CA2470742A1 - Process for producing cathode blocks - Google Patents

Abstract

Disclosed is a method for producing graphitized cathode blocks. A carbonized cathode block is used in a longitudinal graphitization method, the cross section of said block being larger at the ends of the block than in the middle thereof. At least part of the graphitized material is removed from the ends upon graphitization. Also disclosed are cathode blocks produced according to the inventive method and the use thereof for the electrolytic production of aluminum.

Description

SGL CARBON AG

Process for producing cathode blocks The invention relates to a process for producing cathode blocks, in particular for the electrolytic production of aluminum.
In the electrolytic production of aluminum by the Hall-Heroult process, use is made of electrolysis cells which have a bottom which is made up of a plurality of blocks and acts as cathode. The electrolyte is a melt comprising a solution of aluminum oxide in cryolite. The working temperature is, for example, about 1 000 °C. The electrolytically generated molten aluminum is deposited on the bottom of the cell under a layer of the electrolyte. The cells are surrounded by a metallic housing (preferably steel) lined with high-temperature-resistant material.
Due to the chemical resistance and thermal stability required, the material of choice for the cathode blocks is preferably carbon which may have been partially or completely graphitized by means of thermal treatment.
Such cathode blocks are produced by mixing pitches, cokes, anthracite and/or graphite in selected particle sizes or particle size distributions for the solids and shaping, firing and, if appropriate, (partially) graphi tizing the mixtures. Firing (carbonization) is usually carried out at temperatures of about 1 200 °C, and the graphitization is usually carried out at temperatures above 2 400 °C.
While graphitized cathodes are preferred because of their higher electrical conductivity, they suffer from increased corrosion during operation, corresponding to a mean annual decrease in their thickness of up to 80 mm.
This corrosion is not distributed uniformly over the length of the cathode blocks (corresponding to the width of the cell), but the surface of the cathode blocks is changed to a W-shaped profile. Due to the nonuniform removal of material, the useful life of the cathode blocks is limited by the areas having the greatest erosion.
One possible way of making the erosion more uniform over the length of the cathode block and thus increasing the useful life is to configure the cathode blocks so that their electrical resistance varies over the length in such a way that the current density (and thus the corrosion) is uniform over their length or at least displays a very small deviation from its mean over the length.
One solution for this problem is described in DE 20 61 263, in which composite cathodes are made up of either a plurality of carbon blocks which have different electrical conductivities and are arranged so that a uniform or approximately uniform current distribution results, or of carbon blocks whose electrical resistances increase continuously in the direction of the cathodic terminals. The number of carbon blocks and their electrical resistance depend in each case on the size and type of the cell and have to be adapted individually for each case. Cathode blocks made up of a plurality of individual carbon blocks are complicated to construct;
the joins also have to be sealed well in order to prevent the liquid aluminum flowing out at the joins.

In WO 00/46426, a graphite cathode is described consisting of a single block which has an electrical conductivity which varies over its length, with the conductivity being lower at the ends of the block than in the middle. This nonuniform distribution of electrical conductivity is achieved by bringing the end zones to a temperature of from 2 200 to 2 500 °C during graphitization, while the middle zone is exposed to a temperature of from 2 700 to 3 000 °C. This different heat treatment can be achieved in two ways according to these teachings : on the one hand, the heat loss in the graphitization furnace can be limited differently, or heat sinks can be provided in the vicinity of the end zones so as to increase the heat loss. In the case of a transverse graphitization, the density of, the thermally insulating bed is altered so that the heat loss over the length of the cathodes becomes nonuniform and the desired temperatures are obtained as a result. In the case of longitudinal graphitization, too, the heat loss in the vicinity of the ends can be increased by different configuration of the thermally insulating bed, or bodies which carry away the heat, preferably graphite bodies, are installed for this purpose in their vicinity so as to produce greater outward heat flow to the furnace wall.
According to another method, the difference in heat treatment can be achieved by local changes in the current density, with the result of different heat evolution.
This change in the current density can, according to the teachings, be achieved by different resistances of the conductive bed between two cathodes in an Acheson furnace (transverse graphitization); no such solution is indica-ted for a longitudinal graphitization process.

