Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~5~6~9
This invention relates to cathodes for use in the
electrolytic refining of copper.
For over fifty year~ and up to the present the most
commonly used cathode of the type in question has been in
the form of a copper starter sheet suspended by a pair o~
copper loops on a hanger bar. This prior art cathode and
an example of a cathode according to this invention are
illustrated in the drawings herewith. In those drawings:
Fig. 1 is a perspective view of the mentioned prior
art cathode.
Fig. 2 is a perspective view of a cathode accord-
ing to this invention.
Fig. 3 is a side elevation of the same cathode
when drawn to a presently referred scale.
Fig. 4 is an end elevation projected from Fig. 3.
Fig. 5 repeats, on an enlarged scale, that part of
Fig. 4 enclosed by circle 5
Fig. 6 is a section taken on line 6-6 on a still
further enlarged scale.
It is to be noted that Figs. 1 and 2 are not drawn
to scale, having certain parts disproportionably enlarged
so as more clearly to illustrate details of construction.
Referring to Fig. 1, the prior art cathode con-
sists of a copper hanger bar 7, a copper starter sheet 8
and a pair of copper loops or straps 9 by which the sheet
is hung on the bar. The end portions 10 of bar 7 project
- 2 -
~ ~ '
. ,:
, ,, : , -: . - :
, - .: : : : :
:
gÇi69
beyond the width of sheet ~ so that they may rest, for
support, on the cell sides with one making the usual
electrical contact, and with the starter sheet between a
pair of anode plates of unre~ined copper, both being
immersed in the electrolyte of a refining cell, or
between a pair of insoluble anodes both immersed in the
electrolyte of an electro-winning cell.
Notwithstanding their long use in the industry, it
is well recognised that the prior cathodes are open to
objection in several respects. For example:
~ a) The copper starter sheets are not re-usable.
That is to say, they cannot be stripped of the copper
deposited on them and then put back in the cell to
receive a fresh copper deposit. The present and most
economical practice is to melt the starter sheet copper,
along with the copper deposited on it, and from the melt
produce wirebars, rod, cakes, billets and other copper
stock commodities in
""..--
,--
/~
-- ",- -- .
, /'''' '
- 2a -
. .
.
. . - ~ . . .
.
.
,
marketable condition. A fraction of the melt output could be
used for making fresh starter sheets, experience has shown
however that it is more economic to produce fresh starter
sheets electLolytically; but even so, the man-hour
expenditure is extremely high because it involves stripping
of starter sheets from mother plates, flattening of sheets,
making and attaching the loops, and assembly of the looped
sheets with support bars.
(b) The copper starter sheets are not easily made truly
flat and even when they are substantially flat to start with
they frequently warp in service and this gives rise to short
circuiting between cathode and anode. Even where loss of
flatness is not as bad as that, any departure from
parallelism between cathode and anode causes non-uniformity
of copper deposit, with excess deposit at "peak" or
short-path points accelerating short circuiting at those
points. Short circu~ting is a serious problem in that it is
necessary to use a team of men whose task consists entirely
of patrolling the tank-house to remedy short circuits.
(c) Sometimes the loops 9 make little more than line,
or even point, contact with the support bar 7. This does not
impair the actual copper deposit process~ but it does
increase resistance and consequently power costs are
increased.
Extensive research and experiment have been carried out
in the copper refining industry to imporve the mother plate
from which the starting sheets are produced. Some work was
done with stainless steel but was generally unsuccessful due
to inconsistency in results which gave problems such as
'` ' ~ ' ' ~:-
. ' ' . :
, ~'.
669
passivity, localised corrosion and variable adhesion of t'ne
copper deposits.
Titanium starter blanks have had considerable acceptance
because of the inherent characteristics of titanium which
provide an oxide film which appears well suited to overcoming
the problems of too little or too much adhesion of the copper
deposit and its corrosion resistance is exceptionally good.
The most advanced form of titanium re-usable starter plate or
cathode is complex in consisting of a composite hanger bar
primarily composed of copper and incorporatiny a titanium
component to which a titanium starter plate is weIded. In
one form, the hanger bar is a copper bar encased in a
titanium shell and the top marginal portion of the starter
plate is goose-necked so that that portion may be welded
directly to one side of the hanger bar encasement. In
another form the hanger bar is a copper bar having a core rod
of titanium inside and running longitudinally of it. In this
case, the starter plate is goose-necked as before and is
welded to the core rod by way of short studs of titanium
which extend through holes in the copper bar, each stud
having one end welded to the core rod and its other end
welded to the starter sheet margin.
An example of the prior "titanium" arrangement just
.~ ~
described is provided by ~h~t subject of United Kingdom
Patent 1415793.
