Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1
METHOD FOR THE SOLVENT EXTRACTION OF ZINC
This invention relates to the solvent extraction of zinc
and, more particularly, to a method for the extraction of
zinc from sulfate solutions using organic phosphoric and
thiophosphinic extractants.
BACKGROUND OF THE INVENTION AND PRIOR ART
A large number of extractants are known that are capable of
extracting zinc from solutions that may also contain one or
more other metals such as iron, copper, silver, cadmium,
cobalt, nickel, manganese, magnesium, calcium, sodium,
potassium, arsenic and antimony. Zinc and other metals
have been extracted from solutions with organo phosphorus
compounds such as substituted organic phosphoric acids,
particularly di-2 ethylhexylphosphoric acid (D2EHPA).
Methods for the extraction of zinc with D2EHPA are
disclosed in references that include Hydrometallurgy 3 ( 4 ) ,
327-342, Oct. 1978; Chem, Engg. Res. and Design 61, 62
66, ,Tan. 1983; U.S. Patents 3 989 607, 3 441 372, 4 124
462, 4 423 012, 4 552 629 and 4 618 428; and CA Patents
1 083 830, 1 098 719 and 1 198 290.
It is also known that zinc and many other metals form
complexes with substituted phosphinic acids. Methods that
use substituted phosphinic acids for the extraction of zinc
from solutions are disclosed, for example, in U.S. Patents
3 966 569 and 4 721 605. According to the former, zinc and
iron are removed with a dialkylphosphinic acid in a
multiple-stage solvent extraction process using different
extractants. According to the latter, a metal from a first
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group consisting of zinc, silver, cadmium, mercury, nickel,
cobalt and copper can be extracted from solution
additionally containing a metal from a second group
consisting of calcium and magnesium by contacting an
aqueous solution containing one or more of the metals of
the first group with an organic-soluble substituted
dithiophosphinic acid or salt thereof at a pH of from 0.2
to 3Ø It is apparent from the disclosure of this patent
that, when more than one metal of the first-recited group
is present in the solution, all of these metals are
extracted. The extraction is only selective with respect
to the metals of the second-recited group.
None of these references disclose the selective extraction
of zinc from a zinc sulfate solution with an extractant
mixture that contains both an organic substituted
phosphoric acid and an organic substituted thiophosphinic
acid.
SUMMARY OF THE INVENTION
I have now discovered that zinc may be selectively
extracted from a zinc sulfate solution, containing zinc
sulfate, sulfuric acid, as well as controlled amounts of
other metals, with an extractant mixture that contains an
effective amount of both an organic substituted phosphoric
acid, such as an alkylphosphoric acid, and an effective
amount of an organic substituted thiophosphinic acid.
More specifically, aqueous zinc sulfate-containing solution
that may also contain sulfuric acid and controlled
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concentrations of other metals is mixed in an extraction
step with an extractant mixture for the substantially
selective extraction of zinc from the other metals in the
solution with the formation of an extract phase and a
raffinate phase. After settling, the extract is separated
from the raffinate, and zinc salt solution substantially
free of other metals is stripped from the extract by
contacting the extract in a stripping step with a suitable
stripping solution. The raffinate is removed from the
process, the purified zinc salt solution stripped from the
extract is treated for the recovery of zinc, and the
stripped extractant is returned to the extraction step.
The acid content of solutions in the solvent extraction
must be controlled which can be done by the addition of a
neutralizing agent or by solvent extraction of the acid.
Using solvent extraction, a multi-step process of dual-
circuit, alternating zinc and acid extraction steps may be
used. According to a preferred embodiment, the method of
the invention is carried out in a dual-circuit, side-by-
side simultaneous extraction of zinc and sulfuric acid. In
the acid extraction circuit, acid is extracted from the
aqueous feed solution and from the raffinate from the zinc
extraction circuit with an extractant suitable for
extracting sulfuric acid for the formation of an acid
extract and an acid extraction raffinate. The acid extract
is strigped of its sulfuric acid content, and the stripped
organic phase is returned to the extraction step. The acid
extraction raffinate is recirculated to the zinc extraction
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where zinc is extracted with a suitable organic extractant
for zinc and with the formation of a zinc extract and a
zinc extraction raffinate. The zinc extraction raffinate
is recirculated to the acid extraction, and a portion of
the recirculating raffinate is removed from the process as
a residual solution substantially free of zinc. The zinc
extract is stripped for the removal of zinc. Removed zinc
is recovered as a zinc salt solution such as zinc. sulfate
or zinc chloride solution. Each of the steps of zinc
extraction, zinc stripping, acid extraction and acid
stripping may be carried out in one or more stages.
The extractant mixture for zinc consists of a mixture of an
effective amount of one or more organic substituted
phosphoric acids and an effective amount of an organic
substituted thiophosphinic acid :in an appropriate ratio
and, preferably, in admixture with a suitable diluent. The
substituted phosphoric acids are preferably
ethylhexylphosphoric acids chosen from the group consisting
of mono-2-ethylhexylphosphoric acid (M2EHPA), di-2-
ethylhexylphosphoric acid (D2EHPA) and mixtures thereof.
The substituted thiophosphinic acids are preferably chosen
from the group consisting of bis- 2, 4, 4 -
trimethylpentylmonothiophosphinic acid (CyanexTM 302) and
bis- 2, 4, 4 -trimethylpentyldithiophosphinic acid
(CyanexTM 301). The use of an extraatant mixture of D2EHPA
and CyanexTM 302, or a mixture of M2EHPA, D2EHPA and CyanexTM
302 is preferred. Hy using a mixture of the phosphoric and
thiophosphinic extractants, the extraction of zinc is
enhanced, and is greater than can be obtained with either
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the phosphoric acids or the thiophosphinic acids alone.
