CA2862277C

CA2862277C - Method for producing a cathode block for an aluminium electrolytic cell

Info

Publication number: CA2862277C
Application number: CA2862277A
Authority: CA
Inventors: Martin Kucher; Frank Hiltmann; Janusz Tomala
Original assignee: SGL Carbon SE
Current assignee: Tokai Cobex GmbH
Priority date: 2012-02-01
Filing date: 2013-01-31
Publication date: 2016-10-25
Anticipated expiration: 2033-01-31
Also published as: CN104126032A; EP2809833B1; RU2666806C2; JP2015511273A; JP6018227B2; RU2014135212A; UA112676C2; CA2862277A1; EP2809833A1; DE102012201468A1; WO2013113837A1

Abstract

The invention relates to a method for producing a cathode block for an aluminum electrolytic cell and to a cathode block produced by means of said method.

Description

WO 2013/113837 Al Method for producing a cathode block for an aluminium electrolytic cell The present invention relates to a method for producing a cathode block for an aluminium electrolytic cell and to a cathode block produced by this method.
A known method for producing metal aluminium is the Hall-Heroult process. In this electrolytic method, the base of an electrolytic cell is typically formed by a cathode surface consisting of individual cathode blocks. The cathodes are contacted from below via steel bars, which are introduced into corresponding elongate recesses in the underside of the cathode blocks.
Cathode blocks are conventionally produced by mixing coke with carbon-containing particles such as anthracite, carbon or graphite, compressing and carbonising. This may optionally be followed by a graphitisation step at higher temperatures at which the carbon-containing particles and the coke are converted into graphite at least in part. A carbon cathode consisting at least in part of graphite is obtained.
The service life of the cathode blocks is limited by a range of factors. In particular corrosion and erosion by liquid aluminium and electrolyte, in particular cryolite, disrupt the cathode blocks from the upper face over time.
Various measures have been taken in the past to increase the wear resistance of the cathode blocks. For example, it has been attempted to increase the bulk density of the cathode blocks, and this should increase the strength and thus the wear resistance thereof.
However, only bulk densities of up to 1.68 g/cm3 can be obtained in this way in fully graphitised, non-impregnated cathode blocks, meaning that the wear resistance still remains sub-optimal.
On the other hand, carbon cathodes have been coated with titanium boride (TiB2) (disclosed in CN 1062008) or with a TiB2/carbon mixture, as disclosed for example in DE
112006004078. TiB2 can clearly improve the wetting properties of aluminium on the cathode, and additionally contributes to a higher hardness and wear resistance.
Nevertheless, the wear resistance of a TiB2 layer on a carbon cathode and on a composite layer of carbon and

2 TiB2 is still too low, and so the wear resistance of cathode blocks provided with layers of this type is also too low.
Therefore, the object of the invention is to provide a carbon-based cathode block having a high, improved wear resistance and a method for the production thereof.
The present invention therefore relates to a method for producing a cathode block, the method comprising: (a) providing a mixture of starting materials comprising coke and pitch, wherein the coke comprises two coke types which have different volume-change properties during carbonisation, graphitisation, cooling, or a combination thereof; (b) shaping the mixture into a green body; and (c) carbonising the green body to give a carbonised green body, graphitising the carbonised green body without it being previously impregnated to obtain a graphitised body, and cooling after the graphitising.
The present invention further relates to a cathode block produced by the method as defined herein, wherein the bulk density in at least one layer of the cathode block is greater than 1.68 g/cm3 up to 1.75 g/cm3 based on the carbon portion.
In this context, according to the invention the coke comprises two coke types, which have different volume-change properties during carbonisation and/or graphitisation and/or cooling.
Further, unlike in conventional methods for producing a cathode block, the carbonised green body is not impregnated prior to graphitisation, and in particular is not impregnated with pitch, tar or artificial resins. In the graphitisation step, at least a portion of carbon in the cathode block is converted into graphite.
It has surprisingly been found that the service life of the cathode blocks produced by a method according to the invention is much higher than for cathode blocks produced by conventional methods. This is all the more surprising given that, unlike in conventional methods, the carbonised green body is not impregnated prior to graphitisation to produce a cathode block. For example, in US 4,308,115, to produce a cathode, a green mixture of coke and pitch is prepared, and subsequently undergoes a shaping step to produce a green body.
Subsequently, the green body is compacted in that it is repeatedly impregnated with pitch and subsequently burnt.
Impregnated cathodes of this type are expensive to produce because of the many repeated impregnation and burning steps. In this context, the impregnation is carried out so as to compact the cathode green body, making it possible to reduce the penetration of molten aluminium into pores in the cathode and thus to increase the service life of cathodes of this type.

