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Ion-exchange equilibria are presented for [Au(CN)2]-/Cl- and [Au(CN)2]-/SCN- in dimethyl sulfoxide (DMSO) + water and N-methyl-2-pyrrolidone (NMP) + w...
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7496

Ind. Eng. Chem. Res. 2005, 44, 7496-7504

Ion-Exchange Equilibria for [Au(CN)2]-/Cl- and [Au(CN)2]-/SCN- on Purolite A500 in Mixed Solvents at 303 K Nivari S. Jayasinghe, Frank P. Lucien,* and Tam Tran School of Chemical Engineering and Industrial Chemistry, The University of New South Wales (UNSW), Sydney, NSW 2052, Australia

Ion-exchange equilibria are presented for [Au(CN)2]-/Cl- and [Au(CN)2]-/SCN- in dimethyl sulfoxide (DMSO) + water and N-methyl-2-pyrrolidone (NMP) + water mixed solvents at 303 K, using Purolite A500 as the ion-exchanger. The effects of mixed-solvent composition and the type of counterion on the selectivity of the ion-exchange resin for [Au(CN)2]- is discussed in terms of the degree of solvation of the various anions in the mixed solvents. The effect of the mixed solvent on the exchange capacity of the resin is also considered. The experimental data are correlated using the law of mass action, modified with activity coefficients, to determine the equilibrium constant for each binary system. It is shown that the selectivity of the resin for [Au(CN)2]- decreases significantly with an increase in the composition of organic solvent in the mixed solvent. The particular combination of Cl- in NMP-water mixtures is remarkably effective for reducing the loading of [Au(CN)2]- on Purolite A500. The fitted values of the equilibrium constants are consistent with the trends observed in the corresponding ion-exchange isotherms. Introduction Carbon adsorption is the principal means by which gold, in the form of its cyanide complex, is recovered from leach solutions in commercial mining operations. In the past decade, however, ion exchange has gradually emerged as a viable alternative to carbon adsorption.1,2 Ion-exchange resins have a much higher capacity than carbon for loading gold and other base-metal cyanide complexes and also offer the possibility of recycling cyanide. The nonselective nature of ion-exchange resins is one of the key issues that must be resolved to enable their more widespread use in the gold industry. The concentration of base-metal cyanide complexes in leach solutions is considerably higher than that of the gold cyanide complex, [Au(CN)2]-. The base-metal cyanide complexes compete strongly with [Au(CN)2]- for sites on the resin and, consequently, the capacity of the resin for gold decreases dramatically.3 Research aimed at mitigating this problem has focused on making the resin more selective for gold and on the development of selective elution procedures for recovering the complexes from the resin. The main factors that control the selectivity of the resin for [Au(CN)2]- are the nature of the functional groups, the ionic density of the resin, and the hydrophilicity of the polymer matrix.4,5 Strong-base anionexchange resins with triethylammonium groups, instead of the conventional trimethylammonium groups, exhibit selectivity for [Au(CN)2]- and bivalent cyanide complexes over multivalent complexes such as [Fe(CN)6]4and [Cu(CN)4]3-. Multivalent complexes require functional groups in close proximity to accommodate the high charge density of these complexes. The low charge density of [Au(CN)2]- reduces its hydration requirements and renders it less hydrophilic than the multivalent complexes. Consequently, resins with a low ionic * To whom correspondence should be addressed. Tel.: +612-9385-4302. Fax: +61-2-9385-5966. E-mail: f.lucien@ unsw.edu.au.

density and a low degree of hydrophilicity are more selective for [Au(CN)2]-. The selective elution of cyanide complexes from the resin is normally accomplished with sequential elution steps in which the base-metal cyanide complexes are eluted first using dilute acid solutions or aqueous cyanide solutions.3 Highly concentrated chloride solutions are also effective at selectively eluting base-metal cyanide complexes.6,7 In conventional practice, the elution of [Au(CN)2]- is achieved most easily with an aqueous solution containing another cyanide complex, such as Zn(CN)42-, as the counterion. A disadvantage in all of these procedures is the formation of HCN during the elution and subsequent regeneration of the resin. From an environmental perspective, HCN formation is undesirable and necessitates further processing to eliminate the presence of this species in waste streams. Nonaqueous solvents and mixed solvents have been investigated for many years as a way of altering selectivities in both cation and anion exchange processes.8-12 Here, the term “mixed solvent” is used to describe a mixture of water and an organic solvent. The composition of the mixed solvent provides an additional variable that can be manipulated to improve separation efficiencies. The effect of modifying the solvent composition on selectivity is often more dramatic than the effect of changing variables such as ionic strength and temperature in an aqueous system. This phenomenon is attributed to the changes in the activities of the ions resulting from the solvation process in the mixed solvent.10,13,14 Mixed solvents have been shown to be highly effective for the elution of [Au(CN)2]- from activated carbon.15-17 In comparison to carbon, fewer studies have been devoted to the use of mixed solvents for the recovery of the complex from ion-exchange resin. Burstall et al.18 first demonstrated the elution of [Au(CN)2]- from anionexchange resin with mixed solvents containing simple alcohols or acetone and 5%-10% HCl. Law et al.19 later

