WO2016065433A1 - Process for recovery of yttrium and europium from lamp phosphor waste - Google Patents

Abstract

This invention is related to the recycling of rare earths, and more particularly to the recovery of rare earths such as yttrium and europium from end-of-life fluorescent lamps. A new process is described to selectively recover the most valuable components of waste lamp phosphor powder. By means of an ionic liquid, the red phosphor Y₂O₃:Eu³⁺ (YOX) can be selectively dissolved without affecting the other components in the waste fraction. Of particular interest is the possibility to dissolve Y₂O₃:Eu³⁺ (YOX) without bringing the halophosphate phosphor in solution.

Description

PROCESS FOR RECOVERY OF YTTRIUM AND EUROPIUM FROM LAMP PHOSPHOR WASTE

FIELD OF THE INVENTION

This invention is related to the recycling of rare earths, and more particularly to the recovery of rare earths from end-of-life fluorescent lamps. A new process is described to selectively recover the most valuable components of waste lamp phosphor powder. By means of an ionic liquid, the red phosphor Y203: Eu³⁺ (YOX) can be selectively dissolved without affecting the other components in the waste fraction. Of particular interest is the possibility to dissolve Y₂03: Eu³⁺ (YOX) without bringing the halophosphate phosphor in solution.

BACKGROUND OF THE INVENTION

The rare earths or rare-earth elements (REEs) include the 15 lanthanides plus yttrium and scandium. They are used in many high-tech applications including NiMH batteries, wind turbines, electric vehicles, hard disk drives and fluorescent lamps. Efficient recycling of these elements could help to create a closed-loop system and to solve the balance problem which is caused by the unwanted co-production of other rare-earth elements during primary mining. This has led to the accumulation of large stocks of certain elements (La, Ce) while facing shortages of others (Y, Eu, Tb, Nd, Dy). New recycling technologies are becoming increasingly selective, efficient and sustainable and they are opening up new possibilities for urban mining and rare-earth waste revalorization. Originally, the collection of end-of-life fluorescent lights was put in operation to assure the safe disposal of the mercury contained in the lamps; however, the lamp phosphors were discarded or stockpiled. In recent years, the recovery of the REEs contained in these powders received a lot of attention due to the increasing supply risk for some of these elements. Lamp phosphor waste powder contains around 20 wt% of REEs including critical (Y, Eu, Tb) and less critical (La, Ce, Gd) REEs. These powders are used in fluorescent lamps to convert ultraviolet radiation into visible light. They consist of fine particles (1-10 μιτι) coated on the inside of the glass. A blend of red Y20₃: Eu³⁺ (YOX), blue BaMgAlioOi₇: Eu²⁺ (BAM) and green LaP0₄:Ce³⁺,Tb³⁺(LAP), (Ce,Tb)MgAluOi₉ (CAT) or (Gd,Mg)B₅Oio: Ce³⁺,Tb³⁺ (CBT) phosphors is used to get the desired color rendering index. Besides rare-earth phosphors, lamp phosphor waste also often contains large amounts (40-50 wt%) of non-valuable halophosphate (HALO) which emits cold white light and does not contain any REEs. The recycling value of the different phosphor components greatly varies. The waste fraction with the highest economic value is the red Y₂03: Eu³⁺ (YOX) phosphor. This phosphor consists almost entirely of the two critical rare earths yttrium and europium as opposed to the other phosphors which are only doped with critical REEs. This explains why YOX holds 80 wt% of the REEs present in the lamp phosphor waste powder even though YOX only accounts for 20 wt% of the lamp phosphor waste.

Many approaches have been proposed for the recycling of lamp phosphors (Binnemans, K. et al. J. Clean. Prod. 2013, 51, 1-22). Physical separation methods like magnetic separation, flotation and centrifugation could allow the direct reuse of the phosphors, but so far they are not used due to the high purity requirements and the deterioration of the phosphor powders during their lifetime. Chemical methods can be used to dissolve the phosphors completely or selectively based on the increasing difficulty to dissolve some of the phosphors: HALO < YOX < < LAP/BAM/CAT (Innocenzi, V. et al.. Waste Manage. 2014, 34, 1237-1250). The halophosphate (HALO) phosphor is easily dissolved in dilute hydrochloric acid solutions at room temperature. The dissolution of Y203: Eu³⁺ (YOX) requires more acidic conditions (e.g. 1 M HCI, 60-90 °C). LAP requires the use of very strong acidic conditions (e.g. 18 M H2SO4, 120-230 °C), while the aluminate phosphors BAM and CAT are best dissolved in strong alkaline conditions (35 wt% NaOH, 150 °C) in an autoclave or by molten alkali (e.g. Na₂C03, 1000 °C). Ionic liquids have been proposed for the selective extraction of previously dissolved rare earths, but so far they have not been used yet as a tool for the dissolution of phosphors (Yang, F. et al. J. Hazard. Mater. 2013, 254- 255, 79-88). New recycling schemes often focus on the recovery of yttrium and europium from the red phosphor Y2O3 : Eu³⁺ (YOX) phosphor, because of its high value and the relative ease to dissolve this phosphor compared to BAM, CAT and LAP. The main problem with these processes is that the halophosphate phosphor (HALO) is often ignored as they focus mostly on the so-called tri-band phosphors which all contain rare earths (YOX, BAM, LAP, CAT). However, real lamp phosphor waste contains up to 50 wt% of HALO and therefore has to be considered when trying to develop an industrially applicable recycling method (EP 2419377, US2012/0027651). Unfortunately, halophosphate (Sr,Ca)io(P04)6(CI,F)₂:Sb³⁺,Mn²⁺ (HALO) is very easily dissolved in dilute acids even at room temperature. HALO contains no rare earths and has a very low intrinsic value; therefore dissolving this phosphor leads to considerable pollution of the leachate and introduces a large amount of unwanted exogens (Sr, Ca, F, Sb, Mn, CI) in the waste water. This is greatly complicating the further processing (e.g. solvent extraction). The dissolution of HALO also consumes considerable amounts of acid (18 protons per formula unit of HALO) and leads to the formation of large amounts of H3PO4 (6 molecules per formula unit of HALO) which forms very insoluble YP0₄ and Eu P0₄ precipitates with the dissolved Y³⁺ and Eu³⁺ ions from the YOX. The desig n of a selective d issolution method for YOX without dissolving HALO wou ld therefore g reatly increase the efficiency and profitability of a recycling process for lamp phosphor waste, but, such a process has not been descri bed yet, nor has any of the prior art documents suggested such a process as described in the present i nvention .

