UA112676C2 - A METHOD OF MANUFACTURING THE CATODE UNIT FOR THE ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM - Google Patents

Abstract

The present invention relates to a method of manufacturing a cathode block for an electrolytic cell to produce aluminum and a cathode block made by this method.

Description

The present invention relates to a method of manufacturing a cathode block for an electrolytic cell intended for the production of aluminum, and a cathode block manufactured by this method.

One well-known method of obtaining metallic aluminum is the Hall-Ehrue process. In this electrolytic method, the bottom of the electrolytic cell is usually formed by the cathode surface, which consists of separate cathode blocks. The contact of the cathodes is carried out from below with the help of steel rods, which are installed in the corresponding oblong grooves on the underside of the cathode blocks.

The production of cathode blocks is usually carried out by mixing coke with carbon-containing particles, such as anthracite, carbon or graphite, compaction and carbonization. If necessary, a stage of graphitization is added at higher temperatures, at which carbon-containing particles and coke are at least partially converted into graphite. A carbon cathode is obtained, which at least partially consists of graphite.

The service life of cathode blocks is limited due to a number of influences. Corrosion and erosion caused by liquid aluminum and electrolyte, especially cryolite, over time destroy the cathode blocks from the upper side.

Various measures have been taken in the past to increase the wear resistance of cathode blocks.

For example, attempts were made to increase the volume density of cathode blocks, which should increase their strength and, thus, wear resistance. At the same time, however, it was possible to achieve bulk densities that only reach 1.68 g/cm" in the case of fully graphitized, non-impregnated cathode blocks, as a result of which the wear resistance remains below optimal, as before.

On the other hand, carbon cathodes were coated with titanium diboride (TiVg) (as described in Chinese patent 1062008) or a TiVg-carbon mixture coating, as described, for example, in

Germany 112006004078. TIV? can clearly improve the wetting behavior of aluminum on the cathode and additionally contributes to higher stiffness and wear resistance. However, the wear resistance of the TiVg2 layer on the carbon cathode and the composite layer of carbon and TiV" is still very low and, therefore, the wear resistance of the cathode blocks equipped with the corresponding layers is also very low.

The object of the present invention, therefore, is to propose a cathode block based on

From carbon, which has high, improved wear resistance, and the method of its manufacture.

The problem is solved by the method according to item 1 of the claims.

The method of manufacturing such a cathode block includes the following stages: a) preparation of a mixture of raw materials, which includes coke and pitch, p) formation of raw material from the mixture, and c) carbonization of the raw material and graphitization of the carbonized raw material to obtain a graphitized body (product), as well as cooling after graphitization

At the same time, according to the invention, coke includes two types of coke, which during carbonization and/or graphitization and/or cooling have different behavior in terms of volume change. In addition, unlike the traditional method of manufacturing the cathode block, the carbonized raw material is not impregnated before graphitization, especially not impregnated with pitch, tar or synthetic resins. At the stage of graphitization, at least part of the carbon in the cathode block is transformed into graphite.

Unexpectedly, it was found that the service life of the cathode blocks made by the method according to the invention is clearly higher than that of the cathode blocks made by the conventional method. This is all the more unexpected, because unlike the traditional method for manufacturing the cathode block, the carbonized raw material is not subjected to impregnation before graphitization. In the US patent 4308115, for example, for the manufacture of the cathode, an initial mixture of coke and pitch is obtained, which is further processed at the stage of formation to obtain raw material. In the future, the raw material is compacted with its repeated impregnation with pitch and then fired.

The production of such impregnated cathodes is expensive due to the repeated stages of impregnation and firing. At the same time, impregnation is done in order to compact the raw material for the manufacture of the cathode, thanks to which the penetration of molten and liquid aluminum into the pores of the cathode can be reduced and, thereby, the service life of such cathodes increases.

Despite the absence, according to the invention, of this impregnation stage, the penetration of molten and liquid aluminum into the pores of the cathode is clearly reduced and, thereby, the service life of the cathodes produced by the method according to the invention is increased, approximately due to the use according to the invention of two grades of coke, which during carbonization and/or graphitization and/or cooling have different behavior in terms of volume change.

The advantage can be provided by mechanical processing of the graphite body to obtain a cathode block.

Preferably, the cathode block produced by the method according to the invention has a volume density of carbon particles greater than 1.68 g/cm3, particularly preferably greater than 1.71 g/cm3, especially up to 1.75 g/cm3,

Roughly speaking, a higher bulk density primarily contributes to a longer service life. The reason for this may lie, on the one hand, in the fact that there is more mass per unit volume of the cathode block, which at a given removal of mass per unit of time leads to a higher final mass after a given duration of removal. On the other hand, it can be predicted that the higher bulk density, together with the corresponding lower porosity associated with it, complicates the impregnation of the electrolyte, which acts as a corrosive medium.