10.1021/ie0504304 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/18/2005

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considered the use of a number of dipolar aprotic solvents, including dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP), with SCN- as the counterion. All of the mixed solvents considered were more effective than aqueous solutions for the separation of [Au(CN)2]- from the resin. These early studies have only considered the effect of mixed solvents over a narrow range of composition. Although the feasibility of using mixed solvents for gold elution has been established, there is limited knowledge on the selectivity of the resin for [Au(CN)2]in mixed solvents. In addition, prior studies have only considered the effect of mixed solvents over a narrow range of composition. The principle means of assessing the selectivity of the resin for a given species is through the measurement of binary ion exchange equilibria. We have previously reported binary ion-exchange equilibria for [Au(CN)2]-/Cl-, and [Au(CN)2]-/SCN- in aqueous solution using commercially available Purolite A500 as the ion-exchanger.20 In this work, binary ion-exchange equilibria are presented for the same systems in DMSOwater and NMP-water mixtures over a wide range of solvent composition. Experimental Section Chemicals. Potassium aurocyanide (99%) and potassium thiocyanate (99%) were supplied by EBS and Associates and Sigma-Aldrich, respectively. The anionexchange resin, Purolite A500, was supplied by Purolite International and consisted of 0.5-mm spherical beads. Purolite A500 is a Type 1 strong base, divinylbenzene, macroporous resin in chloride form. Prior to use, the resin was soaked in distilled water for 24 h to stabilize resin swelling. DMSO was supplied by Asia Pacific Specialty Chemicals (>99% purity), whereas NMP was supplied by Sigma-Aldrich (>99% purity). Procedure. Ion-exchange equilibria were determined using a batch experimental method. A detailed description of the apparatus and procedure has been presented in earlier work on ion-exchange equilibria in aqueous solution.20 A 5-mL sample of the resin (wet and settled volume in water) was repeatedly contacted with 100 mL of a mixed-solvent stock solution containing a fixed concentration of the loading anion. For each contact, the resin and solution phases were allowed to reach equilibrium. By gradually loading the resin in this way, it was possible to obtain ion-exchange equilibria encompassing a wide range of composition for each phase. For the [Au(CN)2]-/Cl- binary system, the Cl- form of the resin was contacted with a stock solution containing [Au(CN)2]-. For the [Au(CN)2]-/SCN- binary system, the SCN- form of the resin was contacted with a stock solution containing [Au(CN)2]-. The SCN- form of the resin was prepared by contacting the as-supplied resin (Cl- form) with an aqueous solution containing an excess concentration of SCN-. A mass balance was used to confirm that the as-supplied resin was fully converted to the SCN- form. Using knowledge of the change in concentration of the loading anion in the solution phase, it was possible to calculate the composition of [Au(CN)2]- on the resin phase at equilibrium. Because the same sample of resin was gradually loaded with [Au(CN)2]-, it was not possible to analyze the resin phase for gold directly using conventional incineration methods after each contact with the stock solution. A mass balance was also

