Note: Descriptions are shown in the official language in which they were submitted.
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LOW RESISTANCE ELECTRODE ASSEMBLIES FOR PRODUCTION OF METALS
TECHNICAL FIELD
[0001] The present subject matter relates to electrode assemblies
for use in
the electrolytic reduction of refined materials, including, but not limited
to, use in
aluminum reduction cells. The subject matter more specifically relates to the
reduction of electrical resistance and improvement of the current distribution
through the anode and cathode assemblies to reduce power consumption, to
improve reduction cell performance and to improve operational life of the
anodes
and cathodes.
BACKGROUND
[0002] The Hall-Heroult process is a well known process for
aluminum
synthesis by electrolytic reduction of alumina. The process uses either pre-
baked
or in-situ baked carbon anodes paired in a reduction cell with opposing carbon
cathodes that are separated from the anodes by a molten electrolyte (bath')
that
contains dissolved alumina, and a molten metal pad of conductive aluminum
overlying the top of the cathode. During the process an electrical current is
passed
between the anode and cathode to reduce the dissolved alumina into molten
aluminum and to evolve carbon monoxide/or dioxide from the bottom of the
anode.
The conductive molten aluminum sinks to the aluminum layer or 'metal pad' on
top
of the carbon cathode, thereby becoming part of the electrical circuit.
Alumina is
added to the bath from an overhead hopper, while the molten aluminum is
withdrawn from the reduction cell by intermittent siphoning to a mobile
crucible.
[0003] The pre-baked carbon anode is attached to, and supported by,
the
lower end of a vertical conductor rod, and the upper end of the conductor rod
is
clamped to an electric buss beam. The vertical conductor rod is made of
aluminum
or copper and is typically joined to the carbon anode through a lower,
horizontal
,
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steel yoke and from one to eight (or more) round bar steel stubs attached to
the
yoke.
[0004] The carbon anodes are formed with prepared recesses (stub holes)
in
which the corresponding stubs are fitted. The stubs and anode are then joined
by
filling the space between them with cast iron or with carbon paste adhesive.
When
using cast iron for the bond the solidified iron annulus between the stub and
carbon
anode is referred to as a 'thimble'. The carbon anodes are mostly consumed in
the
reduction process by combining with the evolved oxygen from the reduced
alumina.
The anode assemblies have an operational duty life of typically 20-30 days in
the
reduction cell. A typical anode assembly of this type is disclosed in US
Patent No.
3,398,081 to BonfiIs et al.
[0005] Pre-baked cathode assemblies include a pre-baked carbon
(carbonaceous or graphitic) cathode connected by cast iron or carbon paste to
one
or more steel current collector bars that are positioned in slots in the
underside of
the cathode. The collector bars are longer than the cathode and protrude
through
the reduction cell sidewall for connection to an electric buss. The cathodes
have an
operational duty life of typically 5-10 years, during which period the carbon
corrodes, preferentially in electrical paths of lowest resistance and highest
current
density. The cathode blocks undergo sodium absorption from the bath, aluminum
infiltration and vertical heaving from thermal stresses due to uneven
temperature
profile throughout the carbon, which conditions gradually increase the
electrical
resistance of the cathode assembly. When the resistance becomes too high or
the
drawn aluminum contains too much iron, that indicates dissolving collector
bars,
the reduction cell is removed from service to be relined with new material.
[0006] The electrical energy required to reduce alumina to aluminum and
to
heat the incoming alumina to the reduction temperature is a minor fraction of
the
total electrical energy typically consumed between the anode buss to cathode
buss
connections. The balance of energy consumed is from heat generated due to
electrical resistance through the various components and connection interfaces
of
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the reduction cell circuit, which heat is removed to the environment. The
factors
influencing electrical resistance at the interfaces include the electrical
resistance of
the adjoining materials, the surface area and cleanliness of the interface,
and the
contact pressure between the adjoining materials. Although the cast iron
connections used in the anode and cathode assemblies are electrically
conductive,
they still exhibit significant electrical resistance which generates ohmic
heating that
does not directly contribute to the electrolytic reduction process. The high
electrical
resistance is at least partly due to the shrinkage which the iron undergoes
during
solidification. This problem can be exacerbated by differential thermal
expansion of
the various materials of the electrode assemblies when the cell is heated to
operating temperature.
[0007] Past efforts to improve the efficiency of electrolytic processes
for
metal production have either focused on improving the electrical current
distribution through the anodes and cathodes, and/or reducing resistance
between
various components of the electrode assemblies. For example, a number of
patents
and published patent applications disclose thimbles having extensions to
increase
surface contact area with the anode, improving current distribution and
reducing
resistance. Examples of such constructions are disclosed in US Patent Nos.
4,552,638, 4,557,817 and 4,824,543. Others have attempted to change vertical
current distribution through the thimble by adding undercuts or lateral
extensions
to the thimble (eg. US 4,621,674). Others have attempted to modify the contact
pressure between the carbon electrodes and other components of the electrode
assemblies, for example by providing an expanded graphite lining in the
cathode
collector bar slots (eg. US Patent No. 7,776,190 to Hiltmann et al.), or
providing
tapered, straight, or threaded connectors which are assembled with a low
pressure
fit with stress relief slots in the connection pin, or assembled with a loose
fit, which
pin expands and tightens only with increasing temperature (eg. US Patent Nos.
3,179,736, 3,390,071, 3,489,984 and 3,499,831). Others have used alternate
materials and methods to provide separate mechanical and electrically
conductive
connections between the stub and anode (e.g. International Publication No. WO
2009/099335).
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[0008] Despite the past efforts to improve the efficiency of
electrolytic
processes, there remains a need for anode and cathode assemblies which will
help
to achieve further reductions in the amount of energy consumed per kilogram of
produced metal, reduced carbon emissions and carbon losses, and longer
electrode
life.
SUMMARY OF THE DISCLOSURE
[0009] In an embodiment, there is provided an electrode assembly
for use in
a reduction cell for the production of metal, the electrode assembly
comprising: (a)
an electrically conductive carbon electrode block having a first surface and a
second
surface, wherein the first surface faces an interior of the reduction cell
when the
electrode assembly is in use; (b) an electrically conductive metal member
having a
first end and a second end, wherein the first end of the metal member is
connected
to the carbon electrode block in an electrically conductive manner, and the
second
end of the metal member is adapted for connection to a buss bar in an
electrically
conductive manner; (c) a solid, conductive metal insert at least partly
received in
the carbon electrode block, wherein the insert extends into the carbon
electrode
block from the second surface thereof; and wherein the metal insert is
received in
the carbon electrode block with an interference fit, such that the insert
exerts a
lateral force on the carbon electrode block.
[00010] According to one aspect, the electrode is a pre-baked carbon
anode,
wherein the first surface of the carbon electrode block is a bottom surface
thereof,
and wherein the electrically conductive metal member comprises a vertical
conductor rod. The electrically conductive metal member may further comprise a
vertical stub at its first end, the carbon electrode block having a top
surface
opposite the bottom surface, a recess formed in the top surface, with an end
of the
vertical stub being received in the recess. The second surface in which the
insert
extends may comprise an inner surface of the recess, the inner surface being
selected from a bottom surface and a side surface of the recess. For example,
the
insert may extend into the bottom surface of the recess and extend vertically
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downwardly therefrom, and/or the insert may extend into the side surface of
the
recess and extend radially outwardly therefrom, with a portion of each said
insert
optionally protruding from the bottom surface or the side surface of the
recess, the
protruding portion optionally including an enlarged head. The insert may be
inclined downwardly and outwardly from said second surface. A plurality of
said
inserts may be provided in the bottom surface and/or the side surface of the
recess, with each of said inserts being at least partly received in the carbon
electrode block.
[00011] According to another aspect, the recess may be provided with
a
conductive metal lining through which an electrically conductive connection is
formed between the stub and the carbon electrode block, and wherein the insert
or
the plurality of inserts is in direct, electrically conductive contact with
the
conductive metal lining of the recess. At least a portion of the conductive
metal
lining may comprise a cast portion which is formed in situ between the stub
and the
carbon electrode block. For example, a portion of the conductive metal lining
may
comprise a solid preform which is combined with the cast portion during
formation
of the metal lining, and wherein the insert or the plurality of inserts is in
direct,
electrically conductive contact with the preform prior to formation of the
cast
portion. The preform may comprise a bottom plate which is in contact with a
bottom surface of the recess.
[00012] According to yet another aspect, the electrode assembly may
further
comprise a plurality of said vertical stubs at its first end, the stubs being
spaced
apart from one another, with the carbon electrode block having a plurality of
said
recesses formed in its top surface, with an end of each of the vertical stubs
being
received in a corresponding one of the recesses; and wherein each of the
vertical
stubs is secured to the vertical conductor rod through an electrically
conductive
metal yoke. The electrode assembly may further comprise a plurality of
electrically
conductive bypass members, each of which bypasses the yoke and one of the
vertical stubs, wherein each of the bypass members has a first end connected
to
the vertical conductor rod through an electrically conductive connection, and
a
,
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second end connected to the carbon electrode block through an electrically
conductive connection. The carbon electrode block is provided with a plurality
of
said inserts in the top surface thereof, and wherein the second end of each of
the
bypass members is connected to at least one of the inserts. The second end of
each bypass member may be secured to the top surface of the electrode by at
least
one of the inserts, and wherein the second end of each bypass member includes
an
expandable or flexible portion.
