Towards a more environmentally friendly process for
gold: models on gold adsorption onto activated carbon
from ammoniacal thiosulfate solutions
Patricio Navarroa, Cristian Vargasa, Manuel Alonsob,
Francisco Jose Alguacilb*
a
Departamento de Ingenieria Metalurgica, Facultad de Ingenieria, Universidad de
Santiago de Chile, Avda. L.B. O´Higgins 3363, Casilla 10233, Santiago, Chile.
b
Centro Nacional de Investigaciones Metalurgicas (CSIC), Avda. Gregorio del Amo 8,
Ciudad Universitaria, 28040 Madrid, Spain. E-mail: fjalgua@cenim.csic.es
*
Corresponding author
2
Abstract
In the context of gold extraction, leaching is the dissolution of the metal or even a mineral
in a liquid phase. Thus, it is of a primary concern the dissolution of gold in an aqueous
solution, operation which requires both a complexant and an oxidant to achieve
acceptable leaching rates and yields. A limited number of ligands form complexes of
sufficient stability and at a suitable rate for their use in gold leaching operations. Cyanide
is mostly used because of its relatively low cost and great efficiency for gold dissolution.
The main disadvantage associated with its use is the extremely toxic character of the
reagent. Thus, several other ligands had been considered for gold extraction from ores,
regarding to environmental pressures and even restrictions to the use of cyanide,
potential of having faster gold leaching kinetics than cyanide, possibility of their use in
acidic media, which is suitable in the treatment of refractory ores, and a greater degree of
selectivity than cyanide for gold over other metals. Nevertheless, some of them presented
some disadvantages which limited their practical use. Ammoniacal thiosulfate leaching
has a considerable potential as an effective and less hazardous procedure (than cyanide)
for gold extraction from auriferous ores. Once gold is dissolved, separation of the
precious metal from the solution can be achieved using different procedures (i.e.
adsorption onto activated carbon). In the present investigation, up to fivel models were
used to correlate the adsorption of gold onto the carbon, being the Fleming´s model the
one which best fit the experimental results on gold adsorption.
Keywords: Gold; Adsorption; Carbon; Modelling; Ammoniacal thiosulfate
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1. Introduction
A relative interest has attracted to the use of alternatives to the conventional cyanidation
process to extract gold from gold ores. This interest in the use of other alternatives to
cyanide for the dissolution of this precious metal arises from concerns relative to the
toxicity of cyanide and the lack of effectiveness of cyanidation process to leach, in an
effective form, carbonaceous or complex ores.
Several alternatives to cyanide have been considered [1], and thiosulfate leaching
emerges as one of the more attractive [2-4], because thiosulfate is considered a non-toxic
reagent and can leach gold faster than cyanide. On the other hand, the main
disadvantage of thiosulfate processing is the reagent consumption and the lack of a
suitable gold recovery method.
The dissolution of gold can be described by the following reaction:
4Au  8S 2 O 32  O 2  2H 2 O  4Au (S 2 O 3 ) 32  4OH 
(1)
However, the presence of ammonia in the system is needed in order to prevent
passivation of the precious metal and to stabilise copper (II), of which presence is also
required to increase the rate of gold dissolution.
The gold-ammine complex is converted to thiosulfate complex, since measurements at
pH values greater than 9.0 have shown that gold rest potentials change with thiosulfate
concentration rather than with ammonia concentration according to following equation:
Au ( NH 3 ) 2  2S 2 O 32  Au (S 2 O 3 ) 32  2 NH 3
(2)
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it was also described that under certain conditions gold can be present in solution as the
ammine complex.
Most of the literature devoted to the gold-thiosulfate processing has been concerned with
the leaching of the precious metal, with limited research effort on the process of
recovering gold from solution. The various alternatives proposed for gold recovery from
thiosulfate media can be found elsewhere [2,5].
Activated carbon has the advantages that it can be added to the pulp and avoid soluble
losses in tailings, but apparently, it has a low affinity for the gold-thiosulfate complex.
From the view of point of engineering, before scaling up the technology, a theoretical
model of the adsorption operation is needed in order to design an efficient recovery
process. In the present investigation, several adsorption models [6-8], proposed for the
adsorption of gold from cyanide medium, are tested in order to look for a suitable one to
describe the adsorption of gold from a synthetic solution onto activated carbon from
ammoniacal thiosulfate solution.
2. Experimental
The gold thiosulfate complex used in this investigation was obtained from Alfaaesar, a
stock gold solution was prepared by dissolution of the reagent in distilled water. Working
gold solutions were prepared by dissolving a known quantity of the stock solution into the
corresponding ammoniacal solution. All other chemicals were of reagent grade.
