Methods for recovering rare earth elements

Abstract

The objective is to provide a novel method for recovering REE from blast furnace slag that allows for REE extraction while suppressing the leaching of Si from the blast furnace slag. A method for recovering rare earth elements from blast furnace slag, The blast furnace slag is crystalline, and the process includes an acid leaching step in which the blast furnace slag is brought into contact with an acid to obtain a rare earth element-containing leaching solution containing rare earth elements leached from the blast furnace slag, A solid-liquid separation step for separating solid components from the aforementioned rare earth element-containing leachate, Includes, A method for recovering rare earth elements, wherein the contact time between the blast furnace slag and the acid is 1 minute or more and 20 minutes or less.

Description

Technical Field

[0001] The present invention relates to a method for recovering rare earth elements, and more particularly to a method for recovering rare earth elements from crystalline blast furnace slag.

Background Art

[0002] Rare earth elements, also called REE (Rare Earth Element), 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] As prior art, there are inventions of REE recovery methods targeting tin slag, fly ash, Ni ore, bauxite residue, etc. [[ID=

[0004] Patent Document 1 discloses, for the purpose of efficiently recovering high-grade scandium from nickel oxide ore, subjecting nickel oxide ore together with sulfuric acid to solid-liquid separation into a leachate and a leach residue under high temperature and high pressure, etc.

[0005] Patent Document 2 aims to separate both precious metals and rare earth elements contained in fly ash, and discloses dissolving precious metals in a first extraction process and dissolving and separating the rare earth elements in a second extraction process, etc.

[0006] Patent Document 3 aims to provide a method for recovering rare earth metals from tin slag containing rare earth metals, etc., which can easily and reliably recover rare earth metals. It discloses a method in which tin slag is dissolved with an inorganic acid, and an oxidizing agent and a neutralizing agent are added to the solution so that radioactive materials precipitate, while rare earth metals and iron do not precipitate. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 5652503 [Patent Document 2] Patent No. 6159731 [Patent Document 3] Patent No. 5825074 [Patent Document 4] Special Publication No. 2018-530673 [Patent Document 5] Japanese Patent Publication No. 2022-110887 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Incidentally, steel production accounts for the majority of total metal production, and the amount of steel slag produced as a by-product is also very large. For example, in Japan, more than 100 million tons of crude steel are produced annually, and as by-products, approximately 23 million tons of blast furnace slag, approximately 12 million tons of steelmaking slag, and approximately 3 million tons of electric furnace slag are produced annually. The iron ore, coal, limestone, and iron scrap that are the raw materials for these steel slags contain trace amounts of rare earth elements, and the steel slag obtained through the steelmaking process also contains trace amounts of rare earth elements. If rare earth elements can be recovered from such a large amount of by-product steel slag, a large amount of rare earth elements can be recovered, which is desirable.

[0009] Generally, to industrially utilize REE in blast furnace slag, it is necessary to leach the REE from the blast furnace slag with an acid solution, and then separate and concentrate the REE from the main components of the blast furnace slag (Ca, Si, Al, Mg) using solvent extraction or ion exchange methods.

[0010] In the separation and concentration of REE by solvent extraction and ion exchange methods, a lower concentration of unintended components in the leachate, typically Ca, Si, Al, and Mg (the main components of blast furnace slag), is advantageous for the separation and concentration of REE. In other words, a lower concentration of Ca, Si, Al, and Mg in the leachate is desirable. Si, in particular, forms precipitates called gel-like silica, Ca, and Al, known as CASH gel or CSH gel, during the pH adjustment process required in solvent extraction and ion exchange operations, hindering leachate manipulation, separation of residue from leachate, and wastewater filtration. Therefore, it is desirable to keep the concentration of Si and other components in the leachate as low as possible.

[0011] The leaching characteristics of the raw materials targeted by the aforementioned prior art (e.g., nickel oxide ore, fly ash, tin slag) differ significantly from those of blast furnace slag, and the types and concentrations of the main constituent elements to be separated also differ. Therefore, it has been difficult to efficiently recover REE from blast furnace slag using the aforementioned development technologies (Patent Documents 1-3).

[0012] Patent Document 4 describes a method for recovering rare earth elements from steel slag by magnetic separation and wet refining. The rare earth element leaching method described in Patent Document 4 utilizes saline solution, chelating agents, or 0.1 mol / L hydrochloric acid or nitric acid. In Patent Document 4, the reaction time for acid leaching is 2 hours or more.

[0013] Patent Document 5 describes a method for recovering rare earth elements from steel slag, and discloses that in the leaching process, "the pH of the leached solution obtained by contacting it with an acid is adjusted to a predetermined final pH" or "the amount of additional acid or base added is controlled while monitoring the pH of the leached solution obtained by contacting it with an acid." In Patent Document 5, the contact time with the acid is approximately 24 hours or more.

[0014] A more appropriate method is needed for the industrial recovery of REE from blast furnace slag. The present invention aims to provide a novel method for recovering REE from blast furnace slag that can extract REE while suppressing the leaching of Si from the blast furnace slag. [Means for solving the problem]

[0015] The inventors of the present invention conducted acid leaching on multiple blast furnace slags and found that the crystalline / amorphous state of the blast furnace slag significantly affects the leaching behavior of Si.

[0016] Specifically, blast furnace slag can be broadly classified into two types: amorphous blast furnace granulated slag, obtained by pulverizing molten slag by spraying it with water, and crystalline blast furnace slowly cooled slag, obtained by releasing molten blast furnace slag into a yard and gradually lowering its temperature to solidify it. When the latter type of blast furnace slowly cooled slag was targeted for acid leaching, it was possible to selectively leach REE while suppressing the leaching of Si.

