Method for recovering rare earth elements

Abstract

The purpose of the present invention is to provide a novel method for recovering rare earth elements, with which it is possible to separate scandium (Sc) from the other rare earth elements contained in steel slag and recover those rare earth elements.　Disclosed is a method for recovering rare earth elements contained in steel slag, the method including: a leaching step in which steel slag is brought into contact with an acid so as to obtain a leachate 1; a solvent extraction step 1 in which the pH of the leachate 1 is adjusted and an organic solvent is added thereto so that the leachate 1 is separated into an organic phase 1 and an aqueous phase 1 and extracted; a back extraction step 1 in which an aqueous phase that is back-extracted from the organic phase 1 is separated and extracted as a recovery liquid 1; a precipitation step in which the pH of the aqueous phase 1 is adjusted so as to precipitate a precipitate in the aqueous phase 1; a solid-liquid separation step in which the precipitate is solid-liquid separated from the aqueous phase 1 so as to obtain an aqueous phase 1'; a solvent extraction step 2 in which the pH of the aqueous phase 1' is adjusted and an organic solvent is added thereto so that the aqueous phase 1' is separated into an organic phase 2 and an aqueous phase 2 and extracted; and a back extraction step 2 in which an aqueous phase that is back-extracted from the organic phase 2 is separated and extracted as a recovery liquid 2.

Description

Method for recovering rare earth elements

[0001] The present invention relates to a method for recovering rare earth elements, and more particularly to a method for recovering rare earth elements capable of separating Sc (scandium), which is a rare earth element contained in steel slag, from other rare earth elements and recovering these rare earth elements.

[0002] Rare earth elements are also called REE (Rare Earth Element), and are a general term for a total of 17 elements including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Rare earth elements are used as additives for various materials such as hydrogen storage alloys, secondary battery raw materials, optical glass, rare earth magnets, phosphors, abrasives, and aluminum, and are a group of elements with high industrial value. However, since the ore deposits of rare earth elements are unevenly distributed and have a high scarcity value, the supply volume is globally small, and there is also a problem of drastic price fluctuations due to changes in the social situation. Therefore, establishing a method for stably and abundantly supplying rare earth elements is important for the development of the industry.

[0003] By the way, the production volume of steel accounts for the majority of the total metal production, and the production volume of steel slag produced as a by-product is also very large. For example, in Japan, about 100 million tons of crude steel are produced annually, and as by-products, blast furnace slag is produced about 23 million tons annually, steelmaking slag is produced about million tons annually, and electric furnace slag is produced about 3 million tons annually. The iron ore, coal, limestone, and iron scrap that are the raw materials for these steel slags contain rare earth elements, and the steel slag also contains rare earth elements through the ironmaking process.

[0004] There are various prior arts related to REE recovery.

[0005] Patent Document 1 relates to a method for recovering rare earth elements from steel slag, and discloses that in the leaching process, the pH in the second leaching step is made lower than the pH in the first leaching step. Thereby, while recovering rare earth elements from steel slag, rough separation of rare earth elements is also performed.

[0006] Non-patent document 1 studies the separation of Sc from other rare earth elements and discloses that scandium and other rare earth elements can be separated by using a carboxyl-functionalized ionic liquid as a solvent extractant and by controlling the pH of the aqueous phase.

[0007] Furthermore, Patent Document 2 proposes a method and system for regenerating useful products from steel slag. It proposes a process for recovering various valuable materials, including not only rare earth elements but also water glass, calcium carbonate, and other rare earth elements.

[0008] International Publication No. 2024 / 053596, Japanese Patent Publication No. 2018-530673

[0009] Yuehua Chen, Huiyong Wang, Yuanchao Pei, Jianji Wang, Separation and Purification Technology, 178 (2017), 261-268

[0010] As mentioned above, tens of millions of tons of steel slag are produced annually in Japan as a by-product, and this steel slag contains rare earth elements, albeit at low concentrations. It would be desirable to separate and recover these rare earth elements from such a large amount of by-product steel slag.

[0011] While steel slag contains trace amounts of rare earth elements, its main components are silicon (Si), iron (Fe), aluminum (Al), magnesium (Mg), calcium (Ca), etc. A process is needed to separate these main components from the rare earth elements and recover the rare earth elements.

[0012] In addition, rare earth elements consist of 17 different elements, and their separation is required depending on the final application. While separation and purification are possible by utilizing the differences in the chemical properties of each element, streamlining this separation and purification process is extremely important from the perspective of industrial production management, including cost reduction.

[0013] In particular, steel slag contains scandium (Sc), which has high industrial value, and it is desirable to efficiently separate and recover it.

[0014] Patent Document 1 relates to a method for recovering rare earth elements from steel slag, and while recovering rare earth elements from steel slag, it also performs rough separation of the rare earth elements. However, the separation efficiency of the rare earth elements, for example, the separation efficiency of Sc (Sc concentration in the final extract / total rare earth element concentration) is 0.58 to 0.79.

[0015] Non-patent document 1 studies the separation of Sc from other rare earth elements, but the experimental subjects were solutions prepared using reagent-grade rare earth oxides and nitric acid, and its applicability to steel slag containing large amounts of Si, Al, etc. is unknown. In fact, when the inventors attempted to separate Sc from other rare earth elements from steel slag using the method described in non-patent document 1, precipitates derived from Si and Al, which are components of steel slag, were generated. These precipitates adsorbed the rare earth elements in the leachate, hindering the migration of rare earth elements from the leachate to the organic phase, and it was found that the recovery rate of rare earth elements decreased.

[0016] Furthermore, while Patent Document 2 aims to separate not only rare earth elements but also metal salts other than rare earth elements from steel slag, the specific process conditions are not optimal for that purpose. Specifically, in the leaching process from steel slag, "saline solution, chelating agent, acid (0.1 mol / L HCl or HNO)" is used. ３ Although only ) is used, steel slag is basic, and the degree of its basicity changes, so with a fixed acid concentration (0.1 mol / L), there is a problem that the leaching efficiency changes greatly depending on the ratio of steel slag to acid.

[0017] As described above, there is room for improvement in the industrial method of recovering rare earth elements from steel slag, and a more appropriate method is needed. The present invention aims to provide a novel method for recovering rare earth elements that can separate Sc (scandium) and other rare earth elements contained in steel slag while recovering those rare earth elements.

