Steel slag modification method and steel slag

Abstract

[Problem] The purpose of the present invention is to provide: an iron and steel slag modification method which is capable of modifying iron and steel slag so that a specific mineral phase of the iron and steel slag contains a rare earth element-enriched phase, wherein a rare earth element is concentrated, when the rare earth element in the iron and steel slag is recovered; and iron and steel slag which has a rare earth element-enriched phase. [Solution] An iron and steel slag modification method which comprises a step for melting iron and steel slag that contains a rare earth element, and a step for adding a phosphate to the iron and steel slag; and iron and steel slag which has a rare earth element-enriched phase.

Description

Technical Field

[0001] The present invention relates to a method for modifying steel slag, and typically includes a step of melting steel slag containing rare earth elements and a step of adding phosphates, and relates to a method for modifying slag having a phase containing rare earth elements at a high concentration (sometimes referred to as a REM enrichment phase) and to steel slag.

Background Art

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

[0003] Therefore, various technologies for developing and improving the recovery of rare earth elements from rare earth element-containing substances have been carried out.

[0004] Patent Documents 1 and 2 relate to a method for recovering rare earth elements from rare earth-containing substances such as a motor using a rare earth magnet, and examine a method for easily and efficiently recovering rare earth elements.

[0005] Specifically, Patent Document 1 proposes adding a melting point depressant, an oxidizing agent, and sodium borate to a waste product or semi-finished product containing a rare earth magnet and steel material, heating and melting them, and separating the waste into a two-phase of a rare earth enrichment phase in which rare earth elements in the waste are concentrated in the sodium borate and an Fe-C phase for recovery.

[0006] Patent Document 2 proposes a method that includes the steps of: heating and melting a rare earth element-containing material; adding an oxidizing agent to the melted solution; adding B2O3 to the solution; and cooling the solution to separate it into three phases: a B2O3 phase, a rare earth-enriched phase, and an Fe-containing phase, and then extracting the rare earth-enriched phase. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2016-186121 [Patent Document 2] Japanese Patent Publication No. 2016-186122 [Non-patent literature]

[0008] [Non-Patent Document 1] Keita Tanada et al., "Crystallization Behavior of Slag in the Continuous Solidification Process of Blast Furnace Slag," Iron and Steel, Japan, Iron and Steel Institute of Japan, July 19, 2018, Vol. 104, No. 11, pp. 708-716. [Non-Patent Document 2] Kazuo Fujita et al., "A Thoroughly Easy Book on Rare Earth Elements," First Edition, Japan, Nikkan Kogyo Shimbun, August 30, 2012, pp. 12-13. [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, various technologies are being developed and improved to recover rare earth elements from rare earth element-containing materials.

[0010] 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 these trace amounts of rare earth elements are also contained in the steel slag after the steelmaking process. 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.

[0011] Patent documents 1 and 2 attempt to recover rare earth elements from materials containing relatively large amounts of rare earth elements, such as motors that use rare earth magnets. On the other hand, steel slag is mainly composed of lime (CaO) and silica (SiO2), and there was a problem that the methods described in patent documents 1 and 2 could not be applied directly to recover sufficient rare earth elements.

[0012] Furthermore, Patent Documents 1 and 2 propose a method for efficiently recovering rare earth elements by melting a rare earth element-containing material to produce a rare earth element-enriched phase, an Fe-containing phase, and (in Patent Document 2, an additional B2O3 phase), and then recovering the rare earth element-enriched phase. However, since steel slag generally has an iron content of less than 10% by mass, the methods described in Patent Documents 1 and 2 cannot be directly applied in this respect either.

[0013] As mentioned above, steel slag is mainly composed of lime (CaO) and silica (SiO2). More specifically, the main components of steel slag are silicon (Si), iron (Fe), aluminum (Al), magnesium (Mg), calcium (Ca), etc. It is necessary to develop a process that can separate these main components from rare earth elements and recover the rare earth elements.

