Extraction and separation of rare earth metals from waste resources

Abstract

Disclosed herein is a method for recovering a rare earth metal from a material comprising the rare earth metal, the method comprising: leaching the material with an aqueous leach solution comprising an acid in the presence of pulsed ultrasound to leach the rare earth metal from the material.

Description

EXTRACTION AND SEPARATION OF RARE EARTH METALS FROM WASTE RESOURCESField

[0001] The present invention relates to a process for recovering rare earth elements from a source material comprising the rare earth elements.Background

[0002] There is significant commercial interest in the production of rare earth elements (REE). REE are generally recovered from minerals like monazite-xenotime placer deposits, carbonatites (e.g. bastnasite), placer deposits and ion-adsorption clays in which the REEs are adsorbed on the surface of alumino-silicate minerals like kaolinite. Unfortunately, current mining and mineral extraction practices for recovering REE are expensive and environmentally unsustainable since these REE recovery processes generally involve the use of harsh and toxic chemical reagents for extracting REEs from concentrated mineral ores. By way of example, to produce 1 tonne of REE from the mineral ores at the Bayan Obo mine in Mongolia, which hosts a mixture of REE bearing bastnasite (REEFCO3) and monazite (REEPO4) minerals), requires: 50 tonnes of raw material, approximately 4.41 tonnes of sulfuric acid, 12.32 tonnes of sodium chloride, 1.64 tonnes of sodium hydroxide, 1.17 tonnes of hydrochloric acid, 1.9 tonnes of water, and significant energy input. Further, processing primary REE ores results in large quantities of tailings containing toxic radionuclides such as Th-232 and U-238 and their decay products (e.g., radium), heavy metals in the mine sewage, toxic acids in the water discharge, air pollutant emissions (e.g., particulate matter formation during rock blasting operations).

[0003] Various attempts have been made to develop processes that have high REE recoveries, that have improved energy efficiency, and that are more environmentally friendly.

[0004] Research has been conducted into the use of alternative sources of REE, such as abundant secondary resources including coal fly ash, e-waste, and mine tailings. Coal fly ash (CFA), in particular, is estimated to have an average REE concentration ranging from about 267 to about 843 ppm. The current state of the art suggests that utilizing concentrated inorganic acids like nitric acid (HNO3), sulfuric acid + hydrofluoric acid (H2SO4 + HF), and hydrochloricacid (HC1) is necessary to extract >80% REEs from CFA. There have also been attempts at using more environmentally friendly leaching agents, such as organic acids including acetic acid, oxalic acid, citric acid, and organosulfonic acids. However, these typically yield low recovery rates, typically around 55-77% depending on the source of CFA. It is thought that this low recovery rate is because >60% of the REEs are locked within refractory alumino-silicate phases of the CFA, and that strong mineral acids are required to break these phases to recover the REEs. Given this, current commercial efforts use strong mineral acids.

[0005] Non-chemical approaches to breaking these refractory alumino-silicate phases have also been investigated. One such process investigated the use of a fast electrochemical process based on flash joule heating to improve REE extractability from CFAs using mild acids, such as 0.1M HC1. The idea was to target decomposition of the refractory REE phosphates, zircon and alumino-silicate glass phases. The thermal decomposition of these refractory phases at ~3000°C permits access to the REEs. In this case, an increase in the extraction of REEs of -206% for class F-type CFA and -187% for class C-type CFA, when compared to directly leaching the CFAs with concentrated mineral acids.

[0006] Application of a continuous source of ultrasound has also been considered as a method for improving extractability of REEs from CFA. The underlying principle is that the continuous application of low ranging ultrasound (e.g., with frequency typically in the range of 20 kHz to 2 MHz) causes acoustic cavitation from the formation and collapse of micro-bubbles. The collapse of these micro-bubbles creates local hotspots where extreme temperatures (e.g., up to 10,000 K) and high pressures (up to 10,000 atm) can be generated which may assist in breaking refractory alumino-silicate phases. Although continuous application of ultrasound has been found to assist with the extraction of REEs from man-made sources of REEs (such as scrap magnets containing neodymium), it has been found ineffective for materials such as CFA.