[00013] According to yet another aspect, the carbon electrode block
has a top
surface opposite the bottom surface, and is provided with a plurality of said
inserts
in the top surface thereof, wherein the electrode assembly further comprises a
collar-shaped connector having a side wall to receive the first end of the
electrically
conductive metal member and to provide an electrically conductive connection
between the electrically conductive metal member and the carbon electrode
block,
and wherein the collar-shaped connector further comprises at least one
attachment
portion which is connected to the side wall and extends outwardly therefrom,
each
of the attachment portions being connected to at least one of the inserts to
provide
an electrically conductive connection between the attachment portion and the
at
least one of said inserts. The plurality of said inserts may be distributed
across the
top surface of the carbon electrode block, with the attachment portion(s)
connected
to each of the inserts. The electrically conductive metal member may further
comprise a vertical stub at its first end, wherein each of the vertical stubs
is
secured to the vertical conductor rod through an electrically conductive metal
yoke;
the electrode assembly further comprising a plurality of said collar-shaped
connectors, and the sidewall of each collar-shaped connector receiving an end
of
one of the vertical stubs; and each of the attachment portions is conductively
connected to all of the vertical stubs through the collar-shaped connectors.
[00014] According to yet another aspect, carbon electrode block has
a top
surface opposite the bottom surface, and is provided with a plurality of said
inserts
in the top surface thereof, wherein the electrode assembly further comprises a
yoke
assembly through which an electrically conductive connection is provided
between
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the electrically conductive metal member and the carbon electrode block;
wherein
the yoke assembly comprises a plurality of curved metal struts, each having an
upper end and an opposed lower end, the upper end being secured by an
electrically conductive connection to the lower end of the electrically
conductive
metal member, the lower end being secured to the carbon electrode block with
an
electrically conductive connection by at least one of said inserts. The lower
ends of
the struts may extend outwardly away from one another and from the
electrically
conductive metal member, and the yoke assembly may include a pair of said
struts,
oppositely disposed relative to one another.
[00015] According to yet another aspect, the electrode is a pre-baked
carbon
cathode, wherein the first surface of the carbon electrode block is a top
surface
thereof and the carbon electrode block has a bottom surface opposite the top
surface, and wherein the electrically conductive metal member comprises a
current
collector bar having an end received in a slot in the bottom surface, the
current
collector bar and the bottom surface being substantially parallel. A cast iron
layer
may be provided in the slot, between the current collector bar and the carbon
electrode block. The second surface in which the insert extends may comprise
an
inner surface of the slot, the inner surface of the slot being selected from a
top
surface and a side surface of the slot. The insert may have a flat head which
is
received between the cast iron layer and the carbon electrode block, and a
conductive metal liner and/or a conductive metal washer may be received
between
the flat head of the insert and the carbon electrode block.
[00016] According to yet another aspect, the current collector bar is
provided
with one or more collector bar anchors, each said anchor having a first end
attached to the collector bar and a second end embedded in the cast iron
layer. For
example, the first end of each said anchor comprises a threaded shank which is
received in a threaded bore in the collector bar.
[00017] According to yet another aspect, the second surface in which the
insert
is received comprises the bottom surface of the carbon electrode block, the
current
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collector bar having a flat bottom surface which is substantially co-planar
with the
bottom surface of the carbon electrode block. An electrically conductive metal
connector may be attached to the bottom surface of the carbon electrode block
and
to the flat bottom surface of the current collector bar, to provide an
electrically
conductive connection between the current collector bar and the carbon
electrode
block. The electrically conductive metal connector may be attached to the top
surface of the carbon electrode block by said insert, and is in electrically
conductive
contact with said insert. The electrically conductive metal connector is in
the form
of one or more layers of flat metal strap. The strap may have an expandable
portion to permit deformation of the electrically conductive metal connector
in
response to differential thermal expansion of the current collector bar and
the
carbon electrode block, along an axis defined by the current collector bar.
Alternatively, the electrically conductive metal connector may comprise a
flexible
cable connector, which may be provided with lug ends.
[00018] According to yet another aspect, the electrode assembly may
comprise
a plurality of said inserts provided in the bottom surface of the carbon
electrode
block, with each of said inserts being at least partly received in the carbon
electrode block, and the electrode assembly may further comprise a plurality
of said
electrically conductive metal connectors, each providing an electrically
conductive
connection between the current collector bar and at least one of the inserts.
The
inserts may be spaced apart along a length of the carbon electrode block,
and/or
the inserts may be of different dimension to control the resistance of the top
surface of the carbon block relative to the external portion of the collector
bar.
[00019] According to yet another aspect, a thickness of the insert
and the
width of the bore are sized relative to one another such that an interface
contact
pressure between the insert and the adjoining carbon electrode block is at
least
about= 1 kPa. For example, the interface contact pressure may be less than
about
MPa, and/or the interface contact pressure may be between about 1 MPa and
about 10 MPa. A maximum interface contact pressure may be less than about one
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half of the breaking pressure required to break the surrounding carbon
electrode
block.
[00020] According to yet another aspect, the second end of the metal
member,
which is adapted for connection to the buss bar, includes a connecting surface
which is adapted to mate with the buss bar, and wherein the connecting surface
is
electroplated or clad with corrosion resistant conductive material.
[00021] According to yet another aspect, the insert is received in a bore
either
pre-drilled or formed in the carbon electrode block.
[00022] According to yet another aspect, the electrode is pre-baked and
the
insert is inserted into the carbon electrode block either before or after the
electrode
is pre-baked.
[00023] According to yet another aspect, the current collector bar is
supported
within the slot of the carbon electrode block by at least one hanger assembly
comprising a tongue portion slidably received in a slot portion. The slot
portion
may be secured to the carbon electrode block by one or more of said conductive
metal inserts, and the tongue portion is secured to the current collector bar.
BRIEF DESCRIPTION OF THE DRAWINGS
[00024] In order that the claimed subject matter may be more fully
understood, reference will be made to the accompanying drawings, in which:
[00025] Figure 1 is a cross section through a reduction cell as described
herein;
[00026] Figures 2 and 3 are perspective views showing the stubs of an
anode
assembly before and after insertion into the stub holes of the anode block;
[00027] Figures 4 and 5 are perspective views showing the casting of the
thimble in the anode of Figures 2 and 3;
[00028] Figure 5a is an enlarged, partial vertical cross-sectional view
through
the stub, thimble and anode block of the anode of Figure 5;
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[00029] Figures 6 and 7 are perspective views showing the electrode of
Figure
5 provided with electrically conductive bypass members;
[00030] Figure 8 is a partial, perspective cross-sectional view, showing a
recess for receiving a stub according to an embodiment described herein;
[00031] Figure 8a is a close-up, cut-away view of an insert embedded in
the
wall of the stub hole shown in Figure 8;
[00032] Figure 9 is a partial, perspective cross-sectional view, showing a
recess for receiving a stub according to another embodiment described herein;
[00033] Figure 9a shows a preform for formation of a thimble according to
an
embodiment described herein;
[00034] Figure 10 is a cut-away side elevation view showing an electrode
with
thimble anchors;
[00035] Figures 11 and 12 show an electrode assembly including an assembly
for attaching the stubs to the top surface of the anode block;
[00036] Figure 13 is a perspective view of an electrode assembly having a
low
resistance yoke assembly as described herein;
[00037] Figures 14a to 14e are side views of various types of conductive
inserts and connections as described herein;
[00038] Figures 15 and 16 are perspective views showing the casting of a
cast
iron layer in a cathode assembly;
[00039] Figure 17 is a perspective view showing a cathode block in which
the
connection to the collector bar is provided by a plurality of strap
connectors;
[00040] Figure 18 is a side view of the cathode block of Figure 17;
[00041] Figure 19 is a partial transverse cross-section through a cathode
block
showing an embodiment in which the connection to the collector bar is provided
by
an alternate form of strap connector;
[00042] Figure 20 is a partial perspective view of a cathode block, partly
in
cross-section, showing another embodiment in which the connection to the
current
collector bar is provided by an alternate form of strap connectors;
[00043] Figure 21 is a perspective view of a steel cathode collector bar
with a
conductive metal surface cladding;
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[00044] Figure 22 is a partial view of the slot of a cathode assembly
according
to an embodiment disclosed herein;
[00045] Figure 23 is a perspective view showing a portion of a conductive
lining
for a cathode slot;
[00046] Figure 24 is a partial view of the slot of a cathode assembly
according
to another embodiment disclosed herein; and
[00047] Figures 25 to 28 are views of cathode assemblies, showing
alternate
methods for connecting the current collector bar to the cathode block.
DETAILED DESCRIPTION OF EMBODIMENTS
[00048] Certain embodiments are now described below with reference to the
Hall-Heroult process for aluminum synthesis by electrolytic reduction of
alumina.
However, it will be appreciated that the embodiments described herein may be
modified for use in other electrolytic reduction or electrolysis processes for
the
production of metals or chemicals that may also use metal electrodes joined to
carbon anodes or cathodes, including but not limited to the electrolytic
reduction of
Lithium, Sodium and Magnesium.
[00049] Although pre-baked anode technology has developed over time to
larger anodes with larger or more stubs, and to larger reduction cells with
higher
applied amperage, the subject matter disclosed herein may be applied to all
forms
and configurations of pre-baked carbon anodes and pre-baked carbon cathodes.
