JP7528581B2 - How to recover scandium - Google Patents

Description

The present invention relates to a method for recovering scandium. More specifically, the present invention relates to a method for recovering scandium that selectively recovers scandium from a sulfuric acid solution containing impurities.

Scandium is known to be extremely useful as an additive element in aluminum alloys and as an electrolyte in fuel cells. However, it has not been widely used due to its low production volume and high cost.

Scandium has a particularly small ionic radius among rare earth elements, and is hardly present in ordinary rare earth minerals. Instead, it is known to exist widely and in trace amounts in oxide ores of aluminum, tin, tungsten, zirconium, iron, nickel, etc.

In recent years, the HPAL process has been put into practical use, in which nickel oxide ore is placed in a pressurized vessel together with sulfuric acid and heated to high temperatures to separate the nickel-containing leachate from the leach residue. A neutralizing agent is added to the leachate obtained in the HPAL process to separate impurities, and a sulfurizing agent is added to recover nickel as nickel sulfide. The recovered nickel sulfide can then be processed in an existing nickel smelting process to obtain nickel salt compounds such as electrolytic nickel, nickel sulfate, and nickel chloride.

When using the HPAL process as described above, the scandium contained in the nickel oxide ore is leached together with nickel and contained in the leachate, as shown in Patent Document 1, for example. Then, when a neutralizing agent is added to the leachate to separate impurities and then a sulfurizing agent is added, nickel is recovered as nickel sulfide, and scandium is contained in the acidic solution (sulfuric acidic solution) after the addition of the sulfurizing agent without forming a sulfide. In this way, it is also known that nickel and scandium can be effectively separated by using the HPAL process.

However, the concentration of scandium in the acidic sulfuric acid solution is low (several tens of mg/L), and other impurities, especially iron and aluminum, are contained in much larger quantities.

Therefore, in order to concentrate and purify scandium, methods are used to selectively recover and concentrate scandium using adsorption methods using ion exchange resins or solvent extraction methods. After that, to further improve the scandium quality, it is recovered by carrying out processes such as adding an alkali to the concentrated scandium solution to cause a crystallization reaction to form hydroxides, or adding oxalic acid to crystallize oxalates.

For example, among the above-mentioned methods, the method of crystallizing scandium as hydroxide has the problem that it is difficult to selectively separate scandium because the starting liquid contains various impurity metals. In addition, since the obtained hydroxide is in a gel state, it is difficult to handle and has low productivity, as solid-liquid separation processes such as filtration take time.

Therefore, as described above, a crystallization reaction using oxalic acid ((COOH) ₂ ) (hereinafter referred to as "oxalation") is often used as a method for selectively recovering scandium.

For example, the oxalation method is as shown in Patent Document 2. That is, scandium is extracted from a scandium-containing solution by solvent extraction, and the back-extracted solution is crystallized as scandium oxalate using oxalic acid to separate and purify the scandium. Specifically, the oxalation process involves adding an oxidizing agent such as hydrogen peroxide to an acidic solution containing scandium and iron as an impurity to control the oxidation-reduction potential (ORP) of the acidic solution within a certain range, and adding an oxalic acid solution to recover scandium in the form of oxalate while suppressing the formation of precipitation of iron oxalate (II).

In the above-mentioned treatment, the acidic solution containing scandium and iron contains iron in the form of divalent ions, and if oxalation is carried out directly, the solubility of the resulting iron oxalate (II) is low (solubility in water: 0.022 g/100 g), and the oxalic acid reaction will result in the precipitation of iron oxalate (II).

Therefore, in order to prevent the precipitation of iron (II) oxalate, a method is used in which an oxidizing agent is added to oxidize the ORP of the acidic solution to about 700 mV with a silver/silver chloride electrode as the reference electrode, and the divalent iron ions are oxidized to trivalent iron ions. Iron (III) oxalate, which is produced by the oxalation of trivalent iron ions, has high solubility, so it is possible to produce scandium oxalate while preventing the precipitation of iron (II) oxalate.

As another method in the oxalic oxidation process, for example, Patent Document 3 discloses a method for recovering scandium in the form of high-purity scandium oxide from a solution containing scandium and iron. Patent Document 3 also discloses a method for adding a scandium sulfate acid solution to an oxalic acid solution in order to reduce the amount of oxalic acid added and save industrial equipment space (hereinafter referred to as the "reverse addition method").

This patent document 3 also discloses that in addition to performing the reverse addition method with oxalation, the pH of a solution containing scandium and iron is adjusted to a range of -0.5 or more and less than 1, and then the pH-adjusted scandium-containing solution is added to an oxalic acid solution to obtain scandium oxalate. In this way, it is possible to prevent the formation of precipitation of iron (II) oxalate without adding an oxidizing agent such as hydrogen peroxide, and further, it is possible to prevent the precipitation of impurities even when the concentration of impurities in the solution is high.

However, when oxalation is performed using the reverse addition method, the recovered material can vary in quality, resulting in problems such as an increase in the quality of impurities and the final quality falling outside the specifications.

