EP2598675B1 - Cathode block for an aluminium electrolysis cell and a process for the production thereof - Google Patents

Description

The present invention relates to a cathode block for an aluminum electrolytic cell and a method for its production.

One known method of producing metallic aluminum is the Hall-Heroult process. In this electrolytic process, the bottom of an electrolytic cell is typically formed by a cathode surface consisting of individual cathode blocks. From below, the cathodes are contacted via steel ingots, which are placed in corresponding elongated recesses in the bottom of the cathode blocks.

Cathode blocks are conventionally made by mixing coke with carbonaceous particles such as anthracite, carbon or graphite, compacting and carbonizing. Optionally, a graphitizing step at higher temperatures follows, at which the carbonaceous particles and the coke are at least partially converted to graphite.

Graphitized carbon and graphite, however, are poorly or not wetted by liquid aluminum. This increases the power consumption and thus also the energy consumption of an electrolysis cell.

In order to solve this problem, in the prior art TiB _{2 is introduced} into an upper layer of a cathode block. This is for example in the DE 112006004078 T described. Such a top layer, which is a TiB ₂ graphite composite, is in direct contact with the aluminum melt and thus crucial for the current injection from the cathode into the molten aluminum. TiB ₂ and similar hard materials cause an improvement in the wettability of the cathode in the graphitized state and thus better energy efficiency of the electrolysis process. Hard materials can also increase the bulk density and hardness of Cathodes increase, which has a better wear resistance, especially compared to aluminum and Kryolitschmelzen result.

However, TiB ₂ powders and similar hard material powders (also called refractory hard material (RHM)) are difficult to process. In addition, cathode blocks made with them, which form a TiB ₂ graphite composite layer completely or in their upper layer, tend to be inhomogeneities.

The object of the present invention is therefore to provide a TiB ₂ graphite composite cathode which is readily wettable to aluminum melts, has good wear properties and is easy to produce, and a process for their preparation.

The object is achieved by a cathode block according to claim 1.

A cathode block for an aluminum electrolytic cell according to the invention, which comprises a composite layer containing graphite and a hard material such as TiB ₂ , is characterized in that the hard material is in a monomodal particle size distribution, the mean particle size of the distribution d _{50 being} between 10 and 20 microns, in particular between 12 and 18 microns, in particular between 14 and 16 microns.

• Staubneigung, beispielsweise beim Einfüllen in einen Mischbehälter oder beim Transport des Pulvers,
• Agglomeratbildung, insbesondere beim Mischen, wie etwa Nassmischen mit Koks (Nassmischen bedeutet in diesem Zusammenhang insbesondere Mischen mit Pech als flüssiger Phase),
• Entmischung aufgrund unterschiedlicher Materialdichten von Hartmaterial und Koks.

Surprisingly, it has been found within the scope of the invention that in such a d _50, the hard material powder on the one hand has a large active surface, which causes a very good wettability of the cathode block after graphitization, but on the other hand does not have the disadvantages, the processing of the hard material powder as Adversely affect composite component in a graphite hard material composite. These possible disadvantages, which the hard material powder used according to the invention does not have, are:

• Dusting, for example when filling into a mixing container or during transport of the powder,
• Agglomerate formation, in particular during mixing, such as wet mixing with coke (wet mixing means in this context in particular mixing with pitch as the liquid phase),
• Demixing due to different material densities of hard material and coke.

Apart from eliminating these disadvantages, the hard material powder used according to the invention has a particularly good flowability or flowability. This makes the hard material powder particularly well with conventional conveyors, for example, conveyed to a mixing apparatus.

Due to the good processability of the hard material powder with the d ₅₀ between 10 and 20 microns and a monomodal particle size distribution, the production of Hartmaterialpulverkompositen for cathode blocks is greatly simplified. The obtained cathode blocks have a very good homogeneity with respect to the distribution of the hard material powder in the coke in the green body and in the graphite in the graphitized cathode body.

The d _{90 of} the refractory hard material is preferably between 20 and 40 μm, in particular between 25 and 30 μm. This has the advantageous consequence that wetting and processing properties of the hard material powder are even better.

Advantageously, the d _{10 of} the refractory hard material is between 2 and 7 microns, in particular between 3 and 5 microns. This has the advantageous consequence that wetting and processing properties of the hard material powder are even better.

