Note: Descriptions are shown in the official language in which they were submitted.
2278-261
1
METH10D P'OR STRIPPING tdETALS TN SOLVENT EXTRACTION
'this invention relates to the solvent extraction of metals
and, more particularly, to a method for the removal of metal
cation species in solvent extraction by galvanic stripping with
added rnetals.
Background of the Invention
Aqueous solutions that contain one or more dissolved metals
in ionic Corm may be subjected to solvent extraction for the
recovery of one or more desired metals. The desired metal ions
are usually extracted from aqueous solution into an organic
solvent containing an extractant and are recovered from the
loaded solvent by stripping with a suitable aqueous strip
solution. The other metals, present as ions in the aqueous
solution as impurities, must often be removed from the process
as they may cause difficulties in the stripping of the desired
metal, and often increase in concentration in the circulating
solvent to an extent that affects the efficiency of the
extraction process.
Methods that are used alone and in combinations for removing
desired and impurity cat.ions present in solvent extraction
processes include the conventional stripping or selective
stripping with acidic or basic solutions and the more recently
developed hydrogen reductive stripping, hydrolytic stripping
and electrolytic stripping.
2 ~ ,~ .' in ". !) .~
~ ;:' . , i~ ;_~ _,: ,~.
Stripping is often accomplished with acids or bases under
ambient or elevated conditions. Hydrogen reductive stripping
is carried out at temperatures between 150~and 350°C under
elevated pressure and usually i.n the presence of seed metal.
particles to produce a metal powder. In hydrolytic stripping,
loaded solvent is subjected to elevated temperatures
(100-250°C) in the presence of water whereby metal oxides or
hydroxides are Formed. Elydrogen reductive stripping and
hydrolytic stripping have been reviewed by Monh emius, A.J.,
Mintek 50, pp. 599-609. >;lectrolytic stripping has been
applied to a loaded solvent by subjecting the solvent to
electrolysis with electrodes placed in the loaded solvent (Wan,
R.Y., etal., J. of Metals, Dec. 198b, pp. 35-X10).
Ferric iron may also be stripped From various loaded organic
solvents into the aqueous phase with an acid alone or combined
with the introduction of sulfur dioxide or hydrogen sulfide to
reduce ferric to Ferrous. The stripp7.ng may be carried out at
ambient or elevated temperatures and pressures. It is noted
that iron is usually present in solvent extraction processes
as ferric and that, in many cases, ferrous is the stable form
in the aqueous phase.
The above prior art methods have a number of disadvantages.
In conventional stripping, high concentrations of the strip
solution are often required. Where lower concentrations are
used, the processes are complicated by, for example, the use
of combinations of extractants. Hydrogen precipitation and
f ~ ' ~ )j ;.'r -''~~:
3
gaseous and/or hydrolytic stripping, especially under elevated
pressure and at higher temperatures, are expensive and complex.
In aqueous hydrometallurgical processes use is often made of
galvanic reactions between metals that cause reduction of a
metal cation and precipitation, i.e. 'cementation, onto an added
solid metal. This has not been applied to solvent extraction
processes. Any methods disclosed for the reduction of metal
cations to reduce and cement metal cations onto an added solid
metal have been applied before carrying out the solvent
extraction. According to Canadian Patent 1 250 210, a solution
containing iron and zinc is treated in tcvo stages caith metallic
iron and zinc to reduce ferric to ferrous iron and cementation
of copper, arsenic, antimony and bismuth on the iron, followed
by precipitation of a sludge of tin, cadmium and lead in the
second stage treatment with zinc duet. After this two-stage
pre-treatment in an aqueous system, zinc chloride is extracted
with an organic liquid. The reduction stages are, therefore,
essentially separate t:rom the solvent extraction process. It
is noted that no metal is actually deposited onto the zinc
powder in the second stage.
Shibata Junji etal. reported that ferric iron can be stripped
from di-2-ethyl-hexylphosphoric acid (D2EFFPA) with mineral
acid and iron powder (Proc. Symp. Solvent >;xtr. 1986, 139-192).
Shibata et al. only disclose the stripping of ferric iron from
D2>;HPA with iron pocader. Shibata et al, do not disclose the
galvanic stripping with metals other than iron, or the
J
deposition of metals onto added metals, or stripping of metals
other than iron from organics other than D2EHPA.
Taking the above-mentioned teachings according to the Canadian
Patent and Shibata et al., one could not presume a priori that
the Shibata et al. method is operable with zinc pocader or
metals other than iron and zinc. Similarly, it could not be
presumed that the Shibata et al. method is operable with
organics other than D2Ef~PA, or that actual deposition of a
metal species dissolved in an organic liquid would occur in
the organic phase onto an added solid metal. It could also not
be presumed that addition of a solid metal to an organic phase
would make it possible to reduce metal ions other than ferric
ions in the organic phase From a higher to a lower state of
oxidation.
Summary of the :Invention
The present invention seeks to provide an excellent and simple
way to separate a coanted ration species from another one in an
organic liquid, the species being either a desired or an
undesired species. Thus, it has been found that many metal
rations can be readily reduced in a loaded organic liquid by
contacting loaded organic liquid with a suitable solid metal
or solid metal alloy causing a galvanic reaction. More
particularly, aqueous solutions containing metal rations, that
may include rations of both desired metal and impurity or
secondary metal, are treated with an organic liquid extractant
or solvent suitable for the extraction of rations of the
c", ~ ;1 a~ ';' ;'~ A
~~ '.~ .r .
r.r i; -. ~ .. ..
