Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1:1'7~9;~5
The present invention relates to apparatus for electro-
lytically treating various electroactive species in solu-
tions thereof, and, more particularly, to such apparatus
for extracting small concentrations of metallic species
from solutions, such as liquid waste waters and effluents
from industrial processes, e.g., metal plating operations.
There are, at present, a variety of so-called
"electrochemical" apparatus and processes in which an input
of electrical power is employed in order to bring about
activity at a working electrode. These electrochemical pro-
cesses and apparatus are generally employed to treat solu-
tions, such as waste water and plant effluents, in order to
reduce the concentration of metal contaminants to levels
which are acceptable, particularly in view of the present
stringent environmental regulations, and to recover these
metal contaminants.
There are two general categories of such
electrochemical processes depending on their most signi-
ficant limiting factor. The first group includes processes
whose reaction rates are kinetically controlled, i.e., the
reaction rates are limited by the speed of the reactions at
a working electrode. In these processes, the solution or
electrolyte being treated contains high concentrations of
electro-active species. An example of one such process is
the electro-refining of zinc, where there is inherently a
high concentration of zinc in the electrolyte.
p
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11789~5
The second group of electrochemical processes includes
those in which the reaction rates are controlled by mass
transfer considerations, rather than by kinetic require-
ments, i.e., the reaction rates are limited by how much of
the contaminants can be brought into contact with a working
electrode in a given time. In contrast to the electrodes
used in kinetically controlled processes, the working
electrodes used in these mass transfer controlled processes
must exhibit characteristics which enhance the obtainable
mass transfer rates. One such characteristic is a large
surface area to volume ratio. Attempts have been made to
achieve acceptable surface area to volume ratios by
utilizing packed beds of fibrous or granular material (see,
for example, U.S. Patent Nos. 2,563,903; 3,450,622;
3,457,152; and 3,827,964), as well as active beds which can
move in a flow of electrolyte. These attempts have
suffered, however, from distinct disadvantages based
primarily on the difficulty of providing a uniform and
controlled electrical potential throughout the electrode to
make full use of the surface area. The use of granular or
fibrous beds is also disadvantageous because the electro-
lyte can channel around the granules or fibers, thereby
bypassing the effective portion of -the electrode and,
consequently, deleteriously affecting the effectiveness of
the electrode. Thus, two general disadvantages of the prior
art mass transfer controlled processes are low current
efficiency and low conversion completeness. As a result
of these major drawbacks, none of the prior art mass
transfer controlled processes has achieved significant
acceptance.
11~89Z5
In both the kinetically controlled processes and
the mass transfer controlled processes, one of the prime
considerations is the method of recovering the electro-
active material removed from the electrolyte and deposited
on a working electrode. It is generally necessary to
conduct a stripping operation to remove the deposited
material from the working electrode prior to the subsequent
use thereof. The working electrodes used in these processes
are sometimes made from the same material that is to be
stripped therefrom, so that the resulting product can be
used directly. More commonly, however, these electrodes are
designed for mechanical stripping. In addition, in other
cases, the electrode must meet other requirements, such as
those described in U.S. Patent No. 3,953,312, where the
prime consideration is that the electrode be combustible so
that silver deposited on the electrode can be recovered by
~elting during combustion.
More recently electrodes and reactors have been
developed which employ carbon fibers in a manner so as to
both provide a large surface area to volume ratio and at
the same time limit fluctuations in the electrical poten-
tial throughout the electrode. Such electrodes and reactors
are described, for example, in U.S. Patent Nos. 4,046,663;
4,046,664; 4,108,754; 4,108,755; and 4,108,757. These elec-
trodes and reactors suffer, however, from the same channel-
ing and bypass problems which plaque the granular or
fibrous bed electrodes described above.