However, titanium is not without its shortcomings. It
is expensive and its production in a practical form is
complicated The original object of the present invention
~' was to overcome or ameliorate at least some of the mentioned
-- 4 --
'
-: :
669
shortcomings and in a preferred embodiment, all of those
shortcomings, in a particularly simple and inexpensive manner
by the provision of a cathode which is virtually entirely of
stainless steel, is indefinitely re-usable, maintains its
flatness, provides an unimpaired current path between bus-bar
and hanger bar and between that bar and the plate, provides a
permanent oxide coating acting as an effective parting layer
facilitating stripping of deposit copper from the starter
blank while providing sufficient adhesion for retention of
deposited copper during build-up, and is particularly
amenable to mechanical handling in its manufacture and use.
Experimental use of invention has proved so successful
that it is considered economic to extend the use of this
cathode throughout applicants electrolytic copper refining
tank house.
The conventional method of refining makes use of starter
blanks in a limited number of cells to produce two thin
sheets of refined copper from each plate each day. These
sheets are then assembled into the cathode as shown in
Figure 1.
The special starter sheet manufacturing operation
involving additional control, higher power consumption per
tonne of product, and considerable expenditure on labour can
be eliminated by the use of the present invention in all
cells and for a longer time growth (a week or more instead of
only 24 hours) before stripping. It also provides a suitable
electrode for meehanical handling and mechanieal stripping of
whieh the product is eommereial refined eathode copper.
In addition the aeeurate dimensions of the starter
:
.
~5~9
blank, when coupled with accurately dimensioned anodes,
provide the facility to operate at closer spacing of
electrodes thereby considerably reducing power costs, and
most importantly largely eliminating short circuits, thus
further reducing labour costs and offering a better facility
for computer monitoring of short circuits. Furthermore the
more intensive operation made possible with closer spacing,
and the higher current density at which the refining
operation can be performed because of the elimination of
short circuits, means that the plant for a given annual
capacity can be housed in a smaller building, thus offering
considerable savings in capital investment for that building.
The elimination of starter sheets and their requirement
of special, larger area, anodes and particularly the ability
to take more "pulls" from a loading of anodes all reduce the
amount of "working" copper in the process at any time. As
this reduction can be of the order of 20% of say 10,000
tonnes, the amount of money invested in the copper in process
is considerably reduced.
The invention provides a cathode for use in the
electrolyte refining of copper, comprising:
A stainless steel hanger bar having a flat undersurface
with end portions adapted to rest upon supports and
electrical contacts.
A flat stainless steel starter sheet welded by its upper
edge to said undersurface between said end portions ~ so as
~.
to extend perpendicularly from said undersurface,
a copper cladding which envelops said bar and at least
s~ ~ cJ sl~c.~
the marginal upper portion of said sheet by which ~ is
.
:. :' : '
-. '
6~
welded to said undersurface, and
means to mask at least the upright side ed~es of said
sheet.
It will be appreciated that terms used herein, such
as "undersurface", "upper edge" and others implying a
specific orientation for a cathode or the parts thereof,
are used for descriptive convenience. Such terms are
literally applicable to a cathode when in normal use sus-
pended in an electrolytic cell.
A preferred embodiment of the invention is illustra-
ted in Figs. 2 to 6 of the drawings herewith.
Referring to Figs. 2 to 6, a stainless steel hanger
bar 11 is preferably of RSJ or I-beam cross-sectional
shape as shown. It could be of some other shape, but that
shown is preferred because it is of light weigh~ and
hence economical of the steel employed, it is structural-
ly sound and it provides a relatively large surface area
; for the current to be transmitted. Bar 11 has a flat
undersurface 12 and end portions 13 adapted to rest on
support and electrical contacts as well understood,
.
. '.
.
.~
, , ~ . . .
, . , . :
,
. .
.: ' ~ .
A flat stainless steel starter sheet 14 is welded, as
indicated at 15 in Fig. 5, by its upper edge to undersurface
12 so that the sheet extends perpendicularly from the
undersurface 12. Sheet 1~ preferably has a pair of holes 16
gapped from it prior to welding. These holes facilitate mass
handling of a number of the cathodes by support rods or
prongs extended through the holes.
A range of stainless steels may serve effectively as
starter plates according hereto, for ease of copper
stripping however we prefer to use that grade of stainless
steel marketed as "AISI 316 ELC" and having what is known as
a "2B" standard of surface finish. This steel is one having
an approximate analysis of:
Carbon 0.03% by weight
Nickel 12.0% by weight
Chromium 17.0~ by weight
Molybdenum 2.25~ by weight
and the 2B surface finish is one intermediate bright and
dull, being a silvery-grey, semi-bright surface produced by
cold rolling, softening and descaling, and then final rolling
lightly with polished rolls.
The indicated steel and surface finish are preferred, as
experiment has shown that they provide a sufficient tenacity
of attachment between the steel sheet and the copper
deposited therein to prevent the copper from peeling or
slumping from the steel on its own accord; yet, this tenacity
is not such as to impede ready stripping of the copper from
the steel sheet. Stripping may be performed by use of
knife-like blades or knife-edge wedges inserted between the
' ' ' ' ~ '
6~9
steel sheet and the deposited copper at the upper edge of the
copper; however, using stainless steel of the kind and surface
finish indicated above, our experiments have shown that effec-
tive stripping may be carried out automatically by passing
the copper laden cathodes through a hammering station in
which the deposited copper is smartly rapped near its upper
edge from both sides, this loosens the copper upper edge and
stripping is then finished by directing one or more streams
of air into the minute space between the steel and the loosen-
ed upper edge of the copper.
