Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS FOR RECOVERY
OF CYANIDE AND METALS
Field of the Invention
The invention relates to processes and apparatus for recovery of
cyanide and/or rnetals. The processes and apparatus may involve
electrochemical and membrane processes. An electrochemical cell useful in
such processes forms a further aspect of the invention.
Backgiround to the Invention
Cyanide is used extensively in gold processing plants for the leaching
of gold from a milled ore containing it. Certain ores, concentrates and
oxidation residues also contain copper and/or other base or precious metals
that, like gold and silver, form complexes with cyanide. As a result, mixtures
of dissolved complexes, including Cu(CN)2 , Cu(CN)32~ and Cu(CN)43~ may also
be formed in a gold cyanide leach solution.
When this occurs, large excesses of cyanide may be required to ensure
that sufficient "free" cyanide is present to leach the gold. This results in
high
cyanide consumption, which is costly. Moreover, the complexed species are
not useful for the further leaching of gold, and cannot be readily recycled to
the leach. In certain situations, such complexes cannot be discharged to a
tailings dam or alternative disposal system due to prohibitions under
environmental regulations.
As cyanide is a potent contaminant in waste streams, particularly those
associated with gold processing operations, technology has been developed
for managing cyanide.
Oxidation may be used to destroy cyanide species known as weak acid
dissociable cyanide from waste waters. Such oxidation techniques commonly
involve use of an oxidant to destroy cyanide. Hydrogen peroxide has been
widely used for this purpose with weak acid dissociable cyanide being
oxidised to cyanide. Any metal ions released during the oxidation are
precipitated, in an alkaline environment, as hydroxide. However, ferricyanide
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is not destroyed during oxidation. As well as destruction of cyanide, such a
process is not suitable for destruction of the thiocyanate ion (SCN-).
An alternative process involves destruction of weak acid dissociable
cyanide by oxidation with a mixture of sulphur dioxide and air. Again, cyanide
is oxidised to cyanate and the process is not suitable for removing
thiocyanate.
Further options may involve ion exchange or acidification-volatilisation-
re-absorption (AVR) processes. An AVR treatment circuit involves
acidification of cyanide liquors or slurries to lower the pH from an alkaline
range to pH 3-5 resulting in conversion of free cyanide and weak complexes
such as zinc, cadmium and nickel complexes to HCN which is then volatilised
by passing air bubbles through the liquor or pulp. An air/HCN stream may
then be scrubbed in a lime slurry or NaOH solution to convert HCN back to
sodium cyanide for recycling.
While the AVR process is an option for treating moderate or strong
cyanide liquors (over 500ppm weak acid dissociable cyanide), copper is
precipitated as CuCN during the acidification stage. Such product is
unsaleable and binds up a third of the available cyanide complexed with the
copper, lowering the cyanide recovery.
The Cyanisorb process is similar to the AVR process above but differs
in that clear solutions or slurries are processed at near neutral pH.
The MNR process was developed by Metalgesellschaft Natural
Resources and involves sulphidisation using NaSH and acidification, to pH
less than 5, of copper/cyanide rich liquors precipitate synthetic chalcocite
(Cu2S).
The liquor may then be re-causticised to produce caustic cyanide or
acidified further to form HCN gas which may be recovered by absorption. The
copper by-product is saleable. Nevertheless, material handling of the
chalcocite and potential co-precipitation of CuCN and CuSCN may be issues
in plant design.
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Solvent extraction and ion exchange techniques are also possible
alternatives for treatment of liquors containing free cyanide. However, they
do
not address recovery of cyanide from cyanide complexes such as copper
cyanide complexes.
Summarlr of the Invention ,
It is the object of the present invention to provide a process for dealing
with liquors containing cyanide compounds and complexes, particularly base
metal and precious metal cyanide complexes. The process may be applied to
recovery of cyanide for use in a metallurgical or industrial process.
Advantageously, the process ought not to be chemically intensive in
comparison with prior art processes.
