Facilitated supported liquid-membrane transport of gold (I) and gold (III) using Cyanex®921F.J.Alguacila, M.Alonsoa, A.M.Sastreb

aCentro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain. E-mail: fjalgua@cenim.csic.esbDepartment of Chemical Engineering, ETSIB, Universitat Politecnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain

Abstract

The commercially available extractant Cyanex® 921 (phosphine oxide) was studied to be applied in the carrier-facilitated transport of gold (I) (cyanide media) and gold (III) (chloride media), across a flat-sheet supported liquid membrane (FSSLM). Gold (I) is transported from alkaline pH values. The presence of lithium salts in the aqueous media improves the transport. In chloride media, the carrier is able to transport gold (III), decreasing the permeability as the initial HCl concentration is increased. From both aqueous media, a model is presented that describes the transport mechanism, consisting of diffusion through a feed side aqueous layer, a fast interfacial chemical reaction, and diffusion of carrier and metal complexes through the organic membrane. The organic membrane diffusional resistance (Δo), aqueous diffusional resistance (Δa) were calculated from the proposed model, and their values were 1.1x104 and 9.1x108 s/cm (Δo, cyanide and chloride media) and 5.3x102 and 2.6x102 s/cm (Δa, cyanide and chloride media). The values of the bulk diffusion coefficient (Do,b) and diffusion coefficient (Do) calculated from the model were Do,b (cyanide)= 4.1x10-6 cm2/s, Do,b

(chloride)= 5.2x10-11 cm2/s; Do (cyanide)= 1.1x10-6 cm2/s, Do (chloride)= 1.4x10-11 cm2/s.

Keywords: Supported liquid membrane; Gold; Cyanide; Chloride

1. Introduction

Nearly twenty years ago, Danesi began considering supported liquid membrane processes [1]. After this time, scientists began finding applications for supported liquid membrane technologies in two configurations for the recovery and separation of metal species from aqueous solutions [2-4]. The term supported liquid membrane is used to describe a process separation which does not rely on chemical characteristics of a thin, solid (semi permeable) barrier. The closest equivalent non-membrane separation process is liquid-liquid extraction and consequently supported liquid membrane processes are referred to as liquid pertraction though other names are used, i.e. carrier-mediated extraction and facilitated transport. The equivalent between supported liquid membranes and liquid-liquid extraction arises from the use of a multiple phases system.

The principle of supported liquid membranes operation is relatively simple; two homogeneous, miscible liquids, one the donor the other the acceptor are spatially separated by a third phase, the membrane. This phase consisting of a diluent and

extractant or carrier is immiscible and insoluble in the donor and acceptor solutions. Transport of solute from the donor to the acceptor solution is due to favourable conditions created at the two interfaces between the three phases. The thermodynamics at the donor/membrane interface favour extraction of solute into the membrane while simultaneously the thermodynamics at the membrane/acceptor interface favour the reverse transport, i.e. stripping.

The use of supported liquid membranes for the removal of metals from various resources and liquid effluents has long been targeted both from industrial or scientific point of views; applications of this technology had been reviewed in the literature [5-7].

Referring to gold transport, supported liquid membranes have been used in the separation of this precious metal mainly from cyanide or chloride media [8-28].

In the present investigation, the transport of gold from these two aqueous media through a FSSLM using the phosphine oxide Cyanex® 921 as carrier is investigated. The aim was to optimise various operational parameters, and thus obtain efficient supported liquid membranes.

2. Experimental

2.1. Reagents and solutions

The extractant Cyanex® 921 was used as supplied by the manufacturer (CYTEC Ind., Canada), the active substance of the reagent being tri-n-octylphosphine oxide (near 99%) [29]. Other chemicals used were AR (analytical reagent) grade, except the complex K2Ni(CN)4 which was prepared accordingly to the literature [30]. Gold cyanide solutions were prepared by dissolving KAu(CN)2 in distilled water. The different concentrations used in the experiments were prepared from a stock solution of 5x10-3 M in gold (I). The pH of the solution was adjusted by addition of sodium hydroxide solutions. During the experiments, the pH was continuously controlled using a 605 pH-meter (Crison, Spain). In chloride media, the gold (III) solutions were prepared in a similar manner using HAuCl4.

2.2. Membrane support and FSSLM measurements

The transport experiments were carried out using the cell, membrane support, and procedure as described in the literature [31]. The characteristics of the support were: 12.5x10-3 cm thick microporous polyvinylidenedifluoride (Durapore®) film, 75% porosity, 2.2x10-5 cm effective pore size and tortuosity 1.67. The effective membrane area was 11.3 cm2.

