Slotted cathode collector bar for electrolyte reduction cell

An electrolytic reduction cell for the production of aluminum having a slotted cathode collector bar. The slots are filled with insulating material thereby directing the electrical current flow through the cathode collector bar in a manner which reduces the horizontal current components in the cell.

This invention relates generally to improvements in electrolytic cells, and 
more specifically, to improvements in the cathode collector bars of 
electrolytic reduction cells used in the production of aluminum. 
BACKGROUND OF THE INVENTION 
Aluminum is produced by the electrolytic reduction of alumina from an 
electrolyte. The electrolyte comprises primarily molten cryolite 
containing alumina and possibly other materials such as fluorspar, 
dissolved therein. The classic prebaked anode and Soderberg anode aluminum 
reduction cells comprise an anode structure suspended in a cryolite bath. 
Beneath the cryolite bath is a molten aluminum metal pad which forms the 
cathode and collects on the carbon blocks in the bottom of the cell. 
The cathode blocks conduct the electric current from the molten metal pad 
to the external electric bus system of the cell. In a typical cathode 
design, multiple steel collector bars extend from the external bus bars 
through each side of the cathode shell into the carbon cathode blocks. The 
cathode collector bars are usually tightly attached to the cathode blocks 
with cast iron or carbon cement to enhance the electrical contact between 
the carbon cathode blocks and the cathode collector bars. 
Modern aluminum manufacturing plants often have hundreds of electrolytic 
cells with over one-hundred cells connected in series. With such a large 
number of cells, cell maintenance is an ongoing operation involving 
numerous personnel and heavy equipment, such as cranes, to move the heavy 
carbon cathode blocks and cathode collector bars. Aluminum reduction cells 
of this type are operated at low voltages (e.g. 4-4.5 volts) and high 
currents (e.g. 70,000-275,000 amps). The current enters the reduction cell 
through the anode structure and then passes through the cryolite bath, 
down through the molten aluminum pad where it then enters the carbon 
cathode blocks and is carried out of the cell by the cathode collector 
bars. 
As the electrolyte bath is traversed by electric current, alumina is 
reduced electrolytically to aluminum at the cathode, and carbon is 
oxidized to carbon dioxide at the anode. The aluminum thus produced, 
accumulates in a molten aluminum pad and, in a conventional cell, is 
tapped off periodically. 
The alumina-cryolite bath is typically maintained on top of the metal pad 
at a set depth. There is a voltage loss as the current passes through the 
cryolite bath. This voltage loss is directly proportional to the length of 
the current path and is typically about 1 volt per inch of gap between the 
anode and molten aluminum pad, i.e. the interpolar distance. Therefore, 
any restriction on the reduction of the anode to cathode spacing restricts 
the achievement of maximum power efficiency and limits the ability to 
improve the electrolytic cell operation. The molten aluminum pad acts as a 
liquid metal cathode in common commercial cells. 
Commercial aluminum reduction cells are generally operated by maintaining a 
minimum depth of liquid aluminum on the floor of the cell which is "seen" 
by the bath as the true cathode. This minimum aluminum depth is usually at 
least 2 inches and may be 20 inches. 
The high currents passing through the electrolytic cell produce powerful 
magnetic fields that induce excessive circulation in the molten aluminum 
pad leading to problems such as reduced electrical efficiency and "back 
reaction" of the molten aluminum with the electrolyte. 
The flow of electrical current through the aluminum pad and carbon cathode 
naturally follows the path of least resistance. The electrical resistance 
in a conventional cathode collector bar is proportional to the length of 
the current path from the point at which electric current enters the 
cathode collector bar to the nearest external bus. As generally depicted 
in FIG. 1, the lower resistance of the current path starting at points on 
the cathode collector bar closer to the external bus causes the flow of 
current through the molten aluminum pad and carbon cathode blocks to be 
skewed in that direction. The horizontal components of the flow of 
electric current interact with the vertical component of the magnetic 
field, adversely affecting efficient cell operation. These interactions 
increase the motion of the metal pad, sometimes violently stirring the 
molten pad and generating vortices, and causing localized electrical 
shorting to the anode. The magnetic fields also lead to unequal depths in 
the molten aluminum pad and the cryolite bath. 
Metal pad turbulence can also increase the "back reaction", or reoxidation, 
of cathodic products, thereby lowering cell efficiency. Furthermore, metal 
pad turbulence tends to accelerate distortion and degradation of the 
cathode bottom liner through attrition and penetration of the cryolite. 
The depth variations also restrict the reduction of the anode to cathode 
gap and produce a loss in current efficiency since power is lost to the 
electrolyte interposed between the anode and cathode blocks. Movement of 
the molten aluminum metal pad also tends to cause uneven wear on the 
carbon cathode blocks and may result in early cell failure. 
