Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to recovery of dissolved
species from aqueous solutions thereof, employing a
so-called "liquid membrane" technique.
"Liquid membrane" techniques were pioneered by
Norman L. Li and his co-workers and have been described
in a series of patent specifications including United
States Patent Specification No. ~,779,907. This dis-
closes a process for removal of a dissolved species
by contacting the solution with a "water-in-oil" emulsion
comprising an aqueous interior phase surrounded by a
surfactant-containing hydrophobic exterior phase, the
exterior phase being permeable to the dissolved species
and the interior phase comprising a reactant capable of
converting the dissolved species to a non-permeable
species. It is said that in this way the dissolved
species permeates the exterior phase and is converted
to a non-permeable species in the interior phase.
Following such contact the emulsion is then separated
from the aqueous solution, which is now depleted in the
dissolved species, and the emulsion may thereafter be
regenerated. One method of regeneration of the emulsion
is to break the emulsion, this being mentioned at column
7 line 29. However no method of breaking the emulsion
is mentioned in United States Patent Specification
No. 3,779,907-
~ .rr ~ .... , _ _ , _ ~_, ~ _ ~ _ __ _,_ _ ,_,~__ ~ _ ,, _, ~_ __ _, _ ~ __,~_ _ _ _ _, _, _ _ _ _ _ , _ ,
ll~{;~Q94
- 2 -
In the aforementioned United States Patent Specifi-
cation No. 3,779,907 it is suggested that copper can be
separated from a~ueous solution by using a water-in-oil
emulsion comprising an aromatic or olefinic solvent as
the exterior phase (column 3 lines 29 to 33). One method
of converting the ions into the desired non-permeable
form in the interior phase is to precipitate copper ions
with sulphide ions (column 5 lines 1 to 6). A variant of
this process involves maintenance of a pH differential
between the interior phase and the aqueous solution,
whereby the pH at the aqueous solution - exterior phase
interface promotes the solubility of the ions in the
ion exchange compound-containing exterior phase, and the
pH at the exterior phase - interior phase interface
promotes the desorption of the ions from the exterior
phase and subsequent solubilising in the interior phase
(column 5 lines 12 to 21). Amongst the surfactants
suggested for use is Span 80 which is said to be a fatty
acid ester of anhydro sorbitol condensed with ethylene
oxide (column 5 lines 63 to 65).
The use of a liquid surfactant membrane in the de-
salination of water is disclosed by Li and co-workers
in United States Patent Specification No. 3,454,489. In
this process the salt solution is emulsified with a hydro-
2~ phobic surfactant solution so that the salt solution to
~lV~Q9
-- 3 --
be treated forms the interior phase of an emulsion which
is then contacted with a washing solvent. Water permeates
through the surfactant membrane from the salt solution
into the washing solvent. The emulsion, whose interior
phase is now depleted in water, is separated from the
washing solvent and passed to a demulsifier, which can
take the form of an electrostatic precipitator. The
process is said to be applicable to desalination of
seawater "as well as extraction of ore or minerals from
thelr aqueous solutions" (column 5 lines 34 to 36).
Other patents in the name of Li and his co-workers
include United States Patent Specification Nos. 3,389,078,
3,410,794, 3,617,546, 3,637,488, 3,650,091, 3,696,028,
3,719,590, 3,733,776, 3,740,315, 3,740,329, 3,897,308,
3,942,527, 3,959,173, 3~969,265, and 4,014,785, as well
as U.S. reissue patents Nos. 27888 and 28002.
Papers by Li and his co-workers on the subject
of liquid membranes have also appeared in the literature;
see, for example, Ind. ~ng. Process Des. Develop., Vol. 10,
No. 2, 1971, pp 215-221 and Separation Science, 9 (6),
pp 505-519, 1974.
Also in the name of Li et al is United States Patent
Specification No. 4,001,109. This describes a process for
dem~lsi~ying an emulsion whi&h can be either of the water-
in-oil or oil-in-water type. This process is said to be
. , , . _, _
94
particularly useful for breaking emulsions in li~uid
membrane emulsion systems which are defined in the above-
mentioned United States Patent Specification No. ~,779,907.
The emulsions used in the ~arious liquid membrane processes
are said to be difficult to break. Li and his co-workers
go on to state (column 2 line 63 to column 3 line 23):
"The liquid membrane water treating process to function
effectively requires a water-in-oil emulsion wherein the
oil maintains its integrity as the continuous phase of the
emulsion under various conditions of heat, pressure and
agitation in order to function as a membrane. Thus the
emulsions useful in this process are designed to be
especially stable. The formation of stable liquid membrane
emulsions is an art in itself and the difficulties and
solutions to said difficulties may be found in U.S. Pat.
No. 3,779,907.... These emulsions, while suitable for
liquid membrane processes because of their stability are
to a great extent problematic when it comes to the breaking
thereof to separate the oil and water phases for separate
reclamation of the components present therein. Prior art
emulsion breaking processes which include heating,
polyvalent flocculating salts, electrostatic precipitation,
centrifuging, are completely unsuitable for breaking these
emulsions." Thus the teaching in the art is that the
emulsions used in li~uld membrane processes such as des-
~lO~Q94
-- 5 --
cribed in United States Patent Specification No. 3,779,907cannot be broken by electrostatic precipitation.
In a liquid membrane process the emulsion is
separated from the aqueous feed solution and must then be
broken in order to recover the interior phase. In U.S.
Patent No. 4001109 Li and his co-workers teach that
emulsions for use in liquid membrane techniques, such as
are described in U.S. Patent No. 3779907, must be formulated
to be especially stable but that this means that breaking of
the emulsion becomes problematic. The solution that is
therefore proposed involves addition of a mixture of
solvents which in turn means that such solvents must be
recovered~ e.g. by distillation, from the separated phases
of the emulsion. Such a proposal is thus relatively
complicated and, since distillation is i~volved, it requires
a considerable energy consumption.
There is thus a need for a liquid membrane process
in which emulsion breaking can be simply effected without
addition of further solvents or other chemicals to the
2~ system and without the expenditure ot excessive amounts of
energy.
. . . .
It has now surprisingly been found that, if the emul-
sion is suitably tailored, it is possible to utilise
successfully a liquid membrane process in the extraction
f dissolved species from aqueous solutions thereof and to
)94
recover such species by a process which involves breaking
the emulsion by means of electrostatic precipitation.
