JP2016108583A - Method for recovering rare earth element from noble metal smelting slag - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover high-purity rare-earth elements from a noble metal smelting slag through a simple and inexpensive process without using an expensive organic solvent extraction method.SOLUTION: A method for recovering rare-earth elements from a noble metal smelting slag includes continuously carrying out the following processes: an inorganic acid leaching process of dissolving the noble metal smelting slag including rare-earth elements in the aqueous solution of inorganic acids excluding sulfuric acid and phosphoric acid to obtain an acid leachate including the rare-earth elements; a cementation process of adding a metallic iron to the acid leachate to precipitate heavy metals replaceable and precipitable with iron, and removing the heavy metals by solid-liquid separation; an oxidation process of adding an oxidant to the heavy metal-removed acid leachate to convert the divalent Fe ions to the trivalent Fe ions by oxidation; a neutralization process of adjusting the pH of the acid leachate after oxidation to 5 to 7 to precipitate the trivalent Fe ions and aluminum ions as hydroxides to remove by solid-liquid separation; and a recovery process of adjusting the pH of the separated acid leachate to 8 to 10 and precipitating the rare-earth elements as hydroxides to recover by solid-liquid separation.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for recovering a rare earth element, which is a useful substance, from a noble metal smelting slag containing rare earth elements and heavy metals generated in each step of noble metal smelting.

  In recent years, rare earth elements have been used in various fields such as solid electrolytes, rare earth magnets, phosphors, and abrasives, taking advantage of their unique physical properties. However, as the name suggests, rare earth elements are low in reserves, and the production areas themselves are unevenly distributed around the world. Recovery of rare earth elements has been strongly desired.

The technology disclosed for the method of recovering rare earth elements from used materials is compared with the content of rare earth elements in used materials such as abrasives and rare earth magnets using rare earth elements. In many cases, the types of impurities contained at the same time are limited.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 2004-175852) discloses a technique for recovering a rare earth oxide from polishing waste liquid, and Patent Document 2 (Japanese Patent Laid-Open No. 2012-237053) discloses a raw material containing a rare earth magnet alloy. Techniques for recovering rare earth elements using bis are disclosed.

  On the other hand, when compared from the viewpoint of the rare earth element content and the diversity of impurities contained therein, although it is inferior to the above-mentioned used materials, the smelting slag discharged in each process of precious metal smelting also contains a considerable amount of rare earth elements. Since it is contained, it is considered as a promising source for collecting rare earth elements. Note that the precious metal smelting slag is a by-product generated when a base containing precious metal (for example, an automobile waste gas catalyst or petrochemical catalyst), a reducing agent and a flux are mixed and reduced in an electric furnace, In addition to oxides such as Si, Ca, Al, and Fe derived from raw materials and flux, heavy metals such as Cu, Ni, and Pb are contained. Considering the use after recovery, when recovering rare earth elements from precious metal smelting slag, it is necessary to separate these heavy metals, particularly Cu, in addition to the flux components.

As a method for recovering rare earth elements from slag containing impurities, for example, Patent Document 3 (International Publication No. 2012/137495) discloses recovery of rare earth elements from Sn smelting slag using a solvent extraction method. A method has been proposed. However, the solvent extraction method is suitable for the precise separation of rare earth elements, but the organic solvent used for the separation is expensive and requires special equipment for fire prevention, which increases the cost. It is not suitable for the purpose of collecting the elements at a low cost in a batch.
In addition, since the chemical | medical agent used for a process is expensive and the chemical | medical agent used for a process is high and the COD (chemical oxygen demand) of a process liquid is high, the separation method of the rare earth elements by the oxalate precipitation formation currently disclosed by patent document 3 is used. The load becomes high.
The method described in Patent Document 3 is mainly intended for separation of radioactive substances such as U and Th, and there is no disclosure about separation of heavy metal elements such as Cu and Pb.

