ALUMINUM CATHODE BASE PLATE WITH INSERT OF DIELECTRIC MATERIAL

PLAQUE DE BASE EN ALUMINIUM POUR CATHODE, COMPRENANT INSERTION DE MATERIEL DIELECTRIQUE

English Abstract

There is provided an aluminum cathode base plate which has an insert of dielectric material at an upper edge thereof, in the vicinity of the solution line of the electrolytic cell when the cathode is placed into such cell for the purpose of electrodeposition of zinc. The insert has a flat surface which is raised or projected above the surface of the aluminum base plate by a distance of 0.5 mm to 1.5 mm on both sides of the base plate. In this manner, when the stripping knife strikes the dielectric insert during stripping of the electrodeposited zinc sheets and then penetrates between the zinc sheets and the aluminum base plate, it will not hit and damage the perpendicular interface between the dielectric insert and the aluminum base plate. This will avoid rapid deterioration of the base plate and will significantly improve its life span.

French Abstract

Ici est fournie une plaque de base en aluminium pour cathode qui a une insertion de matériau diélectrique sur son bord supérieur, à proximité de la surface de la solution de la cellule électrolytique lorsque la cathode est placée dans cette cellule aux fins de l'électrodéposition de zinc. L'insertion a une surface plane qui est soulevée ou projetée au-dessus de la surface de la plaque de base en aluminium d'une distance de 0,5 mm à 1,5 mm de chaque côté de la plaque de base. De cette manière, lorsque le couteau à dénuder frappe l'insertion diélectrique pendant le dénudage des feuilles de zinc électrodéposées puis pénètre entre les feuilles de zinc et la plaque de base en aluminium, il ne frappera pas et n'endommagera pas l'interface perpendiculaire entre l'insertion diélectrique et la plaque de base en aluminium. Cela permettra d'éviter une détérioration rapide de la plaque de base et permettra d'améliorer considérablement sa durée de vie.

Claims

CLAIMS
1. An aluminum cathode base plate having an insert of
dielectric material at an upper edge thereof, in the
vicinity of a solution line of the electrolytic cell when
the base plate is placed in such cell, said insert having a
flat surface raised above the surface of the base plate by
a distance such that a stripping knife will penetrate
between the base plate and zinc sheets electrodeposited
thereon without hitting the perpendicular interface between
the aluminum cathode base plate and the dielectric insert,
said insert being so raised at least in the area where said
stripping knife strikes the insert prior to penetrating
between the base plate and the zinc sheets electrodeposited
thereon.
2. An aluminum cathode base plate according to claim 1,
in which the distance by which the surface of the
dielectric insert projects over the surface of the base
plate on each side of said base plate is between about
0.5 mm and 1.5 mm.
3. An aluminum cathode base plate according to claim 2,
in which said distance is about 0.8 mm.
4. An aluminum cathode base plate according to claims 1,
2 or 3, in which the raised edge of the dielectric insert
is chamfered in the region where it meets the aluminum base
plate.
5. An aluminum cathode base plate according to claim 4,
in which said raised insert is chamfered at an angle of
45°.
-9-

6. An aluminum cathode base plate according to any one
of the preceding claims 1 to 5, in which the raised
dielectric insert is curved and its curvature where it
meets the aluminum cathode has the shape of a tear-drop,
and the stripping knife is adjusted to strike said insert
in the area where it projects furthest into the base
plate.
7. An aluminum cathode base plate according to any one
of claims 1 to 6, in which the insert is made of an
epoxy-based vinyl ester polymer.
-10-

Description

CA 02224790 1997-12-12
ALUMINUM CATHODE BASE PLATE WITH
INSERT OF DIELECTRIC MATERIAL
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to an aluminum cathode base
plate used in the electrodeposition of zinc sheets and
subsequent stripping or peeling of the electrodeposited
zinc sheets therefrom. In particular, the invention
relates to a cathode base plate provided with an insert
of dielectric or insulating material at an edge of the
base plate in the vicinity of the solution line of the
electrolytic cell.
2. Brief Description of the Prior Art
It is generally known in the art to provide aluminum
cathode base plates with inserts of dielectric material
so that the blades of the stripping knife would first
strike the dielectric surface and then penetrate between
the zinc sheets and the cathode base plate in order to
strip the zinc sheets from the latter. This is disclosed,
for example, in applicant's Canadian patent application
No. 2,178,776 filed June 11, 1996 entitled "Method and
Apparatus for Automated Stripping of Zinc Sheets from
Aluminum Cathode Base Plates".
Such prior art base plates use dielectric or
insulating inserts which are essentially flush with the
adjacent surface of the plate as disclosed, for example,
in Canadian patent No. 1,169,025 issued June 12, 1984.
-1-