[0007] It is an object of the invention to address at least one shortcoming of the prior art and / or provide a useful alternative.Summary of Invention

[0008] In one aspect of the invention, there is provided a method for recovering a rare earth metal from a material comprising the rare earth metal, the method comprising:leaching the material with an aqueous leach solution comprising an acid in the presence of pulsed ultrasound to leach the rare earth metal from the material.

[0009] In an embodiment, the acid is an organic acid. Preferably, the acid is a carboxylic acid, such as a mono-, di-, or tri-carboxylic acid. More preferably, the carboxylic acid is tricarboxylic acid, such as citric acid.

[0010] In an embodiment, a concentration of the acid in the aqueous leach solution is from about IM to about 2M. Preferably, the concentration is from about 1.1M. More preferably, the concentration is from about 1.2M. Most preferably, the concentration is from about 1.25M. Alternatively or additionally, it is preferred that the concentration of the acid is up to 1.8M. More preferable, the concentration is up to 1.6 M. Most preferably, the concentration is up to about 1.4 M. In one embodiment, the concentration is about 1.3 M + / - 10%.

[0011] In an embodiment, the pulsed ultrasound has a frequency in the range of from about 15 kHz to about 100 kHz. Preferably, the pulsed ultrasound has a frequency of from about 18 kHz. More preferably, from about 20 kHz. Alternatively, or additionally, the pulsed ultrasound has a frequency of up to about 80 kHz. More preferably, up to about 60 kHz. Most preferably, up to about 40 kHz.

[0012] In an embodiment, the pulsed ultrasound is applied for a time of from 0.5 hours up to 4 hours. Preferably up to 2 hours. More preferably, up to 1.5 hours. Most preferably, for up to about 1 hour.

[0013] In an embodiment, the ultrasound is at an amplitude sufficient to generate cavitation bubbles within the aqueous solution at the surface of the material.

[0014] In an embodiment, the pulsed ultrasound has a power of from about 20 Watts to about 400 Watts. Preferably, the power is from about 40 Watts. More preferably, the power is from about 60 Watts. Most preferably, the power is from about 70 Watts. Alternatively, or additionally, it is preferred that the power is up to about 380 Watts. More preferably, the power is up to about 360 Watts. Most preferably, the power is about 350 Watts. In one embodiment, the power is about 300 Watts + / - 10%.

[0015] In an embodiment, the pulsed ultrasound has an energy flux of from about 10 W / cm2up to about 80 W / cm2. Preferably, the power is from about 12 W / cm2. More preferably, the power is from about 20 W / cm2. Most preferably, the power is from about 40 W / cm2. Alternatively, or additionally, it is preferred that the power is up to about 70 W / cm2. More preferably, the power is up to about 60 W / cm2. Most preferably, the power is about 50 W / cm2. In one embodiment, the power is about 42 W / cm2+ / - 10%.

[0016] In an embodiment, the pulsed ultrasound is applied by modulating the amplitude of the ultrasound between a peak amplitude and a minimum or zero amplitude.

[0017] In one form of the above embodiment, there is a first duration between successive minimum or zero amplitudes and a second duration between successive peak amplitudes, and a ratio of the first duration to the second duration is from about 1 : 1 up to about 10:1. Preferably, the ratio is up to about 8:1. More preferably, the ratio is up to about 6: 1. Most preferably the ratio is up to about 4:1. Additionally, or alternatively, it is preferred that the ratio is from about 2: 1. In one embodiment, the ratio is about 3:1.

[0018] In an embodiment, the pulsed ultrasound is applied by switching the ultrasound on and off.

[0019] In one form of the above embodiment, the ultrasound is switched on for a first duration and switched off for a second duration, wherein a ratio of the first duration to the second duration is from about 1 : 1 up to about 10:1. Preferably, the ratio is up to about 8:1. More preferably, the ratio is up to about 6:1. Most preferably the ratio is up to about 4:1. Additionally, or alternatively, it is preferred that the ratio is from about 2: 1. In one embodiment, the ratio is about 3:1.

[0020] In various forms of the above embodiments, the first duration is from about 1 second up to about 60 seconds. Preferably the first duration is from about 5 seconds. More preferably, from about 10 seconds. Even more preferably, from about 20 seconds. Most preferably, from about 25 seconds. Alternatively or additionally, the first duration is up to about 50 seconds. More preferably, the first duration is up to about 40 seconds. Most preferably, up to about 35 seconds. In one embodiment, the first duration is about 30 seconds + / - 10%.

