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Solid-liquid extraction studies of Zn(II), Cu(II) and Cd(II) from chloride media with impregnated resins containing mixtures of organophosphorus compounds immobilized on to Amberlite XAD2
hydmmetallurgy ELSEVIER
Hydrometallurgy 37 (1995) 301-322
Solid-liquid extraction studies of Zn ( II ) , Cu ( II ) and Cd (II) from chloride media with impregnated resins containing mixtures of organophosphorus compounds immobilized on to Amberlite XAD2 J.L. Cortina*, N. Miralles, A.M. Sastre, M. Aguilar Chemical Engineering Department, ETSEIB, UniversitatPolitknica de Catalunya, Diagonal 647, E-08028, Barcelona, Spain
Received 17 December 1993; revised version accepted 23 April 1994
Abstract The solid-liquid extraction of Zn (II), Cu (II) and Cd (II) from chloride medium at 0.1 and 0.5 M ionic strength and 25°C by impregnated resins containing mixtures of di (2ethylhexyl)phosphoric acid (DEHPA = HL) and tri-n-octylphosphine oxide (TOPO= S ) was studied. Impregnated resins containing TOP0 and mixtures of DEHPA and TOPO, by direct adsorption of both extractants into a styreneldivinylbenzene macroporous support, Amberlite XAD2, using the dry impregnation method, were prepared. The distribution coefficients of Zn (II ), Cu (II) and Cd (II) were determined as a function of pH, chloride concentration in the aqueous phase and extractant concentration (DEHPA and TOPO) in the resin phase and the data were analyzed graphically using the slope analysis method in terms of synergistic effect. Analysis of the results showed that the extraction of these metal ions with resins containing mixtures of DEHPA and TOP0 (XAD2-DEHPA-TOPO) can be explained assuming the formation of metal complexes in the resin phase with a general composition MCl,L,( HL)& where p, t, s and q take different values depending on the metal and the extractant concentration ratio in the resin phase ( [HL],/ [S],). Finally, a comparison between the extraction of Zn(II), Cu( II) and Cd(I1) in terms of the separation factors with Amberlite XAD2-DEHPA-TOP0 resins and using Amberlite XAD2-DEHPA resins was made.
* Author for correspondence. 0304-386X/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO304-386X(94)00029-3
302
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1. Introduction The need for more specific systems for dilute metal recovery, from both ecological and economic aspects, has lead to the development of new chemical techniques. In this sense, new ion exchange resins, solid adsorbents and metal extractants used in solvent extraction processes have been developed for different applications [ 11. Although the use of ion exchangers in industry implies simple equipment, easy operation and no problems with phase disengagement and reagent losses [ 2,3], these resins have limited potential for the recovery of transition metals, due to their selectivity. A bridge between solvent extraction and ion exchange was the introduction in hydrometallurgical applications of Solvent Impregnated Resins (SIR) by Warshawsky [ 45 1. In this approach, the extractant molecules are adsorbed onto a high surface area, macroporous polymeric support to produce a SIR. These are interesting because they maintain the specificity of the metal extractants used in liquid-liquid extraction systems and avoid chemical functionalization of the polymeric supports. A different, more limited, approach was presented by Kroebel and Meyer [ 6,7 1, who proposed as impregnation procedure the polymerization of styrene and divinylbenzene in the presence of the extractant to produce Levextrel resins. Since these pioneering works, the development and application of these systems in metal extraction processes has been intensively investigated for the purpose of hydrometallurgical [ 8- 12 ] and analytical [ 13- 17 ] applications for separation and recovery processes. Recent work in this laboratory has shown that metal transition ions are efficiently extracted from nitrate solutions by solvent impregnated resins containing organophosphorus extractants adsorbed into Amberlite XAD2. The organophosphorus compounds used have been di (2-ethylhexyl)phosphoric acid (DEHPA ) [ 18,191, di (2,4,4_trimethylpentyl)-phosphinic acid (DTMPPA) [ 20,2 1] and O’methyl-dihexyl-phosphine oxide O’hexyl-2-ethyl phosphoric acid [ 22 1. As a part of this series, the extraction of the above metal ions with impregnated resins containing DEHPA and Levextrel Resins (Lewatit 1026 Oc), containing DEHPA as active component [ 231, have been thoroughly studied and the systems have been described in terms of species and extraction constants, to establish, as in liquid-liquid extraction systems, if the speciation in the resin phase could provide certain differences in composition which could be utilized for the design of separation methods. The increasing concern towards the recovery of transition metal ions from such secondary-grade sources, such as electronic scrap, spent catalysts, anode slimes and industrial waste waters [ 24,25 1, usually from liquid eMuents with high chloride concentrations, requires the development of advanced separation systems. Earlier work in this laboratory has shown that impregnated resins containing mixtures of DEHPA and TOP0 with different [ HL] ,/ [S ] r can be prepared and they have shown a high efficiency in the extraction of Zn (II), Cu( II) and Cd( II) from nitrate solutions [ 261. Furthermore these studies in nitrate solutions indi-
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
303
cate that the presence of a second extractant produces a large increase in the separation factors in comparison with the previously used resins containing DEHPA [191. In this context, the objective of the present work is to investigate the solidliquid extraction of Zn (II), Cu (II) and Cd (II) from aqueous chloride solutions with solvent-impregnated resins containing mixtures of DEHPA and TOPO. This work presents fundamental studies on the extraction of Zn (II), Cu (II) and Cd( II) from 0.1 M and 0.5 M NaCl at 25 “C with impregnated resins containing mixtures of DEHPA and TOPO. In addition, we examine the extraction in terms of the synergistic or antisynergistic effect of the extractant mixture in the metal extraction reactions by graphical methods. Finally, a comparison between the extraction behaviour of XAD2-DEHPA-TOP0 resins from chloride media and nitrate media was performed.
2. Experimental 2. I. Reagents and solutions The di (2-ethylhexyl ) phosphoric acid (DEHPA= HL) was supplied by BDH (UK). Its purity was determined by potentiometric titration in an 75% ethanol solution of the acid with 0.1 M NaOH. The purity was found to be 98.2%. Tri-noctylphosphine oxide (TOP0 = S), supplied by Merck p.a., was used without further purification. Stock solutions of Zn (II), Cu (II) and Cd (II) ( 1 g*drnp3 ) were prepared by dissolving the corresponding salts (Merck a.r. grade) in water. Sodium chloride, sodium hydroxide and hydrochloric acid (Merck a.r. grade), were used for the preparation of the different solutions and potassium bromide (Merck, for spectroscopy) for the preparation of the pressed discs for FTIR. Amberlite XAD-2 Resin, supplied by Rohm and Haas, size 0.3-0.9 mm, was used. XAD-2 was kept in contact for 12 h with a 50% methanol-water solution containing 4 M HCl in order to eliminate impurities. Ethanol and acetone, supplied by Probus p.a, were used without further purification and methanol was used in the chromatographic system. 2.2. Impregnation process
The XAD2-DEHPA-TOP0 and XAD2-TOP0 impregnated resins were prepared according to the modified version of the dry impregnation method, as described previously [ 201. The amount of DEHPA ( [ HL],) and TOP0 ( [S],) impregnated was evaluated after washing a known amount of resin with ethanol, which completely elutes both extractants, and subsequent determination of the ligands. For resins containing mixtures of DEHPA and TOPO, after elution of both extractants with ethanol, the DEHPA content was determined by NaOH potentiometric titration. The TOP0 content was determined by spectrophotometry
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
304
measurements at 230 nm after the chromatographic separation of TOP0 from DEHPA. The separation process was based on the procedure developed by Kolosky et al. [ 271 in the determination of TOPO. For resins containing TOPO, after the elution of TOP0 with ethanol, the TOP0 content was determined by spectrophotometry measurements at 230 nm with a Shimadzu UV-240 spectrophotometer. The impregnated resins containing mixtures of both organophosphorus extractants and the concentration in both derivatives are described in Table 1. 2.3. Metal distribution with xAD2-DEHPA-TOP0
and XAD2-TOP0
resins
The extraction of Zn( II), Cu( II) and Cd( II) was carried out with batch experiments at 25 ‘C. Samples of 0.2 g of XAD2-DEHPA-TOP0 and XAD2-TOP0 resins were mixed mechanically in special glass-stoppered tubes with a 20 ml of a 0.1 M or 0.5 M aqueous solution of (M 2+ H+, Na+ )Cl- until equilibrium was reached. According to Fig. 1, where the disthbution of Zn (II), Cu (II) and Cd( II) Table 1 Impregnated
resins prepared (XAD2-DEHPA-TOPO)
Impregnated
resin
iSIr
[HLI, (molekg-‘)
DT 19/l DT 10/l DT l/l DT l/4 DTO/l
0.059 0.109 0.631 0.608 0.494
1.085 1.082 0.646 0.158
. . *
0
. .
100
Wlr/[Slr
(molekg-*)
.
.
19/l 10/l l/l l/4 O/l
: .
.
200
300
400
t(min)
Fig. 1. Variation in the distribution coefficients of Zn( II), Cu (II ) and Cd ( II ) as a function of time at fixed pH values for XAD2-DEHPA-TOP0 resins (HL/S = 1/ 1) .
J.L. Cortina et al. /Hydrometallurgy 37(1995) 301-322
305
as a function of time at constant pH are plotted, a time of 2 h was enough to reach equilibrium. After phase separation with a high-speed centrifuge, the equilibrium pH was measured using a Metrohm AG 9 100 combined electrode connected to a CRISON digital pH meter, model Digilab 517. The metal content was determined by atomic absorption spectrophotometry. A Perkin-Elmer 2380 AAS with air-acetylene flame was used. 2.4. FTIR spectroscopic studies FTIR spectra of metal complexes of Zn (II), Cu (II) and Cd (II) with XAD2DEHPA-TOP0 (DT 1/ 1) resins were recorded with a BOMEM MB 120 Fourier Transform Infrared Spectrometer (4000-700 cm- ’) ( 32 co-added interferograms were scanned at 2 cm-’ resolution).