Furthermore, the subject matter disclosed herein for the thimble anchors or
preform may be applied either before or after the anode is baked, the carbon
material of the anode being referred to herein as "green" prior to baking.
[00050] Figure 1 is a cross-sectional view showing the components of a
reduction cell 10 for aluminum production. The reduction cell 10 includes a
plurality of opposed electrode assemblies, including a plurality of anode
assemblies
12, of which two are shown in Figure 1, and a plurality of cathode assemblies
14, of
which one is shown in Figure 1.
,
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[00051] Each electrode assembly 12, 14 comprises an electrically
conductive
carbon electrode block having a first surface and a second surface, wherein
the first
surface faces the interior 16 of the reduction cell 10 when in use. In this
regard,
each anode assembly 12 includes a carbon anode electrode block 18 (also
referred
to herein as the "anode block 18") and each cathode assembly includes a carbon
cathode electrode block 20 (also referred to herein as the "cathode block
20"),
which are described in more detail below. The first surface of each anode
block 18
is the lower surface 22, and the first surface of each cathode block 20 is the
upper
surface 30. As shown in Figure 1, the surfaces 22, 30 face one another and
face
the interior 16 of the reduction cell 10.
[00052] The interior 16 of reduction cell 10 contains a molten electrolyte
bath
26 which contains dissolved alumina, and a molten metal pad 28 of conductive
aluminum overlying the top surface 30 of the cathode block 20. As alumina is
reduced to aluminum, it sinks into the molten pad 28 and becomes part of the
electrical circuit. Alumina is added to the bath 26 from an overhead hopper
32,
while the molten aluminum is withdrawn from the metal pad 28 by intermittent
siphoning to a mobile crucible (not shown). The interior 16 of reduction cell
10 is
enclosed by sidewall refractories 34 and bottom refractory 36.
[00053] Each electrode assembly 12, 14 further comprises an electrically
conductive metal member having a first end and a second end, wherein the first
end of the metal member is connected to the carbon electrode block 18 or 20 in
an
electrically conductive manner, and the second end of the metal member is
adapted
for connection to a buss bar in an electrically conductive manner.
[00054] In this regard, the electrically conductive metal member 37 of
each
anode assembly 12 comprises a vertical conductor rod 38 which is typically
made of
aluminum or copper, the first (lower) end 40 of conductor rod 38 being
connected
to the top surface 42 of the anode block 18, and the second (upper) end 44 of
which is connected to an anode buss bar 46, for example by clamping or the
like.
It will be appreciated that the electrically conductive metal member 37
carries
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electric current from the anode buss bar 46 to the anode block 18, and also
suspends the anode block 18 in the bath 26.
[00055] The electrically conductive metal member 37 of each anode assembly
12 further comprises one or more vertical stubs 48 at its first end 40. In the
reduction cell shown in Figure 1 the anode assemblies 12 each have two stubs
48
which are spaced apart from one another along the top surface 42 of the anode
block 18. However, it will be appreciated that the anode assembly may comprise
a
single stub 48, or more than two stubs 48. The stubs 48 are typically
comprised of
steel and typically have a cylindrical shape, although the stubs 48 may have
other
cross-sectional shapes, such as square or rectangular.
[00056] The electrically conductive metal member 37 of each anode assembly
12 further comprises an electrically conductive metal yoke 50 through which
the
stubs 48 are connected to the lower end 40 of the vertical conductor rod 38.
The
yoke 50 comprises a horizontal member such as a thick conductive plate of
steel or
other conductive metal. Alternatively, the yoke 50 may be integrally formed
with
the stubs 48.
[00057] The electrically conductive metal member of each cathode assembly
14 comprises a current collector bar 52. A section of the collector bar 52 is
located
within the cell 10 and is connected to the cathode block 20. One or both ends
of
the collector bar 52 are located external of the cell 10 and connected to a
cathode
buss 24 located outside the cell 10. Therefore, as shown in Figure 1, the
collector
bar 52 extends through the sidewall refractory 34 on one or both sides of the
cell
10. The cathode block 20 has a bottom surface 54 opposite the top surface 30.
The collector bar 52 may be split in two pieces (not shown) and separated in
the
middle of the cathode block 20, each piece having one end and a portion of the
collector bar 52 connected to the cathode block 20 and the opposite end
protruding
through the refractory sidewall 34 of reduction cell 10 and connected to the
cathode
buss 24 located outside the cell 10.
1
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[00058] As shown most clearly in the isolated views of Figures 15 and 16,
the
bottom surface 54 of the cathode block 20 has an elongate slot 56, open at the
bottom surface 54, in which the end of the current collector bar 52 is
received. The
cathode block 20 and the current collector bar 52 may be connected by a layer
of
cast iron 57 or carbon paste provided between the interior surfaces of the
slot 56
and the current collector bar 52.
[00059] During operation, the cathode blocks 20 undergo sodium absorption
from the bath 26, aluminum infiltration from the metal pad 28, and thermal
stresses between the top and bottom surfaces 30, 54 due to uneven temperature
profile throughout the carbon, which conditions gradually cause an upward
bowing
of the cathode block 20 which increases the electrical resistance of the
cathode
assembly 14. When the resistance becomes too high, or the cathode block 20
corrodes to the extent of allowing aluminum contact with the collector bars
52, the
reduction cell 10 is removed from service to be relined with new material.
[00060] Figures 2 to 5 illustrate a conventional anode assembly 12 having
an
anode block 18 of generally rectangular shape, a vertical conductor rod 38 of
rectangular cross-section and a horizontal yoke 50 integrally formed with a
pair of
vertical stubs 48, the yoke 50 being conductively connected to the lower end
of the
conductor rod 38 by welding, brazing or the like and typically on either side
of a
bimetallic transition joint 51. After the anode assembly 12 is clamped and
suspended from the anode buss 46, the top surface 42 of the anode block 18 is
covered with a layer consisting of a combination of frozen crystalline bath
and
powdered alumina, called a "bath cover" 25, (Fig. 1) to prevent air contact
with the
top surface 42 of the anode block 18 and to insulate the anode 12 from excess
heat
loss.
[00061] Figures 2 and 3 illustrate the insertion of the lower ends of
vertical
stubs 48 into recesses 58 formed in the top surface 42 of the anode block 18,
the
recesses 58 sometimes being referred to herein as "stub holes". As best seen
in
the enlarged views of Figures 8, 8a and 9, the recesses 58 have a generally
,
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cylindrical shape to match that of the stubs 48, the inner surface of each
recess 58
comprising a flat, horizontal, circular bottom surface 60, and a generally
cylindrical
side surface 62, which may be tapered to have a smaller diameter at its bottom
than at its top. The side surface 62 may be provided with helical grooves 66,
as
disclosed in above-mentioned US Patent No. 3,398,081 to Bonfils et al.
[00062] The stubs 48 and recesses 58 have diameters such that an annular
gap is provided between the vertical side surface of each stub 48 and the
cylindrical
side surface 62 of each recess 58. Figures 4 and 5 illustrate the formation of
a
conductive metal lining 64, also referred to herein as a "thimble", in the gap
between the stub 48 and the inner surface of the recess 58, to provide an
electrically conductive connection between each stub 48 and the anode block
18.
The metal lining 64 is at least partially cast in situ, i.e. with the stub 48
received in
the recess 58, and with the bottom of the stub either in contact with or in
close
proximity to the bottom surface 60, where molten metal 27 such as cast iron is
poured into the annular recess between the stub 48 and the inner surface of
recess
58. The metal lining 64 is typically formed from cast iron, the brittle nature
of which
makes it easy to remove from the steel stubs 48 during anode 12 recycling
procedures.
[00063] Upon rapidly freezing the thimble 64 in situ from molten iron, the
iron
undergoes solidification shrinkage. As shown in Figure 5a, the iron solidifies
with
freeze isotherms 29 primarily parallel to the vertical surface of the stub 48,
which
acts as a heat sink, and further undergoes three dimensional thermal shrinkage
of
the solid iron upon cooling of the thimble 64 from its freezing temperature
down to
its lower operating temperature. The shrinkage of the iron causes a vertical
gap 31
and a loose fit between the outer surface of the thimble 64 and inner surface
of
recess 58.
[00064] Similarly, with reference to Figures 15 and 16, the cathode
collector
bar 52 is received in the slot 56 of the cathode block 20 with a space
provided
between the collector bar 52 and the walls of slot 56. This space is filled
with a
CA 02838113 2013-12-16
16
layer 57 of cast iron formed in situ against the collector bar 52 by pouring
molten
metal 27 into the space between the collector bar 52 and the walls of slot.
For the
same reasons described above with reference to the anode 12, a gap may form
between the surfaces of the slot 56 and the outer surface of the cast iron
layer 57
due to shrinkage of the metal of layer 57. During initial heatup of the
reduction cell
10, the steel stub 48, collector bar 52, cast iron connections 57, 64 and
carbon
electrode blocks 18, 20 expand at different rates, and a high carbon cast iron
may
expand slightly by undergoing phase transition, all providing a tighter fit of
the
assembly than when cold, however still with considerable electrical resistance
through each of the electrode assemblies 12 and 14.
