Minerals Engineering, Vol. 7, No. 10, pp. 1301-1311, 1994 Copyright ~)1994 Elsevier Science Ltd

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0892-6875(94)00068-9

RECOVERY OF HEAVY METALS BY A COMBINATION OF TWO PROCESSES: CEMENTATION AND LIQUID MEMBRANE PERMEATION

M. T. A. REIS and J. M. R. CARVALHO

Chemical Engineering Department, Instituto Superior T~nico, Av. Rovisco Pals, 1096 Lisboa Codex, Portugal

(Received 8 February 1994," accepted 19 May 1994)

ABSTRACT

A Zinc Plant effluent was treated by applying two processes: Cementation and Liquid Membrane Pe~eation. The effluent was a synthetic aqueous solution with 1700 ppm of Zn, 600 ppm of Cd and 100 ppm of Cu. Due to the high contents of Cd and Cu, it was proposed to apply Cementation with zinc powder before applying Emulsion LiquM Membranes in order to remove these metals. The influence of several variables on zinc pelrneation was studied. The use of nonylphenol in the membrane was tested. The presence of this reagent promotes the stril)ping step but a concentration higher than 1% wt. is not beneficial because promotes the occurrence of osmosis and occlusion.

Keywords Permeation, emulsion liquid membranes, cementation, heavy metals recovery, effluents

INTRODUCTION

The effluent studied in this work was a synthetic aqueous solution which simulates an expected waste to be obtained in a Zinc Plant processing zinc oxides and scrap as raw material. The chemical composition of this effluent is shown in Table 1.

TABLE 1 Chemical composition of the aqueous feed (sulphuric media).

Zn Cd Cu AI Na Ca pH

(ppm)

1700 600 100 160 3800 500 1.1

Due to the characteristics of this stream, especially the high contents of copper and cadmium, it is not possible to apply the Emulsion Liquid Membranes (ELM) technique as a single step of treatment. Thus, it was decided to use a combination of two processes: Liquid Membrane and Cementation.

The Emulsion Liquid Membranes separation technique is a highly selective method of separating organics, inorganics and metal ions from dilute streams. It was invented by Li in 1968 [1] and his first work dealt with the separation of hydrocarbons [2]. In the opposite of a conventional solvent extraction process, in liquid membranes the extraction and re-extraction are carried out in one single step. A primary emulsion is prepared which is dispersed in the effluent to be treated. In the case of metals separation, the

1301

1302 M . T . A . REIS and J. M. R. CARVALHO

continuous phase of the emulsion drops consists of an organic phase, which contains a diluent, an extractant and a surfactant to stabilise the emulsion. The internal phase of the emulsion is the receiving phase to the solute to be recovered. The organic phase separates the aqueous external phase (effluent phase) and the emulsified internal phase (stripping phase) and behaves like a liquid membrane, allowing the permeation of solutes. When permeation is over, the effluent phase is settled. The emulsion is broken usually by applying high voltage and the enriched aqueous phase is recovered. Figure 1 illustrates the mechanism of metal ions transport (e. g. Zn2+). At the outer interface (phase III / phase II), the extractant selectively combines with the solute giving a metal-carrier complex, which is only soluble in the organic phase, e.g.

Zn2+ aq + 2 HR org -' ZnR2 org + 2 H + aq (extraction reaction)

The organic complex diffuses through the membrane phase and is decomposed at the inner interface (phase II/phase I). The zinc ions are released into the stripping phase by the reversal of the equilibrium reaction and the regenerated carrier goes back into the membrane phase, e. g.

ZnR2 org + 2 H + aq -' Zn2+ aq + 2 HR org (stripping reaction)

The mass transfer driving-force is the concentration gradient of protons between the two aqueous phases and zinc ions can be transported even if the concentration of zinc in the internal phase is already much higher than in the effluent phase.

