DESC:FIELD OF THE INVENTION 5

[001] The present invention relates to a cathode carbon block resulting in an enhanced
aluminium reduction cell performance in a smelting process. Specifically, the invention relates
to a composite cathode carbon block used in the smelting process and more particularly to a
cathode carbon block that results in an enhanced aluminium reduction cell performance in the
smelting process. The present invention also relates to a process of preparing the same. 10

BACKGROUND OF THE INVENTION
[002] Aluminium is produced conventionally by the Hall-Héroult process, by the electrolysis
of alumina dissolved in a cryolite-based molten electrolyte. During the production of
aluminium, large number of individual electrolytic cells are employed. Each cell has a
carbonaceous lining forming the cathode, with cathode collector bars buried therein. Suspended 15

above each cell on iron rods are a plurality of carbon anodes. Worldwide, the aluminium
industry is seeking for ways to increase the productivity and efficiency of the aluminium
reduction cell while reducing the specific energy consumption.
[003] The cathode blocks are usually made of an anthracite based pre-baked carbon material
containing graphite that is homogenously distributed across the cathode block. The problems 20

associated with the such cathode blocks are stated below.
[004] First, during the smelting process, strong magnetic fields are produced as a result of high
current (in the range of 70 – 600kA depending upon the smelter technology) flowing in the
busbar system. The magnetic fields present in the cell, interact with the horizontal currents in
the molten metal region of the cell. This interaction leads to the generation of volumetric 25

Lorentz force, which is responsible for flow in the molten fluids as well as it deforms the metal
bath interface thereby affecting the cell performance.
3


[005] Vertical component of magnetic field and horizontal component of current are the two 5

critical factors that are responsible for instability in the metal-bath interface. Modification in
the busbar configuration attempts to reduce the vertical component of magnetic field, however,
it is cost and time intensive.
[006] Second, reduction in horizontal currents reduces the gradient of vertical current density
over the cathode surface resulting in a uniform current distribution over the cathode carbon 10

block. Researchers have been trying to minimize the horizontal currents by modifying the
cathode shape/design and by using copper-insert collector bars or copper collector bars.
Cathode design change affects the operational activities and a non-uniform cathode surface can
result in uneven stress generation that might lead to pot failure. Nevertheless, minimization of
horizontal currents in the molten metal remains an area of interest for researchers. 15

[007] Third, electromagnetic forces cause motions in the molten aluminium and the electrolyte,
causing the metal-electrolyte interface to be unstable. Therefore, in order to maintain stable
cell operation, the anode-to-cathode distance is increased which results in high energy
consumption. Aim is to have a vertical current so the interface is stable and the inter-electrode
gap can be minimized. Also, a stable metal-bath interface aids in reducing the inter-electrode 20

gap thereby resulting in reduced energy consumption.
[008] Fourth, cathode erosion depends on current density flowing through the cathode, and
since current density is maximum along the path of least resistance, i.e. at ends of the cathode,
the resultant non-uniform cathodic erosion reduces pot life.
[009] A typical illustration of such kind of carbon blocks is showed in Figure 1 which depicts 25

the conventional aluminium smelter showing the electrical resistive path. Conventionally, the
material properties of the cathode carbon block are constant over the entire length of the block.
Current flow in the molten aluminium is affected by the electrical resistance path of cathode
4


collector bar assembly, as depicted in the figure. Since the current follows the least resistive 5

path i.e. Path 2 (R1’ – R3) as compared to Path 1 (R1 – R2 – R3), the current density in the molten
metal is high towards the exit of the current i.e. at the collector bar which leads to the generation
of horizontal currents. This horizontal component of current interacts with the vertical
component of the magnetic field resulting in instability at the metal-electrolyte interface.
Electromagnetic forces cause motions in the molten aluminium and the electrolyte causing the 10

metal-electrolyte interface to be unstable and this in turn increases the resistance at the metal
electrolyte interface resulting in high electricity consumption.
[010] In some cases, researchers increase the graphitic content of the cathode block to lower
CVD (cathode voltage drop) but this concomitantly increases the gradient of vertical current
density which is undesirable. Thus, the usage of higher graphitic content cathode block also 15

leads to an increased wear of the cathode block.
[011] Hence, a strong need exists for an alternate cathode carbon block and process of its
preparation which solves some of the problems present in the prior art as mentioned above.
SUMMARY OF THE INVENTION
[012] According to an embodiment of the present invention, there is provided a cathode carbon 20

