ALUMINIUM CATHODE FOR ELECTROWINNING ZINC

CATHODE D'ALUMINIUM POUR L'EXTRACTION ELECTROLYTIQUE DU ZINC

English Abstract

A cathode for electrowinning zinc is formed by an aluminum
plate having broad flat faces and at least one narrow edge bridging
the faces and a polyolefin copolymer strip bonded to the plate over
the edge thereof. The polyolefin copolymer strip can include an inner
layer directly bonded to the aluminum plate and formed of the bonding
polyolefin copolymer and an outer layer of standard polyolefin.

Claims

WHAT IS CLAIMED IS:
1. A cathode for electrowinning zinc, the cathode comprising:
an aluminum plate (1) having broad flat faces and a narrow edge bridging the
faces;
and
a strip of polyolefin copolymer (6), consisting of a polymer of olefin and 1-
10% by
weight of adhesive additives of aliphatic carbonic acids or derivatives
thereof, directly
bonded to and surrounding the narrow edge of the aluminum plate (1).
2. A cathode as claimed in Claim 1, wherein the polyolefin copolymer strip (6)
includes
an inner layer (7) directly bonded to the aluminum plate (1) and formed of a
bonding
polyolefin copolymer and an outer layer (8) of polyolefin.
3. A cathode as claimed in either of Claims 1 or 2 where the polyolefin
copolymer is a
copolymer of olefin and from 1% to 10% by weight of one of the group
consisting of
an acrylic acid, a derivative of an acrylic acid, and an ester.
4. A cathode as claimed in either of Claims 1 or 2 where the polyolefin
copolymer is a
copolymer of an olefin with one of the group consisting of acrylic acid, vinyl
acetate
and a mixture of acrylic acid and vinyl acetate.
5. A method of making a cathode for electrowinning zinc, the method comprising
the
steps of:
providing an aluminum plate (1) having broad flat faces and a narrow edge
bridging
the faces;
heating a polyolefin copolymer and extruding it as a strip (6); and

applying the strip (6), while still hot from extrusion, to the narrow edge of
the
aluminum plate (1).
6. The method of making a cathode as claimed in Claim 5, wherein the strip (6)
is
applied to a longitudinally extending aluminum band having two longitudinal
narrow
edges, and including the additional steps of:
applying the strip (6) to both longitudinal narrow edges of said
longitudinally
extending aluminum band; and
cutting up said longitudinally extending aluminum band to form individual
cathode-
plates.
7. The method of making a cathode as claimed in either of Claims 5 or 6
wherein
polyolefin copolymer and polyolefins are co-extruded as the strip (6).
8. A method of making a cathode for electrowinning zinc, the method comprising
the
steps of:
providing an aluminum plate (1) having broad flat faces and a narrow edge
bridging
the faces;
heating a polyolefin copolymer and extruding it as a strip of polyolefin
copolymer (7);
applying the strip of polyolefin copolymer (7), while still hot from extrusion
directly
to the narrow edge of the aluminum plate (1); and
fitting a strip of polyolefin (8) over the strip of polyolefin copolymer (7)
to form a
strip (6).

9. The method of making a cathode as claimed in Claim 8, wherein the strip (6)
is
applied to a longitudinally extending aluminum band having two longitudinal
narrow
edges, and including the additional steps of:
applying the strip (6) to both longitudinal narrow edges of said
longitudinally
extending aluminum band; and
cutting up said longitudinally extending aluminum band to form individual
cathode-
plates.
10. A method as claimed in any one of Claims 5 - 9 including the additional
step of
welding holding pieces (5) on an edge extending between two edges provided
with
the strip (6).
11. A method as claimed in any one of Claims 5 - 10, where the polyolefin
copolymer is
a copolymer of olefin and from 1% to 10% by weight of one of the group
consisting
of an acrylic acid, a derivative of an acrylic acid, and an ester.
12. A method as claimed in any one of Claims 5 - 10, where the polyolefin
copolymer is
a copolymer of an olefin with one of the group consisting of acrylic acid,
vinyl acetate
and a mixture of acrylic acid and vinyl acetate.

