Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MANUFACTURING DIAPHRAGM ELEMENT AND DIAPHRAGM ELEMENT.
FIELD OF THE INVENTION
The invention relates to metallurgy, in particular to metallurgy of
heavy non-ferrous metals, and more particularly to methods of manufacturing
structural components of diaphragm cells used in electroextraction of metals,
such as nickel, cobalt, etc., from aqua solutions.
BACKGROUND OF THE INVENTION
Electrolysis is known to be used as one stage in the metal manufac-
turing process (Yu.V. Baimakov & A.I. Zhurin, Electrolysis in Hydrometallurgy,
Ferrous and Non-Ferrous Metallurgy Publishers, Moscow, 1963, pp.136 - 142,
316 - 327, 362 - 363). Electrolysis generally takes place in a bath, which is
supplied with an electrolyte, positive electrodes, i.e. anodes, and negative
electrodes, i.e. cathodes. By way of example, in electroextraction of nickel,
an-
odes are usually plates cast from lead or other "soluble" anodes, while cath-
odes are thin nickel priming plates. An electric field is applied between the
2o electrodes, and a metal, e.g. nickel released in the process from the
solution or
dissolved from the anode, stratifies on fihe cathode surfaces due to the influ-
ence of the electric field.
Depending on the case, either anodes or cathodes are arranged
within a diaphragm cell comprised of a frame and a diaphragm element. The
2s frame is usually a U-shaped or rectangular piece including a pair of
vertical
beams, and a lower and upper supporting beams. The vertical beams and the
lower supporting beam of the frame are chute-shaped to provide shields
against growth of dendrites over the cathode edges, and can be made of
wood, plastic or plastic-insulated metal.
3o A diaphragm element is generally a bag-like piece made from a dia
phragm fabric and arranged around the frame. Alternatively, sheet-like dia
phragm pieces are arranged on both sides of the frame by suitable fastening
members (D.J. Robinson and F. Day, Cathode Boxes and Anode Bags (Elec
trode Containers) in Electrometallurgy, ALTA 2000 Nickel/Cobalt-6, Technical
35 Proceedings, Technical Session 8, May 15, 2000).
The diaphragm fabric is generally flax tarpaulin, flax/Dacron fabric or
a fabric made of synthetic fibers, such as polypropylene, polyacrylonitrile,
poly-
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vinyl alcohol, etc. The diaphragm fabric must exhibit minimum resistance to
passage of electric current, a predetermined electrolyte flow-through capacity
and chemical resistance.
In electrolysis, a supply solution is delivered through a feed channel
into a space defined by a diaphragm cell. Undesirable particles are removed
from the process into a discharge channel.
The closest prior art related to the present invention is:
- a method of manufacturing a diaphragm element of a cell for elec-
troextraction of metals from aqua solutions, the method comprising the step of
forming from a diaphragm fabric a sheet- or bag-like element comprising at
least one side surface with vertical edges and horizontal edges (D.J. Robinson
and F. Day, Cathode Boxes and Anode Bags (Electrode Containers) in Elec-
trometallurgy, ALTA 2000 Nickel/Cobalt-6, Technical Proceedings, Technical
Session 8, May 15, 2000);
- a diaphragm element of a cell for electroextraction of metals from
aqua solutions, which element is formed from a diaphragm fabric and compris
ing at least one side surface with substantially vertical edges and
substantially
horizontal edges (D.J. Robinson and F. Day, Cathode Boxes and Anode Bags
(Electrode Containers) in Electrometallurgy, ALTA 2000 Nickel/Cobalt-6,
2o Technical Proceedings, Technical Session 8, May 15, 2000).
Practical tests conducted at the cells for electroextraction of nickel
have shown that in the course of electrolysis distortions appear in the
electric
field between the anode and the cathode near the frame, as result of which the
metal stratifies between the frame and the diaphragm element. In addition, af-
ter numerous discharges of cathodes and anodes from the electrolytic bath,
electrically conductive solid particles (anodic slurry, cathode debris, copper
sponge, etc.) accumulate over the frame surfaces, and as early as after 20
days in operation, especially at an increased (above 250 A/m2) current
density,
the frame surface turns to be electrically conductive. As the result, the
frame
3o no longer acts as a shield, and, consequently, dendrites grow along the
cath-
ode edges. Additionally, the stratified metal damages the diaphragm fabric.
The aforementioned deficiencies disturb the production and impair
the product quality. Furthermore, replacement of a damaged diaphragm in-
volves considerable costs.
SUMMARY OF THE INVENTION
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The object of the invention is to provide a method of manufacturing
a unique diaphragm element of a cell for electroextraction of metals from aqua
solutions, which method ensures the disturbance-free process independent of
electric properties of the diaphragm fabric, frame and electrolysis
parameters,
and the production of high-quality cathode metal with dense, smooth edges
that need no cutting.