CA 02862277 2014-0.7-22

3 In spite of the absence according to the invention of this impregnation step, penetration of molten aluminium into pores in the cathode is presumably clearly reduced because of the use according to the invention of two coke types, which have different volume-change properties during carbonisation and/or graphitisation and/or cooling, and the service life of the cathodes manufactured by the method according to the invention is thus increased.
It may be advantageous to machine the graphitised bodies mechanically to obtain the cathode block.
Preferably, a cathode block produced by the method according to the invention has a bulk density of a carbon portion of over 1.68 g/cm3, particularly preferably over 1.71 g/cm3, in particular up to 1.75 g/cm3.
Presumably, a higher bulk density advantageously contributes to a longer service life. On the one hand, this may be because there is more mass per unit volume of a cathode block, and this leads to a higher residual mass after a given erosion duration at a given mass erosion per unit time. On the other hand, presumably a higher bulk density together with a corresponding lower porosity prevents infiltration of electrolyte, which acts as a corrosive medium.
Advantageously, the two coke types include a first coke type and a second coke type, the first coke type exhibiting greater contraction and/or expansion than the second coke type during carbonisation and/or graphitisation and/or cooling. In this context, the greater contraction and/or expansion is an advantageous development of different volume-change properties, which are presumably particularly suitable for leading to stronger compaction than if coke sorts having the same contraction and/or expansion are mixed. In this context, the greater contraction and/or expansion relates to any desired temperature range. Thus there may for example merely be greater contraction of the first coke during carbonisation.
On the other hand, there may for example additionally or alternatively be greater expansion in a transition region between carbonisation and graphitisation. Additionally or alternatively, there may be different volume-change properties during cooling.
Preferably, the contraction and/or expansion of the first coke type during carbonisation and/or graphitisation and/or cooling is at least 10 % greater than that of the second coke type in terms of volume, in particular at least 25 % greater, in particular at least 50 %

4 greater. Thus for example in the case of a 10 % greater contraction of the first coke type, the contraction from room temperature to 2000 C may be 1.0 % by volume for the second coke type but 1.1 % by volume for the first coke type.
Advantageously, the contraction and/or expansion of the first coke type during carbonisation and/or graphitisation and/or cooling is at least 100 % greater than that of the second coke type in terms of volume, in particular at least 200 % higher, in particular at least 300 %
higher. Thus for example in the case of a 300 % greater expansion of the first coke type, the expansion from room temperature to 1000 C may be 1.0 % by volume for the second coke type but 4.0 % by volume for the first coke type.
The method according to the invention also includes the case where the first coke type undergoes contraction but by contrast the second coke type undergoes expansion in the same temperature interval. For example, a 300 % greater contraction and/or expansion thus also includes the case where the second coke type contracts by 1.0 % by volume but the first coke type expands by 2.0 % by volume.
Alternatively, in at least a desired temperature interval of the method according to the invention, the second coke type, rather than the first coke type, may have a greater contraction and/or expansion, as described above for the first coke type.
Preferably, at least one of the two coke types is a petroleum- or coal tar pitch coke.
Preferably, the proportion of the second coke type in the total amount of coke in percent by weight is between 50 % and 90 %, in particular between 50 and 80 %. In these ranges, the different volume-change properties of the first and second coke types presumably have a particularly good effect on the compaction during carbonisation and/or graphitisation and/or cooling. Conceivable ranges for the second coke type may be from 50 to 60 %, but also from 60 to 80% and from 80 to 90 %.
Advantageously, at least one further carbon-containing material and/or additives and/or pulverulent hard material are added to the coke. This may be advantageous both for the processability of the coke and for the subsequent properties of the produced cathode block.