3. Results and discussion 3.1. Treatment of the distribution data The distribution of Zn (II), Cu (II) and Cd( II) between the resin phase containing DEHPA and TOP0 (XAD2-DEHPA-TOPO) and the aqueous phase can be obtained directly from:
DWLS) =
[MUI) lr
(1)
[M(II)]
where [M ( II ) ] r = the total concentration of M* + in the resin phase (in mol. kg- ’) , and [ M( II) ] = the total concentration of M*+ in the aqueous phase. Hence, from the mass balance, Eq. ( 1) can be expressed as: D (HL,S)
=
([M(II)l,-[M(II)l)x(‘C/lm,) [MUI) 1
(2)
where [M (II) ] f= the initial total concentration of metal in the aqueous phase; V= the volume of the aqueous phase; mr= the mass of dry impregnated resin. 3.2. Metal distribution with XADI-DEHPA-TOP0
resins
Metal distribution data with XAD2-DEHPA-TOP0
resins are plotted as log
D versus pH in 0.1 M and 0.5 A4 Cl- in Figs. 2-4. These figures show that the
distribution functions are straight lines with a slope between 2 and 1, depending on the ratio [ HL],/ [S],. Accordingly, the extraction of these metal ions with XAD2-DEHPA-TOP0 resins can be described with the following general reaction: M2++(2+q-t)HL,+sS,+tC1-=MC1,L,2_1,S,(HL),,+(2-t)H+
(3)
306
J.L. Cortina et al. / Hydrometailurgy 37 (1995) 301-322
5
Zn(ll)
4
A
.
0
3 log D
*
urn
??
n
u
2
8.
?? *
?? *
1
..&.d..
A A
AA
A
A ??
0.
.
0,l M NaCl
0 2
4
3
PH 3
Cu(ll) ’ DT 19/l
2 A DT 1011
log D ??
DT l/l
1 A DTll4
0.1 M
NaCl
0 1
2
4
3
5
6
PH Cd(ll)
4
2
3
4
5
PH Fig. 2. Variation in the distribution coeffkients of Zn(II), Cu(II) and Cd(I1) as a functionof PH and total metal concentration in 0.1 M NaCl for XAD2-DEHPA-TOP0 resins. [Zn(II) I = 10 pmol.dm-3, [Cu(II)] =64pmol.dmw3 and [Cd(U)] = 10pmol~dm-3.
Polynuclear metal (II) complexes in the resin phase were excluded because plots of log D versus pH are independent of metal concentration. Following the approach developed by Marcus [ 281 in the study of metal extraction reactions in ion exchangers, the corresponding equilibrium constant of the extraction process (K) is defined as:
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
307
??
DT 19/l
A DT 10/l
logD2.
??
DT111
‘ DT1/4
NaCl 1
0.1 M
--I
PH 3 Cu(ll) = DT19/1
A
d
2
A
log D 1
**
A
*/=A“‘k“‘ 0.1 M
A DT 10/l ??
DT111
‘ DTll4
NaCl
0 1
2
3
4
5
6
PH
1 CW
I
0 A
3 .. /IAm
.*...
------I .
.
logD2--
DT 19/l
A DT 10/l
EQB
. . .
~ !*~Tlit
j
.
1 ‘.
e 0.1 M 2
3
4
NaCl 5
PH Fig. 3. Variation in the distribution coeffkients of Zn( II), Cu(II) and Cd( II) as a function ofPH and total metal concentration in 0.1 M NaCl for XAD2-DEHPA-TOP0 resins. [Zn(II) I =20 pmol*dm-3, [Cu(Il)]=32ymol~dm-3and [Cd(II)]=18pmol~dm-3.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
308
2
3
5
4
PH 3-
Cu(ll) .
DTlQfi
2 .. A DTlO/l
log D ?? DTlll
1 ” A DT114
0.5 M NaCl 0
3
1
5
PH 3-
Cd(ll) PhA R
2 ‘. log
D
. DT 19/l A DT 1011
88 .
1 .. f
.
.
??
??
DTl/l
El 0.5 M NaCl
04 2
3
4
5
PH Fig. 4. Variation in the distribution coeffkients of Zn( II), Cu( II) and Cd(U) as a function of pH and total metal concentration in 0.5 M NaCl for XADZ-DEHPA-TOP0 resins. [Zn(II) = 10 pmol*dm-‘, [C~(II)]=64ymol.dm-~and [Cd(II)]=10pmol~dm-3.
J.L. Cortina et al. /Hydrometallurgy 37 (1995) 301-322
K=aMCI’L~~-‘~SS(HL)ePH+
309
(2--t)
aM2+aj.&?rq-f)a~,a&,-
(4)
where ai and ai,= the activity of the i species in the aqueous and the resin phase, respectively. Eq. (4) can be written as:
where yj and yi,r= the activity coefficients of the i species in the aqueous and the resin phase, respectively. Rearranging Eq. (5 ), the following expression may obtained: K=
Blqtsr
(6)
where j?zqts= the stoichiometric equilibrium constant for the extraction reaction: (7) and r= the term containing all the activity coefficients. As a preliminary hypothesis the term f can be expected to remain constant as long as the ionic strength in the aqueous phase is constant and the variations in resin phase concentrations are small. In order to relate the distribution of the metal with the metal composition in the resin phase the formation of a simplest species may be assumed. So, according to previous results, if only one species of the type ML,,_ tj (Cl) f (HL), is formed the distribution coefficient for ( M2+ ) becomes: ~=ZqZtZJ?;qtJHL]~2+q-“[S]~[Cl-]t[H+]-(2-’)
(8)
From Eq. (8 ) it can be seen that, for a given constant ionic medium and [ HL] ,/ [S ] r ratio, the variation in log D as a function of pH allows the determination of the slope index (2 - t ) for the extracted species in the resin phase. The dependence of the distribution coefficient on proton ion concentration in the aqueous phase at constant extractant concentration in the resin phase can be seen in Figs. 2-4, where the experimental data obtained in the distribution equilibrium studies are plotted as a function of pH. The graphical treatment of the experimental data (plotted in Figs. 2-4) indicates that for [ HL],/ [S],>10, the predominant species in the resin phase are species with t= 0.This means that in this condition the extraction of these metal ions involves the formation of species with the same composition as in the case of resins containing DEHPA as a single component ( ML2 (HL),) [ 191 in both the media studied. In the case of [ HL],/ [S],< 10, the slope of the functions log D versus pH decreases from 2 to 0.5 for resins with ratios [ HL],/ [S],< l/4. These fractional values between 2 and 0.5, indicate that species with t values different to 0 are extracted and the extraction of the above metal ions involves the formation of species with a general composition ML,,_,, (Cl),S,(HL),,.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
310
On the other hand, the slope values decrease with increasing chloride concentration in the aqueous phase and depend on the metal ion. The decrease of the slope follows the order Cd (II ) > Zn (II) > Cu (II). This agrees with the predominance of chloride complexes in the aqueous phase. For Cu (II) in 0.1 M chloride, less than 8% of copper is complexed with chloride and no decrease in the slope is observed for [ HL] ,/ [S ] r = 1/ 1. For Zn (II) and Cd (II), 40% and 70%, respectively, are complexed with chloride in the aqueous phase and the slopes are 1.85 and 1.80, respectively. When the chloride concentration is 0.5 M, the percentage of chloride complexes in the aqueous phase increases; becoming 30% and 50% for copper and zinc, respectively, and, consequently, the slopes decrease compared to the slopes for 0.1 M chloride. 3.2. Metal distribution with XAD2-TOP0
resins
Metal distribution data with XAD2-TOP0 (HL/S = O/ 1) resins are plotted as log D versus pH in 0.1 M and 0.5 M NaCl in Figs. 2-4. Considering to the extraction results with XAD2-TOP0 resins, only Zn( II) is extracted from chloride media and the extraction reaction for Zn (II) ions can be defined as: Zn2++2C1-+sS,=ZnCl
2
S3-J
(9)
and the distribution of Zn (II) becomes: D=C,[S];[C1-]2
(10)
The distribution coefficient is independent of the proton concentration in the aqueous phase at constant extractant concentrations in the resin phase [S ] r and constant chloride concentrations in the aqueous phase, as can be seen in Figs. 24. In this case the plots of log D versus pH are straight lines of slope 0. Accordingly the index (2 - t) = 0, then t = 2 and the extracted species in the resin phase are species with general composition ZnCl,S,. 3.3. Determination
of the synergistic e#ect
Studies on liquid-liquid extraction systems related to the use of couples of metal extractants indicate that they can modify the extraction reactions, increasing the distribution coefficient of the metal ions from the aqueous to the resin phase. The synergistic effect produced using the combination of two extractants, HL and S, could be defined following the approach of Marcus [ 29 ] as: Do-n_,s)=&ii_ +Ds +dD
(11)
where Don_,~)--the distribution coefficient of the extractant mixture; DHL and DS = the corresponding distribution coefficients of the extractants separately under the same conditions; and dD=the synergic effect. When AD>0 there is a synergistic increase and when LID< 0 there is an antisynergistic effect. Following the approach developed previously [ 26 ] in the extraction studies of Zn (II ) , Cu (II ) and Cd (II ) from nitrate media with resins containing mixtures
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
311
of DEHPA and TOP0 (XAD2-DEHPA-TOPO ), from Eq. ( 3 ) the comparison of the experimental metal distribution value obtained with the mixture (DEHPATOPO) (DcHL,sj) with the metal distribution coefficient due to the acidic organophosphorus extractant DEHPA (&) could serve to define the synergistic or antisynergistic behaviour of the mixture of extractants. In order to study this behaviour a graphical procedure was used. In this procedure the metal distribution values for XAD2-DEHPA-TOP0 resins measured (Dons,) from chloride medium were compared with the metal distribution coefficient due to the acidic organophosphorus extractant DEHPA (DHL) with the same [ HL] r concentration. The &,_ values were calculated with the program LETAPL [301, along with the constants calculated previously for the extraction of Zn (II ), Cu (II ) and Cd (II) with XAD2-DEHPA resins, which are given in [ 19 1, taking into account the formation complexes in the aqueous phase at various NaCl concentrations [ 3 11. The experimental metal distribution values of Zn (II), Cu (II) and Cd (II) for XAD2-DEHPA-TOP0 (DoiL,sj ) and calculated metal distribution values XAD2-DEHPA (DHL) as a function of pH are plotted in Figs. 5-7. From these figures, for the case of Zn(II), extraction is practically unmodified by the presence of TOP0 and only when the ratio [ HL],/ [S 1, < 1 is it seen that there is antisynergistic effect, with a slight shift of the metal distribution values ( Dous, ) to higher pH values than the distribution values for XAD2-DEHPA resins (&IL) ). On the other hand, for Cu(I1) and Cd( II) a synergistic effect can be seen in the extraction of both metal ions with a shift in the metal distribution to higher pH values. This antisynergistic effect increases when the [ HL],/ [S 1, ratio decreases. The effect is larger for Cd (II) than for Cu (II ), as can be seen in Figs. 6 and 7 for the XAD2-DEHPA-TOP0 resins (DT 1/ 1) . No extraction of Cd (II) was observed at pH values lower than 6 for XAD2-DEHPA-TOP0 resins with a [HLlr/[Slr= l/4. 3.4. FTIR spectroscopic studies on metal extraction The FTIR spectra of the M (II)-XAD2-DEHPA-TOP0 (DT 1/ 1) complexes were recorded for the three metal ions (Zn (II), Cu (II) and Cd (II) ) and IR absorption frequencies assignments to DEHPA and TOP0 molecules are given in Tables 2-4. It can be seen that the characteristic frequency z+,~ of the P-O bond of both organophosphorus compounds, or the species formed after the interaction of both molecules [ 261, have been shifted from 1267 cm-’ to 1240 cm- ’for Zn(II), to 1230cm-’ for Cu(I1) and to 1237 cm-’ for Cd(I1). The frequency z+,~ of the P=O bond in the infrared spectra of the organophosphorus compounds reflects the charge density of the phosphoryl oxygen atom and can be regarded as a measure of its coordinative activity. For this reason, the observed shifts in P=O frequency modes after the extraction step indicate the formation of a metal complex (DEHPA-TOPO-M (II) ) in the resin phase. These metal complexes involve the coordination between the phosphoryl oxygen atom
J.L. Cortina et al. / Hydrometallurgy 37 (I 995) 301-322
312
1
3
2 PH
4 ”
D-rlo/l
log D
1
3
2
4
PH 5 DT 111 4
IogD
. I
0.1 M Cl-
3 2 1
Zn(ll)
0 3
2
1
4
PH 4.
