A CARBON ELECTRODE AND A METHOD FOR PRODUCING SUCH AN ELECTRODE

Abstract: The present invention concerns an improved carbon electrode and a method for producing such a carbon electrode. In particular, the present invention relates to anodes for use in connection with electrolytic production of aluminium in accordance with the Hall-Héroult process. The anisotropy in a vibrated carbon anode results in partially significant differences in the physical properties depending on how the samples are oriented in relation to the vibration direction, in particular with regard to electrical resistance. For a tested, typical quality electrode, the resistance perpendicular to the vibration direction is 8.3 % lower than in the vibration direction. If this is utilised by placing the nipple or suspension hanger holes so that the direction of electric current flow when the electrode is in use in the electrolysis is substantially 90 DEG to the vibration/compression direction, this can produce a reduction of approximately 0.31 % in power consumption.

Description: A Carbon Electrode and a Method for Producing such an Electrode The present invention concerns an improved carbon electrode and a method for producing a carbon electrode. Carbon electrodes, particularly anodes, produced in accordance with the present invention may expediently be used in connection with electrolytic production of aluminium in accordance with the Hall-Heroult process involving pre-baked anodes. The present invention is based on the observed fact that several physical properties of carbon electrodes will be directional on the basis of the moulding process used. This applies, among other things, to electrodes moulded by vibration moulding, for which differences can be demonstrated between the vertical and horizontal directions. A common method for producing anodes for use for aluminium production is vibration moulding of a "green" mass (a viscous, ductile mass containing carbon particles and binder) in a mould consisting of a box open at the top which has a plumb or a heavy lid designed to slide downwards along the inner walls of the box. Nipple holes or recesses in the anode for fixing it to an anode suspender are usually created by the plumb having downward-facing projections which extend down into the mass. The creation of anodes in this way means that the orientation of the recesses corresponds to the vibration direction (vertical direction). One disadvantage of the above production method is that the physical properties of the anode cannot be exploited in an optimized manner because of limitations in the actual production method. One explanation of the directional difference may be related to how particles inside the material move during the moulding operation. For example, the external geometric dimensions of the mass during vibration will be reduced in the vertical direction, while the dimensions will remain virtually constant in the horizontal direction. Another reason may be that the mass which is vibrated contains carbon particles which, to a large extent, have the form of oblong flakes. During the vibration of the "green" mass, the flakes will tend to be adjusted so that their centre of gravity is located on the lowest possible vertical level. This means that there may be more interfaces between the carbon particles in the vertical direction than in the horizontal direction, which is assumed to be a dominant factor regarding the fact that the physical properties such as mechanical strength, electrical resistance, thermal properties, etc. are directional in relation to the moulding process used. With the present invention, it has become possible for a carbon electrode to be produced so that its physical properties can be utilised optimally. With the present invention, a carbon electrode will be produced with reduced electrical resistance and more favourable thermal conductivity properties. With the present invention, it will also be possible to use simpler materials than previously without having to reduce the requirements for the properties stated. The present invention will be described in the following using examples and figures, where: Figure 1 shows the physical properties of a carbon electrode. Figure 2 shows how sampling is done in relation to a carbon electrode. Figure 3 gives a graphic presentation of the difference between vertical and horizontal resistance in a carbon electrode. Figure 4 shows a comparison between density and resistance in a carbon electrode. The vibration direction will be called the vertical direction (V) in the following. Correspondingly, the horizontal direction (H) is perpendicular to this. Two core samples were drilled out in both directions from 9 areas in typical carbon electrodes, see Figure 2. The areas were in a plane 200 mm above the underside of the carbon electrode, i.e. where the wear surface is located after half the operating life period in an electrolysis process. The points of intersection between this and three vertical planes longitudinally to and three vertical planes transversely to the carbon describe where the samples were taken. The vertical samples had their centre axis in the intersection between the longitudinal and transverse planes and in such a way that the horizontal plane intersected them at half their height. The horizontal samples had their centre axis in the horizontal plane and as close to the others as possible. The samples were tested in relation to a number of parameters, which are shown in Figure 1 : - Reactivity in carbon dioxide, Rco2 Expresses the carbon electrode's (anode's) tendency to react with carbon dioxide at 960°C. A high value of this means high reactivity. - Soot index, Sco2 Expression of selective reaction with carbon dioxide which results in loose particles (soot) in the electrolysis bath. - Density (unit weight, volume weight) Calculated on the basis of the sample's weight and external dimensions. - Specific electrical resistance Calculated on the basis of the measured voltage drop over the sample and its cross-section and length. - Young's modulus, YM Modulus of elasticity, determined by measuring compression in a compression strength test. - Compression strength, CS Calculated on the basis of the force applied in connection with compression to break. - Air permeability, Perm Expression of continuous pores. A high value corresponds to open material. - Coefficient of thermal expansion, CTE Linear expansion as a result of change in temperature. - Reactivity in air, RAIR Expresses the carbon electrode's (anode's) tendency to react with air at 525°C. A high value corresponds to high reactivity. - Porosity, Por Total porosity based on image analysis. The table in Figure 1 indicates typical values for the horizontal and vertical directions. The permeability is slightly higher in the horizontal direction than in the vibration direction. This corresponds with the porosity determined in samples from the centre axis. However, it has not been demonstrated that this can produce a noticeable increase in the internal CO 2 reactivity in the carbon. The other direction-dependent parameters, resistance (converted into thermal conductivity), YM, CS and CTE are subject to considerations of thermal stress. Modelling tests with the values in question give no reason to expect significant changes in these forces in the carbon electrode (anode). Figure 3 shows the directional difference between vertical and horizontal specific electrical resistance in each of the 9 sample points, expressed in a bar chart. It can usually be observed that density and resistance will correspond well (high density produces low resistance), in particular when the raw material and process are generally the same and with standard sampling, i.e. in the vibration direction. The table in Figure 4 shows this, but also that this is not so marked when the resistance is measured in the H direction. The latter tendency probably increases as the density decreases. The last line in the table in Figure 4 indicates that the correlation between density and the difference in resistance between the directions is low, at least for the anode quality in question. If the manufacturing process is such that the nipple holes in an anode are created entirely after moulding, for example by milling or by drilling nipple holes after calcination, it is possible to choose the side on which they are to be placed. It is thus possible to benefit from the anisotropy by ensuring that the direction of electric current flow in the electrolysis coincides with the H direction in connection with vibration. In accordance with commonly used vibration/compression techniques, this will imply that the nipple holes are arranged substantially perpendicular to the direction of vibration/compression of the electrode in its "green state". It should be understood that electrodes produced in a way where the "green mass" is compressed merely in a static manner or by extruding techniques may in the same manner as described above have directional properties which can be exploited in accordance with the present invention. The size of the power saving which can be achieved with this will depend on how the anode is produced. On the basis of a typical anode as described earlier, the total energy saving will be 0.31 % on the basis of the below conditions: Total voltage drop over cell : 4 V Average voltage drop over anode 150 mV Difference in specific electrical resistance 4.5 μΩm Power consumption 14 kWh/kg AI Reduction in resistance in the carbon itself 8.3 % The present invention thus offers a considerable potential for savings in the form of reduced power consumption. The present invention will also make it possible for the carbon electrode, in connection with vibration, to be given a more precise height as the nipples in the finished anode are innstalled in a direction in which the geometric dimensions of the mass during tamping/vibration are kept constant.