[0017] We diligently investigated the possibility of selectively leaching REE from crystalline blast furnace slow-cooled slag while suppressing Si leaching. As a result, we discovered that REE is concentrated in dicalcium silicate (2CaO·SiO2) and monocalcium silicate (CaO·SiO2) produced when blast furnace slag is slowly cooled. When blast furnace slag is slowly cooled, the main crystal constituting the blast furnace slag is melilite (2CaO·Al2O3·SiO2-2CaO·MgO·2SiO2 solid solution), but observation with an electron microscope revealed small amounts of calcium silicate-based mineral phases such as monocalcium silicate and the aforementioned dicalcium silicate. Calcium silicate-based minerals in which REE is concentrated are readily soluble in acid, while melilite, the main mineral phase of blast furnace slag, is poorly soluble in acid. Figure 1 schematically shows the REE leaching process in crystalline blast furnace slag and amorphous water-granulated slag. As shown in Figure 1, by adjusting the leaching conditions (typically leaching time, pH, etc.), calcium silicate minerals that concentrate REE can be selectively dissolved to leach REE, while melilite can be separated as a residue without dissolving it. This makes it possible to roughly separate REE from other components, including Si, in the acid leaching process. On the other hand, since granulated blast furnace slag is a uniform amorphous material, it is necessary to dissolve most of the granulated blast furnace slag in order to increase the leaching rate of REE. In that case, separating Si from REE was difficult.

[0018] Based on the above findings, we conceived the idea of ​​suppressing Si leaching into the acid leaching solution during REE leaching, and thus completed the present invention. Embodiments of the present invention include the following.

[0019] [1] A method for recovering rare earth elements from blast furnace slag, The blast furnace slag is crystalline, and the process includes an acid leaching step in which the blast furnace slag is brought into contact with an acid to obtain a rare earth element-containing leaching solution containing rare earth elements leached from the blast furnace slag, A solid-liquid separation step for separating solid components from the aforementioned rare earth element-containing leachate, Includes, A method for recovering rare earth elements, characterized in that the contact time between the blast furnace slag and the acid is 1 minute or more and 20 minutes or less. [2] In the acid leaching step, while monitoring the pH of the rare earth element-containing leachate obtained by bringing the acid into contact with the blast furnace slag, controlling the additional addition amount of the acid, and keeping the pH of the rare earth element-containing leachate constant, the method for recovering rare earth elements according to [1]. [3] The method for recovering rare earth elements according to [1] or [2], characterized in that the pH of the rare earth element-containing leachate maintained in the acid leaching step is selected from the range of 3.0 or less, and the contact time is selected from 3 minutes or more. [4] The method for recovering rare earth elements according to [1] or [2], characterized in that the pH of the rare earth element-containing leachate maintained in the acid leaching step is selected from the range of 0.5 or more and 3.0 or less, and the contact time is selected from 20 minutes or less. [5] The method for recovering rare earth elements according to any one of [1] to [4], characterized in that the acid consists of one or more inorganic acids. [6] A step of adding a base or an acid to the rare earth element-containing leachate after the solid-liquid separation step to adjust the pH to obtain a pH-adjusted rare earth element-containing leachate, An extraction step of treating the pH-adjusted rare earth element-containing leachate by any one or both of solvent extraction and solid phase extraction to obtain a rare earth element concentrate, A precipitation step of adding a precipitant to the rare earth element concentrate to obtain a rare earth element precipitate, A roasting step of roasting the rare earth element precipitate to obtain an oxide of the rare earth element, further comprising The method for recovering rare earth elements according to any one of [1] to [5]. [7] The solvent extraction agent used in the extraction step is an extraction agent of any one of an amine-based, an organic phosphoric acid-based, and a carboxylic acid-based, The solid phase extraction agent used in the extraction step is a resin having iminodiacetic acid as a functional group A method for recovering rare earth elements as described in [6], characterized by the above. [8] The precipitating agent used in the aforementioned precipitation step is one of oxalic acid, tartaric acid, carbonic acid, or a base. A method for recovering rare earth elements as described in [6] or [7], characterized by the above. [Effects of the Invention]

[0020] According to embodiments of the present invention, when leaching REE from blast furnace slag, it is possible to leach REE while suppressing the leaching of Si from the blast furnace slag. By suppressing the leaching of Si, the precipitation of gel-like substances that hinder the extraction process is suppressed, making it possible to perform the REE extraction process smoothly. Furthermore, since REE can be leached without dissolving the entire blast furnace slag with acid, the amount of Al, Mg, and Ca leached in addition to Si can also be suppressed, which is advantageous in REE separation and concentration processes using solvent extraction or ion exchange methods. As a secondary benefit, only the acid necessary to dissolve the trace amounts of calcium silicate containing REE can be added, which leads to a reduction in acid consumption and contributes to cost reduction in the REE recovery process from blast furnace slag. Furthermore, compared to conventional techniques, it is possible to increase the separation ratio of REE to Si while also increasing the leaching rate of REE. Typically, the separation ratio of REE to Si may be 1.5 or higher, preferably 1.7 or higher, and more preferably 2.0 or higher, and the leaching rate of REE may be 40% or higher, preferably 50% or higher, and more preferably 60% or higher. Here, the leaching rate of each element (including REE) is determined by the following formula (1).