desired metal, rations of at least one secondary metal being
co-extracted. After phase disengagement, the loaded organic
phase containing either rations of a desired~metal or rations
of desired metal together with rations of secondary metal is
contacted with a solid metal or solid metal alloy capable of
reducing in the organic phase either rations of a desired metal
or rations of a secondary metal from a higher to a lower state
of oxidation. Depending on the extracted metals) and the
added solid metal or solid metal alloy, the rations of an
extracted metal are reduced to the lower state of oxidation and
are either deposited (cemented) in the metallic state onto the
solid metal or solid metal alloy, or are partially reduced in
the organic phase to a lower oxidation state, svhich is easily
stripped, with the solid metal or solid metal alloy being
oxidized in part. The method of galvanic stripping is carried
out at ambient pressures and at ambient or slightly elevated
temperatures. It .is understood that the term solid metal will
~e used hereinafter to denote and is hereby defined to include
Both solid metals and solid metal alloys.
galvanic stripping is v.~seEul For the removal of rations of a
esired metal, or cations~ of a secondary metal, From an organic
iquid into which it has been extracted from an aqueous
elution. Cations of a desired metal or rations of a secondary
atal may be galvanically stripped from an organic liquid by
position of such a metal onto an added solid .metal capable
reducing rations of such a metal to its metallic state, and
moving the added solid metal with deposited metal from the
~, .w. ~ .r3 ~,a .~~..
6
organic liquid. Galvanic stripping is also useful For the
partial reduction to a lower state of oxidation and for the
removal of rations of secondary metals from ari organic liquid.
Hy adding a suitable solid metal, rations of the secondary
metal are reduced to a lower state of oxidation and are removed
by stripping into an aqueous phase. Reduction and stripping
may be carried out selectively and separately in two steps or
simultaneously in one step. Where the organic phase contains
rations of both a desired metal and a secondary metal, and
rations of the secondary metal are only partially reduced to
a lower state of oxidation, the stripping of rations of the
desired metal may be carried out either before or after the
contacting with solid metal and the removal of catians of the
reduced secondary metal from the organic phase. The solid
metal is preferably used in particulate form. The use of a
solid metal reductant makes it possible to directly reduce
rations of a metal, especially rations of those metals that may
be difficult to transfer from the org,~nir phase in their normal
oxidation state, into the solid state or in a partially reduced
form (but usually still in cationic formy to allow easy
stripping. The method according to the invention eliminates
the use of high temperatures and/or pressures, and the need to
deal with stripping solutions that are not easily treated, or
axe chemically or environmentally undesirable.
Accordingly, it is an aspect of the present invention to
provide a method for the galvanic'stripping of metal ions from
loaded organic liquids,
CA 02040541 2000-08-21
It is another aspect to provide a method for the solvent
extraction of cations of metals wherein a loaded organic liquid
is treated with solid metal reductant, which may be a single
metal or an alloy.
It is still another aspect to provide a method for the solvent
extraction of cations of metals wherein cations of an extracted
metal are precipitated onto a solid metal reductant.
It is yet another aspect to provide a method for the solvent
extraction of cations of metals wherein cations of an extracted
metal are at least partly reduced.
It is yet a further aspect to provide a method for the solvent
extraction of cations of metals wherein cations of a metal are
simultaneously at least partly reduced in and stripped from the
organic phase.
These and other aspects of the method according to the
invention will become clear from the following detailed
description.
According to the main embodiment of the invention, there is
provided a method for the extraction of cations of at least
'?0 one metal from an aqueous solution with an organic liquid
capable of extracting canons of said at least one metal in a
higher state of oxidation from said solution, said aqueous
solution containing cations of metals chosen from the group
7
CA 02040541 2000-08-21
consisting of a)cations of a desired metal and b)cations of a
desired metal together with rations of at least one secondary
metal, rations of said at least one secondary metal being co
extracted from the aqueous solution by the organic liquid,
said method comprising the steps of:
(1) mixing said aqueous solution with said organic liquid for
the formation of an aqueous raffinate phase and an organic
liquid phase containing rations of said at least one metal in
a higher state of oxidation;
(2)separating said aqueous raffinate phase from said organic
liquid;
(3)contacting separated organic liquid with a solid metal
capable of reducing in said organic phase at least a portion
of said rations of said at least one metal from said higher
state of oxidation into a lower state of oxidation, said solid
metal being chosen from the group consisting of Zn, A1, Cu,
Cd, Mn, Mg, Fe and their alloys to provide organic liquid
having a reduced content of said canons of said at least one
metal;
(4)removing said organic liquid having a reduced content of
said rations of said at least one metal from said solid metal;
and
(5)returning organic liquid having a reduced content of said
rations of said at least one metal to said mixing of step (1);
wherein solid metal is defined as including both solid metals
and solid metal alloys.
According to a first preferred embodiment, there is provided
a method according to the main embodiment wherein said rations
of metals comprise both rations of a desired metal and rations
of at least one secondary metal, rations of said desired metal
and rations of said at least one secondary metal are extracted
into said organic phase. rations of said desired metal are
stripped from separated organic phase with a stripping solution
8
9 ~ ;1 .? F~; ", ~
~, i; ::: C9 ;_1 i'.
capable of stripping rations of said desired metal frorn
separated organic phase prior to said contacting with solid
metal while substantially leaving rations of~'said at least one
secondary metal in said organic phase, and rations of said at
least one secondary metal are reduced to said lower state of
oxidation in said contacting.
According to a second preferred embodiment, there is provided
a method according to the main embodiment, wherein said rations
of metals comprise rations of. a desired metal and rations of
at least one secondary metal, rations of said desired metal and
rations of said at least one secondary metal are extracted into
said organic phase, rations of said at least one secondary
metal are reduced to a lower state of oxidation in said
contacting while substantially leaving rations of said desired
metal in said organic phase to provide organic liquid having
a reduced content of rations of secondary metal and having left
rations of the desired metal herein, and stripping rations of
said desired metal from said organic liquid having a reduced
content of rations of secondary metal with a stripping solution
capable of stripping rations of said desired metal from said
organic liquid prior to returning liquid having a reduced
content of rations of secondary metal to said mixing of step
(1).