[0018] The inventors of this invention conducted extensive research to solve the above-mentioned problems. Through this research, they obtained the following findings.

[0019] When rare earth elements in steel slag were leached (dissolved) with acid to obtain a leachate, and then a basic solution was added to the leachate to adjust the pH to weakly acidic for operations such as solvent extraction, a gel-like precipitate (hereinafter referred to as precipitate) was formed due to Si and Al in the leachate. This is a phenomenon unique to the use of steel slag as a raw material. The reason for this phenomenon is thought to be that when steel slag is leached with acid, it is unavoidable that not only rare earth elements but also Si and Al will leach (dissolve).

[0020] This precipitate has the property of adsorbing REE, and furthermore, the adsorption behavior of rare earth elements differed depending on the pH of the leachate. It is generally known that there are differences in the solubility of elements, and that this also depends on pH. However, SiO ２ Ya Al ２ O ３ It had not been revealed that controlling the pH of a solution obtained by acid leaching of steel slag, which is mainly composed of [unclear], can result in differences in the adsorption rate of rare earth elements onto precipitates.

[0021] The inventors have found that the pH suitable for solvent extraction of Sc, the pH at which the precipitate precipitates, and the pH at which the precipitate adsorbs rare earth elements other than Sc and inhibits solvent extraction (in other words, the pH unsuitable for solvent extraction of rare earth elements other than Sc) are slightly different.

[0022] Using this, we conceived the idea of ​​separating rare earth elements other than Sc into the organic phase by leaching rare earth elements in steel slag with acid, performing solvent extraction at a first pH suitable for solvent extraction of Sc, setting the pH of the aqueous phase (from which Sc has been separated) to a second pH where precipitates precipitate, generating precipitates and separating them by filtration, and then setting the pH of the aqueous phase from which the precipitates have been removed by the filtration operation to a third pH and performing solvent extraction, thereby separating rare earth elements other than Sc into the organic phase, and thus completed the present invention.

[0023] In other words, the present invention includes the following embodiments: [1] A method for recovering rare earth elements contained in steel slag, comprising: a leaching step of contacting the steel slag with an acid to dissolve it and obtain a leaching solution 1 containing the rare earth elements; a solvent extraction step 1 of adjusting the pH of the leaching solution 1 to a first pH, adding an organic solvent to separate and extract a first organic phase (organic phase 1) and a first aqueous phase (aqueous phase 1); a back-extraction step 1 of adding water to the first organic phase (organic phase 1), separating and extracting the back-extracted aqueous phase as a recovery solution 1; a precipitation step of adjusting the pH of the first aqueous phase (aqueous phase 1) to a second pH higher than the first pH, and precipitating precipitates containing Si and Al in the first aqueous phase (aqueous phase 1); and a solid-liquid separation step of separating the precipitates from the first aqueous phase (aqueous phase 1) using solid-liquid separation to obtain an aqueous phase (aqueous phase 1') from which the precipitates have been removed. A method for recovering rare earth elements, comprising: a solvent extraction step 2, which involves adjusting the pH of the aqueous phase (aqueous phase 1') from which the precipitate has been removed to a third pH higher than the second pH, adding an organic solvent to separate and extract a second organic phase (organic phase 2) and a second aqueous phase (aqueous phase 2); and a back extraction step 2, which involves adding water to the second organic phase (organic phase 2), back-extracting the aqueous phase as a recovered liquid 2, wherein the composition ratio of the rare earth elements contained in the recovered liquid 1 and the recovered liquid 2 are different. [2] The method for recovering rare earth elements according to [1], characterized in that the first pH is pH 3.5 or higher and 4.0 or lower. [3] The method for recovering rare earth elements according to [1] or [2], characterized in that the second pH is pH greater than 3.5 and 4.7 or lower. [4] The method for recovering rare earth elements according to any one of [1] to [3], characterized in that the third pH is pH 6.6 or higher. [5] The recovery method according to any one of [1] to [4], characterized in that the extractant used in the solvent extraction steps 1 and 2 is neodecanoic acid or a carboxylic acid-based extractant.

[0024] The present invention provides a method for recovering rare earth elements in steel slag, which allows for the separation of rare earth elements from Sc and other elements, and the recovery of each, through simple processes such as pH adjustment and filtration of the solution. Furthermore, compared to prior art, the separation accuracy of rare earth elements Sc and other rare earth elements can be improved, or the recovery rate of those rare earth elements can be improved.

[0025] Figure 1 is a flowchart illustrating an example of a method for recovering rare earth elements. Figure 2 is a conceptual diagram illustrating an example of a rare earth element recovery flow. Figure 3 is a diagram schematically illustrating the behavior of rare earth elements other than Sc in solvent extraction step 2, depending on the presence or absence of a solid-liquid separation step. Figure 4 is a flowchart illustrating a rare earth element recovery method in an example described in Patent Document 1 (a comparative example to the example of the present invention).

[0026] The following describes specific embodiments of the present invention (hereinafter referred to as "these embodiments") in detail with reference to the drawings. However, the present invention is not limited in any way to the following embodiments, and can be implemented with appropriate modifications without altering the gist of the present invention.

[0027] Although there are multiple definitions of elements included in the term "rare earth elements," the definition of rare earth elements presented in this disclosure refers to a group of 17 elements that include lanthanoids (Ln), consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), plus yttrium (Y) and scandium (Sc).

[0028] The term "steel slag" as used in this disclosure is defined to include blast furnace slag produced in the ironmaking process, steelmaking slag produced in the steelmaking process, and electric arc furnace slag produced in electric arc furnaces.

[0029] The following description of this embodiment may also refer to Figures 1 and 2. Figure 1 is a flowchart illustrating an exemplary method for recovering rare earth elements. Figure 2 is a conceptual diagram illustrating a more schematic example of a rare earth element recovery flow. However, as stated above, the present invention is not limited to the drawings or embodiments.

