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Liquid-liquid and solid-liquid extraction of gold by trioctylmethylammonium chloride (TOMAC1) dissolved in toluene and impregnated on amberlite XAD-2 resin
hydrometallurgy Hydrometallurgy 41 ( 1996) 303-3 I 1
Liquid-liquid and solid-liquid extraction of gold by trioctylmethylammonium chloride ( TOMACl) dissolved in toluene and impregnated on amberlite XAD-2 resin I. Villaescusa aT*, V. Salvad6 b, J. de Pablo ’ aE.QA.T.A. Department, E.P.S., Universitat de Gironu, 17003 Girona, Spain b Chemistry Department, F.C.E.S., Universitat de Girona, 17001 Girona, Spain ’ Chemical Engineering Department, E.T.S.E.I.B., Universitat Politecnica de Catalunya, 08028 Barcelona, Spain Received
16 February
1995; accepted 5 May 1995
Abstract The extraction of A&II) with the extractant trioctylmethylammonium chloride (TOMACI) dissolved in toluene and the same extractant impregnated in a polymeric resin XAD-2 has been studied. The results of gold extraction with both systems were compared. They showed similar gold extraction behaviour (extracted species), although greater amounts of extractant were needed when impregnated resins were used. Liquid-liquid and solid-liquid procedures can be used to extract gold selectively from solutions containing zinc and copper.
1. Introduction
The recovery of gold and other precious metals from secondary grade sources such as electronic scrap, spent catalysts and metal concentrates [1,2] is by an initial treatment with aqua regia. Under such conditions, gold is oxidized to gold(II1) and forms chloro-complexes, the predominant species being AuCl, [3,4], while base metals such as copper and zinc can also form chloro-complexes [5].
* Corresponding
author.
0304-386X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0304-386X(95)00064-X
304
I. Villaescusa et al./Hydrometallurgy
41 (1996) 303-31 I
Solvent extraction is an alternative method for the separation and purification of gold. This method has been applied to extract gold(II1) from chloride solutions using anion-exchange [6,7] and ligand-exchange systems [8]. In this method, commercial quatemary ammonium salts are the most useful anions extractants, in particular tri-octylmethylammonium chloride (TOMACI) has been widely used in metal extraction studies [7,9-l I]. Studies of the extraction of Au(II1) with TOMACl in cyanide medium showed a better extraction of this metal compared to copper and iron extraction results [12,13]. On the other hand, solid-liquid extraction has also been applied to extract Au(II1) from chloride solutions [ 14,151. Solvent-impregnated resins (SIR), introduced by Warshawsky [ 16,171, have been proposed as an alternative method in hydrometallurgical applications [ 18,191. Recently, the extraction of Au(II1) with tri-isobutylphosphine sulfide impregnated in Amberlite XAD-2 resin has been carried out [20]. The aim of the present investigation is to study the extraction of Au(II1) with TOMACl dissolved in toluene and TOMACI impregnated in a polymeric support, Amberlite XAD-2 resin, in order to compare these methodologies. The effect of base metals, such as copper and zinc, during the Au(II1) extraction is also studied, as well as the stripping procedure.
2. Experimental 2.1. Reagents and solutions Stock solutions of Au(II1) were prepared from HAuCl, (s) (Merck A.R.) in HCl (Merck A.R.) 1 mol dmd3. Trioctylmethylammonium chloride (TOMACI), the commercial reagent called Aliquat 336, (Merck A.R.) was used without further purification. Solutions containing 0.5 mol dm -3 of thiourea (Merck A.R.) were used in gold stripping procedures. Methanol (Merck A.R.) was used without further purification. Toluene (Panreac A.R.) was purified by washing with 0.5 mol dm-’ HCl, distilled water, and 0.5 mol dme3 NaOH alternatively. Amberlite XAD-2 was purified with a 4.0 mol dmw3 HCl solution and, after elimination of chlorides by washing with distilled water, with a methanol-water (1:l) solution and washed with water again. Standard solutions of ZnCl, (15.3 mmol dmm3) and CuCl, (15.7 mmol dmm3) (Carlo Hrba) were used. 2.2. Procedure 2.2.1. Liquid-liquid extraction Experiments were performed at 25°C by shaking equal volumes (10 ml) of the organic phase and aqueous solutions in a rotatory mixer for 1 h. Previous experiments showed that after 30 min the equilibrium had been attained. The organic phase consisted of TOMACl solutions in toluene. The total extractant concentration was varied within the range 0.05-0.5 mmol dmm3. The total Au(II1) concentration in the aqueous phase was varied between 0.106 and 0.218 mmol dmm3.