° o o o 0 0

Membrane phase (II) HR, ZnR 2

Feed phase (III) Zn 2+

EMULSION GLOBULE

, R e c e i v i n g p h a s e (I )

H ÷

rt

I I membrane

2 , H+ 2HR

Z ~2~ ZnR2

III

2H +

I

Zn 2+

Fig. 1 Mechanism of zinc-transport.

A problem inherent in all membrane processes is the co-transport of water (osmosis) which causes the swelling of the emulsion and dilutes the internal receiving phase [3-8]. The swelling phenomena may be also due to the occlusion of phase III which occurs during the coalescence of globules after stopping agitation [4]. Usually, this contribution does not appear in the formulae for determining the water transfer into an emulsion. This is not completely correct since occlusion in some cases can be much higher than osmosis [9, 10].

Another deleterious effect in a membrane process is the leakage of the internal phase due to the rupture of the membrane (break-up). This phenomenon leads to the release of solutes from phase I and the consequence is the increase in the effluent concentration. The rate of break-up is usually a function of the surfactant concentration in the membrane and the agitation speed [11, 12].

Recovery of heavy metals 1303

Emulsion Liquid Membranes have been successfully applied to complex feeds containing heavy metals [13]. At low pH values (< 2) of the effluent, the most promising extractants are the organic thio- and dithio-phosphoric and phosphinic acids. Di-(2-ethylhexyl)dithiophosphoric acid (dithio-DEHPA) and the equivalent monothio- form (monothio-DEHPA) have been used in several pilot plants treating feeds with Zn, Cd, Cu, Pb, Ni, etc. [6, 13]. Dithio-DEHPA is a very strong extractant for many cations but nevertheless it is rarely used in Solvent Extraction due to very slow stripping kinetics [14]. Emulsion Liquid Membranes offer a much higher interfacial area (about 106 m2/m 3) and therefore the kinetics of the stripping reaction are much faster than is in Solvent Extraction.

Concerning the presence of copper and cadmium in the feed, it has been observed, for both extraetants, that those metals and also lead are not transferred to the internal phase (2.5 M in H2SO4) and are accumulated in the membrane phase [15]. Thus, in a continuous process, the zinc-transport will be stopped due to the saturation of the carrier. Draxler et al. [15] showed that only sulphurie acid with a concentration higher than 10 M or a solution 10 M in hydrochloric acid can be used for a sufficient stripping of Cd. Unfortunately, neither of these acids can be used directly in the liquid membrane process. The highly concentrated sulphuric acid not only shows a very high osmotic pressure gradient, but also decomposes the extractant very quickly. To solve this problem the authors above mentioned [15] proposed a combination of a liquid membrane and a solvent extraction process. A third of the organic phase was continuously stripped in a mixer-settler by a 5 M hydrochloric acid solution before reusing the membrane for the preparation of the next emulsion.

In this work, the effluent to be treated had a high level of zinc, cadmium and copper (see Table 1). So, it was expected that the ELM could not be used since the extractant should become saturated inhibiting the transport of zinc. It will be necessary to remove part of cadmium and copper before applying ELM and in this case a cementation process with zinc powder was chosen. Cementation involves the reductive precipitation of a noble metal by another less noble metal. The cementation reactions are widely used in hydrometallurgy to remove metals from solutions and there are several variables that are known to influence the cementation process such as: the cell potential, stirring speed, temperature and pH [ 16-18].

EQUIPMENT AND EXPERIMENTAL PROCEDURE

ELM experiments

The emulsions were prepared by emulsifying aqueous solutions of varying acidity (in H2SO 4, p.a.) with an organic phase (membrane). The membrane consisted of a paraffinic solvent (Shellsol T, Shell Chemical Ltd.), a non-ionic surfactant (polyamine ECA 4360, Essochem. Europe Inc.) and an extractant, di-(2-ethylhexyl)dithiophosphoric acid (dithio-DEHPA) (Hoechst AG.) A rotor-stator type high-speed disperser (IKA UltraTurrax T50 with IKA G45FF ultrafine turbine) was used at 7000 rpm and the residence time was 15 minutes. The emulsion was continuously recirculated and cooled.

