SOLVENT EXTRACTION OF Au(III) BY THE CHLORIDE SALT
OF THE AMINE ALAMINE 304 AND ITS APPLICATION TO A
SOLID SUPPORTED LIQUID MEMBRANE SYSTEM
F.J.Alguacil
Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda.
Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain.
E-mail: fjalgua@cenim.csic.es
ABSTRACT
The extraction of Au(III) by the chloride salt of the amine Alamine 304 (R3NH+Cl-) in
xylene from hydrochloric acid solutions has been investigated. The analysis of metal
distribution data by numerical calculations suggested the formation of the species
R3NH+AuCl4- in the organic phase with formation constant log Kext= 5.44. The results
obtained on Au(III) distribution have been implemented in a solid-supported liquid
membrane system, where in NaSCN solutions were found to be the most effective to strip
the metal from the organic solution. Influence of membrane composition, metal
concentration on gold transport, and the selectivity of the system have also been studied.
INTRODUCTION
Extraction of gold from an aqueous halide medium into an organic phase has been
frequently used for both the separation and concentration of this precious metal. In fact,
the technology has been implemented in various flow sheets aimed at the separation and
concentration of gold and platinum group metals (PGM) (1,2). Due to the reactivity of the
gold(III)-chloro complex, several studies had dealt with using various types of extractants
such as amines (3-7), phosphine oxides (8-10), and reagents containing sulphur (11-15) or
oxygen (1,16) as donor atoms.
These extraction systems can also be implemented in a solid-supported liquid membrane,
where the performance of solvent-extraction separation is enhanced by combining the
extraction and stripping processes in one step (17,18). Though scarce, various applications
of these extraction systems to membrane transport of gold have been considered (19-24).
The present investigation was undertaken to obtain quantitative characterization of the
extraction reaction between aqueous Au(III) and the chloride salt of the amine Alamine
304 in xylene. Moreover, a liquid-membrane system has been designed by using the
extraction process mentioned above, and parameters affecting the liquid membrane, i.e.
stripping reagent, composition of the membrane phase and metal concentration have been
studied.
2
EXPERIMENTAL
Reagents and solutions
A 1000 mg L-1 stock solution of Au(III), was obtained by dissolving HAuCl4 (Fluka) in
distilled water. The working solutions, containing various gold(III) concentrations in 6 M
HCl, were prepared by dilution of the stock gold solution in HCl-water solutions.
The chloride salt of the amine Alamine 304 (Cognis) was prepared as described in the
literature (25), diluted in xylene (AR grade, Fluka), and used as the organic phase. Various
concentrations of the organic salt were also used in the investigation.
Apparatus and Procedure
Solvent extraction experiments
Distribution batch experiments were carried out at 20C by shaking (700 rpm) equal
volumes (25 mL) of the organic phase and the aqueous solutions in separatory funnels.
After equilibrium, the metal remaining in the aqueous phase was analysed by atomic
absorption spectrophotometry (AAS) (Perkin Elmer 1100B spectrophotometer). The
amount of metal extracted was obtained by difference with the initial concentration in the
aqueous solution. The validity of the procedure was checked by complete stripping of
several gold-loaded organic phases using a thiocyanate (0.5 M) solution and analyzing the
gold concentration in the stripped phase. After applying the corresponding mass balance,
96% mass balance accuracy were regularly obtained.
From these data, the distribution coefficient, D, was calculated as the ratio
D=
[Au(III) ]org
[Au(III) ]aq
(1)
where [Au(III)]org and [Au(III)]aq are the total gold concentration in the organic and
aqueous phases, respectively.
Liquid membrane system
Permeation experiments were carried out in a two-compartment membrane cell described
elsewhere (26), where 200 mL aqueous solution containing different gold concentrations
in 6 M HCl was used as the source solution. For the stripping phase, the same volume of
NaSCN 0.5 M was used. The support for the liquid membrane was a
polyvinylidenedifluoride (PVDF) film (Millipore GVHP) with thickness (dorg) of 125 μm,
75% porosity (ε), 0.22 μm average pore size, tortuosity (τ) 1.67, and effective membrane
area 11.3 cm2.
3
The liquid membrane was prepared by impregnating the laminar microporous film with
solutions of the chloride salt of the amine Alamine 304 in xylene, whereas both aqueous
phases were pre-saturated with xylene in order to stabilize the membrane phase. In each
experiment, stirring rate in both the source and stripping solutions was kept constant at
1200 rpm.
