JP2010163657A - Process for recovering rare earth element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a recovering process which recovers a useful rare earth element from a rare earth element-iron based alloy, improves the handlability of a by-produced iron residue and decreases the amount of the produced iron residue. <P>SOLUTION: This recovering process includes: heating a slurry prepared by mixing the rare earth element-iron based alloy with an aqueous alkaline solution while blowing air therein to convert the rare earth element to its hydroxide and the iron to triiron tetraoxide; and filtering the resultant slurry. The filtrate is reused as the aqueous alkaline solution for next treatment. The recovering process further includes: adding water to the separated solid matter to make it slurry; adding an acid to the slurry preferably while blowing air therein to leach the rare earth element with the acid; after the leaching operation has been completed, filtering the resultant slurry; and recovering the solution in which the salt of the rare earth element dissolves, and the solid residue containing triiron tetraoxide as a main component. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for recovering a rare earth element from a rare earth element-iron alloy. The method of the present invention has a small amount of by-product iron residues and is easy to process.

  Magnets using rare earth elements-iron alloys, specifically Nd-Fe-B alloys (also called neodymium magnets) exhibit remarkably excellent magnetic performance among mass-produced permanent magnets. In particular, the market is greatly expanding for electronic devices.

  In the manufacturing process of this magnet, a large amount of process scraps of rare earth element-iron alloys are generated. In order to recover useful components, particularly expensive rare earth elements, from this process waste, a method of selectively leaching rare earth elements with an acid while controlling the pH to 3 to 5 in the presence of an oxidizing agent is disclosed in JP-A-5-287405. (Patent Document 1). Rare earth metal is leached into the liquid, and iron is separated as an iron residue from the rare earth element in solution as an insoluble trivalent iron compound. From the leachate which is a rare earth element solution, the rare earth element can be recovered as an insoluble precipitate such as carbonate.

  Since divalent iron ions generated by the dissolution of iron cannot be precipitated under neutral to acidic conditions in which the rare earth element is in a dissolved state, the iron is precipitated as an insoluble compound and separated from the rare earth element by oxidation. Therefore, it is necessary to change to a trivalent iron compound. In Patent Document 1, iron reacts with an oxidizing agent to become iron oxyhydroxide (FeOOH). Therefore, the iron residue separated from the leachate containing rare earth elements is mainly composed of iron oxyhydroxide. However, this iron residue is in the form of fine particles or gel containing a large amount of moisture, and has a problem in handling. In order to recover rare earth elements economically, it is important that the iron residue produced as a by-product is easy to process.

  Since the rare earth element-iron-based alloy serving as the processing raw material contains iron at a high rate exceeding 50%, the amount of residue generated is large. For example, Example 1 of Patent Document 1 describes that the amount of alloy powder to be processed is 3.0 kg and the amount of residue recovered after leaching with hydrochloric acid is 7.8 kg. In Examples 2 and 3 as well, a residue twice as much as the treated alloy is generated.

  The large amount of iron residue generated in this way is preferably used as a raw material for iron making. However, in the case of iron residues that contain a large amount of moisture and are mainly composed of sticky iron oxyhydroxide, solid-liquid separation, handling, and drying There are many problems such as processing, and further deterioration of fuel intensity at the time of making steel.

  Further, in the method described in Patent Document 1, the recovery rate of elements that are not easily leached among rare earth elements such as Dy is somewhat lower than the recovery rate of elements that are relatively easily leached such as Nd, and the entire rare earth element The recovery rate is also somewhat reduced. Since Dy is more expensive than Nd, it is economically advantageous to collect as much Dy as possible.

JP-A-5-287405

  The present invention recovers useful rare earth elements from rare earth element-iron-based alloys and reduces the generation amount of iron residues as a by-product and makes them easy to handle. It is an object of the present invention to provide a method for efficiently and economically recovering rare earth elements from an alloy.

  Another object of the present invention is that there are few restrictions on the liquid pH and liquid temperature during the leaching of rare earth elements with an acid, the leaching rate can be increased as compared with conventional methods, and a rare earth such as Dy that is relatively difficult to leach out. It is to provide a method for recovering rare earth elements from rare earth element-iron alloys, which can recover elements at a high recovery rate.

