JPH05263285A - Electrode for aluminum electrolysis - Google Patents

Abstract

(57) [Summary] [Object] To provide an electrode having excellent wettability with molten aluminum and capable of performing molten salt electrolysis with high current efficiency. [Structure] This aluminum electrolysis electrode is made of a carbonaceous or graphite composite material containing 40 to 70% by weight of zirconium boride. In addition to zirconium boride, one or more boride compounds of titanium, niobium, tantalum and hafnium may be contained. In this case, the total boride content should be 4
The range is 0 to 70% by weight. Alternatively, a carbonaceous or graphitic composite material containing zirconium boride may be laminated on a carbonaceous or graphitic substrate with an intermediate layer, if necessary, to form a bipolar electrode. [Effect] Since the wettability of the cathode surface with the molten aluminum is excellent, the block cathode or the bipolar electrode can be arranged in the electrolytic cell with a small distance between the electrodes. In addition, the anode surface is also consumed uniformly, and molten salt electrolysis can be performed for a long period of time at a high current density and under stable electrolysis conditions. Therefore, the consumption of electric energy required for manufacturing aluminum is significantly reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for aluminum electrolysis, which is used in the production of aluminum by molten salt electrolysis and has excellent current efficiency.

[0002]

2. Description of the Related Art Electrolytic production of aluminum includes a method of using a chloride-based molten salt and a method of using a fluoride-based molten salt.

When aluminum is electrolytically produced by molten salt of aluminum chloride sodium chloride-potassium chloride system,
As shown in FIG. 1, a plurality of electrodes 2 ₁ , 2 ₂ are provided in the electrolytic cell 1.
... 2 _n are arranged in the vertical direction via the spacer 3. A flat carbon plate is usually used for the electrodes 2 ₁ , 2 ₂ ... 2 _n . The spacer 3 is composed of individual electrodes 2 ₁ ,
To keep the distance between 2 ₂ ... 2 _n constant,
A material having excellent durability against electrolytic reaction and erosion by molten salt is used.

The electrodes 2 ₁ , 2 ₂ ... 2 connected to each other
_{The n} is immersed in the electrolytic bath 4 contained in the electrolytic bath 1. Since the electrolytic cell 1 does not function as a current-carrying member, it is constructed of a refractory brick of silicon nitride or the like having excellent durability against corrosion by molten salt or molten aluminum. Electrolysis bath 4
As the material, the one dissolved in the molten salt of aluminum chloride sodium chloride-potassium chloride system charged from the supply port 5 is used.

Electrodes 2 ₁ , 2 ₂ ... Immersed in the electrolytic bath 4
Energize so that the upper part of 2 _n is on the cathode side and the lower part is on the anode side. When carrying out electrolysis while maintaining the electrolytic bath 4 at about 700 ° C., the upper cathode surface of each electrode 2 ₁ , 2 _2, ... 2 _n
An electrolytic deposition reaction of aluminum occurs. The generated molten aluminum flows down in the electrolytic bath 4 along the side surfaces of the electrodes 2 ₁ , 2 ₂ ... 2 _n and collects at the bottom of the electrolytic cell 1 to form a molten metal layer 6.

On the other hand, the generated chlorine gas is generated by the electrodes 2 ₁ , 2
₂ ... goes up along the side surface of 2 _{n in} the electrolytic bath 4 and is exhausted from the reaction system. In order to regulate the discharge of chlorine gas, each electrode 2 ₁ , 2 ₂ ... 2 _n is provided with a downward projection 3 a that regulates the gas flow direction. Downward protrusion 3a
As a result, the flow in the right direction in FIG. 1 is suppressed, and the generated chlorine gas rises from the left side of the electrodes 2 ₁ , 2 ₂ ... 2 _n ,
It is discharged to the outside of the system through the exhaust port 7.