SUMMARY OF THE INVENTION

The process descri bed in the present i nvention allows selective dissolution and regeneration of the valuable red phosphor Y203 : Eu³⁺ (YOX) i n a three-step process that consumes very few chemicals and generates zero additional waste (except CO2) . An ionic liquid is used for the selectively dissolution of Y203 : Eu³⁺ without dissolving the other components in the lamp phosphor waste (HALO, BAM , LAP, CAT, S1O2, AI2O3), and is fu rther described in step (a) of the present i nvention . An ionic liquid is a solvent that consist entirely of cations and anions. Br0nsted acidic ionic liquids with a carboxylic acid group appended to the cation have the ability to dissolve ra re-earth oxides and many transition metal oxides. Silica and alumina cannot be dissolved in such ionic liquids, which is highly relevant to the recycling of lamp phosphor waste si nce these powders can contain significant amou nts of si lica (as fine glass pa rticles) and alumina . The dissolution is d riven by the reactivity of the carboxylic acid group located on the cation of the ionic liquid . In general, the solubility of metal salts (e.g . NaCI, EuC ) is much lower in this type of ionic liquid than the solubility of thei r oxide equivalent (e.g . a20, EU2O3) due to the inefficient solvation of anions. This effect explains the differences in solubility in acidic ionic liquids, compared to acidic aqueous solutions.

In certain embod i ments of the present invention, the ionic liquids as referred to in the present invention, including the use of said ionic l iquids in the methods and processes and uses of the present i nvention, have a structure accord i ng to the fol lowing general formu la :

R¹R²R³R⁴A⁺ X^",

wherein

- A is selected from N, P, As, and Sb;

- R¹, R² and R³ are each independently selected from the g rou p consisting of O- 12 al kyl and C3-12 cycloal kyl ; or each of R¹ and R², or R¹ and R³, or R² and R³ can be taken together to form a su bstituted or unsubstituted cyclic structure; - R⁴ is selected from the group consisting of Ci-12 alkyl-COOH and C3-12 cycloalkyl- COOH; and

- X^" is selected from the group consisting of chloride, bromide, iodide, thiocyanate, nitrate, perchlorate, sulfate, hydrogensulfate, hydrogenphosphate, di(Ci-i2 alkyl)phosphate, organic sulfonates, organic sulfates, organic sulfonylimides and organic carboxylates.

In a preferred embodiment, A is N or P.

In a particular embodiment, each of R¹, R² and R³ are independently selected from Ci-8 alkyl and C3-8 cycloalkyl, yet more in particular are independently selected from Ci-6 alkyl and C3-6 cycloalkyl, still more particularly are independently selected from Ci-4 alkyl, yet more particularly from C1-3 alkyl. In a preferred embodiment, each of R¹, R² and R³ are the same and more particularly are methyl, ethyl, propyl or butyl. In another particular embodiment, R⁴ is selected from O-s alkyl-COOH and C3-8 cycloalkyl-COOH, still more in particular R⁴ is selected from Ci-6 alkyl-COOH and C3-6 cycloalkyl-COOH, yet more particularly R⁴ is C1-4 alkyl-COOH or C1-3 alkyl-COOH and yet more particularly R⁴ is -CH2COOH .

In a more preferred embodiment, R¹R²R³R⁴A⁺ or R¹R²R³R⁴N⁺ is betaine, wherein R¹ is -CH₃, R² is -CH₃, R³ is -CH₃ and R⁴ is -CH2COOH .

