WO2025164758A1 - Rare earth oxide recovery method - Google Patents

Abstract

The present disclosure provides a rare earth oxide recovery method in which a boron-free flux is used. The present disclosure relates to a rare earth oxide recovery method for recovering a rare earth oxide from waste that includes a rare-earth-element-containing material. The recovery method has: a melt preparation step (1) for melting, by heating, the aforementioned waste and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides to prepare a melt containing at least a rare earth oxide, the aforementioned oxide, and an oxide of an easily oxidizable metal; and a separation step (2) for separating, from the melt, a rare-earth-enriched phase in which the rare earth oxide is concentrated in the oxide, and an Fe-C phase.

Description

Method for recovering rare earth oxides

The present invention relates to a method for recovering rare earth oxides. More specifically, it relates to a method for recovering rare earth elements as rare earth oxides from waste containing rare earth elements.

Rare earth elements are used in a variety of products, including display phosphors, fluorescent lights, sensors, permanent magnets, and fuel cells, and are essential materials in the manufacture of high-tech devices such as personal computers, smartphones, and electric vehicles. Demand for rare earth elements has increased in recent years with the widespread use of these high-tech devices; however, rare earth elements are only produced in limited areas, are produced in small quantities, and are soaring in price. For this reason, there is a need to develop and improve technologies for recovering rare earth elements from rare earth-containing materials in discarded high-tech devices.

Known methods for recovering rare earth elements from rare earth-containing materials include the wet method, in which the material is dissolved in acid or a solvent and then separated into individual rare earth elements using solid-liquid separation or solvent extraction, and the dry method, in which the material is heated and melted together with a flux (flux) and the rare earth oxides are extracted into the flux. Of these methods, the wet method requires the use of large amounts of chemicals such as acid and solvent, and has the problem of generating a large amount of waste liquid after processing. Another problem with the wet method is that it takes a long time to elute the rare earth elements from the material into the acid or solvent. On the other hand, the dry method has the advantage that rare earth elements can be easily extracted by simply heating and melting the rare earth-containing material in the presence of flux, and the generation of waste liquid is minimized.

Known fluxes for dry processes include boron compounds such as boron oxide (B ₂ O ₃ ) and borates. For example, Japanese Patent Application Laid-Open Publication No. 2016-186121 discloses a method for recovering rare earth elements from rare earth-containing materials (a method for recovering rare earth elements as oxides), which involves adding a melting point depressant, an oxidizer, _and sodium borate to waste products or semi-finished products containing rare earth magnets and steel, melting the waste, and separating the resulting mixture into two phases: an RE x O _y -B ₂ O ₃ slag (RE: Nd, Pr, Dy, Tb) and an Fe—C phase. According to this publication, the use of sodium borate as a flux reduces the viscosity of the slag compared to conventional methods using boron oxide, thereby enabling easier and more efficient recovery of rare earth elements from rare earth-containing materials.

However, boron and its compounds are subject to regulation as hazardous substances that may be harmful to human health, and there has been a demand to reduce their use.

The present invention therefore aims to provide a method for recovering rare earth oxides using a boron-free flux.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by using an alkali metal or alkaline earth metal oxide as the flux in the dry method, and allowing the easily oxidizable metal in the system to coexist in the form of an oxide when extracting rare earth oxide into the flux, thereby completing the present invention.

In other words, one embodiment of the present invention relates to a method for recovering rare earth oxides, which recovers rare earth oxides from waste containing rare earth elements, and includes a melt preparation step (1) in which the waste and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides are heated and melted to prepare a melt containing at least the rare earth oxide, the oxide, and an oxide of an easily oxidizable metal; and a separation step (2) in which a rare earth-enriched phase in which the rare earth oxide is concentrated in the oxide, and an Fe-C phase are separated from the melt.

FIG. 1 is a pseudo-quaternary phase diagram at 1450° C. when Nd ₂ O ₃ is used as the rare earth element-containing material, Al ₂ O ₃ and SiO ₂ are used as the oxides of easily oxidizable metals, and calcium oxide (CaO) is used as the oxide.

The following describes embodiments of the present invention, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the range "X to Y" means "greater than or equal to X and less than or equal to Y."

One aspect of the present invention is a method for recovering rare earth oxides from waste containing rare earth elements, the method comprising: a melt preparation step (1) of heating and melting the waste and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides to prepare a melt containing at least the rare earth oxide, the oxide, and an oxide of an easily oxidizable metal; and a separation step (2) of separating from the melt a rare earth-enriched phase in which the rare earth oxide is concentrated in the oxide, and an Fe-C phase. This aspect provides a method for recovering rare earth oxides using a boron-free flux.

The following describes in detail each step of the rare earth oxide recovery method according to this embodiment.

<Melt preparation step (1)>
In this step (1), waste containing a rare earth element is heated and melted with at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides, thereby preparing a melt containing at least the rare earth oxide, the oxide, and an oxide of an easily oxidizable metal.

[Waste]
The waste material includes a rare earth element-containing material. The waste material may be a product or semi-finished product containing a rare earth element. The rare earth element-containing material is not particularly limited as long as it contains one or more rare earth elements (i.e., scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium).

Materials containing rare earth elements can take various forms, such as mixtures, compounds, sintered materials, alloys, and combinations of these. Materials containing rare earth elements can also be found in products that use alloys containing rare earth elements, waste products or semi-finished products, and waste materials such as scraps and defective products generated during the manufacturing process.

Among rare earth element-containing materials, rare earth magnets (including sludge generated during their production) rich in rare earth elements are preferred. Rare earth magnets are not particularly limited as long as they are magnets made from alloys containing one or more rare earth elements. Specific examples of rare earth magnets include neodymium magnets, samarium-cobalt magnets, praseodymium magnets, and samarium-iron-nitrogen magnets, with neodymium magnets being particularly preferred. Neodymium magnets made from rare metals such as rare earths (Nd, Pr, Dy, Tb) are used in drive and power-generating motors for environmentally friendly electric vehicles (EVs) and hybrid electric vehicles (HEVs). Rare earths (rare earth elements) pose procurement risks. Given the predicted increase in demand for environmentally friendly vehicles in Japan, a country lacking natural resources, recycling rare earths from neodymium magnets is important. Recycling the rare earths in neodymium magnets is advantageous in that it reduces rare earth procurement risks. Neodymium magnets are generally defined as a type of rare earth magnet whose main components are neodymium, iron, and boron. However, in this specification, neodymium magnets refer to magnets defined as R-T-B alloys containing R, T, and B (where R must contain at least one element selected from the group consisting of Nd, Pr, Dy, and Tb, T is a transition metal that must contain Fe, and B is boron, some of which can be substituted with carbon or nitrogen).

The rare earth oxide recovery method of the present invention can be applied to products or semi-finished products containing rare earth magnets, not only when the rare earth magnet is a single component, but also when the rare earth magnet is included as an integral component. Examples include motors and air conditioning compressors that contain rare earth magnets as integral components. The shape of such products or semi-finished products containing rare earth magnets may be the original shape of the essential parts of the product or semi-finished product, or they may be disassembled. It is extremely difficult to remove a rare earth magnet incorporated into a motor from a discarded motor. In contrast, the recovery method of this embodiment allows the motor itself to be processed without separating the magnet from the discarded motor. This makes it easy to recycle products or semi-finished products containing rare earth magnets. Furthermore, there is no need to crush the rare earth magnets beforehand. The shapes of waste vary depending on the type of product, and in order to extract rare earth magnets, it is necessary to design rare earth magnet extraction equipment suited to the shape of the waste, as described, for example, in "Recovery of Neodymium Magnets in Home Appliance Recycling" (November 29, 2011, Association for Electric Home Appliances; https://www.meti.go.jp/shingikai/sankoshin/sangyo_gijutsu/haikibutsu_recycle/pdf/016_06_00.pdf). The high cost of this has been one of the factors hindering magnet recycling. According to the recovery method of this embodiment, at least one oxide (flux) selected from the group consisting of alkali metal oxides and alkaline earth metal oxides, and optionally an easily oxidizable metal (e.g., aluminum, silicon) and/or its oxide, a melting point depressant (e.g., carbon), and/or an oxidizer (e.g., iron oxide) are added to the waste, and the waste is then heated and melted to separate and recover the rare earth magnets. This eliminates the need to extract the rare earth magnets from the waste, making this a highly economical method.

Furthermore, in conventional technology, rare earth magnets are demagnetized by heating or other methods when processed (see, for example, pages 9-11 of "Recovery of Neodymium Magnets in Home Appliance Recycling," above), but the recovery method of this embodiment makes it possible to use products or semi-finished products that contain rare earth magnets that have not been demagnetized.

