JP2013532772A - Method for producing cathode block for aluminum electrolytic cell and cathode block - Google Patents

Abstract

The invention relates to the preparation of a starting material comprising coke, a hard material powder such as, for example, TiB ₂ -powder, and optionally a carbon-containing material, mixing of the starting material, shaping of the cathode block, carbonization and graphite. The present invention relates to a method for manufacturing a cathode block including steps of conversion and cooling. In the present invention, the graphitization step is performed at a temperature between 2300 ° C. and 3000 ° C., in particular at a temperature between 2400 ° C. and 2900 ° C.

Description

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

  The Hall-Eleu method is known as a method for producing metallic aluminum. In this electrolysis method, the bottom of the electrolysis cell is typically formed by a cathode surface using a plurality of single cathode blocks. The cathode is connected from below through a steel bar housed in an elongated recess of the same length below the cathode block.

  Cathode blocks are conventionally produced by mixing, compressing, and carbonizing coke with carbon-containing particles such as anthracite, carbon or graphite. Optionally, a graphitization step at high temperature follows, whereby the carbon-containing particles and coke are at least partially converted to graphite.

  By graphitization, the thermal conductivity of the cathode material is significantly increased, and the specific electrical resistance is significantly decreased.

  Graphitized carbon and graphite, however, are difficult to wet or not wet at all to liquid aluminum. As a result, the required current of the electrolysis cell increases, and the energy demand increases accordingly.

In order to solve the above problem, TiB ₂ has been included in the upper layer of the cathode block. This is described, for example, in DE112006004078. Such an upper layer, which is a TiB ₂ -graphite-composite, is in direct contact with the molten aluminum and is therefore critical for current injection from the cathode into the molten aluminum. TiB ₂ and similar hard ceramic materials provide improved wettability of the cathode in the graphitized state, and as a result, improved energy efficiency of the electrolysis process. In addition, the ceramic hard material improves the bulk density and hardness of the cathode, resulting in improved wear resistance, particularly for molten aluminum and cryolite melts. The hard material is also referred to as RHM (heat resistant hard material).

TiB ₂ -powder and similar hard material powders, however, lose the effect of partially improving their wettability and wear resistance during the graphitization process.

Therefore, the object of the present invention is to provide a simple method for manufacturing a TiB ₂ -graphite-composite-cathode having good wettability to molten aluminum and having excellent wear resistance, and a corresponding cathode block. is there.

  The above problem is solved by the method according to claim 1.

The method for producing a cathode block according to the present invention comprises the preparation of a starting material comprising coke, a hard material powder such as TiB ₂ -powder, and optionally a further carbon-containing material, mixing of the starting material, cathode Including the steps of block shaping, carbonization and graphitization and cooling, the graphitization step being carried out at a temperature between 2300 ° C. and 3000 ° C., in particular at a temperature between 2400 ° C. and 2900 ° C. It is characterized by being.

Since conventionally used TiB ₂ does not melt below 2900 ° C., it has been demonstrated that temperatures below 2900 ° C. are particularly preferred. Although it is believed that melting does not cause TiB ₂ chemical changes, in fact, after melting and subsequent cooling, TiB ₂ in the cathode block was identified by X-ray diffraction. However, due to melting, the dispersed TiB ₂ -particles can aggregate into larger particles. There is also a certainty that the liquid TiB ₂ will move through the open pores in an uncontrolled state.

  According to the temperature range of the present invention, since the graphitization process is proceeding, the thermal conductivity and electrical conductivity of the carbon-containing material is high.

  Preferably, the graphitization step is performed at an average heating rate of between 90 K / h and 200 K / h. Alternatively or additionally, the graphitization temperature is held for a time between 0 and 1 hour. This heating rate or holding time achieves particularly good results in graphitization and acquisition of hard materials.

  Advantageously, the duration of the temperature treatment up to the start of cooling is 10 to 28 hours.

  Advantageously, the composite comprising hard material and graphite or graphitized carbon forms the entire cathode block. This is advantageous because only a single raw composition is required and likewise only a single mixing step is required.

  Alternatively, advantageously, the cathode block may have 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.

  Preferably, the cathode block has at least one further layer (hereinafter referred to as the first layer) that contains less hard material powder or no hard material powder than the upper layer. . Thereby, the quantity of the expensive hard material powder used can be reduced. The further layer does not need to be in direct contact with molten aluminum when the cathode is used in an aluminum electrolysis cell, and as a result does not have to exhibit excellent wettability and wear resistance.

  Preferably, the second layer has a height of 10 to 50%, in particular 15 to 45% of the total height of the cathode block. It is advantageous if the height of the second layer is small, such as about 20%, because less expensive hard material is required.