I. Villaescusa et al./ Hydrometallurgy 41 (1996) 303-31 I
305
From the different reagents for stripping of gold, thiourea was found to be the most effective. Equal volumes (7 ml> of the organic phase containing gold were contacted with a 0.5 mol dme3 thiourea solution for 30 min. The separation of gold from solutions containing Z&I) and Cu(I1) was carried out by shaking equal volumes (10 ml) of the organic and aqueous phase for 1 h. The aqueous phase contained a fixed concentration of gold (0.164 mmol dmm3) and the concentration of zinc(H) and copper(R) was varied between 6.0 and 12.0 mmol dme3 and 6.3 and 11.0 mmol dme3, respectively. The organic phase was a 0.5 mmol dme3 TOMACl solution in toluene. 2.2.2. Solid-liquid extraction 2.2.2.1. Impregnation procedure. Amounts of 0.1 g of dry XAD-2 were contacted with different concentrations of TOMACl (0.1-500 mmol drnm3) dissolved in methanol for 3 h until equilibrium was reached. The impregnated resins were separated from the organic solutions by filtration through a porous filter using a water pump. The TOMACl 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 the mass balance. 2.2.2.2.
Gold extraction. Gold extraction was carried out by putting in contact 0.1 g of impregnated resin with 10 ml of A&II) solutions for 1 h. In this case preliminary experiments showed that after 45 min the equilibrium had been reached. The resins had been impregnated with different concentrations of TOMACl in methanol (81-1200 mmol dmm3). After filtration of the resins the gold concentration in the remaining aqueous solutions was determined by atomic absorption spectrometry. In these experiments the initial gold concentration was varied between 0.1 and 5.0 mmol dmm3. The stripping of gold was performed by shaking 0.1 g of impregnated resin containing gold with 20 ml of a 0.5 mol dm-3 thiourea solution for 30 min. The extraction of Au(III), Cu(I1) and Zn(I1) by the system TOMACl/XAD-2 was studied by shaking 0.1 g of impregnated resin with 10 ml of the metals in 1.0 mol dm - 3 HCl until equilibrium was reached. The concentration range was Au(II1): 0.0508-1.52 mmol dmm3; Zn(I1): 0.153-4.59 mmol dme3; and Cu(I1): 0.157-4.72 mmol dmm3.
3. Results and discussion 3.1. TOMACl/XAD-2
impregnation
Results of impregnation of the XAD-2 as a function of the load concentration of TOMACl in methanol are shown in Fig. 1. It can be seen that, for a concentration of TOMACl of 0.15 mol dme3, the capacity of the resin reaches saturation. The maximum concentration in the resin was 0.033 mol TOMACl kg-’ dry resin. Therefore, this concentration was used to study the gold extraction in this system.
I. Villaescusa
306 0,035
et al./Hydrometallurgy
I
41 (1996)
303-311
.
.
0,03 r 9 Z .E qJ 3 ,E C 0 2
0,025
.
0,02
.
0,015
. 0,Ol
.
ot 0,005
t
.
0
I I
0
0,05
0,15
021
0.2
0.25
[TOMACI] in methanol (mol dm-3) Fig. 1. Impregnation
of XAD-2 with TOMACI.
3.2. Gold extraction The distribution XAD-2) containing
coefficient of the metal between the organic phase (toluene or TOMACl and the aqueous phase can be defined in both cases as:
D = [Au(III)l.&Au(III)l,,
(1)
where [Au(III)],,, denotes the total concentration of Au(II1) in the resin in moles per kilogram or in toluene in moles per cubic decimeter and [Au(III)I, is the total concentration in the aqueous phase. In the case of the impregnated resin, due to the different phase relationship the distribution coefficient can be expressed as: D =
{(b(~m, - b~mlaq) x wm)}/[Awhl
(2)
are the initial and the equilibrium concentrations of where [Au(III)], and [Au(III)],, metal in the aqueous phase, respectively; V is the volume in litres of the aqueous phase; and m is the mass in kilograms of dry impregnated resin. Gold distribution data with both systems were plotted as log D versus log [~0h4.4,,, , as shown in Figs. 2 and 3. 3.2.1. Treatment of data The extraction of gold can be described for the experimental conditions used in this work, high chloride concentration and low pH, taking into account only the tetrachlorocomplex AuCl, by using the following reaction: AuCl;
+ nTOMACl(
org) CJ AuCl,(TOMACl)
Jorg)
+ Cl-
(3)
I. Villaescusa et al./Hydrometallurgy
41 (1996) 303-311
307
25 L__
[Au]=O,lOGmmol dm-3
n [Au]=O,164 mmol dm-3
2 --
1,s
0
1
E 03
[Au]=O,216 mmol dm-3
??
.
.
I .
0 .
0
i
.
.*
:
u
t
m
.
0
-1 ’
I
-4,4
-433
-492
-4,l
4
-3,9
-3,6
-3,7
-3,6
log~OMACl(mol dm-3)]tot Fig. 2. Liquid-liquid extraction of Au(III) in 1.O mol dme3 TOMACl concentration at various initial Au(II1) concentrations.
1,6 __
’ [Au]=O,506 mmol dm-3
HCl. Log D plotted
n [Au]=0,264 mmol dm-3
as a function
A [Au]=O,lOl mmol dm-3
194 --
.
