Scandium recovery method

Abstract

The purpose of the present invention is to provide a method for recovering rare earth elements, whereby, when recovering scandium from steel slag, scandium is recovered from the steel slag and, at the same time, scandium is roughly separated from rare earth elements other than the scandium. The method is characterized in that the scandium-containing rare earth element is leached from the steel slag multiple times in different pH ranges. Further, the method is characterized in that scandium is efficiently recovered by combining the leachate obtained by the method with an element separation step.

Description

Technical Field

[0001] The present invention relates to a method for recovering scandium (Sc), and more specifically, to a method for recovering scandium from blast furnace slag, steelmaking slag or electric furnace slag while also roughly separating scandium from other rare earth elements (sometimes also referred to as REE). Background Art

[0002] Scandium is used as an additive material for various materials such as high-strength alloys and electrode materials for fuel cells, and is an element with high industrial value. In addition, the burial area of ​​scandium ore is uneven, and its supply is small worldwide, and scandium has a high rarity value. However, scandium has a problem of drastic price fluctuations due to changes in social conditions. Therefore, it is important to establish a method for stable and large-scale supply of scandium in the development of the industry.

[0003] However, the production of steel accounts for the majority of the total metal production, and the production of steel slag produced as a by-product is also very large. For example, Japan produces about 100 million tons of crude steel each year, and as its by-products, blast furnace slag is produced about 23 million tons per year, steelmaking slag is produced about 12 million tons per year, and electric furnace slag is produced about 3 million tons per year. Iron ore, coal, limestone and iron waste, which are the raw materials of these steel slags, contain trace amounts of rare earth elements including scandium, which are contained in the steel slag through the ironmaking process.

[0004] Various studies have been conducted so far in order to efficiently obtain scandium.

[0005] In Patent Document 1, for the purpose of recovering scandium with high grade and high efficiency, it is proposed to use nickel steel oxide as the starting material, separate the nickel, aluminum, chromium and scandium contained in the raw material, and then further perform separation operations such as extraction, back extraction, precipitation, drying and roasting from the separated Sc eluate to obtain oxidized Sc.

[0006] Patent Document 2 aims to separate and recover the precious metal elements and rare earth elements containing scandium contained in the fly ash, and proposes separating the precious metals in a first process and separating the rare earth elements in a second process.

[0007] Patent document 3 aims to recover rare earth metals containing scandium from tin slag discharged from the tin refining process, and proposes a method specifically for separating components other than rare earth metals contained in the tin slag, namely radioactive substances, iron, aluminum and other metals, to ultimately obtain salts or oxides of rare earth metals.

[0008] Patent Document 4 discloses a method for recovering rare earth elements containing scandium from steel slag by magnetic separation and wet refining. The rare earth element leaching method described in Patent Document 4 utilizes salt water, a chelating agent, 0.1 mol / L hydrochloric acid or nitric acid, and the like.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent No. 5652503

[0012] Patent Document 2: Japanese Patent No. 6159731

[0013] Patent Document 3: Japanese Patent No. 5825074

[0014] Patent Document 4: Japanese Patent Application Publication No. 2018-530673 Summary of the invention

[0015] Problem that the invention aims to solve

[0016] Iron and steel slag contains trace amounts of rare earth elements including scandium, but the main components of iron and steel slag are silicon (Si), iron (Fe), aluminum (Al), magnesium (Mg), calcium (Ca), etc. It is necessary to establish a process that can separate these main component elements and recover scandium.

[0017] In addition, as described in detail later, rare earth elements include 17 elements including scandium, and it is required to separate scandium from rare earth elements other than scandium. Scandium can be separated / purified by utilizing the difference in chemical properties of each rare earth element, but from the perspective of industrial production management including cost, it is very important to make the separation / purification process efficient.

[0018] That is, steel slag contains scandium, which is industrially valuable, but also contains rare earth elements other than Sc, which are elements that become main components of slag. It is desired to efficiently separate and recover only scandium from steel slag.

[0019] Patent Documents 1 to 3 relate to the separation of rare earth elements containing scandium, but the starting raw material is not steel slag. The raw materials and steel slag used in these prior arts have different acid leaching characteristics, types of main component elements to be separated, and concentrations. Therefore, it is difficult to efficiently recover rare earth elements, especially scandium, from steel slag using these prior arts.

[0020] In addition, the purpose of Patent Document 4 is to separate not only rare earth elements containing scandium but also metal salts other than rare earth elements from steel slag, but the specific process conditions cannot be said to be optimal for this purpose. Specifically, in the leaching process from steel slag, only "salt water, chelating agent, acid (0.1 mol / L HCl or HNO3)" is used, but steel slag has alkalinity and the degree of its alkalinity also changes. Therefore, at a fixed acid concentration (0.1 mol / L), there is a problem that the leaching efficiency varies greatly depending on the amount ratio of steel slag to acid.

[0021] As described above, it cannot be said that the method of recovering scandium from steel slag industrially is sufficiently established, and a more appropriate method is required.

[0022] An object of the present invention is to provide a method for recovering scandium which recovers scandium from steel slag and also performs rough separation of scandium from other rare earth elements.

[0023] Solutions for solving problems

[0024] The inventors of the present invention have repeatedly conducted in-depth research to solve the above-mentioned problems. As a result, it was found that the scandium contained in the steel slag can be leached with acid while the pH is controlled, and the dissolution behavior of scandium and rare earth elements other than scandium is different depending on the pH range of the leaching. Taking advantage of this, it was thought that by leaching scandium from the steel slag multiple times within different pH ranges, it is possible to recover scandium from the steel slag while performing a rough separation of scandium and rare earth elements other than scandium, thereby completing the present invention. It should be noted that, in general, it is known that there are differences in the solubility of elements, which also depends on pH, but in the leaching of steel slag with CaO and SiO2 as the main components using acid, it is not clear that there are differences in leaching rates between rare earth elements by controlling to a specified pH.

[0025] Furthermore, the scandium-containing leachate obtained in the above-mentioned leaching step is provided to an extraction step including one or both of a solid phase extraction step and a solvent extraction step, and scandium is separated from the main components of steel slag such as iron, aluminum, magnesium, calcium, silicon, etc. to produce a scandium concentrate. A precipitate of scandium is produced in a precipitation step of adding a precipitant thereto, and the precipitate is subjected to a calcination step of calcining to obtain a scandium oxide, thereby recovering high-purity scandium.