Furthermore, to characterize the monomodal particle size distribution, its distribution width can be described by the so-called span value, which is calculated as follows: $$\mathrm{chip}=\left({\mathrm{d}}_{\mathrm{90}}-{\mathrm{<figure-callout\; id="10"\; label="d"\; filenames="imgf0001.png"\; state="\{\{state\}\}">d</figure-callout>}}_{\mathrm{10}}\right)/{\mathrm{<figure-callout\; id="50"\; label="d"\; filenames="imgf0001.png"\; state="\{\{state\}\}">d</figure-callout>}}_{\mathrm{50}}$$

Span of the refractory hard material powder is advantageously between 0.65 and 3.80, in particular between 1.00 and 2.25. This has the advantageous consequence that wetting and processing properties of the hard material powder are even better.

It can be advantageously provided that the composite layer forms the entire cathode block. This has the advantage that for the preparation of the cathode block a only green composition is necessary and accordingly only a single mixing step.

Alternatively, it may be advantageous for the cathode block to have at least two layers, wherein the composite layer forms the upper layer of the cathode block. This top layer is in use of the cathode block according to the invention in direct contact with the melt of the electrolysis cell.

The cathode block preferably has at least one further layer which has less hard material powder than the upper layer or no hard material powder. This can reduce the amount of expensive hard material powder used. When using the cathode in an aluminum electrolysis cell, the further layer is not in direct contact with the aluminum melt and therefore does not have to have good wettability and wear resistance.

Advantageously, the top layer may have 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 topsheet, such as 20%, may be advantageous because a small amount of expensive hard material is needed.

Alternatively, a high height of the topsheet such as 40% may be advantageous because a layer having hard material has high wear resistance. The greater the height of this highly wear-resistant material relative to the overall height of the cathode block, the higher the wear resistance of the entire cathode block.

Preferably, a cathode block according to the invention is prepared by a method comprising the steps of providing starting materials comprising coke, a hard material such as TiB ₂ , and optionally another carbonaceous material, forming the cathode block, carbonizing and graphitizing, and cooling. According to the invention, the coke comprises two types of coke, which have a different volume change behavior during carbonation and / or graphitization and / or cooling.

In the graphitizing step, at least a portion of carbon in the cathode block is converted to graphite.

Surprisingly, it has been shown that the lifetime of the cathode blocks produced by such a method is significantly higher than in the case of the cathode blocks produced by conventional methods.

Preferably, a cathode block produced by a method according to the invention has a bulk density of a carbon content of more than 1.68 g / cm ³ , particularly preferably more than 1.71 g / cm ³ , in particular up to 1.75 g / cm ³ .

Presumably, a higher apparent density advantageously contributes to a longer service life. This may be due to the fact that more mass is present per unit volume of a cathode block, resulting in a given mass removal per unit time to a higher residual mass after a given removal period. On the other hand, it can be assumed that a higher bulk density with a corresponding corresponding lower porosity hampers an infiltration of electrolyte, which acts as a corrosive medium.

The second layer may have a bulk density of more than 1.80 g / cm ³ , for example, because of the addition of RHM after graphitization.

Advantageously, the two types of coke comprise a first type of coke and a second type of coke, the first type of coke having a greater shrinkage and / or expansion during carbonation and / or graphitization and / or cooling than the second type of coke. Here, the increased shrinkage and / or expansion is an advantageous embodiment of a different volume change behavior, which is probably particularly well suited to lead to a greater compression than when coke are mixed, which have an equal shrinkage and / or expansion. The stronger shrinkage and / or expansion refers to any temperature range. Thus, for example, only a stronger shrinkage of the first coke during carbonization can be present. On the other hand, for example, in addition or instead, a greater extent in a transition region between carbonization and graphitization available. Instead or in addition, a different volume change behavior may be present during cooling.

Preferably, the shrinkage and / or expansion of the first type of coke during carbonation and / or graphitization and / or cooling based on the volume is at least 10% higher than that of the second coke, in particular at least 25% higher, in particular at least 50% higher. Thus, for example, in the case of a 10% higher shrinkage of the first type of coke, the shrinkage from room temperature to 2000 ° C for the second type of coke 1.0 vol .-%, in the first coke variety, however, 1.1 vol .-%.

Advantageously, the shrinkage and / or expansion of the first type of coke during carbonation and / or graphitization and / or cooling based on the volume at least 100% higher than that of the second coke, in particular at least 200% higher, in particular at least 300% higher. Thus, for example, in the case of a 300% higher expansion of the first type of coke, the expansion from room temperature to 1000 ° C is 1.0% by volume for the second type of coke, and 4.0% by volume for the first type of coke.

