Journal of Minerals & Materials Characterization & Engineering, Vol. 3, No.1, pp 53-63, 2004
jmmce.org Printed in the USA. All rights reserved
Continuous Recovery of Gold(III) via Foam Separation with Nonionic
Surfactant
T. Kinoshita
a,1
, S. Akita
a
, S. Ozawa
b
, S. Nii
b
, F. Kawaizumi
b
,
K. Takahashi
b
a
Nagoya Municipal Industrial Research Institute, 3-4-41 Rokuban, Atsuta-ku, Nagoya 456-
0058, Japan
address
1
Fax: [phone].
e-mail:
b
Department of Chemical Engineering, Nagoya University, Chikusa-ku,
Nagoya 464-8603, Japan
A continuous recovery of Au(III) via foam separation from its hydrochloric acid
solutions was studied 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 successfully played a dual role for foam-
production and metal-collection in the system. Experimental parameters examined
were the feed flow rate, the foam height and the depth of bulk liquid phase. Under
a standard condition, the percent recovery and the enrichment ratio of Au(III)
were 54 % and 4.5, respectively. Using the adsorption isotherm of Au(III)
obtained in a batch operation, these two values could be estimated against the feed
flow rate which are in good agreement with the experimental ones. It was also
found that about 90 % of Au(III) recovered in the foamate phase was transferred
onto the foam surface and the rest was conveyed to the interstitial water between
the foam. Highly selective recovery of Au(III) was achieved from multi-metals
solution.
Key Words: foam separation, nonionic surfactant, gold(III), continuous operation,
surface excess, Freundlich’s adsorption isotherm
Introduction
Foam fractionation plays an important role in diverse fields both for recovering valuable
solutes and for rejecting impurities and pollutants in dilute solutions. The advantage of this
separation technique is its low cost in energy and operation as well as small installation space. To
date various separations have been carried out; hazardous metal ions [1,2], proteins [3,4],
mineral ore [1,2,5] and surfactants [6]. In foam fractionation processes, the main roles are played
by a foam producing agent, i.e. surfactant and a collecting agent. The former enables stable
formation of foam by bubbling aqueous solutions, while the latter collects the target solutes
adsorbed on the surface of foam. During a rise of foam through a drainage section of the column,
the interstitial water between foam is drained out by gravity, and solutes are concentrated in the
foamate phase along with surfactant. There are a number of reports concerning the combination
of such collecting agents and target solutes under various conditions [7-10].
A series of polyoxyethylene nonyl phenyl ethers (PONPEs) have a strong interaction
with Au(III) [11,12]. This reagent would be an excellent collector and also performs as a
surfactant. Using a multifunctional reagent would simplify the fractionation process. In batch

54 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.3, No. 1
operation, the percent recovery of Au(III) increased with an increase in the surfactant
concentration and the air flow rate, while the enrichment ratio improved by decreasing air flow
rate and increasing the length of drainage section of the column. Based on the difference in
affinity of the surfactant to several metals, selective foam separation of Au(III) in a multi-metals
solutions could be attained. Moreover, the adsorption of Au(III) on the foam surface was well
predicted by Freundlich’s adsorption isotherm (Γ x 10
7
= 10.3 [M]
ret
0.39
) under the experimental
conditions studied.
In this study, a continuous mode operation was carried out to separate Au(III) from
hydrochloric acid solutions using PONPE. Several experimental parameters such as the feed low
rate, the foam height and the depth of bulk liquid phase were varied to examine their effects on
the recovery and enrichment of Au(III). Furthermore, using the adsorption isotherm obtained in
the batch mode [12], the process efficiency in continuous operation has been estimated and
compared with experimental results.
Materials
Nonionic surfactant, polyoxyethylene nonyl phenyl ether (PONPE20), with average
ethylene oxide units of 20 (HO(CH
2
CH
2
O)
20
C
6
H
4
C
9
H
19
) was obtained from Tokyo Kasei Kogyo
Co., Ltd. and used without further purification. Aqueous feed solutions were prepared by
dissolving prescribed amounts of metal chlorides and the surfactant in hydrochloric acid
solutions. All the chemicals used were of reagent grades.
Procedure for Continuous Foam Separation
A schematic diagram of continuous foam separation apparatus is shown in Figure 1. The
bubble column is composed of a cylindrical glass tube (500 mm in height and 30 mm in inner
diameter) with a sintered glass filter (G3) mounted at the bottom of the column as a gas
distributor. Air was introduced at 40 cm
3
/ min through the distributor. A feed solution was
brought into the column through an inlet port and an effluent was discharged from the bottom of
the column. A pump was used to drain the effluent to keep the liquid depth of 100 mm from the
glass filter, which corresponds to the volume of 71 cm
3
, unless otherwise stated. The length of
the drainage section of the column was 400 mm, except when measurements were conducted on
its effect. Foam was collected through a froth collection device equipped at the top of the
column.
Initial concentrations of Au(III), the surfactant and hydrochloric acid in feed solutions
were set at 20 ppm, 0.05 wt% and 2.0 M, respectively. Aliquots of the foamate and effluent
solutions were withdrawn at intervals of 10 minutes and analyzed for metal concentrations by
inductively coupled plasma spectroscopy (ICP) after appropriate dilution. All the experiments
were carried out at 298
o
K.