In addition ~to the aforementioned object, the present invention re-
duces materials and labor spent for manufacture and operation of diaphragm
cells of electrolytic bath, and improves quality of the cathode metal due to
re-
duced metal crops.
The above objects are accomplished in a method of manufacturing
a diaphragm element of a cell for electroextraction of metals from aqua solu-
tions, the method comprising the step of forming from a diaphragm fabric a
sheet-like or bag-like element comprising at least one side surface with
vertical
~5 edges and horizontal edges, wherein according to the present invention at
least one edge of at least one side surface of the diaphragm element is formed
with an electrically insulating edge portion having a predetermined width and
specific electrical resistance of at least 5-fold compared to the specific
electri-
cal resistance of the middle portion of the side surface.
2o The term "specific" used herein with respect to the electrical resis-
tance of a diaphragm element refers to the electrical resistance related to
the
area of the measured portion of the diaphragm element.
The term "fabric" used herein refers to a woven or non-woven mate-
rial made from individual threads or fibers; a woven material (i.e. woven
fabric)
25 is used in a preferred embodiment.
In a preferred embodiment of the invention, the electrically insulating
edge portions having a predetermined width are formed at at least one of the
vertical edges and at the horizontal edges of at least one side surface of the
diaphragm element, so that an electrically conductive window defined by the
3o edge portions is formed in the middle portion of the side surface.
In another preferred embodiment of the invention, the edge portion
(portions) of the diaphragm element is/are treated with an electrically
insulating
material on at least one surface of the diaphragm fabric.
In another preferred embodiment of the invention, the edge portion
s5 (portions) of the diaphragm fabric is are provided with threads or
individual fi
bers having at least external surface made of a material with a poorer
electric
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conductivity than that of the material of the th reads or fibers in the middle
por-
tion.
In another preferred embodiment of the invention, one or more dia-
phragm preforms provided with the edge portions is/are forming prior to said
s forming of the diaphragm element.
In another preferred embodiment of the invention, prior to forming
the sheet-like or bag-like element, an initial elongated web is formed from
the
diaphragm fabric, several successive diaphragm preforms are formed in the
initial web, and individual diaphragm preforrns are detached from the initial
web.
In another preferred embodiment of the invention, the bag-like dia
phragm element having at least two side surfaces is formed by substantially
tightly attaching the vertical edges of the side surfaces to each other by a
seam being formed by melting the material of preform edge portions or the ma
15 terial provided (deposited) in the edge portions of the side surFaces.
In another preferred embodiment of the invention, the diaphragm
element is formed by a thermal welding method.
In another preferred embodiment of the invention, the diaphragm
element intended for a cathode cell is formed by providing an electrically
insu
20 lating edge portion having a specific electrical resistance, which is at
least 50
fold compared to the resistance of the middle portion of the side surFace.
In addition, the aforementioned objects are attained in a diaphragm
element of a cell for electroextraction of metals from aqua solutions, the dia-
phragm element being formed from a diaphragm fabric and comprising at least
2s one side surface with vertical edges and horizontal edges, wherein
according
to the present invention at least one edge of at least one side surface of the
diaphragm element is formed with an electrically insulating edge portion
having
a predetermined width and specific electrical resistance of at least 5-fold
com-
pared to the specific electrical resistance of the middle portion of side
surface.
3o In a preferred embodiment of the diaphragm element, the vertical
edges and the horizontal edges of the side surface of the diaphragm element
have electrically insulating edge portions with a predetermined width, so that
a
window having a better electrical conductivity is defined in the middle
portion of
the side surface.
35 In another preferred embodiment of the diaphragm element, the
electrically insulating edge portion or portions is/are denser in respect of
liquid
permeability than the middle portion of the side surface.
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In another preferred embodiment of the diaphragm element, at least
one edge portion is formed by treating the diaphragm fabric with an
electrically
insulating material.
In another preferred embodiment of the diaphragm element, said at
5 least one edge portion includes threads or individual fibers having at least
ex-
ternal surface made of a material having a poorer electric conductivity than
that
of the material of threads or fibers in the middle portion. Said at least one
edge
portion is preferably formed by adding threads made of an electrically insulat-
ing material (i.e. a material having a poor electrical conductivity).
In another preferred embodiment, the diaphragm element is a bag
having one open edge and assembled by a thermal welding method.
In another preferred embodiment of the diaphragm element, when
the diaphragm element is used in a cathode cell, the specific electrical resis-
tance of the edge portion of the diaphragm element is at least 50-fold com-
pared to the specific electrical resistance of the middle portion.
Practical tests have demonstrated that the use of a diaphragm cell
with the diaphragm element made as described above contributes to maximum
electric insulation of cathode edges in electrolysis process.
