Towards zero-consumption of acid and alkali recycling rare earths from scraps: A precipitation-stripping-saponification extraction strategy using CYANEX®572

Accepted Manuscript Towards zero-consumption of acid and alkali recycling rare earths from scraps: A precipitation-stripping-saponification extraction strategy using CYANEX®572

Fujian Li, Yanliang Wang, Xiang Su, Xiaoqi Sun PII:

S0959-6526(19)31415-5

DOI:

10.1016/j.jclepro.2019.04.318

Reference:

JCLP 16674

To appear in:

Journal of Cleaner Production

Received Date:

28 December 2017

Accepted Date:

24 April 2019

Please cite this article as: Fujian Li, Yanliang Wang, Xiang Su, Xiaoqi Sun, Towards zeroconsumption of acid and alkali recycling rare earths from scraps: A precipitation-strippingsaponification extraction strategy using CYANEX®572, Journal of Cleaner Production (2019), doi: 10.1016/j.jclepro.2019.04.318

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ACCEPTED MANUSCRIPT

Graphical abstract

ACCEPTED MANUSCRIPT Towards zero-consumption of acid and alkali recycling rare earths from scraps: A precipitation-stripping-saponification extraction strategy Fujian Li a, c, d, Yanliang Wang a, b, c, Xiang Su a, b, c, and Xiaoqi Sun a, b, c* a. Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China; b. Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, P. R. China; c. University of Chinese Academy of Sciences, Beijing, 100039, P. R. China; d. Ganzhou Rare Earth Group Co., Ltd., China Southern Rare Earth, Ganzhou, 341000, P. R. China *Corresponding author, E-mail: [email protected]. Tel.: +86592637637

Abstract: Recycling schemes from rare-earth-containing scraps have been developed rapidly due to the increasing supply risk for rare-earth elements (REEs). This article aimed to reduce the ecological impact and reagent costs for separating and recycling REEs from scraps, i.e., spent ceriabased polishing powders and lutetium-based crystal waste. A simultaneous process was proposed using an industrial extractant Cyanex®572, which allowed efficient recycling of REEs from the wastes with less acid and alkali consumption than traditional hydrometallurgy process. Based on thermodynamic and kinetic analyses, common acid scrubbing was avoided by employing REEs scrubbing. Additionally, the alkali saponification, acid stripping and oxalic acid precipitation were substituted by one step of precipitation-stripping-saponification with ammonium fluoride which contained abundant F- in the solution with weak acidity. F- was easy to complex with REEs on the extractant as precipitate while NH4+ bonded with anion of the extractant. Using this technique, the vast consumption of chemicals and million tons of wastewater in REE-industry may be avoided. Totally, the chemical consumption could be reduced over 80% and the wastewater was decreased more than 90%. REEs recovery yield also increased considerably using the novel process. Moreover, the ammonium fluoride was in containment system and was reused with no emission to the

1

ACCEPTED MANUSCRIPT environment. This article provides a sustainable and efficient alternative process to REEs waste treatment industry. Keywords: rare-earth elements; recycling; metal scrubbing; precipitation-stripping-saponification extraction; CYANEX®572 1. Introduction Environmental care and resource management are basic for sustainable development. Rare-earth elements (REEs) are critical in high-tech applications as polishing materials, catalysts, permanent magnets and so on (Binnemans, 2009; Tanaka et al., 2013; Zhang et al., 2012). With no decisive substitutes of these critical elements, potential REEs shortages will hinder the development of involved industrial fields (Alonso et al., 2012). On the other hand, there are huge amount of industrial solid waste (about 3.2 billion tons in 2014) in China (Mathews and Tan, 2016), which contained considerable content of valuable resources in this so called “urban ore”, such as rareearth-containing scraps (Jha et al., 2016). Discarding REEs containing wastes was wasteful, the recovery of REEs help with both valuable resources recycling and environmental burden relieving (Binnemans et al., 2013; Binnemans et al., 2015; Tan et al., 2015). Though many REEs-recycling methods have been developed, the recycled REEs is still an extremely minimum part of the supply chains (Jordens et al., 2013; Massari and Ruberti, 2013). Thus, environment friendly and costeffective methods for REEs recovery are highly desirable to be developed. Recently, many attractive methods were investigated to recover REEs from spent REE sources including selective reduction (Bogaert et al., 2015), ionic liquid extraction (Sun et al., 2012; Dupont and Binnemans, 2015a, 2015b), traditional extraction (Wang et al., 2013; Ippolito et al., 2017; Chen et al., 2016; Huang, et al., 2017a), and adsorption (Dupont et al., 2014; Chen et al., 2017), et al. Neverthless, hydrometallurgy is still the widely used processes, which was established very well in REEs extraction industry. As specially designed extractant, the organophosphorus acids used for REEs extraction process was widely studied recently (Yan et al., 2006; Xie et al., 2014). It should be noted that the main drawbacks of traditional REEs extraction process lied at large volumes of chemicals consumed and lots of waste (sometimes radioactive wastes) produced (Liao et al., 2013; Sholl and Lively, 2016; Wang et al., 2017a). A/B (represent of two or two groups REEs) 2

ACCEPTED MANUSCRIPT separation process can be briefly expressed as Fig.1. Before extracting, vast alkali should be added as saponification reagent to improve the extracting ability. Then a large amount of acid should be employed to scrub the over extracted metal B (as less-extractable metal) for purification and strip the metal A (as more-extractable metal) loaded on organic phase for extractant regeneration. To precipitate REEs ion, oxalic acid was always introduced as the precipitation reagent. The separation of all 15 elements in the typical ion-adsorbing concentrate was reported to require approximately 2-3 tons of NaOH and 10 tons of HCl concentrate (31 wt.%) for the production of per ton of RE oxide (Liao et al., 2013). 96,900 tons of rare earth oxides were produced in China in 2011, which were made up of more than ninety percent of the total output all over the word (Wang et al., 2013). Additionally, over 20 million tons of discharged waste water and 0.8 million tons of salts were produced undesirably. According to the Chinese government, some rare earth manufactures must be shut down when meeting the stricter environmental emission standards (GB 26451-2011). A technique of controlling equilibrium acidity by water diluting was employed to avoid alkali consumption and achieve ammonia-free emissions in REEs extraction process (Wang et al., 2013). But it still needed the consumption of acid and alkali to balance the pH of the waste water. Additionally, the heavy REEs (Tb, Dy, Ho, Er, Tm,Yb, Lu and Y) interacted very strongly with the extractant. As a consequence, the totally stripping was always required high acidities solution (Wang et al., 2017b). Cyanex®572 was produced to limit the stripping acidities by the manufacturer Cytec Industries Inc. and it was optimized over traditional phosphonic acid extractants. (https://www.cytec.com/sites/default/files/files/CYTEC_CYANEX_572_FINAL.pdf).