In a particular embodiment, X^" is selected from the group consisting of fluorinated organic sulfonates, fluorinated organic sulfates, fluorinated organic sulfonylimides and fluorinated organic carboxylates. Examples of X^" include methylsulfonate (CH3SO3 ), bis(trifluoromethylsulfonyl)imide (Tf2N ), bis(nonafluorobutylsulfonyl)imide, triflate (TfO ) and pentafiuorobenzoate (C6F6COO ). In another particular embodiment, X^" is selected from perfluoro-Ci-i2alkyl-sulfonylimide (or more particularly from perfluoro- Ci ealkyl-sulfonylimide), and in yet another more specific embodiment X^" is bis(trifluoromethylsulfonyl)imide

In a more preferred embodiment, the organic salt is betainium bis(trifluoromethylsulfonyl)imide. The structure of betainium bis(trifluoromethylsulfonyl)imide (in protonated form) is according to formula I:

In the present invention, the ionic liquid betainium bis(trifluoromethylsulfonyl)imide is also referred to as [Hbet] [Tf2N] .

[Hbet] [Tf2N] can selectively dissolve the Y203: Eu³⁺ (YOX) in a lamp powder waste fraction obtained from end-of-life fluorescent lamps. Such lamp (phosphor) waste fraction is typically obtained by crushing and sieving end-of-life fluorescent lamps. An interesting feature of the process is that the (Sr,Ca)io(P04)6(CI,F)2:Sb³⁺,Mn²⁺ phosphor is not dissolved by the ionic liquid. This non-valuable broad-band white phosphor known as "halophosphate" (HALO) can make up as much as 50 wt% of the lamp phosphor waste fraction. In previous dilution methods, using dilute mineral acids, dissolution of HALO is unavoidable, especially in the conditions required for the dissolution of Y203: Eu³⁺. The yttrium and europium dissolved in the ionic liquid [Hbet][Tf₂N] can then be stripped (i) with an HCI solution, as further described in an alternative step (b) or b' in the present invention; or (ii) by adding pure (solid) oxalic acid directly to the ionic liquid, as further described in step (c) of the present invention. The oxalic acid route precipitates the rare earths as a mixed (yttrium, europium) oxalate salt, which can be transformed into a new Y203: Eu³⁺ phosphor by a calcination step at 950 °C, as further described in step (d) of the present invention. The ionic liquid is automatically regenerated during the stripping step and can be reused. This innovative approach offers selectivity, mild conditions and reusability which turns this invention into a promising green technology for the targeted recovery of Y203: Eu³⁺ and the valorization of lamp phosphor waste.

Ionic liquids can be heated in a very energy-efficient way by microwave irradiation. Ionic liquids consist entirely of ions as opposed to water or organic solvents which explains the high adsorption of microwave radiation. Microwaves do not interfere with the dissolution process and provide a highly economical way to heat an ionic liquid in an industrial context. The immediate on/off behavior of microwave irradiation is also useful for process control.

Thus the present invention relates to the process for the separation of red lamp phosphor (or europium-doped yttrium oxide Y203: Eu³⁺ or YOX) from other lamp phosphors, by mixing the lamp phosphors in an ionic liquid which selectively dissolve the red lamp phosphor, such as the ionic liquid betainium bis(trifluoromethylsulfonyl)imide. In said process, the selective dissolution of the lamp phosphor in said ionic liquid is an essential feature of the claimed process, and selective is in the meaning that other compounds like HALO, but also BAM, LAP and CAT are not dissolved in said ionic liquid. The present invention furthermore relates to the recovery or selective enrichment of the red lamp phosphor (or europium-doped yttrium oxide Y₂03: Eu³⁺ or YOX), more specifically the recovery or selective enrichment of the REEs (being Y and Eu) from the waste fraction of fluorescent lamps, more specifically end-of-life fluorescent lamps. Said separation/recovery/enrichment process means that as much as possible of said REE's (Y and Eu) are recovered, being at least 80%, 90%, preferably more than 95%, more than 98% or even more preferably more than 99% or ideally 100% of said REE's are recovered from the original (waste) fraction; whereas the impurities or unwanted compounds, such as HALO, BAM, LAP and CAT are preferably still present in the waste fraction (solid fraction at the end of step (b) of the present invention) in more than 80%, or more than 90% or more preferably more than 95%; more than 98% or more than 99% or ideally 100% after the separation/recovery/enrichment process of the present invention. An important advantage of the present invention is that the halophosphate or HALO is separated/removed (for at least 80, 90, 95, 98, 99 or ideally 100%) from the valuable REE's (mainly Y and Eu) in a single (REE) dissolution step followed by a separation step (HALO comprising solid/REE comprising liquid).

The present invention relates to the above mentioned process wherein the red lamp phosphor or YOX is recovered by a method which is environmentally more favorable compared to all currently known methods in the art. The method of the present invention contains less steps, and is less energy consuming. Therefore the present invention, and more particularly the described method steps (a), (b), (c), (d) and (e) allow for an industrial process and easy recycling plant design which (i) makes use of basic and compact installations; (ii) does not require solvent extraction; and (iii) allows for the on-site processing for recyclers. Additional advantages are the complete reuse/recovery of the ionic liquid, no use of acid and thus no acid waste (when step (b) is used instead of (b')), and very little waste overall. A schematic overview of a flow sheet for a potential recycling plant design is depicted in Figure 9.

