the course of the reaction and the standard deviation of the measurements was found to be 0.005. The results indicate that nearly all the carrier molecules are "free" and no unstripped metal-carrier transient complexes were accumulated in the membrane phase. This finding is consistent with the previous one (Abou-Nemeh, 19911, where the maximal cobalt concentration in the membrane was found to be 0.126g/L after 15 min emulsion residence time. However, after 40 min nearly the total metal accumulated in the membrane phase was stripped and encapsulated in the inner phase. 4. Conclusions Throughout this study, insights concerning membrane recycling and its decay were gathered and the following conclusions can be drawn: 1. Recycling of the membrane has revealed to have a significant impact on the kinetics of metal permeation due to emulsion instability. 2. The splitting efficiency of the emulsion was found to be proportional to the number of cycles. The higher the number of cycles, the easier the emulsion breakdown. 3. During permeation the carrier concentration was found to be effectively constant for this specific system. It is not excluded that the usage of other carriers or even the same, however, for other metal extractions will not influence the kinetics of the stripping reaction and as a result alter the "free" carrier concentration. 4. One of the major precursors of the emulsion instability is the surfactant decomposition during the course of permeation. More than 10% of the surfactant (Span 80) was decomposed within 20 min, consequently destabilizing the emulsion. This finding is of prime importance to avoid the use of Span 80 in LSM. Furthermore, ECA 4360, which is a polyamine, also

* RCOO-NH:

i-

RCONH,

(7)

This result throws light on a wider issue of the susceptibility of such surfactants in the LSM process. Therefore, it is more likely that ether-based surfactants are chemically more stable in various media than ester, polyamine surfactants. However, the need for surfactants compatible with the LSM process is intimately related to the advances of chemistry. Meanwhile, kinetic modeling of the surfactant decomposition seems to be inevitable and vital to the whole aspect of mass transfer and emulsion stability in the LSM process.

Nomenclature (HX12: dimer of bis(2-ethylhexy1)phosphoricacid (D2EHPA)k: rate constant, L/(mol.s) ~ T E R :kinetic rate of sorbitan ester (mono-, di-, tri-, and tetraoleate) hydrolysis rACID: kinetic rate of oleic acid formation S,: splitting efficiency, 3'% Vem,0: initial volume of the emulsion, mL Vem,t: volume of the emulsion after permeation, mL Vi,o: initial internal aqueous phase volume, mL V~,H~O: volume of the transported water due to osmosis, mL V~,T:total volume of the internal aqueous phase at the end of the permeation experiment, mL Vi,t: internal aqueous phase volume obtained upon emulsion splitting at time t, mL Literature Cited Abou-Nemeh, I. A Study on Liquid Surfactant Membranes Process For Metals Separation. Ph.D. Dissertation, University of Gent, Gent, 1991; Part I, pp 57-209. Abou-Nemeh, 1.;Van Peteghem, A. P. Cobalt Recovery From Waste Water by Liquid Surfactant Membranes. Proceedings of the E r s t Znternutionul Conference on Environmental Protection, Envirotech'89; Westarp Wbsenschaften: Wien, 1989a; Vol. 2, pp 125134. Abou-Nemeh, I.; Van Peteghem, A. P. Extraction of Cobalt and Manganese From an Industrial Effluent by Liquid Emulsion Membranes. Proceedings of the SecondZnternational Conference on Separations Science and Technology ZCSST 8S;Canadian Society for Chemical Engineering: Hamilton, 1989b; Vol. 2, pp 416-423. Abou-Nemeh, I.; Van Peteghem, A. P. Some Aspecta of Emulsion Instability on Using Sorbitan Monooleate(Span 80)as a Surfactant in Liquid Emulsions Membranes. Chem.-Zng.-Tech. 1990,62 (S), 420-423. Abou-Nemeh, I.; Van Peteghem, A. P. Kinetic Study of the Emulsion Breakage During Metals Extraction by Liquid Surfactant Membranes (LSM) From Simulated and Industrial Effluenta. J. Membr. Sci. 1992a, 70,65-73. Abou-Nemeh,I.; Van Peteghem, A. P. Extraction of Multicomponent System of Metals From Simulated and Industrial Effluenta by Liquid Surfactant Membranes (LSM). Hydrometallurgy 1992b, 31, 14S162. Abou-Nemeh, I.; Van Peteghem, A. P. Electrostatic Splitting of the Emulsion Used in Liquid Surfadant Membranes Process (LSM) For Metals Separation. Sep. Sci. Technol. 1992c, 27 (lo), 13191335. Abou-Nemeh, I.; Van Peteghem, A. P. Sorbitan Monooleate (Span 80) Decomposition During Membrane Ageing. A Kinetic Study. J. Membr. Sci. 1992d, 74,9-17. Baird, R. 5.;Bunge, A. L.; Noble, R. D. Batch Extraction of Amines Using Emulsion Liquid Membranes: Importance of Reaction of Reversibility. AZChE. J. 1987,33 (l),43-53. Bamford, C. H.; Tipper, C. H. F. Comprehensiue Chemical Kinetics. Ester Formation and Hydrolysis and Related Reactions; Elsevier: Amsterdam, 1972; pp 121-135. Bart, H. J., Wachter, R.; Marr, R. Copper Permeation From Natural Leach Solutions. In Separation Processes in Hydrometallurgy; Davies, G. A., Ed.; Ellis Horwood: Chichester, 1987; pp 347-354.

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