Further, a method of conducting electric current between a horizontally arranged block-shaped consumable electrode and an electrode installed on the bottom of an electrolytic cell to perform molten salt electrolysis in a fluoride-based electrolytic bath, a plurality of horizontally opposed electrodes There is also adopted a method of immersing the electrode of No. 2 in a molten salt, electrolyzing the molten salt between the electrodes, and causing aluminum produced on the cathode side to flow down to the bottom of the electrolytic cell along the surface of the cathode.

Anthracite blocks, graphite blocks and the like have been conventionally used as materials for the cathode on which aluminum is deposited or for the bipolar type electrode shown in FIG. However, the carbonaceous or graphite electrode has poor wettability with the molten aluminum. Therefore, the molten aluminum deposited by the electrolytic reaction becomes spherical and easily stays on the cathode surface.

Due to the retention of droplets, the distance between the poles locally changes. For example, the distance between the electrodes is reduced at the place where the droplets are retained, and the resistance between the electrodes is reduced. Therefore, the entire surface of the electrode is not effectively used for the electrolytic reaction, and the electrolytic current flows intensively to the droplet retention location. As a result, the current efficiency is lowered, and the droplet may grow to a size exceeding 10 mm. When a bridge that causes a short circuit is formed between the electrodes due to the large-sized droplet, it becomes inoperable.

In this respect, in an electrolysis furnace using a fluoride-based electrolytic bath, a molten metal layer having a certain depth is left on the cathode, and electrolysis is performed using the molten metal layer as the cathode surface. However, due to the influence of the magnetic field, phenomena such as flow, wave, and uplift occur in the molten metal layer. In order to avoid the adverse effect of such a phenomenon, the distance between the electrodes is currently set to 40 mm or more even if the electrolytic energy is sacrificed to some extent.

From the viewpoint of improving the electrolytic energy, it is effective to set the distance between the electrodes as small as possible. However, when the distance between the electrodes is set to be small, the electrolytic conditions become unstable due to the above-mentioned droplets generated on the cathode surface, the flow of the molten metal layer, the wave motion, and the ridge. As a result, the current efficiency tends to decrease. Therefore, use of an electrode having improved wettability of the cathode surface with the molten aluminum has been studied.

On the cathode surface having good wettability, the molten aluminum deposited by the electrolytic reaction flows along the cathode surface without growing into large droplets and is collected in the molten metal layer. Therefore, the effective inter-electrode distance does not change due to the retention of droplets, and electrolytic smelting can be performed under stable conditions. Further, since the distance between the electrodes can be set small, the current efficiency itself is improved, and the electric energy consumed for molten salt electrolysis is reduced.

As an electrode having good wettability to molten aluminum, it is possible to use a carbonaceous composite material containing titanium boride as an ALUMINIUM (June 1990 issue), pages 573-582, "Von flussigigem Aluminum benetz."
bare Kathoden fur Aluminum elektrolysezellen ”. In addition, ALUMINIUM (October 1980 issue), pages 642-648, “Inert cathodes for a
In "luminium electrolysis in Hall-Heroult cells", an investigation report using various metal borides, carbides, nitrides and the like as cathode materials is introduced. Further, European Patent Publication No. 0169152 also discloses a carbonaceous electrode in which titanium boride is compounded.

[0014]

Even if a carbonaceous or graphite electrode compounded with titanium boride is used as the cathode, the current efficiency could not be increased to 60% or more of the target value in the laboratory evaluation. .. Further, when the compounding amount of titanium boride is increased, the bulk specific gravity of the electrode is increased and the mechanical strength such as transverse rupture strength is decreased due to the weak bond between the carbon material and titanium boride. Moreover, the composite of titanium boride increases the coefficient of thermal expansion of the cathode, causing a problem such as peeling when a bipolar electrode or the like is manufactured.

The present invention has been devised to solve such a problem, and by combining zirconium boride instead of titanium boride, high current efficiency is provided in aluminum electrolysis, and mechanical An object is to provide an electrode for aluminum electrolysis having excellent thermal characteristics.

[0016]

In order to achieve the object, the aluminum electrolysis electrode of the present invention comprises a carbonaceous or graphitic composite material containing 40 to 70% by weight of zirconium boride. .. In addition to zirconium boride,
1 boride of titanium, niobium, tantalum and hafnium
It is also possible to contain one kind or two or more kinds. In this case, the components are adjusted so that the total boride content is 40 to 70% by weight.