The provision of at least one side surface of the diaphragm element
2o with the electrically insulating edge portion having a predetermined width
and
the claimed specific electrical resistance makes possible to avoid distortions
of
the electric field and ion movement caused by the frame. Acting as shields,
the
electrically insulating edge portions inhibit growth of dendrites over the
cathode
edges. The electrically insulating edge portions prevent metal from depositing
(stratifying) between the frame and the diaphragm, in which case damage to
the diaphragm element can be avoided.
The formation of electrically insulating edge portions having a pre-
determined width at at least the vertical edges and horizontal edges of at
least
one side surface of the diaphragm element to form an electrically conductive
so window defined by the edge portions and arranged in the middle portion of
the
side surface, confines the effective area of the side surface of the diaphragm
element so that ions moving in the electrical field permeate only the middle
portion of the side surface, i.e. the current is "focused", this essentially
improv-
ing efficiency of electrolysis. The quality of metal released at the electrode
will
also improve.
Practical tests have shown that the width of the edge portions can
be selected depending on each particular case. The edge portion generally ex-
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tends to 5 - 15 cm inside from the external edge of the diaphragm element,
i.e.
towards the middle portion thereof.
It has been found advantageous to make one or more diaphragm
preforms with the aforementioned electrically insulating edge portions before
s the formation of the diaphragm element.
Practical tests have shown that the edge portions of the diaphragm
element can be made electrically insulating by making them denser in respect
of liquid permeability than the middle portion of the side surface, and by
treat
ing the edge portions at least at one side of the diaphragm fabric with an
elec
trically insulating material. The electrically insulating material can be, for
ex
ample, one of the following materials: polyvinylchloride, polyvinyl resin,
poly
urethane, silicone, neoprene, polychloroprene, etc. The edge portions may be
treated, for example, by spraying, impregnating, rolling, brushing or using
print
ing methods such as the screen printing technique. The treatment may be car
~s tied out on one side of the fabric or on its both sides.
In addition, it has been found that the electrically insulating edge
portions can be formed in the process of producing the diaphragm fabric.
In this case the edge porkions include threads or individual fibers
having at least external surFace made of a material with a poorer electrical
2o conductivity than that of the material of threads or fibers in the middle
portion.
The edge portions can be fully or partly made of the threads having a surface
with a poorer electrical conductivity. The edge portions can also differ from
the
middle portion by the presence of additional threads having a surface with a
poorer electric conductivity. In the latter case, the edge portions further
exhibit
2s a better density in respect of liquid permeability.
The phrase "at least external surface" used herein means that the
threads or fibers can have heterogeneous composition in transition from the
core to the surface. By way of example, a diaphragm fabric (woven fabric) can
be made (woven) from fibers or threads having a core with a relatively good
so electric conductivity and with an insulating surface layer, which has a
poor
electric conductivity and covers the core. When the surface layer is then re-
moved from the threads in the middle portion by a chemical reagent or by heat-
ing, the cores of the threads are exposed, so that a portion with a very good
conductivity will be formed in the middle of the diaphragm element. In this
case
3s the edge portions will have a poor electric conductivity since the
insulating sur-
face layer will still cover the threads in the edge portions.
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If necessary, different manufacturing methods can naturally be
combined.
An optimal sequence of forming a diaphragm element has been
found during the practical tests, including the steps of forming an initial
elon-
gated web before forming a sheet-like or a bag-like element from the dia-
phragm fabric; forming several successive diaphragm preforms in the initial
web; detaching the individual diaphragm preforms from the initial web; and
forming the diaphragm element from one or more diaphragm preforms, this al-
lowing a substantial reduction in material and labor costs. To facilitate
further
~o installation of the diaphragm element on the frame, longitudinal edge
portions
can be located at a distance from the outer longitudinal edges of the initial
elongated web.
The tests have also demonstrated that formation of the bag-like dia-
phragm element by tightly attaching the vertical edges of the side surfaces
and
15 forming a seam by melting the material of preform edges or the material de-
posited on the edge portions of the side surfaces substantially increases life
time of a cell for electroextraction of metals.
The seam can be formed by heating/melting the edge portions using
hot air blowing or rolling between heated rolls. An additional component can
be
2o positioned in the seam region during the attachment to provide a desired
strength and density of the seam.
It is easier to form the seam if the material for the diaphragm fabric
and/or the edge/seam portions is selected according to the attachment tech-
nique to be used.
2s The bag-like diaphragm element can be assembled using methods
of thermal welding, e.g. high-frequency or ultrasonic welding.
It has been further found that the edge portions extending to the ex-
ternal edges of the web are preferably treated with an electrically insulating
material that not only exhibits desired electrically insulating properties,
but also
3o is suitable to form the seams. Strength of the seam substantially depends
on
the selected electrically insulating material.