The emulsion was dispersed in the aqueous effluent solution, in a batch-type baffled thermostatic glass reactor (85 mm int. diameter, round bottom, 1 1 capacity). This reactor had 4 vertical baffles (165 x 10 x 1.5 mm). Agitation was effected by a stainless steel paddle (45 x 45 x 1 mm) centred at 77 mm from the bottom. A Heidolph RZR1 driver at 280 rpm was used and the temperature was 298 K. The three-phase dispersion was stirred for 10 minutes or less.

After contacting the loaded emulsion phase was separated from the rafflnate phase by settling. The separation of phases was effected 10 minutes after stopping agitation. The emulsion was broken in a coaxial electrostatic coalescer by applying a voltage of 7-12 kV AC and a frequency of 50 Hz. The electrocoalescer prototype used is made of 2 concentric glass tubes, the annular space between them being filled with a strong ionic solution connected to the earth. Other details are published elsewhere [19].

1304 M.T.A. REIS and J. M. R. CARVALHO

Stripping experiments

To analyse the level of copper, cadmium and zinc in the organic phase, a volume phase ratio 2:1 (aqueous/organic) was used in three stages. The separator funnels were vigorously shaked during 30 minutes. Sulphuric and hydrochloric acids were used in several concentrations as stripping solutions.

Cementation

The cementation experiments were carried out in a 500 ml round glass reactor. Agitation was effected by an anchor paddle with a diameter of 65 mm (and maximum height = 20 mm) and leaving a small clearance between the agitator and the bottom of the reactor. A Heidolph RZR1 driver was used at several rotational speeds and the value 500 rpm was chosen as the most adequate level of agitation. Temperature was kept constant by means of a thermostatic water bath. Combined redox and pH electrodes were used for monitoring the solution conditions and the pH was automatically controlled by the addition of a sulphuric acid solution. Zinc powder was used to cementate the metals from solution.

Analyses

All the analyses of metals (raffinate and enriched phase) were performed on an atomic absorption spectrophotometer (PERKIN-ELMER 3100).

The internal phases obtained after demulsification were automatically titrated on a TITROPROCESSOR METROHM 682 in order to analyse the level of acid.

RESULTS AND DISCUSSION

General

Some preliminary data were obtained in order to confirm the inhibition of zinc-transport in ELM due to the high contents of cadmium and copper in the effluent. The results showed that the percentage of zinc transferred into the phase I was very low (< 2 %), copper was totally accumulated in the solvent phase and cadmium was partially accumulated in the membrane and some was transferred into phase I. To understand the behaviour of copper, zinc and cadmium during permeation, it was decided to analyse the capacity of the solvent for these three elements at different pH values. The results and other useful information are presented in Tables 2 and 3. It is shown that cadmium and copper are the elements that are more extracted and the solvent capacity is not affected by the pH of the aqueous phase. However the more stable complex is the one with copper. Concerning the extraction of zinc, it is shown that the capacity of the solvent starts to decrease for values of pH smaller than 2.8. To analyse the level of zinc in the loaded organic phase, a stripping solution of 2.5 M in sulphuric acid was used. In the case of the stripping of cadmium it was necessary to use a 5 M solution of hydrochloric acid while the stripping of copper from the solvent phase needs a 10 M solution in hydrochloric acid. Table 3 shows the results of the stripping experiments which confirm that only strong acid solutions are able to strip cadmium and copper as other authors also published [6, 15]. As it was already mentioned, these conditions are not feasible in a liquid membrane process. For example, only values between 1.5 - 3 M in sulphuric acid must be used in order to maximise zinc-transport into phase I and to avoid the occurrence of osmosis. Thus, a set of cementation experiments was carried out in order to remove copper and cadmium from the effluent.