9ZS
Carbon fiber electrodes and reactors therefor
have also been proposed, at least on a laboratory scale, by
D. Yaniv and M. Ariel in an article appearing in the
Journal of Electroanalytical Chemistry, Volume 79 (1977),
pages 159 to 167. The structure disclosed in this article
includes an electrode of graphite cloth positioned in a
frame defining an opening having an area of 2.4 cm2. The
article states that the results obtained confirm the fea-
sibility of exploiting graphite cloth as a practical elec-
trode material suited for flow-through configurations. How-
ever, the article goes on to indicate that, although the
laboratory reactor worked well, it would be necessary to
undertake further work to optimize a reactor using a
graphite cloth electrode.
A more recent approach to an electrode for use in
mass transfer controlled environments, such as in connec-
tion with dilute electrolyte solutions, is disclosed in
Japanese Patent No. 67267/76 which was published on
June 10, 1976 and assigned to Mitsui Petrochemical
Industries Ltd. This patent discloses the use of a porous
carbon electrode in connection with an electrode base mater-
ial which the patent discloses can be any one of a number
of well-known electrode materials, such as platinum, iron,
copper, nickel, silver, lead and certain alloys thereof.
The patent also discloses the use of carbon fibers in
various forms, such as cloths, fabrics, felts and carbon
fiber papers, to cover a base material in the form of a
p].ate, tube, mesh or plate with holes therein. Furthermore,
in Example l of this patent, the cathode employed comprises
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a titanium plate which is plated with platinum and then
covered with a layer of carbon fiber fabric. Thus, in
effect, a platinum cathode is provided. This patent does
not deal with the question of how metals can be recovered
from such electrodes so that the concentration of metallic
ions can be reduced to extremely low levels in real time in
an economical manner.
In accordance with the present invention, there
is provided new and improved apparatus for waste treatment
equipment. The apparatus are especially effective in prac-
ticing mass transfer controlled electrochemical processes.
One aspect of the invention involves a thin
porous electrode having a substantially uniform pore distri-
bution which is unchanged by fluid flow through the elec-
trode, thereby preventing fluid flowing through the elec-
trode from making undesired channels therein. A support,
such as a frame, positions the electrode across a fluid
flow path. All edges of the electrode are sealed by the
support so that fluid flowing along the flow path must pass
through an effective portion of the electrode, thereby pre-
venting the fluid from channeling around or bypassing the
electrode.
For a portion of the electrode to be effective,
the electric potential difference between the effective
portion of the electrode and electrolyte in the immediate
vicinity of the effective portion must be greater than or
at least equal to a measured value which varies from
reaction to reaction. Competitive side-reactions, such as
the kinetically controlled hydrogen or oxygen evolution
reactions in ~queous media or the oxygen reduction reac-
tion, can often occur in portions of the electrode which
are ineffective in promoting the desired mass transfer con-
trolled reactions. It has been found that a useful
empirical effectiveness of the various portions of a porous
electrode can be obtained by promoting a reaction which
results in the ireversible deposition of a reaction product
at a reaction site of the electr~de. In particular, reduc-
tion of copper ions to copper metal from a very dilute
acidic copper sulphate solution is a good tracer for deter-
mining the relative effectiveness of various portions of an
electrode constructed in accordance with the present inven-
tion.
Space-time yields are standard indicators of the
performance of a heterogeneous catalytic reactor. In
electrochemical engineering a convenient parameter is the
amount of current carried by an electrode at high current
efficiencies per unit volume of that electrode. This compar-
ative measure of electrode efficiency can be used with a
given electroactive species having a known concentration
and conductivity. For a copper solution having a concentra-
tion of 640 p.p.m. at a current efficiency of 52% the
following space-time yields were obtained for the various
electrodes shown below:
11'7892~
Reactor_TypeSpace-Time
yield mA/cm-
Restrained Packed Bed 57
Fluidized Bed 4 to 60
Filter Press, Capillary
gap systems etc.1 or less
Present electrodeGreater than 1280
It should also be noted that at increased flow velocities
electrodes constructed in accordance with the present inven-
tion have demonstrated space-time yield results as high as
6800 mA/cm3 and at very low flow rates space-time yields
have been recorded in the range of 500 mA/cm3. Thus, the
present electrode is much more effective than any of the
prior electrodes, such as those disclosed in, for e~ample,
U.S. Patent Nos. 3,450,622; 3,457,152; 3.953,313;
4,046,663; and 4,108,755.