block resulting in an enhanced aluminium reduction cell performance in a smelting process and
formed of a mixture of a carbonaceous material including graphite, a metal and a binding agent,
the cathode carbon block comprising a first and second lateral zones having in between a
middle zone, each of the zones containing an equal amount of the mixture to define the cathode
carbon block, wherein the graphite and the metal content of the mixture in the middle zone is 25

substantially greater than the graphite and metal content of the mixture in the first and second
lateral zones, and wherein the graphite and metal content of the mixture gradually decreases
from the centre of the middle zone towards the ends of the first and second lateral zones.
5


[013] According to another embodiment of the present invention, there is provided a process 5

of producing a cathode carbon block resulting in an enhanced aluminium reduction cell
performance in a smelting process, comprising the steps of mixing a carbonaceous material
including graphite, a metal and a binding agent to obtain a first, second and third type of
homogenous mixture, the first and second type of mixture having a lower graphite and metal
content than that of the third mixture; simultaneously pouring an equal amount of the three 10

mixtures in a cathode carbon block mold that is defined by a first and second lateral zones
having in between a middle zone, in a manner that the first, second and third type of mixtures
are received in the first lateral zone, the second lateral zone and the middle zone respectively
thereby resulting in the concentration of graphite and metal decreasing gradually from the
centre of the middle zone towards the ends of the first and second lateral zones; compacting 15

the mixtures in the cathode carbon block mold with the help of an extrusion press or a vibro
compactor to make a green body; taking the green body out of the mold; and heat treating the
green body to a temperature in the range of 1000?C to 1500?C.
BRIEF DESCRIPTION OF THE DRAWINGS:
[014] Figure 1 depicts a cross sectional view of a conventional aluminium smelter showing an 20

electrical resistive path between an anode and cathode in an electrolytic solution;
[015] Figure 2 illustrates a cross sectional view of an aluminium smelter showing half portion
of the cathode carbon block and the flow of current therein, according to an embodiment of the
present invention;
[016] Figure 3 depicts a flow chart showing the process steps for preparing the cathode carbon 25

block, according to an embodiment of the present invention; and
6


[017] Figure 4 depicts a cross sectional view of an aluminium smelter as shown in Figure 2 5

showing half portion of the cathode carbon block, according to another embodiment of the
present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[018] As set out in the claims, the present invention eliminates or reduces the aforementioned
problems of the prior art by varying the material properties of the cathode carbon block along 10

its length thereby reducing the horizontal currents and achieving uniform current distribution
over the said cathode carbon block or in other words to have vertical current in the cathode
carbon block so that the interface will be stable and the inter-electrode gap can be minimized.
[019] An electrolytic cell illustrating one embodiment of the present invention is shown in
Figure 2, which shows only one half of the aluminium reduction cell, the dotted line 15

representing symmetry such that the mirror image of the other half of the reduction cell lies to
the left of the dotted line. The electric current enters through anode (110), molten electrolyte
(111), molten aluminium (112), cathode carbon block (113) and exits from collector bar (114)
which is above the bottom insulation (115). The collector bar (114) is connected to the anode
of the next cell in the series. The side insulation is (116) and the side ledge comprising frozen 20

electrolyte is (117). The electrolyte is usually a universal solvent like cryolite fluoride based
solvent.
[020] According to the present embodiment, the current distribution is improved by varying
the material properties of the cathode carbon block, along its length. The present cathode
carbon block is formed of a mixture of a carbonaceous material including graphite, a metal and 25

a binding agent. Furthermore, the cathode carbon block comprises a first and second lateral
zones having in between a middle zone, each of the zones containing an equal amount of the
7


mixture to define the cathode carbon block. It is understood by a person skilled in the art from 5