Also,

Cyanex®572 was employed to separate heavy REEs because of the better extracting performance and the lower required stripping acidity by many researchers (Wang et al., 2015; Tunsu et al., 2016; Wang et al., 2017c).

(Insert Fig. 1)

Known as the king of polishing powder, ceria-based polishing powder was one of the typical REEs products, (Tanaka et al., 2013; Lu et al., 2017). In 2008, China produced about 10,000 tons of this polishing powder, which was nearly 15% of all the REEs product (Xu and Peng, 2009). 3

ACCEPTED MANUSCRIPT Recycling of the spent polishing powders were meaningful to both environment and resource. Recently, an energy saving hydrometallurgical method was introduced for REEs recovery from polishing powder wastes (Um and Hirato, 2016). A recovery of 91.23% by response surface methodology was also used for this common waste (Lu et al., 2017). Meanwhile, waste crystal containing lutetium drew great attention because lutetium was irreplaceable and critical to optical material with low-abundance and high-cost of extraction (Chen et al., 2017). It was less than 1 ppm in the ion-absorbed rare-earth mineral in Ganzhou city, which was the main source area of this lutetium. Hence, waste crystal containing lutetium is a significant resource. The benefits of recycling supplies of these metals are pronounced. However, simultaneous treatment of these wastes was not reported to best of our knowledge. Because of the different bond strength of REEs in extractant, one metal loaded on the organic phase may be exchanged by another (Li et al., 2017). Then, the acid scrubbing and stripping for one metal can be avoided by introducing interest metal scrubbing which should be extracted next in the process. But how to strip the last metal loaded on extractant for regeneration requires more efficient reagent. Solvent extraction route for REF3 nanoparticles was reported (Zhang et al., 2006; Guo et al., 2009; Zhao et al., 2013). Because ammonium fluoride solution contains abundant F- which is easy to complex with REEs on the extractant as precipitate with weak acidity while NH4+ bonded with anion of the extractant. Ammonium fluoride solution as a common commercial reagent is feasible to strip and precipitate metals on the organic phase. Furthermore, it is similar that cation NH4+ in NH4F solutions can be compounded with oxygen in organophosphoric acid extractants as that in alkali solutions (NH4OH, NaOH, et al) for traditional saponification. In this way, one step of precipitation-strippingsaponification can be achieved. Therefore, the vast consumption of alkali and acid in traditional waste treatment for REEs recovery in traditional process ((Liao et al., 2013; Sholl and Lively, 2016; Wang et al., 2017a) may be avoided (Fig. 2).

(Insert Fig. 2)

In this work, an industrial extractant Cyanex®572 was employed to simultaneously recycle REEs from spent ceria-based polishing powders and lutetium-based crystal waste. Metal scrubbing was 4

ACCEPTED MANUSCRIPT investigated via thermodynamic and kinetic methods to limit the acid consumption. One step of precipitation-stripping-saponification extraction was evaluated by introducing the fluoride series solution. The reaction of precipitation-stripping-saponification was elucidated by XRD, 1H-NMR, 31P-NMR

and IR spectra. A novel process toward zero-consumption of acid and alkali of rare earth

scraps recycling was proposed.

2. Experimental 2.1 Reagents and apparatus Cyanex®572 was obtained from Cytec Industries Inc. N-heptane purchased from Aladdin was employed as the diluent of the organic phase. YCl3 (99.999%), LuCl3 (99.995%), CeCl3 (99.99%) and LaCl3 (99.999%) solutions were kindly provided by Ganzhou Rare Earth Group Co., Ltd., (China). The required pH value of aqueous phase was adjusted with HCl or NaOH solution. Lutetium-based crystal waste was kindly provided by DigiPET Ltd., (China). Spent ceria-based polishing powder was kindly provided by Asianmetal Ltd., (China). The pHs value of aqueous phase were measured by A model PHSJ-4F pH meter (Leici shanghai, China). NMR spectra were recorded by spectrometer (AVANCEIII 500 MHz, Bruker). Infrared spectra were obtained by infrared spectrometers (Nicolet IS 50, 3500-750 cm-1). REEs in solutions were analyzed by ICP-AES (Thermos scientific iCAP 6500). The morphologies of obtained crystals were observed by scanning electron microscopy (FESEM, S4800, Hitachi, Japan). The crystalline phase present was identified by Rigaku Miniflex 600 XRD system, generating monochromated CuKα radiation (10°/min, from 0° to 90°). The XRD patterns were obtained in the operating conditions of 40 kV and 15 mA. 2.2 Extraction process Extraction experiments were carried out in a mechanical stirrer, mixing the same volume of organic phase and feed solution for 30 min. The phases were settled and separated thoroughly after equilibration (30 min). Stripping experiments was carried out using the selected stripping solutions. All the experiments were conducted at 25 ± 1℃. 5

ACCEPTED MANUSCRIPT 2.3 Scrubbing process Scrubbing experiments were carried out by mixing with the same volume of the metal loaded organic phase and metal scrubbing reagents or corresponding concentration of HCl solution. Ce loaded on Cyanex®572 was mixed with different concentration of pure YCl3 or LuCl3 solutions while Y loaded on Cyanex®572 was mixed with different concentration of pure LuCl3 solutions for 30 min and then the phases were settled and separated for 30 min. The aqueous phase was collected for further analysis and the scrubbed organic phase was deemed to be totally striping by mixing with 6 mol/L HCl solution for 30 min. The stripping liquor was collected for further analysis. The scrubbing efficiency was calculated via mass balance. 2.4 Precipitation-stripping-saponification process Metal loaded organic phase was mixed with NH4HF2 or NH4F solution for 30 min. Next, a centrifuge separates organic, aqueous and solid phase at 4000 rpm was duration for 10 min. Then the organic phase was additionally stripped by HCl solution (6 mol/L) for 30 min. The stripping solution was collected and analyzed. The scrubbing efficiency was calculated via mass balance. Solid phase was analyzed via XRD. The extraction rate (E), saponification degree (Sd), scrubbing rate (Sc), distribution ratio (D), separation factor (β) and stripping rate (St) are defined as following: 𝐸% = 𝐷 = 𝛽 =

