DESC:FORM 2
TH PATENT ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE OF THE INVENTION:
A cathode assembly resulting in an enhanced aluminium reduction cell performance in a smelting process.

APPLICANT:
Aditya Birla Science and Technology Company Private Limited, Aditya Birla Centre, 2nd Floor, ‘C’ wing, S.K. Ahire Marg, Worli- 40030, Mumbai, Maharashtra, India.

PREAMBLE TO THE DESCRIPTION:
The following specification describes this invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[001] The present invention relates to a cathode assembly resulting in an enhanced aluminium reduction cell performance in a smelting process. Specifically, the invention relates to a cathode block that results in an increased magnetohydrodynamic (MHD) stability in an aluminium reduction cell along with reduced cathode voltage drop.
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, electric current passes through various components of the cell as shown in Figure 1. Electrical current passing through the cell takes least resistant path. The liquid metal offers much less electrical resistance as compared to bath, cathode, anode or any other component of the cell. Therefore, in order to take least resistance path, current changes its direction majorly in liquid metal as shown in Figure 2. The bending of path of the current happens due to design and material of cell components.
[003] Cathode assembly generally comprises of cathode block, single or multiple metallic collector bar joined with cast iron as shown in Figure 3. During the joining process, the pre-heated cathode blocks are generally kept in inverted position and then preheated collector bars are kept into the cathode slots. Subsequently, molten cast iron is poured into the gap formed in the cathode slot after placing the collector bar. Due to cast iron pouring at high temperature, collector bar gets further heated up resulting in differential thermal expansion along its height. This phenomenon takes place due to the higher rate of heat loss and cooling through the surface of the collector bar which is exposed to air, in comparison to the surface facing the carbon cathode.
[004] Therefore, the surface of the collector bar exposed to air remains at lower temperature as compared to the surface facing towards the cathode slot. Due to this temperature gradient, thermal expansion of the collector bar varies along its height, i.e. linear expansion of the surface exposed to air will be lesser as compared to the other surface and leads to bending of the collector bar as shown in Figure 4. Subsequently, cooling of the cathode and collector bar assembly results in attaining original shape of collector bar, however it leads to the formation of gap at the horizontal contact between cathode slot surface and cast iron as shown in Figure 5. The figure shows the gap present between cast iron and cathode block at horizontal location. Due to this air gap, there is very less contact of cast iron with the cathode block and hence very less current (~5-10 %) passes through the said horizontal joint affecting the path of current in metal as shown in Figure 6. Also, this gap leads to higher cathode voltage drop due to reduced area for current passage as most of the current (90-95%) passes through only vertical joints.
[005] 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 field present in the cell interacts with the horizontal currents in the molten metal region of the cell. This interaction leads to the generation of volumetric 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. The present design of the cathode block leads to bending of current in the metal which is responsible for generation of horizontal component of the current. Subsequently, this horizontal component of current interacts with the vertical component of magnetic field that leads to instability in the metal-bath interface.
[006] Moreover, the electromagnetic forces cause motion 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 uniform vertical current so that the metal-bath interface is stable and the inter-electrode gap can be minimized. Also, a stable metal-bath interface aids in reducing the inter-electrode gap thereby resulting in reduced energy consumption.
[007] Hence, a strong need exists for an alternate cathode block which solves some of the problems present in the prior art as mentioned above.
SUMMARY OF THE INVENTION
[008] According to an embodiment of the present invention, there is provided a cathode assembly for an aluminium electrolysis cell, comprising:
at least one cathode block having at least one slot along its length, on one side;
the slot in the cathode block having one or more grooves along the length of the slot, adjacent grooves defining a projection of the cathode block between said grooves; and
a collector bar placed inside the slot of the cathode block and joined with the cathode block by molten cast iron,
wherein the projections ensure contact between the collector bar and cathode block thereby establishing uniform current distribution,
provided that each groove has an air gap allowing for expansion of the projections during operation of the aluminium electrolysis cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[009] Figure 1 depicts a schematic representation of a conventional aluminium electrolysis cell and the path of electrical current passing through it;
[010] Figure 2 illustrates the bending of current in the metal portion of a conventional aluminium electrolysis cell, due to disturbances in the cell or the design of the cell;
[011] Figure 3 depicts a pictorial representation of a standard cathode assembly;
[012] Figure 4 shows the expansion or bending of collector bar downwards during the pouring of cast iron, in a standard inverted cathode assembly.
[013] Figure 5 illustrates the gap formation between the cathode block and the collector bar after their cooling, in a standard inverted cathode assembly.
[014] Figure 6 depicts the segregation and bending of electrical current in the metal portion and that passage of current through the horizontal joint is very less, in a conventional aluminium electrolysis cell.
[015] Figure 7 shows a side view and end view of an inverted cathode assembly wherein the cathode block has grooves which helps in establishing contact between cathode block and the collector bar despite of the presence of gaps in the cathode slot as well as in the grooves, according to an embodiment of the present invention.
[016] Figure 8 illustrates a uniformity in current distribution due to the presence of grooves in the cathode block, according to an embodiment of the present invention.
[017] Figure 9 depicts an end view of the inverted cathode assembly showing an increase in contact area at the joint due to the presence of grooves in the cathode block, according to an embodiment of the present invention.
[018] Figure 10 shows an isometric view of an inverted cathode assembly with grooves in the slot along the length of the cathode block, according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[019] The embodiments of the present invention eliminate or reduces the aforementioned problems of the prior art by allowing cast iron to come in contact with the cathode block in spite of the existing gaps at the horizontal location of the joint. This results in reduction of the cathode voltage drop and uniform current distribution over the said cathode block and subsequently in the metal portion of the aluminium electrolysis cell.
[020] A cathode assembly illustrating an embodiment of the present invention is shown in Figure 7, which shows a side and an end view of an inverted cathode assembly. The figure shows presence of groves in the cathode block. During the joining process, after pouring of cast iron, the collector bar bends as usual and after cooling when the collector bar regains its shape, gaps will be formed in the cathode slots as well as in the grooves. However, due to the profile of the grooves, vertical contact between cast iron and cathode block gets established inside the grooves in spite of the presence of gap.
[021] In accordance with all the embodiments of the present invention, the cathode assembly for an aluminium electrolysis cell, comprises at least one cathode block which has at least one slot along its length, on one side. The slot in the cathode block has one or more grooves along the length of the slot, adjacent grooves define a projection of the cathode block between said grooves. A collector bar placed inside the slot of the cathode block and joined with the cathode block by molten cast iron. The projections ensure contact between the collector bar and cathode block thereby establishing uniform current distribution, provided that each groove has an air gap allowing for expansion of the projections during operation of the aluminium electrolysis cell.
[022] In an embodiment, the current passes from the cathode block to the collector bar through the projections.
[023] A good contact at the top of the cathode slot significantly modifies the current path in the metal portion of the electrolytic cell, thus compensating the consequences of gap formed during the joining process. Presence of these grooves in the horizontal surface ensures contact at the vertical sides of the grooves. This alters the resistive path in cathode-collector bar assembly. Increased contact leads to a reduction in resistance in the portion of the cathode block having gaps thereby allowing more current to flow through it as compared to the conventional cathode assembly design, which results in comparatively a uniform current distribution over the cathode surface and reduction of horizontal currents in molten metal region. This eventually leads to a stable molten metal-bath interface and an improved cell performance, as more clearly shown in Figure 8.
[024] In an embodiment, the shape and/or profile of the grooves can be any regular or irregular shape which can increase the contact area between collector bar and the cathode block at the horizontal joint. As depicted in Figure 9, due to the vertical contacts inside the grooves, the electric current passes through that location, thus making current distribution uniform. Usage of alternatives of shape and/or profile of the grooves are considered to be within the scope of the present invention.
[025] In an embodiment, the grooves can be along the length of the cathode block or along a direction perpendicular to the cathode block length. Figure 10 shows an isometric view of an inverted cathode assembly showing the placement of grooves along the length of the cathode block. In another embodiment, the grooves can follow a straight path or an irregular path in the cathode block. Usage of alternatives of position of the grooves are considered to be within the scope of the present invention.
[026] In another embodiment, the number of grooves can vary from 1 to ‘n’, where n will depend on the overall cathode assembly design and can go up to maximum value of 20. In a yet another embodiment, the depth of the grooves can vary from 0.5 cm to 10 cm and width of the groove can vary from 0.2 cm to 5 cm. In an embodiment, the grooves have a variable depth and the depth and/or width of the grooves can be similar or different from each other. Usage of alternatives of height and/or width of the grooves are considered to be within the scope of the present invention.
[027] In an embodiment, the grooves can be present in the cathode assembly with single, multiple or split collector bars.
[028] The increasing electrical conductivity towards the middle zone of the cathode 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 consumption. The reduced gradient in vertical current distribution also helps in improving the pot life.
[029] Experimental Data:
Advantages and benefits of the embodiments of the present invention would become more apparent from the below experimental details to a person skilled in the art.
[030] Example 1
Table compares the current density for horizontal joints of new design and existing design, data set 1- 8 are the values along the width of the slot, it is clear that current density for new design has significantly increased.
Current density from horizontal joint (A/m2)
S. no. Present invention Prior art
1 5203 4126
2 5681 4109
3 6283 3850
4 6757 3828
5 6700 3822
6 6189 3947
7 5523 4205
8 4982 4237
Average 5915 4015