[0014] In addition, wet refining is generally used to recover rare earth elements in high purity. Generally, wet refining includes processes such as pre-treating raw materials containing rare earth elements, leaching, separating, and precipitating phases containing rare earth elements, and roasting the precipitate containing rare earth elements to reduce them back to rare earth elements. The higher the concentration of rare earth elements in the raw materials introduced in the pre-treatment stage, the more efficient the refining process becomes. This is because a higher concentration of rare earth elements means that less raw material needs to be introduced into the wet refining process, reducing the amount of chemicals and equipment used, thus enabling lower-cost operation. Furthermore, if a phase with concentrated rare earth elements exists in a specific mineral phase within the steel slag, that phase can be extracted to obtain raw materials with a high concentration of rare earth elements.

[0015] The present invention aims to provide a method for modifying steel slag and steel slag having a rare earth element-enriched phase, which allows for the recovery of rare earth elements from steel slag by modifying the steel slag so that it contains a phase in which rare earth elements are concentrated in a specific mineral phase of the steel slag, i.e., a rare earth element-enriched phase. [Means for solving the problem]

[0016] The inventors of this invention conducted extensive research to solve the aforementioned problems. As a result, they discovered that while rare earth elements contained in steel slag are usually diffused almost uniformly and dilutely (at low concentrations) throughout the slag, mixing a predetermined amount of phosphate with molten slag generates a mineral phase mainly composed of phosphate, and rare earth elements are concentrated in this mineral phase. It was known in the field of geosciences, which deals with rare earth elements, that minerals containing rare earth elements can occur as phosphates in addition to carbonates, oxides, and clays. However, the concentration of rare earth elements in a phosphate-based mineral phase in steel slag containing tens of mass percent, typically 30 mass percent or more, of CaO, etc., is a completely novel finding. Based on these findings, the inventors have completed the present invention, which can concentrate dilutely present rare earth elements in slag into a specific mineral phase within the slag.

[0017] (1) A step of melting steel slag containing rare earth elements, and A step of adding phosphate to the steel slag, which includes A method for modifying steel slag. (2) Characterized by adding phosphate to the molten steel slag, The method for modifying steel slag according to (1). (3) Characterized in that the step of adding the phosphate is included in the iron-making process, the method for modifying steel slag according to (1) or (2). (4) Characterized in that the step of adding the phosphate is included in the slag discharging step of the iron-making process, the method for modifying steel slag according to (3). (5) The step of adding the phosphate to the steel slag and After the adding step, a step of heating and melting the steel slag and the phosphate, which includes The method for modifying steel slag according to any one of (1) to (4). (6) Characterized in that the step of heating and melting is included in the iron-making process, the method for modifying steel slag according to (5). (7) After the step of adding the phosphate, a step of mixing the steel slag and the phosphate, which includes The method for modifying steel slag according to any one of (1) to (S). (8) The phosphate is calcium phosphate, the method for modifying steel slag according to any one of (1) to (7). (9) The steel slag is blast furnace slag, the method for modifying steel slag according to any one of (1) to (8). (10) A steel slag having a phase containing at least 0.90 mass% of one or more elements from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and at least 0.04 mass% of P. (11) The steel slag according to (10), having a phase containing 0.90% by mass or more of La and 0.04% by mass or more of P. (12) The steel slag described in (10) or (11), modified by the method described in any one of items (1) to (9). [Effects of the Invention]

[0018] The present invention provides a method for modifying steel slag so that, in order to recover rare earth elements from steel slag, the steel slag can be modified to contain a phase in which rare earth elements are concentrated in a specific mineral phase of the steel slag, i.e., a rare earth element-enriched phase. In other words, a method for producing modified steel slag is provided. Steel slag containing a rare earth element-enriched phase is also provided.

[0019] Modified steel slag contains a rare earth element-enriched phase, and this phase can be extracted from the steel slag. This allows for the production of an extract with a high concentration of rare earth elements. A wet refining process can be used to recover the rare earth elements, and in this case, the extract with a high concentration of rare earth elements is a suitable raw material for the wet refining process. This is because the higher the concentration of rare earth elements in the raw material, the less raw material can be fed into the wet refining process, reducing the amount of chemicals and equipment used, thus enabling lower-cost operation.