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The activated coconut-shell carbon, obtained from Anglo Chilena SA, was washed with
deionized water, dried at 60º C and kept in a dessicator under vacuum. Table 1 shows
the main physical characteristics of the activated carbon.
Gold adsorption tests were carried out in a 500 mL glass reactor equipped with a stirring
device. The reactor was placed in a water bath. Experiments were run at 20º C and at
500 min-1 with 250 mL of the gold solution and the experiments were initiated by placing
the weighed amount of the activated carbon into the reactor. The experimental conditions
used to correlate the various model´s fit to the experimental data were summarized as
shown in Table 2, at these conditions, gold adsorption performance can be considered as
representative of the present system.
Samples were taken periodically for gold concentration analysis by atomic absorption
spectrometry using a Perkin Elmer 1100B spectrophotometer. The effectiveness of the
adsorption process was estimated from the percentage of gold adsorption as the
following:
%Au ads 
Au 0  Au t   100
Au 0
3. Results
3.1. The model fitting
The five models applied to fit the experimental data are described as the following:
First order model
(3)
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The first order model can be represented by the equation (4):
r  k 1 Au t  k 2 Au c, t
(4)
By linearization of the equation (4), a plot of r/[Au]t versus [Au]c,t/[Au]t can be built-up and
a straight line should be obtained with slope k2 and intercept k1. The values of k1 and k2
obtained from the present experimental data are given together with the other models in
Table 3.
It should be noted here that data obtained from the 35 mg /L gold solution do not fit
properly to the model (r2 < 0.9). In this model k1 can be related to a rate constant of the
adsorption process, whereas k2 to the desorption process.
The Dixon model
In this model the corresponding equation is given by the following expression of:
r  k 1 Au t Au c,m  Au c, t   k 2 Au c, t
(5)
As in the above case, a linearization of the equation and by plotting r/[Au] t versus
[Au]t([Au]c,m-[Au]c,t)/[Au]c,t, a straight line with slope k2 and intercept k1 should be obtained.
As seen in Table 3, data experimentally obtained with the 35 mg/L gold solution do not fit
to the model.
The Nicol model
The equation (6) can be used to represent the Nicol model:
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r  k 1 KAu t  k 1 Au c, t
(6)
Similarly to all the above, a plot of r/[Au]t against [Au]c,t/[Au]t should results in a straight
line with slope k1 and ordinate k1K. According to results shown in Table 3, the
experimental data obtained using the 35 mg/L gold solution are not given due to the poor
(R2<0.9) fit with this model.
The Fleming model
The model proposed by Fleming is given by the expression of:
Au c,t
 k1 Au 0 t n
(7)
The experimental data were fitted to the model by linearization of the above by the plot
log [Au]c,t versus log t. From this plot the coefficient n is given by the corresponding slope,
whereas from the intercept the value of k1 can be calculated. The experimental data for
35 mg/L also fitted this model (R2>0.9) as seen in Table 3.
The La Brooy model
In this model, the following equation had been proposed to explain the adsorption of gold
onto activated carbon:
Auc,t  k1 Aut t n
(8)
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Similar to the above model a plot of log ([Au]c,t/[Au]t) versus time should results in a
straight line with slope n and ordinate to calculate k 1. Also this model fitted well (R2>0.9)
the data obtained for 35 mg/L of gold concentration.
4. Discussion and Conclusions
Five models have been examined in order to explain the adsorption of gold onto activated
carbon from ammoniacal thiosulfate medium. Three of the above models (the first order,
Dixon and Nicol models) seemed to be only suitable to explain the adsorption of gold at
lower metal concentrations, whereas with the Fleming and La Brooy models the fit of the
experimental data to the respective model can be broaded to all the range of the gold
concentrations tested. However the fit of the experimental data to the Fleming model is
slightly better than that obtained with the La Brooy model (Table 3). Thus, this model can
best represent the adsorption of gold onto activated carbon within the experimental
conditions using in this investigation. Fig. 1 compares the experimental data on gold
adsorption (see Table 2 for details) with those obtained using the theoretical data
estimated from the present model. It can be seen that reasonable good agreement is
obtained between each set of data.
On the other hand, from results obtained within this work it can be concluded that the
values of kn tends to decrease as the initial gold concentration in the aqueous solution
increases.
Results of this work also allow to conclude that activated carbon has a low affinity for
Au(S2O3)23- species, however the presence of the Au(NH3)2+ in the aqueous phase can
not be completely neglected, and this species will help in the adsorption of the precious
metal. Moreover, the presence of AuNH3S2O3- ion had been proposed [2], though it has
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not been confirmed experimentally. A possible mixed gold-ammine-thiosulfate complex
has also been mentioned in the literature [9]. As it is stated in the literature [4], there is a
need for more thermodynamic data for the Au/NH3/S2O32- system.