[0030] The general method for recovering rare earth elements includes a leaching step, a solvent extraction step 1, a back extraction step 1, a precipitation step, a solid-liquid separation step, a solvent extraction step 2, and a back extraction step 2. In the leaching step, steel slag is contacted with acid and dissolved to obtain leaching solution 1 containing rare earth elements. In the solvent extraction step 1, the pH of leaching solution 1 is adjusted to a first pH, and an organic solvent is added to separate and extract a first organic phase (organic phase 1) and a first aqueous phase (aqueous phase 1). In the back extraction step 1, water is added to the first organic phase (organic phase 1), and the back-extracted aqueous phase is separated and extracted as recovery solution 1. In the precipitation step, the pH of the first aqueous phase (aqueous phase 1) is adjusted to a second pH higher than the first pH, and precipitates containing Si and Al are precipitated from the first aqueous phase (aqueous phase 1). In the solid-liquid separation step, the precipitate formed in the precipitation step is separated from the first aqueous phase (aqueous phase 1) by solid-liquid separation to obtain an aqueous phase (aqueous phase 1') from which the precipitate has been removed. In the solvent extraction step 2, the pH of the aqueous phase (aqueous phase 1') from which the precipitate has been removed is adjusted to a third pH higher than the second pH, and an organic solvent is added to separate and extract the second organic phase (organic phase 2) and the second aqueous phase (aqueous phase 2). In the back-extraction step 2, water is added to the second organic phase (organic phase 2), and the back-extracted aqueous phase is separated and extracted as recovery liquid 2. Note that the composition ratio of rare earth elements contained in recovery liquid 1 and recovery liquid 2 are different.

[0031] The gist of this invention is as follows. First, rare earth elements are leached from steel slag with acid. Si and Al, which are the main components of the slag, also dissolve in the leachate. The dissolution or precipitation behavior of each element changes with pH. The pH suitable for solvent extraction of Sc, the pH suitable for precipitation of the main components of the slag, and the pH suitable for solvent extraction of rare earth elements other than Sc are slightly different. Therefore, the pH of the leachate is set to a first pH suitable for solvent extraction of Sc, and solvent extraction is performed to separate Sc into the organic phase. The pH of the aqueous phase from which Sc has been removed is set to a second pH suitable for precipitation of precipitates such as Si and Al, and precipitates are generated and separated by a solid-liquid separation operation. The pH of the aqueous phase from which precipitates have been removed by the solid-liquid separation operation is set to a third pH suitable for solvent extraction of rare earth elements other than Sc, and a solvent extraction operation is performed to separate rare earth elements other than Sc into the organic phase. This allows for the separation of rare earth elements from steel slag into Sc and other elements, while also enabling the recovery of each element.

[0032] Each step is described below. <Leaching Step> In the leaching step, the steel slag is brought into contact with an acid to leach and dissolve the main components, such as Si and Al, and rare earth elements, from the steel slag into the acidic solvent. More specifically, the steel slag is brought into contact with an acid to leach it and dissolve the Si and Al, and rare earth elements contained in the steel slag into the acidic solvent. A leachate 1 is obtained in which the steel slag has been leached and Si and Al, and rare earth elements have been dissolved. The acid may be selected to leach and dissolve the steel slag. If at least a portion of the steel slag is leached, the effects of the present invention can be obtained with respect to the leached and dissolved leachate 1. The more of the steel slag that is leached by the acid, the less slag residue there is, and the less effort is required for the subsequent separation treatment of the slag residue, which is preferable. In this respect, the entire amount of steel slag may be leached and dissolved by the acid.

[0033] The amount of additional acid added can be controlled while monitoring the pH of the leachate 1 obtained by contacting steel slag with acid. From the viewpoint of sufficiently leaching or dissolving the steel slag, a low pH of leachate 1 is preferable, and may be 4.5 or lower. Examples of acids include inorganic acids and organic acids, with inorganic acids being preferred. Specific examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, or mixtures thereof.

[0034] The following is an example of a specific method for carrying out the leaching process. Steel slag is charged into the leaching reaction vessel, and acid is added. At this time, the solid-liquid ratio of steel slag to acid is preferably 1:1 to 1:100. The pH of the added acid may be 1.0 or less. Steel slag contains approximately 10 to 70% oxides and hydroxides of silicon (Si), aluminum (Al), calcium (Ca), magnesium (Mg), etc., so the amount of acid to be initially added and its pH should be determined taking into account that these will neutralize the slag. One method for monitoring the pH is to immerse a pH sensor in the leaching solution 1 obtained by contacting the steel slag with acid and monitor the pH of the leaching solution 1 as needed. It is also preferable to stir the solution as appropriate to ensure that the pH of the leaching solution 1 becomes uniform. Furthermore, adjusting the pH to a target value means that an acid or base is added continuously for at least one minute so that the pH does not deviate by ±0.5, preferably ±0.3, and more preferably ±0.1 from the target pH. In addition, while rare earth elements can be sufficiently leached even under conditions where the temperature of the leachate 1 is at room temperature (15 to 35°C) and the pressure is at atmospheric pressure (approximately 10¹³ hPa), the leaching of rare earth elements may be carried out while heating or pressurizing to increase the leaching rate.

[0035] If the pH of leachate 1 is higher than the target pH, the pH can be adjusted to the target pH by adding small amounts of acid while monitoring the pH value. On the other hand, if the pH of leachate 1 is lower than the target pH, the pH can be adjusted to the target pH by adding small amounts of steel slag while monitoring the pH value.

[0036] The target pH can be appropriately determined depending on the degree of leaching or dissolution of the steel slag. From the viewpoint of sufficiently leaching or dissolving the steel slag, a lower pH of leaching solution 1 is preferable, and may be 4.5 or lower. The lower limit of pH is not particularly limited and may be selected from -0.5, 0.0, 1.0, 1.5, 2.0, 2.5, 3.0, or 3.5.

[0037] Although not mandatory, the steel slag may be crushed to increase its surface area prior to the leaching process. Alternatively, the molten steel slag may be subjected to a water granulation treatment during cooling to produce fine-grained granulated slag with a large surface area. Increasing the surface area of ​​the steel slag can increase the leaching rate of rare earth elements in the steel slag.

[0038] <Solvent Extraction Step 1> In solvent extraction step 1, the pH of leachate 1 is adjusted to a first pH, and an organic solvent is added to separate and extract the first organic phase (organic phase 1) and the first aqueous phase (aqueous phase 1).

[0039] (pH adjustment step 1) In pH adjustment step 1, the pH of the leachate 1 is adjusted to a first pH. The first pH may be 3.5 or higher and 4.0 or lower, preferably 3.6 or higher and 3.9 or lower. Within this pH range, in the subsequent extraction step 1, mainly Sc among the rare earth elements is extracted, which is preferable. In solvent extraction step 1, the first pH may be adjusted so that precipitates do not form. Generally, when the pH is high, precipitates such as Si and Al are more likely to form, and these precipitates may adversely affect the subsequent extraction step 1 and aqueous phase / organic phase separation step 1. Typically, among the rare earth elements, Sc is easily adsorbed by the precipitates, which can reduce the extraction efficiency of Sc. To suppress the amount of precipitates, the upper limit of the first pH may be adjusted to a range of 4.0 or lower.

