Note: Descriptions are shown in the official language in which they were submitted.
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COMPONENT CATHODE COLLECTOR BAR
The present invention relates to cathode assemblies for use in
Hall-Heroult aluminum reduction cells. Such cathode assemblies include a
cathode
block into which is fitted a collector bar. More particularly, the invention
relates to
cathode assemblies having multiple collector bars.
Aluminum is commonly manufactured via a smelting process in an
electrolytic cell of the established Hall-Heroult design. A conventional Hall-
Heroult
electrolytic cell, known as a pot and shown in Fig. 1, includes a cell C
defining a
chamber H in which are received carbonaceous anodes A. The anodes A are
suspended in a bath B of electrolytic fluid containing alumina and other
materials.
Electric current is supplied to the anodes A via anode rod assemblies R to
provide a
source of electrons for reducing the alumina to aluminum which accumulates as
a
molten aluminum pad P. The molten aluminum pad P forms a liquid metal cathode.
A cathode assembly CA, shown in detail in Fig. 2 from the underside thereof,
is
positioned in the bottom of the chamber H and completes the cathodic portion
of
the cell C. The cathode assembly CA includes a carbonaceous cathode block CB
having an upper surface which supports the molten aluminum pad P and a lower
surface which defines a groove or slot S extending between the ends of the
cathode
block CB. A collector bar BA, typically formed from hot rolled or cast mild
steel,
is received within the slot S and is secured in the slot S with a layer of a
conductive
material CM such as cast iron, carbonaceous glue, rammed carbonaceous paste or
the like. The conductive material layer CM is disposed between the collector
bar
BA and the cathode block CB along the entire length of the slot S. The
collector
bar BA is longer than the cathode block CB and extends out of the chamber H.
The exposed end of the collector bar BA is connected via a bus bar (not shown)
to
the current supply in a conventional manner to complete the circuit. The
cathode
assembly CA may include a pair of opposing collector bars BA as shown in Fig.
2
which are separated by a filler material F which fills the gap between the
collector
bars BA. The filler material F may be a crushable material or a piece of
carbon or
a carbonaceous paste, commonly referred to as seam mix or ramming paste (an
unfired mixture of anthracite or graphite and anthracite and pitch binder), or
a
combination thereof.
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These electrolytic cells are typically operated at high temperatures
(about 940 to 980 C) which, when combined with the corrosive nature of the
electrolytes, creates a harsh environment. Collector bars conventionally are
formed
from hot rolled or cast mild steel. Mild steel has relatively poor
conductivity
compared to aluminum, but has a high melting point and relatively low cost.
The
cathode blocks have historically been formed from a mixture of anthracite and
pitch
binder and exhibit relatively high electrical resistivity, high sodium
swelling, low
thermal shock resistance and high abrasion resistance. As aluminum producers
have
sought to increase productivity, the operating amperages for such cells have
been
increased; hence the need for reduced power losses in the smelting process has
increased. In an effort to reduce the electrical resistivity, graphite has
been
substituted for some of the anthracite in the cathode blocks, however with
concomitant loss in abrasion resistance, increased erosion rates and higher
cost of
materials. Moreover, cathode blocks with high graphite content and cathode
blocks
that have undergone a graphitizing process are subject to uneven cathode
current
distribution along the length of the cathode block and high localized erosion
rates.
An electrical current passing through an object naturally follows the
path of least resistance. In the case of a Hall-Heroult cell, this is believed
to be
through the outer one-third of the cathode block CB. The lines D in Fig. 1
depict
the uneven distribution of current passing through the cathode block CB and
the
high concentration of current passing through the outer one-third of the
cathode
block CB. This high concentration of current in the cathode block CB results
in
increased localized erosion rates in that portion of the cathode block CB.
Accordingly, a need remains for a device for and a method of
improving the current distribution in cathode blocks of a Hall-Heroult
electrolytic
cell which permits high graphite content and graphitized cathode blocks to be
operated at high amperage with an improved pot life expectation.
This need is met by the cathode assembly of the present invention
which is designed for use in a Hall-Heroult electrolytic cell for the
production of
aluminum. The cell includes a shell defining a chamber, an anode received in
the
chamber and a current bus positioned outside the shell and connected to the
cathode
assembly.
