Recovery of Rare Earth Oxide from Waste NiMH Batteries by Simple Wet Chemical Valorization Process

: Nickel metal hydride (NiMH) batteries contain a signiﬁcant amount of rare earth metals (REMs) such as Ce, La, and Nd, which are critical to the supply chain. Recovery of these metals from waste NiMH batteries can be a potential secondary resource for REMs. In our current REM recovery process, REM oxide from waste NiMH batteries was recovered by a simple wet chemical valorization process. The process followed the chemical metallurgy process to recover REM oxides and included the following stages: (1) H 2 SO 4 leaching; (2) selective separation of REM as sulfate salt from Ni / Co sulfate solution; (3) metathesis puriﬁcation reaction process for the conversion REM sulfate to REM carbonate; and (4) recovery of REM oxide from REM carbonate by heat treatment. Through H 2 SO 4 leaching optimization, almost all the metal from the electrode active material of waste NiMH batteries was leached out. From the ﬁltered leach liquor managing pH (at pH 1.8) with 10 M NaOH, REM was precipitated as hydrated NaREE(SO 4 ) 2 · H 2 O, which was then further valorized through the metathesis reaction process. From NaREE(SO 4 ) 2 · H 2 O through carbocation, REM was puriﬁed as hydrated (REM) 2 CO 3 · H 2 O precipitate. From (REM) 2 CO 3 · H 2 O through calcination at 800 ◦ C, (REM) 2 O 3 could be recovered. ﬁltration. for


Introduction
Owing to the strategic, economic, and industrial importance of rare earth metals (REMs), the EU has classified them as critical raw materials (CRMs). A criticality assessment of metals based on economic importance and supply risk done in 2017 resulted in the EU considering heavy rare earth metals (HREMs), light rare earth metals (LREM), and Sc as CRMs [1,2]. The EU, along with the US Department of Energy (DOE) [3] and the American Physical Society (APS), [4] have reported REM as critical for energy and emerging technologies. The global supply chain of critical metals like HREMs, LREMs, and Sc is mainly dominated by China, which has a market share of up to 95% [2]. The global distribution of primary resources for these critical REMs is quite uneven and currently dominated by China. Hence, the recovery of these CRMs from secondary resources is very important in order to secure these metals for industrial economies like Korea, where natural resources are scarce. The nickel metal hydride (NiMH) battery could be a secondary resource for REMs like Ce, La, and Nd, as it contains a significant amount of these elements (which are all REMs). By efficiently recycling NiMH waste batteries, the REMs can be recovered and circulated in the industrial ecosystem. This could simultaneously address issues like urban mining, environmental directive, and CRM supply chain challenges, and could secure the supply of these metals for industrial development. Table 1. Summary of battery recycling processes by various companies around the world (data from [5,6] In our current investigation, we developed a simple yet industrially feasible and sustainable REM oxide recovery process. The complete process is depicted in a flowsheet in Figure 1. As shown in the figure, the developed process comprises sequential H 2 SO 4 acid leaching followed by selective NaREE(SO 4 ) 2 ·H 2 O precipitation and purification of (REM) 2 CO 3 ·H 2 O by carbocation-mixed REM oxide isolation by calcination. As shown in Figure 1, the process follows the integration of physical dismantling and classification followed by the chemical metallurgy process. The physical separation process comprises the following mechanical steps: (1) residual stored energy discharge; (2) grinding and calcination; and (3) classification and access of the electrode material. This is followed by the chemical metallurgy process, which has the following stages: (1) H 2 SO 4 leaching; (2) separation of the REMs as sulfate salt from the Ni/Co sulfate solution; (3) the metathesis synthesis reaction process for conversion of REM sulfate to REM carbonate; and (4) REM oxide recovery by heat treatment. The novelty of this process is described below. i.
All battery recycling processes by various companies around the world that have been reviewed mostly follow pyrometallurgical or pyrometallurgical-dominated processes, in contrast to our developed process which is hydrometallurical. ii.
It is a simple acid leaching and precipitation process for the recovery of REMs. iii.
Recovered REM sulfate is value-added through a simple carbocation reaction. In our current investigation, we developed a simple yet industrially feasible and sustainable REM oxide recovery process. The complete process is depicted in a flowsheet in Figure 1. As shown in the figure, the developed process comprises sequential H2SO4 acid leaching followed by selective NaREE(SO4)2 H2O precipitation and purification of (REM)2CO3.H2O by carbocation-mixed REM oxide isolation by calcination. As shown in Figure 1, the process follows the integration of physical dismantling and classification followed by the chemical metallurgy process. The physical separation process comprises the following mechanical steps: (1) residual stored energy discharge; (2) grinding and calcination; and (3) classification and access of the electrode material. This is followed by the chemical metallurgy process, which has the following stages: (1) H2SO4 leaching; (2) separation of the REMs as sulfate salt from the Ni/Co sulfate solution; (3) the metathesis synthesis reaction process for conversion of REM sulfate to REM carbonate; and (4) REM oxide recovery by heat treatment. The novelty of this process is described below. i.
All battery recycling processes by various companies around the world that have been reviewed mostly follow pyrometallurgical or pyrometallurgical-dominated processes, in contrast to our developed process which is hydrometallurical. ii.
It is a simple acid leaching and precipitation process for the recovery of REMs. iii.
Recovered REM sulfate is value-added through a simple carbocation reaction.