As a method for recovering a rare earth element without using a solvent extraction method, for example, Patent Document 4 (Japanese Patent Application Laid-Open No. 2013-104098) discloses that a rare earth element is produced by precipitation of a sulfate double salt of a rare earth element in an aqueous solution. Techniques for recovery are disclosed.
However, in the case of this method, it is necessary to set the anion concentration of the treatment liquid to a high concentration of 4 to 10 mol / L in advance in relation to the solubility product of the rare earth element sulfate double salt precipitation, and this is used for the treatment. In this method, since the cost of the chemical becomes very high and sulfate ions are used for precipitation formation, in the case of noble metal smelting slag containing Ca, poorly soluble Ca sulfate (gypsum) is produced. It is not suitable for the recovery of rare earth elements from smelting slag.

  As an example of a method for recovering rare earth elements from iron ore, Patent Document 5 (International Publication No. 2013/085052 pamphlet) discloses a method using an Al smelting bauxite residue. In order to dissolve the perovskite crystal that is hardly soluble in acid, it is a special method in which leaching with an acid is performed at high temperature and pressure, and in Patent Document 5, no consideration is given to the separation of heavy metal elements. Absent.

Japanese Patent Laid-Open No. 2004-175552 JP 2012-237053 A International Publication No. 2012/137495 Pamphlet JP 2013-1004098 A International Publication No. 2013/085052 Pamphlet

  In view of the problems of the prior arts described above, the present invention provides a flux-derived component and a heavy metal component that coexist in a large amount in a precious metal smelting slag without using an expensive organic solvent extraction method and by a simple and inexpensive process. The purpose is to collect and remove high-purity rare earth elements.

In order to solve the above problems, the present invention provides the following rare earth element recovery method.
In the first method, a precious metal smelting slag containing rare earth elements is dissolved in an aqueous solution of an inorganic acid excluding sulfuric acid and phosphoric acid, and an acid-soluble component contained in the precious metal smelting slag is leached and then an acid-insoluble solid. Inorganic acid leaching step for obtaining an acid leaching solution containing rare earth elements by solid-liquid separation, adding iron in a metallic state to the acid leaching solution, and heavy metal ions capable of substitution precipitation with iron such as Cu in the metallic state A cementation step in which the precipitated metal is removed by solid-liquid separation; and an oxidant is added to the acid leaching solution from which heavy metals have been removed to oxidize divalent Fe ions in the acid leaching solution to form trivalent Fe ions. An oxidation treatment step, a neutralization treatment step of adjusting the pH of the acid leaching solution after the oxidation treatment to 5 to 7 to precipitate trivalent Fe ions and aluminum ions as hydroxides, and removing them by solid-liquid separation, and , The pH of the acid leaching solution from which the hydroxides of l and Fe have been removed is adjusted to 8 to 10, the rare earth element is precipitated as a hydroxide, and the recovery step of recovering the rare earth element hydroxide by solid-liquid separation is continued. And do it. There is provided a method for recovering rare earth elements from precious metal smelting slag, wherein the recovered rare earth element hydroxides are washed by a known means, and then dried or fired to obtain rare earth element hydroxides or oxides.
As a second method, the oxidation treatment step and the neutralization treatment step in the first method described above are carried out by first separating the Al hydroxide and then oxidizing the divalent Fe ions to obtain the trivalent Fe. A two-stage treatment for separating ions and trivalent Fe hydroxide may be performed.
In these recovery methods, it is preferable to use a gas containing oxygen, such as air, as the oxidant.

In the third method, the precious metal smelting slag containing rare earth elements is dissolved in an aqueous solution of an inorganic acid excluding sulfuric acid and phosphoric acid, and the acid-soluble component contained in the precious metal smelting slag is leached, followed by solid-liquid separation. The inorganic acid leaching step for obtaining an acid leaching solution containing a rare earth element, and adjusting the pH of the acid leaching solution from which heavy metals have been removed to 5 to 7, causing Fe ions and Al ions to precipitate as hydroxides, and solid-liquid separation A neutralization treatment step and a separation step to be removed; a complexation treatment step in which an ammonium salt or aqueous ammonia is added to the acid leaching solution from which Al and Fe hydroxides have been removed to form Cu as a soluble copper-ammine complex; The pH of the leachate is adjusted to 8 to 10, the rare earth element is precipitated as a hydroxide, and the recovery step of recovering the rare earth element hydroxide by solid-liquid separation is continuously performed. After the hydroxide wash, give a hydroxide or oxide of a rare earth element by drying or calcining a rare earth element method for recovering a precious metal smelting slag is provided.
In these recovery methods, it is preferable to use one or two of hydrochloric acid and nitric acid as the inorganic acid.
The rare earth element recovery method of the present invention can also be used for recovery of rare earth elements from ore smelting slag such as tin refining slag.