DT 114
3 ‘.
O1
0.5 M CI-
0.1 N NO3-
. l-----l Cl- 0.1
IogD 2 .. 1”
.
0
00
0
0.5M CI-
.
0.1 M N03-
J
2
3
4
5
PH
Fig. 5. Variation in the distribution coeffkient of Zn(I1) as a function of pH at several total concentration ratios, [ HL] ,/ [S ] T,in the resin phase from 0.1 M NaN03, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XADZ-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
J.L. Cortina et al. / Hydrometaliurgy 37 (1995) 301-322
4
313
DT 19/l
.
3
0.1CI-
.
0.5MCI-
0
0.1M N03-
.
0.1CI-
.
0.5MCI-
0
0.1M NO3-
.
0.1M CI-
.
0.5MCI-
0
0.1M NO3-
logD2 1 Cu(ll)
I
OJ
2
3
4
PH
47
DT 1011
3 ”
3
PH .2
I
DT Ill
4
log D
3
K, .n”
2 1
Cu(ll) 2
4
3
PH 4
DT 114
/
3 IogD 2
fl-em
#
.
1 I
Cu(ll) 3
4
5
6
PH
Fig. 6. Variation in the distribution coeffkient of Cu (II) as a function of pH at several total concentration ratios, [ HL] ,/ [S] ~, in the resin phase from 0.1 M NaNOx, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XAD2-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
314
J.L. Cortina et al. / Hydrometallurgy
37 (1995) 301-322
4
DT 19ll 3 logD2
A
0.5 M CI-
0
0.1 M NO3-
A
0.1 M Cl-
0
0.5 M CI-
.
0.1 M N03-
1
v-
~
3.5
2.5
1.5
PH 4y
DT 10/l
3 ‘. log D 2 .. 1 ..
1
I
O1
4
3
2
1
4
I
DT l/l
3 log
??
0.1 M CI-
A
0.5 M CI-
0
0.1 M N03-
D 2
AA A
1
I
I
Cd(ll) 2
4
3
5
PH Fig. 7. Variation in the distribution coefficient of Cd( II) as a function of pH at several total concentration ratios, [ HL] ,/ [S] r, in the resin phase from 0.1 M NaN03, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XAD2-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
J. L. Cortina et al. / Hydrometallurgy Table 2 Some fundamental
3 7 (1995) 301-322
315
frequencies of DEHPA in the resins prepared
DEHPA=
XAD2-DEHPA
XAD2-DEHPA-TOP0
Assignments
2961 2861 1460 1381 1228
2961 2861
2963 2863
1382 1230
1034
1031
903
903
1379 1268 1243 1028 987 902
C-H stretching of CH, Aliphatic C-H stretching P-CH, and C-H bending C-H deformation of CH, P=O stretching (-0-P=O) P=O stretching (-C-P=O) P-O-C stretching P-O-C stretching P-O-C stretching
All frequencies in cm- ‘. a DEHPA assignments from the spectrum of DEHPA as liquid component.
Table 3 Some fundamental frequencies of TOP0 in the resins prepared TOPO”
XAD2-TOP0
XADZ-DEHPA-TOP0
Assignments
2958 2859 1457 1373 1280 1243 1145 1120
2960 2862 1456 1379 1274
2963 2863
C-H stretching of CHg Aliphatic C-H stretching P-CH1 and C-H bending C-H deformation of CH3 P=O stretching (-0-P=O) P=O stretching (-0-P=O) P=O stretching (-C-P=O) P=O stretching (-C-P=O) P-O-C stretching
1379 1268 1243
1146 1125 1028
All frequencies in cm-‘. a TOP0 assignments from the spectrum of TOP0 in Nujol.
Table 4 Some fundamental frequencies in the 1600-900 cm-’ region of DEHPA and TOP0 in XAD2DEHPA-TOP0 resins after complexation with Zn (II ) , Cu ( II) and Cd (II ) XAD2-DEHPA-TOP0 Zn(II)
XADZ-DEHPA-TOP0 Cu(II)
XADZ-DEHPA-TOP0 Cd(H)
Assignments
1240 1200 1101 1062 1029
1230 1199 1119 1060 1033
1237 1201 1114 1062 1029
P=O stretching (-0-P=O) P=O stretching (-0-P=O) P=O stretching (-C-P=O) P-O-C stretching P-O-C stretching
All frequencies in cm-‘.
J.L. Cortina et al. / Hydrometallurgy 37 (199s) 301-322
316
and the central ions in addition to the ionic bonds between oxygen and the central ions. 3.5. Evaluation of the separation factors of Zn(II), Cu(II) and Cd(II) with XAD2DEHPA- TOP0 resins The results obtained in the extraction of Zn( II), Cu( II) and Cd( II) with XAD2-DEHPA-TOP0 resins show that the presence of a second extractant produces a modification of the reactions responsible for the extraction of these metal ions by the resin phase. Furthermore, this modification, which depends on the metal ion, chloride concentration and extractant concentration ratio in the resin phase, produces the variation in the separation factors of these metal ions. For the situation where the separation of two metals is to be made a useful indication as to whether this can be achieved is given by the separation factors [ 321. In metal extraction, for practical purposes the separation factors are delined from the pHSo, or the pH at which 50% of the metal is extracted. The separation factors of Zn( II), Cu( II) and Cd( II) have been expressed in terms of dpHso from experimental curves of the metal extraction percentage (%Ex) for different metals as a function of pH. The extraction percentage of M(I1) is delined as: %Ex=$lOO
(12)
where nM,rand n t = the total moles of metal in the resin phase and total moles of metal in the aqueous phase, respectively. Taking into account the definition of the metal distribution coefficient, metal extraction percentage can be expressed as: %Ex =
D D+ (NV)
xl00
(13)
Plots of %Ex versus pH in Figs. 8 and 9 show that the extraction of M2+ increases with increasing pH. From Figs. 8 and 9 values of pHso and dpHSo for the extraction of Zn (II), Cu (II) and Cd (II) with XAD2-DEHPA-TOP0 have been calculated and are given in Tables 5 and 6. In these tables the separation pH values for which the maximum separation factors could be obtained (pH,,) are also given, together with the extraction percentages of the three metal ions for these pH values for the different resins used. Given that the differences in pHso for the couple Cu (II) /Cd (II) are small, the pH,, values are given for the couples Zn (II) /Cu (II) and Zn (II) /Cd( II). The use of mixtures of DEHPA with TOP0 improve the separation factors of these three metal ions when [ HL] ,/ [S] r G 1 and the chloride concentration in the aqueous phase is higher. As can be seen in Tables 5 and 6, XAD2-DEHPA-TOP0 resins with [ HL I,/ [S],= 1 give recoveries of Zn (II) of 95% with a contamination of Cd( II) and Cu (II) of 3% from 0.1 M NaCl aqueous solutions. On the other hand, the extraction data for a 0.5
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322 100
DT 16/l
60
. .
??
.
.
%EX
40
Y, f
J. ID.
60
I
2) .: :*’
.’
??
20
NaCI0.1M
0A 1
317
3
2
zn(ll)
??
Cu(ll)
A
cd(ll)
4
PH 100 60 ..
DTlWl
.=-
.
c 0.
. %EX
60 ..
. .’
m 8” Na Cl O.lM
M
O1
3
2
4
PH 100 60 .I
DT 111 .
60 ” %EX
. .. .
.
.
40 ”
1
NaClO.1 M
2
4
3
DT 114
. .
60 ‘1 .
20 ‘.
1
.m.
.
.
0
+
.
40 ‘.
04
5
PH
60 ‘. %Ex
*
*A
r*i
fl
_,
100.
.
. #
20 ” 0
-
.
.
s’* .
.
??
zn(ll)
??
Cu(ll)
N&l 0.1 M
_ e_,;
3
??
5
7
PH
Fig. 8. Variation in the extraction percentage (%E) of Zn ( II), Cu ( II) and Cd (II) as a function of pH at several concentration ratios, [ HL],/ [S],, in the resin phase from 0.1 M NaCl solutions.
318
J.L. Cortina et al. /HydrometaNurgy 37 (1995) 301-322
r 60
.