The pH during zinc extraction is controlled at a value in
the range of about 1.3 to 5.
Accordingly, there is provided a method for the selective
5 extraction of zinc from an acidic aqueous solution
containing zinc sulfate and essentially free of ferric iron
and co-extractable metals including the steps of contacting
said solution with ~an extractant mixture containing an
effective amount of at least one organic substituted
phosphoric acid, and an effective amount of an organic
substituted thiophosphinic acid for the formation of a zinc
extract containing said zinc and of a zinc extraction
raffinate; controlling the acid content during said
extraction of zinc such that the pH in said extraction is
in the range of about 1.3 to 5; separating said extract
from said raffinate; contacting separated extract in a zinc
stripping with a suitable aqueous stripping solution for
removing said zinc from said extract into said stripping
solution; and recovering said zinc as a zinc salt solution.
Accordingly, there is further provided a method for the
selective extraction of zinc from an acidic aqueous
solution containing zinc sulfate including the steps of
contacting said solution with an extractant mixture
containing an extractant for zinc chosen from the group
consisting of mono-2-ethylhexylphosphoric acid, di-2-
ethylhexylphosphoric acid, a mixture of mono-2-
ethylhexylphosphoric acid and di-2-ethylhexylphosphoric
acid, and an extractant for zinc chosen from the group
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consisting of bis- 2, 4, a_ -trimethylpentyl-
monothiophosphinic acid and bis- 2, 4, 4 -
trimethylpentyldithiophosphinic acid for the formation of
a zinc extract containing said zinc and of a zinc
extraction raffinate; controlling the acid content during
said extraction of zinc such that the pH in said extraction
is in the range of about 1.3 to 5; separating said extract
from said raffinate; contacting separated extract in a zinc
stripping with a suitable aqueous stripping solution for
ZO removing said zinc from said extract into said stripping
solution; and recovering said zinc as a zinc salt solution.
Preferably, said extractant mixture contains mono-2-
ethylhexylphosphoric acid, di-2- ethylhexyl phosphoric acid
and bis- 2, 4, 4 -trimethylpentylmonothiophosphinic acid.
Preferably, Said extractant mixture contains di-2-
ethylhexylphosphoric acid and bis-2-, 4, 4-
trimethylpentylmonothiophosphinic acid.
It is an aspect of the present invention to extract zinc
selectively from zinc sulfate-containing solutions. It is
another aspect to selectively extract zinc from zinc
sulfate solutions with an extractant mixture containing
organic substituted phosphoric acid and organic substituted
thiophosphinic acid. It is a further aspect to selectively
extract zinc and sulfuric acid from acidic zinc sulfate
solutions using dual-circuit, side-by-side simultaneous
extraction of zinc and acid. It is yet another aspect to
selectively extract zinc and sulfuric acid from acidic zinc
sulfate solutions using dual-circuit, alternating
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extraction of zinc and acid. These and other aspects will
become apparent from the following detailed description of
preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 depicts a schematic flow scheme of a dual--circuit
alternating extraction of zinc and acid;
Figure 2 is a schematic flowsheet of a dual-circuit, side-
by-side, simultaneous solvent extraction of zinc and
sulfuric acid; and
Figure 3 is a schematic flowsheet of a multi-stage method
of the dual-circuit method of Figure 2.
DETAILED DESCRIPTION
Solutions that may be treated according to the method of
the present invention are zinc sulfate - containing
solutions which may contain ZnSOq up to its saturation
cancentration and up to about 150 g/L HzSOq. Practically,
feed solutions may contain from about 1 to 300 g/L ZnS04 and
from about 10 to 150 g/L H2S04. The feed solutions should
be substantially free of ferric iron. In addition, the
feed solutions may also contain at least one metal chosen
from the group consisting of ferrous iron, calcium,
magnesium, manganese, sodium, potassium, arsenic and
antimony. The feed solution should be essentially free of
co-extractable metals such as copper, cadmium, germanium
and indium. The co-extractable metals must be present in
concentrations below the values at which they would be co-
extracted. The concentrations must, therefore, each be
considerably less than the concentration of zinc in the
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feed solutions. For example, a solution containing 1 g/L
zinc should not contain more than about 0.1 g/L of each of
co-extractable metals. If feed solutions contain higher
concentrations of co-extractable metals, the concentrations
should be reduced prior to selective extraction of the zinc
using methods known in the art. Any ferric iron in the
feed solutions should be removed or reduced to the ferrous
state by usual means prior to extraction. Amounts of
metals other than those mentioned above may be present in
the feed solution in incidental or trace amounts.
The method of the invention is carried out in a
conventional manner using known equipment. The zinc
sulfate solution is treated by liquid-liquid extraction
including extraction, stripping and scrubbing steps using
either mixer-settlers or extraction columns, both with the
appropriate use of agitation. Means are also provided to
maintain the amount of acid in the solvent extraction of
zinc at a controlled level, as will be described.
The extractant mixture used in the extraction of zinc
consists of a mixture of an effective amount of at least
one organic substituted phosphoric acid and an effective
amount of an organic substituted thiophosphinic acid
dissolved in an appropriate amount of a suitable diluent.