JP 2000-313928 A JP 2014-12901 A JP 2016-141839 A

The present invention has been proposed in light of the above-mentioned circumstances, and aims to provide a method for recovering scandium, which includes a step of obtaining scandium oxalate by oxalation treatment from a scandium-containing solution containing impurity metal elements, that effectively reduces the impurity quality of the resulting scandium oxalate and suppresses the variation in the impurity quality, thereby making it possible to recover high-purity scandium.

After extensive research, the inventors discovered that the above-mentioned problems could be solved by performing the oxalation treatment using the reverse addition method in which a scandium-containing solution is added to an oxalic acid solution during the oxalation treatment process, and by adjusting and maintaining the reaction temperature of the oxalation reaction within a specific range, thus completing the present invention.

(1) The first aspect of the present invention is a method for recovering scandium from an acidic solution (scandium-containing solution) containing scandium and impurity metal elements, which includes a step of performing an oxalation treatment by a reverse addition method in which the scandium-containing solution is added to an oxalic acid solution contained in a container to obtain scandium oxalate, in which the reaction temperature in the oxalation treatment is 20°C or higher and 35°C or lower.

(2) The second aspect of the present invention is a method for recovering scandium according to the first aspect of the present invention, in which the scandium-containing solution contains at least one or more impurity metal elements selected from the group consisting of iron, nickel, and manganese.

(3) The third invention of the present invention is a method for recovering scandium according to the first or second invention, in which the scandium-containing solution is a solution obtained by separating sulfides produced by adding a sulfurizing agent to a leachate obtained by leaching nickel oxide ore with sulfuric acid.

(4) The fourth aspect of the present invention is a method for recovering scandium according to any one of the first to third aspects, in which the pH of the scandium-containing solution is adjusted to a range of -0.5 or more and less than 1.0 to perform the oxalation treatment.

(5) The fifth aspect of the present invention is a method for recovering scandium according to any one of the first to fourth aspects of the present invention, in which an oxidizing agent is not added to the scandium-containing solution subjected to the oxalation treatment.

The present invention provides a method that effectively reduces the impurity content of the resulting scandium oxalate and suppresses the variation in the impurity content, making it possible to recover high-purity scandium.

FIG. 1 is a flow diagram showing an example of a nickel oxide ore hydrometallurgical process flow. FIG. 1 is a flow diagram showing an example of a flow of an ion exchange treatment performed by an ion exchange reaction using a chelating resin. 1 shows a flow diagram of the oxalation treatment. FIG. 1 is a graph showing a comparison of the relationship between reaction temperature and Ni content in the oxalic oxidation treatment at two different oxalic acid equivalents in Test Examples 1 to 16. FIG. 1 is a graph showing a comparison of the relationship between reaction temperature and Mn content in the oxalic oxidation treatment at two different oxalic acid equivalents in Test Examples 1 to 16. FIG. 1 is a graph showing a comparison of the relationship between the reaction temperature and the Fe content in the oxalic oxidation treatment at two different oxalic acid equivalents in Test Examples 1 to 16.

A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail below. Note that the present invention is not limited to the following embodiment, and various modifications are possible without departing from the gist of the present invention. In addition, in this specification, the expression "X to Y" (X and Y are any numerical values) means "X or more and Y or less."

≪1. Overview≫
The method for recovering scandium according to the present embodiment is a method for recovering scandium from an acidic solution containing scandium and impurity metal elements (hereinafter also referred to as a "scandium-containing solution").

Specifically, this scandium recovery method includes a step of subjecting a scandium-containing solution containing scandium and impurity metal elements to an oxalation treatment in which a reaction is caused to oxalate scandium using oxalic acid. In particular, in the scandium recovery method according to this embodiment, the oxalation treatment is carried out by a so-called "reverse addition" method in which a scandium-containing solution is added to an oxalic acid solution contained in a container to obtain scandium oxalate.

The scandium-containing solution containing impurity metal elements to be subjected to the oxalation treatment can be a solution (post-sulfurization solution) obtained by sulfurizing the leachate obtained by high-pressure acid leaching (HPAL) treatment of nickel oxide ore to separate nickel. The post-sulfurization solution can also be further subjected to ion exchange treatment and/or solvent extraction treatment to separate impurities and produce a solution in which scandium is concentrated, and this concentrated solution can be used. Note that in the HPAL process, nickel is converted into sulfide by the sulfurization treatment, while scandium can be left in the solution, allowing nickel and scandium to be effectively separated.

The scandium recovery method according to this embodiment is characterized in that in the step of performing the oxalation treatment by the reverse addition method, the reaction temperature is adjusted to a specific range to cause the oxalation reaction. Specifically, the reaction temperature in the oxalation treatment is set to 20°C or higher and 35°C or lower to cause the oxalation reaction.

This method prevents impurity metal elements contained in the scandium-containing solution from precipitating as oxalates, effectively reducing the impurity content of the resulting scandium oxalate crystals. In addition, it is possible to reduce the variation in impurity content in scandium oxalate, making it possible to stably obtain high-purity scandium oxalate.