Vol.3, No.1 Continuous Recovery of Gold(III) via Foam Separation with Nonionic Surfactant 55
Results and discussion
Continuous foam separation of Au(III) from hydrochloric acid media
The recovery percent (R) and the enrichment ratio (E) are defined by the following
equations, and used for evaluating the separation efficiency, where F and L are the volumetric
flow rates of the feed and foamate solutions, respectively. M represents the metal and the
subscripts, fm and ini, refer to the foamate and feed phases, respectively.
R
fm
= 100 (L / F) (1)
R
M
= 100 (L [M]
fm
/ F [M]
ini
) (2)
E = [M]
fm
/ [M]
ini
= R
M
/ R
fm
(3)
A time course of the recovery percent and the enrichment ratio of Au(III) is shown in
Figure 2. The feed concentrations of the metal, surfactant and hydrochloric acid were 20 ppm,
0.05 wt% and 2.0 M respectively. Within retentate solution in the column, the feed liquid and air
bubble contact counter-currently. The feed solution and air were supplied at 2.5 cm
3
/min and 40
cm
3
/min respectively. During the run, the discharging rate of the effluent from the column was
adjusted to keep a constant height of the retentate solution. After starting to supply air, the foam
was produced and the froth grew to fill the column. At the outlet of the column, the foam

56 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.3, No. 1
ruptured spontaneously and the liquid was recovered. Due to the high affinity of Au(III) to the
surfactant [11-15], the metal was accumulated in the foamate phase; this could be ascertained by
color change to yellow due to Au(III) enrichment in the phase.
The percent recovery of Au(III) decreases gradually from 73 % with time. Since the
foamate recovery shows a similar decreasing trend, their ratio, i.e. the enrichment ratio, gives a
nearly constant value of 4.5. Stable operation is attained after 120 min, where the recovery of
Au(III) converges to ca. 55 %, which is lower than that (86 %) obtained in the batch mode under
the same experimental condition [12]. In foam separation, solutes are recovered as the
components in foamate phase by being transferred either on the foam surface or in the interstitial
water between foams. In continuous mode, the foamate recovery (13%) is much smaller than the
batch mode (22%). Therefore, the foamate recovery has a large effect on the metal recovery
under a similar enrichment ratio. This is the main reason for the difference of metal recovery in
batch modes.
Figure 3 shows the effect of the feed flow rate on the percent recovery and the
enrichment ratio of Au(III) under steady state condition. As the flow rate increases from 1.5 to
4.5 cm
3
/min, the foamate recovery declines from 13 to 3.8 %. It is thought that increasing the
feed flow leads to decrease the rising velocity of bubbles. The foamate generation rate will be
decreased and this causes a decrease in the foamate recovery. The significant decrease of metal
recovery from 74 to 28 % with increasing feed flow rate is due to shortening of the residence
time of the retentate solution in the column. On the other hand, the enrichment ratio of Au(III)

