WO2008120993A1

WO2008120993A1 - Improvements relating to electrolysis cells connected in series and a method for operation of same

Info

Publication number: WO2008120993A1
Application number: PCT/NO2008/000087
Authority: WO
Inventors: Frank Ovstetun; _Christian Droste
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2007-04-02
Filing date: 2008-03-10
Publication date: 2008-10-09
Anticipated expiration: 2009-10-02
Also published as: BRPI0809671B1; NZ579815A; AU2008233392A1; NO331318B1; EP2150639A1; CA2681205A1; EA016404B1; CN101663422A; EP2150639B1; CN101663422B; ZA200906503B; CA2681205C; NO20071766L; EP2150639A4; EA200901330A1; BRPI0809671A2; AU2008233392B2

Abstract

A method and improvements relating to an electrolysis cell, where the cell comprises a substantially horizontal cathode of an electronic conducting material and further have current leads such as horizontal collector bars embedded therein. The cell further comprises a bus bar system. The cell can be operated by improved distribution of electrical current, while said bus bar system conduct current from at least one intermediate position of the cathode. The invention can be applied for cells arranged both in an end-to-end manner and in a side-by-side manner.

Description

Improvements relating to electrolysis cells connected in series and a method for operation of same

The present invention relates to improvements to electrolysis cells connected in series and a method for operating the same. In particular the invention relates to a bus bar system and followingly electrical current distribution in cells of the Hall-Heroult type for production of aluminium.

TECHNICAL FIELD OF THE INVENTION For good understanding of the invention, it should first be remembered that the industrial production of aluminium is made by electrolysis in cells, which are connected electrically in series, with a solution of alumina in molten cryolite brought to a temperature typically between 940 and 980 ⁰C, by the heating effect of the current traversing through the cell.

Each cell is constituted by an insulated parallelepiped steel container supporting a cathode containing prebaked carbon blocks in which there are sealed some steel rods known as cathode current collector bars, which conduct the current out of the cell, traditionally approximately 50% from each of the long sides of the cell. The outlets of the cathode current collector bars are connected to the busbar system, which serve to conduct the current from the cathodes towards the anodes of the following cell. The anode system, composed of carbon, steel and aluminium, is fixed on a so-called "anode frame", with anode rods adjustable in height and electrically connected to the cathode rods of the preceding cell.

The electrolyte, that is the solution of alumina in a molten cryolite mixture at 940-980 ⁰C, is located between the anode system and the cathode. The aluminium produced is deposited on the cathode surface. A layer of liquid aluminium is kept permanently on the bottom of the cathode crucible. As the crucible is rectangular, the anode frame supporting the anodes is generally parallel to its large sides, whereas the cathode rods are parallel to its small sides known as cell heads.

The main magnetic field in the cell is created by the current flow in the anode and the cathode system. All other current flows will give perturbations to this created main field.

The cells are arranged in rows and can be disposed transversely in a side-by-side orientation; their short side is parallel to the axis of the potline. Alternatively, disposed longitudinally in an end-to-end orientation, their long side is parallel to the axis of the potline. Commonly, one potline is represented by two rows of cells. The current has opposite directions in the two rows. The cells are connected electrically in series, the ends of the series being connected to the positive and negative outputs of an electric rectification and control substation. The electric current traversing the various conducting elements: anode system, electrolyte, liquid metal, cathode system and its corresponding connecting conductors, creates large magnetic fields. These fields, together with the electrical current in the liquid electrolyte and metal, form the basis for the Magneto Hydro Dynamic (MHD) behaviour in the electrolyte and in the liquid metal contained in the crucible. The so-called LaPlace forces, which create electrolyte and metal flow, are also harmful to the steady operation (stability) of the cell. Further, the design of the cell and its bus bar configuration, will also influence upon how the electric current traversing the cell is distributed. It should be understood that the invention can be implemented in side-by-side as well as end-to-end arranged cells. The weight and correspondingly the costs related to the bus bar system is of importance to present a price competitive melter technology.