In addition, for example, the scandium oxalate crystals obtained by such oxalation treatment can be roasted to recover them in the form of scandium oxide.

≪2. How to recover scandium≫
The scandium recovery method according to the present embodiment will be described in more detail below.

<2-1. Nickel oxide ore hydrometallurgy process>
In the method for recovering scandium according to the present embodiment, as described above, the solution containing scandium and impurity metal elements (scandium-containing solution) can be a solution (post-sulfurization solution) obtained by subjecting nickel oxide ore to high-pressure acid leaching treatment and separating nickel as sulfide from the leachate. Also, a solution obtained by removing impurity components from the post-sulfurization solution by ion exchange treatment or solvent extraction treatment can be used.

These scandium-containing solutions contain impurity metal elements such as iron (Fe), manganese (Mn), aluminum (Al), chromium (Cr), and magnesium (Mg). Nickel (Ni) that was not separated as a sulfide may also be included as an impurity.

Below, we first explain the hydrometallurgical process of nickel oxide ore to obtain the starting scandium-containing solution.

Figure 1 is a flow diagram showing the flow of the hydrometallurgical process of nickel oxide ore. The hydrometallurgical process of nickel oxide ore includes a leaching process S11 in which nickel oxide ore is leached with sulfuric acid under high temperature and high pressure to obtain a leached slurry, a solid-liquid separation process S12 in which the leached slurry is separated into a leaching liquid and a leaching residue, a neutralization process S13 in which a neutralizing agent is added to the leaching liquid to obtain a neutralized precipitate containing impurities and a neutralized liquid, and a sulfurization process S14 in which a sulfurizing agent is added to the neutralized liquid to obtain nickel sulfide and a sulfurized liquid.

(1) Leaching Step The leaching step S11 is a step in which, for example, using a high-temperature pressurized vessel (autoclave), sulfuric acid is added to a slurry of nickel oxide ore, and high-pressure steam and high-pressure air are supplied, and stirring is performed at a temperature of 240° C. to 260° C. to generate a leaching slurry consisting of a leachate containing nickel and a leach residue containing hematite. Note that scandium is contained in the leachate together with nickel.

Here, nickel oxide ores mainly include so-called laterite ores such as limonite ore and saprolite ore. The nickel content of laterite ore is usually 0.8 to 2.5% by weight, and is contained as hydroxide or magnesia (magnesium silicate) mineral. These nickel oxide ores also contain scandium.

(2) Solid-liquid separation step The solid-liquid separation step S12 is a step of performing multi-stage washing of the leaching slurry produced in the above-mentioned leaching step S11 to separate into a leachate containing nickel and cobalt and a leaching residue which is hematite.

In the solid-liquid separation step S12, the leaching slurry is mixed with the washing liquid, and then a solid-liquid separation process is carried out using a solid-liquid separation device such as a thickener. Specifically, the slurry is first diluted with the washing liquid, and then the leaching residue in the slurry is concentrated as a sediment in the thickener. This makes it possible to reduce the amount of nickel adhering to the leaching residue according to the degree of dilution. In actual operation, thickeners with this function are used in multiple stages connected together.

(3) Neutralization Step In the neutralization step S13, a neutralizing agent is added to the leachate to adjust the pH to obtain a neutralized precipitate containing impurity elements and a neutralized liquid. As a result of the neutralization treatment in the neutralization step S13, valuable metals such as nickel, cobalt, and scandium are contained in the neutralized liquid, and most of the impurities including aluminum become the neutralized precipitate.

In the neutralization step S13, known neutralizing agents can be used, such as limestone, slaked lime, and sodium hydroxide. In the neutralization process, it is preferable to adjust the pH to a range of 1 to 4 while suppressing oxidation of the separated leachate, and it is more preferable to adjust the pH to a range of 1.5 to 2.5. If the pH is less than 1, neutralization will be insufficient, and separation into the neutralized precipitate and the neutralized liquid may not be possible. On the other hand, if the pH exceeds 4, not only impurities such as aluminum but also valuable metals such as scandium and nickel may be contained in the neutralized precipitate.

(4) Sulfurization Step The sulfurization step S14 is a step of adding a sulfurizing agent to the neutralized liquid obtained in the neutralization step S13 to obtain nickel sulfide and a sulfurized liquid. By the sulfurization treatment in the sulfurization step S14, nickel, cobalt, zinc, etc. are recovered as sulfides, and scandium, etc. remain in the sulfurized liquid. Therefore, nickel and scandium can be effectively separated by the sulfurization treatment in such a hydrometallurgical process.

Specifically, in the sulfurization step S14, a sulfurizing agent such as hydrogen sulfide gas, sodium sulfide, or sodium hydrosulfide is supplied to the resulting neutralized liquid to produce a sulfide (nickel sulfide) containing nickel with few impurities, and a sulfurized liquid containing scandium and the like with the nickel concentration stabilized at a low level.

In the sulfurization process S14, the nickel sulfide slurry is separated using a sedimentation device such as a thickener, and the nickel sulfide is separated and recovered from the bottom of the thickener, while the post-sulfurization liquid, which is an aqueous solution component, is allowed to overflow and recovered.