With this object in view, the present invention provides a process for
recovery of cyanide and metals from a liquor containing a compound or
complex of cyanide and/or thiocyanide and metal generated by a metallurgical
process including the steps of:
(a) electrochemically treating the liquor in an electrochemical cell for
electrochemically dissociating the complex present in an electrolyte stream;
(b) recovering the metal by electrowinning;
(c) passing electrolyte through a membrane treatment step for
separation of cyanide by a membrane process; and, optionally,
(d) recycling free cyanide to the metallurgical process.
The process is suitable for recovery of cyanide and/or metal from
solutions containing very low concentrations of these components whether in
cyanide or thiocyanide form. Thiocyanide compounds or complexes are also
considered herein as compounds or complexes of cyanide electrochemically
dissociable in accordance with the process of the invention. As to solution
concentration, the liquor containing the complexes of cyanide and metal may
contain significantly less than 10g/1 metal, a very low concentration.
An important feature of the process is step (a), the electrochemical
treatment step for electrochemical dissociation of the metal-cyanide or metal-
thiocyanide complex in an electrochemical cell to form free cyanide which
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itself forms a further inventive aspect. This step allows higher free cyanide
recovery, notably from thiocyanide present in the liquor. Thiocyanide may
also be oxidised to free cyanide in the cell.
The process can be applied to recovery of many different metals and
may be particularly suitable for recovery of base metals such as copper, zinc
and nickel, particularly from complexes of these with cyanide (base-metal
cyanide complexes). In the case of copper cyanide complexes, often
encountered as a by-product from precious metals leaching, the Cu(CN)2 ,
Cu(CN)32- and Cu(CN)43- complexes are the most significant. All may be
present in a liquor from a gold leaching operation for gold ores containing
copper or base metals. All the processes may also be used to recover
precious metals such as silver, which are complexed with cyanide in precious
metal-cyanide complexes.
Metals may be recovered from the cathode or from the process in a
form suitable for recovery by a suitable solid/liquid separation process. For
example, copper, silver or other metals may be recovered in a powder or
granulated form by conducting solid-liquid separation in a hydrocyclone or
similar device.
The membrane process may be pressure or diffusion driven, being
characterised as a process of ultrafiltration, nanofiltration or reverse
osmosis,
differences between which are described in Lein, L, "Nanofiltration: Trend of
the Future!", Water Conditioning and Purification, September 1992, pp 24 to
27 and Cheryan, M et al, "Consider Nanofiltration for Membrane Separations",
Chemical Engineering Progress, March 1994, pp 68 to 74, both of which are
incorporated herein by reference. These references further describe specific
characteristics of membranes for use in these processes though many
membranes are proprietary in nature.
Nanofiltration membranes prevent passage of materials, such as ions,
that have size in excess of 10 to 20 angstroms. Ultrafiltration membranes
prevent passage of ions having size in excess of 50 to 200 angstroms.
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Reverse osmosis membranes prevent passage of smaller ions and
material having size greater than about 2 to 5 angstroms. Membrane
processing for recovery of free cyanide may take place in one or more stages
involving membrane modules of spiral, fibre, flat plates, tubular or other
5 module design known in the art.
Such processes, as nanofiltration, ultrafiltration or reverse osmosis,
may further be applied to the step of concentrating the recovered cyanide
stream to enable re-use if desired. A typical pressure range would be 500 kPa
to 8,000 kPa. The recovered cyanide stream or permeate may contain a very
low concentration of cyanide in comparison to that required in a metallurgical
process, for example a gold cyanide leaching process.
Therefore, the concentration of free cyanide may be upgraded by
further membrane modules in which the cyanide is selectively retained by
membranes of decreasing pore size or increasing "tightness". A number of
stages may be required to achieve the requisite degree of concentration for
recycling of cyanide to the metallurgical process. Such a process may be
feasible in two to three stages.