According to a third preferred embodiment there is provided a
method according to the main embodiment, wherein rations of
metal comprise rations of a desired metal, and wherein rations
lu a .? ~~ ;.~ ~~~.
of said desired metal are reduced to said locaer state of
oxidation and are deposited onto said solid metal, and said
solid metal with deposited desired metal is removed from said
organic liquid.
Detailed Description
Solvent extraction or liquid-liquid extraction is a versatile
method for the selective separation of metals. The separation
involves the mass transfer of metal cations across boundary
interfaces between two contacting, insoluble phases, i.e. an
organic phase and an aqueous phase.
The degree of extraction is influenced by the selectivity among
various canons in the solution and the pkk. In many cases, the
process functions very well using straight-forward, reversible
loading and stripping. In other cases reactions are less
easily attained by simple shifts in chemical equilibria, and
it is in these cases that the galvanic stripping rnethod of the
present invention finds particular application. Although the
method of the present invention is applicable in many instances
as an alternative to ,or even preferable over hydrogen
reduction, or gaseous or hydrolytic stripping, the galvanic
stripping of this invention is particularly valuable far the
recovery of certain desired cations and for the removal of
co-extracted cations of impurity, unwanted or secondary metals,
especially iron.
11
In the galvanic stripping according to the invention, an added
solid metal reluctant provides an electrochemical driving force
to alter the oxidation state or, in general, modify the
equilibrium of inorganic ions dissolved in organic liquids.
These alterations are important because certain separations
or recoveries, that are not normally attainable using a
standard chemical driving force or technique, become possible
using electrochemical reactions. The following simplified
equations represent two possible reactions:
( R-) nMln+(org) + xM2(S) -___> x ( R_ )mM2m+(org) .t. M1(5) ( 1 )
( R ) nMln+(org) + xM2(S) -___~ ( R-) n_mxMl(n-xm)t(org) + x ( R )mM2m+(org)
(2)
wherein (R-) is the organic and M~ and MZ are different metals
or their complexes in the organic pt7ase. equation (1) shows
the reduction of.the M~ ion to metal M~~S) by added solid metal
MZ(S). This reduction is similar to but by no means the same as
the displacement (or cementation) reactions commonly
encountered in aqueous chemical processes. In equation (2),
the M1 ion is only partially reduced by solid metal M2~5) to a
lower oxidation state, altering the equilibrium and allowing
easier stripping into an aqueous phase. In both cases, .M2
remains in the organic phase as an oxidized stable species.
The suitability of M2 as a reactant for Ml in such a system
then requires that a) added solid metal MZ is capable of
forming a stable R-M2 species, and b) Lhe potential of the half
cell Mz/R-M2 is less noble than that of M1/R-M~. These
conditions are necessary, but not necessarily sufficient, to
6w ~; .~ ? ~-.. -
f.~ ':A i~ ,'.' '',: ~IR.
12
insure reaction, As is often the case for similar reactions
occurring in an aqueous solution, the magnitude of the
differences in potentials as well as the. reaction kinetics
must be considered. Generally, the added solid metal must
be capable of reducing a metal extracted into an organic liquid
to a lower state of oxidation.
The method is carried out using the conventional process steps
in solvent extraction. Aqueous solution containing one or more
dissolved metal species, in the form of rations or, in some
cases, complexed as anions, is mixed with an amount of an
organic liquid capable of extracting rations or complexed
anions of a desired metal. The organic liquid is usually
premixed with a diluant, a modifier may be added, and the
organic liquid may be equilibrated ar conditioned prior to
mixing with aqueous solution. Aqueous solution and organic
liquid are mixed to form a loaded organic raffinate phase and
an aqueous phase, which are subsequently separated, If
desired, the loaded organic phase rnay be scrubbed with a scrub
solution. Conventionally, a scrub raffinate is separated and
loaded (scrubbed) organic phase is stripped with a strip
solution to form a strip liquor and a stripped organic liquid.
Cations of the desired metal are recovered from the separated
strip liquor and the stripped organic liquid is regenerated,
purified and recycled to the extraction.
To strip rations of a metal galvanically, cahether rations of
a desired metal or rations of a secondary metal, from loaded
13 ~A ~ ,~ Ei' ;~.~ ::3. ~'.
organic phase, an amount of suitable solid metal is added to
the loaded organic phase, after separation from the aqueous
raffinate phase. The suitable salid metal must be capable of
reducing cations of the metal to a lower oxidation state,
either to its elemental Form, i.e. the metallic state, or to
a partly lowered oxidation state. The amount of added solid
metal should be at least stoichiometric to accomplish the
reactions, but is added preferably in excess. of the
stoichiometrical amount required to effect the reduction.
The solid metal may be added in the form of sheets or coupons
but is preferably added in a particulate form such as chips,
pellets, granules or powders. Although coarse reductant
effects the reduction, small particle sizes increase the rate
and efficiency considerably. A broad range of particle sizes
may be used, such as in the range of From about 94 to 6000
microns. The particle sizes are preferably in the range of
from about 9A to 600 microns. 'fhe CE:duCtiOn may be carried out
in an oxidizing, neutral or reducing atmosphere to suit the
needs of the desired reaction. The reduction is preferably
carried out in the absence of oxygen, as, in the case of some
metals, oxygen (air) tends to re-oxidize cations of the reduced
metal and to lower the effic3.ency. If desired, the reduction
may be carried out in the presence of nitrogen, cvhich is
essential in some cases to avoid re-oxidation.
The galvanic stripping is carried out at ambient pressures and
at ambient temperatures. As the reduction is temperature
14 ft ~ t ~ :' " ,~ -~:
~, .::~,'?.:
dependent, the rate of reduction may be increased by using
elevated temperatures. Preferably, slightly elevated
temperatures such as, for example, up to about 60°C may be
used. The temperature may, therefore, be in the range of from
ambient to about 60°C. Efficiency is increased with good
mixing during the contacting and stripping steps.
