REDUCTION CELL IN A SMELTING PROCESS.

FIELD OF THE INVENTION

[001] The present invention relates to an apparatus for enhancing the performance of an aluminium reduction cell in a smelting process. More particularly, the invention relates to an apparatus with current collector bar having metal insert and cell refractory lining to enhance the performance of the aluminium reduction cell.

BACKGROUND OF THE INVENTION

[002] Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in a cryolite -based molten electrolyte. Specifically, the current enters the cell through the anode and then passes through the molten cryolite bath (electrolytic bath), molten aluminium and enters the carbon cathode before being collected by collector bars.

[003] The electrical current is carried out of the cell by collector bars. The flow of electrical current through the carbon cathode and collector bar follows the path of least resistance. The resistance of the current path between the collector bar and the nearest external bus is lower because of which the flow of current through the molten aluminium, cathode and collector bar gets concentrated towards the exit of the collector bar thereby generating a horizontal current component as illustrated in Fig. 1 (Resistance: Ri < R2). Such horizontal current component interacts with the vertical component of the magnetic field results in MagnetoHydroDynamics (MHD) instability and adversely affects efficient cell operation, thus limiting the reduction in inter-electrode distance. [004] Generally, steel collector bars extending from the external bus bars through each side of the electrolytic cell into the carbon cathode blocks are used in the industry. In order to increase the flow of current through these collector bars, an insert formed of a secondary material, having a higher electrical conductivity as compared to the primary material of the collector bar, is also used. Use of a collector bar along with the metal insert results in reduction of energy consumption through reduced voltage drop in the inter-electrode gap as well as in the cathode and collector bar assembly. Moreover, a reduction in generation of horizontal current is also noted in such a design. However, there is a need for further reduction of energy consumption and voltage drop and a novel design of collector bar with the metal insert, which reduces generation of horizontal current component thereby providing uniform current distribution in the cell. Figure 1 shows a cross sectional view of a conventional aluminium smelter showing the electrical resistive paths (R2 and Ri) from an anode (1) to the exit of collector bar (6), while passing through an electrolyte bath (2), molten metal (3) and cathode (4), which are housed inside a steel shell enclosure (7) along with refractory lining (5). Current is usually made to exit from both sides of the electrolytic cell and carried to the next cell using external aluminium busbars. This lead to voltage drop and energy consumption.

[005] Moreover, the electric current in the cell helps in electrolysis as well as it generates heat required to operate the cell at high temperature. In case of blackout/power outage, electrolytic cells start freezing and all types of collector bars being highly heat conductive, increases the heat loss. It results in faster shutdown of the cells. Huge investment is needed to replace the frozen cells with the new one. This is also applicable to the existing designs of collector bars with a metal insert. In the existing apparatus design, the metal insert is not able to reduce heat loss effectively. Therefore, there is also a need of a new design of collector bar and cell refractory lining, which slows down the cooling of electrolytic cells during power outages.

[006] Therefore, there is a need a new design of the apparatus for aluminium reduction cell which solves some of the problems of the prior art.

SUMMARY OF THE INVENTION [007] According to an embodiment of the present invention, there is provided an apparatus for enhancing performance of an aluminium reduction cell in a smelting process, the apparatus comprising: one current collector bar (6) and at least one insert (8) located therein, the insert (8) tapering towards one end of the collector bar (6), such the tapering is in continuous mode or in steps and such that the current is collected at the tapered end, provided that the material of construction of the insert (8) is different from the material of construction of the collector bar (6).