, . . .
- It has also surprisingly been found that, by makinE
the emulsion so as to produce a relatively small droplet
size of interior phase and a relatively narrow distribution
of droplet sizes, the stability of the emulsion during
contact with the aqueous feed solution is improved without
the need to add excessive amounts of surfactant or to add
viscosity-increasing additives and at the same time the
kinetics of mass transfer through the liquid membran~ are
significantly lmp~oved.
According to the present invention there is provided
a process for recovery of a dissolved species from an
aqueous solution thereof which comprises: contac+ing the
aqueous solution with an emulsion, which emulsion comprises
droplets of an aqueous interior phase surrounded by a hydro-
phobic exterior phase which is immiscible with the aqueous
solution and is permeable to said species, the aqueous
interior phase comprising a component capable of rendering
. _ _ _ ~ . . .. . ..
said species non-permeable, and the droplet size of the
interior phase in the emulsion lying in the range of from
about 0.3 to about 10 micrometres with the ma~ority of the
droplets lying in the range of from about 0.8 to about 3
micrometres, whereby s id species permeates the exterior
phase and is rendered non-permeable in the i.nterior phase,
1~0~094
separating emulsion from the aqueous solution now
depleted in said species, passing separated emulsion to
an electrostatic coalescence zone in which an electro-
static field is maintained, whereby coalescence of drop-
lets of the aqueous interior phase is promoted, andrecovering from the electrostatic coalescence zone
coalesced interior phase containing said species.
The in~ention further provides a continuous process for
the recovery of a dissolved species from an aqueous feed
solution thereof comprising supplying to an emulsification
zone a hydrophobic exterior phase which is immiscible with
the aqueous solution and is permeable to said species and
an aqueous interior phase which comprises a component adapted
to promote the desorption of said species from the exterior
phase into the interior phase, mixing the interior and
exterior phases in said emulsification zone so as to form
therein a water-in-oil type emulsion having an interior
phase droplet size in the range of from about 0.3 micrometres
up to about 10 micrometres with the majority of the droplets
lying in the range of from about 0.8 to about 3 micro-
metres, contacting the aqueous solution with the water-in-
oil type emulsion in a contact zone, whereby said species
permeates the exterior phase and is rendered non-permeable
in the interior phase, separating emulsion from the aqueous
-solution in a separation zone, the aqueous solution now
ll~Q094
- 8 -
being depleted in said species, passing separated emulsion
to an electrostatic coalescence zone in which an
electrostatic field corresponding to a voltage drop of at
least 1 kilovolt per centimetre is maintained, whereby
coalescence of droplets of the interior phase is promoted,
recovering from the electrostatic. coalescence zone
exterior phase and coalesced interior phase containing
said species, and recycling exterior phase to the emulsifica-
tion zone.
It will be seen that the invention utilises a li~uid
membrane process for the recovery of the dissolved species
from the aqueous feed solution, the internal phase
containing a component capable of rendering the species
non-permeable. Such a component can be a reagent that
reacts with the species to transform this in the interior
phase into a non permeable form. For example, if the
dissolved species comprises ammonia or an amine, the
interior phase may be an acidic solution having a pH less
than that of the aqueous solution, the ammonia or amine
reacting with the hydrogen ions to form ammonium or
substituted ammonium ions. If the dissolved species
comprises a metal ion, the interior phase may contain an
anion that forms a precipitate with the metal ion.
Alternatively the component capable of rendering the
species non-permeable can be one that promotes desorption
Q94
of the species from the exterior phase into the interior phase.
Thus, for example, if the dissolved species comprises a metal,
such as copper, and the feed solution is a cupric sulphate
solution, the interior phase can be an acidic solution
having a lower pH than that of the aqueous feed solution.
The hydrophobic exterior phase of the emulsion is
chosen to be permeable to the dissolved species and to
provide a water-in-oil type of emulsion that is
sufficiently stable to survive the contacting step
with the aqueous feed solution containing the dissolved
species essentially intact, but not so stable an emulsion
that it cannot readily be broken by electrost~tic coal-
escence in the presence of the electrostatic field main-
tained in the electrostatic coalescence zone. Thus extreme
stability of the emulsion is desirably avoided in the
process of the invention.
Generally speaking the hydrophobic exterior phase will
usually comprise an oil-soluble, water-insoluble surfactant
as emulsifier. Usually it will also comprise a transport
agent for the dissolved species. Additionally it may
comprise an inert hydrophobic solvent and one or more
further minor additives such as an accelerator for the
transport agent.
The process is applicable to any dissolved species
(solute) that can be rendered permeable in the hydrophobic
- -- -
1~ 94
-- 10 --
water-immiscible exterior phase and can be rendered non-
permeable in the aqueous interior phase. Preferably the
species is ionic. Thus the species may be any ionic
species that is recoverable by ion exchan~e processes.
The species may be anionic or cationic in nature.
As examples of cationic species that can be recovered
using the process of the present invention, there can be
mentioned silver, cadmium, chromium,lead, tin, mercury,
copper, calcium, zinc, uranium, cobalt, iron and nickel. The
process can be used in the removal of chromium ions from
.
cooling water, and for the removal of iron from clay
slurries, for example. The process is of particular
importance in the recovery of copper from solutions
thereof.
Using the process of the invention, copper can be
reco~ered from a wide variety of copper-containing liquors.
The process is, however, of particular advantage in the
recovery o~ copper from ore leach liquors and from vat
leach liquors as well as from raffinates from solvent
extraction plants and other waste liquors. Typically,
copper-containing ore leach liquors are generated by
contacting a copper-containing ore with an acid solution,
for example a solution of dilute sulphuric acid, or with
an ammoniacal solution, for example an ammoniacal solution
of ammonlum carbonate. Such copper-containing ore leach
ll~OQ94
liquors may contain from about 4 parts per million or less
up to about 3000 parts per million or more of copper
(correspond~ng to about 10 parts per million of copper
sulphate or less up to about 7500 parts per million or
more of copper sulphate, in the case of a copper-containing
sulphuric acid leach liquor). Vat leach liquors may
contain for example up to about ~0,0~0 parts per million
of copper (or up to about 75,000 parts per million of
copper sulphate, in the case of a sulphuric acid-containing
vat leach liquor). Ammonium salt solutions, such as
ammonium carbonatescan alternatively be used as the leach-
ing solution in place of sulphuric acid ~n either case.
