Separation method of rare earth element and iron and rare earth element-containing slag

Abstract

The present invention provides a separation method of a rare earth element and iron including: forming alkali silicate slag incorporating a rare earth element, by melting a rare earth-iron-containing material together with an alkali silicate flux in a metallic silicon melt or an iron-silicon alloy melt; and separating rare earth-containing slag from an iron-silicon alloy, in which volatilization of alkaline components contained in the flux is suppressed by performing heating and melting under an oxidizing atmosphere, and the rare earth-containing slag having a SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less is formed.

Claims

1. A separation method of a rare earth element and iron, comprising: forming alkali silicate slag (referred to as rare earth-containing slag) incorporating a rare earth element, by melting a treatment object containing a rare earth element and iron (referred to as a rare earth-iron-containing material) together with an alkali silicate flux in a metallic silicon melt or an iron-silicon alloy melt; separating the rare earth-containing slag from an iron-silicon alloy; leaching an alkali silicate from the slag separated from the iron-silicon alloy, with water; and recovering the rare earth element from a rare earth element concentrate of a leached residue, wherein volatilization of alkaline components contained in the flux is suppressed, by performing heating and melting under an oxidizing atmosphere, to form the rare earth-containing slag having a SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less.

2. The separation method of a rare earth element and iron according to claim 1, wherein the rare earth-containing slag is formed by suppressing a volatilization rate of the alkaline components from the alkali silicate flux to be 25.5% or lower.

3. The separation method of a rare earth element and iron according to claim 2, wherein the rare earth-containing slag, in which the SiO.sub.2/Na.sub.2O molar ratio is 2.1 or less and a SiO.sub.2 content is 50 wt % or less, is formed by suppressing a volatilization rate of Na to be 25.5% or lower, by using sodium silicate as the alkali silicate flux.

4. The separation method of a rare earth element and iron according to claim 1, wherein the rare earth-containing slag, in which the SiO.sub.2/Na.sub.2O molar ratio is 2.1 or less and a SiO.sub.2 content is 50 wt % or less, is formed by suppressing a volatilization rate of Na to be 25.5% or lower, by using sodium silicate as the alkali silicate flux.

5. The separation method of a rare earth element and iron according to claim 1, further comprising: leaching an alkali silicate from the slag separated from the iron-silicon alloy, with water; recovering a leached residue to perform leaching with hydrochloric acid; forming an oxalate by adding oxalic acid to leachate; and recovering and calcining the oxalate to recover a rare earth oxide.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a process chart of a treatment of separating and leaching rare earth from a rare earth-iron-containing material.

(2) FIG. 2 is a process chart of treatments from heating and melting the rare earth-iron-containing material to recovering a rare earth oxide.

DESCRIPTION OF EMBODIMENTS

(3) A treatment method of the present invention includes: forming alkali silicate slag (referred to as rare earth-containing slag) incorporating a rare earth element, by melting a rare earth-iron-containing material together with a flux of an alkali silicate in a metallic silicon melt or an iron-silicon alloy melt; and separating the rare earth-containing slag from an iron-silicon alloy, in which volatilization of alkaline components contained in the flux is suppressed, by performing heating and melting under an oxidizing atmosphere, to form the rare earth-containing slag having a SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less.

(4) An outline of the treatment method of the present invention is shown in the process chart of FIG. 1.

(5) Weight in the present specification is used synonymously with mass. Accordingly, the weight ratio is the mass ratio and the wt % is mass %.

(6) In the treatment method of the present invention, the rare earth-iron-containing material is melted together with the alkali silicate flux in the metallic silicon melt or the iron-silicon alloy melt, the volatilization of the alkaline components contained in the flux is suppressed, thereby forming and separating the slag incorporating the rare earth element.

(7) Examples of the rare earth-iron-containing material include machining chips (sludge) or scraps of rare earth magnet, scraps of a motor rotor having a rare earth magnet, and the like. A motor having a rare earth magnet is used for electric vehicles, hybrid vehicles, electronic devices, or home appliances. In addition, a material containing the rare earth element is also used for VCMs or speakers of electronic devices. In the separation method of the present invention, these rare earth-iron-containing materials can be used as raw materials.