Numbered statements of this invention are:

1. A method for recovering rare-earth elements from a solid mixture comprising halophosphate and at least one compound comprising one or more rare-earth element the method comprising the steps of:

(a) mixing said solid mixture in an ionic liquid,

(b) separating from the mixture of a) the ionic liquid phase from the solid phase, wherein the solid mixture comprises the halophosphate. 2. The method according to statement 1, wherein the compound comprising one or more rare-earth elements is red phosphor or Y203: Eu³⁺ (YOX).

3. The method according to statement 1 or 2, comprising additional steps: (c) wherein the ionic liquid phase resulting from step b) is contacted with oxalic acid and mixed, thereby forming a rare-earth oxalate; and (d) wherein the rare-earth oxalate containing solid phase resulting from step c) is separated from the ionic liquid phase. 4. The method according to statement 3, wherein the oxalic acid in step c) is added as a solid or powder form.

5. The method according to statement 3 or 4, wherein the solid phase separated in step (d) comprises the rear-earth oxalate (Yn,Eui-_n)2(C204)3, wherein n is between 0 and 1, including 0 and 1.

6. The method according to any of statements 3 to 5, comprising additional step (e) of calcining the solid phase resulting from step (d). 7. The method according to statement 6, wherein the calcined product obtained in step (e) is YOX.

8. The method according to statement 1 or 2, wherein after step a) the ionic liquid phase is contacted with an aqueous acid solution, and mixed; and wherein in step b) an aqueous phase comprising a rare-earth metal is separated from the ionic liquid phase.

The method according to statement 8 wherein the aqueous acid solution is a HCI solution.

The method according to statement 8 or 9, wherein the rare-earth metal in the aqueous phase comprises the water soluble YC and/or EuCta salts.

The method according to any of the previous statements, wherein the ionic liquid is according to the general formula :

R¹R²R³R⁴A⁺ X^",

wherein independently from each : - A is selected from the group consisting of N, P, As, and Sb;

R¹, R² and R³ are each independently selected from the group consisting of Ci-12 alkyl and C3-12 cycloalkyl; or each of R¹ and R², or R¹ and R³, or R² and R³ form a substituted or unsubstituted cyclic structure;

R⁴ is selected from the group consisting of Ci-12 alkyl-COOH and C3-12 cycloalkyl-COOH; and

X^" is selected from the group consisting of chloride, bromide, iodide, thiocyanate, nitrate, perchlorate, sulfate, hydrogensulfate, hydrogenphosphate, di(Ci-i2 alkyl)phosphate, organic sulfonate, organic sulfate, organic sulfonylimide and organic carboxylate.

12. The method according to statement 11, wherein each of R¹, R² and R³ are methyl

The method according to statement 11 or 12, wherein X^" is selected from the group consisting of fluorinated organic sulfonates, fluorinated organic sulfates, fluorinated organic sulfonylimide anions and fluorinated organic carboxylates.

The method according to any of statements 11 to 13, wherein X^" is a perfluoro- Ci-i2alkyl-sulfonylimide.

The method according to any of statements 1 to 14, wherein the ionic liquid betainium bis(trifluo ulfonyl)imide with the structure of formula I:

(I)

16. The method according to any one of statements 1 to 15 wherein the solid mixture is a mixture comprising a red lamp phosphor and other lamp phosphors. 17. Use of an ionic liquid for separating red lamp phosphor or europium-doped yttrium oxide Y203: Eu³⁺ (YOX) by selective dissolution from other lamp phosphors. 18. The use according to statement 17, wherein the ionic liquid is betainium bis(trifluoromethylsulfonyl)imide.

19. The use according to statement 17 or 18 for the recovery of europium-doped yttrium oxide Y203". Eu³⁺ (YOX) from the lamp phosphor powder waste fraction from end-of-life fluorescent lamps.

20. The use according to any of statements 17 to 19, wherein oxalic acid is used for the stripping of dissolved yttrium and/or europium from the ionic liquid comprising said rare-earth elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Dissolution of YOX (wt%) in [Hbet] [Tf2N] at 90 °C with varying amounts of water in the ionic liquid (0, 1, 2.5 and 5 wt%). The solutions were stirred at 600 rpm. No dissolution of BAM and LAP was observed.

Figure 2. Leaching (wt%) of HALO by [Hbet][Tf₂N] at 90 °C after 24 h and 100 h, with increasing amounts of water in the ionic liquid : 0, 1, 2.5, 5 ,7.5, 10 and 13 wt% (= water-saturated). Leaching is also shown in an aqueous solution containing [HbetjCI (1 M). The solutions were stirred at 600 rpm.

Figure 3. Dissolution (wt%) of YOX in water-free [Hbet] [Tf2N] and water-containing (5 wt% H2O) [Hbet] [Tf2lM] as function of time and temperature.

Figure 4. Dissolution (wt%) of YOX in [Hbet][Tf₂N] (5 wt% H2O) as function of initial YOX concentration (mg/g). The dissolutions were carried out at 90 °C during 40 h and 100 h. Figure 5. Stripping of Y³⁺ and Eu³⁺ from [Hbet] [Tf2N], previously loaded with 10 mg/g of YOX. The influence of the HCI concentration on the stripping efficiency for Y³⁺ and Eu³⁺ is shown. The biphasic system was shaken (1700 rpm) at 25 °C for 1 h.