  As described above, by using the recovery method of the present invention, it has become possible to recover a high-purity rare earth element from a noble metal smelting slag containing a heavy metal element as an impurity by an inexpensive process.

It is a flowchart which shows one Embodiment of the rare earth element collection | recovery method of this invention. It is a flowchart which shows other embodiment of the rare earth element recovery method of this invention. It is a flowchart which shows other embodiment of the rare earth element recovery method of this invention.

FIG. 1 is a flowchart showing a first embodiment of the rare earth element recovery method of the present invention. In this embodiment, impurities such as Cu that have been leached into the acid leaching solution are replaced with metal Fe, precipitated in a metallic state, and then separated and removed from the acid leaching solution by solid-liquid separation. To do.
FIG. 2 is a flowchart showing a second embodiment of the rare earth element recovery method of the present invention. In this embodiment as well, impurities such as Cu leached into the acid leaching solution are replaced with metal Fe and precipitated in a metallic state, and then separated and removed from the acid leaching solution by solid-liquid separation. To do.
FIG. 3 is a flowchart showing a third embodiment of the rare earth element recovery method of the present invention. In this embodiment, the impurity Cu is separated from the rare earth element by finally being left in the acid leaching solution by forming a soluble Cu-ammine complex. Pb is separated and removed from the acid leaching solution by being precipitated as a hydroxide or adsorbed on an Fe hydroxide.
Details of the rare earth element recovery method of the present invention will be described below with reference to the flowcharts of FIGS.

[Inorganic acid leaching process]
In the present invention, noble metal smelting slag is used as a rare earth element recovery source. In the present invention, the particle size of the precious metal smelting slag is not particularly specified, but considering the leaching rate in the aqueous inorganic acid solution, it is preferably 0.3 mm or less, preferably 0.15 mm or less. The temperature of the inorganic acid aqueous solution is preferably 20 to 70 ° C. In order to adjust the particle size of the slag, it is preferable to perform water granulation when the slag is solidified or to mechanically crush the solidified slag.

An inorganic acid is preferably used for the leaching of the acid-soluble component from the noble metal smelting slag. Even if an organic acid is used, it is possible to leach out an acid-soluble component. However, in that case, an organic substance is contained in the drainage liquid, the COD value is increased, and the wastewater treatment cost is increased, which is not preferable.
As the acid used in the inorganic acid leaching step, any inorganic acid such as hydrochloric acid, nitric acid, perchloric acid or the like may be used, but sulfuric acid and phosphoric acid are not preferable. Precious metal smelting slag contains a large amount of flux-derived Ca and Al, and when sulfuric acid is used in the leachate, sulfate ions react with Ca to form insoluble Ca sulfate (gypsum). The slag surface is covered with poorly soluble gypsum, which inhibits the leaching of acid-soluble components. Further, the leached Ca reacts with sulfate ions to form a gypsum precipitate, which hinders uniform stirring of the leachate and impairs filterability when filtration is used as a means for solid-liquid separation. When phosphoric acid is used for the leaching solution, in this case, hardly soluble Al phosphate is formed, and thus a phenomenon similar to Ca sulfate in sulfuric acid occurs.
Considering availability and price, it is preferable to use hydrochloric acid or nitric acid and a mixture thereof as the acid used in the inorganic acid leaching step.
In the acid leaching reaction in the inorganic acid leaching step, and in the hydroxide precipitation reaction in the neutralization treatment step and recovery treatment step described later, the treatment liquid is mechanically stirred from the viewpoint of promoting the reaction. Is preferred.