[𝑀]𝑜𝑟𝑔

[𝑀]𝑜𝑟𝑔 + [𝑀]𝑎𝑞 [𝑀]𝑜𝑟𝑔

× 100%

[𝑀]𝑎𝑞 𝐷1 𝐷2

𝑆𝑑% =

[𝑀]𝑁𝐻4𝑂𝐻 × 𝑉𝑎𝑞

× 100% [𝑀]Cyanex®572 × 𝑉org [𝑀]𝑎𝑞,𝑎1 × 𝑉𝑎𝑞 𝑆𝑐% = × 100% [𝑀]𝑜𝑟𝑔,𝑡1 × 𝑉𝑜𝑟𝑔 [𝑀]𝑎𝑞,𝑎2 × 𝑉𝑎𝑞 𝑆𝑡% = × 100% [𝑀]𝑜𝑟𝑔,𝑡2 × 𝑉𝑜𝑟𝑔

(1) (2) (3) (4) (5) (6)

where D1 and D2 represent the distribution ratios of metal 1 and 2, respectively. [M]NH4OH is the concentration of saponification NH4OH solution, [M]CYANEX®572 is the concentration of saponified 6

ACCEPTED MANUSCRIPT Cyanex®572 in organic phase. [M]aq, a1 is equilibrium concentration of the metal (less-extractable) scrubbed in aqueous phase, and [M]org, t1 is the initial concentration of the metal (less-extractable) in loaded organic phase. [M]aq, a2 is the equilibrium concentration of metal a stripped in aqueous phase, and [M]org, t2 is the initial concentration of metal a in loaded organic phase. 2.5 Treatment of the scraps Characterization of spent polishing powders and Lutetium-based crystal waste was employed XRD and FESEM. The leaching experiments were followed by the references (Poscher et al., 2016; Lu et al., 2017).

3. Results and discussion 3.1 Treatment of the scraps 3.1.1 Characterization of spent polishing powders As shown in Fig. 3, the spent ceria-based polishing powders were 1~5 μm, which were conglomerated significantly, always containing some glass scarps (Fig. 3(a)). Ce in the dried waste polishing powder formed as CeO2 with the content of 48.75 wt. %, while La was in form of the compound LaOF with a lower content of 22.93 wt% (Fig. 3(b) and Table 1). The elements F, La, O, and Ce were 89.17 wt% in total in average (Table 1). The main impurity elements were found to be C, Si, Fe and Ca which came from glass substrates when polishing (Poscher et al., 2016; Lu et al., 2017).

(Insert Fig. 3) (Insert Table 1) 3.1.2 Characterization of Lutetium-based crystal waste As shown in Fig. 4, Lutetium-based crystal waste is crushed and dried and the particle size is about 2~5μm (Fig. 4(a)). From Fig. 4(b), the XRD showed that it was almost one phase of Lu2SiO5, 7

ACCEPTED MANUSCRIPT in which peak shifting was 0.3-0.5°due to Y and Ce doping. The main component Lu was 74.23 wt.% in average (Table 2). Impurity metal elements were mainly Al, Si, Fe and Y, Ce.

(Insert Fig. 4) (Insert Table 2)

3.3.3 Leaching experiments Thiourea dosage and hydrogen peroxide were introduced to reduce tetravalent cerium state to promote leaching (Poscher et al., 2016; Lu et al., 2017). LuYSiO crystal waste roasted at high temperature of 750 °C for 5 min with NaOH and/or Na2O2. Then the roasted sample was dissolved by 6 mol/L HCl solution. The most important parameters were provided in Table 3. The concentration of raw waste and undissolved solid residue were measured and analyzed, and REEs recycling yield was calculated by mass balances. REEs recycling yield was given in Table 3. The recovery yields of spent polishing powder were all above 98% by thiourea dosage and H2O2. In this work, H2O2 was chosen due to more green and sustainable. As for LuYSiO crystal waste, NaOH and/or Na2O2 were employed to leach, and the recycling yields were high than 99%. Considering security, NaOH was employed to treat the waste in the further experiments.

(Insert Table 3)

3.2 Thermodynamics analysis of metal scrubbing The extraction equilibrium of REEs (Ce3+, Y3+, and Lu3+) in organophosphoric acid extractants can be expressed as following (Li et al., 2017): 3+

𝐶𝑒

+ 3𝐻2𝐴2 = 𝐶𝑒(𝐻𝐴2)3 + 3𝐻

𝑌3 + + 3𝐻2𝐴2 = 𝑌(𝐻𝐴2)3 + 3𝐻 +

+

𝐾1 =

𝐾2 = 8

[𝐶𝑒(𝐻𝐴2)3] × [𝐻 + ]3 [𝐶𝑒3 + ] × [𝐻2𝐴2]3

[𝑌(𝐻𝐴2)3] × [𝐻 + ]3 [𝑌3 + ] × [𝐻2𝐴2]3

(7)

(8)

ACCEPTED MANUSCRIPT

3+

𝐿𝑢

+ 3𝐻2𝐴2 = 𝐿𝑢(𝐻𝐴2)3 + 3𝐻

+

𝐾3 =

[𝐿𝑢(𝐻𝐴2)3] × [𝐻 + ]3 [𝐿𝑢3 + ] × [𝐻2𝐴2]3

(9)

Ce3+ loaded on the organic phase scrubbed by Y3+ can be expressed as Eq. (8-7): 𝐸𝑞.(8 ― 7) = 𝐸𝑞.(8) ― 𝐸𝑞.(7): 𝑌3 + + 𝐶𝑒(𝐻𝐴2)3 = 𝑌(𝐻𝐴2)3 + 𝐶𝑒3 +

𝛽𝑌/𝐶𝑒 =

𝐾2 𝐾1

=

[𝑌(𝐻𝐴2)3] × [𝐶𝑒3 + ] [𝐶𝑒(𝐻𝐴2)3] × [𝑌3 + ]

Ce3+ loaded on the organic phase scrubbed by Lu3+ can be expressed as Eq. (9-7): 𝐸𝑞.(9 ― 7) = 𝐸𝑞.(9) ― 𝐸𝑞.(7): 3+

𝐿𝑢

3+

+ 𝐶𝑒(𝐻𝐴2)3 = 𝐿𝑢(𝐻𝐴2)3 + 𝐶𝑒

𝛽𝐿𝑢/𝐶𝑒 =

𝐾3 𝐾1

=

[𝐿𝑢(𝐻𝐴2)3] × [𝐶𝑒3 + ] [𝐶𝑒(𝐻𝐴2)3] × [𝐿𝑢3 + ]

Y3+ loaded on the organic phase scrubbed by Lu3+ can be expressed as Eq. (9-8): 𝐸𝑞.(9 ― 8) = 𝐸𝑞.(9) ― 𝐸𝑞.(8): 3+