[0026] That is, the present invention includes the following aspects. (1)

[0028] A method for recovering scandium, characterized in that it is a method for recovering scandium (Sc) from steel slag generated by an ironmaking process, comprising the following steps:

[0029] Leaching step 1, adjusting the pH of a leachate obtained by contacting a first acid with the steel slag to a first pH range to obtain a first leachate;

[0030] a first solid-liquid separation step of separating solid components from the first leachate; and

[0031] In a leaching step 2, the pH of the leaching solution obtained by contacting the solid component separated in the first solid-liquid separation step with a second acid is adjusted to a second pH range lower than that in the leaching step 1 to obtain a second leaching solution,

[0032] Here, the first acid and the second acid include nitric acid or sulfuric acid,

[0033] The first pH range is greater than 1.1 and less than 6.7, and the second pH range is greater than -1.0. (2)

[0035] The method for recovering scandium according to (1), wherein in the leaching step 1 and / or the leaching step 2, an acid or a base is added to adjust the pH while monitoring the pH of the first leachate and / or the second leachate. (3)

[0037] The method for recovering scandium according to (1) or (2), characterized in that the second pH range in the leaching step 2 is greater than or equal to -0.5 and less than or equal to 3.3. (4)

[0039] The method for recovering scandium according to any one of (1) to (3), wherein sulfuric acid is used as the first acid. (5)

[0041] The method for recovering scandium according to (4), characterized in that the final pH of the leaching step 1 is 3.6 or more and less than 6.7. (6)

[0043] The method for recovering scandium according to (4), characterized in that the final pH of the leaching step 1 is 3.6 or more and 5.3 or less. (7)

[0045] The method for recovering scandium according to any one of (1) to (3), wherein nitric acid is used as the first acid. (8)

[0047] The method for recovering scandium according to (7), characterized in that the final pH of the leaching step 1 is 3.4 or more and less than 6.0. (9)

[0049] The method for recovering scandium according to (7), characterized in that the final pH of the leaching step 1 is 3.4 or more and 4.0 or less. (10)

[0051] The method for recovering scandium according to any one of (1) to (9), comprising the following steps:

[0052] A step of adding alkali or acid to the first leachate and / or the second leachate to adjust the pH to obtain a first pH-adjusted leachate and / or a second pH-adjusted leachate; and

[0053] The extraction step is to process the first pH-adjusted leachate and / or the second pH-adjusted leachate by either or both of solvent extraction and solid phase extraction to obtain a first element concentrated solution and / or a second element concentrated solution. (11)

[0055] The method for recovering scandium according to (10) comprises: a precipitation step of adding a precipitant to the first element concentrated solution and / or the second element concentrated solution to obtain a first precipitate and / or a second precipitate. (12)

[0057] The method for recovering scandium according to (11) comprises: a calcination step of calcining the first precipitate and / or the second precipitate to obtain the first oxide and / or the second oxide. (13)

[0059] The method for recovering scandium according to (12), comprising: a drying step of drying the first precipitate and / or the second precipitate, respectively. (14)

[0061] The method for recovering scandium according to any one of (10) to (13), wherein the solvent extractant used in the extraction step is any one of an amine-based, organic phosphoric acid-based and carboxylic acid-based extractant. (15)

[0063] The method for recovering scandium according to any one of (10) to (14), characterized in that the solid phase extractant used in the extraction step is a resin having iminodiacetic acid as a functional group. (16)

[0065] The method for recovering scandium according to any one of (10) to (15), characterized in that the precipitant used in the precipitation step is any one of oxalic acid, carbonic acid, tartaric acid or alkali. (17)

[0067] The method for recovering scandium according to any one of (1) to (16), wherein the steel slag is blast furnace slag. (18)

[0069] The method for recovering scandium according to any one of (1) to (17), wherein scandium is concentrated relative to rare earth elements other than scandium in the second leachate.

[0070] Effects of the Invention

[0071] According to the present invention, a method for recovering scandium in steel slag is provided, which can recover scandium from steel slag while roughly separating scandium from other rare earth elements. By controlling the pH in multiple leaching steps, it is possible to control the leaching of different rare earth elements (scandium and other rare earth elements), thereby reducing the burden of the purification step following the leaching step. Furthermore, by selecting the pH of the leachate from a specific range, the recovery rate of scandium from steel slag and / or the rough separation rate (purification rate) of scandium from other rare earth elements can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a flow chart illustrating an example of a method for recovering rare earth elements.

[0073] Figure 2 This is a flow chart illustrating an example of a solid phase extraction step.

[0074] Figure 3 This is a diagram showing an example of the relationship between the pH of the leachate and the leaching rate of the rare earth element.

[0075] Figure 4 This is a graph showing the particle size distribution of steel slag used in Examples.

[0076] Figure 5 It is a diagram schematically showing the leaching process of Examples and Comparative Examples.

[0077] Figure 6 The diagram shows the steps of a method for confirming the effect of separating scandium from other rare earth elements. DETAILED DESCRIPTION

[0078] Hereinafter, a specific embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail with reference to the accompanying drawings, but the present invention is not limited to the following embodiments and can be implemented by adding appropriate changes within the scope of the gist of the present invention.

[0079] There are multiple definitions of elements included in the term rare earth elements, but the definition of the term rare earth elements shown in the present disclosure is set to include a group of elements including lanthanide elements (Ln) including 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) and adding yttrium (Y) and scandium (Sc) to a total of 17 elements.

[0080] The definition of iron and steel slag described in the present disclosure is a term that includes blast furnace slag produced in an ironmaking process, steelmaking slag produced in a steelmaking process, and electric furnace slag produced in an electric furnace.

[0081] Figure 1 and Figure 2 This is a flow chart for explaining an exemplary method for recovering scandium. In the method for recovering scandium, the leachate obtained by leaching steel slag with an acid such as sulfuric acid, nitric acid, or hydrochloric acid is introduced into a purification step to separate the elements such as aluminum, magnesium, calcium, and silicon that are the main components of the steel slag from the rare earth elements other than scandium, thereby recovering scandium.