The concentration of acid used in the inorganic acid leaching step, for example, pH = 1 following solutions in H ⁺ is at least twice the molar amount of CaO contained in slag, and 2 times or more the molar amount of MgO, and Al ₂ O More than 6 times the molar amount of ₃ .
The time required for the inorganic acid leaching reaction varies depending on the crystal grain size of the precious metal smelting slag, which is the starting material, and the ratio of the contained components, but is usually 1 to 2 hours. It is desirable to take 2 hours or more to perform the leaching reaction more completely.
In the inorganic acid leaching step, the reaction solution is preferably stirred by a known stirring means from the viewpoint of promoting the leaching reaction.

In the inorganic acid leaching step, mainly SiO ₂ remains as an insoluble component in a solid state, so that it is separated and removed from the acid leaching solution using a solid-liquid separation means.
As the solid-liquid separation means, any known solid-liquid separation means such as decantation, filtration, and centrifugation may be used.

[Cementation process]
The term cementation has various meanings such as carbonization and agglomeration, but in this specification, due to the difference in oxidation-reduction potential, a metal ion having a noble reduction potential in an aqueous solution has a low reduction potential. It is used as a term that refers to a reaction that reduces and precipitates with a metal.
In 1st embodiment of this invention, Fe is used for the metal as a reducing agent. When metallic Fe is added to the acid leaching solution obtained in the above-described inorganic acid leaching step, Cu ions, Ni ions, Pb ions, etc. having a noble reduction potential than Fe are substituted and deposited on the surface of Fe. The metal Fe on which Cu, Ni and Pb are precipitated is separated and removed from the acid leaching solution using a known solid-liquid separation means.
The amount of metal Fe to be added to the acid leaching solution in the cementation step is sufficient if it is necessary for displacement precipitation of the total amount of Cu ions and Pb ions contained in the acid leaching solution, but the displacement precipitation reaction is completed quickly. Therefore, it is preferable to add more than the sum of the amount of Cu ions, Ni ions, Pb ions and Fe ions contained in the acid leaching solution.
In the present invention, the shape of the metal Fe to be added is not particularly limited, but it is preferable to use Fe powder having a large specific surface area in order to increase the displacement precipitation reaction rate. Moreover, when implementing this invention on an industrial scale, it is also possible to use a fine Fe scrap piece.
The temperature of the substitutional precipitation reaction in the cementation step is not particularly specified in the present invention.For example, after completion of the inorganic acid leaching step, which is the previous step, without heating or cooling the obtained acid leaching solution, Metal Fe may be added as it is.
The time for the substitutional precipitation reaction is not particularly specified in the present invention, but if it is too long, the added metallic Fe is chemically dissolved, which is not preferable.

[Oxidation treatment process, neutralization treatment process]
In the present invention, Fe and Al leached in the acid leaching solution are precipitated and precipitated as hydroxides, and then separated and removed by solid-liquid separation.
Part of Fe in the precious metal smelting slag exists as a divalent oxide, dissolves as a divalent Fe ion in the acid leaching process, and metal Fe becomes a divalent Fe ion in the cementation process. Since it dissolves, divalent Fe ions are present in the acid leaching solution. In the neutralization treatment, since trivalent Fe ions are more likely to precipitate as hydroxides, divalent iron ions are oxidized to trivalent in advance. When an alkali is added to the acid leaching solution, Al (OH) ₃ and Fe (OH) ₃ are precipitated as the pH increases. In these processes, the oxidation reaction may be performed after adjusting the pH of the acid leaching solution.
In addition, when performing the complexation process mentioned later, without performing a cementation process, since there is little quantity of a bivalent Fe ion, it is possible to remove an iron ion, without performing oxidation. In this case, the divalent Fe ions are considered to coprecipitate or adsorb with the precipitation of Al (OH) ₃ .
As the oxidizing agent, water-soluble oxidizing agents such as hydrogen peroxide, permanganate, perchlorate, and oxygen can be used. Oxygen may be blown into the acid leachate in the form of oxygen gas, air, oxygen-containing gas, or the like.
This is because when Ce ions are oxidized from trivalent to tetravalent, the solubility decreases, and Ce coprecipitates with Fe, making it difficult to recover. Therefore, by using air as the oxidizing agent, it is dissolved in the acid leachate. This is because rapid oxidation of rare earth element ions can be suppressed.
The time required for the oxidation reaction of divalent Fe ions varies depending on the amount of divalent Fe ions contained in the acid leaching solution. The temperature of the oxidation reaction is preferably 20 to 70 ° C.