𝐿𝑢

3+

+ 𝑌(𝐻𝐴2)3 = 𝐿𝑢(𝐻𝐴2)3 + 𝑌

𝛽𝐿𝑢/𝑌 =

𝐾3 𝐾2

=

[𝐿𝑢(𝐻𝐴2)3] × [𝑌3 + ] [𝑌(𝐻𝐴2)3] × [𝐿𝑢3 + ]

Where, A refers to the anion of Cyanex®572. H2A2 refers to the extractants in n-heptane exist as a dimer. Series experiments were performed in a vibrating mixer at the temperature 25 ± 1℃ by contacting the volumes of with aqueous solution organic phase for 30 min to ensure the extraction equilibrium achieved. As shown in Table 4, the concentration of Ce3+, Y3+, and Lu3+ solutions were 0.431, 0.451 and 0. 459 mmol/L, respectively. The average Gibbs free energy (△G) of extraction of Ce3+, Y3+, and Lu3+ were 16.84, -3.33 and -9.30 KJ/mol, respectively. It indicated that it tended to the reverse extraction of Ce3+at equilibrium under the experimental conditions. As the cases of Y3+ and Lu3+, △G was less than zero. It was corresponding to the extraction order in the paper (Wang et al., 2017c). As for the metal scrubbing, △G of scrubbing equilibriums (Ce3+ scrubbed by Y3+, Ce3+ scrubbed by Lu3+, Y3+ scrubbed by Lu3+) were -20.16, -26.14 and -5.97 KJ/mol, respectively. All of them were less than zero, meaning that the scrubbing reaction could spontaneously occur at the given temperature and pressure. In addition, the separation factors βY/Ce, βLu/Ce and βLu/Y were 3539, 44788 and 12, respectively. Similar results can be inferred from the paper (Wang et al., 2017c). As for the organophosphoric extractants in traditional extraction processes, three separation factors were always greater than one (Xiong et al., 2005; Wang et al., 2015). Interestingly, the large 9

ACCEPTED MANUSCRIPT separation factors indicated that the more extractable elements such as Lu and Y could be used as scrubbing agent to substitute the scrubbing acid of the less extractable element, which was named metal scrubbing. Then, ΔG can be obtained as shown in Table 4, indicating less than 0 in the experimental condition. Furthermore, lanthanide contraction is generally known to us. It indicates that the ionic radius is given as Lu3+ < Y3+ < Ce3+. Therefore, the bond strength of REEs in extractant lies as Lu-O > Y-O > Ce-O due to electron affinity. Hence, the exchange between [Y3+]aq and [Ce3+]org, [Lu3+]aq and [Ce3+]org, and [Lu3+]aq and [Y3+]org can be expressed as Eq. (8-7), Eq. (9-7) and Eq. (9-8). (Insert Table 4) 3.3 Kinetic analysis of metal scrubbing The scrubbing kinetics was comparatively studied at room temperature (25°C ± 1). Ce3+ loaded on organic phase were scrubbed by 0.0599 mol/L LuCl3, 0.0604 mol/L YCl3 and 0.1864 mol/L HCl solutions, respectively. Y3+ loaded on organic phase were scrubbed by 0.0599 mol/L LuCl3 and 0.1864 mol/L HCl solutions. Because Lu3+ and Y3+ were trivalent ions, while H+ was monovalent ion in solution, the concentration of HCl solution was set three folds of stripping metal ion. As shown in Fig. 5, less than 1min was needed to achieve Ce3+ scrubbing equilibrium of all the three solutions (HCl, YCl3 and LuCl3). More than 80% of Ce3+ loaded on the organic phase was exchanged by H+, Y3+ and Lu3+. It indicated that metal solutions scrubbing was as quickly as acid solution under the experiments condition. However, Y3+ on the loaded organic phase scrubbed by LuCl3 and HCl solution was less efficient, which were about 55% (10 min equilibrium) and 14% (1min equilibrium), respectively. It showed that Lu3+ was more efficient than H+ to scrub Y3+ loaded on the organic phase with the relative stoichiometric ratio, but it was slower to achieve equilibrium than that scrubbed by H+. The results showed that metal scrubbing in REEs separation process using Cyanex®572 as an extractant was feasible in kinetics. Compared our previous work (Li et al, 2017), thermodynamics and kinetic analysis certified the feasibility of metal scrubbing. The optimized process integrated metal scrubbing (step of Ce3+ scrubbed/stripped from extractant by Y3+) to replace acid and saponification (step of extractant saponified by alkali to promote Y3+ extractability from aqueous) to avoid alkali. Additionally, The 10

ACCEPTED MANUSCRIPT efficiency of Ce3+ scrubbed by Lu3+ and Y3+scrubbed by Lu3+ is much better than that La3+, Ce3+ and Pr3+ scrubbed by Nd3+.

(Insert Fig. 5) 3.4 Effect of metal concentration on scrubbing efficiency To evaluate metal scrubbing efficiency, series experiments were carried out additionally. As shown in Fig. 6 (a), the metal (Ce3+) on organic phase were scrubbed thoroughly by both Y3+ and Lu3+ above the concentration of 0.06 mol/L, while the scrubbing metals (Lu3+, Y3+) loaded on the organic phase increased as adding the scrubbing concentration. However, Y3+ on organic phase was decreased as the concentration of scrubbing metal (Lu3+) increasing from 0.02 to 0.06 mol/L. Then kept a slight change at about 0.0165 mol/L with adding concentration. The phenomenon can be explained by △G in Table 5. It was shown that △G9-7 (Ce3+ scrubbed by Lu3+, -26.14 KJ/mol) < G8-7 (Ce3+ scrubbed by Y3+, -20.16 KJ/mol) < △G9-8 (Y3+ scrubbed by Lu3+, -5.97 KJ/mol). The thermodynamics theory held that the lower value of △G, the more sensitive of the concentration of scrubbing reagent. As a result, Ce3+ were almost completely scrubbed from loaded organic phase by both Y3+ and Lu3+ at the concentration of 0.06 mol/L. But only 48.98% of Y3+ was scrubbed by Lu3+even the concentration reached to 0.1 mol/L (Fig. 6 (b)). It indicates that Lu3+ cannot completely scrub Y3+ on organic phase in single stage due to the △G approaching zero.