[0082] The first feature of the present invention is that scandium is recovered from steel slag by leaching scandium from steel slag multiple times in different pH ranges, thereby performing rough separation of scandium from rare earth elements other than scandium. Typically, in leaching step 1, acid is brought into contact with steel slag, and the pH of the obtained leachate is controlled, thereby leaching at least a part of rare earth elements (mainly rare earth elements other than scandium). In leaching step 2, acid is brought into contact with solid components (residues) produced in leaching step 1, and the pH of the obtained leachate is controlled in a pH range different from that of leaching step 1 (lower than the pH of leaching step 1), thereby leaching rare earth elements different from the rare earth elements leached in leaching step 1 (mainly scandium). Thus, scandium can be recovered while performing rough separation of scandium from rare earth elements other than scandium. It should be noted that, here, "mainly leached" means that one of scandium and rare earth elements other than scandium is leached at a higher leaching rate than the other.

[0083] Although not necessarily required, a second feature of the present invention is that the elements (typically scandium and other rare earth elements) in the leachate obtained in the leaching step 1 and the leaching step 2 are purified in extraction steps M and M' including either or both of the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2'.

[0084] By using the method of the present invention, rare earth elements containing scandium can be leached from steel slag. Therefore, rare earth elements containing scandium in steel slag can be recovered, and rare earth elements containing high purity scandium can be recovered even from raw materials such as steel slag containing a large amount of impurities. In addition, in leaching process 1 and leaching process 2, scandium and other rare earth elements can be roughly separated at the same time. Thus, the burden of the purification process can be reduced, and rare earth elements can be recovered at a lower cost. In particular, Sc, which has a high industrial value among rare earth elements, can also be roughly separated in the leaching process as the initial stage of the overall rare earth element recovery process.

[0085] It should be noted that, although not necessarily required, the steel slag may be crushed to increase the surface area before the leaching step K. Alternatively, fine-particle pulverized slag with a large surface area may be produced by pulverizing the molten steel slag when it is cooled. By increasing the surface area of ​​the steel slag, the leaching rate of the scandium-containing rare earth elements in the steel slag can be increased.

[0086] <Leaching Step 1, K>

[0087] Leaching process 1( Figure 1 In the process of the present invention, the first acid is brought into contact with the steel slag, and the first acid is brought into contact with the steel slag, wherein the first acid is brought into contact with the steel slag, and the pH of the leachate obtained by the first acid is adjusted to a first pH range to obtain a first leachate. The pH adjustment can be implemented by controlling the additional amount of acid or alkali added while monitoring the pH of the leachate obtained by bringing the acid into contact with the steel slag. As an acid, inorganic acids and organic acids can be listed, preferably inorganic acids. As specific examples of inorganic acids, sulfuric acid, nitric acid, and hydrochloric acid can be listed. From the viewpoint of the leaching rate of rare earth elements, hydrochloric acid and nitric acid are preferred. From the viewpoint of the separation ratio of rare earth elements, typically the separation ratio of Sc and rare earth elements other than Sc, sulfuric acid and nitric acid are preferred. Figure 3 It is a graph showing the relationship between the final pH of the leachate and the leaching rate of the rare earth elements, and the rare earth elements are plotted as Sc and other than Sc. As the leachate, the graphs of three kinds of mixed acids (nitric acid volume: sulfuric acid volume = 9:1, 5:5, 1:9) are shown. It should be noted that Tables 1 to 5 are tables that extract the numerical data in each figure.

[0088] [Table 1]

[0089] Table 1 Leaching rate data when nitric acid was used

[0090]

[0091] [Table 2]

[0092] Table 2 Leaching rate data when sulfuric acid was used

[0093]

[0094] [Table 3]

[0095] Table 3 Leaching rate data when using mixed acid (9 nitric acid: 1 sulfuric acid)

[0096]

[0097] [Table 4]

[0098] Table 4 Leaching rate data when mixed acid (5 nitric acid: 5 sulfuric acid) is used

[0099]

[0100] [Table 5]

[0101] Table 5 Leaching rate data when using mixed acid (nitric acid 1: sulfuric acid 9)

[0102]

[0103] based on Figure 3 Based on the data in Figures 1 to 5 and Tables 1 to 5, the inventors of this case obtained the following insights. First, regardless of the type of acid, i.e., nitric acid, sulfuric acid, and their mixed acids, they all showed roughly the same behavior. Specifically, in the range of pH approximately less than 6.7, the difference in leaching rate between Sc and rare earth elements other than Sc began to increase, and the difference in leaching rate was confirmed in the range of pH approximately above 1.1. Furthermore, when the pH is in the range of 1.1 or more and less than 6.6, the difference in leaching rate between Sc and rare earth elements other than Sc is approximately 0.1% or more, which is preferred. In addition, when the pH is in the range of 3.0 or more and 5.3 or less, the difference in leaching rate is approximately 5.0% or more, which is more preferred.

[0104] Based on these insights, by controlling the pH of the leachate, at least a portion of the rare earth elements with similar chemical properties, for example, only Sc, can be left in the solid phase (residue), while other rare earth elements, for example, rare earth elements other than Sc, can be leached into the liquid phase (solvent). From the perspective of obtaining a high leaching rate of rare earth elements, the acid is preferably a strong acid. From the same perspective, the pK of the acid is a Preferably, the pH value is -2 or less. One or more acids may be used. The concentration of the acid may be appropriately selected within the range in which the desired pH value can be obtained, and is not particularly limited. In consideration of availability and operability, the acid concentration of nitric acid may be 0.1 mol / L or more and 17.1 mol / L or less, and the acid concentration of sulfuric acid may be 0.1 mol / L or more and 18.7 mol / L or less.

[0105] In this embodiment, the first acid includes nitric acid or sulfuric acid, and the first pH range is greater than 1.1 and less than 6.7. The first pH range can be appropriately adjusted to obtain a desired difference in leaching rate. Specific values ​​are described later.

[0106] The following example illustrates a specific method for performing the leaching step 1 (K). Iron and steel slag is placed in a leaching reaction container and a first acid is added. At this time, the solid-to-liquid ratio (mass ratio) of the iron and steel slag to the acid is preferably 1:1 to 1:100. The pH of the added acid may be 1.0 or less. The iron and steel slag contains 10 to 70% by mass of oxides and hydroxides of calcium and magnesium, so it is expected that they will be neutralized by them, and the amount of acid added initially and the pH may be determined. As a method for monitoring pH, there can be cited a method in which a pH sensor is immersed in the leachate obtained by contacting the acid with the iron and steel slag, and the pH of the leachate is monitored at any time. In addition, it is preferred to perform appropriate stirring, etc. to make the pH of the leachate uniform. In addition, rare earth elements can be fully leached under the conditions of a leachate temperature of 15 to 35°C and a pressure of about 1013 hPa, but in order to increase the leaching rate, rare earth elements may also be leached while heating or pressurizing.

