JPH05247679A - Electrode for fused salt electrolysis of aluminum and electrolysis method - Google Patents

Abstract

(57) [Summary] [Objective] By incorporating an electrode group in which a plurality of electrodes are arranged face-to-face into a molten salt electrolysis furnace, production capacity is improved and equipment is downsized. [Constitution] As a molten salt electrolysis electrode, a bipolar electrode in which a ceramic-carbon composite b is integrated with a carbonaceous substrate a is used. As ceramics, TiB ₂ , ZrB ₂ or the like, which has good wettability to molten metal and is conductive, is used. An electrode group is formed by arranging a plurality of electrodes so as to face each other via a spacer c such that the surface of the composite b is the cathode surface and the surface of the carbonaceous substrate a is the anode surface. The electrode group is arranged inside the molten salt electrolysis furnace such that the cathode surface and the anode surface are in the vertical direction. [Effect] Since the surface of the composite b is always wet by the molten metal electrolytically deposited, the electrolytic reaction proceeds with excellent current efficiency. Further, since the bipolar electrodes are arranged vertically, the total surface area of the electrode group can be made large, and an electrolytic furnace having a high production capacity and a small size can be constructed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for molten salt electrolysis used for electrolytic smelting of aluminum and an electrolysis method.

[0002]

2. Description of the Related Art Aluminum is electrolytically smelted in an electrolytic bath of cryolite-aluminum fluoride system or cryolite-aluminum fluoride-calcium fluoride system in which alumina is dissolved, or sodium chloride in which aluminum chloride is dissolved.
It is manufactured by electrolytic smelting in a potassium chloride-based electrolytic bath. As an electrode used for electrolytic smelting, a consumable carbon electrode is used in a fluoride-based electrolytic bath, and a bipolar carbon electrode is used in a chloride-based electrolytic bath.

When a consumable carbon electrode is used, the carbon electrode 3 is immersed in an electrolytic bath 2 contained in a carbonaceous container 1 as shown in FIG. 1, for example. Using the carbonaceous container 1 as a cathode and the carbon electrode 3 as an anode, an electric current is passed through the electrolytic bath 2 heated to about 1000 ° C. At this time, the side wall is insulated by the solidified layer 2a so that the current does not flow to the side wall.

On the bottom surface of the carbon electrode 3, aluminum oxide dissolved in the molten salt undergoes an electrolytic reaction. The generated molten aluminum is molten metal layer 4 at the bottom of the carbonaceous container 1.
To form. Oxygen generated by the electrolytic reaction of aluminum oxide reacts with the carbon electrode 3 at the anode and becomes carbon dioxide gas.
The carbon dioxide gas rises along the side surface of the carbon electrode 3 and is exhausted from the reaction system.

The carbon electrode 3 is consumed as the electrolytic reaction progresses. Therefore, the carbon electrode 3 is lowered according to the consumption, and the inter-electrode distance between the carbon electrode 3 and the molten metal layer 4 is kept constant. As a result, the electrolysis conditions are kept constant and the aluminum electrolytic deposition reaction proceeds smoothly. The molten aluminum deposited on the molten metal layer 4 is taken out of the furnace by an appropriate suction container.

When a bipolar type electrode is used, as shown in FIG. 2, a plurality of carbon electrodes 3 ₁ , 3 ₂ ... 3 _{n are used.}
Are vertically connected via the non-conductive spacer 5.
Flat carbon plates are used for the carbon electrodes 3 ₁ , 3 _2, ... 3 _n . The spacer 5 is made up of individual carbon electrodes 3 ₁ , 3 _2.
A material for maintaining a constant distance between 3 _n and having excellent durability against electrolytic reaction and erosion by molten salt is used. In this case, the carbon electrodes 3 ₁ , 3 _2, ... 3
_n is not consumed.

Carbon electrodes 3 ₁ , 3 ₂ ... Connected to each other
3 _n is immersed in the electrolytic bath 2 contained in the smelting vessel 6. Since the smelting vessel 6 does not work as a current-carrying member,
It is constructed with a refractory brick such as silicon nitride, which has excellent durability against corrosion by molten salt or molten aluminum. As the electrolytic bath 2, a solution in which aluminum chloride charged from the supply port 7 is dissolved in a molten salt of sodium chloride-potassium chloride system is used.

