WO2008033034A1

WO2008033034A1 - Electrolysis cell and method for operating the same

Info

Publication number: WO2008033034A1
Application number: PCT/NO2007/000323
Authority: WO
Inventors: Frank ØVSTETUN; Elin Haugland; Jørn RUTLIN; Jørund HOP; Stanislaw Jarek; Sara Thömblad MATHISEN
Original assignee: Norsk Hydro Asa
Priority date: 2006-09-14
Filing date: 2007-09-12
Publication date: 2008-03-20
Also published as: EA200900443A1; EP2066831A4; CA2660998C; CA2660998A1; WO2008033034A8; BRPI0716951B1; ATE507325T1; NO20064165L; CN101680102A; AU2007295188A1; CN101680102B; EA014744B1; EP2066831B1; EP2066831A1; DE602007014229D1; ZA200901822B; NO332480B1; AU2007295188B2; NZ575484A; BRPI0716951A2

Abstract

A method and improvements relating to an electrolysis cell, where the cell comprises a substantially horizontal cathode (4) 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 cathode comprises at least one substantial vertical electrical current outlet (6).

Description

Electrolysis cell and method for operating the same

The present invention relates to improvements to an electrolysis cell and a method for operating the same. In particular the invention relates to electrical current distribution in a cell 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 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, electrolyte, liquid metal, cathode and 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.

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.

STATEMENT OF THE PROBLEM

The design of the cathode current distribution and the corresponding busbar system for aluminium production cells 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 of today there is not any proven solution for the realisation of such a concept with vertical current outlets in the bottom of the cathode.

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. The suggested material can consist either of a carbonaceous mass or may be a solidified poured metal such as iron. 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. Thus, this solution is based upon the substitution of the horizontal collector bars by means of plural nipples of relatively small diameter. At the time the above patent application was filed, a cell requiring current of 100,000 amperes was defined as a large cell. Today, a cell will commonly be defined as large if it requires approximately 2,5 times that amount of amperage. Therefore, due to the relatively small area represented by the nipples, there will be a prohibitive high current density between each individual nipple and the cathode, even if a substantial amount of nipples are applied. Further, this publication does not define how to arrange the nipples in an optimal manner to obtain an even current distribution, other than one regular, symmetrical pattern as shown in Figs. 4-6 involving the application of 132 nipples. Still further, due to thermal induced forces and expansion/contraction the solution using vertically arranged nipples in accordance with this publication, will experience increased resistance due to the above mentioned limited current transfer area and the corresponding high spot current densities. The vertical arranged bores in the carbon blocks can serve as weakening points where crack formation may occur, and increasing the number of nipples to cope with the current demand of large cells of today, will even worsen this situation. In accordance with the present invention the above shortcomings can be avoided. The present invention includes the application of vertical current leads of an optimised design. 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, tapered (wedge shaped or conical) design of the current leads has shown to be optimal with regard to expansion and bending of the collector bar elements, which normally is of a current leading metal. The angle of the tapered outlet is chosen based on considerations of mechanical strength, voltage drop and heat loss, and is preferably in the range of 5-15° relative to the vertical plane.

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 cathode current distribution 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 cathode current distribution.

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

Figure 1 discloses a collector bar design of a electrolysis cell having current outlet in its bottom part, Figure 2 discloses details related to vertical collector bar outlets,

Figures 3a-e disclose various configurations of collector bar arrangements. The purpose of the described designs is to obtain a low cathode voltage drop and an even or flat current distribution at the cathode block surface. The corresponding collector bar design will also give possibility for a simplified busbar system (less weight and thereby cheaper) compared to a conventional collector bar design. A key factor for success is the details around the vertical current outlets. During operation the cathode block will bend and heave upwards. The vertical collector bars must then also be allowed to slide upwards, otherwise the vertical outlets will be torn off the horizontal collector bars.

In accordance to Figure 1 , there is shown a collector bar design of an electrolysis cell 1 with anode arrangements 2, 3 and a cathode block 4. The Figure discloses current outlets in the bottom part of the cell. As shown in this embodiment of the invention, the cell may have both horizontal 5, 5' and vertical 6, 6' current outlets.

In accordance to Figure 2 details related to vertical collector bar outlets are disclosed. As shown, the outlet has one vertical outlet 25 to be connected with the cell's bus bar system

(not shown). The vertical outlet 25 is connected to one horizontal collector part 23 that is embedded in one cathode block 4. The vertical and the horizontal parts can be made out of one piece for instance by casting, or it can be produced out of two separate parts interconnected by welding or similar joining methods that ensure good electrical conducting properties. The parts can consist of steel or any other appropriate material.