Furthermore, some finished or semi-finished products have various plating applied to the surface of steel materials such as magnetic steel sheets or magnets in order to improve rust resistance and corrosion resistance. From the perspective of recovering rare earth elements, plating is undesirable because it increases impurities, and in conventional methods, this is removed beforehand by polishing or other methods. However, with the recovery method of this embodiment, the material can be recovered as is without removing the plating.

The waste may contain materials other than rare earth element-containing materials. Examples of materials other than rare earth element-containing materials include steel and copper. That is, according to a preferred embodiment of the present invention, the waste further contains at least one or both of steel and copper.

Steel materials include, for example, steel plates and screws that are integrated with products and contain iron as a primary component, as well as product cases and chassis. There are no particular restrictions on the iron content. The steel material may be magnetic steel, and its composition may include various elements such as Ni, Cr, Si, and Co. When rare earth magnets and steel are included, the material can be separated into two phases: a rare earth-rich phase and an Fe-C phase, by appropriately adjusting the amount of carbon relative to the total amount of iron. On the other hand, when steel is not included, for example, when only the rare earth magnet contains iron, the material can be separated into two phases: a rare earth-rich phase and an Fe-C phase, by appropriately adjusting the amount of carbon relative to the amount of iron in the rare earth magnet.

Furthermore, in this embodiment, if the waste contains copper, it can be separated into three phases: a rare-earth-enriched phase, an Fe-C phase, and a Cu phase. In conventional technology, aluminum and silicon are mixed into the Fe-C phase, making it difficult to efficiently recover only Cu. However, the recovery method of this embodiment can improve the efficiency of Cu recovery. This point will be explained in more detail in the section on separation process (2) below.

[Alkali metal oxide and/or alkaline earth metal oxide]
In the recovery method according to the present embodiment, at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides (hereinafter also simply referred to as "oxide (AMO/AEMO)") is used as a flux.

Examples of alkali metal oxides and alkaline earth metal oxides include sodium oxide (Na ₂ O), lithium oxide (Li ₂ O), potassium oxide (K ₂ O), rubidium oxide (Rb ₂ O), cesium oxide (Cs ₂ O), barium oxide (BaO), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), etc. Among these, sodium oxide (Na ₂ O), barium oxide (BaO), and calcium oxide (CaO) are preferred, with calcium oxide (CaO) being more preferred, from the viewpoint of being less susceptible to reduction.

The amount of oxide (AMO/AEMO) added (the total amount when two or more oxides are used) is sufficient to form a homogeneous melt (liquid phase region) between the rare earth element contained in the rare earth-containing material and the oxide of the easily oxidizable metal. In other words, the amount of oxide (AMO/AEMO) added is an amount equal to or greater than the liquidus line in a phase diagram determined by the mass of rare earth oxide in the rare earth-containing material and the content of the easily oxidizable metal oxide. Here, in this specification, "mass of rare earth oxide in the rare earth-containing material" refers to the total mass of rare earth oxide when _the rare earth elements (e.g., Nd, Pr, Dy, _Tb ) contained in _the rare earth-containing material (e.g., _rare earth magnet) are considered to be oxidized to rare earth oxides (e.g., _Nd2O3 , _Pr2O3 , _Dy2O3 , _Tb2O3 ). Furthermore, the content of easily oxidizable metal oxides refers to the content of each type. For _example , when only _Al2O3 is contained as an easily oxidizable metal oxide, the content of easily oxidizable metal oxides refers to the content of _Al2O3 . When _{Al2O3 and SiO2} are contained as easily oxidizable metal oxides, the content of easily oxidizable metal _oxides refers to the content of _each of _Al2O3 and _SiO2 .

Hereinafter, the "amount above the liquidus in a phase diagram determined by the mass of rare earth oxide in a rare earth element-containing material and the content of easily oxidizable metal oxide" will be described with reference to FIG. _1. FIG. 1 is a pseudo-quaternary phase diagram at 1450°C when _Nd2O3 is used as the rare earth element-containing material, _Al2O3 and _SiO2 are used as easily oxidizable metal oxides, _and calcium oxide (CaO) is used as the oxide (AMO/AEMO). Each edge of the regular tetrahedron shown in FIG. 1 represents the concentration of the four components. For example, consider the edge of the regular tetrahedron shown in FIG. _{1 with Nd2O3} and CaO as vertices. The composition ratio (mass ratio) of _Nd2O3 :CaO at any point A on this edge corresponds to the "distance from point A to the vertex (CaO)" and the "distance from point A to the vertex _{( Nd2O3} )" (leverage principle). Here, each side of the equilateral triangle located to the left of the regular tetrahedron represents the concentrations of _Nd2O3 , _{Al2O3 , and SiO2} , and one point (for example, point W) shown on the _face of the equilateral triangle represents the composition of these three components. More specifically, consider an equilateral triangle in the regular tetrahedron shown in FIG. ₁ with _Nd2O3 , _Al2O3 , and _SiO2 as its vertices. The composition ratio (mass ratio) of _Nd2O3 : _Al2O3 : _SiO2 at point _W on this face corresponds to the "distance of the _{perpendicular} from the line segment connecting _{Al2O3 and SiO2} to point W," the "distance of the perpendicular from _the line segment connecting _Nd2O3 and _SiO2 to point W," and the "distance of the perpendicular from _the line segment _{connecting Nd2O3} and _Al2O3 to point W." The composition of the three components is uniquely determined by the amounts _{of Nd2O3} , _{Al2O3 ,} and _SiO2 added in step (1). The line drawn from point W to the apex (CaO) represents the concentrations of the three components and CaO when CaO is added to the composition of the three components indicated by point W. The closer to the apex (CaO), the higher the CaO concentration. As the CaO concentration is increased relative to the composition of the three components indicated by point W, the solid-liquid mixed region changes to a liquid region (a region where a homogeneous melt is formed) at a certain concentration. The boundary from the solid-liquid mixed region to the liquid region is called the "liquidus," and on the liquidus, the liquid phase and a portion of the solid phase usually exist in equilibrium. In the pseudo-quaternary phase diagram shown in FIG. 1, the boundary between the solid-liquid mixed region and the liquid region exists as a surface, and in such cases, the boundary is sometimes also called the "liquidus surface." However, in this specification, the term "liquidus" will be used consistently regardless of whether the boundary is a line or a surface. The oxide (AMO/AEMO) content being "amount above the liquidus" refers to the concentration corresponding to the liquidus or the concentration in the liquidus region. In the case shown in Figure 1, the "amount above the liquidus" corresponds to the CaO concentration represented by the line segment (excluding the vertex (CaO)) connecting the intersection (not shown) of the straight line and the liquidus line to the vertex (CaO).

In the above, we used a pseudo-quaternary phase diagram to explain the "amount above the liquidus in a phase diagram determined by the rare earth oxide mass of a rare earth element-containing material and the content of easily oxidizable metal oxides." However, this "amount above the liquidus" can be determined without creating a phase diagram. In other words, if the rare earth oxide mass and easily oxidizable metal oxide content of a rare earth element-containing material that may be present in a system are determined in advance, the oxide (AMO/AEMO) concentration corresponding to the liquidus can be experimentally determined by increasing the oxide (AMO/AEMO) concentration for this composition. This makes it possible to determine the composition that produces a homogeneous melt, regardless of the composition of the rare earth elements and easily oxidizable metals (oxides) contained in the rare earth element-containing material. Therefore, the recovery method of this embodiment can be applied to waste with a variety of elemental compositions.

[Easily oxidizable metals and/or their oxides]
In this step (1), in addition to the above-mentioned waste containing a rare earth element and the oxide (AMO/AEMO), an oxidizable metal and/or its oxide may be added. In this specification, the term "easily oxidizable metal" refers to a metal or metalloid that is more easily oxidized than an oxidizing agent (e.g., iron oxide (Fe ₂ O ₃ )) described below. Examples of the easily oxidizable metal include aluminum (Al), silicon (Si), titanium (Ti), and zirconium (Zr). From the viewpoint of improving phase separation, the easily oxidizable metal is preferably at least one or both of aluminum (Al) and/or silicon (Si), and more preferably aluminum (Al) and silicon (Si) are used in combination. Furthermore, these easily oxidizable metals may be added in the form of oxides. Examples of oxides of easily oxidizable metals include aluminum oxide (Al ₂ O ₃ : alumina), silicon dioxide (SiO ₂ : silica), titanium oxide (TiO ₂ : titania), and zirconium oxide (ZrO ₂ : zirconia). Among these, from the viewpoint of improving phase separation, at least one or both of aluminum oxide (Al ₂ O ₃ : alumina) and/or silicon dioxide (SiO ₂ : silica) are preferred, and it is more preferable to use aluminum oxide (Al ₂ O ₃ : alumina) and silicon dioxide (SiO ₂ : silica) in combination. Note that, if a sufficient amount of easily oxidizable metal and/or its oxide is contained in the waste containing a rare earth element, no additional addition is necessary. Therefore, according to a preferred embodiment, the waste contains a rare earth element-containing material and an easily oxidizable metal. According to a more preferred embodiment, the waste contains a rare earth element-containing material, aluminum, and silicon.

