JPS6137341B2 - - Google Patents

Abstract

Rare earth oxides can be reduced to rare earth metals by, a novel, high yield, metallothermic process. The oxides are dispersed in a suitable, molten, calcium chloride-based bath (44) along with calcium metal. The bath (44) is agitated and calcium metal reduces the rare earth oxides to rare earth metals. The metals collect in a discrete layer (43) in the reaction vessel (22).

Description

[Detailed description of the invention]

The present invention relates to a novel metallothermic process for the direct reduction of rare earth oxides, particularly neodymium oxide, to rare earth metals. This method is particularly useful for low-cost production of neodymium metal for neodymium-iron-boron magnets. To date, the best permanent magnets produced commercially have been made from sintered powder of an alloy of samarium and cobalt (SmCo ₅ ). Recently even stronger magnets have been made from alloys of lighter rare earth elements, especially neodymium and praseodymium, iron and boron. The above alloys and methods of processing them into magnets are disclosed in European Patent Applications Nos. 0108474, 0125752, 0133758 and
0144112. Rare earth (RE) elements Atomic numbers 57 to 71 in the periodic table
The sources of yttrium with atomic number 39 are bastnesite and monazite. Rare earth mixtures can be extracted from the ores by several known beneficiation techniques. The rare earths can then be separated from each other by conventional processes such as elution and liquid-liquid extraction. Once the rare earth metals have been separated from each other, they must be reduced from their oxides to relatively pure forms of each metal (95 atomic percent purity or higher, depending on the type of inclusions) useful in permanent magnets. There is.
Up until now, the final reduction process described above has been complex and expensive, adding a significant burden to the cost of rare earth metals. Both electrolytic and metal thermal (non-electrolytic) processes have been employed for the reduction of rare earths. The electrolytic process includes (1) the decomposition of anhydrous rare earth chlorides dissolved in molten alkali or alkaline earth salts and (2) the decomposition of rare earth oxides dissolved in molten fluoride salts. Disadvantages of both electrolytic processes include the use of expensive electrodes that eventually wear out, the use of anhydrous chloride or fluoride salts to prevent the formation of undesirable RE-oxysalts (e.g. NdOCl), and high temperatures. These include electrolyzer operation (generally above 1000°C), low current efficiency combined with high energy costs, and low salt-to-metal yields (less than 40% of metal can be recovered). The RE/chloride reduction process generates corrosive chlorine gas, and the fluoride process requires careful control of the temperature gradient in the electrolytic salt bath to solidify rare earth metal nodules. An advantage of the electrolytic process is that the process can be run continuously if provision is made to remove the reduced metal and replenish the salt bath. Metal thermal (non-electrolytic) processes include (1) reduction of RE and fluoride (calciosa) using calcium metal powder;
Mitsuku process (calciother-mic)
(2) reduction and diffusion of RE/oxide by calcium hydride (C ₂ H ₅ ) or calcium metal (Ca). Disadvantages are that both processes must be carried out in batches, in a non-oxidizing atmosphere, consume a lot of energy, and in the case of the reduction-diffusion method, the product is a powder, which must be hydrated before use. It is something that must be refined. Both processes consist of many steps. One advantage of metal thermal reduction processes is that the yield of metal from oxide or fluoride is generally better than 90%. Processes involving RE fluoride or chloride require pretreatment of the RE oxide to form a halide. This additional step increases the final cost of the rare earth metal. With the invention of light rare earth iron permanent magnets, the demand for low cost, relatively high purity rare earth metals has increased significantly. However, none of the existing rare earth compound reduction methods has shown much promise in reducing costs or increasing the availability of magnet grade metals. Accordingly, it is one of the objects of the present invention to provide a new, efficient, and low cost method for producing rare earth metals. The above and other objects will be achieved by the preferred embodiments of the invention described below. A reaction vessel is prepared that can be heated to the desired temperature with an electric resistance heater or other heating means.
