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Solid-liquid extraction of Au(III) from aqueous chloride solutions by tri-n-dodecylammonium chloride impregnated in amberlite XAD-2 resin
REACTIVE
Reactive & Functional Polymers 32 (1997) 125-130
F”NC&NAL POLYMERS
Solid-liquid extraction of Au(II1) from aqueous chloride solutiods by tri-n-dodecylammonium chloride impregnated in Amberlite XAb-2 resin I. Villaescusa a,*, V. Salvado b, J. de Pablo’ aE.Q.A.T.A. Department, E.P.S., Universitat de Girona, Avda. Santa16 s/n, 17003 Girona, Spain b Chemistry Department, EC.E.S.. Universitat de Girona, ha. Hospital. 17001 Girona, Spain ’Chemical Engineering Department, E.T.T.E.I.B., Universitat PolitLcnica de Catalunya, Avda. Diagonal, 647, 08028 Barceloha, Spain Accepted I8 August 1996
Abstract The extraction of Au@) from 1.0 mol dm” HCI solutions with the extractant tri-n-dodecylammonium chloride (TLAHCI) impregnated in a polymeric resin XAD-2 has been studied. The results of gold extraction showed that AuC13TLAHCl and AuC13(TLAHCl)2 were the extracted species in the resin. There was no loss of extraction resin capacity when thiourea solutions were used as stripping agent. It was also found that the system TLAHCUXAD-2 can be used to extract selectively gold from solutions containing zinc and copper. Keywords:
Gold(II1); Tri-n-dodecylammonium
chloride; Amberlite XAD-2; Solvent-impregnated
resin; Solid-liquid
ex-
traction
1. Introduction
the last few years, solvent-impregnated resins (SIR) introduced by Warshawsky [ 1,2] have been proposed as alternative for metal extraction, separation and recovery with the idea that these new materials combine the versatility and potential of the solvent extraction (SX) reagents with the advantage of using a solid phase as an extractant system [3-51. Recently, solvent-impregnated resins have been used in the extraction of gold(II1) from aqueous hydrochloric solutions. Two different systems, employing Amberlite XAD-2 impregDuring
* Corresponding author.
nated with either triisobutyl phosphine sulfide (TIBPS) [6] or trioctylmethylammonium chloride (TOMACl) [7] have been studied, showing the possibilities of this methodology. In gold(III) extraction with TOMACl dissolved in toluene and impregnated in Amberlite XAD-2 [7], it has been proved that the extracted species is the same, AuClsTOMACl, by using both methodologies, even taking into account that more extractant is needed in the resin than in toluene to obtain the same extraction. This result indicates that an impregnated solid support has chemical properties similar to those of the corresponding solvent extraction system. No information is available concerning the extraction of gold(II1) by SIR using extrac-
1381-5148/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII S1381-5148(96)00079-X
126
I. Villaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
tants which extract more than one species in liquid-liquid systems. This is the case in the liquid-liquid extraction of gold from aqueous hydrochloric acid solutions by tri-ndodecylammonium chloride dissolved in toluene, as it was recently reported [8]. For this system, it has been shown that the extraction process occurs via the formation of the species AuCls (TLAHCl) and AuCls(TLAHC1)2 in the organic phase [8]. We have therefore undertaken a study of the extraction of gold with a SIR comprised of XAD-2 impregnated with methanol solutions of TLAHCl procedure. The object of this work is to study both the impregnation procedure and the extraction of Au(II1) as well as to determine the extracted species. In addition, the conditions required for the selective extraction and recovery of gold from solutions containing Zn(I1) and Cu(I1) have been studied. 2. Experimental 2. I. Reagents and solutions
lution for 3 hours until equilibrium was reached. The concentration of the extractant was varied between 1.8 x 10e3 and 1.8 x 10-i mol dm”. The impregnated resins were separated from the organic solutions by filtration through a porous filter using a water pump. The TLAHCl content of the organic phase was determined by evaporation of the methanol. In some cases, the extractant was stripped from the resin with pure methanol to verify mass balance. Gold extraction was carried out by contacting a series of 0.1 g of impregnated resin with 10 ml of Au(II1) solutions of different concentration (1.0 x 10e4-3.0 x low3 mol dm”), for 1 hour. After 45 minutes the equilibrium was reached but in order to be sure that the equilibrium was attained in all cases the contacted time was 1 hour. After removal of the resins by filtration, the gold concentration in the remaining aqueous solutions was determined by Atomic Absorption Spectrometry. Extracted gold was stripped by shaking 0.1 g of the gold-loaded resin with 20 ml of a 0.5 mol dm” thiourea solution for 30 minutes. The extraction of Au@), Cu(I1) and Zn(I1) by the TLAHCVXAD-2 resin was studied by shaking 0.1 g of the impregnated resin with 10 ml of a solution of the metals in HCl 1.0 mol dmm3,until equilibrium was reached.
