NZ529979A - Arrangement of anode for utilisation in an electrolysis cell provided that its working surface area is larger than its projected area - Google Patents

Abstract

An anode for utilisation in an electrolysis cell for production of aluminium metal from an aluminium containing component in a molten salt electrolyte is described. The aluminium containing component is mainly alumina and the molten salt electrolyte is based on mixtures of NaF and AIF3 and CaF2. The invention relates to improvements of anodes useful for retrofit of existing electrolysis cells, in which the anodes remain inert during operation. The anode is characterised by being shaped in a manner to increase the area of the electroactive surface.

Description

5299 WO 02/088432 PCT/N002/00157 Arrangement of anode for utilisation in an electrolysis cell The present invention relates to an arrangement of anode for utilisation in an electrolysis cell. More specific it relates to improvements of anodes useful for retrofit of existing electrolysis cells, in which the anodes remains inert during operation.

Prior art: Aluminium is presently produced by electrolysis of an aluminium containing compound dissolved in a molten electrolyte, and the electrowinning process is performed in cells of conventional Hall-Heroult design. These electrolysis cells are equipped with horizontally aligned electrodes, where the electrically conductive anodes and cathodes of today's cells are made from carbon materials. The electrolyte is based on a mixture of sodium fluoride and aluminium fluoride, with smaller additions of alkaline and alkaline earth fluorides. The electrowinning process takes place as the current passed through the electrolyte from the anode to the cathode causes the electrical discharge of aluminium containing ions at the cathode, producing molten aluminium, and the formation of carbon dioxide at the anode.

During production of aluminium metal in accordance with the Hall-Heroult principles, carbon based anodes are used. The carbon anodes are consumed in the electrolytic process, through reactions in which the carbon material in the anodes combine with the/ oxygen in the added alumina feed stock to form carbon dioxide gas. The currently used process displays several shortcomings and weaknesses, but it is still the only industrial process for aluminium production. The environmental impact from the Hall-Heroult process is unwanted due to production of pollutant greenhouse gases like CO2 and CO in addition to the so-called PFC gases (CF4, C2F6, etc.). The traditional aluminium production cells also utilise carbon materials as the electrically conductive cathode. Since carbon is not wetted by molten aluminium, it is necessary to maintain a deep pool of molten aluminium metal above the carbon cathode, and it is in fact the surface of the aluminium pool that is the "true" cathode in the present cells.

PCT/N002/00157 2 The environmental impact from electrolytic aluminium production could be reduced if inert (or dimensionally stable) anodes were utilised. If the process could be operated without consumable anodes, i.e. using inert anodes, oxygen gas would be evolved at the anode in stead of carbon dioxide gas. As demonstrated by Keniry (Keniry, J.: "The economics of inert anodes and wettable cathodes for aluminium reduction cells", JOM, pp. 43-47, May 2001), also possible operational cost savings imply that the retrofit of conventional Hall-Heroult electrolysis cells remain an attractive option if one could retain to the highest possible extent the cell superstructure, cathode shell, bus-bar system and other cell features of the present technology, in order to minimise the cost of the retrofitting.

Over the times, numerous material technical solutions aimed at solving the problems related to inert anodes have been suggested, however, to the present day none of which have proven commercially feasible.

Field of invention: The present invention relates to an improved anode design mainly for retrofit of Hall-Heroult cells, where the anode of a principally inert material is fabricated in a specific manner to overcome one of the most important obstacles of utilisation of inert anodes in retrofit of Hall-Heroult cells; The purity of the produced aluminium metal. A reduction in the contamination of anode components in the produced aluminium metal can be achieved by increasing the electroactive surface of the anode, i.e. increasing the cathodic current density with respect to the anodic current density in the electrolysis cell. This feature can be obtained by optimising the shape of the anode surface and the overall anode structure.