  According to the present invention, the rare earth element-iron-based alloy is first heated in an alkaline aqueous solution while blowing air (that is, in the presence of an oxidizing agent), and then the resulting solid is subjected to an acid leaching treatment. By carrying out, an iron residue which is easy to handle and contains triiron tetroxide instead of iron oxyoxide as a by-product is produced as a by-product, and the recovery rate of Dy is improved, and all of the above problems can be solved.

The present invention is a method for recovering a rare earth element from a rare earth element-iron-based alloy characterized by including the following steps:
(A) heating a rare earth element-iron alloy slurry in an alkaline aqueous solution in the presence of an oxidizing agent;
(B) The slurry obtained in step (a) is subjected to solid-liquid separation to recover a solid,
(C) acidifying the slurry of the solid recovered in step (b) in water, preferably in the presence of an oxidizing agent, to leach the rare earth element into the liquid; and (d) in step (c) The obtained slurry is subjected to solid-liquid separation to be separated into a solution containing a rare earth element and a residue mainly composed of triiron tetroxide.

  In the alkali treatment step (a), the rare earth element is converted into a hydroxide by treating the rare earth element-iron-based alloy with an alkaline aqueous solution while heating in the presence of an oxidizing agent such as air blowing (eg, molecular oxygen). In addition, iron is oxidized to triiron tetroxide. However, both remain solid.

  In the acid leaching step (c), the solid material consisting of the rare earth element hydroxide and triiron tetroxide recovered in the step (b) is slurried in water, and the oxidant is preferably blown into the slurry by blowing air. In the presence of acid, the acid is added and acidified to turn the rare earth element into a soluble salt that is leached into the liquid while iron remains insoluble triiron tetroxide. Thereafter, when the slurry is subjected to solid-liquid separation in step (d), a rare earth element solution substantially free of iron is obtained. Thereafter, rare earth elements such as oxalate, carbonate, hydroxide and the like can be recovered from this solution by a known method. On the other hand, iron can be recovered as a non-sticky, easy-to-handle residue containing triiron tetroxide as a main component, so that post-treatment such as conversion to a steelmaking raw material becomes extremely easy.

  The alkaline aqueous solution used in the step (a) can be reused as the alkaline aqueous solution in the operation after the second time, so that the solution remaining after collecting the solid in the step (b) can be reused. .

  According to the present invention, when the rare earth element is recovered from the rare earth element-iron-based alloy by leaching with an acid, it is treated with an alkali in the presence of an oxidizing agent (eg, air blowing) before the acid leaching. The boron dissolves and the iron in the alloy is oxidized to stable ferric tetroxide. Therefore, after the acid leaching step, the iron residue obtained after separation from the leaching solution containing the rare earth element is also composed mainly of triiron tetroxide. This iron residue has no stickiness and can be easily used as a raw material for iron making, and the amount of by-product is significantly less than that of the method described in Patent Document 1.

  Furthermore, it is possible to shorten the time of the acid leaching process, and depending on the type of acid used and the raw material to be treated, the generation of a large amount of bubbles that makes handling difficult at the time of acid leaching is suppressed, and the treatment operation becomes easy. . In addition, the recovery rate of Dy in the rare earth elements is improved, and it can be recovered at a rate substantially close to Nd. Then, most of the boron in the rare earth element-iron-based alloy can be separated from the rare earth element and iron by alkali treatment, and the incorporation of boron into the rare earth element solution and iron residue recovered in step (d). Can be reduced.

The XRD measurement result of the collection | recovery solid substance after the alkali treatment of Example 1, 2 is shown. The XRD measurement result of the leaching residue of Examples 1 and 2 and Comparative Example 1 is shown.

The rare earth element recovery method from the rare earth element-iron alloy according to the present invention includes the following steps:
(A) an alkali treatment step of heating a rare earth element-iron-based alloy slurry in an alkaline aqueous solution in the presence of an oxidizing agent;
(B) a recovery step for solid-liquid separation of the slurry obtained in step (a) to recover a solid,
(C) an acid leaching step of acidifying the slurry of the solids recovered in step (b) in water, preferably in the presence of an oxidizing agent to leach rare earth elements into the liquid; and (d) step (c) And separating the slurry obtained in step 1) into a solution containing a rare earth element and a residue mainly composed of triiron tetroxide.