[Melting Point Depressants]
Waste may contain a large amount of iron. For example, discarded motors contain a much larger amount of iron, derived from the electromagnetic steel plate portion of the motor, compared to rare earth magnets alone. When waste contains a large amount of iron, the melting point of iron is as high as 1538°C. Therefore, considering the efficiency of phase separation and the reduction of energy required for melting, it is preferable to melt the waste in the presence of a melting point depressant. Carbon is preferably used as the melting point depressant. Carbon prevents iron oxidation and prevents it from migrating to the rare earth-enriched phase, thereby improving separability. Examples of carbon sources include using a carbon crucible in a heating furnace (melting furnace), coating the furnace walls with carbon, and adding Fe-C alloys such as pig iron, coke, graphite, commercially available recarburizers, plastics, organic materials, etc., to the reaction system as additives. Other examples include blowing in a gaseous carbon source such as carbon dioxide or a hydrocarbon gas. Furthermore, as will be described later, when carbon is used as a melting point depressant, high-purity iron such as electrolytic iron added for the purpose of producing an Fe—C alloy is also included in the melting point depressant. Also, when the heating furnace or its furnace wall is used as the carbon supply source (melting point depressant) as described above, the carbon material on the surface of the furnace wall dissolves in the molten waste, etc., and is added as a melting point depressant.

Furthermore, when adding a melting point depressant to waste electrical steel sheets containing rare earth magnets and then heating and melting them, electrolytic iron can be added. Electrolytic iron is not necessarily required when heating and melting electrical steel sheets containing rare earth magnets at high temperatures of 1500°C or higher, but when carbon is added as a melting point depressant, the electrolytic iron reacts with the carbon in the melting point depressant to produce an Fe-C alloy. In this way, by producing an Fe-C alloy that melts at a temperature of around 1200°C prior to heating and melting the electrical steel sheets, the melting of the electrical steel sheets can be promoted, and the molten state of the electrical steel sheets can be achieved in a shorter time and at a lower temperature.

The amount of melting point depressant added is preferably near the eutectic point composition, as this results in the lowest melting temperature. When using carbon as a melting point depressant, it is preferable to heat and melt the waste in a carbon-saturated state, i.e., when no more carbon dissolves in the melt, from the perspective of melting point reduction and oxidation prevention effects. As a general guideline, the amount of melting point depressant added should be in the range of 5% to 10% by mass relative to the iron element content of the waste. This range is intended solely for cases where melting point depressants are added to the reaction system as additives, such as Fe-C alloys such as pig iron, coke, graphite, commercially available recarburizers, plastics, or organic materials, or when gaseous carbon sources such as carbon dioxide or hydrocarbon gases are injected. When using a carbon crucible or a carbon-coated furnace wall as a carbon source or in combination with the melting point depressant, the melting point depressant is not limited to the above range. However, when using part of such a heating furnace (melting furnace) as a carbon source, it is necessary to periodically repair the furnace wall and replace the crucible, so it is preferable to use another carbon source.

[Oxidizing agent]
In the recovery method according to this embodiment, an oxidizing agent for oxidizing the rare earth element can be added in step (1) to convert the rare earth element contained in the rare earth element-containing material into a rare earth oxide form before phase separation. Furthermore, when an easily oxidizable metal (e.g., Al and/or Si) is contained, the metal is converted into an easily oxidizable metal oxide form by the oxidizing agent, followed by phase separation. Preferably, at least 90 mol % of the easily oxidizable metal in the system is converted into an easily oxidizable metal oxide form, more preferably 95 mol % or more, even more preferably 99 mol % or more, and preferably 100 mol %. Increasing the proportion of the easily oxidizable metal in the oxide form can further reduce the amount of oxide (AMO/AEMO) used (particularly, the increase in the amount of oxide (AMO/AEMO) used due to the reduction of the oxide (AMO/AEMO) can be more effectively suppressed). Furthermore, the recovery of the easily oxidizable metal can be more efficiently performed. The oxidizing agent is added to the heated and melted rare earth element-containing material and the easily oxidizable metal to supply sufficient oxygen for the oxidation of the rare earth element and the easily oxidizable metal. Accelerating the oxidation of the rare earth element and the easily oxidizable metal is preferable from the viewpoint of improving the phase separation property and the recovery rate of the rare earth oxide.

Examples of oxidizing agents include oxidizing gases such as air, oxygen, and carbon dioxide, as well as iron oxide and composite oxides containing iron oxide. Among these, iron oxide is preferred because it not only provides sufficient oxygen for the oxidation of rare earth elements and easily oxidizable metals, but also reduces impurities in the recovered iron.

The amount of iron oxide added is preferably such that the molar ratio of oxygen to the amount of rare earth elements and easily oxidizable metals in the rare earth element-containing material (e.g., rare earth magnet) in the waste is 1.5 to 2.0 times. Note that if iron oxide is not added in an inert atmosphere, the added oxides (AMO/AEMO) may react with the rare earth elements in the magnet and be reduced, potentially reducing their function as a flux. Therefore, if iron oxide is not added, it is preferable to carry out step (1) in the presence of an oxidizing gas such as air, oxygen, or carbon dioxide.

As mentioned above, in the recovery method of this embodiment, since boron and its compounds are subject to regulation as hazardous substances, it is preferable to avoid their use as much as possible. That is, it is preferable that step (1) does not include the addition of boron-containing substances other than waste. Furthermore, the boron (B) content in the melt is preferably low. That is, the boron (B) content in the melt is preferably 4.3 mass% or less, relative to the total mass of rare earth elements. This makes it possible to reduce the boron content in the waste liquid discharged when recovering rare earth element oxides from the rare earth-enriched phase in steps (3a) to (3c) described below. Note that if boron is contained in the waste, the melt will contain boron derived from the waste. In such cases, the boron (B) content in the melt may be 2.4 mass% or more and 4.3 mass% or less, relative to the total mass of rare earth elements.

In this process (1), as explained above, waste containing rare earth elements and oxides (AMO/AEMO) are heated and melted. The heating temperature is preferably 1250°C to 1700°C. At 1250°C or higher, a homogeneous melt (liquid phase region) is easily produced. Furthermore, from the viewpoint of the durability of the refractories used in the melting furnace used for heating and melting, the heating temperature is preferably 1700°C or lower. Furthermore, from the viewpoint of improving the two-phase separation between the rare earth-rich phase and the Fe-C phase, the heating temperature is more preferably 1400°C to 1600°C. At a heating temperature of 1400°C or higher, the rare earth-containing material (e.g., rare earth magnet) is more easily melted. Furthermore, the melting point of pure iron is 1535°C; tilting facilitates separation of the rare earth-rich phase and the Fe-C phase due to the density difference. Therefore, the heating temperature is more preferably 1600°C or lower.

Heating to temperatures higher than the above temperature range is preferably avoided as it will worsen the two-phase separation properties. However, heating to a temperature higher than the above temperature range before maintaining it at that temperature range is effective in dissolving rare earth elements mixed in high-melting-point substances such as iron. For this reason, the temperature change during heating and melting may involve heating to the above temperature range that is suitable for two-phase separation, followed by cooling. Alternatively, to form a homogeneous melt, heating may be performed to a temperature higher than the above temperature range that is more suitable for two-phase separation (for example, above 1600°C to 1700°C), then the temperature may be lowered and maintained at the above temperature range that is more suitable for two-phase separation, followed by cooling.

From the perspective of improving phase separation, it is preferable to hold the heated and melted melt within the above temperature range for 10 minutes or more, and more preferably for 60 minutes or more. However, even if the holding time is too long, the effect will not exceed the theoretical distribution ratio, so from an economical perspective, it is preferable to hold the melt for 180 minutes or less.

The preferred heating temperature range when the waste further contains copper and is separated into three phases: a rare earth-enriched phase, an Fe-C phase, and a Cu phase, is the same as above.

The step (1) preferably includes the following steps (1a) to (1c) in this order:
Step (1a): adding a melting point depressant to the waste containing a rare earth element-containing material, aluminum, and silicon, and then heating and melting the waste to obtain a melt (1a);
Step (1b): contacting the melt (1a) with an oxidizing agent to obtain a melt (1b);
Step (1c): The oxides (AMO/AEMO) are added to the melt (1b) to obtain a melt (1c).

These steps (1a) to (1c) make it possible to prepare a melt suitable for phase separation with simpler procedures.