Inert anodes utilised in existing Hall-Heroult cells have to satisfy several demands. The most important demand is to contribute to the production of commercial purity aluminium metal, as pointed out by Thonstad and Olsen (Thonstad, J. and Olsen, E.: "Cell operation and metal purity challenges for the use of inert anodes", JOM, pp. 36-38, May 2001), without the need for new, costly purification processes. This requirement put demands on the electrochemical integrity of the inert anode material under the prevailing circumstances in the electrolyte. Additionally , however, also the design and/or electrode design can be utilised to contribute to maintain acceptable metal purities in retrofitted Hall-Heroult cells.

WO 02/088432 PCT/N002/00157 The electrolyte (bath) in the aluminium electrolysis cell can for all practical purposes be considered to be saturated with inert anode components as dissolved oxides. The accumulation of anode material elements in the aluminium produced is then governed by the mass transfer coefficient for the species from the bath to the aluminium metal pool. A major drawback of inert anode retrofit of Hall-Heroult cells is that there are limited possibilities for reducing the large area of the metal pool cathode exposed to the electrolyte, without costly rebuilds of the cell (i.e. drained cell concepts). Hence, optional ways of reducing the metal contamination should be sought after, and one seductive possibility is to increase the electroactive surface of the anode.

During electrolysis alumina containing species diffuse towards the anode and are discharged. In a thin layer (diffusion layer) toward the anode, the alumina concentration is different from the bulk electrolyte due to this discharge. By increasing the anodic current density the alumina concentration will decrease in the diffusion layer, due to the discharge rate at the anode being higher than the diffusion rate of the alumina species into the diffusion layer. Hence, the solubility of anode species (as oxides) will increase in the layer compared to the bulk electrolyte. It is well known that the solubility of inert anode material components, as oxides, decrease as the alumina concentration in the electrolyte increase. Diffusion of anode species from the layer close to the anode surface and into the bulk electrolyte will lead to precipitation of anode species in the bulk electrolyte due to super-saturation, and consequently a destruction of the inert anode material. However, by increasing the anode surface area, the anodic current density will decrease (if the current load is maintained unchanged) and as a result, the alumina concentration in the diffusion layer will increase. This will reduce the solubility of inert anode species (as oxides) in the diffusion layer and also reduce the concentration of these species in the bulk electrolyte. As a result, the contamination of the produced aluminium metal by anode material components will be reduced and a commercial quality aluminium can be produced with inert anodes. This approach will also increase the durability of the oxide-ceramic (or metals or cermets) inert anodes in the electrolysis cells.

PCT/N002/00157 4 However, since the reduction of the metal pool surface area is not practically feasible during retrofit of existing Hall-Heroult cells, the angle of attack will be to increase the anode surface area. This is amongst others described in US 4,392,925, 4,396,481, 4,450,061, 5,203,971, 5,279,715 and 5,938,914 and in GB 2 076 021. Increased anode surface area is amongst others described in US 4,707,239 and 5,286,359 in addition to NO 176189 and 308141.

NO 176189 involves a novel cell design for an aluminium electrolysis cell involving the use of a horizontal, wetted cathode and several vertically aligned inert anodes. The purpose of the novel cell design is to increase the total anode surface area by inserting several vertical, planar anodes above the cathode, but maintained within the outlined outer circumference of the cathode, so that a low anodic current density can be maintained. The low anodic current density is necessary to operate the low temperature cell to prevent formation of fluorine containing species due to the low solubility of alumina in the suggested electrolyte. Such an electrolyte is not feasible to use in existing Hall-Heroult cells with retrofitted inert anodes.

US 4,707,239 describes an electrode assembly for production of lead from a chloride based electrolyte. In the proposed assembly, the anodes (and cathodes) are designed with saw tooth pattern and spacers to maintain stable ACD and the anodes are also equipped with holes for gas release. The purpose of the patented increased electrode area is to decrease voltage and energy requirements, increase metal production, increase effective inter electrode electrolyte area, enhance rapid gas removal, and reduce the overall metal production costs. The proposed anode design will have limited benefits in a retrofitted Hall-Heroult cell with inert anodes and a horizontal metal pool introducing variations in the effective ACD, without substantial changes made to the anode (electrical) properties).

