JPS5848634B2 - Aluminum manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an industrially advantageous method for producing aluminum by subjecting aluminum chloride to molten salt electrolysis.

? Since then, aluminum has been industrially produced by the so-called Hall-Heroux method, in which aluminum oxide (alumina) is dissolved in a metal fluoride molten salt electrolytic bath mainly composed of molten cryolite, and electrolyzed using carbon as an anode.

However, in principle, the Hall-Heroux method requires a large amount of electrical energy for electrolytic reduction of alumina, and in fact, the electric power required to produce one ton of alumium, that is, the electric power consumption rate, is 14,000 kWh/t or more. Therefore, there is a strong demand for the development of an aluminum manufacturing technology that can reduce electrical energy consumption.

As a promising electrical energy-saving aluminum manufacturing method that replaces the Hall-Helloux method, aluminum chloride and N
An electrolytic method of aluminum chloride is known in which aluminum chloride is dissolved in a molten salt bath of alkali metal chloride such as aCl or KCI and electrolyzed.

In this aluminum chloride melting electrolysis method, the electrolysis temperature is
It has many advantages, such as being able to operate at a temperature of around 700°C, which is approximately 300°C lower than the Elou method, and because the anode reaction is a chlorine generation reaction, the graphite electrode used in the anode is non-consumable. Despite this, it has remained unused industrially for a long time due to the difficulty of handling high-temperature aluminum chloride and generated chlorine gas, and the inability to obtain suitable bath-resistant materials industrially. However, in recent years, Alcoa Corporation of the United States has developed an aluminum chloride electrolytic method (Alcoa method) using a new electrolyzer and an electrolytic bath with a new bath composition.
was proposed (Japanese Patent Application Laid-Open No. 48-36006 and
-50910), suddenly attracted industrial attention.

This Alcoa method uses AICl containing LiCl at a high concentration.
3 Using a LiCI NaCl-based molten salt electrolytic bath, carbon (graphite) electrode plates are stacked in layers with appropriate gaps in a tank lined with a nitride-based refractory material to form a horizontal double electrode. Chlorine? This method produces molten aluminum on the anode surface and molten metal aluminum on the cathode surface.
A special material with excellent fire resistance and corrosion resistance is used for the electrolytic cell container, and Li, which has high electrical conductivity, is used to reduce the voltage drop in the bath.
It is characterized in that Cl is introduced as a component of the molten salt electrolytic bath, and the distance between the electrodes is narrowed in order to further reduce the electric power consumption.

However, the lithium chloride used in the electrolytic bath is industrially expensive, and the current efficiency when using this type of electrolytic bath is about 85% at most, so there is a limit to improving the power consumption rate. Therefore, it is desired to develop an industrially more advantageous method for producing aluminum.

As a result of various studies, particularly regarding the composition of the electrolytic bath, the inventors found that A
It has been found that when aluminum chloride is electrolyzed using an ICI3 MgCl2NaCl mixed molten salt as an electrolytic bath, aluminum can be produced with extremely high current efficiency.

That is, the present invention melts and electrolyzes aluminum chloride together with a metal halide in an electrolytic bath to apply chlorine gas to the anode surface.
In addition, in a method for producing metallic aluminum by generating molten aluminum on the cathode surface, an electrolytic bath A
ICl32-20% by weight, MgCl220-70% by weight
, NaC 178 to 10% by weight.

According to the present invention, metallic aluminum can be produced by electrolyzing aluminum chloride with a high current efficiency of about 90% or more and close to 97%, so that the electric power consumption can be significantly reduced.

AIC l 3 -MgC used in the present invention
l The composition range of the 2-NaCl molten salt electrolytic bath is AIC1
32 to 20% by weight, MgC12 20 to 70% by weight, and Nail 78 to 10% by weight.

It is not clear why a high current efficiency is obtained when aluminum chloride is electrolyzed using a bath containing such a relatively large amount of MgC1z.

However, the most important reason for the decrease in current efficiency in aluminum chloride electrolysis is that part of the aluminum deposited on the cathode surface dissolves in the electrolytic bath, and this reacts with the chlorine gas generated on the anode surface, resulting in a reverse reaction that returns to AICl3. AICI3? has the above composition range from the viewpoint of the solubility of aluminum in the bath, the viscosity of the bath, the wettability of aluminum to the bath, etc. MgC 1
It is presumed that the 2 NaCl mixed molten salt has the effect of effectively suppressing the above-mentioned reverse reaction.