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In one aspect of the invention, there is provided, in an electrolytic cell for
the
production of aluminum comprising a shell defining a chamber and an anode
received in the
chamber, the improvement comprising: a cathode block positioned in the chamber
below the
anode, the cathode block defining at least two first slots and at least one
second slot; at least
two primary collector bars, each primary collector bar being received in one
of the first slots
and having a primary interface for electrical connection to the cathode block;
and at least one
secondary collector bar being received in the second slot and having a
secondary interface for
electrical connection to the cathode block, wherein a combination of the at
least two primary
interfaces is larger than the secondary interface. Preferably, each of the
primary interfaces
are larger than the secondary interface. More preferably, the cross-sectional
area of the
primary collector bars is greater than the cross-sectional area of the
secondary collector bar
and each of the primary collector bars has a width greater than its height.
The primary
interface preferably includes a connected portion of an exterior surface of
the primary
collector bar which is electrically connected to the cathode block whereas an
unconnected
portion of the primary collector bar exterior surface is electrically
disconnected from the
cathode block. The secondary interface preferably includes an exterior surface
of the
secondary collector bar which extends substantially the full length of the
portion of the
secondary collector bar received in the second slot. In this manner, current
may pass from the
cathode block to the entire secondary collector bar but current can only pass
from the cathode
block to the primary collector bar at the interior of the cathode block.
This arrangement is preferably accomplished by including a layer of an
electrically
conductive material along the exterior surface of the primary collector bar
adjacent the
interior portion of the cathode block and along substantially the entire
exterior surface of the
secondary collector bar which is received in the second slot. The electrically
conductive
material may be cast iron, carbonaceous glue or rammed carbonaceous paste or
the like.
Preferably, the connected portion of the primary collector bar exterior
surface extends
between an interior of the cathode block and a position spaced from the
external end of the
cathode block. More preferably, the connected portion extends along about two-
thirds of the
length of the first slot.
In another aspect of the invention, there is provided a cathode assembly
configured
for use in an Hall-Heroult electrolytic cell for the production of aluminum,
the assembly
comprising: a cathode block defining at least two first slots and at least one
second slot; at
least two primary collector bars, each primary collector bar being received in
one of the first
slots and having a primary interface for electrical connection to the cathode
block; and at
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least one secondary collector bar being received in the second slot and having
a secondary
interface for electrical connection to the cathode block, wherein a
combination of the at least
two primary interfaces is larger than the secondary interface.
The cathode assembly of the present invention is particularly useful for
producing
aluminum in a cell having a chamber containing an electrolytic bath and an
anode suspended
in the bath, where the current distribution through the cathode assembly is
uniform.
In another aspect of the invention, there is provided a method of producing
aluminum
in a cell having a chamber containing an electrolytic bath and an anode
suspended in the bath,
the method comprising: providing a cathode assembly in the chamber below the
anode, the
cathode assembly having (1) a cathode block having an external end, the
cathode block
defining at least two first slots and at least one second slot; (2) at least
two primary collector
bars, each primary collector bar being received in one of the first slots and
having a primary
interface for electrical connection to the cathode block and (3) at least one
secondary
collector bar being received in the second slot and having a secondary
interface for electrical
connection to the cathode block, wherein a combination of the at least two
primary interfaces
is larger than the secondary interface; and passing an electric current from
the anode to the
cathode assembly, thereby controlling the amount of current passing to the
primary collector
bars relative to the amount of current passing to the secondary collector bar.
An even current distribution may be accomplished by passing current from the
anode
to only a connected portion of the exterior surface of the primary collector
bars adjacent the
interior portion of the cathode block and to substantially all of the exterior
surface of the
secondary collector bar. This method further includes steps of electrically
connecting
substantially all of the exterior surface of the secondary collector bar to
the cathode block and
electrically connecting the connected portion of the exterior surface of the
primary collector
bar to the cathode block. In this manner, current is prevented from passing
from the cathode
block to the exterior surfaces of the primary collector bars adjacent the
external ends of the
cathode block.
A complete understanding of the invention will be obtained from the following
description when taken in connection with the accompanying drawing figures
wherein like
reference characters identify like parts throughout.