[0107] When the pH of the leachate is higher than the target final pH, the acid can be added in small amounts each time while monitoring the pH value to adjust to the final pH. On the other hand, when the pH of the leachate is lower than the target final pH, the iron slag or alkaline substance can be added in small amounts each time while monitoring the pH value to adjust to the final pH. It should be noted that the final pH of the leachate in the present disclosure refers to the pH at the moment when the rate of change of the pH after the addition of the acid or alkali reaches 1 / minute or less. The alkaline substance is not particularly limited, and sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, etc. can be listed.

[0108] The final pH of the leaching step 1 (K) can be appropriately determined according to the purpose, and can be selected from the first pH range, for example, from a range consisting of any combination of an upper limit of less than 7.0, 6.0, 5.5, 5.0, 4.5 or 4.0 and a lower limit of 0.0, 1.0, 1.5, 2.0, 2.5 or 3.0. From the viewpoint of separation of scandium from other rare earth elements, the final pH can be appropriately adjusted according to the type of the first acid.

[0109] In addition, if Figure 3As shown, regardless of the type of acid, i.e., nitric acid, sulfuric acid, and their mixed acids, the separation behavior of scandium and other rare earth elements is roughly the same. Therefore, nitric acid, sulfuric acid, and their mixed acids can be used to make the pH range greater than 1.1 and less than 6.7. Within this pH range, the leaching rates of rare earth elements other than Sc are different, and only Sc can be left in the solid phase (residue), and rare earth elements other than Sc can be leached into the liquid phase (solvent). Furthermore, the pH is preferably in the range of greater than 1.1 and less than 6.6. This pH range can make the difference in leaching rate between Sc and rare earth elements other than Sc approximately 0.1% or more. In addition, the pH is further preferably in the range of greater than 3.0 and less than 5.3. This pH range can make the difference in leaching rate approximately 5.0% or more. When nitric acid is used as the first acid, the pH can be greater than 1.2 and less than 6.0, and within this pH range, the difference in leaching rate between Sc and rare earth elements other than Sc can be approximately 1.0% or more. In addition, the pH is more preferably in the range of greater than 3.4 and less than 4.0. Within this range, the difference in leaching rate between Sc and the rare earth element other than Sc can be made approximately 10.0% or more.

[0110] When sulfuric acid is used as the first acid, the pH may be in the range of 1.1 or more and less than 6.7. In this pH range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 1.0% or more. In addition, the pH is more preferably in the range of 3.6 or more and 5.3 or less. In this range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 10.0% or more.

[0111] When a mixed acid (nitric acid 9: sulfuric acid 1) is used as the first acid, the pH may be 1.2 or more and less than 6.0. Within this pH range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 1.0% or more. In addition, the pH is more preferably in the range of 3.4 or more and 4.0 or less. Within this range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 10.0% or more.

[0112] When a mixed acid (5 nitric acid: 5 sulfuric acid) is used as the first acid, the pH may be 1.2 or more and less than 6.3. Within this pH range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 1.0% or more. In addition, the pH is more preferably in the range of 3.4 or more and 4.0 or less. Within this range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 10.0% or more.

[0113] When a mixed acid (nitric acid 1: sulfuric acid 9) is used as the first acid, the pH may be 1.1 or more and less than 6.6. Within this pH range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 1.0% or more. In addition, the pH is more preferably in the range of 3.6 or more and 5.3 or less. Within this range, the difference in leaching rate between Sc and the rare earth elements other than Sc may be approximately 10.0% or more.

[0114] The mixing ratio of nitric acid to sulfuric acid can be varied depending on the desired degree of separation, and the range of the final pH can be further limited.

[0115] In one embodiment, from the viewpoint of improving the leaching rate of rare earth elements, the final pH is usually preferably low and can be selected from less than 7.0, 6.7 or less, 6.5 or less, 6.0 or less, 5.5 or less, and 5.0 or less. The final pH can be appropriately selected so that the leaching rate of the rare earth elements shown in the following (Formula 1) is within the desired range.

[0116] [Leaching rate of rare earth elements (mass %)] = [leached rare earth elements (μg)] ÷ [rare earth elements in steel slag (μg)] × 100 (Formula 1)

[0117] It should be noted that, in Formula 1, when only focusing on the leaching step 1, the scandium separated in the residue is excluded for consideration. Alternatively, when calculating the leaching rate including the scandium separated in the residue, the leaching amount in the leaching step 2 described later and the content contained in the steel slag are considered.

[0118] <Leaching Step 2, K2>

[0119] Leaching process 2( Figure 1 In the leaching step 2 (wherein K2 is K2), the residue produced in the leaching step 1 is immersed in an acid, and the rare earth elements (mainly scandium) not leached in the leaching step 1 are leached into the solvent. In more detail, the pH of the leachate obtained by contacting the second acid with the residue produced in the leaching step 1 is adjusted to a second pH range lower than that in the leaching step 1, to obtain a second leachate. In the leaching step 2, by adjusting to achieve a second pH range lower than the first pH range in the leaching step 1, the rare earth elements (mainly scandium) not leached in the leaching step 1 can be leached. The pH adjustment can be implemented by controlling the additional amount of acid or alkali added while monitoring the pH of the leachate obtained by contacting the acid with the residue. As the acid, inorganic acids and organic acids can be listed, preferably inorganic acids. Specific examples of inorganic acids include sulfuric acid, nitric acid, and hydrochloric acid. From the viewpoint of obtaining a high leaching rate of rare earth elements, the acid is preferably a strong acid. From the same viewpoint, the pK of the acid is preferably 0.04. aPreferably, the pH value is -2 or less. One or more acids may be used. The concentration of the acid may be appropriately selected within the range in which the desired pH value can be obtained, and is not particularly limited. In consideration of availability and operability, the acid concentration of nitric acid may be 0.1 mol / L or more and 17.1 mol / L or less, and the acid concentration of sulfuric acid may be 0.1 mol / L or more and 18.7 mol / L or less.

