Separation of zinc(II) from cobalt(II) solutions using supported liquid
membrane with DP-8R (di(2-ethylhexyl) phosphoric acid) as a carrier
Francisco José Alguacil and Manuel Alonso
Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8,
Ciudad Universitaria, 28040 Madrid, Spain. E-mail: fjalgua@cenim.csic.es
Abstract
Selective transport of zinc(II) from cobalt(II) sulphate solutions using a supported liquid
membrane impregnated with DP-8R extractant was studied. Several variables were
investigated: stirring speed of the feed phase, stirring speed and composition of the
receiving phase, diluent of the organic phase, metal concentration and pH of the feed
phase, extractant concentration and the lifetime of the membrane. Conditions for the
separation of both metals were established: i) aqueous feed: pH 3.0±0.02, [Co]/[Zn] molar
concentration ratio near 22; ii) receiving phase: 0.5 M sulphuric acid; iii) membrane phase:
DP-8R 10% v/v ib Exxsol D100.
Keywords: DP-8R, membrane transport, separation, zinc(II), cobalt(II)
1.INTRODUCTION
It is described in the literature [1] that the recovery of cobalt from various leach solutions
needs a series of separation/purification steps in order to maintain a lean cobalt solution
from which the metal can be finally obtained by an electrolytic procedure. Zinc appeared
in the solution, and it is removed by sulphide precipitation but this also leads to significant
cobalt losses. Solvent extraction had been considered as an alternative for the purification
of the cobalt solution.
Besides solvent extraction, supported liquid membrane technology offers the advantage
that the extraction, stripping and regeneration operations are combined in a single
stage[2,3]. The present works investigates the possibilities of flat-sheet supported liquid
membrane technology for the separation of zinc(II) from cobalt(II) sulphate solutions
using DP-8R in Exxsol D100 as mobile carrier phase. It should be pointed out that flat
membrane geometry is very useful for first laboratory results, but for industrial purposes
this geometry is not effective since the ratio of surface area to volume is too low; hollow
fiber and spiral wound configurations are then used to provide high surface area to volume
ratio.
The literature has reported the use of flat-sheet supported liquid membrane configuration
in the transport of zinc(II) [4-11], and also of cobalt(II) [12-23], and since relatively few
studies concerning the separation of two or more competitive solutes are encountered in
2
the literature, more work is required in this field in order to approach more practical
separation problems [24], whereas the transport of a metal through a supported liquid
membrane containing an acidic extractant (such as DP-8R) as a mobile carrier is
described, in the literature, by a number of steps [2, 25-27]
In the present investigation, a flat membrane configuration has been implemented to
obtain data on the transport and selective Zn-Co separation using DP-8R
(organophosphoric acid) as a carrier. Various experimental conditions which influence the
metal transport were studied, and conditions for the zinc-cobalt separation were also
established.
2.EXPERIMENTAL
2.1.Materials
DP-8R extractant, which is the active substance is di(2-ethylhexyl) phosphoric acid, was
supplied by Daiachi (Japan) and was used as received. Its purity was checked to be >97%
by potentiometric titration with sodium hydroxide in ethanol media [4].
Exxsol D100 (ExxonMobil Chem. Iberia, Spain) was employed as diluent for the organic
phase; other diluents used were: Solvesso 100 and Escaid 100 (ExxonMobil Chem.
Iberia), Iberfluid (CS, Spain), and Toluene (Fluka). Stock metal solutions were prepared
by dissolving the required amount of ZnSO41H2O and CoSO47H2O (AR grade, Fluka) in
distilled water, and metal concentrations were analysed by standard atomic absorption
spectrometry (AAS) using a Perkin Elmer 1100B spectrophotometer. All other chemicals
used in the present study were of AR grade. In all the experimental work, unless stated
otherwise, a [Co(II)]/[Zn(II)] molar concentration ratio of 22 was maintained in the
aqueous feed; this represents a typical leach solution [1].
2.2.Procedure
2.2.1.Liquid-liquid extraction experiments
Metal extraction experiments were carried out by mechanical shaking (800 min-1) in
separatory funnels of the appropriate organic and aqueous solutions. Experiments were
performed at 20C using and organic:aqueous (O:A) phase ratio of 1, whereas previous
test indicated that equilibrium was reached within 10 min of contact. The metal
distribution coefficients were calculated according to:
D=
[M ]org
[M ]aq
(1)
3
where [M]org and [M]aq represents the total metal analytical concentrations in the
equilibrated organic and aqueous phases, respectively. Thus, the zinc/cobalt separation
factor was calculated as:
Zn/Co =
DZn
DCo
(2)
2.2.2.Liquid membrane experiments
The organic membrane phase was prepared by dissolving the corresponding volume of
DP-8R in the organic diluent to obtain carrier solutions of different concentrations. The
polymeric support was impregnated with the carrier solution by immersion for 24 h
(previous experiments had shown that longer immersion times do not enhance metal
transport, i.e. zinc permeation coefficients of 4.5x10-3 4.7x10-3 and 4.4x10-3 cm s-1 for
24h, 36h and 48h, respectively), then leaving them to drip for ten seconds before being
placed in the flat-sheet supported liquid membrane cell. The support used was Millipore
Durapore GVHP 4700, which characteristics are given elsewhere [28].