Another aspect of the invention involves a cell
which utilizes the above-described electrode as a first
electrode. The cell may further include a second electrode,
which is positioned on one side of the first electrode, and
a third electrode positioned on the other side of the first
electrode. A first inlet is in fluid communication with a
first chamber positioned between the first and second elec-
trodes, so that fluid can be supplied to the first chamber
through the first inlet. A second chamber, positioned
between the first and third electrodes, communicates with a
first outlet, whereby fluid can be discharged from the
second charnber through the first outlet. By this arrange-
ment, fluid flowing from the first inlet to the first out-
let flows through the first electrode in a first direction.
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A second outlet and a second inlet may be pro-
vided in fluid communication with the first and second
chambers, respectively, so that fluid can flow through the
first electrode in a second direction opposite the first
direction. Thus, upon termination of flow of a first fluid,
such as waste water, in the first direction, the first elec-
trode can be back flushed, for stripping and cleaning pur-
poses, by the reverse flow of a second fluid, such as a
suitable stripping electrolyte, through the first electrode
in the second direction. Inasmuch as the first electrode is
preferably thin, e.g., about l/4-15 millimeters thick, back
flushing of the electrode can be especially effective in
removing particulate matter, such as dirt, sand and
insoluble foreign material, which has been previously
deposited on the electrode. In thicker electrodes, such
particulate matter becomes entrapped deep in the electrodes
where back flushing is generally ineffective in dislodging
and removing it.
In one embodiment of the cell, the first and
second inlets and the first and second outlets are formed
in the support for the first electrode. The cell can be
; made more compact by forming these inlets and outlets in
the support.
The first chamber may be delimited by a first
diaphragm disposed between the first and second
electrodes and cooperating with the second electrode to
delimit a third chamber. Similarly, a second diaphragm
can be disposed between the second and third electrodes
to delimit the second chamber and a fourth chamber,
39~5
positioned between the second diaphragm and the third
electrode. By this arrangement, a third fluid, such as a
suitable anolyte, may be supplied to the third and
fourth chambers through third and fourth inlets,
respectively. Fluid supplied to the third and fourth
chambers can be discharged therefrom through third and
fourth outlets, respectively.
The first elec~rode can be designed so
that it normally operates as a cathode onto which
metallic species are plated. The second and third
electrodes normally operate as anodes. By changing the
polarity of the first, second, and third electrodes, the
second and third electrodes can operate as cathodes,
while the first electrode operates as an anode for
stripping the plated metallic species therefrom. When
the first electrode operates as a cathode in the
embodiment described in the preceding paragraph, waste
water flowing from the first inlet to the first outlet
flows through the first electrode, while anolyte flows
through the third and fourth chambers. No electrolyte is
permitted to flow into the second chamber through
the second inlet as long as the waste water continues
to be supplied to the first chamber and, hence,
the second chamber. When the flow of the waste water
through the first and second chambers ceases, the first
electrode can operate as an anode by permitting the
electrolyte to flow through the first and second
chambers, while a catholyte flows through the third and
fourth chambers, whereby the plated metallic ionic
species is mechanically and electrochemically removed
from the first electrode.
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A plurality of the above-described cells can be
combined to form a reactor in accordance with the present
invention. The reactor can, therefore, be adapted to
receive three different fluids, all of which are trans-
ported through the reactor.