It is possible to reduce molten aluminum metal pad stirring by modifying 
the bus system on an existing cell line to reduce the overall magnetic 
effects. However, it is normally very expensive to modify the bus system. 
In recognition of the adverse effects that horizontal current components 
have on cell efficiency, cell designs have been proposed which attempt to 
reduce the horizontal component of current by changing the basic design of 
the cathode collector bars. The proposals found in the literature, 
however, often fail to account for the practical necessity of 
preassembling cathode blocks onto the iron collector bars so that the 
carbon cathode blocks can be reassembled in the bottom of the cell. They 
also fail to provide designs which are amenable to safe handling by 
maintenance crews using heavy equipment such as cranes. 
One example of a design of an aluminum reduction cell which attempts to 
increase cell efficiency by reducing horizontal current components is 
found in U.S. Pat. Nos. 4,194,959 to Hudson, et al. and 4,592,820 to 
McGreer. The Hudson et al. patent teaches the use of one or more collector 
bars. Each collector bar is provided with one or more connector bars. The 
connector bars carry the current from the collector bars to the external 
bus system. The connector bars are of a lighter gauge material than the 
collector bars and are connected to the collector bars at points distant 
from the ends of the collector bars. The resistances of the disclosed 
connector bars are chosen so that preselected currents are drawn from each 
corresponding collector bar section. This design fails to account for the 
practical necessity of preassembling cathode blocks onto the iron 
collector bars so that the cathode shells can be relined. In addition, 
this design calls for major changes in the design of conventional cathode 
bars mandating a new cathode shell and current bus requiring major capital 
investments. Such a design would also be inherently weak due to the 
lighter gauge material used in the connector bars and probably could not 
safely be handled by workers and cranes during maintenance operations. 
U.S. Pat. No. 2,528,905 of Ollivier, et al. teaches disposing "current lead 
bars" perpendicular to the bottom of the electrolytic cell. This design, 
which necessitates providing passages through other portions of cell 
lining, i.e. the concrete vault and the layer of insulating bricks, would 
require extensive capital expenditures since it entails a significant 
departure from the conventional rectangular-block collector bar design. 
U.S. Pat. No. 3,787,311 to Wittner et al. attempts to equalize the current 
flow through the cell bottom by providing a plurality of carbon blocks 
with different resistivities. The carbon blocks are arranged such that 
blocks with higher resistivities are closer to the sides of the cell where 
the current flow would otherwise be greater. It is generally known to 
those skilled in the art that high resistivity blocks will convert to 
lower resistivity blocks during the high operating temperatures. 
Accordingly, the use of different resistivity blocks does not provide the 
desired result. 
U.S. Pat. No. 2,868,710 to Pontremoli discloses a design modification for 
the cathodic bottom of electrolysis furnaces which results in a portion of 
the (cathode) block being situated under the cathode conductor (collector 
bar). This design would provide a cell that is very difficult to construct 
and maintain in comparison to typical cells wherein the carbon cathode 
blocks are provided with grooves that allow the relatively simple lowering 
of the carbon cathode block onto the cathode collector bar for subsequent 
sealing. 
Prior attempts to solve the recognized current distribution problem in 
aluminum electrolytic reduction cells fail to proiide a practical design 
which can be implemented without major capital expenditures and which is 
safe to handle by maintenance operators using heavy equipment. 
The present invention provides a more practical approach by using 
specifically-designed cathode collector bars to minimize the horizontal 
electrical currents in the metal pad. The improved cathode collector bar 
of the present invention is advantageously employed in existing cell 
designs using standard carbon cathode blocks or carbon cathode blocks 
having new improved molten alumina wettable surfaces such as described in 
U.S. Pat. No. 4,526,911 to Boxall et al. The improved collector bar design 
of the present invention can also be advantageously employed in new low 
energy, drained sloped cathode cells such as described in U.S. Pat. No. 
4,602,990 to Boxall et al. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to improve the design of 
elecrolytic reduction cells by improving the cathode collector bar design 
to obtain a more uniform current distribution across the cathode bottom. 
It is a further object of the present invention to provide uniform current 
distribution through an aluminum electrolytic reduction cell by making 
only minor modifications to cathode collector bars entering the side of 
the cell and embedded in the carbon cathode blocks. 
It is another object of the present invention to produce preassembled 
cathode structures resulting in more uniform current distribution in an 
aluminum reduction cell that can also safely be handled by workers using 
heavy equipment, such as cranes, during relining operations. 
It is yet a further object of the present invention to provide such an 
improved cathode collector bar design which can be used with existing 
conventional cathode shells and external current buses. 
These and other objects of the present invention will become apparent from 
the following description and claims in conjunction with the drawings. 