Preferably, the container body is constructed of a metal or refractory material that is substantially inert or non-toxic to the reaction components. A predetermined amount of RE/oxide is charged in advance into a reaction vessel containing a salt mixture of about 70% by weight or more of calcium chloride and about 5 to 30% by weight of sodium chloride (NaCl). The salt serves as a medium for the reduction reaction. A stoichiometric excess of calcium metal is added relative to the amount of rare earth oxide. A eutectic alloy is formed with the reduced rare earth metal, the reaction is carried out at a low temperature, and a certain amount of another metal such as iron or zinc is added so that the RE/metal product is obtained in liquid form. It would be convenient. To proceed with the reaction, the container is heated to a temperature above the melting point of the constituent components (approximately 675°C). The molten components in the vessel are stirred rapidly to maintain contact with each other during the course of the reaction. Calcium chloride (CaCl ₂ ) is replenished into the bath as needed to maintain 70% by weight relative to the total weight of CaCl ₂ and NaCl. _CaCl2 concentration is 70
If the reaction proceeds below %, the yield will drop sharply. Although several different competitive chemical reactions occur within the container, the reduction reaction of RE/oxide is thought to proceed according to the experimental reaction formula below. REnOm＋mCa→mCaO＋nRE In the formula, “n” and “m” are the number of moles of the constituent components, and
The relationship between n and m depends on the oxidation state of the rare earth element. The density of the reduced metal is about 7 grams/cc and the density of the salt bath is about 1.9 grams/cc. When stirring is stopped, the reduced metal forms a complete layer at the bottom of the reaction vessel. This layer can also be extracted in its molten state,
Alternatively, it can be separated from the salt layer after solidification. Thus, the method of the present invention has a number of advantages over prior art methods. This process is carried out at relatively low temperatures of about 700° C., especially when the rare earth metals are recovered in the form of eutectic alloys with zinc or iron. This method uses relatively inexpensive RE/oxides,
CaCl ₂ , NaCl and Ca metal are used as reactants. There is no need to pre-convert the RE oxide to chloride or fluoride, and there is no need to use expensive reducing agents pure Ca or CaH ₂ . Since it is not an electrolytic process, it consumes less energy and is conveniently carried out at temperatures of about 700°C and atmospheric pressure. The method can be carried out in either a batch or continuous process, and the by-products CaCl ₂ , NaCl and calcium oxide (CaO) are easy to dispose of. Moreover, rare earth metals can be alloyed by adding iron into the reaction vessel if they are made in the form of RE-Fe eutectic alloys, or RE-Fe can be alloyed without subsequent expensive refining treatments. It can also be alloyed for use in magnets. The present invention relates to an improved method for reducing compounds of rare earth elements to obtain the corresponding elemental metals. Rare earth metals include elements 57 to 71 of the periodic table (lanthanum,
Cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium,
These include terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium) and atomic number 39, yttrium.
Rare earth oxides are commonly colored powders produced in metal separation processes. The term "light rare earths" in Japanese refers to lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd). In the process of the present invention, the RE oxide is generally used as it comes out of the separator, but may be calcined to remove excess absorbed moisture or carbon dioxide. In the examples below, the RE oxide was oven dried at 1000°C for approximately 2 hours prior to use. CaCl ₂ and NaCl for the salt bath were of reagent grade and dried at 500° C. for approximately 2 hours prior to use. In the initial work, care was taken to ensure that no moisture was introduced into the reaction vessel to avoid harmful reactions with Na or Ca. Adding calcium to a NaCl-containing bath can generate some sodium metal in the reaction described below. 2NaCl + Ca → CaCl ₂ + 2Na When CaCl ₂ is mixed with Nd ₂ O ₃ in a molten bath, oxychloride is produced by the following reaction Nd ₂ O ₃ + CaCl ₂ → 2NdOCl + CaO Presence of this type of RE/oxychloride is known to reduce yield in prior art electrolytic processes. However, in the present invention, RE oxide is also used.