The loaded organic phase .is contacted with the solid metal for
a period of time sufficent to reduce at least a portion of
rations of the metal in the organic phase. Contact times may
be in the range from about 1 to 90 minutes, preferably about
15 to 60 minutes. The contacting may be carried out
continuously or intermittently in a column loaded with
reductant by passing the loaded organic phase through the
column. Alternatively, the contacting may be carried out by
mixing reductant with the loaded organic phase in a suitable
vessel provided with agitation.
In the galvanic stripping with a suitable solid metal, whereby
rations of the metal extracted into an organic liquid are
reduced to the metallic state and deposited onto the added
solid metal, rations of certain metals (extracted as rations
of desired metal or rations of secondary metal) can be
deposited onto suitably selected added solid metals from
certain solvents. Cations of more than one secondary metal
may be ca-extracted, and rations of at least one of the co-
extracted secondary metals may be reduced to a lower state of
oxidation and deposited in the metallic state onto the solid
~3 ~. '~ 'Y,
f.~ , . ..: Y,f .~~ '~%: .,..
metal. The reduction and deposition are also dependent on
the type of anion originating from the aqueous feed
solution fed to the solvent extraction process. The
following Table I lists extracted metal cations that were
extracted from aqueous feed solution with a suitable
organic liquid, and were reduced to elemental form in the
organic phase from either a sulfate, chloride or cyanide
anion system by solid metals.
TABLE T
Organic Extracted Solid
Liauid Anion Cation Metal
D2EHPA sulfate Cu2+ An,Cd,iron
sulfate Ag+ Al,Zn,Cu
sulfate Co2-t- Zn*, Mn*
chloride Sn4+ Zn*
chloride Pb2+ Zn
AliquatTM sulfate Cu2-H Zn,Al,Cd
336 chloride Cu2+ Zn,Al,Cd
sulfate Ni2+ Zn*,iron*
cyanide Au3+ Zn
LIXmM622 chloride Cu2+ Zn
LTXrM864 chloride Cu2-E Zn, iron
* Nitrogen must be present
wherein D2EHPA is di-2-ethylhexylphosphoric acid, AliquatTM
336 is tri- (CeClo) methylammonium chloride, LIXTM 622 is a
mixture of LIXTM 860 (5-dodecylsalicylaldoxime) with tride-
canol, LIXTM 864 is a mixture of LIX.rM 64N, which is 1 vol
LIXTM 63 (5, 8-diethyl-7-hydroxy-6-dodecanone oxime) in
LIXTr~ 65N (2-hydroxyl-5-nonylbenzophenone oxime), and LIXTM
860, and iron denotes low carbon steel or electrolytic
iron.
:.' ~ ,, ~'~ ,.>ui .7
16
As, unfortunately, the majority of common organic extractants
also remove undesirable cations,i.e. rations of secondary
metals, from an aqueous solution in addition to the rations
of the desired metal, the present method is also useful to
remove rations of such secondary metals from the organic phase.
For example, a reaction that may occur in the loading of the
extractant with iron as secondary metal in the form of ferric
ions from a sulfate solution may be represented by the
following equation:
Fe3+(aq) + 3IIR(org) -___> Fe R3(or9) + 3IiF(a~) ( 3 )
The extraction frorn a chloride solution, using a tertiary
amine, may be represented by the equation:
Fe3~(a~l) v HCl(aq) -H 3C1-(aO) 't NR3(or9) -__~ FeCl,~-fINR3(or~) ( 9 )
Thus, when aqueous solution containing rations of desired metal
and rations of iron as secondary metal is contacted with an
organic liquid extractant, the resulting organic phase
contains, in addition to rations of the desired metal, the
above-noted ferric complexes which are strongly stable. After
separation of the loaded organic phase From the aqueous
raffinate phase, the loaded organic phase is contacted with a
suitable solid metal that is capable of reducing the ferric
to the ferrous form. Suitable solid metals are selected t:rom
the group consisting of zinc, manganese and magnesium, the use
of Zinc being preferred. Certain alloys, such as, for example,
low carbon steel or zinc containing a small amount of lead
(e. g. 0.2~) may also be used. Small amounts of certain
17 f ~.' ::i~ i ' ~''.
alloying metals favorably affect the reactivity of the so
activated zinc. The use of activated zinc is most preferred.
Ca n ons of the desired metal may be stripped from the organic
phase with a stripping solution capable of stripping cations
of the desired metal from the organic phase, either before or
after the contacting caith solid metal, and removing cations of
the secondary metal in its partially reduced state of
oxidation.
For example, the galvanic stripping reactions of iron with the
preferred solid metal, i.e. activated zinc, may be represented
by the equations (5), and (6) and (7):
FeR3(or9) + 3E1+(~~~) -H 1/2 Zn° __~ re2~(a~~) (-h 1/2 Zn2~(a~) ) * -f
3HR(or9)
(5)
and
FeR3(or9) v 1/2 Zn° ____~ 1/2 ZnRZ(°r~) v FeRZ(°r~)
(
the organic complexes reacting with hydrogen ions (acid) as
follows
1/2 ZnR2(°r9) + FeR2(°r9) + 3Ht(°r9)
_____> 3HR(org) ( + 1/2 ZnZ+(~~) ) * + Fe2+(~~) ( 7 )
*Zinc may remain in the organic. phase if solution is dilute.
Equation (5) illustrates the reduction and stripping being
accomplished simultaneously in one step, and equations (6) and
(7) sequentially in two separate steps.