(8) As the alkali silicate flux, a compound containing oxides of alkali metal and silicon can be used. For example, sodium orthosilicate (2Na.sub.2O—SiO.sub.2), sodium metasilicate (Na.sub.2O—SiO.sub.2), sodium disilicate (Na.sub.2O-2SiO.sub.2), potassium metasilicate (K.sub.2O—SiO.sub.2), potassium disilicate (K.sub.2O-2SiO.sub.2), and the like can be used. Further, the alkali silicate flux may contain a small amount of oxide, for example, 5 wt % or less of a calcium oxide, a boron oxide, a chromium oxide, a manganese oxide, an aluminum oxide, and a magnesium oxide. The alkali silicate flux can be reused by recovering a rare earth-containing flux separated after the heating and melting treatment.

(9) The rare earth-iron-containing material and the alkali silicate flux are mixed with metallic silicon or an iron-silicon alloy. The mixture is heated to 1250° C. to 1550° C. under the oxidizing atmosphere, by using a ceramic container, to form a metallic silicon melt or an iron-silicon alloy melt. The rare earth-iron-containing material is melted in the melt. Alternatively, the rare earth-iron-containing material and the alkali silicate flux are added to the metallic silicon melt or the iron-silicon alloy melt inside the ceramic container and heated to 1250° C. to 1550° C. under the oxidizing atmosphere to melt the rare earth-iron-containing material in the melt.

(10) When the rare earth-iron-containing material is heated and melted together with the flux, in the metallic silicon melt or the iron-silicon alloy melt, the iron in the rare earth-iron-containing material tends to react with silicon rather than becoming an iron oxide, in a coexisting system with the flux. Therefore, the iron of the rare earth-containing material reacts with silicon in the melt to form an iron-silicon alloy. The generated iron-silicon alloy melt is accumulated at the bottom of the container, and the amount of the iron-silicon alloy melt increases gradually, along with the progress of the melting reaction.

(11) Since the metallic silicon melt reacts with the iron of the rare earth-iron-containing material to become an iron-silicon alloy melt, the iron-silicon alloy may be used from the beginning. The iron-silicon alloy can be reused by recovering the iron-silicon alloy separated after the melting treatment. Since when the iron concentration in the iron-silicon alloy is close to 90 wt %, a capacity for receiving iron in the rare earth-iron-containing material becomes low, the iron-silicon alloy having the iron concentration lower than 90 wt %, for example, 75 wt %, may preferably be used.

(12) In the iron-silicon alloy melt, when an iron content is within the range of 40 wt % to 50 wt %, a melting temperature thereof is approximately 1250° C., and when the iron content is within the range of 60 wt % to 70 wt %, the melting temperature thereof becomes approximately 1300° C. or higher. When the iron content is in around 80 wt %, the melting temperature falls again to approximately 1250° C., thereafter, the melting temperature increases to approximately 1550° C., along with the iron content. Accordingly, in order to obtain an iron-silicon alloy melt, according to a component ratio between iron and silicon, heating may be performed to 1250° C. to 1550° C. to form a melt. A heating time may be a time during which a melt is formed.

(13) The alkali silicate flux becomes a slag incorporating the rare earth element, by the heating and melting treatment. The rare earth element in the rare earth-iron-containing material is different from the iron in the rare earth-iron-containing material, and is oxidized by heating the rare earth-iron-containing material under an oxidizing atmosphere to be incorporated into the slag. For example, a sodium silicate flux becomes a slag containing a rare earth oxide, together with a sodium oxide and silica.

(14) In the treatment method of the present invention, volatilization of the alkaline components contained in the flux is suppressed by performing heating and melting under the oxidizing atmosphere, and the rare earth-containing slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less is formed. In general, in the heating and melting treatment, since the alkaline components of the flux volatilize to be reduced and the silicon in the melt is slightly oxidized to be incorporated into the slag, the silicon concentration in the slag increases. Therefore, the SiO.sub.2/Na.sub.2O molar ratio in the slag tends to be larger than that of flux at the beginning. In the treatment method of the present invention, the volatilization of the alkaline components is suppressed by heating and melting the slag under the oxidizing atmosphere. Specifically, the volatilization rate of the alkaline components from the flux is suppressed to 25.5% or lower, preferably 10% or lower, thereby forming rare earth-containing slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less, preferably having a SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less and a SiO.sub.2 content of 50 wt % or lower.