[0120] In this embodiment, the second acid includes nitric acid or sulfuric acid, and the second pH range is above -1.0. In addition, the second pH range is lower than the pH of the leachate in the leaching step 1, that is, lower than the final pH of the leachate in the leaching step 1. The second pH range can be appropriately adjusted to obtain a desired leaching rate. The specific values ​​are described later.

[0121] The following example illustrates a specific method for performing the leaching step 2 (K2). The residue produced in the leaching step 1 is placed in a leaching reaction container, and a second acid is added. At this time, the solid-liquid ratio (mass ratio) of the residue to the acid is preferably 1:1 to 1:100. The pH of the added acid can be 1.0 or less. The residue sometimes contains oxides and hydroxides of calcium and magnesium contained in the steel slag, which are expected to be neutralized by them, and the amount and pH of the acid added initially can be determined. As a method for monitoring pH, a method of immersing a pH sensor in the leachate obtained by contacting the acid with the steel slag and monitoring the pH of the leachate at any time can be listed. In addition, it is preferred to stir appropriately to make the pH of the leachate uniform. In addition, rare earth elements can be fully leached under the conditions of a leachate temperature of 15 to 35°C and a pressure of about 1013 hPa, but in order to increase the leaching rate, rare earth elements can also be leached while heating or pressurizing.

[0122] When the pH of the leachate is higher than the target final pH, the acid can be added in small amounts each time while monitoring the pH value to adjust to the final pH. On the other hand, when the pH of the leachate is lower than the target final pH, the iron slag or alkaline substance can be added in small amounts each time while monitoring the pH value to adjust to the final pH. It should be noted that the final pH of the leachate in the present disclosure refers to the pH at the moment when the rate of change of the pH after the addition of the acid or alkali reaches 1 / minute or less. The alkaline substance is not particularly limited, and sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, etc. can be listed.

[0123] The final pH of the leaching step 2 (K2) can be appropriately determined depending on the purpose, and can 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 or 4.0 and a lower limit of -1.0, -0.5, 0.0, 1.0, 1.5, 2.0, 2.5 or 3.0.

[0124] In one of the embodiments, from the viewpoint of improving the leaching rate of rare earth elements, generally, the final pH is preferably low and can be selected from less than 7.0, 6.7 or less, 6.5 or less, 6.0 or less, 5.5 or less, 5.0 or less. The leaching rate of scandium in the leaching step 2 can be obtained by the following (Formula 1').

[0125] [Leaching rate of scandium (mass%)] = [Scandium leached (μg)] ÷ [Scandium in the residue generated in leaching step 1 (μg)] × 100 (Formula 1')

[0126] <Solid-liquid separation steps L, L'>

[0127] The solid-liquid separation steps L, L' are steps of separating the solid components (residues) from the first leachate and the second leachate obtained from the aforementioned leaching steps K, K2. In the solid-liquid separation steps L, L', the respective leachates obtained from the leaching steps K, K2 can be physically separated from the leaching residues. As the separation method, known solid-liquid separation devices such as a continuous concentrator, a deep cone concentrator, a layered concentrator, a drum filter, a disc filter, a horizontal belt filter, a filter press, a pressure filter, and centrifugal separation can be used.

[0128] By performing an arbitrarily selected purification step on the first leachate and the second leachate obtained through the leaching steps K, K2 and the solid-liquid separation steps L, L', the purity of the elements (typically rare earth elements other than scandium and scandium) contained in each leachate can be further improved. As the arbitrarily selected purification step, a pH adjustment step, extraction steps M, M', precipitation steps N, N', roasting steps O, O' can be cited. Hereinafter, exemplary methods of each step will be described. It should be noted that unless otherwise specified, in each purification step, "first" means that the first leachate has been treated, and "second" means that the second leachate has been treated.

[0129] <pH adjustment step>

[0130] Although not a necessary method, an alkali or an acid can also be added to the first leachate and the second leachate that have passed through the aforementioned leaching steps K, K2 or the solid-liquid separation steps L, L' to adjust the pH and obtain respective pH-adjusted leachates. This is because by making the pH of the leachate between 0.5 and 8, in the subsequent solid-phase extraction steps M1, M1', the adsorption rate of rare earth elements to the solid phase is improved, and separation from the main components of steel slag such as calcium and magnesium can be achieved.

[0131] <Extraction steps M, M'>

[0132] The extraction steps M and M' are steps for extracting rare earth elements from the leachate after the solid-liquid separation steps L and L', respectively, to obtain an element concentrated solution (typically a rare earth element concentrated solution other than scandium, or a scandium concentrated solution). The extraction steps M and M' preferably include either or both of the solid phase extraction steps M1 and M1' for concentrating the rare earth elements using the principle of solid phase extraction and the solvent extraction steps M2 and M2' for concentrating the rare earth elements using the principle of solvent extraction. When both the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2' are used, the order of treating the rare earth element-containing leachate after the solid-liquid separation step may be either of them performed first. In the aforementioned extraction steps M and M', when only one of the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2' is used, there is an advantage that the equipment can be simplified and rare earth elements can be recovered at low cost, but the purity of the rare earth elements obtained is lower than that of the case where both the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2' are used. On the other hand, when the rare earth elements are recovered using both the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2', there is an advantage that the rare earth elements can be recovered with high purity, but compared with the case where only one of the solid phase extraction steps M1 and M1' and the solvent extraction steps M2 and M2' is used, the equipment becomes larger and the equipment cost becomes higher. The above advantages and disadvantages can be used according to the purpose. For example, when rare earth elements are used in applications where the mixing of some impurities will not be a problem, such as mixed rare earth alloys, one extraction process is sufficient. On the other hand, in applications such as the electronics industry where the purity of the product is particularly important, an extraction step combining the solid phase extraction step M1 or M1' and the solvent extraction step M2 or M2' is preferred.