The effectiveness of the gold adsorption operation onto carbon can be improve by the
use of a more effective operational device, i.e. columns, or also by adding a small amount
of cyanide to the system [2].
Acknowledgements
The authors wish to acknowledge CONICYT (Chile) for project FONDECYT 7040009 and
CSIC (Spain) for support to carry out this work.
Nomenclature
[Au]0 initial gold concentration in the aqueous phase
[Au]t gold concentration in the aqueous phase at an elapsed time
[Au]c,t mass of gold adsorbed onto the carbon at an elapsed time
[Au]c,m maximum mass of gold adsorbed onto the carbon
[NH4OH] initial ammonium hydroxide concentration in the aqueous phase
[S2O32-] initial thiosulfate concentration in the aqueous phase
kn, K constants for the different models
n time coefficient for the Fleming and La Brooy models
r gold adsorption velocity onto the carbon
t
time
R correlation coefficient
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References
[1] J. Marsden and I. House, The Chemistry of Gold Extraction, Ellis Horwood, New York,
1992.
[2] M.G. Aylmore and D.M. Muir, Thiosulfate leaching of gold-a review, Min. Eng., 14
(2001) 135-174 and references therein.
[3] P. Navarro, C. Vargas, A. Villarroel and F.J. Alguacil, On the use of
ammoniacal/ammonium
thiosulphate
for
gold
extraction
from
a
concentrate,
Hydrometallurgy 65, (2002) 37-42.
[4] E. Molleman and D. Dreisinger, The treatment of copper-gold ores by ammonium
thiosulfate leaching, Hydrometallurgy, 66 (2002) 1-21.
[5] P. Navarro, R. Alvarez, C. Vargas and F.J. Alguacil, On the use of zinc for gold
cementation from ammoniacal-thiosulphate solutions, Min. Eng., 17 (2004) 825-831.
[6] J.S. Van Deventer, Kinetic model for the reversible adsorption of gold cyanide on
activated carbon, Chem. Eng. Comm., 44 (1986) 257-274.
[7] J.D. Le Roux and A.W. Bryson, A comparison of several kinetic models for the
adsorption of gold cyanide onto activated carbon, J. South African IMM, 91 (1991) 95103.
[8] M.N. Vegter and R.F. Sandenberg, Rate-determining mechanisms of the adsorption
of gold di-cyanide onto activated carbon, J. South African IMM, 96 (1996) 109-118.
[9] P.L. Breuer and M.I. Jeffrey, An electrochemical study of gold leaching in thiosulfate
solutions containing copper and ammonia, Hydrometallurgy, 65 (2002) 145-157.
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Table 1
Characteristics of the activated carbon.
Shape
Cylinder
Length (cylinder)
5.6 mm
Diameter (cylinder)
3 mm
Surface area
925 m2/g
Micropores
96%
Table 2
Experimental conditions used in the investigation.
[Au]0 (mg/l)
[S2O32-]0 (mg/l)
[NH4OH]0 (M)
pH±0.05
Carbon added
(g)
10
11.2
1
10.5
0.16
25
28.0
1
10.5
0.16
35
39.2
1
10.5
0.16
In this table, the concentrations of gold and thiosulfate responded to these given by the
Au(S2O3)23- stoichiometry
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Table 3
Values of the constants and correlation coefficient of the models tested
R2
[Au]0 (mg/l)
aFirst
10a
k1: 9.0x10-5 s-1
k2: 6.4x10-5 kg/L s
0.911
25a
k1: 1.2x10-5 s-1
k2: -3.0x10-6 kg/L s
0.991
10b
k1: 1.4x10-6 kg/L s
k2: 3.2x10-5 kg/L s
0.999
25b
k1: 1.6x10-5 kg/L s
k2: 1.3x10-5 kg/L s
0.992
10c
k1: 6.4x10-5 kg/L s
K: 1.4 L/kg
0.911
25c
k1: -3.0x10-5 kg/L s
K: -3.4 L/kg
0.991
10d
k1: 0.026 L/kg s
n: 0.36
0.929
25d
k1: 6.5x10-4 L/kg s
n: 0.71
0.998
35d
k1: 1.7x10-4 L/kg s
n: 0.83
0.991
10e
k1: 0.012 L/kg s
n: 0.51
0.910
25e
k1: 3.8x10-4 L/kg s
n: 0.79
0.995
35e
k1: 1.0x10-4 L/kg s
n: 0.91
0.985
order, bDixon, cNicol, dFleming, eLa Brooy
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Fig 1. Percentage of gold adsorption onto activated carbon. Experimental data (points),
predicted values according to the Fleming model (lines).
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Fig.1