1 --
: .
7 ;
0,6 --
n m -P 0,2
of total
. .
’
:. * .
4,2 --0,6 --1 -I,2
-1
-0,9
-0,6 log ~OMACl(mol
-0,4
-0,2
0
032
dm-3)]tot
Fig. 3. Solid-liquid extraction of A&II) in 1.0 mol dmm3 HCI. Log D plotted as a function of total TOMACl concentration at various initial Au(II1) concentrations.
I. Villaescusa et al./Hydrometallurgy
308 0,5
I 1
??
[Au]=O,lO6mmol dm-3
41 (1996) 303-31 I
o [Au]=O,164 mmol dm-3
??
[Au]=O,218 mmol dm-3
094 093 0,2 091 n 0 E -0,l -0,Z -0,3 -0,4 -0,5 -7-2
-7,1
-7
-6,s
-6,8
-6,7
log p’OMACl(mol
-6,s
-6,5
-6,4
-6,3
dm-3)]
Fig. 4. Liquid-liquid extraction of A&II) in 1.O mol dm -3 HCl. Log D plotted as a function of free TOMACI concentration at various initial A&II) concentrations. Line corresponds to a slope equal to 1.
considering ideal behaviour in the organic phase and assuming the activity coefficients of the aqueous phase to be constant, the equilibrium constant of reaction (3) can be written as: K = [AUCI,(TOMACI),,][C~-] n [AUC~;] [TOMACI]”
(4)
where no polynuclear species have been considered in the organic phase or in the aqueous phase, due to the low concentration of metal used. Taking into account the definition of D, the expression can be written: logs = log~,J~l-I-’
+ ~~~~[TOMACI]
(5)
So, a plot of log D against log [TOMACI] should give a straight line with the ordinate axis equal to KJCl-]’ and slope of n. The concentration of free TOMACl can be calculated from the mass balance equation: [TOMACI] = [TOMACI],,, - [AU],,%
(6)
In Figs. 4 and 5, the plots of log D versus the calculated log [TOMACI] for different gold concentrations and [HCl] = 1.0 mol dmm3 in both systems are presented. In the case of impregnated resin, it should be pointed out that [TOMACI] = [TOMACI],,,, since gold concentration is very low compared to the TOMACl impregnated concentration. As seen in these figures the adjustment of the points to a slope of 1 in both cases can be made. Therefore, experimental data can be explained assuming the extraction of the species AuCl,(TOMACl) in both systems.
I. Villaescusa et al./Hydrometallurgy
??
18
[Au]=0,508 mmol dm-3
41 (1996) 303-311
. [Au]=O,Z64 mmol dm-3
309
r\ [Au]=O,lOl mmol dm-3
i’
1
7 ’ c! 0 -
0,6 I
-0,6
t -1
-1,2
-0,6
-0,6
-094
-092
0
6,2
log pOMACl(mol dm-3)] Fig. 5. Solid-liquid extraction of Au(IfI) in 1.0 mol dmm3 HCI. Log D plotted as a function of free TOMACl concentration at various initial A&II) concentrations. Line corresponds to a slope equal to I.
Comparing the results, it is clear that, in order to obtain similar very high concentration of TOMACl is needed using the solid-liquid 3.3. Stripping
gold extraction, system.
a
results
With thiourea solutions the extent of stripping found in the liquid-liquid system was more than 95%, independent of the initial gold concentration in the organic phase. In the case of TOMACl/XAD-2, 85% of gold recovery was obtained in the same time. In both cases, after the stripping the capacity of the extractant was the same. 3.4. Gold extraction
in the presence
of base metals Zn and Cu
3.4. I. Liquid-liquid extraction There was no influence on gold extraction when Cu(I1) and Zn(I1) were present in the same solution since the extraction of gold was more than 98%. On the other hand, the extraction of Cu(I1) and Z&I) by TOMACl was in all cases less than l%, in spite of being a non-selective extractant. The reason could be that the concentration of extractant in this study was not enough to form the stoichiometric species for C&I) and Z&I) to be extracted. 3.4.2. Solid-liquid extraction The results of the extraction of gold, zinc and copper are shown in Fig. 6. There was no extraction of copper and the greatest amount of zinc extracted was 4.59 mmol kg-’
I. Villaescusa
310
et al./Hydrometallurgy
41 (1996)
303-311
0,16
=
0,06 .
3
-E
.
0,04 0,02
.
I
0
. ??
.
L
i
L
c
.
.
:
0,0002
0
3
I:
.
0,0004
0,0006
0,0008
0,001
0,0012
.
0,0014
0,0016
0,0018
[metalli (mol dm-3) Fig. 6. Extraction
of A&II),
Zn(I1) and Cu(II) by XAD-2 impregnated
with TOMACI.
when the initial concentration of this metal was 1.5 mmol dme3. Other experiments performed with solutions containing zinc and gold demonstrated that gold can be selectively extracted from solutions containing zinc by controlling the stirring time (Table 1) because the kinetics of zinc have been proved to be slower than for gold [21-231. After 15 min gold is totally extracted and the extraction of zinc is less than 7%. In a second extraction step, the zinc recovery decreased to less than 2%.