The metal concentrations in the aqueous solutions were periodically determined by means
of AAS. The metal flux, J, was calculated as
(2)
J = P [Au(III) ]TOT
where [Au(III)]TOT is the metal concentration in the source phase, and P is the permeability
coefficient which was calculated as (27)
Cdt   A Pt
ln
(3)
Cd0 V
RESULTS AND DISCUSSION
Solvent extraction system
A preliminary investigation was carried out in order to determine the time needed to obtain
equilibrium. From experiments carried out using an aqueous phase containing 50 mg L-1
Au(III) and 6 M HCl and the amine salt 3x10-3 M in the organic solution, it was observed
that equilibrium is reached after 1 minute of mixing. Nevertheless, the contacting time was
fixed at 10 minutes.
The metal distribution ratio, D, at 6 M HCl, was determined for different amine salt
concentrations at different Au(III) concentrations. Results are plotted in Fig.1. It can be
seen that log D depends on the gold concentration, and increased as the initial metal
concentration decreased. Taking into consideration that Au(III) exists mainly as
tetrachloroaurate(III) ion (28,29), AuCl4-, it can be assumed that gold(III) is extracted by
the chloride amine salt (R3NH+Cl-) according to the following ion exchange reaction
AuCl 4aq + R3 NH Clorg  R3 NH AuCl 4org + Claq
-
+
-
+
-
-
(4)
Assuming ideal behaviour in the organic phase and constant activity coefficient in the
aqueous phase, the equilibrium constant for eq.(4) can be written as
K ext =
[R 3 NH+ AuCl -4 ]org [Cl- ]aq
[AuCl -4 ]aq [R 3 NH+ Cl- ]org
(5)
It is known that amines and quaternary ammonium salts tend to form aggregates in organic
4
phases (30). Aggregation depends strongly on the organic diluent and the extractant
concentration. In this investigation, very dilute solutions of reagent have been used, so it
could assume that, though the amine salt aggregates (31), the monomer form of the amine
salt predominates in the organic phase, and the behaviour shown in Fig.1 may be
explained taking into account the following set of equations.
The total concentration of the amine chloride in the organic phase is
[R3 NH+ Cl- ]total = [R3 NH+ Cl- ]org + [R3 NH+ AuCl -4 ]org
(6)
and from eq.(5), the next expression is obtained
+
K ext [AuCl 4 ]aq [R 3 NH Cl ]org
[R 3 NH AuCl ] =
[Cl- ]aq
+
4 org
(7)
Substitution of eq.(7) in eq.(6) leads, after re-arranging to
[R 3 NH+ Cl- ]org =
[R 3 NH+ Cl- ]total
K ext [AuCl 4 ]aq
1+
[Cl- ]aq
(8)
From eq.(5), considering the definition of the distribution coefficient (eq.(1)), taking
logarithms and re-arranging, the following expression is obtained
log D = log K ext + log [R3 NH+ Cl- ]org - log [Cl- ]aq
(9)
From eq.(8), as the gold concentration in equilibrium is increased, the concentration of the
amine chloride in the equilibrated organic phase becomes lower, and thus in eq.(9), the
value of log D also becomes lower (Fig.1).
To determine the composition of the extracted species and their extraction equilibrium
constants, extraction data were also numerically treated by using the LETAGROP-DISTR
program (32); in the program the error square sum U over all Np ( 27 experimental points)
defined as
U =  N p (log D cal - log D exp )2
(10)
was used in the minimization process. Dexp represents the experimental distribution
coefficient and Dcal is the value calculated by the program for the model tested after
solving the mass balance equations of the components of the system. Therefore, the best
model is the one which gives the lowest value of U. The calculations were performed
taking a set of species and extraction constants as the starting input and considering the
5
influence of the minimized function when partially varying or adding new species to the
model. Furthermore, data on the formation of trimers by the amine salt (log K3 = 3.18) (31)
were also considered in the numerical treatment of the data. The results of computer
analysis showed that the extraction system is best fitted to the existence of one species,
with stoichiometry R3NH+AuCl4- (as shown in eq.(4)), in the organic phase with log Kext =
5.44±0.12 (σ(log Kext)= 0.04) and U= 0.19 (σ= 0.13).
Liquid membrane system
The extraction system of tetrachloroaurate(III) ion by the chloride salt of amine Alamine
304 has been implemented in a solid-supported liquid membrane, where the transport of
metal species across the liquid membrane depends not only on the equilibrium distribution
ratios but also on the kinetics of extraction and stripping processes.
The transport of Au(III) through the liquid membrane has been studied by varying the
chemical conditions of the three phases: stripping and source aqueous solutions and the
membrane phase. The selectivity of the system was also investigated.
Best stripping conditions are essential to apply the extraction system studied to a
membrane transport system. Several reagents were tested as stripping solutions for Au(III)
from the loaded organic phase. Coordinating ligands, as thiourea and thiocyanate, reducing
agents like NaHSO3, and other salts (NaClO4) or even water have been used. As can be
seen in Fig.2, the stripping ability for Au(III) decreases in the order
NaSCN>NaClO4>water>thiourea>NaHSO4.