Precipitation of the Al and Fe hydroxides described above is carried out by neutralizing the acid leaching solution with alkali and adjusting its pH to 5-7. As the alkali used for neutralization, Na, K, Ca and the like, alkali metals and alkaline earth hydroxides, carbonates and the like are used alone or in combination. The alkali may be added to the acid leaching solution as it is, or the solid state chemical may be added as it is, or it may be added after dissolving in water to make an aqueous solution. The case where ammonia water (ammonium hydroxide) is used will be described later.
The reaction temperature of the hydroxide by neutralization is preferably 20 to 70 ° C.
Al and Fe hydroxides precipitated by the neutralization reaction are separated and removed from the acid leaching solution by a known solid-liquid separation means.

  The above-described oxidation treatment step and neutralization treatment step separate and remove impurities Al and Fe in a single step process, but this step can also be performed in multiple steps. In that case, as a first step, by adjusting the pH of the acid leaching solution to 5 to 7 by adding an alkali, Al ions and trivalent Fe ions are precipitated as hydroxides, and these hydroxides are converted into known solid liquids. After separation / removal by the separation means, as a second step, an oxidant is added to the acid leaching solution to oxidize the divalent Fe ions in the acid leaching solution to form trivalent Fe ions. After precipitation as an oxide, the hydroxide is separated and removed by a known solid-liquid separation means. When this recovery method is used, the hydroxide of Al and the hydroxide of Fe can be separated and separated, and can be reused as a by-product.

[Recovery treatment process and drying process]
When an alkali is further added to the acid leaching solution from which Al and Fe have been separated and removed to adjust the pH to 8 or more, rare earth element hydroxide is finally deposited. The upper limit of the pH is not particularly specified, but is preferably 10 or less from the viewpoint of cost. The precipitated rare earth element hydroxide is separated and recovered from the acid leachate using a known solid-liquid separation means, washed with water, dried or fired by an electric furnace or other known heating means, and the collected rare earth element To obtain a hydroxide or oxide.
At this stage, Ca, Mg, and Ni remain in the remaining solution of the acid leaching solution.

[Complexing process]
In the first and second embodiments of the present invention described above, the impurity Cu contained in the acid leaching solution is separated and removed by cementation. In the third embodiment of the present invention, complexation is performed. The agent is coordinated to form a soluble complex and is finally separated from the rare earth element by leaving Cu in the acid leaching solution.
As the complexing agent, it is preferable to use ammonium salts containing ammonium ions, ammonium salts such as ammonium carbonate, or aqueous ammonia (ammonium hydroxide) from the viewpoint of availability and cost. Since ammonium ions usually form a four-coordinate soluble complex with Cu ions, the amount of ammonium ions must be 4 equivalents or more of the amount of Cu ions.
Addition of ammonium ions to the acid leaching solution is performed simultaneously with the neutralization treatment step or after separating and removing Fe and Al by the neutralization treatment step.
Since ammonia water also acts as an acid neutralizing agent, pH adjustment may be performed using ammonia water in place of the neutralizing agent such as alkali hydroxide in the neutralization treatment step and the recovery treatment step. Absent. When ammonium is used as a neutralizing agent, it is added in an amount of 30 equivalents or more, preferably 40 equivalents, of the Cu ion amount.

[PH measurement]
In the present invention, the pH of the reaction system is an important factor affecting the acid leaching rate of various metals, separation due to the formation of hydroxides, and the like. In the present invention, pH is defined as follows.
The pH values described in this specification are based on JIS Z8802, using glass electrodes, and as pH standard solutions, oxalate buffer solution (pH = 1.68) and phthalate buffer solution (pH = 4.01) is a value measured with a pH meter calibrated at three points using a neutral phosphate buffer (pH = 6.86) in the neutral range.
The pH described in the present specification is a value obtained by directly reading a measured value indicated by a pH meter compensated by a temperature compensation electrode under reaction temperature conditions.