4. Conclusions In this study, the same thermodynamic approach to determine with TOMACl in toluene and in XAD-2 was successfully AuCl,TOMACl is the extracted species.
Table 1 Gold and Zinc extraction Time
(min)
[Au], (mm01 dm
15 30 60 180 300 420
0.254 0.254 0.254 0.254 0.254 0.254
as a function 3,
gold extracted species used. In both cases,
of time
lZn1,
[A&g
[znl,,
(mmol dm- 3,
(mmol kg- ‘1
(mmol kg- ‘)
0.765 0.765 0.765 0.765 0.765 0.765
25.13 25.38 25.38 25.38 25.38 25.38
5.05 12.39 19.43 21.57 24.62 24.93
I. Villaescusa
et al./Hydrometallurgy
41 (1996)
303-311
311
The concentration of TOMACl in toluene can be much lower than in XAD-2 for obtaining the same gold recovery. This behaviour was also observed during the stripping procedure, for which a higher recovery was obtained in the liquid-liquid system for the same contact time. Both systems allow the selective extraction of gold in the presence of Zn(I1) and Cu(II), although, in order to separate A&II) and Zn(I1) using XAD-2, it was necessary to use two extraction steps.
Acknowledgements
This work has been supported by CICYT MAT 93-6212 (Ministerio de Education y Ciencia, Spain). The assistance of Mr. Jordi Corrales and Miss M” Angels Martos with the laboratory work is gratefully acknowledged.
References [l] Edwards, RI. and Te Riele, W.A.M., In: T.C. Lo, M.H.I. Baird and C. Hanson (Editors), Handbook of Solvent Extraction. Wiley, New York (1983). [2] Morris, M.H.I. and Ali Khan, M., Talanta, 16 (1968): 1301-1308. [3] Bjerrum, N., Bull. Sot. Chim. Beige, 57 (1938): 432-438. [4] Johnston, H.L. and Leland, H.L., J. Am. Chem. Sot., 60 (1938): 1439-1445. [5] Hligfeldt, E., Stability constants of metal-ion complexes. Part A: Inorganic Ligands (IUPAC Chemical Data Series, 2 1.) Pergamon, Oxford ( 1982). [6] Villaescusa, I., Miralles, N., De Pablo, J., Salvad6, V. and Sastre, A., Solvent Extr. Ion Exch., 1 l(4) (1993): 613-626. [7] Seeley, F.G. and Grouse, D.-J., J. Chem. Eng. Data, 1 l(3) (1966): 424-429. [8] Salvad6, V., Hidalgo, M., Masana, A., Muiioz, A., Valiente, M. and Muhammed, M., Solvent Extr. Ion Exch., 8(3) (1991): 491-502. [9] Edwards, I., Haines, A.K. and Te Riele, W.A.M., In: M. Streat (Editor), Theory and Practice of Ion Exchange. SCI, London ( 1976). [lo] Grate, M. and Kettrup, A., Anal. Chim. Acta, 172 (1985): 223-239. [ 111 Sato, T., J. Inorg. Nucl. Chem., 34 (1970): 3835-3843. [12] Sato, T. and Watanabe, H., Anal. Chim. Acta, 49 (1970): 463-471. [13] Bagreev, V.V., Fischer, C., Yudushkina, L.M. and Zolotov, Yu. A., J. Inorg. Nucl. Chem., 40 (1978): 553-5.57. [14] Irving, H.M.N.H. and Damodaran, A.D., The extraction of complex cyaniges by liquid ion exchangers. Anal. Chim. Acta, 53 (1971): 267-275. [15] Shivrin, G.N., Basov, AS., Laskorin, B.N. and Shivrina, E.M., Extracting noble metals from cyanide solutions with quaternary ammonium compounds. Tsvet. Metal., 39 (1966): 19-23. [16] Warshawsky, A., Talanta, 21 (1974): 624-626. [l7] Warshawsky, A., Talanta, 21 (1974): 962-965. [18] Warshawsky, A., Trans. Inst. Min. Metall., 83 (1975): 101-104. [19] Warshawsky, A., In: J.A. Marinsky and Y. Marcus (Editors), Ion Exchange and Solvent Extraction. Marcel1 Dekker, New York (1981), pp. 229-281. [20] Villaescusa, I., Salvadb, V., De Pablo, J., Valiente, M. and Aguilar, M., Reactive Polymers, 17 (1992): 69-73. [21] Riveros, P.A. and Cooper, W.Ch., Solvent Extr. Ion Exch., 6(3) (1988): 479-503. [22] Kar-On, L.B. and Hudson, M.J., Solvent Extr. Ion Exch., 10(l) (1992): 173-190. [23] Riveros, P.A., Studies on the solvent extraction of gold from cyanide media. Hydrometallugy, 24 t 1990): 135- 156.
