Note: Descriptions are shown in the official language in which they were submitted.
11~137f~
.~
EXPLOSION BONDING OF BIPOLAR ELECTRODE BACKPLATES
BACKGROUND OF THE INVENTION
The present invention relates generally to an
electrolytic cell of the filter press type wherein a series
of bipolar electrodes with diaphragms or membranes sandwiched
in between can be used for electrochemical production of
alkali metal hydroxides and halogens. ~ore particularly, the
present disclosure relates to an improved method for connecting
the backplates of the bipolar electrodes by an explosion bonding
process which provides the essential electrical and mechanical
connection therebetween while leaving sufficient air space to
allow hydrogen gas to escape from within the cell thereby pre-
venting hydrogen embrittlement of the titanium anode backplate.
Chlorine and caustic (sodium hydroxide) are essential
and large volume commodities which are basic chemicals required
in all industrial societies. They are produced almost entirely
electrolytically from aqueous solutions of alkali metal chlorides,
with a major proportion of current production coming from the
diaphragm type electrolytic cells. These cells generally have a
plurality of electrodes disposed within the cell structure to
present a plurality of rows of alternatively spaced anodes and
cathodes. These electrodes are generally foraminous in nature
and made of a screen or mesh material so that a hydraulically
permeable diaphragm may be formed over the cathode. This com-
partmental cell will allow fluid flow through the cell structure.
Brine (sodium chloride) starting material is continuously fed
into the cell.through the anode compartment and flows through
the diaphragm backed by the cathode. To minimize back-diffusion
and migration through the hydraulically permeable diaphragm, the
flow rate is always maintained in excess of the conversion rate
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378
so that resulting catholyte solution has unreacted alkali metal
chloride present. This catholyte solution, containing sodium
hydroxide, unreacted sodium chloride, and certain other impur-
ities, must then be concentrated and purified to obtain a
marketable sodium hydroxide commodity and a sodium chloride
solution to be reused in the diaphragm electrolytic cell. This is
a serious drawback since the costs of this concentration and
purification process are rising rapidly.
With the advent of technological advances such as the
dimensionally stable anode which permits ever narrowing gaps
between the electrodes, and the hydraulically impermeable membrane,
other electrolytic cell structures are being considered. The
geometry of the diaphragm cell structure makes it inconvenient
to place a planar membrane between the electrodes, hence the filter
press electrolytic cell structure with planar electrodes has been
proposed as an alternate electrolytic cell structure.
A filter press electrolytic cell is a cell consisting
of several units in series, as in a filter press, in which each
electrode, except the two end electrodes, acts as an anode on
one side and a cathode on the other, and the space between these
bipolar electrodes is divided into an anode and cathode compart-
ments by a membrane. In a typical operation, alkali metal halide
is fed into the anode compartment where halogen gas is generated
at the anode. Alkali metal ions are selectively transported
through the membrane into the cathode compartment, and combine
with hydroxyl ions generated at the cathode by the electrolysis
of water to form the alkali metal hydroxides. In this cell the
resultant alkali metal hydroxide is sufficiently pure to be
commercially marketable, thus eliminating an expensive
salt recovery step of processing. Cells where the bipolar
3~
electrodes and the diaphragms or membranes are sandwiched into
a filter press type construction may be electrically connected
in series, with the anode of one connected with the cathode of
an adjoining cell through a common structural member or
partition. This arrangement is generally known as a bipolar
configuration. A bipolar electrode is an electrode without
direct metallic connection with the current supply, one face
of which acts as an anode and the opposite face as a cathode
when an electric current is passed through the cell.
While the bipolar configuration provides a certain
economy for electrical connection of these electrodes in series
there is a serious problem with the corrosion of cell components
in contact with anolyte. The anolyte normally contains highly
corrosive concentrations of free halide, and the use of base
metals such as iron to contain the solution have proven to be
ineffective.
Proposals to overcome this problem include utilizing
valve metals or alloys thereof to contain anolyte, either by
fabricating an entire electrode from such a corrosion resistant
material or by bonding a coating of valve metal onto a base metal
within the anolyte compartment. The use of large quantities of
expensive valve metals in commercial cell construction, though,
has proven to be economically undesirable. The coated base
metals on the other hand are prone to disintegration by pealing
off of the protective layer and have also proven ineffective. It
has been found that use of an air space between the backplates
will act as an insulator against hydrogen embrittlement since
the hydrogen ions combine to form harmless molecular hydrogen,
which can be vented off, more readily than they move through
the air space. This then provides a convenient means for solving
37~
the embrittlement problem but leaves the problem of properly
connecting the backplates. Resistance welding would be ideal
except that there are only insufficient methods available for
welding different metallic materials together such as steel,
copper and titanium.