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[0021] [Figure 1] This diagram schematically illustrates the REE leaching process from crystalline blast furnace slow-cooled slag and amorphous blast furnace granulated slag. [Figure 2] This figure shows the time course of the leaching rate in crystalline blast furnace slowly cooled slag. [Figure 3] This figure shows the time course of leaching rate in amorphous blast furnace granulated slag. [Figure 4] This figure shows the relationship between the separation ratio of REE and Si and the leaching time. [Figure 5] This diagram schematically illustrates the precipitation of gel-like substances in an acid leachate. [Figure 6] This diagram schematically shows an example of an apparatus for performing an acid leaching operation while maintaining a constant pH. [Figure 7] This graph plots the REE leaching rate against leaching time for each pH level. [Figure 8] This figure shows the relationship between the separation ratio of REE and Si and the leaching time for each pH level. [Figure 9] This graph plots the separation ratio of REE to Si against the leaching pH for each leaching time. [Figure 10] This figure shows the amount of acid consumed during the acid leaching process. [Modes for carrying out the invention]

[0022] The following describes in detail specific embodiments of the present invention (hereinafter also referred to as "this embodiment"), but the present invention is not limited in any way to the following embodiments, and can be implemented with appropriate modifications without changing the gist of the present invention.

[0023] This embodiment is characterized by including an acid leaching step in which crystalline blast furnace slag and an acid are brought into contact for a specific period of time to obtain a rare earth element-containing leaching solution containing rare earth elements leached from the blast furnace slag.

[0024] (Blast furnace slag) Blast furnace slag is a by-product of the steelmaking process, typically formed in blast furnaces that produce molten pig iron, where non-iron components from iron ore and ash from auxiliary raw materials such as limestone and coke are melted, separated, and recovered together.

[0025] Blast furnace slag mainly consists of lime (CaO) and silica (SiO2), and may also contain alumina (Al2O3), magnesium oxide (MgO), and small amounts of sulfur (S). Blast furnace slag may also contain iron oxide (FeO), and its content may be approximately 10% by mass or less, 5% by mass or less, 3% by mass or less, or 1% by mass or less.

[0026] In addition, iron ore, coal, limestone, and iron scrap, which are the raw materials for blast furnace slag, contain trace amounts of rare earth elements (REE), and blast furnace slag obtained through the steel manufacturing process also contains trace amounts of rare earth elements.

[0027] Although there are multiple definitions of elements included in the term "rare earth elements," the definition of rare earth elements (REE) presented in this disclosure is a group of 17 elements that include the lanthanides (Ln), which consist 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] (crystalline) Furthermore, blast furnace slag can be broadly classified into two types: amorphous blast furnace granulated slag, obtained by pulverizing molten slag by spraying water onto it, and crystalline blast furnace slowly cooled slag, obtained by releasing molten blast furnace slag into a yard and gradually lowering the temperature to solidify it. In this embodiment, crystalline blast furnace slag is used. As described above, this can be obtained by slowly cooling the blast furnace slag from a molten state. The conditions for slow cooling are not particularly limited as long as crystalline slag can be obtained. To change to a crystalline state, the cooling rate (for example, less than 35°C / sec) up to approximately 900°C, when the slag solidifies from a molten state, may be appropriately adjusted. If necessary, heat retention or reheating may be performed.

[0029] Furthermore, whether the slag is crystalline or amorphous can be determined by microscopy, specifically using a polarizing microscope. More specifically, the sample of steel slag or steel slag powder of interest is dried, and then the particle size is adjusted by fine grinding and classification. Samples with a particle size of 63 μm to 45 μm are collected, and the collected samples are examined under a polarizing microscope.

[0030] The above-mentioned fine grinding process is not particularly limited and can be carried out using known grinders such as disc mills, rod mills, ball mills, and vertical mills, by adjusting conditions such as grinding time and air flow rate. Similarly, the above-mentioned classification process is not particularly limited and may be carried out using a sieve, or using wind power, magnetism, etc.

[0031] For the sample obtained as described above, the total number of particles is counted under open nicols and the number of crystalline particles is counted under crossed nicols. This operation is performed with approximately 50 particles per field of view, and measurements are continued until the total number of counted particles reaches 500 or more. After that, the amorphousness rate can be calculated from the total number of counted particles and the number of crystalline particles based on the following formula. Generally, a material with an amorphousness rate of 10% or less can be called "crystalline". Amorphization rate (%) = {(Total number of particles - Number of crystalline particles) / Total number of particles} × 100

[0032] Furthermore, in samples with a high degree of blackness, there is a possibility of misidentifying vitrified particles as crystalline particles during microscopic examination under crossed nicols. This is because, when observing whether or not a particle transmits light through a polarizing plate, in samples with a high degree of blackness, it may appear as if no light is transmitting. The higher the degree of blackness, the more likely this misidentification is to occur. If all or most of the particles are determined to be crystalline particles, the vitrification rate (amorphization rate) will be calculated to be 0% or close to 0%. In cases where this is a concern, X-ray diffraction can be used to determine the amount of amorphous material using Rietveld analysis, and this can be used as the vitrification rate to prevent underestimation due to such misidentification.

[0033] In such cases, the amount of amorphous material relative to the mass of the sample to be measured becomes the vitrification rate (mass %). A standard substance is added to the sample to be measured in a predetermined proportion, and then measurement and analysis are performed by X-ray diffraction. When X-ray diffraction is performed, amorphous material is not observed as a clear peak, but rather as behaving in a way that pushes up the background in the resulting X-ray diffraction spectrum. Therefore, by focusing on the percentage decrease from the quantitative value that should have been obtained relative to the amount of standard substance added, it is possible to identify the amount of amorphous material. In this case, a sample with an amorphous content of 10% or less of the total can be called "crystalline."