%Ex 40
.
??
/ .*
+
DT loll .
60
.
%Ex
m
.
Cu(ll)
9’ *
.
.
A A
.
:
20 ..=
N&l
0.5 M 4
3
2
1
??
A Cd(H)
’
/i.
I
-1 Zn(ll)
??g
40
0
4 .
??
PH 100 60
. Zn(ll)
60 %Ex
??
CU(ll)
40 A Cd(ll)
20 NaC10.5 M 0
3
1
5
7
PH DT l/4
I
60 ..
II
.
60 .d
%Ex
9 Zn(ll)
.
??
1
2
3
4
5
CU(ll)
6
PH
Fig. 9. Variation in the extraction percentage (Ok&) of Zn (II), Cu (II ) and Cd (II) as a function of pH at several concentration ratios, [ HL ] ,/ [ S] r, in the resin phase from 0.5 MNaCl solutions.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
319
Table 5 pHSo, A pH5a values and extraction ratios for pH values in which the maximum separation factor is obtained (PH.,,) for XADZDEHPA-TOP0 0.1 NaCl Resin
DT DT DT DT
19/l 10/l l/l l/4
PHSO
APH,o
%Ml/M2
(PH,,,)
Zn(II)
Cu(II)
Cd(I1)
Zn/Cd
Zn/Cu
Cd/Cu
Zn/Cd
Zn/Cu
1.87 1.95 2.21 3.92
2.85 2.88 3.66 5.19
2.95 2.89 3.80 -
1.08 0.94 1.20 -
0.98 0.93 1.03 1.27
-0.17 0.01 -0.17 -
9513 (2.2) 9513 (2.5) 95/5 (3.1) -
9515 (2.2) 9513 (2.5) 95/6 (3.0)
Table 6 PHIL, ApHSo values and extraction ratios for pH values (IMl /M2) in which the maximum separation factor is obtained (pH,,) for XADZDEHPA-TOP0 0.5 NaCl Resin
DT DT DT DT
19/l 10/l l/l l/4
PHSO
‘/oM1/M2 (PH,,, )
PHSO
Zn(I1)
Cu(I1)
Cd(I1)
Zn/Cd
Zn/Cu
Cd/Cu
Zn/Cd
Zn/Cu
2.16 2.24 2.56 3.28
3.02 3.06 3.65 5.23
3.35 3.40 4.51 -
1.21 1.12 1.95 -
0.86 0.93 1.09 1.93
0.30 0.01 0.32 -
98/5 (2.8) 9513 (2.7) 97/3 (3.5)
85/10 (2.7) 9017 (2.5) 85/10 (3.0)
M chloride medium indicate an increase in the separation factors, which allows the quantitative separation of Zn (II) from Cd (II) with XAD2-DEHPA-TOP0 resinswith [HL],/[S],< 1. This improvement in the separation factors for the couples Zn (II) /Cu (II ) and Zn (II) /Cd (II) using resins containing equimolecular mixtures of DEHPA and TOPO, or with higher concentrations of TOPO, is due to the antisynergistic effect of the mixture for Cu (II) and Cd (II), which produces the shift in the extraction of these metals to higher pH values. Furthermore, in a 0.5 M chloride medium, the complexing nature of chloride ions has an important effect on the extraction of Cu (II) and Cd (II), while the extraction of Zn (II) is practically unaffected.
4. Discussion
The resins containing mixtures of DEHPA and TOP0 (XAD2-DEHPATOPO), show good performance in the extraction of Zn (II ), Cu (II ) and Cd (II ) from chloride media. The metal extraction shows a dependence on pH, with a general trend towards higher extraction as the pH increases and the chloride concentration decreases. The graphical treatment of the data by the slope analysis method gives us some information about the composition of the metal complexes extracted by the resin
320
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
phase. Using slope analysis of the lines log D versus pH for [ HL] ,/ [S ] r > 10, the predominant species in the resin phase are species with t=O and the extraction of the above metal ions involves the formation of species with the same composition as in the case of resins containing DEHPA as a single component (ML,(HL),) [ 191. In the case of [ HL] ,/ [S ] r < 10 the slopes of the functions log D/pH decrease from 2 to 0.5 for resins with ratios of [ HL] ,/ [ S ] r < l/4 and the extraction of the above metal ions involves the formation of species with general composition ML,,_,, (Cl),S,( HL),,. On the other hand, the slope values decrease with increasing chloride concentration in the aqueous phase and depend on the metal ion. The slopes decrease in the order Cd (II) > Zn (II) > Cu (II), which is the order of increasing stability of the metal ion complexes in the aqueous phase. The extraction behaviour of resins containing TOP0 as a single component (XAD2-TOPO) indicates that although Cu( II) and Cd( II) are not extracted Zn (II) is extracted. Its extraction independent of the pH but dependent upon the chloride concentration, with a general trend that the higher the chloride concentration, the higher is the extraction. The graphical treatment of the data by the slope analysis method indicates that the extraction of Zn(I1) can be explained assuming the formation of species with a composition ZnCl$,. The FAIR spectroscopic studies of the XAD2-DEHPA-TOPO-M (II) resin complexes indicate the existence of these metal extractant complexes in the resin phase (Table 4). The expected structure of the DEHPA-M (II) complexes is similar to those proposed for the same systems in nitrate media [ 261. Concerning the extraction selectivity of XAD2-DEHPA-TOP0 resins towards Zn (II), Cu (II) and Cd (II) from a NaCl medium, some differences have been found in comparison with the selectivity of DEHPA in XAD2-DEHPA resins [ 191. From Figs. 8 and 9 the extraction ability of XAD2-DEHPA-TOP0 resins decreases as a function of pH in the order Zn (II) > Cd (II) > Cu (II). However, the extraction ability of DEHPA in organic solvents decreases as a function of pH in the order Zn (II) > Cu (II) > Cd (II ) . This different behaviour observed in the separation factors of the three metal ions, especially for Cu (II) and Cd( II), shows a change in the extraction ability of DEHPA molecules adsorbed in the macroporous support by the presence of TOP0 in the resin phase and chloride in the aqueous phase. This fact seems to indicate that the extraction reactions can be explained in terms of a competitive process between the polymeric network, the extractant molecules, the metal and chloride ions, opening a possible new way of modifying the separation factors depending on the polymer structure. Similar results were obtained previously with SIR prepared by direct adsorption of mixtures of DEHPA and TOP0 molecules into Amberlite XAD2 resin [ 261. The separation factors for XAD2-DEHPA-TOP0 resins in the extraction of these metal ions depends on the DEHPA/TOPO ratio in the mixture, as can be seen in Table 5. The separation factors, expressed in terms of pHsO differences, indicate higher separation factors with decreasing [HL],/ [ S], ratio. In particular, when the ratio is 1/O the maximum separation factors are obtained. As can
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
321
be seen in Table 5 with XAD2-DEHPA-TOP0 (HL/S = 1/ 1) resins a quantitative separation of Zn (II) from Cd (II) can be obtained. As a general rule, the use of resins containing mixtures of DEHPA and TOP0 in a chloride medium has a great influence in the extraction of Cu (II) and Cd ( II), shifting its extraction to higher pH values and, as a consequence, increasing the separation factors towards Zn (II).
Acknowledgement
This work was supported by CICYT Project MAT 93-62 12 (Ministerio de Educacion y Ciencia de Espafia) . The authors are indebted to Prof. A. Warshawsky, Weizmann Institute of Science, Israel, for his help and suggestions for this work and to Dr. Maria Martinez for helping in PTIR spectroscopic measurements.
References [ I] Grossi, G. and Cecille, L., In: L. Cecille, M. Casarci and L. Pietrelli (Editors), New Separation Chemistry Techniques for Radioactive Waste and other Specific Applications. Elsevier, Amsterdam(1991),pp. 11-19. [2]Tavlarides, L.L., Bae, J.H. and Lee, C.K., Sep. Sci Technol., 22 (1987): 581-617. [ 3]Cloete, F.L.D., In: D. Naden and M. Streat (Editors), Ion Exchange Technology. Sot. Chem. Ind., Ellis Horwoord, Chichester ( 1984). [4]Warshawsky, A., Trans. Inst. Min. Metal.,0 83 (1974): 101-104. [ 51Warshawsky, A., Berkovitz, H. and Kalir, R., In: M. Streat (Editor), The Theory and Practice of Ion Exchange. Sot. Chem. Ind., London ( 1976). [6]Kroebel, R. and Meyer, A., West German Pat. Appl., 2,162,951 ( 1971). [ 7]Kauczor, H.W. and Meyer, A., Hydrometallurgy, 3 (1978): 65-73. [ 81 Warshawsky, A., In: J.A. Marinsky and Y. Marcus (Editors), Ion Exchange and Solvent Extraction. Marcel Dekker, New York, Vol. 8 ( 198 1 ), pp. 229-28 1. [ 9]Yoshizuka, K., Sakomoto, Y., Baba, Y. and Ionue, K., Distribution equilibria in the adsorption of colbalt(I1) and nickel( II) on Levextrel resin containing Cyanex 272. Hydrometallurgy, 23 (1990): 309-318. [ lO]Gonzalez-Luque, S. and Streat, M., Hydrometallurgy, 11 (1983): 207-225. [ 1 l]Muscatello, A.C. and Navratil, J.D., J. Radioanal. Nucl. Chem., 128(6) (1988): 463-477. [ 12]Glatz, J.P., Bokelund, H. and Ougier, M., J. Less-Common Met., 122 (1986): 419-423. [ 13]Porta, V., Mentasti, E., Sarzanini, C. and Gennaro, M.C., Talanta, 35 (1988): 167-171. [ 14]Gennaro, MC., Mentasti, E. and Sarzanini, C., Talanta, 33 (1986): 620-622. [ 1S]Louis, R.E. and Duyckaerts, G., J. Radioanal. Nucl. Chem., 81 ( 1984): 305-315. [ 16]Louis, R.E. and Duyckaerts, G., J. Radioanal. Nucl. Chem., 90 (1985): 105-l 12. [ 17]Koshima, H., Anal. Sci., 2 (1986): 255-260. [ 18]Cortina, J.L., Miralles, N., Aguilar, M. and Sastre, A.M., In: T. Sekine and S. Kusakabe (Editors), Solvent Extraction 1990. Elsevier, Amsterdam (1992), pp. 159-167. [ 19]Cortina, J.L., Miralles, N., Aguilar, M. and Sastre, A.M., Solvent Extr. Ion Exch., 12 (1994): 371-391. [ZO]Cortina, J.L., Miralles, N., Sastre, A.M., Aguilar, M., Profumo, A. and Pesavento, M., Reactive Polymers, 18 (1992): 67-75. [ 21 ]Cortina, J.L., Miralles, N., Sastre, A.M., Aguilar, M., Profumo, A. and Pesavento, M., Reactive Polymers, 21 (1993): 89-101.