[0021] In various forms of the above embodiments, the second duration is from about 1 second up to about 30 seconds. Preferably the second duration is from about 2 seconds. More preferably, from about 3 seconds. Most preferably, from about 4 seconds. Alternatively or additionally, the second duration is up to about 25 seconds. More preferably, the first duration is up to about 20 seconds. Even more preferably, up to about 15 seconds. Most preferably, up to about 10 seconds. In one embodiment, the second duration is about 5 seconds + / - 20%.

[0022] In an embodiment, the pulsed ultrasound is applied in the form of a square wave.

[0023] In an embodiment, the material is selected from the group consisting of fly ash, bottom ash, waste electronics, battery waste, waste magnet, mineral tailings, or mineral ores.

[0024] In an embodiment, the rare earth metal is one or more rare earth metals selected from the group consisting of: Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.Preferably, the rare earth metal is two, three, four, or five or more rare earth metals. It is preferred that the rare earth metal is two rare earth metals, and the at least two rare earth metals comprises one or more metals selected from the group consisting of Ce, Sc, Y, La, Nd, Eu, Tb, Dy, and Er.

[0025] In an embodiment, the rare earth metal comprises at least one rare earth metal selected from the group consisting of Ce, Sc, Y, La, Nd, Eu, Tb, Dy, and Er. Preferably, the rare earth metal comprises one or more rare earth metals selected from the group consisting of Ce.

[0026] In an embodiment, the leaching step is conducted at atmospheric pressure and / or at a temperature less than the boiling point of the aqueous leach solution. Preferably, the temperature is from about 40 °C. More preferably, from about 50 °C. Alternatively or additionally, the temperature is up to about 90 °C. More preferably, up to about 80 °C. Most preferably, up to about 70 °C. In one embodiment, the temperature is about 60 °C + / - 10%.

[0027] In an embodiment, the leaching step provides a leachate comprising the rare earth metal dissolved therein and a solid residue. Preferably, the method further comprises separating the leachate and the solid residue. Any suitable solid-liquid separation step known to those skilled in the art may be used.

[0028] In an embodiment, wherein leaching the rare earth metal from the material comprises: dissolving the rare earth metal into solution in the form of a soluble ion thereof; or reducing the rare earth metal into a soluble ion thereof and dissolving the soluble ion thereof into solution; oroxidizing the rare earth metal into a soluble ion thereof and dissolving the soluble ion thereof into solution.

[0029] In an embodiment, the solid-liquid ratio of the material to the leach solution is about 1 / 20 to about 1 / 100 g / mL.

[0030] In an embodiment, the material has a D[4,3] particle size in the range of from about 50 pm to about 1.5 mm. Preferably, the D[4,3] particle size is from about 60 pm. More preferably, the D[4,3] particle size is from about 70 pm. Most preferably, the D[4,3] particle size is from about 75 pm. Alternatively, or additionally, the D[4,3] particle size is up to about 1.4 mm. More preferably, the D[4,3] particle size is up to about 1.2 mm. Most preferably, the D[4,3] particle size is up to about 1 mm.

[0031] In an embodiment, the material has a D50 particle size that is in the range of from about 50 pm to about 1.5 mm. Preferably, the D50 particle size is from about 60 pm. More preferably, the D50 particle size is from about 70 pm. Most preferably, the D50 particle size is from about 75 pm. Alternatively, or additionally, the D50 particle size is up to about 1.4 mm. More preferably, the D50 particle size is up to about 1.2 mm. Most preferably, the D50 particle size is up to about 1 mm.

[0032] In an embodiment, the material has a D10 particle size of at least 50 pm. Preferably, the D10 particle size is from about 60 pm. More preferably, the D10 particle size is from about 70 pm. Most preferably, the D10 particle size is from about 75 pm.

[0033] In an embodiment, the material has a D90 particle size of up to 1.5 mm. Preferably, the D90 particle size is up to about 1.4 mm. More preferably, the D90 particle size is up to about 1.2 mm. Most preferably, the D90 particle size is up to about 1 mm.

[0034] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that thisprior art could reasonably be expected to be understood, regarded as relevant, and / or combined with other pieces of prior art by a skilled person in the art.

