AN ELECTROLYTIC REDUCTION CELL AND COLLECTOR BAR

Abstract: An electrolytic reduction cell for the production of a metal is disclosed. The cell includes a plurality of collector bars (21). Each collector bar includes an elongated first section (27) that contacts the cathode (15) and at least one end section (29) that extends through one of the cell side walls (5) and is electrically connected to the electrical current carrier. The cell is characterised in that, for the purpose of controlling current distribution, the first section of each collector bar includes a core (31) of relatively high electrical conductivity material and an outer housing (33) of a more mechanically and chemically resistant material than the core material and the end section of each collector bar is formed from relatively low thermal conductivity material.

Description: AN ELECTROLYΉC REDUCTION CELL AND COLLECTOR BAR The present invention relates to an electrolytic reduction cell for the production of a metal, such as aluminium. The present invention relates particularly to a collector bar construction for use in such cells. Aluminium metal is generally produced in an electrolytic reduction cell by the Hall-Heroult process in which electrical current is passed through an electrolytic bath comprising alumina dissolved in molten cryolite to cause the electrodeposition of molten aluminium as a metal pad on the cell cathode. An electrolytic reduction cell comprises an outer steel shell that is lined with a layer of insulating material, such as refractory bricks. Blocks of carbonaceous material are placed on top of the insulating layer on the base of the cell and these blocks form the cathode of the cell. The blocks are hereinafter referred to as "cathode blocks" . The cathode must last for the expected operating life of the cell, which is typically 1000 to 2000 days. A number of consumable anodes are located a short distance above the metal pad that forms above the cathode. In an operating cell, an electrolytic bath is located between the metal pad and the anodes, and the passage of electrical current through the electrolytic bath breaks down the dissolved alumina in the electrolytic bath into aluminium and oxygen and the molten aluminium collects in the metal pad on the cathode. The molten aluminium is periodically drained from the metal pad, typically on a daily basis. Electrolytic reduction cells are arranged in potlines in which a large number of cells are connected in series. Electrical current enters a cell through the anodes, passes through the electrolytic bath and pad of molten metal and into the cathode. The current in the cathode is collected and passes to an external current carrier, such as an external bus bar, and then along to the next cell. In conventional aluminium reduction cell technology, collector bars that are embedded in the cathode blocks are used to collect electrical current from the cathode and conduct it to an external ring bus . In conventional embedded collector bar technology, the bar is made from steel and is either cast or glued into a channel formed in the underside of a cathode block. In an operating cell, the cathode current density distribution along the length of cathode blocks is uneven with the outermost portions of the blocks drawing current at up to three to four times higher density compared to the inner portions of the blocks . Current travels unevenly through the cathode blocks as it finds the least resistance path from the cell. Specifically, current tends to travel through the cathode blocks towards the ends of the collector bars rather than directly down through the cathode into the collector bars, thus increasing the average current path length in the cathode. Poor conductivity of steel collector bars and the use of high conductivity cathode material contribute to the uneven current density. One consequence of the uneven current density is an uneven current distribution on the surface of the cathode blocks . It is highest near to the outer edge of the anode shadow or ledge toe . The uneven cathode current distribution has a dual effect on cell operation: on the one hand it increases the rate of erosion of carbonaceous material by increasing the chemical activity of sodium (this drives the aluminium carbide-forming reaction) in the affected region; and on the other hand it increases the rate of transport of dissolved aluminium carbide by inducing circulation of metal and catholyte. This increased circulation can result either from increased metal pad heave due to interaction in the metal pad of horizontal currents with vertical magnetic fields or from the Marangonni effect (i.e. circulation induced by uneven interfacial tension between catholyte and aluminium due to uneven cathode current density distribution at the interface) . The rate of erosion of carbon is therefore directly related to the current density and the rate of circulation of metal and catholyte. As neither the horizontal currents in the metal pad nor the vertical magnetic fields are even, balanced, or static, their coupling can lead to hydrodynamic instability of the metal-bath interface. The circulation of the metal, the deformation of its surface and the instability of the metal-bath interface are the three most significant limitations of the current technology aluminium reduction cells which affect potlife (cathode and sidewall erosion) and operating efficiency. Moreover, these limitations make it difficult to reduce the anode/cathode spacing. This spacing has a major impact on the power requirements of aluminium reduction cells. In conventional aluminium reduction cell technology it is difficult to have a completely uniform cathode current density distribution throughout the cell. The best outcome which can be achieved to date is to reduce the variation of current density distribution by constructing relatively narrow but long cells having relatively deep, high resistivity, anthracitic cathode blocks and large steel collector bars. The problem of metal heave and metal pad stability (product of field current interaction) is then addressed through the modification of bus bars to control the vertical