Method for recovering rare earth elements

Abstract

The purpose of the present invention is to provide a novel method for recovering REE in blast furnace slag whereby it is possible to leach REE while suppressing the leaching of Si in the blast furnace slag when leaching REE from the blast furnace slag.　The present invention is a method for recovering rare earth elements from blast furnace slag, wherein the blast furnace slag is crystalline, the method for recovering rare earth elements comprises an acid leaching step for bringing the blast furnace slag into contact with an acid to obtain a rare earth element-containing leachate containing a rare earth element leached from the blast furnace slag and a solid-liquid separation step for separating solid content from the rare earth element-containing leachate, and the contact time between the blast furnace slag and the acid is 1-20 minutes.

Description

Methods for recovering rare earth elements

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

[0002] Rare earth elements, also known as REE (Rare Earth Element), are a collective term for 17 elements: 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 in a wide variety of materials, including hydrogen storage alloys, secondary battery raw materials, optical glass, rare earth magnets, phosphors, abrasives, and aluminum, making them a group of elements with high industrial value. However, rare earth element ore reserves are unevenly distributed, making them scarce and resulting in low global supply. Furthermore, their prices fluctuate wildly due to changes in social conditions. Therefore, establishing a method for the stable and large-scale supply of rare earth elements is crucial for industrial development.

[0003] Prior art includes inventions for REE recovery methods targeting tin slag, fly ash, Ni ore, bauxite residue, etc.

[0004] Patent Document 1 discloses a method for efficiently recovering high-grade scandium from nickel oxide ore by separating the nickel oxide ore into a leachate and leachate residue under high temperature and high pressure together with sulfuric acid.

[0005] Patent Document 2 aims to separate both precious metals and rare earth elements contained in fly ash, and discloses a method in which precious metals are dissolved in a first extraction process and rare earth elements are dissolved and separated in a second extraction process.

[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.

[0007] Japanese Patent Publication No. 5652503, Japanese Patent Publication No. 6159731, Japanese Patent Publication No. 5825074, Japanese Patent Publication No. 2018-530673, Japanese Patent Publication No. 2022-110887

[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 from 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 target 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 C-A-S-H gel or C-S-H gel, during the pH adjustment process required in solvent extraction and ion exchange operations, hindering the handling of the leachate, the separation of residue from the leachate, and the filtration of wastewater. 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 relates to 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.

[0015] The inventors of this 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 is subjected to acid leaching, it is possible to selectively leach REE while suppressing the leaching of Si.

[0017] We diligently investigated how to selectively leach REE (Rich Oxide) from crystalline blast furnace slag while suppressing Si leaching. As a result, we found that when blast furnace slag is slowly cooled, dicalcium silicate (2CaO·SiO₂) is produced. ２ ) and monocalcium silicate (CaO・SiO ２ It was discovered that REE is concentrated in ) . When blast furnace slag is slowly cooled, the main crystals constituting the slowly cooled blast furnace slag are melilite (2CaO·Al ２ O ３ SiO ２ -2CaO・MgO・2SiO ２ Although it is a solid solution, observation with an electron microscope revealed a small amount of calcium silicate-based mineral phases, such as monocalcium silicate and the aforementioned dicalcium silicate. Calcium silicate-based minerals that concentrate REE are readily soluble in acid, while melilite, the main mineral phase of blast furnace slow-cooled slag, is poorly soluble in acid. Figure 1 schematically shows the REE leaching process in crystalline blast furnace slow-cooled slag and amorphous granulated slag. As shown in Figure 1, by adjusting the leaching conditions (typically leaching time and pH, etc.), calcium silicate-based minerals that concentrate REE are selectively dissolved to leach REE, while melilite is separated as a residue without dissolving. 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 REE leaching rate. In that case, separation of Si and 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, comprising: an acid leaching step of contacting the blast furnace slag with an acid to obtain a rare earth element-containing leachate containing rare earth elements leached from the blast furnace slag, wherein the blast furnace slag is crystalline, and the method comprises: a solid-liquid separation step of separating solid components from the rare earth element-containing leachate, wherein the contact time between the blast furnace slag and the acid is 1 minute or more and 20 minutes or less. [2] The method for recovering rare earth elements according to [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 leachate obtained by contacting the blast furnace slag with the acid, thereby maintaining a constant pH of the rare earth element-containing leachate. [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 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 [1] or [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 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 method for recovering rare earth elements according to any one of [1] to [5], further comprising: 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 of treating the pH-adjusted rare earth element-containing leachate by solvent extraction and solid-phase extraction, or both, 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; and a roasting step of roasting the rare earth element precipitate to obtain an oxide of a rare earth element. [7] A method for recovering rare earth elements according to [6], characterized in that the solvent extractant used in the extraction step is an amine-based, organophosphoric acid-based, or carboxylic acid-based extractant, and the solid-phase extractant used in the extraction step is a resin having iminodiacetic acid as a functional group.[8] The method for recovering rare earth elements according to [6] or [7], characterized in that the precipitating agent used in the precipitation step is oxalic acid, tartaric acid, carbonic acid, or a base.