[00065] The electrical current distribution through the anodes and cathodes
follow the paths of least resistance, passing from the power connection points
at
the anode buss 46 to the external ends of the collector bars 52. This
electrical
current behavior results in uneven electrical resistance and current density
across
the first surfaces of the blocks 22, 30 to the bars 38 and 52 respectively,
which in
turn may cause uneven anode consumption and uneven cathode wear and
corrosion. The uneven electrical resistance across the top surface 30 of the
cathode
block 20 may also contribute to horizontal electrical flow through the
aluminum
metal pad 28 due to the aluminum's very low electrical resistance, which
horizontal
electrical flow generates electromagnetic flow currents and turbulence (waves)
in
the metal pad 28. This turbulence may force a larger than normal anode to
cathode distance (ACD) to be maintained to avoid short circuits between the
bottom surface of anode block 18 and the top surface 30 of cathode block 20,
which
additional ACD incurs extra electrical resistance through the bath 26,
consuming
more electrical energy than otherwise required for the reduction process.
[00066] The present subject matter addresses the above problems by
providing solid conductive metal inserts 76 received in the carbon electrode
blocks
18 and/or 20 of the electrode assemblies 12, 14, with an interference fit, and
with
or without accompanying bores 68 (also referred to herein as "boreholes") in
the
carbon electrode blocks 18 and/or 20. One end of each insert 76 is embedded
into
CA 02838113 2013-12-16
17
the carbon of the electrode block 18 or 20, with a controllable lateral
interface
contact pressure between the insert 76 and the carbon, which pressure provides
a
low resistance electrical path into the carbon, thereby reducing power
consumption.
The application of the interference fit inserts 76 varies between the anode 12
and
cathode 14, but follows the same general principle.
[00067] As defined herein, an interference fit is one which produces a
high
'lateral' interface pressure between the insert 76 and the carbon material of
the
electrode block 18 or 20, i.e. the pressure being directed radially outwardly
against
the carbon material. The high lateral interface pressure reduces the
electrical
resistance across the 'lateral' interface between the insert 76 and the carbon
material. Alternatively, or in addition to the high lateral interface
pressure, the
interference fit may provide an axial (i.e. along an axis defined by insert
76)
compression or interference fit with high contact pressure between the outer
surface of the insert 76 and the carbon material. To produce an interference
fit the
inserts 76 must be inserted into the solid carbon material, as opposed to
being
formed by solidifying a molten metal poured or injected into a recess or
cavity,
which would be subject to shrinkage upon solidification and which, in any
event,
would not achieve significant interface pressure to provide an interference
fit.
[00068] The controlled, high contact pressure between the insert 76 and
the
carbon electrode block 18 or 20 may cause minor carbon material failure local
to
the interface (by crushing or scaling), which limited carbon failure is
acceptable
provided that such carbon failure does not extend to cracks through the carbon
electrode block 18 or 20 which may introduce high electrical resistance areas
across
the crack, or which cracks may propagate during operation causing the anode or
cathode electrode block 18 or 20 to physically break apart. Use of inserts 76
providing an interference fit therefore requires consideration of the
mechanical
stresses induced by thermal expansion of the insert within the carbon anode or
cathode electrode block 18 or 20 when placed in operation, and the strength of
the
anode or cathode electrode block 18 or 20 relative to the location of the
insert 76
determined by the lateral thickness of carbon surrounding the insert 76. The
CA 02838113 2013-12-16
18
interface pressure may change during the operational life of the anode 12 or
cathode 14 due to material creep, depending on the temperature and thermal
behaviour of the adjoining materials. The breaking strength of the anode 12 or
cathode 14 may also change due to its temperature and condition over its
operational life. The interface pressure and resulting electrical resistance
of the
connection can be controlled by adjusting the external dimensions of the
insert 76
and/or the dimensions of bores 68 (e.g. drilled pilot holes, etc.) in the
carbon
material, in order to control the amount of interference fit, contact area and
resulting contact pressure.
[00069] The interference fit between the insert 76 and the carbon material
of
the electrode block 18 or 20 is such that an interface contact pressure
between the
insert 76 and the carbon material of the adjoining electrode block 18 or 20 is
at
least about 0.1 kPa. For example, the interface contact pressure is typically
up to
about 10 MPa, and/or between about 1 MPa and about 10 MPa. Interface contact
pressures in excess of 10 MPa, although possible, do not significantly further
reduce
the electrical resistance across the interface and increase the risk of
cracking of the
carbon substrate.
[00070] To avoid cracking the carbon electrode block 18 or 20, the
pressure
applied by the interference fit may be less than about one half of the stress
required to break the carbon electrode block 18 or 20. The breaking pressure
depends on the strength of the carbon material of the carbon electrode block
18 or
20 and the minimum width and thickness of carbon material surrounding the
insert
76. It will be appreciated that the breaking pressure may vary in different
regions
of the electrode block 18 or 20, being higher in the middle of the block than
near its
edges.
[00071] The total applied pressure caused between the insert and the
surrounding carbon material of the electrode block 18 or 20 depends on the
contact
area between them. Limiting the surface area of the insert 76 within the
carbon
material, with or without a bore 68, can enable a high interface pressure with
a
CA 02838113 2013-12-16
19
limited total applied pressure. Selecting a maximum applied pressure of less
than
approximately one half of the minimum yield strength of the surrounding carbon
should provide an adequate margin of error, such that the anode or cathode
electrode block 18 or 20 does not break from extra pressure that may result
during
anode and cathode operation, possibly due to greater thermal expansion of the
inserts 76 versus the carbon of the carbon electrode block 18 or 20, and the
potential weakening of the carbon electrode block 18 or 20 from bath
absorption
and air burn during its operational cycle. In the alternative, the interface
pressure
for each insert 76, with or without a bore 68, may be theoretically determined
using
Lames equation for interference fit. The selection of dimensions, locations
and
quantity of inserts 76 are to take into consideration an acceptable current
density
through the inserts 76 and desired current distribution through the anode or
cathode electrode block 18 or 20. The maximum interface pressure may also be
tested experimentally for each insert 76 location prior to implementation in
the
reduction cell 10.
[00072] The inserts 76 will typically be inserted into the electrode block
18 or
20 after it has been formed and baked to a hardened state, producing an
interference fit as described above. However, it will be appreciated that the
inserts
76 may be inserted into the electrode block 18 or 20 after it has been formed,
and
while it is in a green state, i.e. prior to baking. Where an insert 76 is
installed into
the carbon of the electrode block 18 or 20 prior to baking, the carbon is
relatively
soft and the interface contact pressure between the insert 76 and the
surrounding
carbon will initially be low. The electrode block 18 or 20 will then be
hardened by
baking, with the insert 76 installed therein, and a good electrical bond
between the
insert 76 and the surrounding carbon of the electrode block 18 or 20 will
result.
[00073] The current density and resistance through the inserts 76 and
between
the inserts 76 and the surrounding carbon material of electrode block 18 or 20
will
generate ohmic heating that will heat the inserts 76, which temperature should
be
maintained below the point at which the yield strength of the heated insert 76
falls
below two times the desired interface contact pressure between the insert 76
and
CA 02838113 2013-12-16
=
the surrounding carbon material, taking into account any additional load that
any
insert 76 may carry, for example due to the mass of the suspended carbon block
18
in the case of anode 12.
[00074] When the inserts 76 are used within the anode assembly 12,
specifically between the anode block 18 and the cast iron thimble 64, the
insert 76
is sometimes referred to herein as a 'thimble anchor'. The thimble anchor 76
enhances the mechanical and electrical connections between the thimble 64 and
the
anode block 18. A portion of the thimble anchor 76 is embedded in the anode
block
18 with an interference fit, as defined above, between the thimble anchor 76
and
the surrounding carbon material of the anode block 18. The total interface
contact
area between the insert and the anode block 18 depends on the diameter,
embedded length and quantity of the inserts 76 used. When using large diameter
inserts 76, then a predrilled borehole 68 may be provided for the insert 76 to
be
embedded into, in order to limit the maximum lateral interface contact
pressure to
avoid fracture of the carbon of the anode block 18. The inserts 76 may be
impact
driven if nail style, or screwed into the carbon if screw or bolt style,
either with or
without a borehole 68 to provide the desired interface contact pressure. The
inserts
76 may also be in the form of expansion anchors that are loose fit inserted
into
predrilled boreholes 68 and then tightened to impose the desired lateral
interface
contact pressure between the insert 76 and the inner surface 74 of the
borehole 68
[00075] When used as thimble anchors, the inserts 76 perform two
functions,
firstly that of providing an electrical conductor between the cast iron
thimble 64,
which is cast in-situ from molten iron around the non-embedded end of the
thimble
anchor 76, and the anode block 18. Secondly, because one end of the thimble
anchor 76 protrudes from a surface of the anode block 18 into the stub hole
58, the
thimble anchor 76 modifies the freeze profile of the cast iron thimble 64 by
providing a heat sink on the inner wall of stub hole 58 which promotes the
solidification or freezing of the iron across the full width of the space
between the
stub 48 outer surface and the inner wall of stub hole 58 at the locations of
the
CA 02838113 2013-12-16
21
inserts 76, while there is molten metal above the insert 76 to fill in for the
solidification shrinkage. This modified freeze profile will reduce or
eliminate the
usual shrinkage gap between the thimble 64 and the inner wall of stub hole 58
at
those locations, thereby enabling a tighter contact and lower electrical
resistance
between the stub 48, thimble 64 and wall of stub hole 58 when the assembly 12
heats up in the reduction cell 10.