[0020] Furthermore, the steel slag modification method according to the present invention can be carried out with relatively simple operations and can be easily incorporated into existing steelmaking processes. In this respect as well, it is desirable as it enables low-cost operation. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a flowchart illustrating an example of a method for modifying steel slag containing rare earth elements. [Figure 2] Figure 2 shows optical microscope images and X-ray microscope mapping images of lanthanum in typical (unmodified) steel slag. [Figure 3] Figure 3 shows an optical microscope observation image and an X-ray microscope mapping image of lanthanum in (modified) steel slag according to one embodiment of the present invention. [Figure 4] Figure 4 shows lanthanum mapping images of the steel slag used in the examples and comparative examples, obtained using an X-ray microscope. [Modes for carrying out the invention]

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

[0023] Although there are multiple definitions of elements included in the term "rare earth elements," the definition of rare earth elements (REM) 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).

[0024] Steel slag is a by-product generated during the steelmaking process and is generally classified into blast furnace slag and steelmaking slag. Blast furnace slag is typically produced in blast furnaces where molten pig iron is manufactured, and the non-iron components of the iron ore, along with the ash from the auxiliary raw materials such as limestone and coke, are melted, separated, and recovered together. Steelmaking slag is typically a by-product of the steelmaking process, where the composition of steel is adjusted from pig iron and scrap to produce "steel" with excellent toughness and workability. Generally, slag produced from pig iron in a converter is called converter slag, and slag produced from scrap in the electric furnace steelmaking process is called electric furnace slag. These steel slags are mainly composed of lime (CaO) and silica (SiO2), and may also contain alumina (Al2O3), magnesium oxide (MgO), and small amounts of sulfur (S). Due to differences in the manufacturing process, steelmaking slag may have a relatively high iron oxide (FeO) content, sometimes exceeding 10% by mass, while blast furnace slag may have an iron oxide (FeO) content of approximately 10% by mass or less, 5% by mass or less, 3% by mass or less, or even 1% by mass or less.

[0025] Figure 1 is a flowchart illustrating an exemplary method for modifying steel slag containing rare earth elements, particularly blast furnace slag. A typical blast furnace slag production process involves adding coal, limestone, and iron ore to the top of a blast furnace, melting these raw materials at high temperatures while blowing oxygen onto them to produce a molten product (pig iron process F), separating the heavier pig iron from the lighter slag, and discharging the slag (slag removal process G). Phosphate is added to the discharged molten slag (phosphate addition process H). Since the slag is in a molten state, the added phosphate diffuses into the slag. The slag cools over time, changing from a molten state to a solid state (molten slag cooling process I). Here, the phosphate and rare earth elements diffused in the slag aggregate and solidify, forming a rare earth element-enriched phase.

[0026] Here, the phosphate in this embodiment is not particularly limited as long as it has the property of coagulating and solidifying with rare earth elements. However, since the phosphate is added to the slag and the slag is melted, it is preferable that the phosphate has a melting point equivalent to that of the slag. Typically, the phosphate may be tripotassium phosphate, tricalcium phosphate, magnesium diphosphate, trisodium phosphate, etc., or mixtures thereof. Regarding calcium phosphate, Ca(H2PO4)2, CaHPO4, etc., decompose thermally to become phosphates, so these may also be used. Iron phosphate or hydrogen phosphate can also be used. However, it should be noted that these generally have low melting points and tend to volatilize easily when added to slag, resulting in a lower yield. Furthermore, the phosphate mentioned above is not limited to the pure substance of the phosphate, but may also include substances containing the phosphate, such as ores.

[0027] In the above flow (Figure 1), if the phosphate addition step H is omitted, i.e., in a typical blast furnace slag manufacturing process, rare earth elements are dispersed in a nearly uniformly dilute state within the cooled and solidified blast furnace slag. Figure 2 shows an optical microscope observation image and an X-ray microscope mapping image of lanthanum (La) in such blast furnace slag. The mapping image is almost entirely black, but the areas where lanthanum is present are mapped as tiny white dots. These tiny white dots are dispersed throughout the mapping image, indicating that lanthanum is dispersed in a nearly uniformly dilute state within the cooled and solidified blast furnace slag.