[Component analysis]
The smelting slag was measured using a fluorescent X analyzer, the solution was measured using ICP-AES (emission spectroscopic analysis), and the recovered product was measured using SEM-EDS.

[Example 1]
Table 1 shows the composition of the precious metal smelting slag used for the recovery of rare earth elements.

20 g of slag of the test material pulverized by a disk mill was weighed, 250 mL of an aqueous HCl solution of pH 0.05 was added thereto, and acid leaching was performed at 70 ° C. for 2 hours while stirring at 450 rpm using a magnetic stirrer and allowed to stand. Then, it filtered using the filter paper and obtained the acid leaching solution (inorganic acid leaching process).
To the obtained acid leaching solution, 0.3 g of Fe powder (Soekawa Riken Co., Ltd., purity 99.5%, particle size 300 mesh (<50 μm)) was added, stirred at 50 ° C. for 1 hour at 450 rpm and allowed to stand. Then, it was filtered using a filter paper (cementation step).
With the filtrate after cementation maintained at 50 ° C., air was blown for 12 hours to oxidize Fe ions (oxidation treatment step).
While maintaining the solution after oxidation at 50 ° C., aqueous ammonia (NH ₄ OH aqueous solution 7 mol / L) is added, pH is adjusted to 5, and Fe (OH) ₃ and Al ₂ O ₃ are precipitated. (Neutralization treatment step and separation step).
While maintaining the neutralized solution at 50 ° C., aqueous ammonia (NH ₄ OH aqueous solution 7 mol / L) is added to adjust the pH to 8, and the rare earth hydroxide is precipitated. The precipitate is separated and collected using filter paper. (Recovery process).
The collected precipitate was a hydroxide mainly composed of rare earth elements. The precipitate obtained in the recovery step was washed with deionized water and then baked in an electric furnace at 800 ° C. for 30 minutes in an air atmosphere to obtain a final recovery product (drying step).

Table 2 shows the mass (mg) of each component contained in the acid leaching solution after the inorganic acid leaching step, the cementation step and the neutralization treatment step, and the residual solution after the recovery treatment step. By performing cementation, the Cu ion concentration in the acid leaching solution was reduced to 1% or less of the initial value.
Table 3 shows the composition of the recovered product after firing.
It has been found that by the recovery method of the present invention, an oxide of a rare earth element having a high purity and a small amount of heavy metal impurities can be obtained.

[Example 2]
FIG. 3 shows a flowchart of a technique in which noble metal smelting slag having the composition shown in Table 1 as in Example 1 is used and no cementation operation and oxidation treatment are performed.

20 g of slag of the test material pulverized by a disk mill was weighed, 250 mL of an aqueous HCl solution of pH 0.05 was added thereto, and acid leaching was performed at 70 ° C. for 2 hours while stirring at 450 rpm using a magnetic stirrer and allowed to stand. Then, it filtered using the filter paper and obtained the acid leaching solution (inorganic acid leaching process).
With the acid leaching solution kept at 50 ° C., ammonia water (NH ₄ OH aqueous solution 7 mol / L) was added to adjust to pH = 5 to precipitate Fe (OH) ₃ and Al ₂ O ₃ , and using filter paper Removed neutralization treatment step and separation step).
While maintaining the neutralized solution at 50 ° C., ammonia water (NH4OH aqueous solution 7 mol / L) is added (complexation process) to precipitate rare earth hydroxide, and the precipitate is separated and collected using filter paper. (Recovery process).
The collected precipitate was a hydroxide mainly composed of rare earth elements. The precipitate obtained in the recovery step was washed with deionized water and then baked in an electric furnace at 800 ° C. for 30 minutes in an air atmosphere to obtain a final recovery product (drying step).

Table 4 shows the mass (mg) of each component contained in the acid leaching solution after the inorganic acid leaching step, the neutralization treatment step, and the residual liquid after the recovery treatment step.
Table 5 shows the composition of the recovered product after firing.
It has been found that by performing the complexing treatment in the recovery method of the present invention, an oxide of a rare earth element having a high purity and a small amount of heavy metal impurities can be obtained.

