Journal of Minerals & Materials Characterization & Engineering, Vol. 2, No.2, pp 71-82, 2003, printed in the USA. All rights reserved
A Study on Gold(III) Recovery Via Foam Separation
with Nonionic Surfactant in Batch Mode
T. Kinoshita
, S. Akita
, S. Ozawa
, S. Nii
, F. Kawaizumi
, K. Takahashi
Nagoya Municipal Industrial Research Institute, 3-4-41 Rokuban, Atsuta-ku,
Nagoya 456-0058, Japan
Fax: +81-52-654-6788. e-mail:
Department of Chemical Engineering, Nagoya University, Chikusa-ku,
Nagoya 464-8603, Japan
Foam separation of Au(III) from its hydrochloric acid solutions was
studied in a batch mode using a nonionic surfactant: polyoxyethylene nonyl
phenyl ether having 20 ethylene oxide units (PONPE20). The surfactant showed a
strong affinity to Au(III) in HCl media and played a double role of foam-producer
and metal-collector. Effects of experimental parameters, such as the length of
drainage section of the column, concentration of the surfactant and the metal, air
flow rate and solution temperature, were discussed in terms of the recovery and
the enrichment of Au(III). The recovery increased with an increase in the
concentration of surfactant and in air flow rate, while the enrichment improved
with decreasing air flow rate and increasing the length of drainage section. The
behavior of Au(III) adsorption onto the foam was also analyzed in terms of the
surface excess, and the Freundlichs adsorption isotherm was successfully applied
to the system. Moreover, the selective separation of Au(III) from several heavy
metals and the application of cloud point extraction to the present foamate
solution were also carried out with the resultant enrichment ratio of 59.
Key Words: foam separation, nonionic surfactant, gold(III), surface excess,
Freundlichs adsorption isotherm, cloud point extraction
Conventional hydrometallurgical processes for recovering precious metals generally
consist of multiple steps of dissolution (leaching), conditioning and precipitation. These are not
only labor-intensive but also time-consuming, and much work has been conducted on the
development of alternative methods including solvent extraction and ion exchange. In
hydrochloric acid, gold exists exclusively as tetrachloro-anion (AuCl
) [1]. Ion pair extractants
such as amine salts are utilized to extract this anionic species [2,3], while solvating extractants
including methyl isobutyl ketone [4] and dibutylcarbitol (diethyleneglycol dibutyl ether) [1] are
adopted in the extraction of its parent acid, tetrachloroauric acid (HAuCl
). Among other
extractants examined are di(2-ethylhexyl) phosphoric acid, guanidine-based compound and
alkyliminodimethylene phosphonic acid [3-5].
72 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.2, No.2
Polyoxyethylene nonyl phenyl ethers (PONPEs) are commonly and widely used
nonionic surfactants with their relatively benign nature. The surfactants have a number of
electron-donating oxygen atoms in their ethylene oxide (EO) chains, and form complexes with
various metal ions [6,7] in the same manner as solvating extractants. In our previous studies,
gold was found to be strongly complexed with the nonionic surfactants in hydrochloric acid
media, and we have examined the separation of gold in such unit operations as solvent extraction
[8], cloud point extraction [9] and micellar-enhanced ultrafiltration [10]. Cloud point extraction
utilizes spontaneous coacervation of aqueous PONPE solutions at temperatures above the cloud
point; the metal is concentrated into the small coacervate (surfactant-rich) phase from the bulk
aqueous (surfactant-poor) phase. In micellar-enhanced ultrafiltration, gold is entrapped in the
surfactant micelles and concentrated in the residual phase since the micelles can not pass through
the ultrafiltration membrane having pores smaller than the micelles. In these methods, however,
the complete recovery of gold is difficult to attain since a small amount of the surfactant remains
in the bulk aqueous phase in the monomeric form below the critical micelle concentration.