Figure 6. Stripping of yttrium and europium from [Hbet] [Tf₂N], using pure (solid) oxalic acid. The influence of oxalic acid concentration on the stripping efficiency is shown. A ratio of 1.5: 1 is the stoichiometric amount of oxalic acid required to precipitate all rare-earth ions (REE = Y + Eu) in solution as (Y,Eu)₂(C20₄)3 and results in a stripping efficiency of 100%.

Figure 7. Stripping of yttrium and europium from [Hbet] [Tf2N], using a stoichiometric amount (3 : 2) of pure (solid) oxalic acid. The influence of time and temperature on the total stripping efficiency (Y+Eu) is shown.

Figure 8. Heating of different solvents in a microwave oven (100 W), equipped with an infrared in-situ temperature probe.

Figure 9. Flowsheet of the process.

DETAILED DESCRIPTION OF THE INVENTION An "ionic liquid" is a solvent that consists entirely of ions (cations and anions). In this invention ionic liquids or organic salts are those solvents that consist of organic cations and anions that are organic or inorganic.

The ionic liquid presented in formula I is also named betainium bis(trifluoromethylsulfonyl)imide or [Hbet][Tf2lM] . Here we represent the cation betaine by Hbet rather than by bet, because in [Hbet][Tf2N] the carboxylate group is protonated. The anion bis(trifluoromethylsulfonyl)imide is also known as

, PMS, BTFSI, TFSI or Tf₂N\ [Hbet] [Tf₂N] is sometimes also called betaine bis(trifluoromethylsulfonyl)imide. However, it should be clear to a person skilled in the art that the ionic liquids or organic salts described in the present invention can be in different forms, protonated or unprotonated or with different counter ions.

As used herein and unless otherwise stated, the term "rare earths" or "rare-earth elements" or "REEs" or "rare-earth metal" means the 15 lanthanides (lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu)) plus yttrium and scandium.

As used herein and unless otherwise stated, the term "YOX" means europium-doped yttrium oxide or Y₂03: Eu³⁺.

As used herein and unless otherwise stated "red phosphor" means the product that almost entirely consists of YOX. Red phosphor can contain some impurities next to the YOX fraction, and therefore red phosphor is a more general term which, in most cases can be replaced by YOX, but is also used for an impure YOX fraction. As used herein and unless otherwise stated, the term "HALO" or "halophosphate" or

"halophosphate phosphor" means (Sr,Ca)io(P04)6(CI,F)₂:Sb³⁺,Mn²⁺.

As used herein and unless otherwise stated, the term "BAM" or "blue phosphor" means

BaMgAlioOi₇:Eu²⁺.

As used herein and unless otherwise stated, the term "LAP" means LaP04:Ce³⁺,Tb³⁺. LAP can be present in the green phosphor fraction of fluorescent lamps.

As used herein and unless otherwise stated, the term "CAT" means (Ce,Tb)MgAlnOi9. CAT can be present in the green phosphor fraction of fluorescent lamps.

As used herein and unless otherwise stated, the term "CBT" means (Gd,Mg)BsOio : Ce³⁺, Tb³⁺. CBT can be present in the green phosphor fraction of fluorescent lamps.

In each of the following definitions, the number of carbon atoms represents the maximum number of carbon atoms generally optimally present in the substituent or linker; it is understood that where otherwise indicated in the present application, the number of carbon atoms represents the optimal maximum number of carbon atoms for that particular substituent or linker.

The term "Ci-12 alkyl" as used herein means normal, secondary, or tertiary hydrocarbon having 1 to 12 carbon atoms. Examples are methyl, ethyl, 1-propyl, 2- propyl, 1-butyl, 2-methyl-l-propyl(i-Bu), 2-butyl (s-Bu) 2-methyl-2-propyl (t-Bu), 1- pentyl (n-pentyl), 2-pentyl, 3-pentyl, 2-methyl-2-butyl, 3-methyl-2-butyl, 3-methyl- 1-butyl, 2-methyl-l-butyl, 1-hexyl, 2-hexyl, 3-hexyl, 2-methyl-2-pentyl, 3-methyl-2- pentyl, 4-methyl-2-pentyl, 3-methyl-3-pentyl, 2-methyl-3-pentyl, 2,3-dimethyl-2- butyl, and 3,3-dimethyl-2-butyl. As used herein and unless otherwise stated, the term "C3-12 cycloalkyl" means a monocyclic saturated hydrocarbon monovalent radical having from 3 to 12 carbon atoms, such as for instance cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and the like, or a C7-12 polycyclic saturated hydrocarbon monovalent radical having from 7 to 12 carbon atoms such as, for instance, norbornyl, fenchyl or adamantyl.

For the avoidance of doubt, when the terms Ci-8, Ci-6, C1-4 or C1-3 are used herein they respectively refer to structures having from 1 to 8, 1 to 6, 1 to 4 or 1 to 3 carbon atoms; and when the terms C3-8 or C3-6 are used herein they respectively refer to structures having from 3 to 8 or 3 to 6 carbon atoms.