Liquid-liquid and solid-liquid extraction of gold by trioctylmethylammonium chloride ( TOMACl) dissolved in toluene and impregnated on amberlite XAD-2 resin I. Villaescusa aT*, V. Salvad6 b, J. de Pablo ’ aE.QA.T.A. Department, E.P.S., Universitat de Gironu, 17003 Girona, Spain b Chemistry Department, F.C.E.S., Universitat de Girona, 17001 Girona, Spain ’ Chemical Engineering Department, E.T.S.E.I.B., Universitat Politecnica de Catalunya, 08028 Barcelona, Spain Received
16 February
1995; accepted 5 May 1995
Abstract The extraction of A&II) with the extractant trioctylmethylammonium chloride (TOMACI) dissolved in toluene and the same extractant impregnated in a polymeric resin XAD-2 has been studied. The results of gold extraction with both systems were compared. They showed similar gold extraction behaviour (extracted species), although greater amounts of extractant were needed when impregnated resins were used. Liquid-liquid and solid-liquid procedures can be used to extract gold selectively from solutions containing zinc and copper.
1. Introduction
The recovery of gold and other precious metals from secondary grade sources such as electronic scrap, spent catalysts and metal concentrates [1,2] is by an initial treatment with aqua regia. Under such conditions, gold is oxidized to gold(II1) and forms chloro-complexes, the predominant species being AuCl, [3,4], while base metals such as copper and zinc can also form chloro-complexes [5].
* Corresponding
author.
0304-386X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0304-386X(95)00064-X
304
I. Villaescusa et al./Hydrometallurgy
41 (1996) 303-31 I
Solvent extraction is an alternative method for the separation and purification of gold. This method has been applied to extract gold(II1) from chloride solutions using anion-exchange [6,7] and ligand-exchange systems [8]. In this method, commercial quatemary ammonium salts are the most useful anions extractants, in particular tri-octylmethylammonium chloride (TOMACI) has been widely used in metal extraction studies [7,9-l I]. Studies of the extraction of Au(II1) with TOMACl in cyanide medium showed a better extraction of this metal compared to copper and iron extraction results [12,13]. On the other hand, solid-liquid extraction has also been applied to extract Au(II1) from chloride solutions [ 14,151. Solvent-impregnated resins (SIR), introduced by Warshawsky [ 16,171, have been proposed as an alternative method in hydrometallurgical applications [ 18,191. Recently, the extraction of Au(II1) with tri-isobutylphosphine sulfide impregnated in Amberlite XAD-2 resin has been carried out [20]. The aim of the present investigation is to study the extraction of Au(II1) with TOMACl dissolved in toluene and TOMACI impregnated in a polymeric support, Amberlite XAD-2 resin, in order to compare these methodologies. The effect of base metals, such as copper and zinc, during the Au(II1) extraction is also studied, as well as the stripping procedure.
2. Experimental 2.1. Reagents and solutions Stock solutions of Au(II1) were prepared from HAuCl, (s) (Merck A.R.) in HCl (Merck A.R.) 1 mol dmd3. Trioctylmethylammonium chloride (TOMACI), the commercial reagent called Aliquat 336, (Merck A.R.) was used without further purification. Solutions containing 0.5 mol dm -3 of thiourea (Merck A.R.) were used in gold stripping procedures. Methanol (Merck A.R.) was used without further purification. Toluene (Panreac A.R.) was purified by washing with 0.5 mol dm-’ HCl, distilled water, and 0.5 mol dme3 NaOH alternatively. Amberlite XAD-2 was purified with a 4.0 mol dmw3 HCl solution and, after elimination of chlorides by washing with distilled water, with a methanol-water (1:l) solution and washed with water again. Standard solutions of ZnCl, (15.3 mmol dmm3) and CuCl, (15.7 mmol dmm3) (Carlo Hrba) were used. 2.2. Procedure 2.2.1. Liquid-liquid extraction Experiments were performed at 25°C by shaking equal volumes (10 ml) of the organic phase and aqueous solutions in a rotatory mixer for 1 h. Previous experiments showed that after 30 min the equilibrium had been attained. The organic phase consisted of TOMACl solutions in toluene. The total extractant concentration was varied within the range 0.05-0.5 mmol dmm3. The total Au(II1) concentration in the aqueous phase was varied between 0.106 and 0.218 mmol dmm3.