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[22]Cortina, J.L., Miralles, N., Aguilar, M. and Warshawsky, A., In: D.H. Longsdail and M.J. Slater (Editors), Solvent Extraction in the Process Industries. Elsevier Applied Science, Essex, UK (1993), pp. 962-969. [23]Cortina, J.L., Miralles, N., Aguilar, M. and Sastre, A.M., Extraction studies of Zn(II), Cu(I1) and Cd(I1) with impregnated and Levextrel resins containing di( 2-ethylhexyl)phophoric acid. Hydrometallurgy, 36(2) (1994): 129-140. [ 24]Dalton, R.F., Burgess, A. and Quan, P.M., ACORGA ZNXSO-a new selective reagent for the solvent extraction of zinc from chloride leach solutions. Hydrometallurgy, 30 ( 1992): 385-400. [25]Reinhardt, H., In: D.H. Longsdail and M.J. Slater (Editors), Solvent Extraction in the Process Industries. Elsevier Applied Science, Essex, UK (1993), pp. 1625-1632. [26]Cortina, J.L., Miralles, N., Aguilar, M. and Sastre, A.M., Solvent Extr. Ion Exch. (1994) (submitted). [27]Kolosky, M., Vialle, J. and Cotel, T., J. Chromatogr., 299 ( 1984): 436-444. [ 28]Marcus, Y., In: J.A. Marinsky (Editor), Ion Exchange, A series of Advances. Marcel Dekker, New York, Vol. 1, Chap. 3 ( 1966). [ 29]Marcus, Y., Kertes, A.S., In: Solvent Extraction and Ion Exchange of Metal Complexes. Wiley Interscience, New York ( 1969), p. 8 16. [ 30]Cortina, J.L., Ph.D. Thesis, Univ. Barcelona (1992). [ 3 1 ] Smith, R.M. and Martell, A.E., In: Critical Stability Constants. Plenum Press, New York, Vol. 4 (1976), pp. 105-108. [32]Ritcey, GM. and Ashbrook, A.M., Solvent Extraction. Principles and Applications to Process Metallurgy. (Process Metallurgy, 1. ) Elsevier, Amsterdam, Part 1 ( 1984), 362 pp.
Hydrometallurgy 37 (1995) 301-322
Solid-liquid extraction studies of Zn ( II ) , Cu ( II ) and Cd (II) from chloride media with impregnated resins containing mixtures of organophosphorus compounds immobilized on to Amberlite XAD2 J.L. Cortina*, N. Miralles, A.M. Sastre, M. Aguilar Chemical Engineering Department, ETSEIB, UniversitatPolitknica de Catalunya, Diagonal 647, E-08028, Barcelona, Spain
Received 17 December 1993; revised version accepted 23 April 1994
Abstract The solid-liquid extraction of Zn (II), Cu (II) and Cd (II) from chloride medium at 0.1 and 0.5 M ionic strength and 25°C by impregnated resins containing mixtures of di (2ethylhexyl)phosphoric acid (DEHPA = HL) and tri-n-octylphosphine oxide (TOPO= S ) was studied. Impregnated resins containing TOP0 and mixtures of DEHPA and TOPO, by direct adsorption of both extractants into a styreneldivinylbenzene macroporous support, Amberlite XAD2, using the dry impregnation method, were prepared. The distribution coefficients of Zn (II ), Cu (II) and Cd (II) were determined as a function of pH, chloride concentration in the aqueous phase and extractant concentration (DEHPA and TOPO) in the resin phase and the data were analyzed graphically using the slope analysis method in terms of synergistic effect. Analysis of the results showed that the extraction of these metal ions with resins containing mixtures of DEHPA and TOP0 (XAD2-DEHPA-TOPO) can be explained assuming the formation of metal complexes in the resin phase with a general composition MCl,L,( HL)& where p, t, s and q take different values depending on the metal and the extractant concentration ratio in the resin phase ( [HL],/ [S],). Finally, a comparison between the extraction of Zn(II), Cu( II) and Cd(I1) in terms of the separation factors with Amberlite XAD2-DEHPA-TOP0 resins and using Amberlite XAD2-DEHPA resins was made.
* Author for correspondence. 0304-386X/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO304-386X(94)00029-3
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1. Introduction The need for more specific systems for dilute metal recovery, from both ecological and economic aspects, has lead to the development of new chemical techniques. In this sense, new ion exchange resins, solid adsorbents and metal extractants used in solvent extraction processes have been developed for different applications [ 11. Although the use of ion exchangers in industry implies simple equipment, easy operation and no problems with phase disengagement and reagent losses [ 2,3], these resins have limited potential for the recovery of transition metals, due to their selectivity. A bridge between solvent extraction and ion exchange was the introduction in hydrometallurgical applications of Solvent Impregnated Resins (SIR) by Warshawsky [ 45 1. In this approach, the extractant molecules are adsorbed onto a high surface area, macroporous polymeric support to produce a SIR. These are interesting because they maintain the specificity of the metal extractants used in liquid-liquid extraction systems and avoid chemical functionalization of the polymeric supports. A different, more limited, approach was presented by Kroebel and Meyer [ 6,7 1, who proposed as impregnation procedure the polymerization of styrene and divinylbenzene in the presence of the extractant to produce Levextrel resins. Since these pioneering works, the development and application of these systems in metal extraction processes has been intensively investigated for the purpose of hydrometallurgical [ 8- 12 ] and analytical [ 13- 17 ] applications for separation and recovery processes. Recent work in this laboratory has shown that metal transition ions are efficiently extracted from nitrate solutions by solvent impregnated resins containing organophosphorus extractants adsorbed into Amberlite XAD2. The organophosphorus compounds used have been di (2-ethylhexyl)phosphoric acid (DEHPA ) [ 18,191, di (2,4,4_trimethylpentyl)-phosphinic acid (DTMPPA) [ 20,2 1] and O’methyl-dihexyl-phosphine oxide O’hexyl-2-ethyl phosphoric acid [ 22 1. As a part of this series, the extraction of the above metal ions with impregnated resins containing DEHPA and Levextrel Resins (Lewatit 1026 Oc), containing DEHPA as active component [ 231, have been thoroughly studied and the systems have been described in terms of species and extraction constants, to establish, as in liquid-liquid extraction systems, if the speciation in the resin phase could provide certain differences in composition which could be utilized for the design of separation methods. The increasing concern towards the recovery of transition metal ions from such secondary-grade sources, such as electronic scrap, spent catalysts, anode slimes and industrial waste waters [ 24,25 1, usually from liquid eMuents with high chloride concentrations, requires the development of advanced separation systems. Earlier work in this laboratory has shown that impregnated resins containing mixtures of DEHPA and TOP0 with different [ HL] ,/ [S ] r can be prepared and they have shown a high efficiency in the extraction of Zn (II), Cu( II) and Cd( II) from nitrate solutions [ 261. Furthermore these studies in nitrate solutions indi-
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
303
cate that the presence of a second extractant produces a large increase in the separation factors in comparison with the previously used resins containing DEHPA [191. In this context, the objective of the present work is to investigate the solidliquid extraction of Zn (II), Cu (II) and Cd (II) from aqueous chloride solutions with solvent-impregnated resins containing mixtures of DEHPA and TOPO. This work presents fundamental studies on the extraction of Zn (II), Cu (II) and Cd( II) from 0.1 M and 0.5 M NaCl at 25 “C with impregnated resins containing mixtures of DEHPA and TOPO. In addition, we examine the extraction in terms of the synergistic or antisynergistic effect of the extractant mixture in the metal extraction reactions by graphical methods. Finally, a comparison between the extraction behaviour of XAD2-DEHPA-TOP0 resins from chloride media and nitrate media was performed.
2. Experimental 2. I. Reagents and solutions The di (2-ethylhexyl ) phosphoric acid (DEHPA= HL) was supplied by BDH (UK). Its purity was determined by potentiometric titration in an 75% ethanol solution of the acid with 0.1 M NaOH. The purity was found to be 98.2%. Tri-noctylphosphine oxide (TOP0 = S), supplied by Merck p.a., was used without further purification. Stock solutions of Zn (II), Cu (II) and Cd (II) ( 1 g*drnp3 ) were prepared by dissolving the corresponding salts (Merck a.r. grade) in water. Sodium chloride, sodium hydroxide and hydrochloric acid (Merck a.r. grade), were used for the preparation of the different solutions and potassium bromide (Merck, for spectroscopy) for the preparation of the pressed discs for FTIR. Amberlite XAD-2 Resin, supplied by Rohm and Haas, size 0.3-0.9 mm, was used. XAD-2 was kept in contact for 12 h with a 50% methanol-water solution containing 4 M HCl in order to eliminate impurities. Ethanol and acetone, supplied by Probus p.a, were used without further purification and methanol was used in the chromatographic system. 2.2. Impregnation process
The XAD2-DEHPA-TOP0 and XAD2-TOP0 impregnated resins were prepared according to the modified version of the dry impregnation method, as described previously [ 201. The amount of DEHPA ( [ HL],) and TOP0 ( [S],) impregnated was evaluated after washing a known amount of resin with ethanol, which completely elutes both extractants, and subsequent determination of the ligands. For resins containing mixtures of DEHPA and TOPO, after elution of both extractants with ethanol, the DEHPA content was determined by NaOH potentiometric titration. The TOP0 content was determined by spectrophotometry
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
304
measurements at 230 nm after the chromatographic separation of TOP0 from DEHPA. The separation process was based on the procedure developed by Kolosky et al. [ 271 in the determination of TOPO. For resins containing TOPO, after the elution of TOP0 with ethanol, the TOP0 content was determined by spectrophotometry measurements at 230 nm with a Shimadzu UV-240 spectrophotometer. The impregnated resins containing mixtures of both organophosphorus extractants and the concentration in both derivatives are described in Table 1. 2.3. Metal distribution with xAD2-DEHPA-TOP0
and XAD2-TOP0
resins
The extraction of Zn( II), Cu( II) and Cd( II) was carried out with batch experiments at 25 ‘C. Samples of 0.2 g of XAD2-DEHPA-TOP0 and XAD2-TOP0 resins were mixed mechanically in special glass-stoppered tubes with a 20 ml of a 0.1 M or 0.5 M aqueous solution of (M 2+ H+, Na+ )Cl- until equilibrium was reached. According to Fig. 1, where the disthbution of Zn (II), Cu (II) and Cd( II) Table 1 Impregnated
resins prepared (XAD2-DEHPA-TOPO)
Impregnated
resin
iSIr
[HLI, (molekg-‘)
DT 19/l DT 10/l DT l/l DT l/4 DTO/l
0.059 0.109 0.631 0.608 0.494
1.085 1.082 0.646 0.158
. . *
0
. .