Figure 2 that the figure shows only the second lateral zone and half of the middle zone because
as noted above, Figure 2 shows only half of the aluminium reduction cell. The other half which
is not shown contains the first lateral zone and other half of the middle zone.
[021] The middle zone of cathode carbon block is made by adding intrinsically, high electrical
conductive material like graphite and/or metallic compounds in such a way that the electrical 10

conductivity decreases from the middle of the block towards the lateral zones (i.e. R’1 > R1)
as shown, by the direction of the dark arrow within the cathode carbon block, in Figure 2. This
alters the resistive path in cathode-collector bar assembly. Reduction of R1 allows more current
to flow via Path 1 as compared to the conventional cathode design, which results in
comparatively uniform current distribution over the cathode surface and reduction in horizontal 15

currents in molten metal region, as shown in Figure 2. This eventually leads to a stable metal
bath interface and improved cell performance.
[022] According to an embodiment of the invention, there is provided a process for preparing
a cathode carbon block giving an enhanced aluminium reduction cell performance in a smelting
process. The process comprises the following sequential steps: (1) raw material processing, (2) 20

mixing, (3) shaping, (4) compacting, (5) baking and (6) inspection, as more clearly shown in
Figure 3.
[023] (Step 1) The carbonaceous material including graphite is screened to a defined grain size
distribution to control the properties of the finished product. (Step 2) Subsequently, the
carbonaceous material is mixed with a metal and a binding agent at elevated temperatures. This 25

also includes the homogenization of the ‘mixed raw materials’ for different graphitic contents.
A first, second and third type of homogenous mixture is obtained, the first and second type of
mixture having a lower graphite and metal content than that of the third mixture.
8


[024] (Step 3) An equal amount of the three mixtures are then simultaneously poured in a 5

cathode carbon block mold. The cathode carbon block mold is defined by a first and second
lateral zone having in between a middle zone. While molding/shaping, the graphitic content is
adjusted along the cathode length in such a way that cathode carbon block results in having a
varying electrical resistivity, along the length. The three mixtures are simultaneously poured
in a manner that the first, second and third type of mixtures are received in the first lateral zone, 10

the second lateral zone and the middle zone respectively thereby resulting in the concentration
of graphite and metal decreasing gradually from the centre of the middle zone towards the ends
of the first and second lateral zones.
[025] (Step 4) After the shaping process, the mixtures are compacted in the cathode carbon
block mold with the help of an extrusion press or a vibro compactor to form a ‘green body’. 15

[026] (Step 5) Thereafter, the green body is baked at a temperature of around 1200 °C (with
the range of 1000?C to 1500?C) in a furnace, depending on the raw materials used in preparing
the cathode carbon block. The baking process could be performed in continuous or batch
furnace, depending on the type of materials, dimensions and required material properties. (Step
6) Subsequently, the cathode carbon block is inspected for its quality and desired properties. 20

[027] In an embodiment, the graphite and the metal content of the mixture in the middle zone
is substantially greater than the graphite and metal content of the mixture in the first and second
lateral zones, and wherein the graphite and metal content of the mixture gradually decreases
from the centre of the middle zone towards the ends of the first and second lateral zones.
[028] The electrical conductivity of the middle zone of the cathode carbon block becomes 25

relatively higher due to composite material or high graphite and metal content in the middle
zone of the cathode carbon block than that of the lateral zones. This results in a variable
9


electrical conductivity profile along the length of cathode carbon block. This reduces the 5

generation of horizontal currents in the molten metal thereby helping in reducing the energy
consumption of the cell.
[029] In an embodiment, the concentration of graphite in the first and second lateral zones is
in the range of (0 – 50) % and in the range of (50 – 100) % in the middle zone, by weight of
the mixture, and wherein the concentration of graphite gradually decreases from the centre of 10

the middle zone towards its lateral ends and the concentration of graphite in the first and second
lateral zones, when present, gradually decreases from their ends that are adjacent to the middle
zone towards their respective opposite ends.
[030] In another embodiment, the concentration of metal in the first and second lateral zones
is in the range of (0 – 10) % and in the range of (10 – 50) % in the middle zone, by weight of 15

the mixture, and wherein the concentration of metal gradually decreases from the centre of the
middle zone towards its lateral ends and the concentration of metal in the first and second
lateral zones, when present, gradually decreases from their ends that are adjacent to the middle
zone towards their respective opposite ends.
[031] In an embodiment, the binding agent is coal tar pitch. In another embodiment, the 20

carbonaceous material is selected from the group consisting of petroleum coke, pitch coke,
anthracite, graphite or a mixture thereof. Usage of all these alternatives of carbonaceous
materials and others are considered to be within the scope of the present invention.
[032] In another embodiment, the metal is selected from the group consisting of titanium,
copper, manganese, tungsten, zirconium, chromium, molybdenum, nickel or a mixture thereof. 25