In the scandium recovery method according to the present embodiment, for example, the post-sulfurization liquid, which is a sulfuric acidic solution obtained through each step in the hydrometallurgical process of nickel oxide ore as described above, can be recovered and used as the starting material. In addition, the post-sulfurization liquid can be subjected to the ion exchange treatment and solvent extraction treatment described below to obtain a solution, which can be used as the starting material. In the scandium recovery method, scandium oxide is produced from such a solution.

<2-2. Ion exchange treatment, solvent extraction treatment>
In the method for recovering scandium according to the present embodiment, as described above, the scandium-containing solution can be a solution obtained by subjecting a post-sulfurization liquid obtained through a sulfurization step in a hydrometallurgical process for nickel oxide ore to an ion exchange treatment and a solvent extraction treatment. In this way, by subjecting the post-sulfurization liquid to an ion exchange treatment and/or a solvent extraction treatment, it is possible to separate and remove impurities in the solution and concentrate scandium.

Below, we will explain the ion exchange process and the solvent extraction process. Note that the solvent extraction process is explained using an example in which the eluent obtained through the ion exchange process is treated, but it is also possible to perform only the solvent extraction process without performing the ion exchange process.

(1) Ion exchange treatment The post-sulfurization solution contains aluminum, chromium, and other impurities. For this reason, when recovering scandium in the solution as scandium oxide, it is preferable to remove these impurities and concentrate the scandium. One method for concentrating scandium is an ion exchange treatment using a chelating resin.

Figure 2 is a flow diagram showing an example of the flow of ion exchange processing performed by an ion exchange reaction using a chelating resin. Note that Figure 2 also shows the flow up to subjecting the scandium eluate obtained by the ion exchange processing to a solvent extraction process. In the ion exchange processing, the post-sulfurization liquid obtained through the sulfurization step S14 (Figure 1) in the hydrometallurgical process of nickel oxide ore is brought into contact with a chelating resin to adsorb the scandium in the post-sulfurization liquid onto the chelating resin, thereby obtaining a scandium (Sc) eluate.

The mode (each step) of the ion exchange treatment is not particularly limited, but for example, as shown in FIG. 2, an example includes an adsorption step S21 in which the post-sulfurization liquid is brought into contact with a chelating resin to adsorb scandium to the chelating resin, an aluminum removal step S22 in which the chelating resin is brought into contact with sulfuric acid to remove aluminum adsorbed to the chelating resin, a scandium elution step S23 in which the chelating resin that has been through the aluminum removal step S22 is brought into contact with sulfuric acid to obtain a scandium eluate, and a chromium removal step S24 in which the chelating resin that has been through the scandium elution step S23 is brought into contact with sulfuric acid to remove chromium adsorbed to the chelating resin in the adsorption step S21.

[Adsorption process]
In the adsorption step S21, the post-sulfurization liquid is brought into contact with a chelating resin to adsorb scandium to the chelating resin. The type of chelating resin is not particularly limited, and for example, a resin having iminodiacetic acid as a functional group can be used.

[Aluminum removal process]
In the aluminum removal step S22, the chelating resin that has adsorbed scandium in the adsorption step S21 is contacted with sulfuric acid of 0.1 N or less to remove the aluminum adsorbed to the chelating resin. When removing the aluminum, the pH is preferably maintained in the range of 1 to 2.5, more preferably 1.5 to 2.0.

[Scandium elution step]
In the scandium elution step S23, the chelating resin that has been subjected to the aluminum removal step S22 is contacted with sulfuric acid of 0.3 N or more and less than 3 N to obtain a scandium eluate. When obtaining the scandium eluate, it is preferable to maintain the normality of the sulfuric acid used in the eluate in the range of 0.3 N or more and less than 3 N, and more preferably in the range of 0.5 N or more and less than 2 N.

[Chromium removal process]
In the chromium removal step S24, the chelating resin that has been subjected to the scandium elution step S23 is contacted with sulfuric acid of 3N or more to remove the chromium adsorbed on the chelating resin. When removing chromium, if the normality of the sulfuric acid used in the eluent is less than 3N, chromium is not properly removed from the chelating resin, which is not preferable.

By this type of ion exchange process, impurities such as aluminum and chromium are removed, and a scandium eluate in which scandium is concentrated can be obtained. The concentration of the scandium eluate can be increased by repeating the same ion exchange process on the resulting scandium eluate. The more times the process is repeated, the higher the concentration of the recovered scandium will be, but even if the process is repeated too many times, the degree of increase in the concentration of the recovered scandium will be small, so industrially it is preferable to repeat the process no more than about eight times.

(2) Solvent Extraction Treatment In the solvent extraction treatment, the scandium (Sc) eluate obtained through the above-mentioned ion exchange treatment is brought into contact with a predetermined extractant to extract scandium.