Sodium thiocyanate was found to be the most efficient stripping agent. The transport of
Au(III) through the membrane against its concentration gradient was achieved, recovering
practically all the metal in the stripping compartment after 3 hours. Thiocyanate ion strips
gold due to the strong complex ability of thiocyanate (log ß3= 42) (28) towards the metal.
The influence of the membrane composition on gold transport was studied using different
carrier concentrations in the membrane phase.
Figure 3 shows metal flux values for the transport of gold through a supported liquid
membrane impregnated with solution 0.01-0.7 M of the amine salt in xylene. Lower
concentrations of carrier were investigated, and no transport was achieved. The metal flux
value increased with the amine salt concentration and then levels off, this being typical of
a process controlled by diffusion in the stagnant film of the source phase. In this condition
(33)
J lim =
Daq
[Au(III) ]TOT
d aq
(11)
6
-6
2 -1
and assuming a value of 7.2x10 cm s for the aqueous diffusion coefficient of the goldcontaining species (Daq) and Jlim equal to 7.7x10-10 mol cm-2 s-1, the thickness of the
aqueous diffusion film (daq) estimated from the above equation is 1.0x10-3 cm.
The influence of the initial Au(III) concentration at 6 M HCl in the source phase on
transport across the supported liquid membrane containing 0.1 M amine salt in xylene is
shown in Fig.4. At low gold concentrations, the average metal flux is a function of the
initial metal concentration in the source phase. Hence, the permeation process is controlled
by diffusion of gold species in the lower range of metal concentrations. However, beyond
a certain limiting concentration, the flux tends to be independent of the metal
concentration, the most probable reason for this may be the rate-determining step for the
transport process. Under the limiting condition the total concentration of the amine salt
becomes equivalent to [R3NH+AuCl4-]org, and on the basis of the next equation (34)
+
-
Dorg [R 3 NH AuCl 4 ]org
J lim =
d org n
(12)
where n is the stoichiometric coefficient of the reaction and dorg is the thickness of the
membrane, the value of the membrane diffusion coefficient, Dorg, is estimated to be
9.6x10-8 cm2 s-1. The diffusion coefficient of the gold complex in the bulk organic phase,
Dorg,b, can be evaluated from the diffusivity (Dorg) in the membrane phase (35)


2
Dorg,b = Dorg
(13)
the value of Dorg,b was calculated to be 3.6x10-7 cm2 s-1. It should be noted that in the
present system, Dorg presents a lower value than that of the bulk diffusion coefficient, this
is attributable to diffusional resistance caused by microporous thin membrane placed
between the source and stripping phases.
The selectivity of the Au(III)-amine salt over Fe(III), Cu(II), Zn(II) and Ni(II) has also been
analyzed. Transport experiments have been carried out using a liquid membrane of 0.1 M
amine salt dissolved in xylene and a source phase which contained 10 mg L-1 of each
metal in 6 M HCl. Table 1 shows the values of the selectivity factors, ß, obtained for the
present investigation. These values were estimated according to the following (36)
=
J Au [M ]0
J M [Au ]0
(14)
where J represents the metal flux and [Au]0 and [M]0 are the respective metal
concentrations in the source phase at time zero. The values indicate that gold is transported
7
by the amine salt, preferably to the other metals with quantitative separation over Cu(II)
and Ni(II).
ACKNOWLEDGEMENTS
This work has been carried out under the support of CSIC (Spain). Technical assistance by
Mr.Bascones and Mr.López is also thanked.
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TABLE 1. SEPARATION FACTORS USING THE AMINE SALT AS CARRIER
___________________________________________________________________
ßAu/Zn
ßAu/Fe
ßAu/Cu
ßAu/Ni
__________________________________________________________________
21
10
quantitative
quantitative
___________________________________________________________________
Stripping phase: 0.5 M NaSCN
FIGURE 1. Experimental distribution data, log D vs log [R3NH+Cl-]org at different metal
concentration.
FIGURE 2. Investigation of stripping conditions for Au(III). Source phase: 10 mg l-1
Au(III) and 6 M HCl. Membrane phase: 0.1 M amine salt in xylene. Stripping phase: 0.5
M stripping reagent.
FIGURE 3. Metal flux versus carrier concentration. Source phase: 10 mg l-1 Au(III) and 6
M HCl. Stripping phase: 0.5 M NaSCN.
FIGURE 4. Effect of metal concentration on metal flux. Membrane phase: 0.1 M amine
salt in xylene. Stripping phase: 0.5 M NaSCN.