Preferably, the two grades of coke include a first grade of coke and a second grade of coke, and the first grade of coke exhibits greater shrinkage and/or expansion during carbonization, and/or graphitization, and/or cooling than the second grade of coke. Here, greater shrinkage and/or expansion represents a preferred form of different volume behavior that is approximately particularly well-suited to lead to stronger compaction compared to mixing coke grades of equal shrinkage and /or extension. At the same time, stronger contraction or expansion applies to any temperature range. Thus, for example, there can only be a stronger shrinkage of the first coke during carbonization. On the other hand, there may be, for example, additionally or instead a stronger expansion in the transition region between carbonization and graphitization. Instead, or in addition, during cooling, there may be a different behavior in terms of volume change.

Preferably, the shrinkage and/or expansion of the first grade of coke during carbonization and/or graphitization and/or cooling by volume exceeds that of the second grade of coke by at least 10 95, especially exceeds by at least 25 95, especially exceeds by at least 50 95. Thus, for example, in the case of 10 95 higher shrinkage of the first grade of coke in the range from room temperature to 2000 "C, the shrinkage for the second grade of coke is 1.0 vol. 95, for the first grade of coke, on the contrary, 1, 1 volume 9.

The advantage is provided when the shrinkage and/or expansion of the first grade of coke is in progress

From carbonization and/or graphitization and/or cooling, on a volume basis, exceeds by at least 100 95 such/such second grade coke, especially exceeds by at least 200 95, especially exceeds by at least 300 95. Thus, for example, in the case of 300 95 higher expansion of the first grade of coke in the range from room temperature to 1000 C, the expansion for the second grade of coke is 1.0 vol. 95, for the first grade of coke, on the contrary, 4.0 vol. 90

Also, the method according to the invention covers the case when the first grade of coke is subject to shrinkage, and the second grade of coke, on the contrary, is subject to expansion in the same temperature range. By 300 95 higher shrinkage and/or expansion covers, thereby, for example, also the case when the second grade of coke gives a shrinkage of 1.0 00.95, and the first grade of coke, on the contrary, expands by 2.0 vol. 9.

Alternatively, at least in any temperature range of the method according to the invention, instead of the first grade of coke, the second grade of coke may exhibit stronger shrinkage and/or expansion, as described above for the first grade of coke.

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

Preferably, the quantitative share in percent by weight of the second grade of coke in the total amount of coke is from 50 95 to 90 95, especially from 50 to 80 95. In these ranges of quantities, different behavior in terms of volume changes of the first and second grades of coke, approximately, it has a particularly good effect on compaction during carbonization, and/or graphitization, and/or cooling.

Possible ranges of second grade coke amounts may be 50 to 60 95 as well as 60 to 80 95 as well as 80 to 90 95.

An advantage is provided when at least one additional carbonaceous material and/or additives and/or powdered solid material is added to the coke. This can provide an advantage with respect to both the processability of the coke and the subsequent properties of the manufactured cathode unit.

Preferably, the additional carbon-containing material comprises a graphite-containing material; in particular, the additional carbon-containing material consists of a graphite-containing material, such as, for example, graphite. Graphite can be synthetic and/or natural graphite. With the help of such an additional carbonaceous material, it is achieved that the necessary shrinkage of the cathode mass, which is dominated by coke, is reduced.

The advantage is provided when the additional carbon-containing material is present based on the total amount of coke and additional carbon-containing material in the amount of 1 to 40 wt. 95, especially from 5 to 30 wt. 95.

Preferably, pitch can be added in amounts from 5 to 40 wt. 95, especially from 15 to 30 wt. to (based on the mass of the entire initial mixture). Peck acts as a binder and serves to form a body that has a stable shape during carbonation.

Additives that provide an advantage may be an oil, such as an auxiliary compression oil, or stearic acid. They facilitate the mixing of coke and, if necessary, additional components.

In particular, Tiv powder is used as a powdery solid material. Using such a solid material increases the wetting ability of the cathode relative to the aluminum melt. The share of this solid material in the mixture of starting materials is in the range of 15 wt. 95 to 60 wt. 95, especially from 20 wt. 95 to 50 wt. 9.

Advantage is provided when the cathode block is made in the form of a multi-layer block, the first layer as starting materials containing coke and, if necessary, additional carbonaceous material, and the second layer as starting materials containing coke and refractory solid material, especially TiVg", and also, with if necessary, additional carbon-containing material.