The melt obtained in step (1) contains at least rare earth oxides, oxides (AMO/AEMO), and oxides of easily oxidizable metals. Here, when the oxides of easily oxidizable metals are aluminum oxide and silicon dioxide, and the oxides (AMO/AEMO) are calcium oxide (CaO), the composition of these components preferably falls within the following ranges: The ratio of the aluminum oxide content to the sum of the mass of rare earth oxides, the content of easily oxidizable metal oxides, and the content of oxides (AMO/AEMO) in the rare earth element-containing material is 15.7 to 17.5 mass%, the content of silicon dioxide is 50.0 to 55.9 mass%, and the content of oxides (AMO/AEMO) is 5.0 to 15.0 mass%; the ratio of the mass of rare earth oxides, the content of easily oxidizable metal oxides, and the content of oxides (AMO/AEMO) in the rare earth element-containing material is 15.7 to 17.5 mass%. The ratio of the content of aluminum oxide to the sum of the content of the rare earth oxide of the rare earth element-containing material, the content of the oxide of the easily oxidizable metal, and the content of the oxide (AMO/AEMO) is 27.5 to 29.3 mass%, the content of silicon dioxide is 32.1 to 34.2 mass%, and the content of the oxide (AMO/AEMO) is 20.0 to 25.0 mass%; the ratio of the content of aluminum oxide to the sum of the mass of the rare earth oxide of the rare earth element-containing material, the content of the oxide of the easily oxidizable metal, and the content of the oxide (AMO/AEMO) is 10.9 the ratio of the aluminum oxide content to the total of the mass of rare earth oxides in the rare earth element-containing material, the content of easily oxidizable metal oxides, and the content of oxides (AMO/AEMO) is 11.6 to 12.9 mass%, and the ratio of the silicon dioxide content is 49.5 to 54.9 mass%. The oxide (AMO/AEMO) content is 10.0 to 18.9 mass%; the aluminum oxide content is 17.0 to 22.5 mass% relative to the sum of the rare earth oxide mass of the rare earth element-containing material, the easily oxidizable metal oxide content, and the oxide (AMO/AEMO) content, the silicon dioxide content is 35.9 to 47.6 mass%, and the oxide (AMO/AEMO) content is 10.0 to 32.1 mass%. By adding the rare earth element-containing material, oxide (AMO/AEMO), and easily oxidizable metal or its oxide to achieve this composition, a homogeneous melt can be more easily obtained.

<Separation step (2)>
In step (2), a rare-earth-enriched phase in which rare earth elements are concentrated in the oxides (AMO/AEMO) and an Fe—C phase are separated from the melt obtained in step (1). In the melt state, the relatively dense Fe—C phase separates into a lower layer and the relatively low-density rare-earth-enriched phase separates into an upper layer. Here, the rare-earth-enriched phase may contain, in addition to the rare earth oxides extracted by the oxides (AMO/AEMO), oxides of easily oxidizable metals (e.g., Al ₂ O ₃ and/or SiO ₂ ). According to the recovery method of this embodiment, by allowing the oxides of easily oxidizable metals to coexist when extracting rare earth oxides in the oxides (AMO/AEMO) as a flux, it is possible to obtain a homogeneous melt without using boron compounds such as boron oxide (B ₂ O ₃ ) or borates, as in conventional techniques.

Once the rare earth-rich phase and Fe-C phase have been formed, they can be separated and recovered by separating the phases while they are in a liquid state. According to the recovery method of this embodiment, the rare earth-rich phase has a low viscosity due to the use of oxides (AMO/AEMO) as a flux, which allows the rare earth-rich phase to be removed from the top of the furnace by tilting, facilitating separation.

Another method of separation is to discharge the phases from the bottom of the furnace in order of density. Another method is to cool the melt to solidify it, and then cut it along the boundary between the phases with a cutter or similar tool. When cooling, it is preferable to cool slowly until it solidifies in order to improve separability, but it is also possible to solidify it by rapid cooling.

When the waste contains copper, the melt separates into three phases: a rare-earth-enriched phase, an Fe-C phase, and a Cu phase. That is, according to a preferred embodiment of the present invention, the waste further contains copper, and in separation step (2), a rare-earth-enriched phase in which rare-earth elements are concentrated in oxides (AMO/AEMO), an Fe-C phase, and a Cu phase are separated from the melt. When the melt contains copper, the melt separates into a rare-earth-enriched phase (upper layer), an Fe-C phase (middle layer), and a Cu phase (lower layer), in descending order of density. As with two-phase separation, these phases can be extracted by any of the following methods: tilting, discharging from the bottom of the furnace, or solidifying and cutting. When easily oxidizable metals (aluminum, silicon) are not converted to oxide form, as in conventional technology, the easily oxidizable metals are contained in the Fe-C phase, which can make separation of the Fe-C and Cu phases difficult. According to the recovery method of this embodiment, by converting the easily oxidizable metal into an oxide form, the oxide of the easily oxidizable metal is contained in the rare earth-enriched phase. As a result, the easily oxidizable metal is hardly contained in the Fe-C phase, which improves the phase separation between the Fe-C phase and the Cu phase and makes it possible to recover Cu efficiently. Therefore, in this respect as well, the recovery method of this embodiment has an advantageous effect compared to conventional techniques.

The rare earth-enriched phase (rare earth-enriched product) separated in separation step (2) contains rare earth oxides, oxides of easily oxidizable metals, and oxides (AMO/AEMO). That is, according to another aspect of the present invention, a rare earth-enriched product is provided that contains rare earth oxides, oxides of easily oxidizable metals, and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides. Furthermore, according to the present invention, since the rare earth-enriched phase (rare earth-enriched product) is obtained using a boron-free flux, the rare earth-enriched product has the characteristic of having a low boron content. This allows for a reduction in the boron content in the waste liquid discharged when recovering oxides of rare earth elements from the rare earth-enriched product in steps (3a) to (3c) described below. According to one embodiment of the present invention, the boron content in the rare earth-enriched product is preferably 4.3 mass% or less, relative to the total mass of rare earth elements, and may be 2.4 mass% to 4.3 mass%.

<Steps (3a) to (3c)>
The recovery method according to this embodiment may further include steps (3a) to (3c) of recovering rare earth oxides from the rare earth-enriched phase after the separation step (2). That is, according to a preferred embodiment of the present invention, the recovery method includes the following steps (3a) to (3c) in order after the separation step (2):
Step (3a): The rare earth-enriched phase obtained in step (2) is leached with an acid to obtain a rare earth element leachate.
Step (3b): Precipitating the rare earth elements in the rare earth leach solution as salts to obtain a precipitate.
Step (3c): The precipitate is heated to recover the rare earth elements as oxides.

Examples of acids used in the leaching treatment in step (3a) include oxalic acid, hydrochloric acid, and sulfuric acid. After acid leaching in step (3a) dissolves the rare earth elements, an alkali (e.g., ammonium hydroxide, ammonium sulfate, or sodium hydroxide) is added in step (3b) to adjust the pH (e.g., to pH 1.5 to 2) to precipitate a precipitate (salt of the rare earth elements). Since components derived from the oxides (AMO/AEMO) remain dissolved in the leachate, the precipitate (salt of the rare earth elements) can be recovered by solid-liquid separation. Then, in step (3c), the precipitate (salt of the rare earth elements) is calcined at 600°C to 1000°C for 30 to 90 minutes, allowing it to be recovered as rare earth oxides.

Furthermore, in the recovery method according to this embodiment, after steps (3a) to (3c), the obtained rare earth oxides can be reduced to and recovered as simple rare earth elements using known methods such as molten salt electrolysis (molten salt reduction) and Ca reduction (calcium reduction), which are existing methods for reducing rare earth oxides to metals.

The following embodiments are also within the scope of the present invention: the recovery method according to claim 1 having the characteristics of claim 2; the recovery method according to claim 1 or 2 having the characteristics of claim 3; the recovery method according to any one of claims 1 to 3 having the characteristics of claim 4; the recovery method according to any one of claims 1 to 4 having the characteristics of claim 5; the recovery method according to any one of claims 1 to 5 having the characteristics of claim 6; the recovery method according to any one of claims 1 to 6 having the characteristics of claim 7; the recovery method according to any one of claims 1 to 7 having the characteristics of claim 8; the recovery method according to any one of claims 1 to 8 having the characteristics of claim 9; the recovery method according to claim 6 having the characteristics of claim 10; the recovery method according to claim 6 having the characteristics of claim 11; the recovery method according to claim 6 having the characteristics of claim 12; the recovery method according to claim 6 having the characteristics of claim 13; the recovery method according to claim 6 having the characteristics of claim 14; the recovery method according to any one of claims 1 to 14 having the characteristics of claim 15; the recovery method according to claim 15 having the characteristics of claim 16; the recovery method according to any one of claims 1 to 16 having the characteristics of claim 17; and the rare earth-enriched product according to claim 18 having the characteristics of claim 19.

The present invention will be explained in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples.