[00076] In the embodiments shown in Figures 8, 8a, 9 and 10, the inserts
76
are similar in appearance to nails, having a pointed end, a smooth shank and a
single, enlarged head. It will be appreciated that the inserts 76 do not
necessarily
have this configuration, and that heads and pointed ends are not necessarily
required. However, to enhance the function of the protruding portion of
thimble
anchor 76 as a heat sink, it is beneficial to provide the thimble anchor 76
with an
enlarged head, as further discussed below. The thimble anchors 76 may, for
example, have a single head or a duplex head.
[00077] Figure 8 is a sectioned view through a portion of anode block 18,
showing the interior of one of the stub holes 58, and with the lower end of a
stub
48 shown above the block 18. As shown, the anode electrode block 18 is
provided
with a plurality of impact driven, nail style thimble anchors 76 which, in
this
embodiment, are of cylindrical shape, and extend partially into the anode
block 18
from a second surface thereof. In this embodiment, the second surface is the
inner
surface of the stub hole 58, comprising the flat bottom surface 60, the side
surface
62, and/or in the helical grooves 66. Although only a single thimble anchor 76
may
be required, improved results may be obtained by providing a plurality of
thimble
anchors 76. As shown in Figure 8a, a first end 78 of each thimble anchor 76
protrudes from the inner surface of the stub hole 58, and a second end 80 of
each
thimble anchor 76 is embedded in the inner surface of stub hole 58 with an
interference fit. The protruding first end 78 of the thimble anchor 76 may
have an
enlarged head to enhance its function as a heat sink during the pouring and
solidification of the molten metal in the gap between the stub hole 58 and
stub 48,
to form the thimble 64. It will be appreciated that the additional surface
area
i
CA 02838113 2013-12-16
22
provided by the enlarged head at the first end 78 of thimble anchor 76 will
provide
an enhanced mechanical bond with the thimble 64, as well as an enhanced
electrical connection with the thimble 64. Thus, with the provision of thimble
anchors 76 as disclosed herein, better mechanical and electrical connections
are
formed between the stubs 48, the thimble 64, and the inner surface of stub
hole
58.
[00078] Depending on the diameter and type of thimble anchor 76, it may be
necessary to provide the inner surface of the stub hole 58 of Figure 8 with
boreholes 68 corresponding in number to the thimble anchors 76, in order to
achieve an interference fit with the desired amount of lateral interface
contact
pressure discussed above. This possibility is also illustrated in the close-up
of
Figure 8a, which shows one of the impact driven, nail style thimble anchors 76
of
Figure 8 partially embedded in a pre-formed borehole 68 in the side surface 62
of
the stub hole 58. The borehole 68 is pre-drilled in the substrate and has a
first
open end 70 at the second surface (i.e. the inner surface) of the anode block
18,
and a second closed end 72 inside the anode block 18. The borehole 68 also has
an
inner wall 74, a width (in the case of the cylindrical bore 68, the same as
diameter), and a length extending from the first end 70 to the second end 72
of
bore 68 Except where the thimble anchor 76 is an expansion type anchor, the
diameter of the borehole 68 is sized less than the diameter of the thimble
anchor
76 in order to provide the desired lateral interface contact pressure between
the
thimble anchor 76 and the inner wall 74 of the borehole 68. As shown in Figure
8a,
the first end 78 of thimble anchor 76 is proximate to the first end 70 of
borehole 68
and spaced therefrom, while the second end 80 of thimble anchor 76 received
inside borehole 68 proximate to the second end 72 thereof. It can be seen from
Figure 8a that the initial diameter of borehole 68, as shown at the second end
72 of
borehole 68, is smaller than the diameter of the thimble anchor 76, the
relative
diameters of borehole 68 and thimble anchor 76 being selected to provide an
interference fit having the desired amount of lateral interface contact
pressure.
1
CA 02838113 2013-12-16
23
[00079] As shown, the thimble anchors 76 embedded partially in the bottom
surface 60 of stub hole 58 may extend vertically downwardly from the bottom
surface 60, although the thimble anchors 76 in bottom surface 60 may instead
be
sloped relative to the vertical. The partially embedded thimble anchors 76 in
the
side surface 62 may extend horizontally and radially outwardly from the side
surface 62. Alternatively, the thimble anchors 76 in the side surface 62 may
be
sloped relative to the horizontal, extending downwardly and outwardly from the
side surface 62, and one such sloped anchor 76 is shown in Figure 10.
[00080] In this embodiment, where the inserts 76 comprise thimble anchors,
they will be formed of a material such as cast iron or carbon steel which can
be
recycled with the metal comprising the thimble 64.
[00081] The thimble anchors 76 provide the electrical connection between
the
thimble 64 and the anode block 18, while the cast iron thimble 64 will bond
the
anode block 18 to the stub 48. The thimble anchors 76 and stub 48 together
create
a good electrical connection through those components of the electrode
assembly
12 even when it is cold. Further, the provision of at least one thimble anchor
76 in
the bottom surface 60 of stub hole 58 with a cast iron bond will enable
current
distribution through the bottom surface 60 of the stub hole 58 through the
stub 48.
This typically does not occur when the stubs 48 sit directly on the bottom
surface
60 of the stub hole 58 during iron pouring, which prevents a cast iron
connection
forming between the bottom of the stub 48 and the bottom surface 60 of stub
hole
58.
[00082] Care must be taken if the thickness of the cast iron under the
stub 48
is too great, thereby preventing the thimble stripping press from breaking and
stripping the thimble 64 from the stub 48. To prevent this from happening, the
bottom surface of the thimble 64 can be provided with a weaker breaking area.
For
example, as shown in Figure 9, the bottom surface 60 of stub hole 58 may be
provided with one or more raised ridges 86 of carbon, extending wholly or
partially
across the diameter of bottom surface 60. The height of ridges 86 is below the
tops
CA 02838113 2013-12-16
24
of the inserts 76 provided in the bottom surface 60, such that the ridges 86
will
allow the cast iron 27 to flow over the ridges during casting to fill the
space
between the bottom surface 60 and the bottom of the stub 48 while creating the
necessary weakness in that layer to enable the thimble stripping press to
break and
strip the cast iron metal lining 64 from the stub 48. The ridges 86 may be
formed
by altering the shape of the anode forming press mold by removing material
from
the bottom of the stub hole form in the shape and orientation suited to the
needs of
the thimble stripping press.
[00083] In another aspect, also shown in Figure 9a, only a portion of
thimble
64 comprises a cast structure, and the remainder comprises a preform 82. The
preform 82 is in the form of a circular disc, of the same material as the
thimble 64,
which is sized to fit inside the bottom of stub hole 58, against the bottom
surface
60 thereof. The preform 82 becomes incorporated in the structure of the
thimble
64 when the molten iron is cast, and helps to maintain a desired spacing
between
the bottom of stub 48 and the bottom surface 60 of stub hole 58. As shown in
Figure 9a, the preform 82 may have apertures through which inserts 76 can be
driven or threaded into the bottom surface 60 of stub hole 58. The preform 82
may
be formed with grooves 84 on its lower or upper surface to allow the preform
to be
easily fractured when it is desired to remove the stub 48 and the thimble 64
from
the stub hole 58 during anode recycling.
[00084] The preform 82 described above will typically be fastened to the
bottom surface 60 of stub hole 58 after baking of the anode block 18. However,
according to an alternate embodiment, the preform 82 may be inserted into the
stub hole 58 while the anode block 18 is in a relatively soft, green state, in
which
case the preform 82 may be partially embedded in the bottom surface 60 of stub
hole 58. In such an embodiment, the preform 82 may be formed into the anode
block 18 during the anode forming process, i.e. at the time that the stub hole
58 is
formed. Alternatively, the preform 82 may be inserted into the stub hole 58
after it
is formed. Where the preform 82 is inserted into the green anode block 18, the
inserts 76 may optionally be integrally formed with the preform 82.
i
CA 02838113 2013-12-16
. .
[00085] Although Figures 8 to 10 illustrate the use of impact
driven, nail style
thimble anchors 76, other types of inserts 76 may be used in this embodiment,
or
in other embodiments described herein. For example, the inserts may be similar
in
appearance and/or function to conventional fasteners, such as friction fit or
press fit
nails, rods or spikes, screws or lag bolts, expansion anchors (including but
not
limited to lag shields, sleeve or wedge type expansion anchors, etc.), or
other
mechanically installed fasteners that produce a controlled lateral and/or
axial
pressure. For example, Figures 14a to 14c show three forms of inserts,
labelled
76a, 76b and 76c. Insert 76a is in the form of a nail having a pointed tip,
smooth
shank and circular, enlarged head, similar to that shown in Figures 8 to 10.
Insert
76b is in the form of a lag screw or lag bolt, having a hexagonal head, a
pointed tip
and a threaded shank. Figures 14a and 14b each shows insert 76a or 76b
optionally being inserted into a pre-drilled bore 68 which is smaller in
diameter than
the shank of the insert 76a or 76b, the bore 68 being deformed radially
outwardly
by the insertion of insert 76a or 76b. However, it will be understood that the
bores
68 are not always necessary, depending at least partly on the diameter of the
insert
76a or 76b.
[00086] Insert 76c shown in Figure 14c is in the form of an
expansion anchor
having an inner threaded screw portion and an outer split sleeve portion. The
insert 76c is initially inserted into a pre-drilled bore 68 with a relatively
loose fit
and, when the screw portion is threaded into the sleeve portion and tightened,
the
sleeve portion is forced outwardly against the inner wall 74 of bore 68 as the
insert
76c, causing radially outward deformation of the bore 68.
