JPS5948957B2 - Improved electrolytic production of aluminum - 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.

Conventionally, aluminum has been produced industrially 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 with carbon as an anode. There is.

However, in principle, the Hall-Heroux method requires a large amount of electrical energy for the electrolytic reduction of alumina, and in fact, the electric power required to produce one ton of aluminum, that is, the electric power consumption rate, is 14,000 kWh/or less. Therefore, there is a strong demand for the development of aluminum manufacturing technology that can reduce electrical energy consumption. An aluminum chloride electrolytic method is known as a promising electrical energy-saving method for producing aluminum as an alternative to the Hall-Heroux method, in which aluminum chloride is dissolved in a molten salt bath of an alkali metal chloride such as NaCLKCI and then 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 production reaction, the graphite electrode used in the anode is non-consumable. However, it is difficult to handle high-temperature aluminum chloride and generated chlorine gas, and furthermore, suitable bath-resistant materials cannot be obtained industrially.
Although it was left unused for a long time industrially, in recent years Alcoa Corporation of the United States proposed an aluminum chloride electrolytic method (Alcoa method) using a new electrolyzer and an electrolytic bath with a new bath composition (Japanese Patent Laid-Open No. 48 -36006 and JP-A-4
8-J50910), which suddenly attracted industrial attention. This Alcoa method uses an AlCl3-LiCl-NaCl molten salt electrolytic bath containing a high concentration of LiCl, which has high electrical conductivity, and a carbon (graphite) electrode plate is placed inside the bath lined with a nitride-based refractory material. Using an electrolytic bath in which horizontal double electrodes were formed by stacking them in layers with appropriate gaps, the bath temperature was approximately 700°C, the distance between the electrodes was approximately 15 mm, and the current density was IA.
By electrolyzing near /cm・, chlorine gas is generated on the anode surface and molten metal aluminum is generated on the cathode surface.A special material with excellent fire resistance and corrosion resistance is used for the electrolytic cell container. It is characterized in that LiCl, which has high electrical conductivity, is introduced as a component of the molten salt electrolytic bath to reduce the voltage drop in the bath, and the distance between the electrodes is narrowed to reduce the power consumption rate. However, the lithium chloride used in the electrolytic bath is industrially expensive, and the current efficiency when using this type of electrolytic bath is approximately 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, especially regarding the composition of the electrolytic bath, the inventors found that A
It has been found that when aluminum chloride is electrolyzed using a lCl3-CaCl2-NaCl mixed molten salt as an electrolytic bath, aluminum can be produced with extremely high current efficiency. That is, the present invention is a method for manufacturing aluminum metal 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. Weight %, CaCl2lO
The present invention provides a method for producing aluminum, characterized in that the mixed composition consists of ~40% by weight and 88~40% by weight of NaCl.

According to the present invention, metal aluminum can be produced by electrolyzing aluminum chloride with a high current efficiency of about 90% or more and close to 95%, so that the electric power consumption can be significantly reduced.

AlCl3-CaCl2-N used in the present invention
The composition range of the aCl-based molten salt electrolytic bath is AlCl32-20
Weight%, CaCl21O~40wt%, NaCl88~
It is 40% by weight. In this way, a relatively large amount of Cacl2
It is not clear why a high current efficiency is obtained when electrolyzing aluminum chloride with a bath containing . However, the biggest reason for the decrease in current efficiency in aluminum chloride electrolysis is due to the reverse reaction in which part of the aluminum deposited on the cathode surface dissolves in the electrolytic bath, reacts with chlorine gas generated on the anode surface, and returns to AlCl3. AlCl3-CaCl2-Na having 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.
It is presumed that the Cl-based mixed molten salt has the effect of effectively suppressing the above-mentioned reverse reaction. AlCl3 in the present invention
-CaCl2-NaCl-based molten salt electrolytic bath, if the AlCl3 concentration in the bath exceeds 20% by weight, the electrical conductivity of the bath will decrease significantly, and the vapor pressure of the bath will become excessive, resulting in an increase in sliding voltage and furnace operation. Therefore, it is desirable to keep the AlCl3 concentration to a maximum of 20% by weight, preferably 15% by weight or less.

Furthermore, if the concentration is less than 2% by weight, the concentration is too low and there is a risk that electric power will be consumed locally for purposes other than the production of aluminum, and it is also difficult to control the supply of AlCl3 to maintain the concentration. In addition, when the CaCl2 concentration in the bath is higher than 10% by weight, the current efficiency during electrolysis tends to improve, but when it exceeds 40% by weight, the bath separates into two layers and becomes heterogeneous, making it difficult to perform normal electrolytic work. It is desirable to keep it below this level since it will not be possible to do it continuously.

A more optimal bath composition range is AlCl35-15
Weight%, CaCl22O~40wt%, NaCl75~
45% by weight is recommended. The electrolytic conditions for stable operation of aluminum chloride electrolysis using the electrolytic bath of the present invention described above vary depending on the type and capacity of the tank, but generally the bath temperature is 680 to 780°C and the current density is 0.5. ~2.0
By setting the A/d distance between the electrodes to 10 to 25 mm and carrying out electrolysis within this range, it is possible to continue electrolytically producing aluminum with a current efficiency of approximately 90% or more.

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. Electrolysis is carried out using a closed electrolytic cell that has a raw material supply port and a chlorine gas discharge port at the top, a metal storage tank at the bottom, and a graphite electrode inside.

The electrodes may consist of a pair of graphite plates or graphite rods arranged in parallel at appropriate intervals, or they may consist of three or more graphite plates or graphite rods arranged in parallel on a stack to form a double electrode. But that's fine. The electrodes can be arranged horizontally as in the Alcoa method, but in general, in this type of electrolytic method, the distance between the electrodes is relatively small, so the amount of aluminum produced on the cathode surface is small. If this increases, there is a risk of short-circuiting of the electrodes, and the chance of reaction with chlorine also increases, so it is preferable to quickly eliminate this. For this purpose, it is desirable to tilt the plate or rod-shaped electrode. The inclination of the electrode is at an angle of 6 with the horizontal plane.
0° or less, preferably 5 to 45°, and the inclination angle is 6
If it exceeds 0°, the current efficiency will be extremely reduced, which is rather disadvantageous. Furthermore, in the method of the present invention, it is advantageous to add a small amount of LiCl to the electrolytic bath in order to increase the electrical conductivity of the bath. Next, examples of the present invention will be presented.

Example 1 AIC 1310.0% by weight, CaCl 238.4% by weight
, NaCl5l. AlCl3-Ca with a bath composition of 6% by weight
In an electrolytic bath lined with an alumina refractory material using a Cl2-NaCl mixed molten salt as an electrolytic bath, an inclined graphite electrode plate (effective reaction surface 80 mm x 25 mm) with an angle of 30° to the horizontal was used to measure the distance between the electrodes. 14mm, bath temperature 750℃, current 20A, current density 1A/Cm2, sliding voltage 3.20V.
．． Electrolysis continued for 5 hours to obtain 29.0 g of aluminum.

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, Electrolytic bath A
An improved method for electrolytically producing aluminum, characterized in that the mixed composition consists of 32-20% by weight of lCl_2 10-40% by weight of CaCl_2 and 88-40% by weight of NaCl. 2. A patent claim characterized in that plate-shaped or rod-shaped graphite electrodes are used as both the negative and negative electrodes, and the longitudinal axes of the electrodes are provided in parallel in an inclined manner so that the angle is 60° or less with respect to the horizontal plane. The improved method for electrolytically producing aluminum according to item 1.