As shown in the Figure, the vertical outlet is penetrating the bottom part of the cathode structure. The cathode structure comprises (from above) one cathode block 4, two or more layers of bricks 20-21 having the appropriate thermal and chemical properties, and the pot shell 22, normally made out of steel plates. The pot shell may have a lowered section in the region of the outlet (not shown). The vertical outlet penetrates the various layers through one hole or channel. Outside the vertical outlet, which may have a tapered shape, there is arranged a protective layer of a carbonaceous material 27 with good resistance to electrolyte and electrolyte reactant products. The space between the protected vertical outlet and the cathode structure can be filled with a castable material 26 with good resistance to chemical attack by electrolyte and electrolyte reactant products.

One important feature relating to the vertical outlet design is that the current outlet is enclosed by the carbonaceous layer 27 that aids the vertical sliding of the outlet inside the hole or channel filled with castable material.

In Figure 3a-e there are disclosed various collector bar designs. In Fig. 3a one cathode block 4 is shown schematically. There are disclosed three collector bars 30, 31 and 32 embedded in the cathode block 4. There are two horizontal outlets 30', 31 'and one vertical outlet 33.

In Fig. 3b there are shown two collector bars 35, 36 embedded in a cathode block 4. The collector bars have horizontal outlets 35' and 36'. In addition, collector bar 36 has one vertical outlet 37.

In Fig. 3c there are shown four collector bars 40, 41 , 43 and 45 embedded in a carbon block 4. Collector bar 45 and 40 have one horizontal outlet 45' and 40' respectively. Collector bars 41 and 43 have vertical outlets 42 and 44 respectively.

In Fig. 3d there are shown just one collector bar 50 embedded in one carbon block 4. The collector bar have one horizontal outlet 50' and one vertical outlet 51.

Fig. 3e discloses a collector bar design where a collector bar 60 is embedded in a cathode block 4. The collector bar 60 have two horizontal outlets 61 ', 61 " and one centrally arranged vertical outlet 62.

It should be understood that further combinations and arrangements of horizontal and vertical collector bar outlets can be achieved by the teachings of the present invention.

By the arrangement mentioned above, it is possible to arrange the collector bars wholly or partly in each individual cathode block in a manner that combines vertical and horizontal current outlets in an advantageous manner with regard to achieve an even current distribution in the cell's cathode structure.

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

Claims

1. A method for operating an electrolysis cell, where electrical current is lead into the 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 (4), characterised in that electric current is lead out of the cell by at least one substantial vertically arranged current outlet (25).

2. A method in accordance with claim 1 , characterised in that electric current is collected in the cathode by one inner, horizontally extending part (23) of the said current outlet (25).

3. A method in accordance with claim 1 , where the cathode comprises 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 vertical outlet is a pre-calculated proportion of that of the horizontal end of the collector bar.

4. A method in accordance with claim 1 -3, characterised in that the amount of current conducted out of the vertical outlet is in the range of 20-100 % of the total current, where 100 % represent a design with only vertical outlets.

5. Improvements relating to 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, the cell further comprising a bus bar system, characterised in that said cathode structure comprises at least one substantial vertical electrical current outlet (25) connected to the bus bar system.

6. Improvements in accordance with claim 5, characterised in that the outlet is provided with at least one horizontal current lead portion (23) that is in electrical contact with the cathode material (4).

7. Improvements in accordance with claim 6, characterised in that the said at least one horizontal current lead portion is embedded in a pre-formed slot in the cathode material by a paste material or the like.

8. Improvements in accordance with claim 6, characterised in that the said at least one horizontal current lead portion is protruding outside the cathode material, and is further electrically connected to the bus bar system.

9. Improvements in accordance with claim 8, characterised in that the horizontal current lead portion is protruding outside the cathode material with two ends, said both ends being electrically connected to the bus bar system

10. Improvements in accordance with claim 8 or 9, characterised in that the horizontal current lead portion is protruding outside the cathode material to a minor extend, thus allowing easy removal and replacement of the cathode in the cathode shell.

11. Improvements in accordance with claim 10, characterised in that the horizontal current lead portion are connected to the bus bar system by means of flexible electrical couplings.

12. Improvements in accordance with claim 10, characterised in that the connection between the bus bar system and the horizontal current lead portion is situated inside the cathode shell.

13. Improvements in accordance with claim 5, characterised in that the said at least one vertical electrical current outlet is at least partly enclosed by a carbonaceous layer.

14. Improvements in accordance with claim 5 or 13, characterised in that the said at least one vertical electrical current outlet is at least partly embedded in a castable material with good resistance to chemical attack.

15. Improvements in accordance with claim 5, characterised in that the said at least one vertical electrical current outlet has a tapered section.

16. Improvements in accordance with claim 15, characterised in that the tapered section is of a conical shape, preferably with an angle in the range of 5-10° relative to the vertical plane,

17. Improvements in accordance with claim 15, characterised in that the tapered section is wedge shaped.