The phosphoric acids are, preferably, chosen from the group
consisting of mono-2-ethylhexylphosphoric acid (M2EHPA),
di-2-ethylhexylphosphoric acid (D2EHPA) and mixtures
thereof (EHPA). The thiophosphinic acids are, preferably,
chosen from the group consisting of dialkylthiophosphinic
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acids such as bis-2,4,4-trimethylpentylmonothiophosphinic
acid, sold under the name of CyanexT~ 302 or bis-2,4,4-
trimethylpentyldithiophosphinic acid, sold under the name
CyanexTM 301. It is understood that other dithiophosphinic
acids such as those mentioned in U.S. Patent 4 721 605, and
that can be selective for the extraction of zinc, may also
be used. The ratio of phosphoric acid to phosphinic acid
in the extractant mixture is in the range of about 1:1 to
6:1 by volume, and is preferably in the range of about 1:1
to 3:1 by volume. The phosphoric and phosphinic
extractants are dissolved in a suitable water-immiscible
organic diluent such as, for example, ExxsolTM D80 in a
volume ratio in the range of about 1:1 to l:9 extractants
to diluent. The most preferred extractant mixture contains
D2EHPA or M2EHPA + D2EHPA and CyanexTM 302, diluent in
amounts of about 5 to 30% by volume of D2EHPA or M2EHPA +
D2EHPA and about 5 to 20% by volume of .Cyanex 302, the
balance being substantially diluent, for example, ExxsolTM
D80.
The extractant mixture is contacted with the aqueous acidic
zinc sulfate-containing solution in an organic to aqueous
volume phase ratio (o/a ratio) selected on the basis of the
zinc concentration in the aqueous phase. The contacting is
carried out for the formation of an extract, i.e. loaded
organic phase, and a raffinate, i.e. aqueous phase, with
appropriate Zn concentrations. If desired, a small amount
of a modifier such as, for example, an alcohol with a long
hydrocarbon chain (e. g. decanol) may be used to facilitate
the separation, enhance the extraction or both. The zinc
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sulfate is selectively extracted into the extract while the
other metals as defined hereinabove substantially remain in
the raffinate. The extraction may be carried out at
temperatures in the range of about 0°C to 60°C and,
5 preferably, at ambient temperatures such as in the range of
about 20°C to 50°C.
In the extraction of zinc from zinc sulfate, an amount of
sulfuric acid is formed that is equivalent to the amount of
sulfate in the zinc sulfate. It is necessary that this
10 amount and any amount of free sulfuric acid present in the
zinc sulfate solution be controlled. The acid content can
be controlled by any one of a number of methods either
alone or in combination.
Generally, the pH during zinc extraction is controlled at
a value in the range of about 1 " 3 to 5. Below a pH of
about 1.3, the extraction efficiency is low, while at a pH
above about S, basic zinc sulfate or zinc hydroxide may
precipitate. When additional metals are present, the pH is
preferably in the range of about l.7 to 3.
Control may be exercised by either removal, neutralization,
extraction or a combination of these methods of at least a
portion of the acid prior to or during the solvent
extraction of zinc. Removal may be carried out by such
methods as dialysis and electrodialysis. Neutralization
may be carried out prior to or during the extraction of
zinc by adding a suitable alkaline neutralizing agent such
as potassium hydroxide, sodium hydroxide or ammonium
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hydroxide solution. As the increase in aqueous acid
concentration due to the generation of- acid in the zinc
extraction step limits the zinc extraction, the acid
content in the zinc extraction is controlled by
neutralization of at least a portion of the acid with an
alkaline solution such as, preferably, sodium hydroxide or
ammonium hydroxide solution. Alternatively, by only
extracting a small amount of zinc from a neutral solution,
the generation of a limited (equivalent) amount of acid is
achieved. This small amount of zinc is in the range of
about 3 to 5 g/L. Of course, extraction of zinc may be
carried out with simultaneous neutralization of a portion
of the acid such that the free acid content of the
raffinate does not exceed an amount of free acid in the
range of about 5 to 8 g/L.
Extraction of acid is carried out with a suitable organic
extractant, as will be explained. Hecause of the need for
extracting only a limited amount of zinc in the absence of
simultaneous neutralization, many extraction steps may be
required, the number being dependent on the concentration
of zinc in the feed solution.
The extraction of zinc, or zinc and acid, is carried out in
mixer-settlers with agitation and in co-current, counter-
current or cross-current fashion. Continuous counter-
current zinc extraction and cross-current acid extraction
are preferred. Aqueous feed solution is mixed with
extractant in the mixer-settler and the two-phase liquid
mixture is settled with the formation of an extract and a
raffinate.
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Suitable extractants for sulfuric acid are amine-type
extractants which include primary, secondary, tertiary and
quaternary amines, such as, for example, AlamineTM336
(tricaprylamine) or PrimeneTM JMT (C16-C22 tert alkyl
primary amine). The acid extractant is mixed with a
suitable diluent to give an amine concentration of about 4
to 255 by volume. If desired, a modifier may be added at
a concentration of, for, example, about 1 to 8~ by volume.
In one embodiment, the extraction of zinc and the
extraction of sulfuric acid are carried out in a series of
alternating extraction steps wherein zinc sulfate-
containing solution is contacted with an extractant for
zinc in a zinc extraction step and acid is extracted from
the raffinate from the zinc extraction in an acid
extraction step.
