Uphill Transport of Au(CN)2- with Mixtures of the Amine Primene JMT and the Phosphine Oxide

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Transport of Au(CN)2- across a supported liquid membrane using mixtures of amine Primene JMT and phosphine oxide Cyanex 923

F.J.Alguacil, M.Alonso

Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain. E-mail:

Abstract

The transport of Au(CN)2- between aqueous solutions and organic phases of the mixture of the amine Primene JMT and the phosphine oxide Cyanex 923 in xylene was studied at various experimental conditions i.e. extractant mixtures and gold concentrations. The behaviour of the system with different organic diluents, aqueous ionic strength and the selectivity of the system with respect to the transport of different metal-cyano complexes were also investigated. The transport of gold (I) can be adscribed to a mechanism which consists of i) a diffusion process through the feed aqueous diffusion layer, ii) a fast interfacial chemical reaction and iii) a diffusion of HAu(CN)2·R·L and HAu(CN)2·R·L2 (R= the amine, L= the phosphine oxide) through the membrane.

Keywords: Gold cyanide; Supported liquid membrane transport; Primene JMT; Cyanex 923

1.Introduction

Past years had shown a grown interest in the investigation of new alternatives for the processing of liquid solutions which contained valuable or toxic metals. Liquid membrane technologies have been proposed as a real challenge for the treatment of such solutions, having supported liquid membranes (SLMs) the claimed advantage (i.e. versus conventional solvent extraction) of that the extraction, stripping and regeneration of the organic phase are reduced to a single stage.

Although gold-cyanide processing from hydrometallurgical procedures is well established (Marsden and House, 1992), there is a continuous need in the development of new technologies for the treatment of more diluted and/or more complex solutions, and here is when SLMs can find a direct and real application field due to the reasons mentioned above.

Relatively no much data can be found in the literature about the use of SLMs technologies in gold-cyanide processing (Alguacil, 2000; Kumar and Sastre, 2000; Alguacil and Martin, 2003; Alguacil, 2004); however, solvent extraction references shown that the use of mixtures of amines and salvation extractants can be used advantageaously in the recovery of Au(CN)2- from alkaline solutions (Mooiman and Miller, 1986; Alguacil et al., 1990; Martin and Alguacil, 1998).

Little systematic study has been conducted to evaluate the effectiveness of such mixtures for permeation of gold (I) across supported liquid membranes. Thus, carrier-facilitated membrane transport of gold across a supported liquid membrane using as organic reagent the mixture of primary amine Primene JMT and phosphine oxide Cyanex 923 was investigated with a view to optimising several parameters and so obtaining efficient supported liquid membranes.

2.Experimental

The characteristics and properties of both extractants used in the present investigation, the primary amine Primene JMT and the phosphine oxide Cyanex 923, have been published elsewhere (The Rohm and Haas Co., 1992; Dziwinski and Szymanowski, 1998). These extractants were kindly supplied by Roham and Haas Co., and CYTEC Ind., respectively and were used without further purification. Others chemicals used in this work were of AR grade, except the organic diluents Solvesso 100 and Escaid 100, which were obtained from ExxonMobil Chem. Iberia (Spain), Iberfluid, which was obtained from CS (Spain), and some of the metal-cyanide complexes, which were prepared according to the methods described in the literature: nickel and cobalt (Caravaca and Alguacil, 1994).

The organic membrane phase was prepared by dissolving the corresponding volume of Primene JMT and Cyanex 923 (unless stated otherwise) in xylene to obtain carrier solutions of different concentrations. The polymeric support was impregnated with the carrier solution by immersion for 24 h, then leaving to drip for 15 s before being placed in the FSSLM cell. Previous experiments shown than prolonged immersion times (i.e. 36 or 48 h) do not affect the results obtained using 24 h. The flat-sheet membrane used was Millipore-Durapore GVHP 4700, its characteristics being obtained elsewhere (Alguacil and Martinez, 2000).

Single stage FSSLM measurements were carried out in a cell which characteristics had been described before (Alguacil and Martinez, 2000). The feed and receiving phases were stirred mechanically at 1000 min-1 (unless stated otherwise) at 20º C to avoid concentration polarisation conditions at the membrane interfaces and in the bulk of the solution. Membrane permeabilities were determined by monitoring gold or metal concentrations by AAS in the feed phase as a function of time. The gold concentration in the various phases was found to be reproducible within 97 % accuracy. The permeation coefficient (P, within ±3 % error) was calculated by:

(1)

where V is the volume of the feed phase solution, A is the effective membrane area and [C]t and [C]0 are the concentrations of metal ions in the feed phase at a given time and time and time zero, respectively, and t the elapsed time.