Gold or metal concentrations in the feed phase were analyzed by atomic absorption spectrometry, using a Perkin Elmer 1100B spectrophotometer, though in several experiments, the metal concentrations in the receiving phase were also analyzed to verify active metal transport. The gold (metal) concentration in the aqueous solution was found to be reproducible within ±2%. The permeation coefficient (P) was computed using eq. (1):

2

(1)

where V is the volume of the feed phase solution, A is the effective membrane area, [M]0 and [M]t are the concentrations of metal in the feed phase at time zero and at a given time, and t is the elapsed time. It can be point out that the above expression was obtained under condition of the fully intermix in the feed chamber.

3. Results and discussion

3.1. Gold transport from cyanide media

In the pH range of 6 to 11, the extraction of gold (I) cyanide by Cyanex® 921 is represented by the general equilibrium [32]:

(2)

which is pH-independent and where aq and org denote species in the aqueous and organic phases, respectively. L represents the active substance of the extractant. In this reaction, M+ represents a cation, such as sodium, potassium or lithium (preferably). The value of the extraction constant (Kext) for the above equilibrium is calculated as 100.

3.1.1. Influence of stirring speeds on the feed and receiving phases

In all the transport experiments, stirring speeds of 1400 and 1000 min -1 were used for the feed and receiving phases, respectively. Previous experiments, carried out with a feed phase containing 2.5x10-5 M gold (I) at pH 10.5±0.05, receiving phase water and a membrane phase of 0.52 M Cyanex® 921 in xylene (mixed isomers, it should be noted here that throughout all the work, the word xylene is used to describe this mixture), showed that the permeability coefficient becomes virtually independent (1.9x10 -3 cm/s) of the stirring speed in these ranges, which also indicates that a minimum value of the thickness of the feed phase boundary layer is reached.

3.1.2. Effect of Ionic Strength

Table1shows the results obtained for the transport of Au(CN)2- in the presence of

different lithium salts. The organic phase used in the experiments contained Cyanex®

921 (0.52 M) in xylene. The aqueous phase contained 2.5x10-5 M of gold (I) at pH 10.5±0.05. If it is considered that when no salts were added to the feed phase the permeability coefficient was 2x10-4 cm/s, results shown in Table 1 demonstrate that the presence of the inorganic lithium salts in the feed phase increases the transport of gold with little influence of the counter anion of the salt on the permeability coefficient.

To study the influence of the presence of lithium salts in the feed phase on gold permeation in more detail, a set of experiments was carried out at a constant LiCl concentration of 1 M and varying pH values.

3

In Fig. 1, the variation of the gold permeation coefficient is plotted vs. aqueous pH of the feed phase for the transport of Au(CN)2

- from aqueous solutions containing 2.5x10-5

M gold. Organic phases contained 0.52 M Cyanex® 921 in xylene. It can be noted that in the absence of LiCl, the transport of gold (I) tends to decrease as the pH increases; whereas in the presence of LiCl, the transport is higher at all pH values, and remains almost constant. This effect should be attributable to the fact that the transport occurs by solvation of ion pairs Li+Au(CN)2

- by the organic reagent as described in the gold-solvent extraction research using phosphine oxides [32,33].

3.1.3. Effect of extractant concentration on gold transport

In Table 2, the obtained gold permeation coefficients, at different concentrations of Cyanex® 921 in the organic phase, are given in order to study the effect of extractant concentration variation on gold (I) transport. Experiments were carried out at a constant pH value of 10.5±0.05. Aqueous phases contained 2.5x10-5 M gold (I) and 1 M LiCl. In the organic phase, different extractant concentrations in xylene were used.

Results obtained revealed no significant change in the metal permeability at higher carrier concentrations. This constant permeation coefficient value, or limiting permeability (Plim), can be attributed to the assumption that diffusion in the membrane is negligible compared with the aqueous diffusion and the permeation process is controlled by the diffusion in the stagnant film of the feed phase. Thus:

(3)

and assuming a value of 10-5 cm2/s for Daq (average diffusion coefficient) [34], then daq= 5.3x10-3 cm. This value (daq) is the minimum thickness of the stagnant aqueous diffusion layer in the present experimental conditions.

3.1.4. Effect of initial metal concentration on gold transport

Table 3 shows the variation in the gold permeation coefficient and the initial flux (J= P[Au]0) against the concentration of gold ranging from 2.5x10-5 to 2.5x10-4 M in the feed phase solution. It can be observed that with the present experimental conditions the metal flux increased with the increase of the initial gold concentration in the feed phase.