The results obtained in the cementation tests showed that'copper cementation with zinc powder was always completed after 5 minutes of reaction and was independent on the variables such as stirring speed, temperature and pH. However, the cementation of cadmium was significantly affected by the presence of copper, pH and zinc excess. It was noticed that copper increased the rate of cadmium cementation but also favoured the redissolution of the deposit. The increase in the pH improved the rate of cementation (99.9 % of extraction at pH=6.0) and stabilised the cement but zinc hydroxide starts to precipitate for a pH greater than 5.

Recovery of heavy metals 1305

TABLE 2 Capacity of a solvent phase (dithio-DEHPA - 3% wt. in Shellsol T) and percentage of extractant complexed. The solvent is loaded with an aqueous phase of copper, zinc and cadmium

(1 g/! of solute) at different pH values at equilibrium (T - 293 K).

Aq. phase

pH

2.8

1.0

0.5

Solvent capacity (g/1) and % of extractant complexed

g/l

Zn

%

1.62 68.9

1.38 58.6

0.98 41.7

g/l

Ca

%

1.53 67.0

1.55 68.1

1.53 67.0

g/l

Cd

%

2.54 62.8

2.46 60.7

2.56 63.2

TABLE 3 Stripping experiments of loaded solvents with aqueous phases (1 g/l of solute) at pH=2.8 (T- 293 K).

Element

Zn

Cd

Cu

Capacity

(g/l)

1.62

2.54

1.53

Stripping

solution

H2SO 4 (2.5 M 1

HCI (5 M)

HCI (10 M)

Solute removed (mg) from org. phase (10 ml)

Stage 1 Stage 2 Stage 3 Total

15.98 0.21 0.01 16.21

25.16 0.26 0.00 25.42

15.06 0.15 0.06 15.27

The results of cementation proved that it was possible to obtain a pre-treated effluent with a very low content of copper and cadmium (below 1 ppm for pH=4.7 and zinc excess= 0 %) . It was therefore decided to produce another synthetic solution without these metals and containing 2.15 g/l of Zn (initial concentration of Zn plus the amount of Zn powder for cementation of copper and cadmium). The ionic strength was 0.3 M (Na2SO4).

ELM tests

A first set of experiments was carried out in order to investigate the transport of zinc and water into the internal phase. Dithio-DEHPA was used as carrier. The permeation time was 3 minutes due to the high kinetics exhibited by this extractant [19, 20]. The parameters studied were the % of extractant in the membrane, the ratio of VIII/Vt and the molarity of H2S04 in phase I. The ratio of Vu/V I and the concentration of surfactant were kept constant ( 5.5 and 2 % wt., respectively). The experiments were designed with the aid of factorial method and the corresponding levels of the parameters are listed in Table 4.

TABLE 4 Maximum and minimum level for the variables studied.

Variable Min. level Max. level

Extractant cone. in membrane phase (% wt.)

Effluent phase/phase I (Viii/Vi)

H2SO 4 molarity in phase I (M)

4.0 7.0

I0 40

2.5 3.0

1306 M.T.A. REIS and J. M. R. CARVALHO

Table 5 presents the concentrations of zinc obtained in the raffinate and receiving phase. Also the change in the volume of internal phase (AV I) is presented. This measure was effected by titration of acid existing in phase I (before permeation and after demulsification). All the consumption of acid was due to the zinc-transport. Thus, the difference between the content of acid due to the reaction with zinc and the real value achieved in the internal phase is caused by water entrance. The break-up of the emulsion was assumed negligible and was not taken into account which is not completely correct. The experimental results presented in Table 5 show that the increase in the ratio VIII/V I from 10 to 40 clearly leads to a raffinate with a higher concentration of zinc. The variance analysis confirmed that this variable is significant ( > 95 % confidence level). Also, this variable is significant ( > 99 % confidence level) for the enrichment of zinc in phase I. Concerning the values of water transport, no variable studied is significant.

TABLE 5 Results of experiments programmed according to Table 4 (initial concentration of zinc in the effluent= 2.15 g/I, ionic s trength=0.3 M (Na2SO4), surfactant=2.0 % wt., VII/VI=5.5,

permeation t ime=3 min, T= 298 K).