[0133] <Solid Phase Extraction Steps M1, M1′>

[0134] The solid phase extraction steps M1 and M1' are steps included in the extraction steps M and M', respectively, and are steps including, for example, rare earth element adsorption steps M11 and M11', cation removal steps M12 and M12', and rare earth element elution steps M13 and M13', respectively. As required, in the solid phase extraction steps M1 and M1', solid phase washing steps M14 and M14' may be respectively implemented. In the rare earth element adsorption steps M11 and M11', for example, the leachate may be brought into contact with a solid phase extractant such as a solid phase composed of a resin having iminodiacetic acid as a functional group, so that cations are adsorbed on the solid phase to obtain a cation adsorbed solid phase. Next, in the cation removal steps M12 and M12', for example, the cation adsorbed solid phase may be brought into contact with an inorganic acid of less than 0.3N, so that cations other than rare earth elements are eluted from the solid phase to obtain a rare earth element adsorbed solid phase. In the rare earth element elution steps M13 and M13', for example, the rare earth element adsorption solid phase can be brought into contact with an inorganic acid of 0.3N or more and less than 3N to elute the rare earth element, thereby obtaining a rare earth element solid phase eluate. In the solid phase cleaning steps M14 and M14', for example, an inorganic acid of 3N or more can be brought into contact with the solid phase that has undergone the rare earth element elution steps M13 and M13' to elute impurity cations. It should be noted that in M1', in principle, only the scandium contained in the residue produced in the leaching step 1 is targeted, so that the consumption of the solid phase extractant, cation adsorption solid phase, inorganic acid, etc. used can be suppressed, which is preferred.

[0135] Although not a necessary method, the solid phases that have been through the solid phase washing steps M14 and M14' can be reused as the solid phases of the rare earth element adsorption steps M11 and M11', respectively. By repeatedly using the solid phase, the consumption of the resin having iminodiacetic acid as a functional group can be reduced, which is economically advantageous. However, if the same solid phase is repeatedly used, the resin having iminodiacetic acid as a functional group deteriorates, the performance is reduced, and the separation ability of impurities and rare earth elements is reduced. In this case, by exchanging with a new resin having iminodiacetic acid as a functional group, the performance is restored to the original state.

[0136] <Solvent Extraction Steps M2, M2'>

[0137] The solvent extraction steps M2 and M2' are steps included in the extraction steps M and M', respectively, and are, for example, steps including extraction steps M21 and M21' and back-extraction steps M22 and M22'. In the aforementioned extraction steps M21 and M21', for example, an organic solvent containing a solvent extractant can be mixed with the leaching solution, and the rare earth elements can be distributed in the organic solvent to obtain an organic phase containing rare earth elements. In the aforementioned back-extraction steps M22 and M22', for example, the organic phase containing rare earth elements obtained in the extraction steps M21 and M21' can be mixed with water, and the rare earth elements can be distributed from the organic phase to the aqueous phase for back extraction. In the aforementioned back-extraction steps M22 and M22', pH adjustment can be performed as needed. The pH can be set to a pH suitable for the organic solvent used.

[0138] In the extraction steps M21 and M21' and the back extraction steps M22 and M22', when the organic phase and the aqueous phase are mixed, a well-known solvent extraction apparatus such as a centrifugal extractor and a pulse column may be used.

[0139] The solvent extractant used in the extraction steps M21 and M21' can be a conventionally known solvent extractant, for example, carboxylic acid extractants such as neodecanoic acid, organic phosphoric acid extractants such as di(2-ethylhexyl) phosphate, tributyl phosphate, tetrabutyl methylene diphosphonate, trioctylphosphine oxide, and amine extractants such as triisooctylamine. The solvent extractant can be used without a solvent, or can be used by dissolving in an organic solvent such as kerosene, xylene, toluene, etc. that is not miscible with water. The solvent extractant can be added to the aqueous phase.

[0140] <Precipitation Steps N, N'>

[0141] The precipitation steps N and N' are steps for obtaining a precipitate containing the target element by adding a precipitant to the element concentrate obtained by the extraction steps M and M', respectively. As the precipitant, alkali and acid can be cited. As the alkali, metal-containing alkaline salts such as sodium hydroxide and potassium hydroxide, and organic bases such as tetramethylammonium hydroxide (TMAOH) can be cited. From the viewpoint of obtaining the target element with higher purity, organic bases are preferred. As the acid, acids such as tartaric acid, carbonic acid, and oxalic acid can be cited, and oxalic acid is preferred.

[0142] In the above-mentioned precipitation steps N and N', when oxalic acid is used as the precipitant, there is an advantage that impurities such as uranium, aluminum, and iron contained in the above-mentioned element concentrate can be separated from rare earth elements efficiently, but there is a disadvantage that the separation ability of magnesium and calcium from rare earth elements is low. On the other hand, when alkali is used as the precipitant, there is an advantage that impurities such as uranium, magnesium, and calcium contained in the above-mentioned element concentrate can be separated from rare earth elements efficiently, but there is a disadvantage that the separation ability of iron and aluminum from rare earth elements is low. The impurities contained in the above-mentioned element concentrate vary depending on the composition of the steel slag. In addition, the impurity elements that become problems vary depending on the element to be precipitated, so the precipitant can be used according to the purpose.

[0143] The obtained precipitate and the post-precipitation solution can be physically separated. As a separation method, a known solid-liquid separation device such as a continuous concentrator, a deep cone concentrator, a layered concentrator, a drum filter, a disc filter, a horizontal belt filter, a filter press, a pressure filter, a centrifugal separation, etc. can be used.

[0144] <Calcination Step O, O'>

[0145] The calcination steps O and O' are steps for obtaining oxides of the target element by calcining the precipitates obtained in the above-mentioned precipitation steps N and N', respectively. The calcination steps O and O' may include cleaning steps O1 and O1' and heating steps O2 and O2', respectively. In the cleaning steps O1 and O1', the precipitates obtained in the above-mentioned precipitation steps N and N', respectively, may be cleaned with water to remove impurities. In the heating steps O2 and O2', the precipitates that have undergone the above-mentioned cleaning steps O1 and O1', respectively, may be heated to remove water, and then volatile elements such as carbon, phosphorus, and nitrogen may be gasified and removed while reacting with oxygen to obtain oxides of the target element.

[0146] In the calcination steps O and O', the calcination conditions are not limited, and for example, heating at about 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 calcination can be performed in the same device, so that oxides can be produced industrially with high efficiency.

[0147] Example

[0148] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these descriptions at all.

[0149] (Example 1)

[0150] Before the leaching process, 1 cm 3 ~4cm 3 An example of the particle size distribution of steel slag having a particle size of about 1000 Å is shown in FIG. Figure 4(particle size 0.5 μm or more and 500 μm or less) In addition, the element composition obtained by measuring the steel slag used in the experiment by fluorescent X-ray analysis is shown in Table 6 as an example. It should be noted that in this example, blast furnace slag among steel slags was used as the object.