[00087] Although the inserts 76 described and shown herein generally
have
cylindrical shanks, it will be appreciated that this is not necessary. Rather,
the
inserts May be of any convenient cross-section, including square-shaped,
rectangular-shaped, star-shaped, fluted, etc.
[00088] In another embodiment now described with reference to
Figures 6 and
7, the conductive metal inserts 76 are used to augment the existing cast iron
,
CA 02838113 2013-12-16
26
connected assemblies, by adding one or more additional electrical paths
between
the vertical conductor rod 38 and the anode block 18 using one or more
electrically
conductive bypass members, resulting in lower overall electrical resistance of
the
anode assembly 12.
[00089] The embodiment shown in Figures 6 and 7 includes a plurality of
"external" electrically conductive bypass members 88, meaning that they form
an
electrically conductive connection from the vertical conductor rod 38 to the
carbon
material of the anode block 18, without forming a connection through the metal
lining 64 of recess 58. Each of the bypass members 88 has a first end 90
connected to the vertical conductor rod 38 through an electrically conductive
connection, and a second end 92 connected to the anode block 18 through an
electrically conductive connection.
[00090] The anode block 18 is provided with a plurality of conductive
metal
inserts 76 in its top surface 42, forming an interference fit as defined above
with
the carbon material of the anode block 18. As discussed above, the inserts 76
are
provided with or without bores 68, depending on the diameters of the inserts
76
and the strength of the substrate material.
[00091] The inserts 76 are at least partly received in the top surface 42.
It can
be seen from the drawings that the inserts 76 are located in top surface 42
such
that the second ends 92 of the bypass members 88 are connected to the anode
block 18 by at least one of the inserts 76. In the illustrated embodiment,
each of
the second ends is connected to the anode block 18 by two of the inserts 76.
High
pressure contact between the second ends 92 and the inserts 76 may be provided
by using inserts 76 provided with compression washers 77 to maintain interface
pressure, or using other conductive connections, including but not limited to
brazing, welding or use of a nut 79 welded to the second end 92, or a locking
thread 81 in the second end 92, ensuring an electrical connection with the
insert
76. These latter two options are illustrated in Figures14d and 14e,
respectively.
CA 02838113 2013-12-16
27
[00092] It will be appreciated that the attachment of the bypass members
88
to inserts 76 also results in the second ends 92 of the bypass members 88
being
secured to the top surface 42 of the anode block 18 with an electrically
conductive
connection.
[00093] The first ends 90 of the bypass members 88 are fastened to the
vertical conductor rod 38 by means of fusion welding, soldering, brazing,
interference fit fastener, screw, bolt, rivet, clamp or other mechanical or
fusion
connection which forms an electrical conduction path from the vertical
conductor
rod component 38 to the bypass member 88. In the embodiment of Figures 6 and
7, the first ends 90 of bypass members 88 are connected to the conductor rod
38
by mechanical fastening means 83a and 83b, including nuts and bolts, each pass
through or adjacent to the conductor rod 38.
[00094] Although Figures 6 and 7 show bypass members having a specific
configuration, it will be appreciated that the bypass members may instead
comprise
flexible conductors such as wire cables with lug ends for the attachment of
the
inserts 76.
[00095] According to another embodiment, the inserts 76 are used in
combination with conductive connectors similar to the bypass members 88
described above, to replace and eliminate the conventional iron connection
without,
however, changing the main basis of the shapes of the electrodes, to allow a
user
to transition from traditional assemblies to the low resistance assemblies
disclosed
herein. Small changes to the shape of the carbon electrode block 18, 20 may be
included in this embodiment to enable the use of the inserts 76, with or
without
bores 68.
[00096] Figures 11 and 12 illustrate an anode assembly 12 according to
this
embodiment, including a vertical conductor rod 38, yoke 50 and stubs 48
similar to
those illustrated in the embodiments described above. However, in the present
CA 02838113 2013-12-16
28
embodiment, the lower end of each stub 48 is secured to the top surface 42 of
the
anode block 18 in an electrically conductive manner by a collar-shaped
connector
94. Each connector 94 has a vertical sidewall 96 which receives the lower end
of a
stub 48. Because the stub 48 is cylindrical, the sidewall 96 in the
illustrated
embodiment of connector 94 is also cylindrical, and has an inside diameter
slightly
greater than the outside diameter of the stub 48, so as to closely receive the
lower
end of stub 48 in its hollow interior. The sidewall 96 is electrically
connected to the
stub 48 by welding, brazing or electrically conductive mechanical fasteners.
It will
be appreciated that the sidewall 96 may have any desired shape which provides
an
electrically conductive connection with the end of stub 48. Although the
sidewall 96
is shown as being continuous, this is not necessarily the case. It will be
appreciated that the sidewall 96 may instead be discontinuous, or comprise a
plurality of separate pieces each attached mechanically and electrically to
the stub
48.
[00097] The connector 94 also has at least one attachment portion 98 which
is
connected to the sidewall 96 in an electrically conductive manner, and may be
integrally formed therewith. Each attachment portion 98 extends outwardly from
the sidewall 96 of connector 94 and is secured and conductively connected to
the
top surface 42 of the anode block 18 by one or more conductive inserts 76,
with or
without a bore 68. High pressure contact between the connector 98 and the
insert
76 may be provided by the same means described above with reference to Figures
6 and 7 and 14a to 14e, for example by using inserts 76 provided with
compression
washers 77, to maintain interface pressure, or using other conductive
connections,
including but not limited to brazing, welding or use of a backing plate,
ensuring an
electrical connection.
[00098] In the embodiment of Figures 11 and 12, the collar-shaped
connector
94 includes a plurality of attachment portions 98, in the form of radially
projecting
tabs, each of which is secured to the top surface 42 by at least one insert
76. Each
one of the attachment portions 98 includes means for permitting differential
thermal expansion thereof in relation to the anode block 18. In the
illustrated
CA 02838113 2013-12-16
29
embodiment, the means for permitting expansion comprise folds or bends 100 in
the attachment portion 98, so as to permit the attachment portion 98 to
expand,
contract or flex in response to differential thermal expansion or contraction
while
supporting the load of the anode. Although bends 100 are shown as the means
for
permitting thermal expansion, other means may be used instead. For example,
the
attachment portions 98 may have expansion slits cut or formed into the
attachment
portion 98, providing a serpentine electrical path that enables different
thermal
expansion of the attachment portions 98 between inserts 76 relative to the
thermal
expansion of the top surface 42 of the anode block 18 between inserts 76.
[00099] As discussed above, the inserts 76 are used to connect components
such as the collar-shaped connector 94 to the top surface 42 of the anode
block 18,
and also to conduct electricity into the block 18 through the inserts 76
themselves.
The use of inserts 76 allows for modification of electrical resistance and
current
distribution through the anode assembly 12 by adjusting the material of
inserts 76,
as well as the length, diameter, contact surface area, quantity, location and
interference fit or contact pressure between the insert 76 and the anode block
18.
These adjustments may enable a more consistent electrical resistance from the
vertical conductor rod 38 to any point on the bottom surface 22 of the block
18,
which promotes consistent current density, lower overall electrical
resistance, more
consistent anode consumption resulting in a flatter bottom surface 22 of the
block
18 during its operational life. The flatter anode bottom surface 22 may enable
a
higher portion of the block 18 to be consumed before it is necessary to remove
the
anode 12 from operation, thus reducing recycle volumes and reducing the
ongoing
replacement costs of anode replacement.
[000100] The use of inserts 76, with or without bores 68, may also be applied
in
a cathode assembly 14, between the cathode block 20 and the current collector
bar
52. The collector bar 52 is typically bonded to the cathode block 20 by a
layer of
cast iron 57 between the collector bar 52 and the corresponding recess 56
(also
referred to in this embodiment as "slot 56"), in the bottom surface 54 of
carbon
cathode collector block 20. The current collector bar 52 is bonded to the
carbon
CA 02838113 2013-12-16
=
cathode collector block 20 with molten cast iron 27 while it is upside down
(See Fig.
15) prior to assembly into the reduction cell 10. Bonding may instead be done
using a carbon adhesive paste. The following embodiments apply to both cast
iron
and carbon adhesive collector bar connections.
[000101] As will be appreciated, there may be multiple current collector bars
52
connected to each cathode block 20. Each block 20 is installed across the
width of
the reduction cell 10 and multiple blocks 20 are installed beside each other
to line
the bottom floor of the reduction cell 10. As shown in Figures 15 and 16, the
current collector bar 52 has a flat bottom surface 53 which is substantially
co-
planar with the bottom surface 54 of the cathode block 20.
[000102] In accordance with an embodiments shown in Figures 17 to 19, there
may be provided one or more electrically conductive metal connectors 112 in
the
form of a flat, elongated strap which is attached to the bottom surface 54 of
carbon
cathode collector block 20 by inserts 76, with or without bores 68. The
inserts 76
are received in the bottom surface 54, adjacent to slot 56. Each strap
connector
112 extends substantially transversely across the bottom surface 54, extending
across the slot 56 and having each of its ends secured to the bottom surface
54 by
at least one insert 76. The middle portion of each strap connector 112 is
electrically connected to the current collector bar 52 by means of fusion
weld,
solder, braze, friction fit pin, screw, bolt or other mechanical or fusion
connection
which forms an electrical conduction path from the collector bar 52 to the
strap
connector 112. The strap connectors 112 of Figure 17 are connected to the
bottom
surface 53 of each collector bar 52 by a fusion connection, whereas Figure 19
shows a variant in which mechanical connections are formed between the strap
connectors and collector bar 52. If protrusion of strap connector 112 below
the
plane of bottom surface 54 is undesirable, it is possible to machine grooves
114
(Figure 19) or clearance cavities into the bottom surface 54 of the cathode
block 20
to provide the necessary clearance.