[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 operation is suppressed, making it possible to perform the REE extraction operation 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 the REE separation and concentration process using solvent extraction or ion exchange methods. As a secondary effect, only the acid necessary to dissolve the trace amount of calcium silicate containing REE can be added, which leads to a reduction in acid consumption and contributes to a reduction in the cost of the REE recovery process from blast furnace slag. In addition, compared to conventional technology, it is possible to increase the REE leaching rate while increasing the separation ratio of REE and Si. 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). The leaching rates of other major components of blast furnace slag (Si, Ca, Al, Mg, Fe, etc.) can be determined in the same way. Furthermore, the separation ratio of REE and Si is determined by the following formula (2). The separation ratio of other major components of blast furnace slag (Ca, Al, Mg, Fe, etc.) to REE can also be determined in the same way.

[0021] It is a diagram schematically showing the REE leaching process from crystalline blast furnace slowly cooled slag and amorphous blast furnace granulated slag. It is a diagram showing the time course of the leaching rate in crystalline blast furnace slowly cooled slag. It is a diagram showing the time course of the leaching rate in amorphous blast furnace granulated slag. It is a diagram showing the relationship between the separation ratio of REE and Si and the leaching time. It is a diagram schematically showing the state of gel-like substance precipitation in the acid leaching solution. It is a diagram schematically showing an example of an apparatus for performing an acid leaching operation while keeping the pH constant. It is a diagram in which the REE leaching rate is plotted against the leaching time for each pH. It is a diagram showing the relationship between the separation ratio of REE and Si and the leaching time for each pH. It is a diagram in which the separation ratio of REE and Si is plotted against the leaching pH for each leaching time. It is a diagram showing the consumption amount of acid consumed in the acid leaching step.

[0022] Hereinafter, specific embodiments of the present invention (hereinafter also referred to as "the present embodiment") will be described in detail. However, the present invention is not limited to the following embodiments, and can be appropriately modified and implemented within the scope of not changing the gist of the present invention.

[0023] The present embodiment is characterized by including an acid leaching step of bringing crystalline blast furnace slag into contact with an acid for a specific 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 generated as a by-product in the steel manufacturing process. Typically, in a blast furnace for producing molten pig iron, components other than iron contained in iron ore and ash in by-products such as limestone and coke are melted, separated, and recovered together.

[0025] Blast furnace slag has lime (CaO) and silica (SiO ２ ) as main components. As other components, it can contain alumina (Al ２ O ３ ), magnesium oxide (MgO), and a small amount of sulfur (S). Note that blast furnace slag may contain iron oxide (FeO), and its content may be generally 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 the term "rare earth element" as 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) Blast furnace slag can be broadly classified into two types: amorphous blast furnace granulated slag, which is obtained by pulverizing molten slag by spraying water onto it, and crystalline blast furnace slowly cooled slag, which is 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. In order to change to a crystalline state, the cooling rate (for example, less than 35°C / sec) up to about 900°C, when the slag solidifies from a molten state, may be appropriately adjusted. If necessary, the slag may be kept warm or reheated.

[0029] Furthermore, whether the slag is crystalline or amorphous can be determined by microscopy using a polarizing microscope. More specifically, the particle size is adjusted by drying the sample of steel slag or steel slag powder of interest, followed 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, count the total number of particles under open nicols and count the number of crystalline particles under crossed nicols. Perform this operation with approximately 50 particles per field of view, and continue the measurement until the total number of counted particles reaches 500 or more. After that, the amorphous ratio can be calculated from the counted total number of particles and the number of crystalline particles based on the following formula. Generally, a material with an amorphous ratio of 10% or less can be called "crystalline". Amorphous ratio (%) = {(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 a case, the amount of amorphous in the mass of the sample used for measurement becomes the vitrification rate (mass %). For the sample used for measurement, a standard substance is added at a predetermined ratio, and then measurement and analysis by the X-ray diffraction method are carried out. When the X-ray diffraction method is carried out, the amorphous is not observed as a distinct peak, but is observed as a behavior that pushes up the background in the obtained X-ray diffraction spectrum. Therefore, by focusing on the ratio decreased from the quantitative value that should originally be obtained with respect to the addition amount of the standard substance, it is possible to specify the amount of amorphous. In this case, those with an amorphous amount of 10% or less of the whole can be referred to as "crystalline".