On the other hand, the percentage of metal transport (% M) was calculated as:

(2)

at the corresponding elapsed time.

As receiving phase, solutions of 0.1 M NaOH were used, as previous solvent extraction experiments shown that this phase is effective to remove gold (I) from loaded organic solutions (Alguacil et al., 1997).

3.Results and discussions

The optimum concentration of Primene JMT and Cyanex 923 in the mixture was first studied with the objective of obtaining higher metal transport. These experiments were carried out using feed solutions of 2.5x10-5 M gold (I) at pH 9.5±0.05 and organic phases with different amine and phosphine oxide ratios (expressed as % v/v) in xylene. Total reagent concentration was 40 % v/v.

Figure 1 shows the variation in gold transport versus elapsed time; best transport results were obtained when the reagent mixture solution was used.

Fig.1. Gold transport using Primene JMT, Cyanex 923 or a mixture of both reagents. Feed and receiving phases stirring speeds: 1000 min-1.

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Furthermore, best results were obtained when the 0.5:0.5 (amine:phosphine oxide) ratio was used instead of the 0.25:0.75 or 0.75:0.25 ratios for the extractants mixture (though data are not presented in the work). The 0.5:0.5 mixture was used in all subsequent studies.

3.1.Influence of the stirring speed in the feed and receiving phases

Conditions to establish adequate hydrodynamic conditions were determined. The permeability of the membrane was studied as a function of the stirring speed on the feed and receiving solutions sides. Results obtained are shown in Table 1, best permeability

Table 1

Gold permeation at various stirring speeds

values were obtained at 1000 min-1 stirring speeds in both sides. Thus, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized and the diffusion contribution of the aqueous species to the mass transfer process is assumed to be constant.

3.2.Effect of change of gold concentration

The influence of initial gold concentration on transport of the metal was studied using feed phases which contained different gold concentrations at pH 9.5±0.05 and organic phases of Primene JMT-Cyanex 923 (20 %- 20 % v/v) in xylene. As can be seen from Figure 2, the variation in the initial gold concentration influences the transport of the metal, decreasing this as the initial metal concentration in the feed phase increases. This decrease should be explained as being due to a saturation and, thus, a lower effective membrane area, when higher gold concentrations are used, in the supported liquid membrane and also to maximisation as a result of saturation of membrane pores with metal-carrier species and build-up of the carrier layer on the membrane interface, enhancing the retention of the separating constituents on the entry side and thus causing the permeability to decrease (El Aamrani et al., 1998).

Fig.2. Gold transport by the mixture Primene JMT-Cyanex 923 at different inicial gold concentrations.

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3.3.Dependence on extractant mixture concentration

Figure 3 shows the variation of gold transport with time for experiments carried out with feed solutions of 2.5x10-5 M gold (I) at pH 9.5±0.05 and organic solutions of the mixture Primene JMT-Cyanex 923 in xylene.

It can be seen that the transport of gold increases with the increasing of the extractant mixture concentration, though no significant change in the permeability was found at concentrations above 20 %-20 % v/v. This limiting permeability value could be attributable to a permeation process controlled by the diffusion in the stagnant film of the aqueous feed phase; thus Plim= 1/Δaq= 6.7x10-3 cm/s and assuming a value of Daq= 10-5 cm2/s (Daq being the average aqueous diffusion coefficient of the metal-containing species) (Bermejo et al., 2000), then daq (the thickness of the aqueous boundary layer) is calculated to be 1.5x10-3 cm. A similar trend was obtained using various initial gold concentrations (1.3x10-5-5.1x10-5 M).

Fig. 3. Variation in gold transport at different extractant mixture concentrations.

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3.4.Influence of organic phase diluent

It is recognized that the organic diluent influences the performance of many liquid membrane systems (Izatt et al., 1990; Shukla et al., Hill et al., 1996, Fontas, 2003). To determine their effect on the present system, experiments were carried out with solutions of Primene JMT-Cyanex 923 (20 %-20 % v/v) in each diluent and feed phases of 2.5x10-5 M gold at pH 9.5±0.05. Table 2 shows the corresponding percentage of gold transport after 3 h for these experiments.

Table 2

Percentage of gold transport by the mixture Primene JMT-Cyanex 923 in different diluents

a After 3 h

The change in the organic diluent influences the transport of gold, this transport using the carrier mixture of Primene JMT and Cyanex 923 is enhanced using organic diluents with lower viscosity and of aromatic nature, though it is difficult to adscribe the transport of the metal to one or another property of the organic diluent.