used to determine the equilibrium compositions of the resin and solution phases, with respect to Cl- or SCN-. The concentration of [Au(CN)2]- in solution was determined by analysis for gold using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The standard uncertainty of the analysis was (1%. In the ICP-AES analysis, calibration curves were established by preparing standards representative of the mixedsolvent composition in the samples. Ion-exchange equilibria are expressed in terms of equivalent ionic fractions for the solution and resin phases (see eqs 3 and 4). The experimental ionic fraction data that are reported represent the average of duplicate runs, with an uncertainty of (5%. A much longer duration of time was required to attain equilibrium in the mixed-solvent systems, in comparison with ion-exchange equilibria determined in aqueous media. In preliminary work, it was established that a duration of 1-4 days was required for the attainment of equilibrium between the resin phase and the solution phase, although the majority of the loading of gold was achieved within the first 24 h. It was determined that the equilibration time was strongly dependent on the initial resin loading and the composition of the mixed solvent. Generally, mixed solvents that contain high levels of organic solvent and partially loaded resin lead to the longer equilibration times. In view of the influence of these factors, a duration of 4 days was used in all of the subsequent experiments in DMSO-water and NMPwater mixtures. Because the purpose of this work was to examine the effect of mixed-solvent composition on the selectivity of the resin for [Au(CN)2]-, we used mixed-solvent stock solutions that contained the same initial concentration of [Au(CN)2]- in all experiments. In our previous work, it was also demonstrated that the ion-exchange isotherms for the given binary systems in aqueous solution are independent of the total solution concentration, within the range of concentration of 0.0025-0.0066 M. Therefore, in this work, ion-exchange equilibria were obtained with stock solutions that contained [Au(CN)2]at a nominal concentration of 0.005 M. The effect of mixed-solvent composition on the ionexchange isotherm for a given binary system was examined by varying the amount of organic solvent in the stock solution. To simplify the preparation process, stock solutions were initially prepared on the basis of an unmixed volume of water and organic solvent. The nominal volume of all stock solutions was 1000 mL. The compositions of the stock solutions were subsequently converted to mole percentage. The compositions of DMSO-water stock solutions prepared in this way were 5.91, 14.3, 27.4, and 69.3 mol %, with respect to DMSO. The compositions of NMP-water stock solutions were 4.46 and 62.7 mol %, with respect to NMP. The loading anion was incorporated into the stock solution as follows. A known mass of KAu(CN)2 was weighed into a volumetric flask. The salt was initially dissolved in distilled water. After the salt had completely dissolved, the required amount of DMSO or NMP was added to the flask. During the addition of the organic solvent, some heat was released from the mixture, followed by a slight contraction in the volume of the solution (1%-2%). The reduction in volume of the solution was taken into account in the calculation of the concentration of the loading anion in each stock solution.

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Data Correlation When an ion-exchanger (r) of form A is placed in a solution (s) of ion B, resulting in the displacement of ion A for ion B, an equilibrium is eventually reached in which each species is present in both phases. The equilibrium condition can be described by βRββARr + RBs S βAs + RBr

(1)

The proportions of ions A and B in the two phases is primarily governed by the selectivity of the resin. The selectivity exhibited by an ion exchanger can be represented by an equilibrium constant in accordance with the law of mass action modified with activity coefficients.21,22 In relation to eq 1, the equilibrium constant is

KAB )

(γAcA)β(γ j Bq B) R (γ j AqA)β(γBcB)R

(2)

where ci is the molar concentration of ion i in the solution phase, qi is the molar concentration of ion i in the resin phase, and KAB refers to the equilibrium constant for B entering the resin phase and displacing A. The activity coefficients of ion i in the solution j i, phase and resin phase are denoted by γi and γ respectively. Here, the molar concentrations in the resin phase are calculated with respect to the wet and settled volume of the ion-exchange resin. The use of eq 2 to describe the equilibrium constant assumes that swelling-pressure effects are negligible.23 As a rule, the equilibrium concentrations of ions in the resin and solution phases are expressed in terms of equivalent ionic fractions. Equivalent concentrations take into account the charge of the exchanging species and, therefore, better represent an ion-exchange system. However, for competing ions of equal charge, the equivalent ionic fraction is equal in value to the mole fraction. The molar concentration terms in eq 2 can be replaced by equivalent ionic fractions, using the following relations:

xi ) yi )

zici N ziqi Q

(γAxA)(γ j B yB ) (γ j AyA)(γBxB)

c

5.91 14.3 27.4 69.3

77.47 74.72 70.42 56.66

DMSO-Water 0.51 0.54 0.59 0.81

3.282 3.342 3.443 3.838

4.46 62.7

73.72 40.29

NMP-Water 0.55 1.35

3.365 4.552

A ((mol/L)-1/2)

a Parameters calculated according to Robinson and Stokes.26 DMSO ) dimethyl sulfoxide; NMP ) N-methyl-2-pyrrolidone. b Composition refers to the amount of organic solvent. c Data are taken from ref 27.

loading anion in the solution phase was observed to be negligible. In aqueous systems, the apparent capacity of the resin was in close agreement with the theoretical capacity of the resin of 1.3 equiv/L. In this work, however, we were unable to satisfactorily determine the apparent capacity of the resin in mixed solvents containing in excess of 25 mol % organic solvent. This aspect is discussed further in the Results and Discussion section. For these mixed solvents, the resin loading did not approach the theoretical capacity of the resin with repeated exposure of the resin to the stock solution. Furthermore, it was not clear from the experimental data whether the loading of the counterion in each case approached a limit that was lower than the theoretical exchange capacity. In view of this uncertainty, the capacity of the resin was set to 1.3 equiv/L for the construction of ion-exchange isotherms in all of the DMSO-water and NMP-water mixtures. Individual ionic activity coefficients appearing in eq 5 are usually estimated with appropriate assumptions. Kielland24 suggested that individual ionic activity coefficients in sufficiently dilute solutions maybe calculated from an extended form of the Debye-Hu¨ckel limiting law. Compared to the Pitzer25 method, the DebyeHu¨ckel approach requires fewer parameters, all of which are available for the ionic species relevant to this study. The extended form of the Debye-Hu¨ckel limiting law is expressed as follows:

log γi ) (4)