The zinc extract is contacted with an acidic solution to
strip the zinc into a zinc salt solution. A solution
containing sulfuric acid may be used for stripping to give
a strip liquor containing zinc sulfate. Alternatively, a
hydrochloric acid solution may be used to give a zinc
chloride strip liquor. In a zinc production plant using
electrolysis, the spent acid solution from the cell house
may be conveniently used for zinc stripping. The acid
extract may be stripped with water to give sulfuric acid,
but preferably the extract is stripped with an alkali
solution to give a sulfate. An ammonium hydroxide solution
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is used to strip the acid producing an ammonium sulfate
solution.
With reference now to the schematic of Figure 1 for dual-
circuit, alternating zinc and acid extractions, aqueous
acidic zinc sulfate solution is fed to a first acid
extraction 10, wherein solution is mixed with a suitable
extractant for sulfuric acid. After phase separation, the
extract is passed to an acid stripping 11, and the
raffinate is passed to a first zinc extraction 12. In
first zinc, extraction 12, raffinate from first acid
extraction 10 is mixed with zinc extract from a second zinc
extraction 13. After phase separation, the extract is
passed to a first zinc stripping 14, and the raffinate is
passed to a second acid extraction 15. In second acid
extraction 15, the raffinate from first zinc extraction 12
is mixed with more of the acid extractant, and, after phase
separation, the extract is passed to the acid stripping 11
while the raffinate is passed to a second zinc extraction
13. The raffinate from second acid extraction 15 is mixed
in second zinc extraction 13 with xecycled zinc extractant
from a second zinc stripping 16. After phase separation,
the raffinate is removed from second zinc extraction l3 as
residual solution substantially depleted in both zinc and
acid, and the extract is fed to first zinc extraction 12.
The zinc is stripped countercurrently from the extract from
first zinc extraction 12 in a first and a second zinc
stripping 14 and 16, respectively, with a suitable
stripping solution and a solution containing the stripped
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zinc is recovered as a zinc salt solution. Although a
number of stripping solutions may be used, the use of '
sulfuric acid is preferred, resulting in the recovery of a
zinc sulfate solution. Alternatively, hydrochloric acid
may be used as a stripping solution for the recovery of a
zinc chloride solution.
The acid is stripped in the first acid stripping 11 with
water for the recovery of sulfuric acid or, preferably,
with ammonium hydroxide far the recovery of sulfate as
ammonium sulfate, as shoran. The acid extractant is
scrubbed with water in an acid scrubbing 17. The scrubbed
acid extractant is recycled to acid extractions 10 and 15.
The scrubbing liquor from scrubbing 17 is passed with or
without added ammonium hydroxide to acid stripping 11.
When ammonium hydroxide is used, a portion of the recovered
ammonium sulfate solution may be recycled as scrubbing
solution to acid scrubbing 17. It is understood that,
although Figure 1 depicts a two-stage acid and two-stage
zinc extraction process, the process may include as many
extraction and stripping stages as necessary to
substantially remove acid and zinc from the feed solution.
According to a preferred embodiment, aqueous acidic zinc
sulfate solutions are subjected to a dual-circuit, side-by-
side simultaneous solvent extraction as shown in Figure 2
and generally indicated with 20. Dual-circuit side-by-side
extraction 20 is preferred especially when the
concentration of zinc in the feed solution exceeds about 5
g/L. Aqueous acidic zinc sulfate solution is fed as feed
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solution to acid extraction 21. The concentration of acid
in solutiom fed to solvent extraction 20 should preferably
be as low as possible in order to reduce the need for acid
removal during solvent extraction. A portion of the acid
may be removed prior to feeding solution to extraction 20,
as described. In the acid extraction 21 of the acid
extraction circuit, acid is extracted from the raffinate 25
from zinc extraction 22 combined with feed solution by
mixing the combined aqueous solution with an amine organic
extractant, described hereinabove, for extracting sulfuric
acid with the formation of an acid extract and an acid
extraction raffinate 26. The acid extract, i.e. the acid
loaded organic phase, is passed to acid stripping 23, and
the acid extraction raffinate 26 is passed to zinc
extraction 22 of the zinc extraction circuit.
In acid stripping 23, the sulfuric acid is stripped from
the acid extract, and stripped organic phase (acid
extractant) is returned to acid extraction 21. The
sulfuric acid is stripped with a compound chosen from water
and suitable alkaline substances. Stripping with water
makes it possible to recover sulfuric acid directly, while
stripping with an alkaline substance forms a sulfate salt.
Stripping with alkali is moxe efficient than stripping with
water, and is preferred. One of a number of alkaline
substances such as alkali metal or ammonium hydroxide
solutions may be used, but the use of ammonium hydroxide is
preferred because a saleable byproduct, i.e. ammonium
sulfate, is obtained. The formed sulfuric acid or sulfate,
preferably ammonium sulfate, is recovered. Both acid
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extraction 21 and acid stripping 23 are carried out at
ambient conditions.
The acid extraction raffinate 26 from acid extraction 21 is
passed to zinc extraction 22, where zinc is selectively
extracted from the solution with a mixed extractant,
described hereinabove. The organic phase is mixed with
acid extraction raffinate 26 from acid extraction 21. The
mixing is carried out for a time sufficient to extract the
zinc from acid extraction raffinate 26 into the organic
phase, i.e. the zinc extract, and with the formation of
zinc extraction raffinate 25. The pH in zinc extraction 22
is maintained in the range of about 1.3 to 5.0, and
preferably in the range of about 1.7 to 3Ø Zinc
extraction raffinate 25 from zinc extraction 22, which also
contains the acid generated during zinc extraction, is
passed to acid extraction 21. The recirculation of the
raffinates 25 and 26 between the two extractions 21 and 22
results in substantially complete extraction of the zinc.