Carbon electrodes 3 ₁ , 3 ₂ · immersed in the electrolytic bath 2
・ Energize so that the upper part of 3 _n is on the cathode side and the lower part is on the anode side. When electrolysis is performed by holding the electrolytic bath 2 at about 700 ° C., the upper surface of each carbon electrode 3 ₁ , 3 _2, ... 3 _n serves as an anode, and an electrolytic deposition reaction of aluminum occurs.
The generated molten aluminum is used for the carbon electrodes 3 ₁ , 3 ₂ ...
・ Smelting vessel 6 by flowing down in the electrolytic bath 2 along the side of 3 _n
To form a molten metal layer 4 on the bottom of the.

On the other hand, the generated chlorine gas is generated by the carbon electrode 3
It goes up in the electrolytic bath 2 along the side surfaces of ₁ , 3 ₂ ... 3 _n , and is exhausted from the reaction system. In order to regulate the discharge of chlorine gas, the individual carbon electrodes 3 ₁ , 3 _2, ... 3 _n are provided with downward projections 3 a that regulate the gas flow direction. The downward protrusion 3a suppresses the rightward flow in FIG.
The generated chlorine gas rises from the left side of the carbon electrodes 3 ₁ , 3 _2, ... 3 _n and is discharged to the outside of the system through the exhaust port 8.

[0010]

In electrolytic smelting using a molten salt of alumina-cryolite-aluminum fluoride system,
The influence of the electromagnetic force acting on the molten metal layer 4 appears. The molten metal layer 4 is agitated by the electromagnetic force, and causes a bulge, a flow, and a wave phenomenon. In order to maintain the electrolysis conditions constant without being affected by the movement of the molten metal layer 4, the distance between the molten metal layer 4 and the carbon electrode 3 is set to about 40 mm or more. Therefore, the power loss is large and the power source unit cannot be 10,000 kWh / ton or less.

When the current capacity is increased, the molten metal layer 4
The agitation becomes active and the distance between the poles fluctuates significantly. The change in the distance between the electrodes not only makes the electrolysis conditions unstable, but also
In extreme cases it also leads to inoperable situations. In this respect, the structure shown in FIG. 1 has a current capacity of 250 to 300K.
It is not possible to design a furnace with a capacity of A or higher, in other words, an aluminum production capacity of 2 tons / reactor.

Since the carbon electrodes are horizontally arranged, the larger the furnace, the larger the floor area. As a result, hundreds of electric furnaces are installed and operating in a building with a length of several hundred meters. The installation of a large number of electric furnaces in this way causes a great burden of equipment and increases the manufacturing cost of aluminum in combination with the smelting process which consumes a large amount of electric power.

On the other hand, in the molten salt electrolysis using the bipolar type electrode of the chloride-based molten salt shown in FIG. 2, chlorine is generated, so that it is necessary to completely seal the electrolytic furnace. Further, as a source of aluminum to be charged into the electrolytic bath, a pre-process for converting alumina into aluminum chloride is required, which is economically disadvantageous. In addition, since the food material that withstands the high temperature chloride-based molten salt is expensive, the burden on the facility also increases.

The present invention has been devised to solve such a problem, and uses a bipolar type electrode in which a composite of ceramics and carbon is integrated on the cathode side with the reaction surface being vertical. By doing so, it is an object to enhance the current consumption efficiency and perform molten salt electrolysis without increasing the size of electrolysis equipment.

[0015]

In order to achieve the object, a molten salt electrolysis electrode of the present invention has a carbonaceous anode part and a heat-resistant / conductive ceramic and carbon excellent in wettability to molten aluminum. It is characterized in that it is integrated with the cathode part made of the composite of, and is installed with the reaction surface vertical.