[008] According to another embodiment of the present invention, there is provided an apparatus for enhancing performance of an aluminium reduction cell in a smelting process, the apparatus comprising: at least one current collector bar (6) and at least one insert (8) therein, the insert (8) being placed within the collector bar (6), such that the ends of each inserts (8) having the largest cross-section are placed in the middle of the aluminium reduction cell and the tapering ends of each insert (8) are near the respective ends of the collector bar(s) (6), similar ends of each insert (8) are equidistant from the corresponding lateral ends of the collector bar(s) (6), and the ends of each insert (8) in the middle of the aluminium reduction cell face each other so as to define a gap between the inserts (8), provided that the material of construction of the inserts (8) is different from the material of construction of the collector bar(s) (6) and the current is collected from both side of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS [009] Figure 1 depicts a cross sectional view of a conventional aluminium smelter, in accordance with the embodiments of the present invention;

[010] Figures 2(a), 2(b) and 2(c) illustrate an isometric view of various designs of the metal insert (8) inside a current collector bar (6), having a greater cross sectional area on one side of the collector bar, in accordance with the embodiments of the present invention; [Oil] Figure 3 depicts cross sectional view of the aluminium smelter, having asymmetrical thermal insulation strips (9) with thickness 1.2 to 5 times higher along the current exit side as compared to the opposite side, in accordance with an embodiment of the present invention;

[012] Figures 4(a) and 4(b) illustrate an isometric view of two current collector bars (6) having at least one insert (8), having a greater cross sectional area in the middle of the collector bar (6), in accordance with the embodiments of the present invention;

[013] Figure 5 depicts cross sectional view of the present aluminium smelter, having symmetrical thermal insulation strips (9) on both current exist sides of cell, in accordance with an embodiment of the present invention; [014] Figure 6 illustrates the cross-sectional view of the collector bar (6) and the position of metal insert (8) within the collector bar (6), in accordance with an embodiment of the present invention;

[015] Figure 7(a) and 7(b) shows a comparison of cell stability between prior art apparatus and apparatus of the present invention, in accordance with an embodiment of the present invention; and [016] Figure 7(c) shows a graph showing the operating electric current during power outage and a comparison of electrolyte temperature of prior art apparatus and the apparatus of the present invention, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[017] For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the disclosed process and system, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates. [018] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

[019] Reference throughout this specification to “one embodiment” “an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[020] As set out in the claims, the present invention eliminates or reduces the aforementioned problems of the prior art by providing a novel design of the apparatus for aluminium reduction cell comprising current collector bar (6) having at least one insert (8) located therein and having a tapering design. Moreover, the apparatus provides thermal insulation strips (9) placed along the bottom of the electrolytic cell and along the lower sides of the electrolytic cell from the bottom thereof up to the top of the cathode (4), such that heat escape is reduced from the said cathode (4) from the side from where the current is collected. Lastly, the present invention demonstrates collection of current from one side of the apparatus in order to save energy in the external aluminium busbar network without adversely affecting the cell MHD stability.

[021] According to one embodiment of the present invention, there is provided an apparatus for enhancing performance of an aluminium reduction cell in a smelting process comprising one current collector bar (6) and at least one insert (8) located therein. Placement of the metal insert (8) within the collector bar results in reduction in horizontal currents in metal region, thus reducing the MHD instability for energy reduction, without enhancing the heat loss from the collector bar region. Hence, sustaining the longevity of cell operation during power outages. Also, the current is collected at the tapered end. The cross sectional area of the insert(s) (8) tapers towards one end of the collector bar (6), as shown in Figs. 2(a), 2(b) and 2(c), in cases where the requirement is to collect current at one end only. As shown in the figures, the tapering is in continuous mode or in steps. The tapering helps in ensuring similar resistance path in the whole cathode (4) and collector bar (6) assembly to provide higher MHD stability. [022] Preferably, the ratio of the largest cross-section of the inserts (8) to the cross-section of the current collector bar (6) is in the range of 0.1:0.6. The ratio of the cross-section of the tapered ends of the inserts (8) to the cross-section of the current collector bar (6) is in the range of 0.05:0.4. The ratio of the length of the inserts (8) to the length of the current collector bar (8) is in the range of 0.5:0.8. [023] In an embodiment, the electrical conductivity of the material of construction of the inserts