(Insert Fig. 6) 3.5 Effect of times on scrubbing efficiency Because Y3+ was remaining largely on the loaded organic phase after scrubbing by Lu3+ solution in one stage. Additional experiments were performed to evaluate scrubbing efficiency by multiple scrubbing of Lu3+. Y3+ loaded on the organic phase (0.0428 mol/L) was mixed with the same volume of 0.0676 mol/L LuCl3 solution for 20 mins at room temperature (25°C ± 1). As shown in Fig. 7, almost 99% Y3+ was scrubbed from the Cyanex®572 after 3 times of scrubbing. Obviously, less than

11

ACCEPTED MANUSCRIPT 5 stages scrubbing strategy was effective for the exchange of [Lu3+]aq and [Y3+]org, which revealed great potential for industrial application.

(Insert Fig. 7) Y3+ can be almost totally scrubbed by Lu3+ as Ce scrubbed by Lu3+ and Y3+ in less than 5 stages. Therefore, acids in scrubbing steps were avoided by the interest metal (Lu3+ and/or Y3+). What should be mentioned, a noticeable over-extracting of less-extractable elements above 10% occurs normally to ensure the purity of interest metal in the process of solvent extraction. It is possible in an extreme situation with low separation factor of the two metals that the over-extracting ratio arrived at 30%. Therefore, the scrubbing acid consumption is considerable and highly desirable to upgrade and update, such as using interest metal scrubbing instead. Simultaneously, the scrubbing metal (Lu3+ and/or Y3+) was extracted on the organic phase via exchanging of the metal loaded on organic phase (Y3+ and/or Ce3+), which should be in virtue of saponification by alkali initially in traditional process (Fig. 1). Then the alkali consuming in saponification can be also reduced in this strategy. 3.6 Stripping Lu3+ on loaded organic phase for regeneration Lu3+ solution was effective for scrubbing Ce3+ and Y3+, but Lu3+ was loaded on the organic phase finally. Due to the biggest electron affinity, lutetium-oxygen bond was the strongest of the three REE-oxygen bonds. The stripping of Lu3+ and regeneration of the organophosphoric acid extractants had drawn great attention (Wang et al., 2015; Chen et al., 2016). Because of the strength bond of Lu-O, concentrated acid with high acidity was always employed to strip totally (Chen et al., 2016). However, it may cause several problems inevitably. Firstly, more acid must be added, and cost may be enlarged consequently. Secondly, amount of acid will leave as residual in the stripping solution due to incompletely reaction. That will enhance the difficulty of REEs precipitation and reduce the REEs recovery yield, because solubility of REEs deposit in solution escalates continuously as the acidity increasing. Last but not the least, high acidity aggravates the volatility of acid, which is pollution for the environment, harmful for worker’s health, and corrosive for the equipment. Therefore, efficient, safe and green stripping reagent should be developed with high desirable. 12

ACCEPTED MANUSCRIPT Precipitation-stripping-saponification was evaluated in this article. Organic phase loaded Lu3+ was mixed with NH4HF2 and NH4F solutions, respectively. The samples were then shaken to form Lu3+ fluoride complex as a solid phase. After centrifuging for 10 mins at 4000 rpm, organic phase floated on the top due to smaller density, while solid phase deposited under the aqueous phase which was in the middle of the three phases (Fig. 8(a)). It should be noted that the interface was clear among the three phases which revealed that the organic, aqueous and solid phase could be separated thoroughly. Then the regeneration of organic phase and product of lutetium were realized in one step with less loss and higher yield. As shown in Fig. 8(b), 10% (by wt%) of NH4HF2 was sufficient to obtain >99% stripping. While under NH4F concentration of 10 wt%, stripped Lu3+ was almost at 0%. With the increasing concentration of NH4F to 30% wt %, it grew to 73.38%. However, more than 98.15% Lu3+ was stripped from the loaded organic phase after seventh scrubbing (Fig. 9).

(Insert Fig. 8) (Insert Fig. 9)

In additional, the solid phase was analyzed by XRD analysis. The crystals produced by NH4HF2 and NH4F were NH4Lu2F7 and (NH4)3Lu2F9, respectively (Fig. 10(a)). The stripping process can be expressed by the following formulae: 2[𝐿𝑢(𝐻𝐴2)3] (𝑜) + 4[𝑁𝐻4𝐻𝐹2](𝑎) = [𝑁𝐻4𝐿𝑢2𝐹7] (𝑠) + [𝑁𝐻4𝐹](𝑎) + 2[𝑁𝐻4𝐻𝐴2] (𝑜) + 4[(𝐻𝐴)2](𝑜)

(10)

2[𝐿𝑢(𝐻𝐴2)3] (𝑜) + 9[𝑁𝐻4𝐹](𝑎) = [(𝑁𝐻4)3𝐿𝑢2𝐹9] (𝑠) + 6[𝑁𝐻4𝐻𝐴2] (𝑜)

(11)

Interestingly, Cyanex®572 stripped lutectum was automatically compounded with NH4+ partly (by NH4HF2, Eq. (10)) and entirely (by NH4F, Eq. (11)), forming the compound NH4HA2 which had extractability as that produced in traditional saponification process by NH4OH. Precipitationstripping with oxalic acid treating REEs in the ionic liquid were reported in these papers (Dupont and Binnemans, 2015a, Dupont and Binnemans, 2015b). However, it integrated with precipitation, stripping and saponification here with high efficiency. The extractability of the regenerated organic phase was further evaluated.

(Insert Fig. 10) 13

ACCEPTED MANUSCRIPT

3.7 1H NMR, 31P NMR and IR spectra analysis NMR and IR transmittance spectra were employed to characterize the chemical environment of extractant in extraction process (Lumetta et al., 2016; Huang et al., 2017b). To further gain deep understanding of the extractant in different extraction steps, the 1H NMR, 31P NMR and IR spectra of extractant of (1) fresh Cyanex®572, (2) saponified by NH4OH, (3) loading Lu3+after NH4OH saponification, (4) stripped Lu3+ by NH4F and (5) loading Ce3+ after NH4F stripping were analyzed. As can be seen in Figure 11(a), there is a pronounced chemical shift of hydrogen atom in POOH from fresh Cyanex®572 at 10.788 ppm, while it disappears in the samples of (2), (3), (4) and (5). It indicates that hydrogen atom in POOH is substituted by the cation ions (NH4+, Lu3+, NH4+ and Ce3+, respectively), which reveals the extractant had been saponified by NH4F and successfully extracted Ce3+. Five 31P NMR spectra were collected and shown in Fig. 11(b). The two main peaks reveal Cyanex®572 have two P groups or two compounds which have one P group. After loaded metals, there are four broad peaks around 26, 27.25, 47.5 and 50 ppm (sample (3) and (5)), respectively. The literature reported the similar results (Quinn et al., 2015). As for Lu3+ stripped by NH4F, the broad peaks disappear which means the metal was stripped efficiently. However, the main peaks shift significantly from 59.97 to 53.24 and 35.66 to 29.24 ppm, respectively. The similar shift is also observed in the NH4OH saponification situation, but it is smaller due to the lower saponification degree. Because ammonium fluoride solution contained abundant F- which was easy to compound with REEs on the extractant as precipitate with weak acidity while NH4+ bonded with anion of the extractant. It can be explained as the strong interaction between the bond P = O and NH4+ in NH4F solution. Hence, NH4F solution can be used to strip and saponify the extractant loaded with Lu3+ as NH4OH did in the traditional saponification process. From the IR spectra in Fig. 12, The shift of H−O−(P) at 1668 cm-1 is appeared in (1), (2) and (4), but disappears in (3) and (5). The shift of P=O around 1150 cm-1 is also changed (1172 cm-1) after extracted metals. Summarily, it obviously revealed that Lu3+ loaded on the organic phase was stripped by NH4F and Cyanex®572 was regenerated as NH4-C572 which had considerably extractability of Ce3+. 14