As examples of anionic species that can be recovered
using the proc~ss of the present invention there can be
mentioned cyanide, nitrate, sulphate, chloride ions and
complex metal-containing anions (such as ~U02(S04)3~" ",
CuC12', CuC13', ZnC13', FeC13' and FeC14'), and the like.
The process is applicable, for example, to removal of
cyanide ions from waste waters.
The surfactant must be an oil-soluble, water-
insoluble surfactant. Preferably the surfactant has an HLB
(hydrophilic-to-lyophilic balance) ratio in the range of fro~
about 1.75 up to about 7. HLB ratios can be calculated by
one of two methods. The first method is the so-called "group
contributions method" which is described by Davies in
... . .
11~0094
-- 12 --
"Proceedings of the Second International Congress on Surface
Acti~lty" Vol. 1, page 426 (1957). The HLB ratio of surfact-
ants can also be determined by an experimental method
described by Griffin (see J. Soc. of Cosmetic Chemists, Vol.
1, page 311 (1949) and Vol. 5, page (195~)). For most sur-
factants there is normally good agreement between the HLB
ratios determined by the group contributions method and by
the above-mentioned experimental method described by Griffin.
Preferably the HLB ratio of the surfactant lies in the
ran~e o~ from about 3.5 to about 6.
Typical oil-soluble surfactants that can be used in
the process of the invention include sorbitan monolaurate,
sorbitan mono-oleate, the sorbitol monoester sold under
the trade designation "Crill 4" by Croda Chemicals Limited,
and the materials sold under the trade designations Span
20, Span 80 and Tween. According to the manufacturers'
literature the Span materials are sorbitan fatty acid esters.
The material sold as Span 80 is a fatty acid ester of anhydro
sorbitol, whilst the material sold as Tween is an ethylene
oxide adduct of such an ester, it is believed.
Other oil-soluble surfactants having an HIB ratio
lying within the range of from 1.75 to about 7 include
sorbi,tan monopalmitate, glycerol monostearate and
propylene ~lycol monostearate. Sorbitan monopalmitate
has an HIB ~alue of about 6.7. Propylene glycol mono-
;
)094
stearate is somewhat anomalous giving an HlB ratio of
3.4 by the experimental method of Griffin and an HLB
- ratio o~ 1.8 determined by the group contributions
method.
The exterior phase of the emulsion usually comprises
a water-immiscible inert solvent, as well as one or more
oil-soluble surfactants. Typical water-immiscible inert
solvents include hydrocarbons, halogenated hydrocarbons,
ethers, higher oxygenated compounds such as alcohols,
ketones, and esters and the like. Preferably the solvent
comprises a hydrocarbon or a halogenated hydrocarbon.
Typical hydrocarbons include both aliphatic and aromatic
hydrocarbons which may be saturated or may contain one or
more unsaturated groups. Thus essentially any liquid
hydrocarbon solvent can be used thàt will dissolve the
surfactant and the transport agent. Amongst suitable
hydrocarbons there can be mentioned hexane, hexene, octane,
octene, cyclohexane, benzene, toluene, xylenes, mesitylene,
n-butyl benzene, and mixtures of two or more thereof, as
well as kerosene and the various co~mercially available
hydrocarbon mixtures produced for example by distillation
of petroleum. As typical of such solvents can be mentioned
t'Shell Solvent L" (an aliphatic non-polar hydrocarbon
mixture), "Shell Solvent T" (an aromat~c non-polar hydro-
carbon mixture), "Napoleum 470", "Esso Escaid", the refined
ll~OQ94
-- 14 --
isoparaffins sold by Exxon Chemical Company such as SolventNeutral 100, Solvent Neutral 150, Solvent Neutral 600 and
the various grades in between, and the like.
As examples of halogenated hydrocarbons there can
be mentioned "Arklone P" (i.e. trifluorotrichloroethane),
chloroform, carbon tetrachloride, fluorobenzene, chloro-
benzene, bromobenzene, o- and p-dichlorobenzene, hexa-
chloroethane, perchloroethylene, and trichloroethylene,
and the like, and mixtures thereof.
The solvent, of course, must be liquid at the con-
ditions at which the process is operated, and also must
be capable, in conjunction with a surfactant, of forming
a suitable water-in-oil emulsion with the interior phase.
The transport agent will be chosen to be suitable
for the nature of the dissolved species that it is desired
to recover from the aqueous solution. ~uch transport
agents are soluble in the water-immiscible exterior phase
and are capable of interaction with the dissolved species
at the aqueous solution-exterior phase interface to solubi-
lise the dissolved species and are added in order topromote the permeation of the species through the exterior
phase.
If it is desired to remove cations from an aqueous
~ solution, a suitable ion exchange material may advantage-
ously be incorporated in the exterior phase. Such an ion
. ~ . ~ ~
Q94
-- 15 --
exchange material will usually be a compound selected from
the group consisting of sulphonic acids, organic phos-
phonic acids, carboxylic acids, diketones and oximes.
Preferably such materials are selected from polyfunctional
sulphonic acids, polyfunctional carboxylic acids, poly-
functional organic phosphonic acids,oximes of a-hydroxy
aliphatic ketones, and oximes of orthohydroxy aromatic
ketones. Such ion exchange compounds generally have a
molecular weight of from about 200 to about 10,000 and
have a ratio of carbon atoms to functional groups of grea-
ter than 5. Examples of ion exchange compounds which are
useful for transfer of ions through the exterior phase
include sulphonated styrene co-polymers, petroleum
sulphonic acids, naphthenic acids, sulphonated phenyl
formaldehyde co-polymers, styrene maleic acid co-polymers,
styrene-acrylio acid co-polymers, and the like.