Fig. 1 is a cross-sectional view of a portion of an aluminum electrolytic
reduction cell
of the prior art employing a conventional cathode block with collector bars;
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Fig. 2 is a plan view of the underside of the cathode block and
collector bars shown in Fig. 1;
Fig. 3 is a perspective view of the underside of a cathode assembly
made in accordance with the present invention having a cathode block and
multiple
collector bars;
Fig. 4 is a plan view of the cathode assembly shown in Fig. 3;
Fig. 5 is a perspective view of the cathode block shown in Fig. 3;
Fig. 6 is a cross-sectional view of the cathode assembly shown in
Fig. 4 taken along line 6-6;
Fig. 7 is a cross-sectional view of the cathode assembly shown in
Fig. 4 taken along line 7-7;
Fig. 8 is a perspective view of a second embodiment of a cathode
assembly made in accordance with the present invention;
Fig. 9 is a perspective view of a third embodiment of a cathode
assembly made in accordance with the present invention;
Fig. 10 is a perspective view of a fourth embodiment of a cathode
assembly made in accordance with the present invention;
Fig. 11 is a cross-sectional view of the cathode assembly shown in
Fig. 10 taken along line 11-11;
Fig. 12 is a cross-sectional view of the cathode assembly shown in
Fig. 11 taken along line 12-12; and
Fig. 13 is a cross-sectional view of the cathode assembly shown in
Fig. 11 taken along line 13-13.
For purposes of the description hereinafter, the terms "upper",
"lower", "right", "left", "vertical", "horizontal", "top", "bottom" and
derivatives
thereof relate to the invention as it is oriented in the drawing figures.
However, it
is to be understood that the invention may assume various alternative
variations and
step sequences, except where expressly specified to the contrary. It is also
to be
understood that the specific devices and processes illustrated in the attached
drawings, and described in the following specification, are simply exemplary
embodiments of the invention. Hence, specific dimensions and other physical
characteristics related to the embodiments disclosed herein are not to be
considered
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as limiting.
As shown in Fig. 3, the cathode assembly 2 of the present invention
is intended for use in a Hall-Heroult electrolytic cell for the production of
aluminum
and includes a cathode block 4 and a multi-piece collector bar assembly 6. For
ease
of description, Figs. 3-7 show the cathode assembly of the present invention
and its
components as viewed from the underside of the cathode assembly 2.
In the embodiment shown in Fig. 5, the cathode block 4 defines at
least two first slots 8 extending between the external ends of the cathode
block 4
and at least one second slot, preferably two second slots, 10 extending
between one
external end of the cathode block 4 and an interior portion of the cathode
block 4.
As shown in Figs. 3 and 4, a primary collector bar 12 is received within each
of the
first slots 8 and a secondary collector bar 14 is received in each second slot
10.
Each multi-piece collector bar assembly 6 includes two primary collector bars
12
and one secondary collector bar 14. The primary and secondary collector bars
12
and 14 extend out of the cathode block 4 and each has an exposed end which is
connected to a common member 16 which in turn is connected to a bus bar in a
conventional manner. In use, the exposed ends of the primary and secondary
collector bars 12 and 14 and the common member 16 are positioned outside the
chamber of an electrolytic cell and are connected via a bus bar (not shown) to
a
current supply in a conventional manner.
Each of the primary collector bars 12 have a primary interface for
electrical connection to the cathode block 4 and the secondary collector bar
14 has a
secondary interface for electrical connection to the cathode block 4. The
electrical
connection may be achieved by various mechanisms as described hereinafter. The
combination of the primary interfaces taken together is sized to be larger
than the
secondary interface. In this manner, more surface area of electrical contact
is
achieved between the total of the primary collector bars 12 and the cathode
block 4
than is achieved between the secondary collector bar 14 and the cathode block
4.
A preferred way of accomplishing this relative sizing of the combined
primary interfaces and the secondary interface is shown in Figs. 4, 6, and 7,
wherein
the primary collector bars 12 are larger than the secondary collector bars 14.
By
this it is meant that the cross-sectional area and/or length of the primary
collector
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bars 12 is larger than that of the secondary collector bar 14. More
preferably, the
primary collector bars 12 each have a cross-sectional area which is at least
as large
as the cross-sectional area of the secondary collector bars 14, and the
portion of the
primary collector bars 12 received in the slots 8 are longer than the portion
of the
secondary collector bars 14 received in the slots 10.