The solid material is also referred to as Refractory Solid Material. Additional carbon-containing material may be present as described above for the monolithic cathode block. In this version of the multilayer block, the advantages of the multilayer block, in which the layer returned to the aluminum melt contains solid material, are combined with the use of two grades of coke with different behavior in terms of volume change.

Since the second layer after graphitization always exhibits a high bulk density of, for example, greater than 1.82 g/cm3 due to the addition of a hard material resistant to high temperatures, an advantage is provided when the first layer after graphitization also exhibits a high vol. bulk density, which is mostly more than 1.68 g/cm. Small differences in the thermal behavior in terms of expansion and in bulk densities during the stages of heat treatment reduce the time spent on production and the proportion of defective cathode blocks, since strong differences in the properties of the layers during the heat treatment can lead to thermal stresses. Except

Therefore - and this provides an advantage - the resistance to thermal stresses and defects that arise during application, which are their result, also increases.

Preferably, the coke of the first and/or second layer includes two grades of coke, which, having different behavior in terms of volume change during carbonization and/or graphitization and/or cooling, lead to the volume density of the graphite formed, more than 1.70 g/cm.

Preferably, in addition, at least one of both layers is made with a volume density of carbon particles greater than 1.68 g/cm3. If desired and/or necessary, thus, according to the invention, both layers or one of the two layers are made with two different grades of coke.

Thus, there is an opportunity to adjust volume densities and ratios of volume densities as needed or desired. For example, according to the invention, only the first layer can be made with two grades of coke, while the second layer is made with only one grade of coke, but it additionally contains TiHg as a ceramic solid.

If necessary, the advantage can be provided when the multilayer block has more than two layers. In this case, more than two layers can be produced, according to the invention, any number of layers in each case with two grades of coke, which have different behavior in terms of volume change.

Preferably, the second layer can have a height that is from 10 to 50 95, especially from 15 to 45 95 of the total height of the cathode block. A small height of the second layer, such as 20 95, can provide an advantage because a small amount of expensive hard ceramic material is required. Alternatively, a high second layer height, such as 4095, may provide an advantage because a layer having a hard ceramic material has high wear resistance. The greater the height of this highly wear-resistant material relative to the total height of the cathode block, the higher the wear resistance of the entire cathode block. An advantage can be provided when the solid material has a unimodal particle size distribution, with the average particle size distribution d5o lying between 10 and 20 μm, especially between 12 and 18 μm, especially between 14 and 16 μm.

The value of k indicates the average particle size, and here 50 k of particles have a size smaller than the given value. According to this, the value of siyu, respectively (oo) indicates the average size of particles, at which 10 95, respectively 90 9o, particles have a size smaller than the given value.

Unexpectedly, within the scope of the invention, it turned out that with such a method, although the powder of solid material has, on the one hand, a large active surface, which causes very good wettability of the cathode block after graphitization, it, on the other hand, does not have disadvantages that adversely affect the workability of solid material powder as a composite component in the graphite-solid material composite. These possible disadvantages, which the powder of a solid material used according to the invention does not have, are: - a tendency to dust, for example, when loading into a mixing tank or when transporting the powder, - the formation of agglomerates, especially during mixing, such as, for example, wet mixing with coke (wet mixing means in this respect especially mixing with pitch as a liquid phase), - stratification due to different density of materials in solid material and coke.

In addition to the absence of these disadvantages, the solid material powder used according to the invention has particularly good fluidity, correspondingly - flowability. This leads to the fact that the powder of solid material is particularly well transported by traditional transport mechanisms, for example, to mixing equipment.

As a result of the good workability of solid material powder with a value of from 10 to 10 microns and the unimodal distribution of particles by size, the production of solid material composites for cathode blocks is greatly simplified. The resulting cathode blocks show very good homogeneity in terms of the distribution of solid material powder in coke in the case of crude and in graphite in the case of graphitized cathode body.

Preferably, the value of the refractory solid material lies between 20 and 40 μm, especially between 25 and 30 μm. Providing an advantage, this leads to the fact that the wettability and workability properties of the solid material powder are further improved.

Advantageously, the value for the refractory solid is between 2 and 7 μm, especially between 3 and 5 μm. Providing an advantage, this leads to the fact that the wettability and workability properties of the solid material powder are further improved.

In addition, to characterize the unimodal particle size distribution, the width of this distribution can be described by the so-called value range value, which

Zo is calculated as follows: zZrap--(two-y10)/d5o

Advantageously, the brap value of the solid refractory powder is between 0.65 and 3.80, especially between 1.00 and 2.25. Providing an advantage, this leads to the fact that the wettability and workability properties of the solid material powder are further improved.

