Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

• the present invention relates to a method for producing a cathode block for an aluminum electrolysis cell and a cathode block produced by this method.
• One known method of producing metallic aluminum is the Hall-Heroult process.
• the bottom of an electrolytic 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, carbonaceous particles such as anthracite, carbon or graphite, densifying and carbonizing.
• a graphitizing step at higher temperatures follows, at which the carbonaceous particles and the coke are at least partially converted to graphite. It will get a carbon cathode, which consists at least partially of graphite.
• the life of the cathode blocks is limited by a number of influences.
• TiB 2 can obviously improve the wetting behavior of aluminum on the cathode and additionally contributes to higher hardness and wear resistance. Nevertheless, the wear resistance of a TiB 2 layer on a carbon cathode and a composite layer of carbon and TiB 2 is still too low, and thus the wear resistance of cathode layers provided with respective layers is too low.
• Object of the present invention is therefore a carbon-based
• Cathode block having a high, improved wear resistance and to provide a method for its production.
• the object is achieved by a method according to claim 1.
• a method for producing such a cathode block comprises the steps of a) providing a mixture of starting materials comprising coke and
• the coke according to the invention comprises two types of coke, which during the
• Carbonising and / or graphitizing and / or cooling have a different volume change behavior. Furthermore, the carbonized
• Cathode blocks not impregnated before graphitization in particular not impregnated with pitch, tar or synthetic resins.
• the graphitizing step at least a portion of carbon in the cathode block is converted to graphite.
• Such impregnated cathodes are expensive to manufacture because of the many repetitive impregnation and firing steps.
• the impregnation is carried out here in order to densify the cathode green body, whereby penetration of molten aluminum into pores of the cathode can be reduced and thus the service life of such cathodes is increased.
• a cathodic block produced by a process according to the invention preferably has a bulk density of a carbon content of more than 1.68 g / cm 3 , more preferably of more than 1.71 g / cm 3 , in particular of up to 1.75 g / cm 3 .
• the two types of coke comprise a first type of coke and a second type of coke, the first type of coke exhibiting a greater degree of shrinkage and / or expansion than the second type of coke during carbonation and / or graphitization and / or cooling.
• 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.
• a different volume change behavior may be present during cooling.
• the shrinkage and / or expansion of the first type of coke during carbonation and / or graphitization and / or cooling is at least 10% higher than that of the second coke variety, in particular at least 25% higher, in particular at least 50% higher.
• 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.
• the expansion from room temperature to 1000 ° C. for the second type of coke 1 is 0% by volume, whereas for the first type of coke it is 4.0% by volume.
• the case that the first type of coke undergoes shrinkage, the second coke variety, however, an expansion in the same temperature interval, is characterized by the inventive method comprises.
• a 300% higher shrinkage and / or expansion also includes the case that the second type of coke shrinks by 1, 0 vol .-%, the first Koksorte, however, by 2.0 vol .-% expands.
• the second type of coke instead of the first 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.
• the percentage by weight of the second type of coke in the total amount of coke is between 50% and 90%, in particular between 50 and 80%.
• the different volume change behavior of the first and second types of coke probably has a particularly positive effect on compaction during carbonation 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%.
• At least one further carbonaceous material and / or additives and / or powdery hard material are added to the coke. This can be both in terms of the processability of the coke and the later
• the further carbonaceous material contains graphite-containing material;
• the further carbonaceous material is graphite-containing material, such as graphite.
• the graphite may be synthetic and / or natural graphite.
• the further 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 further carbonaceous material.
• pitch can be added in amounts of from 5 to 40% by weight, in particular from 15 to 30% by weight (based on the weight of the entire green mixture).
• 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.
• TiB 2 powder is used as powdered hard material.
• the use of such a hard material increases the wettability of the cathode with respect to the aluminum melt.
• the proportion of this hard material in the mixture of starting materials is between 15 wt .-% and 60 wt .-%, in particular between 20 wt .-% and 50 wt .-%.
• the cathode block is produced as a multi-layer block, wherein a first layer contains coke as starting materials and optionally another carbonaceous material and a second layer contains as starting materials coke and a refractory hard material, in particular TiB 2 , and optionally a further carbonaceous material.
• Hard material is also called RHM
• the further carbonaceous material may be as described above for a monolithic cathode block.
• the advantages of a multilayer block in which the layer facing the molten aluminum layer contains a hard material are combined with the use of two coke varieties with different volume change behavior. Since the second layer always has a high bulk density of, for example, more than 1.82 g / cm 3 due to the addition of high-temperature-resistant hard material after graphitization, it is advantageous if the first layer after graphitization likewise has a high bulk density of advantageously more than 1.68 g / cm 3 .
• the coke of the first and / or second layer comprises two types of coke which have a different volume change behavior during the course of the process
• Carbon represents and / or graphitizing and / or cooling lead to a bulk density of the resulting graphite of about 1, 70 g / cm 3 .
• both layers or one of the two layers can thus be produced according to the invention with two different types of coke.