3.1.5. Separation of gold (I) from metal-cyanide complexes

In order to investigate the effect of other metal-cyanide complexes accompanying Au(CN)2

-, a study was made about their interference with the overall transport of gold (I). The metal-cyanide complexes studied, Ni(CN)4

2- and Fe(CN)63-, were investigated in

the form of a mixture with Au(CN)2-. Thus, the feed phase contained 2.5x10-5 M Au(I),

1.8x10-4 M Ni(II) and 1.8x10-4 M Fe(III) in 1 M LiCl at pH 10.5±0.05, the organic phase was of 0.26 M Cyanex® 921 in xylene and the receiving phase water.

4

The results obtained indicated negligible Ni(II) and Fe(III) transport, whereas the permeation coefficient for gold is 1.4x10-3 cm/s; if this value is compared with the corresponding data shown in Table 2, it can be seen that though gold (I) is transported selectively with respect to nickel (II) and iron (III), the presence of these metals in the feed phase slightly decreased the value of the gold permeation coefficient, this probably being due to a multi-ion competition or crowding effect [5].

3.2. Gold transport from chloride media

The solvent extraction of Au(III) from HCl solutions by Cyanex® 921 was also studied previously [35]. The extraction equilibrium is described by the next general following reaction:

(4)

where L is the organic extractant and n represented an stoichiometric coefficient. Thus, from 3 M HCl concentration in the aqueous media, the extraction of gold can be represented by the formation of two species in the organic phase: HAuCl4·L and HAuCl4·L2, whereas the values of the extraction constants at 6 M HCl (representative of solutions used in the treatment of jewelry scrap and printed circuit boards) are 1.1·104

and 6.6·108, respectively.

3.2.1. Influence of the stirring speed in the feed phase

Experiments were performed to establish adequate hydrodynamic conditions. The permeability of the membrane was studied as a function of the stirring speed on the feed phase solution side as previous tests had shown that the variation of the stirring speed on the receiving solution side had little influence on gold (III) transport.

Results obtained are shown in Fig. 2. Near constant permeability for stirring speeds higher than 1100 min-1 was obtained. Consequently, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized. The diffusion contribution of the aqueous species to the mass transfer process is assumed to be constant. Stirring speeds of 1200 min-1 and 700 min-1 were maintained throughout all the investigation for the feed and receiving phases, respectively.

3.2.2. Influence of the initial gold concentration

A series of experiments was performed using feed solutions with different contents of gold and hydrochloric acid, which varied from 7.6x10-5 to 3.0x10-4 M for gold and 6 M for HCl. The organic phase contained 0.52 M Cyanex® 921 in xylene.

Table 4 shows the variation of gold permeation coefficient and flux for different gold concentrations. This show that the increase of the initial gold concentration increased the initial metal flux.

3.2.3. Influence of the initial hydrochloric acid concentration

5

The variation in transport, as a function of the initial acid concentration, at 0.52 M Cyanex® 921 in xylene in the organic phase, has been studied. The experimental conditions were: aqueous feed phases containing 3.0x10-4 M gold at different HCl concentrations and water as receiving phase. Results are shown in Table 5, in which a decrease (though small from 2 m HCl) in gold transport occurs as the initial HCl concentration increases. This result can be tentatively explained by competition between gold and hydrochloric acid to be transported by the phosphine oxide. It is known that Cyanex® 921 extracts hydrochloric acid [35].

3.2.4. Influence of the carrier concentration on permeability of gold

The results concerning transport of gold (III) from a feed phase containing 7.6x10-5 M Au(III) in 6 M HCl, the receiving phase being water, and varying concentrations of Cyanex® 921 in the range 0.13 to 0.52 M dissolved in xylene on a Durapore®

GVHP4700 support, revealed no change in the permeation coefficient (3.9x10-3 cm/s) at higher carrier concentrations (0.26-0.52 M) against the value of 2.7x10-3 cm/s obtained for a 0.13 M Cyanex® 921 solution in xylene. Accordingly to Eq (3) and considering the value of 3.9x10-3 cm/s as the limiting permeability value, the thickness of the aqueous diffusion film is calculated as 2.6x10-3 cm.