Vol.3, No.1 Continuous Recovery of Gold(III) via Foam Separation with Nonionic Surfactant 57
improves by increasing the flow rate, and reaches as high as 7.3 at 4.5 cm
3
/min. This can be
ascribed to the facilitated drainage of interstitial water by reducing the upward flow rate of foam.
It should be noted, that there can be a trade-off relationship between the recovery of Au(III) and
the enrichment ratio at a higher flow rate.
In Table 1 are summarized the effects of both height of the retentate solution and length
of the drainage section on the separation efficiency. The latter height was altered by inserting
extending tubes on the column top. The feed flow rate was set at 2.5 cm
3
/min. At a fixed height
of solution (10 cm), extension of the drainage section leads to a significant decrease in the
foamate recovery due to facilitated drainage. As for the metal recovery, a constant value of ca. 55
% was obtained from 40 to 70 cm of drainage section. The enrichment ratio of Au(III) improves
and reaches as high as 8.9 at the length of 70 cm. Subsequently, the height of the retentate
solution was changed for the constant drainage length (40 cm). By increasing the solution
volume, the percent recovery of Au(III) increases up to 64 %. While the foamate recovery
decreases from 12 to 18 %. These trends cause slight deterioration of the metal enrichment from
4.6 to 3.5. In the following experiments, the length of drainage section and the height of solution
were fixed at 10 cm and 40 cm, respectively.

58 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.3, No. 1
Table 1. Effect of length of drainage section and depth of retentate
solution.
Depth of retentate solution (cm) 10 10 10 10 20 30
Length of drainage section (cm) 40 50 60 70 40 40
ReM (%) 54 54 54 56 60 64
Refm (%) 12 8 7 6 14 18
En (-) 4.6 6.6 8.0 8.9 4.2 3.5
Experiments on the continuous recovery of Au(III) from a multi-metals solution of
Co(II), Cu(II), Ni(II), Pd(II), Pt(IV) and Zn(II) have been carried out. Table 2 shows the recovery
percent for each metal in a steady state operation. Initial concentrations of each metal and
hydrochloric acid were 20 ppm and 2.0 M, respectively. Sixty-four % of Au(III) was recovered
from the feed solution, and the enrichment ratio is as high as 4.1. The recovery of the foamate
solution was 16 %, which corresponds to other metals recovery. This remarkable difference in
behavior of Au(III) and the other metals is ascribed to their difference in affinity to the
surfactant. Metals except for Au(III) are not adsorbed onto the foam surface but transferred
within the interstitial water into the foamate phase. A similar tendency was also observed in the
batch foam separation [12], where the percent recoveries of Au(III) and the other metals were
found to be 86 and 22 %, respectively.
Table 2. Result for continuous foam separation from multi-metals
solution.
Element Au Co Cu Ni Pd Pt Zn
Percent Recovery (%) 64 17 16 16 16 17 17
Enrichment ratio (-) 4.1 1.1 1.0 1.1 1.0 1.1 1.1
Mass balance for continuous foam separation of Au(III)
In foam separation, transfer of a dissolved solute into the foamate phase follows two
distinct paths: A) on the surface of foam stabilized by a surfactant, B) the interstitial water
between foam. The degree of adsorption of a solute on the foam surface is generally expressed
by the surface excess, Γ, which is determined from the mass balance on the foamate phase
[3,9,12]:
L [M]
fm
= S Γ + L [M]
ret
(4)
where S is the surface generation rate and the subscript, ret, denotes the retentate phase. The first
term in the right hand side of Equation (4) corresponds to the above mentioned path A, and the
second one to the path B. In the present continuous operation, a mass balance for the metal is
given by the following relation:
F [M]
ini
= L [M]
fm
+ G [M]
ret
(5)

Vol.3, No.1 Continuous Recovery of Gold(III) via Foam Separation with Nonionic Surfactant 59
where G denotes the volumetric flow rate of the effluent solution, i.e. drainage rate. At steady
state, the metal concentration in the retentate solution, [M]
ret
, is constant. Equation (5) can be
rewritten using Equation (4) and the relationship, F=L+G, as follows:
F [M]
ini
= S Γ + (L+G) [M]
ret
= S Γ + F [M]
ret
(5)’
In our previous study [12], following Freundlich’s adsorption isotherm (6) was found to
express the results of batch foam separation of Au(III) using PONPE20 through the plot of the
surface excess against the mean metal concentration in the retentate solution.
Γ x 10
7
= 10.3 [M]
ret
0.39
(6)
On the other hand, the relationship between the volumetric flow rates of the feed (F) and foamate
(L) solutions is given in Figure 4. The following equation (7) is obtained from the present data.
L = -0.383 + 0.614F – 0.171F
2
+ 0.014F
3
(7)
By combining Equations (5)’, (6) and (7), the modified forms of Equations (2) and (3)
are obtained as a function of the feed flow rate.
R
M
= 100 (L [M]
ret
+Γ S) / (F [M]
ini
) (2)’
E = ( (L [M]
ret
+Γ S) / L) / [M]
ini
(3)’