[031] Example 2
a) Grooves of 1 cm width and 2 cm depth was cut throughout the length of the cathode slot.
b) Cast iron was poured in the slot after keeping the steel collector bar to complete the joining process, the joints were checked individually to ensure good quality joints.
c) The cathode assemblies were installed in the pot for experiment.
The table below shows the results of the experiment. The cathode voltage for pots with groove was found significantly less as compared to the pots without groove.
Pot Cathode Voltage (mV)
Pots without groove 1 522
2 348
3 354
4 329
5 397
6 623
7 315
Pot with groove 1 276

[032] Observations:
It was observed that current density increases with the presence of grooves, i.e., horizontal joint allows more current. This increase in current from horizontal joint leads to reduced current from vertical joint. Hence more uniform current distribution.
[033] It was also observed from initial measurements, that the voltage drop across cathode assembly with grooves was lesser when compared with the cathode assembly without grooves, shown in the table.
[034] 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.
[035] The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application thereby enabling others, skilled in the art, to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated.
,CLAIMS:We Claim:
1. A cathode assembly for an aluminium electrolysis cell, comprising:
at least one cathode block having at least one slot along its length, on one side;
the slot in the cathode block having one or more grooves along the length of the slot, adjacent grooves defining a projection of the cathode block between said grooves; and
a collector bar placed inside the slot of the cathode block and joined with the cathode block by molten cast iron,
wherein the projections ensure contact between the collector bar and cathode block thereby establishing uniform current distribution,
provided that each groove has an air gap allowing for expansion of the projections during operation of the aluminium electrolysis cell.
2. A cathode assembly as claimed in claim 1, wherein the grooves in cathode block are of regular or irregular shape.
3. A cathode assembly as claimed in claim 1, wherein the grooves have a variable depth.
4. A cathode assembly as claimed in claim 1, wherein current passes from the cathode block to the collector bar through the projections.