[0034] When blast furnace slag is slowly cooled and crystallized, the main crystals constituting the crystalline blast furnace slag are melilite (2CaO·Al2O3·SiO2-2CaO·MgO·2SiO2 solid solution), but it also contains small amounts of calcium silicate mineral crystalline phases such as dicalcium silicate (2CaO·SiO2) and monocalcium silicate (CaO·SiO2). Furthermore, the inventors have found that REE is concentrated in dicalcium silicate (2CaO·SiO2) and monocalcium silicate (CaO·SiO2). Concentration or enrichment of a certain element containing REE means that the concentration in the enriched phase is higher than the concentration in other parts of the slag. The types and abundances of these crystalline phases and the elements that constitute them are confirmed by combining observation with electron microscopy and analysis with energy-dispersive X-ray spectrometer.

[0035] (Acid leaching process) In the acid leaching process, blast furnace slag is brought into contact with an acid (acidic solvent). By bringing the blast furnace slag into contact with the acid, at least a portion of the slag dissolves in the acid and leaches out. A liquid containing components leached from the blast furnace slag (including rare earth elements) is obtained, and this is called a rare earth element-containing leaching solution.

[0036] The solubility of a substance (solute) in an acid varies depending on the type of substance (solute). Calcium silicate-based crystalline minerals with concentrated REE are readily soluble in acid, while melilite, the main mineral phase of blast furnace slow-cooled slag, is poorly soluble in acid. The inventors of this invention have also discovered this.

[0037] Immediately after the start of the acid leaching process, the dissolution of calcium silicate concentrated with easily soluble REE proceeds (relatively), while the dissolution of melilite, the main mineral phase of slowly cooled blast furnace slag which is difficult to dissolve, does not proceed (relatively) and is even suppressed. In other words, the leaching rate of REE is relatively high, and the leaching of Si, the main component of blast furnace slag, is suppressed.

[0038] Generally, the longer the contact time between the acid and blast furnace slag, the more the slag components dissolve or leach out, and the higher the REE leaching rate. However, Si leaching also progresses. Furthermore, the REE leaching rate may saturate.

[0039] On the other hand, generally, the shorter the contact time between the acid and blast furnace slag, the less Si leaching occurs. However, the leaching rate of REE may not be sufficient.

[0040] From the above perspective, the leaching rate of REE or Si in the rare earth element-containing leached liquid can be adjusted by adjusting the contact time between the acid and the blast furnace slag. The contact time can be appropriately adjusted according to the desired REE leaching rate and the degree of Si leaching suppression.

[0041] (solid-liquid separation process) Furthermore, this embodiment is characterized by including a solid-liquid separation step for separating solids from the obtained rare earth element-containing leachate.

[0042] The solid-liquid separation process is a process of separating the solid components from the rare earth element-containing leachate obtained in the aforementioned acid leaching process. In the solid-liquid separation process, the rare earth element-containing leachate obtained in the acid leaching process can be physically separated from the leaching residue. 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 can be used.

[0043] The leachate containing rare earth elements, after separating the solid components, contains rare earth elements. In other words, the rare earth elements that were contained in the blast furnace slag are recovered in this liquid.

[0044] (Contact time between blast furnace slag and acid) The contact time between blast furnace slag and acid should be between 1 minute and 20 minutes. If the contact time is less than 1 minute, the REE leaching rate may be insufficient. If the contact time exceeds 20 minutes, the Si leaching may become excessive, or the REE leaching rate may become saturated. Within the range of 1 minute to 20 minutes, the contact time can be appropriately adjusted according to the desired REE leaching rate and the degree of Si leaching suppression. The lower limit may be 2 minutes, 3 minutes, 4 minutes, or 5 minutes, and the upper limit may be 15 minutes or 10 minutes.

[0045] Here, contact time refers to the time during which the acid (acidic solvent) and the blast furnace slag are in contact. In this embodiment, it refers to the time from the start of contact between the acid and the blast furnace slag in the acid leaching process until the separation of solids from the rare earth element-containing leachate in the solid-liquid separation process is completed. In other words, the start of contact time is the point when the acid (acidic solvent) and the blast furnace slag first come into contact in the acid leaching process. The end of contact time is the point when the acid (acidic solvent) and the blast furnace slag (or its residue) are separated in the solid-liquid separation process.

[0046] In actual operation, a continuous process capable of processing large quantities of blast furnace slag may be used. In this case, the contact time is the time that each individual blast furnace slag is actually in contact with the acid (acidic solvent). The start and end points are as described above. Typically, the start point may be the time when the acid (acidic solvent) first comes into contact with each individual blast furnace slag by spraying, scattering, coating, etc., or the time when each individual blast furnace slag is placed in a container containing the acid (acidic solvent) and first comes into contact with it. The end point may be the time when the individual blast furnace slag is separated from the acid (acidic solvent) by removal, filtration, etc.

[0047] Although not an essential embodiment, in the acid leaching process described above, The pH of the rare earth element-containing leachate obtained by contacting blast furnace slag with acid may be monitored, and the amount of additional acid added may be controlled to maintain a constant pH of the rare earth element-containing leachate.

[0048] REE can also be leached by monitoring the pH of the leachate and adding acid as needed to maintain a constant pH. This reduces the amount of acid consumed. More specifically, a rare earth element-containing leachate is obtained by adjusting the pH of the leachate obtained by contacting blast furnace slag with acid to a constant pH. pH adjustment can be performed by monitoring the pH of the leachate obtained by contacting blast furnace slag with acid and controlling the amount of acid added. Note that adjusting the pH to a target constant value means that the acid or base is added so that the pH does not deviate by ±0.5, preferably ±0.3, and more preferably ±0.1 or more from the target pH for at least 1 minute continuously.