A residual solution substantially free of acid and zinc but
containing substantially all other metals that were
contained in the feed solution to the process is removed
from the process. The residual solution may be passed to
further treatment for the removal of environmentally
harmful substances before being discarded.
The zinc extract from zinc extraction 22 is passed to zinc
stripping 24 for the removal of zinc from the zinc extract
and for the recovery of a zinc salt solution and a stripped
organic phase. The o/a ratio is selected to give
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substantially complete stripping of the zinc and yield a
desired concentration of zinc salt in the strip solution.
The strength of the acid used in zinc stripping 24 is
dependent on the zinc concentration desired in the zinc
salt solution recovered from zinc stripping 24. Removal of
traces of organic extractants in the recovered zinc salt
solution may be desirable, and removal may be done by a
treatment with activated carbon.
The stripped organic phase is recycled from zinc stripping
24 to zinc extraction 22, Fresh organic phase may be added
to make up for any losses that may have occurred.
The simultaneous, side-by-side, dual-circuit countercurrent
method is much more efficient than the alternating zinc and
acid extraction as described with reference to Figure 1.
In the dual-circuit process, sulfuric acid and zinc axe
each separated by a suitable extractant, and zinc
extraction raffinate 25 from each zinc extraction circuit
is exchanged with acid extraction raffinate 26 from a
corresponding acid extraction circuit.
Fllthough Figure 2 illustrates the dual-circuit side-by-side
extraction method for one stage each of the zinc and acid
extraction and stripping steps, it is understood that each
of these steps may be carried out in one or more stages.
Figure 3 illustrates the dual-circuit side-by-side
simultaneous extraction method for multiple extraction
stages for zinc and for acid. The four zinc .extraction
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18
stages axe indicated with ZE 1, ZE 2, ZE 3 and ZE 4, the
latter being optional and without a corresponding acid
extraction stage. The three acid extraction stages are
indicated with AE 1, AE 2 and AE 3. The zinc circuit also
includes three zinc stripping stages indicated with ZS 1,
ZS 2 and ZS 3. The acid circuit also includes an acid
stripping stage marked AS 1 and an acid scrubbing stage
marked ASc 1. In the zinc circuit the flows of aqueous and
organic phases are essentially counter-current as are the
flows of raffinates between the acid extraction stages and
the zinc extraction stages. These flows of liquids are
similar to those described with reference to Figure 2. The
scrubbing of the stripped acid extractant in scrubbing ASc
1 is carried out with water, while the stripping of acid in
AS 1 is carried out with the scrubbing liquid from
scrubbing ASc 1 to which a second strip solution may be
added. The second strip solution is water or an alkaline
solution such as ,sodium hydroxide solution or ammonium
hydroxide solution, the latter being preferred. The use of
water enables the recovery of the stripped acid as a
sulfuric acid solution, while the use of ammonium or sodium
hydroxide enables the recovery of a sodium or ammonium
sulfate solution. Aqueous phase recycle is included in the
zinc extraction and stripping steps and in the acid
stripping and scrubbing steps to maintain a volumetric
phase ratio of about unity, as is a common practice to
obtain optimum contacting.
The use of an extractant mixture that contains both an
organic substituted phosphoric acid and an organic
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19
substituted thiophosphinic acid has a number of important
advantages over using either one- of the zinc extractants
alone. Zinc extraction is improved and a recovery of 99a
or better of the zinc from feed solution may be achieved.
The zinc extraction has good selectivity, and phase
separations are better.
The invention will now be illustrated with the following
non-limitative examples. In the examples, the following
abbreviations are used:
D2EHPA = di-2-ethylhexylphosphoric acid
EHPA - a mixture of di-2-ethylhexylphosphoric acid and
mono-2-ethylhexylphosphoric acid
CNX 302 = CyanexTM 302.
All extractants and extractant mixtures were dissolved in
ExxsolTM D80 diluent. .
Example 1
This example illustrates phase disengagement
characteristics of various extractant solutions during
contacting with acidic sulfate solutions. A 280 mL portion
of organic solution was agitated for two minutes with a 140
mL portion of two aqueous solutions (a) and (b) containing
225 g/L H2S04, and 35 g/L H2S04, 47 g/L Zn, 7.5 g/L Mg and
1.2 g/L Mn, respectively, and left to separate. The
results are given in Table I.
20
TABLE I
wSeparatiom Time (seconds)
(a) (b)
Acidic sol'n
containing
Extractant 225 g/1 H Zn
S0 Mg & Mn
2 ,
4
30% D2EHPA 75 120
30% EHPA 130 210
30% CNX 302 60 70
15% D2EHPA, 15% CIVX 65 90
302
15% EHPA, 15% CNX 302 180 330
The CNX 302 solution had the fastest phase separation,
followed by the mixed CNX 302 and D2EHPA solution. The
latter gave faster separation than D2EHPA by itself.
Example 2
This example illustrates the extraction performance of the
various extractant solutions when treating acidic metal
solutions with varying acid content.
Spent acid from the cell house of a zinc production plant
was treated by dialysis to remove most of its acid content.
The treated solution was used as feed for zinc extraction
using various extractant solutions. A 280 mL portion of
extractant was contacted with two 140 mL portions of the
feed solution for two minutes. The separated aqueous feed
was analyzed for Zn, Mg and Mn. The two feed solutions
used were:
a) raw dialyzate from dialysis (35 g/L HZSO4, 50 g/L Zn,
8 g/L Mg and 1.2 g/L Mn)
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~.I ~. ': ~ (.,,7 1.