(5)

The calculation of yi in eq 4 requires the capacity of the resin. In our previous work, the apparent capacity was determined by repeatedly contacting the same sample of resin with a fresh quantity of stock solution until no further loading of counterion occurred. This condition was confirmed when the change in concentration of the

10-7 B ((mol/L)-1/2 cm-1)

compositionb (mol %)

(3)

where xi and yi are the equivalent ionic fractions in the solution phase and resin phase, respectively, zi is the valence of ion i, N is the total normality of exchanging ions in the solution phase, and Q is the capacity of the resin (equiv/L). Substitution of eqs 3 and 4 into eq 2, and noting that R ) β ) 1 for all species in this study, leads to the following expression for the equilibrium constant:

KAB )

Table 1. Debye-Hu 1 ckel Parameters for Mixed Solvents at 303 Ka

I)

1 2

Azi2xI 1 + BaixI

∑i zi2ci

(6)

(7)

where A and B are the Debye-Hu¨ckel parameters, ai is the effective diameter of the hydrated ion, and I is the ionic strength of the solution phase (in molar units). This semiempirical approach is often used to estimate the mean ionic activity coefficients, which are measurable. Values of ai pertinent to this study are given elsewhere.20 The Debye-Hu¨ckel parameters appearing in eq 6 (A and B) are equivalent to those in the corresponding equation for the mean ionic activity coefficient and are dependent on temperature (T) and the dielectric constant () of the solvent.26 The dielectric constants and Debye-Hu¨ckel parameters for the DMSO-water and NMP-water mixtures are listed in Table 1. The Wilson equations are commonly used to calculate the resin phase activity coefficients. This method has

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been applied successfully in the correlation of binary ion-exchange equilibria for cationic systems in nonaqueous and mixed solvents.8 The relevant equations for the exchange of ions A and B are

ln γ j A ) 1 - ln(yA + yBΛAB) yBΛBA yA (8) yA + yBΛAB yB + yAΛBA ln γ j B ) 1 - ln(yB + yAΛBA) yAΛAB yB (9) yB + yAΛBA yA + yBΛAB where ΛAB and ΛBA are the Wilson parameters. In view of the fact that ΛAB and ΛBA are adjustable parameters, it was not possible to calculate the equilibrium constant directly from eq 5. Instead, KAB, ΛAB, and ΛBA for each binary system were regressed from the experimental data (three-parameter regression). The working equation for this calculation procedure is obtained by rearranging eq 5 as follows:

yB )

KABγ j AxBγB KABγ j AxBγB + (1 - xB)γAγ jB

(10)

Equation 10 was used to generate calculated values of yB from the experimental values of xB and for given values of KAB, ΛAB, and ΛBA. The optimum values of KAB, ΛAB, and ΛBA, for a given isotherm, were obtained by minimizing the sum of squared relative deviations (SSRD), with respect to the resin phase composition: M

SSRD )

∑ i)1

(

)

exp ycalc B - yB

yBexp

2

(11)

i

exp where M is the number of data points and ycalc B and yB , respectively, are the calculated and experimental values of the equivalent ionic fraction of ion B in the resin phase. In the discussion that follows, the average absolute relative deviation (AARD) is defined as

AARD )

1

M

∑

Mi)1

|

|

exp ycalc B - yB

yexp B

(12) i

Mehablia et al.28 have described a method in which the equilibrium constant is first calculated independently and then used in the regression of ΛAB and ΛBA from the experimental data (two-parameter regression). The advantage of this method is that it decouples the intercorrelation between KAB and the pair of Wilson parameters. However, experimental data are required over the entire range of resin composition (0 < yB < 1) to calculate KAB. If this is not the case, then KAB has a tendency to be underestimated. In addition, the twoparameter regression does not necessarily produce more-accurate results than the three-parameter regression.20 In view of the limited range of resin-phase composition in some of the mixed solvent systems, this method was not applied in this work. Results and Discussion Capacity of the Resin in Mixed Solvents. There is some evidence in the literature that the use of