r CA 02224790 1997-12-12
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide
an improved aluminum cathode base plate wherein the
surface of the insert of dielectric material is raised
with reference to the adjacent surface of the plate.
Another object of the present invention is to
provide an insert which has a predetermined shape and
protrusion, particularly in the area where the knifes
penetrate between the zinc sheets and the cathode.
Other objects and advantages of the present
invention will become apparent from the following
description of the invention.
In conventional aluminum cathode base plates, where
the dielectric inserts are flush with the base plate, the
base plate has a life span of less than 200 strips and
thus remains in service approximately 12-13 months. Thus,
such base plates must be replaced approximately once a
year at a cost usually exceeding one million dollars,
depending on the size of the zinc plant.
The failure of aluminum cathode base plates with
flush dielectric inserts has been identified as being
produced by a gradual opening of the interface between
the insert and the adjacent aluminum surface: this is the
interface which is perpendicular to the surface of the
base plate. When the base plates with flush inserts are
stripped by the stripping machine, the knife travels
across the interface and, over a period of cycles, pushes
back the aluminum and opens up the interface. Zinc is
-2-

- CA 02224790 1997-12-12
electrodeposited in the interface after every strip and
when it is subsequently stripped, it further damages the
interface. Eventually the interface becomes so wide that
the stripping knives are prevented from peeling the zinc
deposit. The cathode base plate must thus be removed from
service prematurely because of its inability to be
stripped by the automated stripper.
The applicant has surprisingly found that this
deficiency can be readily obviated by raising the level
of the dielectric insert slightly above the surface of
the aluminum cathode, so that the knife will still
penetrate between the cathode and the zinc sheet, but
will not strike the perpendicular aluminum-dielectric
interface when doing so. This protrusion of the flat
surface of the dielectric insert is usually between 0.5
mm and 1.5 mm over the aluminum base plate surface and
most preferably about 0.8 mm. Although, to simplify the
installation of the dielectric insert, the entire insert
surface may be so raised, it is sufficient to raise only
the area where the knifes penetrate between the cathode
base plate and the zinc sheets, while the remainder of
the insert may be essentially flush with the cathode
plate.
Although a dielectric insert of any suitable shape
can be used in conjunction with the present invention, it
has been found that an insert having the shape of a tear-
drop is especially advantageous. The area where it
extends furthest into the base plate is made to
-3-

CA 02224790 1997-12-12
correspond with the spot where the knife penetrates
between the plate and the zinc sheets to strip the latter
and at least this area is made to project above the
surface of the aluminum cathode as indicated above.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will further be described with
reference to the accompanying drawings in which:
Fig. 1 shows a plan view of an edge portion of an
aluminum cathode base plate according to an embodiment of
the present invention;
Fig. 2 shows a section view along line A-A of Fig.
1;
Fig. 3 shows a plan view of an edge portion of an
aluminum cathode base plate according to another
embodiment of the present invention;
Fig. 4 is a section view along line D-D of Fig. 3;
and
Fig. 5 shows a stripping machine for automatically
stripping zinc sheets from the base plates of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
In all figures, the same elements are identified by
the same reference numbers.
Portion 10 of the aluminum cathode base plate
illustrated in Fig. 1 and Fig. 3 is in the area also
shown in Fig. 5 as portion 10 of the aluminum cathode
base plate 12. According to one embodiment of the
-4-