The extractant used in the solvent extraction is not particularly limited, and amine extractants, phosphoric acid extractants, etc. can be used. Depending on the type of extractant, the scandium to be extracted can be extracted into an organic solvent containing the extractant, or the extractant can be made to selectively extract impurity components, leaving scandium in the raffinate. For example, when an amine extractant is used, since it is an extractant with low selectivity for scandium, impurity components are selectively extracted into the organic solvent, and scandium is concentrated in the raffinate.

The mode (each step) of the solvent extraction process is not particularly limited, but examples include an extraction step S31 in which the scandium eluent and the extractant are mixed to extract impurities and separate the resulting organic solvent and raffinate containing scandium, a scrubbing step S32 in which the post-extraction organic solvent is mixed with a hydrochloric acid solution or a sulfuric acid solution to separate the scandium contained in trace amounts in the post-extraction organic solvent, and a back-extraction step S33 in which the post-wash organic solvent is mixed with the back-extraction starting liquid to back-extract the impurities from the post-wash organic solvent and obtain a back-extracted liquid. In this way, the purity of scandium contained in the scandium eluent can be further increased by performing the solvent extraction process.

[Extraction process]
In the extraction step S31, the scandium-containing solution is mixed with an organic solvent containing an extractant to selectively extract impurities into the organic solvent, thereby obtaining an organic solvent containing impurities and a raffinate in which scandium is concentrated.

For example, an amine-based extractant is used as the extractant. Examples of amine-based extractants that can be used include those known by trade names such as Primene JM-T, which is a primary amine, LA-1, which is a secondary amine, and TNOA (Tri-n-octylamine) and TIOA (Tri-i-octylamine), which are tertiary amines.

During extraction, it is preferable to use an extractant such as an amine extractant diluted with, for example, a hydrocarbon organic solvent. The concentration of the extractant in the organic solvent is not particularly limited, but considering phase separation during extraction and back extraction (described later), it is preferably about 1% by volume to 10% by volume, and more preferably about 5% by volume. In addition, the volume ratio of the organic solvent to the scandium-containing solution during extraction is not particularly limited, but it is preferable to set the organic solvent molar amount to about 0.01 times to 0.1 times the metal molar amount in the scandium-containing solution.

[Scrubbing (cleaning) process]
In the extraction step S31, when a small amount of scandium coexists in the organic solvent used to extract impurities from the scandium-containing solution, it is preferable to subject the organic solvent (organic phase) to a scrubbing (washing) treatment before stripping the resulting extract, and separate scandium into the aqueous phase and recover it from the extractant (scrubbing step S32). By providing the scrubbing step S32 to wash the organic solvent and separate the small amount of scandium extracted by the extractant, it is possible to separate scandium in the washing liquid, and thus to further increase the recovery rate of scandium.

As the solution (cleaning solution) used for scrubbing, a sulfuric acid solution, a hydrochloric acid solution, etc. can be used. Also, a solution containing water-soluble chlorides or sulfates can be used. Specifically, when using a sulfuric acid solution as the cleaning solution, it is preferable to use one with a concentration range of 1.0 mol/L or more and 3.0 mol/L or less.

The number of washing stages (number of times) depends on the type and concentration of impurity elements, and can be changed appropriately depending on the extraction agent and extraction conditions. For example, if the phase ratio of the organic phase (O) to the aqueous phase (A) is O/A = 1, scandium extracted into the organic solvent can be separated to below the lower detection limit of the analytical device by using about 3 to 5 stages.

[Back extraction process]
In the back extraction step S33, impurities are back extracted from the organic solvent from which the impurities were extracted in the extraction step S31. Specifically, in the back extraction step S33, a back extraction solution (starting back extraction liquid) is added to an organic solvent containing an extractant and mixed, thereby causing a reaction opposite to the extraction treatment in the extraction step S31 to back extract the impurities, and a post-back extraction liquid containing the impurities is obtained.

The concentration of the carbonate-containing solution used as the back-extraction solution is preferably, for example, about 0.5 mol/L to 2 mol/L in order to prevent excessive use.

In addition, if a scrubbing process is performed on the organic solvent containing the extractant in the scrubbing step S32, a back extraction process can be performed by adding and mixing the back extraction solution to the extractant after scrubbing.

In this way, a carbonate solution such as sodium carbonate is added to the extracted extractant or the scrubbed extractant, and the extractant after the back-extraction process is carried out to separate the impurities can be reused as the extractant in the extraction process in extraction step S31.

<2-3. Oxidation treatment>
The method for recovering scandium according to the present embodiment includes a step of converting scandium into oxalate (scandium oxalate) by subjecting a scandium-containing solution to an oxalation treatment. Specifically, an oxalation treatment is performed to generate crystals of scandium oxalate using a scandium-containing solution that is a reverse extract obtained through the above-mentioned solvent extraction treatment. In this way, by converting scandium into oxalate by the oxalation treatment, it is possible to improve handling properties such as filterability, and to efficiently recover scandium.

The scandium-containing solution obtained through the solvent extraction process is a solution in which scandium is concentrated, with most of the impurity metal elements having been removed by the above-mentioned ion exchange process and solvent extraction process. However, it still contains a certain amount of impurity metal elements that were not removed by these processes (see the composition of the oxalation starting solution shown in Table 1 in the examples described below). Specifically, such impurity metal elements include at least iron, nickel, manganese, aluminum, chromium, magnesium, etc.