ACCEPTED MANUSCRIPT

(Insert Fig. 11) (Insert Fig. 12)

3.8 Effect of stripping reagents on extractability of the regenerated Cyanex®572 To evaluate the extractability of regenerated organic phase, additional experiments were carried out by mixing the regenerated Cyanex®572 with 0.5810 mol/L Ce3+. When contacted with NH4F solution, the extractant was compounded NH4-C572 as that mixing with NH4OH solutions. So, the extraction efficiency of the regenerated Cyanex®572 was given as saponification degree which defined as molar ratio of the mixing NH4OH and Cyanex®572 in organic phase. As adding NH4F concentration from 1 wt% to 30 wt%, the saponification degree grew steadily from 13.11% to 34.91% (Fig. 13). It revealed that 30 wt.% NH4F was adaptive to saponify Cyanex®572 when stripping the Lu3+ loaded organic phase. However, when increasing the concentration of NH4HF solution, saponification degree climbed up and then declined due to acidity increased in high NH4HF concentration solution. The highest level was at 8.19%, which was relatively low and not suitable for the REEs extraction. Therefore, NH4F solution was employed in the precipitation-strippingsaponification process.

(Insert Fig. 13)

3.9 Process of recycling REEs from spent ceria-based polishing powders and lutetium-based crystal waste Conventional REEs solvent extraction process using acidic extractants includes the procedures of organic saponification, REEs extraction, less-extractable element scrubbing and more-extractable element stripping (extractant regeneration). A certain base substance of alkali was always needed to increase and stabilize the extracting capacity. In addition, acid was usually used for scrubbing and stripping the loaded REEs (Fig. 1). As for recycling REEs from spent ceria-based polishing powders 15

ACCEPTED MANUSCRIPT (1kg) and lutetium-based crystal waste (1kg), about 3.7 kg of hydrochloric acid (31 wt.%) and 1.1 kg of sodium hydroxide oxide were required to obtain all the individual REEs. What is worse, about 10 kg wastewater containing high salinity would be produced. Therefore, it is critical to improve measure for green, less chemicals consumed process. Based on the above study, a proposed process was developed as Fig. 14 for recycling REEs from spent ceria-based polishing powders and lutetium-based crystal waste. Common acid solution for scrubbing and stripping was manipulated by interest metal solution (Y3+ or Lu3+), which also contributed to extracting the interest metal on the organic phase with no saponification. The key point of the technology is to use REE-loaded extractants and REE-contained water as the substitutes of the saponified extractant and the stripping acid, respectively. Furthermore, the last metal on the loaded organic phase (Lu3+) was precipitated and stripped by 30 wt% NH4F solution, which was a common commercial chemical used to treat glass surfaces (Houston and Maboudian, 1995) and was employed to prepare REEs particles (Zhang et al., 2006; Guo et al., 2009; Zhao et al., 2013). The solubility of NH4F in water is 45.3 g/100 ml at 25°C, which is about 31% by weight. Moreover, the references described that an organic solvent (D2EHPA, TBP, D2EHPDTA, et al) containing extracted metal ions (Mg, Co, La and the like) mixed and contacted with a similar fluoride series stripping solution to deposit a fluorinated metal complex crystal. Then, the deposited crystal, organic solvent as well as the stripping solution were individually recovered. The yield of metal recovered was up to 100% and the cost of producing high-purity metal was decreased considerably in industrial practice (Uchino et al., 1989; Watanabe et al., 1983). An additional benefit was that the extractant was regenerated and saponified simultaneously, then one step of precipitation-stripping-saponification was achieved. Hence, the consumption of alkali for saponification and acid for scrubbing and stripping was zero, which only consumed 0.78kg NH4F per kilogram of the two wastes (Table 5). The traditional saponification wastewater which usually contained high salinity was also reduced intrinsically. To improve the utilization and avoid the emission of NH4F, a containment system for fluorine circulation was designed with no diffusion into the environment.

(Insert Fig. 14) 3.10 Economic evaluation and environmental impacts 16

ACCEPTED MANUSCRIPT Economic evaluation of traditional process and proposed process were based on the data of Table 1 and Table 2, together with the authors’ manufacturing experiences. In the traditional REEs extraction process, the less extractable elements would be over extracted 25% of the more extractable elements. Aiming for high recovery yield, the stripping acid and precipitating oxalic acid should be over 20% and 100% of the target metal, respectively. However, there were no chemicals wastes in the proposed process because of using metal scrubbing and precipitation-strippingsaponification of NH4F which circulated in a closed system. The prices of the chemicals were provided from both the internet and Ganzhou Rare Earth Group Co., Ltd. In summary, the costs of REEs separation in traditional process and proposed process was 10.09 CNY/Kg waste and 4.32 CNY/Kg waste, respectively (Table 5). Totally, the chemical consumption was reduced over 80% and the wastewater was decreased more than 90% in this process. However, more details of economic analysis about the costs of the man power, the fixed and variable costs of the process should be investigated in larger scale experiments. The proposed process contributes to reducing the use of environmentally harmful chemicals such as acid, alkali and oxalic acid which dominate most of the environmental impacts and biggest proportion (about 50%~70%) of operational costs at solvent extraction facilities in REEs industry (Liao et al., 2013; Sholl and Lively, 2016; Vahidi and Zhao, 2017). The limitation of this paper lies in no details of economic analysis and more investigations are underway in the lab. Additionally, the environmental impacts of the proposed process lie in the following aspects: (a) Metal scrubbing and precipitation-stripping-saponification strategy has skillfully avoided using acid, alkali and oxalic acid reagents. Chemicals consumption or reagent use, which was deemed to be the most significant source of pollution in REEs separation industry (Liao et al., 2013; Das et al., 2018), have dramatically reduced over the traditional extraction processes. (b) REEs recycling from industrial scraps not only have treated the huge waste of ‘city ore’ but also have broaden the resources of the critical metals, especially lutetium (one of the scarcest metals in the earth). That leads to directly decrease the vast solid and liquid wastes in REEs minerals exploiting. As reported in the papers (Edahbi et al., 2019; Hurst, 2010), it would produce mining waste (9600 to 1200 tons), radioactive waste (1 tons), acidic water (75 m3), fluorite (8.5 kg) and