[0028] Here, lanthanum, one of the rare earth elements, is the target of mapping. However, it is known that rare earth elements have similar properties (Non-Patent Literature 2), and other rare earth elements (lanthanoids (Ln) consisting of 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)), as well as yttrium (Y) and scandium (Sc), are dispersed in blast furnace slag in a nearly uniformly diluted state, similar to lanthanum (La).

[0029] On the other hand, Figure 3 shows the optical microscope observation image and the X-ray microscope mapping image of lanthanum (La) of the blast furnace slag (J) modified according to this embodiment, when the phosphate addition step H is performed in the above flow (Figure 1). In Figure 3, as in Figure 2, the locations where lanthanum is present are mapped as white dots, but the white dots are shown to be larger than in Figure 2. This indicates that the lanthanum is present in an aggregated state, or in other words, that a lanthanum-enriched phase is present. Note that enrichment of a certain element means that the concentration in the enriched phase is higher than the concentration in other parts of the slag. For rare earth elements, including lanthanum, the concentration in general steel slag as a whole is originally low, generally 0.20% or less, so the concentration of rare earth elements in the enriched phase may be greater than 0.20 mass%. From the viewpoint of recovering rare earth elements, a higher concentration of rare earth elements in the enriched phase is preferable, and the concentration may be 0.30% by mass or more, 0.50% by mass or more, 0.70% by mass or more, 0.90% by mass or more, or 0.96% by mass or more.

[0030] Furthermore, X-ray diffraction analysis of the aggregated or concentrated phase of lanthanum confirms that the phase is a mineral phase composed of phosphates. Therefore, although we do not wish to be bound by any particular theory, it is considered that the addition of phosphates causes the rare earth elements, which were uniformly and dilutely present throughout the slag, to aggregate and solidify with the phosphates, forming a rare earth element-enriched phase. Generally, in the rare earth element-enriched phase, i.e., the mineral phase, the P concentration increases in proportion to the amount of phosphate added, and typically the P concentration in this phase may be 0.04 mass% or higher. The P concentration in the rare earth element-enriched phase may also be 0.10 mass% or higher. In this embodiment, the elemental concentration in the rare earth element-enriched phase is measured using an X-ray microscope (XGT-7200V, Horiba, Ltd.) with a measurement area (X-ray irradiation diameter) of approximately 100 μm. If the rare-earth-enriched phase being measured is larger than the X-ray irradiation diameter, measurements are taken at at least three points within the phase, and the measured concentrations are averaged. In this embodiment, unless otherwise specified, the elemental concentrations in the rare-earth-enriched phase are quantitatively analyzed using the fundamental parameter method. Furthermore, in this fundamental parameter method, iron oxide is assumed to be FeO.

[0031] Furthermore, as mentioned above, since rare earth elements have similar properties, in the lanthanum-enriched phase of blast furnace slag modified according to this embodiment, other rare earth elements (lanthanoids consisting of 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)), as well as yttrium (Y) and scandium (Sc), are also enriched in the same way as lanthanum (La).

[0032] Thus, the blast furnace slag modified according to this embodiment contains a lanthanum-enriched phase, that is, a phase enriched with rare earth elements. Subsequently, the rare earth element-enriched phase can be extracted from the blast furnace slag and recovered efficiently through a wet refining process or the like.

[0033] For extracting the rare earth element-enriched phase from blast furnace slag, known techniques can be used. For example, the phosphate, which is the main component of the rare earth element-enriched phase, and the silicate minerals contained in the blast furnace slag have different specific gravities and ease of solubility in acids. Therefore, the rare earth element-enriched phase can be selectively dissolved and extracted using a known selective dissolution method, that is, a method for selectively dissolving the phosphate phase. Alternatively, the phosphate phase, i.e., the rare earth element-enriched phase, can be extracted by crushing the cooled and solidified steel slag and separating it by specific gravity using a known method.