Practical tests have shown that the specific electrical resistance of
the edge portion of a diaphragm element intended for a cathode cell -has to be
at least 50-fold compared to the specific electrical resistance of the middle
por-
35 tion of its side surface.
Increasing the specific electrical resistance of the edge portions of
the diaphragm element less than 50 times leads to the cathode edges being
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incompletely shielded and to increased growth of dendrites, and, as a conse-
quence, the cathode metal has a worse quality and the cells fail more fre-
quently.
The magnitude of the specific electrical resistance of the edge por-
n tion of the side surFace of a diaphragm element intended for an anode cell
has
to be at least 5-fold compared to the magnitude of the specific electrical
resis-
tance of the middle portion thereof so that the desired efFects can be
achieved
in respect of ion movement, metal recovery and disturbance-free process.
By way of example, when the d iaphragm element is made from a
polyester woven fabric, its normal specific electrical resistance is 10
Ohm/cm2.
In that case, the specific electrical resistance of the edge portions of the
dia-
phragm element in the cathode cell has to be at least 500 Ohm/cm2 so that the
ratio 50:1 is achieved. Correspondingly, when the anode diaphragm element is
made from the polyester woven fabric, the specific electrical resistance of
the
15 edge portions of the diaphragm element in the anode cell has to be at least
50
Ohm/cm2.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail with reference
2o to the accompanying drawings in Figures 1 to 11, in which
Figure 1 schematically illustrates an arrangement for electroextrac-
tion of metals from aqua solutions;
Figure 2 is a schematic view of a frame on which a diaphragm ele-
ment can be arranged;
25 Figure 3 is a schematic view of a sheet-like diaphragm element;
Figure 4 is a schematic view of a diaphragm cell for electroextrac-
tion of metals from aqua solutions;
Figure 5 schematically illustrates a cross section of the arrangement
of Figure 4 at A - A;
3o Figure 6 is a schematic view of a bag-like diaphragm element;
Figures 7 and 8 are schematic views of diaphragm preforms pro-
vided with edge portions;
Figure 9 is a schematic view of an embodiment of the present inven=
tion;
35 Figure 10 schematically illustrates a process of forming a bag-like
diaphragm element from a diaphragm preform, and
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Figure 11 is a schematic view of a bag-like diaphragm element
made in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 illustrates an arrangement for electroextraction of metals, in
particular nickel, from aqua solutions. The arrangement comprises an elec-
trolysis bath 1 where a nickel sulphate mixture is used as electrolyte 2.
Positive
electrodes, i.e. anodes 3, and negative electrodes, i.e. cathodes 4, are alter-
nately arranged in the bath 1. In the embodiment shown in Fig.1 the cathodes
4 are arranged in a diaphragm cell 5. Nickel is supplied from a feed channel 6
as nickel sulphate solution into a space defined by the diaphragm cell 5
around
a priming plate functioning as the cathode 4.
In an alternative embodiment, the anode 3 may be arranged in the
space defined the diaphragm cell 5, with the cathode 4 being outside the cell
5.
The treated solution is guided into a discharge channel 7, from
which it returns to the solution cycle of the system.
A frame 8 illustrated in Figure 2 is substantially a U-shaped mem-
ber. The frame 8 consists of vertical beams 9 and 10 and a horizontal beam
11, which connects the lower parts of the vertical beams. Upper parts of the
vertical beams 9 and 10 can support each other through a supporting beam
12.
Figure 3 illustrates a sheet-like diaphragm element 13 of the cell 5,
made in accordance with the present invention and having electrically insulat-
ing edge portions 14, 15 and 16.
Figure 4 illustrates the diaphragm cell 5 comprising the sheet-like
diaphragm element 13 attached to the frame 8 by a fastening member 17.
The fastening member 17 may be, for example, a strip-like piece
made of elastic material, which can be pushed into a groove 18 provided in the
3o side surface of the frame 8 together with the diaphragm element 13, in
which
case said member presses the diaphragm element 13 into place. Other fasten-
ing members 17 based on friction or form clamping can be naturally used. In
some cases, the fastening member 17 may be. integrated into the diaphragm
element.
Figure 5 is a cross-sectional side view of the arrangement shown in
Figure 4.
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The sheet-like diaphragm element 13 comprises a first side surface
13a and a second side surface 13b. The diaphragm element 13 is attached to
the groove 18 provided in the side surface of the frame 8 by the elastic
fasten-
ing element 17.
5 Figure 6 illustrates a bag-like diaphragm element 13 in accordance
with the invention, where separate side elements 13a and 13b are substantially
tightly attached to each other by stitches 19a, 19b and 19c. Vertical edges of
the diaphragm element 13 are provided with edge portions 14 and 15, and the
lower edge is provided with an edge portion 16. The edge portions 14, 15 and
16 are electrically insulating and define the electrically conductive middle
por-
tion 20. The upper part of the diaphragm element 13 is open so that the frame
8 and the electrode 3 or 4 can be arranged inside it.