[0040] The pH adjustment can be carried out by controlling the addition amount of an acid or a base while monitoring the pH of the leaching solution 1. When the pH of the leaching solution 1 is higher than the target pH, while monitoring the pH value, a small amount of acid can be added to adjust it to the target pH. On the other hand, when the pH of the leaching solution 1 is lower than the target pH, while monitoring the pH value, a small amount of base can be added to adjust it to the target pH if necessary.

[0041] Examples of the acid include inorganic acids and organic acids, and inorganic acids are preferred. Specific examples of the inorganic acid include sulfuric acid, nitric acid, hydrochloric acid or a mixed acid thereof. The acid used in the leaching step may be used. Also, the pH at the end of the leaching step may be within the range of the first pH. The basic substance is not particularly limited, and examples thereof include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, etc. or a mixed base thereof.

[0042] As a method for monitoring the pH, a method of immersing a pH sensor in the leaching solution 1 and monitoring the pH of the leaching solution 1 at any time can be mentioned. Also, it is preferable to appropriately perform stirring or the like so that the pH of the leaching solution 1 becomes uniform. Note that adjusting the pH to a target constant value means that an acid or a base is added so as not to deviate from the target pH by more than ±0.5, preferably ±0.3, more preferably ±0.1 for 1 minute or more continuously. Also, the temperature and pressure of the leaching solution 1 are not particularly limited, and conditions of normal temperature of 15 to 35 °C and normal pressure of about 1013 hPa may be used.

[0043] (Extraction Step 1) In Extraction Step 1, an organic solvent is added to the leaching solution 1 adjusted to the first pH to obtain a first organic phase (organic phase 1) and a first aqueous phase (aqueous phase 1). In the previous pH adjustment step 1, it is adjusted to the first pH suitable for mainly extracting only Sc among the rare earth elements. Therefore, in the first organic phase (organic phase 1), mainly only Sc among the rare earth elements is distributed or extracted, and it becomes a phase rich in Sc. The first aqueous phase (aqueous phase 1) is a phase in which Sc is hardly distributed or extracted and Sc is relatively removed. Thereby, it is preferable that Sc and rare earth elements other than Sc among the rare earth elements can be separated.

[0044] In the extraction step 1, by mixing the organic phase 1 and the aqueous phase 1, the distribution or extraction of Sc into the organic phase 1 may be promoted. When mixing, a known solvent extraction apparatus such as a centrifugal extractor or a pulsed column may be used.

[0045] The organic solvent used in the extraction step 1 may contain a solvent extraction agent. As the solvent extraction agent, conventionally known ones can be used. For example, carboxylic acid-based extraction agents such as neodecanoic acid, organic phosphoric acid-based extraction agents such as di(2-ethylhexyl)phosphoric acid, tributyl phosphate, and trioctylphosphine oxide, and amine-based extraction agents such as triisooctylamine can be mentioned. The solvent extraction agent may be diluted with kerosene, xylene, toluene, etc., or can be used without dilution. The solvent extraction agent may be added to the aqueous phase.

[0046] (Aqueous phase - organic phase separation step 1) In the aqueous phase - organic phase separation step 1, the organic phase 1 and the aqueous phase 1 obtained in the extraction step 1 are separated. Using a separating funnel or the like, the organic phase 1 and the aqueous phase 1 can be separated. In the separated organic phase 1, mainly only Sc among the rare earth elements is distributed or extracted, and through the subsequent back-extraction step 1 and the like, Sc can be separated and recovered from other rare earth elements. In the separated aqueous phase 1, Sc is hardly distributed or extracted, and through the subsequent precipitation step and the like, rare earth elements other than Sc can be recovered.

[0047] <Reverse extraction step 1> In the reverse extraction step 1, water as a back-extraction agent is added to the organic phase 1 separated in the aqueous phase - organic phase separation step 1, and the back-extracted aqueous phase is separated and extracted as the recovery liquid 1. In the organic phase 1, mainly only Sc among the rare earth elements is distributed or extracted, and the Sc can be back-extracted from the organic phase 1 into the aqueous phase (recovery liquid 1). In the reverse extraction step 1, pH adjustment may be performed as necessary. For pH adjustment, an acid or a base may be added as exemplified in the pH adjustment step 1. The pH may be a pH suitable for the organic solvent used in the organic phase 1.

[0048] In the back-extraction step 1, the organic phase and the aqueous phase may be mixed to promote the distribution or extraction of Sc into the aqueous phase (recovery solution 1). When mixing the organic phase and the aqueous phase, known solvent extraction devices such as centrifugal extractors or pulsed columns may be used.

[0049] <Precipitation Process> In the precipitation process, the pH of the first aqueous phase (aqueous phase 1) is adjusted to a second pH higher than the first pH, causing precipitates containing Si and Al in the first aqueous phase (aqueous phase 1) to precipitate. The pH can be adjusted by adding a base to aqueous phase 1 obtained in solution extraction process 1, particularly aqueous phase / organic phase separation process 1. The pH rises due to the addition of a base. When the pH rises and is adjusted to the second pH, a gel-like precipitate (hereinafter referred to as precipitate) containing Si and Al, etc., that were dissolved in aqueous phase 1 is generated. The base is not particularly limited, but examples include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, etc., or mixed bases thereof.

[0050] The second pH may be greater than 3.5 and less than or equal to 4.7. Within this pH range, the precipitate is easily precipitated, which is preferable. Although various elements may be adsorbed onto the precipitate, within this pH range, rare earth elements other than Sc are less likely to be adsorbed onto the precipitate. Furthermore, since Sc is hardly distributed or extracted into the first aqueous phase, Sc is hardly adsorbed onto the precipitate. In other words, within this pH range, precipitates are formed, while rare earth elements are hardly adsorbed onto the precipitate, thus promoting the separation and recovery of rare earth elements.