  Alternatively, if the height of the second layer is as great as about 40%, the layer with the hard material is advantageous because it has a high wear resistance. The higher the height of the material having high wear resistance in relation to the total height of the cathode block, the greater the wear resistance of the entire cathode block.

  Preferably, the coke comprises two types of coke that exhibit different volume change behavior during carbonization and / or graphitization and / or cooling.

  Surprisingly, it has been shown that the lifetime of cathode blocks made by such a method is clearly longer than cathode blocks made by conventional methods.

Preferably, the carbon component of the cathode block has a bulk density of greater than 1.68 g / cm ^3, particularly preferably, greater than 1.71 g / cm ^3, in particular, the upper limit is 1.75 g / cm ³ So that it is compressed.

  Larger bulk density is thought to favorably contribute to longer life. The reason is, on the one hand, that there is more mass per unit volume of the cathode block, which results in a larger residual mass after a given wear time in a given amount of wear per unit time. On the other hand, a higher bulk density would indicate a correspondingly low porosity, which would prevent the penetration of the electrolyte acting as a corrosive medium.

  In this variant, the graphitization temperature advantage in the range between 2300 ° C. and 3000 ° C. of the present invention is combined with an increase in the bulk density of the cathode block. This advantageously at least partially offsets the result of incomplete graphitization.

Since the second layer always shows a bulk density greater than, for example, 1.80 g / cm ³ after graphitization due to the addition of a hard material, the first layer also presents the present invention after graphitization. It is advantageous to exhibit a bulk density of greater than 1.68 g / cm ³ . Slight differences in thermal expansion behavior and bulk density during the heat treatment process reduce cathode block production time and rejection. Thus, more advantageously, the durability against thermal stresses and consequent damage in use is also increased.

  Preferably, the two types of coke include a first coke and a second coke, wherein the first coke is more than the second coke during carbonization and / or graphitization and / or cooling. Exhibit stronger contraction and / or expansion. Thereby, the stronger contraction and / or expansion mentioned above causes an advantageous different volume change behavior, which is different than when coke with similar shrinkage and / or expansion is mixed. It is considered particularly suitable for achieving stronger compression. At this time, the stronger contraction and / or expansion is related to an arbitrary temperature region. Thus, for example, during carbonization, only stronger contraction of the first coke may occur. On the other hand, for example, additionally or alternatively, a stronger expansion may occur in the transition region between carbonization and graphitization. Alternatively or additionally, there may be different volume change behaviors upon cooling.

  Preferably, the shrinkage and / or expansion of the first coke during carbonization and / or graphitization and / or cooling is at least 10% greater than the second coke in terms of volume, in particular at least 25%. Large, in particular at least 50% larger. Thus, for example, if the shrinkage of the first coke is 10% greater, the shrinkage from room temperature to 2000 ° C. is 1.0% by volume for the second coke and 1.1% by volume for the first coke. .

  Advantageously, the shrinkage and / or expansion of the first coke during carbonization and / or graphitization and / or cooling is at least 100% greater in volume than the second coke, in particular at least 200. % Larger, in particular at least 300% larger. Thus, for example, if the shrinkage of the first coke is 300% greater, the shrinkage from room temperature to 1000 ° C. is 1.0% by volume for the second coke and 4.0% by volume for the first coke. .

  In addition, a case where the first coke contracts and the second coke expands at the same temperature interval is included in the method of the present invention. 300% greater shrinkage and / or expansion thus includes, for example, the case where the second coke shrinks 1.0% by volume and the first coke expands 2.0% by volume relative thereto.

  Alternatively, in at least one arbitrary temperature interval of the method of the invention, instead of the first coke, the second coke has a stronger contraction and / or expansion as described above for the first coke. May be indicated.

  Preferably, at least one of the two types of coke is petroleum coke or coal tar pitch coke.

  Preferably, the ratio of the amount of second coke to the total amount of coke is between 50% and 90% in weight percent. Within this amount range, the different volume change behavior of the first coke and the second coke works particularly well for compression during carbonization and / or graphitization and / or cooling. The possible amount range of the second coke may be 50 to 60%, 60 to 80%, or 80 to 90%.

  Advantageously, at least one carbon-containing material and / or pitch and / or additives are added to the coke. This is advantageous both in terms of coke processability and in properties after the cathode block to be produced.

  Preferably, said further carbon-containing material comprises a graphite-containing material; in particular, said further carbon-containing material consists of a graphite-containing material such as, for example, graphite. The graphite may be synthetic graphite and / or natural graphite. According to the further carbon-containing material, reduction of inevitable shrinkage of the volume of the cathode whose main component is coke is realized.

  Advantageously, the carbon-containing material comprises 1 to 40% by weight, in particular 5 to 30% by weight, based on the total amount of coke and carbon-containing material.