The alkali treatment step (a) and the acid leaching step (c) are the main steps.
In order to avoid combustion, wastes of rare earth element-iron alloys such as magnet scraps to be treated in the present invention are generally stored in a submerged state (as sludge). Therefore, the wet cake from which the water has been removed incompletely by the sludge as it is or by filtration may be treated according to the present invention.

  In the alkali treatment step (a), a slurry obtained by mixing a rare earth element-iron alloy in an alkaline aqueous solution is heated in the presence of an oxidizing agent to react with alkali and oxygen. By this alkali treatment, the rare earth element is changed to hydroxide and iron is changed to triiron tetroxide, but both remain solid.

At least a part of boron contained in the rare earth element-iron-based alloy in an amount of about 1% by mass is eluted as an alkali metal borate in the alkaline aqueous solution in this alkali treatment and separated from the rare earth element and iron.

  A preferable oxidizing agent is air, and it is preferable to carry out the alkali treatment while blowing the oxidizing agent air into the slurry from the viewpoint of economy and no introduction of impurities. However, it is also possible to use chemical oxidants.

  The alkali treatment step (a) can be performed, for example, by starting to raise the temperature to a predetermined heating temperature while blowing air into a slurry of a rare earth element-iron alloy and water, and subsequently adding alkali. it can. As another method, as described later, an alkaline aqueous solution and a rare earth element-iron-based alloy can be mixed and the mixture can be heated. The heating temperature is preferably 50 ° C. or higher, and more preferably 60 to 90 ° C. from the viewpoint of promoting the reaction.

  The alkali is preferably sodium hydroxide from the viewpoint of price, but other alkali metal hydroxides such as potassium hydroxide can also be used. The amount of alkali added is preferably about 1 to 3 times, and particularly preferably about 2 times the total number of moles of elements in the raw material alloy. The liquid pH is 12 or more. The addition of alkali is necessary for the first alkali treatment of the rare earth element-iron alloy, but as will be described later, in this alkali treatment, the alkali consumed in the reaction reacts with molecular oxygen and is regenerated. Therefore, in the alkali treatment of the rare earth element-iron-based alloy for the second and subsequent times, the solution obtained by solid-liquid separation in the step (b) after the alkali treatment is used as an alkaline aqueous solution in the step (a). The addition of (eg, sodium hydroxide) is substantially unnecessary or can be significantly reduced.

  When sodium hydroxide is added to a slurry of rare earth element-iron alloy and water, hydrogen is generated and the slurry turns black. The alkali treatment is continued at least until the generation of hydrogen stops. Even after the generation of hydrogen stops, the treatment is preferably continued for about 30 minutes to 3 hours. When air is blown as an oxidant, the generation of hydrogen is difficult to identify with the naked eye, but sometimes the generation or termination of hydrogen gas can be detected by stopping the blowing of air or by appropriate chemical means. .

  After the alkali treatment, for example, solid-liquid separation is performed by filtration to separate into a solid and a solution. The separated solid content was dried and examined with an X-ray diffractometer as shown in FIG. The main components of the solid content were rare earth element hydroxide and triiron tetroxide. These are all hardly soluble compounds, and if the liquid pH is 12 or more, substantially all of iron and rare earth elements are recovered as solids.

  The presumed chemical reaction is the following formula (1) for rare earth elements. In the case of iron, hypoferrite with an iron valence of 2 is produced according to the following formula (2), which reacts with molecular oxygen in the blown air, and formula (3) It is presumed that triiron tetroxide will be produced. At this time, since sodium hydroxide is produced from sodium hypoferrite, the entire amount of sodium hydroxide consumed in the reaction is regenerated. Therefore, the filtrate obtained by solid-liquid separation can be used for the next alkali treatment process.

2Re + 6H ₂ O → 2Re (OH) ₃ + 3H ₂ (1)
Fe + 2NaOH + 2H ₂ O → Na ₂ [Fe (OH) ₄ ] + H ₂ (2)
6Na ₂ [Fe (OH) ₄ ] + O ₂ → 2Fe ₃ O ₄ + 12NaOH + 6H ₂ O (3)
In the above formula, Re represents a rare earth element such as Nd.