When it is desired to recover copper from an aaueous
solution thereof, there may be used any of the reagents
known to be suitable therefor-, for example the reagents
sold under the trade name LIX by General Mills Chemicals,
Inc. Typical ion exchange materials that can be used in
the recovery of copper from aqueous solutions thereof are
described in papers by R. Price and J. Tumilty and by
A.J. van der Zeeuw in "Hydrometallurgy", edited by
G.A. Davies and J.B. Scuffham and published by The
9 4
- 16 -
Institution of Chemical Engineers, being the proceedings
of a Symposium held at the University of Manchester,
Institute of Science and Techno]ogy on 2nd to 4th April,
1975. Such copper transport agents include the material
known as Shell SME ~29 copper extractant which is said to be
a hydrocarbon solution of 2-hydroxy-5-nonyl acetophenone
oxime. Also worthy of mention are LIX 34, LIX 63, LIX 64N,
LIX 65 and Acorga series 5000 agents, e.g. Acorga 5100.
In the recovery of dissolved anions from aqueous feed
solutions thereof the exterior phase will again usually
comprise an ion exchange material. Such ion exchange
materials can be, for example, compounds containing one
or more basic groups capable of solubilizing the anion in
the hydrophobic exterior phase at the aqueous feed
solution-exterior phase inter~ace`to form a ~ermeable
species in the exterior phase. Examples of suitable ion
exchange materials fsr use in recovery of anions include
water- msoluble a~ines and polyamines such as those dis-
closed in United States Patent Specification No. 3779907.
Certain phosphonium salts, e.g. alkyl triphenyl phosphonium
salts9 are soluble in water-immiscible solvents such as chloro-
form and can be considered for use as anion transport agents.
Generally speaking,simple ionic species, such as the
cupric ion (Cu++~, are insoluble in hydrophobic, water-
2~ i~miscible liquid media and are hence non-permeable in the
1100C~94
exterior phase. When the exterior phase contains a suitable
transport agent, such as Shell SME 529 copper extractant (des-
ignated for convenience as RH), reaction can take place
at the aqueous feed solution-exterior phase interface
according to the following equation:
Cu++ + 2RH ~ CuR2 + 2H
; The complex CuR2 is soluble in hydrocarbon solvents and
is thus permeable in the exterior phase. At the interior
phase-exterior phase interface the following reaction takes
place when the interior phase is acidic, e.g. a sulphuric
acid solution:
CuR2 + 2H+ ~ Cu++ + 2RH (2)
The cupricion dissolves in the interior phase and becomes
non-permeable with respect to the exterior phase again.
The overall effect of both reactions is the mass transfer
o~ copper from the aqueous feed solution through the
exterior phase into the interior phase and the mass
transfer of hydrogen ions from the interior phase through
the exterior phase to the aqueous feed solution. These
processes will continue while there exists a pH differ-
ential between the interior phase and the aqueous feed
solution and the interior phase has the lower pH value.
In the case of anion recovery, such as fluoride
reco~ery, analogous processes take place. In this case
the interior phase may comprise a source of anions, e.g.
llO~Q94
_ 18 -
FeC13, while the feed solution contains fluoride ions.
Whilst there is an appropriate ionic activity differential
between the interior phase and the feed solution, the pro-
cess will continue to transfer fluoride ions from the feed
solution to the interior phase.
The above-mentioned cation and anion mass transfer
processes will occur while there exists an appropriate
ionic activity differential, for example a pH differential
where cation mass transfer is involved, across the "liquid
membrane" and are substantially independent of the concen-
tration in either the interior phase or in the feed solution
of the species being recovered. Hence the process of the
invention is applicable both to dilute solutions, such
as raffinates and other waste waters containing only a few
parts per million of the dissolved species, and to more
cor.centrated solutions, such as depleted electrolysis
tank-house liquors. Thus, for example, the process can
be applied in copper extraction to recovery of copper from
a raffinate from a conventional solvent extraction plant
containing aboutlOOparts per million of copper, using as
the interior phase an acidic copper sulphate spent liquor
from the electrolysis tank-house containing (say) about
30 grams per litre of copper, in order to raise the
copper concentration of the interior phase to a level
suitable ~or recirculation to the electrolysis tanks,
ill)O094
-- 19 --
(say) about 50 grams per litre of copper. The process
- is therefore applicable to the recovery of dissolved
species from extremely dilute solutions, including
solutions which are not amenable to treatment by con-
ventional solvent extraction techniques, as well as tosolutions which are amenable to conventional solvent
extraction techniques.
In the formation of the emulsion the volume ratio
of the interior and exterior phases can vary within wide
limits. ~hus, for example, the interior phase may comprise
from about 20~ by volume of the emulsion up to about 80%
by volume of the emulsion. Generally the aqueous phase
comprises from about 20 to about 60% by volume of the
emulsion. The proportion of aqueous phase in the emulsion
is determined at least to some extent by the nature and
quantity of surfactant in the exterior phase, as well as
by the nature of the solvent and of the transport agent.
The interior phase/exterior phase volume ratio in the
emulsion determines at least to some extent the viscosity
of the emulsion. Usually the higher this ratio is, the
more viscous is the emulsion.
The emulsion must be sufficiently stable to prevent
widespread rupture, during the contacting of the emulsion
with the aqueous solution to be extracted, of the "liquid
membr~ne" formed by the coating of exterior phase on the
droplets o~ interior phase. If rupture of this "liquid
)Q94
- 20 -
membrane" occurs, bleeding of the components of the
interior phase into the aqueous solution occurs. This
can result in "bleeding back" into the aqueous solution
of already extracted ionic species and loss of the acid
or other components of the interior phase. However the
emulsion should not be so stable that it cannot readily
be broken by electrostatic coalescence.
The emulsion used in the process of the present
invention may be prepared by ~arious methods. Thus, for
example, in the extraction of copper from aqueous solutions
thereof, the hydrophobic solvent, the surfactant, and the
transport agent or agents are blended and the appropriate
volume of sulphuric acid solution is emulsified
using a homogeniser or similar mixing device. Suitable
mixing de~ices include high speed stirrers, colloid mills,
valve homogenisers~ ultrasonic generators and mixing jets.
In the case of copper recovery, the aqueous interior
phase may comprise a solution containing sulphuric acid.