100
Wlr/[Slr
(molekg-*)
.
.
19/l 10/l l/l l/4 O/l
: .
.
200
300
400
t(min)
Fig. 1. Variation in the distribution coefficients of Zn( II), Cu (II ) and Cd ( II ) as a function of time at fixed pH values for XAD2-DEHPA-TOP0 resins (HL/S = 1/ 1) .
J.L. Cortina et al. /Hydrometallurgy 37(1995) 301-322
305
as a function of time at constant pH are plotted, a time of 2 h was enough to reach equilibrium. After phase separation with a high-speed centrifuge, the equilibrium pH was measured using a Metrohm AG 9 100 combined electrode connected to a CRISON digital pH meter, model Digilab 517. The metal content was determined by atomic absorption spectrophotometry. A Perkin-Elmer 2380 AAS with air-acetylene flame was used. 2.4. FTIR spectroscopic studies FTIR spectra of metal complexes of Zn (II), Cu (II) and Cd (II) with XAD2DEHPA-TOP0 (DT 1/ 1) resins were recorded with a BOMEM MB 120 Fourier Transform Infrared Spectrometer (4000-700 cm- ’) ( 32 co-added interferograms were scanned at 2 cm-’ resolution).
3. Results and discussion 3.1. Treatment of the distribution data The distribution of Zn (II), Cu (II) and Cd( II) between the resin phase containing DEHPA and TOP0 (XAD2-DEHPA-TOPO) and the aqueous phase can be obtained directly from:
DWLS) =
[MUI) lr
(1)
[M(II)]
where [M ( II ) ] r = the total concentration of M* + in the resin phase (in mol. kg- ’) , and [ M( II) ] = the total concentration of M*+ in the aqueous phase. Hence, from the mass balance, Eq. ( 1) can be expressed as: D (HL,S)
=
([M(II)l,-[M(II)l)x(‘C/lm,) [MUI) 1
(2)
where [M (II) ] f= the initial total concentration of metal in the aqueous phase; V= the volume of the aqueous phase; mr= the mass of dry impregnated resin. 3.2. Metal distribution with XADI-DEHPA-TOP0
resins
Metal distribution data with XAD2-DEHPA-TOP0
resins are plotted as log
D versus pH in 0.1 M and 0.5 A4 Cl- in Figs. 2-4. These figures show that the
distribution functions are straight lines with a slope between 2 and 1, depending on the ratio [ HL],/ [S],. Accordingly, the extraction of these metal ions with XAD2-DEHPA-TOP0 resins can be described with the following general reaction: M2++(2+q-t)HL,+sS,+tC1-=MC1,L,2_1,S,(HL),,+(2-t)H+
(3)
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J.L. Cortina et al. / Hydrometailurgy 37 (1995) 301-322
5
Zn(ll)
4
A
.
0
3 log D
*
urn
??
n
u
2
8.
?? *
?? *
1
..&.d..
A A
AA
A
A ??
0.
.
0,l M NaCl
0 2
4
3
PH 3
Cu(ll) ’ DT 19/l
2 A DT 1011
log D ??
DT l/l
1 A DTll4
0.1 M
NaCl
0 1
2
4
3
5
6
PH Cd(ll)
4
2
3
4
5
PH Fig. 2. Variation in the distribution coeffkients of Zn(II), Cu(II) and Cd(I1) as a functionof PH and total metal concentration in 0.1 M NaCl for XAD2-DEHPA-TOP0 resins. [Zn(II) I = 10 pmol.dm-3, [Cu(II)] =64pmol.dmw3 and [Cd(U)] = 10pmol~dm-3.
Polynuclear metal (II) complexes in the resin phase were excluded because plots of log D versus pH are independent of metal concentration. Following the approach developed by Marcus [ 281 in the study of metal extraction reactions in ion exchangers, the corresponding equilibrium constant of the extraction process (K) is defined as:
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
307
??
DT 19/l
A DT 10/l
logD2.
??
DT111
‘ DT1/4
NaCl 1
0.1 M
--I
PH 3 Cu(ll) = DT19/1
A
d
2
A
log D 1
**
A
*/=A“‘k“‘ 0.1 M
A DT 10/l ??
DT111
‘ DTll4
NaCl
0 1
2
3
4
5
6
PH
1 CW
I
0 A
3 .. /IAm
.*...
------I .
.
logD2--
DT 19/l
A DT 10/l
EQB
. . .
~ !*~Tlit
j
.
1 ‘.
e 0.1 M 2
3
4
NaCl 5
PH Fig. 3. Variation in the distribution coeffkients of Zn( II), Cu(II) and Cd( II) as a function ofPH and total metal concentration in 0.1 M NaCl for XAD2-DEHPA-TOP0 resins. [Zn(II) I =20 pmol*dm-3, [Cu(Il)]=32ymol~dm-3and [Cd(II)]=18pmol~dm-3.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
308
2
3
5
4
PH 3-
Cu(ll) .
DTlQfi
2 .. A DTlO/l
log D ?? DTlll
1 ” A DT114
0.5 M NaCl 0
3
1
5
PH 3-
Cd(ll) PhA R
2 ‘. log
D
. DT 19/l A DT 1011
88 .
1 .. f
.
.
??
??
DTl/l
El 0.5 M NaCl
04 2
3
4
5
PH Fig. 4. Variation in the distribution coeffkients of Zn( II), Cu( II) and Cd(U) as a function of pH and total metal concentration in 0.5 M NaCl for XADZ-DEHPA-TOP0 resins. [Zn(II) = 10 pmol*dm-‘, [C~(II)]=64ymol.dm-~and [Cd(II)]=10pmol~dm-3.
J.L. Cortina et al. /Hydrometallurgy 37 (1995) 301-322
K=aMCI’L~~-‘~SS(HL)ePH+
309
(2--t)
aM2+aj.&?rq-f)a~,a&,-
(4)
where ai and ai,= the activity of the i species in the aqueous and the resin phase, respectively. Eq. (4) can be written as:
where yj and yi,r= the activity coefficients of the i species in the aqueous and the resin phase, respectively. Rearranging Eq. (5 ), the following expression may obtained: K=
Blqtsr
(6)
where j?zqts= the stoichiometric equilibrium constant for the extraction reaction: (7) and r= the term containing all the activity coefficients. As a preliminary hypothesis the term f can be expected to remain constant as long as the ionic strength in the aqueous phase is constant and the variations in resin phase concentrations are small. In order to relate the distribution of the metal with the metal composition in the resin phase the formation of a simplest species may be assumed. So, according to previous results, if only one species of the type ML,,_ tj (Cl) f (HL), is formed the distribution coefficient for ( M2+ ) becomes: ~=ZqZtZJ?;qtJHL]~2+q-“[S]~[Cl-]t[H+]-(2-’)
(8)
From Eq. (8 ) it can be seen that, for a given constant ionic medium and [ HL] ,/ [S ] r ratio, the variation in log D as a function of pH allows the determination of the slope index (2 - t ) for the extracted species in the resin phase. The dependence of the distribution coefficient on proton ion concentration in the aqueous phase at constant extractant concentration in the resin phase can be seen in Figs. 2-4, where the experimental data obtained in the distribution equilibrium studies are plotted as a function of pH. The graphical treatment of the experimental data (plotted in Figs. 2-4) indicates that for [ HL],/ [S],>10, the predominant species in the resin phase are species with t= 0.This means that in this condition the extraction of these metal ions involves the formation of species with the same composition as in the case of resins containing DEHPA as a single component ( ML2 (HL),) [ 191 in both the media studied. In the case of [ HL],/ [S],< 10, the slope of the functions log D versus pH decreases from 2 to 0.5 for resins with ratios [ HL],/ [S],< l/4. These fractional values between 2 and 0.5, indicate that species with t values different to 0 are extracted and the extraction of the above metal ions involves the formation of species with a general composition ML,,_,, (Cl),S,(HL),,.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
310
On the other hand, the slope values decrease with increasing chloride concentration in the aqueous phase and depend on the metal ion. The decrease of the slope follows the order Cd (II ) > Zn (II) > Cu (II). This agrees with the predominance of chloride complexes in the aqueous phase. For Cu (II) in 0.1 M chloride, less than 8% of copper is complexed with chloride and no decrease in the slope is observed for [ HL] ,/ [S ] r = 1/ 1. For Zn (II) and Cd (II), 40% and 70%, respectively, are complexed with chloride in the aqueous phase and the slopes are 1.85 and 1.80, respectively. When the chloride concentration is 0.5 M, the percentage of chloride complexes in the aqueous phase increases; becoming 30% and 50% for copper and zinc, respectively, and, consequently, the slopes decrease compared to the slopes for 0.1 M chloride. 3.2. Metal distribution with XAD2-TOP0
resins
Metal distribution data with XAD2-TOP0 (HL/S = O/ 1) resins are plotted as log D versus pH in 0.1 M and 0.5 M NaCl in Figs. 2-4. Considering to the extraction results with XAD2-TOP0 resins, only Zn( II) is extracted from chloride media and the extraction reaction for Zn (II) ions can be defined as: Zn2++2C1-+sS,=ZnCl
2
S3-J
(9)
