JPS581192B2 - Aluminum hardware - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a prefired carbon electrode for an aluminum molten salt electrolytic furnace having a layer for corrosion protection on the terminal supporting the anode.

In a multi-polar aluminum molten salt electrolytic furnace that uses a pre-fired carbon electrode as an anode, the carbon electrode is generally suspended by a steel rod (hereinafter referred to as a terminal) which also serves as a conductive terminal.

When an electrolytic bath at a temperature of 950 to 1000° C. is placed in an electrolytic furnace and direct current is applied with the carbon electrode as an anode and the aluminum metal at the bottom of the furnace as a cathode, oxygen is electrolytically generated at the anode and aluminum is generated at the cathode.

Aluminum produced at the bottom of the furnace is taken out in a timely manner and used as a product, but the oxygen produced at the anode reacts with the anode, which consumes the carbon that is the electrode.

The amount of carbon consumed is proportional to the amount of electricity that can pass through the anode, and in actual operation of an electrolytic furnace, the electrode is replaced with a new one after a predetermined number of days of use.

FIG. 1 is a schematic diagram showing the relationship between the electrolytic bath and the electrodes when the electrodes are new.

In the figure, 1 is a pre-sintered electrode 1 which is suspended by a steel bar or terminal 2 and is in contact with an electrolytic bath 3, and 4 is a cathode made of molten aluminum.

FIG. 2 is a schematic diagram showing the relationship between the electrode and the electrolytic bath immediately before replacement after use for a predetermined number of days.

When the electrode is first used, the upper surface of the electrode is positioned well above the surface of the electrolytic bath, as shown in FIG. 1, and there is no danger of the terminal portion coming into contact with the electrolytic bath.

However, as the days of use pass, the anode gradually wears out due to the oxidation from the bottom.

The distance between the anode and the cathode must always be maintained within a certain range, and the position of the anode is gradually lowered.

The position of the terminal also gradually lowers until it comes into contact with the electrolytic bath and is further immersed in the bath, until a considerable portion of the terminal is present in the electrolytic bath as shown in FIG.

In the electrolytic bath, the terminal is electrolytically oxidized, e.g.
It is eroded as shown in .

When the terminal is eroded, iron is leached into the electrolytic bath and deposited at the bottom of the furnace, reducing the purity of the aluminum product.

Conventionally, in order to prevent the terminal part from being eroded by the electrolytic bath, a mixture of coke and pitch was used to cover the part that might be immersed in the electrolytic bath when the terminal deteriorated as the anode wore out, as shown in Figure 3. was.

In FIG. 3, 6 indicates a mixture of coke and pitch to prevent corrosion of the terminal.

A terminal corrosion prevention material is required to satisfy the following three conditions.

(1) Resistance to air oxidation during use.

(2) Do not react with electrolytic oxidation, oxidation by electrolytically generated gas, or electrolytic bath until the end of use.

(3) The preventive material itself does not contain substances that reduce the purity of aluminum in the product during electrolysis.

Although the above-mentioned mixture of coke and pitch satisfies some of the terms (3) and (2), since carbon itself is easily oxidized, (
Items 1) and (2) are not fully met, and terminal erosion often occurs during operation.

The present invention provides that a mixture of coke, pitch, alumina and cryolite is superior to a mixture of coke and pitch as a terminal corrosion inhibitor, and that a mixture of coke, pitch, alumina and cryolite is superior to the mixture of coke and pitch.
This was done as a result of elucidating that it satisfies the requirements.

That is, the mixture of coke, pitch, alumina, and cryolite satisfies (1) of the three conditions as the terminal corrosion prevention material.
, For (2), the oxidation resistance of alumina and cryolite effectively acts, and for (3), coke, pitch, alumina, and cryolite are all raw materials or sub-raw materials for aluminum production. Therefore, there is no need to worry about lowering the purity of aluminum.

Based on the above facts, the present inventor completed a pre-fired carbon electrode for an aluminum molten salt electrolytic furnace having a layer of a composition consisting of coke, pitch, alumina and cryolite on the surface of the terminal supporting the anode. did.