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Preferably, the further carbon-containing material contains graphite-containing material; in particular, the further carbon-containing material consists of graphite-containing material, such as graphite. The graphite may be synthetic and/or natural graphite.
Further carbon-containing material of this type ensures that the required contraction of the cathode mass, which is dominated by the coke, is reduced.
Preferably, the further carbon-containing material is present at 1 to 40 % by weight, in particular 5 to 30 % by weight, based on the total amount of coke and further carbon-containing material.
Preferably, pitch can be added in amounts of 5 to 40 % by weight, in particular 15 to 30 % by weight (based on the weight of the total green mixture). Pitch acts as a binder and serves to create a dimensionally stable body during carbonisation.
Advantageous additives may be oil, such as compression oil, or stearic acid.
These facilitate the mixing of the coke and if applicable of the further components.
In particular TiB2 powder is used as the pulverulent hard material. The use of a hard material of this type increases the wettability of the cathode in relation to the aluminium melt. The proportion of this hard material in the mixture of starting materials is between 15 % by weight and 60 % by weight, in particular between 20 A by weight and 50 % by weight.
Advantageously, the cathode block is produced as a multi-layer block, a first layer containing coke and optionally a further carbon-containing material as starting materials, and a second layer containing coke and a fireproof hard material, in particular TiB2, and optionally a further carbon-containing material as starting materials. Hard material is also known as RHM
(refractory hard material). The further carbon-containing material may be present as described above for a monolithic cathode block. This variant of a multi-layer block combines the advantages of a multi-layer block, in which the layer facing the aluminium melt contains a hard material, with the use of two coke types having different volume-change properties.
Since the second layer always has a high bulk density of for example over 1.82 g/cm3 after graphitisation because of the addition of high-temperature-resistant hard material, it is advantageous for the first layer also to have a high bulk density of advantageously over 1.68 g/cm3 after graphitisation. The small differences in the thermal expansion properties and bulk densities during the thermal treatment steps reduce the production times and rejection rates of the cathode blocks, since large differences in the layers during temperature treatment can lead to thermal stresses. Further, the resistance to thermal stresses and damage resulting therefrom during use is therefore also advantageously increased.
Preferably, the coke of the first and/or second layer comprises two coke types, which have different volume-change properties during carbonisation and/or graphitisation and/or cooling whilst leading to a bulk density of over 1.70 g/cms of the resulting graphite.
Further, preferably at least one of the two layers is produced with a bulk density of over 1.68 g/cm3 of a carbon portion. Thus, depending on what is desired and/or required, one or both layers may be produced according to the invention using two different coke types. This results in the possibility of setting bulk densities and bulk density ratios as required or desired. For example, merely the first layer may be produced according to the invention using two coke types, whilst the second layer is produced with merely one coke type but additionally contains TiB2 as a ceramic hard material.
It may potentially be advantageous for the multi-layer block to comprise more than two layers. In this case, any desired number of the more than two layers may be produced according to the invention using two coke types having different volume-change properties in each case.
Advantageously, the second layer may be of a height which is 10 to 50 %, in particular 15 to 45 %, of the total height of the cathode block. A small height of the second layer, such as 20 %, may be advantageous, since a small amount of cost-intensive hard ceramic material is required. Alternatively, a large height of the second layer, such as 40 %, may be advantageous, since a layer which has hard ceramic material has a high wear resistance.
The greater the height of this highly wear-resistant material in relation to the total height of the cathode block, the greater the wear resistance of the cathode block as a whole.
It may be advantageous for the hard material to be in a monomodal particle size distribution, the average particle size d50 of the distribution being between 10 and 20 pm, in particular between 12 and 18 pm, in particular between 14 and 16 pm.