17

ACCEPTED MANUSCRIPT dust (13 kg) to obtain one-ton REO from the minerals in China. Environmental benefits are significant for REEs recycling from scraps. (c) Though some undesirable environmental impacts of using phosphonic acid extractants (P507, P204 and Cyanex®572) such as skin irritation and toxic to aquatic life were introduced in this process, Cyanex®572 has better stripping performance than the widely used phosphonic acid extractants (P507 and P204) in REEs industry, decreasing the stripping regents and reducing environmental impacts with the original design intent (MSDS of Cyanex®572) and the evidences of many investigations (Wang et al., 2017; wang et al., 2015; Innocenzi et al., 2018). In a word, the proposed strategy shows fewer chemical costs and much cleaner over the widely used REEs recycling progresses.

(Insert Table 5)

4. Conclusions An industrial extractant Cyanex®572 was successfully used to extract La3+, Ce3+, Y3+and Lu3+ from spent ceria-based polishing powders and lutetium-based crystal waste with zero consumption of alkali and acid. The highly efficient extraction process was based on the metal scrubbing but not acid and one step of precipitation-stripping-saponification via employing NH4F solution. The novel extraction process was developed which only consumes NH4F solution. The proposed process contributes to reducing the use of environmentally harmful chemicals such as acid, alkali and oxalic acid while they are also the main operational costs of REEs separation and extraction processes. Totally, the chemical consumption was reduced over 80% and the wastewater was decreased more than 90% in this strategy. The chemical costs of REEs separation in this proposed process was only about 43% of that in traditional process. The REEs (LaCl3, CeCl3, YCl3 and (NH4)3Lu2F9) were obtained. The extraction process presented in this paper served as model systems for the recovery of REEs from spent ceria-based polishing powders and lutetium-based crystal waste. Also, the process is suitable for other REEs recycling from REEs scraps such as lamp phosphor waste and magnet waste, as well as the rare earth ore. The simplified model system presented in this paper will be used in the near future as the basis for the development of environment-friendly extraction processes in REEs industry. 18

ACCEPTED MANUSCRIPT Acknowledgments This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences, Science and Technology Major Project of Fujian Province

(2015HZ0001-3),

Science

and

Technology Service Network Initiative from Chinese Academy of Sciences and Science and Technology Major Project of Ganzhou (2017-8). References Alonso, E., Sherman, A.M., Wallington, T.J., Everson, M.P., Field, F., Roth, R., Roth, R., Kirchain, R.E., 2012. Evaluating rare earth element availability: A case with revolutionary demand from clean technologies. Environ. Sci. technol. 46, 3406-3414. Binnemans, K., 2009. Lanthanide-based luminescent hybrid materials. Chem. Rev. 109, 4283-4374. Binnemans, K., Jones P.T., Blanpain, B., Gerven, T.V., Pontikes, Y., 2015. Towards zero-waste valorisation of rare-earth-containing industrial process residues: A critical review. J. Clean. Prod. 99, 17-38. Binnemans, K., Jones P.T., Blanpain, B., Gerven, T.V., Yang, Y., Walton, A., Buchert, M., 2013. Recycling of rare earths: A critical review. J. Clean. Prod. 51,1-22. Bogaert, B.V., Havaux, D., Binnemans K., Gerven, T.V., 2015. Photochemical recycling of europium from Eu/Y mixtures in red lamp phosphor waste streams. Green Chem. 17, 2180-2187. Chen, L., Chen, J., Jing, Y., Li, D., 2016. Comprehensive appraisal and application of novel extraction system for heavy rare earth separation on the basis of coordination equilibrium effect. Hydrometallurgy 165, 351-357. Chen, P., Yang, F., Liao, Q., Zhao, Z., Zhang, Y., Zhao, P., Guo, W., Bai, R., 2017. Recycling and separation of rare earth resources lutetium from LYSO scraps using the diglycol amic acid functional XAD-type resin. Waste manage. 62, 222-228. China Ministry of Environmental Protection, 2011. Emission standards of pollutants from rare earths industry, GB 26451-2011(in Chinese). Cytec Industries Inc. 2013. Product data sheet of CYANEX® 572 solvent extraction reagent. https: //www.cytec.com/sites/default/files/files/CYTEC_CYANEX_572_FINAL.pdf. 19

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23

ACCEPTED MANUSCRIPT Table 1 Composition of spent polishing powder (wt.%) Elt.

Sample1

Sample2

Sample3

Average

C

8.65

2.78

3.64

5.02

O

11.74

15.12

14.97

13.94

F

2.80

3.79

4.05

3.55

Al

0.57

0.80

0.86

0.74

Si

2.18

4.51

4.17

3.62

Ca

0.33

0.35

0.34

0.34

Fe

0.69

2.13

0.52

1.11

La

23.51

22.31

22.97

22.93

Ce

49.55

48.21

48.48

48.75

24

ACCEPTED MANUSCRIPT Table 2 Composition of LuYSiO crystal waste (wt.%) Elt.

sample1

sample2

sample3

Average

O

16.21

17.50

16.93

16.88

Al

1.77

0.77

1.27

1.27

Si

4.85

4.79

4.77

4.80

Fe

0.26

0.25

0.25

0.25

Y

2.10

1.70

1.97

1.92

Ce

0.90

0.40

0.62

0.64

Lu

73.92

74.59

74.18

74.23

25

ACCEPTED MANUSCRIPT Table 3 Basic parameters for spent polishing powder and LuYSiO crystal waste leaching Waste

Spent polishing

Duration

Chemicals

Chemicals

time

(1st step)

(2nd step)

Thiourea

HCl (100ml,

Temperature

80 °C

Yield

150 min

>98% dosage (10 g)

31wt%)

H2O2 (100 g,

HCl (100ml,

30 wt%)

31wt%)

powder (50 g)

80 °C

240 min

25 °C to 750 °C

HCl (100ml, 5 min

LuYSiO crystal waste (5 g)

>98%

NaOH (15 g)

within 80 min

>99% 6 mol/L)