Description

CA 02041055 2000-09-18
ALUMINUM CATHODE FOR ELECTROWINNING ZINC
SPECIFICATION
Field of the Invention
The present invention relates to the electrowinning of zinc. More particularly
this
invention concerns an aluminum cathode for zinc electrowinning.
Background of the Invention
In the electrowinning of zinc, plate cathodes of aluminum are alternated with
plate
anodes of lead in a bath of zinc calcine (Zn0) in sulfuric acid (HZS04).
Voltage is applied
across the electrodes and the zinc plates out on the aluminum cathodes while
oxygen is
generated at the lead anodes. In this manner it is possible to produce high-
purity metallic zinc
from the zinc-sulfate solution.
While deposition of the zinc on the faces of the cathode is of course
desirable, the
edges of the cathode are normally masked in order to prevent buildup at these
regions of
higher current density as such buildup could bring the zinc into contacts with
the walls of the
cells or with the anodes. In the standard systems, edge strips of wood,
rubber, or plastic are
used as described in German patent application 3,219,300, published November
24, 1983.
The main problem in such systems is that the sulfuric acid of the extraction
bath gets
under the edge strips and corrodes the aluminum cathode. As a result the edge
strip can come
loose, zinc can rapidly build up on the cathode edge, and the system can short
out to the
anode or the wall of the cell.

Objects of the Invention
It is therefore an object of the present invention to
provide an improved cathode for electrowinning zinc.
Another abject is the provision of such an improved cathode
for electrowinning zinc which overcomes the above-given disadvantages,
that is whose edges are protected against premature failure.
Yet another object is to provide an improved method of
making such an electrode.
Summary of the Invention
1o A cathode far electrawinning zinc according to this inven-
tion is faraned by an aluminum plate having broad flat faces and at
least one narrow edge bridging the faces and a polyolefin copolymer
strip bonded to the plate over the edge thereof. This combination of
materials -- polyolefin copolymer and aluminum -- makes it possible to
form a very tight bond between the insulating edge strip and the
conductive aluminum plate so that no acid can get between the two and
erode the plate.
According to another feature of this invention the poly-
olefin copolymer strip comprises an inner layer directly bonded to the
20 aluminum plate and formed of the bonding palyolefin copolymer and an
cuter layer of standard polyolefin. It is also within the scope of
this invention to make the entire strip of the high-bonding polyolefin
copolymer.
The cathode according to this invention is made by heating a
bonding polyolefin copolymer and extruding it as a strip and then
applying this strip while still hot from extrusion to the edge of an
aluminum plate. The strip is applied at a temperature of between
100pC and 200°C, preferably between 160°~ and 180°C, far
lmin to 5min.
The strip can be applied to both longitudinal edges of a longitudinal-
30 1y extending aluminum band and this band can subsequently be cut into
2

the individual plates. It is also possible according to the method of
this inventian to coextrude high-bonding (that is polyolefin-copoly-
mer) and standard polyolefins as the strip and apply them to the plate
edge with tire high-bonding polyolefin copolymer in direct contact with
the plate edge.
Tt is also possible to make the cathode according to this
invention by applying an inner layer of a high-bonding polyolefin
copolymer directly to the edge of an aluminum plate and then fitting a
strip of normal polyolefin over the layer of high-bonding polyolefin
copolymer on the plate edge.
According to the invention the polyolefin copolymers are mixed poly-
mers of olefin and bonding additives for metallic surfaces. These
additives are as a rule aliphatic carboxylic acids in a ration of 1%
to 10% by weight, for example acrylic acids or their derivatives.
Instead of acrylic acid esters can be employed, for example vinyl
acetate. Vinyl acetate can also be used simultaneously with acrylic
acids and even other terpolymers can be used. The invention can also
employ melt-type glues of a polyolefin base so long as they are
sufficiently corrosion resistant.
Brief Description of the Drawing
The above and other objects, features, and advantages will
become more readily apparent from the following, reference being made
to the accompanying drawing in which:
Fig. 1 is a small-scale largely diagrammatic view in vert.i-
cal cross section illustrating a zinc electrowinning cell equipped
with the cathode of this invention:
Fig. 2 is a large-scale horizontal section through a detail
of the cathode of Fig. 1f and
Fig. 3 is a view like Fig. 2 of another cathode according to
the invention.
3

Specific Description
As seen in Figs. 1 and 2 a zinc electrowinning cell 3
contains a bath 2 of a zinc-sulfate solution and an aluminum cathode
plate 1 suspended from a hanger 5 riding on electrical feeds 4 and 4°.
This cathode 1 has vertical edges provided with polyolefin copolymer
insulating strips 6 which, as shown in Fig. 2, are U-shaped in section
so as not only to cover edges 1' of the plate 1, but to reach
over slightly to faces 1" thereof.
Fig. 3 shows an alternative arrangement where an edge strip
6° is formed of an inner U-shaped bonding layer 7 and a U-section
outer strip ~. The inner layer 7 is of a bonding polyolefin copoly-
mer.
The system of Fig. 3 is made by extruding two such inner
layers 7 and applying them while still hat from the extrusion to
opposite edges of an aluminum band. This band is then cut into
individual plates and the outer layers & are fitted to them to serve
as hangers for the cathode plates thus produced.
4

Representative Drawing