  The solid-liquid separation step (b) is usually performed by filtration, but other methods such as centrifugation can also be employed. The separated solid is preferably subjected to acid leaching treatment in step (c) after removing the adhering alkalinity by washing with water. On the other hand, at least a part of the filtrate (which is an alkaline aqueous solution) is preferably reused as the alkaline aqueous solution in the subsequent step (a). This eliminates the need for waste liquid treatment or significantly reduces the burden.

  The alkaline aqueous solution recovered in step (b) contains boron. Even if the recovered alkaline treatment liquid containing boron is recycled to the alkaline treatment step (a), the inclusion of boron has little adverse effect on the alkaline treatment. However, since the boron content in the liquid increases as the recirculation is repeated, a part of the alkaline aqueous solution that is recirculated so that the boron content in the alkaline aqueous solution used in the alkali treatment step is kept almost constant. Is preferably replaced with water and / or a new aqueous alkaline solution. When the water is replaced, the insufficient alkali is replenished in the next alkali treatment step. As will be apparent to those skilled in the art, the alkaline aqueous solution may be completely renewed if the impurity concentration increases to an unacceptable level by repeated reuse of the alkaline aqueous solution. That is, as in the first time, the rare earth element is slurried with water, and the temperature is increased to add the alkali.

  In the acid leaching step (c), the solid recovered in the step (b) (which is mainly composed of rare earth element hydroxide and triiron tetroxide as described above) is slurried in water. Acidify with acid. Thereby, the rare earth element (hydroxide) dissolves in the acid to form a soluble salt and is leached into the liquid. On the other hand, iron (triiron tetroxide) does not react with the acid and does not change in a solid state except for specific conditions as described later. In this way, the rare earth element and iron can be separated.

  If the rare earth element and iron in the rare earth element-iron alloy are completely oxidized in the alkali treatment process, there is no need for oxidation in the acid leaching process. Therefore, in the present invention, unlike the conventional method, acid leaching of rare earth elements proceeds without the presence of an oxidizing agent. However, considering the possibility that rare earth elements and iron that have not been oxidized in the alkali treatment step remain, the recovery rate of rare earth elements is increased, and iron is reliably precipitated as an insoluble trivalent iron compound. At least the last part of the leaching process is preferably carried out in the presence of an oxidant, for example molecular oxygen supplied by air. The entire acid leaching process may be performed while blowing air.

  In the acid leaching step (c), first, the solid obtained by the solid-liquid separation in the step (b) is mixed with water to form a slurry. Acid is added to the slurry to acidify the slurry. The type of acid used for acid leaching is not particularly limited as long as it can dissolve rare earth hydroxide and does not dissolve triiron tetroxide, but it is desirable to use hydrochloric acid that is inexpensive and can increase the concentration of rare earth elements. Unlike iron hydroxide, ferric tetroxide is stable in acid and does not dissolve except for excess hydrochloric acid and concentrated nitric acid whose pH is lower than 2. Therefore, acid can be added in a relatively short time. There is no problem even if the pH in the liquid is less than 3, but in the case of hydrochloric acid, the pH is preferably 2 or more.

The leaching of rare earth elements with hydrochloric acid follows the following equation (4). Since rare earth elements are oxidized to hydroxides by alkali treatment, they are easily converted into chlorides.
Re (OH) ₃ + 3HCl → ReCl ₃ + 3H ₂ O (4)
(Re represents a rare earth element such as Nd)
In the acid leaching process, when acid is added until the pH of the slurry is lower than 3, the acid is added, and after the addition of the acid is completed, the pH is set to 3 in order to eliminate the mixing of iron into the rare earth solution. It is desirable to continue the acid leaching with the above adjustment. The pH at this time is more preferably in the range of 3 to 5, particularly preferably in the range of 3 to 4. This pH adjustment is preferably performed with ammonia in order to prevent metal contamination of rare earth elements and iron residues. Ammonia gas can also be used, but the use of ammonia water is simple. It is also possible to raise the pH using an alkali metal hydroxide such as sodium hydroxide. However, for example, when sulfuric acid is used as the acid for acidification, the alkali metal hydroxide may be precipitated by double chlorination. The use of acid and base combinations with the potential for such precipitation is not preferred.