The oxalation treatment here is carried out by the so-called "reverse addition" method. Specifically, a scandium-containing solution is gradually added to an oxalic acid solution contained in a specified container, causing solid crystals of scandium oxalate to precipitate. This type of oxalation treatment based on the reverse addition method can suppress the oxalation of iron, an impurity metal element, and prevent the precipitation of iron (II) oxalate, without the need to add an oxidizing agent such as hydrogen peroxide to adjust the oxidation-reduction potential (ORP).

The oxalation treatment using the reverse addition method is characterized in that the reaction temperature is within a specific range, specifically, 20°C or higher and 35°C or lower. It is preferably 20°C or higher and 30°C or lower, and more preferably 23°C or higher and 30°C or lower.

Conventional oxalation treatment is generally carried out by the so-called "normal addition" method (standard addition method), in which an oxalic acid solution is added to a scandium-containing solution, and the reaction temperature is also left to the course of events. However, although this oxalation treatment preferentially produces scandium oxalates and can suppress the precipitation of impurity metal elements, the impurity quality in the resulting scandium oxalate tends to vary depending on the treatment conditions, making it impossible to consistently obtain scandium oxalate that meets the specified specifications.

In the oxalic oxidation process, metal elements other than the rare earth element scandium also react with oxalic acid to form complexes, but impurity metal elements with less stable complexes are adsorbed into the scandium oxalate precipitate and coprecipitated. The stability of such complexes depends not only on the type of metal element but also on the conditions of the oxalic oxidation process, which is thought to result in variation in the impurity grade in the resulting scandium oxalate.

In this embodiment, as described above, the oxalation process is performed by the "reverse addition" method in which a scandium-containing solution is added to an oxalic acid solution contained in a specified container to cause an oxalation reaction, and the reaction temperature is set to 20°C or higher and 35°C or lower. This makes it possible to effectively reduce the impurity quality contained in the scandium oxalate crystals by suppressing the oxalation of impurity metal elements contained in the scandium-containing solution. In addition, it is possible to suppress the variation in the impurity quality contained in the crystals regardless of the conditions of the oxalation process. In particular, it is possible to effectively suppress the variation in the quality of iron (Fe), nickel (Ni), and manganese (Mn) among the impurity metal elements contained in the scandium-containing solution.

The mechanism by which setting the reaction temperature within a specific range reduces the variation in impurity content in scandium oxalate is unclear, but it is possible that the number of water molecules of hydration changes depending on the temperature during the oxalation reaction, which affects the solubility of the impurities.

This type of oxalation treatment method makes it possible to obtain scandium oxalate with stable impurity content specifications and a high purity of scandium, and by roasting this scandium oxalate, it is possible to obtain high-quality scandium oxide.

Figure 3 shows a flow diagram of the oxalation process. As described above, this embodiment is characterized in that the reaction temperature during the oxalation process by the reverse addition method is 20°C or higher and 35°C or lower. Specifically, the oxalation process includes a temperature adjustment step S41 for adjusting the temperature of the scandium-containing solution, and a crystallization step S42 for adding the temperature-adjusted solution (oxalation starting solution) to an oxalic acid solution contained in a container to precipitate scandium oxalate crystals. The resulting scandium oxalate crystals can be recovered by passing through a filtration and washing step S43 in which a filtration and washing process is performed.

In addition, it is preferable to adjust the pH of the scandium-containing solution to a range of -0.5 to 1.0 prior to the treatment in the crystallization step S42. That is, although not shown in FIG. 3, a pH adjustment step is provided to adjust the pH of the scandium-containing solution to be treated in the crystallization step S42 to the above range in advance. This increases the solubility of iron (II) oxalate and the like, and allows the precipitation of impurities to be more effectively suppressed without using expensive oxidizing agents, etc., to recover scandium of higher purity. The pH adjustment of the scandium-containing solution may be performed before or after the temperature adjustment step S41, or may be performed at the same time as the temperature adjustment.

(Temperature adjustment process)
In the temperature adjustment step S41, the temperature of the scandium-containing solution to be subjected to the oxalation treatment is adjusted to 20° C. or more and 35° C. or less, preferably 20° C. or more and 30° C. or less, and more preferably 23° C. or more and 30° C. or less. In this way, by adjusting the temperature of the scandium-containing solution to a range of 20° C. or more and 35° C. or less and maintaining that temperature to cause a reaction in the next crystallization step S42, the impurity quality contained in the obtained scandium oxalate crystals can be reduced and the variation in the impurity quality can be suppressed.

The method for adjusting the temperature of the scandium-containing solution is not particularly limited, and can be adjusted, for example, using a heater, etc.