NO 308141 relates to the insertion of shapes (contours) on the cathode surface to "in situ" produce a rounding of the anode surface. The patent is based on the shapes (contours) being placed on the cathode of an Hall-Heroult cell, in which the cathodes are at least partially operated under drained conditions. This means that no horizontal metal pool is present as a continous surface across the whole cathode panel area. The "in situ" PCT/N002/00157 formation of the rounded anodes for enhanced gas release and reduced cell voltage is based on the use of carbon consumable anodes, and is as such not applicable to retrofit of existing Hall-Heroult cells with inert anodes, maintaining a horizontal metal pool in the cell.

US 5,286,359 concerns the use of pyramid shaped anodes and cathodes in existing Hall-Heroult cells. Both electrode types are made from inert materials and the cell is operated at low ACDs with a metal pool located below the active cathode surfaces. The invention obtains increased anode and cathode surface area, although the proposed anode design would most likely operate at increased anodic current densities if deployed in a retrofitted cell with a horizontal metal pool due to the relative high electrical conductivity of the electrolyte.

Detailed description of the present invention: The present invention relates to an arrangement of anode for utilisation in an electrolysis cell. More specific it relates to improvements of anodes useful for retrofit of existing Hall-Heroult electrolysis cells, in which the anodes remains inert during operation. The proposed anode design takes into consideration the increase of the anode electroactive surface area in order to obtain one or more of the features listed below, whereof the two main features is: - Reduced contamination of the produced aluminium metal in the cell by increasing the anode to cathode surface area. Reduced contamination in produced metal will lead to reduced dissolution of anode material in the electrolyte, and as such contribute to prolong the lifetime of the anodes by maintaining its structural integrity.

- Anodic current density can be kept lower than in existing cells, or be maintained at the same level through an amperage increase.

Other features obtained by the invention, and as also pointed out in US 4,392,925, 4,396,481, 4,450,061, 4,707,239, 5,203,971, 5,279,715, 5,286,359 and 5,938,914, in NO 176189 and 308141, as well as in GB 2 076 021, are: PCT/N002/00157 6 - Possibilities for reduced voltage and energy requirements during aluminium production.

- Possible increase in metal production through increase in effective inter electrode electrolyte area.

- Enhanced (and rapid) gas removal and there through reduced voltage drops.

The combined result of these effects will represent possible reductions in the overall production cost of aluminium metal.

Based on the desire to accomplish these features, an invention with respect to the design of the anode surface has been proposed in order to enhance the electroactive surface area of the anode. Advantages as mentioned above and further improvements can be achieved in accordance with the present invention as defined in the accompanying claims.

The invention is in the following described by examples and figures, where: Figure 1: shows a first design of an anode surface with increased surface area, Figure 2: shows a second proposed design of an anode surface with increased surface area, Figure 3: shows a third possible design of an anode surface with increased surface area, Figure 4: shows a fourth possible design of an anode surface with increased surface area, Table 1: presents a comparison of different anode surface areas with a mainly horizontal underside with an extent of 700x1000 mm2 with respect to alternative anode surface design modifications.

PCT/N002/00157 7 In figure 1 there is shown an anode surface design (1), in which the surface area is increased through the introduction (forming, shaping) of a series of pyramidal elements (2).

In figure 2 is shown another an anode surface design (10), in which the surface area is increased through the introduction (forming, shaping) of a series of (upward) protruding elements (11) with a pyramidal shape and rounded tops. To illustrate the design of the elements, a separate element (12) is also shown in perspective in the figure.

In figure 3 is shown a third possible design of an anode surface (20), in which the surface area is increased through the introduction (forming, shaping) of a series of (upward) protruding elements (21). To illustrate the design of the elements, a separate element (22) is also shown in perspective in the figure. As can be seen from the figure, this particular element is designed with a plurality of recesses/ steps (23, 24, 25, 26) that will actively contribute to the increase of the anode surface area.

In figure 4 there is illustrated a fourth possible design of an anode surface (30), in which the surface area is increased through the introduction (forming, shaping) of a series of (upward) protruding elements (31). The figure shows the anode surface increasing measures applied in the length wise direction, although it may be applied both length wise and crosswise. To illustrate the design of the elements, a separate element (32) is also shown in perspective in the figure. As can be seen from the figure, this particular element is designed with first a series of waves defined by a sinus function (33). Thereafter, a second series of sinus waves (34) are superimposed on the first, creating what is called a double sinus function. This design will actively contribute to the increase of the anode surface area.