[0034] In the above explanation, we have used the example of blast furnace slag according to the flow chart in Figure 1. However, other types of steel slag, such as steelmaking slag and electric furnace slag, can also be modified to have a rare earth element-enriched phase by adding phosphates, similar to the case of blast furnace slag.

[0035] Furthermore, there are no particular restrictions on the amount of phosphate added. However, if the amount added is too small, it may be difficult to mix with the slag, or the rare earth element-enriched phase may not solidify properly. For this reason, depending on the properties of the slag and the process conditions, the amount of phosphate added may be 1.0% by mass or more, 2.0% by mass or more, 3.0% by mass or more, 4.0% by mass or more, 5.0% by mass or more, or 10.0% by mass or more relative to the weight of the slag. Also, if the amount added is too large, the effect of solidifying the rare earth element-enriched phase will saturate, so the upper limit of the amount added may be 20.0% by mass or less, 19.0% by mass or less, 18.0% by mass or less, 17.0% by mass or less, 16.0% by mass or less, or 15.0% by mass or less relative to the weight of the slag.

[0036] Furthermore, the cooling rate when cooling and solidifying the slag is not particularly limited. Generally, the critical cooling rate at which slag vitrifies when producing granulated blast furnace slag is several hundred degrees Celsius / second. For example, even when cooled at 100 degrees Celsius / second, crystalline slag can be obtained. However, some of it may become glassy. Therefore, the upper limit of the cooling rate may be 100 degrees Celsius or less or less. Non-patent document 1 discloses that when the cooling rate is 35 degrees Celsius or higher, blast furnace slag may become amorphous. Accordingly, the upper limit of the cooling rate may be 35 degrees Celsius or less or less. At the above cooling rates, the blast furnace slag becomes crystalline, and the rare earth element-enriched phase can also solidify. Also, if the slag is cooled rapidly, the added phosphate may not diffuse sufficiently. Therefore, from the viewpoint of ensuring time for phosphate diffusion, the cooling rate may be lowered, and the upper limit of the cooling rate may be 30 degrees Celsius or less, 25 degrees Celsius or less, or 20 degrees Celsius or less. On the other hand, there is no particular lower limit to the cooling rate, and the cooling rate may be set within an appropriate range considering the cooling equipment capacity and productivity. Typically, it may be 0.1°C / min or more, 0.5°C / min or more, 1°C / min or more, 3°C / min or more, or 5°C / min or more.

[0037] Furthermore, there are no particular restrictions on the temperature range in which the cooling rate is controlled, but it is sufficient to continue until the slag solidifies. Since slag generally solidifies at around 1000°C, the cooling temperature can be controlled within the temperature range from when the slag is molten until the slag temperature reaches 1000°C.

[0038] The steel slag modification method according to this embodiment is A process for melting steel slag containing rare earth elements, and The process includes adding phosphate to the aforementioned steel slag, but the chronological order of these processes is irrelevant. Here, "melting" includes at least one of turning a solid into a liquid, or keeping something that has been turned into a liquid in a liquid state. Therefore, it also includes remelting steel slag containing rare earth elements in a solid state. Furthermore, it includes generating steel slag containing rare earth elements in a steelmaking process, such as a pig ironmaking process, and maintaining the molten steel slag containing rare earth elements in a molten state.

[0039] In one embodiment, the process may include adding phosphate to the steel slag after the process of melting the steel slag. That is, in this embodiment, phosphate may be added to the molten steel slag. When the phosphate is added, since the steel slag is already in a molten state, the added phosphate is easily diffused or mixed into the steel slag. Subsequently, as the slag cools and solidifies, the phosphate and rare earth elements diffused in the slag aggregate and solidify, forming a rare earth element-enriched phase in the slag.