Batch liquid membrane measurements were carried out with a two-compartment
permeation cell (Figure 1) which consisted of a feed phase (200 cm3) separated from a
receiving phase chamber (200 cm3) by a liquid membrane having an effective membrane
area of 11.3 cm2. The feed and receiving phases were stirred mechanically at 1500 and
1000 min-1, respectively (unless stated otherwise) at 20C to avoid concentration
polarisation conditions at the membrane interfaces and in the bulk of the solutions.
Agitation was performed in both compartments by using cylindrical Teflon impellers
having a diameter of 2.4 cm. Membrane permeabilities were determined by monitoring
metal concentrations by standard atomic absorption spectrometry (AAS) in the source
phase as a function of time. After appropriate dilution of the samples with distilled water,
the conditions (concentrations range, wavelength, etc.) for the metal analysis were those
established for the spectrophotometer (Perkin Elmer 1100B). The metal concentration in
the various phases was found to be reproducible within 98% accuracy. The permeation
coefficient (P, within ±3% error) was calculated by equation (3) [25,26]:
ln
A
Ct
=- Pt
V
C0
(3)
where Ct and C0 are concentrations of metal ions in the source phase at a given time and
time zero, respectively, A is the effective membrane area, V is the volume of the feed
phase solution and t the elapsed time. The zinc/cobalt separation factor were defined as the
4
ratio of permeabilities [29].
3.RESULTS AND DISCUSSION
3.1.Liquid-liquid system
The extraction/stripping mechanism for both metals can be represented by the general
equilibrium:
n

M aq
 n (HR ) 2org  M(R·HR ) n org  nH ac
(4)
where (HR)2 refers to the dimmer form of the extractant and aq and org to the species in
the aqueous and organic phases, respectively. It is evident that metal extraction occurs
when the equilibrium is shifted to the right, whereas in excess of acid in the aqueous
phase, metal stripping occurs and the equilibrium is shifted to the left.
Previous liquid-liquid extraction experiments were performed in order to investigate the
extraction of zinc and cobalt using DP-8R dissolved in Exxsol D100. Results obtained are
shown in Table 1. The data indicates that extraction at pH near 3.0 gives the best
zinc/cobalt separation factor value for the present experimental conditions; thus, this pH
value was selected for the membrane permeation studies since transport at pH 3.0 will be a
good compromise to obtain a significant zinc removal from the feed solution and keeping
the cobalt co-transport as low as possible.
3.2.Liquid membrane system
3.2.1.Influence of the stirring speed in the feed phase
Previous experiments were carried out to establish adequate hydrodynamic conditions.
The permeability of the membrane was studied as a function of the stirring speed on the
feed solution side. The agitation of the receiving phase was kept constant at 1000 min-1.
Feed and receiving conditions being maintained as: 7.7x10-5 M Zn(II), 1.7x10-3 M Co(II) at
pH 3.0±0.02 and 0.5 M H2SO4, respectively. The DP-8R concentration was 20 % v/v in
Exxsol D100 immobilised on the Durapore microporous support.
Constant zinc permeability (3.510-3 cm s-1) for stirring speeds higher than 1300 min-1 was
obtained. Consequently, the thickness of the aqueous diffusion layer and the aqueous
resistance to mass transfer were minimised and the diffusion contribution of the aqueous
species to the mass transfer process is assumed to be constant [29,30].
5
3.2.2.Influence of the stirring speed and composition of the receiving phase
The influence of the stirring speed of the receiving phase on zinc transport was studied
using the same aqueous and organic phases as described in Section 3.2.1., and using a
stirring speed in the feed phase of 1500 min-1. Results obtained shown that the variation of
the stirring speed in the 1000-1500 min-1 range does not influence zinc transport.
Furthermore, experiments carried out using sulphuric acid solutions in the 0.15-0.5 M
range as receiving phases also showed no influence on zinc transport under the present
experimental conditions (aqueous feed and organic phases as described above, stirring
speed of the feed and receiving phases 1500 and 1000 min-1, respectively).
