EP2598674B1 - Process for producing a cathode block for an aluminium electrolysis cell - Google Patents

Description

The present invention relates to a method of manufacturing a cathode block for an aluminum electrolytic cell. One known method of producing metallic aluminum is the Hall-Heroult process. In this electrolytic process, typically, the bottom of an electro-lyse cell is 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 graphitization step follows at higher temperatures where the carbonaceous particles and coke are at least partially converted to graphite.

By graphitization, the thermal conductivity of the cathode material is greatly increased and the specific electrical resistance is greatly reduced. 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 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 ceramic materials cause an improvement in the wettability of the cathode in the graphitized state and thus a better energy efficiency of the electrolysis process. Ceramic hard materials can also increase the density and increase the hardness of cathodes, which results in better wear resistance, in particular compared with aluminum and cryolite melts. Hard materials are also referred to as RHM (refractory hard material).

However, TiB ₂ powders and similar hard material powders lose some of their wettability and wear resistance during a graphitization process. Other known from the prior art method for producing a cathode block are in the publications US Pat. No. 4,308,115 . US 4,376,029 A and CN 101 158 048 A described. The object of the present invention is therefore to provide a simple process for producing a TiB ₂ graphite composite cathode, which is readily wettable to aluminum melts and has good wear properties, and a corresponding cathode block.

• Verfahren zur Herstellung eines Kathodenblocks als Mehrfachschichtblock, aufweisend die Schritte Bereitstellen von Ausgangsmaterialien, umfassend Koks und ein Hartmaterialpulver, wie etwa TiB₂, sowie gegebenenfalls ein kohlenstoffhaltiges Material, wobei eine erste Schicht als Ausgangsmaterial Koks enthält und eine zweite Schicht als Ausgangsmaterial Koks und ein Hartmaterial, insbesondere TiB₂, enthält, Mischen der Ausgangsmaterialien, Formen eines Kathodenblocks, Carbonisieren und Graphitieren sowie Abkühlen, wobei der Schritt des Graphitierens bei Temperaturen zwischen 2300 und 3000 °C, insbesondere zwischen 2400 und 2900 °C durchgeführt wird, und wobei die zweite Schicht mit einer Dicke hergestellt wird, die 10 bis 50 %, insbesondere 15 bis 45 % der Gesamtdicke des Kathodenblocks beträgt.

The object is achieved by a method according to claim 1. The method according to the invention has the following features:

• A method of manufacturing a cathode block as a multi-layer block, comprising the steps of providing raw materials comprising coke and a hard material powder such as TiB ₂ , and optionally a carbonaceous material, wherein a first layer contains coke as a raw material and a second layer as a raw material coke and a hard material , in particular TiB ₂ , mixing the starting materials, forming a cathode block, carbonizing and graphitizing and cooling, wherein the step of graphitizing is carried out at temperatures between 2300 and 3000 ° C, in particular between 2400 and 2900 ° C, and wherein the second layer is made with a thickness which is 10 to 50%, in particular 15 to 45% of the total thickness of the cathode block.

Temperatures below 2900 ° C have proven to be particularly advantageous since conventional TiB ₂ does not melt below 2900 ° C. Although melting is unlikely to result in a chemical change in the TiB ₂ , X-ray diffractometry is also achieved after melting and subsequent cooling TiB ₂ detected in a cathode block. By melting, however, finely divided TiB ₂ particles can agglomerate into larger particles. There is also a certain risk that liquid TiB ₂ moves uncontrollably through open porosity.

In the temperature range according to the invention the graphitization process has progressed so far that a high thermal and electrical conductivity of the carbonaceous material is given.

Preferably, the graphitization step is carried out at an average heating rate between 90 K / h and 200 K / h. Alternatively or additionally, the graphitization temperature is maintained for a period between 0 and 1 h. At these heating rates or holding periods, particularly good results are achieved with regard to graphitization and preservation of the hard material.

Advantageously, a duration of the temperature treatment may be 10 to 28 hours up to the time of commencement of the cooling. Not according to the invention it may be advantageous that the composite with hard material and graphite or graphitized carbon forms the entire cathode block. This has the advantage that a single green mass composition is necessary, and accordingly only a single mixing step. According to the invention, the cathode block has at least two layers, wherein the composite layer forms the second layer of the cathode block. This second layer is in direct contact with the melt of the electrolysis cell.

The cathode block preferably has at least one further layer (referred to below as the first 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 the cathode is used in an aluminum electrolysis cell, the first layer is not in direct contact with the aluminum melt and therefore does not have to have good wettability and wear resistance. According to the invention, the second layer has a height which amounts to 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 about 20%, may be advantageous because a small amount of expensive hard material is needed.