magnetic field. Modern magnetically compensated cells are a good example of this type of engineering within the limitations of the system. However, relatively narrow, but long reduction cells are a disadvantage as they have a high external surface to production volume ratio and hence have a high heat loss. Nevertheless, in conventional cell construction methods, the limitations resulting from embedded collector bar technology have been accepted as inherent to the nature of the aluminium reduction cells cathode and its negative impact has been minimised by focussing on improving the magnetic field aspect of the curren /field interaction. Modern aluminium reduction cells are designed with magnetic compensation in order to improve the hydrodynamic stability of the cells, and therefore achieve reductions in anode/cathode spacing. However, this requires relatively expensive external bus bars . An objective of the present invention is to improve the efficiency of electrolytic reduction cells by improving the spacial current density distribution in the cells cathode and metal pad. According to the present invention there is provided an electrolytic reduction cell for the production of a metal, which cell includes: an outer shell and an inner lining of insulating material which form a base, side walls and end walls for containing an electrolytic bath; an anode; a cathode located on the base of the cell; and a plurality of collector bars which electrically connect the cathode to an electrical current carrier that is external to the cell, wherein each collector bar includes an elongated first section that contacts the cathode and at least one end section that extends through one of the side walls and is electrically connected to the electrical current carrier, and wherein the cell is characterised in that, for the purpose of controlling current distribution, the first section of each collector bar includes a core of relatively high electrical conductivity material and an outer housing of a more mechanically stable and chemically resistant material than the core material and the end section of each collector bar is formed from relatively low thermal conductivity material . The applicant has made the following findings in computer modelling studies and in operation of several test cells. 1. The use of collector bars having a highly electrically conductive core improves the spatial current density and therefore the stability of an electrolytic reduction cell . 2. The use of collector bars having a relatively low thermal conductivity end section avoids excessive heat loss from the cell via the collector bars. 3. Construction of collector bars with the conductive core enclosed in a more mechanically and chemical resistant material than the core material achieves collector bar durability at least equivalent to conventional steel collector bars. More particularly, the applicant has found that the use of relatively high electrical conductivity material, such as copper, as the cores of collector bars does not have the disadvantages that were found with prior art proposals, such as US patent 3,551,319 of Elliot and are likely to arise with the proposal disclosed in the US patent 5,976,333 of Pate. In the Elliot proposal, copper cored bars were originally used to improve voltage losses but were not applied for commercial production purposes. The copper extended all the way to the ends of the collector bars, ie outside the cell, and the high thermal conductivity copper extracted much more heat than conventional steel bars and resulted in an overall increased cell heat loss, excessive cell instability and long term thermal cycling. The result was a reduced performance and overall higher voltages. The applicant has realised that a significant proportion of the voltage savings that were thought to be possible with copper cored collector bars can be achieved without having to form the collector bars with highly electrically (and thermally) conductive end sections outside the cathode. As a consequence, with the present invention the applicant has been able to achieve reduced cell overall heat loss and maintain correct cathode heat balance to permit stable operation. The net effect has been a greater overall energy saving through lower voltage requirements and lower energy consumption due to required current efficiency and controlled current distribution. Preferably the core material is copper or a copper alloy. Preferably the outer housing material is a relatively low electrical conductivity material compared to the core material . Preferably the outer housing material is steel. Preferably the end section material is steel. Preferably the cathode is in the form of a plurality of blocks that are positioned side by side on the base of the cell. More preferably the cathode blocks extend side by side along the length of the cell with the ends of the blocks contiguous with the side walls of the cell. In one embodiment there is one collector bar per cathode block, with the first section extending along the length of the block and the end sections of the bar being formed from relatively low thermal conductivity material and extending through opposite side walls. In another, although not the only other, embodiment there are two collector bars per block, with the first section of one bar extending substantially half way along the length of the block with an end section extending through one side wall and the first section of the other bar extending substantially half way along the length of the block with an end section extending through the other side wall. Preferably the undersurface of the block includes a channel which receives the first section of the collector bar. Preferably the first section of the collector bar is cast or glued in the channel . Preferably the cell includes a means for increasing the effective surface area of electrical contact between the cathode and the relatively high electrical conductivity material core of each collector bar. Preferably the cell also includes a means for improving both the longitudinal and transverse distribution of current in the cathode. In one embodiment the electrical contact