JP2761001B2 - Molten salt electrolytic bath - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for producing a rare earth metal or an alloy thereof by a molten salt electrolysis method, and in particular, Nd—Fe—B which has recently attracted attention as a high-performance magnet. Nd metal and Nd-
The present invention relates to a molten salt electrolysis apparatus suitable for producing an Fe alloy.

(Prior Art and Problems to be Solved) As one of methods for producing a rare earth metal or an alloy thereof, a molten salt electrolysis method is known, which includes a method using a chloride electrolytic bath and a method using a fluoride electrolytic bath. Is used.

On the other hand, regarding a molten salt electrolysis apparatus, conventionally, LiF is mainly used, a rare earth compound is added thereto to form a molten salt, and an inert atmosphere is used as a bath-resistant material when producing Nd metal or Nd-Fe alloy by electrolysis. Method using graphite in E
Maurice et al., USBur.Min.Rep.Invest, Nos. 6957, 196
7 years) and a method using iron (JP-A-61-87888, etc.)
It has been known.

However, in such a dissolved salt electrolysis method, there is a problem of corrosion of an electrolytic bath, and a portion to be corroded by a molten salt is roughly classified into a liquid state (part in a bath) and an interface between a liquid and a gas (bath). There is corrosion in three parts, the surface part) and the gaseous state (upper part of the bath).

The inventor has previously proposed an electrolysis method in an oxidizing atmosphere as a method of producing an Nd-Fe alloy at low cost (Japanese Patent Application No.
However, under these conditions, there is a problem that the corrosion conditions are more severe than in an inert atmosphere.

Therefore, as a material capable of withstanding such corrosion and improving the quality of the product, the present inventor replaces the above-mentioned bath-resistant materials (graphite, iron) with a low cost, workability and workability. Has proposed that austenitic stainless steel be used as a durable material without deteriorating the quality of the produced product (Japanese Patent Application No. 61-233697).

However, of the corrosion caused by the molten salt, the corrosiveness is particularly strong when the contacting partner is in a gaseous state (upper part of the bath), and considering the life of the entire bathtub, the upper part of the bathtub is preferentially corroded by the gaseous state. Corrosion will eventually occur and the life of the bathtub as a whole will be shortened.In the case of an industrial-scale production facility, the equipment cost will increase due to the shortening of the service life, and the production volume will decrease due to the stoppage of the production for replacing the bathtub. Therefore, there is a strong demand for the development of a material exhibiting even greater corrosion resistance because it is an obstacle to inexpensive and stable production of products.

The present invention has been made in order to meet such a demand, and in an electrolytic bath for producing a rare earth metal or an alloy thereof by a molten salt electrolysis method, a bathtub portion in contact with a gas is sufficiently resistant to corrosion by molten salt. It is an object of the present invention to provide an electrolytic bath having a configuration that can be obtained.

(Means for Solving the Problems) As described above, in an electrolytic bath for producing a rare earth metal or an alloy thereof by a molten salt electrolysis method, corrosion caused by a molten salt is roughly classified into a state in which a contacting partner is in a liquid state. There are three types of corrosion: (in the bath), the interface between liquid and gas (the bath surface), and gas (the upper part of the bath). Corrosion is most severe when in the gaseous state (the upper part of the bath). The part slightly above the bath surface suffers the most severe corrosion.

Regarding this phenomenon, the molten salt is in a liquid state at the part in the bath or at the interface between the liquid and gas (bath surface part),
Although the temperature of the molten salt itself is high, the activity of the molten salt itself is not so high.On the other hand, in the part away from the bath surface, the molten salt is in a gaseous state and its activity is high, but it is not enough to cause corrosion. No temperature. On the other hand, in the part slightly above the bath surface, the molten salt is in a gaseous state, has a high activity of itself, and has a sufficient temperature. It is considered to receive.

For this reason, if the lid is placed on top of the bathtub to keep the bathtub warm, or if the bath surface is lowered extremely, the range of temperature distribution where corrosion is likely to occur will change, and the corroded part will also change. become.

The present inventor has conducted intensive studies on various materials in order to find a material exhibiting strong corrosion resistance against such corrosion. As a result, the present inventors have found that an alloy mainly composed of Ni exhibits greater corrosion resistance to a molten salt in a gaseous state than other materials, and the present invention has been made here.