(8) is greater than the electrical conductivity of the material of construction of the collector bar (6). Preferably, the material of construction of the inserts (8) is copper, copper alloys, gold, silver, platinum or a mixture thereof. Usage of all these alternatives of material of construction of insert

(8) and others are considered to be within the scope of the present invention. [024] In an embodiment, as shown in Figure 3, the electrolytic cell for aluminium reduction showing an enhanced cell performance in a smelting process comprises of thermal insulation strips (9) that are placed along the bottom of the electrolytic cell and along the lower sides of the electrolytic cell from the bottom thereof up to the top of the cathode (4), such that heat escape is reduced from the said cathode (4) from the side from where the current is collected. This heat loss results in a thermal balance during blackouts thereby avoiding freezing of the electrolytic cell. Preferably, the thermal insulation strip (9) placed on the current exist side is thicker than the insulation strip (9) placed on the other side. Since current is made to exit from one side of cell using metallic collector bar (6), it acts as a thermal window as compared to opposite non-current exit end. Hence, a thicker insulation strip

(9) near to current exit region help in maintaining the heat flux balanced from both the long sides of the cell for efficient thermal balance and cell performance. [025] In yet another embodiment of the resent invention, there is provided an apparatus for enhancing performance of an aluminium reduction cell in a smelting process, the apparatus comprising at least one current collector bar (6) and at least one insert (8) therein. The insert (8) is placed within the collector bar (6), such that the ends of each insert (8) having the largest cross-section is placed in the middle of the aluminium reduction cell and the tapering ends of each insert are near the respective ends of the collector bar(s) (6), such that similar ends of each insert (8) are equidistant from the corresponding lateral ends of the collector bar(s) (6), and the ends of each insert (8)in the middle of the aluminium reduction cell face each other so as to define a gap between the insert (8).

[026] In case at least one insert (8) is placed within one or more collector bar(s) (6), the ends of each insert (8) in the middle of the aluminium reduction cell face each other so as to define a gap between the inserts (8), as shown in Fig. 4(a) and Fig. 4(b). The metal inserts (8) in the middle of the aluminium reduction cell attract least current and thus a small gap is not only economically feasible but aids in allowing current to pass through the ends of the inserts (8) which are placed in the middle of the cell thus reducing the horizontal component of current. In other words, due to greater cross sectional area of the metal insert (8) in the middle of the aluminium reduction cell, electrical conductivity increases at center as compared to that of at sides. This allows more current to pass through center, thus reducing the horizontal component of current in molten aluminium deposited on the cathode (4). Preferably, the collector bar (6) may also be single or multiple to compensate the stress generated due to thermal expansion of the collector bar (6) at operating temperature. [027] Preferably, the ratio of the largest cross-section of the inserts (8) to the cross-section of the current collector (6) is in the range of 0.1:0.6. The ratio of the cross-section of the tapered ends of the inserts (8) to the cross-section of the current collector (6) is in the range of 0.05:0.4. The ratio of the length of the inserts (8) to the length of the current collector bar (6) is in the range of 0.5:0.8.

[028] In an embodiment, the electrical conductivity of the material of construction of the inserts (8) is greater than the electrical conductivity of the material of construction of the collector bar (6). Preferably, the material of construction of the inserts (8) is copper, copper alloys, gold, silver, platinum or a mixture thereof. Usage of all these alternatives of material of construction of insert (8) and others are considered to be within the scope of the present invention.

[029] In an embodiment, as shown in Figure 5, the electrolytic cell for aluminium reduction showing an enhanced cell performance in a smelting process comprises of thermal insulation strips (9) that are placed along the bottom of the electrolytic cell and along the lower sides of the electrolytic cell from the bottom thereof up to the top of the cathode (4), such that heat escape is reduced from the said cathode (4) from both sides of the cell from where the current is collected. The reduced heat loss results in a thermal balance during blackouts thereby avoiding freezing of the electrolytic cell. Preferably, the thermal insulation strip (9) placed on both the sides are of equal thickness as current is collected from both the sides, as collector bars (6) with inserts (8) act as thermal window during power outages.