[0034] When blast furnace slag is slowly cooled and crystallized, the main crystal constituting the crystalline blast furnace slowly cooled slag is merlite (2CaO·Al ２ O ３ ·SiO ２ -2CaO·MgO·2SiO ２ solid solution), but slightly contains calcium silicate-based mineral crystalline phases such as dicalcium silicate (2CaO·SiO ２ ) and monocalcium silicate (CaO·SiO ２ ). Furthermore, the inventors have found that REE is concentrated in dicalcium silicate (2CaO·SiO ２ ) and monocalcium silicate (CaO·SiO ２ ). Note that concentrating or enriching a certain element containing REE means that the concentration in the enrichment phase is higher than the concentration at locations other than the enrichment phase in the slag. Also, the types and abundance ratios of these crystal phases and the elements constituting them are confirmed by combining observation with an electron microscope and analysis with an energy dispersive X-ray spectrometer.

[0035] (Acid leaching step) In the acid leaching step, an acid (acidic solvent) is brought into contact with blast furnace slag. By bringing blast furnace slag into contact with an acid, at least a part of the blast furnace slag dissolves in the acid and leaches into the acid. A liquid containing the components leached from the blast furnace slag (this also includes rare earth elements) is obtained, and this is referred to as a rare earth element-containing leachate.

[0036] The solubility of a substance (solute) in an acid varies depending on the type of the substance (solute). Crystalline minerals of calcium silicate enriched with REE are easily soluble in an acid, while merlite, which is the main mineral phase of blast furnace slowly cooled slag, is hardly soluble in an acid. The present inventors have also found this fact.

[0037] Immediately after the start of the acid leaching step, the dissolution of calcium silicate enriched with easily soluble REE proceeds (relatively), while the dissolution of merlite, which is the main mineral phase of blast furnace slowly cooled slag and is hardly soluble, does not proceed (relatively), but rather is suppressed. In other words, relatively, the leaching rate of REE is high, and the leaching of Si, which is the main component of blast furnace slag, is suppressed.

[0038] Generally, the longer the contact time between the acid and the blast furnace slag, the more the dissolution or leaching of the slag components proceeds, and the higher the leaching rate of REE. However, the leaching of Si also proceeds. Also, the leaching rate of REE may reach saturation.

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

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

[0041] (Solid-liquid separation step) Further, this embodiment is characterized by including a solid-liquid separation step of separating solid components 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 shall be between 1 minute and 20 minutes. If it is less than 1 minute, the leaching rate of REE may not be sufficient. If it is more than 20 minutes, the leaching of Si may be excessive, or the leaching rate of REE may become saturated. Within the range of 1 minute to 20 minutes, it may be adjusted as appropriate according to the desired REE leaching rate and the degree of suppression of Si leaching. 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 the 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 the 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 leaching solution obtained by contacting the 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 leaching solution.

[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 one 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 where the leachate temperature is 15 to 35°C and the pressure is 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. Also, by performing acid leaching for a short time, it is possible to suppress the leaching of Si, which is 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 less, and preferably to 1.5 or less.

[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 shown in the following formula (Equation 1), can be set to, for example, 40% or more.

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

[0056] The pH of the leachate (leachate containing rare earth elements) and the contact time between the acid and the blast furnace slag may be adjusted as appropriate. This allows for obtaining the desired REE leaching rate and degree of Si leaching suppression.

[0057] In the aforementioned solid-liquid separation process, the purity of the rare earth elements can be further increased by subjecting the rare earth element-containing leachate, from which the solid components have been separated, to an optional purification process. Optional purification processes include pH adjustment, extraction (which may include solid-phase extraction and / or solvent extraction), precipitation, and roasting. Illustrative methods for each process are described below.