In a particular embodiment, the said alkyl-groups and cycloalkyl-groups are optionally substituted with one or more substituents such as a halogen and an OH group. In another particular embodiment said cycloalkyl-groups can additionally contain a heteroatom such as N, P, O and S which qualifies said groups as heterocyclic groups. In certain embodiments said cyclic group can be an aromatic group. Said heterocyclic and aromatic groups can be subsituted with one or more substituents such as a halogen and an OH group.

As used herein and unless otherwise stated, the term halogen means any atom selected from the group consisting of fluorine, chlorine, bromine and iodine.

Any substituent designation that is found in more than one site in a compound of this invention shall be independently selected.

In certain embodiments of the present invention, the ionic liquids as referred to in the present invention, including the use of said ionic liquids in the methods and processes of the present invention, are organic salts having the general formula :

R^!R²R³R⁴A⁺ X

wherein

- A is selected from N, P, As, and Sb, more preferably A is N or P;

- each of R¹, R² and R³ are independently selected from Ci-12 alkyl and C3-12 cycloalkyl, wherein said Ci-12 alkyl and C3-12 cycloalkyl can optionally be substituted ; or each of

R¹ and R², or R¹ and R³, or R² and R³ can be taken together to form a substituted or unsubstituted cyclic structure;

- R⁴ is selected from a Ci-12 alkyl-COOR⁵; and C3-12 cycloalkyl-COOR⁵; wherein R⁵ is selected from hydrogen and Ci-12 alkyl;

- X^" is selected from chloride, bromide, iodide, thiocyanate, nitrate, perchlorate, sulfate, hydrogensulfate, hydrogenphosphate, di(Ci 12 alkyl)phosphate, organic sulfonates; organic sulfates; organic sulfonyl imides or organic carboxylates.

In a preferred embodiment, the ionic liquids of the present invention are quaternary ammonium or quaternary phosphonium structures according to the following formulas:

R¹R²R³R⁴N⁺ X-, or R¹R²R³R⁴P⁺ X^",

wherein R¹, R², R³, R⁴ and X^" can have any of the values as described in the present invention.

The substituents that can be present on R¹, R² and R³ comprise halogens, OH, and any further substituent described in the present invention. In alternative embodiments, the ionic liquids or organic salts that can be used within the present invention, including the use of said ionic liquids in the methods and processes of the present invention, are the ionic liquids described in WO2007147222, such as the ionic liquids described in claim 1, formula I, II, III, IV, V, VI, VII, VIII and IX of the description of WO2007147222, which is hereby incorporated by reference. These organic salts are ionic liquids and can be used as such. More preferably, the present invention provides for the ionic liquid betainium bis(trifluoromethylsulfonyl)imide. In a search for cheap and easily accessible ionic liquids, we turned our attention to betaine. Betaine is a trivial name for l-carboxy-/V_//V_//V-trimethylmethanaminium hydroxide. It is also known as Λ/,Λ/,/V-trimethylglycine, /V-trimethylglycine or trimethylglycine. Betaine has a zwitter- ionic structure, and it is thus an inner salt. Betaine melts with decomposition at 310 °C. It readily reacts with mineral acids or organic acids whereby the carboxylate group gets protonated and the anionic part of the acid becomes the anion in the betaine salt. For instance betaine reacts with hydrochloric acid to form betaine hydrochloride, a salt with melting point of 232 °C. Betaine hydrochloride is a water-soluble salt that can be used to prepare hydrophobic salts by a metathesis reaction. The carboxylate group of betaine is a very good coordinating group towards metal ions. The most important form of betaine on the market is the hydrochloride salt, betaine hydrochloride. An aqueous solution of betaine hydrochloride reacts with an aqueous solution of lithium bis(trifluoromethylsulfonyl)imide to form betaine bis(trifluoromethylsulfonyl)imide, [Hbet] [Tf2N] . The present invention comprises the use of betainium bis(trifluoromethylsulfonyl)imide,

(see formula I), for selective solubilization of Y₂03: Eu³⁺ (YOX) in a lamp powder waste fraction.