3.2.5. Separation of gold (III) from metal-chloride complexes

Since base metals are normally found in the gold-HCl bearing solutions, the selectivity of the present transport system against the presence of various metals in the feed phase was investigated by using a membrane phase of 0.26 M Cyanex® 921 in xylene, and the feed phase containing 7.6x10-5 M Au(III), 1.8x10-4 M Fe(III), 1.8x10-4

M Ni(II) and 1.8x10-4 M Cu(II) in 1M HCl media. From the results obtained (Table 6), it is inferred that gold was preferably transported over these base metals. The selectivity with respect to gold seems to be enough to separate this precious metal selectively from the base metals; it can be also noted here that in the present experimental conditions, the permeation coefficient for gold is not affected by the presence of other metals in the solution (5.3x10-3 cm/s when only gold (III) is present in the feed solution against the value given in Table 6), thus, in the present system the crowding effect seems not to influence the transport of gold (III).

3.3. Estimaton of the diffusional parameters

Before scaling up any supported liquid membrane configuration to a hollow fibers or spiral wound operations, a theoretical model of the membrane system is needed in order to design an efficient recover process in terms of improved stability [5,7]. Thus, theoretical models which account for the experimental result are required to complete the understanding of supported liquid membrane transport mechanisms.

Different models have been proposed for flat-sheet supported liquid membrane transport, among them, modellization proposed in the literature [36] had been and still is

6

successfully applied to a number of systems because the experimental conditions usually employed allow satisfactory application of the equations [15,27,37-40].

In the present investigation, the mass transfer of gold across the membrane is described considering diffusional parameters. It is assumed that the resistance of interfacial reaction to the overall transport is negligible in flat-sheet supported liquid membranes [41-43]. Thus, the interfacial flux due to the chemical reaction has not been considered as the chemical reactions taking place at the aqueous feed phase-membrane (and membrane-receiving solution) interfaces are fast [44], and it is suggested that rapid chemical reactions can be considered to occur instantaneaously relative to the diffusion processes [45,46].

Such a simplified type of data analysis is used in the present work, this is reasonably valid because of the relatively thick membrane support used in this investigation [47].

Accordingly to the model derived [36], the mass transfer of gold considered the diffusion of the metal through the aqueous feed boundary layer, the reversible chemical reaction at the interface and the diffusion of the metal-extractant complex species in the membrane.

The extraction equilibrium of gold (I) by Cyanex® 921 dissolved in xylene can be described by the reaction shown in Eq. (2) and the extraction constant expressed as:

(5)

The gold (I) transport rate is determined by the rate of diffusion of gold-containing species through the feed phase diffusion layer and the rate of diffusion of gold (I)-Cyanex® 921 species through the membrane. Then, the flux of gold (I) crossing the membrane can be derived by applying Fick´s first diffusion law to the diffusion layer at the feed phase side and to the membrane.

The diffusional fluxes in the feed phase boundary layer (Ja) and in the membrane phase (Jo) can be expressed by the next Equations, where Δa and Δo are the diffusional resistances caused by the feed phase boundary layer and due to diffusion through the membrane, respectively.

(6)

(7)

Since the distribution coefficient of gold between the membrane and the receiving phase is much lower than that between the feed phase and the membrane, the concentration of the metal-extracted complex in the membrane phase at the receiving

7

side may be negligible compared with that at the feed solution side. Then Eq. (7) can be rewritten as:

) (8)

If the chemical reaction expressed by Eq. (2) is assumed to be fast compared to the diffusion rate, local equilibrium at the interface is reached and concentrations at the interface are related through Eq. (5). At steady state, Ja=Jo=J and by combination of Eqs. (5), (6) and (8), the following expression can be obtained:

(9)

The permeability coefficient (P=J/[Au(I)]TOT) is given by:

(10)

This expression combines the equilibrium and diffusion parameters involved in gold (I) transport process through a supported liquid membrane containing Cyanex® 921 in xylene as carrier. To determine the value of the resistance to the mass transfer, from Eq. (10) the next expression is obtained:

(11)

By plotting 1/P as a function of 1/Kext[L]3org for various extractant concentrations in

xylene, one should obtain a straight line with slope Δo and ordinate to calculate Δa. The values of the various diffusional parameters calculated from the model are given in Table 7. The diffusion coefficient of the gold complex in the bulk organic phase can be estimated from the diffusivity in the membrane by the following expression [45]:

(12)

Where τ is the membrane tortuosity and ε is the membrane porosity. The value of Do,b

is also given in Table 7.

In the case of the gold (III)-Cyanex® 921 system, the next equations give the reactions and extraction constants of the corresponding extraction equilibria at 6 M HCl in the aqueous phase:

8

(13)

(14)

(15)

(16)

Taking into account the same assumptions and development as in the case of the Au(I)-Cyanex® 921 system, a final expression for the permeability coefficient can be written as:

(17)

and:

(18)

As in the above case, considering various extractant concentrations and by plotting 1/P as a function of the denominator of the second term of Eq. (18), a straight line with slope Δo and ordinate to calculate Δa may be obtained. The values of the various parameters affecting gold(III) transport, together with the corresponding diffusion coefficient in the bulk organic phase, are given in Table 7.

