Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to an insoluble
anode for use in the electrowinning of copper.
This appllcation is a divisional application of
Canadian Patent Application Serial No. 239,754, filed
November 17, 1975.
The Electrowin_ing Process:
A brief descript-Lon of t'he electrowinning
process for the recovery of copper is set forth below
to permit a better understanding of the present invention.
Generally, a solution of concentrated copper in sulfuric
acid is formed, usually by leaching of copper ore.
An acid concentration in the range of 100-200 gms. per
liter of sulfuric acîd is generally necessary in order
to place sufficient copper in solution. A corresponding
copper concentration may be in the range of approximately
30-50 gms. per liter for a pregnant electrolyte to be
introduced into the electrowinning process.
The copper-laden solution is then introduced
into one or more electrolytic cells each containing
a series of anodes and cathodes.
Within the electrolytic cells 9 the anodes are sub-
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stantially insoluble wi-th an electrical potential impressed
between the anodes and cathodes -tending to cause migration of
copper from -the electrolyte toward the cathode with metallic
copper being deposited or built up on each of the cathodes.
Fresh electroly-te is constantly supplied to the cells. Sul-
furic acid solution is recycled from the cells for the leach-
ing or solution of additional copper which is then again in-
troduced into the electrowinning cells.
The cathodes containing the build-up of metallic
copper are periodically removed from the cells and replaced
by fresh electrodes or starter sheets to permit continued de-
position. The cathodes removed from the cells contain rela-
tively pure copper, for example in the range of 99~ purity.
A portion of the copper resulting from the electrowinning
process is accordingly used directly in copper consuming ap-
plications. However, since many applications require copper
of even higher purity, it is also common to further refine
copper obtained from the electrowinning process by conven-
tional electro-refining techniques.
Insoluble Anodes Used In Electrowinning:
The theoretically insoluble anodes are a particular
source of concern within the electrowinning process and have
been the object of substantial developmen-t efforts throughout
the relatively long history of the electrowinning process.
Problems arising in connection with the anodes tend to de-
velop because of the infeasibility of providing a completely
insoluble anode which is still capable of adequa-te electrical
performance within the cell. It has been commonly found tha-t
material from the anode tends to become dissolved in small
quantities within the electrolyte with a portion of the dis-
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solved anode maLerial being collected or trapped upon thecathode together with metallic copper.
For some time, insoluble anodes used in the elec-
trowinning of copper have been formed Erom lead or lead al-
loys. Relatively limited amoun-ts of lead have been found
within the copper deposit on the ca1:hodes. However, in many
copper consumin~ applications, the acceptable limits for lead
as an impurity are very low, commonly in the range of 10-20
parts per million. Much of the effort directed toward devel-
opment of improved anodes has therefor concerned techniquesfor making lead or lead alloy anodes having high hardness and
resistance to corrosion or exfolia-tion while also maintaining
adequate mechanical and dimensional stability in the anodes
to permit their continued use in electrowinning cells over
substantial periods of time.
In the recent past t the most common lead alloy em-
; ployed in electrowinning anodes was one containing substan-
tial quantities of antimony, for example, 5-15~ Sb by wèight.
In addition to such binary alloys, ternary and quaternary -~
alloys including lead and antimony have also been commonly
employed with the additional alloying agents being selected
-~ from a broad group including arsenic, bismuth, tin, cadmium,
thallium, tellurium, mercury, cobalt, barium, strontium,
selenium, tantalum, smooth platinum, etc. More recently,
lead-silver alloys have been investigated, particularly those
in ternary form including a third alloy such as arsenic or
bismuth.
Generally, anodes formed from lead alone do not
have sufficient hardness or resistance to corrosion or exfol-
iation to permit their use in electrowinning because of ex-
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cessive lead migratioll with -the lead being deposited or
entrapped upon the cathocles together with metallic copper.
The various alloying agents ten~ to increasc hardness and
corrosion resistance while also con-tributing to rnechanical
and dimensional stability, all of -these being particular:Ly
desirable characteristics for insoluble anodes in the electro-
winning process.
Calcium is an addi-tiona:L material of particular
interest within such lead alloys. Although calcium may be
employed wi-th binary, -ternary or even quaternary alloys in
widely varying amounts, the most useful concentrations for
calcium in such alloys are believed to be within the range
of 0.01 to 0.1% by weight.
The attractive corrosion resistance and superior
mechanical properties of lead alloys which contain calcium
have been known for many years as is demonstrated to some
degree by the use of calcium alloys in the manufacture of
battery grids. However, it is to be particularly noted that
manufacturing techniques and operating performance require-
ments for battery grids are different from the requirementsfor insoluble anodes such as are employed in the electrowin-
ning process. Experience in the battery field may be taken
- to reinforce the conclusion that lead-calcium alloys are more
difficult to cast or otherwise form into a usable configura-
tion.
This appl~cation describes one or more casting tech-
niques, which either alone or in combination, permit the casting
of a lead-calcium anode having grea-tly superior properties of cor-
rosion resistance and mechanical and dimensional stabili-ty, parti-
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cular]y for use in electrowinning processes. ~lowever, it is
emphasiY.ed that the casting techniques are not limited
merely to lead-calcium al]oys employed in lnsoluble anodes
for use in the copper e]ectrowining process. On the other
hand, because of the particular efEec-tiveness of these
techniques for forming such anocles, the preferred embodiment
and examples as described below, are directed in large
part towards lead-calcium alloys.
It is also noted at this point that the techniques
and apparatus described are also specifically applicable to
more complex lead-calcium alloys, for example ternary alloys
which include silver or tin, for example, as well as calcium.