(15) For example, in a case where the sodium metasilicate (SiO.sub.2/Na.sub.2O molar ratio of 1.0) is used as the alkali silicate flux, when the volatilization rate of Na from the flux is suppressed to 10% or lower, the rare earth-containing slag having a SiO.sub.2/Na.sub.2O molar ratio of 1.07 to 1.17 (Examples 1 and 2) can be formed. In addition, in a case where the sodium disilicate (SiO.sub.2/Na.sub.2O molar ratio of 2; measured value of 1.919), when the volatilization rate of Na from the flux is suppressed to 10% or lower, the rare earth-containing slag having a SiO.sub.2/Na.sub.2O molar ratio of 1.924 (Example 3) can be formed.

(16) On the other hand, when the heating and melting treatment is performed in a non-oxidizing atmosphere such as an inert gas, since the alkaline components contained in the flux are reduced, the volatilization of the alkaline components increases. When the heating and melting is performed using a carbon container, since reduction of the alkaline components proceeds due to a reaction with carbon, the amount of volatilization of the alkaline components further increase, and the SiO.sub.2/Na.sub.2O molar ratio in the slag greatly increases.

(17) In the treatment method of the present invention, the heating under the oxidizing atmosphere may preferably be performed in the air or performed in an inert atmosphere by blowing air onto a surface of a melt. As the melting container, a ceramic crucible of magnesia type or alumina type, or a graphite type crucible of C—SiO.sub.2—SiC type is preferably be used.

(18) According to the heating and melting treatment, the iron-silicon alloy melt is accumulated at the bottom of the container and the rare earth-containing slag is formed on the melt. Since the slag is in a molten state, the slag can be easily extracted from the container. The iron-silicon alloy melt contains little rare earth element and almost the whole amount of rare earth element is incorporated in the slag. Accordingly, the slag is separated from the iron-silicon alloy melt, whereby almost the whole amount of rare earth element and the iron contained in the rare earth-iron-containing material can be separated from each other.

(19) In the treatment method of the present invention, the rare earth-containing slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less is formed. In general, in the slag having the SiO.sub.2/Na.sub.2O molar ratio of more than 2.1, since the SiO.sub.2 content is large and Na.sub.2O content is small, it is difficult to leach the alkali silicate contained in the slag, with water. Therefore, in order to separate the silica component and the rare earth element in the slag, it is necessary to perform alkali leaching or acid leaching for a long time and wet process takes time and effort.

(20) On the other hand, in the treatment method of the present invention, since the slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less is formed, water soluble alkali silicate can be easily leached from the slag, with water. As a result, the rare earth elements concentrated in the leached residue can be efficiently separated and recovered. In the rare earth-containing slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less and SiO.sub.2 content of 50 wt % or lower, water leaching becomes easier.

(21) Examples of a method of recovering the rare earth element from the rare earth element concentrate include a method including: adding hydrochloric acid to the rare earth element concentrate; leaching the rare earth element under the liquidity of pH 1 or lower; adding oxalic acid to the rare earth element leachate; forming an oxalate of the rare earth element under the liquidity of pH 1 or lower; recovering the rare earth element oxalate by a solid-liquid separation to perform calcining at 900° C.; and obtaining a rare earth oxide. FIG. 2 shows a process of treatments from heating and melting the rare earth-iron-containing material to recovering a rare earth oxide. As the method of recovering the rare earth from the rare earth element concentrate, a solvent extraction method also is used, whereby the rare earth element may be separated to be recovered each rare earth oxide.

EXAMPLES

(22) Examples of the present invention and Comparative examples will be shown together. In Examples and Comparative examples, a composition of the iron-silicon alloy was quantitatively analyzed using X-ray fluorescence spectrometry (XRF method) and electron probe micro analysis (EPMA). In addition, the composition of the recovered matter containing the rare earth element was quantitatively analyzed using a chemical method.