For a more complete understanding of the present
invention, reference may be had to the following descrip-
tion of the exemplary embodiments, considered in conjunc-
tion with the accompanying figures of the drawings, in
whi.ch:
Fig. 1 is a front perspective view of a reactor
produced from a number of electrochemical cells constructed
in accordance with the present invention;
Fig. 2 is an exploded perspective view of a por-
tion of the reactor shown in Fig. l;
Fig. 3 is a partially broken away front per-
spective view of a flow divider employed in connection with
the reactor of Fig. l;
Fig. 4 is a partial horizontal, cross-sectional
view of the reactor illustrated in Fig. l;
Fig. 5 is a schematic representation of a process
employing the reactor of the present invention;
Fig. 6 is a graphical representation of results
obtained employing the reactor of the present invention;
Fig. 7 is a graphical representation of further
results obtained employing the reactor of the present inven-
tion; and
Fig. 8 is a graphical representation of still
further results obtained using the reactor of the present
invention.
- 1 1 -
9;~
By utilizing the present invention it is now pos-
sible, for example, to recycle all or a major portion of a
treated solution continuously so as to effectively elim-
inate the need to discharge effluent, such as in plant pro-
cesses, waste water treatment and the like. Because of the
economics of the present invention, as well as its extreme
reliability, it is possible to conduct such closed cycle
treatments while, at the same time, substantially avoiding
the need to suspend the process in order to service or
repair the treatment facility. This can be accomplished in
accordance with the present invention by using polarity
reversal in such electrochemical processes. At the same
time, it is also possible to now reduce the concentration
of metal contamination in dilute streams to levels which
are acceptable in terms of the most stringent environmental
regulations presently in effect.
Polarity reversal itself has primarily been used
in various forms. No practical system has previously been
developed, however, which lends itself both to continuous
cyclic operation in a mass transfer controlled process and
at the same time avoids significant electrode damage during
the stripping cycle. In the past, when such processes
employing polarity reversal have been contemplated, signifi-
cant problems have arisen from the fact that during anodic
operation the electrode itself becomes subject to attack
and, in fact, can simply dissolve. Thus, with electrodes of
the type disclosed in the aforementioned Japanese Patent
No. 62767/76, for example, the electrode base material, or
so-called "feeder", as well as the carbon fibers them-
selves, would be subject to such attack during the anodic
-L2-
1:1'7~5
stripping cycle. While the feeder or electrode base mater-
ial can be made of platinum or metal coated with platinum
(such as is disclosed in the aforesaid Japanese patent) to
thus avoid degradation thereof, this approach is not only
quite expensive but in no way solves the problem of anodic
attack upon the carbon fibers themselves. This anodic
attack is basically the result of the production of anodic
gases during stripping.
The metals employed in connection with a
secondary electrode component of the present invention,
however, have a number of unexpected advantages in this
regard. For example, it has been discovered that during the
stripping cycle when these electrodes are operating as
anodes, nonconductive substances are formed before sig-
nificant amounts of corrosive agents are produced. It is
therefore possible to sense termination of the stripping
operation and thus prevent attack on a primary electrode
component or carbon fibers by sensing a drop in current in
the anode caused by the presence of this nonconductive
material. Even more significant, however, is the discovery
that upon further reversal of the polarity of these elec-
trodes so that they operate again as cathodes, the
secondary electrode component again becomes conductive and
normal cathodic operation can continue just as before.
The exact nature of the nonconductive coatings
formed in connection with the metals employed as the
secondary electrode component of the present invention
during their use as an anode is not entirely understood. In
the case of titanium, for example, it appears that a
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11'~8~;~S
resistive oxide coating is produced during anodic opera-
tion. However, chemically induced oxide coatings of
titanium are sufficiently resistive so as to prevent their
use as a cathode. These oxide coatings produced in
accordance with the present invention, however, are
quickly reduced during subsequent cathodic use, and it must
therefore be presumed that although the electrochemically
induced coatings which are formed on the titanium component
are most probably oxides, they must nevertheless somehow be
different from chemically induced titanium oxide coatings.
While not wishing to be bound by any particular theory, it
appears that a hydrated form of titanium dioxide is formed
in connection with the present invention, and that this is
a reversible form of titanium dioxide which is reduced
during subsequent cathodic operation.