It was indicated above that the purpose of employ-
ing lead alloys is to improve resistance to corrosion or ex-
foliation as well as to enhance both dimensional and mechan-
ical stability. These requirements are relatively complex
for insolubie anodes of the type employed within the electro-
winning process. ~o provide additional background in this
connection, it is noted that the "insoluble" or "inert''
anodes are immersed in sulfuric acid solution contained by
electrowinning cells. Wi-thin the electrolytic process or
under generally similar conditions commonly employed to sta-
bilize or precondition the anode surface, lead within the
anode tends to react with the sulfuric acid and also with air
bubbles generated by electrolysis upon the surface of the
anode. Interaction of these materials under electrolytic or
similar stabilizing conditions tends to cause forma-tion of a
film upon the lead or lead alloy anode. The film principally
-5-
consists of lead dioxide (PbO2) which acts as a semiconductor,
thus enabl.ing electrical conductance throu~h -the anode and
particularly enhancin~ i-ts corros:ion resis-tance. Lead oxide
(PbO) and lead sulfate (PbSO~) are also present during various
s-tages of the film formation and are characterized as being
generally poor conductors.
It is theorized that the phenomenon of initial film
formation and subsequent film reforma-tion is important in
maintaining the corrosion resistance o:E the anode. In any
event, it has been found that the chemical and physical
characteristics of the anode are important factors affec-ting
formation of the above noted film and accordingly are important
in achieving maximum corrosion resistance of the anode.
Important Characteristics For Insoluble Anodes:
.
In addition, certain chemical and physical charac-
teristics of the anode are also important in determining its
mechanical and dimensional stability as was suggested above.
These chemical and physical properties of the anode, which
may affect corrosion resistance and/or mechanical and dimen-
- 20 sional stability are summarized below.
Initially, chemical composition of the anode is of
critical importance as suggested by the preceding discussion
of the various lead alloys which have been developed. For
example, it was clearly indicated above that certain lead
alloys, particularly those including calcium as well as other
binary, ternary and quaternary alloys, contribute both to the
corrosion resistance of the anode as well as i-ts mechanical
strength and dimensional stability. It is also believed
; importan-t to maintain in uncombined form the basic lead com-
ponent except for the presence of precipitates formed with
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and between -the various alloyinq ayen-ts. ~or example, in a
lead-calci~m alloy, a precipi-ta-te oE lead calcium (Pb Ca3)
is believed to be an importan-t fac-tor contributing to -the
improved characteris-tics of anodes formed from such alloys.
In respect to one aspect of the presen-t invention,
it is theorized tha-t other compound forma-tions of lead, in
particular, wi-thin the anode, may be undesirable. In this
regard it is particularly believed that the combination of
lead with oxygen to form either lead oxide or lead dioxide
within the anode may undesirably interfere with subse~uent
formation of a stabilizing or conditioned film, as noted
above.
Uniform precipitate distribution is an additional
desirable characteristic to be considered in connection with
alloy composition of the type discussed immediately above.
Particularly with alloy compositions such as lead-calcium it
is believed that precipitate formation and uniform distribu-
tion of the precipitate throughout a matrix of lead contri-
butes particularly to corrosion resistance and the related
characteristic of surface hardness.
The characteristics of high density and low poro-
sity are believed to be interconnected and jointly contribute
again to the characteristics of corrosion resistance and
surface hardness for the anode. These characteristics may
also effect in part mechanical and dimensional stability
of the anode.
Grain size is another characteristic which is
believed to provicle an important contribu-tion to both corrosion
resistance and mechanical charac-teristics such as ha~dness.
Generally, it is believed that for a non-ferrous metal such
:;;- ~ , '' ' '
as lead, i-t is desirable to malntcLin a relatively large or
coarse grain size. Here again, -the charactcris-tic of coarse
or large grain size is particularly important for the lead
ma-trix in an alloy such as lead~calcium. I-t is assurned tha-t
relatively large or coarse grain size in such an alloy
con-tributes both -to film formation, as discussed above, and
possibly also ~o uniEorm precipitate distribution. This
supposition again illustrates that the many characteristics
discussed herein are interrelated or interdependent upon
each other.
Finally, it is believed desirable to form a surface
upon the anode which may be characterized as uniform,
continuous or generally smooth. It is believed that the
nature of the initial surface formed upon the anode is of
substantial importance. It is again theorized that the surface
characteristics contribute importantly to proper film forma-
tion, as discussed above. Possibly, the combined characteris-
tics of a smooth surface and relatively large grain size
tend to promote development of a uniform film upon an anode
which importantly minimizes corrosion within the electrolytic
bath. It will be noted below that in conjunction with the
present invention, anodes formed according to the procedure
described below tend to have the appearance of being "rolled"
- or "galvanized." In any event, the surface characteristics
of the anode formed accoxding to the present invention are
believed to contribu-te importantly to its value within an
electrowinning process.
In summary, the preceding comments have been directed
to~ard a discussion of basic anode characteristics including
30 good resistance to corrosion or exfoliation, mechan- ~
::
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z~s
ical stability or "strength", dimensional stabllity
and relative hardness, both on the anode surface and
within the anode interior. In this connection, it is
important to note that as material is eventually lost
from the anode, those portions originally within the
anode interior then form its surface.
In addition, the preceding discussion
emphasized the characteristics of chemical composition,
uniform precipitate distribution, high density and
low porosity, large or coarse grain size and initial
surface characteristics of the anode.
This discussion of anode characteristics is
set forth above in some detail in order to emphasize
advantages of the present invention. With the e~ception
of chemical composition, the other basic anode
characteristicsdiscussed above are believed to be
primarily dependent upon the method of casting or forming
the anode, either alone or in conjunction with the
` chemical composition.
Objects of the_I_vention
A basic object of the present invention is
~o provide a novel and particularly useful insoluble lead
alloy anode of a type useful in electrowinning processes
~; for metals such as copper, the anode preferably lncluding
calcium as an alloying agentO
It is a more specific and related object of
the invention to provide such a lead alloy anode,
including calcium as one of the alloying agents, wherein
the anode is characterized by maximum density, minimum
porosity, uniform precipitate distribution and further
by a uniformly smooth and hard anode surface.
mb/J~ 9
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~ s an even further part.i.cularity, it is an object to
provide such a lead alloy anode havinc3 a calciurn content
within the approxlmate range of 0.01 to 0.1% by wei~ht.