Advantage is provided when the graphitization stage is carried out at temperatures between 2550 and 3000 °C, especially between 2600 and 2900 °C.

Temperatures below 2900 C turned out to be particularly preferable, since the traditionally used TiV?g does not melt below 2900 "C. Melting, approximately, does not cause a chemical change in TiVg, since after melting, as well as subsequent cooling, the presence

TiIV2 in the cathode block is determined by X-ray diffraction. However, due to melting, finely dispersed TiV" particles can agglomerate to form larger particles. There is also a certain danger of uncontrolled movement of liquid TiHg due to open porosity.

In the temperature range according to the invention, the graphitization process proceeds to such an extent that the result is high thermal and electrical conductivity of the carbon-containing material.

The advantage is provided when the graphitization stage is carried out with an average heating rate in the range of 90 K/h. up to 200 K/h. Alternatively or additionally, the graphitization temperature is maintained for a period of time from 0 to 1 hour. At these heating rates, in accordance with the given duration of maintaining the temperature, particularly good results are achieved in terms of graphitization and production of solid material.

Preferably, the duration of the temperature treatment until the start of cooling can be from 10 to 28 hours.

The problem of the invention, in addition, is solved by the cathode block according to clause 15 of the claims.

The cathode block is manufactured by the method according to the invention, which provides an advantage. According to the invention, the bulk density exceeds 1.68 g/cm3, especially exceeds 1.70 g/cm, especially at least exceeds 1.71 g/cm, especially is up to 1.75 g/cm3. At the same time, the bulk density is given based on the calculation of the entire layer, when the refractory solid material is not taken into account, that is, on the net fraction of carbon. In the case where the layer contains a solid ceramic material such as TiV", the bulk density is the calculated volume density of the layer without taking into account the proportion of refractory solid material.

Additional options which provide an advantage and improvement of the invention are explained below with the help of a preferred embodiment and drawings.

At the same time: in Fig. 1 shows the measurement curve obtained using a dilatometer as a function of the temperature of the first and second grades of coke for the method according to the invention, in Fig. 2 presents a schematic representation of the formation of the cathode block according to the invention in the form of a multilayer block.

To manufacture the cathode block according to the invention, the first and second cokes are ground separately from each other, divided into fractions according to grain size and mixed together with resin.

The mass fraction of the first coke in the total amount of coke can be, for example, from 10 to 20 wt. 95 or from 40 to 45 wt. 95. The cathode block can be made from the original mixture by extrusion. Alternatively, for example, the mixture can be used to fill a mold that, in addition, corresponds to the future shape of the cathode blocks, and vibrocompacted or pressed into a block in this form. The raw material that is formed is heated to a final temperature in the range of 2550 to 3000 C, and the carbonization stage and then the graphitization stage are carried out without intermediate impregnation, for example, with pitch, tar or synthetic resin, and then cooled. The resulting cathode block has a bulk density of 1.71 g/cm and very high wear resistance relative to liquid aluminum and cryolite.

In Fig. 1 shows the measurement curve of the first grade of coke (dashed line) obtained using a dilatometer during the graphitization process. In addition, in Fig. 1 shows the corresponding measurement curve (solid line) for the second grade of coke. It can be seen that both grades of coke have different behaviors in terms of volume change.

In Fig. 1, the first coke shows, starting from the zero line, at the beginning of the temperature regime up to a temperature of 28007 C, first expansion, and up to approximately 1200 C, an increase in volume can be observed, and after approximately 1400 "C, a temporary decrease in volume is detected. Then, approximately up to 2100 "С, you can see the maximum increase in volume relative to the initial volume.

When measuring the second coke with a dilatometer, in principle, the course of the curve can be observed similar to that for the first coke, and the whole curve, in general, grows stronger.

Accordingly, at approximately 2100 "С, the maximum increase in volume can also be identified for the second coke, which, however, is clearly smaller than that of the first coke.

Only during further cooling for both grades of coke, shrinkage is found, which occurs more strongly in the second grade of coke than in the first.

Alternatively, two types of coke are used, the first of which shrinks already during the heating phase at the stage of carbonization and/or graphitization. The second of both grades of coke has clearly stronger shrinkage (relative to shrinkage after carbonization, graphitization and cooling compared to the initial volume) than the other grade of coke.

In an additional variant of the implementation example, graphite powder or carbon particles are added to the coke mixture.

In an additional version of the example, the mold 1 is first partially filled with a mixture 2 of two grades of coke, graphite and TiV" and vibrated, as explained in Fig. 2a. Then, a mixture 5 of two grades of coke and graphite is applied to the obtained initial layer 4, which is the upper layer in the future cathode, which is turned to the anode and, thus, will be in direct contact with molten aluminum, and is compacted again (see Fig. 26).