nonaqueous solvents in ion exchange affects the exchange capacity of the resin. In most cases, capacities in excess of the maximum or theoretical exchange capacity have been reported in both anion and cation exchange.29-32 This phenomenon is principally due to the nonionic adsorption of electrolytes. For example, as many as four molecules of acetic acid are adsorbed, per functional group, from benzene-oil solutions by strong base anion-exchange resins. In situations where capacities lower than the theoretical exchange capacity are reported, it is often assumed that the system has not reached a true equilibrium, because of the slow rates of ion exchange encountered in nonaqueous media.23 de Lucas et al.8 have reported capacities for the cation-exchange resin, Amberlite IR-120, in various pure alcohols. In the same study, they also have presented ion-exchange equilibria for Na+/K+ in the various solvents in which equilibrium was achieved in 3 days and with vigorous stirring of the resin/solution mixture. They concluded that the capacity of the resin decreased as the polarity of the solvent decreased. However, it is noteworthy that, in their work, there is no clear indication that these capacities were obtained under true equilibrium conditions. Furthermore, the specific procedure used to determine the capacity of the resin, as opposed to the procedure used for measuring ion exchange equilibria, was not described. In the present study, the capacity of the resin was determined by repeatedly contacting the same sample of resin with a fresh quantity of stock solution until no further loading of counterion occurred. This condition was confirmed when the change in the concentration of the anion in the solution phase was determined to be negligible. For mixed-solvent systems that contained in excess of 25 mol % organic solvent, this procedure was not satisfactory for determining the capacity of the resin. In these systems, the total loading of the anion on the resin increased relatively slowly, in response to the repeated exposure of the resin to the stock solution. This characteristic is illustrated in Figure 1 for the loading of [Au(CN)2]- onto the Cl- form of the resin in the various mixed solvents. For mixed solvents that contained 4-6 mol % organic solvent, the loading of [Au(CN)2]- seemed to become invariant after ∼15 contacts between the resin and the stock solution. The maximum loading of [Au(CN)2]achieved is also very similar to the theoretical exchange capacity of the resin (1.3 equiv/L). As the composition of the organic solvent increases, the loading of [Au(CN)2]decreases for a given number of contacts; i.e, the resin becomes less selective for this species. However, it is not clear from the data whether the loading of [Au(CN)2]in each case approaches a limit that is lower than the theoretical exchange capacity. In addition, the projected trends for mixed solvents that contain in excess of 25 mol % organic solvent suggest that an exceedingly large number of contacts would be required to achieve the theoretical exchange capacity. The maximum loadings of [Au(CN)2]- achieved on Purolite A500 in the mixed solvents are presented in Table 2. For mixed solvents that contain 60 mol % organic solvent, the maximum loadings are well below 1.3 equiv/ L. For these systems, the number of contacts with the

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Figure 2. Ion-exchange isotherms for [Au(CN)2]- (B)/Cl- (A) in various DMSO-water mixtures at 303 K and a total solution concentration of 0.005 M. The solid curve represents the correlation of the data with eq 10. Symbol legend is as follows: (0) 5.91 mol %, (]) 14.3 mol %, (O) 27.4 mol %, and (×) 69.3 mol %.

Figure 1. Loading of [Au(CN)2]- onto the Cl- form of the resin in various mixed solvents: (A) dimethyl sulfoxide (DMSO)-water ((0) 5.91 mol %, (O) 27.4 mol %, and (×) 69.3 mol %) and (B) N-methyl-2-pyrrolidone (NMP)-water ((0) 4.46 mol % and (×) 62.7 mol %). Table 2. Maximum Loadings of [Au(CN)2]- on Purolite A500 in Various Mixed Solvents composition (mol %)a

a

Maximum Loading (equiv/L) [Au(CN)2]-/Cl[Au(CN)2]-/SCN-

5.91 14.3 27.4 69.3

DMSO-Water Solvent 1.24 1.25 1.18 0.37

1.41 1.42 1.31 0.65

4.46 62.7

NMP-Water Solvent 1.26 0.08

1.23 0.30

Figure 3. Ion-exchange isotherms for [Au(CN)2]- (B)/SCN- (A) in various DMSO-water mixtures at 303 K and a total solution concentration of 0.005 M. The solid curve represents the correlation of the data with eq 10. Symbol legend is as follows: (0) 5.91 mol %, (]) 14.3 mol %, (O) 27.4 mol %, and (×) 69.3 mol %.

Composition refers to the amount of organic solvent.

stock solution was insufficient to ensure complete resin loading. Effect of Mixed-Solvent Composition. Ionexchange equilibria for [Au(CN)2]-/Cl- and [Au(CN)2]-/ SCN- in DMSO-water and NMP-water mixtures at 303 K are presented in Figures 2-4. The numerical values of the equivalent ionic fraction data for the solution and resin phases may be found in the Supporting Information. The equivalent ionic fractions are plotted with respect to ion B, the loading anion, in accordance with the ion-exchange process presented in eq 1. The resin is considered to be selective for ion B if the isotherm lies above the diagonal. For the [Au(CN)2]-/Cl- binary system, it can be observed that the resin is generally selective for [Au(CN)2]- (ion B) in mixed solvents that contain 60 mol % organic solvent, the selectivity of the