Within the process for produc:ing an anode of the
-type descrlbed above, a nurnber o~ process objects or elements
have been determined as being of interrela-ted importance.
However, it is believed that no-t all of these objects must
necessarily be employed together in order to produce a
novel insoluble anode. These process objects are set
forth immediately be:Low.
Summary of the Inven-t.ion
._ __ __ __
This invention relates to an insoluble anode for .
use in the electrowinning of copper, -the anode having a
substantially flat configuration with an effective surface
area on either side thereof of at least approxima-tely 5 square feet,
the anode being formed by a casting process from a lead
: alloy including approximately 0.01-0.1~ by weight calcium
the anode being further characteri~ed by substantially a
maximum density and minimum porosity, a uni~orm precipitate
distribution of PbCa3 in a lead matrix, the surface of the
: anode being continuous and smooth and further characterized
as having a galvanized or rolled appearance.
The method of producin~ the anode
contemplates limiting the amount of time during which
the lead alloy remains in a molten state after being
introduced into a suitable mold. This time limit is
preferably achieved through selectively controlling
the temperature of the molten alloy in a melting pot :.
or furnace and in a ladle prior to introduction to the
mold. Additionally, the temperature of the mold is
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also controlled within selected limits at least upon
commencement of pouring the lead alloy into the mold.
This feature is believed to contribute to
both chemical and physical characteristics of the
anode. In addition, it is thereby possible to remove
the anode from the mold within a relatively short
time, for example within two to three minutes, in
order to facilitate further treatment of the anode
and to achieve operating economy within the casting
operation.
-It is particularly contemplated that the anode
be suspended in a vertical condition immediately
after being removed from the mold. The vertically
suspended anode is then rapidly cooled. This step
is believed to "freeze" the grain size and precipitate
distribution throughout the anode and particularly
adjacent its surface while also contributing to
di=enslo=al and/or mechl:nlcal s~ability.
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~ n additiona:L elemen-t o:E the casting method
contemplates limiting the amount o.E :E:Low for the molten
lead alloy wit~in the mold. Thi.s i.s believed to contribu-te
-to the achievement maximum densi-ty and minimum porosi-ty
for the anocle while also con-tributing to -the :Eo.rma-tion of a
smooth or continuous surface. M.inimum flow of the molten
alloy wi-thin the mold is achieved by preferably disposing
the mold in a vertical position with the top o:E the mold
cavity being completely open to receive molten lead alloy
along its length. The molten lead alloy is then "streamed"
into the mold along the length of -the molcl cavi-ty wi-th the
amount of lead alloy being introcluced into each portion of
the mold in proportion to the volume of the mold cavity.
: In this manner/ all portions of the mold tend to be filled
in approxima-tely the same amount of time with minimum flow
of the alloy being necessary within the mold to achieve
complete filling of the mold cavity. The alloy material
thereby enters into continuous and intimate contact with all
surfaces of the mold cavity. In addition to con-tributing
toward the development of maximum density and minimum porosity,
minimum flow in the mold is also believed to contribute
toward other chemical and physical features of the finished
anode such as, for example, uniform precipitate distribution
particularly adjacent the anode surfaces, as well as the
25 surface finish itself. ~.
It is another particular element of the
casting method to preven-t access of oxygen
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32q~
to the molten lead alloy, particularly for an alloy
includincJ calclum as an alloying agent, or even after the
molten lead alloy solidifies within the mold -to form an
insoluble anode. I'his step is believed impor-tant both to
regu:Late chemical composition of the lead alloy within the
finished anode and also to limit the loss oE calcium from
a lead alloy including calcium as an alloying agent.
Additional elemen-ts of the method are also directed
-towards main-taining the calcium conten-t of such a lead alloy,
for example by the imposi-tion of an electrical potential,
preferably a low DC voltage, upon the molten lead alloy
within the furnace or mel-ting pot.
Finally, i-t is also an element of the method -to
select a lead alloy including calcium as an alloying agent.
Calcium may be present within -the lead alloy to form a
binary alloy and other alloying agents may also be added to
provide either a terna~y or quaternary alloy, for example,
from which an insoluble anode may be produced within the
scope of the present invention.
Calcium is particularly selec-ted as a preferred
alloying agent within the lead alloy because of its tendency
to-increase alloy hardness, improve corrosion resistance
and enhance mechanical or dimensional strength. The calcium
; alloying agent may be present in a relatively wide range
of 0.01 -to 0.1% by weight. It is further noted -that calcium
is a particularly desirable alloying agent for lead alloy
anodes because of i-ts -tendency -to reac-t wi-th sulfuric acid
and produce salts of low solubili-ty. This characteris-tic is
believed to greatly con-tribute toward the forma-tion of
pro-tective, compact or thick films of a type par-ticularly
sui-table
- 13 -
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for minimizing corrosion resistance of -the anode under
elec-trolytic condi-tions such as those encountered in an
elec-trowinnin~ process.
- Adcli-tional objec-ts and advantages oE -the presen-t
inven~ion are made apparent in the following description
having reference to the accompanying drawings.
Desc lpt~
To facilitate unders-tanding of the invention,
casting apparatus of -the type employed to produce such an
insoluble anode is illu~trated in Fig. 1 which is a side
view in elevation of apparatus including a mold, a ladle,
and a furnace or mel-ting pot.
Fig. 2 is an additional view taken from the rlght
side of Fig. 1, to particularly illustrate the configuration
and arrangement of an internal cavity within the mold.
Fig. 3 is a view of an insoluble anode produced
by the method of the presen-t invention.