Further, it is also used as a bipolar electrode in which a surface layer made of a carbonaceous or graphitic composite material containing zirconium boride is formed on a carbonaceous or graphitic substrate. in this case,
It is preferable to form an intermediate layer having a reduced boride content from the surface layer toward the substrate between the substrate and the surface layer.

[0018]

[Operation] Zirconium boride has better wettability to binders such as coal tar, pitch, and resin than titanium boride. Therefore, when zirconium boride is blended with a carbonaceous or graphitic material to produce a block compact having a large bulk specific gravity. When this block compact is fired or graphitized, a composite having a large binding force between zirconium boride and carbon and excellent in mechanical strength such as transverse rupture strength can be obtained. The difference between zirconium boride and titanium boride in terms of bulk specific gravity and transverse rupture strength becomes more remarkable as the content of zirconium boride or titanium boride increases.

Further, since the bond between zirconium boride and the carbon material is strong, the coefficient of thermal expansion of the composite can be suppressed to a small value, and excellent thermal shock resistance is exhibited. The effect of zirconium boride and titanium boride on the thermal expansion also appears remarkably as the content of zirconium boride or titanium boride increases. Therefore, even when this composite is laminated on the carbonaceous or graphite substrate as the cathode portion of the bipolar electrode,
The occurrence of defects such as peeling is reduced.

The resulting zirconium boride-carbonaceous or graphite composite has a high bulk specific gravity and a dense structure with few pores. Therefore, the effect of improving the wettability of zirconium boride with respect to the molten aluminum works effectively, and the electrode has high current efficiency. Further, since the penetration and diffusion of the electrolytic bath components into the inside of the cathode are suppressed, the cathode is less deteriorated and the durability is improved.

A composite was produced by firing a mixture of this zirconium boride in carbon and graphite. About the obtained composite, machinability, durability,
We investigated various characteristics such as electrical resistance and wettability with molten aluminum. As a result, zirconium boride 40-7
It has been found that a carbonaceous or graphitic composite material containing 0% by weight has the properties required for an electrode for aluminum electrolysis. In addition to zirconium boride, when one or more borides of titanium, niobium, tantalum, and hafnium are mixed with carbonaceous or graphite, the total boride content is 40 to 70% by weight. It was found that by maintaining the content within the range, a composite suitable for an electrode for aluminum electrolysis can be obtained as well.

The present invention will be specifically described below. When manufacturing an electrode block for an aluminum electrolysis furnace, roasted anthracite, graphite, petroleum coke, coal-based pitch, etc. are used as an aggregate, and coal tar, resin, etc. are used as a binder. After preparing the respective raw material components so that the aggregate particle size distribution, the binder content, the composition, etc. reach the target values, they are sufficiently kneaded and molded into a block shape. 100 molding blocks
A primary firing block is obtained by firing at 0 to 1300 ° C., and a graphitization block is obtained by further firing at a temperature of 2500 ° C. or higher.

In this firing process, the weight loss of the aggregate hardly occurs. Since the binder contains a large amount of volatile components,
Reduce the amount by heating during firing. The degree of weight loss depends on the type of binder and the maximum temperature reached during firing. Roasted anthracite and graphite aggregate in the primary firing block for the cathode,
Aggregates such as petroleum coke and coal-based pitch coke are generally used for the primary firing block for the anode. The aggregate of the graphitized block includes petroleum coke, coal-based pitch coke, etc., and is used for both the cathode and the anode.

When the block after firing is used as a monopolar electrode, the cathode is not consumed, but the anode is oxidized and consumed by oxygen generated in the electrolytic reaction. Therefore, it is necessary to supplement the electrode material to compensate the consumed amount. There is. On the other hand, when used as a bipolar electrode, both sides of the block act as an anode and a cathode, respectively, and the anode side is consumed. Therefore, it is necessary to replace with a new electrode according to the consumption.