25 °C to 750 °C

NaOH (7.5 g)

HCl (100ml,

Na2O2 (7.5 g)

6 mol/L)

5 min within 80 min

26

>99%

ACCEPTED MANUSCRIPT Table 4 Gibbs free energy and separation factors of metal scrubbing Cyanex®572

logk1

logk2

logk3

logk2-1

logk3-1

logk3-2

βY/Ce

βLu/Ce

βLu/Y

0.012 (mol/L)

-2.166

1.256

1.884

3.421

4.050

0.629

2638

11222

4

0.015 (mol/L)

-2.347

0.959

1.870

3.306

4.217

0.911

2024

16486

8

0.018 (mol/L)

-2.745

0.815

1.779

3.560

4.524

0.964

3634

33423

9

0.021 (mol/L)

-2.599

0.917

1.757

3.517

4.356

0.840

3288

22721

7

0.024 (mol/L)

-3.072

0.647

1.736

3.719

4.808

1.089

5240

64282

12

0.027 (mol/L)

-2.858

0.606

1.697

3.463

4.555

1.092

2905

35891

12

0.03 (mol/L)

-2.917

0.501

1.683

3.418

4.599

1.181

2618

39731

15

0.036 (mol/L)

-3.244

0.346

1.456

3.590

4.700

1.110

3891

50144

13

0.048 (mol/L)

-3.417

0.191

1.421

3.607

4.837

1.230

4049

68762

17

0.06 (mol/L)

-3.523

0.123

1.294

3.646

4.816

1.171

4423

65507

15

0.072 (mol/L)

-3.576

0.050

1.351

3.626

4.927

1.301

4223

84503

20

average (mol/L)

-2.951

0.583

1.630

3.534

4.581

1.047

3539

44788

12

△G (KJ/mol)

16.84

-3.33

-9.30

-20.16

-26.14

-5.97

-

-

-

27

ACCEPTED MANUSCRIPT Table 5 Economic evaluation of traditional process and proposed process ( / Kg waste)

Contents

REEs (g)

Ceria polishing

LuYSiO crystal

Chemical

powders waste

waste

consumption

La

Ce

Ce

Y

Lu

/Waste water (kg)

229.3

487.5

6.4

19.2

742.3

-

0.87

3.48

-

1.06

4.24

-

0.32

-

-

0.39

-

Extracted (over 25%) (mol) Scrubbing acid (kg) 4.09 Stripping acid -

1.52

-

-

1.86

0.10

0.42

-

0.13

0.51

1.16

-

-

-

-

1.15

1.15

0.31

1.40

0.00

0.37

52.35

54.43

(over 20%) (kg) Saponification alkali (kg ) Traditional process

Oxalic acid (over 100%) (kg ) Waste water (kg ) 4.09 kg × 0.4 CNY/kg (30 wt% HCl）+ 1.16 kg × 1.4

Costs (CNY)

CNY/kg ( NaOH ) + 1.15 kg × 5 CNY /kg ( H2C2O4）+ 54.43 kg × 0.02 CNY/kg（Costs of waste water treatment） = 10.09 CNY

NH4F needed Proposed

(kg )

process

Waste water

-

-

-

-

0.78

0.78

-

-

-

-

2.59

2.59

(kg )

28

ACCEPTED MANUSCRIPT 0.78 kg × 5.5 CNY/kg (NH4F） + 2.59 kg × 0.02 CNY/kg Costs (CNY) （Costs of waste water treatment）= 4.32 CNY

29

ACCEPTED MANUSCRIPT Captions of Illustrations Fig. 1 Traditional extraction process for REEs separation. Fig. 2 Schematic of proposed REEs separation process in this work. Fig. 3 Characterization of spent polishing powder. (a) SEM (b) XRD (c) EDS Fig. 4 Characterization of LuYSiO crystal waste. (a) SEM (b) XRD (c) EDS Fig. 5 Effect of contact time on scrubbing efficiency. A/O= 1:1, 0.0494 mol/L Ce3+ loaded on organic phase, 0.0430 Y3+mol/L loaded on organic phase. (1) [Ce3+]org scrubbed by 0.1864 mol/L H+. (2) [Ce3+]org scrubbed by 0.0604 mol/L Y3+, pH= 2.061. (3) [Ce3+]org scrubbed by 0.0599 mol/L Lu3+, pH= 2.074. (4) [Y3+]org scrubbed by 0.0599 mol/L Lu3+, pH= 2.074. (5) [Y3+]org scrubbed by 0.1864 mol/L H+. Fig. 6 Effect of metal concentration on scrubbing efficiency. Fig. 7 Effect of times on scrubbing efficiency. Loaded organic phase: 30% saponified 0.6 mol/L Cyanex®572 with 0.0428 mol/L of Y3+; diluent: n-heptane; scrub feed: 0.0676 mol/L of pure Lu3+ solution. Fig. 8 Precipitation-stripping-saponification progress. (a) Photograph of organic, aqueous and solid phase in NH4F stripping system. (b) Effect of concertation of striping reagents on stripping efficiency. [C572] = 0.6 mol/L, A/O= 1:1, 0.0536 mol/L Lu3+ loaded on organic phase. Organic phase (Op), Aqueous phase (Ap). Solid phase (Sp). Fig. 9 Effect of times on stripping efficiency. Loaded organic phase (LO): 30% saponified 0.6 mol/L Cyanex®572 with 0.0684 mol/L of Lu; diluent: n-heptane; stripping feed: 30 wt% NH4F solution. Fig. 10 XRD image of the solid phases. Fig. 11 1H NMR and 31P NMR spectra and IR transmittance spectra of extracting agent in different steps. (1) fresh Cyanex®572, (2) saponified by NH4OH, (3) loading Lu3+ after NH4OH saponification, (4) stripped Lu3+ by NH4F and (5) loading Ce3+ after NH4F saponification. Fig. 12 IR transmittance spectra of extracting agent in different steps. (1) fresh Cyanex®572, (2) saponified by NH4OH, (3) loading Lu3+after NH4OH saponification, (4) stripped Lu3+ by NH4F and (5) loading Ce3+ after NH4F saponification.

30

ACCEPTED MANUSCRIPT Fig. 13 Effect of striping reagents on extraction efficiency of regenerated Cyanex®572 A/O= 1:1, [C572] = 0.6 mol/L，[Ce3+] = 0.5810 mol/L, pH= 3.84. Fig. 14 Schematic illustration of novel process for REEs separation.

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Fig.1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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(a)

(b) Fig. 8

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Fig. 9

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Fig. 10

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(a)

(b) Fig.12

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Fig.12

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Fig.13

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Fig.14

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