Fig. 4 is a fragmentary view taken along section -~
line IY-IV in Fig. 3.
Fig. 5 is a detailed, isometric view of the ladle
- for the casting apparatus of Fig. 1 to illustrate a preferred ~
configuration therefor. ~-
Fig. 6 illustrates ano-ther preferred configuration
of the ladle.
Figs. 7 and 8 are photomicrographs, at different
degrees of magnification, of a typical insoluble anode
produced from a lead alloy including calcium as an alloying
agen-t.
-14-
i~ig. 9 illustra~es the su:rEace f:inish o~ a lead
alloy ~nocle procluced by the method o:~ -the present invention.
D sc ri~t_on or t.he Pre_erred l,mbocllmen_
As noted above, the present inven-tion is directed
towarcl an anode having a p:referred compositic-n as well
as other characteristic features. In view o:E -the rela-tively
large number of fea-tures invo:Lved ~or the insoluble anode the
following descrip-tion includes fixst a discussion of suitable
alloys for use in such an insolub:Le anode followed by a
description of the anode configura-tion, a description oE
the casting apparatus, a descrip-tion of the casting process
followed by a number of specific examples setting forth
various anodes produced by the present method with pertinent
method parameters for each example.
Suitable Alloy Compositions:
As indicated above, the present invention is
particularly directed toward lead alloys including calcium as
an alloying ayent. However, the invention is not intended .~-
to be limited to lead alloys including calcium as a single
:20 alloying agent or even to alloys necessarily including calcium.
For example, a lead alloy including calcium as an alloying
agent might also include one or more other alloying agents
such as silver or tin. Accordingly, a lead alloy of the type
contemplated by the present invention could be, for example, .
a binary, ternary or quaternary alloy.
Furthermore, it is generally accepted thak lead
alloys including calcium as an alloying agent are particularly
difficult to form or cast. This even further indicates the
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value and novelty of the present inven-tion in providing a
me-thod for successfully casting such ma-terials. ~lowever,
the success of the present invention in providing cas-t lead
alloys inclucling calcium also indica-tes i-ts value in cas-ting
other ma-terials particularly other generally similar lead
alloys, such as those including, for example, silver or
tin, with -the present inven-tion not necessarily being limi-ted
to the additional~inclusion of calcium.
Moreover, alloying agents such as calcium have
a tendency to evaporate or otherwise be lost Erom -the alloy
when i-t is in molten condition. Accordingly, the composition
of the alloy as originally produced may vary from the alloy
composition within a finished anode.
Generally, one preferred composition for the lead
alloy anode includes calcium as an alloying agent either
alone or in combination with other alloying agents. More
particularly, it is con-templated that a lead alloy anode
with calcium as an alloying agent include calcium within
the proximate range of 0.01 to 0.1% by weight. An even
more limited concentration for calcium,within the range of
approximately of 0.02 to 0.07% by weight, is believed to
be of particular importance.
One typical composition for a lead calcium alloy
to be employed within the present method, for example as
~- 25 an alloy ingot, inclucles approximately 0.0~-0.06~ by weight
calcium, the remainder essentially pure lead except for
normal impurities. In view of the tendency for calcium
to be lost from the alloy, it is accordingly possible that
an alloy ingot to be employed within the presen-t casting
method may include calcium above 0.1%.
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~ s a Eurther exclmple oE the application for -the
method of -the present inven-tion, it is also belicved appli-
cable -to leacl-an-timony alloys including antimony as an alloying
agent within the approxima-te range of 5-15% by weight. One
typical composition for such an alloy is set forth below:
Pure, soft lead 90% by wg-t.
Pure, antimony 10~ by wgt.
Maximum impurity content: Ag-0.0004% by wgt.
Cu-0.0009~ by wgt.
Zn-0.0005% by wgt.
As-0.0003~ by WCJt.
Fe-0.0002~ by wgt.
Bi-0.00014~ by wgt. .-
Many additional alloy compositions may also be
employed in conjunction with the method of the present
invention. Such alloys may include alloying agents of the
type summarized above. It is believed that silver and tin have
particular value as alloying agents either alone, in combina-
tion with each othert and/or in com~ination with calcium,for
example.
Insoluble Anode Configuration:
- As indicated above, the present invention is par-
ticularly direc-ted -toward techniques for casting an insoluble
anode of a type used in an electrowinning process such as for
the recovery of copper. A typical configuration for such an
insoluble anode is illustrated in Fig. 3. Referrïng to Fig.
3, it may be seen that the anode is relatively large, for
example having overall dimensions of approximately 48" x 36",
; a thickness of about 1/2 inch, with an eEEective surface area
on either side of approximately 8-9 sq. :Et. The effective
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area of the anode is inclica-ted at 11 and includes a generally
continuous rectancJular portion which is immersed wi.-thin an
electrolyte solu-tion for an electrowinning pxocess. The
effectlve portion 11 of the anode may be either continuous or
perfora-ted as i.llustrated :Eor a portion of the anode at lla.
Insoluble anodes are commonly formed wit.h such perforations
in order to reduce the weigh-t of the anode and to increase
its effective surface.
Each insoluble anode inclucles a rela-tively heavy
copper bar or conductor 12 which ex-tends transversely beyond
the edges of -the anode. The extended ends of the copper bar
12 may be conventionally disposed on paralle]. spaced apart
supports (not shown) for suspending the anode within the
electrolyte solution. The copper bar 12 also provides a
conductor path and accordingly must be in intimate conductive
relation with the anode 11. For that purpose, it may be seen
that the lead alloy encompasses the copper bar and has a
relatively increased thickness (indicated at 13 in Fig. 4) to
provide proper support upon the copper bar. A window, or
opening 14, is commonly formed along a central portion of the
anode adjacent the copper bar mainly for the purpose of
reducing weight of the anode. Accordingly, the weight of the
anode is supported by a s-trip of lead alloy which extends
upwardly along either side of the anode for engagement with the
25 copper bar 12. These strips are indicated a-t 16. The overall
thickness of the anode is increased to approximately 1 in.
where the anode surrounds the copper support bar.