[0040] Another embodiment may include a step of melting the steel slag after the step of adding phosphate to the steel slag. That is, in this embodiment, phosphate may be added to the steel slag before melting, and the steel slag may be heated and melted together with the phosphate. As the slag melts, the phosphate diffuses or mixes into the molten slag. Subsequently, as the slag cools and solidifies, the phosphate and rare earth elements that have diffused into the slag aggregate and solidify, forming a rare earth element-enriched phase in the slag.

[0041] In one embodiment, the steel slag and phosphate may be intentionally mixed. Known techniques such as stirring can be used for mixing. Mixing promotes the diffusion of phosphate into the steel slag, and consequently, the enrichment of rare earth elements is also promoted. The timing of mixing is not particularly limited as long as it is done after the addition of phosphate to the steel slag, and may be done before, after, or during the melting of the slag, or may be carried out over multiple stages. When heating and melting steel slag together with phosphate, it is preferable to mix the steel slag and phosphate before heating and melting. This is because, since the slag and phosphate are mixed, even during the melting process, some of the molten slag can easily come into contact with the phosphate, thus promoting the diffusion of phosphate. It is also preferable to add the phosphate to the steel slag while it is molten. This can promote the mixing of the phosphate and the steel slag.

[0042] Furthermore, in one embodiment, the steelmaking process may include a step of adding or mixing phosphate. The steelmaking process is a method of producing iron, and in a broad sense, it may include the pig ironmaking process, which involves reducing and smelting iron ore to produce pig iron, the steelmaking process, which involves refining pig iron to produce steel, and the rolling and processing process of steel. Since steel slag is produced in the steelmaking process, by incorporating this embodiment into the steelmaking process, it is possible to operate the entire process (including the steel slag modification process of this embodiment) efficiently. In a typical steelmaking process, there is a step in which the steel slag is heated and in a molten state, and it is preferable that this step overlaps (simultaneously) with at least a part of the steel slag melting step of this embodiment, as this can reduce the cost of heating and melting.

[0043] Furthermore, the steelmaking process may include a step of heating and melting slag and phosphate. That is, it is preferable to be able to heat and melt not only the slag but also the phosphate simultaneously, thereby improving the efficiency of the entire process, including not only the steelmaking process but also the slag modification process. For example, in the case of blast furnace slag, phosphate may be loaded into the upper part of the blast furnace along with the slag raw material (limestone, silica, etc.) and the blast furnace may be operated.

[0044] In one embodiment, the slag removal step of the steelmaking process may include a step of adding or mixing phosphate. A typical steelmaking process includes a step of discharging steel slag (slag removal step). Normally, during the slag removal step, the slag is in a molten state, and the added phosphate can easily diffuse or mix into the slag. Furthermore, since the slag becomes fluid due to removal, the diffusion or mixing of the added phosphate into the slag is further promoted.

[0045] In another embodiment, a steel slag is provided having a phase containing 0.90 mass% or more of one or more elements from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and 0.04 mass% or more of P, particularly a steel slag having a phase containing 0.90 mass% or more of La and 0.04 mass% or more of P. Typically, the steel slag obtained by the steel slag modification method according to the above embodiment can have a phase containing rare earth elements and P at the above concentrations. Substantially, the steel slag modification method can be considered a method for producing modified slag. In other words, according to another embodiment, a method for producing modified steel slag is provided. This phase containing rare earth elements and P at predetermined concentrations is a rare earth element-enriched phase, and by performing a wet refining process or the like using this enriched phase, it is possible to efficiently recover the rare earth elements. As an example of wet smelting, the desired rare earth element may be dissolved from the concentrated phase using a suitable solvent such as an acid or alkaline solution, and then the rare earth element may be extracted from the solution chemically or electrochemically. [Examples]

[0046] 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.