I. Villaescusa et al./ Hydrometallurgy 41 (1996) 303-31 I
305
From the different reagents for stripping of gold, thiourea was found to be the most effective. Equal volumes (7 ml> of the organic phase containing gold were contacted with a 0.5 mol dme3 thiourea solution for 30 min. The separation of gold from solutions containing Z&I) and Cu(I1) was carried out by shaking equal volumes (10 ml) of the organic and aqueous phase for 1 h. The aqueous phase contained a fixed concentration of gold (0.164 mmol dmm3) and the concentration of zinc(H) and copper(R) was varied between 6.0 and 12.0 mmol dme3 and 6.3 and 11.0 mmol dme3, respectively. The organic phase was a 0.5 mmol dme3 TOMACl solution in toluene. 2.2.2. Solid-liquid extraction 2.2.2.1. Impregnation procedure. Amounts of 0.1 g of dry XAD-2 were contacted with different concentrations of TOMACl (0.1-500 mmol drnm3) dissolved in methanol for 3 h until equilibrium was reached. The impregnated resins were separated from the organic solutions by filtration through a porous filter using a water pump. The TOMACl 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 the mass balance. 2.2.2.2.
Gold extraction. Gold extraction was carried out by putting in contact 0.1 g of impregnated resin with 10 ml of A&II) solutions for 1 h. In this case preliminary experiments showed that after 45 min the equilibrium had been reached. The resins had been impregnated with different concentrations of TOMACl in methanol (81-1200 mmol dmm3). After filtration of the resins the gold concentration in the remaining aqueous solutions was determined by atomic absorption spectrometry. In these experiments the initial gold concentration was varied between 0.1 and 5.0 mmol dmm3. The stripping of gold was performed by shaking 0.1 g of impregnated resin containing gold with 20 ml of a 0.5 mol dm-3 thiourea solution for 30 min. The extraction of Au(III), Cu(I1) and Zn(I1) by the system TOMACl/XAD-2 was studied by shaking 0.1 g of impregnated resin with 10 ml of the metals in 1.0 mol dm - 3 HCl until equilibrium was reached. The concentration range was Au(II1): 0.0508-1.52 mmol dmm3; Zn(I1): 0.153-4.59 mmol dme3; and Cu(I1): 0.157-4.72 mmol dmm3.
3. Results and discussion 3.1. TOMACl/XAD-2
impregnation
Results of impregnation of the XAD-2 as a function of the load concentration of TOMACl in methanol are shown in Fig. 1. It can be seen that, for a concentration of TOMACl of 0.15 mol dme3, the capacity of the resin reaches saturation. The maximum concentration in the resin was 0.033 mol TOMACl kg-’ dry resin. Therefore, this concentration was used to study the gold extraction in this system.
I. Villaescusa
306 0,035
et al./Hydrometallurgy
I
41 (1996)
303-311
.
.
0,03 r 9 Z .E qJ 3 ,E C 0 2
0,025
.
0,02
.
0,015
. 0,Ol
.
ot 0,005
t
.
0
I I
0
0,05
0,15
021
0.2
0.25
[TOMACI] in methanol (mol dm-3) Fig. 1. Impregnation
of XAD-2 with TOMACI.
3.2. Gold extraction The distribution XAD-2) containing
coefficient of the metal between the organic phase (toluene or TOMACl and the aqueous phase can be defined in both cases as:
D = [Au(III)l.&Au(III)l,,
(1)
where [Au(III)],,, denotes the total concentration of Au(II1) in the resin in moles per kilogram or in toluene in moles per cubic decimeter and [Au(III)I, is the total concentration in the aqueous phase. In the case of the impregnated resin, due to the different phase relationship the distribution coefficient can be expressed as: D =
{(b(~m, - b~mlaq) x wm)}/[Awhl
(2)
are the initial and the equilibrium concentrations of where [Au(III)], and [Au(III)],, metal in the aqueous phase, respectively; V is the volume in litres of the aqueous phase; and m is the mass in kilograms of dry impregnated resin. Gold distribution data with both systems were plotted as log D versus log [~0h4.4,,, , as shown in Figs. 2 and 3. 3.2.1. Treatment of data The extraction of gold can be described for the experimental conditions used in this work, high chloride concentration and low pH, taking into account only the tetrachlorocomplex AuCl, by using the following reaction: AuCl;
+ nTOMACl(
org) CJ AuCl,(TOMACl)
Jorg)
+ Cl-
(3)
I. Villaescusa et al./Hydrometallurgy
41 (1996) 303-311
307
25 L__
[Au]=O,lOGmmol dm-3
n [Au]=O,164 mmol dm-3
2 --
1,s
0
1
E 03
[Au]=O,216 mmol dm-3
??
.
.
I .
0 .
0
i
.
.*
:
u
t
m
.
0
-1 ’
I
-4,4
-433
-492
-4,l
4
-3,9
-3,6
-3,7
-3,6
log~OMACl(mol dm-3)]tot Fig. 2. Liquid-liquid extraction of Au(III) in 1.O mol dme3 TOMACl concentration at various initial Au(II1) concentrations.
1,6 __
’ [Au]=O,506 mmol dm-3
HCl. Log D plotted
n [Au]=0,264 mmol dm-3
as a function
A [Au]=O,lOl mmol dm-3
194 --
.
1 --
: .
7 ;
0,6 --
n m -P 0,2
of total
. .