  Preferably, if the total amount of coke and optionally carbon-containing material is 100% by weight, these are additionally pitched, in addition 5 to 40% by weight, in particular 15 to 30% by weight (100% total raw mixture). %).

  Advantageously, the additive may be an oil such as a compression aid oil or stearic acid. These facilitate the mixing of the coke with optional further components.

Preferably, the coke is carbonized and / or graphitized and / or cooled in at least one of the two layers described above, i.e. the first layer and / or the second layer. Including two types of coke that exhibit different volume change behaviors. Thus, the graphite to produce the 1.70 g / cm ^3, is believed to be compressed to particular greater than 1.71 g / cm ^3. Thus, as desired and / or required, both layers or one of both layers is produced according to the present invention using two different cokes. Thereby, it is possible to adjust the bulk density and the bulk density ratio as desired or necessary. For example, the second layer is manufactured using only one type of coke, but additionally includes TiB ₂ as a hard material, while exclusively using the first layer according to the present invention using two types of coke. Can be manufactured. This makes it possible to adapt the expansion behavior of both layers, which is advantageous in that it extends the life of the layers.

  In some cases it may be advantageous for the multilayer block to have more than one layer. In this case, any number of two or more layers can be produced according to the invention using two types of coke, each exhibiting different volume change behavior.

  Further advantageous developments and developments of the invention are described below by means of preferred examples.

In order to produce the cathode block of the present invention, the first coke and the second coke are crushed separately into powders, separated into particle size fractions, and each is combined together, for example 15 To 25% by weight, for example 20% by weight of TiB ₂ and mixed with the pitch. The weight ratio of the first coke to the total amount of coke is, for example, 10 to 20% by weight, or 40 to 45% by weight. The mixture is filled into a mold that closely matches the shape of the subsequent cathode block and vibration compacted or compressed into a block. The resulting shaped product is heated to a final temperature in the range of 2300 to 3000 ° C, for example 2600 to 2800 ° C. Thereby, a graphitization process is realized and then cooled. The resulting cathode block has a bulk density of 1.68 g / cm ³ and very good wear resistance against liquid aluminum and cryolite melt. According to the average graphitization rate obtained, the thermal conductivity and electrical conductivity are high. The disappearance of TiB ₂ was not confirmed by the X-ray diffraction method. The wettability of the cathode block to liquid aluminum is very good.

  Alternatively, a single type of coke is used. The wettability of the obtained cathode block is as good as in the first example. The thermal conductivity and electrical conductivity are in a range similar to the first embodiment.

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

  All of the features recited in the specification, examples and claims contribute to the invention in any combination. The invention is not, however, limited to the examples given, and variations which are not specifically described herein are also included in the invention.

Claims (10)

Preparation of a starting material comprising coke and a hard material powder, for example TiB ₂ -powder, and possibly further carbon-containing material, mixing of the starting material, shaping of the cathode block, carbonization and graphitization, Including the step of cooling,
The method for producing a cathode block, wherein the graphitization step is performed at a temperature between 2300 ° C and 3000 ° C, in particular at a temperature between 2400 ° C and 2900 ° C.   The graphitization step is performed at a heating rate between 90 K / h and 200 K / h and / or at a graphitization temperature between 2300 ° C. and 2900 ° C. Item 2. A method for producing a cathode block according to Item 1.   Cathode block according to claim 1 or 2, characterized in that the coke comprises two types of coke that exhibit different volume change behavior during carbonization and / or graphitization and / or cooling. Production method. Bulk density greater than 1.68 g / cm ^3, in particular, is characterized in that 1.71 g / cm ³ greater than the cathode block is obtained, the production method of the cathode block of claim 3.   5. The method of manufacturing a cathode block according to claim 1, wherein the entire cathode block is manufactured as a composite containing graphite and a hard material. 6. The cathode block is manufactured as a multilayer block,
The first layer contains coke as a starting material,
6. The method of manufacturing a cathode block according to claim 1, wherein the second layer includes coke as a starting material and a hard material, particularly TiB ₂ .   7. The method of manufacturing a cathode block according to claim 6, wherein the cathode block includes at least one additional carbon-containing material as a starting material as the first and / or second layer.   8. The production of a cathode block according to claim 6 or 7, characterized in that the second layer is produced with a thickness of 10 to 50%, in particular 15 to 45% of the total thickness of the cathode block. Method.   9. The ratio of graphite and / or graphitized carbon to the total carbon content in at least one layer of the cathode block is at least 60%. A method for producing a cathode block according to 1.   The method for producing a cathode block according to claim 9, characterized in that the proportion of graphite and / or graphitized carbon is at least 80%.