Betainium bis(trifluoromethylsulfonyl)imide, [Hbet] [Tf₂l\l], is accessible via different synthetic routes. The best method is by reaction of the zwitterionic betaine with the acid hydrogen bis(trifluoromethylsulfonyl)imide, Tf₂NH. This reaction involves a simple proton transfer from the bis(trifluoromethylsulfonyl)imide to the more basic carboxylate group, so that the betaine will be protonated. This method is general applicable for the preparation of other betaine salts. For instance, we prepared by this method protonated betaine hexafluorophosphate, protonated betaine triflate, and protonated betaine pentafluorobenzoate. However, all these salts have melting points above 100 °C and can therefore not be considered as ionic liquids. A second synthetic route to the protonated betaine bis(trifluoromethylsulfonyl)imide ionic liquid is by the metathesis reaction of betaine hydrochloride and lithium bis(trifluoromethylsulfonyl)imide (in 1 : 1 molar ratio) in aqueous solution. An aqueous solution of betaine hydrochloride reacts with an aqueous solution of lithium bis(trifluoromethylsulfonyl)imide to form protonated betaine bis(trifluoromethylsulfonyl)-imide, [Hbet][Tf₂N]. The ionic liquid easily separates from the aqueous layer (due to its hydrophobicity). The ionic liquid prepared by the metathesis reaction has a melting point of 57 °C and the compound can easily be supercooled to room temperature. [Hbet] [Tf2N] is a hydrophobic ionic liquid at room temperature; after addition of sufficient water ( > 13 wt%), two separate phases are formed . Heating treatment of a mixture induced the formation of a one-phase-system at a critical temperature of 56 °C. Cooling of the one-phase mixture resulted again in phase separation. A similar temperature-dependent miscibility was observed for mixtures of [Hbet] [Tf₂N] with toluene. [Hbet] [Tf₂N] is miscible with ethanol, 1-octanol (and other higher alcohols), benzonitrile, acetonitrile, DMSO, acetic acid and ethyl acetate, but also with other ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion like [C6mim] [Tf2N] . [Hbet] [Tf₂N] is immiscible with hexane, heptane, dichloromethane (DCM), chloroform, benzene, toluene and diethyl ether.

The protonated functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide [Hbet] [Tf₂N] has the ability to dissolve certain metal oxides, including rare-earth oxides like Y₂03 and EU2O3, so it has also the ability to dissolve the red Y₂03 : Eu³⁺ (YOX) phosphor. The addition of small amounts of water to [Hbet] [Tf₂N] accelerates the dissolution of Y₂03: Eu³⁺, but too large amounts of water cannot be used because in these conditions the HALO would easily dissolve, as it does in acidic aqueous solutions. The pK_a of the betaine hydrochloride salt [Hbet]CI is 1.83. An aqueous solution of [Hbet]CI (1 M) quickly dissolves both HALO and YOX; therefore, no selectivity is expected . BAM and LAP are not dissolved in these conditions because they require very concentrated acidic conditions to be dissolved (e.g . 18 M H₂S0₄, 100°C) . When using the ionic liquid [Hbet] [Tf₂N] it is evident that BAM and LAP cannot be dissolved. However, only a very small amount of HALO is leached in the ionic liquid [Hbet] [Tf₂N] when less than 5 wt% of water was present, even when chloride ions were added (NaCI) . YOX on the other hand can be dissolved in [Hbet] [Tf₂N] rather easily as it is a rare-earth oxide. The control of the water content in the ionic liquid therefore allows the selective dissolution of YOX, without dissolving HALO. The optimal leaching system is a compromise between the fast dissolution of YOX and keeping the leaching of HALO as low as possible. [Hbet] [Tf₂N] with 5 wt% of H₂0 (90 °C, 24 h) is the best compromise between speed and selectivity. In these conditions, the leaching of YOX and HALO is 100 wt% and 0.23 wt% respectively, and BAM and LAP leaching below detection limit. Therefore, step (a) of the present invention optimally comprises a small amount of water such as about 5 wt%, or between 2.5 and 7.5 wt%. Step (a) of the present invention is further optimal performed at higher temperatures such as more than 70°C or at about 90°C; for a prolonged time such as 24h to 48h, said timing being shorter, the higher the temperature in step (a). Higher temperatures furthermore lead to lower viscosity, providing for a more optimal and faster process. At the end of step (a) the rare earth elements-containing liquid/ionic phase is separated (in step (b)) from the solid phase (comprising the waste) by any suitable method for solid/liquid separation, for example centrifugation, filtration, decanting.

The dissolved Y³⁺ and Eu³⁺ (YOX) in [Hbet] [Tf₂N] can be recovered by a stripping step. This can be achieved either by contacting the loaded ionic liquid with an acidic water phase to extract the rare-earth ions (method 1, comprising an alternative step (b) or (b'), with the addition of an aqueous acid solution), or by directly precipitating the rare earths from the ionic liquid using pure (solid) oxalic acid (method 2, comprising step (b) and (c)) . Stripping method 1 has limitations due to the loss of ionic liquid to the water phase. Stripping with pure oxalic acid (method 2) is clearly a better alternative since no ionic liquid is lost and the YOX phosphor can be immediately regenerated from the yttrium/europium oxalate salt in a simple calcination step.

The yttrium/europium oxalate that is generated in step (c) is depicted as (Yn,Eui-n)2(C204)3, wherein n can have any values between 0 and 1, including 0 and 1. Said yttrium/europium oxalate is a big complex wherein both yttrium and/or europium can be present. Typically both REEs are present in said oxalate in the same ratio as they were present in the starting material or waste fraction from fluorescent lamps, typically end-of-life fluorescent lamps. Usually yttrium is present in an amount of 90% or more, typically 95% or more, or even more than 98%; whereas europium is usually present in less than 10%, typically less than 5% or even less than 2%. Nevertheless, this stripping step also works for pure yttrium or pure europium, and Y2(C20₄)3 respectively Eu2(C20₄)3 would then be formed by said oxalic acid precipitation.