[0051] Generally, precipitates mainly composed of Si and Al begin to form when the pH exceeds approximately 3.5. Furthermore, the amount of precipitate increases as the pH rises. Therefore, from the viewpoint of obtaining (precipitating) a sufficient amount of precipitate, the second pH may be set to above 3.5. Preferably, the lower limit of the pH here may be selected from 4.0 or 4.5.

[0052] Rare earth elements dissolved in liquid phase 1 may also be adsorbed onto this precipitate. Here, each of the 17 rare earth elements exhibits different adsorption behavior towards the precipitate. For example, at a certain pH, some rare earth elements may be adsorbed onto the precipitate, while others may not. Therefore, by adjusting the pH, it is possible to selectively adsorb or prevent the adsorption of rare earth elements onto the precipitate. Generally, as the pH increases, rare earth elements become more readily adsorbed onto the precipitate. Therefore, by increasing the pH, it is possible to adsorb almost all of the rare earth elements onto the precipitate. Alternatively, by limiting the upper limit of the pH, the adsorption of rare earth elements can be suppressed. From the viewpoint of preventing rare earth elements other than Sc from being adsorbed onto the precipitate, the second pH may be set to 4.7 or less. Preferably, the upper limit of the pH here may be selected from 4.6 or 4.5.

[0053] The following is an example of a specific method for carrying out the precipitation process. The aqueous phase 1 obtained in the solution extraction process 1, particularly the aqueous phase / organic phase separation process 1, is charged into the precipitation reaction vessel, and a base is added. At this time, the liquid-to-liquid ratio of aqueous phase 1 to the base is preferably 1:1 to 100:1. The pH of the added base may be 10.0 or higher. Aqueous phase 1 contains an acid used to leach and dissolve the steel slag, or to extract mainly Sc from the rare earth elements, and the type and amount of base to be added initially should be determined with the expectation that it will be neutralized by this acid. As a method for monitoring the pH, one method is to immerse a pH sensor in the aqueous phase 1 to which the base has been added and monitor the pH as it occurs. It is also preferable to stir as appropriate to ensure that the pH becomes uniform. Note that adjusting the pH to a target constant value means that the acid or base is added so that it does not deviate from the target pH by ±0.5, preferably ±0.3, and more preferably ±0.1 or more for 1 minute or more continuously from the target pH. Furthermore, even under conditions where the temperature of aqueous phase 1 is room temperature (15-35°C) and the pressure is atmospheric pressure (approximately 1013 hPa), precipitates can be generated by adjusting the second pH. However, the pH may also be adjusted while heating or pressurizing to increase the precipitation rate.

[0054] If the pH of aqueous phase 1 is lower than the target final pH, the pH can be adjusted to the target pH by adding a small amount of base while monitoring the pH value. The base to be added is not particularly limited, but examples include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, or mixtures thereof.

[0055] <Solid-Liquid Separation Process> In the solid-liquid separation process, the precipitate precipitated in the precipitation process is separated from the first aqueous phase (aqueous phase 1) by solid-liquid separation to obtain an aqueous phase (aqueous phase 1') from which the precipitate has been removed. The solid-liquid separation process is a process of separating the precipitate, which is the solid component, from the aqueous phase 1 obtained in the precipitation process. In the solid-liquid separation process, the precipitate that appeared in the precipitation process can be physically separated from the liquid portion of aqueous phase 1. As for the separation method, known solid-liquid separation devices such as continuous thickeners, deep cone thickeners, lamellar thickeners, drum filters, disc filters, horizontal belt filters, filter presses, pressure filters, and centrifugal separators may be used.

[0056] The aqueous phase from which precipitates have been removed by the solid-liquid separation process is called aqueous phase 1' and is distinguished from aqueous phase 1 obtained in the aforementioned solution extraction process 1, particularly the aqueous phase-organic phase separation process 1. Generally, aqueous phase 1 contains almost no Sc among the rare earth elements, either distributed or extracted. Furthermore, aqueous phase 1' has had precipitates removed, and these precipitates can be mainly composed of Si and Al, and hardly adsorb any rare earth elements. Therefore, aqueous phase 1' can contain rare earth elements other than Sc while being separated from Sc, Si, and Al. In other words, it can promote the separation and recovery of rare earth elements other than Sc.

[0057] <Solvent Extraction Step 2> In Solvent Extraction Step 2, the pH of aqueous phase 1' from which precipitates have been removed is adjusted to a third pH, and an organic solvent is added to separate and extract the second organic phase (organic phase 2) and the second aqueous phase (aqueous phase 2).

[0058] (pH adjustment step 2) In pH adjustment step 2, the pH of aqueous phase 1' is adjusted to a third pH. The third pH may be 6.6 or higher. This pH range is suitable for the extraction of rare earth elements other than Sc in the subsequent extraction step 2.

[0059] pH adjustment can be performed by controlling the amount of acid or base added while monitoring the pH of aqueous phase 1'. If the pH of aqueous phase 1' is higher than the target pH, the acid can be added in small amounts while monitoring the pH value to adjust it to the target pH. On the other hand, if the pH of leachate 1 is lower than the target pH, the base can be added in small amounts while monitoring the pH value to adjust it to the target pH.

[0060] Examples of acids include inorganic acids and organic acids, with inorganic acids being preferred. Specific examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, or mixtures thereof. The acid used in the leaching process may also be used. Basic substances are not particularly limited, but examples include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, or mixtures thereof.

[0061] One method for monitoring pH is to immerse a pH sensor in the aqueous phase 1' and monitor the pH of the leachate 1 as needed. It is also preferable to stir the mixture as appropriate to ensure that the pH of the aqueous phase 1' becomes uniform. Adjusting the pH to a target value means that an acid or base is added so that the pH does not deviate by more than ±0.5, preferably ±0.3, and more preferably ±0.1 from the target pH for at least one minute continuously. The temperature and pressure of the aqueous phase 1' are not particularly limited and may be at room temperature of 15 to 35°C and atmospheric pressure of about 1013 hPa.