(23) The compositions of objects to be treated (I) and (II) used in Examples and Comparative examples were shown in Table 1. Treatment conditions and the recovered amounts of slag and an alloy were shown in Table 2. A composition of the recovered rare earth-containing slag and a SiO.sub.2/Na.sub.2O molar ratio were shown in Table 3. A composition of an iron-silicon alloy was shown in Table 4. Transition rates of rare earth element and iron to slag and Na volatilization rate were shown in Table 5.

(24) The Transition rate of the rare earth element to the slag is a weight ratio [RE(S)/RE(M)×100%] of a rare earth element content RE(S) in the slag to a rare earth element content RE(M) in the object to be treated. The Transition rate of iron is a weight ratio [Fe(S)/Fe(M)×100%] of an iron content Fe(S) in the slag to an iron content Fe(M) in the object to be treated. The Na volatilization rate is a weight ratio [[Na(F)—Na(S)]×100%/Na(F)] of the difference between a Na content [Na(F)] in the flux and a Na content [Na(S)] in the slag to the Na content [Na(F)] in the flux.

Example 1

(25) 38.0 g of the object to be treated (I) shown in Table 1 and 4.6 g of metallic silicon (purity of 99%) were put into a magnesia crucible. Further, 48.0 g of flux including sodium metasilicate (Na.sub.2O—SiO.sub.2) were added thereto. Heating was performed at 1300° C. in the air to form a melt and the melt was held for 60 minutes. Thereafter, a sample was cooled with water, 31.0 g of the iron-silicon alloy and 64.0 g of slag were recovered.

Example 2

(26) 20.0 g of the object to be treated (I) and 2.4 g of metallic silicon were put into an alumina crucible. 24.0 g of sodium metasilicate as in Example 1 was added thereto. Heating and melting treatment was performed under the same conditions as in Example 1, except that the heating time was set to 30 minutes, 45 minutes, and 60 minutes. 16.0 g of iron-silicon alloy and respectively 32.0 g, 32.5 g, and 33.0 g of slag were recovered.

Example 3

(27) 43.0 g of the object to be treated (I) and 5.0 g of metallic silicon were put into a graphite crucible. 45.0 g of sodium disilicate [Na.sub.2O-2SiO.sub.2] was added thereto. The heating temperature was set to 1350° C. and the heating time was set to 20 minutes. Melting treatment was performed by blowing air onto the melt while maintaining an argon gas atmosphere. 35.0 g of iron-silicon alloy and 59.0 of slag were recovered.

Example 4

(28) 10.0 g of the object to be treated (I) shown in Table 1 and 1.2 g of metallic silicon were put into an alumina crucible. Further, 12.0 g of flux including sodium orthosilicate (2Na.sub.2O—SiO.sub.2) were added thereto. Heating was performed at 1300° C. in the air to form a melt and the melt was held for 5 minutes. Thereafter, a sample was cooled with water, 7.5 g of the iron-silicon alloy and 15.0 g of slag were recovered.

(29) As shown in Tables 2 to 5, in all the slags of Examples 1 to 3, the Transition rate of the rare earth element to the slag was 100%. On the other hand, the Transition rate of the iron to the slag was 1.2% or lower, and the separability between the rare earth element and the iron was good. In addition, in all the slags of Examples 1 to 3, the Na volatilization rate was 10% or lower and volatilization of Na was suppressed, and the slag having a SiO.sub.2/Na.sub.2O molar ratio of 2.1 or less was formed.

(30) In addition, as shown in Table 5, the Transition rate of the rare earth element to the slag was 100%. On the other hand, the Transition rate of the iron to the slag was 1.20% or lower, and the separability between the rare earth element and the iron was good. In addition, the Na volatilization rate was 25.5% or lower, and preferably, the Na volatilization rate was 10% or lower (Examples 1 to 3), and the Na volatilization rate is greatly suppressed as compared with Comparative Example 2. In Example 4, the Na volatilization rate was high, that is, 25.4%; however, since the SiO.sub.2/Na.sub.2O molar ratio was 2.1 or less and SiO.sub.2 content was 50 wt % or lower, water leaching became easier. Accordingly, in all the rare earth-containing slags of Examples 1 to 4, the alkali silicate in the slag can be leached with water and the rare earth elements concentrated in the leached residue can be efficiently separated and recovered.