These known methods have considerable disadvantages for industrial use. A difference of 500 °C between the desired graphitization temperatures in the middle and at the ends of the cathodes cannot be achieved by means of heat sinks alone. The required difference in heat conduction to the outside results in a considerable energy loss which significantly increases the costs of manufacture. The higher heat loss towards the furnace walls also means a higher thermal load which makes the construction of the furnace more expensive or reduces its life. Finally, an inhomogeneity in the thermally insulating bed or the conductive bed is not very practical, since the bed material would have to be introduced in a plurality of steps and have to be classified again according to its thermal or electrical conductivity after the furnace cycle is terminated and the cathodes are removed.
It is therefore an object of the present invention to provide a practical process for producing cathodes with an electrical conductivity carrying along their length.
This object is achieved according to the invention by increasing the temperature during graphitization in the middle zone compared to the temperature in the end zones by generating greater Joule heat in the middle zone.
During the investigations leading to the present invention, it was found that the introduction of a heat sink increases the specific energy consumption, i.e.
adversely affects the energy consumption. However, it is desirable to produce cathodes having the desired properties without increasing the energy consumption. The Joule heat which is generated in a body through which an electric current flows is proportional to the tatter's electrical resistance and the square of the current. In longitudinal graphitization, a constant current flows through a cathode. At a given current, more heat is therefore generated in a zone having a higher resistance.
The invention accordingly provides a process for producing graphitized cathode blocks which can be used for the electrolytic production of aluminum, which comprises using in a longitudinal graphitization process, a carbonized cathode block whose cross section at the ends of the block is greater than in the middle, and removing at least part of the graphitized material at the ends after the graphitization.
The invention is illustrated by the drawings. In -the drawings, Fig. 1 shows a side view of a cathode block which is narrowed in the middle, and Fig. 2 shows a cathode block having a stepped profile.
Fig. 1 depicts a cathode block 4 which is electrically heated by means of a direct current in a longitudinal graphitization. The current is drawn from the current source via the leads 1. For simplification only a single block is depicted in the side view, with other details being left out. The cathode block 4 has a cross section which corresponds to two symmetrical trapezoids having adjoining short sides and an additional rectangle on each of the bases. The corresponding three-dimensional body can correspond to two truncated pyramids with cuboidal base plates or preferably two truncated cones joined at the smaller flat sides and having base plates in the form of circular disks. The latter shape can be produced simply by, for example, turning on a lathe of cylindrical blank.
Fig. 2 depicts a cathode block having a stepped profile, with the cross section of the adjoining volume elements (circular disks or cuboids) decreasing monotonically to the middle from 41 to 46. The electric current required for generation of the Joule heat for graphitization is drawn via the leads 1.
If the material of the carbonized cathodes has a homogeneous composition, the electrical resistance is inversely proportional to the cross-sectional area of the individual volume elements. Appropriate choice of the number, length and cross section of the successive volume elements enables the Joule heat generated in a given volume element and thus the temperature in the graphitization to be matched precisely to the desired electrical conductivity profile in the cathode.
The graphitized material at the ends can readily be removed by machining, in particular milling off, after the finished graphitized cathodes have been taken from the furnace and cooled.
According to the invention, it is possible for the carbonized cathode block to have a stepped design corresponding to the shape depicted in fig. 2. Here, the cathode block consists of at least three zones, with the two outer zones preferably having the same cross section.
The electrical conductivity profile after graphitization substantially follows this geometric profile, with a maximum in the middle. However, it is preferred that the cross section of the carbonized cathode block increases continuously from the middle to the ends, so that the electrical conductivity likewise has a continuous profile.
Particular preference is given to an embodiment in which the cross section at the ends of the cathode block is at least 10 o greater than that in the middle.
The graphitized cathode blocks can be used in the produc-tion of aluminum by electrolytic reduction of aluminum oxide in a bath of molten cryolite and in such a process have an increased life compared to cathode blocks having a homogeneous conductivity.

Claims (6)

Claims

1. A process for producing graphitized cathode blocks which can be used for the electrolytic production of aluminum, which comprises using in a longitudinal graphitization process a carbonized cathode block whose cross section at the ends of the block is greater than in the middle and removing at least part of the graphitized material at the ends after graphitization.

2. The process as claimed in claim 1, wherein the cross section of the carbonized cathode block increases stepwise in at least one step on each side towards the end of the cathode block.

3. The process as claimed in claim 1, wherein the cross section of the carbonized cathode block increases continuously towards the ends of the cathode block.

4. The process as claimed in claim 1, wherein the cross section at the ends of the cathode block is at least 10 % greater than that in the middle.

5. The process as claimed in claim 1, wherein the graphitized material at the ends is removed by machining.

6. A graphitized cathode block for producing aluminum by electrolytic reduction of aluminum oxide in a bath of molten cryolite, which cathode block has been produced by the process as claimed in one or more of the preceding claims.