It is preferable to form or cast the anodes in
substantially the shape indicated in the drawings in order to
conserve the lead alloy and to minimize further finishing
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work. The copper suppor-t bar 12 is initially arranged within
the base oE -the mold prior -to introduction of the lead alloy
and thus ~ecomes an intimate por-t:ion of the cas-t anode.
C3~ ~ bus
The castin~J appara-tus employed in practice of the
method of the present lnvention is of a generally conven-tional
type found in many foundry operations. The casting apparatus
does not form a particularly importan-t elemen-t of the -
present invention excep-t to enable practice of the preferred
method as described below, with one e~ception as to specific
cons,ruction of the ladle. Accordingly, the casting apparatus
is only indicated generally in Figs. 1 and 2 and includes
a conventional furnace or melting pot 21, a ladle 22 and a
mold 23 forming an internal cavity as indicated at 24 in
Fig. 2.
The furnace is of a general type sui-ted for the
heating of a lead alloy to molten condition in a temperature
range generally 700-800F. The capacity of the furnaee is
selected to greatly exceed the volume of the mold eavity 24
so that the lead alloy may be maintained in a molten condi-
; tion within the furnaee after each pour without an interval
-being required for reheating. Preferably, the capacity of
the furnace is selected so that only about 5% of its eontent
is required to fill the mold cavity. Accordingly, additional
lead alloy ingo-ts may be added to the furnace after each pour
with the molten alloy in the furnace then being immediately
ready to commence pouring a new anode.
The combination of the configuration of the ladle
22 and the orientation for the mold 23 are important to the
present invention. The importance of those two elemen-ts is
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summarized immedia-tely below and is desc~ibed in grea-ter
- detail below in terlns of the pref~rred process or rnethod~
Initially, the mold 23 is preferably forrned by
separate mold sections 26 and 27 which divide around the
periphery oE -the mold cavi-ty in order to permi-t rapid removal
of the anode. :[n addi-tion, the mold is arrangecl with the
cavity 24 in vertical alignmen-t to facilitate a preferred
method of filling the mold cavi-ty as described in greater
detail below. The mold sections 26 and 27 are also preferably
formed to provide a top opening 28 along the entire transverse
dimension of the cavity 2~ in order to permit simultaneous
fillinq along the length of the anode as is also described
in greater detail below.
In conjunction with the fea-tures of the mold as
described above, the ladle 22 is preferably formed as a
cylinder which is closed at each end and has approximately a
quadrant of its cylindrical surface removed (at 29) in order
to permit filling of the ladle from the furnace.
The ladle 22 includes gating in the form of perfor-
ations or apertures 31 extending completely along the trans-
verse length of the cylindrical ladle, -the leng-th of the ladle
generally conforming with the length of the mold cavity 2~.
The angular location of the gating upon the cylindrical ladle
is selected so that, with the ladle in the position illustrated
in-Fig. 1, the ladle may be filled with molten alloy from
the furnace. The cylindrical ladle is then rapidly rotated
to bring the gating apertures 31 into alignment with the
.
opening 29, the molten lead alloy from -the ladle streaming
downwardly to simultaneously fill all portions of the mold
along its length. As may be seen in Figs. 5 and 6, the
.
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. .
~, . . . , :: , ~
apertures are configured or arrangecl to permit increased flow
into selected portions oE the mo:ld. Referrlny again -to Fig. 3,
it may be seen that the copper bar 12 is ini-tially disposed
in a hase portion of the molcl cavity 24 so that the anode is
actually formed in inverse or upside-down relation within the
mold. Relatively larger amoun-ts of lead alloy are then required
adjacent the ends of the mold cavity 2~ to initially fill
the enlarged por-tions 13 oE the mold cavity which form the
sections surrounding the copper bar 12. The window 1~ formed
along the central portion of -the anode fur-ther minimizes
the amount of lead alloy required along the center of the
mold cavity. Accordingly, the gating apertures 31 may be
of a larger size adjacent the ends of the mold cavity as
is illustrated in the embodiment of Fig. 5. Alterna-tively,
the gating may include a larger number of openings or slots
31' of longer length may be provided adjacent the longitudinal
ends of the mold cavity as illustrated in the embodiment of
Fig. 6.
The ladle of Fig. 6 is employed for filling a mold
cavity having no structure to impede or interfere with the
streaming of molten alloy toward the bottom of the cavity.
Such a mold configuration is employed to form an insoluble
anode of continuous cross-sections. The ladle of Fig. 5 is
employed to fill a mold configured to form a perfora-ted anode.
For this purpose, one of the mold sec-tions may be formed
- with a large number of lugs arranged in vertical rows. The
luys, as indicated at 32 in Fig. 2, are arranged substan
tially across the effec-tive area and extend outwardly to
abou-t with the other mold section. The gating apertures in
the ladle of Fig. 5 are disposed a3ong its length to conform
` -21-
. . .
~' ' ' . ~ .
2~5;
with the spaces between the lugs. q~hus, moLten alloy may
pass in con-tinuous strealns toward -the bottom of -the mold
cavity in accordance wi.th a basic object of the present invention.
soth of these embodiments have the purpose of
enab:Ling -the mold cavity to be comple-tely ancl uni.formly filled
with the lead alloy entering into close intimate con-tact with
all surface portions of -the cavity while requiring minimum
flow of the molten lead alloy. This feature of the invention
is initially believed to contribute importan-tly to maximum
clensity for the anode. In addition, the minimiza-tion of
molten flow wi-thin the mold cavity permits -the anode to be
solidified more rapidly, an additional important objective of
the method of the present invention as described below.