CA 02838113 2013-12-16
31
[000103] Although the strap connectors 112 are shown in Figure 17 as
extending across both sides of the slot 58, this is not necessarily the case.
Rather,
as shown in Figure 19, the strap connectors 112 may be shorter pieces, having
one
end electrically connected by an insert 76 to the bottom surface of cathode
block 20
and the other end electrically connected to the collector bar 52. For example,
Figure 19 shows an insert 76 in the form of a bolt with a lock washer 77
compressed between the bolt head and one end of the strap connector 112. The
other end of strap connector 112 is bolted to the bottom surface 53 of the
current
collector bar 52 by a bolt and washer, which are similarly labelled 76 and 77,
but
which are not necessarily the same as the insert 76 and lock washer 77.
[000104] Alternately, as shown in Figure 20, the connectors 112 may be
comprised of flexible electrical conductors such as wire cable 113 with lugs
115 at
one or both ends thereof, to attach to the inserts 76. A small amount of extra
wire
cable 113 may be provided to allow for differential expansion between the
collector
bar 52 and the cathode block 20.
[000105] It will be appreciated that the use of strap connector 112 allows the
layer 57 of cast iron or carbon paste to be bypassed or eliminated, as the
strap
connector provides a direct, electrically conductive connection between the
current
collector bar 52 and the bottom surface 54 of the cathode block 20. As shown
in
Figure 17, a plurality of spaced strap connectors 112 may be provided along
the
length of block 20, each secured by inserts 76, so as to provide multiple
electrical
connections between the block 20 and the current collector bar 52. This
provides
an improved current distribution across the length and the width of the block
20.
[000106] As with the anode assemblies discussed above, the use of inserts 76
allows modification of the electrical resistance and current distribution of
the
cathode assembly 14 from the end of the collector bar 52, where it exits the
reduction cell 10, to the top surface 30 of the cathode block 20, by adjusting
the
insert material, length, diameter, quantity, position and interference fit or
contact
CA 02838113 2013-12-16
32
pressure relative to the carbon material of the cathode block 20. For example,
the
current distribution and resistance profile of the cathode assembly 14 may be
made
more even across the top surface 30 by using longer or more inserts 76 towards
the centre of block 20, in relation to its ends as illustrated in Figure 18.
The actual
lengths and positions of the inserts 76 may be determined during cathode
assembly
outside of the reduction cell 10, using a suitable ohmmeter to measure the
resistance from the top surface 30 to the end of the collector bar 52. These
modifications promote consistent current density, lower overall electrical
resistance,
more even cathode wear, a longer cathode life, and a flatter cathode top
surface 30
during its operational life.
[000107] Due to differential thermal expansion of the current collector bar 52
it
is also desirable to provide strap connectors 112 with means for permitting
differential thermal expansion. As shown in Figures 18 and 19, the means for
permitting expansion comprise folds or corrugations 100 in the connector 112,
so
as to provide a bellows-like arrangement which can expand or contract in
response
to differential thermal expansion or contraction.
[000108] It is appreciated that the strap connectors 112 may not be limited to
singular pieces of conductor but may be comprised of multiple layers of thin
straps
which flex more easily than solid pieces while providing a similar electrical
resistance. For example, the strap connectors 112 in Figure 19 are shown as
comprising two layers 117.
[000109] According to another embodiment, illustrated in Figure 13, a low
resistance yoke assembly 126 is provided for forming a connection between the
lower end of a vertical conductor rod 38 and the top surface 42 of an anode
block
18.
[000110]
The yoke assembly 126 comprises a pair of curved metal struts 128,
which may be identical to one another. Each of the struts has an upper end 130
and an opposed lower end 132. The upper end 130 of each strut 128 is bonded
CA 02838113 2013-12-16
33
through mechanically and electrically conducting attachment to opposing side
surfaces of the lower end of the vertical conductor rod 38, which is shown as
having
a rectangular cross section with four vertical side surfaces. The mating
surface of
the struts 128 and the rod 38 may be electroplated, or bonded with suitable
surface
material to enable subsequent bonding of the strut 128 to the rod 38 by
welding,
brazing or other electrical connection. Following attachment of the struts 128
to the
rod 38, one or more mechanical through fasteners 136, such as but not limited
to
bolts with washers and nuts, are inserted through the struts 128 and rod 38,
and
are adequately tightened to remove cyclical physical stress on the electrical
joint
due to loading and unloading the weight of suspended anode block 18 from the
rod
38.
[000111] The two struts 128 are arranged in opposed, face-to-face relation
with
each other. In this embodiment they are joined together by one curved
connector
or brace 134, which contributes to the mechanical strength of the struts 128
while
enabling some flexure of the struts 128 under thermal expansion stress. The
struts
128 may be made of a single alloy of metal or clad or cored with different
conductive metal.
[000112] The lower ends 132 of the struts 128 are curved outwardly away from
the vertical conductor rod 38 and may be formed with multiple sections,
separated
by slits 138. The lower ends 132 mate with the top surface 42 of the anode
block
18, and for this purpose the lower ends 132 are provided with one or more
holes to
enable the installation of one or more inserts 76 through the lower ends 132
and
into the top surface 42 of the anode block 18, with or without bores 68. The
inserts
76 therefore carry the weight of the anode block 18 from the struts 128 and
provide an electrical connection from the vertical conductor rod 38 to the
anode
block 18. To provide added resistance against withdrawal of the inserts 76, at
least
some of the bores 68 may be angled from the vertical, toward the vertical
conductor rod 38, so that the inserts 76 received in these bores 68 will be
"toed in"
toward one another. Other inserts 76 may be angled from the vertical in other
directions to provide improved current distribution within the anode block 18.
If, as
CA 02838113 2013-12-16
34
shown in Figure 13, multiple holes are provided within the same section of
strut
128, such that one section is secured by two or more inserts 76, then an
expansion
fold 101 is provided in between the holes and inserts 76 to enable
differential
expansion of the strut 128 versus the anode block 18, with minimal stress
imposed
on the anode block 18 due to the flexing of the curved struts 128.
[000113] It is appreciated that the lower ends 132 of the struts 128 may be
connected to the vertical conductor rod 38 by other means than mechanical
fasteners 136, provided that electrical conduction is maintained between the
two
parts during operation.
[000114] As compared to the traditional rod assembly, the configuration of the
low resistance yoke assembly 126 eliminates one fusion weld on the bimetallic
transition joint, increases the electrical contact surface area of the strut
128 to rod
38 connection through connection on both sides of the rod 38, it removes the
physical stresses in the bimetallic connection from the weight of the
suspended load
by use of the through bolt connections, it eliminates the yoke to stub welded
connection, it eliminates the condition of stub toe-in that the traditional
yoke & stub
assembly suffers due to repeated thermal stress and material creep at high
temperatures, it enables the use of inserts 76 with toed in orientation to
carry high
loads through the cross section of the inserts 76. These benefits provide a
low
electrical resistance configuration with long life.
[000115] According to convention, the upper end 44 of the aluminum or copper
vertical conductor rod 38 is temporarily attached to anode buss bar 46 with a
buss
clamp (not shown). The electrical resistance through the mating surface(s) of
the
rod 38 and buss 46 is dependent on the cleanliness, surface area and clamp
pressure between the mating surfaces. Over repeated use the surfaces of the
rod
38 and buss 46 may become oxidized or pitted from arcing which introduces
surface roughness and a surface oxide layer of relatively high electrical
resistance.
The electrical resistance of the rod 38 to buss 46 connection may be reduced
by
cladding or electroplating the mating surface 140 of the buss 46 (Figure 11)
and/or
CA 02838113 2013-12-16
the mating surface 142 of rod 38 (Figure 11) with an electrically conductive
corrosion resistant metal such as, but not limited to, nickel, platinum or
gold.
Although this surface treatment may add a small amount of electrical
resistance
compared to a clean metal interface (aluminum rod to aluminum bus or copper
rod
to aluminum bus) this clad or electroplated surface will maintain its
electrical
resistance at levels less than those of oxidized aluminum or oxidized copper
over
the life of the rod assembly.
[000116] One or both ends of each steel current collector bar 52 are connected
by a bolted connection to the cathode buss 24 flex connectors (not shown).
Also, a
portion of each collector bar 52 is in electrical contact with the cathode
block 20
through the cast iron layer 57, as explained above with reference to Figure 3.
The
mating surface(s) of collector bar 52 which are in electrical contact with the
cast
iron layer 57 may oxidize and develop an electrically resistant oxide layer
due to
the high temperature of the collector bar during operation. Similarly, the
mating
surface(s) of collector bar 52 which are in contact with the flex connectors
of the
cathode buss 24 may develop oxidation. For this reason, the mating surface(s)
of
the current collector bar 52 which are in contact with the cathode buss flex
connectors and/or the cast iron layer 57 be clad or electroplated with
electrically
conductive corrosion resistant material, such as any of the electrically
conductive
corrosion resistant metals mentioned above.