Leaching and REM Separation
The leaching reactor used for leaching of the NiMH battery powder is shown in Figure 2. The reactor vessel was a three-necked round-bottom flask of 1 L capacity. The flask was jacketed for heating and controlling the reaction temperature. The reactor was equipped with an overhead agitator, and was driven by a variable-speed motor, a digital thermocouple to measure the temperature during continuous operation of the reactor, and a condenser for cooling. Before starting the leaching, the requisite volume of H 2 SO 4 solution was added to the reaction flask and heated to the target temperature. Waste NiMH battery powder was added to the reaction vessel, which was then closed. After the reaction was complete, the leaching liquor was separated from residue through filtration. From the leach liquor, NaREE(SO 4 ) 2 ·H 2 O was separated by precipitation, controlling pH using NaOH while stirring at 400 rpm under pH control. Isolated NaREE(SO 4 ) 2 ·H 2 O was dried. For the metathesis reaction, the NaREE(SO 4 ) 2 ·H 2 O was added to the Na 2 CO 3 solution and the reaction was allowed to complete. After reaction completion was achieved, the (REM) 2 CO 3 was precipitated out and recovered through filtration. After drying, the (REM) 2 CO 3 was calcined for (REM) 2 O 3 recovery.

Leaching and REM Separation
The leaching reactor used for leaching of the NiMH battery powder is shown in Figure 2. The reactor vessel was a three-necked round-bottom flask of 1 L capacity. The flask was jacketed for heating and controlling the reaction temperature. The reactor was equipped with an overhead agitator, and was driven by a variable-speed motor, a digital thermocouple to measure the temperature during continuous operation of the reactor, and a condenser for cooling. Before starting the leaching, the requisite volume of H2SO4 solution was added to the reaction flask and heated to the target temperature. Waste NiMH battery powder was added to the reaction vessel, which was then closed. After the reaction was complete, the leaching liquor was separated from residue through filtration. From the leach liquor, NaREE(SO4)2 H2O was separated by precipitation, controlling pH using NaOH while stirring at 400 rpm under pH control. Isolated NaREE(SO4)2 H2O was dried. For the metathesis reaction, the NaREE(SO4)2 H2O was added to the Na2CO3 solution and the reaction was allowed to complete. After reaction completion was achieved, the (REM)2CO3 was precipitated out and recovered through filtration. After drying, the (REM)2CO3 was calcined for (REM)2O3 recovery.

Characterization of Waste NiMH Battery Material
Prior to characterization, the waste NiMH battery material module was discharged inside the brine. Grinding was followed by classification, and electroactive material of 100 mesh-size powder was recovered using a pulverizer. XRD (Figure 3) of the waste NiMH battery powder was analyzed. From XRD analysis, major phases such as Ni(OH) 2 , NiO, Ni, Ni 7 Ce 2 , Co 5 Nd, and La 2 O 3 and minor phases such as NiO, NiOOH, Ni 7 Nd 2 , and Co 3 Nd were observed. XRF analysis for the same sample indicated Ni, Co, Ce, La, and Nd were the primary constituents in battery material with significant Ni content found in the waste NiMH battery. The composition of the recovered NiMH battery powder was also analyzed by MP-AES (Microwave Plasma Atomic Emission Spectroscopy 4200, Agilent). It was confirmed that Ni, Co, and REMs (La, Ce, Nd) comprised 45.8%, 8.5%, and 20.2% of the recovered battery powder, respectively ( Table 2). As Table 2 indicates, the NiMH battery waste contained a significant amount of REMs (20.2%), which would be valuable CRMs for Korean industries.