[0035] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.Brief Description of Drawings

[0036] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

[0037] Figure 1: Graph showing recovery of REEs with different organic acids, tests conducted at 50% ultrasound amplitude, pulse sequencing on / off of 15s / 5s and for an effective time of 120 minutes.

[0038] Figure 2: Graph showing the effect of ultrasound amplitude on REE recovery, pulse sequence on / off of 15s / 5s, for effective times of 60 mins and 120mins, 1:50 g / ml and 1.31M citric acid.

[0039] Figure 3: Graph showing the effect of ultrasound pulses on recovery of REEs. Legend shows ultrasound power (%), pulse sequence on / off (s), and effective time (mins).

[0040] Figure 4: Graph showing the effect of ultrasound pulses on recovery of REEs. Legend shows ultrasound power (set at 13%), effective time (set at 120 mins), and pulse sequence on / off (s).Description of Embodiments

[0041] The inventors have found that application of pulsed ultrasound, as opposed to e.g., continuous ultrasound, can enhance the recovery of REEs from a source material comprising the REEs during a leaching process. The source material may for example be, a REE containing ore, and as will be discussed below, is preferably an REE containing waste material such as fly ash, bottom ash, waste electronics, battery waste, waste magnets, mineral tailings and the like.

[0042] Broadly, the process comprises leaching the REE containing source material with a leach solution comprising a leaching agent, and during the leaching process, applying a source of pulsed ultrasound to the REE containing source material. Without wishing to be bound by theory, the inventors are of the view that the application of pulsed ultrasound promotes cavitation effects by generating and collapsing cavitation bubbles within the leach solution at the surface of the REE containing source material. In particular, the inventors have found that pulsed ultrasound is more effective than continuous ultrasound since the high-low / on-off sequence of pulses promotes bubble generation and bubble collapse respectively. The collapse of cavitation bubbles releases energy into the system which causes micro-reactions at the surface of the REE containing source material to liberate REE and thus enhances the leaching process.

[0043] The process can be practiced with a wide range of leaching agents, for example, various mineral acids (e.g., sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, combinations thereof, and the like). However, the inventors have also found that the process can advantageously provide high REE recovery rates using leaching agents that are more environmentally benign, such as organic acids and in particular, acetic acid, oxalic acid, malic acid, or citric acid.

[0044] Previous efforts using organic acids to recover REEs from, for example, brown coal fly ash (BCFA) using organic acids achieved an extraction efficiency of about 55% which is far lower than the recovery rate required to provide an economically viable commercial process. In contrast, the use of mineral acids generally yields a recovery of 90% or greater. Based on these findings, the skilled person understands that harsh mineral acids are required to break refractory alumino-silicate phases, which host typically greater than 60% of REEs in source materials such as BCFA, and as such, extraction techniques utilizing these harsh inorganic acids are essential to achieve commercial viability. That is, the skilled person appreciates that organic acids are generally unsuitable for use in extracting REEs, particularly from materials such as BCFA since a significant portion of REEs are hosted in refractory alumino-silicate phases and are not extractable with organic acids. However, in contrast with conventional approaches, the inventors have found that application of pulsed ultrasound greatly improves the recovery rate of REEs even when using organic acid leaching agents.

[0045] The invention will be discussed in more detail below.Example

[0046] This example reports a series of experiments investigating extraction of REEs from brown coal fly ash (BCFA).Sample preparation

[0047] Three BCFA samples were collected, in 20L pail containers, from bag filters in power stations located in Loy Yang, Morwell and Yalloum. The samples were used in as received conditions for total metal quantification. A portion of the sample was separated from different parts of the bucket, sieved using a 75 pm mesh in a Tyler sieve shaker and homogenized overnight using a 3 -bar variable speed rotary tumbler.Ultrasonication method

[0048] Ultra-sonication experiments were conducted with the Q700 sonicator from Qsonica with a U inch diameter horn with solid probe. The amplitude can be controlled over its power output range of 700 Watts and frequency fixed at 20 kHz.