Foam fractionation is another promising surfactant-based separation process which
features minimum requirements for the energy input and the operational costs. The technique is
particularly attractive for treating dilute solutions; a variety of applications have been reported
from mineral ore [11-13], hazardous metal ions [11,12], proteins [14,15] to surfactants [16]. In
the process, foam produced by bubbling travels upward in a column while draining the
interstitial water between the foam and is collected in a reservoir; solutes to be removed or
recovered are adsorbed onto the foam surface and concentrated in the collapsed foam liquid
(foamate) phase. A foam producing agent, i.e. surface active reagent, is added to feed solution to
stabilize the foam, while a collector is often introduced into the system to make target solutes
surface active and collectable. Numerous researches were conducted on the combination of such
additives and target solutes under various conditions [17-20].
In this study, foam separation of gold(III) from hydrochloric acid solutions was carried
out in a batch mode using the nonionic surfactants. Effects of several experimental parameters
were investigated on the recovery and enrichment of the metal. The surfactants are expected to
work as both foam-producing and collector agent, thus, reducing a reagent inventory.
Nonionic surfactants, polyoxyethylene nonyl phenyl ethers (PONPEs), with average
ethylene oxide units of 7.5, 10 and 20 were obtained from Tokyo Kasei Kogyo Co., Ltd. and
used without further purification. The general formula is HO(CH
, where n is
the ethylene oxide number. Aqueous metal solutions were prepared by dissolving each metal
chloride in dilute hydrochloric or nitric acid solutions. All the chemicals used were of reagent
Vol.2, No. A study on gold(III) recovery via foam separation with nonionic surfactant in batch mode 73
Figure 1. Schematic diagram of foam separation apparatus.
Procedure for Foam Separation Experiments
A schematic diagram of foam separation apparatus employed in this study is shown in
Figure 1. The bubble column is composed of a cylindrical glass tube (450 mm in height and 30
mm in inner diameter) with a sintered glass filter (G3) mounted at a lower part of the column as
a gas distributor. Initial concentrations of metal, surfactant and acid were set at 20 ppm, 0.05
wt% and 2.0 M, respectively, except for the experiments on their effects. The feed solution was
charged into the column to the height of 10 cm from the glass filter (ca. 78 ml), and air bubbles
were introduced through the distributor by an air compressor at 40 ml/min, unless otherwise
stated. The foam was collected through a froth collector equipped at the top of the column
connected to a foamate reservoir. Air flow was stopped when no more foam appeared from the
bulk solution. Experiments were carried out at 298 K.
Metal concentrations in the foam liquid (foamate) and residual (retentate) phases were
determined by inductively coupled plasma spectroscopy (ICP) after appropriate dilution. A
residual amount of the surfactant in the retentate phase was determined by UV
spectrophotometry at 276.0 nm. An average diameter of the foam was estimated by
photographing the column content and measuring the radius of each foam.
Experiments on cloud point extraction were also carried out using both specially
prepared and actual foamate solutions after the foam separation. A prescribed amount of
PONPE7.5 was added to the feed solution (20 ml) containing Au(III), PONPE20 and HCl. After
mixing, the solution was kept overnight in a incubator to attain the phase separation. Then, the
volume of the coacervate phase was measured; the metal concentration in the aqueous
(surfactant-poor) phase was measured by ICP and that in the coacervate phase was determined
from a mass balance.
74 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.2, No.2
Results and Discussion
Foam separation of Au(III) from hydrochloric acid media
In this study, the efficiency of foam separation was evaluated in terms of the recovery
percent (R) and enrichment ratio (E) defined by the following equations:
= 100 (V
/ V
) (1)
= 100 (V
/ V
) (2)
E = [M]
/ [M]
= R
/ R
where M represents metal, and the subscripts, fm and ini, refer to the foamate phase and the feed
solution, respectively.
Figure 2. Time course of foam separation of Au(III) using 0.05 wt% PONPE 20 in
2.0M HCl.