and the distribution of Zn (II) becomes: D=C,[S];[C1-]2
(10)
The distribution coefficient is independent of the proton concentration in the aqueous phase at constant extractant concentrations in the resin phase [S ] r and constant chloride concentrations in the aqueous phase, as can be seen in Figs. 24. In this case the plots of log D versus pH are straight lines of slope 0. Accordingly the index (2 - t) = 0, then t = 2 and the extracted species in the resin phase are species with general composition ZnCl,S,. 3.3. Determination
of the synergistic e#ect
Studies on liquid-liquid extraction systems related to the use of couples of metal extractants indicate that they can modify the extraction reactions, increasing the distribution coefficient of the metal ions from the aqueous to the resin phase. The synergistic effect produced using the combination of two extractants, HL and S, could be defined following the approach of Marcus [ 29 ] as: Do-n_,s)=&ii_ +Ds +dD
(11)
where Don_,~)--the distribution coefficient of the extractant mixture; DHL and DS = the corresponding distribution coefficients of the extractants separately under the same conditions; and dD=the synergic effect. When AD>0 there is a synergistic increase and when LID< 0 there is an antisynergistic effect. Following the approach developed previously [ 26 ] in the extraction studies of Zn (II ) , Cu (II ) and Cd (II ) from nitrate media with resins containing mixtures
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
311
of DEHPA and TOP0 (XAD2-DEHPA-TOPO ), from Eq. ( 3 ) the comparison of the experimental metal distribution value obtained with the mixture (DEHPATOPO) (DcHL,sj) with the metal distribution coefficient due to the acidic organophosphorus extractant DEHPA (&) could serve to define the synergistic or antisynergistic behaviour of the mixture of extractants. In order to study this behaviour a graphical procedure was used. In this procedure the metal distribution values for XAD2-DEHPA-TOP0 resins measured (Dons,) from chloride medium were compared with the metal distribution coefficient due to the acidic organophosphorus extractant DEHPA (DHL) with the same [ HL] r concentration. The &,_ values were calculated with the program LETAPL [301, along with the constants calculated previously for the extraction of Zn (II ), Cu (II ) and Cd (II) with XAD2-DEHPA resins, which are given in [ 19 1, taking into account the formation complexes in the aqueous phase at various NaCl concentrations [ 3 11. The experimental metal distribution values of Zn (II), Cu (II) and Cd (II) for XAD2-DEHPA-TOP0 (DoiL,sj ) and calculated metal distribution values XAD2-DEHPA (DHL) as a function of pH are plotted in Figs. 5-7. From these figures, for the case of Zn(II), extraction is practically unmodified by the presence of TOP0 and only when the ratio [ HL],/ [S 1, < 1 is it seen that there is antisynergistic effect, with a slight shift of the metal distribution values ( Dous, ) to higher pH values than the distribution values for XAD2-DEHPA resins (&IL) ). On the other hand, for Cu(I1) and Cd( II) a synergistic effect can be seen in the extraction of both metal ions with a shift in the metal distribution to higher pH values. This antisynergistic effect increases when the [ HL],/ [S 1, ratio decreases. The effect is larger for Cd (II) than for Cu (II ), as can be seen in Figs. 6 and 7 for the XAD2-DEHPA-TOP0 resins (DT 1/ 1) . No extraction of Cd (II) was observed at pH values lower than 6 for XAD2-DEHPA-TOP0 resins with a [HLlr/[Slr= l/4. 3.4. FTIR spectroscopic studies on metal extraction The FTIR spectra of the M (II)-XAD2-DEHPA-TOP0 (DT 1/ 1) complexes were recorded for the three metal ions (Zn (II), Cu (II) and Cd (II) ) and IR absorption frequencies assignments to DEHPA and TOP0 molecules are given in Tables 2-4. It can be seen that the characteristic frequency z+,~ of the P-O bond of both organophosphorus compounds, or the species formed after the interaction of both molecules [ 261, have been shifted from 1267 cm-’ to 1240 cm- ’for Zn(II), to 1230cm-’ for Cu(I1) and to 1237 cm-’ for Cd(I1). The frequency z+,~ of the P=O bond in the infrared spectra of the organophosphorus compounds reflects the charge density of the phosphoryl oxygen atom and can be regarded as a measure of its coordinative activity. For this reason, the observed shifts in P=O frequency modes after the extraction step indicate the formation of a metal complex (DEHPA-TOPO-M (II) ) in the resin phase. These metal complexes involve the coordination between the phosphoryl oxygen atom
J.L. Cortina et al. / Hydrometallurgy 37 (I 995) 301-322
312
1
3
2 PH
4 ”
D-rlo/l
log D
1
3
2
4
PH 5 DT 111 4
IogD
. I
0.1 M Cl-
3 2 1
Zn(ll)
0 3
2
1
4
PH 4.
DT 114
3 ‘.
O1
0.5 M CI-
0.1 N NO3-
. l-----l Cl- 0.1
IogD 2 .. 1”
.
0
00
0
0.5M CI-
.
0.1 M N03-
J
2
3
4
5
PH
Fig. 5. Variation in the distribution coeffkient of Zn(I1) as a function of pH at several total concentration ratios, [ HL] ,/ [S ] T,in the resin phase from 0.1 M NaN03, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XADZ-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
J.L. Cortina et al. / Hydrometaliurgy 37 (1995) 301-322
4
313
DT 19/l
.
3
0.1CI-
.
0.5MCI-
0
0.1M N03-
.
0.1CI-
.
0.5MCI-
0
0.1M NO3-
.
0.1M CI-
.
0.5MCI-
0
0.1M NO3-
logD2 1 Cu(ll)
I
OJ
2
3
4
PH
47
DT 1011
3 ”
3
PH .2
I
DT Ill
4
log D
3
K, .n”
2 1
Cu(ll) 2
4
3
PH 4
DT 114
/
3 IogD 2
fl-em
#
.
1 I
Cu(ll) 3
4
5
6
PH
Fig. 6. Variation in the distribution coeffkient of Cu (II) as a function of pH at several total concentration ratios, [ HL] ,/ [S] ~, in the resin phase from 0.1 M NaNOx, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XAD2-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
314
J.L. Cortina et al. / Hydrometallurgy
37 (1995) 301-322
4
DT 19ll 3 logD2
A
0.5 M CI-
0
0.1 M NO3-
A
0.1 M Cl-
0
0.5 M CI-
.
0.1 M N03-
1
v-
~
3.5
2.5
1.5
PH 4y
DT 10/l
3 ‘. log D 2 .. 1 ..
1
I
O1
4
3
2
1
4
I
DT l/l
3 log
??
0.1 M CI-
A
0.5 M CI-
0
0.1 M N03-
D 2
AA A
1
I
I
Cd(ll) 2
4
3
5
PH Fig. 7. Variation in the distribution coefficient of Cd( II) as a function of pH at several total concentration ratios, [ HL] ,/ [S] r, in the resin phase from 0.1 M NaN03, 0.1 M NaCl and 0.5 M NaCl solutions. The lines correspond to XAD2-DEHPA resins and were calculated by the LETAPL program using the constants given in [ 191.
J. L. Cortina et al. / Hydrometallurgy Table 2 Some fundamental
3 7 (1995) 301-322
315
frequencies of DEHPA in the resins prepared
DEHPA=
XAD2-DEHPA
XAD2-DEHPA-TOP0
Assignments
2961 2861 1460 1381 1228
2961 2861
2963 2863
1382 1230
1034
1031
903
903
1379 1268 1243 1028 987 902
C-H stretching of CH, Aliphatic C-H stretching P-CH, and C-H bending C-H deformation of CH, P=O stretching (-0-P=O) P=O stretching (-C-P=O) P-O-C stretching P-O-C stretching P-O-C stretching
All frequencies in cm- ‘. a DEHPA assignments from the spectrum of DEHPA as liquid component.
Table 3 Some fundamental frequencies of TOP0 in the resins prepared TOPO”
XAD2-TOP0
XADZ-DEHPA-TOP0
Assignments
2958 2859 1457 1373 1280 1243 1145 1120
2960 2862 1456 1379 1274
2963 2863
C-H stretching of CHg Aliphatic C-H stretching P-CH1 and C-H bending C-H deformation of CH3 P=O stretching (-0-P=O) P=O stretching (-0-P=O) P=O stretching (-C-P=O) P=O stretching (-C-P=O) P-O-C stretching
1379 1268 1243
1146 1125 1028
All frequencies in cm-‘. a TOP0 assignments from the spectrum of TOP0 in Nujol.
Table 4 Some fundamental frequencies in the 1600-900 cm-’ region of DEHPA and TOP0 in XAD2DEHPA-TOP0 resins after complexation with Zn (II ) , Cu ( II) and Cd (II ) XAD2-DEHPA-TOP0 Zn(II)
XADZ-DEHPA-TOP0 Cu(II)
XADZ-DEHPA-TOP0 Cd(H)
Assignments
1240 1200 1101 1062 1029
1230 1199 1119 1060 1033
1237 1201 1114 1062 1029
P=O stretching (-0-P=O) P=O stretching (-0-P=O) P=O stretching (-C-P=O) P-O-C stretching P-O-C stretching
All frequencies in cm-‘.