3.5.Effect of aqueous ionic strength (feed phase)

To study the effect of aqueous ionic strength in the feed phase, experiments were carried out with organic phases of Primene JMT-Cyanex 923 (20 %-20 % v/v) in xylene and aqueous solutions which contained 2.5x10-5 M gold (I) at pH 10.0±0.05 and different salts at 1 M concentration. From the results obtained (Table 3), it can be deduced that the presence of these salts in the aqueous solution had different influence on gold transport. While with NaCl the transport is increased, the presence of the lithium salts decreases the percentage of gold transported. On the other hand, the presence of sodium cyanide does not appreciable affect the transport of the metal with respect to the results obtained when this salt is not present in the feed phase.

Table 3

Percentage of gold transport at different ionic strengths in the feed phase

a After 3 h

3.6.The selectivity of the system Primene JMT-Cyanex 923-Au(CN)2- versus other metal-cyano complexes

This study was carried out using an organic phase of Primene JMT-Cyanex 923 (20 %-20 % v/v) in xylene and an aqueous phase containing 2.5x10-5 M (each) of the corresponding metal at pH 9.5±0.05. Table 4 shows the percentage of metal transported obtained from this series of experiments. Only the gold-cyanide complex is effectively transported and, consequently can be separated selectively from other metal-cyano complexes present in the aqueous feed phase at this pH range.

Table 4

a After 1 h. a After 3 h

The variation in the metal transport should be explained according to the different easiness of extraction of such complexes using the Primene JMT-Cyanex 923 mixture (the extraction of the gold (I)-cyanide complex favoured over that of the other metal-cyano complexes at these alkaline pH values) and the chemistry of the organic carrier mixture-transported metal-cyano complexes.

3.7.Evaluation of the diffusional parameters

The extraction of Au(CN)2- by the mixture Primene JMT-Cyanex 923 in xylene can be described by the following reactions and extraction constants (Alguacil et al., 1997):

K1 (3)

K2 (4)

(5)

(6)

where R and L represented the amine and the phosphine oxide, respectively. The values of K1 and K2 were found to be 8.7x1011 and 2.8x1012, respectively.

According to the literature (Danesi et al., 1983; Cianetti and Danesi, 1983; El Aamrani et al., 1998; Rovira and Sastre, 1998; Alguacil and Alonso, 2000; Sastre et al., 2000a; Castillo et al., 2003), and at steady state the following expression for the flux can be obtained:

(7)

and the permeability coefficient P= J[Au(I)]0 can be written as:

(8)

In this expression, the equilibrium and diffusion parameters involved in the Au(CN)2- transport process through the flat-sheet supported liquid membrane using the mixture of Primene JMT and Cyanex 923 as carrier are combined in one equation.

To determine the value of the resistances to the mass transfer, the next expression, obtained from the above equation has been used:

(9)

In the present investigation, experiments on gold transport have been carried out at pH value of 9.5, if the parameters A and B are defined as:

(10)

(11)

By substituting in Eq.(9), the expression is re-written as:

(12)

Thus, by plotting 1/P vs. 1/AB a straight line should be obtained with slope Δorg (transport resistance due to diffusion through the membrane) and intercept Δaq (transport resistance due to diffusion by the aqueous feed boundary layer). The values of Δorg and Δaq obtained from the present experimental data was found to be 11067 and 150 s/cm, respectively. The calculated value of the diffusion coefficient was Dorg= 1.1x10-6 cm2/s.

The diffusion coefficient of the gold complexes in the bulk organic phase (Dorg,b) can be evaluated from the diffusivity in the membrane from the next ratio (Huang and Juang, 1988):

(13)

where τ is the membrane tortuosity and ε is the porosity. The value of Dorg,b was calculated to be 4.1x10-6 cm2/s.

4.Conclusions

The transport of Au(CN)2- using the mixture of amine Primene JMT and the phosphine oxide Cyanex 923 under various experimental conditions has been studied and a mechanism of the gold-cyanide complex transport considering the aqueous film diffusion of the complex, fast chemical reaction at the interface and diffusion of HAu(CN)2·R·L and HAu(CN)2·R·L2 through the membrane is proposed. For concentrations of the extractant mixture above 20 %-20 % v/v (amine-phosphine oxide), a limiting value of 6.7x10-3 cm/s for permeability is obtained and the transport process is controlled by the diffusion of the aqueous stagnant film. Mass transfer coefficients in the membrane and in the feed phase are found to be 9x10-5 and 6.7x10-3 cm/s, respectively. The transport of Au(CN)2- is enhanced using the extractans mixture if compared with the two separated reagents. More than 95 % of the Au(I) could be separated using the mixture of Primene JMT and Cyanex 923 in the presence of various metals such as Ni(II), Co(III) and Fe(II).

Acknowledgement

Authors thank to Mr. Bascones and Mr. Lopez for technical assistance troughout the experimental work. Also to CSIC (Spain) for support.

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