Typical results for the time course of foam separation of Au(III) with 0.05 wt%
PONPE20 from 2.0 M hydrochloric acid solution are shown in Figure 2. Initial concentration of
the metal was set to be 20 ppm. On starting bubbling (40 ml/min), stable foam was produced and
the generation of foam continued for ca. 50 min until the surfactant became insufficient in the
bulk solution. The foam collected in the reservoir defoamed spontaneously. As the bubbling
proceeds, Au(III) also accumulates in the foamate phase with the final recovery as high as 86 %,
implying the strong interaction between the metal and the surfactant. Since Au(III) exists in the
form of AuCl
in hydrochloric acid, the adsorption should take place through the coordination of
the surfactant EO unit to the chloroauric acid, HAuCl
, akin to the case of solvent extraction with
solvating extractants. Thus, the surfactant successfully plays a double role of foam-producer and
collector, and the separation of Au(III) can be attained with this simple technique. On the other
hand, a slight decrease in the enrichment ratio of Au(III) from 5.2 to 4.0 is observed with time.
This can be ascribed to an increase in the volume of the foamate phase, caused by the interstitial
water existing between foams. The final recovery of foamate is 22 %. In nitric acid media under
the same experimental conditions, though the data are not shown here, the foam was less stable
Vol.2, No. A study on gold(III) recovery via foam separation with nonionic surfactant in batch mode 75
than in hydrochloric acid; the final values were 36 % in the recovery and 3.1 in the enrichment
ratio of Au(III).
TABLE I. Effect of length of drainage section and heig
ht of feed solution on efficiency of foam
Height of feed solution (cm) 10 20 30 10 10
Length of drainage secction (cm) 35 35 35 65 85
(%) 86 83 84 79 72
(%) 22 21 21 10 8.6
E (-) 4.0
3.9 7.9 8.5
Run time (min) 50
157 66 65
In foam separation, the interstitial water between foams is drained out in the drainage
section in the column during upward travel of the foam from the bulk liquid surface to the top of
the column. The degree of water drainage strongly affects the enrichment of target solutes and is
a crucial factor in the process. In Table I are summarized the effects of both height of feed
solution and length of the drainage section, the latter being altered by inserting extending tubes
on the top of the column. When the length of drainage section is kept constant, an increase in the
height of feed solution has little effect on both recovery of Au(III) and foamate, though the run
time defined as the duration for the surfactant in the bulk solution to deplete increases from 50
min (at 10 cm) to 157 min (30 cm) due to an increased volume of the feed solution. On the
contrary, much improvement in the enrichment is observed by extending the drainage section. A
slight decrease in the metal recovery is canceled out by smaller volume of the foamate phase,
and the enrichment ratio reaches as high as 8.5 at the length of 85 cm compared with 4.0 at 35
cm. In the following experiments, the heights of feed solution and the length of drainage section
were set at 10 cm and 35 cm, respectively.
Figure 3. Effect of initial PONPE concentration on recovery percent and
enrichment ratio.
76 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.2, No.2
Results for the effect of the initial concentration of PONPE20 on the recovery and
enrichment of Au(III) are shown in Figure 3. By increasing PONPE20 concentration to 0.05
wt%, the recovery of Au(III) increases dramatically to 86 % and reaches plateau at higher
surfactant concentrations. However, increasing volume of the foamate phase leads to a decline in
the enrichment ratio of Au(III) from 8.9 to 4.0. Taking into account the contradicting influences
on the recovery and the enrichment, the optimum concentration for PONPE20 is determined to
be 0.05 wt% for Au(III) separation in the present system. At this concentration, the molar ratio of
the surfactant to the metal corresponds to 4.5. When the surfactant concentration exceeded 0.10
wt%, stable operation could not be attained due to excessive formation of foam from the onset of
bubbling. Results for the system using PONPE10 are also given in Figure 3 by the dotted lines,
indicating slightly lower efficiency than that of PONPE20. The difference in performance
between the two surfactants is ascribed to the difference in their foamability rather than the
extractability of Au(III) since the total amounts of ethylene oxide units existing in the two
systems are similar.