[0049] The acid may be in the form of a liquid, i.e., an acidic solvent, since it is used in contact with the blast furnace slag to leach the elements. The type of acid is not particularly limited as long as it can dissolve the slag and adjust the pH. As the acid, one or more inorganic acids can be used. Specific examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, or a mixture thereof. Hydrochloric acid is preferred from the viewpoint of the leaching rate of rare earth elements and the separation ratio between rare earth elements and silicon. The acid is preferably a strong acid from the viewpoint of obtaining a high leaching rate of rare earth elements. The purity or concentration of the acid is not particularly limited and may be selected as appropriate.

[0050] A typical method for maintaining a constant pH is illustrated below. Blast furnace slag is charged into the acid leaching reaction vessel, and acid is added. At this time, the solid-liquid ratio of blast furnace slag to acid is preferably 1:1 to 1:100. The pH of the added acid may be 1.0 or lower.

[0051] One method for monitoring pH is to immerse a pH sensor (pH probe) in the leachate (rare earth element-containing leachate) obtained by contacting blast furnace slag with acid, and monitor the pH of the leachate as needed. It is also preferable to stir the leachate as appropriate to ensure that the pH of the leachate becomes uniform. Rare earth elements can be sufficiently leached even under normal conditions of a leachate temperature of 15-35°C and a pressure of approximately 1013 hPa, but the leaching of rare earth elements may be carried out while heating or pressurizing to increase the leaching rate.

[0052] If the pH of the leachate is higher than the target pH, the pH can be adjusted to the target value by adding small amounts of acid while monitoring the pH value. In this disclosure, the pH of the leachate refers to the pH of the leachate read by a pH meter or the like during the leaching process.

[0053] The constant pH can be appropriately determined depending on the purpose, and may be selected from a range consisting of any combination of an upper limit of less than 7.0, 6.0, 5.5, 5.0, 4.5, 4.0, 3.5, or 3.0 and a lower limit of less than 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, or 3.0. Generally, a lower pH is preferable because it allows for faster leaching of REE. Furthermore, a shorter acid leaching time can suppress the leaching of Si, which is also preferable. The pH can be appropriately adjusted according to the desired REE leaching rate and the degree of Si leaching suppression. From the viewpoint of increasing the REE leaching rate, typically from the viewpoint of achieving a REE leaching rate of 80% or more, the pH during acid leaching may be set to 2.0 or lower, and preferably to 1.5 or lower.

[0054] In one embodiment, the pH can be selected from 3.0 or lower from the viewpoint of improving the leaching rate of rare earth elements. In addition, the contact time between the acid and blast furnace slag can be selected from 3 minutes or more. If the pH is selected from 3.0 or lower and the contact time between the acid and blast furnace slag is 3 minutes or more, the leaching rate for rare earth elements, as expressed by the following formula (Equation 1), can be set to, for example, 40% or more.

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[0055] In other embodiments, from the perspective of suppressing the leaching of Si or from the perspective of improving the separation ratio between rare earth elements (REE) and silicon (Si), the pH is selected from 0.5 or more and 3.0 or less, and the contact time between the acid and the blast furnace slag can be selected from 20 minutes or less. If the pH is selected from 0.5 or more and 3.0 or less and the contact time between the acid and the blast furnace slag is 20 minutes or less, the leaching of Si can be suppressed, and the separation ratio represented by the following (Formula 2) can be, for example, 1.5 or more.

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[0056] The pH of the leachate (rare earth element-containing leachate) and the contact time between the acid and the blast furnace slag may be adjusted as appropriate. Thereby, a desired REE leaching rate and a degree of suppression of Si leaching can be obtained.

[0057] In the above solid-liquid separation step, by subjecting the rare earth element-containing leachate from which the solid content has been separated to an optional purification step, the purity of the rare earth elements can be further increased. Examples of the optional purification step include a pH adjustment step, an extraction step (which may include a solid phase extraction step and / or a solvent extraction step), a precipitation step, a roasting step, and the like. Hereinafter, exemplary methods for each step will be described.

[0058] <pH Adjustment Step> Although not an essential aspect, a base or an acid may be added to the rare earth element-containing leachate that has undergone the solid-liquid separation step to adjust the pH to obtain a pH-adjusted rare earth element-containing leachate. By setting the pH of the leachate between 0.5 and 8, the adsorption rate of rare earth elements to the solid phase in the subsequent solid phase extraction step is improved, and separation from the main components of the blast furnace slag such as calcium and magnesium becomes possible. <Extraction Step> The extraction step is a step of extracting rare earth elements from a rare earth element-containing leachate after a solid-liquid separation step to obtain a rare earth element concentrate. The extraction step preferably includes one or both of the following: a solid-phase extraction step (sometimes referred to as M1) that concentrates rare earth elements using the principle of solid-phase extraction, and a solvent extraction step (sometimes referred to as M2) that concentrates rare earth elements using the principle of solvent extraction. When both the solid-phase extraction step (M1) and the solvent extraction step (M2) are used, the order in which the rare earth element-containing leachate after the solid-liquid separation step is processed does not matter. When only one of the solid-phase extraction step (M1) and the solvent extraction step (M2) is used in the extraction step, there is an advantage in that the equipment can be simplified and rare earth elements can be recovered at a low cost, but the purity of the rare earth elements obtained will be lower compared to when both the solid-phase extraction step (M1) and the solvent extraction step (M2) are used. On the other hand, when rare earth elements are recovered using both solid-phase extraction (M1) and solvent extraction (M2), there is the advantage of recovering rare earth elements with high purity. However, compared to using only one of the two processes, the equipment becomes larger and the equipment cost is higher. The advantages and disadvantages of these processes should be considered and used appropriately depending on the purpose. For example, when using rare earth elements in applications where the inclusion of some impurities is not a problem, such as in mischmetal, one extraction process is sufficient. On the other hand, for applications where the purity of the product is particularly important, such as in the electronics industry, an extraction process combining solid-phase extraction (M1) and solvent extraction (M2) is desirable.