Elsctrical and mechanical connection of these bipolar
electrodes has been accomplished by internal bolting systems
wherein the electrode is bolted through one pan, providing a
spaced relation by use of a spacer of some sort, and through the
second pan to the other electrode. Another method employs the
use of an external busbar, outside of the electrolytic cell
structure. Electrical connections made by the internal bolting
systems are undesirable because elaborate sealing schemes are
necessary to prevent electrolyte leakage which could result in
an extreme corrosion of the cathode compartment. This increases
the cell costs and necessitates frequent maintenance. Electrical
connections made externally are also not desirable since larger
power losses are occasioned by the added structural voltage drops.
Thus, it has become exceedingly advantageous to
provide a method for connecting the bipolar electrode backplates
in a spaced relation at a commercially viable cost.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention
to provide a bipolar electrode which is capable of insertion
into a filter press electrolytic cell that will have a greatly
simplified means of connecting the two plates to provide a
bipolar electrode capable of withstanding commercial electro-
chemical production, at a significantly reduced manufacturing
cost.
3~i3
. It is another object of the present invention to
:~ provide an improved method for electrically and mechanically
connecting the anode and cathode backplates of a bipolar
electrode wherein a good current efficiency is achieved such
that commercial electrochemical production would be facilitated
thereby.
These and other objects of the present invention,
together with the advantages thereof over existing and prior
art forms which will become apparent to those skilled in the
art from the detailed disclosure of the present invention as set
forth hereinbelow, are accomplished by the improvements herein
shown, described, and claimed.
It has been found that the anode and cathode back-
plates of a bipolar electrode for use in a filter press
electrolytic cell can be connected mechanically and electrically
by placing a spaced series of strips of a solid metallic
electrical conductor in a spaced relation of at least 0.001
inch (0.0254 mm) from one of the backplates parallel thereto;
placing a layer of a detonating explosive having a detonation
velocity of less than 120 percent of the sonic velocity of the
metal having the highest sonic velocity in the system, on the
outside surface of one of the metallic layers; initiating said
explosive so that detonation is propogated parallel to said
strips of solid metallic electrical conductor such that the
pressure upon collision of the series of strips with the back-
plate is greater than the elastic limit of the metal having the
lowest elastic limit in the system; and connecting the other
backplate to the series of strips of solid metallic electrical
conductor.
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J ~3'78
BRIEF DESCRIPTION OF THE DRA~INGS
FIGURE 1 is a perspective view of the anode and
cathode backplates of a bipolar electrode with the mechanical
and electrical connection effected therebetween by explosion
bonded solid metallic strips therebetween according to the
concepts of the present invention.
FIGURE 2 is a perspective view of an alternate
embodiment of a bipolar electrode according to the concepts
of the present invention.
FIGURE 3 is a side section view of the second
embodiment of the bipolar electrode taken substantially along
line 3-3 of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, numeral 10 generally refers
to a bipolar electrode assembled by a method according to the
concepts of the present invention. The bipolar electrode lO
has an anode 12 which is generally foraminous in nature and can
be made of a screen or mesh material of an appropriate metallic
substance. Such foraminous anodes 12 may be made of any conven-
tional electrically conductive electrolytically active material
resistant to electrolyte and preferably, a valve metal such as
20 titanium or tantalum or alloys thereof bearing on the surface a
noble metal, and noble oxide (either alone or in combination
with the valve metal oxide), or other glectrocatalytically active
corrosive resistant material. Anodes of this preferred class
are called dimensionally stable anodes and are well known and
widely used in the industry. Foraminous anodes such as anode
12 shown in FIG. 1 are generally preferred because of their
378
greater electrolytically active surface areas which facilitate
the èlectrochemical reaction and flow within the compartments
of an electrolytic cell. Each bipolar electrode 10 also has a
cathode 14 on the reverse side thereof. The cathode 14
similarily may be made of any conventional electrically conductive
material resistant to the catholyte, examples including iron,
mild ~teel, stainless steel, and nickel. The cathodes are
preferably foraminous, similar to the anodes. The bipolar
electrode 10 has an anote backplate 16 and a cathode backplate
18 each of which acts as a supporting base for the anode 12 and
the cathode 14 respectively. Generally the anode backplate 16
will be made of the same material as the anode 12 such that
conventional resistance weldments may be accomplished between
the anode 12 and the anode backplate 16. Similarly, the cathode
backplate 18 and cathode 14 are generally made of the same
material for ease of connection therebetween. The bipolar
electrode 10 8~ shown in FIG. 1 when fitted into a filter press
electrolytic cell will have either a frame surrounding the
peripheral edBe of the bipolar electrode or the backplates 16
and 18 may be pan shaped so as to present clamping flanges so
that a liquid tight engagement between a series of these bipolar
electrodes 10 can be accomplished. For ease of illustration,
these supporting structures or frames have not been shown. The
anode 12 and cathode 14 are connected respectively to the anode
backplate 16 and cathode backplate 18 by riser posts or current
distr$butors 20 which are also made of a material corresponding
to the materials of the anode 12 and anode backplate 16 and the
cathode 14 and cathode backplate 18 respectively. This facilit-
ates use of conventional welding techniques for attaching the
anode 12 and cathode 14 to their respective backplates.