Claims (5)

Precious metal smelting slag containing rare earth elements is dissolved in an aqueous solution of inorganic acid (excluding sulfuric acid and phosphoric acid), and acid-soluble components contained in the precious metal smelting slag are leached, and then acid-insoluble solids are separated into solid and liquid. And an inorganic acid leaching step for obtaining an acid leaching solution containing rare earth elements,
A cementation step of adding metallic iron to the acid leaching solution, depositing metal ions capable of substitution precipitation with iron in a metallic state, and then removing the deposited metal by solid-liquid separation;
An oxidation treatment step of adding an oxidizing agent to the acid leaching solution from which the deposited metal has been removed to oxidize divalent iron ions in the acid leaching solution to form trivalent iron ions;
A neutralization treatment step of adjusting the pH of the acid leaching solution after the oxidation treatment to 5 to 7 and precipitating trivalent iron ions and aluminum ions as hydroxides;
A separation step of removing the precipitated iron and aluminum hydroxide by solid-liquid separation;
Adjusting the pH of the acid leaching solution from which the hydroxide has been removed to 8-10, precipitating the rare earth element as a hydroxide, and recovering the precipitated rare earth element hydroxide by solid-liquid separation;
A method for recovering rare earth elements from precious metal smelting slag. Precious metal smelting slag containing rare earth elements is dissolved in an aqueous solution of inorganic acid (excluding sulfuric acid and phosphoric acid), and acid-soluble components contained in the precious metal smelting slag are leached, and then acid-insoluble solids are separated into solid and liquid. And an inorganic acid leaching step for obtaining an acid leaching solution containing rare earth elements,
A cementation step of adding metallic iron to the acid leaching solution, depositing metal ions capable of substitution precipitation with iron in a metallic state, and then removing the deposited metal by solid-liquid separation;
A neutralization treatment step of adjusting the pH of the acid leachate from which the deposited metal has been removed to 5 to 7 to precipitate aluminum ions as hydroxides;
An aluminum separation step of removing the precipitated aluminum hydroxide by solid-liquid separation;
An oxidant is added to the acid leaching solution from which the aluminum hydroxide has been removed to oxidize divalent iron ions in the acid leaching solution to form trivalent iron ions, and to deposit trivalent iron ions as hydroxides. An oxidation process;
An iron separation step of removing the precipitated iron hydroxide by solid-liquid separation;
A recovery step of adjusting the pH of the acid leachate from which the iron hydroxide has been removed to 8 to 10, precipitating the rare earth element as a hydroxide, and recovering the precipitated rare earth element hydroxide by solid-liquid separation; ,
A method for recovering rare earth elements from precious metal smelting slag.   The method for recovering rare earth elements from precious metal smelting slag according to claim 1 or 2, wherein the oxidizing agent is air. Precious metal smelting slag containing rare earth elements is dissolved in an aqueous solution of inorganic acid (excluding sulfuric acid and phosphoric acid), and acid-soluble components contained in the precious metal smelting slag are leached, and then acid-insoluble solids are separated into solid and liquid. And an inorganic acid leaching step for obtaining an acid leaching solution containing rare earth elements,
Adjusting the pH of the acid leaching solution to 5 to 7, and neutralizing the iron ions and aluminum ions as hydroxides;
A separation step of removing the precipitated iron and aluminum hydroxide by solid-liquid separation;
A complexation treatment step of adding one or two ammonium salts and aqueous ammonia to the acid leaching solution from which the hydroxide has been removed to convert the copper ions in the acid leaching solution into a soluble copper-ammine complex;
Adjusting the pH of the acid leaching solution after the complexing treatment step to 8-10, precipitating the rare earth element as a hydroxide, and collecting the precipitated rare earth element hydroxide by solid-liquid separation;
A method for recovering rare earth elements from precious metal smelting slag.   The method for recovering rare earth elements from precious metal smelting slag according to any one of claims 1 to 4, wherein the inorganic acid is one or two of hydrochloric acid and nitric acid.

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