Conditions Results

No. Extract. V iii/V I M C III C I AV I

% wt. mol/l ppm g/1 %

1 4.0 i0 2.5

2 7.0 I0 2.5

3 4.0 40 2.5

4 7.0 40 2.5

5 4.0 I0 3.0

6 7.0 I0 3.0

7 4.0 40 3.0

8 7.0 40 3.0

5.8 13.3 47.6

2.8 12.5 16.0

177.5 57.5 22.9

157.5 55.0 19.3

4.6 15.0 28.2

6.2 14.5 16.2

152.0 60.0 25.6

60.0 56.0 26.1

A second set of experiments was carried out by varying the ratio Vlli/V I in order to check how the zinc-transport depends on. The concentration of extractant in the membrane was 7 % wt. and the molarity in phase I was 2.5 M. Figure 2 illustrates the results obtained. As it is observed, the enrichment in the internal phase is approximately linearly related to the VIII/V I ratio in the range below 60. Also the zinc level in the raffinate is greatly influenced by the variable Vlii/V I and only a ratio of 15 or lower leads to an acceptable concentration in the raffinate (< 5 ppm). The swelling of the emulsion is not significantly affected by the Vm/V I ratio and values between 5 and 20 % for z~V I are obtained. It was noticed that the occlusion(*) phenomenon was very low (< 3 %) for this set of experiments.

Previous work [19, 20] showed that is essential to establish an adequate permeation time in order to avoid non-idealities. Figure 3 illustrates the influence of residence time on zinc transport and shows that the permeation must be stopped around 3 minutes. The data for the swelling of the internal phase is presented in Figure 4. The water transport increases approximately linearly with the permeation time. This behaviour is similar to the one found by other authors [8, 10]. Since the occlusion is negligible the decrease in the zinc concentration of phase I is due to the osmosis effect.

(*) The amount of occlusion is estimated by analysing the trace of sodium in the internal phase. Sodium sulphate (I=0.3 M) is presented in phase III.

R e c o v e r y o f h eav y meta ls 1307

1000

~: 100 Q.

10 4 - '

N

1 0 0 160

-G- IZnl raff inate

IZnl internal phase

I I I I I - - I

20 40 so so 100 120 140

Volume ratio phases (Vizi/ V I )

140

120

100 ,.. D~

03 80 (a

=.-

60 ~

.E lID

4 0 .,.-

.E "2 N

2 0 ~

Fig.2 Concentrations of zinc in the raffinate and internal phase versus volume ratio phases (V]N/V]), (Initial cone. of zinc in the effluent=2.15 g/l, extraetant=7 % wt., surfaetant=2 % wt.,

VII/VI-5.5, 2.5 M of H2SO 4 in phase I, permeation time=3 min, T=298 K).

1 0 0 0

E Q- 100 r,

._= q . ,

l e -

~" 10

0 I 2 3 4 5 6 7 8 9 10 11

Permeation time (rain)

2 0

1 5

1 0

u~

~o e - Q.

C

o ~

e- 4 . 1

e- ° ~

"E N

Fig.3 Concentrations of zinc in the raffinate and internal phase versus permeation time, (initial cone. of zinc in the effluent=2.15 g/l, extractant=7 % wt., surfaetant-2 % wt.,

VII/VI=5.5, 2.5 M of H2SO 4 in phase I, T=298 K).

1 3 0 8 M . T . A . REIs a n d J . M . R . CARVALHO

50 50

-G - osmosis*occlusion - B - occlusion

40 © - 40

30 30 e" 0

-'I i c O O O o 20 20 "~

+ "6 (n o '~ 0 0 E (n 0 lO lO

o ~ - 0 0 1 2 3 4 5 6 7 8 9 10 11

Permeation time (min)

Fig.4 Influence of permeation time on water transport (initial conc. of zinc in the effluent=2.15 g/l, extractant=7 % wt., surfactant=2 % wt., V,/VI=5.5, 2.5 M of H2SO 4 in phase I, T=298 K).