The pH of the interior phase is maintained less than that
of the aqueous solution to be extracted. The pH of the
aqueous feed solution may lie in the range of from about
1 to about 12, such pH being maintained by addition of
suitable quantities of sulphuric acid or ammonia, for
example. If appreciable quantities of iron are present
then the pH of the aqueous feed solution is desirably
,
094
-- 21 --
maintained below about 2.5. The concentration of acid
in the interior phase may range from about 10 grams per
litre or less of sulphuric acid up to about 250 grams
per litre or more. However it is preferred to use as
the interior phase a sulphuric acid solution containing
not more than about 150 grams per litre. Concentrations
of sulphurlc acid in excess of about 250 grams per litre
tend to suffer from the disadvantage that osmosis through
the "liquid membrane" can occur leading to dilution of the
interior phase and swelling of the emulsion. Such swelling
of the emulsion is usually accompanied by an undesirable
increase in viscosity of the emulsion leading to increased
difficulty in stirring and in breaking of the emulsion
electrostatically.
~he quantity of surfactant i~ the emulsion may
similarly vary within wide limits. Typically it will con-
tain from about 0.01 to about 10% by weight of the surfact-
ant based upon the weight of the exterior phase. Preferably,
however, the exterior phase will contain at least 0.5% by
weight, and usually at least about 1% by weight, of sur-
factant based upon the weight of exterior phase up to
about 4/0 by weight. However the use of large amounts of
surfactant is to be avoided so as not to make the emulsion
so stable that it cannot be broken at an acceptable rate
by electrostatic coalescence. Qn additional reason for
:
llO~Q94
avoiding the use of large amounts of surfactant is that
in the contacting zone the surfactant will mainly be
present, it is believed, at the exterior phase - interior
phase and - feed solution interfaces (i.e. the inner ~nc
outer interfaces respectively) so that if too much sur-
factant is present it may hinder mass transfer across the
interfaces as, for example, by tending to block access of
the transport agent and complex to the outer and inner
interfaces respectively. Thus it will usually be preferred
to use from about 1 to about 2% by weight of surfactant
based up~n the weight of exterior phase.
The transport agent or agents may comprise from
about 1 to about 99.9% by weight of the exterior phase.
However it will usually be preferred to operate in the
range from about 1 to about 10% by weight of transport
agent based upon the weight of the exterior phase. Since
the transport agent is usually a relatively ex~ensive
chemical it is preferred to operate at the lowest possible
concentration of transport agent that gives acceptable
recovery of the dissolved species under the reaction
conditions employed and at short contact times.
The droplet size of the interior phase in the
emulsion u~ed in the process of the invention lies in
the range of from about 0.3 micrometres to about 10
micrometres, usually from about 0.5 to about ~
llOOQ94
micrometres, with the majority of the dr~plets lying in
the range of from about 0.8 to about 3 micrometres,
preferably in the range of from about 0.8 to about 1.5
micrometres. To achieve such droplet size it has been
found preferable to use a homogeniser or emulsifying
mill rather than a high speed turbine or similar mixer.
The emulsion and the aqueous solution to be treated
by the process of the invention are contacted in a con-
tacting zone. Such a contacting zone can be provided by
a static mixer through which the emulsion and the solution
to be treated are flowed. Alternatively the emulsion and
the solution can be mixed in a conventional mixer such
as is used in conventional solvent extraction plants.
~he contact time may vary within wide limits from about
1 second or less up to about 60 minutes or more.
Preferably however the residence time in the contacting
zone lies in the range of from about ~ seconds to about
25 minutes.
The optimum residence time in the contacting zone
will be determined at least in part by the nature of the
exterior phase, and in particular by the nature of any
transport agent present, and by the nature of the dissolved
species. This optimum period will be influenced by the
reaction kinetics involved in the mass transfer process
through the liquid membrane of the exterior phase, which
llOOQ94
are in turn influenced by the interior phase droplet size
and the effective thickness of the liquid membrane. Thus
the optimum residence time is dependent at least in part on
the method used for formation of the emulsion, and on
features such as the interior phase:exterior phase ~olume
ratio. It is of advantage to reduce the residence time as
far as possible in the contacting zone consistent with
efficient extraction of the desired dissolved species from
the feed solution. The longer this residence time is, the
larger the scale of the equipment must be that is needed for
handling a given quantity or flow rate of the feed solution.
Furthermore the longer the residence time is in the con-
tacting zone the more risk there is of rupture of the
liquid membrane of exterior phase and of loss of interior
phase into the feed solution. Not~only may such loss of
interior phase result in loss of sulphuric acid or any
other chemical added to the interior phase but it also
results in "bleed back" of the already extracted species
into the feed solution. ~oth factors reduce the overall
efficiency of the process. Thus, overall, short residence
times help to reduce the scale of the equipment and the
.,
associated running costs of the plantr
In the contacting zone the emulsion and aqueous
solution to be treated are mixed at a shear rate sufficient
to cause dispersion of the emulsion as "globules" in the
.. .. . ... ~ . . . . . .. .
ll(~OQ94
- 25 -
aqueous solution to be treated so as to form a dispersion
therein, but not so high a shear rate that extensive break-
down of the emulsion occurs. The size of the "globules"
will be dependent on the method and rate of shearing as
well as on the interior phase droplet size of the emulsion.
The "globule" slze may vary, for example, from about 100
micrometres or less up to a~out 10000 micrometres or more.
However, usually the "globules" will be from about 500
micrometres up to about 1500 micrometres in diameter under
suitable mixing and shearing conditions in the contacting
zone. Typically they are about 1000 micrometres in diameter.
Preferably the emulsion is so formulated and the conditions
in the contacting zone are so chosen that the breakdown of
emulsion is less than about 1% (and preferably less than
about 0.05%) at a residence time of about 18 minutes in the
contacting zone. Where the interior phase comprises zn
acidic solution, breakdown of the emulsion can be monitored
from measurements of the amount of the ionic species
transferred to the interior phase and the decrease in pH
of the aqueous solution being extracted after disengagement
of the emulsion from the aqueous solution.
The volume ratio of feed solution to emulsion in the
contacting zone can vary within wide limits, for example
from about 1:50 or less up to about 50:1 or more. Ho~ever
it will usually be preferred to operate at a feed solution:
emulsion volume ratlo of from about 1:10 to about 10:1, for
,
l~OOQ94
-- 26 --
example about 5:1.
From the contacting zone the dispersion of emulsion
in the aqueous solution is passed to a phase disengagement
zone. This may take the form of a conventional gravity-
settler.
~ ere the hydrophobic exterior phase comprises a
hydrocarbon, the emulsion may float on the disengaged
aqueous raffinate. However, if the hydrophobic exterior
phase comprises a solvent having a specific gravity greater
than 1, for example a chlorinated hydrocarbon having a
specific gravity greater than 1, the emulsion may be heavier
than the aqueous raffinate.