Alternatively, a larger height of the second layer, such as 40%, may be advantageous since a layer having a 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, the coke comprises two types of coke, which have a different volume change behavior during carbonation and / or graphitization and / or cooling.

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.

The carbon content of the cathode block preferably condenses to a bulk density of more than 1.68 g / cm ³ , in particular of 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.

With this variant, the advantages of the graphitization temperature according to the invention in a range between 2300 and 3000 ° C are combined with the increase in the bulk density of the cathode block. As a result, a consequence of the incomplete graphitization is advantageously at least partially compensated.

Since the second layer always has a high apparent density of more than 1.80 g / cm ³ , for example, because of the addition of hard material after graphitization, it is advantageous if the first layer also has a high density after graphitization Bulk density of according to the invention over 1.68 g / cm ³ . The small differences in thermal expansion behavior and bulk densities 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.

Advantageously, the two types of coke include 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, there may be a greater extent in a transition region between carbonization and graphitization. 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% increase in the first type of coke, the expansion from room temperature to 1000 ° C for the second type of coke 1.0 vol .-%, in the first coke variety, however, 4.0 vol .-%.

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.

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 advantageous quantitative 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 advantageous both in terms of the processability of the coke and the subsequent properties of the cathode block produced.

Preferably, the further carbonaceous material contains graphite-containing material; In particular, the further carbonaceous material is graphite-containing material, such as graphite. The graphite can be synthetic and / or natural graphite be. 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 auxiliary 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, which have a different volume change behavior during carbonation and / or graphitization and / or cooling. This can presumably lead to a compression of the resulting graphite of more than 1.70 g / cm ³ , in particular more than 1.71 g / cm ³ . 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 bulk 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 expansion behavior of the two layers are adjusted, which can advantageously increase the life of the layers.

Optionally, it may be advantageous that the multilayer block has more than two layers. In this case, of the more than two layers one can any number of layers are produced according to the invention each with two types of coke different volume change behavior.

Further advantageous embodiments and further developments are explained below with reference to a preferred embodiment.

To make a cathode block, first and second cokes are separately ground, separated into grain size fractions, and mixed together with pitch together with, for example, 15 to 25 weight percent, such as 20 weight percent TiB ₂ . The weight fraction of the first coke may be, for example, 10 to 20% by weight or 40 to 45% by weight of the total amount of coke. The mixture is filled into a mold that largely corresponds to the later shape 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, such as 2600 or 2800 ° C, with a graphitization step, and then cooled. The resulting cathode block has a bulk density of 1.68 g / cm ³ and a very high wear resistance to liquid aluminum and cryolite. Due to the average degree of graphitization obtained, thermal and electrical conductivity are high. A loss of TiB ₂ could not be determined by X-ray diffractometry. The wettability of the cathode block by liquid aluminum is very good.

Alternatively, a single coke variety is used. The wetting behavior of the resulting cathode block is largely the same as in the first embodiment. The thermal and electrical conductivity are similar to those in the first embodiment.

In a further variant of the exemplary embodiment, graphite powder or carbon particles are added to the coke mixture.

All features mentioned in the description, examples and claims may contribute to the invention in any combination. The invention is defined by the subject matter of the following claims.

Claims (8)

1. Method for producing a cathode block as a multi-layered block, comprising the steps of preparing starting materials, including coke and a hard material powder, such as TiB₂, and optionally a carbonaceous material, a first layer containing coke as the starting material and a second layer containing coke and a hard material, in particular TiB₂, as the starting material; mixing the starting materials; forming a cathode block; carbonising; and graphitising, as well as cooling, the step of graphitising being carried out at temperatures of between 2300 and 3000°C, in particular of between 2400 and 2900°C, characterised in that the second layer is produced with a thickness of from 10 to 50%, in particular from 15 to 45%, of the entire thickness of the cathode block.
2. Method according to claim 1, characterised in that the graphitising step is carried out at a heating rate of between 90 and 200 K/h and/or at the graphitising temperature of between 2300 and 2900°C.
3. Method according to either claim 1 or claim 2, characterised in that the coke comprises two types of coke that exhibit different volume change behaviour during carbonising and/or graphitising and/or cooling.
4. Method according to claim 3, characterised in that the cathode block is obtained so as to have a bulk density of more than 1.68 g/cm³, in particular of more than 1.71 g/cm³,
5. Method according to one or more of claims 1 to 4, characterised in that the entire cathode block is produced as a composite of graphite and hard material.
6. Method according to claim 5, characterised in that the cathode block contains, as the starting material, at least one additional carbonaceous material as the first and/or second layer.
7. Method according to one or more of claims 1 to 6, characterised in that a proportion of graphite and/or graphitised carbon in at least one layer of the cathode block is at least 60%, based on the total carbon content.
8. Method according to claim 7, characterised in that the proportion of graphite and/or graphitised carbon is at least 80%.