The resulting upper initial layer b is the future cathode lower layer, which is not returned to the anode. This two-layer block is carbonized and graphitized, as in the first embodiment.

All features listed in the description, examples and claims can be used in the invention in any combination. The invention is not limited to the specified examples, but can also be implemented in the form of variants that are not specifically described here. In particular, different behaviors in terms of volume change also cover behaviors other than shrinking behavior. For example, at least on the sections of the heating and cooling cycle, there may be an increase in volume, which provides an advantage in sealing the cathodes. Thus, the invention may be applicable to two grades of coke which ultimately exhibit the same shrinkage after carbonization, graphitization and cooling, but exhibit different shrinkage or volume expansion at some intermediate temperature.

Different grades of coke can include, in addition to grades of coke from different manufacturers, also coke from the same manufacturer, but with different preliminary processing, such as, for example, differently roasted coke.

Claims (15)

FORMULA OF THE INVENTION 1. The method of manufacturing a cathode block, which includes the stages of: a) preparation of a mixture of raw materials, which includes coke and pitch, and coke includes two types of coke, which in the course of carbonization and/or graphitization, and/or cooling have different behavior in relation to the change in volume, p) formation of crude from the mixture and c) carbonization of crude and graphitization of carbonized crude, without its prior impregnation, with obtaining a graphitized body (product), as well as cooling after graphitization. 2. The method according to claim 1, which is characterized by the fact that the cathode block is obtained with a volume density of the carbon part that is more than 1.68 g/cm3, especially preferably more than 1.71 g/smuU, in particular up to 1.75 g /cm3. 3. The method according to claim 1 or 2, which differs in that the two types of coke include the first type of coke and the second type of coke, and the first type of coke during carbonization and/or graphitization and/or cooling exhibits stronger shrinkage and/or expansion than the second grade of coke. 4. The method according to claim 3, which is characterized by the fact that the shrinkage and/or expansion of the first grade of coke during carbonization and/or graphitization, and/or cooling on a volume basis exceeds that of the second grade of coke by at least 10 95, in particular exceeds at least 25 95, especially exceeds at least 50 95. 5. The method according to one or more of claims 1-4, which differs in that the quantitative share in percentage by weight of the second grade of coke based on the total amount of coke is from 50 to 90 95. b. The method according to one or more of claims 1-5, which is characterized by the fact that additional carbonaceous material and/or additives and/or powdered solid material are added to the coke. 7. The method according to claim 6, which is characterized by the fact that the proportion of solid material, in particular, such as TiV", in the mixture is in the range from 15 to 60 wt. 95, in particular from 20 to 50 wt. 95. 8. The method according to one of claims 1-7, which is characterized by the fact that the cathode block is made in the form of a multilayer block, and the first layer contains coke as a starting material and, if necessary, an additional carbonaceous material, and the second layer contains coke as a starting material and refractory solid material, in particular TiVg2, and, if necessary, additional carbon-containing material. 9. The method according to claim 8, which differs in that the coke of the first and/or second layer includes two types of coke, which, due to different behavior with respect to the change in volume during carbonization and/or graphitization, and/or cooling, lead to the volume density of the formed graphite, more than 1.70 g/cm3. 10. The method according to claim 8 or 9, which is characterized by the fact that the second layer has a height that is from 10 to 50 95, in particular from 15 to 45 95, of the total height of the cathode block. 11. The method according to one or more of claims 7-10, which is characterized by the fact that the solid material has a unimodal particle size distribution, and the D value lies between 10 and 20 μm, in particular between 12 and 18 μm, in particular between 14 and 16 μm . 12. The method according to one or more of claims 7-11, which is characterized by the fact that the value of the refractory solid material lies between 20 and 40 μm, in particular between 25 and 30 μm. 13. The method according to one or more of claims 7-12, which is characterized by the fact that the value of a:o of the refractory solid material lies between 2 and 7 μm, in particular between 3 and 5 μm. 14. The method according to one or more of claims 1-13, which differs in that the graphitization stage is carried out at temperatures in the range from 2550 "C to 3000 "C, in particular in the range from 2600 to 2900 76. 15. Cathode block, in particular manufactured by the method according to one or more of claims 1-14, which is characterized in that the bulk density relative to the carbon part for at least one layer of the cathode block exceeds 1.68 g/cm3, in particular exceeds 1.70 g /cm, in particular at least exceeds 1.71 g/cm3, in particular is up to 1.75 g/cm3.