37~3
It is desirable to use a valve metal for the anode 12
and the anode backplate 16 since this compartment contains an
anolyte which normally has highly corrosive concentrations of
free halide which can cause corrosion of the anode 12 and anode
backplate 16. The anode backplate 16 will generally have a
-; thickness of 0.040 to 0.080 inch (1.016 to 2.032 m~) when
titanium is used. The cathode 14 and cathode backplate 18 need
not be of such an expensive valve metal since the catholyte is
not nearly so corrosive and generally steel will be used for the
cathote 14 and cathode backplate 18. The cathode backplate 18
will generally have a thickness of 0.080 to 0.50 inch (2.032 to
12.7 ~m) w`ith a preferred thickness of 0.25 inch (6.35 m~) when
steel is used. Since it is believed that hydrogen ions generated
at the cathode can migrate to the anode backplate and anode of
prior art constructions causing hydrogen embrittlement it is
necessary to leave so~e kind of barrier to these ions between
` the anode backplate 16 and cathode backplate 18. Any insulative
- material can be used which will resist the flow of atomic
hydrogen therethrough and it has been found tha~ air provides
such an insulative property very inexpensively since the atomic
hydrogen generally combines to form~molecular hydrogen which is
vented off before the atomic hydrogen reaches the anode backplate
16. Copper also provides a good ~arrier to atomic hydrogen flow
but would be rather expensive if solid copper sheet was used ~ `
between the backplates. Copper does provide excellent electrical
properties though, so to provide this kind of insulative barrier
at a lower cost, a spaced series of metallic electrical conducto s
22 such as copper strips is placed between the anode backplate 16
and cathode backplate 18 so as to conduct an electrical current
and yet provide an insulative zone between the anode backplate
16 and cathode backplate 18 to prevent hydrogen embrittlemene
_
7c3
of the anode backplate 16. This metallic electrical conductor
22 can be of any substance capable of carrying the necessary
amount of electrical current while providing an insulator
against hydrogen ion movement with copper being the preferable
form because of the cost and electrical conductivity thereof.
The method of the present invention employs an
explosion bonding technique to bond the metallic electrical
conductor 22 to the anode 'oackplate 16 and cathode backplate 18
in either a one step or two s'tep process. This process can be
generally achieved by supporting a layer of one material parallel
to the surface of the other material, the inside surfaces being
spaced apart slightly and placing on the outer surface of one
layer a detonating explosive having a velocity of detonation
less than 120 percent of the velocity of sound in that metal
in the system having the highest sonic velocity and thereafter
initiating the explosive layer. Usually it is desirable to use
an explosive having a detonation velocity not greater than the
velocity of sound in that metal with the higher sonic velocity.
The metal layers must be separated from each other a distance
et least sufficient for the explosive propelled layer to achieve
an adequate velocity before impact with the stationary layer,
a spacing of 0.001 inch (0.0254 mm) between the facing surfaces
of the two layers represents the minumum spacing to produce
consistently adequate results. The maximum separation will
almost depend almost entirely upon the reduction of the velocity
of the propelled layer caused by the air layer between the two
metals. By increasi,ng the explosive loading or evacuating the
space between the layers, spacings much greater than 0.001 inch
(0.0254 mm) are feasible. In general, however, separation of
more than 0.5 inch (12.7 mm) is not convenient or necessary.