As for the primary electrode component of the
present invention, this comprises a highly porous con-
ductive material which is in electrical contact with the
aforementioned secondary electrode component. Most
preferred are the various forms of carbon fibers discussed
above. These carbon fibers must meet certain requirements
in order to be useful in mass transfer controlled pro-
cesses. Thus, they must provide substantially continuous
electrical conductivity throughout the electrode in order
to minimize voltage and current variations. Further, the
surface area of this porous conductive material should be
available to the electrolyte and the material must thus
have a maximum surface area to volume ratio so as to
provide a high percentage of usable surface area. Prefer-
ably such ratio should exceed about 100 cm /cm .
In addition, the overall flow path which existswithin the porous conductive material is quite significant.
There must be a minimum of blind or dead end passages in
the flow through the electrode structure, again to provide
contact for the solution being treated. In connection with
carbon fibers, for example, ideally the pores between the
fibers will define tortuous paths through the electrode in
order to minimize laminar flow and to encourage the
break-up of boundary layers around the surfaces. The
average pore size, which is of course related to voidage,
should be in the range of from about 0.1 to 3000 ~m and the
voidage should be in the range of from about 30 to 99% of
the total volume of the electrode. These figures are also
related to the pore size distribution, and it has been
found that about 80% of the pores should lie within the
range of from about 1 to 100 ~m.
When a fibrous material is used as the porous
conductive material, it is necessary to restrain the
fibers within the electrode. In some cases, the fibers are
similar to yarn rather than thread, so that each fiber is
made up of many smaller fibers. An example of a suitable
material would be a woven cloth made up of carbon fiber
yarn which is spun quite loosely but woven quite tightly.
As a result, larger spaces between adjacent yarns will be
minimized while the elements or fibers themselves which
make up the yarn are free to move slightly in the flow of
electrolyte while being restrained enough to maintain the
pore size required as well as the necessary pore size
distribution.
-15-
9~S
Reference is next made to the drawings, in which
Fig. 1 shows a reactor 20 which includes a plurality of
individual cells 22 arranged for operation in parallel
between a pair of end plates 24, 26. Bolts 28 restrain the
cells 22 between the end plates 24, 26. The parts used to
make up each of the cells are aligned by a pair of bolts
30, 32 which pass through the parts in a manner to be
explained hereinbelow. For the purposes of this description
the reactor 20 will be described in the position shown in
Fig. 1, but it is understood that it can be used in a num-
ber of different orientations.
Electrical connection to the individual cells 22
is made through electrically conductive bars 34 provided at
both sides of the reactor (one side being shown in Fig. 1)
and by electrically conductive bars 36 provided at the top
of the reactor. As will be described more fully with refer-
ence to Fig. 2, an electrolyte solution to be treated, such
as waste water, is fed frorn behind and at the bottom of the
reactor as shown in Fig. 1 and exits by way of outlet 38.
Anolyte is also fed from the bottom of the reactor, and
exits through another outlet 40. These outlets are used
during the plating or metal removing cycle. Afterwards,
when deposits on a working electrode, which during any such
plating operation acts as a cathode, are to be stripped,
the flow of waste water ceases and is replaced by a flow of
a suitable electrolyte, which again enters from the bottom
and behind the reactor and, in this case, exits through out-
let 42. As will become evident from the description below,
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11 '7~5
the electrolyte is made to back flush through the working
electrode, which during any such stripping operation acts
as an anode, to provide some mechanical cleaning action as
well as an electrochemical removal of the plated metal.
Reference is next made to Fig. 2 to illustrate
some of the mechanical details of the reactor shown in
Fig. 1, and, in particular, par~s which make up the indi-
vidual cells. As seen in Fig. 2, a frame 44 is positioned
for electrochemical action relative to adjacent sides of
lead counter electrodes 46, 46'. In effect, a complete cell
consists of the parts shown in Fig. 2, although only the
sides of the counter electrodes 46, 46' facing the frame 44
are active in that cell. Opposite sides of the counter elec-
trodes 46, 46' are active in adjacent cells, except at the
ends of the reactor where sides of the corresponding
counter electrodes adjacent the end plates 24, 26 (see Fig.