means includes a plurality of electrical contact plugs mounted in electrical contact to the cathode and to the collector bars . Preferably the collector bar is cylindrical and the diameter of the core is 60-80%, more preferably 70%, of the diameter of the collector bar. The present invention is based on thermal, electrical and stress modelling studies on a proposed aluminium reduction cell design and on the results of operation of test cells based on the cell design at the smelter of the applicant situated at Bell Bay, Tasmania, Australia. The cell design is based on the use of collector bars having a copper core housed in an outer steel sleeve. The cell design is described in more detail in section D in relation to the figures. A. Thermal Modelling of Cell Design Thermal modelling of the cell design with a preferred form of copper-cored collector bars in accordance with the present invention predicted the following: 1. The cell design would not incur any thermal penalty because of the use of the low thermal conductivity end design of the bars. 2. When operated at standard Bell Bay operating conditions with a metal level of around 150 mm there could be a small voltage benefit. 3. At a lower metal level higher voltage savings could be achieved, B. Electrical Modelling Electric modelling of current distribution in the test and conventional cells established that significant improvements in current density distribution can be achieved through the use of copper-cored collector bars. Table 1 contains a compilation of the expected current distribution data obtained through electrical (3- D) modelling which shows that the cell design ("the Test Cell") had a significantly more uniform cathode current density distribution and significantly reduced horizontal currents compared to two standard cells ("Std" and "Graphitic Std") . Table 1: Vertical and Horizontal Current Distribution in Cells Stress Modelling The expansion coefficient of copper is higher than that of steel, leading to differential expansion of copper . Consideration of a situation where the copper- core perfectly fits the steel tubing indicated the possibility of high hoop stresses developing on the outer surface of the steel hollow. However, the modelling showed that, even under the worst case assumptions, the stresses which could be generated would not exceed the tensile strength of mild steel. Hence, the modelling showed that cracking of steel is unlikely to be a problem. Under operating conditions and temperature of 900°C both copper and steel are ductile and would easily deform to relieve these stresses. Physical modelling of this worst-case scenario - using a sample in the form of a 150mm long copper core tightly-fitted into a steel tube and heated to 1000°C and held at temperature for 2 weeks - showed that cracking is not a problem. Electrical resistance testing of the interface between the copper and steel of the sample indicate a low contact resistance of about 0.05Ωmm 2 (<lmV) . The sample was cut open and the interface between the copper and the steel was examined using SEM and Microprobe analysis. The examination showed the following: 1. The interface between copper and steel was subject to oxide penetration to a distance of 10- 20mm from the ends; 2. The oxide combined with the alloying elements in steel (Si and Mn) to cause precipitation of oxide particles and grain boundary embrittlement in steel; There was mutual migration of copper and iron across the copper/steel interface and a metallurgical bond formed in the regions of interface which were not affected by oxide penetration; Regions affected by oxygen penetration did not form this metallurgical bond. The work established the need for care to exclude the possibility of air access to the copper/steel interface to avoid deterioration of contact resistance. Also, the work also showed that, if exclusion of air is successful, there is a likelihood that a metallurgical bond may form between the copper and steel to make this interface more resistant to any attack by sodium in subsequent service. D. Cell Design The test cells were constructed with collector bars of half-cell length. It is noted that the present invention is not restricted to such arrangements and extends to full cell collector bars. Figures 1 to 6 illustrate the construction of one test cell. Figure 1 is a vertical cross-section along the length of the cell, Figure 2 is an enlargement of the right hand end of the cell shown in Figure 1, Figure 3 is a vertical cross-section across one half of the cell, Figures 4 and 5 are longitudinal cross-sections of the collector bar used in the cell, and Figure 6 is a perspective view of the collector bar. The cell has parallel side walls 5 (Figure 3), parallel end walls 7 (Figures 1 and 2), and a base 9 (Figures 1 to 3) . As with conventional aluminium reduction cells, the test cell is relatively long and narro . The side walls 5, end walls 7 and base 9 include an outer steel shell 11 and an inner lining 13 of suitable refractory material . The cell also includes a plurality of cathode blocks 15 located on the refractory lining 13 of the base 9 and arranged to extend across the cell to the side walls 5 and side-by-side along the length of the cell. The cell also includes a plurality of anodes (not shown) . Each cathode block 15 is formed with a channel 19 in the undersurface of the block 15. The channels 19 extend along the whole length of the blocks . The cell further includes collector bars 21 which electrically connect each cathode block 15 to an external ring bus (not shown) . Each collector bar 21 includes an elongated section 27 that is cast or glued in one of the channels 19 in a cathode block 15 and an end section 29 that extends through one of the side walls 5 and is connected to the ring bus . The elongated section 27 is generally cylindrical and has a central core 31 of copper and an outer sleeve 33 of steel. The terminal end of the elongated section 27 is closed by a steel disc 35. The end section 29 is generally blocked-shaped and is formed from steel. A preferred method of constructing the collector bar 21 (of preferred dimensions) is described below. 