With the arrangement described above, it is possible
that lead exiting through the apertures 31 of the ladle may
initially tend to contact a lateral surface of the rnold
cavity during rotation of the ladle to place the gating in
register with -the mold cavity. Even this amount of contact
between the molten alloy and -the cavity surfaces could be
minimized or substantially eliminated, for example, by the
arrangement of a stationary gating member direc-tly beneath
the ladle. Through the use of such an element, molten lead
alloy from the ladle would tend to be prevented from passing
through the gating of the ladle into the mold cavity until
-the gating apertuxes 31 are directly above the mold cavity.
The casting apparatus necessary for -the method of
the presen-t invention may include cextain supplemental compo-
nents, particularly such as -tempexature sensors, in order to :
facilitate proper contxol of the casting process. However,
such sensors are believed to be sufficiently conventional
-22-
,.
~, , ~ , - .' ' . . - . '
- . .
: . .
~)3~
Wit}lill the casting art that they need not be illustrated
herein. In addit:ion, the present inven-tlon particularly
contemplates means for selectively coolincJ the mold sections
21 and 22. This cooling means, which is also no-t illustrated,
could comprise condui-t m~ans integrally Eormed within the
mold sec-tions or could merely comprise separate means such as
a nozzle arrangement Eor spraying a fluid or liquid upon -the
surfaces of the mold.
Method of Casting:
.. . .
Novelty of the presen-t invention is believed to
priMarily reside wi-thin -the me-thod of cas-ting as desGribed
immediately below. The use of various alloy agents has long
been recognized as providing a means for achieving desired
charac-teristics in resul-tant castings. In particular, it has
long been recognized that lead alloys, especially those
including calcium as an alloying agent, are capable of contri-
buting subs-tan-tially to desired chemical and physical charac~
teristics. Accordingly, the present inven-tion is believed to
be of substantial importance since it provides a method of
casting which is believed to approach maximum utilization of
tne performance capabilities for-alloying agents such as calcium.
As was indicated above, the cas-ting method of the
present invention includes a number of important elemen-ts.
Initially, it has been found impor-tant -to regulate tempera-
ture or temperature differential throughout much of thecasting process. In addition, and/or simultaneously, another
importan-t feature has been the minimization of necessary molten
flow within the mold before solidifica-tion. Addi-tional impor-
tan-t elements of the presen-t process include the preferred
:
2 3
exclusion of ox~gen from -the alloy whi.le it .is in a rnolten
state an~ the selection of the alloy m~-terial, particularly
-the selection o:~ leacl alloys with calcium as an alloying
agent. However, it is once again stressed that the method
of the presen-t invention is not lim:i-ted -to lead alloys
necessarily including calcium as an alloying agent. Further,
it is possible tha-t the casting method oE the present invention
may be applied to other non-ferrous metals as well as lead.
In describing -the various elemen-ts o:E the present
casting method, it is noted that -the selection of alloy
composition is discussed above and will be illust:rated in even
greater detail within the specific examp:les set forth below.
The various specific steps within the casting method or
process are described in sequential order below.
15 Initially, a selected alloy composition is intro-
duced into the furnace and heated to a molten condition. As
indicated above, a substantlal excess of molten alloy is
preferably maintained within the furnace in order to facilitate
continuous casting. For example, where each insoluble anode
may weigh, for example, in the range of 150 lbs., the total
amount of molten lead alloy within the furnace is maintained
a-t approximately twenty times that amount. Accordingly, the
total amount o~ molten lead alloy within the furnace may
approximate 3000 lbs. Thus,~approximateIy 150 lbs. of lead
alloy, for example, in the form of ingots, may be added to the
furnace after each anode is poured. The figures set forth
; immediately above are primarily for purposes of example and .
; illustrate the proportions necessary to permit generally
continuous casting.
The temperature of molten lead alloy is also closely
c ~2~-
; ~ . ' ': . ~:
32~
controlled within the furllace. Althou~h optimu~l values may
be estab:Lished ~or -temperature of the molten alloy within the
furnace, those values tend ~o vary depending upon various
external factors such as ambient temperature, humidity, e-tc.
Accordirlgly, .i-t .is to be understood that the discussion of
tempera-ture limi-ts Eor the present cas-tiny me-thod es-tablish
approximate values or ran~es. It is to be further understood
that these approximate values or ranges may be varied within
the scope of the present i.nven-tion depending upon factors
such as those set forth above and upon add:itional factors
such as selection of the par-ticul.ar alloy composit.ion, size
of the anode or casting, etc.
The molten lead alloy within the furnace is
. maintained at a minimum of about 50F above its melting point
~ 15 in order to assist in minimizing the time during which the
alloy remains molten within the mold. The various types of
alloy composition considered at this time exhibit a range of .
` melting points within the approximate range of 500-625F.
The temperature of the molten lead alloy within the furnace
is more particularly maintained at a temperature differential
of approximately 50-100F above the melting point. For the
lead alloys under exemplary consideration, the temperature
within the furnace may thus vary from approximately 550F
~ to approximately 725F.
:~ 25 As was also indicated above, it is desirable to
prevent or minimize oxidation o~ the molten a:Lloy. The
excess quantity of alloy maintained in a molten state within
the furnace assists towards this end since the exposed
surface area for the molten lead alloy within the furnace
necessarily becomes of lesser significance. In addition, it
g
-25-
.- ~, . , ~
has been found that oxygen contact with the surface of -the
molten ]ead alloy may ~e further limited by Eloa-ting charcoal
upon the surface oE the moltell a]loy Or by maintaininy a
blanket of a relatively heavy, inert gas such as argon above
the mol~en lead alloy within the furnace. It is also noted
tha-t such steps are believed -to Eurther minimize the loss of
calcium from lead al:Loys where calcium is an alloying agent~
It has further been found tha-t, when calcium is
employed as an alloying agent in combination with lead, the
calcium conten-t can be Eurther stabilized by impressing an
electrical po-tential across the molten alloy in the ~urnace.