<Checking the liquidus line>
[Reference example 1]
A mixture of 0.155 g of 99.9 mass% pure Nd ₂ O ₃ , 0.250 g of CaO, 0.274 g of Al ₂ O ₃ , and 0.321 g of SiO ₂ (total 1 g) was used as the sample of this reference example. The mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ in the mixture was Nd ₂ O ₃ : Al ₂ O ₃ : SiO ₂ = 20.6: 36.6: 42.8. This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated to 1450 ° C in a Kanthal furnace in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched samples were subjected to structural observation using an optical microscope and a scanning electron microscope (SEM), and phase identification using an X-ray diffractometer (XRD). Based on these results, the dissolving ability of CaO flux _{in Nd2O3} was investigated. As a result, only a vitrified structure formed by cooling the homogeneous melt was observed in the quenched samples, confirming that the samples of this reference example produced a homogeneous melt at high temperatures.

[Reference Examples 2 to 4, Comparative Reference Examples 1 to 4]
Nd ₂ O ₃ with a purity of 99.9% by mass, CaO, Al ₂ O ₃ , and SiO ₂ were weighed and mixed (1 g total) to obtain the composition shown in Table 1 below. In Reference Examples 2 to 4 and Comparative Reference Examples 1 to 4, the mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ was approximately the same as that in Reference Example 1 (with an error of ±0.1% by mass). This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated in a Kanthal furnace at 1400°C or 1450°C (see Table 1) in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched sample was subjected to structural observation using an optical microscope and SEM, and phase identification using XRD. Based on these results, the dissolving ability of CaO flux in _Nd2O3 was investigated. Then, depending on whether or not only a vitrified structure was observed in the rapidly cooled sample, _it was determined whether a homogeneous melt was formed at high temperature or whether a mixture of liquid and solid phases was formed. The results are shown in Table 1 below. In the table below, "L" indicates that a homogeneous melt was formed, and "L+S" indicates that a mixture of liquid and solid phases was formed.

The results shown in Table 1 reveal that when the mass ratio of _Nd2O3 , _{Al2O3 , and SiO2} is _Nd2O3 : _Al2O3 : _SiO2 = _20.6 : _36.6 :42.8 and the melt temperature is 1450°C, the liquidus exists in a range where the proportion of CaO relative to the total mass _{of Nd2O3} , _CaO , _Al2O3 , and _SiO2 is greater than 15.0 mass% and less than or equal to 20.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least _20.0 mass% or more (the proportion of CaO relative to the total mass of _Nd2O3 and CaO is at least 54.8 mass% or more).

Furthermore, from the results shown in Table 1, it _can be _seen that when the mass ratio of _Nd2O3 , _Al2O3 , and _SiO2 is _Nd2O3 : _Al2O3 : _SiO2 = _20.6 : _36.6 :42.8 and the melt temperature is 1400°C, the liquidus exists in a range where the proportion of CaO relative to the total mass of _Nd2O3 , _CaO , _Al2O3 , _{and SiO2} is greater than 22.5 mass% and less than or equal to 23.5 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least 23.5 mass% or more (the proportion of CaO relative to the total mass _{of Nd2O3} and CaO is at least 59.8 mass% or more). The compositions in Reference Examples 1 to 4 and Comparative Reference Examples 1 to 4 correspond to the compositions on the line segment connecting point W and the vertex (CaO) in the pseudo-quaternary phase diagram shown in FIG.

[Reference example 5]
A sample of this reference example was prepared by mixing 0.204 g of Nd ₂ O ₃ with a purity of 99.9 mass%, 0.100 g of CaO, 0.166 g of Al ₂ O ₃ , and 0.530 g of SiO ₂ (total 1 g). The mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ was Nd ₂ O ₃ : Al ₂ O ₃ : SiO ₂ = 22.7: 18.4: 58.9. This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated in a Kanthal furnace at 1450 ° C in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched samples were subjected to structural observation using an optical microscope and a scanning electron microscope (SEM), and phase identification using an X-ray diffractometer (XRD). Based on these results, the dissolving ability of CaO flux _{in Nd2O3} was investigated. As a result, only a vitrified structure formed by cooling the homogeneous melt was observed in the quenched samples, confirming that the samples of this reference example produced a homogeneous melt at high temperatures.

[Reference Examples 6 to 8, Comparative Reference Examples 5 to 7]
Nd ₂ O ₃ with a purity of 99.9% by mass, CaO, Al ₂ O ₃ , and SiO ₂ were weighed and mixed (1 g total) to obtain the composition shown in Table 2 below. In Reference Examples 6 to 8 and Comparative Reference Examples 5 to 7, the mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ was approximately the same as that in Reference Example 5 (with an error of ±0.1% by mass). This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated in a Kanthal furnace at 1400°C or 1450°C (see Table 2) in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched sample was subjected to structural observation using an optical microscope and SEM, and phase identification using XRD. Based on these results, the dissolving ability of CaO flux in _Nd2O3 was investigated. Then, depending on whether or not only a vitrified structure was observed in the rapidly cooled sample, _it was determined whether a homogeneous melt was formed at high temperature or whether a mixture of liquid and solid phases was formed. The results are shown in Table 2 below.

The results shown in Table 2 reveal that when the mass ratio of _Nd2O3 , _{Al2O3 , and SiO2 is Nd2O3} : _Al2O3 : _SiO2 = _22.7 :18.4:58.9 and the melt temperature is 1450°C, the liquidus exists in a range where the proportion of CaO relative to the total mass _{of Nd2O3} , _CaO , _Al2O3 , and _SiO2 is greater than 0.0 mass% and less than or equal to 5.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least _5.0 mass% or more (the proportion of CaO relative to the total mass of _Nd2O3 and CaO is at least 18.8 mass% or more).

Furthermore, from the results shown in Table 2, it _can be _seen that when the mass ratio of _Nd2O3 , _Al2O3 , and _SiO2 is _Nd2O3 : _Al2O3 : _SiO2 = _22.7 :18.4:58.9 _and the melt temperature is 1400°C, the liquidus exists in a range where the proportion of CaO relative to the total mass of _Nd2O3 , _CaO , _Al2O3 , _{and SiO2} is greater than 5.0 mass% and less than or equal to 10.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least 10.0 mass% or more (the proportion of CaO relative to the total mass _{of Nd2O3} and CaO is at least 32.9 mass% or more). The compositions in Reference Examples 5 to 8 and Comparative Reference Examples 5 to 7 correspond to compositions on the line segment connecting point Z and the vertex (CaO) in the pseudo-quaternary phase diagram shown in FIG.

[Reference example 9]
A mixture of 0.250 g of 99.9% by mass Nd ₂ O ₃ , 0.177 g of CaO, 0.109 g of Al ₂ O ₃ , and 0.464 g of SiO ₂ (total 1 g) was used as the sample of this reference example. The mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ in the mixture was Nd ₂ O ₃ : Al ₂ O ₃ : SiO ₂ = 30.4: 13.2: 56.4. This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated to 1450 ° C in a Kanthal furnace in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched samples were subjected to structural observation using an optical microscope and a scanning electron microscope (SEM), and phase identification using an X-ray diffractometer (XRD). Based on these results, the dissolving ability of CaO flux _{in Nd2O3} was investigated. As a result, only a vitrified structure formed by cooling the homogeneous melt was observed in the quenched samples, confirming that the samples of this reference example produced a homogeneous melt at high temperatures.

[Reference Example 10, Comparative Reference Examples 8 to 9]
Nd ₂ O ₃ with a purity of 99.9% by mass, CaO, Al ₂ O ₃ , and SiO ₂ were weighed and mixed (1 g total) to obtain the composition shown in Table 3 below. In Reference Example 10 and Comparative Reference Examples 8-9, the mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ was approximately the same as that in Reference Example 9 (with an error of ±0.1% by mass). This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm and heated in a Kanthal furnace at 1450°C in an air atmosphere for 24 hours. After the specified time, the sample was quenched by water cooling. The quenched sample was subjected to structural observation using an optical microscope and SEM, and phase identification using XRD. Based on these results, the solubility of CaO flux in Nd ₂ O ₃ was investigated. Then, depending on whether or not only a vitrified structure was observed in the rapidly cooled sample, it was determined whether a homogeneous melt had been produced at high temperature or whether a mixture of liquid and solid phases had been produced.

The results shown in Table 3 reveal that when the mass ratio of _Nd2O3 , _{Al2O3 , and SiO2} is _Nd2O3 : _Al2O3 : _SiO2 = _30.4 : _13.2 :56.4 and the melt temperature is 1450°C, the liquidus exists in a range where the proportion of CaO relative to the total mass _{of Nd2O3} , _CaO , _Al2O3 , and _SiO2 is greater than 10.0 mass% and less than or equal to 14.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least _14.0 mass% or more (the proportion of CaO relative to the total mass of _Nd2O3 and CaO is at least 34.9 mass% or more). The compositions of Reference Examples 9 and 10 and Comparative Reference Examples 8 and 9 correspond to the compositions on the line segment connecting point G and the vertex (CaO) in the pseudo-quaternary phase diagram shown in FIG.