[0049] The samples were tested under continuous and pulsed ultra-sound fields to observe any potential variations in the acoustic cavitation generated, and consequently on the recovery results. Solid / liquid ratio was maintained at 1 / 50 g / mL throughout these studies. Temperature was continuously monitored using a temperature probe and controller. The sample beaker was immersed in an ice bath and temperature maintained at ~ 60 °C.Results and discussionEffect of reagent and concentration on REE recovery

[0050] Preliminary experiments were conducted to identify the best choice of reagent for use as the leaching agent. A range of organic acids, including a mono-carboxylic acid (acetic acid), a di- carboxylic acid (malic acid), and a tri- carboxylic acid (citric acid) were tested. The experiments were carried out at 50% amplitude using 15s / 5s on-off pulse sequences. The results are presented in Figure 1.

[0051] The results show that for similar concentrations of both acetic and malic acids, the recovery of select REEs (Sc, Y, Nd, Dy and Ce used as experimental probes) was < 70 %, although malic acid exhibited higher recovery rates than acetic acid. In contrast, significantly higher recovery rates were obtained when using citric acid as the leaching agent. The results show a clear trend that recovery of REEs increases with the number of carboxyl acid groups in the leaching agent. Without wishing to be bound by theory, inventors are of the view that tricarboxylic acids outperform mono- and di-carboxylic acids due to their higher chelating ability with rare earth elements resulting in the formation of strong bonds with lanthanides and weak bonds with alkali and alkaline earth metals.

[0052] The inventors also tested citric acid at IM under similar reaction conditions using a conventional mixing process and in the absence of any ultrasound. In this experiment, about 42% Nd and 51% Dy was recovered compared with 84% Nd and 97% Dy using pulsed ultrasound as shown in Figure 1.

[0053] To overcome chemical reaction kinetic limitations, the concentration was fixed at 1.31 mol / L leaching agent for subsequent experiments.Effect of amplitude and time on REE recovery

[0054] The effect of ultra-sound amplitude was tested on the recoverability of REEs from BCFA to determine optimal sonication amplitude for the recovery of REE. The inventors hypothesise that higher sonication amplitude will result in higher REE recovery due to higher energy input into the system. The results are presented in Figure 2.

[0055] Figure 2 shows an increase in the recovery of REEs as the applied energy intensity (amplitude) is increased to an amplitude of 43%. Surprisingly, and contrary to the inventors’ hypothesis, increasing the amplitude beyond a certain level, which in this example is 43%, causes the recovery efficiency to significantly drop. Furthermore, applying ultrasonic pulses for longer periods, e.g., greater than 60 minutes, did not result in a significant improvement in REE recovery.

[0056] Without wishing to be bound by theory, the inventors are of the view that the amplitude has an impact on the quality of ultra-sound cavitation generation. It has been previously found that using high power and low frequency ultra-sonic energy provides a low number of powerfulcavitation bubbles with each pulse. It has also been found that acoustic bubble size distribution from pulsed ultrasound increases with increasing acoustic power and decreases with increasing ultrasound frequency for single bubble systems. However, in multi -bubble systems due to strong non-linear pulsations, at higher acoustic power the bubble radius is expected to decrease with an increase in acoustic pressure. The effects of amplitude are more prominent at high power range than at lower power range. It has also previously been observed that the size of the bubbles saturates after a tipping point over the range of the applied acoustic power (0-30W). Based on the foregoing, inventors conclude that in multibubble systems and at high amplitude operations maximizing REE extraction is an optimization problem requiring identification of a suitable power range to maximise the cavitation effects.Effect of pulse sequency of REE recovery

[0057] The effect of ultra- sonication pulsing on REE recovery was also investigated. In particular experiments were conducted using on / off pulses of 5s / 5s, 15s / 5s, and 30s / 5s for a total time of 1 hr.

[0058] The results are presented in Figure 3. From the results, it can be seen that that varying the ultrasound pulses at fixed frequency has a significant impact on recovery of REEs. The results show that as the effective time increases, the recovery is lower at a fixed amplitude, which is analogous to continuous ultrasonication. The term ‘effective time’ is intended to refer to the time of exposure to ultrasound. This contrasts with ‘total time’ which refers to the total duration of the experiment. By way of example, an effective time of 1 hr for an on / off pulse sequence of 30s / 5s has a total time of 1 hr and 10 minutes.

[0059] Inventors believe that during ultrasonication at higher effective times, the cavitation bubbles become trapped within the ultrasonic energy field, which leads to an increase in bubbles coalescing and growing rather than collapsing as compared with shorter effective times. To test this hypothesis, experiments were conducted at 13% amplitude and similar pulse ratios. The conditions were tested for 120 minutes, to avoid time as a constraint and to purely see the effect of amplitude vs pulse ratio. The results are presented in Figure 4. As can be seen, shorter pulse times provide better results than longer pulse times.