1. Construction of end section 29 (i) Cut a 370mm long 100 x 100mm steel bar, drill and prepare ends; (ii) Centrally drill a 70mm die hole (37 in Figures 4 and 5) to a depth of 55mm; (iii)Cut a 45° external bevel (39 in Figures 4 and 5) to create a groove for welding; 2. Construction of elongate section 27 (i) Cut a 70mm diameter X 1150mm long copper rod 31. (ii) Slide fit the copper rod 31 into a 1045mm long steel tubing 33 with 100mm OD and 70mm ID. Bevel the edges at 45° at a depth of 10mm on one end. 3. Assembly of collector bar (i) Insert the copper rod 31 into the hole in the 370mm steel collector bar and weld copper to steel . (ii) Place the assembly into a 200 tonne press and push the copper into the hollow steel tube until the pressure increases reaches the press maximum. The copper core 31 should end up being 30-70mm shorter than the outer steel tube 33. (iii)To this end of the assembly weld a steel disc 35, 10mm thick and 100mm diameter. Appendix 9 contains the drawings which describe the CCCB assemblies. The first test cell was operated for 876 days until it was deliberately cut out for autopsy. At the completion of the autopsy the cell was reconstructed and restarted successfully and operates as the second test cell . The autopsy results indicate that the performance of the test cell was favourable when compared with standard operating cells of the applicant. Specifically, the test cell had a statistically lower voltage (lOOmV on average for the majority of the operating period) than that of the standard operating cell, a similar current efficiency to the standard operating cell, and the noise was lower or similar to that of the standard operating cell . Many modifications may be made to the preferred embodiment without departing from the spirit and scope of the present invention. By way of example, whilst the preferred embodiment of the collector bar 21 shown in the figures includes a generally cylindrical copper-cored elongated section 27 located within the cell and a generally block- shaped steel end section 29 that extends through the side walls 5 and from the cell, the present invention is not limited to this construction. The collector bar 21 may be of any suitable configuration. By way of example, the collector bar may be generally flat rather than cylindrical and block shaped. Moreover, the flat collector bar may have a relatively wide section located in the cell and a relatively narrow section extending through and outwardly from the side walls of the cell.

Claims: Claims : 1. An electrolytic reduction cell for the production of a metal, which cell includes: an outer shell and an inner lining of insulating material which form a base, side walls and end walls for containing an electrolytic bath; an anode; a cathode located on the base of the cell; and a plurality of collector bars which electrically connect the cathode to an electrical current carrier that is external to the cell, wherein each collector bar includes an elongated first section that contacts the cathode and at least one end section that extends through one of the side walls and is electrically connected to the electrical current carrier, and wherein the cell is characterised in that, for the purpose of controlling current distribution, the first section of each collector bar includes a core of relatively high electrical conductivity material and an outer housing of a more mechanically and chemically resistant material than the core material and the end section of each collector bar is formed from relatively low thermal conductivity material. 2. The cell defined in claim 1 wherein the core material is copper. 3. The cell defined in claim 1 or claim 2 wherein the outer housing material is a relatively low electrical conductivity material compared to the core material. 4. The cell defined in claim 3 wherein the outer housing material is steel. 5. The cell defined in any one of the preceding claims wherein the end section material is steel. 6. The cell defined in any one of the preceding claims wherein the cathode is in the form of a plurality of blocks that are positioned side by side on the base of the cell. 7. The cell defined in claim 6 wherein the cathode blocks extend side by side along the length of the cell with the ends of the blocks contiguous with the side walls of the cell. 8. The cell defined in claim 7 wherein there is one collector bar per cathode block, with the first section extending along the length of the block and the end sections of the bar being formed from relatively low thermal conductivity material and extending through opposite side walls. 9. The cell defined in claim 7 wherein there are two collector bars per block, with the first section of one bar extending substantially half way along the length of the block with an end section extending through one side wall and the first section of the other bar extending substantially half way along the length of the block with an end section extending through the other side wall. 10. The cell defined in any one of claims 6 to 9 wherein the undersurface of the block includes a channel which receives the first section of the collector bar. 11. The cell defined in claim 10 wherein the first section of the collector bar is cast or glued in the channel . 12. The cell defined in any one of the preceding claims includes a means for increasing the effective surface area of electrical contact between the cathode and the relatively high electrical conductivity material core of each collector bar. 13. The cell defined in any one of the preceding claims includes a means for improving both the longitudinal and transverse distribution of current in the cathode . 14. The cell defined in claim 10 wherein the electrical contact means includes a plurality of electrical contact plugs mounted in electrical contact to the cathode and to the collector bars.

Inventors: JURIC DRAGO DRAGUTIN, SHAW RAYMOND WALTER, JURIC, DRAGO, DRAGUTIN, SHAW, RAYMOND, WALTER
Applicants: COMALCO ALU, JURIC DRAGO DRAGUTIN, SHAW RAYMOND WALTER, COMALCO ALUMINIUM LIMITED, JURIC, DRAGO, DRAGUTIN, SHAW, RAYMOND, WALTER
Application year: 2001
Main classification: C25C3
IPC codes: C25C3