From the disengagementzone the emulsion is passed to
an electrostatic coalescence zone in which the emulsion is
sub~ected to an electrostatic field, thereby to cause
"breaking" of the emulsion and agglomeration of the droplets
of interior phase. Preferably the voltage gradient in the
electrostatic coalescence zone exceeds at least about 1
kilovolt per centimetre. The ~oltage gradient may be as
high as about 7.5 kilovolts per centimetre or more.
Usually voltage gradients of the order of about 3 to about
5 kilovolts per centimetre suffice. Conveniently the
electrostatic field is provided by imposing a high A.C.
voltage across the emulsion. However D.C. voltages can be
used if desired. One suitable form of apparatus comprises
llOOQ94
-- 27 --
a pair of substantially horizontal parallel plate elec-
trodes. Of this pair of electrodes one may be connected
to earth and will usually lie in the coalesced interior
phase. The other non-grounded, high voltage electrode of
the pair will usually be so positioned that it does not lie
in the coalesced interior phase. Thus where the exterior
phase is lighter than the interior phase the high voltage
electrode is the upper electrode and is positioned in the
coqlesced exterior phase. ~en the exterior phase is
heavier than the interior phase the lower electrode is made
the high voltage electrode and is again positioned in the
coalesced exterior phase.
In the electrostatic coalescence zone rapid
"breaking" of the emulsion can be achie~ed when the emulsion
is suitably tailored. Using parallel plate electrodes the
coalescence rate for commercial operation should desirably
be at least 0.1 U.S. gallons per square foot of the grounded
electrode per minute ~assuming a phase ratio of about 1:1
in the emulsion), and preferably at least 1 U.S. gallon
per square foot of the grounded electrode per minute, at a
voltage gradient of about 7.5 kilovolts per centimetre. If
coalescence is slower than desired, some adjustment may be
desirable to the composition of the exterior phase (e.g. a
lower surfactant concentration should be used). ~eduction
of the interior phase/exterior phase volume ratio of the
_ . _ _ . , , . . _ .. .. _ _ . . .. . . . .
94
- 28 -
emulsion may be of assistance in increasing the rate of
coalescence in the electrostatic field. This can be
achieved by recycling exterior phase to the electrostatic
coalescence zone.
Another suitable form of apparatus for providlng
the electrostatic coalescence ~one comprises a vertical`
portion of tube with an axial high voltage A.C. or D.C.
electrode and with a grounded electrode electrically
insulated ~rom the emulsion If the tube itself is made
of non-conductive material~ e.g. glass, the grounded
electrode can comprise a wire wound helically around the
tube or a conductive layer or sheet wrapped round the tube.
The small droplet size (i.e. about 0.3 to about
10 micrometres, preferably about 0.5 to about 5 micro-
metres~ of the interior phase in the emulsion and thenarrow range of droplet sizes (i.e. the majority of the
droplets lying in the range of about 0.8 micrometres to
about 3 micrometres, preferably in the range of about
0.8 micrometres to about 1.5 micrometres) are important
features o~ the present invention and have a~ extremely
beneficial effect on the kinetics of mass transfer of
the desired species from the aqueous feed solution
through the liquid membrane into the interior phase
-- 29 --
and also on the stability of the emulsion. Since the
droplet size of the interior phase in the emulsion is
uniformly small, the surface area of the exterior phase -
interior phase interface is extremely large. The
surfactant in the exterior phase preferentially migrates
to and is adsorbed at any interface between the hydrophobic
exterior phase and an aqueous phase. Thus in the contact
zone, in which the exterior phase - interior phase emulsion
is contacted with the aqueous solution containing the
species to be recovered, the effective "concentration" of
the surfactant at the interior phase - exterior phase inter-
face and at the exterior phase - aqueous solution interface
is reduced, compared with an emulsion with larger interior
phase droplets, for a gi~en initial concentration of
surfactant in the bulk exterior phase. This effective
reduction of the "concentration" of surfactant adsorbed at
. . _ .,
the interfaces means that the molecules of the transport
agent can more readily reach the interfaces for acceptance
of, or release of, the species to be transported across the
liauid membrane into or from the interior phase. Since
there is less blocking at the interfaces by surfactant the
transfer of the species across the interfaces to and from
the transport agent is facilitated, ~rhich means in turn tha+
the kinetics of the reaction are favourably affected.
_, . . . ..
- 30 ~
; Furthermore the small interior phase droplet size
means that, compared with an emulsion having the same
phase ratio but larger droplets, the e~fective membrane
thickness is reduced. Since the distance across which
;the species to be recovered has to be transported is reduced,
the speed of transfer is increased, leading once again to a
favourable ef~ect on the reaction kinetics.
! Since the kinetics of transfer are improved by use
o~ small interior phase droplets, the residence time of the
emulsion in the contact zone can be reduced. By appropriate
choice of transport agent, sur~actant, sur~actant level and
the like ~actors, it is possible in many cases to e~fect
~e~ficient recovery o~ the desired species ~rom the aqueous
~eed solution thereo~ using con-tact times of 20 minutes or
less. In ~avourable cases contact times of only 1 or 2
minutes are sufficient. Because the residence time o~ the
emulsion in the contact zone is short, it is not necessary
to take any special measures to increase the emulsion
., . .. , . _ . _ .. .
~- stability, e.g. by addition of viscosity increasing
additives to the exterior phase~ Indeed the use o~
viscosity increasing additives is to be avoided in the
pre~erred practice of the invention since such additives
tend to hinder the rapid transfer of the species through the
liquid membrane.
'' ' ,~ .
-
~10{)094
-- 31 --
A further advantage of the use of the uniformly
small droplet size of the dispersed interior phase of the
emulsion that is adopted accordin~ to the invention is that,
because residence time of the emulsion in the contact zone
can be reduced, a corresponding reduction in emulsion swell
due to osmosis can be achie~ed.