[0059] <Solid phase extraction process (M1)> The solid-phase extraction step (M1) is a step included in the extraction step, and for example, it includes a rare-earth element adsorption step (sometimes referred to as M11), a cation removal step (sometimes referred to as M12), and a rare-earth element elution step (sometimes referred to as M13). If necessary, a solid-phase washing step (sometimes referred to as M14) can be performed in the solid-phase extraction step (M1). In the rare-earth element adsorption step (M11), for example, the leachate may be brought into contact with a solid-phase extractant, such as a solid phase made of a resin having iminodiacetic acid as a functional group, to adsorb cations onto the solid phase and obtain a cation-adsorbed solid phase. Subsequently, in the cation removal step (M12), for example, an inorganic acid of less than 0.3 N may be brought into contact with the cation-adsorbed solid phase to elute cations other than rare-earth elements from the solid phase and obtain a rare-earth element-adsorbed solid phase. In the rare earth element elution step (M13), for example, the rare earth elements may be eluted by contacting the rare earth element adsorbed solid phase with an inorganic acid of 0.3 N or more and less than 3 N to obtain a rare earth element solid phase eluent. In the solid phase washing step (M14), for example, impurity cations may be eluted by contacting the solid phase that has gone through the rare earth element elution step (M13) with an inorganic acid of 3 N or more.

[0060] <Solvent extraction process (M2)> The solvent extraction step (M2) is a step included in the extraction step, and for example, it includes an extraction step (sometimes referred to as M21, which is an extraction step performed before the back-extraction step described later) and a back-extraction step (sometimes referred to as M22, which is an extraction step performed following the aforementioned extraction step (M21)). In the extraction step (M21), for example, an organic solvent containing a solvent extractant may be mixed with a rare earth element-containing extract to partition the rare earth element into the organic solvent to obtain a rare earth element-containing organic phase. In the back-extraction step (M22), for example, the rare earth element-containing organic phase obtained in the extraction step (M21) may be mixed with water to partition the rare earth element from the organic phase to the aqueous phase and perform back-extraction. In the back-extraction step (M22), the pH may be adjusted as needed. The pH may be set to a pH suitable for the organic solvent used.

[0061] In the extraction step (M21) and the back-extraction step (M22), when mixing the organic phase and the aqueous phase, known solvent extraction devices such as centrifugal extractors and pulsed columns may be used.

[0062] The solvent extractant used in the extraction step (M21) can be any conventionally known extractant, such as carboxylic acid-based extractants like neodecanoic acid, organophosphate-based extractants like di(2-ethylhexyl)phosphate, tributyl phosphate, or trioctylphosphine oxide, or amine-based extractants like triisooctylamine. The solvent extractant can be used without a solvent, or it can be dissolved in an organic solvent that does not mix with water, such as kerosene, xylene, or toluene. The solvent extractant may also be added to the aqueous phase.

[0063] <Precipitation process> The precipitation step is a step in which a precipitating agent is added to the rare earth element concentrate obtained through the extraction step, for example, to obtain a 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), but organic bases are preferred from the viewpoint of obtaining rare earth elements of higher purity. Examples of acids include tartaric acid, carbonic acid, and oxalic acid, with oxalic acid being preferred.

[0064] 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 uranium, aluminum, and iron contained in the rare earth element concentrate, but it has the disadvantage of low separation efficiency from magnesium and calcium. On the other hand, using a base as the precipitant also has the advantage of efficiently separating the rare earth elements from impurities such as uranium, magnesium, and calcium contained in the rare earth element concentrate, but it has the disadvantage of low separation efficiency from iron and aluminum. The impurities contained in the rare earth element concentrate vary depending on the composition of the blast furnace 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.

[0065] The obtained rare earth element precipitate and the post-precipitation solution can be physically separated. 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 centrifuges can be used for separation.

[0066] <Roasting process> The roasting process is a process of roasting the rare earth element precipitate obtained in the precipitation process to obtain an oxide of the rare earth element. The roasting process may include a washing process (sometimes called O1) and a heating process (sometimes called O2). In the washing process (O1), for example, the rare earth element precipitate obtained in the precipitation process may be washed with water to remove impurities. In the heating process (O2), for example, the rare earth element precipitate that has gone through the washing process (O1) 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 an oxide of the rare earth element.

[0067] In the roasting process described above, the roasting conditions are not limited, but for example, heating at approximately 900°C for about 2 hours in a tubular furnace is sufficient. Alternatively, by using a continuous furnace such as a rotary kiln, drying and roasting can be performed in the same apparatus, enabling industrially efficient production of rare earth element oxides. Furthermore, in this embodiment, the leaching of Si can be suppressed in the acid leaching process to obtain the rare earth element-containing leachate, thereby suppressing the precipitation of gel-like substances that may interfere with optional purification processes (pH adjustment, extraction, precipitation, roasting, etc.) to increase the purity of the rare earth elements. [Examples]

[0068] 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.