27.
b) dialyzate adjusted to pH 2.5 (49.6 g/L Zn, 8.0 g/L Mg,
1.2 g/L Mn)
The equilibrium acidity (ar pH) teas not controlled in these
tests. The results with feed solution a) are given in
Table TI.
TABLE II
Loading after Loading
1st contact agter
xtractant q~L n 2nd contact
0 n M~ Mn Z a~
D2EHPA 2.6% 3.2 Mc~ Mn
30% EHPA 1.80 0.05 2.2 0 0.05
30% CNX 302 4.20.15 0.1 5.2 0.25 0.2
15% D2EHPA, 15% 5.10.1 0 6.S O.1S 0
15 151 CNX 302 4.10.05 0 5.6 0.05 p
EHPA, 15% CNX 0.1 O.OS 0.1 0.1
302
These results show that at 30% total extractant
concentration, the mixed extractants EHPA/CNX 302 and
D2EHPA/CNX 302 gave superior performance to any one of
EHPA, D2EHPA or CNX 302 by itself. The combination of
D2EHPA and CNX 302 gave especially surprising superior
2U performance. Moreover, Zn extraction was selective over Mg
and Mn.
the results with feed solution b) are given in Table III.
TABLE
III
25 Loading Loading after
after
1st contact 2nd contact
~L a/L
Extractant Zn ~ Mn
Z n ~ Mn
30% DZEHPA 7.1 0
15
. 0.059.8 0.2 0.1
301 EHPA 4.4 0
40
. 0.2 8.4 0.6 0.3
30A CNX 302 6.B 0
1
. 0.0511.6 0.1 0.05
1St D2EHPA, 151 8.4 0
CNX 302
O.OS12.1 0.1 0.05
15% EHPA. 1S% CNX 7.7 0
302 1
. 0.0512.1 0.15
0.05
U1 i,)
!,~ ;;:~ ;.;, i~ 'c3 t:~ a:
22
Again the mixed extractants, EHPA/CNX 302 and D2EHPA/CNX
302 gave superior results, with the latter being especially
selective.
Example 3
This example demonstrates the ease of stripping Zn from the
various extractant solutions. Extractant solution
containing Zn from extraction contacts described in Example
2 were contacted with a zinc stripping solution of 225 g/L
HzS04. Extractant solution (280 mL) was contacted with two
fresh 40 mL portions of the stripping solution. The phases
were separated and the aqueous phase analyzed for Zn, Mg
and Mn. The results are given in Table IV.
TABLE IV
Metals in partially
loaded extractantMaximum
_ o/L Zn Recovery
xtractant Zn Mac Mn 1 Zn
conc.
in
co~kact
2 contacts
strip,
o/L
30 D2EHPA 9.8 0.2 0.1 100 11079
301 EHPA 0.4 0.6 0.3 !E2 10054,5
2~ 30 CNX 302 11:6 0.1 0.05 63 79 50.9
151 D2EHPA, 09 10075
151 CNX 302
12.1 0.1 0.05
15 EHPA, 15 302 12.1 0.15 84 1007p,8
CNX 0.05
Example 4
This example illustrates by of relative ranking shown
means
in Table V how comparefrom
the extraotants the
results
given in Examples l-3.
TABLE V
Phase Zn Selectivity2n Overall
Extractant SeparationExtraction stsippina
D2EHPA alone 80 49 10-0 100 329
EHPA alone 46 34 50 96 226
CNX 302 alone 100 80 70 65 315
D2EEiPA/CNX 302 92 100 9S 95 382
ENPA/CNX 302 33 85 80 87 285
ca r~, t'~ ;~ ~-~ c~ r~
~s ':!' a~:W .~ . " ~a ~.a
23 '
Thus, for acidic Zn-containing solutions, the findings from
tests showed the following:
1) When extractants were used by themselves, Cyanex 302
gave the best zinc extraction with the fastest phase
separation.
2) D2EHPA gave the best selectivity and was most easily
stripped.
3) Cyanex 302 was the most difficult to strip.
Surprisingly, the mixtures of D2EHPA/Cyanex 302 and
" EHPA/Cyanex 302 gave better Zn extraction than the single
extractant solutions. The superior performance of
D2EHPA/Cyanex 302 was particularly surprising, in that the
result of that mixture was superior to the calculated,
weighted sum of the individual components, as shown in
Table VI.
xAeLE m
Performance measure
P:Tase Zn Zn Overa
Sep'n. Ext'n. Seleetivity Steip~ina
2~ Expected CNX/D2EHPA: 90 65 85 B3 323
Actual CNx/DZEHPA: 92 100 95 95 382
eased on performance of individual components.
D2EHPA/Cyanex 302 gave an unexpectedly superior result.
EHPA/Cyanex 302 was also slightly superior to the sum of
Cyanex 302 and EHPA, with an actual overall performance
measure of 285 compared to the expected 270. Moreover Zn
extraction by Cyanex 302/EHPA mixture ranked 85 compared to
an expected 57.
Example 5
This example compares the Zn extraction performance of
solutions containing different proportions of two mixed
extractants with a total extractant concentration of 40$ in
each:
a) 30$ D2EHPA, 10$ CNX 302
b) 25$ D2EHPA, 15$ CNX 302
A 280 mL portion of freshly prepared extractant solution
was contacted with two fresh 140 mL portions of a 225 g/L
H2S04 solution as a conditioning step. The conditioned
extractant solution was contacted with two fresh 140 mL
portions of an acidic zinc feed solution containing 42.6
g/L Zn and 37 g/L H2504. The raffinates were analyzed for
Zn and H2S04. The partially loaded extractant solutions
'wer.e then contacted with two 40 mL fresh portions of a 225
g/L H2S04 solution in order to strip the zinc.