Comparative Example 1

(31) 43.4 g of the object to be treated (I) and 5.0 g of metallic silicon were put into a graphite crucible. 45.0 g of sodium disilicate [Na.sub.2O (35 wt %)-SiO.sub.2 (65 wt %)] was added thereto. The heating temperature was set to 1350° C. and the heating time was set to 20 minutes. A heating and melting treatment was performed under the same conditions as in Example 1, except for an argon gas atmosphere. 35.0 g of iron-silicon alloy and 58.0 g of slag were recovered.

Comparative Example 2

(32) 2.4 g of the object to be treated (II) and 9.6 g of iron silicon (Si 25 wt %) were put into a carbon crucible. 6.0 g of sodium metasilicate [Na.sub.2O (50 wt %)-SiO.sub.2 (50 wt %)] was added thereto. The heating temperature was set to 1300° C. and the heating time was set to 5 hours. A heating and melting treatment was performed under the same conditions as in Example 1, except for an argon gas atmosphere. 11.2 g of iron-silicon alloy and 7.2 g of slag were recovered.

Comparative Example 3

(33) 10 g of the slag recovered in Comparative Example 1 was taken, and 0.1 L of 4 g/L concentration caustic soda solution was added thereto. Heating was performed at 50° C. for 3 hours to attempt leaching of the water-soluble silica and Na content. As a result, decomposition of the slag was not confirmed. Next, as a result of performing autoclave leaching under the conditions of caustic soda concentration of 4 g/L at 150° C. for 6 hours, a SiO.sub.2 leaching rate was 60% and decomposition of the slag was insufficient.

(34) As shown in Tables 2 to 5, since the slags of Comparative Examples 1 and 2 was heat-treated under the argon gas atmosphere, the Na volatilization rate was 23.1% to 40.9%, which is greatly higher than the Na volatilization rate of Examples 1 to 3. Therefore, the Na concentration in the slag was lowered and the SiO.sub.2/Na.sub.2O molar ratio of the slag was 2.1 or more. In addition, as shown in Comparative Example 3, the slag having the SiO.sub.2/Na.sub.2O molar ratio of 2.1 or more was low in water solubility, and the alkali silicate was difficult to be leached with water.

(35) TABLE-US-00001 TABLE 1 Composition of object to be treated Fe (wt %) Nd (wt %) Dy (wt %) Pr (wt %) B (wt %) Object to be 67 18 7 5 1 treated (I) Object to be 68 20 10 — 1 treated (II)

(36) TABLE-US-00002 TABLE 2 Treatment conditions and recovered amounts of slag and alloy Example 1 Example 2 Example 3 Kinds of treatment object Object to be treated (I) Object to be treated (I) Object to be treated (I) Treated amount 38.0 g 20.0 g 43.0 g Silicon source Metallic silicon Metallic silicon Metallic silicon 4.6 g 2.4 g 5.0 g Flux Sodium metasilicate Sodium metasilicate Sodium disilicate 48.0 g 24.0 g 45.0 g SiO.sub.2/Na.sub.2O molar ratio of 1.0 1.0 2.0 flux Container (crucible) Magnesia Alumina Graphite material (MgO based) (Al.sub.2O.sub.3 based) (C—SiO.sub.2—SiC) Treatment Temperature 1300° C. 1300° C. 1350° C. Heating atmosphere The air The air Argon Blowing air onto melt Time for high temperature 60 min 30 min 45 min 60 min 20 min treatment Iron-silicon alloy recovered 31.0 g 16.0 16.0 g 16.0 g 35.0 g amount Rare earth-containing slag 64.0 g 32.0 g 32.5 g 33.0 g 59.0 g recovered amount Comparative Comparative Example 4 Example 1 Example 2 Kinds of treatment object Object to be treated (I) Object to be treated (I) Object to be treated (II) Treated amount 10.0 g 43.4 g 2.4 g Silicon source Metallic silicon Metallic silicon Iron-silicon alloy 1.2 g 5.0 g (Si 25%) 9.6 g Flux Sodium orthosilicate Sodium disilicate Sodium metasilicate 12.0 g 45.0 g 6.0 g SiO.sub.2/Na.sub.2O molar ratio of 0.5 2.0 1.0 flux Container (crucible) Alumina Graphite Graphite material (Al.sub.2O.sub.3 based) (C—SiO.sub.2—SiC) (Carbon) Treatment Temperature 1300° C. 1350° C. 1300° C. Heating atmosphere The air Argon Argon Time for high temperature 5 min 20 min 5 hr treatment Iron-silicon alloy recovered 7.5 g 35.0 g 11.2 g amount Rare earth-containing slag 15.0 g 58.0 g 7.2 g recovered amount