Both RE and oxychloride are easily reduced with calcium metal. In reality, RE/oxyoxide floats on the molten layer of reduced RE/metal, so
Its generation is convenient. On the other hand, since RE/oxide has a density close to that of reduced RE/metal, there is a risk that it will be retained in the form of impurities in the molten layer of reduced RE/metal. may cause inconvenience. The RE metal reduced by the method according to the invention was essentially free of oxygen. The melting point of unalloyed Nd metal is approximately 1025°C. Other rare earth metals also have high melting points. If you want to proceed with the target reaction at this level of salinity, it would be possible to do this and obtain pure metal in high yield. However, in order to form an alloy of recovered rare earth metals that melts at lower temperatures, it is preferred to add some amount of other metals such as iron, zinc, or other non-rare earth metals to the reduction vessel. For example, iron forms a low melting point eutectic alloy with neodymium (Fe 11.5% by weight; mp approximately 640°C), as does zinc (Zn 11.9% by weight, mp approximately 630°C). _{Nd2O3 _}
When enough iron is added to the reducing system, the reduced metal forms a liquid puddle at about 640°C. The Nd-Fe eutectic alloy is alloyed with the addition of iron and boron to achieve the optimal
It can be made into a magnet with Nd ₂ Fe ₁₄ B magnet phase. It is preferable to lower the melting point of the recovered rare earth metal, but if it is undesirable to leave the metal added in this way, a metal with a boiling point lower than the boiling point of the recovered rare earth metal may be added to the reaction vessel. It's okay. For example, Zn boils at 907℃, and Nd boils at 3150℃.
Boil at °C. The low melting point metal can then be easily separated from the rare earth metal by a simple distillation operation. The materials used for the reaction vessel or its lining must be carefully selected due to the corrosive nature of molten rare earth metals, especially rare earth metals left in salt flux environments. Refractory materials such as alumina and boron totride linings are non-reactive and are generally acceptable. It is also possible to use refractory linings made of essentially inert metals such as tantalum or consumable but non-hazardous metals such as iron. The iron lining can be used to contain reduced RE metal, which can then be alloyed with RE for use in magnets. In accordance with the present invention, a new method of using calcium metal for the reduction of rare earth oxides has been discovered. This method involves bringing molten calcium and RE oxide together to cause the following reaction: REnOm + mCa → nRE + mCaO If the reaction vessel is not pressurized, an excess of Na will be generated by the reaction between Ca and NaCl. It is desirable to keep the temperature around 910°C to avoid significant losses. The most preferred operating temperature range is approximately 650-750℃
It's around. At such temperatures, the wear and tear on the reaction vessel is not significant. Since the melting temperatures of Nd-Fe and Nd-Zn donors are below 700°C, the above temperature range is suitable for reducing Nd ₂ O ₃ to Nd metal. Furthermore, at about 700°C, the solubility of Ca metal in the salt bath is about 1.3 mol%. This value is sufficient to quickly reduce RE oxide to RE metal. If successful separation of the reduced RE metal from the flux is required, the reaction temperature may be lower than the reduced RE metal or the reduced RE metal alloyed, i.e. reduced together with other metals.
Must be higher than the melting point of the metal. The relatively dense RE metal or alloy described above collects at the bottom of the reaction vessel as it settles. This allows them to be taken out during melting or taken out after solidification. Table 1 shows the molecular weight (mw), density (μ) (g/cc) at 25°C, and the molecular weight (mw) of the simple substance and compound used in the present invention.
Melting points (mp) and boiling points (bp) are shown.