Usage of all these alternatives of metal and others are considered to be within the scope of the
present invention.
10


[033] In an embodiment, variation of electrical conductivity in the cathode block could also be 5

achieved by utilizing the split cathode block pieces of different electrically conductive material,
the material being composite material comprising metals like titanium, copper, manganese,
tungsten, zirconium, chromium, molybdenum, nickel or a mixture thereof. Figure 4 shows one
configuration to use the split pieces of different electrically conductive material to achieve the
desired variable conductivity along the cathode carbon block. While it is shown in the figure 10

that the cathode carbon block utilizes three split pieces (201, 203 and 204) however it is not
limited to three pieces and the cathode carbon block can be made by utilizing 2, 4, 5, 6 pieces
and so on using carbonaceous material for the joints (202).
[034] Preferably, the cathode block has a varying electrical conductivity in a gradient along its
length, the conductivity being highest at the middle zone of the block and the conductivity 15

being lowest at the lateral zones of the block. Preferably, the cathode block is unjointed.
[035] The increasing electrical conductivity towards the middle zone of the cathode carbon
block helps in establishing more uniform current distribution in the molten metal region with
reduction in horizontal currents. Reduced horizontal currents provides stable metal-bath
interface that gives the potential of lowering inter-electrode gap, thus reducing the energy 20

consumption. The reduced gradient in vertical current distribution also helps in improving the
pot life.
EXAMPLES [036] Advantages and benefits of cathode carbon block according to the embodiments of the
present invention would become more apparent from the below experimental details to a person 25

skilled in the art.

11


[037] Experimental Data 1: 5

a) A cathode carbon block was taken by conventional means having constant electrical
conductivity along its length as well as a cathode carbon block with anisotropic (varying
along the length) electrical conductivity was taken according to the embodiments of the
present invention.
b) The cathode carbon block taken conventionally contained 30% graphite concentration 10

distributed homogenously throughout the said block.
c) Case-1 (as shown in Table-1) shows the results for a cathode carbon block with
anisotropic electrical conductivity having 30% graphite content towards its lateral
zones and 100% graphite concentration at the middle zone of the said block.
d) Case-2 (as shown in Table-1) shows the results for a cathode carbon block with 15

anisotropic electrical conductivity and additionally containing electrically conductive
metal such as TiB2 in the middle zone.
e) It was seen that the cathode voltage drop (CVD) had improved in Case-1 as compared
to the conventional 30% graphite cathode block due to the improved electrical
conductance at the middle zone of the said block. 20

f) It was also seen that a reduction in vertical current density (VCD) results in lower
horizontal currents in the molten metal region, which subsequently leads to improved
MHD (Magneto Hydro Dynamic) stability and cell performance.
Table-1
Cases Cathode
Voltage Drop
(CVD) - mV
Vertical current density – A/m2
Cathode carbon
block (middle)
Cathode carbon
block (lateral ends)
Gradient (?)
12


30%
graphite
254 3641 13519 9878
Case-1 234 4754 12256 7502
Case-2 228 5865 12020 6155
5

[038] Experimental Data 2:
a) A cathode carbon block was taken by conventional means having constant electrical
conductivity along its length as well as a cathode carbon block with anisotropic (varying
along the length) electrical conductivity was taken according to the embodiments of the
present invention. 10

b) The cathode carbon block taken conventionally contained 100% graphite concentration
distributed homogenously throughout the said block.
c) Case-1 (as shown in Table-2) shows the results for a cathode carbon block with
anisotropic electrical conductivity having 30% graphite content towards its lateral ends
and 100% graphite concentration at the middle of the said block. 15

d) Case-2 (as shown in Table-2) shows the results for a cathode carbon block with
anisotropic electrical conductivity and additionally containing electrically conductive
metal such as TiB2 in the middle zone.
e) It was seen that the cathode voltage drop (CVD) had improved in Case-1 as compared
to the conventional 100% graphite block due to the improved electrical conductance at 20

the middle zone of the said block.
f) It was also seen that a reduction in vertical current density (VCD) results in lower
horizontal currents in the molten metal region, which subsequently leads to improved
MHD (Magneto Hydro Dynamic) stability and cell performance.
25

13


Table-2 5

Cases Cathode
Voltage Drop
(CVD) - mV
Vertical current density – A/m2
Cathode carbon
block (middle)
Cathode carbon
block (lateral ends)
Gradient (?)
100%
graphite
176 1986 18729 16743
Case-1 234 4754 12256 7502
Case-2 228 5865 12020 6155