[0060] Generally, it is understood that for liquid-liquid systems, increased ultrasonic energy results in higher sono-chemical effects. Mechanistically, this is attributed to (a) residual acousticpressure and active bubbles in the off period, (b) enlargement of chemically active zone in the irradiated solution mixture, (c) shielding effects being lowered at higher energy. However, in the present system, higher amplitudes did not essentially result in improved REE recovery. The inventors are of the view that in solid-liquid systems, the rate of active bubble generation and collapses are key factors for higher extraction performance since these are thought to break or fracture Al-Si glassy structures found in coal fly ash which liberates the REEs.

[0061] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

Claims1. A method for recovering a rare earth metal from a material comprising the rare earth metal, the method comprising:leaching the material with an aqueous leach solution comprising an acid in the presence of pulsed ultrasound to leach the rare earth metal from the material.

2. The method of claim 1, wherein the acid is an organic acid.

3. The method of claim 2, wherein the acid is a carboxylic acid, such as a mono-, di-, or tricarboxylic acid.

4. The method of any one of the preceding claims, wherein a concentration of the acid in the aqueous leach solution is from about IM to about 2M.

5. The method of any one of the preceding claims, wherein the pulsed ultrasound has a frequency in the range of from about 15 kHz to about 100 kHz.

6. The method of any one of the preceding claims, wherein the pulsed ultrasound is applied for a time of from 0.5 hours up to 4 hours.

7. The method of any one of the preceding claims, the ultrasound is at an amplitude sufficient to generate cavitation bubbles within the aqueous solution at the surface of the material.

8. The method of any one of the preceding claims, wherein the pulsed ultrasound has a power of from about 20 Watts to about 400 Watts.

9. The method of any one of the preceding claims, wherein the pulsed ultrasound is applied by modulating the amplitude of the ultrasound between a peak amplitude and a minimum or zero amplitude.

10. The method of claim 9, wherein there is a first duration between successive minimum or zero amplitudes and a second duration between successive peak amplitudes, and a ratio of the first duration to the second duration is from about 1:1 up to about 10:1.

11. The method of any one of claims 1 to 8, wherein the pulsed ultrasound is applied by switching the ultrasound on and off.

12. The method of claim 11, wherein the ultrasound is switched on for a first duration and switched off for a second duration, wherein a ratio of the first duration to the second duration is from about 1:1 up to about 10:1.

13. The method of claim 10 or 12, wherein the first duration is from about 1 second up to about 60 seconds.

14. The method of claim 10 or 12 wherein the second duration is from about 1 second up to about 30 seconds.

15. The method of any one of the preceding claims, wherein the pulsed ultrasound is applied in the form of a square wave.

16. The method of any one of the preceding claims, wherein the material is selected from the group consisting of fly ash, bottom ash, waste electronics, battery waste, waste magnet, mineral tailings, or mineral ores.

17. The method of any one of the preceding claims, wherein the rare earth metal is one or more rare earth metals selected from the group consisting of: Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

18. The method of claim 17, wherein the rare earth metal is two, three, four, or five or more rare earth metals.

19. The method of any one of the preceding claims, wherein the leaching step is conducted at atmospheric pressure and / or at a temperature less than the boiling point of the aqueous leach solution.

20. The method of any one of the preceding claims, wherein the leaching step provides a leachate comprising the rare earth metal dissolved therein and a solid residue.

21. The method of claim 20, wherein the method further comprises separating the leachate and the solid residue.

22. The method of any one of the preceding claims, wherein leaching the rare earth metal from the material comprises:dissolving the rare earth metal into solution in the form of a soluble ion thereof; or reducing the rare earth metal into a soluble ion thereof and dissolving the soluble ion thereof into solution; oroxidizing the rare earth metal into a soluble ion thereof and dissolving the soluble ion thereof into solution.

23. The method of any one of the proceeding claims, wherein the solid-liquid ratio of the material to the leach solution is about 1 / 20 to about 1 / 100 g / mL.

24. The method of any one of the preceding claims, wherein the material has a D[4,3] particle size in the range of from about 50 pm to about 1.5 mm.

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