1) will be insulated from these end plates and have no elec-
trochemical effect.
The frame 44 is made from molded polyurethane and
contains peripheral conductors 48 which grip a conductive
mesh 50 made up of interwoven titanium wires as can best be
seen in Fig. 4. The peripheral conductors 48 are attached
to the bars 34 to ensure good electrical continuity from
the bars 34 to the mesh 50.
The mesh 50 forms a secondary electrode component
of the working electrode, and two primary electrode com-
ponents are attached to either side of the mesh 50 to form
the working electrode. One primary electrode component can
11'7892~
be seen in Fig. 2, and consists of a sheet 52 of carbc)n
fiber cloth of the type known as Morganite 7401 G and sold
by Morganite Modmor Ltd. of England. This sheet 52 is laid
in surface-to-surface contact with the mesh 50, and is held
in place by a series of titanium wire staples similar to
those used in conventional stapling equipment. The staples
are not shown in the drawings, but are distributed over the
sheet 52 where needed to hold the sheet in place. As will
be described more fully below, the edges of the sheets 52
are restrained by pressin~ them against the mesh 50.
The working electrode can have a total thickness
in the range of from about l/4mm to about 15mm. In one par-
ticular embodiment, the mesll has a thickness of about lmm
and each of the sheets 52 has a thickness in the range of
from about 1 mm to about 2 mm.
The frame 44 also inc'udes a series of top and
bottom openings to transport li~uids as indicated with
reference to the outlets 40, 42 and 38 shown :irl Fig. 1. For
instance, waste water to be treated enters through central
bottom opening 54, arld a portion thereof is distributed by
one of a number of inlets 56 into a space bordered on one
side by an adjacent one of the sheets 52, so that the waste
water flows through the working electrode to an opposite
side thereof, from which it exits through one of a number
of outlets 58 associated with central top opening 60, and
eventually leaves the reactor through the outlet 38 (see
Fig. 1). This f'io,i l:akes place during the treatment of
waste water (i.e., with the working electrode operating as
a cathode) in order to remove metallic ionic species from
the electrolyte solution.
-18-
When it then becomes necessary to strip the
deposited metal from the primay electrode components or
sheets 52 of the working electrode, the flow of waste water
is discontinued and a stripping electrolyte is made to flow
through the working electrode (which will now operate as an
anode by reversing the polarity of the working electrode
and the counter electrodes). This electrolyte enters
through bottom opening 62 and a number of inlets 64, and
leaves by way of one of a number of top outlets 66
associated with top opening 68, before finally exiting from
the outlet 42 (see Fig. 1). In this case, the flow is thus
again through the working electrode, but in the opposite
direction to that of the waste water during the preceding
cathodic operation, so as to enhance the flushing action of
the stripping electrolyte.
The frame 44 further includes bottom opening
70 and top opening 72, both of which are used for anolyte.
These openings simply provide passage through the frame 44.
In addition, two srnall openings 71, 73 are provided for
receiving the bolts 30, 32 (see Fig. 1) :in order -to
align the parts.
A flow chamber for the electrolyte solution, such
as waste water, is defined on the inlet side of the working
electrode by space within the frame 44 itself as well as by
a neoprene gasket 74 adjacent the face of the frame 44, as
can be seen in Fig. 4. Openings in the gasket 74 are
provided in alignment with the openings described with
reference to the frame 44, and spacer strips 76 are
compressed between an adjacent surface of the gasket 74 and
the face of an adjacent one of the sheets 52 at the
-19-
periphery of the sheet. These strips 76 ensure that the
edges of the sheets 52 are held tightly against the mesh
50. The inlet chamber is completed by a diaphragm 78 nipped
between the gasket 74 and a further neoprene gasket 80,
which has openings in alignment with the openings described
with reference to the frame 44. A similar outlet chamber is
provided by similar parts labelled correspondingly using
primed reference numerals.