It should be noted that in both systems, Do presents a lower value than that of the bulk diffusion coefficient, this is attributable to diffusional resistance caused by the microporous membrane placed between the feed and receiving phases.

4. Conclusions

Experimental results indicated that it is possible to use Cyanex® 921 to transport Au(CN)2

- species at alkaline pH values. In this medium, the transport is influenced by the presence in the feed phase of lithium cations, which enhance the transport, whereas the counter anion (chloride, nitrate or sulphate) has little influence in the transport of the precious metal. From experimental data, the mass transfer coefficient and the thickness aqueous boundary layer were estimated as 1.9x10-3 cm/s and 5.3x10-3 cm, respectively. From chloride media, this extractant can also be used to transport gold (III) at acidic feed solutions, decreasing the transport as the initial aqueous acidity is increased. In this

9

media, the mass transfer coefficient was calculated as 3.9x10-3 cm/s and the thickness of the aqueous boundary layer as 2.6x10-3 cm. A mechanism of gold (I) and gold (III) transport using Cyanex® 921 considering the aqueous film diffusion of metal ions, fast chemical reaction at the interface and diffusion of the various transported species through the membrane is proposed. However, from certain Cyanex® 921 concentrations, a limiting permeability value is obtained in both cases, suggesting that in these conditions the transport processes are controlled by the diffusion in the aqueous stagnant film. Moreover, a supported liquid membrane technology has been developed for the selective separation of this precious metal from commonly and less valuable accompanying metals in the two aqueous solutions.

Acknowledgements

The authors wish to thank to CSIC and UPC (SPAIN) for support. Also to Mr. Bascones and Mr. López for technical assistance.

References

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Table 1. Effect of the presence of different salts on Au(CN)2- transport.

Salt Px103 (cm/s)LiNO3

LiClLi2SO4

2.31.92.0

Feed phase: salts at 1 M. Membrane support: Durapore® GVHP4700. Receiving phase: water.

14

Table 2. Gold permeation coefficients versus various Cyanex® 921 concentrations.Cyanex® 921 (M) Px103 (cm/s)

0.130.260.52

0.021.91.9

Membrane support: Durapore® GVHP4700. Receiving phase: water.

15

Table 3. Variation in the metal permeation and in the metal flux with the initial gold concentration.

Au(I)x104 (M) Px103 (cm/s) Jx109 (mol/cm2 s)0.250.511.32.5

1.94.44.24.5

0.050.230.541.1

Feed phase: 1 M LiCl at pH 10.5±0.1. Membrane phase: 0.52 M Cyanex® 921 in xylene on a Durapore®

GVHP4700 support. Receiving phase: water.

16

Table 4. Influence of initial gold concentration on gold permeability and metal flux.Au(III)x104 (M) Px103 (cm/s) Jx109 (mol/cm2 s)

0.761.53.0

3.93.53.4

0.300.531.0

Membrane support: Durapore® GVHP4700. Receiving phase: water.

17

Table 5. Influence of HCl concentration on gold permeability.HCl (M) Px103 (cm/s)

1246810

5.83.93.73.43.33.2

Membrane support: Durapore® GVHP4700. Receiving phase: water.

18

Table 6. Selectivity of the system.Metal Px103 (cm/s) βAu/M

Au(III)Fe(III)Ni(II)Cu(II)

5.20.230.17

no transport

-22.630.6

quantitative

19

Table 7. Estimated diffusional parameters.System Δo

-1 (cm/s) Do (cm2/s) Do,b (cm2/s) Δa-1 (cm/s)

Au(I)-Cyanex® 921Au(III)-Cyanex® 921

8.9x10-5

1.1x10-91.1x10-6

1.4x10-114.1x10-6

5.2x10-111.9x10-3

3.9x10-3

20

Fig.1. Effect of the pH variation on gold (I) permeation using Cyanex® 921 in xylene on a Durapore® GVHP4700 support. Receiving phase: water.Fig. 2. Influence of stirring speed on permeability of gold (III). Feed phase: 0.015 g/l gold in 6 M HCl. Membrane phase: 20% w/v Cyanex® 921 in xylene on a Durapore®

GVHP4700 support. Receiving phase: water.

21

FIGURE 1

22

FIGURE 2

23