The contacting may be carried out as described above and with
similar conditions of time, temperature, pressure and particle
~
.n ~ , ,~ "~ . .
f 1 L;:
a E ~ ,
sizes to reduce at least a portion of cations of the secondary
metal in the organic phase from a higher to a partially reduced
lower state of oxidation, i.e. ferric to ferrous ion. The
organic liquid may be one of a number of extractants into which
iron is co- extracted such as, for example, phosphoric acids
such as di-2-ethylhexylphosphoric acid (D2EHPA),
mono-2-ethylhexylphosphoric acid (M2EHPA) and mixtures thereof,
phosphonics such as the mono-2-ethylhexyl ester of 2-
ethylhexylphosphonic acid (PC88A, tradename), phosphinics such
as bis (2,4,4-trimethylpentyl) phosphinic acid (CyanexjM272),
and (LIX 69N) which is a mixture of LIX 65N (2-hydroxy-5-
nonylbenzophenone oxime) and 1~ LIX 63 (5, B -diethyl-7-
hydroxy-6-dodecanone oxime).
In the two steps of first reducing ferric to ferrous and then
stripping the ferrous, the loaded organic phase is contacted
with particulate zinc under the appropriate conditions, as
described. After contacting, the solution is stripped with an
acid such as sulfuric or hydrochloric acid. For example, the
strength of the acid may range from 20 to 100 g/L H2S04 or 30
to 100 g/L HC1, dependent on the organic. For example, the
reduction of ferric in the loaded organic phase with zinc
powder then allows stripping with a sulfuric acid solution at
a value of the pFE of about 3. At this pH value the zinc
remains in the organic phase, providing a selective separation
from the iron. At a pH below about 3.5, no iron or zinc
hydroxides precipitate. Using HC1 as stripping acid, the pH
should also have a value of below about 3.5.
19 ,.
In tile Olle-Step galvanic stripping, zinc metal and acid
stripping solution are both added to the loaded organic phase.
After the desired contact time under the desired conditions,
as described, the phases are separated, the ferrous iron
reporting to the aqueous phase. If desired; using the
appropriate conditions, a portion or virtually all of the iron
can be removed from the organic phase using either the one-step
or the two-step method. It is noted that the reduction
substantially takes place in the organic phase, as ferric ions
do not readily strip into the aqueous phase.
According to the embodiment wherein an aqueous solution
contains rations of a desired metal together with rations of
a secondary metal, the method is carried out as follows. The
aqueous solution is mixed with an organic liquid capable of
extracting rations of the desired metal and co-extracting
rations of the secondary metal to form an aqueous phase and a
loaded organic phase containing both rations of the desired
metal and rations of the secondary metal. The secondary metal
is substantially present in a higher oxidation state. The
loaded organic phase is separated from the aqueous raffinate
phase. Cations of the desired metal are then stripped, if
desired after scrubbing, from the loaded organic phase with a
stripping solution capable of stripping rations of the desired
metal from the organic phase while substantially leaving
rations of the secondary metal in the higher oxidation state
in the organic phase. The organic liquid is subsequently
it :'z i~ J ~'.3 ~ ':
contacted, batch-wise, continuously or intermittently, with
a solid metal for the reduction of at least a portion of
rations of the secondary metal from the~:higher state of
oxidation to a partially reduced state of oxidation. Cations
of the secondary metal in the partially reduced state of
oxidation are stripped from the organic liquid caith a stripping
solution capable of stripping rations of partially reduced
secondary metal from the organic liquid with the formation of.
a regenerated organic liquid having a reduced content of
rations of the secondary metal. 'Phe regenerated organic liquid
is returned to the extraction step. Alternatively, rations of
the secondary metal in the higher state of oxidation in the
separated loaded organic phase are first partially reduced and
stripped from the organic liquid without substantially reducing
or stripping rations of the desired metal in the organic, and
rations of the desired metal are then stripped from the organic
phase, leaving a regenerated organic liquid with a reduced
content of rations of secondary metal for return to the mixing
step for extraction of metal.
This embodiment may, for example, be applied to a solution
containing indium as desired metal and iron as secondary metal,
and using commercial grade di-2-ethylhexylphosphoric acid,
which contains mono-2-ethylhexylphosphoric acid, dissolved in
kerosene. Cations of both metals are substantially extracted
into the organic liquid. The indium is stripped from the
loaded organic phase with dilute hydrochloric acid (1-3 normal)
substantially leaving the iron in the organic phase. The
iv ~ b ~! ~~~
21
organic phase is then contacted, as described, with zinc having
particle sizes from about 49 to 6000 microns, preferably about
79 to 150 microns, in the presence of 'nitrogen and at
temperatures of from ambient to about 60°C. After allowing
adequate contact time and a following solids-liquid
separation, the ferrous iron is stripped from the organic phase
with a sulfuric acid stripping solution containing 20 to 100
g/L sulfuric acid. The organic liquid with a reduced iron
content is returned to the extraction. As explained above, the
reduction and the stripping of ferrous iron may be carried out
simultaneously in one step or separately in two steps.
Although the partial reduction of a metal ration to a lower
state of oxidation has been described mostly with reference
to iron, this embodiment may also be used for the partial
reduction of other multivalent metal rations such as Ce, Mn
and Cr. 'Phe method of the invention may be used in a number
of applications. ~Phe method is suitable for the selective
removal of rations of a metal from an aqueous solution by
solvent extraction, the metal ration being of a desired metal
or of a secondary metal; For the removal of rations of a metal
from an organic liquid that is difficult to remove in its
normally occurring oxidation state; for the removal of rations
of one or more co-extracted secondary metals from an organic
liquid being used for the recovery of a desired metal; or for
the coating of a metal on the added solid metal as a substrate.
In these and modified embodiments, solvent-solid metal
22
r,'~ '.a~'~3~
combinations especially designed to effect such applications
may be used. ,
The invention will now be illustrated by the following non-
limitative examples.