[Reference example 11]
A mixture of 0.200 g of 99.9% by mass Nd ₂ O ₃ , 0.189 g of CaO, 0.116 g of Al ₂ O ₃ , and 0.495 g of SiO ₂ (total 1 g) was used as the sample of this reference example. The mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ in the mixture was Nd ₂ O ₃ : Al ₂ O ₃ : SiO ₂ = 24.7: 14.3: 61.0. This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated to 1450 ° C in a Kanthal furnace in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched samples were subjected to structural observation using an optical microscope and a scanning electron microscope (SEM), and phase identification using an X-ray diffractometer (XRD). Based on these results, the dissolving ability of CaO flux _{in Nd2O3} was investigated. As a result, only a vitrified structure formed by cooling the homogeneous melt was observed in the quenched samples, confirming that the samples of this reference example produced a homogeneous melt at high temperatures.

[Reference Example 12, Comparative Reference Example 10]
_Nd2O3 with a purity of 99.9% by _mass , CaO _{, Al2O3} , and _SiO2 were weighed and mixed (1 g total) to obtain the composition shown in Table 4 below. In Reference Example 12 and Comparative Reference Example 10, the mass ratio of _Nd2O3 , _Al2O3 , and _SiO2 was approximately the same as that in Reference Example 11 (with an error of ±0.1% by mass). The sample was inserted into an iron crucible with an inner diameter of 8 _mm , a thickness of 1 mm, and a height of ₅₀ mm and heated in a Kanthal furnace at 1450°C in an air atmosphere for 24 hours. After the specified time, the sample was quenched by water cooling. The quenched sample was subjected to structural observation using an optical microscope and SEM, and phase identification using _XRD . Based on these results, the solubility of CaO flux in _Nd2O3 was investigated. Then, depending on whether or not only a vitrified structure was observed in the rapidly cooled sample, it was determined whether a homogeneous melt had been produced at high temperature or whether a mixture of liquid and solid phases had been produced.

The results shown in Table 4 reveal that when the mass ratio of _Nd2O3 , _{Al2O3 , and SiO2 is Nd2O3} : _Al2O3 : _SiO2 = _24.7 :14.3:61.0 and the melt temperature is 1450°C, the liquidus exists in a range where the proportion of CaO relative to the total mass _{of Nd2O3} , _CaO , _Al2O3 , and _SiO2 is greater than 0.0 mass% and less than or equal to 10.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least _10.0 mass% or more (the proportion of CaO relative to the total mass of _Nd2O3 and CaO is at least 31.1 mass% or more). The compositions in Reference Examples 11 and 12 and Comparative Reference Example 10 correspond to compositions on the line segment connecting point F and the vertex (CaO) in the pseudo-quaternary phase diagram shown in FIG.

[Reference example 13]
A mixture of 0.150 g of 99.9% by mass Nd ₂ O ₃ , 0.321 g of CaO, 0.170 g of Al ₂ O ₃ , and 0.359 g of SiO ₂ (total 1 g) was used as the sample of this reference example. The mass ratio of Nd ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ in the mixture was Nd ₂ O ₃ : Al ₂ O ₃ : SiO ₂ = 22.1 : 25.0 : 52.9. This sample was inserted into an iron crucible with an inner diameter of 8 mm, a thickness of 1 mm, and a height of 50 mm, and heated to 1450 ° C in a Kanthal furnace in an air atmosphere for 24 hours. The sample held for the specified time was quenched by water cooling. The quenched samples were subjected to structural observation using an optical microscope and a scanning electron microscope (SEM), and phase identification using an X-ray diffractometer (XRD). Based on these results, the dissolving ability of CaO flux _{in Nd2O3} was investigated. As a result, only a vitrified structure formed by cooling the homogeneous melt was observed in the quenched samples, confirming that the samples of this reference example produced a homogeneous melt at high temperatures.

[Reference Example 14, Comparative Reference Example 11]
_Nd2O3 with a purity of 99.9% by _mass , CaO _{, Al2O3} , and _SiO2 were weighed and mixed (1 g total) to obtain the composition shown in Table 5 below. In Reference Example 14 and Comparative Reference Example 11, the mass ratio of _Nd2O3 , _Al2O3 , and _SiO2 was approximately the same as that in Reference Example 13 (with an error of ±0.1% by mass). The sample was inserted into an iron crucible with an inner diameter of 8 _mm , a thickness of 1 mm, and a height of ₅₀ mm and heated in a Kanthal furnace at 1450°C in an air atmosphere for 24 hours. After the specified time, the sample was quenched by water cooling. The quenched sample was subjected to structural observation using an optical microscope and SEM, and phase identification using _XRD . Based on these results, the solubility of CaO flux in _Nd2O3 was investigated. Then, depending on whether or not only a vitrified structure was observed in the rapidly cooled sample, it was determined whether a homogeneous melt had been produced at high temperature or whether a mixture of liquid and solid phases had been produced.

The results shown in Table 5 reveal that when the mass ratio of _Nd2O3 , _{Al2O3 , and SiO2} is _Nd2O3 : _Al2O3 : _SiO2 = _22.1 :25.0:52.9 _and the melt temperature is 1450°C, the liquidus exists in a range where the proportion of CaO relative to the total mass of _Nd2O3 , _CaO , _Al2O3 , _{and SiO2} is greater than 0.0 mass% and less than or equal to 10.0 mass%. Therefore, it was confirmed that at this composition and temperature, a homogeneous melt is produced as long as the proportion of CaO is at least _10.0 mass% or more (the proportion of CaO relative to the total mass of _Nd2O3 and CaO is at least 33.4 mass% or more). The compositions in Reference Examples 13 and 14 and Comparative Reference Example 11 correspond to compositions on the line segment connecting point H and the vertex (CaO) in the pseudo-quaternary phase diagram shown in FIG.

<Recovery of rare earth oxides>
[Example 1]
A graphite crucible (model number: No. 8, maximum processing capacity per run: 8 kg) manufactured by Nippon Crucible Co., Ltd. was used to heat 945.8 g of a rotor and 1,219.5 g of a stator as waste containing rare earth elements and steel, as well as 99.3 g of a recarburizer as a melting point depressant, using a high-frequency induction furnace. The rotor contained 110.7 g of a neodymium magnet (Magnet 1) as the rare earth element-containing material, with a composition of 21.0% by mass of Nd, 5.0% by mass of Pr, 2.5% by mass of Dy, 0.4% by mass of Tb, 0.95% by mass of B, and 70.15% by mass of Fe. The composition of the neodymium magnet (Magnet 1) was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The rotor and stator contained a total of 45.2 g of silicon and 16.0 g of aluminum. After heating to 1400°C and melting, 244 g of _iron oxide ( _Fe2O3 ) was added as an oxidizer, and the molten metal (melt) was stirred with a carbon rod in the atmosphere to sufficiently oxidize the rare earth elements, silicon, and aluminum with the iron oxide and oxygen in the air. Then, 18.3 g of calcium oxide (CaO) was added as an alkaline earth metal oxide, and the molten metal (melt) was stirred with a carbon rod.

After 30 minutes of holding and air-cooling, the crucible was cut to separate the rare-earth-enriched _RExOy - _CaO slag (RE: Nd, Pr, Dy, and Tb) and the Fe—C phase. The mass of rare earth oxides in the rare-earth element-containing material _{(assuming that the Nd, Pr, Dy, and Tb contained in Magnet 1 are oxidized to Nd2O3 , Pr2O3 , Dy2O3 , and Tb2O3} ) is the sum of _the masses _{of Nd2O3} , _Pr2O3 , _{Dy2O3 ,} and _{Tb2O3 )} , the amount of easily oxidizable metal oxides ( _SiO2 and _Al2O3 ), and _the amount of alkaline earth metal oxide (CaO) are shown in Table 6 below. The mass ratio of rare earth oxide to Al ₂ O ₃ to SiO ₂ was approximately the same as in Reference Examples 5 to 8 and Comparative Reference Examples 5 to 7, that is, rare earth oxide mass:Al ₂ O ₃ :SiO ₂ = 22.7:18.4:58.9. The content of alkaline earth metal oxide was 32.9 mass% of the total mass of alkaline earth metal oxide and rare earth oxide.

The results of the component analysis of the recovered Fe-C phase are shown in Table 7 below. The amount of residual RE (RE: Nd, Pr, Dy, and Tb) in the Fe-C phase was a total of 0.03 mass% relative to the total mass of the Fe-C phase. The Fe-C phase contains almost no rare earth elements, and it is believed that the rare earth components in the neodymium magnet have migrated to the slag phase (rare earth-enriched phase). The composition of the Fe-C phase was determined using ICP-AES.