TABLE II. Effect of air flow rate on efficiency of foam separation.
Air flow rate (ml/min) 30 40 50 60
(%) 77 86 84 92
(%) 11 22 24 31
E (-) 7.3 4.0 3.5 3.0
Run time (min) 75 50 44 38
The effect of air flow rate on the system is tabulated in Table II. By rising the flow rate,
the run time is reduced from 75 min at 30 ml/min to 38 min at 60 ml/min. While the metal
recovery is relatively high, a rise in the flow rate leads to a significant decrease in the enrichment
from 7.3 to 3.0. The decline can be attributed to an increased amount of the interstitial water
transferred to the reservoir as reflected in an increase in the recovery of foamate.
TABLE III. Effect of initial Au(III) concentration on efficiency of foam separation.
Initial Au(III) concentration (ppm) 10 20 40 80 120 200
(%) 91 86 66 51 44 36
(%) 18 22 18 19 19 19
E (-) 5.0 4.0 3.6 2.7 2.3 1.9
Molar ratio 9.0 4.5 2.2 1.1 0.75 0.45
Results for the effect of initial Au(III) concentration is given in Table III, along with the
molar ratio of PONPE20 to the metal in each system. At the metal concentration of 10 ppm, at
which the molar ratio corresponds to 9.0, the recovery reaches over 90 % with the enrichment
ratio of 5.0. Increase in the initial Au(III) concentration leads to a considerable decrease in the
metal recovery as the result of depletion of the surfactant; the recovery is reduced to as low as 36
% at 200 ppm. It is to be noted that the residual metal could be recovered by adding the
surfactant into the retentate phase and conducting the bubbling again. Since only a limited
Vol.2, No. A study on gold(III) recovery via foam separation with nonionic surfactant in batch mode 77
amount of the surfactant can be introduced into the system for the reason described above, the
present system is more appropriate in handling solutions containing a small or trace amounts of
TABLE IV. Effect of solution temperature on efficiency of foam separation.
Temp. (K) 298 312 320 330
(%) 86 72 58 48
(%) 22 21 16 8.4
E (-) 4.0 3.4 3.6 5.7
Residual surfactant conc. (wt%) 0.006 0.012 0.016 0.019
The nonionic surfactants are known to lose their surface activity at high temperatures.
From this point, the effect of solution temperature on the foam separation of Au(III) using
PONPE20 has been investigated; the results are listed in Table IV. As the temperature rises, the
percent recovery of Au(III) decreases considerably; its value falls below 50 % at 330 K.
Increased temperature prevents the stable formation of foam and the subsequent adsorption of
the metal on the foam surface, as ascertained by a decrease in the foamate volume recovered.
These results indicate that careful control of temperature must be introduced in the foam
separation process with nonionic surfactants. For the treatment at higher temperatures, the use of
more hydrophilic nonionic surfactant having longer ethylene oxide units might be promising.
Each residual concentration of PONPE20 in the retentate phase is also tabulated in Table IV. At
298 K, less than one-tenth, 0.006 wt%, of the initial surfactant concentration remains in the bulk
solution. Although the value is well below the critical micelle concentration (CMC) of the
surfactant (0.015 wt% at 298 K) [21], the retentate solution is recommended to be recycled or
treated in downstream process, such as activated carbon treatment. The residual concentration
increases by a rise in the solution temperature, and reaches 0.019 wt% at 330 K.
Separation of Au(III) and cloud point extraction
Figure 4. Effect of HCl concentration on foam separation using 0.05 wt%
PONPE 20 from multi metal solution.