Stock solutions of Au(III) were prepared from solid HAuC14 (Merck A.R.) in HCl (Merck A.R.). Tri-n-dodecylammonium chloride (TLAHCI) was prepared from tri-n-dodecylamine (Merck A.R.) and purified as described elsewhere [9]. Organic solutions of TLAHCl in methanol were used as impregnation solution. Methanol (Merck A.R.) was used without further purification. Amberlite XAD-2 was purified with a 4.0 mol dm” HCl solution and, after elimination of chlorides by washing with distilled water, with a methanol-water (1 : 1) solution and washed with water again. A 0.5 mol dme3 solution of thiourea (Merck A.R.) was used in gold stripping procedures. Standard solutions of ZnCl;! (1.53 x 10m2 mol dm”) and CuC12 (1.57 x 10m2mol dmm3)(Carlo Erba) were used.
3.1. TLAHCLJYAD-2 impregnation
2.2. Experimental procedure
3.2. Gold extraction
Amounts of 0.1 grams of dry XAD-2 were contacted with 10 ml of a TLAHCl methanol so-
In analogy with liquid-liquid extraction, the distribution coefficient of the metal between the
3. Results and discussion
The results of impregnation of the XAD-2 as a function of the load concentration of TLAHCl in methanol are shown in Fig. 1. It can be seen that at a concentration of TLAHCl of 0.1 mol dms3, the resin reaches saturation. The maximum TLAHCl concentration in the resin was 0.025 mol kg-’ dry XAD-2 resin.
I. Villaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
.
.
127
.
.
0
0.05
0.1 0,15 ITIAHCIIin madmnol(mol dm-31
0.25
0.2
Fig. 1. Impregnation of XAD-2 with TLAHCl dissolved in methanol.
organic phase (XAD-2) containing TLAHCl and the aqueous phase can be defined as: D
=
([Au(III)lt
-
[Au(III)]aq) x (V/m)
[AuUWaql
(1)
where [Au(III)]t and [Au(III)]aq are the initial and the equilibrium concentrations of metal in the aqueous phase, respectively, V is the volume in dm3 of the aqueous phase and m in kg the mass of dry impregnated resin. Gold distribution data plotted as 1ogD versus log[TLAHCl],,,, can be shown in Fig. 2.
No polynuclear species have been considered in either phase due to the low concentration of metal used. By taking into account the definition ofi D, this expression can be rewritten as: 1ogD = logK,[Cl-‘1
+ nlog[TLAHCl]
(4)
Thus, a plot of 1ogD vs log[TLAHCl] ~should give a straight line with an intercept equal to K, [Cl-]-’and slope equal to n. The concentration of free TLAHCl in this equation can be calculated from the mass balance equation:
3.3. Treatment of data
[TLAHCl] = [TLAHCl],, - [Au],,
The extraction of gold can be described at the experimental conditions used in this work (i.e. high chloride concentration and low pH) by taking into account only the tetrachloro complex, AuCld- [ 10,111, according the following reaction:
As shown in Fig. 2, however, in the extraction of gold by TLAHCl impregnated in XAD-2, a curve is obtained. This indicates that two species, AuC13(TLAHCl), and AuCls (TLAHCl), , are formed in the resin. The distribution coefficient can be written as follows:
AuCl; + nTLAHCl(org) e
D=
AuC13(TLAHCl), (org) + Cl-
(2)
Assuming ideal behaviour in the organic phase and constant activity coefficients in the aqueous phase, the equilibrium constant of reaction 2 can be written as: K = [AuCls WAHCl), 1[Cl- 1 n [AuCl,][TLAHCl],
(3)
[AuC13(TLAHC1),] + [AuC13(TLAHC1),] [AuCl,]
(5)
Substituting Eq. 3 into Eq. 5 yields the following equation: D[Cl-] = KJTLAHCl],
+ K,[TLAHCl], (6)
I. Wlaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
128
-3,5
-3
-2,5
-2
-1,5
-1
log lTLAHClltotilin resin
Fig. 2. IogD plotted as a function of total TLAHCl concentration at two different initial Au@) concentrations: ., 2.5 x 10m4mol dm-3; m, 5.1 x 10S4 mol dm”
Since metal concentration is very low, the aqueous chloride concentration can be considered constant. Values of p, q and the constants were determined graphically by means of normalized curves [ 121. Defining two new variables y and u and a parameter L as:
and Kq was calculated from the L value which fits the experimental data. The values of these constants were 1ogKi = 3.52 and logK2 = 5.56. The experimental data were also numerically treated by using the program LETAGROPDISTR [ 131. In this treatment the error square sum defined as:
logy = 1ogD + log[Cl_]
(7)
logu = logK, + plog[TLAHCl]
(8)
is minimized. The standard deviation defined as: a(logD) = (U/Np)‘/2, where Np is the total number of experimental points, is also calculated. The best fit was obtained when the two species determined in the graphical treatment were considered, the constants values calculated were 1ogKr = 3.540f0.15andlogK2 = 5.780f0.2, which are in fair agreement with the graphical calculations. Therefore, the experimental data can be explained by the extraction of the species AuClsTLAHCl and AuCls (TLAHCl)z . These species have been shown to be the predominant extracted species in the analogous liquidliquid system [8]. However, it should be pointed out that in order to obtain the same gold extraction percentage in both liquid-liquid and solid-liquid systems the TLAHCl concentration in the XAD-2 has to be at least twice higher than the used dissolved in toluene. The same result was obtained by using the tri-
(9) and substituting in Eq. 6, we obtain: logy = log@ + Luq’p)
(10)
By using Eq. 10, a set of theoretical curves of the form logy = f(logu) were constructed for different p, q and L values, then compared to the experimental 1ogD results. The best fit was obtained for values of p = 1, q = 2 and L = 0.033. This fit is shown in Fig. 3. The value of Kp was obtained using expression 8 by reading off the difference: logu - plog[TLAHCl] = logK,
I. Wlaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
129
3.5 3
OS
-3,5
-3
-2
-2,s log
-I,5
-1
ITLAHCII
Fig. 3. 1ogD plotted as a function of free TLAHCl concentration at two different Au(II1) concentrations: ?? , 5.1 x 10e4 mol dme3. Full line corresponds to the function log@ + L&/J’) vs. logu, with L = 0.033.
Table 1 Results of the stripping with thiourea 0.5 mol dme3 Initial Au in SIR (mg)
1st contact (mg) in thiourea
2nd contact in thiourea
(mg) Total efficiency (%)
1.99 4.36 10.68
1.09 2.56 6.75
0.85 1.72 3.88
97.5 98.2 99.5
octylmethylammonium
chloride as an extractant
?? ,
2.5 x low4 mol dm-.‘;
Table 2 Extraction percentage of Au(III), Cu(I1) and Zn(I1)
[Auli
[CUli
Vnli
(mol dm-3)
(mol dmm3)
(mol dmm3)
Au (%)
Cu (S)
Zn @IO)
5.10 x 1.10 x 2.53 x 3.81 x 5.07 x 1.01 x
1.53 x 3.15 x 7.87 x 1.18 x 1.57 x 3.15 x
1.57 x 10-h 3.06 x lO-4 7.65 x 1O-4 1.15 X 10-s 1.53 x 10-s 3.06 x 1O-3
98.0 99.0 98.8 99.7 99.9 98.0
-
71.8 49.0 23.5 24.4 27.5 15.0
10-s 10-d 1O-4 1O-4 10-4 lO-3
10-4 1O-4 1O-4 1O-3 10-s 1O-3
[71. 3.4.Stripping results
Two contacts of the gold-loaded resin with 0.5 mol dmm3thiourea solution were carried out. During the first contact the recovery was no more than 60%; after the second contact the total stripping of sorbed gold yielded ~97% (Table 1). When, the resin was loaded to the same initial gold concentration and stripped again, a 100% extraction was achieved. This operation was repeated 10 times and no extraction loss was observed. 3.5. Gold extraction in the presence of base metals Zn and Cu The results of the gold extraction in presence
of zinc and copper are presented in terms of extraction percentage in Table 2. As can be seen,
there was no measurable extraction of copper. Other experiments performed with solutions containing gold and zinc demonstrated that gold can be selectively extracted from solutions containing zinc simply by controlling the stirring time (Table 3) since the kinetics of zinc extraction have been shown to be slower [ 14-161. 4. Conclusions In this study, the thermodynamic agproach which is normally used for determining extracted species in liquid-liquid extraction was successfully used to determine the species extracted in the TLAHWXAD-2 system. The extracted species, AuClsTLAHCl and AuC~~(TLAHC~)~, proved to be the same as those found in the liquid-liquid extraction of gold by TLAHCl in toluene [8].
I. Wllaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
130
Table 3 Influence of stirring time in the gold extraction in presence of zinc Stirring time (mm)
AU (%)
Zn (%)
15 30 60 120 180 240 300 360 420
90.9 94.9 99.8 99.8 99.8 99.8 99.8 99.8 99.8
3.0 5.8 10.4 10.6 11.0 13.4 14.8 16.7 19.1
[Au]i = 2.53
x
10m3mol dmv3; [Zn]; = 7.60
x
10e3 mol dme3.
The TLAHCVXAD-2 system was shown to provide a means for the selective extraction and recovery of gold from solutions containing copper and zinc. Acknowledgements This work has been supported by CICYT MAT93-6212 (Ministerio de Educacidn y Ciencia, Spain).
References [l] [2] [3] [4]
A. Warshawsky, Talanta, 21,644 (1974). A. Warshawsky, Talanta. 21,962 (1974). A. Warshawsky, Trans. Inst. Min. Metall., 83 (1975) 101. A. Warshawsky, Ion Exchange and Solvent Extraction. Marcel Dekker, New York, N.Y., 1981, p. 229. [5] J.L. Cortina, N. Miralles, A. Sastre and M. Aguilar, Solvent Extraction, 1990, Ed. T. Sekine (Elsevier, The Netherlands), 1992, p. 159. [6] I. Villaescusa, V. Salvad6, J. de Pablo, M. Valiente and M. Aguilar, React. Polym., 17 (1992) 69. [7] I. Villaescusa, V. Salvad6 and J. de Pablo, Hydrometallurgy, 41 (1996) 303. [8] I. Villaescusa, N. Miralles, J. de Pablo, V. Salvad6 and A.M. Sastre, Solvent Extr. Ion Exch., ll(4) (1993) 613. [9] M.Muhammed, J. Szabon, E. Hogfeldt, Chem. Ser., 6 (1974) 61. [lo] N. Bjerrum, Bull. Sot. Chim. Belge, 57 (1938) 432. [ll] H.L. Johnston and H.L. Leland, J. Am. Chem. Sot., 60 (1938) 1439. [ 121 LG. Sillen, Acta Chem. Scand., 10 (1956) 186. [13] D.H. Liem, Acta Chem. Stand., 25 (1971) 147. [14] PA. Riveros and WC. Cooper, Solvent Extr. Ion Exch., 10(l) (1992) 173. [15] L.B. Kar-On, M.J. Hudson, Solvent Extr. Ion Exch., 10(l) (1992) 173. [16] PA. Riveros, Hydrometallurgy, 24 (1990) 135.
Reactive & Functional Polymers 32 (1997) 125-130
F”NC&NAL POLYMERS
Solid-liquid extraction of Au(II1) from aqueous chloride solutiods by tri-n-dodecylammonium chloride impregnated in Amberlite XAb-2 resin I. Villaescusa a,*, V. Salvado b, J. de Pablo’ aE.Q.A.T.A. Department, E.P.S., Universitat de Girona, Avda. Santa16 s/n, 17003 Girona, Spain b Chemistry Department, EC.E.S.. Universitat de Girona, ha. Hospital. 17001 Girona, Spain ’Chemical Engineering Department, E.T.T.E.I.B., Universitat PolitLcnica de Catalunya, Avda. Diagonal, 647, 08028 Barceloha, Spain Accepted I8 August 1996
Abstract The extraction of Au@) from 1.0 mol dm” HCI solutions with the extractant tri-n-dodecylammonium chloride (TLAHCI) impregnated in a polymeric resin XAD-2 has been studied. The results of gold extraction showed that AuC13TLAHCl and AuC13(TLAHCl)2 were the extracted species in the resin. There was no loss of extraction resin capacity when thiourea solutions were used as stripping agent. It was also found that the system TLAHCUXAD-2 can be used to extract selectively gold from solutions containing zinc and copper. Keywords:
Gold(II1); Tri-n-dodecylammonium
chloride; Amberlite XAD-2; Solvent-impregnated
resin; Solid-liquid
ex-
traction
1. Introduction
the last few years, solvent-impregnated resins (SIR) introduced by Warshawsky [ 1,2] have been proposed as alternative for metal extraction, separation and recovery with the idea that these new materials combine the versatility and potential of the solvent extraction (SX) reagents with the advantage of using a solid phase as an extractant system [3-51. Recently, solvent-impregnated resins have been used in the extraction of gold(II1) from aqueous hydrochloric solutions. Two different systems, employing Amberlite XAD-2 impregDuring
* Corresponding author.