[0151] [Table 6]

[0152] Table 6 Composition of steel slag (mass %)

[0153] CaO <![CDATA[SiO2]]> <![CDATA[Al2O3]]> MgO T-Fe 42.2 35.2 16.8 5.6 0.2

[0154] Next, as leaching step 1, the iron and steel slag is brought into contact with an acid (nitric acid or sulfuric acid) at a solid-liquid mass ratio of 1 to 100 to leach the rare earth elements in the iron and steel slag. During the leaching, the pH of the leachate is monitored so as to be within the range of 1.1 to 10.2. When the pH is higher than the target pH (in other words, the final pH of the leaching step 1), an acid is added, and when the pH is lower than the target pH, an alkali is added.

[0155] While referring to the schematic diagram of the leaching process of the embodiment and the comparative example, Figure 5 As explained above, in the leaching step 1, a liquid phase is generated in which rare earth elements other than Sc (i.e., Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) are leached mainly. Then, filtration separation is performed to separate the solid phase mainly containing Sc from the liquid phase. The filtrate collected in the leaching step 1 is used as the leachate A ( Figure 5 ).

[0156] Next, as leaching step 2, the solid phase recovered in leaching step 1 is brought into contact with nitric acid or sulfuric acid to leach Sc in the solid phase. (Similar to leaching step 1) During leaching, the pH of the leachate is monitored, and when the pH is higher than the target pH (in other words, the final pH of leaching step 2), an acid is added, and when the pH is lower than the target pH, an alkali is added. Here, the target pH is set to -0.5.

[0157] One side reference Figure 5 As explained above, in the leaching step 2, a liquid phase mainly leached with Sc is generated, and is filtered and separated from the solid phase. The filtrate collected in the leaching step 2 is used as the leachate B ( Figure 5 ).

[0158] In addition, as a comparative example, the extraction of rare earth elements using only one stage of leaching process as the prior art was also carried out. Specifically, the target pH (final pH) of the leaching process 1 was set to a predetermined value (see Table 8), and filtration separation was performed, and the collected filtrate was used as the leaching solution C ( Figure 5 ).

[0159] It should be noted that the conditions other than the above in the leaching process are leaching for 24 hours, room temperature of 25°C, and air pressure of 1013hPa. In addition, the present invention is not limited by these leaching time, temperature, pressure, quality of steel slag, and the amount of acid and alkali added. In particular, it was found in other experiments that the reaction between blast furnace slag and acid was completed within a few minutes.

[0160] The obtained leachates A, B and C were respectively provided to an inductively coupled plasma mass spectrometer for elemental analysis. In addition, the total amount of rare earth elements (total REE) in the steel slag was determined by completely dissolving the steel slag by acid decomposition and melting, and measuring the sample by the inductively coupled plasma mass spectrometer described above. The results are shown in Table 7. It should be noted that the data collection of Table 7 was based on the slag used in the data collection in the aforementioned Tables 1 and 2, and was implemented in a manner that the test conditions were the same as those in Tables 1 and 2. Therefore, there are slight differences between the values ​​in Table 7 and Tables 1 and 2, but they are roughly consistent.

[0161] [Table 7]

[0162] surface

[0163]

[0164] *1: “Sc leaching rate” indicates the mass ratio of Sc leached into the leachate (leachate A or leachate B) in each leaching step to the mass of Sc contained in the original slag sample.

[0165] *2: "Other leaching rate" represents the mass ratio of the mass of other rare earth elements (rare earth elements other than Sc) leached into the leachate (leachate A or leachate B) in each leaching step 2 to the mass of all other rare earth elements (rare earth elements other than Sc) contained in the original slag sample.

[0166] *3: “Sc separation ratio” means the mass ratio of Sc leached into the leachate (leachate A or leachate B) in each leaching step to the mass of all REEs leached into the leachate (leachate A or leachate B) in each leaching step.

[0167] As can be seen from Table 7, the separation of Sc is promoted by further performing the leaching step 2, compared with performing only the leaching step 1.

[0168] Specifically, the Sc separation ratio (Sc mass concentration in leachate A / total rare earth mass concentration) at the end of the leaching step 1 was 0.005 to 0.058 in the case of sulfuric acid and 0.005 to 0.056 in the case of nitric acid. In contrast, by continuing the leaching step 2, when either sulfuric acid or nitric acid was used, the Sc separation ratio (Sc mass concentration in leachate B / total rare earth mass concentration) was 0.060 or more, the maximum Sc separation ratio was 0.113 in the case of sulfuric acid, and the maximum Sc separation ratio was 0.141 in the case of nitric acid.

[0169] In particular, in the case of sulfuric acid, when the pH in the leaching step 1 is set to 3.6 or more and less than 6.7, the Sc separation ratio in the leaching step 2 is more than 0.06, the leaching rate of Sc is 20% by mass or more, and the leaching rate of rare earth elements other than Sc is 10% by mass or more. Furthermore, when the pH in the leaching step 1 is set to 3.6 or more and 5.3 or less, the Sc separation ratio in the leaching step 2 is 0.089 or more.

[0170] In the case of nitric acid, when the pH in the leaching step 1 is set to 3.4 or more and less than 6.0, the Sc separation ratio in the leaching step 2 is more than 0.06, the leaching rate of Sc is 70% by mass or more, and the leaching rate of rare earth elements other than Sc is 30% by mass or more. Furthermore, when the pH in the leaching step 1 is 3.4 or more and 4.0 or less, the Sc separation ratio in the leaching step 2 is 0.102 or more.

[0171] In order to confirm that the present invention improves the separation effect of scandium from other rare earth elements, verification was performed using leachate B and leachate C. Specifically, the Sc-containing phase was recovered from leachate B and leachate C by solvent extraction, and the extent of rare earth elements other than Sc contained therein was confirmed. The steps of a typical confirmation method are schematically shown in Figure 6 .

[0172] After adjusting the leachate B and leachate C to a nitric acid solution with a nitric acid concentration of 1 mol / L, an organic solvent prepared by dissolving 10 mass % of tetrabutyl methylene diphosphonate in toluene was added to perform solvent extraction, and the mixture was separated into an aqueous phase and an organic phase. Thus, Sc contained in leachates B and C was extracted and separated into the organic phase. Rare earth elements other than Sc were mostly extracted and separated into the aqueous phase, but a part of them was extracted and separated into the organic phase.

[0173] Next, 10 mol / L nitric acid is added to the organic phase (typically a phase mainly containing Sc and also containing a small amount of rare earth elements other than Sc) to perform stripping of Sc from the organic phase.