3.2.3. Organic phase diluent influence
The characteristics of the organic phase diluent, chosen as a water-resistant barrier in
liquid membrane processes, influence the membrane performance and thus the metal
transport [31-34]. In the present work, the use of various diluents for the DP-8R/zinccobalt sulphate system was investigated for permeation studies carried out with feed
solutions of 7.7x10-5 M Zn(II), 1.7x10-3 M Co(II) at pH 3.0±0.02 and organic solutions of
10 % v/v DP-8R.
The results obtained are shown in Figure 2. For the present system it can be seen that the
change of the diluent influences zinc transport, having Exxsol D100 and Solvesso 100 as
the diluents giving the best zinc permeabilities values. On the other hand, Table 2 shows
the corresponding cobalt concentrations in the receiving phase after 3 hours. It seems that
cobalt transport is enhanced using aromatic diluents. From the experimental data, overall
best zinc/cobalt separation is, thus, reached using Exxsol D100 as diluent for the
membrane phase.
3.2.4.Influence of zinc concentration
The influence of the total zinc concentration in the transport of this metal by DP-8R was
investigated. This study was carried out using feed phases which contained various zinc
concentrations at constant cobalt concentration and a membrane phase of 10 % v/v DP-8R
in Exxsol D100. The results are shown in Figure 3 in which the initial flux, J, is plotted
against the total zinc concentration in the feed phase. The initial flux was calculated
according with [35]:
J = P [M ]total
(5)
where, P is the permeation coefficient and [M]total is the totall metal concentration in the
feed phase. It was observed that at low zinc concentrations, the initial flux is a function of
6
the total metal concentration in the feed phase, but, at higher metal concentrations, J value
is less dependent of the zinc concentration. This may be attributed to two reason:
membrane saturation and lower effective membrane area and maximization due to
saturation of the membrane pores with metal-carrier species and in addition, the build-up
of a carrier layer on the membrane interface which assists the retention of the separating
constituent on the entry side and leads to a nearly constant flux [36]. In the case of cobalt
transport, the metal content in the receiving phase after 3 hours seems to increase slightly
with the decrease of the cobalt/zinc molar concentrations ratio in the feed phase (Table 3);
however, good zinc/cobalt separation is generally achieved using DP-8R as carrier.
3.2.5. Influence of the pH
In order to asses the role of the pH of the feed phase solution during the permeation of zinc
(and cobalt), pH variation studies in the range 2.0-5.0 were carried out using a feed
solution of 7.7x10-5 M Zn(II) and 1.7x10-3 M Co(II), a receiving phase of 0.5 M sulphuric
acid and a membrane phase of 20 % v/v DP-8R in Exxsol D100. As seen from Figure 4,
the transport of zinc reaches a maximum at a pH value of 3.0. On the other hand, the
transport of cobalt is increased as the pH value of the feed phase increases from 2.0
through 5.0 (Table 4), thus, decreasing the separation of both metals. Accordingly
transport at pH 3.0±0.02 achieved the best compromise for the separation of both metals,
this result is in accordance with previously obtained solvent extraction results.
3.2.6.Influence of extractant concentration
The results concerning transport of zinc(II) from the feed phase containing 7.7x0-5 M
Zn(II), 1.7x0-3 M Co(II) at pH 3.0±0.02 and the receiving phase 0.5M H2SO4 and varying
concentration of the carrier in the range 5-30 % v/v dissolved in Exxsol D100 are shown
in Table 5. As it can be expected, the permeability value increased with initial DP-8R
concentration, this may be due to an increase in extractability into the liquid membrane. At
higher carrier concentrations the decrease of permeability can be explained in terms of the
increase in solution viscosity that increases membrane resistance.
At near 10 % v/v DP-8R, a limiting permeability value is obtained, this being attributed to
a permeation process controlled by the diffusion in the stagnant film of the aqueous feed
phase [37,38], thus, Plim= 1/Δaq= 4.510-3 cm s-1 and assuming a value of the average
aqueous diffusion coefficient of the zinc-containing species Daq= 10-5 cm2 s-1 [28,39], then
the thickness of the aqueous boundary layer (daq) is calculated to be 2.2x10-3 cm. The
transport of cobalt is increased with the increase in the carrier concentration of the
membrane phase (cobalt concentrations in the receiving phase after 3 h were 2.3x10-5,
7
4.8x10-5, 1.2x10-4 and 2.2x10-4 M for 5, 10, 20 and 30 % v/v DP-8R, respectively). This
can also account for the decrease in zinc permeability from a certain extractant
concentration range.