The gasket 80 also provides access for anolyte
into a flow chamber defined, in part, by the gasket
80, as well as by the diaphragm 78 and the counter
electrode 46. The assembled arrangement is better seen in
Fig. 4. The flow of anolyte is facilitated by a pair of
molded flow diverters 82, 84 made of polyurethane and
arranged to fit in the gasket 80. One such diverter is
shown in Fig. 3. Diverters 82, 84 ensure acccess of anolyte
into the flow chamber adjacent the counter electrode 46 so
as to obtain electrochemical con-tinuity between the
adjacent surface of the anode 46 and the working electrode
contained in the frame 44. A pair of small neoprene gaskets
86, 88 is positioned adjacent the counter electrode 46 in
order to compensate for the thickness of the counter
electrode 46 in the assembly, and to allow the flow of
waste water and electrolyte therethrough. Openings in the
counter electrode 46 permit the flow of anolyte there-
through.
The parts described to the left of the frame 44
as shown in Fig. 2 are also duplicated to the right
thereof, and as mentioned are indicated using primed refer-
ence numerals. Apart from the fact that the spacer strips
-20-
11'7~3925
76' are slightly different because of the arrangement of
inlets and outlets in the frame 44, the parts to the right
are identical to those described on the left of the frame
44.
It will be evident from the foregoing description
that each working electrode is associated with two counter
electrodes, and that the parts are arranged to define a
housing having a waste water flow path through the working
electrode. Also, during the stripping cycle, the flow
passes through the working electrode in the opposite
direction. Electrical distribution is maintained in the
working electrode by a combination of the mesh 50 and the
natural conductivity of the two sheets 52. Because the flow
is through the working electrode, the mesh 50 should have
sufficient strength to reslst flow forces and to prevent
any significant distortion. Also, to ensure electrical con-
tinuity, the staples used to locate the sheets on the
screen should be tight enough to ensure surface-to-surface
contact between the sheets 52 and the mesh 50.
Reference is next made to Fig. 5, which shows the
reactor in use in a typical installation. In practice, a
number of these reactors could be used in parallel, or
possibly in series, with as many reactors as may be
necessary in order to accommodate the volume of effluent
being treated. As seen in Fig. 5, the reactor 20 receives
waste water from a pump 90 by way of inlet 92, and treated
waste water leaves by the outlet 38. While waste water is
being thus fed to the reactor, anolyte is being driven in a
closed loop by pump 94 through inlet 96, to return from the
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li';'~9~5
reactor by way of the outlet 40. The flow of waste water
and anolyte is controlled electrically by a pump control,
system 98 associated with a power supply control 100, which
normally maintains the current at a predetermined level
related to the voltage requirement. After the working elec-
trode has been plated for some time, the pressure drop
between the inlet 92 and the outlet 38 will change and this
is monitored and a signal fed to the pump control system by
way of transducer 102. Once the pressure drop reaches a
predetermined value, the pump control system isolates power
from the pump 90 and causes the power supply control 100 to
reverse the polarity of the working electrode and the
counter electrodes for stripping. At the same time, pump
104 is energized to feed stripping electrolyte into an
inlet 106 in order to back flush the working electrode (now
operating as an anode), and the stripping electrolyte exits
by way of outlet 42, carrying with it a concentrated solu-
tion of the metal being stripped frorn the waste water. The
stripping cycle continues until the voltage drop across the
reactor increases significantly, as caused by the formation
of the highly resistive coating on the secondary electrode
component of the working electrode, as is discussed in
detail above. The power supply control 100 senses this
increase in voltage and again causes reversal of the
polarity of the working electrode and the counter elec-
trodes, at the same time causing the control system to
re-energize the pump 90, and isolate pump 104. The coating
on the secondary electrode component of the working
electrode (again now operating as a cathode) is then
1178~Z~
electro-reduced, and the working electrode is again used to
plate metal from the waste water. The cycle can be repeated
continuously and automatically.