[0069] (Example 1: Acid leaching test without pH control) Three g of powder (particle size 250 μm or less) of amorphous blast furnace slag (granulated blast furnace slag) and crystalline blast furnace slag (slowly cooled blast furnace slag) with the compositions shown in Table 1 were added to 300 mL of 1 mol / L hydrochloric acid (liquid-to-solid ratio 100), stirred for 1 hour, and the concentration changes of Ca, Si, Al, Mg, Fe, and REE in the acid leachate (leachate containing rare earth elements) were measured by ICP-MS (inductively coupled plasma mass spectrometer). Here, the sum of the 17 elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is shown as REE.

[0070] The composition of the blast furnace slag was determined by X-ray fluorescence analysis. The total amount of rare earth elements in the blast furnace slag was determined by completely dissolving the slag using acid decomposition and melting methods, and then measuring the resulting sample using the ICP-MS (inductively coupled plasma mass spectrometer) described above. Furthermore, whether the blast furnace slag was crystalline or amorphous was determined by XRD. XRD measurements showed that the crystalline content of the slowly cooled blast furnace slag was over 90%, while no crystalline peaks were observed in the granulated blast furnace slag.

[0071] [Table 1]

[0072] Figures 2 and 3 show the time course of leaching rates for crystalline blast furnace slag (slowly cooled blast furnace slag) and amorphous blast furnace slag (granulated blast furnace slag), respectively.

[0073] Figure 4 plots the ratio of the leaching rate of REE to Si, the main component of blast furnace slag, against the leaching time for crystalline blast furnace slag (slowly cooled blast furnace slag) and amorphous blast furnace slag (granulated blast furnace slag). Specifically, it shows the separation ratio of REE to Si (leaching rate of REE / leaching rate of Si). In this example and comparative example, the solid-liquid separation step for separating solids from the REE-containing leachate was omitted for convenience in order to measure the leaching rate over time. Therefore, the leaching time corresponds to the contact time between the acid and the blast furnace slag.

[0074] As shown in Figure 4, in the case of amorphous blast furnace slag (granulated blast furnace slag), the separation ratio of REE to each element is approximately 1 regardless of the leaching time (contact time), and REE and Si, the main component of blast furnace slag, are uniformly leached out. On the other hand, when crystalline blast furnace slag (slowly cooled blast furnace slag) is used, the separation ratio of REE to Si is higher as the leaching time (contact time) decreases. Therefore, by using crystalline blast furnace slag (slowly cooled blast furnace slag) as the raw material for REE leaching, it is possible to suppress the leaching of Si and selectively leach out REE, which is the target of leaching.

[0075] As an example, amorphous blast furnace slag (granulated blast furnace slag) was acid-leached for 5 minutes and crystalline blast furnace slag (slowly cooled blast furnace slag) for 1 minute under the aforementioned leaching conditions to obtain a rare earth element-containing leachate. Table 2 shows the leaching rate of Si (A) and REE (B) at the aforementioned leaching time (contact time). The ratio of Si leached to achieve 1% of the target REE (A / B) is also shown.

[0076] [Table 2]

[0077] Table 2 shows that when crystalline blast furnace slag (slowly cooled blast furnace slag), which is an embodiment of the present invention, was used as the raw material and an acid leaching operation was performed for 1 minute, the leaching of Si, which is the main cause of precipitates, was suppressed to about 1 / 3. By suppressing the leaching of Si, the precipitation of gel-like substances in the subsequent extraction operation is suppressed, making it possible to perform the REE extraction operation smoothly.

[0078] As an example, a gel-like substance mainly composed of Si was precipitated by adding ammonia water dropwise to the above-mentioned acid leaching solutions (one made with crystalline blast furnace slag and leaching time (contact time) of 1 minute, and another made with amorphous blast furnace slag and leaching time (contact time) of 5 minutes) under the same conditions, thereby adjusting the pH. Figure 5 schematically shows the precipitation of the gel-like substance in each acid leaching solution.

[0079] In the case where crystalline blast furnace slag (slowly cooled blast furnace slag), which is an example of the present invention, was used, the amount of gel-like Si precipitate formed in the acid leachate was significantly less, making operations such as filtration easier. On the other hand, when amorphous blast furnace slag (granulated blast furnace slag), which was used as a comparative example, a large amount of gel-like Si precipitate was generated, hindering filtration and making the filtration operation extremely difficult.

[0080] (Example 2) We investigated the effectiveness of maintaining a constant pH of a rare earth element-containing leachate by monitoring its pH while controlling the amount of additional acid added, obtained by contacting blast furnace slag with acid.

[0081] Specifically, 8 g of crystalline blast furnace slag (slow-cooled blast furnace slag) shown in Table 1 was crushed to a particle size of 53 μm or less and added to 800 mL of pure water. The pH of the added pure water was read using a pH meter, and 6 mol / L hydrochloric acid was continuously added dropwise to adjust the pH of the solution to a constant value of 3.0, 2.0, 1.0, and 0.5, while performing acid leaching operations for 1, 3, 5, 10, 20, 30, and 60 minutes.

[0082] As a comparative example, 8 g of amorphous blast furnace slag (granulated blast furnace slag) shown in Table 1 was pulverized to a particle size of 53 μm or less, added to 800 mL of pure water, and the pH of the added pure water was read using a pH meter. Acid leaching operations were performed for 1, 3, 5, 10, 20, 30, and 60 minutes while continuously adding 6 mol / L hydrochloric acid to adjust the pH of the solution to 3.0 and 1.0. Figure 6 is a schematic diagram showing an example of an apparatus used to perform the acid leaching operation while keeping the pH constant.