A problem inherent to the use of dithio-DEHPA as carrier is the accumulation of zinc in the membrane phase. So, in a continuous process a part of the organic phase should be stripped with acid. For example, the concentration of zinc in phase II is around 2 g/l under the operating conditions presented in Figures 3 and 4. In an attempt to enhance the stripping step, an auxiliary reagent, nonylphenol, was used in the composition of the membrane. Thus, a final set of experiments was carried out by varying the content of this phenol in the membrane. The results obtained are listed in Table 6. It is noticed that the presence of nonylphenol does not favour the extraction which is in agreement with other authors [21]. This is probably due to a lowering of the activity coefficient of the extractant in solution because of the strong association with the phenol. It was confirmed by infra-red spectroscopy that there is a strong association

TABLE 6 Concentrations of zinc in the raffinate and internal phase for different concentrations of nonylphenol in the membrane (initial conc. of zinc in the effluent=2.15 g/I,

extractant=7 % wt., surfactant=2 % wt., Vn/VI=5.5, 2.5 M of H2SO 4 in phase I, permeation time=3 min.,T=298 K).

Nonylphenol

% wt.

0

1

5

10

C III C I

ppm g/l

3.6 18.4

50 20.2

7.0 17.4

9.6 15.8

Recovery of heavy metals 1309

between nonylphenol and dithio-DEHPA by a hydrogen bonding interaction. It was observed that the accumulation of zinc in the membrane is reduced about 20 % with the presence of nonylphenol but only 1% wt. of this alcohol leads to a more concentrated phase I. This fact is due to the swelling of the emulsion be promoted by this reagent. The rate of water transport increases linearly with the % of nonylphenol in the organic phase (see Figure 5). Also the occlusion is affected by the composition of the membrane. Once more this phenomenon seems to be influenced by active interfacially species [9, 10]. In a previous work a similar effect was found with the % of extractant in the membrane [10]. The occurrence of occlusion is related to the systems that exhibit a lower interfaeial tension. Nonylphenol lows the interfacial tension between the external phase and the emulsion. Consequently the size of the globules is reduced and the separation of phases is more difficult due to the settling is controlled by the coalescence drop-interface. Another interesting point of view is the fact that the splitting of the emulsion is about 3 times faster with the presence of nonylphenol. This is probably caused by an interaction with surfactant.

50 r 7 50

~ osmosis*occlusion ~ occlusion

40 t- ~ -I 40

30 30 C 0 ,-~

"6 " o 0 o 20 20 "~ + _=

o o 0

0 10 10

I F ] ~ i i I I i I I I I 0

0 1 2 3 4 5 6 7 8 9 10 11

Nonylphenol in the membrane (% wt.)

Fig.5 Influence of the presence of nonylphenol in the membrane on water transport (initial cone. of zinc in the effluent=2.15 g/l, extractant=7 % wt.,

surfactant =2 % wt., Vn/VI=5.5, 2.5 M of H2SO 4 in phase I, permeation time=3 rain, T=298 K).

CONCLUSIONS

The effluent studied in this work cannot directly be treated by ELM because the membrane phase becomes saturated with copper and cadmium. The last element is the one of both that is partially transferred into phase I. To remove cadmium and copper from the solvent phase, it was confirmed that it is necessary to use, respectively, a solution of 5 M and 10 M hydrochloric acid as internal phase. To overcome this problem, a cementation process with zinc powder is proposed in order to remove those metals from the effluent.

An effluent with a level of copper and cadmium below 1 ppm is obtained by Cementation and it is possible to apply the Emulsion Liquid Membranes as a second step of treatment.

1310 M . T . A . REIS and J. M. R. CARVALHO

The permeation of zinc with dithio-DEHPA is significantly influenced by the variable Vm/V I and only ratios of VIn/V I ,:15 allow to reduce the level of zinc in the effluent from 2.15 g/1 to values less than 5 ppm.