SUMMARY OF THE INVENTION 
The present invention may be generally summarized as an electrolytic 
reduction cell for the production of aluminum comprising two external 
walls. External bus bars are positioned adjacent to the two external cell 
walls and at least one anode is supported in the cell between the cell 
walls. A carbonaceous cathode block is disposed below the anode and 
extends between the cell walls. A cathode collector bar having a top side, 
a bottom side, and a longitudinal axis is disposed in electrically 
conductive contact with the cathode block and extends from the first cell 
wall to at least near to the cell center. The cathode collector bar is 
electrically connected to the first external bus bar. The cathode 
collector bar has a slot disposed therein; the slot having a first portion 
extending from near the first cell wall toward the cell center 
approximately parallel to the cathode collector bar longitudinal axis and 
terminates at a first interior end between the first cell wall and the 
cell center. The slot has a second portion extending downwardly from the 
top of the cathode collector bar at a location between the first cell wall 
and the cell center and intersects the first slot portion. Insulating 
material is disposed in the first and the second slot portions.

DETAILED DESCRIPTION 
FIG. 1 illustrates the electrical current flow through an aluminum 
reduction cell having a conventional cathode collector bar (10). The 
electrical current enters the cell through the anode (17), passes through 
the electrolytic bath (18) and the molten aluminum pad (19). The 
electrical current then enters the carbon cathode blocks (20) and is 
carried out of the cell by the cathode collector bar (10). As illustrated 
(by lines 90) the electrical current is skewed toward the end of the 
cathode collector bar (10) closest to the external bus (not shown). 
A cathode collector bar typically has a rectangular cross section and is 
fabricated from steel. With reference to FIG. 2, the present invention 
comprises an improved electrolytic reduction cell by providing a cathode 
collector bar (11) having slots (12) which direct the flow of current 
through the electrolytic reduction cell in such a way as to minimize the 
horizontal components of the current flow. Slots (12) are formed in the 
cathode collector bar (11) by any suitable method such as flame cutting. 
The slots (12) extend vertically downward from the top of the cathode 
collector bar (11), which is in contact with the carbon cathode blocks, 
into the cathode collector bar (11) and horizontally, advantageously 
parallel to the longitudinal axis of the cathode collector bar, in a 
direction away from the center of the cathode collector bar, i.e. toward 
the nearest end of the cathode collector bar (11) connected to an external 
bus 41. The slots can range in size, but preferably have a width equal to 
the width of the collector bar and preferably have a vertical height of at 
least 1 inch. 
The horizontal portion of the slots (12) preferably extends beyond the end 
(29) of the corresponding cathode block closest to the external bus system 
(60), and most preferably, beyond the outer shell (42) of the cell and at 
least partially underneath any other electrically conductive carbonaceous 
cathode material. The vertical portion of the slots (12), which typically 
defines the position of the end of the horizontal slot portion closest to 
the center of the cell, is suitably located from about 1/4 to about 3/4 of 
the distance and is preferably located from 1/2 to 3/4 of the distance 
from the end (29) of the corresponding cathode block closest to the 
external bus system, to the center of the cell. 
The vertical and horizontal portions of the slots are filled with a 
refractory cement which, as will be appreciated by those skilled in the 
art, has a higher electrical resistance than the steel cathode collector 
bar (11) and therefore acts as an insulator. An example of a suitable 
refractory cement is PG-23 ES "Extra Strength Castable" made by the 
Pryor-Giggey Co., P.0. Box 739, Whittier, Calif. 90608. The refractory 
cement filled slots (12) prevent the flow of current from the larger 
section (15) of the cathode collector bar which is below the slot (12), to 
the smaller section (16) of the cathode collector bar (11) which is above 
the refractory filled slot (12). To maintain proper electrical separation 
between the smaller sections (16) and the larger sections (15), the sides 
of the larger sections (15) may be reduced by machining, e.g., 
approximately 1 inch from each side, and filling the resulting void with 
insulating material, such as refractory cement (44), as shown in FIG. 4. 
Alternatively, it may be desired to maintain the original width of the 
larger sections (15) and remove portions of the cathode blocks adjacent 
the larger sections (15), and fill the resulting voids with an insulating 
material such as refractory cement. 
The outer cathode blocks (25, 26) make electrical contact with the cathode 
collector bar at only the smaller sections (16) of the cathode collector 
bar above the horizontal slot via, e.g., a cast iron connection (50). See 
FIG. 4. The inner cathode blocks (21, 22) make contact with the entire 
cathode collector bar (11) as illustrated in FIG. 3 by connection thereto 
e.g., by using cast iron (50). The current flowing from the inner blocks 
is therefore carried by a relatively large cross-sectional area and, 
despite the longer current path from the inner cathode blocks to the 
external bus system (41), has an electrical resistance preferably 
approximately equal to the shorter current path of the smaller sections 
(16) of the cathode collector bar. The increased cross-sectional area of 
the longer current path is achieved at the expense of the shorter path. 