Commonly, the current distribution through the anode system is mainly affected by the arrangement of the anodes in the cell, as well as the design of the stub configuration of the anode hanger and their interface with the individual anode.

When it comes to the cathode system, it is normally designed in a manner where collector bars are embedded in individual cathode blocks in a horizontal manner. This technological solution has shown to be very reliable regarding problems with leakages of melt or bath through the cathode system. Further, the collector bars will be protected by the surrounding cathode material (carbon based material) that is highly resistant against high temperatures and corrosive attacks. Commonly, bus bars collect the current outside the cathode shell. One shortcoming by this prior art is that the current distribution in the cathode system will be more intensive in the periphery of the cathode blocks than elsewhere. Further, technology based upon homogenous embedment of collector bars in slots formed in the underside of the cathode blocks, will render the result that the current distribution along the collector bar, inwardly towards the other end of the cathode block, will decrease rather proportional with the distance from the bus bar collector. Therefore, the current should advantageously be distributed in a predefined manner, and at more appropriate areas of the cathode system, to obtain an even current distribution. Further, current that is led out of the cathode system at the so called up-stream side of the cathode have to be led towards the so called down-stream side of the cathode and further to the anode system of the neighbouring cell in the series. This way of conducting current (upstream in parts of the cathode and subsequently downstream in the busbar system, will represent a system where parts of the cell's current is led through a longer distance than strictly necessary.

STATEMENT OF THE PROBLEM The design of the cathode current distribution and the corresponding busbar system for aluminium production cells is acknowledged to represent one of the more qualified key activities in developing a competitive aluminium reduction technology.

The designer should have several degrees of freedom in the process of developing an optimum cathode system, using skill to select a configuration (topology), which can result in an optimum current distribution.

It is recognized that if current could be derived from the cathode system at pre-selected points or areas, assisted by calculations and simulations, it should be possible to improve the current distribution in the cathode system. However, this will imply that the cathode system should be penetrated at lest partially from the bottom up and be preferably connected to horizontal current collector bars, by means of current leads or plugs as described in the Applicants, non published Norwegian Patent Application 20064165.

PRIOR ART

US patent 3,470,083, filed in October 1964, discloses an electrolytic cell cathode bottom with vertically inserted current conductors. Cylindrical nipples are inserted in vertical bores of the cathode, embedded by a poured material. Each individual plug is connected to a current conducting element arranged outside the cathode. The current conducting elements are further extending towards the sides of the cell and are connected to a bus bar system that surrounds the pot. The solution presented in this patent seeks to solve the problems related to conventionally collector bars, among those caused by different heat expansion of the carbon material and the iron rails (collector bars) causing considerable mechanical stresses that lead to formation of transversal cracks in the carbon blocks. However, the solution does not present improvements in the bus bar system as that of the present invention.

In accordance with the present invention the current distribution in the cathode system and correspondingly the lay-out of the bus bar system can be improved, due to the application of at least one current outlet arranged between the ends of the cathode. The present invention includes the application of vertical current leads. Further, the current leads (current outlets) can advantageously be electrically connected to horizontal collector bar elements that may extend partly or wholly through the cathode block. In the latter, its outermost end(-s) can be connected to the bus bar system for the cell.

The preferred cathodic current distribution will depend on characteristic of the busbar system. It can be quite different for retrofitting the invention to existing busbar systems on one hand, or for a new busbar system design on the other hand. Hence, the preferred amount of current conducted out of the vertical outlets can be within the range 20-100 %, with 100 % representing a design with only vertical outlets.

The amount of current leads can be relatively low, for instance in an embodiment applying a commonly used amount of horizontal collector bars. In accordance with the present invention, the MHD effects in an electrolysis cell can be improved, and it is possible to simplify the bus bar design of said cell by reducing its weight. As a consequence the investment costs can be reduced.