Characterization of Waste NiMH Battery Material
Prior to characterization, the waste NiMH battery material module was discharged inside the brine. Grinding was followed by classification, and electroactive material of 100 mesh-size powder was recovered using a pulverizer. XRD (Figure 3) of the waste NiMH battery powder was analyzed. From XRD analysis, major phases such as Ni(OH)2, NiO, Ni, Ni7Ce2, Co5Nd, and La2O3 and minor phases such as NiO, NiOOH, Ni7Nd2, and Co3Nd were observed. XRF analysis for the same sample indicated Ni, Co, Ce, La, and Nd were the primary constituents in battery material with significant Ni content found in the waste NiMH battery. The composition of the recovered NiMH battery powder was also analyzed by MP-AES (Microwave Plasma Atomic Emission Spectroscopy 4200, Agilent). It was confirmed that Ni, Co, and REMs (La, Ce, Nd) comprised 45.8%, 8.5%, and 20.2% of the recovered battery powder, respectively ( Table 2). As Table 2 indicates, the NiMH battery waste contained a significant amount of REMs (20.2%), which would be valuable CRMs for Korean industries.

Leaching Optimization of Waste NiMH Battery Material
For the development of a simple REM valorization process from the waste NiMH battery module, leaching parameters like temperature and reaction time were selected from the literature survey. Pietrelli et al. reported REM recovery from NiMH spent batteries using H 2 SO 4 as a suitable lixiviant [20]. For the extraction of valuable REMs like Ce, La, and Nd from waste NiMH battery modules, important leaching parameters like the lixiviant (H 2 SO 4 ) concentration and pulp densities were optimized, keeping the leaching temperature constant at 90 • C and leaching reaction time of 4 h. Temperature and time were considered from the reported studies by Pietrelli et al. [20]. The leaching optimization experiments were carried out and both parameters were varied at the same time. The lixiviant concentration ranged from 1 to 4 M of H 2 SO 4 , while pulp density ranged from 25 to 200 g/L. The reaction temperature was 90 • C the reaction time was 4 h. Figure 4a depicts the leaching behavior of Al, Co, Fe, Mn, Ni, and Zn, and Figure 4b depicts the leaching behavior of Ce, La, and Nd as a function of lixiviant concentration and pulp density. Figure 4a shows that Al, Co, Fe, Mn, Ni, and Zn extraction gradually increased as a function of lixiviant concentration and pulp density when both were abundant in the leaching process. Hence, higher efficiency, higher pulp density, and higher H 2 SO 4 acid concentration could be suitable for efficient leaching process development. Similarly, as shown in Figure 4b, Ce, La, and Nd extraction increased up to 3 M H 2 SO 4 . Extraction decreased gradually when 4 M H 2 SO 4 was used as a lixiviant. The same figure also indicates a higher amount of Ce, La, and Nd were extracted as function of pulp density. To understand the variable leaching phenomena observed in Figure 4, the leaching chemistry needed to be understood. As the focus of the investigation is only REMs, the possible leaching reaction chemistry is explained below. As XRD indicated, La, Ce, and Nd (REMs) presented mainly as REM oxide and a Co/Ni alloy compound. Therefore the leaching chemistry can be explained using Equations (1)-(3) presented below. For the development of a simple REM valorization process from the waste NiMH battery module, leaching parameters like temperature and reaction time were selected from the literature survey. Pietrelli et al. reported REM recovery from NiMH spent batteries using H2SO4 as a suitable lixiviant [20]. For the extraction of valuable REMs like Ce, La, and Nd from waste NiMH battery modules, important leaching parameters like the lixiviant (H2SO4) concentration and pulp densities were optimized, keeping the leaching temperature constant at 90 °C and leaching reaction time of 4 h. Temperature and time were considered from the reported studies by Pietrelli et al. [20]. The leaching optimization experiments were carried out and both parameters were varied at the same time. The lixiviant concentration ranged from 1 to 4 M of H2SO4, while pulp density ranged from 25 to 200 g/L. The reaction temperature was 90 °C the reaction time was 4 h. Figure 4a depicts the leaching behavior of Al, Co, Fe, Mn, Ni, and Zn, and Figure 4b depicts the leaching behavior of Ce, La, and Nd as a function of lixiviant concentration and pulp density. Figure 4a shows that Al, Co, Fe, Mn, Ni, and Zn extraction gradually increased as a function of lixiviant concentration and pulp density when both were abundant in the leaching process. Hence, higher efficiency, higher pulp density, and higher H2SO4 acid concentration could be suitable for efficient leaching process development. Similarly, as shown in Figure 4b, Ce, La, and Nd extraction increased up to 3 M H2SO4. Extraction decreased gradually when 4 M H2SO4 was used as a lixiviant. The same figure also indicates a higher amount of Ce, La, and Nd were extracted as function of pulp density. To understand the variable leaching phenomena observed in Figure 4, the leaching chemistry needed to be understood. As the focus of the investigation is only REMs, the possible leaching reaction chemistry is explained below. As XRD indicated, La, Ce, and Nd (REMs) presented mainly as REM oxide and a Co/Ni alloy compound. Therefore the leaching chemistry can be explained using Equations (1)-(3) presented below.