Claims: Claims 1. A method for producing a carbon electrode in which a "green" mass comprising particle material containing carbon and a binder undergoes a moulding process which causes the mass to be exposed to externally forced compression in one or more directions and to be subjected to a calcination process before use, characterised in that the carbon electrode is arranged so that, when it is in use, the dominant direction of electric current will mainly be oriented so that it does not coincide with the direction(s) of the forced compression. 2. A method in accordance with claim 1 for production of a carbon electrode, more precisely an anode for use in an electrolysis cell of Hall-Heroult type in which the anode is made with at least one recess for fixing to an anode suspender, characterised in that each recess is arranged directionally so that it mainly coincides with a direction mainly perpendicular to the direction(s) of the forced compression. 3. A method in accordance with claim 2, characterise in that the carbon electrode is calcinated before the recesses are arranged. 4. A method in accordance with claim 3, characterised in that the recesses are arranged by a mechanical milling or drilling process. 5. A carbon electrode produced from a "green" mass comprising particle material containing carbon and a binder where the green mass is exposed to externally forced compression in one or more directions and the carbon electrode is subjected to a calcination process before use, characterised in that the dominant direction of electric current in relation to the carbon electrode, when it is in use, mainly does not coincide with the direction(s) of the forced compression. 6. A carbon electrode in accordance with claim 5, more precisely an anode for use in an electrolysis cell of Hall-Heroult type in which the anode is made with at least one recess for fixing to an anode suspender, characterised in that each recess is arranged directionally so that it mainly coincides with a direction mainly perpendicular to the direction(s) of the forced compression. 7. A carbon electrode in accordance with claim 6, characterised in that it is calcinated before the recesses are arranged. 8. A carbon electrode in accordance with claim 7, characterised in that the recesses are arranged by drilling or by milling the calsinated carbon material.

Inventors: LUNDBERG EGIL, LUNDBERG, EGIL
Applicants: LUNDBERG EGIL, NORSK HYDRO AS, LUNDBERG, EGIL, NORSK HYDRO ASA
Application year: 2000
Main classification: C25B11
IPC codes: C25C3, C25C7, H05B7