The pump 94 which drives the anolyte is also
connected to the pump control system. Consequently, in case
of emergency, the pump control system can be used to switch
off this and the other pumps, while at the same time
disengaging the power used to drive the reactor.
The apparatus shown diagrammatically in Fig. 5 is
particularly useful in stripping nickel from waste water.
When treating nickel, for- example, the anolyte can be a
mixture of sulphuric acid and sodium sulphate, with an addi-
tive of lactic acid. Although the anolyte will become con-
taminated, it has been found that significant working life
can be achieved using this arrangement with a very small
usage of anolyte.
The power supply control described above can thus
maintain a constant current and sense a significant rise in
voltage when the secondary electrode component of the work-
ing electrode hecomes coated. If preferred, a voltage con-
trol can be used, and a sudden decrease in the current
required can thûs be used as a trigger. The system can also
be controlled by either setting the voltage and rnonitoring
the current requirements or by setting the current and moni-
toring the voltage requirements. Figs. 6 to 8 illustrate
some of the results obtainable with apparatus of the type
described. Fig. 6 thus illustrates the results obtained
using a working electrode having 79O/o voidage, an average
pore size of 18 ~m, a pore size distribution of 98% in the
~1~892S
range of from 1 to 100 ~m and a surface area to volume
ratio of 5,600 cm2/cm3. As can be seen from Fig. 6, the
initial nickel content of the waste water was 4,000 parts
per million (p.p.m.). After twenty seconds, that concentra-
tion had diminished to about 2,000 p.p.m., and subsequently
concentrations down to 1 p.p.m. were obtained in about 120
seconds. Such small residence times make the present pro-
cess reasonably viable for use in a real time environment.
This is an extremely important consideration in any com-
merical process, particularly where the treatment is madenecessary by legislation and does not add to the quality of
the finished product being made by a given commerical pro-
cess.
Comparable results to those shown in Fig. 6 are
shown in Fig. 7 for the removal of copper from a solution
thereof. In this case, it can be seen that for very short
fixed residence times of 1.75, 3.45 and 5.15 seconds, the
percentage of copper removed from solution approached 100%
using current densities below about 50 mA/cm2. In all of
these examples, the feed stream had a copper concentration
of about 180 p.p.m.
Further comparable results for the removal of
zinc from solution are shown in Fig. 8. In this case, the
feed contained 10 p.p.m. zinc, and the residence time was
again very short, in this case 3 seconds. It can be seen
that in this case the percentage of zinc removed approached
100% when current densities of below 75 mA/cm2 were used.
It has been found that this completeness conversion of
almost 100% can only be obtained by preventing the solution
from bypassing the electrode, either by passing aroung it
or through a relatively ineffective portion thereof.
--~4-
117~9Z5
Figs. 6 to 8 thus demonstrate some of the results
which can be achieved using the present reactor and cell
therefor. After the materials have been removed from the
waste water, they can thus be quickly stripped from the
working electrode, using a suitable electrolyte. This
yields an output containing a high concentration of the
metal being removed. This output can be either used in
other processes, or can itself be stripped electro-
chemically using a kinetically controlled system. Because
the concentrations of this output can be very high, the
efficiency of the kinetically controlled system provides no
difficulty.
It will be understood by those skilled in the art
that the above-described embodiments are meant to be merely
exemplary and that they are, therefore, susceptible to modi-
fication and variation without departing from the spirit
and scope of the invention. For instance, the flow arrange-
ment can be varied and in general, particularly if the
effects of the secondary electrode component are of para-
~,0 mount importance, any suitable conductive medium can beused in place of a carbon fiber cloth. Also, the secondary
electrode component can be a perforated sheet instead of
the mesh shown in the apparatus illustrated in the figures.
Thus, the invention is not deemed to be limited except as
defined in the appended claims.
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