The primary and secondary collector bars 12 and 14 have
cross-sectional areas that are smaller than the cross-sectional areas of the
corresponding first and second slots 8 and 10. To secure the primary and
secondary
collector bars 12 and 14 within the corresponding first and second slots 8 and
10, a
layer 18 of an electrically conductive material is positioned between each of
the
primary collector bars 12 and the cathode block 4 and between the secondary
collector bar 14 and the cathode block 4. In this embodiment, the layers 18
constitute the primary interface for electrical connection between the primary
collector bars 12 and the cathode block 4 and the secondary interface between
the
secondary collector bars 14 and the cathode block 4. The electrically
conductive
layers 18 are preferably formed from cast iron, carbonaceous glue or rammed
carbonaceous paste and electrically connect the cathode block 4 to the primary
and
secondary collector bars 12 and 14. However, only a connected portion 20 of
the
primary collector bars 12 is secured to the cathode block 4 via the
electrically
conductive layer 18 and is electrically connected thereto.
As shown in Fig. 3, connected portion 20 preferably extends between
the end of the primary collector bar 12 adjacent the center of the cathode
block 4
and a position, noted at reference numeral 22, between the center of the
cathode
block 4 and the end of the cathode block 4. The position 22 is selected to
even out
the current distribution along the length of the cathode block 4. When the
connected portion 20 extends about one half of the length of the primary
collector
bar 12, the position 22 is at about two-thirds of the distance along the first
slot 8
from the center of the cathode block 4. Referring to Fig. 4, non-electrically
connected portions 24 of the primary collector bars 12 which are electrically
disconnected from the cathode block 4 are insulated therefrom via a layer 26
of an
insulating material between the non-electrically connected portion 24 and the
cathode block 4. The insulating material may be refractory mortar or a fibrous
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insulating blanket or other suitable non-electrically conductive material. It
is
preferred that the position 22 at the end of the electrically connected
portion 20 be
aligned with the end of the secondary collector bar 14 in the direction
transverse to
the longitudinal axis of the cathode block 4. The secondary collector bar 14
is
substantially fully secured with a layer of electrically conductive material
18;
although a portion of the secondary collector bar 14 immediately adjacent the
end of
the cathode block 4 may not be secured to the cathode block 4 via the
electrically
conductive material.
It is believed that if the electrical connection from the cathode block
4 to the multi-piece collector bar assembly 6 is minimized at the ends of the
cathode block 4, then the current will be forced to spread more evenly across
the
length of the cathode block 4 than occurs in conventional cathode assemblies.
The
present invention accomplishes this goal by selecting a combination of the
primary
interfaces between the primary collector bars 12 and the cathode block 4 which
is
larger than the secondary interface between the secondary collector bar 14 and
the
cathode block 4. More current will naturally flow to the larger combined
primary
interfaces than to the secondary interface. The amount of current passing
through
the external ends of the cathode block 4 is minimized by preventing current
from
passing through the external ends of the cathode block 4 to the primary
collector
bars 12 and yet allowing current to pass through the external ends of the
cathode
block 4 to the secondary collector bar 14. Accordingly, the cross-sectional
area and
length of each of the primary and secondary collector bars 12 and 14 received
in the
first and second slots 8 and 10 as well as the relative sizes of the primary
and
secondary interfaces are each selected to provide uniform current distribution
through the cathode assembly 2. The electrical current loading of the end of
the
cathode assembly 2 may be minimized despite the presence of the secondary
collector bars 14 by using a smaller cross-sectional area for the secondary
collector
bar 14 than the cross-sectional area for the primary collector bars 12. In
this
manner, not all the current is excluded from passing through the external ends
of
the cathode block 4. A controlled amount of current is permitted to pass
through
cathode block 4 via the secondary collector bar 14 at the external ends of the
cathode assembly 2 in order to even out the current distribution along the
length of
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the cathode block 4.