That is, the present invention relates to an electrolytic bath for producing a rare earth metal or an alloy thereof by a molten salt electrolysis method, wherein at least the upper part of the electrolytic bath surface of the bath is Ni: 50 to 80 wt% and Mo:
A molten salt electrolytic bath characterized by being composed of an alloy containing 10 to 30% by weight.

 Hereinafter, the present invention will be described in more detail.

As described above, the upper part of the electrolytic bath surface of the electrolytic bath is a part that is subject to severe corrosion compared to other parts, and this part is mainly composed of Ni (hereinafter, referred to as “Ni-based alloy”),
Specifically, it is composed of an alloy containing 50 to 80 wt% of Ni and 10 to 30 wt% of Mo. Such alloys are typically Ni-based alloys commonly known as "Hastelloy", for example, a composition of 20.0Mo-20.0Fe-remaining Ni, 28.0
Mo-5.0Fe-composition consisting of residual Ni, 16.5Cr-17.0Mo-5.0Fe
-4.5W-Ni-composition, 5.0Cr-24.5Mo-5.5Fe-
Examples of the composition include Ni. Of course, other elements such as Fe, Cr, W, Si, and Cu may be included as long as Ni and Mo are included in the above range.

However, when the Ni content is lower than 50 wt%, corrosion is liable to occur, and when the Ni content is higher than 80 wt%, corrosion is susceptible to corrosion, though not so much as low. Desirably, Ni is 60 to 78 wt% and Mo is 15 to
Alloys containing 25wt% are suitable. In addition, Mo content is 10wt%
If it is lower than this, the corrosion resistance will be insufficient and 30wt%
If it is higher than this, the effect on corrosion resistance saturates and becomes uneconomical, which is not preferable.

The upper portion of the electrolytic bath surface to be composed of the above-mentioned Ni-based alloy may be only the portion where the corrosion is severe near the bath interface, and may be 100 mm below the normal design interface. A Ni-based alloy is lined from the upper end of the bathtub to 100 mm below the design interface position, and fixed to the bathtub body by dissimilar material welding.
Of course, it is needless to say that the electrolytic bath surface is expected to fluctuate up and down. Further, the constituent material of the bath surface side portion other than the upper portion of the electrolytic bath surface is not particularly limited,
It may be made of the same material or another Ni-based alloy, or may be made of austenitic stainless steel.

The electrolytic bath in which the upper portion of the electrolytic bath surface is made of the Ni-based alloy can be applied to any molten salt electrolysis method. For example, LiF-NdF ₃ system, or this may LiF-NdF ₃ -Nd ₂ O ₃ system obtained by mixing an inexpensive Nd ₂ O _3, more in this Ba
A molten salt to which F ₂ , CaF ₂ or the like is appropriately added may be used. Instead of NdF ₃
NdCl ₃ can also be used.

Regarding the atmosphere, sufficient corrosion resistance can be obtained even when electrolysis is performed in an oxidizing atmosphere, particularly in the air. Of course, a non-oxidizing atmosphere is also possible.

The present inventor conducted a corrosion test in the air on various materials as bathtub materials for holding a molten salt. An example of the result is shown below.

FIG. 1 shows an apparatus used for a corrosion test of various materials with a molten salt, and FIG. 2 shows the results.

First, as shown in FIG. 1, various materials 10 are put in the molten salt 2, and the total corrosion amount of the molten salt, the interface between the molten salt and the atmosphere 5, and the portion extending over the molten salt upper part is investigated every day. The results are shown in FIG.

The experimental conditions were as follows. The bath temperature was maintained at 880 ° C. in the atmosphere using the bath 8 made of various materials shown in FIG.

The molten salt 2, the LiF80mol% -NdF ₃ 20mol% LiF-
Two types of the NdF ₃ system and the LiF-NdF ₃ -Nd ₂ O ₃ system in which 2 wt% of Nd ₂ O ₃ was added to 20 mol% of LiF 80 mol% -NdF ₃ were used, and the results showed the same tendency.