In the process of the invention, free cyanide, cyanide or
thiocyanide complexes and compounds may be present in the liquor. These
species may report at the anode of each electrochemical cell in which
electrochemical dissociation occurs. Advantageously, the anode should be
separated from the cathode at which copper or other metal deposits such that
re-complexing with cyanide is limited. Cyanide can be removed from each
electrochemical cell by conducting the membrane treatment step within the
electrochemical cell to prevent re-complexing with base metal, such as
copper. Free cyanide may also be recovered in a membrane treatment step
conducted externally of the electrochemical cell. The membrane treatment
step may be conducted for spent electrolyte or in a recirculating flow of
electrolyte through the electrochemical cell. A number of electrowinning and
membrane cells may be employed in a suitable electrochemical apparatus.
The cells may be in series or parallel relation.
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In another aspect of the invention, there is provided an electrochemical
apparatus for recovering metals by electrowinning from low concentration
metal containing solutions, the apparatus including:
(a) a housing including at least one electrochemical cell and having
inlet and outlet means to enable electrolyte to pass through at least one
cell;
(b) an anode for each cell;
(c) a cathode for each cell at which metal forms and/or deposits;
and
(d) flow circulating means for circulating a high velocity turbulent
flow of electrolyte through each cell; and
(e) an electrical connection between the anode and cathode of each
cell for completing an electrical circuit between these.
The flow circulating means may include a baffle means located
between the anode and cathode of each cell with the inlet and outlet means
located to promote a circulation within the cell. The baffle could
alternatively,
and advantageously, serve as anode or cathode with a surrounding or
adjacent electrode serving as cathode or anode, respectively. The baffle
would preferably be constituted by the anode.
Alternatively, other means to increase the turbulence, and hence mass
transfer in the cell may be employed. These may include, separately or in
combination, physical modifications to the anode or cathode to induce
turbulence, use of spacers between the anode or cathode, use of gas injection
into the cell, use of increased cross flow velocity, pulsation flow or flow
reversal of the electrolyte.
A number of cells may be located adjacent to each other in series or
parallel relation. The cells may be arranged in a number of banks or stages.
In one arrangement, an electrowinning unit may comprise a number of anodes
and cathodes which may be located in cell units provided with electrolyte
inlets
and outlets. The anodes and cathodes, which may be in the form of plates,
may be retained in supporting frames held in pressure relation by a press. The
press may be opened to recover metal deposited on the cathodes.
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Alternatively, the electrochemical cells may be configured to allow recovery
of
metal in a suitable solid/liquid separation device such as a hydrocyclone.
The inlet means may include one or more port means such as nozzles,
or orifices located, advantageously in spaced relation, to create a flow of
electrolyte past the electrodes which is high in velocity in comparison with
prior art electrowinning cells. The inlet ports may be arranged in banks or
along the length of one side of each cell. Other arrangements are possible.
The narrow diameter of ports, nozzles or orifices relative to an inlet
manifold
supplying electrolyte to the electrochemical apparatus may cause electrolyte
velocity to increase. As a result, electrolyte may be injected or jetted into
each
electrochemical cell. A high degree of turbulence is also induced in the
electrolyte flow and this promotes metal ion mass transfer.
Where the electrodes take the form of flat plates, the inlet manifold may
be connected to one side of a supporting frame being provided with an inlet
gallery through which electrolyte flows, through the nozzles, ports or
orifices
and ultimately across the electrodes. Another side of the supporting frame
may also be provided with outlet port means such as nozzles or orifices
respectively connected to an outlet gallery and ultimately an outlet manifold.
Again the outlet ports may be arranged in banks or along one side of each
cell, typically that side opposed to the inlet ports. Other arrangements,
including a staggered arrangement of inlet and outlet port means, are
possible. Outlet port means may have larger diameter than the inlet port
means.
The diameter of the inlet nozzles, ports or orifices may progressively
decrease toward a centre of the one side of the supporting frame such that the
velocity profile increases toward a centre of the one side of the electrode
supporting frame. This supporting frame may also be electrically connected to
a supply of current in a conventional way. It may incorporate a gas vent for
venting of anode gas, such as hydrogen, from each cell.
More specifically, the anode and cathode of each cell may be
supported by a supporting frame having opposed frame members provided
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with respective inlet and outlet port means, as described above,
communicating each cell with inlet and outlet manifolds, said supporting frame
having internal curved walls forming part of each cell. The curved walls
further
promote the circulating flow of electrolyte within the cell.