When zirconium boride is blended in the cathode block, zirconium boride is uniformly mixed throughout the block in the monopolar electrode. Alternatively, zirconium boride can be mixed only in the surface layer of the block. When forming a surface layer containing zirconium boride in a bipolar electrode, it is preferable to form an intermediate layer having a graded zirconium boride content between the surface layer and the substrate. As the substrate, a block made of aggregate such as petroleum coke or coal-based pitch coke is used because it works as the anode side. When the intermediate layer is provided, the difference in thermal expansion between the substrate and the surface layer is relaxed, and peeling of the surface layer from the substrate due to thermal stress is suppressed.

The intermediate layer can be formed by laying a paste prepared to have an intermediate concentration on the surface of the base paste, laying the surface layer paste on the paste, pressurizing the whole and then firing. Due to the high temperature heating during firing, diffusion occurs between the substrate, the intermediate layer and the surface layer, and discontinuous changes in the concentration gradient of zirconium boride are eliminated. Further, when the intermediate layer paste and the surface layer paste are laid after the surfaces of the base paste and the intermediate layer paste are roughened, the layers are mixed, and the zirconium boride concentration changes from a stepwise change to a continuous change. As a result, when the obtained electrode is heated to a high temperature, the thermal stress generated between the substrate, the intermediate layer and the surface layer is relaxed, and the surface layer is reliably prevented from peeling. Alternatively, it is possible to form the intermediate layer by simply spreading the surface layer paste on the surface-roughened substrate paste and forming a mixed layer of the substrate paste and the surface layer paste at the interface.

The particle size of zirconium boride is not particularly limited to the present invention, but a finer particle size is preferable from the viewpoint of effectively exhibiting the wettability improving action. Further, zirconium boride can be mixed at the same time when the carbonaceous aggregate and the binder are mixed, but after the boron zirconium and the carbonaceous aggregate are well mixed in advance, they are mixed with the binder. Preferably. Other titanium boride, niobium boride, tantalum boride, hafnium boride, etc. are preferably mixed in two stages as well. This two-stage mixing shortens the time required to reach uniform mixing, and can suppress the deterioration of the binder.

The paste obtained by mixing the aggregate and the binder is formed into a block according to a conventional method, and primary firing and graphitization firing are performed to form an electrode. However, the melting points of zirconium boride and other borides are 2850 to 3200 ° C.
Since it is in the temperature range of, the maximum temperature reached during graphitization firing is set to about 2800 ° C.

When zirconium boride is added as a single boride, its content is adjusted to the range of 40 to 70% by weight. When other borides are added together, the total boride content is adjusted to the range of 40 to 70% by weight. When the content of boride is less than 40% by weight, the current efficiency of the obtained electrode becomes insufficient and the consumption of electric energy during aluminum electrolytic smelting increases. Moreover, sufficient wettability with respect to the molten aluminum cannot be obtained, and retention of the molten aluminum on the cathode surface is observed. On the other hand, when the content of the boride exceeds 70% by weight, the wettability with respect to the molten aluminum is sufficient, but the current efficiency tends to decrease conversely as the blending amount increases. Furthermore, the addition of excess zirconium boride also deteriorates the machinability of the fired body.

The wettability of the surface of the cathode with respect to the molten aluminum is generally determined by measuring the contact angle of the molten aluminum placed on the cathode in a vacuum, an inert atmosphere or an electrolytic bath. For example, the contact angle of the molten aluminum retained on the cathode surface in the electrolytic bath with respect to the cathode surface is measured using X-ray radiography. Alternatively, the wettability may be estimated by measuring the force required to pull up the cathode from the electrolytic bath and calculating the interfacial tension from this force. However, both methods are performed in an environment far from the contact state between the cathode and the aluminum molten metal during molten salt electrolysis, and thus the reliability of whether or not the obtained wettability conforms to the actual value is lacking. ..