78 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.2, No.2
Experiments on the selective recovery of Au(III) from multi-metals solution of Co(II),
Cu(II), Ni(II), Pd(II), Pt(IV) and Zn(II) have been carried out via the foam separation using
PONPE20. Figure 4 shows the effect of hydrochloric acid concentration on the recovery percent
of each metal and the enrichment ratio of Au(III). Initial concentration of each metal was 20
ppm, and the acid concentration was varied from 0.1 to 4.0 M. The recovery percent of Au(III)
increases with increasing HCl concentration, and exceeds 85% at acid concentrations more than
2.0 M. The similar trend was observed in the solvent extraction of Au(III) using PONPEs [7],
implying the same complexing mechanism in hydrochloric acid media. On the other hand, the
transfer of the other metals to the foamate phase is much suppressed to below 20 % in the whole
acid range studied; thus, satisfactory separation of Au(III) is attainable. The percent recovery of
foamate, though the data are not shown here, was found to be quite similar to that of the other
metals. This coincidence implies that the metals are not adsorbed onto the surface of foam
through the specific interaction with the surfactant, but move within the interstitial water into the
foamate phase. The following conclusion can also be deduced from the above observation: about
three quarters of Au(III) are entrained on the foam interface while the rest is conveyed with the
interstitial water based on the difference in the recovery percent of Au(III) and the other metals.
The degree of the adsorption of Au(III) onto the surface of foam was analyzed in terms
of surface excess, Γ, determined by a mass balance on the foamate phase:
L [M]
= S Γ + L [M]
where the subscript, ret, refers to the retentate phase, L and S denote the volumetric flow rate of
foamate and the surface generation rate of foam, respectively. The first term in the right side of
Equation (4) corresponds to the metal entrainment on the foam surface, and the second term
corresponds to the metal conveyed in the interstitial water. An air flow rate (F) and an average
diameter of foam (d) were used to calculate the surface generation rate 6F/d, provided that the
foam produced is a perfect sphere and uniform. Under the standard experimental condition, the
foam diameter was determined to be 0.7 mm, and no significant change in the size was observed
during runs.
Figure 5. Time course of surface excess and retentate concentrations of Au(III)
and PONPE.
Vol.2, No. A study on gold(III) recovery via foam separation with nonionic surfactant in batch mode 79
Figure 5 shows time course of the surface excess along with the retentate concentration
of Au(III) and PONPE20. As the run proceeded, the concentrations of metal and surfactant
decreased gradually to the end point, where no more foam was produced. The surface excess also
decreased from 4.9 to 1.0 x10
and there appears an inflection point at around 30 min. The fact
that the residual concentration of surfactant at this point, 0.143 mol/m
, was very close to its
CMC (0.140 mol/m
or 0.015wt%) indicated the strong interaction between the metal-surfactant
complex in the bulk aqueous solution and on the foam surface.
Figure 6. Relation between surface excess and retentate concentration of Au(III).
In Figure 6 the data in Table III are replotted using the as the surface excess against the
concentration of Au(III) in the retentate, i.e. adsorption isotherm. Since the metal concentration
in the retentate varies with time, an arithmetical mean between the initial and residual (final)
values is adopted as [M]
. As expected, the surface excess increased with increasing metal
concentration, and can be well expressed by the following Freundlichs adsorption isotherm:
Γ x10
= 10.3 [M]
The solid line in Figure 6 is a calculated one using Equation. (5). The ranges which the arrows
show are the transition of the concentration of Au(III) during runs.
With an intention to gain more concentrated metal solution, cloud point extraction had
been applied to the foamate obtained in the foam separation. Coacervation, i.e. phase separation,
of aqueous solutions of nonionic surfactant occured at temperatures above the system-intrinsic
cloud point (CP) due to the dehydration of ethylene oxide units in the surfactant. Cloud point
depended on several factors including the length of ethylene oxide units and the concentration of
the surfactant; the longer are the ethylene oxide units, the higher are hydrophilicity of the
surfactant and the cloud point of its aqueous solution. Since the cloud point of PONPE20
solution was well above the boiling point of water, more hydrophobic surfactant, PONPE7.5 was
added to the solution to lower the cloud point.