Figure 7 illustrates part of an elongated web 21 made from a dia
phragm fabric in accordance with the present invention having electrically
insu
lating edge portions: longitudinal edge portions 14 and 15 and transverse edge
portions 22 with respect to a length of the web 21. The edge portions 14, 15,
22 define an electrically conductive window 20 therebetween. Thus, the web
21 comprises several successive diaphragm preforms 24. The longitudinal
edge portions 14, 15 can be spaced at a distance from the longitudinal outer
2o edges of the web 21 if this facilitates the assembly of the diaphragm
element
or its installation in the frame. Alternatively, the edge portions 14, 15 may
ex-
tend up to the edges of the web 21.
In Figure 7, dash lines 23 mark transverse cut-off points where indi
vidual diaphragm preforms 24 can be separated from each other. If separate
diaphragm elements 13 to be arranged only on one side of the frame 8 are
formed, the cutting has to be also performed at a solid line 25. If, on the
other
hand, only one diaphragm preform extends as a sheet-like piece to both sides
of the frame 8 as shown in Figure 5, or a diaphragm bag is formed from the
preform, a fold is formed at the solid line 25, as will also appear from
Figure
3o 10.
The web 21 shown in Figure 8 differs from the web 21 shown in Fig-
ure 7 in that the cut-off line 23 is at a midpoint of the electrically
conductive
window 20. Thus, the edge portions 14, 15 and 16 form a U-shaped edge pat-
tern in the diaphragm preform 24 that is formed. The diaphragm preform 24
may be folded at the solid line 25 into a bag or into a sheet-like element.
Alter-
natively, the preform 24 can be cut at the solid lines 23 and 25 into sheets,
which can be arranged on one side of the frame 8. As appears from Figure 8,
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the outermost longitudinal edges of the web 21 can be provided with seam por-
tions 26 for the formation of seams so that the vertical seams of the
diaphragm
bag can be formed utilizing high-frequency welding or ultrasonic welding.
Other
thermal welding methods may also be used for this purpose. The material of
s the seam portions 26 can also be heated/melted for attachment by hot-air
blowing or by rolling it between heated rolls. If necessary, the seam portions
can be provided with an additional component during attachment so as to en-
sure the desired strength and tightness of the seam.
Figure 9 illustrates an embodiment of the invention using a suitable
device. A web 21 with a predetermined width is made from the diaphragm fab-
ric, and the web is run from a reel 27 onto a first roll 28. The roll 28
comprises
a longitudinal portion 29, which, when the roll 29 is in operation,
sequentially
presses the desired transverse edge portions 22 made of an electrically insu-
lating material against the web 21. The device further comprises one or more
1s second rolls 30, which are constantly in contact with the web 21 and form
lon-
gitudinal electrically insulating edge portions 14 and 15 in the web 21. In
addi-
tion, the device may comprise one or more third rolls 31, which form seam por-
tions 26 (shown in Fig. 8) at the outermost edges of the web 21. After being
treated with a suitable electrically insulating material, the web 21 is heated
with
2o a heating device 32, in which case the coating attaches to the fabric and
hard
ens. The diaphragm preforms 24 (shown in Fig. 8) are detached from the web
21 by a cutting unit 33 which may be a device of any type known from the prior
art. Furthermore, the device may include an automatic assembly unit 34 where
the diaphragm preform 24 can be assembled into a bag and seamed or
2s welded.
An alternative for forming the edge portions 14, 15, 22 of the dia-
phragm preforms 24 in the web is to use a suitable printing method, e.g. the
screen printing technique.
A further alternative is to form the edge portions 14, 15, 22 com
so pletely or partly during the manufacture of the diaphragm fabric. In that
case,
the edge portions 14, 15, 22 can be provided with threads having a surface
with a lower electric conductivity than that of the threads to be arranged at
the
middle portion 20. If necessary, different manufacturing methods can naturally
be combined.
35 Figure 10 illustrates how the diaphragm preform 24 can be folded
into a diaphragm bag. A fold is formed at the lower part of the preform 24.
The
vertical edges of the preform 24 are tightly attached to each other so as to
form
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a substantially liquid-impermeable seam 35. The seam 35 can be formed by
sewing or by a specific fastening member. In addition, the sear~n 35 can be
formed by melting, e.g. using a thermal welding method, or by melting and at-
taching the preform edges to each other. It is easier to form the seam 35, if
materials are selected for the diaphragm fabric and the edge/seam portions
according to the attachment technique to be used.
Figure 11 shows a bag-like diaphragm element made in accordance
with the present invention. The upper edge 36 of the bag is naturally left
open
so that the frame 8 and the electrode 3 or 4 can be arranged inside the bag.