J.L. Cortina et al. / Hydrometallurgy 37 (199s) 301-322
316
and the central ions in addition to the ionic bonds between oxygen and the central ions. 3.5. Evaluation of the separation factors of Zn(II), Cu(II) and Cd(II) with XAD2DEHPA- TOP0 resins The results obtained in the extraction of Zn( II), Cu( II) and Cd( II) with XAD2-DEHPA-TOP0 resins show that the presence of a second extractant produces a modification of the reactions responsible for the extraction of these metal ions by the resin phase. Furthermore, this modification, which depends on the metal ion, chloride concentration and extractant concentration ratio in the resin phase, produces the variation in the separation factors of these metal ions. For the situation where the separation of two metals is to be made a useful indication as to whether this can be achieved is given by the separation factors [ 321. In metal extraction, for practical purposes the separation factors are delined from the pHSo, or the pH at which 50% of the metal is extracted. The separation factors of Zn( II), Cu( II) and Cd( II) have been expressed in terms of dpHso from experimental curves of the metal extraction percentage (%Ex) for different metals as a function of pH. The extraction percentage of M(I1) is delined as: %Ex=$lOO
(12)
where nM,rand n t = the total moles of metal in the resin phase and total moles of metal in the aqueous phase, respectively. Taking into account the definition of the metal distribution coefficient, metal extraction percentage can be expressed as: %Ex =
D D+ (NV)
xl00
(13)
Plots of %Ex versus pH in Figs. 8 and 9 show that the extraction of M2+ increases with increasing pH. From Figs. 8 and 9 values of pHso and dpHSo for the extraction of Zn (II), Cu (II) and Cd (II) with XAD2-DEHPA-TOP0 have been calculated and are given in Tables 5 and 6. In these tables the separation pH values for which the maximum separation factors could be obtained (pH,,) are also given, together with the extraction percentages of the three metal ions for these pH values for the different resins used. Given that the differences in pHso for the couple Cu (II) /Cd (II) are small, the pH,, values are given for the couples Zn (II) /Cu (II) and Zn (II) /Cd( II). The use of mixtures of DEHPA with TOP0 improve the separation factors of these three metal ions when [ HL] ,/ [S] r G 1 and the chloride concentration in the aqueous phase is higher. As can be seen in Tables 5 and 6, XAD2-DEHPA-TOP0 resins with [ HL I,/ [S],= 1 give recoveries of Zn (II) of 95% with a contamination of Cd( II) and Cu (II) of 3% from 0.1 M NaCl aqueous solutions. On the other hand, the extraction data for a 0.5
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322 100
DT 16/l
60
. .
??
.
.
%EX
40
Y, f
J. ID.
60
I
2) .: :*’
.’
??
20
NaCI0.1M
0A 1
317
3
2
zn(ll)
??
Cu(ll)
A
cd(ll)
4
PH 100 60 ..
DTlWl
.=-
.
c 0.
. %EX
60 ..
. .’
m 8” Na Cl O.lM
M
O1
3
2
4
PH 100 60 .I
DT 111 .
60 ” %EX
. .. .
.
.
40 ”
1
NaClO.1 M
2
4
3
DT 114
. .
60 ‘1 .
20 ‘.
1
.m.
.
.
0
+
.
40 ‘.
04
5
PH
60 ‘. %Ex
*
*A
r*i
fl
_,
100.
.
. #
20 ” 0
-
.
.
s’* .
.
??
zn(ll)
??
Cu(ll)
N&l 0.1 M
_ e_,;
3
??
5
7
PH
Fig. 8. Variation in the extraction percentage (%E) of Zn ( II), Cu ( II) and Cd (II) as a function of pH at several concentration ratios, [ HL],/ [S],, in the resin phase from 0.1 M NaCl solutions.
318
J.L. Cortina et al. /HydrometaNurgy 37 (1995) 301-322
r 60
.
%Ex 40
.
??
/ .*
+
DT loll .
60
.
%Ex
m
.
Cu(ll)
9’ *
.
.
A A
.
:
20 ..=
N&l
0.5 M 4
3
2
1
??
A Cd(H)
’
/i.
I
-1 Zn(ll)
??g
40
0
4 .
??
PH 100 60
. Zn(ll)
60 %Ex
??
CU(ll)
40 A Cd(ll)
20 NaC10.5 M 0
3
1
5
7
PH DT l/4
I
60 ..
II
.
60 .d
%Ex
9 Zn(ll)
.
??
1
2
3
4
5
CU(ll)
6
PH
Fig. 9. Variation in the extraction percentage (Ok&) of Zn (II), Cu (II ) and Cd (II) as a function of pH at several concentration ratios, [ HL ] ,/ [ S] r, in the resin phase from 0.5 MNaCl solutions.
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
319
Table 5 pHSo, A pH5a values and extraction ratios for pH values in which the maximum separation factor is obtained (PH.,,) for XADZDEHPA-TOP0 0.1 NaCl Resin
DT DT DT DT
19/l 10/l l/l l/4
PHSO
APH,o
%Ml/M2
(PH,,,)
Zn(II)
Cu(II)
Cd(I1)
Zn/Cd
Zn/Cu
Cd/Cu
Zn/Cd
Zn/Cu
1.87 1.95 2.21 3.92
2.85 2.88 3.66 5.19
2.95 2.89 3.80 -
1.08 0.94 1.20 -
0.98 0.93 1.03 1.27
-0.17 0.01 -0.17 -
9513 (2.2) 9513 (2.5) 95/5 (3.1) -
9515 (2.2) 9513 (2.5) 95/6 (3.0)
Table 6 PHIL, ApHSo values and extraction ratios for pH values (IMl /M2) in which the maximum separation factor is obtained (pH,,) for XADZDEHPA-TOP0 0.5 NaCl Resin
DT DT DT DT
19/l 10/l l/l l/4
PHSO
‘/oM1/M2 (PH,,, )
PHSO
Zn(I1)
Cu(I1)
Cd(I1)
Zn/Cd
Zn/Cu
Cd/Cu
Zn/Cd
Zn/Cu
2.16 2.24 2.56 3.28
3.02 3.06 3.65 5.23
3.35 3.40 4.51 -
1.21 1.12 1.95 -
0.86 0.93 1.09 1.93
0.30 0.01 0.32 -
98/5 (2.8) 9513 (2.7) 97/3 (3.5)
85/10 (2.7) 9017 (2.5) 85/10 (3.0)
M chloride medium indicate an increase in the separation factors, which allows the quantitative separation of Zn (II) from Cd (II) with XAD2-DEHPA-TOP0 resinswith [HL],/[S],< 1. This improvement in the separation factors for the couples Zn (II) /Cu (II ) and Zn (II) /Cd (II) using resins containing equimolecular mixtures of DEHPA and TOPO, or with higher concentrations of TOPO, is due to the antisynergistic effect of the mixture for Cu (II) and Cd (II), which produces the shift in the extraction of these metals to higher pH values. Furthermore, in a 0.5 M chloride medium, the complexing nature of chloride ions has an important effect on the extraction of Cu (II) and Cd (II), while the extraction of Zn (II) is practically unaffected.
4. Discussion
The resins containing mixtures of DEHPA and TOP0 (XAD2-DEHPATOPO), show good performance in the extraction of Zn (II ), Cu (II ) and Cd (II ) from chloride media. The metal extraction shows a dependence on pH, with a general trend towards higher extraction as the pH increases and the chloride concentration decreases. The graphical treatment of the data by the slope analysis method gives us some information about the composition of the metal complexes extracted by the resin
320
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
phase. Using slope analysis of the lines log D versus pH for [ HL] ,/ [S ] r > 10, the predominant species in the resin phase are species with t=O and the extraction of the above metal ions involves the formation of species with the same composition as in the case of resins containing DEHPA as a single component (ML,(HL),) [ 191. In the case of [ HL] ,/ [S ] r < 10 the slopes of the functions log D/pH decrease from 2 to 0.5 for resins with ratios of [ HL] ,/ [ S ] r < l/4 and the extraction of the above metal ions involves the formation of species with general composition ML,,_,, (Cl),S,( HL),,. On the other hand, the slope values decrease with increasing chloride concentration in the aqueous phase and depend on the metal ion. The slopes decrease in the order Cd (II) > Zn (II) > Cu (II), which is the order of increasing stability of the metal ion complexes in the aqueous phase. The extraction behaviour of resins containing TOP0 as a single component (XAD2-TOPO) indicates that although Cu( II) and Cd( II) are not extracted Zn (II) is extracted. Its extraction independent of the pH but dependent upon the chloride concentration, with a general trend that the higher the chloride concentration, the higher is the extraction. The graphical treatment of the data by the slope analysis method indicates that the extraction of Zn(I1) can be explained assuming the formation of species with a composition ZnCl$,. The FAIR spectroscopic studies of the XAD2-DEHPA-TOPO-M (II) resin complexes indicate the existence of these metal extractant complexes in the resin phase (Table 4). The expected structure of the DEHPA-M (II) complexes is similar to those proposed for the same systems in nitrate media [ 261. Concerning the extraction selectivity of XAD2-DEHPA-TOP0 resins towards Zn (II), Cu (II) and Cd (II) from a NaCl medium, some differences have been found in comparison with the selectivity of DEHPA in XAD2-DEHPA resins [ 191. From Figs. 8 and 9 the extraction ability of XAD2-DEHPA-TOP0 resins decreases as a function of pH in the order Zn (II) > Cd (II) > Cu (II). However, the extraction ability of DEHPA in organic solvents decreases as a function of pH in the order Zn (II) > Cu (II) > Cd (II ) . This different behaviour observed in the separation factors of the three metal ions, especially for Cu (II) and Cd( II), shows a change in the extraction ability of DEHPA molecules adsorbed in the macroporous support by the presence of TOP0 in the resin phase and chloride in the aqueous phase. This fact seems to indicate that the extraction reactions can be explained in terms of a competitive process between the polymeric network, the extractant molecules, the metal and chloride ions, opening a possible new way of modifying the separation factors depending on the polymer structure. Similar results were obtained previously with SIR prepared by direct adsorption of mixtures of DEHPA and TOP0 molecules into Amberlite XAD2 resin [ 261. The separation factors for XAD2-DEHPA-TOP0 resins in the extraction of these metal ions depends on the DEHPA/TOPO ratio in the mixture, as can be seen in Table 5. The separation factors, expressed in terms of pHsO differences, indicate higher separation factors with decreasing [HL],/ [ S], ratio. In particular, when the ratio is 1/O the maximum separation factors are obtained. As can
J.L. Cortina et al. / Hydrometallurgy 37 (1995) 301-322
321
be seen in Table 5 with XAD2-DEHPA-TOP0 (HL/S = 1/ 1) resins a quantitative separation of Zn (II) from Cd (II) can be obtained. As a general rule, the use of resins containing mixtures of DEHPA and TOP0 in a chloride medium has a great influence in the extraction of Cu (II) and Cd ( II), shifting its extraction to higher pH values and, as a consequence, increasing the separation factors towards Zn (II).
Acknowledgement
This work was supported by CICYT Project MAT 93-62 12 (Ministerio de Educacion y Ciencia de Espafia) . The authors are indebted to Prof. A. Warshawsky, Weizmann Institute of Science, Israel, for his help and suggestions for this work and to Dr. Maria Martinez for helping in PTIR spectroscopic measurements.
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