[039] The foregoing description of specific embodiments of the present invention has been
presented for purposes of description. They are not intended to be exhaustive or to limit the
present invention to the precise forms disclosed, and obvious modifications and variations are
possible in light of the above teaching. 10
,CLAIMS:We Claim: 5

1. A cathode carbon block resulting in an enhanced aluminium reduction cell performance in
a smelting process and formed of a mixture of a carbonaceous material including graphite,
a metal and a binding agent, the cathode carbon block comprising:
a first and a second lateral zone having in between a middle zone, each of the zones
containing an equal amount of the mixture to define the cathode carbon block, 10

wherein the graphite and the metal content of the mixture in the middle zone is substantially
greater than the graphite and metal content of the mixture in the first and second lateral
zones, and wherein the graphite and metal content of the mixture gradually decreases from
the centre of the middle zone towards the ends of the first and second lateral zones.
2. The cathode carbon block as claimed in claim 1, wherein the binding agent is coal tar pitch. 15

3. The cathode carbon block as claimed in claim 1, wherein the metal is selected from the
group consisting of titanium, copper, manganese, tungsten, zirconium, chromium,
molybdenum, nickel or a mixture thereof.
4. The cathode carbon block as claimed in claim 1, wherein the concentration of graphite in
the first and second lateral zones is in the range of (0 – 50) % and in the range of (50 – 100) 20

% in the middle zone, by weight of the mixture, and wherein the concentration of graphite
gradually decreases from the centre of the middle zone towards its lateral ends and the
concentration of graphite in the first and second lateral zones, when present, gradually
decreases from their ends that are adjacent to the middle zone towards their respective
opposite ends. 25

5. The cathode carbon block as claimed in claim 1, wherein the concentration of metal in the
first and second lateral zones is in the range of (0 – 10) % and in the range of (10 – 50) %
in the middle zone, by weight of the mixture, and wherein the concentration of metal
15


gradually decreases from the centre of the middle zone towards its lateral ends and the 5

concentration of metal in the first and second lateral zones, when present, gradually
decreases from their ends that are adjacent to the middle zone towards their respective
opposite ends.
6. A process of producing a cathode carbon block giving an enhanced aluminium reduction
cell performance in a smelting process, comprising the steps of: 10

- mixing a carbonaceous material including graphite, a metal and a binding agent to
obtain a first, second and third type of homogenous mixture, the first and second type
of mixture having a lower graphite and metal content than that of the third mixture;
- simultaneously pouring an equal amount of the three mixtures in a cathode carbon block
mold that is defined by a first and second lateral zone having in between a middle zone, 15

in a manner that the first, second and third type of mixtures are received in the first
lateral zone, the second lateral zone and the middle zone respectively thereby resulting
in the concentration of graphite and metal decreasing gradually from the centre of the
middle zone towards the ends of the first and second lateral zones;
- compacting the mixtures in the cathode carbon block mold with the help of an extrusion 20

press or a vibro compactor to make a green body;
- taking the green body out of the mold; and
- heat treating the green body to a temperature in the range of 1000?C to 1500?C.
7. The process of producing a cathode carbon block as claimed in claim 6, wherein the step
of pouring an equal amount of the three mixtures in a cathode carbon block mold results in 25

the concentration of graphite in the first and second lateral zones in the range of (0 – 50) %
and in the range of (50 – 100) % in the middle zone, by weight of the mixture, and wherein
the concentration of graphite gradually decreases from the centre of the middle zone
towards its lateral ends and the concentration of graphite in the first and second lateral
16


zones, when present, gradually decreases from their ends that are adjacent to the middle 5

zone towards their respective opposite ends.
8. The process of producing a cathode carbon block as claimed in claim 6, wherein the step
of pouring an equal amount of the three mixtures in a cathode carbon block mold results in
the concentration of metal in the first and second lateral zones is in the range of (0– 10) %
and in the range of (10 – 50) % in the middle zone, by weight of the mixture, and wherein 10

the concentration of metal gradually decreases from the centre of the middle zone towards
its lateral ends and the concentration of metal in the first and second lateral zones, when
present, gradually decreases from their ends that are adjacent to the middle zone towards
their respective opposite ends.