0.5 g of the recovered rare earth-enriched phase (RE _x O _y -CaO-based slag) was acid-leached with 20 ml of 6 mol/L hydrochloric acid and filtered to obtain a filtrate. 10 ml of a 1 mol/L aqueous oxalic acid solution was added to the filtrate, and the pH was adjusted to 1.8 by adding aqueous ammonia. The pH-adjusted solution was stirred and held at 40°C for 1 to 2 hours to obtain a precipitate of rare earth oxalates. The rare earth oxalates were separated by filtration and calcined in a muffle furnace at 900°C for 60 minutes to obtain a powder containing rare earth oxides. The analysis results of the powder are shown in Table 8 below. The amount of rare earth oxides in the powder was 98.6 mass% based on the total mass of the powder. The composition of the powder was determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy).

[Example 2]
In a graphite crucible manufactured by Nippon Crucible Co., Ltd. (model number: No. 8, maximum processing amount per time: 8 kg),
A 945.8 g rotor and 1,219.5 g stator were placed in a high-frequency induction furnace containing 945.8 g of waste material containing rare earth elements, steel, and copper, 99.3 g of recarburizer as a melting point depressant, and 392 g of copper to promote phase separation. The rotor contained 110.7 g of neodymium magnet (Magnet 1) as the rare earth element-containing material, with a composition of 21.0 mass% Nd, 5.0 mass% Pr, 2.5 mass% Dy, 0.4 mass% Tb, 0.95 mass% B, and 70.15 mass% Fe. The rotor and stator contained a total of 45.2 g of silicon, 16.0 g of aluminum, and 323.8 g of copper. After heating to 1400°C and melting, 244 g of _iron oxide ( _Fe2O3 ) was added as an oxidizer, and the molten metal (melt) was stirred with a carbon rod in the atmosphere to sufficiently oxidize the rare earth elements, silicon, and aluminum with the iron oxide and oxygen in the air. Then, 18.3 g of calcium oxide (CaO) was added as an alkaline earth metal oxide, and the molten metal (melt) was stirred with a carbon rod.

After 30 minutes of holding and air-cooling, the crucible was cut to separate the rare-earth-enriched _RExOy - _CaO slag (RE: Nd, Pr, Dy, and Tb), Fe—C, and Cu phases. The mass of rare earth oxides in the rare-earth element-containing material (assuming that the Nd, Pr, Dy, and Tb contained in Magnet ₁ are oxidized to _Nd2O3 , _Pr2O3 , _{Dy2O3 , and Tb2O3} ) is the sum of _the masses _{of Nd2O3} , _Pr2O3 , _{Dy2O3 , and Tb2O3 ) ,} the amount _of easily oxidizable metal oxides ( _{SiO2 and Al2O3} ), and the amount of alkaline earth metal oxide (CaO) are shown in Table 9 below. The mass ratio of the rare earth oxide mass, Al ₂ O ₃ , and SiO ₂ was almost the same as in Reference Examples 5 to 8 and Comparative Reference Examples 5 to 7, that is, rare earth oxide mass:Al ₂ O ₃ :SiO ₂ = 22.7:18.4:58.9.
The content of alkaline earth metal oxides was 32.9 mass % based on the total mass of alkaline earth metal oxides and rare earth oxides.

The results of the component analysis of the recovered Fe-C phase are shown in Table 10 below. The amount of residual RE (RE: Nd, Pr, Dy, and Tb) in the Fe-C phase was a total of 0.02 mass% relative to the total mass of the Fe-C phase. The Fe-C phase contains almost no rare earth elements, and it is believed that the rare earth components in the neodymium magnet have migrated to the slag phase (rare earth-enriched phase). The composition of the Fe-C phase was determined using ICP-AES.

The results of the component analysis of the recovered Cu phase are shown in Table 11 below. The amount of residual RE (RE: Nd, Pr, Dy, and Tb) in the Cu phase was a total of 0.05 mass% relative to the total mass of the Cu phase. The Cu phase contained almost no rare earth elements, and it is believed that the rare earth components in the neodymium magnet had migrated to the slag phase (rare earth-enriched phase). The composition of the Cu phase was determined using ICP-AES.

0.5 g of the recovered rare earth-enriched phase (RE _x O _y -CaO-based slag) was acid-leached with 20 ml of 6 mol/L hydrochloric acid and filtered to obtain a filtrate. 10 ml of a 1 mol/L aqueous oxalic acid solution was added to the filtrate, and the pH was adjusted to 1.8 by adding aqueous ammonia. The pH-adjusted solution was stirred and held at 40°C for 1 to 2 hours to obtain a precipitate of rare earth oxalates. The rare earth oxalates were separated by filtration and calcined in a muffle furnace at 900°C for 60 minutes to obtain a powder containing rare earth oxides. The analysis results of the powder are shown in Table 12 below. The amount of rare earth oxides in the powder was 98.0 mass% in total, based on the total mass of the powder. The composition of the powder was determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy).

[Example 3]
A graphite crucible (model number: No. 8, maximum processing capacity per run: 8 kg) manufactured by Nippon Crucible Co., Ltd. was charged with 945.8 g of a rotor containing rare earth element-containing waste material and steel, 53.6 g of a recarburizer as a melting point depressant, and 30.4 g of silicon dioxide and 4.1 g of aluminum oxide as reagents, and the resulting mixture was heated using a high-frequency induction furnace. The rotor contained 110.7 g of a neodymium magnet (Magnet 1) as a rare earth element-containing material, with a composition of 21.0% by mass of Nd, 5.0% by mass of Pr, 2.5% by mass of Dy, 0.4% by mass of Tb, 0.95% by mass of B, and 70.15% by mass of Fe. The composition of the neodymium magnet (Magnet 1) was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The rotor contained a total of 18.1 g of silicon and 6.4 g of aluminum. The total amount of silicon contained in the rotor and the reagent was 32.3 g, and the total amount of aluminum was 8.6 g. After heating to 1450 ° C and melting, 192 g of iron oxide (Fe ₂ O ₃ ) was added as an oxidizer, and the molten metal (melt) was stirred with a carbon rod in the atmosphere. The rare earth components, silicon, and aluminum were sufficiently oxidized by the iron oxide and oxygen in the air. Then, 26.4 g of calcium oxide (CaO) was added as an alkaline earth metal oxide, and the molten metal (melt) was stirred with a carbon rod.

After 30 minutes of holding and air-cooling, the crucible was cut to separate the rare-earth-enriched _RExOy - _CaO slag (RE: Nd, Pr, Dy, and Tb) and the Fe—C phase. The mass of rare earth oxides in the rare-earth element-containing material (assuming that the Nd, Pr, _{Dy, and Tb contained in Magnet 1 are oxidized to Nd2O3, Pr2O3 , Dy2O3 , and Tb2O3 ) is the} sum of the masses _{of Nd2O3} , _Pr2O3 , _Dy2O3 , and _{Tb2O3 )} , _the amount of easily oxidizable metal oxides ( _SiO2 and _Al2O3 ), and the amount of alkaline earth metal oxide (CaO) _are shown in Table ₁₃ below. The mass ratio of rare earth oxide to Al ₂ O ₃ to SiO ₂ was approximately the same as in Reference Examples 9 to 10 and Comparative Reference Examples 8 and 9, that is, rare earth oxide mass:Al ₂ O ₃ :SiO ₂ = 30.4:13.3:56.4. The content of alkaline earth metal oxide was 41.4 mass% of the total mass of alkaline earth metal oxide and rare earth oxide.

The results of the component analysis of the recovered Fe-C phase are shown in Table 14 below. The amount of residual RE (RE: Nd, Pr, Dy, and Tb) in the Fe-C phase was a total of 0.02 mass% relative to the total mass of the Fe-C phase. The Fe-C phase contains almost no rare earth elements, and it is believed that the rare earth components in the neodymium magnet have migrated to the slag phase (rare earth-enriched phase). The composition of the Fe-C phase was determined using ICP-AES.

0.5 g of the recovered rare earth-enriched phase (RE _x O _y -CaO-based slag) was acid-leached with 20 ml of 6 mol/L hydrochloric acid and filtered to obtain a filtrate. 10 ml of a 1 mol/L aqueous oxalic acid solution was added to the filtrate, and the pH was adjusted to 1.8 by adding aqueous ammonia. The pH-adjusted solution was stirred and held at 40°C for 1 to 2 hours to obtain a precipitate of rare earth oxalates. The rare earth oxalates were separated by filtration and calcined in a muffle furnace at 900°C for 60 minutes to obtain a powder containing rare earth oxides. The analysis results of the powder are shown in Table 15 below. The amount of rare earth oxides in the powder was 98.7 mass% based on the total mass of the powder. The composition of the powder was determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy).