The stainless steel out of which the hanger bar is
made may be the same as that from which the starter sheet is
made. The bar may however, be of any other stainless steel
provided it is one to which the sheet is weldable.
As previously mentioned herein, stainless steel is
not a particularly good conductor, and use of a hanger bar
made only of such steel is not a sufficient vehicle for
passage of current between bus-bar and starter sheet. We
have found that this can be overcome by simply cladding
the hanger bar, and a top margin of the sheet welded to it,
with copper. This cladding may be applied by published
techniques o~ plating to the components and parts thereof
within the circle designated 5 in Fig. 4, and a thickness
of about 1 m.m. is preferred to provide adequate electrical
conductivity and the ability to withstand corrosion and
.
. _g _
.: , .
-
. . ~ - .
'. , : . :
. .
.:
,
~.~5~
mechanical drainage. It will be recalled, as stated earlier
herein, that the presence of an oxide film on the starter
plate is regarded as desirable owing to its ability to act
as a parting layer to facilitate stripping of copper from
the sheet.
The width of the copper pla,ed margin at the top of the
9a-
r~ :
, ... .
~ .
- . : .
i9
starter sheet is not critical provided it stops short of the
top surface o~ the electrolyte. Obviously, deposit of copper
on the plating area must not occur. Similarly, the plating
area on which refined copper will be deposited in the
operating process must not extend onto the lower edges 17
defining holes 16. It follows, that the copper plating may
conveniently stop short of the edges 17 provided it covers
the (preferably sand-blasted) weld metal at 15.
It will be apparent that ease of copper stripping
requires effective masking of the starter sheet edges so that
the deposit of copper will not be continuous about those
edges. This is particularly the case with the upright side
edges of the plate. The bottom edge is less in need of
masking and in the case of that edge masking could be
omitted; however, to improve assurance of clean stripping we
prefer to mask the bottom edge merely by giving it a hot dip,
to a depth of about 10.00 mm, of a high melting point
resilient wax, or other masking material.
With regard to masking of the side edges of the starter
sheet the present invention displays a further important
advantage over the titanium sheets referred to previously
herein.
In titanium starter sheets, owing to the relatively high
conductivity of that metal, there is a proneness for creep o~
deposit copper particles under the masking strip material.
Once that happens copper nodules continue to grow under the
mask strip ~usually of plastics material) thus tending to
lift the strip and so defeat the reason for masking and also
~`- create the need for re-masking before further use.
.
-- 10 --
:, , . . : '
3~5~6~3
With stainless steel starter sheets, the relatively poor
conductivity discourages copper growth under the masking
means and such growth is thereby reduced to such a degree as
to become inconsequential.
Notwithstanding this, the desirability of effective
masking of at least the side edges of the starter s~eet
remains; and, for preference, the form of masking which we
employ is as shown best in Fig. 6.
Referring mainly to that figure, the masking means
comprise a longitudinally slotted plastics beading 18 held on
to the sheet 14 by plastics pins 19. These pins extend
through holes 20 formed in plate 14.
The beading 18 and the pins 19 are preferably made of
the same plastics material, and one suitable material is a
high quality, heat and impact resistant plastic such as that
known as CYCOLOY 800. This being a polymeric alloy of
acrylonitrile-butadiene-styrene, (ABS) and polycarbonate as
described in U.S. patent 3,130,177.
The beading 18 and the pins 19 are secured together by
use of a suitable bonding material such as a 30~ solution of
Cycoloy 800 in methylene chloride.
Such a bonding material sets to an elastic condition and
in addition to acting as a bonding agent it serves a useful
purpose in the following manner.
During the experimentation which led up to the present
invention it was found that on some occasions there was a
tendency for the applied beadings to slightly bulge between
the pin fastenings. This was, of course, objectionable and
it was considered to be due to difference in expansion rates
fr ~ cl c n~ a f~
~ 1 1 --
. ~: - , . , . , . :
~:~
'
669
between the steel and the beading. This situation was
remedied by making the holes 20 oversized as shown in Fig.
6. In applying the beadings and the pins the holes 20 are
filled by the solvent material which sets to an elastic
condition so to act as expansion joints permitting sufficient
longitudinal movement of the beading relative to the starter
sheet as will compensate for the expansion rate difference.
The beadings 18 are preferably first ~ormed by
extrusion, or injection moulding, and in this action it is
desirable for the points 21 defining the aperture of slot 22
to be placed closer to each other than is shown in Fig. 6.
Thus, when the beading is applied to the sheet, the sides of
the slot 22 have an inbuilt resilient loading tending to
press them firmly in contact with the sheet.
As a ~urther precaution the points 21 are preferably
sealed relative to the starter sheet by the application of
; wax or other material sealing strips as indicated at 23.
.~ '' .
- 12 -
: ~ `
~. ~