  In order to promote acid leaching, the slurry can be heated to reduce processing time. The heating temperature for shortening the treatment time is not particularly limited as long as it is 90 ° C. or less. For example, the temperature can be in the range of 50 to 90 ° C, but some effect of promoting treatment can be obtained even by heating at about 40 ° C.

  In the method of the present invention, since rare earth elements and iron are already oxidized in the alkali treatment step (a), the generation of hydrogen due to the addition of acid is negligible. Therefore, the acid addition rate is not particularly limited. However, it is preferable to add the acid gradually in order to avoid dissolution of ferric tetroxide due to local increase in acid concentration. After the addition of the acid, the acid leaching process is continued until leaching is substantially completed (until no increase of rare earth elements is observed in the liquid).

  The acid leaching is preferably performed while blowing air into the slurry at least at the end of the acid leaching in order to reliably precipitate iron. For example, if acidification is performed so that the pH is less than 3 and then acid leaching is continued by raising the pH to 3 or more, air blowing may be performed from the beginning of the acid addition, or the pH may be reduced to 3 You may implement only after raising it above. Alternatively, air may be blown only for the last certain time required for iron precipitation, for example, the last hour after raising the pH.

  In the experiments of the present inventors, when the rare earth element-iron alloy is suddenly acidified and acid leaching treatment is performed without alkali treatment as in the conventional method, foaming is uncontrollable due to air blowing and hydrogen generation. It happens so hard that it is hard to disappear. Therefore, the acid can be added only very slowly, and the processing time is very long. On the other hand, in the method of the present invention, hydrogen is hardly generated in the acid leaching step, and the operation of the acid leaching treatment is easy. Hydrogen is generated in the alkali treatment process, but the foaming is gentle and gradually disappears compared to the foam generated during acid leaching in the conventional method. The reason is unknown.

  Further, in the method of the present invention, acidification can be performed so that the pH is once lower than 3, and acid leaching can be performed under conditions stronger than those of the conventional method. Therefore, Dy that is difficult to leach out of rare earth elements is also Nd and Pr. It can be recovered at the same high rate.

  The slurry obtained in the step (c) is prepared by separating the solution of the rare earth element salt and the iron residue mainly composed of triiron tetroxide by appropriate solid-liquid separation such as filtration in the step (d). Into solids consisting of

  From a solution of the recovered rare earth element salt (eg, hydrochloride), the rare earth element is recovered by changing to an insoluble salt such as oxalate, carbonate, hydroxide, etc. by a known method. Can do. The recovered rare earth compound can be reduced to a metal by a known refining method and reused as a rare earth metal.

On the other hand, the iron residue separated from the solution is a solid material having tritium tetroxide as a main component and having no stickiness and good handleability, and can be easily and effectively used as a raw material for iron making.
In the method of the present invention, since the separation efficiency between iron and rare earth elements is improved, the concentration of iron and boron in the rare earth element solution is low, while the iron residue also has a low content of boron and rare earth elements. Further, as described above, Dy, which tends to have a low leaching rate with acid, can be recovered at a rate close to Nd.

In the following examples,% is% by mass unless otherwise specified.
Example 1
600 mL of water was added to 150 g of Nd—Fe—B alloy sludge to form a slurry. The water content of the sludge was 30%. Therefore, the alloy amount of the raw material was 105 g. The average particle size of the alloy in the sludge was 15 μm, the maximum diameter was 0.7 mm, the alloy component was mass%, the rare earth (Nd, Pr, Dy) was 30%, and the iron was 54%.

  While air was blown into the slurry at 4 L / min, the temperature was raised to 70 ° C., and 48 g of sodium hydroxide was added at the same time. After a while from the start of the addition of sodium hydroxide and the temperature increase, hydrogen generation was observed and the alloy turned black. After completion of the temperature increase, an additional 45 g of sodium hydroxide was further added while continuing to blow air. A total addition amount of 93 g of sodium hydroxide corresponds to 1.95 times the molar amount of the metal element in the raw material alloy. After blowing air and heating at 70 ° C. for 6 hours, the slurry was filtered, and the filter cake was washed with 4 L of water to recover a black solid. The filtrate was stored for the next alkali treatment.