[0062] (Extraction Step 2) In Extraction Step 2, an organic solvent is added to the aqueous phase 1', which has been adjusted to a third pH, to obtain a second organic phase (organic phase 2) and a second aqueous phase (aqueous phase 2). In the preceding pH adjustment step 2, the pH has been adjusted to a second pH suitable for the extraction of rare earth elements other than Sc. Therefore, the second organic phase (organic phase 2) is a phase in which rare earth elements other than Sc are distributed or extracted, and is relatively rich in rare earth elements other than Sc. In the second aqueous phase (aqueous phase 2), rare earth elements other than Sc are hardly distributed or extracted, and is a phase in which rare earth elements other than Sc are relatively removed. This is preferable because it allows for the separation of rare earth elements other than Sc. In addition, the second aqueous phase (aqueous phase 2) may mainly contain Ca, Mg, etc., which were not removed in the above steps. Therefore, the second organic phase (organic phase 2) is preferable because Ca, Mg, etc. are hardly distributed or extracted, and these (Ca, Mg, etc.) can be separated from rare earth elements other than Sc.

[0063] In extraction step 2, the organic phase 2 and the aqueous phase 2 may be mixed to promote the distribution or extraction of rare earth elements other than Sc into the organic phase 2. When mixing, known solvent extraction devices such as centrifugal extractors or pulsed columns may be used.

[0064] The organic solvent used in extraction step 2 may include a solvent extractant. Conventional known solvent extractants can be used, such as carboxylic acid-based extractants like neodecanoic acid, organophosphate-based extractants like di(2-ethylhexyl)phosphate, tributyl phosphate, and trioctylphosphine oxide, and amine-based extractants like triisooctylamine. The solvent extractant may be diluted with kerosene, xylene, or toluene, or it may be used undiluted. The solvent extractant may also be added to the aqueous phase. Furthermore, the organic solvent used in extraction step 2 may be the same as the organic solvent used in extraction step 1, or it may be different.

[0065] (Aqueous Phase / Organic Phase Separation Step 2) In aqueous phase / organic phase separation step 2, the organic phase 2 obtained in extraction step 2 is separated from the aqueous phase 2. The organic phase 2 and aqueous phase 2 can be separated using a separatory funnel or the like. The separated organic phase 2 has rare earth elements other than Sc distributed or extracted, and can be recovered by separating the rare earth elements other than Sc from other elements through a subsequent back-extraction step 2, etc. The separated aqueous phase 2 has almost no rare earth elements other than Sc distributed or extracted, and may mainly contain Ca, Mg, etc., that were not removed in the above steps.

[0066] Figure 3 schematically illustrates the difference in the behavior of rare earth elements other than Sc in solvent extraction step 2, depending on whether the precipitate formed in the preceding precipitation step is separated in a solid-liquid separation step or not. Since the precipitate has the property of adsorbing rare earth elements when the pH of the aqueous phase increases, if the pH of the aqueous phase is increased for solvent extraction without performing a solid-liquid separation step (i.e., with the precipitate remaining), the rare earth elements in the aqueous phase are adsorbed onto the precipitate. As a result, when solvent extraction is performed, a certain amount of rare earth elements remain adsorbed onto the precipitate, reducing the recovery rate. In contrast, in this embodiment, the precipitate formed in the preceding precipitation step is separated in a solid-liquid separation step, and when the pH of aqueous phase 1' is increased for the solvent extraction step, the rare earth elements in the aqueous phase are not adsorbed onto the precipitate. As a result, when extraction step 2 is performed, the recovery rate of rare earth elements other than Sc can be increased.

[0067] <Back Extraction Step 2> In back extraction step 2, water is added as a back extractant to the organic phase 2 separated in aqueous phase / organic phase separation step 2, and the back-extracted aqueous phase is separated and extracted as recovery liquid 2. In organic phase 2, rare earth elements other than Sc are distributed or extracted, while Ca, Mg, etc. are hardly distributed or extracted, and the rare earth elements other than Sc can be back-extracted from organic phase 2 to the aqueous phase (recovery liquid 2). In back extraction step 2, pH adjustment may be performed as needed. pH adjustment may be performed by adding an acid or base, as exemplified in pH adjustment step 2. The pH may be set to a pH suitable for the organic solvent used in organic phase 2. For example, when using neodecanoic acid or a carboxylic acid-based extractant, a pH of 6.6 or higher is suitable, preferably a pH of 6.9 or higher, and more preferably a pH of 7.8 or higher.

[0068] In the back-extraction step 2, the organic phase and the aqueous phase may be mixed to promote the distribution or extraction of rare earth elements other than Sc into the aqueous phase (recovery solution 2). When mixing the organic phase and the aqueous phase, known solvent extraction devices such as centrifugal extractors or pulsed columns may be used.

[0069] The composition ratio of the rare earth elements contained in the recovered liquid 1 and recovered liquid 2 obtained through the above process is different. In other words, recovered liquid 1 obtained through the above process has Sc among the rare earth elements separated and recovered, or recovered liquid 2 has rare earth elements other than Sc separated and recovered. Therefore, the purity of the rare earth elements can be further increased by subjecting recovered liquid 1 or recovered liquid 2 to an optional purification process. Examples of optional purification processes include a precipitation process, a secondary solid-liquid separation process, and a roasting process. Examples of each process are described below.

[0070] <Precipitation Process> The precipitation process involves adding a precipitating agent to a liquid containing rare earth elements, typically recovery solution 1 or recovery solution 2, to obtain a precipitate containing rare earth elements (hereinafter sometimes referred to as rare earth element precipitate). Examples of precipitating agents include bases and acids. Examples of bases include metal-containing basic salts such as sodium hydroxide and potassium hydroxide, and organic bases such as tetramethylammonium hydroxide (TMAOH), or mixed bases thereof, but organic bases are preferred from the viewpoint of obtaining rare earth elements of higher purity. Examples of acids include tartaric acid, carbonic acid, oxalic acid, or mixed acids thereof, with oxalic acid being preferred.

[0071] In the aforementioned precipitation process, using oxalic acid as the precipitant has the advantage of efficiently separating the rare earth elements from impurities such as aluminum and iron that may be present in the liquid containing the rare earth elements (recovered liquid 1 or recovered liquid 2), but it has the disadvantage of having low separation efficiency from magnesium and calcium. On the other hand, using a base as the precipitant has the advantage of efficiently separating the rare earth elements from impurities such as magnesium and calcium present in the liquid containing the rare earth elements (recovered liquid 1 or recovered liquid 2), but it has the disadvantage of having low separation efficiency from iron and aluminum. The impurities contained in the liquid containing the rare earth elements (recovered liquid 1 or recovered liquid 2) vary depending on the composition of the steel slag, and the problematic impurity elements differ depending on the intended use of the rare earth elements, so the precipitant should be selected according to the purpose.