60 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.3, No. 1
In Figure 3 calculated results are given by dotted lines, where the surface generation rate
(S) is calculated as 6W/d, where the air flow rate (W) and an average diameter of foam (d), are
presumed to be the same as the value (0.7 mm) obtained in the batch mode [12]. The calculation
agrees well with the experimental data for both the percent recovery and the enrichment ratio of
Au(III). Thus, the process performance in the continuous foam separation can be predicted by the
adsorption isotherm and the relationship between the flow rates of feed and foamate solutions.
Since Equation (4) was found to be applicable to the present system, we define the
entrainment ratio (Et) as the ratio of the amount of metal transferred on the foam surface to that
recovered in the foamate phase, Γ S / (L [M]
ret
+Γ S). Figure 5 shows the values of Et as a
function of the feed flow rate. Irrespective of the flow rate, the entrainment ratio gives a nearly
constant value of 0.90, indicating the recovered metal is mainly carried on the surface of foam
due to the strong interaction between Au(III) and PONPE20. This finding implies that the metal
enrichment is much affected by dilution of the foamate phase by the interstitial water between
the rising foams rather than the recovered amount of the metal. Thus, a higher enrichment ratio is
attained at a higher feed flow rate, where the recovery of foamate is suppressed, as shown in
Figure 3.

Vol.3, No.1 Continuous Recovery of Gold(III) via Foam Separation with Nonionic Surfactant 61
In Figure 5 are also depicted the calculated curves of the surface excess, Γ, and the metal
concentration in the retentate solution. As can be expected, the retentate concentration increased
with an increase in the feed flow rate. For instance, [M]
ret
rises as high as 15 ppm at F=5.0
ml/min, and most of the metal remained intact and was discharged from the column. Compared
with the retentate concentration, the surface excess increased more gradually, approaching a
constant value of 3.8 mol/m
2
. This implies that the interaction between Au(III) and PONPE20
was so effective that the surface of foam was well occupied by the metal, i.e. saturated, even at a
low feed rate. Use of a glass filter having smaller pore size may improve the process efficiency,
since it leads to the increased surface generation rate, S. With an intention to improve the
enrichment of target solutes one step further in continuous foam separation, a study on the reflux
of foamate phase into the retentate solution is now underway.
Conclusions
Gold(III) recovery from its hydrochloric acid solutions has been investigated. The
method employed is foam separation in a continuous operation using a non-ionic surfactant,
polyoxyethylene nonyl phenyl ether with 20 ethylene oxide units (PONPE20). Experimental
parameters including air flow rate and concentrations of metal, surfactant and HCl were the same
as those determined to be optimum condition in the batch foam separation [12]. After attaining a
stable state, Au(III) was continuously recovered from the foamate formed by the feed solution.
With an increase in the feed flow rate, the recovery percent of Au(III) decreases proportionally,
while the enrichment ratio shows a concave profile with a minimum value at 2.5 ml/min when

62 T. Kinoshita, S. Akita, S. Ozawa, S. Nii, F. Kawaizumi, and K. Takahashi Vol.3, No. 1
plotted against the feed flow rate. The enrichment ratio also improved by extending the drainage
section of the column. The selective recovery of Au(III) from a multi-metals solution was
successfully attained via continuous foam separation. Using Freundlich’s adsorption isotherm
obtained in the batch operation [12], the process efficiency was estimated as a function of feed
flow rate. A satisfactory agreement was observed between the experimental data and the
calculation. Based on the calculation, the entrainment ratio and the surface excess of Au(III)
were estimated, and their effect on the efficiency was discussed.
Nomenclatures
d diameter of foam [m]
E enrichment ratio [-]
Et entrainment ratio [-]
F volumetric flow rate of feed solution [m
3
/min]
G volumetric flow rate of effluent solution [m
3
/min]
L volumetric flow rate of foamate solution [m
3
/min]
R percent recovery [%]
S surface generation rate of foam [m
2
/min]
V volume [m
3
]
W air flow rate [ml/min]
Γ surface excess [mol/m
2
]
[ ] concentration [ppm] or [mol/m
3
]
subscripts
fm foamate phase
ini initial
ret retentate phase
M metal
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