Example 1
This example illustrates that the galvanic stripping of certain
metal rations from certain organic liquids can be carried out
with suitably selected solid metals. In a number of tests, an
organic liquid was loaded with a metal ration extracted From
an aqueous solution containing rations of the metal. The
loaded organic phase was contacted with an excess of solid
metal in particulate form for 30 minutes. Some tests were
carried out in the presence of nitrogen. The solid metal was
removed from the organic liquid. Examination of the removed
solid metal with a scanning electron microscope showed that
rations of the metal had been deposited on the surface of the
added solid metal, the rations of the metal having been reduced
to the metallic state. The rations of the metal extracted Prom
aqueous solutions, the organic liquids, the anion of the
aqueous system and the added solid metal are tabulated in Table
II.
23 sy ~ ~~ n -~ ,y .~
fd i. ''.~ 'v° .~ ~f~. .~~.
TABLE II
Organic Extracted Solid
Liquid*** Anion Cation Metal
D2EHPA sulfate Cu2+ ~ Zn,Cd,iron**
sulfate Ag+ Al,Zn,Cu
sulfate Co2+ Zn*,Mn*
chloride Sn4+ Zn*
chloride Pb2+ Zn
Aliquat 336 sulfate Cu2-~ Zn,Al,Cd
chloride Cu2+ Zn,Al,Cd
sulfate Ni2+ Zn*,iron**
cyanide Au3+ Zn
LIX 622 chloride Cu2-f Zn
LIX 864 chloride Cu2+ Zn,iron**
* Nitrogen must be present
** iron was low carbon steel or electrolytic iron
*** For definitions see TABLE I.
Example 2
This example illustrates that the galvanic stripping can be
carried out in two stages to achieve 100% iron stripping and
removal. An organic liquid loaded with 1.0 g/L iron was mixed
with zinc granules for 30 minutes under nitrogen. Metallic
zinc was then removed, and the resultipg organic phase was
stripped with 10 g/L sulfuric acid for five minutes. Analysis
showed that complete reduction and stripping of iron was
achieved.
Example 3
To examine the effects of reoxidation of the ferrous iron, the
nitrogen was removed from above the organic and aqueous strip
solutions obtained in Example 2. The two phases were allowed
24
.: _. ,
to mix in a vessel open to the air. After 30 minutes, it coas
Found that 20~ o~ the stripped iron was re-extracted back into
the organic phase.
Exams 1p a 4
A number of tests were done to determine the effect of using
nitrogen, an open or a closed reaction vessel, and a one-step
or two-step galvanic stripping. Nitrogen was either used or
not used; when used, nitrogen was bubbled through the liquid
phases, or introduced over the liquid surface. The organic
loading was 5 g/L re, the ratio of aqueous to organic phase
was 1:1, the strip solution was 100 g/L H2S04, the temperature
was ambient and nitrogen (when used) was admitted for 10
minutes. The percentage of iron stripped was determined. The
test conditions and results are given in Table III.
TABLE TII
Test Nitrogen Vessel Zn Metal Steps Fe Stripped
No. Used Used Used No. $
1 none closed none 1 15
2 none open yes 2 38
3 bubbled closed yes 2 63
4. bubbled closed yes 1 7g
over closed yes 1 80
Tt can be seen from the results that mere acid stripping
removed only 15~ of the iron from the organic phase, that the
presence of nitrogen is necessary, and that galvanic stripping
in one step appears to give better results.
b~,, ~'1 .! n ~ ~~ J~ .d
~'.1 :ti ..; ~.., :, ~ ".. .v,
In the following Examples 5 to 7, a leach solution containing
ferric sulfate and zinc sulfate was mixed: with a solvent
consisting of 20 volume percent D2EHPA dissolved in kerosene.
The extraction was done in stages with the pH controlled at a
value of 2.6 to prevent the precipitation of iron or zinc
hydroxides. The volumes of leach solution and solvent were
equal. The organic phase was loaded to metal concentrations
of 1 to 7 g/L iron and 0 to 13 g/L zinc. After settling, the
loaded organic phase was separated from the aqueous raffinate
phase and subjected to galvanic stripping.
The galvanic stripping was carried out in one step with the
addition of 99.0 pure zinc granules having particle sizes of
1000 to 2000 microns and a total surface area in the range
between 15 and 35 cm2. The galvanic stripping was carried out
at temperatures between 20°C and 60°C with the addition of from
2 to 7 g of zinc granules which is in excess of the amount
stoichiometrically required For the reduction of the ferric
present to ferrous, and in a solution containing from 20 to 100
g/L sulfuric acid.
The loaded organic phase was heated to the desired temperature,
sparged with nitrogen for five minutes and zinc and preheated
sulfuric acid solution were added. The mixture was then
agitated in a closed vessel for 30 minutes. The desired number
of samples were taken and analyzed.
nj J, ~ ,, a ~;~ ": '
26
Example 5
Using the procedure described above, eight tests were done to
determine the percentage of iron stripped into the aqueous
phase. The values of the variables and. the results are shown
in Table IV.
TABLE IV
Total re ( ionic)Zn2~ L'e
'PestH2S04 Zn Area Temp Loading Loading Stripped
No. g/L cm2 C g/L g/L $
1 60 15 60 1 0 99
2 60 35 60 5 8 100
3 60 35 20 1 0 86
4 20 15 60 5 0 46
20 35 60 1 8 100
6 60 15 20 5 8 32
7 20 35 20 5 0 84
8 20 15 2U 1 8 68
The results indicate that the galvanic stripping of iron is
strongly dependent on the surFace area of the added zinc metal
and the temperature and, to a lesser c9egree, on the iron
loading.
Example 6
Using the procedure described above, eight tests were done to
determine the percentage iron stripped with varying zinc
loadings, temperatures and zinc metal. areas while maintaining
the iron loading constant at 1.1 g/L. The values of the
variables and percentage iron stripped are given in Table V.
TABLE V
Total
Test Zn Loading Zn Area Temperature Fe Stripped
No g/L cm2 C
.