As described above, in the oxalation treatment, the scandium-containing solution is gradually added to the oxalic acid solution contained in the container to cause the oxalation reaction, so it is preferable to adjust the temperature of the oxalic acid solution used in the oxalation treatment in advance to a range of 20°C to 35°C and maintain that temperature to cause the reaction. This allows the reaction temperature to be appropriately maintained in the range of 20°C to 35°C, and the variation in impurity quality in the resulting scandium oxalate can be more effectively suppressed.

(Crystallization process)
In the crystallization step S42, the reaction temperature is set to 20° C. or higher and 35° C. or lower, and a scandium-containing solution (oxalation starting liquid) is added to the oxalic acid solution to perform an oxalation treatment in which a reaction occurs in which scandium is oxalated, thereby obtaining scandium oxalate (scandium oxalate crystals).

Regarding the reaction temperature of the oxalation reaction, if the reaction temperature is higher than 35°C, the impurity quality in the resulting scandium oxalate will vary. Also, if the reaction temperature is lower than 20°C, the reaction rate will decrease and the solubility of the oxalate will decrease, which may cause problems such as precipitation in undesirable locations such as the supply pipes.

As described above, this is done by the "reverse addition" method, in which an oxalic acid solution is placed in a reaction vessel and a scandium-containing solution is added to the oxalic acid solution to cause a reaction. The amount of oxalic acid solution is preferably at least 1.05 times the equivalent amount required to precipitate the scandium in the scandium-containing solution as oxalate, and more preferably 2.0 times or more. If the amount used is less than 1.05 times the required equivalent amount, there is a possibility that the entire amount of scandium cannot be effectively recovered.

It is preferable to use an oxalic acid solution whose pH is adjusted to the range of -0.5 to 1.0. In particular, when adjusting the pH of the scandium-containing solution to the range of -0.5 to 1.0, the impurity content can be further reduced by adjusting the pH of the oxalic acid solution as well, allowing high-purity scandium to be recovered.

In the above example, the scandium-containing solution is adjusted to a temperature of 20°C or higher and 35°C or lower in advance, and the reaction temperature in the crystallization step S42 is set to 20°C or higher and 35°C or lower while maintaining that temperature. However, this is not limited to this. In other words, it is sufficient that the reaction temperature when the scandium-containing solution is added to the oxalic acid solution to cause the reaction is maintained in the range of 20°C or higher and 35°C or lower.

<2-4. Generation of scandium oxide (roasting)>
In this embodiment, the scandium oxalate crystals obtained by the oxalation treatment as described above are roasted to produce scandium oxide.

In the roasting process, the scandium oxalate crystals obtained by the oxalation process are washed with water, dried, and then roasted. Through this roasting process, scandium can be recovered as scandium oxide.

In particular, in this embodiment, the reaction temperature in the above-mentioned oxalation treatment is set to a specific range, so that the impurity quality of the scandium oxalate produced is effectively reduced and the variation in the impurity quality is also suppressed. Therefore, by roasting such scandium oxalate, it is possible to obtain scandium oxide containing high purity scandium with reduced impurity quality and suppressed variation in impurity quality.

The conditions for the roasting process are not particularly limited, but for example, the material may be placed in a tubular furnace and heated at about 900°C for about 2 hours. From an industrial perspective, it is preferable to use a continuous furnace such as a rotary kiln, since this allows drying and roasting to be carried out in the same device.

The present invention will be described in more detail below with reference to examples and comparative examples. Note that the present invention is not limited to the following examples.

(Hydrometallurgical process for nickel oxide ore)
Nickel oxide ore was leached with sulfuric acid using an autoclave, and the resulting leachate was neutralized by adding hydrated lime. Next, a sulfurizing agent was added to the resulting neutralized liquid to cause a sulfurization reaction, and nickel, cobalt, etc. were separated as sulfides, and a sulfurized liquid containing scandium was obtained.

(Ion exchange treatment, neutralization treatment)
Next, the obtained sulfurized solution was subjected to an ion exchange treatment using a chelating resin to separate impurities in the solution and obtain an eluate (scandium eluate) containing scandium eluted from the chelating resin. After that, a neutralizing agent was added to the scandium eluate to generate a precipitate of scandium hydroxide.

(Solvent extraction treatment)
Next, sulfuric acid was added to the scandium hydroxide precipitate to dissolve it again, thereby obtaining a solution (scandium solution). This scandium solution was then subjected to a solvent extraction process using an amine extractant, and a scandium solution (scandium-containing solution) was obtained as a raffinate, which was purified by removing most of the impurities and in which scandium was concentrated.

(Oxalation treatment)
Next, the obtained scandium-containing solution was used to carry out an oxalation treatment to convert the scandium in the solution into oxalate.

Specifically, sulfuric acid was added to the scandium-containing solution to adjust the pH to 0.28, and water was added to dilute the solution until the scandium concentration in the solution reached approximately 4.5 g/L, preparing an oxalate starting solution. 500 ml of oxalate starting solution was prepared for each test under the conditions shown in Tables 1 and 2 below. The pH was adjusted under the same temperature conditions as the reaction temperature in each test. The oxidation-reduction potential (ORP) of the scandium-containing solution was 500 mV to 560 mV, using a silver/silver chloride electrode as the standard electrode.