-- 10 --
378
A 0.0625 inch (1.5875 mm) layer of copper can be clad
onto a 0.5 inch (12.~ mm) a plate of mild steel in the following
manner. The copper sheet was covered on one side with a one
inch (25.4 mm) thick layer of polystyrene foam and the polystyrene
layer was covered with a layer of an explosive composition having
a weight distribution of 10 grams per square inch (0.155 grams
per square mm.). The explosive employed in this example was a
thin uniform sheet of flexible explosive composition comprising
20 percent very fine pentaerythtritol tetranitrate (PETNj, 70
percent red lead, and as a binder, 10 percent of a fifty-fifty
mixture of butyl rubber and a thermoplastic terpene resin
mixture of polymers of B-pinene of the formula (CloH6)n,
commercially available as PICCOLYTE S-10 (manufactured by the
Pennsylvania Industrial Chemical Corp.). Complete det~ils of
this composition and a suitable method for its manufacture are
disclosed in the U.S. Patent No. 3,093,521. The composition
is readily rolled into sheets and detonates at a velocity of
about 4100 meters per second. The edges of the copper-
polystyrene-explos~ve "sandwich" were sealed with waterproof
tape, and sandwich was placed on the mild steel with a spacing
between the copper layer and the steel layer of 0.0138 inch
(0.35 mm) provided by uniform particles of iron powder. These
are particles which have been screened to pass through a number
45 mesh and held on a 100 mesh. The edges of the completed
assembly were sealed with tape and an electrical initiator was
attached to one corner on the explosive layer. The assembly
was then immersed in water and the explosion initiated. Excellent
bonding of the copper onto the steel resulted.
The nex~ portion of the procedure employed a duplica-
tion of the prior process to prepare a ~itanium on copper
7~3
cladding. The titanium layer was 0.05 inch (1.27 mm) thick and
tbe copper layer was the same as the preceding. The spacing,
which in this case was provlded by particles of titanium powder,
was 0.0138 inch (0.35 mm) and the weight of the explosive was 10
grams per square inch (0.155 grams per square mm). Following the
detonation of the,explosive, the titanium and copper sheets were
firmly and uniformly bonded. This then formed a sandwich of the
cathode backplate 18 made of mild steel to the copper metallic
electrical conductor 22 to the anode backplate 16 made of titaniu~.
A one step process can be accomplished by sandwiching
all three components with the same amount of spacing as in the
prior procedure, using-iron particles and titanium particles as
above hereindescribed, placing the explosi~e to a weight distri-
bution of approximately 15 grams per square inch (0.233 grams per
square mm) on top of the titanium anode backplate 16, and ~he
entire structure being sealed inside a box such that the structure
may be submerged in water and the initiator detonated. A solid
bonding between all three components results from such a process.
xplo~. on
Exr~el~ bonding is further described in detail in
the following patent :
U. S. Patent 3,137,937.
By this technique o~ly about 10 percent of the total
area of the anode backplate 16 or the cathode backplate 18 needs
to be bonded to a metallic conductor 22 to provide excellent
current conducting properties. Also, the air space between the
copper strips provides a means for the hydrogen to vent before
it attacks the titanium.
An alternate embodiment of this concept as pictured in
FIG. 2 embodies the use of a cathode backplate 18 lined with a
rubber liner 24 on the surface thereof with an anode connector
.
~ J - 12 -
111~378
plate 26 in the central portion of the rubber liner 24 backed up
by a copper transition 28 between the cathode backplate 18 and
the anode connector plate 26. The anode connector plate 26 is
explosion bonded with the copper transition 28 to the cathode
backplate 18 in the manner heretofore described to provide a
connector surface for the anode 12 to be connected to. In this
way, as seen in FIG. 2, an even smaller area of anode connector
plate made of titanium 26 may be used in a given bipolar electrode
10, thus saving considerable expense. Corrosion protection i8
provided by the rubber liner 24 or some other suitable material
not of a metallic nature bonded to the cathode backplate 18 as
seen in FIGURES 2 and 3. The rubber liner 24 is formed over the
cathode backplate 18 so as to provide gas ducts 30 between the
cathode backplate 18 and the rubber liner 24. These gas ducts
30 permit gaseous substances such as hydrogen to be vented away
from the cathode backplate 18. In this embodiment the copper
transition 28 protects the titanium from hydrogen embrlttlement.
Thus, it should be apparent from the foregoing
description of the preferred embodiments that the method herein
shown and described accomplishes the objects of the invention
and solves the problems attendant to the art heretofore.
.
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