The permeation time for the system and conditions studied must be kept in 3 minutes in order to avoid water transport.

The use of an auxiliary reagent, nonylphenol, will be beneficial if its content in the membrane does not exceed 1% wt., being possible to concentrate the zinc in phase I from 18.4 to 20.2 g/1. Higher contents of nonylphenol produce osmosis and occlusion in a high degree.

ACKNOWLEDGEMENTS

The financial support of Junta Nacional de Investiga~ 0 Cientffca e Tecn61ogica to Project No. PMCTICENI661/90 is gratefully acknowledged. The authors wish to thank Eng. Aria Paula Vidigal and Eng. Isabel Ribeiro for the valuable help in carrying out part of the experimental programme.

NOMENCLATURE

M Molarity of the acid solution used as internal phase, mol/l

Vm/V p VII/V v VItI/V n Volume ratios of phases

C i Concentration of solute in phase i, M L -3

V i Volume of phase i, 1

aV I change in the volume of phase I due to the transport of water (osmosis + occlusion), %

S u b s c r i p t s

i

I

II

III

I, II or III phase

receiving (internal) dispersed phase

membrane (intermediate) continuous phase

feed (external; effluent) continuous phase

I* 2. 3. 4. 5. 6. 7. 8. 9.

REFERENCES

Li, N.N., U. S. Patent 3410 794 (1968). Li, N.N., bui. Eng. Chem. Process. Des. Dev., 10/2, 215 -221 (1971). Matsumoto, S., Kohda, M., J. ColloM hztetface Sei., 73 (1), 13-20 (1980). Marr, R., Kopp, A., Int. Chem. Eng., 22,1, 44-46 (1982). Colinart, P., Delepine, S., Trouv6 G. & Renon, H., J. Membrane Sci., 20, 167-187 (1984). Draxler, J., Ftirst, W. & Marr, R., J. Membrane Sci., 38, 281-293 (1988). Bart, H.J., Draxler, J. & Mart, R., Hydrometallurgy, 19, 351-360 (1988). Bart, H.J., Rameseder, C., Haselgrubler, T. & Marr R., HydrometaUurgy, 28, 253-267 (1992). Xuan-.cai, D. & Fu-quan, X., J. Membrane Sci., 59, 183-188 (1991).

Recovery of heavy metals 1311

10. 11.

12. 13.

14. 15.

16. 17.

18. 19. 20. 21.

Reis, M.T.A. & Carvalho, J.M.R., Chem. Eng. Tech. (in press). Borwankar, R.P., Chan, C.C., Wasan, D.T., Kurzeja, R.M., Gu, Z.M. & Li, N. N., AIChE J., 34, 5, 753-762 (1988). Fuller, E.J. & Li, N.N., J. Membrane Sci., 20, 251-271 (1984). Marr, R., Bart, H.J. & Draxler, J., Proc. Int. Cot~ Sep. Sci. Technol., 2nd, Vol. 2, 403-410 (1989). Draxler, J. & Marr, R., Proc. Int. Solvent Extraction Cot~ ISEC'90, 38-48 (1992). Draxler, J. & Marr, R., Proc. Int. Solvent Extraction Cot~ 1SEC' 88, Paper 6-1, Moscow (1988). Miller, J.D., Miner. Sci. Eng., 5, 3, 242 (1973). Jackson, E., HydrometallurgicalExtraction atut Reclamation, Ellis Horwood Limited, Chishester, England (1986). Blaser, M.S. & O'Keefe, T.J., ll2th A1ME An. Meet., Atlanta, 587 (March 6-10 1983). Vaseoncelos, J.M.T. & Carvalho, J.M.R., Chem. Eng. Tech., 16, 3, 213-217 (1993). Reis, M.T.A. & Carvalho, J.M.R., J. Memb. Sci., 84, 201-211 (1993). Dalton, R. F., Proc. btternational Solvent Extraction Conference 1SEC 77, C1M Special Vol. 21, 40-48 (1979).