The results are shown in Table VII.
TABLE VII
Loading after 1st contact Loading after 2nd contact
Extraction g/L Zn g/L Zn
30% D2EHPA/10% CNX 302 4.1 6.3
25% D2EHPA/15B CNX 302 5,0
Max. Zn concn.
% Zn stripped in strip liquor
Stripping 1 contact 2 contacts g/L
30% D2EHPA/10% CNX 302 84 100 37.1
25% D2EHPA/15% CNX 302 70 86 45.6
t~~ ~3 F",) !~O a;: CAi i
/,W~i.Y iJ l.~ t ~ ~m l
Acid concentrations in the raffinate were higher than that
of the original feed in both cases due to acid generation
caused by zinc extraction by liquid ion exchange. The 25~
D2EHPA/15o CNX 302 extractant gave superior results as
5 compared to 30o D2EHPA/10°s CNX 302 extractant for zinc
extraction. The stripping of the less-loaded 300
D2EHPA/l0o CNX 302 was more efficient than that of the 25%
EHPA/15o CNX 302 solution. However, the latter gave a
higher zinc concentration in the strip liquor.
10 Example 6
This example illustrates efficient and selective zinc
extraction with pH control. Using a feed solution
containing 52 g/L Zn, 7.1 g/L Mg and 1.1 g/L Mn, a number
of extractions were carried out using 25~ D2EHPA and 15~
15 CNX 302. In each case, the equilibrium pH was adjusted to
2.5 by the addition of sodium hydroxide or ammonium
hydroxide. The use of ammonium hydroxide for pH adjustment
could only be carried out after the zinc concentration in
the aqueous phase had been reduced to low levels or when
20 the required ammonium hydroxide was small enough not to
cause the precipitation of zinc ammonium sulphate.
Multiple contacts with portions of extractant and feed
solution were carried out to establish equilibrium
distribution and maximum loading achievable at pH 2.5. The
25 aqueous phase (raffinate) was analyzed and the composition
of the extractant phase (extract) calculated. The
equilibrium data are given in Table VIII.
~~'l~7Ea,S3~
NJ ~.~ i~,J lJ
26
Raffinate, /L Extract, g/L
_
Zn ~, Mn Zn M~ Mn
0.002 5.5 0.34 0.7 0.8 0,38
0.07 6.1 0.63 8.7 0.5 0.24
1.0 6.6 0.95 22.6 0.25 0.07
2.1 6.4 0.94 25.2 0.35 0.07
2.4 6.8 1.0 37.9 0.15 0.05
4.3 6.6 0.92 39.8 0.25 0.09
The results show the extractants to be selective for zinc,
with a maximum loading of 39.8 g/L Zn achieved in the
extract. From an isotherm derived with the above results,
it can be shown that for the feed solution containing 52
g/L Zn, two countercurrent stages of contacting would be
required to achieve complete zinc extraction.
These tests also show that the addition of large quantities
of alkali would be required to control pH when treating
feed solutions of high zznc concentrations.
Example 7
This example illustrates acid extraction from zinc
containing solutions by amine extractants. The extractions
described in Example 6 required pH control, which was
achieved by means of alkali addition. An alternative use
of liquid-liquid extraction to remove the acid generated
during zinc extraction was demonstrated as follows:
An acidic solution containing 20 g/L Zn and 30 g/L H2S04 was
contacted with 20~ (by volume) solutions of amines at a o/a
~.~ t~~~ ;~3 :~ E~ a
27
volume phase ratio of 1. In each contact, the phases were
allowed to separate, and the aqueous phase was analyzed.
In all cases, no significant change in the zinc
concentration was noted, The extraction of acid by two of
the amines are given in Table IX:
TABLE IX
Extractant Loading after Loading after
1st contact 2nd contact
(g/L H2S04~ ( L H2S04~
20o PrimeneTM JMT 25 31
20~ AlamineTM 336 25 33
Using a.. feed solution containing 47 g/L Zn and 39 g/L H2S04,
Alamine 336 was used to determine the effect of extractant
concentration. Results are given in Table X.
TABLE X
Loading ;after Loading after
1st contact 2nd contact
Extractant ( L H2SClq~ (g/L H2S04~
10~ Alamine 336 16 17
20~ Alamine 3-36 29 35
~lo apparent difference in phase disengagement rate was
observed for the 10% and 20~ extractant solutions.
Therefore, the use of a 20~ solution is preferred for its
higher capacity for acid removal.
Example 8
The equilibrium acid distributions derived from a series of
ten contacts of 20~ amine solutions (five with Alamine 336,
c~ ;' ~ ~~.~ t~ ~? ~7 i'
1'd '.9 ; :! i? z~ s,e :,~
28
five with Primene JMT) and various acidic solutions are
given in Table XI.
TABLE XI
Raffinate Phase Extract
Phase
( 9/L F~2S0q ) ( G/L H2S04 )
20% Alamine 336 20% Primene Alamine 336 Primene
JMT JMT
1,3 1,3 7.0 5.5
2.5 2.5 14 12
5.0 5.0 25 23
10.0 10.0 28 26.5
20.0 20.0 31.5 28.5
These equilibrium data can be used to show that a feed
solution containing 30 g/L H2S0~ can be treated with 20%
Alamine 336 solution in a counter-current contacting at a
phase ratio of 1 to reduce the acid concentration to 2.5
g/L in two stages, or to 0.5 g/L :in three stages.