1 13 15 30 61
2 0 35 50 100
3 13 35 30 88
4 0 1S 30 79
13 15 50 93
6 0 35 30 96
7 13 35 50 100
8 0 15 50 100
The results show that for a given iron loading, increasing the
zinc metal surface area resulted in increased amounts of iron
stripped. Hy comparing the results given in Tab:Les IV and V,
it can be seen that the effect of temperature remains but that
the effect of higher zinc metal area decreased in the presence
of higher zinc loading in the organic. The effect of higher
zinc loadings is more pronounced at the lower temperature.
Example 7
This example illustrates the effects on the percentage of iron
stripped and the iron stripping rate of the iron loading of
the loaded organic and the surface area of the added zinc
metal. Four tests were carried o~.it using the procedure
described above. Samples were taken after 5, 10, 15, 20 and
30 minutes and the iron stripped and stripping rates
determined. The results are given in Table VI.
f;.~ ~.: " Z j' ~ j ; ' .~~.
28
TABLL VI
Total Stripping
Test Pe Loading Zn Surface Time I~'e StrippedRate
No. g/L cmz min, o L
g/ / min.
1 1.1 15' S 13 538
10 30 806
15 46 699
20 59 538
30 74 323
2 1.2 35 5 92 323
10 70 806
15 78 8os
20 83 B60
30 96 538
3 6.8 15 5 7 2096
10 15 2258
15 25 2527
20 34 2419
30 46 1667
9 6.7 35 Gi 13 3441
10 2B 3925
1.i 37 3763
20 50 3495
3(1 84 4570
It follows from the data presented in bitable VT that for a given
iron loading, increasing the zinc metal surface area resulted
in an increase of the amount of iron stripped. At low initial
iron loading and high zinc area, a decline in removal rate
occurs with increasing time. The stripping rate is increased
virtually proportionally with increasing zinc surface area.
example 8
This example illustrates that iron can be effectively removed
in a one-step galvanic stripping from different organic phases
using different solid metals added in particulate form. The
ratio of organic to aqueous phase was 1:1, and the mixing time
was 50 minutes. Test data are presented in Table VTI.
z9
TAHLE VII
Iron
,
Organic Fe3+ loaded HzSO~ Removed
in vol.% in m L Metal added in L in m
L
D2EHPA 760 2.5g Zn 20 750
in kerosene
5 D2EHPA 760 3.5g Fe 20 495
in kerosene
LIX 69N 985 None 10 0.4
15 LIX 64N 985 Cu 10 371
Example 9
This example illustrates that lead can be effectively removed
by galvanic stripping. Lead loaded to 1.5 g/L in 5 vol.%
D2EHPA in 30 mL of organic phase was contacted with 2.5 g of
metallic zinc. Samples were taken from the organic at various
times, and the samples were stripped with 80 g/L HC1. The
amount of lead removed was determined. Examination of the zinc
after the tests with a scanning electron microscope showed that
metallic lead had deposited on the zinc surface. Test results
are given in Table VIII.
TABLE VIII
Pb Removed
Time ______________
in minutes in mg in %
10 519 ' 34
30 855 57
60 963 64
120 1380 92
b~ W
i ~ ~ ~'n ~
30 ...
hxample 10
This example illustrates that an organic liquid loaded with
indium and iron can be selectively stripped of indium, and the
iron remaining in the liquid can then be at least partly
stripped of iron using added solid metal to yield a regenerated
organic liquid with a reduced iron content. In a
countercurrent solvent extraction process, a feed solution
containing iron and 0.94 g/L indium, and an organic extractant
containing one volume $ M21;I~PA, three volume ~ D2l;FiPA, and two
volume ~ TBP in tcerosene were used. A loaded organic phase
was obtained that contained 0.89 g/L indium and 0.52 g/L iron.
The indium was stripped from the loaded organic liquid with 3N
hydrochloric acid. The organic liquid cvas washed faith sodium
sulfate solution to remove chloride and Found to contain <0.003
g/L indium and 0.51 g/L iron. 'Phe washed organic liquid was
split in two portions. The first portion caas treated with 100
g/L sulfuric acid solution in a 1:;1 volume ratio, with 10 g
activated zinc dust,(0.2'k Pb) per litre of organic liquid, at
ambient temperature, and with the addition of nitrogen. All
iron was removed From the organic liquid aFter 15 minutes. The
second portion was similarly treated with 10 g activated zinc
dust (0.2~ Pb) per litre of organic liquid but with the
addition o~ 150 g/L return acid (obtained Erom a zinc
electrowinning process) in a ratio of organic to acid solution
of 30:1. Substantially all iron had been removed after 30
minutes.
~: ; ! '~ p !.,.
3 a f, i,. . ~ ,, ' r1
Example 11
This example illustrates. that iron can be at least partly
stripped from an organic phase in a one-step process by
continuous circulation of organic phase mixed with stripping
solution through a column filled with a solid metal. From a
vessel containing a mixture of 6.5 L of an organic phase
consisting of 4$ EHP,A, 2$ THP and 99$ ExxsolTM D80 by volume,
and 6.5 L of a regenerated raffinate containing sodium sulfate
and 60 g/L sulfuric acid, 0.5 L/min of the rnixture was
continuously circulated through a column containing zinc
granules. The column was 2 m high with a diameter of 1.9 crn,
and was filled with 860 g zinc (0.2$ Pb) granules, the void
volume being 0.47 L. The temperature was ambient. During the
test, nitrogen was sparged into the vessel. Samples were taken
from the vessel and from the column effluent at 15 minute
intervals, and the iron concentration in the solvent in the
aqueous phases of each sample was determined. The results are
given in Table TX.