[0174] The stripped solvent was separated into an aqueous phase and an organic phase. The stripped aqueous phase contained Sc and rare earth elements other than Sc. The stripped aqueous phase was recovered and all rare earth elements including Sc were quantified. In order to evaluate the degree to which Sc and rare earth elements other than Sc can be separated, the Sc purification rate defined by Formula 2 was calculated. The calculation results are shown in Table 8.

[0175] [Sc purification rate] = [Sc mass in the back-extracted aqueous phase] ÷ [rare earth element mass in the back-extracted aqueous phase] (Formula 2)

[0176] [Table 8]

[0177] Table 8 Comparison of Sc purification rates

[0178]

[0179] As can be seen from Table 8, in leachate B, by roughly separating Sc from other rare earth elements in leaching steps 1 and 2, the amount of rare earth elements mixed in other than Sc can be reduced, and the Sc purification rate can be improved compared with leachate C as a comparative example.

[0180] It should be noted that the leachate A collected in the leaching step 1 contains relatively more rare earth elements other than Sc, namely Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (Pm is omitted because it exists stably in nature in very small amounts), so a recovery solution containing rare earth elements other than Sc can be prepared by a known rare earth element purification technique, such as solid phase extraction using iminodiacetic acid or solvent extraction using di-2-ethylhexyl phosphate. Similarly, the leachate B collected in the leaching step 2 contains relatively more Sc, so it was confirmed that a recovery solution containing Sc can be prepared by solvent extraction using a known rare earth element purification technique.

[0181] Furthermore, the recovery liquid containing relatively more rare earth elements other than Sc and the recovery liquid containing relatively more Sc are respectively stripped, and a precipitant is added to the stripped liquid in the same manner as in the known precipitation step, and the precipitate is recovered by filtration, thereby recovering the rare earth elements other than Sc and Sc. In the present embodiment and the comparative example, oxalic acid is used as the precipitant, but an alkaline agent may also be used to precipitate in the form of a hydroxide.

[0182] After washing each precipitate, it was calcined to remove moisture, thereby confirming that oxides of rare earth elements other than Sc and Sc oxide were obtained.

[0183] Description of Reference Numerals

[0184] K leaching process 1

[0185] K2 Leaching Process 2

[0186] L, L' solid-liquid separation process

[0187] M, M' extraction process

[0188] M1, M1' solid phase extraction process

[0189] M2, M2' solvent extraction process

[0190] N, N' precipitation process

[0191] O, O' solid-liquid separation process

[0192] P, P' calcination process

[0193] P1, P1' cleaning process

[0194] P2, P2' heating process

[0195] M11, M11' rare earth element adsorption process

[0196] M12, M12' cation removal process

[0197] M13, M13' rare earth element elution process

[0198] M14, M14' solid phase cleaning process

Claims

1. A method for recovering scandium, characterized in that: The invention relates to a method for recovering scandium (Sc) from steel slag produced by an ironmaking process, which comprises the following steps: Leaching step 1, adjusting the pH of a leachate obtained by contacting a first acid with the steel slag to a first pH range to obtain a first leachate; a first solid-liquid separation step of separating solid components from the first leachate; as well as In a leaching step 2, the pH of a leachate obtained by contacting a second acid with the solid component separated in the first solid-liquid separation step is adjusted to a second pH range lower than that in the leaching step 1 to obtain a second leachate, Here, the first acid and the second acid include nitric acid or sulfuric acid, The first pH range is greater than 1.1 and less than 6.7, and the second pH range is greater than -1.

0.

2. The method for recovering scandium according to claim 1, wherein: In the leaching step 1 and / or the leaching step 2, while the pH of the first leachate and / or the second leachate is monitored, an acid or a base is added to adjust the pH.

3. The method for recovering scandium according to claim 1, characterized in that: The second pH range in the leaching step 2 is greater than or equal to -0.5 and less than or equal to 3.

3.

4. The method for recovering scandium according to claim 1, wherein: Sulfuric acid was used as the first acid.

5. The method for recovering scandium according to claim 4, characterized in that: The final pH of the leaching step 1 is greater than or equal to 3.6 and less than 6.

7.

6. The method for recovering scandium according to claim 4, characterized in that: The final pH of the leaching step 1 is 3.6 or more and 5.3 or less.

7. The method for recovering scandium according to claim 1, wherein: Nitric acid was used as the first acid.

8. The method for recovering scandium according to claim 7, characterized in that: The final pH of the leaching step 1 is greater than or equal to 3.4 and less than 6.

0.

9. The method for recovering scandium according to claim 7, characterized in that: The final pH of the leaching step 1 is 3.4 or more and 4.0 or less.

10. The method for recovering scandium according to claim 1, comprising the following steps: A step of adding alkali or acid to the first leachate and / or the second leachate to adjust the pH to obtain a first pH-adjusted leachate and / or a second pH-adjusted leachate; and The extraction step is to process the first pH-adjusted leachate and / or the second pH-adjusted leachate by either or both of solvent extraction and solid phase extraction to obtain a first element concentrated solution and / or a second element concentrated solution.

11. The method for recovering scandium according to claim 10, comprising: The precipitation step includes adding a precipitant to the first element concentrated solution and / or the second element concentrated solution to obtain a first precipitate and / or a second precipitate.

12. The method for recovering scandium according to claim 11, comprising: The calcination step is to calcine the first precipitate and / or the second precipitate to obtain a first oxide and / or a second oxide.

13. The method for recovering scandium according to claim 12, comprising: The drying step is to dry the first precipitate and / or the second precipitate separately.

14. The method for recovering scandium according to claim 10, characterized in that: The solvent extractant used in the extraction step is any one of an amine-based, organic phosphoric acid-based and carboxylic acid-based extractant.

15. The method for recovering scandium according to claim 10, characterized in that: The solid phase extractant used in the extraction process is a resin having iminodiacetic acid as a functional group.

16. The method for recovering scandium according to claim 11, characterized in that: The precipitant used in the precipitation step is any one of oxalic acid, carbonic acid, tartaric acid or alkali.

17. The method for recovering scandium according to claim 1, wherein: The steel slag is blast furnace slag.

18. The method for recovering scandium according to claim 1, characterized in that: In the second leachate, scandium is concentrated relative to the rare earth elements other than scandium.

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