nated with either triisobutyl phosphine sulfide (TIBPS) [6] or trioctylmethylammonium chloride (TOMACl) [7] have been studied, showing the possibilities of this methodology. In gold(III) extraction with TOMACl dissolved in toluene and impregnated in Amberlite XAD-2 [7], it has been proved that the extracted species is the same, AuClsTOMACl, by using both methodologies, even taking into account that more extractant is needed in the resin than in toluene to obtain the same extraction. This result indicates that an impregnated solid support has chemical properties similar to those of the corresponding solvent extraction system. No information is available concerning the extraction of gold(II1) by SIR using extrac-
1381-5148/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII S1381-5148(96)00079-X
126
I. Villaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
tants which extract more than one species in liquid-liquid systems. This is the case in the liquid-liquid extraction of gold from aqueous hydrochloric acid solutions by tri-ndodecylammonium chloride dissolved in toluene, as it was recently reported [8]. For this system, it has been shown that the extraction process occurs via the formation of the species AuCls (TLAHCl) and AuCls(TLAHC1)2 in the organic phase [8]. We have therefore undertaken a study of the extraction of gold with a SIR comprised of XAD-2 impregnated with methanol solutions of TLAHCl procedure. The object of this work is to study both the impregnation procedure and the extraction of Au(II1) as well as to determine the extracted species. In addition, the conditions required for the selective extraction and recovery of gold from solutions containing Zn(I1) and Cu(I1) have been studied. 2. Experimental 2. I. Reagents and solutions
lution for 3 hours until equilibrium was reached. The concentration of the extractant was varied between 1.8 x 10e3 and 1.8 x 10-i mol dm”. The impregnated resins were separated from the organic solutions by filtration through a porous filter using a water pump. The TLAHCl content of the organic phase was determined by evaporation of the methanol. In some cases, the extractant was stripped from the resin with pure methanol to verify mass balance. Gold extraction was carried out by contacting a series of 0.1 g of impregnated resin with 10 ml of Au(II1) solutions of different concentration (1.0 x 10e4-3.0 x low3 mol dm”), for 1 hour. After 45 minutes the equilibrium was reached but in order to be sure that the equilibrium was attained in all cases the contacted time was 1 hour. After removal of the resins by filtration, the gold concentration in the remaining aqueous solutions was determined by Atomic Absorption Spectrometry. Extracted gold was stripped by shaking 0.1 g of the gold-loaded resin with 20 ml of a 0.5 mol dm” thiourea solution for 30 minutes. The extraction of Au@), Cu(I1) and Zn(I1) by the TLAHCVXAD-2 resin was studied by shaking 0.1 g of the impregnated resin with 10 ml of a solution of the metals in HCl 1.0 mol dmm3,until equilibrium was reached.
Stock solutions of Au(III) were prepared from solid HAuC14 (Merck A.R.) in HCl (Merck A.R.). Tri-n-dodecylammonium chloride (TLAHCI) was prepared from tri-n-dodecylamine (Merck A.R.) and purified as described elsewhere [9]. Organic solutions of TLAHCl in methanol were used as impregnation solution. Methanol (Merck A.R.) was used without further purification. Amberlite XAD-2 was purified with a 4.0 mol dm” HCl solution and, after elimination of chlorides by washing with distilled water, with a methanol-water (1 : 1) solution and washed with water again. A 0.5 mol dme3 solution of thiourea (Merck A.R.) was used in gold stripping procedures. Standard solutions of ZnCl;! (1.53 x 10m2 mol dm”) and CuC12 (1.57 x 10m2mol dmm3)(Carlo Erba) were used.