Separation and Recovery of REM by Metathesis Reaction
Though base metal extraction for Ni and Co was more efficient with higher pulp density and higher lixiviant concentration, REM extraction was not much more efficient at higher lixiviant concentrations. As the main focus of the investigation was REM recovery, conditions such as using 1 M of H2SO4 acid as a lixiviant and a pulp density of 25 g/L were considered for further studies. Sufficient volume of leach liquor was generated using 1 M H2SO4 acid at a pulp density of 25 g/L, at a temperature of 90 °C with a reaction time of 4 h. Followed by solid-liquid separation, REMs were separated the leach liquor through precipitation using NaOH. The initial pH of the leaching liquor was about 0.05 after solid-liquid separation. The pH of leach liquor was adjusted by slowly adding 10 M NaOH to the leach liquor under constant stirring. As the pH of the solution was increased slowly to 1.8, REMs were precipitated, leaving Co, Ni, and other metals in the leach liquor. The precipitated REMs were separated from the solution by simple filtration and the sample was washed several times to remove soluble components. After washing, the samples were dried overnight in an oven at 80 °C, and were then characterized by XRD, ICP-MS, and TGA.

Separation and Recovery of REM by Metathesis Reaction
Though base metal extraction for Ni and Co was more efficient with higher pulp density and higher lixiviant concentration, REM extraction was not much more efficient at higher lixiviant concentrations. As the main focus of the investigation was REM recovery, conditions such as using 1 M of H 2 SO 4 acid as a lixiviant and a pulp density of 25 g/L were considered for further studies. Sufficient volume of leach liquor was generated using 1 M H 2 SO 4 acid at a pulp density of 25 g/L, at a temperature of 90 • C with a reaction time of 4 h. Followed by solid-liquid separation, REMs were separated the leach liquor through precipitation using NaOH. The initial pH of the leaching liquor was about 0.05 after solid-liquid separation. The pH of leach liquor was adjusted by slowly adding 10 M NaOH to the leach liquor under constant stirring. As the pH of the solution was increased slowly to 1.8, REMs were precipitated, leaving Co, Ni, and other metals in the leach liquor. The precipitated REMs were separated from the solution by simple filtration and the sample was washed several times to remove soluble components. After washing, the samples were dried overnight in an oven at 80 • C, and were then characterized by XRD, ICP-MS, and TGA.
The recovered REM powder was analyzed by XRD and the obtained XRD pattern is presented in Figure 5. The powder was identified as a mixture of NaNd(SO 4  will be termed (NaREE(SO 4 ) 2 ·H 2 O). The content of Ce, La, and Nd impurities including Ni and Co was analyzed by MP-AES by dissolving the powder. This is presented in Table 3. The MP-AES confirmed that Ce (17.2%), La (13.1%), and Nd (5.4%) were the main metal components in the precipitate, whereas Co and Ni impurities only made up 0.01%. Hence, selective separation of NaREE(SO 4 ) 2 ·H 2 O using a precipitation technique by adding NaOH to the leachate is an efficient method to recover REMs from leach liquor.
Metals 2019, 9, x FOR PEER REVIEW 8 of 13 Figure 5. XRD pattern of isolated REM (the NaNd(SO4)2 H2O, NaCe(SO4)2 H2O, and NaLa(SO4)2 H2O mixture is referred to as NaREE(SO4)2 H2O)) precipitates. From the isolated NaREE(SO4)2 H2O, a carbonation reaction using Na2CO3 was completed, and the REMs were purified as anhydrous (REE)2(CO3)3‧xH2O. The carbonation reaction was carried out by adding the isolated NaREE(SO4)2‧H2O powder to 200 mL of Na2CO3 solution in 500 mL of reactor, and the reaction was carried out for 5 h. The stoichiometric ratio of 1:1.1 for REM(III) with CO3 2− was maintained for the purification of anhydrous (REE)2(CO3)3‧xH2O. The carbonation reaction was carried out in two different temperatures (i.e., room temperature (RT) and at 70 °C). After the reaction was completed, the isolated sample was dried and analyzed by XRD. The XRD pattern is depicted in Figure 6. The XRD pattern in Figure 6 indicates that there was almost no difference in XRD patterns between the two different samples isolated after the reaction at both room temperature and at 70 °C. The XRD pattern was confirmed to be (CeLa)2(CO3)3‧4H2O, (LaNd)2(CO3)3‧8H2O, and NaNd(CO3)2‧6H2O, which correspond to JCPDS #06-0076, #30-0678, and #30-1223, respectively.  