JPS5930795B2 - Aluminum electrolytic 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.

Traditionally, aluminum has been manufactured 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 using carbon as an anode. has been done.

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 more. Therefore, there is a strong demand for the development of aluminum manufacturing technology that can reduce electrical energy consumption. A promising electrical energy-saving aluminum production method that replaces the Hall-Heroux method is to add aluminum chloride to alkali metal chlorides such as NaCl and KCl, or to these alkali metal chlorides with a very small amount of alkaline earth metal. An aluminum chloride electrolytic method is known in which aluminum chloride is dissolved in a molten salt bath containing chloride and electrolyzed at a temperature equal to or higher than the melting point of aluminum.

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-constituting 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
8-50910) 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 cell stacked in layers with appropriate gaps to form a horizontal double electrode, the bath temperature was approximately 7.
00℃, electrode distance around 15mm, current density IA/cl
By electrolyzing near the IL, chlorine gas is generated on the anode surface and molten metal aluminum is generated on the cathode surface.The electrolytic cell container is made of a special material with excellent fire resistance and corrosion resistance, and the voltage drop in the bath is reduced. It is characterized in that LiCl, which has high electrical conductivity, is introduced as a component of the molten salt electrolytic bath in order to reduce the electrical conductivity, and the distance between the electrodes is narrowed in order to reduce the power unit consumption.

However, when aluminum chloride is electrolyzed using these conventionally used electrolytic baths, the current efficiency is at most about 85 (:!), so there is a limit to the improvement of the power consumption rate, and therefore it is difficult to improve industrially. It is desired to develop a more advantageous method for producing aluminum.

In particular, as a result of various studies on the composition of electrolytic baths, the inventors found that when aluminum chloride is electrolyzed using a mixed molten salt of the AlCl3-CaCl2-LiCl system with a specific composition as an electrolytic bath, aluminum can be electrolyzed with extremely high current efficiency. We discovered that it can be manufactured. 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% Ca
The present invention provides a method for producing aluminum, characterized in that the mixed composition consists of 2 to 42% by weight of Cl2 and 86 to 38% by weight of LiCl.

According to the present invention, metal aluminum can be produced by electrolyzing aluminum chloride with a high current efficiency of about 90% or more, reaching a maximum of around 98 (!), thereby significantly reducing the power consumption rate. be able to. The composition range of the AlCl3-CaCl2-LiCl based molten salt electrolytic bath used in the present invention is 32 to 20% by weight of AlCl, Ca
Cl2l2~42% by weight, LiCl86~38% by weight.

The reason why high current efficiency is obtained when aluminum chloride is electrolyzed using the electrolytic bath of the present invention is not clear. 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- 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.
It is presumed that the LiC-related mixed molten salt has the effect of effectively suppressing the above-mentioned reverse reaction. AlCl in the present invention
In a 3-CaCl2-LiCl-based molten salt electrolytic bath, if the AlCl3 concentration in the bath exceeds 20% by weight, the electrical conductivity of the bath will drop significantly and the vapor pressure of the bath will become excessive, resulting in an increase in sliding voltage and Since this may lead to instability of the furnace, it is desirable to limit the AlCl3 concentration to a maximum of 20% by weight, preferably 15% by weight.

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 aluminum production, and it is also difficult to control the supply of AlCl3 to maintain the concentration. In addition, as the CaCl2 concentration in the bath becomes higher than 12% by weight, the current efficiency during electrolysis tends to improve, but when it exceeds 42% by weight, the volatilization loss of the bath increases, making it difficult to continue normal electrolysis work. It is desirable to keep the temperature below this level.

LlCI forming the remainder of the bath has the effect of improving the electrical conductivity of the bath and reducing the sliding voltage. Electrolytic conditions for stable 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.
．． By performing electrolysis within this range with a distance between 5 to 2.0 A/Cd electrodes, it is possible to electrolytically produce aluminum with significantly higher current efficiency than when using a conventional electrolytic bath. It can be done continuously.

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. good. 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 is less than or equal to, preferably 5 to 45 degrees, and the inclination angle is 6
Exceeding this is actually disadvantageous because the current efficiency will drop significantly. Next, examples of the present invention are listed.

Example 1 134.0% by weight of A1C, 24l. of CaCl. 3% by weight,
AlCl3CaCl with a bath composition of 54.7% by weight of LiCl
2-In an electrolytic bath lined with an alumina refractory material using a LiCl mixed molten salt as an electrolytic bath, an inclined graphite electrode plate with an angle of 3' to the horizontal (effective reaction surface 60 m77! x 33 m77!)
The distance between the poles was maintained at 14mm using a bath temperature of 75°C.
Electrolysis was continued for 4.5 hours at a current of 20 A and a current density of 1 A/CTl to obtain 29.7 g of aluminum.

The current efficiency at this time was 98.4%. Moreover, the sliding voltage was 2.94. Example 2 A1C135.0% by weight, CaCl22O. 9% by weight and LiCl74. AlCl, CaC with a composition of 1% by weight
Using a l2-LiCl mixed molten salt as an electrolytic bath, electrolysis was carried out under the same conditions as in Example 1 to obtain 028.6 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
A method for electrolytically producing aluminum, characterized in that the mixed composition consists of 32 to 20% by weight of lCl, 212 to 42% by weight of CaCl, and 86 to 38% by weight of LiCl. 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 method for electrolytically producing aluminum according to item 1.