3.1. TLAHCLJYAD-2 impregnation
2.2. Experimental procedure
3.2. Gold extraction
Amounts of 0.1 grams of dry XAD-2 were contacted with 10 ml of a TLAHCl methanol so-
In analogy with liquid-liquid extraction, the distribution coefficient of the metal between the
3. Results and discussion
The results of impregnation of the XAD-2 as a function of the load concentration of TLAHCl in methanol are shown in Fig. 1. It can be seen that at a concentration of TLAHCl of 0.1 mol dms3, the resin reaches saturation. The maximum TLAHCl concentration in the resin was 0.025 mol kg-’ dry XAD-2 resin.
I. Villaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
.
.
127
.
.
0
0.05
0.1 0,15 ITIAHCIIin madmnol(mol dm-31
0.25
0.2
Fig. 1. Impregnation of XAD-2 with TLAHCl dissolved in methanol.
organic phase (XAD-2) containing TLAHCl and the aqueous phase can be defined as: D
=
([Au(III)lt
-
[Au(III)]aq) x (V/m)
[AuUWaql
(1)
where [Au(III)]t and [Au(III)]aq are the initial and the equilibrium concentrations of metal in the aqueous phase, respectively, V is the volume in dm3 of the aqueous phase and m in kg the mass of dry impregnated resin. Gold distribution data plotted as 1ogD versus log[TLAHCl],,,, can be shown in Fig. 2.
No polynuclear species have been considered in either phase due to the low concentration of metal used. By taking into account the definition ofi D, this expression can be rewritten as: 1ogD = logK,[Cl-‘1
+ nlog[TLAHCl]
(4)
Thus, a plot of 1ogD vs log[TLAHCl] ~should give a straight line with an intercept equal to K, [Cl-]-’and slope equal to n. The concentration of free TLAHCl in this equation can be calculated from the mass balance equation:
3.3. Treatment of data
[TLAHCl] = [TLAHCl],, - [Au],,
The extraction of gold can be described at the experimental conditions used in this work (i.e. high chloride concentration and low pH) by taking into account only the tetrachloro complex, AuCld- [ 10,111, according the following reaction:
As shown in Fig. 2, however, in the extraction of gold by TLAHCl impregnated in XAD-2, a curve is obtained. This indicates that two species, AuC13(TLAHCl), and AuCls (TLAHCl), , are formed in the resin. The distribution coefficient can be written as follows:
AuCl; + nTLAHCl(org) e
D=
AuC13(TLAHCl), (org) + Cl-
(2)
Assuming ideal behaviour in the organic phase and constant activity coefficients in the aqueous phase, the equilibrium constant of reaction 2 can be written as: K = [AuCls WAHCl), 1[Cl- 1 n [AuCl,][TLAHCl],
(3)
[AuC13(TLAHC1),] + [AuC13(TLAHC1),] [AuCl,]
(5)
Substituting Eq. 3 into Eq. 5 yields the following equation: D[Cl-] = KJTLAHCl],
+ K,[TLAHCl], (6)
I. Wlaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
128
-3,5
-3
-2,5
-2
-1,5
-1
log lTLAHClltotilin resin
Fig. 2. IogD plotted as a function of total TLAHCl concentration at two different initial Au@) concentrations: ., 2.5 x 10m4mol dm-3; m, 5.1 x 10S4 mol dm”
Since metal concentration is very low, the aqueous chloride concentration can be considered constant. Values of p, q and the constants were determined graphically by means of normalized curves [ 121. Defining two new variables y and u and a parameter L as:
and Kq was calculated from the L value which fits the experimental data. The values of these constants were 1ogKi = 3.52 and logK2 = 5.56. The experimental data were also numerically treated by using the program LETAGROPDISTR [ 131. In this treatment the error square sum defined as:
logy = 1ogD + log[Cl_]
(7)
logu = logK, + plog[TLAHCl]
(8)
is minimized. The standard deviation defined as: a(logD) = (U/Np)‘/2, where Np is the total number of experimental points, is also calculated. The best fit was obtained when the two species determined in the graphical treatment were considered, the constants values calculated were 1ogKr = 3.540f0.15andlogK2 = 5.780f0.2, which are in fair agreement with the graphical calculations. Therefore, the experimental data can be explained by the extraction of the species AuClsTLAHCl and AuCls (TLAHCl)z . These species have been shown to be the predominant extracted species in the analogous liquidliquid system [8]. However, it should be pointed out that in order to obtain the same gold extraction percentage in both liquid-liquid and solid-liquid systems the TLAHCl concentration in the XAD-2 has to be at least twice higher than the used dissolved in toluene. The same result was obtained by using the tri-
(9) and substituting in Eq. 6, we obtain: logy = log@ + Luq’p)
(10)
By using Eq. 10, a set of theoretical curves of the form logy = f(logu) were constructed for different p, q and L values, then compared to the experimental 1ogD results. The best fit was obtained for values of p = 1, q = 2 and L = 0.033. This fit is shown in Fig. 3. The value of Kp was obtained using expression 8 by reading off the difference: logu - plog[TLAHCl] = logK,
I. Wlaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
129
3.5 3
OS
-3,5
-3
-2
-2,s log
-I,5
-1
ITLAHCII
Fig. 3. 1ogD plotted as a function of free TLAHCl concentration at two different Au(II1) concentrations: ?? , 5.1 x 10e4 mol dme3. Full line corresponds to the function log@ + L&/J’) vs. logu, with L = 0.033.