In accordance with the present invention as defined in the accompanying claims an optimised bur bar system can be achieved that overcomes main shortcomings of prior art designs. Further, the accompanying claims define a method to operate a cell with improved bus bar system.

The present invention shall in the following be described by figures and examples where:

Figure 1 discloses in perspective a schematic lay-out of a bus bar system in accordance with the present invention, the cells being arranged in a side-by-side manner,

Figure 2 discloses in a top view, the same lay-out as disclosed in Fig. 1.

Figure 3 represents a second embodiment of the invention and discloses in perspective a schematic lay-out of a bus bar system where the cells are arranged in an end-to-end manner,

Figure 4 discloses in a top view, the same lay-out as disclosed in Fig. 3

One purpose of the described design is to obtain a low cathode voltage drop and an even or flat current distribution at the cathode block surface with improved Magnet Hydrodynamic stability. This can be achieved by means of a simplified busbar system (less weight and thereby cheaper), where the design of the individual bus bar elements is optimized.

Fig. 1 and 2 disclose one embodiment of a bus bar system 1 that conduct current from the cathode system in one first electrolysis cell to the anode system of its neighboring cell. The cells are arranged in a side-by-side manner. The bus bar elements of the anode system are indicated as anode beams 2, 3, for connecting electrically the anodic structure of the cell. Individual anodes are indicated at A, A'. Further there are shown anode risers, one of those denoted as reference sign 6. In the cathode system of the first mentioned cell, there are shown some main elements of the bus bar system. First of all, at the downstream side of the cathode system there are arranged connections 7, for conducting current from collector bar outlets of the cathode (not shown) to a downstream arranged collector bus bar 10 which in turn is connected with the above mentioned risers.

For the conductance of current from an intermediate region of the cathode, there is arranged one or more connections 8 which in turn is electrically connected to a intermediate collector bus bar 11. The connection 8 is at the other hand electrically connected to a corresponding current outlet in the cathode (not shown).

At the upstream side of the cathode system there is arranged a collector bus bar 12 having plural connections 9 for conducting current from the cathode collector bar ends.

In addition there are shown bus bar elements such as 13, 15, 16, conducting current from the cathode system to the downstream side of the cathode and further to the corresponding risers 6.

In particular the bus bar element 13, can be arranged outside the cell's foot print to compensate for unwanted magnetic disturbancies. As the bus bar lay-out in this embodiment is of a symmetrical type, a similar bus bar element is arranged at the opposite end of the cell.

Bus bar element 15 and the corresponding elements 16 etc. towards the opposite side of the cell, conduct current from the intermediate collector bus bar 11 arranged in the cathode system and further to the collector bus bar 10.

Similarly, one or more bus bar elements 17 can be arranged beneath the cathode shell, to optimize the magnetic field compensation. Such elements are preferably arranged in a skew-symmetric manner (not shown), to optimize the effect of magnetic field compensation.

By the arrangement mentioned above, the bus bar system can conduct current from both current outlets arranged at the upstream and downstream side of the cathode system together with one or more intermediate positions in an advantageous manner with regard to obtain an even current distribution in the cell's cathode structure, and further to reduce the weight of the bus bar system as a whole.

Figure 3 discloses a second embodiment of the invention, where it in perspective is disclosed a schematic lay-out of a bus bar system, the cells being arranged in an end-to- end manner. Figure 4 discloses in a top view, the same layout as disclosed in Fig. 3.

In the Figures, the bus bar system 100 conducts current from the cathode system in one first electrolysis cell to the anode system of its neighboring cell. The bus bar elements of the anode system are indicated as anode beams 202, 203, for connecting electrically the anodic structure of the cell. Individual anodes are indicated at A, A'. Further there are shown anode risers 206, 206', 206", 206'".

In the cathode system of the first mentioned cell, there are shown some main elements of the bus bar system. At each length side of the cathode structure there are arranged collector bus bars 210, 212 having electrical connections 207, 209 that are electrically connected with the cathode's collector bars (not shown). The collector bus bars 210, 212 are at the other hand connected with the anode risers 206, 206' and via bus bar elements 218, 219 with the anode risers 206", 206'" of the neighboring cell.