PCT/N002/00157 8 Table 1 presents the effect on the anode surface area increase as a function of anode surface design changes. From the calculations in Table 1 it is clear that if the anode surface for instance is formed to a sinus-like shape, the anode surface area is considerably increased. By imposing the sinus function in two dimensions, the overall anode surface area does not increase if the amplitude and frequency is the same in both directions. However, by superimposing a second sinus function on the first one, where the superimposed sinus function has shorter wave length and a shorter amplitude, the surface area will increase even more. A sketch of this "double sinus" function is provided in figure 4. As indicated in Table 1, the double sinus function can increase the surface area of the anode by 240%. This corresponds to a (theoretical) current increase from 200 kA to 480 kA and yet maintaining the anodic current density of the retrofitted cell.

The described shapes/designs of the anode surfaces given above, as well as shown in Figures 1 through 4 and Table 1, represents only a few of the possible modifications to obtain the desired increase in anode surface area. Other embodiments of the proposed designs may also be used.

It should be understood that the anode may be designed so that its electrical conductivity in the outer layer(s) is of the same order of magnitude as in the electrolyte. This can for instance be done by its construction based upon the conductivity of the material composition in the outer layer(s).

Table 1: Effect of surface design modifications on anode surface area. Reference is a horizontal anode with a flat underside (700x1000 mm2), and the table express the percent increase in anode surface area by introducing groves, saw tooth, rows of peaks and valleys, PCT/N002/00157 etc. on the electroactive anode surface, Surface pattern area Extent Dimensions Surface Horizontal, flat 100% Horizontal, jagged Horizontal, jagged Horizontal, jagged Horizontal, jagged width 50 mm, height 10 mm width 25 mm, height 5 mm width 50 mm, height 10 mm width 25 mm, height 5 mm Lengthwise Lengthwise Length and crosswise Length and crosswise 108% 108% 108% 108% Horizontal, sinus Horizontal, sinus Horizontal, sinus Horizontal, sinus radii (1) 5 mm radii (1) 3 mm radii (1) 5 mm radii (1) 3 mm Lengthwise Lengthwise Length and crosswise Length and crosswise 168% 171% 168% 171% Horizontal, double sinus radii (1) 5 mm radii (2) 1 mm Length and crosswise 240% Horizontal, sinus w/protuberance radii (1) 5 mm Length and crosswise 177% radii (2) 1 mm

Claims (7)

Claims:

1. Arrangement of anode for an electrolysis cell with a flat metal pool for production of aluminium metal from an aluminium containing component in a molten salt, where the aluminium containing component mainly consist of alumina and the molten salt electrolyte is based on mixtures of NaF and AIF3 and CaF2, in which the anode principally remains inert (non-consumable) in the process; characterised in that the anode is provided with a dense surface and is further shaped so that its working surface area is larger than its projected area.

2. Arrangement of anode as claimed in claim 1, characterized in that the molten salt contains alkaline and alkaline earth halides,

3. The arrangement of the anode in claim 1 or 2; characterised in that the anode surface has at least one surface feature, the or each surface feature being independently selected from the group consisting of a groove, saw tooth, peak and valley, sinus curve, protruding shape and pyramid as a part of its electroactive surface.

4. The arrangement of the anode in claim 3; characterised in that the or each surface feature present on the electroactive surface of the anode consists of different wave lengths (frequencies) and amplitudes used on top of each other (superimposed).

5. The arrangement of the anode in claim 1; characterised in that the anode is designed so that its electrical conductivity in the outer layer(s) of the anode is of the same order of magnitude as in the electrolyte. 11

6. Use of the arrangement as defined in any one of the previous claims as a retrofit in existing electrolysis cells of Hall-Heroult design for aluminium production.

7. Arrangement of anode substantially as hereinbefore described with reference to and as shown in any one of the accompanying drawings.