A membrane may be incorporated in the electrowinning cell to remove
species formed during electrowinning and, particularly when the
electrochemical cell is to be employed in the recovery of cyanide. The
membrane, of a preferred embodiment of the invention, needs also to be
permeable to cyanide with the cyanide permeate side of the cell connected to
an outlet or outlet manifold such that cyanide can be removed from the cell
without opportunity for re-complexing of cyanide and metal. Retentate is
subjected to electrowinning of metal. Retentate may be recycled to the
apparatus, treated in further stages or disposed of.
If a reverse osmosis, nanofiltration or ultrafiltration membrane is
employed, pressure within the cell can drive the permeate flux.
The outlet or outlet manifold may be connected specifically to receive
permeate from the cell and direct it to a cyanide concentration step as above
described. The retentate side may be connected, optionally through a
separate manifold, to a solid/liquid separation device or hydrocyclone for
collection of metal particles recovered by electrowinning. Otherwise retentate
may be treated as described above.
The process and apparatus of the present invention may be
conveniently and advantageously be used for recovery of free cyanide and
metals from solutions containing complexes of these, such as may be derived
from precious metal leaching processes.
More broadly, metal recovery is also possible from solutions having
much lower metal concentration or tenor than previously known providing the
opportunity to operate without expensive concentration upgrading steps such
as solvent extraction and ion exchange necessarily employed in prior art
processes. For example, solvent extraction is a commonplace step in
recovery of copper from a copper leaching process.
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In a further aspect of the invention, there is more broadly provided a
process for treatment of an aqueous cyanide or thiocyanide bearing stream
including a membrane treatment step for separating free cyanide, a cyanide
complex or a thiocyanide from a stream containing same. Optionally, free
cyanide may be recovered for re-use in a process, particularly a metallurgical
process using it.
Such streams may be waste or other streams derived from leach
plants, other metallurgical plants or waste water streams from other sources.
The membrane processes described above may be directed to this purpose.
Brief Description of the Drawings
The process and apparatus of the present invention may be more fully
understood from the following description of preferred embodiments thereof
made with reference to the accompanying drawings in which:
Figure 1 is a schematic flowsheet for a process for recovering cyanide
from a solution or liquor containing metal complexes thereof;
Figure 2 is a schematic flowsheet for a process for concentrating
cyanide in accordance with the concentration step of Figure 1;
Figure 3 is a schematic diagram in side section of an electrochemical
apparatus for conducting electrolysis of the metal-cyanide complex;
Figure 4 is a schematic diagram, in end section, of the electrochemical
apparatus of Figure 2;
Figure 5 is a schematic diagram along section line A-A of Figure 3;
Figure 6 is a front view of a cell unit or cell supporting frame for use in
an electrochemical apparatus as schematically represented in Figurers 3 and
4;
Figure 7 is a first side view of the cell unit or cell supporting frame of
Figure 6;
Figure 8 is a top view of the cell unit or cell supporting frame of Figures
6 and 7;
Figure 9 is a section view along section line A-A of Figure 6; and
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Figure 10 is a second side view of the cell unit or cell supporting frame
of Figures 6 to 9.
Detailed Description of the Drawingis
Referring now to Figure 1 of the drawings, there is shown a flowsheet
5 for conducting a process in which cyanide and copper metal are recovered
from an alkaline aqueous liquor stream 110 containing copper cyanide
complexes including Cu(CN)2 , Cu(CN)32- and Cu(CN)43- complexes. Stream
110 may itself be derived from a membrane process for separating gold-
cyanide and copper-cyanide complexes as described in US Patent No.
10 5961833, the contents of which are hereby incorporated herein by reference.
Such a liquor stream 110 may be derived from a leach of a copper bearing
gold ore. In step 120, the copper cyanide complexes are dissociated in an
electrochemical process in which cyanide reports to the anode and copper
metal reports to the cathode of electrochemical cells) configured to conduct
the process. One possible embodiment of electrochemical apparatus for
performing this duty is described in further detail below.