1/4 of the solid was collected for XRD investigation. The measurement results are shown in FIG. The solids were rare earth hydroxide and triiron tetroxide.
The remaining 3/4 of the solid was slurried by adding 500 mL of water. While air was blown into the slurry at 4 L / min, the temperature was raised to 70 ° C. After completion of the temperature increase, addition of hydrochloric acid in which concentrated hydrochloric acid was diluted twice with water was started while maintaining the slurry at this temperature. The addition of hydrochloric acid was completed in 131 minutes and the pH of the slurry was 2.5. The amount of hydrochloric acid used was 51.2 g.

  After completion of the addition of hydrochloric acid, 4.3 g of 25% aqueous ammonia was continuously added, air blowing and heating at 70 ° C. were continued for further 152 minutes, and the acid leaching treatment was completed. The slurry pH at the end of this treatment was 3.7.

  The slurry after the acid leaching treatment was filtered using a Buchner funnel and filter paper, and a solution in which the rare earth element was dissolved and an iron residue were recovered. Filtration was very easy and the filter cake was not sticky and drained.

  When the element yield in the collected solution was determined, Fe was 0%, Pr was 98.2%, Nd was 99.9%, Dy was 97.6%, and B was 6.6%. The recovered solid was 111.0 g in the state of a wet cake (water content: 45.6%) after running out of water, and its main component was tetraoxide when examined by an X-ray diffractometer (XRD) after drying. It was triiron (Fig. 2). Table 1 shows the results of elemental analysis of the iron residue after drying.

(Example 2)
This example illustrates the case where the filtrate in the previous alkali treatment process is reused.
The filtrate collected by filtration after alkali treatment in Example 1 was added to 150 g of the same Nd—Fe—B alloy sludge as in Example 1 to make a slurry. While air was blown into the slurry at 4 L / min, the temperature was raised to 70 ° C. Hydrogen generation was observed after a while from the start of heating. Air blowing was continued even after completion of the temperature increase, and heating was continued for 6 hours to complete the alkali treatment.

  Thereafter, the slurry was filtered, and the filter cake was washed with 4 L of water to recover a black solid. 1/4 of the solid was collected for XRD investigation, and the measurement results are shown in FIG. The solids were rare earth hydroxide and triiron tetroxide.

  500 mL of water was added to the remaining 3/4 solids to make a slurry. While air was blown into the slurry at 4 L / min, the temperature was raised to 70 ° C. After completion of the temperature increase, addition of hydrochloric acid obtained by diluting concentrated hydrochloric acid twice with water was started. The addition was complete in 128 minutes and the pH of the slurry was 2.50. The amount of hydrochloric acid used was 52.3 g. Subsequently, 5.3 g of 25% ammonia water was added, and heating at 70 ° C. and blowing of air were continued for 185 minutes to complete the acid leaching treatment. The pH of the slurry at the end of this was 3.6.

  The slurry was filtered to recover the liquid in which the rare earth element was dissolved and the iron residue. The element yield in the collected solution was determined to be 0% for Fe, 97.2% for Pr, 99.7% for Nd, 95.4% for Dy, and 12.3% for B. The amount of iron residue recovered was 121.5 g, and the water content was 49.4%. When the residual iron after drying was examined with an X-ray diffractometer, its main component was triiron tetroxide (FIG. 2).

(Comparative Example 1)
This example illustrates a case where a rare earth element-iron alloy is directly leached with an acid without performing an alkali treatment according to a conventional method.

  To 112.5 g of the same Nd—Fe—B alloy sludge as in Example 1, 1000 mL of water was added to make a slurry. The amount of sludge was set to 3/4 of the amount used in Examples 1 and 2 in order to match the amount used in the acid leaching process in Examples 1 and 2. While air was blown into this slurry at 4 L / min, the temperature was raised to 40 ° C. After completion of the temperature increase, the same addition of hydrochloric acid as used in Examples 1 and 2 was started. After about 8 hours, the temperature was raised to 70 ° C., and the addition was completed in 921 minutes. The pH of the slurry was 1.99, and the amount of hydrochloric acid used was 58.6 g. At this time, the foaming was intense and the foam was likely to overflow from the container, so the addition rate of hydrochloric acid had to be slowed down.