Therefore, the present inventors have adopted a method of determining the wettability of the cathode after actually performing electrolysis with a small experimental apparatus. That is, as shown in FIG. 2, an electrolytic bath 11 is put into a small graphite crucible 10 and melted, and a round rod cathode 14 connected to a conductive rod 12 on the cathode side through an adapter 13 is connected to an electrolytic bath 11. Suspended in the center. Graphite crucible 10
Was supported by an outer shell 15 made of stainless steel, and a conductive rod 16 on the anode side was connected to the outer shell 15.

Round bar cathode 14 using graphite crucible 10 as an anode
A constant current was supplied between and to perform molten salt electrolysis. After completion of electrolysis, the electrolysis device is cooled as it is, the electrolysis device is cut along the center of the round bar cathode 14, and the cross section of the round bar cathode 14 is cut.
The adhered state of aluminum on the surface of the was observed. If the wettability of the round bar cathode 14 to the molten aluminum is good, the molten aluminum hangs down from the round bar cathode 14 leaving a thin film on the surface of the round bar cathode 14. On the other hand, if the wettability of the round bar cathode 14 with respect to the molten aluminum is poor,
The molten aluminum does not adhere to the surface of the round bar cathode 14,
It becomes a spherical droplet and drops to the bottom of the graphite crucible 10.
In the case of exhibiting intermediate wettability, interfacial tension acts between the surface of the round bar cathode 14 and the molten aluminum, and the molten aluminum adheres to the surface of the round bar cathode 14 as hemispherical droplets. Although the method of obtaining the wettability using the apparatus of FIG. 2 is qualitative, it can be said that it faithfully reflects the contact state between the cathode surface and the molten aluminum during actual molten salt electrolysis.

The experimental apparatus of FIG. 2 can also be used for measuring the current efficiency when using a cathode having good wettability with respect to molten aluminum. That is, the round bar cathode 14
The current efficiency is obtained when the aluminum hanging from the lower end of is measured and the measured value is divided by the theoretical production amount. However,
A large amount of aluminum is produced, part of which is a graphite crucible 1
When falling to the bottom of 0, aluminum is dispersed as small spheres and dissolved in the electrolytic bath 11, so that the obtained current efficiency shows a low value. Therefore, when obtaining the current efficiency, it is necessary to reduce the amount of aluminum produced so that the produced molten aluminum does not drop from the round bar electrode 14.

Using anthracite carbonaceous cathodes in which the content of zirconium boride was variously changed in the range of 0 to 90% by weight, FIG.
When the current efficiency was determined by the experimental apparatus shown in FIG. 3, when the zirconium boride content was 53.7% by weight, the maximum current efficiency was 69%. Further, when the content of zirconium boride exceeds 70% by weight, the current efficiency is drastically reduced. From this, the content of zirconium boride is
It can be seen that the upper limit is 70% by weight.

The characteristics of the bipolar electrode were investigated using the experimental apparatus shown in FIG. That is, the electric furnace 20
The flooring brick 21 is laid inside and the electrolytic cell 22 is placed on top of it.
After inserting, the upper opening is closed by the lid 23 of the heat insulating refractory. As the electrolytic cell 22, a graphite base 22a lined with sintered alumina 22b and an outer shell reinforced with a stainless steel outer shell 22c was used. A pair of bipolar electrodes 25 and 26 were immersed in an electrolytic bath 24 charged in the electrolytic cell 22. Bipolar electrodes 25, 26
Were supported by copper support rods 27 and 28 inserted into through holes 23a and 23b formed in the lid 23, respectively. Further, a stainless steel support rod 29 connected to the electrolytic cell 22 was inserted into a through hole 23c formed in the lid 23.

As the bipolar electrodes 25 and 26, intermediate layers 25b and 26 are formed on carbonaceous or graphite substrates 25a and 26a.
A stack of zirconium boride-containing surface layers 25c and 26c via b was used. One bipolar electrode 25 was connected to the cathode and the other bipolar electrode 26 was connected to the anode, and the molten salt in the electrolytic bath 24 was electrolyzed between the bipolar electrodes 25 and 26. Then, as in the case of FIG. 2, the wettability of the surface layer 25c forming the cathode surface to the molten aluminum is determined, and the amount of aluminum adhering to the surface layer 25c and the bottom of the sintered alumina 22b are deposited. The current efficiency was calculated from the amount of aluminum.