The cis() value indicates the average particle size, and in this case 50 % of particles are smaller than the indicated value. Accordingly, dlo or d90 indicates the average particle size when 10 or 90 % of the particles are smaller than the indicated value.
Surprisingly, it has been found in the context of the invention that for a d50 of this type, although on the one hand the hard material powder has a large active surface, which leads to very.good wettability of the cathode block after graphitisation, on the other hand it does not have the drawbacks which negatively influence the processing of the hard material powder as a composite component in a graphite/hard material composite. These potential drawbacks, which the hard material powder used according to the invention does not have, are:
- a tendency to form dust, for example when added to a mixing container or during transportation of the powder, .- agglomerate formation, in particular during mixing, for example wet mixing with coke (in this context, wet mixing means in particular mixing with pitch as the liquid phase), - demixing as a result of the different material densities of the hard material and the coke.
Aside from the elimination of these drawbacks, the hard material powder used according to the invention has particularly good flowability and pourability. This makes the hard material powder particularly easy to convey using conventional conveying devices, for example to a mixing apparatus.
The good processability of the hard material powder having the d50 of between 10 and 20 pm and a monomodal particle size distribution greatly simplifies the production of hard material powder composites for cathode blocks. The cathode blocks obtained have a very good homogeneity in relation to the distribution of the hard material powder in the coke in the green body and in the graphite in the graphitised cathode body.
Preferably, the cis() of the fire-resistant hard material is between 20 and 40 pm, in particular between 25 and 30 pm. This advantageously makes the wetting and processing properties of the hard material powder even better.

Advantageously, the du) of the fire-resistant material is between 2 and 7 pm, in particular between 3 and 5 pm. This advantageously makes the wetting and processing properties of the hard material powder even better.
Further, it is possible to characterise the monomodal particle size distribution by describing its distribution width using what is known as the span value, which is calculated as follows:
span = (do ¨ d10)/c150 Advantageously, the span of the fire-resistant hard material powder is between 0.65 and 3.80, in particular between 1.00 and 2.25. This advantageously makes the wetting and processing properties of the hard material powder even better.
Advantageously, the graphitisation step is carried out at temperatures of between 2550 and 3000 C, in particular between 2600 and 2900 C.
Temperatures below 2900 C have been found to be particularly advantageous, since conventional TiB2 does not melt at less than 2900 C. Presumably melting does not result in a chemical change in the TiB2, since even after melting and subsequent cooling TiB2 can be found in a cathode block by X-ray diffractometry. However, melting can cause finely distributed TiB2 particles to agglomerate to form larger particles. There is also some risk that liquid TiB2 may move in an uncontrolled manner as a result of open porosity.
In the temperature range according to the invention, the graphitisation process has progressed sufficiently far to result in high thermal and electrical conductivity of the carbon-containing material.
Preferably, the graphitising step is carried out using an average heating rate of between 90 K/h and 200 K/h. Alternatively or additionally, the graphitisation temperature is maintained for a period of between 0 and 1 h. At these heating rates or this heating duration, particular good results are achieved as regards graphitisation and obtaining the hard material.
Advantageously, the duration of the thermal treatment until the time when cooling starts may be 10 to 28 hours.

The invention is further achieved by a cathode block according to claim 15.
The cathode block is advantageously produced by a method according to the invention.
According to the invention, the bulk density is greater than 1.68 g/cm3, in particular greater than 1.70 g/cm3, in particular at least greater than 1.71 g/cm3, in particular up to 1.75 g/cm3.
In this context, the bulk density is based on the layer as a whole when no fire-resistant hard material is added, in other words on the pure carbon portion. In the event that the layer contains hard ceramic material such as TiB2, the bulk density is a calculated bulk density of the layer excluding the portion of fire-resistant hard material.
Further advantageous configurations and developments of the invention are explained in the following by way of a preferred embodiment and the drawings, in which:
Fig. 1 is a dilatometer measurement curve as a function of the temperature for a first and second coke type for the method according to the invention, and Fig. 2 is a schematic drawing of the shaping of a cathode block according to the invention in the form of a multi-layer block.
To produce a cathode block according to the invention, a first and a second coke are ground separately from one another, separated into particle-size fractions and mixed together along with pitch. The proportion by weight of the first coke in the overall amount of coke may be for example 10 to 20 % by weight or 40 to 45 % by weight. A cathode block can be produced from the green mixture by extrusion. Alternatively, the mixture may be added for example to a mould, which broadly corresponds to the subsequent shape of the cathode block, and vibration-compacted or block-pressed. The resulting green body is heated to a final temperature in the range of 2550 to 3000 C ¨ a carbonisation step and subsequently a graphitisation step taking place, without impregnation, for example with pitch, tar or synthetic resin, taking place in between ¨ and subsequently cooled. The resulting cathode block has a bulk density of 1.71 g/cm3 and a very high wear resistance against liquid aluminium and cryolite.
Fig. 1 shows a dilatometer measurement curve for the first coke type (as a dashed line) during the graphitisation process. Fig. 1 further shows a corresponding measurement curve (as a solid line) for the second coke type. It can be seen that the two coke types have different volume-change properties.