From the isolated NaREE(SO 4 ) 2 ·H 2 O, a carbonation reaction using Na 2 CO 3 was completed, and the REMs were purified as anhydrous (REE) 2 (CO 3 ) 3 ·xH 2 O. The carbonation reaction was carried out by adding the isolated NaREE(SO 4 ) 2 ·H 2 O powder to 200 mL of Na 2 CO 3 solution in 500 mL of reactor, and the reaction was carried out for 5 h. The stoichiometric ratio of 1:1.1 for REM(III) with CO 3 2− was maintained for the purification of anhydrous (REE) 2 (CO 3 ) 3 ·xH 2 O. The carbonation reaction was carried out in two different temperatures (i.e., room temperature (RT) and at 70 • C). After the reaction was completed, the isolated sample was dried and analyzed by XRD. The XRD pattern is depicted in Figure 6. The XRD pattern in Figure 6 indicates that there was almost no difference in XRD patterns between the two different samples isolated after the reaction at both room temperature and at 70 • C. The XRD pattern was confirmed to be (CeLa) 2 (CO 3 ) 3 ·4H 2 O, (LaNd) 2 (CO 3 ) 3 ·8H 2 O, and NaNd(CO 3 ) 2 ·6H 2 O, which correspond to JCPDS #06-0076, #30-0678, and #30-1223, respectively. The XRD pattern observed is a mixture of all three REM carbonates, so it will be presented as (REM)2CO3. The (REM)2CO3 was analyzed by TGA ( Figure 7). The TGA figure indicates two stages of weight loss as temperature increased to 1000 °C. In the first step, the weight loss was 26.1% between room temperature and 250 °C. The weight loss is associated with loss of water and conversion of hydrated (REM)2CO3 H2O to anhydrous (REM)2CO3. In the second step, the weight loss was 44.7% from room temperature to 800 °C. At 800 °C, (REM)2CO3 was converted to (REM)2O3 by loss of CO2. To understand the (REM)2O3 formation through the metathesis reaction, the (REM)2CO3.H2O sample was calcined at 800 °C for 1 h and characterized by XRD analysis. The XRD pattern is represented in Figure 8. The XRD peak in the XRD analysis indicates that the sample was a mixture of three types of rare earth oxides (i.e., Nd0.5Ce0.5O1.75, La2O3, and Nd6O11). Hence, pure (REM)2O3 can be valorized through heat treatment of (REM)2CO3 at 800 °C. The XRD pattern observed is a mixture of all three REM carbonates, so it will be presented as (REM) 2 CO 3 . The (REM) 2 CO 3 was analyzed by TGA ( Figure 7). The TGA figure indicates two stages of weight loss as temperature increased to 1000 • C. In the first step, the weight loss was 26.1% between room temperature and 250 • C. The weight loss is associated with loss of water and conversion of hydrated (REM) 2 CO 3 ·H 2 O to anhydrous (REM) 2 CO 3 . In the second step, the weight loss was 44.7% from room temperature to 800 • C. At 800 • C, (REM) 2 CO 3 was converted to (REM) 2 O 3 by loss of CO 2 . To understand the (REM) 2 O 3 formation through the metathesis reaction, the (REM) 2 CO 3 ·H 2 O sample was calcined at 800 • C for 1 h and characterized by XRD analysis. The XRD pattern is represented in Figure 8. The XRD peak in the XRD analysis indicates that the sample was a mixture of three types of rare earth oxides (i.e., Nd 0.5 Ce 0.5 O 1.75 , La 2 O 3 , and Nd 6 O 11 ). Hence, pure (REM) 2 O 3 can be valorized through heat treatment of (REM) 2 CO 3 at 800 • C.
A complete flowsheet describing the recovery of (REM) 2 O 3 from a waste NiMH battery via a simple wet chemical valorization process was developed and is shown in Figure 9. The figure depicts the sequential process, i.e., H 2 SO 4 acid leaching, NaREE(SO 4 ) 2 ·H 2 O precipitation, and metathesis reaction for the complete recycling of waste NiMH batteries. The process was completed with the integration of physical separation followed by the chemical metallurgy process. The physical separation process was followed by residual stored energy discharge, grinding and calcination, classification, and access to the electrode active material. In the chemical metallurgy process, the electrode active powder material was extracted by leaching, REMs were separated selectively by precipitation, and purified by carbocation and calcination processes. The chemical metallurgy process sequentially followed (i) 1 M H 2 SO 4 leaching from a pulp density of 25 g/L at 90 • C, (ii) selective separation of REMs as NaREE(SO 4 ) 2 ·H 2 O salt leaving Ni/Co sulfate solution, (iii) conversion of NaREE(SO 4 ) 2 ·H 2 O to (REM) 2 CO 3 ·H 2 O through carbocation, and (iv) synthesis of REM oxide from (REM) 2 CO 3 ·H 2 O by calcination at 800 • C. The mixed REM oxide could be further purified to individual REMs by further investigations, mainly by hydrometallurgy. As this is a complete systematic sequential process for the recovery of mixed critical REMs like Ce, La, and Nd, it could address the circular economy and recycling challenges associated with waste NiMH batteries.  A complete flowsheet describing the recovery of (REM)2O3 from a waste NiMH battery via a simple wet chemical valorization process was developed and is shown in Figure 9. The figure depicts the sequential process, i.e., H2SO4 acid leaching, NaREE(SO4)2‧H2O precipitation, and metathesis  A complete flowsheet describing the recovery of (REM)2O3 from a waste NiMH battery via a simple wet chemical valorization process was developed and is shown in Figure 9. The figure depicts the sequential process, i.e., H2SO4 acid leaching, NaREE(SO4)2‧H2O precipitation, and metathesis selective separation of REMs as NaREE(SO4)2 H2O salt leaving Ni/Co sulfate solution, (iii) conversion of NaREE(SO4)2 H2O to (REM)2CO3.H2O through carbocation, and (iv) synthesis of REM oxide from (REM)2CO3.H2O by calcination at 800 °C. The mixed REM oxide could be further purified to individual REMs by further investigations, mainly by hydrometallurgy. As this is a complete systematic sequential process for the recovery of mixed critical REMs like Ce, La, and Nd, it could address the circular economy and recycling challenges associated with waste NiMH batteries. Figure 9. Process flow sheet for the recovery of rare earth metal oxide from waste NiMH batteries by a simple wet chemical valorization process.