AIC l 3-MgC 12-NaC in the present invention
In an I-based molten salt electrolytic bath, if the AICl3 concentration in the bath exceeds 20% by weight, the electrical conductivity of the bath will decrease significantly;
In addition, the vapor pressure of the bath becomes excessive, leading to an increase in cell voltage and unstable operation of the furnace, so it is desirable to limit the AICl3 concentration to a maximum of 20% by weight.

A Also, in this system of baths, NaCl and MgCl
2 forms a uniform mixed bath ratio over a relatively wide composition range together with AICl3, so it can be used extremely stably.

However, the i current efficiency during electrolysis tends to improve as the content ratio of MgC12 to NaC1 increases, so the content of NaCl and MgC12 should be kept at a weight ratio of MgCl 2 /NaCl=1 or higher. is desirable.

However, if the MgCl2 content exceeds 70 wt%, the conductivity of the bath will drop significantly, so it should be 707 wt% or less, preferably 65 wt% or less, and if it is less than 20 wt%, the effect of improving current efficiency will be poor. 20% by weight
The above will be adopted.

Electrolytic conditions 1 for stable operation of aluminum chloride electrolysis using the electrolytic bath according to the present invention described above vary depending on the type and capacity of the tank, but generally the bath temperature is 680 to 7.
Continuous electrolytic production of aluminum with a current efficiency of approximately 90% or more by performing electrolysis within this range at 80°C, current density 0.5 to 2.0 IJcri, and interelectrode distance 10 to 251 nTlt. I can do it.

Furthermore, when the electrolytic bath of the present invention is used, the cathode collapse phenomenon that is sometimes observed in this type of electrolytic method does not occur.

Note that when performing aluminum chloride electrolysis according to the present invention, horizontal electrodes such as those found in the Alcoa method can be used in the electrolytic cell, but in general, in this type of electrolytic method, the distance between the electrodes is relatively small. In order to improve current efficiency, it is desirable to quickly remove aluminum generated on the surface and prevent it from reacting with chlorine gas generated on the anode surface.

As a result of studying the electrodes in particular from this point of view, it was found that when the electrodes are tilted, the generated aluminum can be easily flowed out from between the electrodes.

Therefore, the inclination of the plate-shaped or rod-shaped electrode is 60 degrees with respect to the horizontal plane.
° or less, preferably 5 to 45 °, and the inclination angle is 60 °
If it exceeds 0, the current efficiency will decrease on the contrary.

Furthermore, in the method of the present invention, a small amount of LiCl is added to the electrolytic bath.
It is also advantageous to increase the electrically conductive layer of the bath by adding .

Next, the present invention will be explained in more detail using specific examples.

Example 1 AIC 138.3% by weight, MgC 1256.7% by weight,
AICl3-? with a bath composition of 35.0% by weight NaCl?
gC 1 2 An inclined graphite electrode plate with an angle of 30° to the horizontal (effective reaction surface 85 x 25
11!711) to maintain the interelectrode distance at 1411Lm, bath temperature 750'C, current 20A, current density IA/c/L.
, electrolyzed continuously for 4.5 hours at a cell voltage of 3.30 V.
．． 6g of aluminum was obtained.

The current efficiency at this time was 98%.

Claims (1)

[Scope of Claims] 1. A method for producing metallic aluminum by melting and electrolyzing aluminum chloride together with a metal halide in an electrolytic bath to produce chlorine gas on the anode surface and molten aluminum on the cathode surface. AIC electrolytic bath
I32-20% by weight MgC1220-70% by weight, N
A method for producing aluminum, characterized in that the mixed composition consists of 78 to 10% by weight of aCl. 2. A patent claim characterized in that plate-shaped or rod-shaped graphite electrodes are used as both the negative and positive electrodes, and the longitudinal axes of the electrodes are provided in parallel in an inclined manner so that the angle is 600 degrees or less with respect to the horizontal plane. A method for producing aluminum according to scope 1.