Using a liquid membrane system formulated for
extraction of copper,experiments have been carried out to
demonstrate the superiority of emulsions produced using a
high speed homogeniser compared with an axial flow turbine
operating at~top speed of 7~0 cm/s. Apart from the method
of emulsification, all other features of the membrane system
were identical. Use of the homogeniser produced droplets
of interior phase of from about 0.3 to about 5 micrometres
with the majority of the droplets lying in the range of
from about 0.8 to 1.5 micrometres; the axial flow turbine
produced some small sized interior phase droplets of the
order of 1 micrometre in diameter but a much broader
distribution o~ droplet sizes with many droplets of larger
size)of the order of 10 to 20 micrometres in diameter.
The resulting emulsions were contacted under identical
conditions with a feed solution containing 300 ppm copper
as copper sulphate at a pH of 1.5 - 2Ø The interior
phase was a solution of sulphuric acid and the exterior phase
contained surfactant an~ a copper transport agent. The
llOQ~94
-- 32 _
results are shown in Figures 1 and 2 o~ the acconpa~yi~-
drawings.
In Figure 2 the curves labelled (a) and (~) p'ot the
copper concentration in the feed solution (shown on the
left hand y-axis) against time (shown on the x-axis) for
the emulsions prepared with the axial flow turbine mixer ar~
with the homogeniser respectively. As can be seen fro~
these curves the extraction efficiency of the homogenised
emulsion is significantly improved. More striking however,
is the improvement in emulsion breakdown characteristics,
as shown by the straight lines (a) and (b) in Fi~ure 2,
Again line (a) illustrates the performance of the emulsion
produced with the axial ~low turbine mixer whilst line (b)
illustrates the performance of the homo~enised em~lsion.
After 18 minutes' extraction the percentage breakdown of
the emulsion produced with the axial flow turbine is more
than 30 times more than the percentage emulsion breakdo~Jn
of the homogenised emulsion. This demonstrates well the
superiority of the small droplet size and the narrow dis~~r, u-
tion OL droplet sizes used in the present invention.
The invention utilises electrostatic coalescence~or breaking the emulsion. This is advantageous co~pa-ed
with other techniques or emulsion breaking such as the
addition c~ solvents (as described in ~.S. Patent ~o.
.,
Q94
-- 33 --
~I~OOllo9) and the use ~ centri~uges; not only is the elec'ro-
static technique simple and economical to operate, but
also there is no need to add solvents or other chemicals
which must be separated from the coalesced phases before
these can be recycled to the emulsification zone. Furthcr-
more the equipment is simple and does not require such a
large capital outlay as would be required for an equivalent
number of centrifuges and the consumption of power is small.
l~en using emulsions with droplets of interior phase
larger than the preferred size, e.g. when using emulsioIls
with droplets of interior phase in the range of from about
10 micrometres up to about 20 micrometres in diameter, the
power required for electrostatic coalescence tends to
fluctuate, thus leading to decreased efficiency. However,
it has been found that emulsions with interior phase drop-
lets in the preferred size range of from about 0.5 micro-
metres to about 5 micrometres undergo electrostatic
coalescence smoothly and efficiently with a small power
consumption. Such emulsions have droplets of a relatively
uniform size; thus, for example, emulsions have been
prepared with an average droplet size of about 1 micrometre
with the majority of all droplets lying within the range o-
~from about 0.8 to about 1.5 micr~metres and only a VeI~'
small fraction of droplets lying outside this range.
llOQQ94
Close control of the shear rate during emulsifi-
cation and of the composition of the exterior phase (e.g.
surfactant concentration) is therefore desirable in order
to produce an emulsion havin~ a droplet size in the preferred
range of from about 0.5 micrometres to about 5 micrometres.
~y and large it is best to avoid paddle mixers and similar
relatively inefficient mixers and to use a homogeniser or
similar efficient mixing device.
The coalesced interior phase from the electrostatic
coalescence zone contains, under appropriate circumstances,
an enhanced content of the dissolved species compared with
the aqueous feed solution, whilst the disengaged aqueous
raffinate is substantially completely depleted in the
dissolved species. Thus, for example, in experiments
using feed solutions containing from about 4 to about
1200 parts per million of copper calculated as copper,
it is possible to recover a coalesced interior phase
having a copper concentration of from about 2,500 to
about 25,000 parts per million of copper or more. Such
solutio~ can be directly used for recove~y of copper by
electrolysis using conventional electrolysis tank-house
techniques.
In a con~ntional sol~ent extraction technique the
aqueous feed solution is contacted with the liquid "ion
exchanger"(for example a LIX reagent) in a conventional
. _ .
11~0094
mixer-settler at a volume ratio of from about 1:1 to
about 1:2 followed by separation of the phases in the
settler part of the mixer-settler and stripping of the
organic phase, again at a volume ratio of from about 1:1
to about 1:2, in a second mixer-settler followed by
settling of the phases and separation of the strip liquor
now loaded with the desired ionic species. This con-
ventional process requires a large inventory of solvent
and ion exchanger and necessitates the use of two mixer-
settlers. In each mixer-settler the concentrations of the
desired species in the organic and aqueous phases are
equilibrium concentrations. Several extraction and strip
stages may be required in order to maximise recovery of
the desired species.
When using the process of the present invention
the volume ratio of the hydrophobic exterior phase to the
aqueous feed solution to be treated may be chosen such
that the reagent:ra~finate ratio lies in the range of from
about 1:250 to about 1:4000. Thus in order to treat the same
volume of feed solution a much smaller inventory of solvent
and ion exchange material is required using such a process ir
accordance with the present invention compared with the con-
ventional solvent extraction techniques.
The process of the present invention may be operated
at any temperature at which the emulsion and the aqueous
.
11~aO94
-- 36 _
feed solution are fluid and stable; conveniently, ambierlt t~m-
peratures areused.-The pressure must likewise be sufficient
to maintain the fluidity of the various phases; conveniently,
amb$ent pressures are used.
The process of the invention may be carried out
batchwise, but is preferably carried out on a continuous basis.
Si~gle or multi-stage operation may be used as appropriate.
Figure 3 of the accompanying drawings illustrate~
diagrammatically a flow sheet incorporating the process
of the invention.
Referring to Figure 3, make-up emulsifier is supplied
by line 1 to holding tank 2. Make-up solvent and make-up
transport agent are supplied via line 3 to holding tank 2.
A recycled mixture of emulsifier, solvent and transport
1~ agent is fed to holding tank 2 by line 4.
Make-up acid is fed ~ia line 5 to holding tank 6,
whilst recycled acid is returned to holding tank 6 via
line 7.