[0072] <Secondary Solid-Liquid Separation Process> Although not shown in Figure 1, the secondary solid-liquid separation process allows for the physical separation of the rare earth element precipitate and the post-precipitation solution obtained in the precipitation process. Known solid-liquid separation devices such as continuous thickeners, deep cone thickeners, lamellar thickeners, drum filters, disc filters, horizontal belt filters, filter presses, pressure filters, and centrifugal separators can be used for the separation. This secondary solid-liquid separation process is referred to as such to distinguish it from the solid-liquid separation process that separates precipitates from the first aqueous phase (aqueous phase 1) described above.

[0073] <Roasting Process> The roasting process is a process of roasting the rare earth element precipitate separated in the secondary solid-liquid separation process to obtain rare earth element oxides. The roasting process may include a washing process and a heating process. In the washing process, for example, the rare earth element precipitate separated in the secondary solid-liquid separation process may be washed with water to remove impurities. In the heating process, the rare earth element precipitate that has undergone the washing process may be heated to remove water, and volatile elements such as carbon, phosphorus, and nitrogen may be vaporized and removed, and reacted with oxygen to obtain rare earth element oxides.

[0074] In the roasting process described above, the roasting conditions are not limited, but for example, heating in a tubular furnace at approximately 900°C for about two hours is sufficient. Alternatively, by using a continuous furnace such as a rotary kiln, drying and roasting can be performed in the same apparatus, enabling the industrially efficient production of rare earth element oxides.

[0075] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these descriptions.

[0076] Table 1 shows the elemental composition of the steel slag (blast furnace slag) used in the experiment. Note that ΣREE-Sc refers to rare earth elements other than Sc.

[0077] As part of the leaching process, blast furnace granulated slag, adjusted to a solid-liquid weight ratio of 1:100 and with a particle size of 250 μm or less, was brought into contact with acid (1 mol / L hydrochloric acid) to obtain a leached liquid containing rare earth elements.

[0078] As solvent extraction step 1, ammonia water was added to the leachate to adjust its pH to 1.9 to 8.2, as shown in Table 2. (pH adjustment step 1) Subsequently, an organic solvent, a mixture of neodecanoic acid and kerosene in a volume ratio of 1:1, was added to the leachate in a volume ratio of 5:1, and Sc from the leachate was extracted into the organic phase. (Extraction step 1) After that, the mixture was separated into aqueous phase 1 and organic phase 1. (Aqueous phase / organic phase separation step 1)

[0079] In the back-extraction step 1, 6 mol / L hydrochloric acid was added to the separated organic phase 1 in a volume ratio of organic phase:6 mol / L hydrochloric acid = 5:1, and Sc from the organic phase 1 was back-extracted into the aqueous phase containing the hydrochloric acid. The aqueous phase obtained by this back-extraction is the recovered liquid 1, from which Sc was recovered from the steel slag.

[0080] Next, as a precipitation step, the aqueous phase 1 separated in solvent extraction step 1 (aqueous phase / organic phase separation step) was adjusted to a second pH by adding hydrochloric acid if the pH of aqueous phase 1 was higher than pH 4.5, or by adding ammonia water if it was lower, thereby adjusting the pH to 4.5 and allowing the precipitate to precipitate in aqueous phase 1.

[0081] As a solid-liquid separation step, the precipitate obtained in the precipitation step was separated from the aqueous phase 1, and an aqueous phase 1' was obtained from which the precipitate had been removed from the aqueous phase 1.

[0082] Subsequently, as solvent extraction step 2, aqueous ammonia was added to the aqueous phase 1' to adjust the pH to 4.6 to 7.8, the third pH range shown in Table 2. (pH adjustment step 2) Next, an organic solvent, a mixture of neodecanoic acid and kerosene in a volume ratio of 1:1, was added to the aqueous phase 1' in a volume ratio of aqueous phase 1':organic solvent = 5:1, and rare earth elements other than Sc in the aqueous phase 1' were extracted into the organic phase. (Extraction step 2) After that, the aqueous phase 2 and organic phase 2 were separated. (Aqueous phase / organic phase separation step 2)

[0083] In the back-extraction step 2, 6 mol / L hydrochloric acid was added to the separated organic phase 2 in a volume ratio of organic phase:6 mol / L hydrochloric acid = 5:1, and the rare earth elements other than Sc in the organic phase 2 were back-extracted into the aqueous phase containing the hydrochloric acid. The aqueous phase obtained by this back-extraction is the recovered liquid 2, from which the rare earth elements other than Sc in the steel slag were recovered.

[0084] Table 2 shows the recovery rates of Sc and other rare earth elements contained in each recovered liquid obtained above. The concentrations of each element were determined by elemental analysis using an inductively coupled plasma mass spectrometer.

[0085]

[0086] As a comparative example, a recovery liquid containing rare earth elements other than Sc was obtained from blast furnace slag using the method described in the example of Patent Document 1. Figure 4 shows the rare earth element recovery flow described in the example of Patent Document 1, which was carried out as a comparative example.

[0087] In the leaching process 1, blast furnace slag with the composition shown in Table 1 was brought into contact with granulated blast furnace slag and acid (1 mol / L nitric acid) with a solid-liquid weight ratio of 1:100 to leach rare earth elements from the blast furnace slag. During leaching, the pH of the leachate was monitored to maintain a range of 3.4 to 6.0. If the pH was higher than the target value, nitric acid was added; if the pH was lower than the target value, ammonia water was added to maintain the target pH. This produced leachate A, which mainly contained rare earth elements other than Sc.

[0088] Furthermore, a filtration separation process was performed to recover a solid phase mainly containing Sc.

[0089] As solvent extraction step 1, the pH of leachate A was adjusted to 7.8. (pH adjustment step 1) Subsequently, an organic solvent mixture of neodecanoic acid and kerosene in a volume ratio of 1:1 was added to the leachate to extract rare earth elements other than Sc from leachate A into the organic phase. (Extraction step 1) After that, the mixture was separated into aqueous phase 1 and organic phase 1. (Aqueous phase / organic phase separation step 1)

[0090] In the back-extraction step 1, 6 mol / L hydrochloric acid was added to the separated organic phase 1, and rare earth elements other than Sc in the organic phase 1 were back-extracted into the aqueous phase containing the hydrochloric acid. The aqueous phase obtained by this back-extraction is the recovered liquid 1 (comparative example) from which rare earth elements other than Sc were recovered from the steel slag.