Table 1 Figure 1 shows an apparatus suitable for carrying out the invention, on which the experimental operations described in some of the Examples were carried out. All experiments were conducted in a long vertical furnace 20 with an inner diameter of 12.7 cm and a depth of 54.6 cm, which was attached to the floor 4 of a dry box with bolts 6. Oxygen (O ₂ ), nitrogen (N ₂ ) and moisture (H ₂ O)
A helium atmosphere containing less than one part per million of each was maintained within the box. The furnace was heated with three cylindrical electric clamshell heating elements 8, 10 and 12 with an inner diameter of 13.3 cm and a total length of 45.7 cm. The sides and bottom of the furnace were completely surrounded by an insulator 14. Thermocouples 15 were attached to the outer wall 16 of the vertical furnace 20 at various positions along its length. One of the centrally located thermocouples was used to connect to a proportional band temperature controller (not shown) to automatically control the central clamshell heater 10. The remaining three thermocouples were monitored using digital temperature readouts, and the top and bottom clamshell heaters 8 and 12 were manually regulated using transformers to maintain a fairly uniform temperature throughout the furnace. Outer diameter held in stainless steel vertical furnace 20
The reduction reaction was carried out in a reaction vessel 22 housed in a stainless steel crucible 18 measuring 10.2 cm by 12.7 cm deep and 0.15 cm thick. Unless otherwise specified in the examples, reaction vessel 22 was made of tantalum metal. A tantalum stirrer 24 was used to stir the melt during the reduction process. The length of the stirrer shaft was 48.32 cm and blades 26 were welded to it. The stirrer has a 100W variable speed motor 28 that can operate at speeds up to 700 rpm.
Driven by. The motor was attached to the bracket 30 so that the depth of the stirring blade 26 in the reaction vessel could be adjusted. The shaft passed through a bushing 32 mounted on an annular support bracket 34. The bracket is held by a collar 35, to which the vertical furnace 20 is attached by bolts 37. Vertical furnace 2
A cold water coil 36 is located near the top of the 0 to promote condensation of volatile reaction components and prevent their escape.
A circular stainless steel baffle 38 was used to reflux the Na vapor. The reflux product falls down through the tube 40 of the bottom baffle 42. At the end of the operation, when the stirring of the components in the furnace is stopped, the components are deposited with the rare earth alloy reservoir 43 at the bottom, the RE/oxychloride, calcium chloride/sodium salt bath 44 above, and any unreacted components above. Calcium metal 45 separates into layers. FIG. 2 is an idealized flow chart of the reduction of Nd ₂ O ₃ to Nd metal according to the present invention. _{Nd2O3 _}
is added to the reaction vessel along with the appropriate proportions of calcium chloride and sodium chloride. Calcium metal and a sufficient amount of a eutectic metal such as iron or zinc are added to form something close to a eutectic Nd alloy. at least 1 hour
The reaction is carried out at a temperature of 700°C with rapid stirring at approximately 300-700 revolutions/min. Preferably, an inert gas atmosphere, such as helium, is maintained over the reaction vessel. After Nd ₂ O ₃ is reduced, about 1 hour
Stirring is continued at a reduced speed to 100 revolutions per minute and then stopped to allow the various liquids in the container to separate into layers. The reduced Nd eutectic alloy gathers at the bottom because it has the highest density. Remaining salts and unreacted Ca
collects on the Nd alloy, and after the container cools and the components solidify, they can be easily broken and removed.
The Nd-Fe alloy thus produced can be alloyed with additional elements to form a permanent magnet composition. This magnetic alloy can be processed by melt spinning, or it can be ground and processed into a magnet by powder metallurgy. Example: 265 grams of 99% pure Nd metal ingot and 99.9% pure Zn
Fifty grams of the metal were placed in a tantalum reactor to produce 315 grams of a near-eutectic alloy. The container was placed in a furnace and heated to 800°C to alloy Nd and Zn. The temperature of the furnace was lowered to about 720° C. and 150 grams of NaCl and 350 grams of CaCl ₂ were added to create a salt bath of 70% by weight CaCl ₂ . 234 grams (0.7 mole) _{of Nd2O3} was added. 104 grams (2.6 moles) of Ca metal were added to the crucible, which was stirred for approximately 2 hours at a speed of 300 revolutions/minute and then for an additional hour at a speed of 60 revolutions/minute. The crucible was removed from the furnace and cooled on the floor of a dry box. By distilling the Nd/Zn alloy that had gathered at the bottom of the container, 189 grams of Nd metal with a purity of over 99% (excluding the 265 grams of Nd metal added at the beginning) was recovered. The yield of Nd metal from the oxide was about 94%. Example: To make something close to 414 grams of eutectic alloy,
350 grams of Nd metal ingot with a purity of 99% and 64 grams of electrolytic iron were placed in a mild steel reaction vessel with a thickness of 6 mm. The container was placed in a furnace and heated to 800°C to alloy Nd and iron. Lower the furnace temperature to about 720℃ to reduce _CaCl2 to 70% by weight
To make a salt bath, 300 grams of NaCl and 700 grams _{of CaCl2} were added. 117 grams (0.35 moles) _{of Nd2O3}
was added. Ca metal 46 grams (1.15 moles) and
Add 10.8 grams (0.47 mol) of Na to the crucible and approx.
These were stirred at a speed of 300 revolutions/min for 135 minutes.
Additionally at this point, 117 grams (0.35 moles) of Nd ₂ O ₃
46 grams (1.15 moles) of Ca metal and 10.8 grams (0.47 moles) of Na were added. The reaction was heated for an additional 114 minutes.
The mixture was stirred at 300 rpm and then for another hour at a stirring speed of 60 rpm. The reaction vessel was removed from the furnace and cooled on the floor of a dry box. A layer of unreacted Ca-Na alloy was formed on top of the salt layer. 594 grams of 97% pure Nd-Fe alloy was recovered. Immediately after recovery, this alloy is combined with iron and boron to create an ideal Nd-Fe for permanent magnet production.
-B alloy can be made. 180 grams of Nd metal with purity greater than 99% was recovered in the form of Nd-Fe alloy. This example shows that calcium and sodium melts in a _CaCl2 -NaCl flux bath can reduce rare earth oxides. Example Using the process described in the example, but stirring the reactants at 300 rpm for 4 hours and then an additional 60 rpm.
The table lists the amounts of the various components used in the metallurgical reduction of about 234 grams of Nd ₂ O ₃ using Ca metal with stirring for 1 hour.