CA 02224790 1997-12-12
invention the upper edge of the base plate 12 is provided
with an elongated, generally parabolic insert 14 of a
dielectric material having a shape of a tear-drop.
Although this is a preferred shape of the dielectric
insert, any other suitable shape may also be used.
As shown in Fig. 2, the insert 14 is so mounted on
plate 12, that it projects over the surface of plate 12
by a distance illustrated by arrows C-C which may, for
example, be 0.8 mm. The dielectric material may, for
instance be a polyester or a vinyl ester polymer, such as
an epoxy-based vinyl ester with added elastomer, however,
any suitable dielectric resin material that can be
properly shaped and mounted would be satisfactory.
Preferably the resin material is used with inert fillers,
such as silicon based fillers. The projection C-C should
be provided at least in the area B of Fig. 1 where the
stripping knife shown by broken lines 30 strikes in order
to strip the zinc sheet deposited by electrolysis on the
aluminum cathode base plate 12. During the
electrodeposition, both sides of the base plate 12 are
coated with zinc except for the portion 14 covered with
the dielectric material and the side strips 16 also made
of appropriate dielectric material and clipped, as is
known in the art, on each edge of the cathode plate to
prevent deposition of zinc on the edges, which would
render automatic peeling or stripping impossible.
Dielectric insert 14 is mounted onto plate 12 in any
suitable manner. One method is to machine the desired
-5-

- CA 02224790 1997-12-12
area of the base plate to a smaller thickness and then
coat this area with the dielectric material by resin
transfer molding or compression molding. So that the
dielectric material will be well anchored, one or a
plurality of holes 18 (see Fig. 2) may be provided in the
thinned down area, thus permitting interconnection of the
dielectric material from both sides. The exact manner in
which the dielectric material is applied is, however, not
important, for the purpose of the present invention, so
long as it is well mounted and fastened and has a
predetermined projection as illustrated by arrows C-C in
Fig. 2 at least in area B shown in Fig. 1. Also, as shown
in Fig. 2, the edge 20 of the raised portion may be
chamfered for instance at an angle of 45°.
Figs. 3 and 4 show another way of providing plate 12
with the dielectric insert 14. Here, a cut-out is made in
the upper edge of plate 12 along line 15 shown as a
broken line. The thinned down section of plate 12 is then
formed only of the strip 17 which runs between the cut-
out line 15 and the edge of the raised portion 20. This
strip 17 is provided with a plurality of holes 18 which
permit interconnection of the dielectric material which
is thereby connected with the plate 12 only within the
area of strip 17 while the rest of the insert 14 remains
a solid dielectric insert. This is particularly well
illustrated in Fig. 4 which represents a cross section
along line D-D of Fig. 3 and where the projection C-C is
the same as in Fig. 2 and the edge 20 of the raised
-6-

CA 02224790 2000-O1-24
portion is also chamfered.
Referring to Fig. 5, it shows an automatic stripping
machine 22, such as described in applicant's Canadian
patent application No. 2,178,776, in which a plurality of
aluminum cathode base plates 12, with zinc sheets
electrodeposited on each side thereof, are suspended in
parallel by their supporting bars 24 within the framework
26 and are then automatically stripped one by one by a
stripping assembly 28 with knives 30 which swing in a
predetermined sequence about a pivot 32. During
stripping, the cathodes 12 are maintained in a straight
position with the help of an arm 34 which is lowered
simultaneously with the stripping assembly 28. Once the
zinc sheets are stripped, they fall into the U-shaped
trough 36 having a conveyor at its bottom which carries
the stripped sheets out of the stripping machine. Area 10
on the aluminum cathode base plate 12 shown in Fig. 5
represents the gist of the present invention. When using
standard plates where dielectric insert 14 is flush with
the surface of the cathode 12, and operating the
stripping machine as outlined above, no more than 200
strips could be achieved before the cathodes 12 had to be
replaced. On the other hand, when using the same machine
on the cathodes of the present invention With a 0.8 mm
protrusion of the dielectric insert 10 above the surface
of the cathode 12, over 360 stripping cycles have been
achieved in 25 months of stripping and the cathodes are
still in operation. Accelerated erosion tests have shown
_7_

CA 02224790 1997-12-12
that with such design, the cathode 12 is able to endure
about 500 stripping cycles before the surface of the
dielectric insert wears down to become flush with the
aluminum surface. Thus, the life of the cathodes of the
present invention may be more than doubled compared to
those having flush surfaces of cathode and dielectric
insert. This is clearly a significant advance in the art.
_g_

Representative Drawing