TABLE IX
Iron Concentration
in q/L
Time solvent con tinuous aqueous continuous
in minutes vessel column vessel column
0 076 0.68
1 0.52 0.55
15 0.69 0.60
16 0.46 0.45
30 0.54 0.52
31 0.36 0.40
45 0.43 0.45
46 0.27 0.30
60 0.34 0.37
61 0.20 0.22
75 0.26 0.29
76 0.15 0.17
b/Z f1 ~ 5' v
~f ;'~
32 ~ - ~; : ... _;
>Jxample 12
This example illustrates that four-valent cerium (Ce'~+) can
be effectively galvanically reduced to the two-valent state
(Ce3r) in solvent extraction and subsequent stripping.
An aqueous nitric acid solution containing 15 g/L Ce'~+ as
ammonium cerium nitrate ( (NHq)2 Ce (N03)6 ) caas mixed with
99~ tributylphosphate (THP) in an aqueous to organic volume
ratio of 1:1. After 10 minutes, the loaded organic phase
was separated From the aqueous phase. The loaded organic
phase was then mixed with zinc either as pieces or as
powder having particle sizes of 74 to 150 microns in an
amount of 1 gram per 1.0 ml of organic phase and an amount
of a dilute acid containing 10 g/h H2SO,t or HN03 added
either together with the addition of zinc or after the
galvanic reduction was completed. The galvanic reduction
was carried out for times ranging from 30 seconds to 30
minutes. The reduction and stripping were carried out in
a closed vessel under a flow of nitrogen. During the
reduction the colour of the solution changes from dark
orange to colourless. The Ce concentrations in the
solutions were determined with x-ray fluorescence, and
additional analyses were made using atomic absorption
spectometry. All cerium analyses have been transformed
into g/L Ce. A series of tests were made including a
comparative non-reductive stripping. The test data and
analytical results were as follows.
n1 r~ v, ,~ .~.
33 h, ;_, ~ !. r ~ s ~:~ '~
Test 1: Comparative nonreductive stripping
The loaded organic phase contained 14.8 g/L Ce, and was
split in two equal portions. One portion was mixed for 30
minutes with an equal volume of a 10 g/L HN03 solution.
The resulting aqueous solution contained 1.1 g/L Ce, and
the stripped organic contained 13.7 g/L Ce for a stripping
efficiency of only 8~. The other portion was mixed for 30
minutes with an equal volume of a 10 g/L Fi2S0,~ solution.
The resulting aqueous solution contained 2.5 g/L Ce, and
the stripped organic contained 12.3 g/L Ce for a stripping
efficiency o.E 17~ into the aqueous phase.
Test 2: One-stage galvanic reduction and separate
stripping
The loaded organic phase containing 19.6 9/L Ce was mixed
with 3 g zinc powder for 30 minutes. After removal of
zinc, the reduced organic phase was split in two equal
portions which were stripped of cerium as in Test 1. After
the nitric acid strip, the aqueous phase contained 7.0 g/L
Ce and the stripped organic phase contained 7.6 g/L Ce for
a 98~ stripping efficiency. For the sulfuric acid
stripping, these figures were, respectively, 11.1 g/L Ce,
3.5 g/L Ce and 76~.
Test 3: Simultaneous one-stage galvanic reduction and
stripping
The loaded organic phase containing 19.1 g/L Ce was
separated in two equal portions. One portion was mixed for
u5 ~ :' rt~ ':w l~ 'i
34
seconds with an equal volume of a 10 g/L HN03 solution
and 1 g Zn powder. The resulting aqueous phase contained
9.4 g/L Ce and the stripped organic phase contained 9.7 g/L
Ce for a 62~ efficiency. The second portion was treated in
the same manner but mixed for 30 minutes. The stripping
efficiency was 60~.
Test 4: Simultaneous multi-stage galvanic reduction and
stripping
The loaded organic phase containing 14.6 g/L Ce was mixed
in a first stage with a volume of a 10 g/L HN03 solution
and zinc pieces with a total surface area of 5.6 cm2.
Mixing was continued until the solution was colourless.
After separation, the aqueous phase was found to contain
12.2 g/L Ce for stripping efficiency of B3~. The separated
organic phase was then treated in a second stage under the
same condition for twice as long a period. The separated
aqueous phase contained 2.05 g/h Ce and the stripped
organic phase contained 0.35 g/L Ce for a stripping
efficiency of 85~. The cumulative efficiency was 97$.
Test 5: Multistage stripping using simultaneous and
separate stages
A first portion of a loaded organic phase containing 14.5
g/L Ce was mixed caith an equal volume of a 10 g/L HN03
solution and zinc pieces with a total surface area of 5.2
cm2 until the solution had turned colourless. After
separation, the aqueous phase contained 12.2 g/L Ce, and
35
the organic phase was mixed in a second stripping stage
with an equal volume of a 10 g/L HN03 solution for twice as
long but no zinc was present. Separated aqueous phase
contained 2.0 g/L Ce and separated twice-stripped organic
phase contained 0.3 g/L Ce.
A second portion of the same loaded organic phase was mixed
with zinc pieces (6.B cm2) until colourless. The reduced
organic phase was then mixed for 10 minutes with an equal
volume of a 10 g/L HN03 solution. After separation, the
aqueous phase contained 12.5 g/L Ce, and the stripped
organic phase containing 2.1 g/L Ce was again treated with
zinc pieces (4.2 cm2) and subsequently mixed for 10 minutes
with an equal volume of a 10 g/L HN03 solution. After
separation the final aqueous phase contained 2.1 g/L Ce and
the twice-reduced and stripped organic contained 0.9 g/L
Ce. It follows from the results of these tests that Cerium
is very difficult to strip with acid solutions in the four
valent state, viz. the low stripping efficiencies of Test
1, but that after a reduction of Ce'~~ to Ce3+ with a solid
metal reductant (zinc), the stripping with dilute acid is
almost quantitative.
It is understood that variations and modifications may be
made in the embodiments of the invention without departing
from the scope and purview of the claims.