From a zero line at the start of the temperature programme to a temperature of 2800 C, the first coke of Fig. 1 initially exhibits an expansion, a rise in volume being observed until approximately 1200 C, and a temporary reduction in volume occurring after approximately 1400 C. Subsequently, until approximately 2100 C, an increase in maximum volume by comparison with the initial volume can be seen.
In the dilatometer measurement for the second coke, a progression which is similar in principle to that of the first coke can be observed, but the curve rises more steeply overall.
Accordingly, at approximately 2100 C, an increase in maximum volume can also be seen for the second coke, but is much smaller than for the first coke.
Only during the subsequent cooling does contraction occur in both coke types, and it is greater for the second coke type than for the first.
Alternatively, two coke types are used, the first of which already exhibits contraction during the heating phase in the carbonisation and/or graphitisation step. The second of the two coke types has a much greater contraction (based on the contraction after carbonisation, graphitisation and cooling by comparison with the initial volume) than the other coke type.
In a further variant of the embodiment, graphite powder or carbon particles are added to the coke mixture.
In a further variant of the embodiment, a mould 1 is initially filled in part with a mixture 2 of the two coke types, graphite and TiB2, and vibration-compacted, as shown in Fig. 2a.
Subsequently, a mixture 5 of the two coke types and graphite is filled onto the resulting starting layer 4, which forms the upper layer facing the anode in the subsequent cathode and will thus be in direct contact with the aluminium melt, and likewise compacted (see Fig. 2b).
The resulting upper starting layer 6 forms the lower layer remote from the anode in the subsequent cathode. This two-layer block is carbonised and graphitised as in the first embodiment.
All of the features mentioned in the description, examples and claims may contribute to the invention in any desired combination. The invention is not limited to the examples given, but can also be configured in modified forms which are not specifically disclosed heroin. In particular, different volume-change properties also include types of properties other than contraction properties. For example, at least in portions of the heating and cooling cycle, a volume increase may be advantageous for compaction of the cathodes. In this way, two coke types which ultimately exhibit the same contraction after carbonisation, graphitisation and cooling, but which exhibit a different contraction or volume increase at an intermediate temperature, may be included in the invention.
As well as coke types from different manufacturers, different coke types may also include cokes from the same manufacturer but with different pre-treatment, such as differently calcined cokes.

Claims

CLAIMS:

1. A method for producing a cathode block, the method comprising:
(a) providing a mixture of starting materials comprising coke and pitch, wherein the coke comprises two coke types which have different volume-change properties during carbonisation, graphitisation, cooling, or a combination thereof;
(b) shaping the mixture into a green body; and (c) carbonising the green body to give a carbonised green body, graphitising the carbonised green body without it being previously impregnated to obtain a graphitised body, and cooling after the graphitising.

2. The method according to claim 1, wherein the cathode block has a bulk density of a carbon portion of over 1.68 g/cm3.

3. The method according to claim 2, wherein the bulk density of the carbon portion is over 1.71 g/cm3.

4. The method according to claim 2 or 3, wherein the bulk density of the carbon portion is up to 1.75 g/cm3.

5. The method according to any one of claims 1 to 4, wherein the two coke types comprise a first coke type and a second coke type, and wherein the first coke type exhibits greater contraction, expansion, or a combination thereof than the second coke type during the carbonisation, the graphitisation, the cooling or the combination thereof.