Table 1 Results of the stripping with thiourea 0.5 mol dme3 Initial Au in SIR (mg)
1st contact (mg) in thiourea
2nd contact in thiourea
(mg) Total efficiency (%)
1.99 4.36 10.68
1.09 2.56 6.75
0.85 1.72 3.88
97.5 98.2 99.5
octylmethylammonium
chloride as an extractant
?? ,
2.5 x low4 mol dm-.‘;
Table 2 Extraction percentage of Au(III), Cu(I1) and Zn(I1)
[Auli
[CUli
Vnli
(mol dm-3)
(mol dmm3)
(mol dmm3)
Au (%)
Cu (S)
Zn @IO)
5.10 x 1.10 x 2.53 x 3.81 x 5.07 x 1.01 x
1.53 x 3.15 x 7.87 x 1.18 x 1.57 x 3.15 x
1.57 x 10-h 3.06 x lO-4 7.65 x 1O-4 1.15 X 10-s 1.53 x 10-s 3.06 x 1O-3
98.0 99.0 98.8 99.7 99.9 98.0
-
71.8 49.0 23.5 24.4 27.5 15.0
10-s 10-d 1O-4 1O-4 10-4 lO-3
10-4 1O-4 1O-4 1O-3 10-s 1O-3
[71. 3.4.Stripping results
Two contacts of the gold-loaded resin with 0.5 mol dmm3thiourea solution were carried out. During the first contact the recovery was no more than 60%; after the second contact the total stripping of sorbed gold yielded ~97% (Table 1). When, the resin was loaded to the same initial gold concentration and stripped again, a 100% extraction was achieved. This operation was repeated 10 times and no extraction loss was observed. 3.5. Gold extraction in the presence of base metals Zn and Cu The results of the gold extraction in presence
of zinc and copper are presented in terms of extraction percentage in Table 2. As can be seen,
there was no measurable extraction of copper. Other experiments performed with solutions containing gold and zinc demonstrated that gold can be selectively extracted from solutions containing zinc simply by controlling the stirring time (Table 3) since the kinetics of zinc extraction have been shown to be slower [ 14-161. 4. Conclusions In this study, the thermodynamic agproach which is normally used for determining extracted species in liquid-liquid extraction was successfully used to determine the species extracted in the TLAHWXAD-2 system. The extracted species, AuClsTLAHCl and AuC~~(TLAHC~)~, proved to be the same as those found in the liquid-liquid extraction of gold by TLAHCl in toluene [8].
I. Wllaescusa et al. /Reactive & Functional Polymers 32 (1997) 125-130
130
Table 3 Influence of stirring time in the gold extraction in presence of zinc Stirring time (mm)
AU (%)
Zn (%)
15 30 60 120 180 240 300 360 420
90.9 94.9 99.8 99.8 99.8 99.8 99.8 99.8 99.8
3.0 5.8 10.4 10.6 11.0 13.4 14.8 16.7 19.1
[Au]i = 2.53
x
10m3mol dmv3; [Zn]; = 7.60
x
10e3 mol dme3.
The TLAHCVXAD-2 system was shown to provide a means for the selective extraction and recovery of gold from solutions containing copper and zinc. Acknowledgements This work has been supported by CICYT MAT93-6212 (Ministerio de Educacidn y Ciencia, Spain).
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