The following electrochemical processes, involving cyanide complexes
and thiocyanide, may occur during the process of the invention:
Cu(CN)43- ~ Cu(CN)32- + CN- (1 )
2o Cu(CN)32- ~-~- Cu(CN)2 + CN- (2)
Cu(CN)2 ~ Cu.~ + 2CN- (3)
SCN-+202 ~- SO42- + CN- (4)
Therefore, as the process proceeds copper is electrowon, as a saleable
product, perhaps in the form of granules or powder which may be recovered in
a suitable solid/liquid separation step 170 including one or more devices such
as hydrocyclones.
In an electrochemical cell operated without a cyanide permeable
membrane copper would, subject to kinetics, re-complex with free cyanide to
form copper complexes in the reverse process to those reflected by reactions
(1) to (3) above.
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However, free cyanide may be recovered from electrolyte in a
membrane process. In that way, the re-complexing problem and cyanide
wastage may be avoided. There are a number of options for this. Broadly,
the process is conducted such that cyanide ions may permeate through the
membrane in a process such as reverse osmosis, ultra-filtration or
nanofiltration, all of which are pressure or diffusion driven membrane
processes and generically described in the literature cited above and
incorporated herein by reference.
In one embodiment, the cyanide permeable membrane may be located
in the electrowinning cell. As copper ions migrate to the cathode, cyanide
ions
pass through a membrane, migrating to a permeate collection manifold. The
permeate stream may then be directed to further membrane treatment step,
such as for concentration of cyanide for re-use in a metallurgical process for
cyanide leaching of copper bearing gold ore.
In another embodiment, a flow of copper depleted electrolyte may be
directed to a membrane treatment step 140 involving reverse osmosis, ultra-
filtration or nanofiltration steps in which cyanide is caused to permeate
through
a suitable membrane under imposition of a pressure gradient across the
membrane. Membranes suitable for the membrane treatment step may be
sourced under the trade marks DOW NF70 or Desal DK. Other membranes
may be suitable noting the importance of pH to membrane selection. The
membrane modules for membrane separation of free cyanide may be of spiral,
fibre, flat plate, tubular or other known membrane unit type operated at a
suitable pressure, for example, in the range 500 kPa to 8,000 kPa (0.5 to 8
MPa). A number of membrane modules, which may be arranged in stages
may be used for this process.
A recirculating flow 125 of part or all of the electrolyte may be
maintained through electrochemical step 120 after treatment of electrolyte in
membrane treatment step 140 for cyanide recovery. Cyanide containing
permeate stream 150 may be directed to a further concentration step 160 for
upgrading concentration of cyanide for re-use in the gold leaching process.
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Stream 154 may be recycled to the leaching process. Lean stream 157, lean
in copper and cyanide, may be directed to further treatment or disposal.
Figure 2 shows a schematic of a cyanide concentration step 160 which
itself involves membrane processes which selectively separate cyanide as
retentate (concentrate) or otherwise. Stream 150, containing for example 1 g/1
cyanide, is directed first to membrane concentration unit 162 with concentrate
stream 152, having concentration 5g/1 cyanide, being directed to further
membrane concentration unit 164 to generate concentrate stream 154 of
concentration sufficient for recycle to a gold leaching process, say 15g/1
concentration free cyanide.
The lean cyanide stream 153 is directed to membrane concentration
unit 166 with concentrate stream 155 being recycled to membrane
concentration unit 162. The lean cyanide stream 157 containing a low
concentration, say 0.01 g/1 cyanide, may be directed to disposal or other
treatment steps.
Cyanide concentration may therefore involve two concentration steps
but other concentration process flowsheets could be developed to perform the
same duty. More concentration or separation steps could be employed as
necessary.
In the embodiment described above, description was made of free
cyanide and copper metal recovery from liquors containing complexes of
these. However, the process of the invention is also suitable for recovery of
cyanide and other base or precious metals from complexes thereof. The
process would also be suitable for recovery of cyanide and silver from
complexes thereof. The process is further suitable for recovery of free
cyanide from other waste water streams.