The case that the first type of coke undergoes shrinkage, the second coke variety, however, an expansion in the same temperature interval, is detected by the inventive method. For example, a 300% higher shrinkage and / or expansion also includes the case that the second type of coke shrinks by 1.0% by volume, whereas the first type of coke expands by 2.0% by volume.

Alternatively, in at least one arbitrary temperature interval of the method according to the invention, instead of the first type of coke, the second type of coke may have a greater shrinkage and / or expansion, as described above for the first coke variety.

Preferably, a cathode block according to the invention is produced by a process comprising the steps of providing starting materials comprising coke, forming the cathode block, carbonizing and graphitizing, and cooling. In this case, the coke preferably comprises two types of coke, with a different Volume change behavior during carbonization and / or graphitization and / or cooling lead to a densification of the cathode block of over 1.68 g / cm ³ . Presumably, different volume change behaviors of the two types of coke result in a compaction process during carbonization and / or graphitization and / or cooling that can prevent interlocking or otherwise blocking of individual coke particles due to similar shrinkage properties. As a result, individual particles can presumably migrate to positions which are more favorable for compaction, and thus a higher packing density of the coke particles or the particles resulting therefrom in the further process than in conventional production processes can be achieved.

With this variant, the advantages of a multi-layer block in which the layer facing the anode comprises a hard material are combined with the use of two types of coke with different volume change behavior. The small differences in thermal expansion behavior during the heat treatment steps reduce production times and reject rates of the cathode blocks. Furthermore, therefore, advantageously the resistance to thermal stresses and resulting damage in the application is also increased.

At least one of the two types of coke is preferably a petroleum or coal tar coke.

Preferably, the weight percent of the second coke variety in the total amount of coke is between 50% and 90%. In these quantitative ranges, the different volume change behavior of the first and second types of coke has a particularly good effect on compression during carbonization and / or graphitization and / or cooling. Conceivable quantity ranges of the second type of coke can be 50 to 60%, but also 60 to 80%, and 80 to 90%.

Advantageously, at least one carbonaceous material and / or pitch and / or additives are added to the coke. This can be both in terms of processability of the coke as well as the later properties of the produced cathode block.

Preferably, the further carbonaceous material contains graphite-containing material; In particular, the further carbonaceous material is graphite-containing material, such as graphite. The graphite may be synthetic and / or natural graphite. By such further carbonaceous material is achieved that the necessary shrinkage of the cathode mass, which is dominated by the coke, is reduced.

The carbonaceous material is advantageously 1 to 40% by weight, in particular from 5 to 30% by weight, based on the total amount of coke and carbonaceous material.

Preferably, in addition to the amount of coke and optionally carbonaceous material, which represents a total of 100 wt .-%, pitch in amounts of 5 to 40 wt .-%, in particular 15 to 30 wt .-% (based on 100 wt .-% the entire green mix). Pitch acts as a binder and serves to create a dimensionally stable body during carbonation.

Advantageous additives may be oil, such as press liquor oil, or stearic acid. These facilitate mixing of the coke and optionally the other components.

Preferably, the coke comprises at least in one of the two layers, ie in the first and / or the second layer, two types of coke with a different volume change behavior during carbonization and / or graphitization and / or cooling to a densification of the resulting graphite of more than 1 , 68 g / cm ³ lead. Depending on desire and / or requirement, both layers or one of the two layers can thus be produced according to the invention with two different types of coke. Thus, there is the possibility to adjust gross densities and raw density ratios, as necessary or desired. For example, only the first layer can be produced according to the invention with two types of coke, while the second layer is produced with only one type of coke, but additionally contains TiB ₂ as hard material. As a result, the bulk densities and / or Expansion behavior of the two layers aligned, which can advantageously increase the resistance of the layer compound.

Further advantageous embodiments and developments of the invention will be explained below with reference to a preferred embodiment and the figure.

The only one shows FIG. 1 a particle size distribution of a TiB ₂ powder used according to the invention: a) as a volume density distribution q 3 and b) as a volume sum distribution Q 3.

To produce a cathode block according to the invention, coke is mixed with pitch, mixed with TiB ₂ powder having a monomodal particle size distribution and a d ₅₀ of 15 μm, a d ₉₀ of 30 μm and a d ₁₀ of 5 μm. The span value for this particle size distribution is 1.67. The weight fraction of TiB ₂ powder on the green mass is for example 10 to 30 wt .-%, such as 20 wt .-%. The mixture is filled in a mold, which largely corresponds to the later form of the cathode blocks, and vibration-compressed or block-pressed. The resulting green body is heated to a final temperature in a range of 2300 to 3000 ° C, in particular 2500 to 2900 ° C, such as 2800 ° C, wherein a carbonation step and then a graphitization occur, and then cooled. The resulting cathode block has a very good wetting behavior and a very high resistance to wear compared to liquid aluminum and cryolite.

