HYDROMETALLURGY
KGP002
SOLVENT EXTRACTION OF Cu2+ USING LIX-984
Theory
Solvent extraction is used for separation and concentration of metal ions in aqueous
solutions. The method is based on that an extraction agent, diluted with an organic
solvent, is mixed by stirring or shaking with an aqueous solution containing the
desired metal. The extraction agent and the organic solvent are both insoluble in
water. By mixing the aqueous and organic phase, the metal ion binds to the
extraction agent and is transferred to the organic phase. Thereafter, the organic and
aqueous phases are allowed to disengage and then the two phases are separated.
The desired metal ion is then transferred (stripped) into a new aqueous phase in a
refined and concentrated form.
Solvent extraction processes are used for solution purification and for extraction of
a large number of different metals within the hydrometallurgical industry. The
following reactions are valid for the extraction and stripping of Cu2+ in sulphate
media with LIX-984 as the extraction agent:
Extraction: 2RH(o) + Cu2+(aq)  R2Cu(o) + 2H+(aq)
Stripping: R2Cu(o) + 2H+(aq)  2RH(o) + Cu2+(aq)
These reactions show that extraction with LIX-984 is pH dependent. High pH
favours extraction while low pH favours stripping. LIX reagents are used
industrially for copper extraction, due to their ability to selectively extract Cu2+
from Fe3+, which is commonly encountered in leaching solutions. The
corresponding reactions for Fe3+ is:
Extraction: 3RH(o) + Fe3+(aq)  R3Fe(o) + 3H+(aq)
Stripping: R3Fe(o) + 3H+(aq)  3RH(o) + Fe3+(aq)
From the reactions above it is apparent that the pH dependence for extraction and
stripping is greater for Fe3+ than for Cu2+, which also is evident from figure 1. By
using a pH between 1.5 and 2.0 Cu2+ can be extracted selectively.
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Figure 1.
pH isotherms for one of the commercial LIX-reagents
McCabe - Thiele chart
The number of steps needed in a mixer-settler to reach desired separation, can be
determined by constructing a McCabe-Thiele chart. The chart is constructed (see
figure 2) by determining the equilibrium isotherm for extraction or stripping via
experiments. The equilibrium isotherm can be obtained by mixing an aqueous
solution and organic solution at different phase ratios (aq/o) until equilibrium is
reached. The time to reach equilibrium is 5 minutes when using LIX-984. After
phase separation the aqueous phase is analysed with respect to copper and the
copper content in the organic phase is calculated. The equilibrium isotherm for
extraction is obtained by plotting the concentration of Cu2+(o) against the
concentration of Cu2+(aq) in a chart.
The equilibrium isotherm obtained is specific for this particular system. Changes in
temperature, pH, and concentrations of LIX-984 or Cu2+ will lead to another
equilibrium curve.
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Figure 2.
McCabe-Thiele chart for extraction of Cu2+ with a LIX-reagent.
(S.O. and L.O. stands for stripped- and loaded organic phase,
respectively)
After the equilibrium isotherm has been plotted into the chart a working line is
drawn, the gradient of this should be equal to the aq/o phase relation in the intended
extraction process. The working line should go from a point on the equilibrium
isotherm corresponding to the copper content in the initial organic phase (S.O. in
figure 2) to the value on the x-axis corresponding to the initial copper concentration
in the aqueous phase (feed in figure 2). Then a horizontal line is drawn from the
point where the working line intersects with the feed line to the equilibrium curve.
A vertical line is then drawn from the equilibrium curve down to the working line
from which point a horizontal line can be drawn again to the equilibrium curve, and
so on. This procedure is repeated until a satisfactory degree of extraction from the
aqueous phase has been obtained. Each horizontal line in the McCabe-Thiele chart
corresponds to one step in the mixer-settler. A McCabe-Thiele chart for stripping is
constructed in a similar way, see figure 3.
3
Figure 3.
McCabe-Thiele chart for stripping of a LIX-reagent.
(S.E. and P.E. is the stripped- and pregnant electrolyte, respectively)
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Exercise I
Task:
Determine the equilibrium isotherm for Cu2+ extraction in sulphate
media using the extraction agent LIX-984. Construct a McCabeThiele chart where the initial Cu2+(aq) = 2,5 g/l and the initial Cu2+(o)
= 0 g/l.
Choose a suitable gradient (aq/o=1-2) of the working line in order to
reach a reasonable concentration in the raffinate. Verify that iron is
not extracted under these conditions. Determine the copper
concentrations in the raffinate, as well as the loaded organic phase, if
the mixer-settler consists of two steps.
Solutions:
CuSO4-solution; 2,5 g/l Cu2+, 1,5 g/l Fe3+, pH=2.0
LIX-984 (10%) in kerosene
Procedure:
Mix the aqueous phase and the organic phase in a separation funnel
with the following phase ratios; aq/o= 10/1, 5/1, 2/1, 1/1, 1/2, 1/5. Use
20 ml of aqueous phase in all cases. Shake the mixtures until
equilibrium is reached (at least 5 minutes). Let the phases separate
and save the aqueous phase for analysis.
Analysis:
Analyse the aqueous phases regarding its copper and iron content with
an atomic absorption spectrometer (AAS).
Note. The samples must be diluted before they are analysed. Take
1000 l of the aqueous phase for the experiments with phase ratios
(aq/o) of 10/1 up to and including 2/1 and 4000 l of the aqueous
phase with aq/o of 1/1 up to and including 1/5. Add 5 ml HNO3 and
10 ml 10% tartaric acid and dilute up to 100 ml in a measuring flask.
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Exercise II
Task:
Determine the equilibrium isotherm for stripping of copper from a
loaded LIX-984 solution with 2M H2SO4. Construct a McCabe-Thiele
chart with the o/aq ratio of at least 4. Use a ratio that is as high as
possible depending on your data. Determine the copper concentration
in the pregnant electrolyte, as well as the stripped organic phase, if the
organic phase is stripped in two steps.
Solutions:
CuSO4-solution: 8 g/l Cu2+ med pH=4
Stripping-solution: 2 M H2SO4
LIX-984 (10%) in kerosene
Procedure:
Load the organic phase by shaking 220 ml LIX-984 with 220 ml
CuSO4-solution (8 g/l). Allow the phases to separate and save the
organic phase. Take a sample of the aqueous phase for analysis.
Shake the loaded organic phase with stripping solutions with the
following phase ratios; o/aq= 1/1, 5/1, 10/1 and 20/1. Use 50 ml of the
organic phase in all cases and shake until equilibrium is reached (at
least for 5 minutes). Allow the phases to separate and save the
aqueous phases for analysis.
Analysis:
Dilute the samples and analyse with the AAS regarding the copper
content. Dilute the aqueous solution obtained after stripping with o/aq
= 1/1 and the aqueous solution obtained after the initial loading of the
organic phase (original concentration of 8 g/l) by taking 200 l of the
aqueous phase, 5ml HNO3, 10 ml 10% tartaric acid and dilute up to
100 ml in a measuring flask.
Take 50 l of the stripping solutions obtained after experiments with
o/aq of 5/1, 10/1 and 20/1. Add 5 ml HNO3 and 10 ml tartaric acid
and dilute up to 100 ml in a measuring flask.
Report
The report should contain the following: Theory, a short summary of the theory
Results
Discussion
Note!! All solutions must be saved until the experiment is
approved.
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