• 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 a ceramic hard material.
• the multilayer block has more than two layers.
• any number of the layers can be produced according to the invention, each with two types of coke of different volume change behavior.
• the second 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 second layer such as about 20%, may be advantageous because a small amount of more costly hard ceramic material is needed.
• a high height of the second layer such as 40%, may be advantageous since a layer that possesses hard ceramic 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.
• the hard material may be present in a monomodal particle size distribution, the mean particle size of the distribution d 5 o being between 10 and 20 ⁇ , in particular between 12 and 18 ⁇ , in particular between 14 and 16 ⁇ .
• the d 5 o value indicates the mean particle size, with 50% of the particles being smaller than the specified value. Accordingly, the dio or dgo value indicates the mean particle size, with 10 or 90% of the particles being smaller than the stated value.
• the invention has been found in the context that o the hard material powder on the one hand, has in such a d 5 a large active surface, which causes a very good wettability of the cathode block after the graphitization, but on the other hand does not have the disadvantages which a
• Dusting for example when filling in a mixing container or during transport of the powder,
• Agglomerate formation in particular during mixing, such as wet mixing with coke (wet mixing in this context means, in particular, mixing with pitch as the liquid phase),
• 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.
• 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 graphite in the graphitized cathode body.
• the dgo of the refractory hard material is between 20 and 40 ⁇ , in particular between 25 and 30 ⁇ . This has the advantageous consequence that
• the dio of the refractory hard material is between 2 and 7 ⁇ , in particular between 3 and 5 ⁇ . This advantageously has the consequence that wetting and processing properties of the hard material powder are even better.
• the span of the refractory hard material powder is between 0.65 and 3.80, in particular between 1.00 and 2.25. This advantageously has the consequence that the wetting and processing properties of the hard material powder are even better.
• the step of graphitizing is carried out at temperatures between 2550 and 3000 ° C, in particular between 2600 and 2900 ° C.
• the graphitization step is carried out at an average heating rate between 90 K / h and 200 K / h.
• 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.
• a duration of the temperature treatment may be 10 to 28 hours up to the time of commencement of the cooling.
• the invention is further achieved with a cathode block according to claim 15.
• the cathode block is advantageously produced by a method according to the invention.
• the apparent density is greater than 1.68 g / cm 3 , in particular greater than 1.70 g / cm 3 , in particular at least greater than 1.71 g / cm 3 , in particular up to 1.75 g / cm 3 .
• the bulk density is based on the entire layer, if no refractory hard material is added, ie on the pure carbon content.
• the bulk density is a theoretical bulk density of the layer without the proportion of refractory hard material.
• FIG. 2 shows a schematic representation of the shaping of a cathode block according to the invention as a multi-layer block.
• a first and a second coke are ground separately, separated into particle size fractions and mixed with each other with pitch.
• the proportion by weight of the first coke in the total amount of coke may be, for example, 10 to 20% by weight or 40 to 45% by weight.
• a cathode block can be made from the green mix by extrusion. Alternatively, for example, the mixture may be filled and vibrationally compressed or block-pressed into a mold which largely corresponds to the later shape of the cathode blocks.
• the resulting green body is heated to a final temperature in a range of 2550 to 3000 ° C, wherein a carbonization step and then a graphitization without intervening impregnation, for example, with pitch, tar or resin, and then cooled.
• the resulting cathode block has a bulk density of 1.71 g / cm 3 and a very high resistance to wear compared to liquid aluminum and cryolite.
• FIG. 1 shows a dilatometer trace of the first type of coke (dashed line) during the graphitization process.
• FIG. 1 further shows a corresponding measurement curve (with a solid line) for the second type of coke. It can be seen that both types of coke have different volume change behaviors.
• the first coke of FIG. 1 shows starting from a zero line at the beginning of the temperature program up to a temperature of 2800 ° C, first an expansion, up to about 1200 ° C, an increase in volume is observed and after about 1400 ° C, a temporary reduction the volume occurs. Up to approx. 2100 ° C, a maximum volume increase compared to the initial volume can be seen.
• two types of coke are used, the first of which already has a shrinkage during the heating phase in the carbonization and / or graphitization step.
• the second of the two coke varieties has a much stronger shrinkage
• a mold 1 is initially partially filled with a mixture 2 of the two types of coke, graphite and TiB 2 and vibrationally compressed, as indicated in Fig. 2a).
• a mixture 5 of the two types of coke and graphite is filled and in turn compacted (see FIG . 2 B).
• the resulting upper starting layer 6 represents at the later cathode the lower layer facing away from the anode.
• This two-layer block is carbonated and graphitized as in the first embodiment.

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• 2012
  • 2012-02-01 DE DE102012201468A patent/DE102012201468A1/de not_active Withdrawn
• 2013
  • 2013-01-31 EP EP13702626.6A patent/EP2809833B1/de active Active
  • 2013-01-31 UA UAA201409529A patent/UA112676C2/uk unknown
  • 2013-01-31 RU RU2014135212A patent/RU2666806C2/ru not_active Application Discontinuation
  • 2013-01-31 CN CN201380007869.2A patent/CN104126032A/zh active Pending
  • 2013-01-31 CA CA2862277A patent/CA2862277C/en active Active
  • 2013-01-31 WO PCT/EP2013/051940 patent/WO2013113837A1/de active Application Filing
  • 2013-01-31 JP JP2014555209A patent/JP6018227B2/ja active Active

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