80 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.2, No.2
TABLE V. Effect of PONPE 7.5 added and settling temperature of cloud point
PONPE 7.5 added (g) Temp. (K) V
(ml) R
E (-)
0.40 303 4.0 96 4.8
0.40 313 3.0 98 6.5
0.40 323 2.3 92 8.0
0.40 333 2.1 86 8.1
0.31 333 1.7 86 10
0.20 333 1.2 82 14
0.12 333 0.99 75 15
0.07 333 0.90 47 11
Results of gold extraction using the cloud point extractive approach are shown in Table
V, which tabulates the percent extraction into the coacervate phase, R
and the enrichment ratio,
E, of Au(III). The concentration of Au(III), PONPE20 and HCl in the feed solution (20 ml) was
adjusted to be 80 ppm, 0.2 wt% and 2.0 M, respectively, according to the standard composition
obtained after the foam separation as described in the previous section. With increasing amount
of PONPE7.5, cloud point decreased and the phase separation was attained at lower
temperatures. To attain the phase separation, over 0.40 g of the surfactant is needed at 303 K,
while 0.07 g is sufficient at 333 K. After the phase separation, adding 0.40 g PONPE7.5 and
followed by heating, the Au(III) was successfully extracted into the small coacervate phase,
giving the percent extraction of around 90 %. The volume of the coacervate phase, which
contains both PONPE20 and PONPE7.5, can be reduced by raising the temperature. The
enrichment ratio of Au(III) as high as 8.1 is attained at 333 K with the volume of 2.1 ml. As also
can be seen in Table V, the lesser the amount of PONPE7.5 that is added, the smaller will be the
volume of the coacervate phased formed at 333 K. Though a slight decline in the percent
extraction of Au(III) was observed with decreasing PONPE7.5, the enrichment ratio significantly
increased and the metal concentration in the coacervate phase reached over 15-fold of that in the
initial solution.
Taking the above results into consideration, the cloud point extraction was applied to the
foamate solution (82 ppm) after the foam separation. With 0.09 g of PONPE7.5 added at 333 K,
Au(III) was extracted into the coacervate phase with the enrichment ratio 14 and the percent
extraction 77%. The concentration in the coacervate phase was 1184 ppm. The total enrichment
of Au(III) from the feed solution (20ppm) was as high as 59 through the foam separation and
subsequent cloud point extraction. Experiments on the foam separation of Au(III) using the
nonionic surfactants in a continuous mode are underway in our laboratory.
Gold recovery from hydrochloric acid solution has been investigated via foam
separation in a batch mode using the non-ionic surfactant, polyoxyethylene nonyl phenyl ether
with 20 ethylene oxide units (PONPE20). On starting the bubbling, stable foams were formed
and rose up through the column to make the small foamate phase, in which Au(III) was
Vol.2, No. A study on gold(III) recovery via foam separation with nonionic surfactant in batch mode 81
successfully concentrated with the surfactant. The recovery percent of Au(III) increased with an
increase in the surfactant concentration, and decreased with an increase in the metal
concentration or the solution temperature. The enrichment of Au(III) was improved by extending
the drainage section of column, but deteriorated by increasing the concentrations of the
surfactant and metal or rising the temperature.
The recovery of Au(III) was found to be based on two distinct paths: one is transfer on
the foam surface and the other is in the interstitial water. The adsorption of Au(III) on the foam
surface was well expressed by the Freundlichs adsorption isotherm in terms of the surface
excess. Some heavy metals were not adsorbed onto the foam stabilized by PONPE20, and the
selective foam separation of Au(III) could be attained from multi-metals solution. Moreover,
cloud point extraction was successfully applied to the foamate solution by adding more
hydrophobic nonionic surfactant, PONPE7.5, and raising the solution temperature, resulting in
further enrichment of Au(III).
d diameter of foam [m]
E enrichment ratio [-]
F air flow rate [ml/min]
L volumetric flow rate of foamate [m
R percent recovery or extraction [%]
S surface generation rate of foam [m
V volume [m
Γ surface excess [mol/m
[ ] concentration [ppm] or [mol/m
fm foamate phase
ini initial
ret retentate phase
sur coacervate phase
M, m metal, i.e. Gold
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