  Subsequently, 4.1 g of 25% aqueous ammonia was added, heating at 70 ° C. and air blowing were continued for 212 minutes, and then the acid leaching treatment was completed. The pH of the slurry at the end was 3.6. The slurry was filtered to recover the liquid in which the rare earth element was dissolved and the iron residue. The iron residue has poor filterability, and even if the filter area is increased by using a larger filter paper than in Examples 1 and 2, filtration takes several times longer than in Examples 1 and 2, and cake recovery is also due to stickiness. It took time.

  The element yield in the collected solution was determined to be 0% for Fe, 97.0% for Pr, 98.8% for Nd, 89.1% for Dy, and 93.4% for B. The amount of iron residue recovered was 226.1 g (water content 67.8%). When examined with an X-ray diffractometer, the main component was γ-FeOOH (FIG. 2). Table 1 shows the results of elemental analysis of the iron residue after drying.

  As can be seen from Table 1, the iron residue obtained in Example 1 had a high iron content, hardly contained B, and the content of rare earth elements was somewhat low. In contrast, the iron residue recovered by the method of Comparative Example 1 according to the conventional method has a low iron content, a high B content and a rare earth element content, and the separation efficiency between iron and rare earth elements is an example. It turns out that it is inferior compared.

  Table 2 summarizes the recovery yields of iron and each rare earth element from the rare earth element solutions recovered after acid leaching in Examples 1 and 2 and Comparative Example 1.

  As can be seen from Table 2, iron did not remain in the recovered solution, and substantially all was recovered as a solid residue. Regarding the recovery rate of rare earth elements, in Comparative Example 1 according to the conventional method, the recovery yield of Dy that is relatively less susceptible to acid leaching is low. On the other hand, in Examples 1 and 2, it can be seen that Dy could also be recovered with a high yield comparable to Nd and Pr. Since most of B was dissolved in the solution during the alkali treatment in Examples 1 and 2, the recovered rare earth element solution contained only a small amount of B. On the other hand, in Comparative Example 1, most of B was contained in the rare earth element solution recovered after acid leaching.

As described above, according to the method of the present invention, it can be seen that the separation efficiency of iron and rare earth elements is improved, and the rare earth elements can be recovered at a higher yield than conventional.
In addition, the iron residue has good filterability and can be easily handled in a smaller amount than conventional methods. On the other hand, in Comparative Example 1 according to the conventional method, the amount of water of the iron residue is large, and the main component is iron oxyhydroxide having a large molecular weight per atom of iron, so the amount of generation is also large. Moreover, since filterability is bad, it takes time to collect the iron residue as the amount increases.

  Comparing the treatment time, in Examples 1 and 2, it was 700 minutes or less including 6 hours of the alkali treatment, and the acid leaching was completed in about 300 minutes. On the other hand, in Comparative Example 1, it took 1100 minutes or more only for acid extraction. Therefore, even if an alkali treatment step is added, the treatment time can be shortened according to the method of the present invention.

Claims (4)

A method for recovering a rare earth element from a rare earth element-iron-based alloy comprising the following steps:
(A) heating a rare earth element-iron alloy slurry in an alkaline aqueous solution in the presence of an oxidizing agent;
(B) The slurry obtained in step (a) is subjected to solid-liquid separation to recover a solid,
(C) acidifying the slurry of the solid matter recovered in step (b) in water to leaching the rare earth element in the liquid; and (d) separating the slurry obtained in step (c) into solid and liquid. Then, the solution is separated into a solution containing rare earth elements and a residue mainly composed of triiron tetroxide.   The method according to claim 1, wherein at least a part of the solution remaining after recovering the solid in the step (b) is reused as an alkaline aqueous solution in the step (a).   The process according to claim 1 or 2, wherein in step (c), an oxidant is present in the slurry at least at the end of leaching.   The method according to any one of claims 1 to 3, wherein the oxidizing agent used in step (a) and step (c) is air.

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