The mixture of emulsifier, solvent and transport
agent is fed from holding tank 2 via line 8 to homogeniser
mill 9 to which is also fed acid via line 10. In this
way the desired water-in-oil emulsion is formed in which
the droplet size of the interior phase of the emulsion lies
in the range of from about 0.3 micrometre to about 10
micrometres, with the ma~or~ty of the droplets lying in
the range of from about 0.8 to about 1.5 m$crometre-~.
llO~Q94
-- 37 --
The aqueous solution to be treated, for example à
copperoreleach liquor,is fed via line 11 to holding tank
12 and thence via line 13 to a mixing compartment 14 of
a conventional mixer-settler 15. Emulsion from homogeniser
mill 9 is also fed to mixing compartment 14 via line 16.
The flow rates of aqueous feed solution through line 13
and of emulsion through line 16 are adjusted to give the
desired residence time in the mixing compartment 14 of mixer-
settler 15 and the desired emulsion:feed solution ratio.
From mixing compartment 14 the resulting dispersion
consisting of globules of emulsion dispersed in a continuous
aqueous phase is fed to settling compartment 17 of mixer-
settler 15. Bulk separation of the phases occurs in settlin~
compartment 17 and the aqueous phase is discharged via line
18. This raffinate can be recycled to the leaching site
after addition of the appropriate amount of make-up acid.
The globules of emuision in the dispersion agglomerat~
and are discharged from settling compartment 17 via line 19,
which leads to an electrostatic coalescer 20. This contai~s
a number of electrodes 21 which are connected to a bus-bar
22 which is maintained at a high positive potential.
Reference numeral 23 illustrates an earthed ground electrode.
In the electrostatic coalescer the droplets of interior
phase coalesce and the coalesced interior phase is withdra~
via line 24 whilst the exterior phase is withdrawn via line
llOOQ94
-- 38 --
4 and returned to holding tank 2 for recycling.
The coalesced interior phase in line 24 then passes to a
conventional electrolysis cell 25 where electrowinning is
carried out in a conventional manner. Spent liquor dis-
charged from cell 25 is recycled to holding tank 6 via line7.
As illustrated the plant is arranged to operate with
a solvent, for example a hydrocarbon solvent which produces
an emulsion that is lighter than water. It is, however,
possible to modify the plant of the drawing so as to permit
operation of the plant with a solvent that produces an
emulsion that is heavier than water, for example a chlori-
nated hydrocarbon solvent such as "Arklone P". In this
case the emulsion is withdrawn via line 18 and the aqueous
raffinate via line 19, line 18 being connected to the
electrostatic coalescer 20 which is itself in~erted so that
the electrodes 21 are in the organic phase rather than in
the conducting coalesced interior phase.
The in~ention is further illustrated in the following
Examples.
Exam~le 1
(a) Synthesis of the emulsion.
A solution was prepared in Napoleum 470 containing
1% by weight of Span 20, a co~mercial name for sorbitan
monolaurate, and 10% v/v Shell SME 529. 20 ccs of this
1100~94
-- 39 --
exterior phase were transferred to a wetted 100 cc beaker.
While stirring at a low speed with an "Ultra-Turràx" homo-
geniser, 30 ccs of 0.375 M H2S04 solution were added
dropwise from a burette. This transfer took about 1~
minutes, after which the homogeniser stirrin~ qpeed was
increased to maximum and the mixture was left to emulsify
for 8 minutes. The resulting water-in-oil emulsion had an
average interior phase droplet size of about 1 micrometre.
Very few droplets were less than 0.8 micrometres or more
than 1.5 micrometres in diameter and virtually none were
larger than 5 micrometres in diameter.
(b) Contact of the emulsion with the aqueous feed
solution.
250 ccs of an aqueous feed solution containing 120
parts per million cupric ions as copper sulphate were
transferred to a wetted 500 cc beaker fitted with baffles.
While stirring with a turbine at 300 r.p.m. the homogenised
emulsion was poured into the stirred aqueous copper-
containing solution and a stopwatch started. The beaker
containing emulsion was allowed to drain into the copper-
containing feed solution for 30 seconds. The stirrer motor
was fitted with a stroboscope and its speed was measured
using a photoelectric cell connected to a portable tacho-
meter. Control of the rotational speed of the turbine
110~094
-- 40 _
was achieved using a ~ariable rheostat. The turbine had
an overall diameter approximately one half of the internal
diameter of the beaker and was located coaxially with thc
b~aker so that the turbine was approxim~tely at the inter-
face b~tween the emulsion ~nd the feed solutlon whenstirring was stopped and the emulsion allowed to separate
under gravity. After stirring for 20 minutes the liquid
mixture was trans~erred to a separating funnel and allow~
to separate under gravity. The raf~inate, which now
contained about 1 part per million of copper, was
discarded.
(c) Breakdown of emulsion.
The emulsion was run into a cylindrical glass con-
tainer having an internal diameter of about 5 centimetres.
This container was arranged with its axis vertical and
was fitted with an axial electrode ~n the form of a copper
wire. The outside of the cylindrical conta~ner was wound
with an earthed copper wire. Upon rai&~-~g tl~e po~e~
of the central electrode to 2Q~000 volts A.C. rapid
coalescence o~ the emulsion occurred, resulting in
complete separation of the phases within about 30 seconds.
The measured current was about 0.3 milliamps. The voltage
gradient was about 7.2 kilovolts per centimetre. The
copper concentration of the coalesced aqueous interior
phase was about 1,000 parts per million in cupric ion.
.
~1~0Q94
-- 41 --
The amount of emulsion breakdown occurring dùring
the contacting step was estimated from pH measurements
and from the amount of copper transferred from the feed
solution to the interior phase of the emulsion to be less
than about 0.05% under the reaction conditions employed.
Example 2
The procedure of Example 1 is repeated using
1.5 M H2S04 solution in place of the 0.375 N H2S04 solution
and an aqueous solution containing 3600 parts per million
of copper ion as copper sulphate as the aqueous feed
solution. The copper concentration of the resulting
coalesced aqueous interior phase is about 30,000 parts
per million. This coalesced aqueous interior phase can
be electrolysed in a conventional manner according to
conventional electrowinning techniques to recover copper
metal at the cathode.