The method for electrolysis of molten aluminum is as follows:
A cathode part made of a composite of heat-resistant / conductive ceramics and carbon showing excellent wettability to molten aluminum is opposed to a carbonaceous anode part, and a plurality of non-conducting and non-consumable spacers are provided between the cathode part. The electrodes are arranged so as to face each other in the horizontal direction, are immersed in a molten salt, and electrolytically generated molten aluminum is caused to flow down in the molten salt.

A carbon plate is used as the carbonaceous anode part. On the other hand, the composite that becomes the cathode part is created by firing a mixture of ceramics and carbon. As ceramics, TiB ₂ , ZrB ₂ or the like, which has good wettability to molten aluminum, and has excellent heat resistance and conductivity, is used. Further, since it is used in an atmosphere in contact with molten salt or molten aluminum, it is preferable that it has excellent corrosion resistance, erosion resistance and the like.

[0018]

[Working] As shown in FIG. 3, the molten salt electrolysis electrode of the present invention comprises a carbonaceous substrate a, a ceramic-carbon composite b.
Are integrally laminated. The composite b has good wettability to molten aluminum, and the entire surface of the composite b is uniformly wet with the molten metal deposited from the molten salt by electrolytic reaction or the like. Therefore, the surface of the complex b is always maintained in a state suitable for the electrolytic reaction, and the current efficiency is improved.

The complex b having good wettability makes it possible to reduce the distance between adjacent electrodes when a plurality of electrodes are arranged via the spacer c as shown in FIG. Further, the carbon contained in the composite b ensures the conductivity, and thus the current loss due to the increase in the conduction resistance is not caused. Therefore, a high-capacity electrolysis furnace can be constructed without increasing the size of the equipment, and it becomes possible to significantly reduce the electric power consumption per unit during molten salt electrolysis.

TiB as a ceramic component of the composite b
_{When 2} is used, since TiB ₂ is an extremely expensive material, it is combined with the carbon component at the minimum content that ensures the wettability of molten aluminum. The disadvantage of TiB ₂ being vulnerable to thermal shock is overcome by its combination with a carbon component. The TiB ₂ content of the composite b is preferably determined in the range of 40 to 80% by weight in consideration of the above. Further, the composite b exhibits a desired function sufficiently with a thickness of about 0.1 to 1 mm.

When a plurality of electrodes are arranged adjacent to each other as shown in FIG. 4, the spacer c inserted between the electrodes is made of Si.
_{3 N 4, BN, AlN,} Al 2 ON and insulating ceramics such as mixed sintered body thereof is used. Spacer c
At the time of molten salt electrolysis, the base end portion is inserted into the hole portion d formed in the composite body b on the cathode side. The tip of the spacer c that contacts the carbonaceous substrate a on the anode side is preferably formed into a hemispherical curved surface so that the contact area with the carbonaceous substrate a is reduced. This suppresses the consumption delay of the anode carbon portion at the contact portion with the spacer c, and the carbonaceous substrate a.
The uniform consumption of

[0022]

Example As the carbonaceous substrate a, a carbon plate having a length of 200 mm, a height of 80 mm and a thickness of 30 mm was used. Mixing ratio of TiB ₂ powder with an average particle size of 2 μm to carbon paste is 5
0% by weight and mixed on one side of the carbonaceous substrate a in an amount of 150 to 20
It was applied to a thickness of 0 μm. And 1 in an inert atmosphere
A composite b integrated with the carbonaceous substrate a was formed by baking at 300 ° C.

In the prepared bipolar electrode composite b,
4 holes 10d with a depth of 10 mm for inserting the spacer c
I drilled one by one. The base end of the spacer c having a length of 30 mm was inserted into the hole d, and the hemispherical end of the spacer c was brought into contact with the carbonaceous substrate a of the adjacent bipolar electrode. Thus, the distance between the electrodes was set to 20 mm to form an electrode group in which two bipolar electrodes were arranged in series so as to face each other.

The electrode group was made into 5% Al ₂ O ₃ -80% Na ₃ Al ₂ F ₆ -1 so that the surface of the composite b was a vertical surface.
It was placed in a 0% AlF ₃ -5% CaF ₂ electrolytic bath. A movable anode and a semi-fixed cathode are placed at both ends of the electrode group, and a current of 160 A is applied.
Was supplied. The current density at this time was 1 A / cm ² . When energized for 24 hours, about 2.86 Kg
Aluminum was collected. In this molten salt electrolysis, current efficiency is 72%, electric power consumption rate is 11,000 kW
It was h / ton-Al.