[Table] When the salt bath ratio is 65.5% by weight of CaCl ₂ and 34.5% by weight of NaCl, the yield is 65.2%. 70% by weight _CaCl2
or higher, in each case the yield of Nd is
Higher than 85% and generally 95% or higher. Figure 3 is
The yield of Nd metal from Nd ₂ O ₃ is plotted against the weight percent of CaCl ₂ in the NaCl-CaCl ₂ binary starting salt bath using Ca metal as the reducing agent. As shown in the table and FIG. 3, it was found that to obtain a high yield, it was necessary to maintain the amount of CaCl ₂ in the salt bath above about 70% by weight. Further, in order to obtain a flux suitable for dispersing the RE/oxide, it is desirable that the volume ratio of the salt to the RE/oxide be at least 2:1. It has been observed that as the volume ratio of the salt bath to RE/oxide increases, the stirring speed can be reduced to obtain the same yield within a given time. The CaCl ₂ -containing bath is an important feature of the invention. Several samples were combined to remove Zn metal by vacuum distillation. The resulting alloy was analyzed and found to have a purity of 99% or higher with 0.4% aluminum and 0.1% silicon.
%, calcium 0.01%, and trace amounts of zinc, magnesium, and iron were found to be present. The Nd metal thus obtained is melted with electrolytic iron and ferroboron in a vacuum furnace to reduce the nominal composition.
An alloy of Nd _0.15 B _0.05 Fe _0.80 was created. This alloy was melt spun as described in the above-mentioned European Patent Application No. 0108474 to produce a very fine crystalline ribbon with a retention strength of approximately 10 megagauss oersted upon quenching. Although the present invention has been described in detail with respect to the reduction of Nd ₂ O ₃ , it is equally applicable to the reduction of other single rare earth oxides or combinations of rare earth oxides.
This is based on the fact that CaO is more stable than any rare earth oxide. A person familiar with this field would have been able to determine the relative free energy of RE oxide and CaO, but
Prior to the present invention, it was not known that RE/oxides could be reduced with Ca metal in a non-electrolytic liquid phase process. If desired, oxides of transition metals such as Fe and Co can be reduced together with RE oxides in the process of the present invention. In summary, a new efficient and low cost method for reducing rare earth oxides to rare earth metals has been developed. The method consists of creating a suitable molten CaCl ₂ -based bath and stirring therein the rare earth oxide with an amount of calcium metal sufficient to reduce it. When stirring is stopped, the components separate into distinct layers that can be broken off after cooling and solidification. Alternatively, the reduced liquid rare earth metal can be withdrawn from the bottom of the reaction vessel. After the RE metal is extracted, the melt bath can be refilled and moved on to the next batch, making the process essentially continuous.