Example 9
This example illustrates how ths~ extracted acid may be
stripped from the amine solution with water or an alkaline
solution in order to allow repeated use of the extractant.
Eight acid loaded amine solutions (four with Alamine 336
and four with Primene JMT) were contacted with water. The
phases were separated, and the aqueous phases were
analyzed. The acid concentration in each extractant phase
was calculated. The results are given in Table XII.
s ~'.~, s~
~~s~.ll=~Gi~jl~l.
29
TABLE 7~II
g/L H2S04 iri extractant phase L H2S04 in strip liauor
Alamine 336 Primene JMT Alamine 336 Primene JMT
21.3 22.0 1.75 1.0
22.5 23.5 3.0 3.0
25.0 25.0 5.5 5.8
27.5 26.3 7.5 8.5
These results show that water stripping can only remove a
fraction of the acid from the extractant phase.
Stripping with ammonium hydroxide solution was carried out
by contacting a partially loaded (30 g/L H2S0~) solution of
20% Alamine 336 with a solution which contained 0.6 normal
NH40H and 3.5 molar (NHq)ZSO~. The presence of ammonium
sulphate enhanced the phase disengagement. The aqueous
phase was analyzed to determine the resulting acid removal
from the extractant. It was found in a number of tests
that, as long as there was sufficient alkali in the strip
solution, acid removal from the amine solutions was
complete in a single contact.
Exam 1p a 10
This example illustrates a continuous operation of the
process using mixer-settlers according to the flow scheme
shown in Figure 3.
Twelve stages of mixer-settlers were assembled to consist
of three zinc extraction stages (ZE 1,2,3) one optional
additional zinc extraction stage (ZE 4), three acid
Ce (~j s'~ J;
r~'~f~~:.'~iC~
extraction stages (AE 1,2,3), three zinc stripping stages
(ZS 1,2,3), one acid stripping stage (AS 1) and one stage
to scrub the acid extractant (ASc 1).
The zinc extractant (organic T) was 25o D2EHPA, l5o Cyanex
5 302, and the acid extractant (organic II) was 20% Alamine
335.
An aqueous feed solution containing zinc was fed to a first
acid extraction stage (AE 1), where it was contacted with
the acid extractant at an o/a volumetric phase ratio of
10 1:4. The raffinate was circulated through a side-by-side
first zinc extraction stage (ZE 1). The zinc extractant
flow was selected to be three times the fresh aqueous feed
rate. Raffinate from ZE 1, was also circulated to AE 1,
with a portion fed to the second acid extraction stage AE
15 2, and so on, as shown in Figure 3. Aqueous and organic
phases were fed counter-currently in the zinc extraction
stages but the acid extractant was fed in a parallel manner
(cross-flow) to the acid extraction stages. The fourth
zinc extraction ZE 4 caas used to achieve nearly complete
20 zinc extraction, but this stage did not require a side-by-
side acid extraction, as the Zn concentration in the
aqueous feed to it was very low.
The zinc extractant leaving extraction was fed through the
zino stripping stages (ZS 1,2,3) countercurrently to a
25 strip solution, which was in one run, a 240 g/L H2S04 or, in
another run, spent acid solution from a zinc production
plant containing about 150 g/L H~S04 and 50 g/L Zn. The
~~ ,~i E ~ .') ~ ~ s1
Fd u~ 1~.~ ~~ =v~' ~d ~.~:
31
strip solution was fed at a rate selected to give the
desired zinc concentration in the resulting strip liquor.
Aqueous recycle was carried out to achieve a volumetric
phase ratio of about 1.
The acid extractants from the three acid extraction stages
(AE1,2,3) were combined and fed to the acid stripping stage
(AS 1), which was also fed with a second strip solution (14
molar NH40H) and a circulating solution from the scrubbing
stage (ASc 1). Moreover, a portion of the aqueous phase
from the stripping stage AS 1 was recirculated to give a
., phase ratio of about 1 in the mixer. The pH of the aqueous
phase was maintained at about 8. The acid extractant was
finally scrubbed in ASc 1 before returning to the
extraction stages. The scrubbing solution consisted of
water and some circulating solution from AS 1. The amounts
of water and ammonium hydroxide fed were chosen such tha t
the acid was completely neutralized .as well as to yield the
desired ammonium sulphate concentration in the resulting
strip liquor. A typical test result of a continuous run is
tabulated in Table XIIT.
TABLE XIII
pH or Composition g/L
A4ueous Stream HZSOa Zn Mg Cl _(NH4~2$O4
Aqueous feed in g/L 49 Mn 0.11 -
Raffinate Erom ZE 3 33 0:069 6.6 _ _
pig 2.1 1.0
6,9
1.5
Raffinate Erom ZE 4 pH 1.7 0.045 6.5 0.96 0.08 -
Fresh Zn stripping 156 Sp
solution - - - -
Zn strip liquor 11 147 - _
Acid strip liquor pH 8 0.035 0.05 0.001 0.015 960
~"~~~n~~~ j~
32 h~ t: ~! 't~ ~ ~~ .~
In this run, the optional zinc extraction stage ZE 4 was
... not necessary to achieve better than 99~ zinc-recovery.
It is understood that changes and modifications may be made
in the methods according to the invention without departing
from the scope and purview of the amended claims.