’
:. * .
4,2 --0,6 --1 -I,2
-1
-0,9
-0,6 log ~OMACl(mol
-0,4
-0,2
0
032
dm-3)]tot
Fig. 3. Solid-liquid extraction of A&II) in 1.0 mol dmm3 HCI. Log D plotted as a function of total TOMACl concentration at various initial Au(II1) concentrations.
I. Villaescusa et al./Hydrometallurgy
308 0,5
I 1
??
[Au]=O,lO6mmol dm-3
41 (1996) 303-31 I
o [Au]=O,164 mmol dm-3
??
[Au]=O,218 mmol dm-3
094 093 0,2 091 n 0 E -0,l -0,Z -0,3 -0,4 -0,5 -7-2
-7,1
-7
-6,s
-6,8
-6,7
log p’OMACl(mol
-6,s
-6,5
-6,4
-6,3
dm-3)]
Fig. 4. Liquid-liquid extraction of A&II) in 1.O mol dm -3 HCl. Log D plotted as a function of free TOMACI concentration at various initial A&II) concentrations. Line corresponds to a slope equal to 1.
considering ideal behaviour in the organic phase and assuming the activity coefficients of the aqueous phase to be constant, the equilibrium constant of reaction (3) can be written as: K = [AUCI,(TOMACI),,][C~-] n [AUC~;] [TOMACI]”
(4)
where no polynuclear species have been considered in the organic phase or in the aqueous phase, due to the low concentration of metal used. Taking into account the definition of D, the expression can be written: logs = log~,J~l-I-’
+ ~~~~[TOMACI]
(5)
So, a plot of log D against log [TOMACI] should give a straight line with the ordinate axis equal to KJCl-]’ and slope of n. The concentration of free TOMACl can be calculated from the mass balance equation: [TOMACI] = [TOMACI],,, - [AU],,%
(6)
In Figs. 4 and 5, the plots of log D versus the calculated log [TOMACI] for different gold concentrations and [HCl] = 1.0 mol dmm3 in both systems are presented. In the case of impregnated resin, it should be pointed out that [TOMACI] = [TOMACI],,,, since gold concentration is very low compared to the TOMACl impregnated concentration. As seen in these figures the adjustment of the points to a slope of 1 in both cases can be made. Therefore, experimental data can be explained assuming the extraction of the species AuCl,(TOMACl) in both systems.
I. Villaescusa et al./Hydrometallurgy
??
18
[Au]=0,508 mmol dm-3
41 (1996) 303-311
. [Au]=O,Z64 mmol dm-3
309
r\ [Au]=O,lOl mmol dm-3
i’
1
7 ’ c! 0 -
0,6 I
-0,6
t -1
-1,2
-0,6
-0,6
-094
-092
0
6,2
log pOMACl(mol dm-3)] Fig. 5. Solid-liquid extraction of Au(IfI) in 1.0 mol dmm3 HCI. Log D plotted as a function of free TOMACl concentration at various initial A&II) concentrations. Line corresponds to a slope equal to I.
Comparing the results, it is clear that, in order to obtain similar very high concentration of TOMACl is needed using the solid-liquid 3.3. Stripping
gold extraction, system.
a
results
With thiourea solutions the extent of stripping found in the liquid-liquid system was more than 95%, independent of the initial gold concentration in the organic phase. In the case of TOMACl/XAD-2, 85% of gold recovery was obtained in the same time. In both cases, after the stripping the capacity of the extractant was the same. 3.4. Gold extraction
in the presence
of base metals Zn and Cu
3.4. I. Liquid-liquid extraction There was no influence on gold extraction when Cu(I1) and Zn(I1) were present in the same solution since the extraction of gold was more than 98%. On the other hand, the extraction of Cu(I1) and Z&I) by TOMACl was in all cases less than l%, in spite of being a non-selective extractant. The reason could be that the concentration of extractant in this study was not enough to form the stoichiometric species for C&I) and Z&I) to be extracted. 3.4.2. Solid-liquid extraction The results of the extraction of gold, zinc and copper are shown in Fig. 6. There was no extraction of copper and the greatest amount of zinc extracted was 4.59 mmol kg-’
I. Villaescusa
310
et al./Hydrometallurgy
41 (1996)
303-311
0,16
=
0,06 .
3
-E
.
0,04 0,02
.
I
0
. ??
.
L
i
L
c
.
.
:
0,0002
0
3
I:
.
0,0004
0,0006
0,0008
0,001
0,0012
.
0,0014
0,0016
0,0018
[metalli (mol dm-3) Fig. 6. Extraction
of A&II),
Zn(I1) and Cu(II) by XAD-2 impregnated
with TOMACI.
when the initial concentration of this metal was 1.5 mmol dme3. Other experiments performed with solutions containing zinc and gold demonstrated that gold can be selectively extracted from solutions containing zinc by controlling the stirring time (Table 1) because the kinetics of zinc have been proved to be slower than for gold [21-231. After 15 min gold is totally extracted and the extraction of zinc is less than 7%. In a second extraction step, the zinc recovery decreased to less than 2%.