The edge portion 37 above the electrically conductive portion 20 forms an elec-
trically insulating portion at the supporting beam 12 of the frame 8, which
pre-
vents metal from stratifying in this section, too.
The drawings and the related description are only intended to illus-
trate the inventive concept. The details of the invention may vary within the
~5 scope of the claims.
Examples
Example 1. Method of determining the specificelectrical resistance
The example describes a method of determining the specific electri-
cal resistance of a diaphragm (particularly from the woven fabric) used in a
2o specific embodiment of the present invention. However, the invention is not
re-
stricted by the described method, and another techniques may be also used.
The specific electrical resistance was measured by the compensa-
tion method, i.e. using comparison of an unknown resistance with a known
one.
25 1. Arrangement
An arrangement used for making the measurements comprised an
ac power supply, an ammeter, a voltmeter and electrodes placed in a vessel
with an electrolyte and a sample to be studied.
The ac power supply was a low-frequency signal generator GZ-102.
3o AC current of sonic frequency (e.g. about 800 Hz) was used to prevent elec
trolysis processes and electrode polarization. The ammeter was B7-35 mul
timeter. The voltmeter was B7-22A multimeter. The electrodes were platinized
electrodes with contacts, clamped between a pair of plates made of organic
glass 4.2 mm thick. One of the plates had an opening 33 mm in diameter (8.55
35 cm2). The electrolyte was 10 % aqueous solution of sodium chloride.
2. Preparation to measurements
2.1 Platinizing of electrodes
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To prevent possible polarization, surfaces of electrodes were plati-
nized, i.e. plated with finely dispersed platinum (generally referred to as
plati-
num black). The solution used for platinizing consisted of 3 g platinum hydro-
chloric acid and 0.25 g lead acetate per 100 ml water. The platinum electrodes
s were connected, via a rheostat, to 4 V accumulator. By reducing the rheostat
resistance, electric current was increased until gas started to abundantly re-
lease at the electrodes. The platinization lasted 10 minutes for fresh elec-
trodes, and 2 - 3 minutes for used electrodes. Direction of current was
switched every 30 seconds. The electrodes were platinized until a thin layer
of
platinum black formed thereon. After the platinizing, the electrodes were care-
fully rinsed in distilled water. Then the electrodes were placed into 10 %
aque-
ous solution of sulfuric acid and after 1 - 2 minutes electric current was
passed
through the solution from the accumulator, where the electrodes were used as
a cathode (-), and platinum wire was an anode (+). Chlorine adsorbed by the
15 platinum black, has been transformed into hydrogen chloride that dissolves
in
water.
2.2. Preparation of electrolyte
For making measurements, a fresh 10 % aqueous solution of so-
dium chloride (NaCI) was prepared.
20 2.3. Preparation of fabric samples
Four samples of woven fabric with diameter 50 mm were held in dis-
tilled water at 20°C and vacuum 0.6 atm during 20 hours, and then in 10
% so-
lution of sodium chloride during 30 minutes.
3. Determination of the specific electrical resistance
25 After energizing the arrangement, measurements were made at
20°C of the sodium chloride solution. The specific electrical
resistance of
woven fabric samples was determined at 800 Hz frequency of the signal gen-
erator with the scale limit key set at 10 V.
The signal generator output was provided to B7-35 multimeter oper
3o ating in the ammeter mode. Measurements were made at 90.0 mA readings
set by the generator output control.
A vessel was filled with a fresh 10 % aqueous solution of sodium
chloride. Electrodes were placed exactly opposite one another in a rectangular
plastic reservoir filled with electrolyte solution and fixed by a retainer.
Voltage
3s readings were taken from the B7-22A multimeter operating in the voltmeter
mode. Measurements were made at the 2 V scale limit.
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First, voltage of a pure aqueous solution of sodium chloride was
measured at 20°C, which was within the range of from 89.7 to 89.8 V. A
pre
pared sample of fabric was clamped between the platinized electrodes, placed
in the reservoir with electrolyte and fixed. Then ammeter and voltmeter read
s ings were taken: The measurements were made at 4 samples.
The specific electrical resistance of the fabric, in Ohm/cm2, was cal-
culated from the formula:
VF Vs
RsP - _ l SF
IF Is
where VF is the voltmeter reading when the woven fabric sample is
present, mV;
IF is the ammeter reading when the woven fabric sample is present,
mA;
Vs is the voltmeter reading in pure solution, mV;
IS is the ammeter reading in pure solution, mA;
15 SF is the cross-section area of the woven fabric sample being
tested, cm2.
The arithmetic average of all of the measurements made at the four
woven fabric samples was taken as the final specific electrical resistance of
the
woven fabric.