Alumina in the composition has a slow elution rate into the electrolytic bath α
It is preferable to use a high quality one.

Furthermore, as for alumina and cryolite, the bottom sediment of the electrolytic cell is substantially homogeneous, and this sediment can be used.

The use of this precipitate is also effective as a method for treating industrial waste.

Furthermore, in the composition, coke, alumina, and cryolite function as aggregates, and pitch functions as a binder for them.

During use in an electrolytic furnace, pitch as a binder is heated by heat transfer from a carbonaceous electrode, high-temperature ambient gas, etc., and undergoes a softening process and a coking process, and is then sintered.

During the softening process, the pinch invades the voids between the aggregates, but if the amount of pitch is too large, the pitch will flow out with the aggregate, making it impossible for the composition to maintain the shape of the protective layer; cannot completely fill the voids in the aggregate, increasing the porosity and reducing the mechanical strength of the protective layer.

During the coking process, the pitch is sintered while creating a crosslinked structure with the coke present as aggregate.

In this case, if there is little coke present as aggregate, a sufficient crosslinked structure will not be formed and the mechanical strength of the protective layer will decrease.
There is a risk that the protective layer will crack, peel, or fall off during operation.

In addition, alumina and cryolite have excellent oxidation resistance, as mentioned above (1).
) It satisfies the conditions of (2), but it dissolves when it comes into contact with an electrolytic bath.

Therefore, it is necessary that the alumina and cryolite be present in the protective layer in sufficient quantities so that, even when dissolved, the protective layer exhibits sufficient corrosion resistance from the time the anode begins contact with the electrolytic bath until the anode is replaced.

The ratio of alumina and cryolite is determined by taking into consideration the equilibrium relationship between the two.

For the above reasons, the practical blending ratio in the protective layer in which each component can fully exert its effect is coke 30 to 6% by weight.
0, pitch 20-30, alumina 20-40, cryolite 2
It ranges from 0 to 40.

The method for forming a protective layer of the composition on the terminal part is a known method, for example, the method of mixing each composition so that it is uniform and then applying it to the required area, or the method of surrounding the terminal part with an aluminum plate, The outer peripheral surface of the terminal part,
A method is used in which the composition is densely filled into the gap between the aluminum plate and the aluminum plate.

Next, the effects of the anode of the present invention will be demonstrated by Examples and Comparative Examples.

Example 1 A layer of the composition shown below as a protective material was formed on the terminal portion under the following conditions and operated for 6 months in a 150KA pre-sintered aluminum electrolytic furnace.

Flowery Alumina 20% by weight Cryolite 20〃 Coke 40〃 Pinch 20〃 The results are shown in the table.

Example 2 The following composition was prepared using precipitates mainly composed of alumina with a high degree of gelatinization and cryolite accumulated at the bottom of an aluminum electrolytic furnace, and the composition was prepared in a 150KA pre-sintered aluminum electrolytic furnace. The test was conducted in the same manner as in Example 1.

Precipitate (44.5% cryolite, fluorides of Ca and Al 8.
3%, highly pregelatinized alumina 47.2%) 50% by weight Coke 30 Pinch 20 The results are shown in the table.

Conventional Example (Comparative Example) A terminal was coated with a mixture of 80% by weight coke and 20% by weight pitch in a 150KA pre-fired aluminum electrolytic furnace and tested in the same manner as in Example 1.

The results are shown in the table.

The numerical values in the table indicate the incidence of melting damage observed on the terminal surface after the test was completed, and the effect of the present invention is remarkable compared to the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the positional relationship between the electrolytic bath and the electrodes when the electrodes are first used. FIG. 2 is a schematic diagram showing the positional relationship between the electrolytic bath, electrodes, and terminals immediately before electrode renewal. Figure 3 shows a pre-fired electrode with a protective layer on the terminal. 1... Carbon anode, 2... Terminal,
3... Electrolytic bath, 6... Mixture for corrosion prevention.

Claims (1)

[Claims] 1. A pre-fired carbon electrode for an aluminum molten salt electrolytic furnace having a protective layer of a composition consisting of coke, pitch, alumina and cryolite on the surface of a steel terminal supporting the anode.