[0091] Next, in leaching step 2, the solid phase recovered in the filtration separation step following leaching step 1 was brought into contact with nitric acid at a pH of -0.5 to leach Sc from the solid phase. Similar to leaching step 1, the pH of the leachate was monitored during leaching, and if the pH was higher than the target value, an acid was added, and if the pH was lower than the target value, a base was added to maintain the target pH. In leaching step 2, a liquid phase mainly composed of leached Sc was produced.

[0092] Furthermore, a filtration separation process was performed to separate the solid phase and obtain leachate B.

[0093] Subsequently, in solvent extraction step 2, the nitric acid concentration of leachate B was adjusted to 1 mol / L. (pH adjustment step 2) Next, an organic solvent containing 10% tetrabutylmethylenediphosate dissolved in toluene was added to leachate B, and the leachate B was extracted into the organic phase. (Extraction step 2) Then, it was separated into aqueous phase 2 and organic phase 2. As a result, the Sc contained in leachate B was extracted and separated into organic phase 2. (Aqueous phase / organic phase separation step 2)

[0094] In the back-extraction step 2, 10 mol / L nitric acid was added to the separated organic phase 2, and Sc was back-extracted from the organic phase 2 into the aqueous phase containing the nitric acid. The solvent from which the back-extraction was performed was separated into the aqueous phase and the organic phase. The aqueous phase obtained by this back-extraction is the recovered liquid 2 (comparative example) from which Sc was recovered from the steel slag.

[0095] Table 3 shows the recovery rates of Sc and other rare earth elements contained in each recovered liquid obtained above. The concentrations of each element were determined by elemental analysis using an inductively coupled plasma mass spectrometer.

[0096]

[0097] The above shows the test results conducted as examples and comparative examples of the present invention.

[0098] In this example, it was confirmed that it is possible to separate the rare earth element Sc (scandium) contained in steel slag from other rare earth elements and recover those rare earth elements.

[0099] In particular, as is clear from Table 2, by setting the first pH of the solvent extraction step 1, which is performed for the purpose of recovering Sc, to 3.5 or higher and 4.0 or lower, it was possible to recover 78.5% by mass or more of the Sc contained in the steel slag.

[0100] In addition, by setting the pH of the third solvent extraction step 2, which is performed for the purpose of recovering rare earth elements other than Sc, to 6.6 or higher, it was possible to recover 78.5% by mass or more of rare earth elements other than Sc.

[0101] Therefore, in this embodiment, by setting the first pH to 3.5 or higher and the third pH to 6.6 or higher, it is possible to efficiently recover Sc and other rare earth elements.

[0102] In a comparative example, when the method described in Patent Document 1 was reproduced, the recovery rate of rare earth elements other than Sc was a maximum of approximately 47% by mass, as shown in Table 3. Furthermore, while the recovery rate of Sc could be 70% by mass or more, in that case the recovery rate of rare earth elements other than Sc was low, at approximately 18% by mass or less, indicating that it was not possible to recover both Sc and other rare earth elements simultaneously at a high recovery rate.

[0103] In other words, it was confirmed that this embodiment allows for the simultaneous recovery of Sc and other rare earth elements with a higher recovery rate compared to the conventional technology.

[0104] Furthermore, Patent Document 1 uses an index called the Sc purification rate to confirm the effect of separating Sc from other rare earth elements. Specifically, it checks how much of other rare earth elements are contained in the recovery liquid intended for Sc recovery. The Sc purification rate can be calculated using the following formula. (Some modifications have been made from the expression in Patent Document 1 to suit the terminology of this specification.) [Sc purification rate] = [Mass of Sc in the recovery liquid] ÷ [Total mass of rare earth elements in the recovery liquid] = ([Sc recovery rate in the recovery liquid] × [Sc concentration in blast furnace slag]) ÷ ([Total rare earth element recovery rate in the recovery liquid] × [Total rare earth element concentration in blast furnace slag])

[0105] The Sc purification rate disclosed in the example of Patent Document 1 is 0.58 to 0.79. In contrast, in the example of this embodiment, typically when the first pH, second pH, and third pH are within a preferred range, the Sc purification rate was 0.97 to 1.00.

[0106] In other words, it was confirmed that, according to this embodiment, Sc can be separated from other rare earth elements with higher separation accuracy compared to the conventional technology.

Claims

1. A method for recovering rare earth elements contained in steel slag, comprising: a leaching step of contacting the steel slag with an acid to dissolve it and obtain a leaching solution 1 containing the rare earth elements; a solvent extraction step 1 of adjusting the pH of the leaching solution 1 to a first pH, adding an organic solvent to separate and extract a first organic phase (organic phase 1) and a first aqueous phase (aqueous phase 1); a back-extraction step 1 of adding water to the first organic phase (organic phase 1), separating and extracting the back-extracted aqueous phase as a recovery solution 1; a precipitation step of adjusting the pH of the first aqueous phase (aqueous phase 1) to a second pH higher than the first pH, and precipitating precipitates containing Si and Al in the first aqueous phase (aqueous phase 1); and a solid-liquid separation step of solid-liquid separating the precipitates from the first aqueous phase (aqueous phase 1) to obtain an aqueous phase (aqueous phase 1') from which the precipitates have been removed. A method for recovering rare earth elements, comprising: a solvent extraction step 2 in which the pH of the aqueous phase (aqueous phase 1') from which the precipitate has been removed is adjusted to a third pH higher than the second pH, an organic solvent is added to separate and extract a second organic phase (organic phase 2) and a second aqueous phase (aqueous phase 2); and a back extraction step 2 in which water is added to the second organic phase (organic phase 2), and the back-extracted aqueous phase is separated and extracted as a recovery liquid 2, wherein the composition ratio of the rare earth elements contained in the recovery liquid 1 and the recovery liquid 2 are different.

2. The method for recovering rare earth elements according to claim 1, characterized in that the first pH is pH 3.5 or higher and 4.0 or lower.

3. The method for recovering rare earth elements according to claim 2, characterized in that the second pH is greater than 3.5 and less than or equal to 4.

7.

4. The method for recovering rare earth elements according to claim 3, characterized in that the third pH is pH 6.6 or higher.

5. The recovery method according to any one of claims 1 to 4, characterized in that the extractant used in the solvent extraction steps 1 and 2 is neodecanoic acid or a carboxylic acid-based extractant.

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