From the results shown in FIG. 2, it can be seen that ordinary steel, SUS-310S, which is an austenitic stainless steel according to the earlier proposal (Japanese Patent Application No. 61-233697), and a Ni-based alloy ( 27.5wt% Mo-67.5wt% Ni), comparing the amount of corrosion with each other, the corrosion resistance of austenitic stainless steel is better than that of ordinary steel, but the case of using a Ni-based alloy is better. It turns out that it has more excellent corrosion resistance.

As for the cathode material, a graphite electrode may be used when manufacturing a rare earth metal, and a cathode made of a material such as iron may be used when manufacturing a rare earth alloy.

 Next, examples of the present invention will be described.

(Example) Nd by molten salt electrolysis using the electrolytic cell shown in FIG.
A continuous operation experiment for producing an Fe alloy was performed.

As shown in FIG. 3, an electrolytic cell is provided with an Ni-based alloy (commonly called "Hastelloy B") (Mo: 28 wt%, Fe: 5 wt%, Ni: : Remaining part) At 9, a lining of 5 mm in thickness was formed from the upper end of the tank to a depth of 30 cm, and the lining to the bottom and the bottom were lined with austenitic stainless steel SUS-310S. The metal receiver 6 was lined with a tantalum plate 11.

In the electrolysis, when the iron cathode 4 and the graphite anode 3 are arranged and energized, the electrolyzed Nd reacts with the cathode 4 and Nd-
Fe alloy droplets 7 are stored in the metal receiver 6,
Precipitates as Nd-Fe alloy 1. The electrolysis was performed in air 5.

There are also two types of electrolytic baths, namely LiF8
And 0mol% -NdF ₃ 20mol% of LiF-NdF ₃ system, Lif80mol% -Nd
Using the F ₃ things 20 mol% of _{2wt% Nd 2 O 3 LiF-} NdF 3 -Nd 2 O 3 system to which added was operated at an electrolytic temperature of either 880 ° C., observed significant change by electrolysis bath composition I couldn't.

Table 1 shows the above experimental results. In the same table, the number of days of continuous use means that the material (thickness 5 mm) used at a depth of 30 cm from the top of the electrolytic cell becomes thinner as the number of days of operation elapses, so there is a risk that the electrolytic bath will flow out. It is expressed by the number of days when it became thin to the extent.

From Table 1, it can be seen that, as shown in the examples of the present invention, by lining at least the upper part of the electrolytic bath surface of the electrolytic bath with a Ni-based alloy, the number of days of continuous use is greatly increased.

(Effects of the Invention) As described above in detail, according to the present invention, when a rare earth metal or an alloy thereof is produced by a molten salt electrolysis method, Ni is mainly contained in an upper portion of the electrolytic bath surface of the electrolytic bath as an electrolytic bath material. Since the alloy is made to be lined, the number of days of use of the electrolytic cell can be greatly increased, maintenance costs can be reduced, and troubles caused by the outflow of molten salt due to corrosion can be significantly reduced. Therefore, high-grade rare earth metals and their alloys can be obtained inexpensively and stably.
In particular, it is suitable for producing raw materials for rare earth magnets.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an apparatus used in a corrosion test, FIG. 2 is a view showing the amount of corrosion of various materials used in the corrosion test and the time, and FIG. 3 is an apparatus used for molten salt electrolysis in Examples. FIG. DESCRIPTION OF SYMBOLS 1 ... Production alloy, 2 ... Molten salt, 3 ... Graphite anode, 4 ... Iron cathode, 5 ... Atmosphere, 6 ... Metal receiver, 7 ... Drop of produced alloy, 8 ... Electrolytic bath, 9 ... Ni alloy, 10 ... Specimen, 11 ... Tantalum plate.

Claims (2)

(57) [Claims] 1. An electrolytic bath for producing a rare earth metal or an alloy thereof by a molten salt electrolysis method, wherein at least an upper portion of the electrolytic bath surface of the bath is Ni: 50 to 80 wt% and Mo: 10 to 30 wt%.
%, Characterized in that the molten salt electrolytic bath is made of an alloy containing 0.1% or less. 2. The molten salt electrolytic bath according to claim 1, wherein a portion of the electrolytic bath other than an upper portion of the electrolytic bath surface is made of austenitic stainless steel.