2o Example 2
Following Examples 2 and 3 of diaphragm elements and methods of
electrolysis using such diaphragm elements are given for electroextraction of
nickel from aqua solutions, where Example 2 relates to the use of such dia-
phragm elements in cathode cells, and Example 3 relates to using them in an-
25 ode cells.
Specifically, a cathode cell operates as follows. A cell with a cath-
ode accommodated therein is placed in an electrolysis bath, and a cathodic
liquor is supplied at a flow fate of 25 - 30 dm3/h to the upper part of the
bath.
The cathodic liquor is mixed in the cell by moving from the top down due to
the
so difference in the specific weights of the electrolyte being supplied and
that al-
ready presented in the cell. The cathodic liquor infiltrates into the anode
space
through the diaphragm element.
The level of the cathodic liquor in the cell is maintained higher than
that in the anode space to prevent reverse passage of the electrolyte. Elec
35 trolysis of nickel is generally conducted at the following process
parameters:
current density 240 - 300 AIm2; inter-electrode space 155 ~ 5 mm; voltage 2.6
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- 3.0 V; electrolyte temperature 75 - 85°C; cathode building-up period
4 - 6
days.
Forty three (43) diaphragm elements, i.e. the number required to
equip a single electrolysis bath, were manufactured for cathode cells of an
5 electrolyser intended for nickel electroextraction from aqua solutions. Each
of
the cells includes a diaphragm bag made in accordance with the present in
vention from technical polyester woven fabric (Mark 2255-V5) of size' 1100 x
1100 mm, put on a frame made of polypropylene (Mark 01005 according to
Spec. 6-05-1105-83) and comprising two beams with 1100 mm length and a
support beam with 950 mm length welded to each other at the lower corner
joints. At the upper part, the beams were joined by a pair of titanium
(titanium
BT-1 ) straps of size 5 x 20 x 950 mm through pins made from titanium wire
with diameter of 3 mm. A shield of polyvinylchloride resin was applied on the
external surface of the diaphragm element along the frame to increase 50
15 times the specific electrical resistance of the diaphragm element.
The resulting cathode cells for electroextraction of nickel were used
as follows. The cells were placed into an electrolysis bath and charged with
nickel priming plates and cathodic liquor charged at the flow rate of 25
dm3/h.
The cathodic liquor was mixed in the cell by moving from the top down due to
2o the difference in the specific weights of the electrolyte being charged and
that
already presented in the cell. The cathodic liquor infiltrated into the anode
space through the diaphragm element. The level of the cathodic liquor in the
cell was maintained 50 mm higher than that in the anode space to prevent the
reverse passage of the electrolyte. The electrolysis process to obtain nickel
was conducted at the standard process parameters: current density 250 A/m2;
inter-electrode space 155 mm; voltage 2.7 V; electrolyte temperature
75°C,
and cathode building-up period 5 days.
When a cathode cell failed due to damage to the diaphragm ele
ment or the frame, the cathode cell was discharged from the electrolysis bath
3o and, after replacement of the damaged component, was used again.
The following table summarized operation results of additional four
exemplary diaphragm elements of the cathode cell, which differ in the pres-
ence of edge portions at the diaphragm elements, the specific electrical resis-
tance and the method of diaphragm element manufacturing.
~ Example 3
An anode cell of an electrolyser for electroextraction of nickel from
aqua solutions operates as follows. The cell with an anode placed therein is
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16
put into an electrolysis bath, and a cathodic liquor is supplied in the
electrolysis
bath at a flow rate of 0.015 - 0.025 dm3/A~h. The cathodic liquor infiltrates
into
the cell through the diaphragm element and turns to anodic liquor, which is
then withdrawn from the cell at the flow rate proportional to the flow rate of
supplying the cathodic liquor into the electrolysis bath and the number of
cells
in the bath. The level of the anodic liquor in the cell is maintained lower
than in
the cathode space to prevent reverse passage of the electrolyte. The electroly-
sis process to obtain nickel is conducted at the following process parameters:
current density 180 - 250 Alm2; inter-electrode space 110 - 155 mm; voltage
3.0 - 5.0 V; electrolyte temperature 65 - 85°C, and cathode building-up
period
4 - 6 days.
Forty five (45) diaphragm elements were manufactured for anode
cells intended for electroextraction nickel from aqua solutions, the number re-
quired to equip a single electrolysis bath. Each of the cells included a dia-
. phragm bag made in accordance with the present invention from technical
polyester woven fabric (Mark 2255-V5) of size 1300 x 1100 mm put on the
frame made of polypropylene (Mark 01005 according to Spec. 6-05-1105-83)
and comprised of a pair of beams viiith length 1300 mm and a support beam
with length 950 mm welded to each other at the lower corner joints. At the up-
2o per part the beams were joined by a pair of titanium (titanium BT-1 )
straps of
size 5 x 20 x 950 mm through pins made from a titanium wire with diameter of
3 mm. A layer of polyvinylchloride resin was applied on the external diaphragm
surface to increase 5 times the specific electrical resistance of the
diaphragm.