[Brief explanation of the drawing]

FIG. 1 is a sectional side view of an apparatus suitable for carrying out the method of reducing RE oxides to RE metal according to the invention. Figure 2 is a flow chart for producing a neodymium eutectic alloy by reducing neodymium oxide (Nd ₂ O ₃ ) with calcium. Figure 3 is
The yield of neodymium (Nd) metal from Nd ₂ O ₃ is plotted against the percentage of CaCl ₂ in the flux bath used in the present invention.

Claims (1)

[Claims] 1. In a molten salt bath in which calcium chloride accounts for the majority, rare earth oxides are dispersed in a volume smaller than that of the molten salt bath, and a stoichiometric excess of calcium metal with respect to the rare earth metal ions present is dispersed. is added to the molten salt bath, the molten salt bath is stirred, and the reaction formula is REnOm+mCa→nRE+mCaO [wherein RE represents one or more rare earth elements with a valence of 2, 3, or 4, and O is oxygen , Ca represents calcium, CaO represents calcium oxide, and n and m represent integers such that n times the valence of the rare earth element is equal to m times the valence of oxygen. ] A metal thermal non-electrolytic reduction method of a rare earth oxide to a base metal, characterized in that the ring element is reduced according to the method. 2. The metal thermal non-electrolytic reduction method according to claim 1, wherein neodymium oxide is reduced to neodymium metal in a molten salt bath according to the reaction formula Nd ₂ O ₃ +3Ca→2Nd+3CaO. 3. The method comprises: creating a molten salt bath consisting of calcium chloride; adding a predetermined amount of rare earth oxide to said molten salt bath; adding an excess of calcium metal to the molten salt bath; maintaining the molten salt bath in a molten state and stirring, so that the calcium metal reduces the rare earth oxide to the rare earth metal. A metal thermal non-electrolytic reduction method according to claim 1. 4. The reduction method according to claim 3, wherein neodymium oxide is reduced to neodymium metal. 5. The method is such that the weight percent of calcium chloride is at least
creating a molten salt bath consisting of 70%; adding a predetermined amount of rare earth oxide to said molten salt bath; a stoichiometric excess of calcium based on the amount of rare earth oxide present; Adding the metal to the molten salt bath; keeping the molten salt bath in a molten state and stirring until the calcium metal reduces the rare earth oxide to the rare earth metal; then discontinuing stirring and adding the rare earth to the molten salt bath; A method according to claim 1, comprising producing a distinct layer containing metal. 6. The metal thermal non-electrolytic reduction method according to claim 5, wherein the rare earth oxide is one or more rare earth oxides selected from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide, and neodymium oxide. 7. The method is such that the weight percent of calcium chloride is at least
Creating a molten salt bath with 70% and the balance being sodium chloride; Adding to the molten salt bath a volume of neodymium oxide Nd ₂ O ₃ not more than 50% of the volume of the molten salt bath; the amount of neodymium oxide therein; adding a stoichiometric excess of calcium metal to the molten salt bath; maintaining the molten salt bath above its melting temperature; stirring the molten salt bath to mix the components together; , continuing the stirring until most of the neodymium oxide has been reduced to neodymium metal; then stopping the stirring while keeping the components in a molten state, so that the molten salt bath essentially does not contain neodymium oxide; A method according to claim 1, comprising the step of producing a distinct layer comprising reduced neodymium metal. 8. The method contains at least 70% by weight of calcium chloride.
creating a molten salt bath with the balance being sodium chloride; adding a volume of rare earth oxide to the molten salt bath equal to or less than 50% of the volume of the molten salt bath; adding enough rare earth oxide to reduce the rare earth oxide present; adding an amount of calcium metal to the molten salt bath; maintaining the molten salt bath at a temperature above its melting temperature; stirring the molten salt bath to mix the components together so that most of the rare earth oxide is Continuing the above stirring until the rare earth metal is reduced; then stopping the stirring while keeping the components in a molten state to form a distinct layer containing the reduced rare earth metal in the molten salt bath. ,
A metal thermal non-electrolytic reduction method according to claim 1, which comprises: 9. The method is such that the weight percent of calcium chloride is at least
70 and the weight percent of sodium chloride is 0 to 30; adding a predetermined amount of rare earth oxide to the molten salt bath; adding an amount of rare earth oxide to the molten salt bath; Adding a stoichiometric excess of calcium metal based on the standard; maintaining the molten salt bath in a molten state and stirring so that the calcium metal reduces the rare earth oxide to the rare earth metal; adding a sufficient amount of non-rare earth metal to the molten salt bath to form a rare earth/non-rare earth metal alloy having a melting temperature well below the melting temperature; then discontinuing stirring and adding the molten salt to the molten salt; A method according to claim 1, comprising allowing the rare earth/non-rare earth metal alloy to collect in a well-defined layer in the bath. 10. The metal according to claim 9, wherein the rare earth oxide is one or more rare earth oxides selected from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide, and neodymium oxide. Thermal non-electrolytic reduction method. 11. The metal thermal non-electrolytic reduction method according to claim 9, wherein the rare earth oxide is neodymium oxide. 12 Claim No. 9 in which the non-rare earth metal is iron
The metal thermal non-electrolytic reduction method according to any one of Items 1 to 11. 13. The metal thermal non-electrolytic reduction method according to any one of claims 9 to 11, wherein the non-rare earth metal is zinc.