For comparison, a carbonaceous substrate a which does not form the composite b was used as an electrode to assemble an electrode group, and molten salt electrolysis was performed under the same conditions. About 2.39 Kg of aluminum was collected by energizing for 24 hours. The current efficiency at this time is 62%, and the power consumption rate is 13,000 kWh /
Ton-Al.

As is clear from the comparison in this lab test, it is found that a large amount of aluminum can be produced with a small amount of power consumption by using the electrode in which the composite body b is integrated with the carbonaceous substrate a. Further, since the distance between the electrodes can be set small, a large number of electrodes are incorporated in a limited space to construct an electrolytic furnace with improved production capacity.

The molten salt electrolysis electrode of this embodiment is incorporated in the molten salt electrolysis furnace shown in FIG. Furnace body 10 of electrolysis furnace
Lining 11 insulating bricks with 12 refractory bricks,
It has a structure reinforced by a steel furnace shell 13. The bottom of the furnace is inclined in one direction, and the metal pool 14 is provided on the lower side. A tap hole 15 for taking out molten aluminum from the metal pool is provided on the side wall of the furnace on the side of the metal pool 14. The refractory bricks 12 other than the metal pool 14 are not lined with the heat insulating bricks 11 and the freeze 61 is formed on the inner surface of the furnace wall. The upper wall 16 of the furnace body 10 is provided with an exhaust port 17 for taking out exhaust gas such as carbon dioxide gas generated by the electrolytic reaction.

The electrode loading portion 20 opened at one end of the upper portion of the furnace body 10 is closed by an electrode insertion lid 21. The current-carrying rod 23 branched from the anode bus bar 22 has an electrode insertion lid 21.
Is inserted into the through hole 24 and inserted into the furnace body 10. The anode bus bar 22 is partially formed with a flexible portion 25 so that the anode bus bar 22 can be slightly expanded and contracted in the arrangement direction of the in-reactor electrodes, that is, in the horizontal direction in FIG. A force in the left direction in FIG. 5 is applied to the anode bus bar 22 having the flexible portion 25, and the entire anode is pushed to the left side.

An electrode extraction lid 3 is provided on the upper side of the furnace body 10 on the opposite side.
The electrode lead-out portion 30 closed by 1 is open. The electrode extraction lid 31 is formed with a through hole 34 into which the energizing rod 33 branched from the cathode bus bar 32 is inserted.

An alumina feeder 40 is provided above the furnace body 10 to replenish the alumina consumed as the electrolytic reaction progresses. The alumina feeder 40 includes a plurality of branch pipes 42 branched from a main pipe 41 in the longitudinal direction of the furnace body 10. The lower end of the branch pipe 42 is exposed to the inside of the furnace through the through hole 18 formed in the upper wall 16. Alumina is replenished from the branch pipe 42 in a substantially even distribution in the cross section of the furnace.

A non-consumable cathode 51 is fixed to the lower end of the current-carrying rod 33 on the cathode side. A bipolar electrode 53 ₁ is arranged to face the non-consumable cathode 51 via a spacer 52. Further, similarly to the bipolar electrode 53 ₁ , the bipolar electrode 53 ₂ is arranged to face the bipolar electrode 53 ₁ with the spacer 52 interposed therebetween. In this way, the n bipolar electrodes 53 ₁ ,
53 _2, the 53 ₃ · · · 53 _n arranged in series in terms opposite, constituting the electrode group 50. The semi-fixed anode 54 on the anode side is supported by the current-carrying rod 23.

Individual bipolar electrodes 53 ₁ , 53 ₂ , 5
3 ₃ ... 53 _n has a specific gravity such that it floats in the electrolytic bath 60. Therefore, the electrode group 50 is sandwiched between the non-consumable cathode 51 and the semi-fixed anode 54, and the bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃ The space between 53 _n is always held at a predetermined interval by the spacer 52 even when the anode side is consumed due to the electrolytic reaction.