[0083] Figure 7 plots the REE leaching rate against leaching time for each pH level. From Figure 7, it was confirmed that for crystalline blast furnace slag (slowly cooled blast furnace slag), the REE leaching rate is approximately 80% or higher when the pH during acid leaching is 2.0 or lower. In this example and comparative example, the solid-liquid separation step to separate solids from the REE-containing leaching solution was omitted for convenience in order to measure the leaching rate over time. Therefore, the leaching time corresponds to the contact time between the acid and the blast furnace slag.

[0084] Figure 8 plots the ratio of the leaching rate of REE to Si, the main component of blast furnace slag, against the leaching time (contact time) for each pH value, i.e., the separation ratio of REE to Si (leaching rate of REE / leaching rate of Si). From Figure 8, it was confirmed that the separation ratio of REE to Si (leaching rate of REE / leaching rate of Si) increases when the leaching time (contact time) is short.

[0085] Figure 9 plots the separation ratio of REE to Si (REE leaching rate / Si leaching rate) against the leaching pH for each leaching time (contact time). The REE leaching rate is noted for each point. Based on Figure 9, it was confirmed that the acid leaching time (contact time) and acid leaching pH can be selected, and the corresponding separation ratio of REE to Si and REE leaching rate can be obtained.

[0086] In addition to a comparative example using amorphous blast furnace slag (blast furnace water-cooled slag), Figure 9 also plots data for leachate obtained by contacting granulated blast furnace slag with acid for 24 hours using the method described in Patent Document 5, as a reference example. It was confirmed that this example had a generally higher separation ratio of REE to Si (REE leaching rate / Si leaching rate) compared to the reference example and comparative example. Furthermore, it was confirmed that the REE leaching rate in this example could be made equivalent to or better than that of the reference example and comparative example by selecting the acid leaching time (contact time) and acid leaching pH.

[0087] Furthermore, Figure 10 shows the amount of acid (hydrochloric acid in this example) consumed during acid leaching. Figure 10 shows the amount of acid consumed in Example 1, where no adjustment was made to keep the pH constant, and in Example 2, where the pH of the rare earth element-containing leaching solution was adjusted to a constant value. Here, since the leaching rate of REE differs under each condition, the amount of acid consumed is expressed as the amount (mol) consumed to leach 1 mass% of REE from the blast furnace slag, which is the raw material, using the following formula (3).

number

[0088] As shown in Figure 10, it was confirmed that the leaching method implemented in Example 2 allowed for a reduction in acid consumption compared to Example 1. In a typical example, when the pH of the acid leaching solution was controlled to pH 1.0 and the leaching operation was performed for 3 minutes, approximately 80% of the REE in the crystalline blast furnace slag (slowly cooled blast furnace slag) could be leached, and the separation ratio of REE to Si in that case exceeded the maximum value of approximately 2.0 in the prior patent (Patent Document 5). Furthermore, it was confirmed that in Example 2, the amount of acid used to leach the same proportion of REE could be reduced to approximately 1 / 3 of that in Example 1.

Claims

1. A method for recovering rare earth elements from blast furnace slag, An acid leaching step is performed using blast furnace slag that is crystalline with an amorphous ratio of 10% or less and contains a calcium silicate-based mineral crystalline phase, by contacting the blast furnace slag with an acid to obtain a rare earth element-containing leaching liquid containing rare earth elements leached from the blast furnace slag. A solid-liquid separation step for separating solid components from the aforementioned rare earth element-containing leachate, Includes, A method for recovering rare earth elements, characterized in that the pH of the rare earth element-containing leachate in the acid leaching step is 3.0 or less, and the contact time between the blast furnace slag and the acid is 1 minute or more and 20 minutes or less.

2. A method for recovering rare earth elements according to claim 1, wherein in the acid leaching step, the amount of additional acid added is controlled while monitoring the pH of the rare earth element-containing leaching solution obtained by contacting the blast furnace slag with the acid, thereby maintaining a constant pH of the rare earth element-containing leaching solution.

3. The method for recovering rare earth elements according to claim 2, characterized in that the pH of the rare earth element-containing leachate maintained in the acid leaching step is selected from a range of 3.0 or less, and the contact time is selected from 3 minutes or more.

4. The method for recovering rare earth elements according to claim 2, characterized in that the pH of the rare earth element-containing leachate maintained in the acid leaching step is selected from a range of 0.5 to 3.0, and the contact time is selected from 20 minutes or less.

5. The method for recovering rare earth elements according to claim 1, characterized in that the acid consists of one or more inorganic acids.

6. A step of adjusting the pH of the rare earth element-containing leachate after the solid-liquid separation step by adding a base or acid to obtain a pH-adjusted rare earth element-containing leachate, An extraction step to obtain a rare earth element concentrate by treating the pH-adjusted rare earth element-containing leachate with either solvent extraction or solid-phase extraction, or both; A precipitation step is performed by adding a precipitating agent to the aforementioned rare earth element concentrate to obtain a rare earth element precipitate, The method further includes a roasting step of roasting the aforementioned rare earth element precipitate to obtain an oxide of the rare earth element. The method for recovering rare earth elements according to claim 1.

7. The solvent extractant used in the extraction step is one of the following: an amine-based, organophosphoric acid-based, or carboxylic acid-based extractant. The solid-phase extractant used in the extraction process is a resin having iminodiacetic acid as its functional group. A method for recovering rare earth elements according to claim 6, characterized by the above.

8. The precipitating agent used in the aforementioned precipitation step is one of oxalic acid, tartaric acid, carbonic acid, or a base. A method for recovering rare earth elements according to claim 6 or 7, characterized by the above.

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