[0037]

EXAMPLES Example 1 : (Block-shaped cathode) Zirconium boride and titanium boride having an average particle size of 1 to 2 μm were blended with roasted anthracite having a particle size of 0.25 mm or less by varying the mixing amount. After the kneading, coal tar pitch having a softening point of 100 ° C. was added as a binder and further kneaded at 150 ° C. The addition amount of the binder was set within the range of 16 to 20% of the total weight according to the blending ratio of the boride so that a predetermined viscous flow could be obtained. The paste obtained by kneading is heated at a temperature of 1
It was pressure molded into a round bar having a diameter of 45 mm at 30 ° C. and a pressure of 200 kgf / cm ² . The molded body is packed in a breeze, and 100
Baked at 0 ° C.

A portion of the fired sample was analyzed to determine the zirconium boride and titanium boride contents. A round bar having a diameter of 40 mm and a length of 100 mm was cut out from the fired sample and used as the round bar cathode 14 of the molten salt electrolysis test apparatus shown in FIG. The electrolytic bath 11 has a NaF / AlF ₃ weight ratio of 1.40, a CaF content of 5 wt% and Al ₂ O.
₃ A bath composition with an initial concentration of 5% by weight was used. The temperature of the electrolytic bath 11 is maintained at 1000 ° C. and the current density is 0.8 A / cm ²
The molten salt electrolysis was continued for 4 hours.

After completion of electrolysis, the test apparatus was cooled as it was,
The cross section was observed, and the current efficiency was determined from the weight of the produced aluminum. As a result, in each of the cathodes containing zirconium boride and titanium boride in the range of 40 to 70% by weight, the produced aluminum left a thin film on the surface of the cathode, and most of it hung down from the lower end of the cathode. In contrast, the content of zirconium boride and titanium boride is 4
With the cathode of less than 0% by weight, it was shown that the produced aluminum became large spheres and adhered to the surface of the cathode, and the wettability with the molten aluminum was poor.

As far as the wettability with respect to the molten aluminum is concerned, there is no particular difference due to the contents of zirconium boride and titanium boride. However, regarding the current efficiency, the difference shown in Table 1 was produced between the zirconium boride-containing cathode and the titanium boride-containing cathode.

When the zirconium boride content and the titanium boride content are in the range of 35.3 to 72.6% by weight, all the cathodes show a current efficiency of 50% or more. However, in the range of 44.4 to 62.2% by weight, the cathode containing zirconium boride exhibits a current efficiency of more than 60%, whereas the cathode containing titanium boride has a maximum current efficiency of 58%. Has stopped at. It is speculated that such a difference between zirconium boride and titanium boride occurs due to the difference in the composite structure such as the denseness due to the difference in the bonding force with the carbon material described above.

[0042]

[Table 1]

Further, both zirconium boride and titanium boride were added together to roasted anthracite, and the effect of zirconium boride and titanium boride on the current efficiency was measured for the similarly fired cathode. As a result, the current efficiency is
As shown in Table 2, it was found that the zirconium boride content and the titanium boride content varied. A similar tendency was observed when niobium boride, tantalum boride, hafnium boride, etc. were added alone or in combination with zirconium boride.

[0044]

[Table 2]

As is clear from Table 2, zirconium boride is significantly superior to titanium boride in improving the current efficiency. In addition, the results of Table 2 are shown in Table 1.
Compared with the above, even if the zirconium boride content is the same, the current efficiency is improved by adding a slight amount of titanium boride. Example 2 : (bipolar electrode)

20% by weight of coal tar pitch having a softening point of 100 ° C. in coal-based pitch coke having a particle size of 0.25 mm or less
Was added and thoroughly kneaded at a temperature of 150 ° C. to prepare a base paste. 150 g of the base paste is evenly spread over a metal mold having a cavity having internal dimensions of 100 mm in length, 130 mm in width and 60 mm in depth, and Example 1 is placed thereon.
The paste having a zirconium boride content of 48.9% by weight prepared above was laid as a surface layer paste. At this time, an intermediate layer was formed at the contact interface by slightly roughening the contact surfaces of the base paste and the surface layer paste.