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Figure 5. Effect of exchange capacity on the ion exchange isotherms of the [Au(CN)2]-/Cl- binary system in DMSO-water mixtures: (A) 5.91 mol %; (B) 14.3 mol %; (C) 27.4 mol %; (D) 69.3 mol %. Symbol legend is as follows: (]) loading of 1.3 equiv/L and ([) maximum loading.

resin changes dramatically in favor of Cl-. The qualitative features of the ion-exchange isotherms for [Au(CN)2]-/Cl- are also evident in the [Au(CN)2]-/SCNbinary system. Thus, for mixtures that contain >60 mol % DMSO or NMP, the selectivity of the resin for a given anion increases in the following order: [Au(CN)2]- < SCN- < Cl-. Note: this is opposite to the selectivity sequence obtained in purely aqueous solutions.20 The extent to which a reversal in the selectivity occurs is dependent, in part, on the value used for the exchange capacity in the calculation of the equivalent ionic fractions in the resin phase (eq 4). This is illustrated in Figure 5 for [Au(CN)2]-/Cl- in DMSO-water mixtures. As noted previously, the exchange capacity of the resin in the various mixed solvents was set at 1.3 equiv/L. However, if the exchange capacity of the resin is set to the value corresponding to the maximum loading (see Table 2), at a given composition of DMSO, the selectivity for Cl- is less significant in the mixed solvent that contains 69.3 mol % DMSO. Regardless of the basis used for constructing the isotherms, it is undoubtedly clear that the composition of the mixed solvent influences the selectivity of the resin. The anions considered in this study exhibit large differences, in terms of their charge densities and polarizabilities. The effect of mixed-solvent composition on selectivity can be explained qualitatively in terms

of the degree of solvation of the anions in the mixed solvents. Generally, the solvation of ions in mixed solvents is achieved by both ion-water and ion-solvent interactions. The ion-water interactions refer simply to the process by which ions become hydrated in the form of hydration shells. The ion-solvent interactions refer to the corresponding processes, with respect to the organic solvent. There are many ways in which anions can interact with water and organic solvents. Hydrogen bonding and interaction via the polarizabilities of anions and solvent molecules (dispersion forces) are only considered in the following discussion. The organic solvents used in this work are examples of dipolar aprotic solvents. Hydrogen-bonding interactions between anions and such solvents are normally absent. Small weakly polarizable anions, such as Cl-, are strong hydrogen-bond acceptors and, therefore, are much more solvated by water than by dipolar aprotic solvents. This type of interaction decreases as the charge density of the anion decreases. Thus, large polarizable anions, such as [Au(CN)2]-, are less solvated than Clin water.14,33 Dipolar aprotic solvents are much more polarizable than water and interact strongly with large polarizable anions. In most cases, however, anion solvation is more significant in water than in dipolar aprotic solvents. An interesting exception to this is the solvation of I3- in DMSO.33 The previous considerations suggest that, in water, the degree of solvation of the anions considered here increases in the following order: [Au(CN)2]- < SCN< Cl-. The opposite sequence is encountered in dipolar aprotic solvents. In relation to the [Au(CN)2]-/Cl- binary system, it was noted previously that the resin is generally selective for [Au(CN)2]- in mixed solvents that contain relatively low levels of organic solvent. In this situation, it is expected that the solvation of the anions occurs mainly via hydration and that [Au(CN)2]- is less solvated than Cl-. Therefore, ion-pair formation between the functional groups of the resin and the anions is energetically more favorable for [Au(CN)2]-. In mixed solvents that contain high levels of organic solvent, the solvation of the anions occurs mainly via ion-solvent interactions. The relatively weak solvation of Cl- in these mixtures significantly enhances the activity of this species and promotes ion-pair formation between Cl- and the resin. This accounts for the reversal in the selectivity of the resin. The transition to a system in which anion solvation is achieved principally by ion-solvent interactions, rather than by hydration, can also be used to explain the reversal in the selectivity of the resin in the [Au(CN)2]-/SCNsystem. It is noteworthy that the enhanced activity of the Cl- species in dipolar aprotic solvents has been used to promote the formation of metal-chloro complexes and, subsequently, to increase the selectivity of extraction of base metals from solution using anion-exchange resin.10 Effect of the Type of Counterion. Based on the ion-exchange equilibria presented in Figures 2-4, it is possible to examine the effect of the type of counterion on the selectivity of the resin for [Au(CN)2]- in a given mixed solvent. This is illustrated more clearly in Figure 6. In mixed solvents that contain relatively low levels of organic solvent ( 0.5), the isotherms in DMSO-water mixtures intersect such that Cl- becomes a slightly more effective counterion than SCN-. The interpretation of this result is that an increase in the concentration of [Au(CN)2]- in solution reduces the interaction between the organic solvent and the counterions. The effect is more pronounced for Cl-, in view of its lower polarizability. As the composition of organic solvent increases, the contribution of ionsolvent interactions to the solvation of the anions increases and the isotherms intersect at a lower concentration of [Au(CN)2]-. This trend is particularly evident for an increase from 14.3 mol % DMSO to 27.4 mol % DMSO (see Figure 6B and C). In mixed solvents that contain relatively high levels of organic solvent (>60 mol %), the contribution of hydrogen bonding to the solvation of the anions is minimal, and, thus, Cl- becomes a more effective counterion than SCN-. These findings suggest that small weakly polarizable anions are the preferred counterions for the elution of [Au(CN)2]- from ionexchange resin in mixed solvents that contain high levels of dipolar aprotic solvents. Modeling Results. The optimized values of KAB, ΛAB, and ΛBA for binary systems in DMSO-water and NMP-water mixtures are presented in Tables 3 and 4,