In the broadest aspect of the invention, as described above, the
process could be conducted by using a membrane process for recovery of
free cyanide in association with a conventional electrochemical cell, in which
electrolyte is recirculated through the cyanide membrane separation unit.
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However, in a preferred embodiment, the electrochemical process for
dissociating copper-cyanide complexes may be conducted in an
electrochemical cell or apparatus as described with reference to Figures 3 to
10.
Referring now to Figure 3, there is schematically shown an
electrochemical apparatus 10 having a housing 30 incorporating a number of
electrochemical cells 21. Housing 30 includes four electrochemical cells 21
each being provided with planar anodes 38 and cathodes 39. Other forms of
anode 38 or cathode 39 could be used, for example cylindrical, rod or
otherwise.
Between each cathode 39 of cell 21 is a planar anode 38 forming a
baffle around which electrolyte circulates in a high velocity turbulent flow
across the anode and cathode surfaces. Cathode 39 may completely
surround anode 38. Alternatively, an electrochemically inert baffle may be
located between anode and cathode plates 38 and 39. In such case, the
baffle may be of polymeric material.
It will be understood that anodes and cathodes may be constructed
from material known to be suitable for electrowinning application. In this
case,
stainless steel anodes and cathodes 38 and 39 may be employed. Type 316
stainless steel may be preferred. Electrical connection is made with anodes
and cathodes 38 and 39 in conventional manner.
The cells 21 are incorporated in cell units 23 held in a press between
end plates 20 which may be of polymeric or other suitable material. In this
embodiment the end plates 20 are of polypropylene. The apparatus may be
assembled or disassembled by adjustment of pressure screw 18.
Electrolyte enters each cell from an inlet manifold 16 by branch pipes
14 havng inlets communicating the inlet manifold 16 with each electrochemical
cell 21. Electrolyte leaves each cell 21 through an outlet communicating with
outlet manifold 18 through branch pipes 15. Outlet manifold 18 may be
connected to membrane treatment step 140 as illustrated in Figures 1 and 2.
However, electrochemical apparatus 10 may be operated in a recirculating
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mode with electrolyte being directed through membrane treatment step 140 for
recovery of free cyanide, and then back to electrochemical apparatus 10 for
further electrochemical treatment. Alternatively, a number of electrochemical
treatment stages could be used.
Leaving electrolyte may be depleted in copper which deposits on planar
cathodes 39 as a powder or granulated deposit. In a turbulent regime, the
metal solid may be transported from cathode 39 surfaces and may be
recovered by solid/liquid separation in hydrocyclones for example.
The leaving electrolyte is then passed to membrane treatment step 140
for recovery of free cyanide which may be concentrated for re-use in the
metallurgical process as previously described. The membrane treatment step
could be conducted within cell 21. For example, anodes 38 could be
separated from the remainder of the cell by a cyanide permeable membrane
to achieve this.
The entering electrolyte enters cells 21 through at least one port
located where each branch pipe 14 enters each cell 21. As such a port is of
lesser diameter then the diameter of the inlet manifold 16, electrolyte
velocity
is increased and turbulence is likewise increased. Electrolyte is therefore
injected or jetted into the cells 21. The walls of the housing 30, the anode
38
(acting as a baffle) and the opposed locations of inlet ports and outlet
port(s),
the latter being located where each branch pipe 15 exits from each cell 21,
act
as a flow circulating means which creates a high velocity turbulent
circulatory
flow of electrolyte that may circulate two or more times about cell 21 before
leaving it. In this way, the mass transfer rate of copper from ionic to
metallic
form is increased with copper recovery being possible even from very low
concentration solutions, having copper tenor 1 g/1 or even lower. This may
also be understood to be the case for other metals.
The circulation effect may be enhanced by orienting the inlet and outlet
ports tangential to curved portions 33 of walls 21 a of cells 21 as shown in
Figure 4. The curved orientation of the walls 21 a enhances the circulation
effect.
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The supporting frames 210 for each cell unit 23 will now be described in
greater detail with reference to Figures 6 to 10.