4. Conclusions In this study, the same thermodynamic approach to determine with TOMACl in toluene and in XAD-2 was successfully AuCl,TOMACl is the extracted species.
Table 1 Gold and Zinc extraction Time
(min)
[Au], (mm01 dm
15 30 60 180 300 420
0.254 0.254 0.254 0.254 0.254 0.254
as a function 3,
gold extracted species used. In both cases,
of time
lZn1,
[A&g
[znl,,
(mmol dm- 3,
(mmol kg- ‘1
(mmol kg- ‘)
0.765 0.765 0.765 0.765 0.765 0.765
25.13 25.38 25.38 25.38 25.38 25.38
5.05 12.39 19.43 21.57 24.62 24.93
I. Villaescusa
et al./Hydrometallurgy
41 (1996)
303-311
311
The concentration of TOMACl in toluene can be much lower than in XAD-2 for obtaining the same gold recovery. This behaviour was also observed during the stripping procedure, for which a higher recovery was obtained in the liquid-liquid system for the same contact time. Both systems allow the selective extraction of gold in the presence of Zn(I1) and Cu(II), although, in order to separate A&II) and Zn(I1) using XAD-2, it was necessary to use two extraction steps.
Acknowledgements
This work has been supported by CICYT MAT 93-6212 (Ministerio de Education y Ciencia, Spain). The assistance of Mr. Jordi Corrales and Miss M” Angels Martos with the laboratory work is gratefully acknowledged.
References [l] Edwards, RI. and Te Riele, W.A.M., In: T.C. Lo, M.H.I. Baird and C. Hanson (Editors), Handbook of Solvent Extraction. Wiley, New York (1983). [2] Morris, M.H.I. and Ali Khan, M., Talanta, 16 (1968): 1301-1308. [3] Bjerrum, N., Bull. Sot. Chim. Beige, 57 (1938): 432-438. [4] Johnston, H.L. and Leland, H.L., J. Am. Chem. Sot., 60 (1938): 1439-1445. [5] Hligfeldt, E., Stability constants of metal-ion complexes. Part A: Inorganic Ligands (IUPAC Chemical Data Series, 2 1.) Pergamon, Oxford ( 1982). [6] Villaescusa, I., Miralles, N., De Pablo, J., Salvad6, V. and Sastre, A., Solvent Extr. Ion Exch., 1 l(4) (1993): 613-626. [7] Seeley, F.G. and Grouse, D.-J., J. Chem. Eng. Data, 1 l(3) (1966): 424-429. [8] Salvad6, V., Hidalgo, M., Masana, A., Muiioz, A., Valiente, M. and Muhammed, M., Solvent Extr. Ion Exch., 8(3) (1991): 491-502. [9] Edwards, I., Haines, A.K. and Te Riele, W.A.M., In: M. Streat (Editor), Theory and Practice of Ion Exchange. SCI, London ( 1976). [lo] Grate, M. and Kettrup, A., Anal. Chim. Acta, 172 (1985): 223-239. [ 111 Sato, T., J. Inorg. Nucl. Chem., 34 (1970): 3835-3843. [12] Sato, T. and Watanabe, H., Anal. Chim. Acta, 49 (1970): 463-471. [13] Bagreev, V.V., Fischer, C., Yudushkina, L.M. and Zolotov, Yu. A., J. Inorg. Nucl. Chem., 40 (1978): 553-5.57. [14] Irving, H.M.N.H. and Damodaran, A.D., The extraction of complex cyaniges by liquid ion exchangers. Anal. Chim. Acta, 53 (1971): 267-275. [15] Shivrin, G.N., Basov, AS., Laskorin, B.N. and Shivrina, E.M., Extracting noble metals from cyanide solutions with quaternary ammonium compounds. Tsvet. Metal., 39 (1966): 19-23. [16] Warshawsky, A., Talanta, 21 (1974): 624-626. [l7] Warshawsky, A., Talanta, 21 (1974): 962-965. [18] Warshawsky, A., Trans. Inst. Min. Metall., 83 (1975): 101-104. [19] Warshawsky, A., In: J.A. Marinsky and Y. Marcus (Editors), Ion Exchange and Solvent Extraction. Marcel1 Dekker, New York (1981), pp. 229-281. [20] Villaescusa, I., Salvadb, V., De Pablo, J., Valiente, M. and Aguilar, M., Reactive Polymers, 17 (1992): 69-73. [21] Riveros, P.A. and Cooper, W.Ch., Solvent Extr. Ion Exch., 6(3) (1988): 479-503. [22] Kar-On, L.B. and Hudson, M.J., Solvent Extr. Ion Exch., 10(l) (1992): 173-190. [23] Riveros, P.A., Studies on the solvent extraction of gold from cyanide media. Hydrometallugy, 24 t 1990): 135- 156.