(37) TABLE-US-00003 TABLE 3 Composition of rare earth-containing slag Na.sub.2O SiO.sub.2 SiO.sub.2/Na.sub.2O Nd.sub.2O.sub.3 Dy.sub.2O.sub.3 Pr.sub.2O.sub.3 MgO Al.sub.2O.sub.3 FeO wt % wt % molar ratio wt % wt % wt % wt % wt % wt % Example 1 33.8 36.3 1.11 12.4 4.8 3.5 7.8 — 0.1 Example 2 30 min 36.1 37.6 1.07 12.9 5.0 3.6 — 4.7 0.1 45 min 35.4 37.5 1.09 12.7 4.9 3.6 — 3.6 0.5 60 min 33.2 37.6 1.17 12.4 4.7 3.5 — 5.8 1.4 Example 3 26.0 48.4 1.92 15.2 5.8 2.5 — — 0.3 Example 4 40.2 33.8 0.87 14.0 5.4 3.9 — 2.4 0.3 Comparative 20.9 51.7 2.55 15.7 6.0 4.4 — — <0.1 Example 1 Comparative 28.2 58.7 2.15 9.0 4.5 — — — <0.1 Example 2

(38) TABLE-US-00004 TABLE 4 Composition of iron-silicon alloy Fe Si Others wt % wt % Wt % Example 1 82.1 15.5 1.6 Example 2 30 min 83.9 13.9 2.2 45 min 83.7 13.8 2.5 60 min 83.4 12.5 4.1 Example 3 82.9 14.9 2.2 Example 4 89.9 9.1 1.0 Comparative Example 1 83.0 13.1 3.9 Comparative Example 2 80.3 18.7 1.0

(39) TABLE-US-00005 TABLE 5 Transition rate of rare earth element and iron to slag, and Na volatilization rate Transition rate (%) to slag Volatilization Rare earth rate (%) of element Fe Na from slag Example 1 100 0.25 9.7 Example 2 30 min 100 0.25 1.6 45 min 100 0.25 3.2 60 min 100 1.20 5.7 Example 3 100 0.61 2.6 Example 4 100 0.52 25.4 Comparative Example 1 100 <0.08 23.1 Comparative Example 2 100 <0.08 40.9

Example 5

(40) 0.6 L of water was added to 60 g of the slag recovered in Example 1. Heating was performed at a room temperature to 50° C. for 3 hours. The water-soluble silica and Na content were sufficiently leached. A solution (pH 12) containing Na and Si was subjected to solid-liquid separation and discharged out of the system. 45.4 g or leached residue (dry) was recovered. The leached residue was stirred in hydrochloric acid solution at pH of 0 to 1 at room temperature for 1 hour to leach the rare earth element. The leached residue (SiO.sub.2 99 wt % or more) was subjected to the solid-liquid separation. The leachate containing the rare earth element was recovered. An oxalic acid solution was added to the leachate, and was stirred at pH of 0 to 1, at a room temperature for 1 hour to precipitate the oxalate. A solution containing hydrochloric acid, Si, and Fe was subjected to solid-liquid separation and discharged out of the system. Oxalate was recovered. The oxalate was calcined at 900° C. for 1 hour to recover 12.4 g of the rare earth oxide. The purity of the recovered material was 99% or higher, and the recovery rate was 97% or higher.

INDUSTRIAL APPLICABILITY

(41) In the separation method of the present invention, separability between iron and rare earth element is good, the amount of volatile substances is less, and a treatment can be performed by using a general furnace in the air. Therefore, the separation method of the present invention is suitable for practical use. In addition, since a rare earth oxide recovered at the end has high purity, the rare earth oxide can be easily reused.