The electric current i during molten salt electrolysis reaches the electrode group 50 from the anode bus bar 22 through the current-carrying rod 23 as shown by the arrow in FIG. The current i depends on the individual bipolar electrode 53.
53 _n passing through ₁ , 53 ₂ , 53 ₃ ... 53 _n, and is sent from the cathode bus bar 32 to the outside of the system via the current-carrying rod 33. As the electrolytic reaction proceeds, aluminum is deposited on the cathode surface. A freeze 61 is formed on the inner surface of the refractory brick 12 in which the molten salt of the electrolytic bath 60 is solidified.

Bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃
.. On the surface (anode side) of the 53 _n carbonaceous substrate, alumina is dissolved in the electrolytic bath 60 and Al ₂ O ₃ → 2Al ³⁺ + 3O ²⁻
Then, carbon dioxide gas is generated by the reaction of C + 2O ^2- → CO ₂ (g) + 4e ⁻ , and the carbonaceous substrate is consumed. Ti
On the surface of the B ₂ -carbon composite (cathode side), Al ³⁺ + 3e →
The reaction of Al occurs and metallic aluminum is deposited. The generated molten aluminum is used for the bipolar electrodes 53 ₁ , 53.
After wetting the cathode surface of ₂ , 53 ₃ ... 53 _n , the electrolytic bath 60 is made to flow down, and is collected in the metal pool 14 along the inclined surface of the freeze 61 formed on the inner wall of the furnace bottom 10. The molten aluminum in the metal pool 14 is the tap hole 1
5 is taken out continuously or intermittently.

By continuing the molten salt electrolysis, the carbonaceous substrate of the bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃ ... 53 _n is consumed and thinned. The bipolar electrode 53 ₁ thinned to about 10 cm is taken out from the electrode group 50 in the furnace by removing the electrode taking-out lid 31 and opening the electrode taking-out portion 30.
On the other hand, the electrode insertion lid 21 is removed to open the electrode insertion portion 20, and the new bipolar electrode is sandwiched between the bipolar electrode 53 _n and the semi-fixed anode 54. This electrode extraction and electrode insertion are performed intermittently, and the bipolar electrodes 53 ₁ and 53 ₂
, 53 ₃ ... 53 _n are sequentially sent from the anode side to the cathode side.

Since the semi-fixed anode 54 is a consumable anode, it needs to be replaced. The current cutoff at the time of replacement is, for example, as shown in FIG.
This can be prevented by dividing the electrode 54 into a plurality of parts and fixing the current-carrying rod 23 to each electrode as shown in FIG. Further, the non-consumable cathode 51 also reduces the wettability in the long term. Therefore, the non-consumable cathode 51 is also replaced in a long cycle. To prevent current interruption during this replacement,
The non-consumable cathode 51 is also preferably divided into a plurality of pieces.

Bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃
.. Since the molten salt electrolysis is carried out while keeping the distance between the electrodes 53 _n constant, the electrolysis conditions are kept constant. In addition, the electrolytically deposited molten aluminum is used to form the bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃
The current efficiency is improved because the cathode surface of 53 _n is constantly wetted.

Bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃
.. The molten aluminum that wets the cathode surface of 53 _n is removed from the electrolytic reaction region by flowing down the electrolytic bath 60. Therefore, the molten metal layer 4 is swelled or waved by the electromagnetic bath described with reference to FIG. Does not interfere with the electrolytic reaction. That is, in the conventional molten salt electrolysis, the distance between the poles is 40 m due to the magnetic field problem.
In the present example, the distance between the electrodes could be set to 30 mm or less, although the distance could not be set to m or less. As a result, it is possible to reduce the furnace voltage and the electric power consumption rate. According to trial calculation, 10,000 kWh / ton-A
It is also possible to reduce the power consumption rate to 1 or less.

Since the electrolysis furnace shown in FIG. 5 is a fixed interelectrode system, the electrolysis temperature cannot be controlled by adjusting the interelectrode distance as in the conventional electrolysis furnace. Therefore, it is preferable to provide a rectifier for each electrolysis furnace and control the current value supplied to each electrolysis furnace to achieve heat balance of the electrolysis furnace.