In one embodiment of the invention, as best shown in Figs. 6 and 7,
the primary collector bars 12 have greater width than height. In this
embodiment,
the primary collector bars 12 of the present invention are about 150 mm wide
and
about 120 mm high and the secondary collector bars 14 are about 80 mm wide and
about 120 mm high. Minimization of the height of the primary and secondary
collector bars 12 and 14 allows the usable portion of the cathode block 4
above the
collector bars 12 and 14 to be thicker (taller) with a corresponding extension
in the
wear life of the cathode block 4. These dimensions are exemplary only; other
dimensions for the cathode assembly 2 are encompassed by the present
invention.
For example, it has been found that relatively wide primary bars 12 (described
above) require that the cathode block 4 have relatively thin portions
surrounding the
slots 8 and 10, particularly at the outer longitudinal edge of the cathode
block 4.
These thin edges may be prone to cracking, hence other dimensions of the
primary
and secondary collector bars 12 and 14 may be more suitable in certain
environments.
The embodiment shown in Figs. 3 and 4 includes a cathode block 4
of the present invention defining two parallel spaced first slots 8 and a
second slot
10 defined at each end of the cathode block 4 positioned between the first
slots 8.
This arrangement allows for the use of an opposing pair of multi-piece
collector bar
assemblies 6. Each slot 8 receives a pair of primary collector bars 12. The
primary
collector bars 12 in a single first slot 8 are maintained spaced apart via
conventional
filler material 30 as described hereinabove. This is not meant to be limiting,
as the
cathode assembly of the present invention includes other arrangements.
For example, in a second embodiment of the invention shown in Fig.
8, a cathode assembly 102 includes primary collector bars 112 which extend the
full
length of the cathode block 4. Alternatively, a single secondary collector bar
(not
shown) may be used which likewise extends the full length of the cathode
block.
In a third embodiment shown in Fig. 9, a cathode assembly 202
includes a split cathode block formed from block 204a and block 204b with a
layer
230 of seam mix or ramming paste therebetween. The split cathode blocks 204a
and 204b each receive a pair of primary collector bars 12 and a secondary
collector
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bar 14. Filler material 30 is positioned between the ends of the primary
collector
bars 12 and the layer 230 and the primary collector bar 12 in each of the
blocks
204a and 204b. Additionally, in each of the above-described embodiments, the
electrically conductive material which connects the collector bars to the
cathode
block may also be positioned at spaced apart locations along the length of the
slots.
The above-described embodiments each utilize some sort of
electrically conductive material positioned between the cathode blocks and the
primary and secondary collector bars. In a fourth embodiment of the invention,
it is
also possible to avoid the use of an electrically conductive material in the
slots of
the cathode block by fitting the connected portion of the primary collector
bar and
substantially all of the secondary collector bar within the slots. This fourth
embodiment, cathode assembly 302, is shown in Figs. 10-13. The cathode
assembly
302 includes a cathode block 304 in which the slots are defined therein as
bores 308
and 310. Preferably, the bores 308 and 310 each have a circular cross section.
A
primary collector bar 312 is received within each bore 308 and a secondary
collector bar 314 is received in each bore 310. The primary and secondary
collector
bars 312 and 314 are preferably also circular in cross-section. Similar to the
first
three embodiments, the primary collector bars 312 each include a connected
portion
322 and an unconnected portion 324. The connected portion is electrically
connected to the cathode block 304 by sizing the connected portion 322 to fit
within
the bore 308. Upon heating of the cathode assembly 302 during use, the primary
collector bar 312 expands to ensure electrical contact with the cathode block
304
along the connected portion 322. The bore 308 includes an enlarged diameter
portion creating a gap 326. The gap 326 is sized so that no electrical contact
is
made between the unconnected portion 324 and the cathode block 304.
Substantially all of the secondary collector bar 314 is fitted within the bore
310 and
likewise expands upon use to ensure electrical connection to the cathode block
304.
The cathode assembly 304 may also be modified by using different geometries
for
the bores and the collector bars and by using a split cathode block or full-
length
primary collector bars as described above with regard to the other embodiments
of
the invention.
It will be readily appreciated by those skilled in the art that
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modifications may be made to the invention without departing from the concepts
disclosed in the foregoing description. Such modifications are to be
considered as
included within the following claims unless the claims, by their language,
expressly
state otherwise. Accordingly, the particular embodiments described in detail
herein
are illustrative only and are not limiting to the scope of the invention which
is to be
given the full breadth of the appended claims and any and all equivalents
thereof.