Referring now to Figure 6, the supporting frame 210 is provided with a
number of frame members 211, 212 and 213 which may be made of non-
5 conductive polypropylene or other materials. Supporting frame 210 is
supportive of, or connected to, plates 238 and 239 which respectively form the
anode and cathode of electrochemical cell 21. Anode and cathode plates 238
and 239 may be of stainless steel or other suitable material. These are
electrically connected to a contactor bar (not shown) and a current supply as
10 known in the electrochemical art.
Anode plate 238 may be located approximately centrally of cell 21.
Centre line 21 a of cell 21 may form the centre line of anode plate 238.
Frame member 211 incorporates a number of inlet ports 240, of
narrower diameter than the diameter of inlet manifold 16 or branch pipe 214.
15 Two zones of inlet ports 240 are shown but ports could extend along the
entire
length of walls of frame member 211 defining cell 21. Electrolyte is pumped to
each cell 21 through inlet manifold 16, branch pipe 214 and corner apertures
242 to inlet gallery 241. Inlet gallery 241 communicates with each cell 21
interior through drilled restrictions or apertures, of circular or other
geometry,
in frame member 211 which form ports 240. In this way, velocity and
turbulence of electrolyte are increased. Ports 240 are conveniently located
offset from a centre axis 21 a of cell 21, and anode 238 to create a turbulent
high velocity circulatory flow of electrolyte transversely across the
electrodes
with the ports 240 acting as jets. Location of anode plate 238 within cell 21,
or
location therein of an inert baffle, creates a circulatory flow of electrolyte
which
leaves cell 21 following electrolysis/electrowinning through outlet ports 250.
Corner apertures 242 may extend through the entire length of electrochemical
apparatus 10. Corresponding apertures may be formed in one or both
electrodes, especially cathode plates 239.
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The supporting frame 210 opposed frame members 211 and 213 are
also provided with internal curved walls 233 which also promote a high
velocity turbulent flow of electrolyte in cell 21.
The leaving flow of electrolyte may be passed through outlet ports 250
to membrane treatment step 140 for recovery of free cyanide while metal can
be recovered from the cathode in any convenient manner after membrane
treatment. Ports 250 may also be located offset from a centre axis 21 a of
each cell 21, advantageously on the opposite side of anode 238 and centre
axis 21 a to ports 240, which further enhances the circulatory effect. The
outlet
ports 250, which may have larger diameter than inlet ports 240, are drilled
apertures, of circular or other geometry, in frame member 213. In the
drawings, outlet ports 250 are centrally located and staggered from the inlet
ports 240. The staggering is apparent from Figures 6 to 10. Figure 9, omitting
anode 238 for clarity of illustration, shows the staggered arrangement in
which
ports 240 and 250 are at different levels within the cell 21. However, the
outlet
ports 250 could extend along the entire length of walls of frame member 213
defining cell 21. Again, the outlet ports 250 communicate with outlet gallery
251 communicating with an outlet manifold 18 through corner apertures 244,
again extending through apparatus 10. In one method, a solid/liquid separator
such as a hydrocyclone may be used to recover metal powder from the cells
21 after membrane treatment step 140.
A number of cell units 23 may be mounted together between end plates
20, all being supported by supporting arms 23a of cell unit 23 on a support
means (not shown). End plates 20 may be polymeric, say of polypropylene.
The cell units 23 may be mounted to be assembled/disassembled by
tightening or loosening screw 18 shown in Figure 3.
The electrochemical apparatus may be operated in recirculating mode
to maximise recovery of cyanide and metal. A number of stages may be used,
each employing apparatus of the same or similar type to that described above.
The electrochemical apparatus is not restricted in its applicability to
metal-cyanide complex electrolysis, as a step in free cyanide recovery, and it
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may also be applied to recovery of metals, particularly base metals, from low
tenor metal ion containing solutions as well as species formed during
electrolytic processes for recovery of metals whether in combination, or not,
with a membrane separation process for such species.
Modifications and variations to the processes and apparatus of the
present invention will be apparent to the skilled reader of this disclosure.
Such
modifications and variations fall within the scope of the present invention.