For example, a relatively low direc-t current voltage of
approximately 6V may be applied across the molten alloy by
creating a potential between the furnace and a separate elec-
trode (not shown) which is floated on the molten alloy.
In order to commence a cycle for formlng an anodeof the type illustrated in Fig. 3, a suitable amount of molten
lead alloy is transferred from the furnace to the cylindrical
ladle. The ladle is maintained in the upright position
illustrated ln Fig. 1 so that the alloy lnitially remains
completely within the ladle.
It is of course desirable to maintain the alloy
temperature within the range discussed above and accordingly
it may be necessary to initially preheat the ladle so that
it does not cause excessive lowering of the temperature for the
molten alloy. However, during a continuous casting process
where the ladle is rapidly being filled with molten lead
alloy which is then poured into the mold, the temperature of
the ladle tends to remain s-tabilized and does not in-terfere
with proper maintenance of the molten alloy temperature.
~ -26
IJI orcler to further prevent excessive contac-t of
o~ygen wi-tl~ the molten alloy, it is conternplated -that the
alloy be preferably blanketed against contact with the
air in the same manner described above for -the Eurn~ce.
Prior to th~ step of pouring molten alloy into the
mold, -the -temperature of the molcl is established at a selec-ted
differential benea-th the tempera-ture maintained for the
molten alloy within both the furnace and the ladle. For
example, i-t is ~enerally preferred -that -the initial tempera-ture
of the mold be approximately 200-250F lower than the molten
alloy temperature. Accordingly, it is generally desirable
to maintain the mold surfaces within a temperature range of
approximately 400-500F. Maintenance of the mold temperature
is particularly subject to variation depending upon ambient
conditions. It has been found through experimentation that
an experienced operator can closely ascertaln and control
the desired temperature of the mold through observation of
the alloy when it is in contact with the mold. For example,
; the optimum temperature differential may be determined by
such an experienced operator through observation of the
rate of shrinkage, the rate of solidification and the amount
of "drag," etc. as the molten alloy commences to solidify
within the mold. The term "drag," as used above, refers to
the rate of movement for the molten alloy relative to the . -
surfaces of the mold. As is clearly observed, such movement
is limited by -the casting me-thod described a~ove.
The mold is also arranged in the vertical position
illustrated in Fig. l with the mold sections being closed
toge-ther to form the mold cavity which is open along its top.
The lad].e is then rapidly rotated so tha-t the perforations are
-27-
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~3;2t3~ii
placed i.n recJiste.L- or vertical alignmen-t wi-th -the mold cavity.
The mol.ten alloy :is thell perm:itted to "stream" in-to the mold
cavity and -to illcrementally fill the mold wi-th llmi-ted
MoVement of the mol-ten a]loy. A t:ime period of approximately
20-30 sec. may be required for fil:ling of the mold for an
insoluble anode. Duri~y that time, the mol-ten alloy commences
solidification because of -the temperature dif:Eerential for
the mold sur~aces. Under certain conditions, i-t may be
necessary to further cool the ou-tside of the mold even during
the pouring s-tage, for example, when the ambient tempera-ture
is relatively high. Once the mold is filled wi-th molten alloy,
it is allowed to remain within the mold for approximately 1-2
min. As soon as the molten alloy is sufficiently solidified
within the mold to permit it to become mechanically self- ~.
supporting, the mold is opened up by separation of the mold
sections 21 and 22. . .
Before proceeding to describe treatment of the anode
~: after its removal from the mold, it is noted that further : -
cooling of the mold may be necessary during the time period
that the molten alloy is solidifying following the pouring
step. Cooling of the mold may preferably be accomplished by ~ :
merely spraying its external surfaces with a mixture of air
and water.
It may also be desirable to further limit contact
of the alloy with oxygen during filling of the mold. Accord-
ingly, it is also contemplated that the heated mold may be.
initially filled with a heavy, inert gas such as argon. As
the molten alloy is then poured into the mold cavity, the
. argon is displaced from the mold and in no way interferes
with the casting operation.
-28-
3;2~S
When -the anocle is removed Erom -the mold, it i5 still
at a re]atively hiqll tempera-tllre, posc;ibly :in the range of
500~. It h~s been Eound ~hat the conEigura-tion oE -the anode
is relatively important from this time until the anode tem-
perat~lre is at a relative:Ly low tempera-ture, possib:Ly 200F,
at which temperature ~he anode -tends to become stable. The
anode -tends to exhibit a "dimensional memory" whereby upon
beiny Ereely suspended, it will tend to return to the shape
in which it was config-lrecl during the final stages of cooling
of such a lower temperature. Accordingly, the anode is
preferably maintclined in a straight configuration and further
cooled, for example, by spraying with an air-water mix-ture
until it reaches the low temperature noted above. ~lterna-
tively and preferably, the anode is suspended in a vertical
position during this cooling step after its removal from the
mold. For example, the anode may be readily suspended by the
copper hanger bar during this step. With the anode being
suspended in a vertical position, substantially no dimensional
stresses are developed in the anode. Thus, after it is
cooled, it tends to maintain its straight alignment or shape.
This of course is importan-t with-in the elec-trowinning bath to
maintain a uniform gap between the anodes and cathodes and
thereby enhance electrolytic action within the process.
It may be noted that, within the preceding method
description, substantial importance is placed upon maintaining
tempera-ture limits during certain stages of the anode forma-
tion. It is believed apparen-t tha-t these temperature controls
are primarily directed toward achieving complete filling
of the mold cavity while limiting the amount of time within
which the lead alloy remains molten after it enters the mold.
~i~c -29-
,.. . . .
z~s
After -the ano~le solidiLies and is removecl from the mold,
rapi~l coolil~q of the a~ode is important both for the purpose
of limiting further grain chancJe as we:Ll as to enhancc dimen-
sional stability as described :immediately above.