The relative cross-sectional areas provided by the slotted collector bar 
are designed to balance the current distribution between the cathode 
blocks. FIGS. 3 and 4 illustrate the cross-sectional areas of the carbon 
cathode blocks and the cathode collector bar along lines B-B and A-A, of 
FIG. 1, respectively. 
In the embodiment, as shown in FIG. 2, four cathode blocks are used in 
conjunction with a cathode collector bar having two slots. The two inner 
blocks (21 and 22) electrically contact the unslotted portions of the 
cathode collector bar and the two outer cathode blocks (25 and 26) 
electrically contact the corresponding smaller portions (16) of the 
cathode collector bar provided by the slots (12). 
The cathode blocks are separated by ram joints (40) which are typically 
made from calcined anthracite and coal tar pitch. The electrical 
resistivity of the baked ram joint is typically about five times that of 
the semi-graphitic cathode block material so that outward horizontal 
electrical currents in the cathode block ram joint structure are 
discouraged in favor of electrical current flowing downward into the 
collector bar which typically has approximately only one-third the 
electrical resistivity of the semi-graphitic cathode block. 
In addition to the two slots previously described, a vertical slot (35) may 
advantageously be cut in the center of the cathode collector bar. The 
vertical slot (35), which may extend more than halfway through the cathode 
collector bar (11), is filled with an insulating material. This vertical 
slot (35) therefore creates an electrical resistance, causing most of the 
current flowing into the cathode collector bars to flow toward the 
external bus (41) located at the nearest side of the cell. This gives a 
better overall current distribution both within the cell and in the 
adjoining current bus of the plant. It also contributes to lower overall 
magnetic fields, less pad stirring, and better cell performance. 
In the preferred embodiments, the ratio of the cross-sectional area of the 
larger section (15) of the cathode collector bar to the smaller section 
(16) is advantageously between 1.5:1 and 3:1 and preferably between about 
1.8:1 and 2.8:1. 
Estimates have been made of the overall decrease in cathode voltage loss 
for anthracitic cathode blocks when the following cathode collector bar 
cross-sections are used: in a cell having 52 cathode blocks set on 13 
collector bars operated at a current of 105,000 amps, with the smaller 
section (16) of the cathode bar having a cross-section of 5.2 inches by 
5.2 inches and the larger portion (15) having a 13.8 inch by 5.2 inch 
cross-section, a savings of 0.190 volts could be achieved; and with 2.25 
inch by 5.2 inch and 4.25 inch by 5.2 inch respective cross-sections, a 
savings of 0.036 volts could be achieved. 
As stated above, when the width of the larger section (15) is equal to the 
width of the smaller section (16), it is necessary to remove a portion of 
the cathode block adjacent to the larger section (15) and fill the void 
with an insulating material. As will be appreciated by those skilled in 
the art, this is necessary to prevent the flow of current from the outer 
blocks (25, 26) to the larger section (15). The amount of cathode block 
removed from each side and filled with insulating material is 
advantageously about 1 inch. 
Within the bottom of the carbon cathode, the cathode blocks are joined to 
the cathode collector bar by a highly conductive material (50) such as 
cast iron, carbon cement, or the like (as shown in FIGS. 3 and 4). 
The present invention is not restricted to cathode blocks constructed of 
segmental blocks as illustrated in FIG. 2. Each collector bar may have 
from one to five or more carbon cathode blocks attached to it. It is also 
possible to replace the single cathode collector with two cathode 
collector bars which meet near the center of the cell and suitably are 
separated by an insulating material. The beneficial effects of the slotted 
cathode collector bar will be achieved in each case by constructing the 
bar so that the larger section electrically contacts that part of the 
cathode block or blocks toward the center of the cell which the smaller 
section contacts the portion of the block or blocks toward the outside of 
the cell. 
The present invention has the potential for near term energy savings of 3 
to 4% (0.2 to 0.3 kWh/lb of aluminum) in conventional cells as well as in 
future applications in newer "drained-cathode" cell designs having sloped 
cathodes. 
The present invention leads to reduced energy consumption without 
sacrificing the strong beam unit of cathode blocks that may be safely 
handled by cell maintenance crews. The cathode designs of the present 
invention reduce energy consumption by creating a more uniform current 
distribution between the molten aluminum pad and the cathode blocks. The 
present invention overcomes problems associated with conventional cell 
designs wherein the electrical current is skewed toward the outside of the 
outer row of blocks, causing large horizontal electrical currents in the 
aluminum pad, potentially violent stirring of the pad, the generation of 
vortices and localized shorting to the anode. 
It will be understood to those skilled in the art that variations of the 
present invention are possible without departing from the spirit and scope 
of the present invention. While several embodiments have been described 
herein, the scope of the present invention is not limited thereby, but is 
defined by the following claims.