composition (mol %)a

KABb

0c 4.46 62.7

224.3 23.76 3 × 10-3

0c 4.46 62.7

14.94 4.133 0.013

ΛAB

ΛBA

[Au(CN)2]-/Cl2.571 4 × 10-5 3.751 0.266 3.635 -0.014 [Au(CN)2]-/SCN0.254 2.028 1.194 0.918 1.092 4.349

ΛAB × ΛBA

AARD (%)

1 × 10-4 0.999 -0.052

8.7 4.6 7.5

0.515 1.096 4.748

2.4 3.3 9.7

a Composition refers to the amount of NMP. b K AB refers to the equilibrium constant for ion B entering the resin phase and displacing ion A. c Data for ion-exchange equilibria in aqueous solution are taken from ref 20.

respectively. The solid lines in Figures 2-4 represent the values of yB calculated using the optimized parameters and the experimental values of xB. The AARD, with respect to yB, ranges from 1% to 10%, and this is similar to the level of accuracy obtained for the ionexchange equilibria of the same species in aqueous solution. However, note that the highest AARDs are observed for the mixed solvents that contain in excess of 60 mol % organic solvent. Generally, the values of KAB for [Au(CN)2]-/Cl- and [Au(CN)2]-/SCN- decrease with an increase in the composition of the organic solvent in the external solution. A decrease in KAB implies a decrease in the selectivity of the resin for the loading anion (B). The decrease in KAB with the addition of increasing amounts of organic solvent is particularly significant for [Au(CN)2]-/Cl- in NMP-water mixtures. For example, the equilibrium constant in 4.46 mol % NMP is approximately an order of magnitude lower than that in aqueous solution. In 62.7 mol % NMP, this difference increases further by several orders of magnitude, indicating that there is virtually no loading of [Au(CN)2]- attained on the resin (also see Figure 4). An equilibrium constant of 60 mol % organic solvent are preferred for the elution of [Au(CN)2]-. Small weakly polarizable anions, such as Cl-, are the preferred counterions in this range of mixed-solvent composition. The particular combination of Cl- in N-methyl-2-pyrrolidone (NMP)-water mixtures is remarkably effective for reducing the loading of [Au(CN)2]- on Purolite A500. The law of mass action, modified with activity coefficients, provides a satisfactory correlation of the ion-exchange equilibria in the range of mixed-solvent composition considered. The optimized values of KAB are consistent with the trends observed in the isotherms of the ion-exchange systems. Acknowledgment This work was funded by the Australian Research Council under Grant No. C00106549. Supporting Information Available: The numerical values of the equivalent ionic fraction data for the solution and resin phases are presented. (PDF.) This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Green, B. R.; Kotze, M. H.; Wyethe, J. P. Developments in Ion Exchange: The Mintek Perspective. JOM 2002, 54, 37-43. (2) Fleming, C. A. Thirty Years of Turbulent Change in the Gold Industry. CIM Bull. 1998, 91, 55-67.