When the electrode group 50 shown in FIG. 5 is used, the furnace current is such that the bipolar electrodes 53 ₁ , 53 ₂ , 53 ₃ ...
-It is almost determined by the total area of 53 _n , and several tens of KA is sufficient. Further, since the distance between the electrodes is set to be small, the bipolar electrodes 53 ₁ , 53 ₂ , incorporated in the electrode group 50,
The number of 53 ₃ ... 53 _n can be increased. Moreover, the cathode surface and the anode surface are made vertical to form the bipolar electrode 53.
_1, 53 _2, 53 ₃ for which a · · · 53 _n is the surface facing inside the furnace structure becomes three-dimensional, space saving can be achieved.

For example, in the conventional molten salt electrolysis equipment, the production amount of 2 tons / reactor / day was limited at a supply current of 300 KA due to a magnetic field problem. Moreover, the number of electrodes arranged horizontally is 3
The size is about m × 12 m. On the other hand, in the design of FIG. 5, the bipolar electrodes 53 ₁ , 53 ₂ , 53 of the same size are used.
_{It is possible} to construct a melting electrolysis facility with a production capacity of about 5 tons / reactor / day using ₃ ... 53 _n , and yet reduce the space per unit to less than half. Thus, since the production capacity per unit is high and the furnace structure itself is small, it is possible to reduce the number of furnaces in the electrolytic plant and shorten the length of the building.

[0042]

As described above, in the present invention, the electrode in which the ceramic-carbon composite is integrated with the carbonaceous substrate is used. The ceramic-carbon composite is
It has good wettability with molten metal, and the surface of the cathode is uniformly wetted by the electrolytically deposited molten metal, and the electrolytic reaction proceeds with excellent current efficiency. Further, when a large number of electrodes are arranged face-to-face with the cathode surface and the anode surface being vertical, an electrode group having a large surface area is formed. Moreover, the electrolytically deposited molten metal is removed from the electrolytic reaction zone by wetting the cathode surface and then flowing down the electrolytic bath. Therefore, the distance between the electrodes can be set to be significantly small, without the electrolytic reaction being hindered by the swelling and wave motion of the molten metal layer found in the conventional molten electrolytic equipment equipped with the horizontally arranged electrodes. Therefore, the furnace itself can be downsized while ensuring high production capacity.

[Brief description of drawings]

FIG. 1 Conventional molten salt electrolysis furnace in which carbon electrodes are horizontally arranged

FIG. 2 Conventional molten salt electrolysis furnace with bipolar electrodes arranged one above the other

FIG. 3 Bipolar electrode according to the invention

FIG. 4 shows a state in which a plurality of the electrodes are arranged face to face.

FIG. 5: Molten salt electrolysis furnace incorporating an electrode group in which a plurality of the electrodes are arranged horizontally in a face-to-face relationship

FIG. 6 is a sectional view of the same molten salt electrolysis furnace as seen from the direction of arrangement of electrode groups.

[Explanation of symbols]

a carbonaceous group (anode side) b ceramics-carbon composite (cathode side) c, 52 spacer 50 electrode group 51
Non-consumable cathode 53 ₁ , 53 _2, ... 53 _n Bipolar electrode 54
Semi-fixed anode

Claims (2)

[Claims] 1. A carbonaceous anode part and a cathode part made of a composite of heat-resistant / conductive ceramics and carbon which are excellent in wettability with respect to molten aluminum are integrated and installed with a reaction surface vertical. An electrode for molten aluminum salt electrolysis, which is characterized in that: 2. A non-conductive and non-consumable spacer in which a cathode part made of a composite of heat-resistant / conductive ceramics and carbon showing excellent wettability to molten aluminum is opposed to a carbonaceous anode part. A method for electrolyzing a molten aluminum salt, characterized in that a plurality of electrodes are horizontally faced to each other via a plate and immersed in the molten salt, and the electrolytically generated molten aluminum is caused to flow down in the molten salt.