3.2.7.Lifetime of the supported liquid membrane
The lifetime of the membrane was evaluated by using the same support in several runs of
3 h duration. Table 6 summarizes the results obtained where percentage extraction of
Zn(II), E(%), was defined as:
E(%) =
[Zn ]0 - [Zn ]t
 100
[Zn ]0
(6)
at an elapsed time of 3 h. After three consecutive experiments, the zinc extraction only
suffers a slight variation, but the membrane efficiency fell after overnight storage, in the
same experimental set-up, in contact with the air. Regeneration of the membrane is
achieved by re-impregnating the support.
4.CONCLUSIONS
On the basis of the flat-sheet supported liquid membrane studies performed, DP-8R shows
a good efficiency for zinc transport and thus its separation from cobalt(II) sulphate
solutions; however, the efficiency of the separation process can be improved by using
more effective membrane unit, i.e. hollow fiber modules. Conditions for best zinc/cobalt
separation are resummed as: i) aqueous feed: pH 3.0±0.02, [Co]/[Zn] molar concentration
ratio near 22; ii) receiving phase: 0.5 M sulphuric acid; iii) organic membrane phase: DP8R 10 % v/v in Exxsol D100. The membrane lifetime was evaluated and stable zinc
transport was observed after continuous use of the same membrane. Once the zinc
transport was suppressed, by re-impregnation of the support, zinc transport ability was
recovered. Under the present experimental conditions, zinc mass transfer coefficient in the
aqueous phase was found to be 4.5x10-3 cm s-1.
ACKNOWLEDGEMENTS
To Mr. Bascones and Mr. López for technical assistance and to the CSIC (Spain) for
support.
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10
Table 1. Values of the zinc/cobalt separation factors (β) at various pH
pHeq
βZn/Co
pHeq
βZn/Co
0.80
1.52
1.67
2.01
2.19
2.55
4.7
7.0
9.6
38.3
53.3
154
2.94
3.03
3.29
3.50
4.00
2800
5700
5000
3700
2000
Organic phase: 20 % v/v DP-8R in Exssol D100.
Aqueous phase: 7.7x10-3 M Zn(II) and 1.7x10-1 M Co(II).
Equilibration time: 15 min.
11
Table 2. Cobalt contents in the receiving phase
Diluent
Density
(kg m-3)
Aromatics
(%)
Viscosity
(cP)
Co
(M)
Solvesso100
Toluene
Escaid 100
Exxsol D100
Iberfluid
877
866
805
824
782
>99
>99
24
0.2
2
0.8
0.6
1.6
2.6
2.3
3.9x10-4
3.1x10-4
8.1x10-5
4.8x10-5
2.7x10-5
12
Table 3. Cobalt concentration in the receiving phase at various [Co]/[Zn] ratios after 3
hours
a
[Co]/[Zn]a
Co (M)
44.5
22.2
11.1
5.6
2.8
4.3x10-5
4.8x10-5
5.3x10-5
5.7x10-5
6.3x10-5
1.9
1.4
1.1
7.1x10-5
7.9x10-5
8.9x10-5
The various ratios were established by variations of zinc concentrations at constant cobalt concentration (see
Figure 3). Other experimental conditions as in Figure 3.
13
Table 4. Cobalt contents in the receiving phase at various pH of the feed phase after 3
hours
pH
Co (M)
2.0
3.0
4.0
5.0
5.1x10-6
1.2x10-4
4.1x10-4
4.2x10-4
14
Table 5. Zinc permeation at various DP-8R concentrations
[DP-8R] (% v/v)
PZnx103 (cm s-1)
5
10
20
30
2.8
4.5
3.5
2.8
15
Table 6. Lifetime of the membrane
Run
Membrane usagea (h)
% EZn
1
2
3
4
5
0
3
6
9
Re-impregantion
93.2
92.6
92.7
56.2
93.0
Feed phase: 7.7x10-5 M Zn(II), 1.7x10-3 M Co(II) at pH 3.0±0.02.
Membrane phase: 10 % v/v DP-8R in Exxsol D100.
Receiving phase: 0.5 M H2SO4.
a
Before the run.
16
Figure 1. Permeation cell. F: feed phase, M: membrane phase, R: receiving phase. I:
impellers.
Figure 2. Influence of the diluent of the membrane phase on zinc transport. Receiving
phase: 0.5 M H2SO4. The chemical nature of the diluent is given in Table 2.
Figure 3. The influence of total concentration of Zn(II) on initial permeability flux (J).
Feed phase: Zn(II) and 1.7x10-3 M Co(II) at pH 3.0±0.02. Receiving phase: 0.5 M H2SO4.
Figure 4. Influence of initial pH on permeability of Zn(II) as a function of ln [Zn]t/[Zn]0.