In addition, a solution containing 100 g of oxalic acid dissolved per 1 L of liquid was prepared as the oxalic acid solution to be used for the oxalation treatment.

In the oxalic acid treatment, 183 ml of oxalic acid solution, which is 2.7 equivalents to the scandium contained in the oxalic acid starting solution used in each test, or 344 ml of oxalic acid solution, which is 5.1 equivalents, was placed in a container, and the oxalic acid starting solution (scandium-containing solution) was added to the oxalic acid solution at a flow rate of 2.3 ml/min to cause a reaction. The temperature during the reaction (reaction temperature) was set to room temperature (23°C), 30°C, 35°C, 42°C, or 50°C for each test, and the temperature was maintained from the start of the addition of the oxalic acid starting solution to the oxalic acid solution until the end of one hour of stirring.

Table 1 below shows the test conditions for each test, such as the equivalent amount of oxalic acid solution and reaction temperature, as well as the analysis results for the concentration of metal components contained in the scandium-containing solution (starting oxalation solution) subjected to oxalation treatment. As shown in Table 1, it can be seen that the scandium-containing solution contains various impurity metal elements.

After stirring, the mixture was left to stand to allow the crystallized scandium oxalate to settle. Next, the supernatant was removed until only 150 ml of liquid remained in the container, and 150 ml of pure water was added and the washing operation of stirring and washing was repeated twice. Each washing operation was performed under the same temperature conditions as the temperature (reaction temperature) when the oxalic acid was added.

Next, after washing, the entire amount was filtered using a suction filtration device and a membrane filter with a pore size of 0.45 μm to separate the scandium oxalate crystals. Furthermore, the separated crystals were placed in 140 ml of pure water to give a slurry concentration of approximately 50 g/L, and this repulp washing process was repeated twice.

After the repulp washing, the entire volume was filtered using the same suction filtration device and membrane filter to recover scandium oxalate crystals. The resulting scandium oxalate crystals were then dried overnight in an atmospheric dryer at 105°C, and the dried crystals were analyzed by ICP atomic absorption spectrometry to confirm the impurity grade.

Table 2 below shows the analysis results of the impurity content of the scandium oxalate obtained in Test Examples 1 to 16.

As shown in Table 2, in Test Examples 1 to 10, in which the reaction temperature during oxalic oxidation was in the range of 20°C to 35°C, impurity quality was effectively reduced compared to Test Examples 11 to 16, in which the reaction temperature was higher than 35°C, and the variation in impurity quality was suppressed even when there were differences in other conditions such as the oxalic acid equivalent under the same reaction temperature conditions. Test Examples 11 to 16 were not only inferior in reducing impurity quality, but also caused variation in impurity quality due to differences in the oxalic acid equivalent conditions under the same reaction temperature conditions.

In order to confirm the variation in the impurity grades contained in the resulting scandium oxalate, the following Table 3 summarizes the relationship between the reaction temperature and the oxalic acid equivalent and the impurity grades, focusing on the grades of Ni, Mn, Fe, and Al as impurity metal elements in all 16 examples from Test Example 1 to Test Example 16. Also, Figures 4 to 6 are graphs comparing the relationship between the Ni grade (Figure 4), Mn grade (Figure 5), and Fe grade (Figure 6) and the reaction temperature, respectively, based on Table 3 below, for two types of oxalic acid equivalent (2.7 equivalents, 5.1 equivalents).

As can be seen from the graphs in Figures 4 to 6, when the oxalic oxidation treatment was performed at a reaction temperature in the range of 20°C to 35°C, there was no variation in the impurity quality regardless of the oxalic acid equivalent condition, and the quality was low. On the other hand, when the reaction temperature was above 35°C, it was clearly seen that the impurity quality varied depending on the oxalic acid equivalent.

Claims (4)

A method for recovering scandium from an acidic solution (scandium-containing solution) containing scandium and impurity metal elements, comprising the steps of:
The scandium-containing solution contains, as the impurity metal element, at least one or more selected from the group consisting of iron, nickel, and manganese,
The method includes a step of performing an oxalation treatment by a reverse addition method in which the scandium-containing solution is added to an oxalic acid solution contained in a container to obtain scandium oxalate;
In the above step, the reaction temperature in the oxalation treatment is set to 20° C. or higher and 35° C. or lower , and the iron content in the generated scandium oxalate is set to 8 ppm or less, the nickel content to 180 ppm or less, and the manganese content to 71 ppm or less.
How to recover scandium. The scandium-containing solution is a solution obtained by leaching nickel oxide ore with sulfuric acid, adding a sulfurizing agent to the leachate, and separating the sulfide produced therefrom.
2. The method for recovering scandium according to claim 1 . In the above step, the pH of the scandium-containing solution is adjusted to a range of −0.5 or more and less than 1.0, and the oxalation treatment is performed.
3. The method for recovering scandium according to claim 1 or 2 . No oxidizing agent is added to the scandium-containing solution subjected to the oxalation treatment.
4. The method for recovering scandium according to claim 1 .

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