Alternatively, instead of a single coke, two coke varieties with different volume change behavior are used. This different volume change behavior of the two cokes leads to a high density of the graphite in the composite and thus to an even higher wear resistance of the cathode blocks obtained than by the TiB ₂ powder alone.

In a further variant, the mold is initially partially filled with a mixture of coke, graphite and TiB ₂ and, if necessary, vibrationally precompressed. Subsequently, reference is made to the resulting starting layer, which at the later cathode represents the upper layer facing the anode and thus making direct contact with the molten aluminum will have a mixture of coke and graphite filled and in turn compacted. The resulting upper starting layer at the later cathode represents the lower layer facing away from the anode. This two-layer brick is carbonized and graphitized as in the first embodiment.

In another alternative, two types of coke with different volume change behavior are used as coke of the lower layer. The wear resistance of the thus obtained cathode block to aluminum is particularly high. This is attributed to the lower density difference between the upper and lower cathode layers than in conventional TiB ₂ composite blocks.

The invention is not limited to the examples given, but can also be carried out in modifications which are not specifically described here. In particular, other hard material powders other than TiB _{2 are} included, such as ZrB ₂ , HfB ₂ , or other transition metal borides.

Claims (15)

1. Cathode block for an aluminium electrolytic cell, comprising a composite layer containing graphite and a hard material, such as TiB₂, characterised in that the hard material is in a monomodal particle size distribution, d₅₀ being between 10 and 20 µm, in particular between 12 and 18 µm, in particular between 14 and 16 µm
2. Cathode block according to claim 1, characterised in that d₉₀ of the hard material is between 20 and 40 µm, in particular between 25 and 30 µm
3. Cathode block according to either claim 1 or claim 2, characterised in that d₁₀ of the hard material is between 2 and 7 µm, in particular between 3 and 5 µm.
4. Cathode block according to one or more of claims 1 to 3, characterised in that the span = (d₉₀ - d₁₀)/d₅₀ of the particle size distribution of the hard material powder is between 0.65 and 3.80, in particular between 1.00 and 2.25.
5. Cathode block according to one or more of claims 1 to 4, characterised in that the composite layer forms the entire cathode block
6. Cathode block according to one or more of claims 1 to 5, characterised in that the cathode block comprises at least two layers, the composite layer forming the upper layer of the cathode block.
7. Cathode block according to claim 6, characterised in that the cathode block has at least one additional layer which comprises less hard material powder than the upper layer, or which comprises no hard material powder.
8. Cathode block according to either claim 6 or claim 7, characterised in that the upper layer is of a thickness which is 10 to 50 %, in particular 15 to 45 %, of the total thickness of the cathode block.
9. Cathode block according to one or more of claims 1 to 8, characterised in that the bulk density in at least one layer of the cathode block is greater than 1.68 g/cm³ based on the proportion of carbon.
10. Cathode block according to claim 9, characterised in that the bulk density is greater than 1.71 g/cm³.
11. Method for producing a cathode block, in particular a cathode block according to one or more of claims 1 to 10, comprising the steps of providing starting materials, including coke and optionally another carbon-containing material, and hard material powder, such as TiB₂ powder, mixing the starting materials, moulding the cathode block, carbonising and graphitising, and cooling, characterised in that hard material powder is used which has a monomodal particle size distribution and a d₅₀ of between 10 and 20 µm, in particular between 12 and 18 µm, in particular between 14 and 16 µm
12. Method according to claim 11, characterised in that a hard material powder is used which has a d₉₀ of between 20 and 40 µm, in particular between 25 and 30 µm.
13. Method according to either claim 11 or claim 12, characterised in that a hard material powder is used which has a d₁₀ of between 2 and 7 µm, in particular between 3 and 5 µm.
14. Method according to one or more of claims 11 to 13, characterised in that a hard material powder is used, the particle size distribution of which has a span = (d₉₀ - d₁₀)/d₅₀ of between 0.65 and 3.80, in particular between 1.00 and 2.25.
15. Method according to claim 14, characterised in that a cathode block having a bulk density of a proportion of carbon of over 1.68 g/cm³, in particular over 1.71 g/cm³, is obtained.

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