(3) Fernando, K.; Tran, T.; Laing, S.; Kim, M. J. The Use of Ion Exchange Resins for the Treatment of Cyanidation Tailings. Part 1sProcess Development of Selective Base Metal Elution. Miner. Eng. 2002, 15, 1163-1171. (4) Lukey, G. C.; van Deventer, J. S. J.; Shallcross, D. C. The Effect of Functional Group Structure on the Elution of Metal Cyanide Complexes from Ion-Exchange Resins. Sep. Sci. Technol. 2000, 35, 2393-2413. (5) Riveros, P. A. Selectivity Aspects of the Extraction of Gold from Cyanide Solutions with Ion Exchange Resins. Hydrometallurgy 1993, 33, 43-58. (6) Lukey, G. C.; Van Deventer, J. S. J.; Shallcross, D. C. Selective Elution of Copper and Iron Cyanide Complexes from Ion Exchange Resins Using Saline Solutions. Hydrometallurgy 2000, 56, 217-236. (7) Leao, V. A.; Ciminelli, V. S. T. Application of Ion Exchange Resins in Gold Hydrometallurgy. A Tool for Cyanide Recycling. Solvent Extr. Ion Exch. 2000, 18, 567-582. (8) de Lucas, A.; Valverde, J. L.; Romero, M. C.; Gomez, J.; Rodriguez, J. F. Ion Exchange Equilibria in Nonaqueous and Mixed Solvents on the Cationic Exchanger Amberlite IR-120. J. Chem. Eng. Data 2001, 46, 73-78. (9) Brajter, K.; Miazek, I. Investigations on the Effect of Aqueous Acetone Medium on Separation of Metal Ions on Chelex 100 Ion-Exchanger. Talanta 1981, 28, 759-764. (10) Fleming, C. A.; Monhemius, A. J. On the Extraction of Various Base Metal Chlorides from Polar Organic Solvents into Cation and Anion Exchange Resins. Hydrometallurgy 1979, 4, 159-167. (11) Phipps, A. M. Anion Exchange in Dimethylsulfoxide. Anal. Chem. 1968, 40, 1769-1773. (12) Fessler, R. G.; Strobel, H. A. Nonaqueous Ion Exchange. II. Univalent Cation Exchange in Alcohols and Methanol-Water, Ethanol-Water, and Methanol-Ethanol Mixtures. J. Phys. Chem. 1963, 67, 2562-2568. (13) Senanayake, G.; Muir, D. M. Competitive Solvation and Complexation of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) in Aqueous Ethanol, Acetonitrile, and Dimethylsulfoxide Solutions Containing Chloride Ion with Applications to Hydrometallurgy. Metall. Trans. B 1990, 21B, 439-448. (14) Muir, D. M.; Singh, P.; Kenna, C. C.; Tsuchida, N.; Benari, M. D. Hydrometallurgical Thermodynamics. II. Solvent Effects on the Activity and Free Energies of Transfer of CN-, Ag(CN)2- and Au(CN)2- in Ethanol-Water and Acetonitrile-Water Mixtures. Aust. J. Chem. 1985, 38, 1079-1090. (15) Vegter, N. M.; Sandenbergh, R. F. The Kinetics of the Organic Elution of Gold Cyanide from Activated Granular Carbon Using an Aqueous Caustic Acetone Solution. Hydrometallurgy 1992, 28, 205-222. (16) Espiell, F.; Roca, A.; Cruells, M.; Nunez, C. Gold Desorption from Activated Carbon with Dilute NaOH/Organic Solvent Mixtures. Hydrometallurgy 1988, 19, 321-333. (17) Muir, D. M.; Hinchliffe, W.; Tsuchida, N.; Ruane, M. Solvent Elution of Gold from C. I. P. Carbon. Hydrometallurgy 1985, 14, 47-65. (18) Burstall, F. H.; Forrest, P. J.; Kember, N. F.; Wells, R. A. Ion-Exchange Process for the Recovery of Gold from Cyanide Solution. Ind. Eng. Chem. 1953, 45, 1648-1658. (19) Law, H. H.; Wilson, W. L.; Gabriel, N. E. Separation of Gold Cyanide Ion from Anion-Exchange Resin. Ind. Eng. Chem., Process Des. Dev. 1985, 24, 236-238. (20) Jayasinghe, N. S.; Lee, K.; Lucien, F. P.; Tran, T. IonExchange Equilibria for [Au(CN)2]-/Cl-, [Au(CN)2]-/SCN-, and SCN-/Cl- in Aqueous Solution at 303 K. J. Chem. Eng. Data 2004, 49, 1279-1284. (21) Martinez, A. de L.; Diaz, J. Z.; Canizares, P. C. IonExchange Equilibrium in a Binary Mixture. Models for Its Characterization. Int. Chem. Eng. 1994, 34, 486-497. (22) Shallcross, D. D.; Herrmann, C. C.; McCoy, B. J. An Improved Model for the Prediction of Multicomponent Ion Exchange Equilibria. Chem. Eng. Sci. 1988, 43, 279-288. (23) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962. (24) Kielland, J. Individual Activity Coefficients of Ions in Aqueous Solutions. J. Am. Chem. Soc. 1937, 59, 1675-1678. (25) Pitzer, K. S. Theory: Ion Interaction Approach. In Activity Coefficients in Electrolyte Solutions; Pytkowicz, R. M., Ed.; CRC Press: Boca Raton, FL, 1979; Vol. 1: pp 157-208.

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Received for review April 8, 2005 Revised manuscript received June 29, 2005 Accepted July 13, 2005 IE0504304