For the conductance of current from an intermediate region of the cathode, there is arranged one or more connections 208 which in turn is electrically connected to a intermediate collector bus bar 211. The connection 208 is at the other hand electrically connected to a corresponding current outlet in the cathode (not shown). The intermediate collector bus bar 211 is further connected with bus bar elements 218, 219 via bus bar elements 220, 221, 222.

By the arrangement mentioned above, the bus bar system can conduct current from both current outlets arranged at both sides of the cathode system together with one or more intermediate positions in an advantageous manner with regard to obtain an even current distribution in the cell's cathode structure, and further to reduce the weight of the bus bar system as a whole.

It should be understood that further combinations and arrangements of bus bar elements could be achieved by the teachings of the present invention.

The amount of current that is distributed through the individual bus bar elements can be pre-calculated and optimized assisted by design software and verification trials.

Claims

1. A method for operating electrolysis cells connected in series, where electrical current is lead into one first cell via an anode arrangement arranged in the upper part of the cell, through an electrical conducting electrolyte and further through a substantially horizontal cathode, and further to an anode arrangement of a neighbouring cell, via one or more risers (6; 206), characterised in that electric current is lead out of the cell from at least one intermediate position of the cathode by means of at least one connection (8; 208) and further downstream said cathode to said one or more risers (6; 206).

2. A method in accordance with claim 1 , where the cells are arranged in a side-by- side manner, characterised in that electrical current collected at one intermediate connection (8) is conducted via at least one bus bar element (17) to a collector bus bar (12) arranged upstream said connection (8).

3. A method in accordance with claim 1 , where the cells are arranged in a side-by- side manner, characterised in that electrical current collected at one intermediate connection (8) is conducted via at least one bus bar element (16) to a collector bus bar (10) arranged downstream said connection (8).

4. A method in accordance with claim 1 , where current is conducted from the cathode via at least one collector bar integrated therein, where current is conducted out of the cathode by at least one horizontal end of said collector bar, characterised in that the amount of current conducted out of the cathode at the at least one intermediate position is a pre-calculated proportion of that of the horizontal end of the collector bar.

5. A method in accordance with claim 4, characterised in that the amount of current conducted out of the intermediate position is in the range of 20-100 % of the total current, where 100 % represent a design with only intermediate current outlets.

6. Improvements relating to a bus bar system (1; 100) of an electrolysis cell comprising a substantially horizontal cathode structure of an electronic conducting material and further having integrated current leads such as horizontal collector bars embedded therein, characterised in that the bus bar system comprises at least one electrical current connection (8; 208) to the cathode structure, said connection being arranged at an intermediate position with regard to the cathode.

7. Improvements in accordance with claim 6, characterised in that the at least one electrical current connection (8; 208) is attached to an intermediate collector bus bar (11 ; 211 )

8. Improvements in accordance with claim 6, characterised in that there are arranged several electrical current connections (8; 208) to the cathode structure.

9. Improvements in accordance with claim 6, characterised in that the bus bar system comprises at least one collector bus bar (10; 210) connected to at least one collector bar outlet at one side of the cathode.

10. Improvements in accordance with claim 6, characterised in that the bus bar system comprises collector bus bars (10, 12; 210, 212) connected with collector bar outlets at the sides of the cathode.

11. Improvements in accordance with claim 6, characterised in that the bus bar system comprises at least one bus bar element (13) outside the cell's head.

12. Improvements in accordance with claim 6, characterised in that the bus bar system comprises at least one bus bar element (17) beneath the cathode shell, that can be arranged in a skew-symmetric manner.

13. Improvements in accordance with claim 6, characterised in that the cells are arranged in a side by side manner.

14. Improvements in accordance with claim 6, characterised in that the cells are arranged in an end-to-end manner.