[030] The electrolytic cell, during power outage starts freezing due to lack of heat, due to which electrolyte temperature decreases, consequently the freeze of electrolyte at the side walls grows. Since, the freeze of the electrolyte has lesser thermal conductivity as compared to that of collector bar, major portion of the heat flux gets diverted towards collector bar region. The reduction in cross-section of the insert (8) as it goes towards end of the collector bar (6) helps in reducing the thermal conductivity at end leading to lesser heat loss. The thermal insulation strips (9) in the cell refractory lining increases the thermal resistance surrounding the collector bar (6), which again reduces the heat loss. Preferably, the insulation can extend from corner of the cell refractory lining up to l/3 ^rd of the cathode length and twice the cathode width. The height of the insulation strip (9) can go up to the cathode top most end starting from the bottom. The thickness of the insulation (9) can vary from one twentieth up to one third of the gap between side of cathode blocks and inner wall of steel shell (7).

[031] In an embodiment, the insert (8) is positioned within the collector bar (6), closer to the top of the collector bar (6), as shown in Fig. 6, such that the cross sectional area of the collector bar (6) above the insert (8) is smaller than the cross sectional area of the collector bar (6) below the insert (8). Positioning the inserts (8), as shown in the figures, at upper portion of collector bar’s (6) cross-section, shifts the heat generation upwards, thus, helps in maintaining higher cathode temperature under normal working condition. The higher cathode temperature facilitates energy efficient process. In an embodiment, there are various ways of the placement of metal insert (8) inside the collector bar (6). The insert(s) (8) may be circular, square, rectangular, trapezoidal, parallelogram, U-shape, V-shape or a combination thereof. [032] Fig. 7(a) and Fig. 7(b) shows that reduced horizontal current results in an increase in level of stability of the cell. It can be observed from the graph that, while performing pot stability test in the aluminium reduction cell with the apparatus of prior art was able to lower the anode-to- cathode distance before reaching the MHD instability at 4.13 volts, whereas the apparatus of the present invention, was able to lower the anode-to-cathode distance before reaching the MHD instability at 3.93 volts. It shows the present invention lower the MHD instability significantly. Fig. 7(c) show an instance of power outage scenario wherein current was zero for over 4 hours due to which average amperage significantly reduced. A significant dip in the electrolyte temperature prior art apparatus can be observed whereas electrolyte temperature for apparatus of the present invention remains unaffected. Unaffected electrolyte temperature indicates very slow freezing rate of the cell.

[033] 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.

[034] Example 1

Different ratios of cross-sectional area of insert (8) to collector bar (6) were analysed and found that ratio close to 0.2 offer optimal result w.r.t reduction in CVD (cathode voltage drop) and horizontal current. The amount of material used for insert (8) is also offer good techno-economic feasibility. Higher ratio than optimal value increases the material amount significantly, however not much saving w.r.t. CVD or horizontal current. On contrary lower ratio does not offer electrical saving.

Table 1 Note - reduction in cost of material in future may lead to shift the techno-economic optimal point towards higher ratio.

[035] Example 2:

Diverse length ratios of insert (8) to collector bar (6) were also analysed and found the optimal ratio at about 0.75, which offer good reduction in CVD and horizontal currents. Higher length ratio than optimal, increases saving in CVD marginally, however it also enhances the collector bar (6) exit temperature, which are detrimental during power outage scenario. Lower ratio does not offer good electrical savings. Additionally, the placement of the insert (8) has to be such that it does not extend much beyond the cathode block (4), as it leads to increase in collector bar (6) exit temperature. From analysis, the optimal extension beyond cathode block (4) was found to be about 150 mm.

Table 2 [036] Example 3:

Table 3 shows the cathode voltage drop and steel shell wall temperature of prior art apparatus and the apparatus of the present invention, in accordance with an embodiment of the present invention.

Table 3

[037] 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.