The base paste, on which the surface layer paste is laminated, is heated to 130 ° C. while being kept in the mold, and the pressure is 200 kgf.
/ Cm ² was used for molding. After removing the molded block from the mold, the block was filled in a breeze, heated to 1000 ° C., and baked.

Observation of the fired block had a laminated structure in which a zirconium boride-containing layer having a thickness of about 3 mm and a carbon layer having a thickness of about 13 mm were integrated via an intermediate layer having a thickness of about 6 mm. Two electrodes each having a width of 50 mm, a length of 130 mm and a thickness of 22 mm were cut out from this firing block. Assembling the two electrodes into the electrolytic cell 22 of the test apparatus shown in FIG. 3 as bipolar electrodes 25 and 26,
The carbon layer and the zirconium boride-containing layer were arranged as an anode and a cathode, respectively, at an interval of 10 mm.

The same fluoride bath as in Example 1 was used as the electrolytic bath 24, and molten salt electrolysis under the same conditions as in Example 1 was continued for 12 hours. As a result, the anode surface was consumed by about 5 mm due to oxygen generated by the electrolytic reaction, but the consumption condition was almost uniform over the entire area of the anode surface. The current efficiency at this time showed an extremely high value of about 60%. Further, when the laminated structure of the bipolar electrodes 25 and 26 after electrolysis was examined, no cracks or peeling were observed between the bases 25a, 26a to the intermediate layers 25b and 26b to the surface layers 25c and 26c. The structure was maintained.

[0050]

As described above, in the present invention, by using a carbonaceous or graphitic composite compound containing zirconium boride as a cathode block or a bipolar electrode for molten salt electrolysis, an extremely high current can be obtained. Molten salt electrolysis can be efficiently performed. Therefore, the consumption of electric energy required for electrolytic production of aluminum is significantly reduced, and the production cost is reduced.

[Brief description of drawings]

FIG. 1 Electrolytic cell using a conventional chloride-based electrolytic bath

FIG. 2 is an experimental electrolysis apparatus for examining the characteristics of the cathode block of the present invention.

FIG. 3 is an experimental electrolysis device for examining the characteristics of a bipolar electrode according to the present invention.

[Explanation of Codes] 10 Graphite crucible 11 Electrolytic bath 14 Round bar cathode 22 Electrolytic cell 25,26 Bipolar electrode 24 Electrolytic bath 25a, 26a Carbonaceous substrate 25b, 26b
Intermediate layer 25c, 26c Surface layer containing zirconium boride

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Isao Sekine 2641 Yamazaki, Noda City, Chiba Prefecture Tokyo University of Science (72) Inventor Makoto Yuasa 2641 Yamazaki, Noda City, Chiba Prefecture Tokyo University of Science (72) Inventor Tsutomu Wakasa Shizuoka Prefecture 5600 Kambara, Kambara-cho, Anbara-gun NIPPON DENKO CO., LTD.

Claims (4)

[Claims] 1. An electrode for aluminum electrolysis comprising a carbonaceous or graphitic composite material containing 40 to 70% by weight of zirconium boride. 2. A carbonaceous or graphite material containing, in addition to zirconium boride, one or more boride compounds of titanium, niobium, tantalum and hafnium, the total boride content being 40 to 70% by weight. Aluminum electrolysis electrode made of composite material. 3. A bipolar electrode for aluminum electrolysis, wherein a surface layer made of the carbonaceous or graphitic composite material according to claim 1 is formed on a carbonaceous or graphitic substrate. 4. A bipolar electrode for aluminum electrolysis, wherein an intermediate layer having a boride content decreasing from the surface layer toward the substrate is formed between the substrate and the surface layer according to claim 3.