6. The method according to claim 5, wherein the contraction, the expansion, or the combination thereof of the first coke type during the carbonisation, the graphitisation, the cooling or the combination thereof is at least 10% greater than that of the second coke type in terms of volume.

7. The method according to claim 6, wherein the contraction, the expansion, or the combination thereof of the first coke type during the carbonisation, the graphitisation, the cooling or the combination thereof is at least 25% greater than that of the second coke type in terms of volume.

8. The method according to claim 7, wherein the contraction, the expansion, or the combination thereof of the first coke type during the carbonisation, the graphitisation, the cooling or the combination thereof is at least 50% greater than that of the second coke type in terms of volume.

9. The method according to any one of claims 5 to 8, wherein the proportion of the second coke type in the total amount of the coke is between 50 and 90 wt%.

10. The method according to any one of claims 1 to 9, further comprising adding a carbon-containing material, an additive, a pulverulent hard material or a combination thereof, to the coke.

11. The method according to claim 10, wherein the hard material is TiB2.

12. The method according to claim 10 or 11, wherein the hard material is present in the mixture of the starting materials in a proportion of between 15 and 60 wt%.

13. The method according to claim 12, wherein the hard material is present in the mixture of the starting materials in a proportion of between 20 and 50 wt%.

14. The method according to any one of claims 1 to 13, wherein the cathode block is produced as a multi-layer block having a first layer and a second layer, wherein the first layer contains coke and optionally a further carbon-containing material as starting materials, and wherein the second layer contains coke, a fire-resistant hard material and optionally a further carbon-containing material as starting materials.

15. The method of claim 14, wherein the fire-resistant hard material is TiB2.

16. The method according to claim 13 or 14, wherein the coke of the first layer, the second layer or the combination thereof, comprises two coke types, which have different volume-change properties during the carbonisation, the graphitisation, the cooling or the combination thereof, whilst leading to a bulk density of over 1.70 g/cm3 of the resulting graphite.

17. The method according to any one of claims 14 to 16, wherein the second layer is of a height which is 10 to 50% of the total height of the cathode block.

18. The method according to claim 17, wherein the second layer is of a height which is 15 to 45% of the total height of the cathode block.

19. The method according to any one of claims 11 to 17, wherein the hard material is in a monomodal particle size distribution with d50 between 10 and 20 µm.

20. The method according to claim 19, wherein the d50 is between 12 and 18 µm.

21. The method according to claim 20, wherein the d50 is between 14 and 16 µm.

22. The method according to any one of claims 14 to 21, wherein the d90 of the fire-resistant hard material is between 20 and 40 µm.

23. The method according to claim 22, wherein the d90 of the fire-resistant hard material is between 25 and 30 µm.

24. The method according to any one of claims 14 to 23, wherein the d10 of the fire-resistant hard material is between 2 and 7 µm.

25. The method according to claim 22, wherein the d10 of the fire-resistant hard material is between 3 and 5 µm.

26. The method according to any one of claims 1 to 25, wherein the graphitisation is carried out at a temperature between 2550 and 3000 C.

27. The method according to claim 26, wherein the graphitisation is carried out at a temperature between 2600 and 2900 C.

28. A cathode block produced by the method according to any one of claims 1 to 27, wherein the bulk density in at least one layer of the cathode block is greater than 1.68 g/cm3 up to 1.75 g/cm3 based on the carbon portion.

29. The method according to claim 28, wherein the bulk density in the at least one layer of the cathode block is greater than 1.70 g/cm3 based on the carbon portion.

30. The method according to claim 29, wherein the bulk density in the at least one layer of the cathode block is greater than 1.71 g/cm3 based on the carbon portion.

31. The method according to any one of claims 28-30, wherein the bulk density in the at least one layer of the cathode block is up to 1.75 g/cm3 based on the carbon portion.