Conclusions
Through the H 2 SO 4 leaching-selective precipitation-metathesis reaction route of waste NiMH batteries, REM oxide could be recovered quantitatively. Through H 2 SO 4 leaching optimization, almost all metal from the electrode active material of waste NiMH batteries was leached out. From the filtered leach liquor with pH managed (pH 1.8) by 10 M NaOH, REMs were precipitated as hydrated NaREE(SO 4 ) 2 ·H 2 O, which was further valorized through a metathesis reaction process. Isolated NaREE(SO 4 ) 2 ·H 2 O was further purified through a carbocation reaction using Na 2 CO 3 . From NaREE(SO 4 ) 2 ·H 2 O through carbocation, REMs were purified as hydrated (REM) 2 CO 3 ·H 2 O precipitate. From (REM) 2 CO 3 ·H 2 O through the calcination process at 800 • C, (REM) 2 O 3 could be recovered. The mixed (REM) 2 O 3 could be further purified through hydrometallurgy, such as dissolution followed by solvent extraction purification. The process is very simple, versatile, and easy to implement in the industry. Advantages of the process are that it is simple, versatile, and flexible, and industrial mass production is feasible. It is also sustainable and eco-efficient, and produces minimal waste emissions. The process addresses several issues concurrently, including waste valorization, environment management, critical REM circular economy, and urban mining.

Conflicts of Interest:
The authors declare no conflict of interest.