[0058] <pH Adjustment Step> Although not an essential feature, a pH-adjusted rare earth element-containing leachate can be obtained by adding a base or acid to the rare earth element-containing leachate obtained after the solid-liquid separation step to adjust the pH. This is because setting the pH of the leachate between 0.5 and 8 improves the adsorption rate of rare earth elements to the solid phase in the solid-phase extraction step described later, and enables separation from the main components of blast furnace slag such as calcium and magnesium. <Extraction Step> The extraction step is a step to obtain a rare earth element concentrate by extracting rare earth elements from the rare earth element-containing leachate after the solid-liquid separation step, for example. The extraction step preferably includes one or both of 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 using both the solid-phase extraction process (M1) and the solvent extraction process (M2), the order in which the rare earth element-containing leachate is processed after the solid-liquid separation process does not matter. When using only one of the extraction processes, the equipment can be simplified and rare earth elements can be recovered at a lower cost, but the purity of the obtained rare earth elements will be lower compared to when both the solid-phase extraction process (M1) and the solvent extraction process (M2) are used. On the other hand, when rare earth elements are recovered using both the solid-phase extraction process (M1) and the solvent extraction process (M2), the equipment can be recovered with high purity, but the equipment will be larger and the equipment cost will be higher compared to when only one of the two processes (solid-phase extraction process (M1) or solvent extraction process (M2) is used). The advantages and disadvantages above should be considered in order to suit 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 a solid-phase extraction process (M1) and a solvent extraction process (M2) is preferable.

[0059] <Solid-phase extraction process (M1)> The solid-phase extraction process (M1) is a process included in the extraction process, and for example, it includes a rare-earth element adsorption process (sometimes referred to as M11), a cation removal process (sometimes referred to as M12), and a rare-earth element elution process (sometimes referred to as M13). If necessary, a solid-phase washing process (sometimes referred to as M14) can be carried out in the solid-phase extraction process (M1). In the rare-earth element adsorption process (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 process (M12), for example, the cation-adsorbed solid phase may be brought into contact with an inorganic acid of less than 0.3 N 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 Step (M2)> The solvent extraction step (M2) is a step included in the extraction step, and for example, includes an extraction step (sometimes called M21, which is an extraction step performed before the back extraction step described below) and a back extraction step (sometimes called M22, which is an extraction step performed following the extraction step (M21) described above). In the extraction step (M21), for example, an organic solvent containing a solvent extractant and a rare earth element-containing extract may be mixed 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 process is a process in which a precipitating agent is added to the rare earth element concentrate obtained through the extraction process, 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 resulting 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 in which the rare earth element precipitate obtained in the precipitation process is roasted 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 two 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.) for increasing the purity of the rare earth elements.

[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) 3 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 was 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 leached solution (leached solution 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 slowly cooled blast furnace slag had a crystalline content of over 90%, while no crystalline peaks were observed in the granulated blast furnace slag.

[0071]

[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 (slow-cooled blast furnace slag) and amorphous blast furnace slag (granulated blast furnace slag). In this example and comparative example, the solid-liquid separation step to separate solids from the REE-containing leachate is 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 leaching solution. Table 2 shows the leaching rate of Si (A) and the leaching rate of 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]

[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 a raw material and an acid leaching operation was performed for one minute, the leaching of Si, which is the main cause of precipitates, was suppressed to about one-third. 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 a leaching time (contact time) of 1 minute, and another made with amorphous blast furnace slag and a 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 precipitates 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 precipitates were generated, hindering filtration and making the filtration operation extremely difficult.

[0080] (Example 2) The effect of controlling the amount of additional acid added while monitoring the pH of the rare earth element-containing leachate obtained by contacting blast furnace slag with acid was verified to keep the pH of the rare earth element-containing leachate constant.

[0081] Specifically, 8 g of crystalline blast furnace slag (slow-cooled blast furnace slag) shown in Table 1 was pulverized 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 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 a constant value of 3.0, 2.0, 1.0, and 0.5.

[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 and 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 in terms of 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).

[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 consumed 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, comprising: an acid leaching step of bringing the blast furnace slag into contact with an acid to obtain a rare earth element-containing leachate containing rare earth elements leached from the blast furnace slag, wherein the blast furnace slag is crystalline, and the method comprises: a solid-liquid separation step of separating solid components from the rare earth element-containing leachate, wherein the contact time between the blast furnace slag and the acid is 1 minute or more and 20 minutes or less.

2. The 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 obtained by contacting the blast furnace slag with the acid, thereby maintaining a constant pH of the rare earth element-containing leaching.

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 method for recovering rare earth elements according to claim 1, further comprising: 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 of treating the pH-adjusted rare earth element-containing leachate by solvent extraction and solid-phase extraction, or both, 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; and a roasting step of roasting the rare earth element precipitate to obtain an oxide of a rare earth element.

7. The method for recovering rare earth elements according to claim 6, characterized in that the solvent extractant used in the extraction step is an amine-based, organophosphoric acid-based, or carboxylic acid-based extractant, and the solid-phase extractant used in the extraction step is a resin having iminodiacetic acid as a functional group.

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

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