The resulting anode cells for electroextraction of nickel were used
2s as follows. The cells were placed in the electrolysis bath and charged with
lead
anodes. Cathodic liquor was charged into the electrolysis bath at the flow
rate
of 360 dm3/h. The cathodic liquor infiltrated into the cells through the dia
phragm element. The level of the anodic liquor in the cell was maintained 30
mm below than in the cathode space to prevent reverse passage of the elec
so trolyte. The anodic liquor was withdrawn from the cells at the flow rate of
8
dm3lh. The electrolysis process to obtain nickel was conducted at the
following
process parameters: current density 200 A/m2; inter-electrode space 135 mm;
voltage 4.0 V; electrolyte temperature 70°C, and cathode building-up
period 6
days.
35 When an anode cell failed due to damage to the diaphragm element
or the frame, the anode cell was discharged from the electrolysis bath and, af-
ter replacement of damaged component, was used again.
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17
The following table summarizes the results of using four additional
exemplary diaphragm elements of the anode cell, which differ in the presence
of edge portions at the diaphragm, the specific electrical resistance and the
method of diaphragm element manufacturing.
Tahla
TestCell DiaphragmPolyesterIncreaseYield Max. DielectricFailed
No. type fabricationthread in specificof thicknessmaterialcells
method spent resistanceH-1/1y of pro-spent dur-
per of edge cathodeduced per ing slurry
diaphragm,portions,metal, cathodecell, period
m times % metal, g (90 days)
mm
1 Cathodesewing 10 50 87 8 60 30
2 Cathodesewing 10 40 80 5 50 42
3 Cathodesewing 10 500 87 8 100 30
4 Cathodewelding 0 50 87 8 60 26
5* Cathodesewing 10 0 76 5 0 45
6 Anode sewing 10 5 95 8 20 20
7 Anode sewing 10 4 95 8 18 22
8 Anode sewing 10 40 95 8 50 20
9 Anode welding 0 5 95 8 20 19.5
10* Anode sewing 10 0 95 8 0 25
* Prior art diaphragm.
As appears from the Table, the use of the cathode cell with the dia-
phragm element whose surface comprises electrically insulating edge portions
providing 50-fold specific electrical resistance of the diaphragm element at
the
edge portions of its side surface as compared to the specific electrical resis
tance of the middle portion of the side surface enables the yield of high-
quality
metal to be increased from 76 % (Test 5) to 87 % (Test 1 ) and the failure of
~ 5 cells to be reduced from 45 % (Test 5) to 30 % (Test 1 ).
If the specific electrical resistance of the diaphragm element of the
cathode cell at edge portions is increased less than 50 times (Test 2), the
cathode edges and cell frames are incompletely electrically insulated, this re-
sults in increased growth of dendrites and formation of an electrically conduc-
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18
tive layer on the frame surface, and as a consequence the yield of high-
quality
metal reduces to 80 % and the failure of cells increases up to 42 %.
If the specific electrical resistance of the diaphragm element of the
cathode cell in the shield area increases more than 50 times (Test 3), the
addi
tional dielectric is spent to form the edge portions (up to 100 g per cell),
i.e. the
production cost increases. At the same time, no noticeable effect is attained
in
improvement of the cathode metal quality and in failure of the cells.
When the diaphragm elements of cathode cells are made using a
thermal welding method (Test 4), failure of cells is further reduced to 26 %,
and
the production and operation costs can be decreased owing to the reduced
consumption of polyester thread.
If an anode cell is used with the diaphragm element whose surface
has electrically insulating edge portions providing the specific electrical
resis-
tance of the diaphragm element at edge portions of its side surface increased
~5 no less than 5 times compared to the specific electrical resistance of the
mid-
dle portion of the side surface, failure of the cells can be reduced from 25
(Test 10) to 20 % (Test 6).
If the specific electrical resistance value is increased less than 5
times (Test 7), the frame is incompletely electrically insulated, which causes
2o formation of a conducting layer on the frame surface, and as a consequence,
increases failure of the cells up to 22 %.
If the specific electrical resistance value is increased more than 5
times (Test 8), the consumption of the electrically insulating material used
to
provide the shield increases up to 50 g per cell, thereby increasing the
produc-
25 tion cost. At the same time, no noticeable effect is attained in reduction
of fail-
ure of the cells.
When the diaphragm elements of anode cells are made using a
thermal welding method (Test 9), failure of the cells is further reduced to
19.5
%, and the production and operation costs can be decreased owing to the re
3o duced consumption of polyester thread.
Consequently, the practical tests have proved that the present in-
vention ensures the increased yield of high-quality cathode metal, reduces the
material and labor costs, and provides a disturbance-free production cycle.