[Example 4]
A graphite crucible (model number: No. 8, maximum processing capacity per run: 8 kg) manufactured by Nippon Crucible Co., Ltd. was charged with 751.6 g of a rotor and 1029.9 g of a stator as waste containing rare earth elements and steel, 77.8 g of a recarburizer as a melting point depressant, and 42.2 g of aluminum oxide as a reagent, and the resulting mixture was heated using a high-frequency induction furnace. The rotor contained 102.8 g of a neodymium magnet (Magnet 2) as the rare earth element-containing material, with a composition of 29.2% by mass of Nd, 0.0% by mass of Pr, 1.6% by mass of Dy, 0.5% by mass of Tb, 0.95% by mass of B, and 67.75% by mass of Fe. The composition of the neodymium magnet (Magnet 2) was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The rotor and stator contained a total of 36.4 g of silicon and 12.9 g of aluminum. The total amount of silicon contained in the rotor and the reagent was 36.4 g, and the total amount of aluminum was 35.2 g. After heating to 1400 ° C and melting, 200.5 g of iron oxide (Fe ₂ O ₃ ) was added as an oxidizer, and the molten metal (melt) was stirred with a carbon rod under atmospheric pressure. The rare earth components, silicon, and aluminum were sufficiently oxidized by the iron oxide and oxygen in the air. Then, 60.6 g of calcium oxide (CaO) was added as an alkaline earth metal oxide, and the molten metal (melt) was stirred with a carbon rod.

After 30 minutes of holding and air-cooling, the crucible was cut to separate the rare-earth-enriched _RExOy - _CaO slag (RE: Nd, Pr, Dy, and Tb) and the Fe—C phase. The mass of rare earth oxides in the rare-earth element-containing material (assuming that the Nd, Pr, _{Dy ,} and Tb contained in magnet ₂ are oxidized to _Nd2O3 , _Pr2O3 , _Dy2O3 , and _{Tb2O3 )} is the sum of _the masses _{of Nd2O3} , _Pr2O3 , _Dy2O3 , and _{Tb2O3 )} , _the amount of easily oxidizable metal oxides ( _SiO2 and _Al2O3 ), and the amount of alkaline earth metal oxide (CaO) _are shown in Table 16 below. The mass ratio of rare earth oxide to Al ₂ O ₃ to SiO ₂ was approximately the same as in Reference Examples 1 to 4 and Comparative Reference Examples 1 to 4, that is, rare earth oxide mass:Al ₂ O ₃ :SiO ₂ = 20.6:36.6:42.8. The content of alkaline earth metal oxide was 61.8 mass% of the total mass of alkaline earth metal oxide and rare earth oxide.

The results of the component analysis of the recovered Fe-C phase are shown in Table 17 below. The amount of residual RE (RE: Nd, Pr, Dy, and Tb) in the Fe-C phase was a total of 0.17 mass% relative to the total mass of the Fe-C phase. The Fe-C phase contains almost no rare earth elements, and it is believed that the rare earth components in the neodymium magnet have migrated to the slag phase (rare earth-enriched phase). The composition of the Fe-C phase was determined using ICP-AES.

One gram of the recovered rare earth-enriched phase (RE _x O _y -CaO-based slag) was acid-leached with 20 ml of 6 mol/L hydrochloric acid, followed by filtration to obtain a filtrate. 20 ml of 1 mol/L oxalic acid solution was added to the filtrate, and the pH was adjusted to 1.9 by adding aqueous ammonia. The pH-adjusted solution was stirred and held at 40°C for 1 to 2 hours to obtain a precipitate of rare earth oxalates. The rare earth oxalates were separated by filtration and calcined in a muffle furnace at 900°C for 60 minutes to obtain a powder containing rare earth oxides. The analysis results of the powder are shown in Table 18 below. The amount of rare earth oxides in the powder was 99.1% by mass relative to the total mass of the powder. The composition of the powder was determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy).

As can be seen from the Reference Examples, Comparative Reference Examples, and Examples shown above, the recovery method according to this embodiment makes it possible to recover rare earth oxides using a boron-free flux.

This application is based on Japanese Patent Application No. 2024-014088, filed February 1, 2024, the disclosure of which is incorporated herein by reference in its entirety.

Claims (19)

A method for recovering rare earth oxides from waste containing rare earth element-containing materials, comprising:
a melt preparation step (1) of heating and melting the waste and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides to prepare a melt containing at least a rare earth oxide, the oxide, and an oxide of an easily oxidizable metal; and a separation step (2) of separating from the melt a rare earth-enriched phase in which rare earth elements are concentrated in the oxide, and an Fe—C phase.
A method for recovering rare earth oxides, comprising: The method for recovering rare earth oxides described in claim 1, wherein the content of the oxide is an amount equal to or greater than the liquidus line in a phase diagram determined by the mass of rare earth oxide in the rare earth element-containing material and the content of the easily oxidizable metal oxide. The waste further comprises copper;
2. The method for recovering rare earth oxides according to claim 1, wherein in the separation step (2), a rare earth-enriched phase in which rare earth elements are concentrated in the oxides, an Fe—C phase, and a Cu phase are separated from the melt. The method for recovering rare earth oxides according to claim 1, wherein the oxide of the easily oxidizable metal is aluminum oxide and/or silicon dioxide. The method for recovering rare earth oxides described in claim 4, wherein the oxides of the easily oxidizable metals are aluminum oxide and silicon dioxide. The method for recovering rare earth oxides described in claim 5, wherein the oxide is calcium oxide (CaO). The method for recovering rare earth oxides described in claim 1, wherein the rare earth element-containing material includes a neodymium magnet. The method for recovering rare earth oxides described in claim 1, wherein the melt preparation step (1) does not include adding any boron-containing substance other than the waste material. The method for recovering rare earth oxides described in claim 1, wherein the boron content in the melt is 4.3 mass% or less, based on the total mass of rare earth elements. The method for recovering rare earth oxides described in claim 6, wherein the ratio of the aluminum oxide content to the sum of the rare earth oxide mass, the easily oxidizable metal oxide content, and the oxide content of the rare earth element-containing material is 15.7 to 17.5 mass%, the silicon dioxide content is 50.0 to 55.9 mass%, and the oxide content is 5.0 to 15.0 mass%. The method for recovering rare earth oxides described in claim 6, wherein the ratio of the aluminum oxide content to the sum of the rare earth oxide mass, the easily oxidizable metal oxide content, and the oxide content of the rare earth element-containing material is 27.5 to 29.3 mass%, the silicon dioxide content is 32.1 to 34.2 mass%, and the oxide content is 20.0 to 25.0 mass%. The method for recovering rare earth oxides described in claim 6, wherein the ratio of the aluminum oxide content to the sum of the mass of rare earth oxides, the content of the easily oxidizable metal oxides, and the content of the oxides in the rare earth element-containing material is 10.9 to 11.4 mass%, the silicon dioxide content is 46.4 to 48.5 mass%, and the oxide content is 14.0 to 17.7 mass%. The method for recovering rare earth oxides described in claim 6, wherein the ratio of the aluminum oxide content to the sum of the mass of rare earth oxides, the content of the easily oxidizable metal oxides, and the content of the oxides in the rare earth element-containing material is 11.6 to 12.9 mass%, the silicon dioxide content is 49.5 to 54.9 mass%, and the oxide content is 10.0 to 18.9 mass%. The method for recovering rare earth oxides described in claim 6, wherein the ratio of the aluminum oxide content to the sum of the mass of rare earth oxides, the content of the easily oxidizable metal oxides, and the content of the oxides in the rare earth element-containing material is 17.0 to 22.5 mass%, the silicon dioxide content is 35.9 to 47.6 mass%, and the oxide content is 10.0 to 32.1 mass%. The method for recovering rare earth oxides according to claim 1, wherein the melt preparation step (1) comprises the following steps (1a) to (1c) in order:
Step (1a): adding a melting point depressant to the waste containing a rare earth element-containing material, aluminum, and silicon, and then heating and melting the waste to obtain a melt (1a);
Step (1b): contacting the melt (1a) with an oxidizing agent to obtain a melt (1b);
Step (1c): The oxide is added to the melt (1b) to obtain a melt (1c). the melting point depressant lowers the melting point of iron and contains carbon;
16. The method for recovering rare earth oxides according to claim 15, wherein the oxidizing agent oxidizes rare earth elements and includes at least one selected from air, oxygen, carbon dioxide, iron oxide, and composite oxides containing iron oxide. The method for recovering rare earth oxides according to claim 1, comprising the following steps (3a) to (3c) in order after the separation step (2):
Step (3a): The rare earth-enriched phase obtained in step (2) is leached with an acid to obtain a rare earth element leachate.
Step (3b): Precipitating the rare earth elements in the rare earth leach solution as salts to obtain a precipitate.
Step (3c): The precipitate is heated to recover the rare earth elements as oxides. A rare earth-enriched material comprising a rare earth oxide, an oxide of an easily oxidizable metal, and at least one oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides. The rare earth-enriched material according to claim 18, wherein the boron content is 4.3 mass% or less, based on the total mass of the rare earth elements.