[000117] Figure 21 shows a section of a current collector bar 52 in which a
cladding 141 of the electrically conductive corrosion resistant metal is
provided on
all four surfaces of the bar 52. Alternatively, the cladding 141 may be
applied only
to the mating surfaces which are in contact with the cast iron layer 57 or the
cathode buss 24. The cladding may have a thickness of about 1 mm.
Alternatively,
the current collector bar 52 may be electroplated with a coating of the
electrically
conductive corrosion resistant metal, the coating having a thickness on the
order of
about 0.1 mm.
CA 02838113 2013-12-16
36
[000118] The embodiments of illustrations in Figures 22 to 26 relate to
reducing
electrical resistance between the current collector bar 52, the cast iron
layer 57 and
the carbon of the cathode block 20. The means for reducing electrical
resistance in
the cathode 14 are similar to the means for reducing electrical resistance in
the
anode 12, discussed above, using conductive inserts 76. However, some
differences are necessitated by the fact that the thermal expansion of stubs
48 and
thimbles 64 in the anode 12 is primarily radial without relative movement
between
the components, whereas the thermal expansion of the collector bar 52 and cast
iron layer 57 in the cathode 14 is primarily axial and with relative movement
between the components due to different coefficients of thermal expansion.
[000119] Figure 22 is a partial, cross-sectional view of a cathode assembly
14,
showing the cathode block 20 having a slot 56 in its bottom surface 54, with a
cast
iron layer 57 and current collector bar 52 received in the slot 56. As shown,
the
interior surfaces of slot 56 are provided with a plurality of conductive
inserts 76
which are received in the carbon of cathode block 20 with an interference fit,
as
discussed above with reference to the anode 12. The above discussion of the
embedding of inserts 76 into the anode block 18 applies equally to the present
embodiments, except where otherwise discussed below.
[000120] The conductive inserts 76 embedded in the surfaces of slot 56 do not
significantly protrude into the cast iron layer 57. Rather, the heads of
inserts 76 in
Figures 24 and 25 are intended to be flat, optionally having rounded edges, so
as to
permit axial expansion movement of the cast iron layer 57 relative to the
cathode
block 20. It will be appreciated that embedment of the heads of inserts 76 in
the
cast iron layer 57 could result in damage to the carbon material when the cast
iron
layer expands axially relative to the cathode block 20. Furthermore, to
prevent a
bond forming between the inserts 76 and the cast iron layer 57, the heads of
inserts may be provided with a thin coating of graphite powder, or other
electrically
conductive non-stick material which will not significantly increase electrical
resistance between the inserts 76 and the cast iron layer 57.
CA 02838113 2013-12-16
37
[000121] As shown in Figure 22, a conductive metal slot liner 144 may be
provided between the cast iron layer 57 and the interior surfaces of slot 56.
The
conductive slot liner comprises a thin sheet of metal through which the
inserts 76
extend into the cathode block 20, the heads of the inserts 76 holding the
liner in
place as shown in Figure 26b. As shown in Figure 23, the liner 144 may have
expansion slits 146 between the insert locations, shown as holes 148, to allow
for
differential thermal expansion of the liner 144 relative to the cathode block
20. The
liner 144 may alternately be comprised of a plurality of close fitting or
overlapping
plates of conductive metal with each piece attached to the inside surface of
the
cathode slot 56 by means of at least one insert 76. It will be appreciated
that the
provision of liner 144 may improve current distribution through the cathode
block
20 by improving the electrical connection between the cast iron layer 57 and
the
surface of the cathode slot 56.
[000122] Instead of using a liner, the current distribution may be enhanced by
providing more inserts 76 or, as shown in Figure 24, by providing the inserts
76
with larger heads and/or conductive metallic washers 150. For example, inserts
76
with enlarged heads 78 having rounded edges 78, which may be partially
embedded in the carbon of block 20, are shown in Figure 25, which will be
discussed below.
[000123] In the case of the anode 12, the inserts 76 embedded in the anode
block 18 have heads which protrude into the stub hole 58 and are embedded in
the
thimble 64, to modify the shape of the freezing iron to reduce shrinkage gap
between the cast iron and the stub hole wall. This reduces the electrical
resistance
between those components. As shown in Figure 25, this effect can be attained
in
the cathode 14 by providing one or more conductive collector bar anchors 152,
the
collector bar anchors 152 having one end attached to the collector bar 52 and
another end protruding into the slot 56. The ends of the collector bar anchors
152
protruding into slot 56 will become embedded in the cast iron layer 57and
provide
heat sinks during casting of layer 57. This will reduce the solidification
shrinkage
gap local to the collector bar anchors, between the cast iron layer 57 and the
inner
CA 02838113 2013-12-16
38
surfaces of slot 56 for the same reasons as discussed above, by modifying the
freeze profile of the cast iron layer 57 to promote solidification of the iron
across
the full width of the space between the collector bar 52 and the cathode block
20,
as illustrated by the freeze isotherms 29 in Figure 26. This provides a
"tighter" fit
of the collector bar 52 and the cast iron layer 57 within the cathode slot 56,
thereby
reducing electrical resistance between the cathode block 20, the collector bar
52
and the cast iron layer 57.
[000124] The quantity, depth, dimensions and locations of the collector bar
anchors 152 may be modified to adjust the electrical resistance between the
top
surface 30 of the cathode block 20 and the collector bar 52 in order to make
the
electrical resistance as consistent as possible. In order to enhance the heat
sink
effect, the collector bar anchors 152 may be provided with enlarged heads, as
shown in Figure 25. Also, the anchors 152 and the holes 154 in which they are
received may be threaded, to permit the amount of protrusion of the anchor 152
to
be adjusted. For example, the degree of protrusion can be adjusted so that the
heads of the anchors 152 contact the heads of the conductive inserts 76
embedded
in the cathode block 20. This will further improve the electrical connection
between
collector bar 52 and cathode block 20.
[000125] Figure 26 illustrates a cathode assembly 14 according to a variant of
the embodiment shown in Figure 25. The cathode assembly of Figure 26 includes
inserts 76 having enlarged, rounded heads 78 which are impact driven into the
carbon of cathode block 20, rather than being driven into pre-formed bores 68.
The right hand side of current collector bar 52 is provided with threaded
collector
bar anchors 152 adjustably received in threaded holes 154, the collector bar
anchors 152 being in the form of threaded studs or rods, not having enlarged
heads, the protruding ends optionally being tapered. The protruding ends of
the
collector bar anchors 152 may or may not be in contact with the heads 78 of
inserts
76.
CA 02838113 2013-12-16
39
[000126] The left hand side of current collector bar 52 is provided with fixed
collector bar anchors 156 which are secured to the outer surface of current
collector
bar by fusion bonding, such as by welding, brazing or soldering. The fixed
anchors
156 may be in the form of cylindrical studs or rods, and the protruding ends
may be
tapered as shown. The fixed anchors 156 will act as heat sinks, and have a
similar
effect on the freeze profile of the cast iron layer 57, as indicated by the
freeze
isotherms 29 on the left hand side of Figure 26.
[000127] Figures 27 and 28 show an alternate embodiment of a cathode
assembly 14 in which no cast iron layer 57 is provided in the space between
the
current collector bar 52 and the cathode block 20. The cathode block 20 is
conductively connected to the collector bar by a strap connector 112
comprising a
wire cable 113 with lugs 115, one of the lug ends 115 being conductively
connected
to the bottom surface 54 of cathode block 20 by a conductive insert 76, the
opposite lug end 115 being conductively connected to the bottom surface 53 of
collector bar 52 by a bolt 176. Alternatively, a plurality of any of the strap
connectors 112 described above may be provided.
[000128] In the embodiment of Figures 27 and 28, the current collector bar 52
is held in place by one or more conductive hanger assemblies 160, one of which
is
shown in the drawings, comprising a tongue portion 162 and a slot portion 164,
the
tongue portion 162 being slidably received in the slot portion 164 along the
longitudinal axis of the collector bar 52, as best seen in the cross-sectional
side
elevation of Figure 29. In the illustrated embodiment, the tongue portion 162
has
an attachment flange 166 which is secured to the top surface of collector bar
52
(opposite the bottom surface 53) by one or more bolts 176, although other
mechanical or fusion connections may be used instead. The slot portion has a
pair
of attachment flanges 168 along its lateral edges, the flanges being attached
to the
inner surface of slot 56 by conductive inserts 76 inserted in the carbon of
cathode
block 20. It will be appreciated that the positions of the tongue portion 162
and
slot portion 164 could be reversed, i.e. with the slot portion 164 attached to
collector bar 52 and the tongue portion 162 attached to the cathode block 20.
CA 02838113 2013-12-16
[000129] With the conductive hanger assembly 160, relative thermal expansion
of the collector bar 52 and the cathode block 20 results in axial longitudinal
movement of the tongue portion 162 within slot portion 164, thereby providing
support for the collector bar 52 while avoiding the creation of thermal
stresses.
[000130] If eliminating the layer 57 of cast iron or paste the gap remaining
between the collector bar 52 and the inner surface of the cathode slot 56 may
be
filled with thermal insulation to reduce heat transfer between the cathode
block 20
and the collector bar 52.
[000131] Although specific embodiments have been described herein, the claims
are not limited to these embodiments. Rather, the disclosure includes all
embodiments which may fall within the scope of the following claims.