Speci~ic Examples:
____ _ _
The followincJ specific examples are intended to set
for-th exemplary limi-ts for various alloy composi-tions together
with the correspondiny process parameters under which each of
the alloy composition is formed in-to an :insoluble anode.
Example No. 1:
An anode of the size described above and having a
configura-tion as illustrated in Fig. 3 (wi-thout perforations)
was formed according -to the preceding method description using
a ladle wi-th a gating arrangement illustrated in Fig. 5.
The lead alloy was selected as representative of a
binary alloy including 0.01-0.1% by wgt. calcium, being
I obtained in alloyed form as ingots containing 0.04-0.06 wgt.
-~ percent calcium,the remainder essentially lead except for
normaI impurities of the type and amount specified above.
This specific alloy, having a melting poin-t of
approximately 621.5F and a theoretical density of 11.34
grams/cc, is a commercial lead alloy obtained from St. Joe
Minerals Corporation.
The alloy was heated in a gas-fired open furnace to
a temperature range of ~75-700F and maintainea a-t that
approximate temperature during a rela-tively shor-t dwell time
(up to approximately 30 seconds) in -the ladle. The ladle was
not heated be-tween pouring cycles bu-t was main-tained at a
high temperature because of the shor-t time duration (approx-
ima-tely 2 1/2-3 minutes) for each cycle.
.~
-30-
2~
~rhc mold, formed oE steel, was ma.inta:ined at a
preheated or precooled -t:emperature of approximately ~00F
prior to pouring of the mol-ten al:Loy. No release agen-t or
other coating ma-terial was employed in the mold cavi-ty.
~he mold was filled, over a time period of approxi-
mately 20-30 seconds, according to the method described above.
The mold was then allowed to stand :Eor approximately one
minute while being sprayed with a water-air mixture to maintain
its relatively low temperature (at leas-t at its outer surfaces1.
The mold was then opened and the anode removed.
The bottom of the anode, which was arranged adjacent the
opening 28 of the mold (see Fig. 2) could then be trimmed if
desired. However, as soon -thereafter as practicable, the
anode was suspended in vertical alignment by the copper hanger
bar and sprayed for approximately one-half minute on each
side with an air-water mixture or until its temperature was
lowered to the approximate range of 200-250F. .
The anode then exhibited a precipitate distribution
and grain structure as illustrated in Figs. 7 and 8 with the
anode surface having a "galvanized" or "rolled" appearance
as illustrated in Fig. 9. :
The anode surface was not treated or conditioned,
beyond the steps described immediately above, prior to
preparation of the photograph in Fig. 9.
The anode was also not etched prior to the photo-
micrographs cf Figs. 7 and 8, at 200x and 800x magnification
respec-tively. However, to better illustrate both grain
structure and precipitate distribution, the anode surface was
polished with a commercial diamond powder. It is believed
.
-31-
32~
that the polishlng powder picked up fine calcium particles
from -the precipita-tes which in turn caused very fine scra-tches
on the anode surface. Such a scratch is par-ticularly no-ted
commencing midway along the leEt side of Fig. 8 and extending
upwardly ancl rightwardly to approximately the center of -the
photomicrograph. Such scratches are not to be confused with
observable grain boundaries which are presen-t to only a
minimum degree in anodes prepared by the method of the present
invention. An observable grain boundary may be detected in
the highly magnified surface of Fig. 8, noting the short and
Eine dark line in the upper right corner.
The finished anode was found to have an average
densi-ty of 11.214 grams/cc and a calcium content of 0.028% by
wgt. calcium.
The grain structure and precipitate distribu-tion
illustrated by Figs. 7 and 8 and the surface finish illus-
trated by Fig. 9, although specifie to the binary anode
deseribed immediately above, are believed applieable to a wide
range of lead alloy compositions, particularly those including
calcium as an alloying agent in the approximate range of
0.01-0.1~ by wgt.
Example No. 2:
This example rela-tes to an anode and method of
preparation differing from the limi-ts of Example No. 1 as
indieated below.
The alloy for Example No. 2 was selee-ted to include
approximately eigh-t percent by weight of anti~ony, the
remainder essentially lead except for normal impurities such
as (in terms of percen-t by weight) silver-0.000~%, copper-
0.0009%, zinc-0.0005%, arsenie-0.0003%, iron-0.0002% and
bismuth-0.00014%.
.. ,~, .~
.. ~. .
;' ' '. ' '
2~ii
The above alloy has a meltincJ point of approximately
520F ancl a theoretical density of 10.7~ grams/cc. Such an
alloy is hea-tecl to an approximate molten -tempera-ture of
600Or~' according to the present invention and thereafter
processed as described above for Example No. 1.
The anode produced according to this ~xample No. 2
would exhibit many of the advantages set Eor-th above.
However, it was indica-ted above that certain characteristics
are particularly due to -the inclusion of calcium as an alloying
agent. Accordingly, even through the anode produced according
to Example No. 2 does not prove to have equal characteristics
as for an anode including calcium,Example No. 2 does illustra-te
applicability to a broad range of alloys.
Example No. 3:
This example relates to an anode prepared according
to the preceding method based on an alloy including approxi-
mately 1.5% by wgt. silver, 1.0~ by wgt. tin, balance essen-
tially lead except for normal impurities.
This alloy has a melting point of approximately
589F and a theoretical density of approximately 11.26
grams/cc.
Here again, the method of preparation differs from
that described above for Example No. 1 only in the following
respects.
The alloy is heated to a molten temperature of
approximately 650~F and thereafter treated as described above
for Example No. 1.
The anode of Example No. 3 is intended to particu-
larly demonstrate applicability of the present method to more
30 complex alloys, such as the ternary composition described above. ~;
- , . .
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