[0047] As a typical example of steel slag, we prepared steel slag (blast furnace slag) with the composition shown in No. 1 of Table 1. The concentration of rare earth elements was left as it was originally contained in the No. 1 steel slag, without any special adjustments. This steel slag with the composition of No. 1 was heated to 1400°C to a molten state, and calcium phosphate (Ca3(PO4)2) was added as a phosphate by spraying. As shown in Table 1, for Nos. 2 to Nos. 4, calcium phosphate was added in an amount of 5.0 to 20.0 mass% of the weight of the molten slag (100.0 mass%), and then the slag was cooled and solidified to 1000°C at a cooling rate of 0.5°C / min, and the slag composition obtained was analyzed. The average composition of the slag shown in Table 1 is the composition before the addition of calcium phosphate, and is the average elemental composition of the entire 1.0g of each steel slag. In this embodiment, quantitative analysis of elemental concentrations was performed using the fundamental parameter method based on measurement results obtained by X-ray microscopy.

[0048] [Table 1]

[0049] Figure 4 shows images of slag samples No. 1 to No. 4 observed under an X-ray microscope. The X-ray irradiation diameter was 100 μm. White areas are present in the images in Figure 4, and the concentrations of lanthanum (La) and phosphorus (P) were measured in the areas indicated by the arrows in Figure 4. The results are shown in Table 2.

[0050] [Table 2]

[0051] As is clear from the image in Figure 4, the area fraction of the white regions was larger in samples No. 2 to No. 4, which had calcium phosphate added. Also, as is clear from Table 2, the concentrations of lanthanum (La) and phosphorus (P) in the white regions were higher than in sample No. 1. From these findings, it was confirmed that samples No. 2 to No. 4 contained a rare earth element-enriched phase (white regions), and that this phase contained phosphates. In Table 2, the measurement results for lanthanum were used as the rare earth element. However, rare earth elements have similar properties, and in the rare earth element-enriched phase (white areas), lanthanoids (Ln), consisting of 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), as well as yttrium (Y) and scandium (Sc), are enriched in the same way as lanthanum (La).

[0052] Separately, steel slag (blast furnace slag) with the composition shown in No. 1 of Table 1 was heated to a molten state, and 5.0% by mass of calcium phosphate (additional amount) relative to the weight of the molten slag (100.0% by mass) was added by spraying. Then, the slag obtained by cooling and solidifying at a cooling rate in the range of 0.1 to 500°C / min was investigated for the presence or absence of rare earth element-enriched phases using an X-ray microscope under the same conditions as above. As a result, it was confirmed that La and P-enriched phases were present in the cooling rate range of 0.1°C / min to 500°C / min, similar to No. 2 to 4 in Table 1. [Explanation of Symbols]

[0053] F: Pig iron production process using blast furnaces G: Process of removing slag from inside the blast furnace H: Step of adding phosphate to molten slag. I: The process of cooling and solidifying molten slag. J: Blast furnace slag suitable for rare earth element recovery

Claims

1. A process for melting steel slag containing rare earth elements, and The process includes adding a phosphate to the aforementioned steel slag, A method for modifying steel slag.

2. The method is characterized by adding a phosphate to the molten steel slag. The method for modifying steel slag according to claim 1.

3. The method for modifying steel slag according to claim 1, characterized in that the steelmaking process includes a step of adding the phosphate.

4. The steel slag modification method according to claim 3, characterized in that the slag removal step of the steelmaking process includes a step of adding the phosphate.

5. A step of adding the phosphate to the steel slag, The process includes, after the addition step, heating and melting the steel slag and the phosphate, The method for modifying steel slag according to claim 1.

6. The steel slag modification method according to claim 5, characterized in that the steelmaking process includes the step of heating and melting.

7. The steel slag modification method according to claim 1, further comprising the step of mixing the steel slag with the phosphate after the step of adding the phosphate.

8. The steel slag modification method according to claim 1, wherein the phosphate is calcium phosphate.

9. The steel slag modification method according to claim 1, wherein the steel slag is blast furnace slag.

10. Steel slag having a rare earth element-enriched phase containing 0.90% by mass or more of one or more elements from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and 0.04% by mass or more of P.

11. The steel slag according to claim 10, having a phase containing 0.90% by mass or more of La and 0.04% by mass or more of P.

12. A method for producing the steel slag described in claim 10 or 11 by the method described in any one of claims 1 to 9.

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