The stripping with an acidic solution (step (b')) can be accelerated by taking advantage of the fact that the [Hbet] [Tf2N]/H20 system shows thermomorphic behavior with an upper critical solution temperature. This means that the biphasic system will become one homogeneous phase above a certain temperature called the critical solution temperature. This is very interesting since the process kinetics will be much faster as they are then no longer limited by the diffusion across an interface. In order to get phase separation, the mixture has to be heated above the critical solution temperature and shaken briefly (1 s) to overcome the metastable state, but the homogeneous state then remains stable as long as the temperature is above the critical solution temperature. Below the critical solution temperature, the phase separation automatically occurs again. The critical solution temperature is dependent on the amount and type of ions present in the water phase, but also on the loading of the ionic liquid and the water to ionic liquid ratio. Stripping can be performed at 80 °C in the homogeneous phase. Thus the stripping step (b') is optimally performed in acidic aqueous environments, for example by adding strong acids such as HCI, optimally in a concentration of at least 0.3M or 0.4M, or about 1M. Step (b) of the present invention is further optimal performed at room temperature while shaking, or at higher temperatures, such as at, or above the critical solution temperature and for a certain amount of time being at least 1 second up to 1 hour or more. At the end of step (b') the temperature can be lowered again, for example to room temperature and the rare earth elements/ions can be extracted from the aqueous phase by conventional methods known to the skilled person.

The problem of the loss of ionic liquid to the water phase can be avoided by adding solid oxalic acid, directly in the ionic liquid phase. This acid forms insoluble oxalate complexes with the rare-earth ions and precipitates them while regenerating the ionic liquid. Thus step (c) essentially comprises the addition of oxalic acid directly to the ionic liquid and mixing followed by a separation of the solid fraction from the liquid fraction. This step (c) is optimally performed at higher temperatures such as more than 50°C or at about 70 °C; for a prolonged time such as for 10 min, 1 hour or more, said timing being shorter, the higher the temperature in step (c). The optimal amount of oxalic acid to be added in this step (c) is a stoichiometric amount of oxalic acid compared to the amount of rare-earth ions (3 : 2). The amount of oxalic acid in step (c) is thus preferably about said stochoimetric amount or higher. At the end of step (c) the rare earth elements-containing solid phase is separated (in step (d)) from the liquid phase by any suitable method for solid/liquid separation, for example centrifugation, filtration, decanting. Thus at the end of step (d) the (ionic) liquid phase can be recovered and reused in step (a) of the present invention.

The rare-earth oxalate precipitate (present in the solid phase at the end of step (d)) can, after it has been separated from the ionic liquid (end of step (d)), be thermally decomposed in an oven at high temperature to form the corresponding rare-earth oxides. This is performed in the calcination step (e) of the present invention. Said calcination step (e) is performed at high temperatures, such as at least 900 °C or about 950 °C. Said calcination step is therefore preferably performed in an oven or furnace which allows for said high temperatures and said step is performed for a prolonged time, such as 1 hour or more, or about 4 or 5 hours.

By carefully choosing the conditions it is also possible to directly resynthesize the Y203: Eu³⁺ phosphor by calcination (in step (c) of the present invention) of the mixed yttrium/europium oxalate. The advantage is that the yttrium and europium are already present in the correct ratio to manufacture the YOX phosphor, since they have been selectively leached from used lamp phosphors in the first place. This stripping system is therefore definitely the preferred choice compared to the stripping system using an acidic water phase. The oxalic acid stripping is much more convenient and can be carried out directly in the ionic liquid, meaning there is no loss of ionic liquid to a water phase.

Typically, the ionic liquid is heated by a conventional heating source. However, since the leaching of YOX in the ionic liquid requires high temperatures over prolonged periods of time, it is interesting to note that ionic liquids can be heated in a very energy-efficient way by microwave irradiation. Ionic liquids consist entirely of ions as opposed to water or organic solvents which explains the high adsorption of microwave radiation. Microwaves do not interfere with the dissolution process and provide a highly economical way to heat an ionic liquid in an industrial context. The immediate on/off behavior of microwave irradiation is also useful for process control. The temperature can be kept constant at a minimal cost.

At the end of the recycling scheme, no additional waste has been created. The rest of the lamp phosphor waste (Si0₂, AI2O3, HALO, BAM, LAP, CAT, CBT) can be discarded or further processed, for example by doing a rough separation of the HALO with physical separation methods, such as magnetic separation to separate LAP, CAT and CBT from HALO and BAM. This will result in a terbium concentrate (« 8 wt% Tb) held in LAP, CAT and CBT. The terbium content in this concentrate is much higher than any commercially exploited ore from primary mining (< 1.3 wt% Tb). The high demand for terbium could therefore make it worthwhile to dissolve the LAP, CAT and CBT phosphors in a final stage, but these phosphors are much more difficult to dissolve and require a lot of energy input. One such method is based on the addition of a strong acid such as H2SO4 in which LAP en CBT are dissolved, for example by using 5 to 10M H₂S04 at 75°C, preferentially for several hours, such as 2 or more hours. CAT can also be dissolved, but higher temperatures are necessary for this purpose and therefore this process might be less economic valuable whereas the above described acid solution of LAP and CAT at low temperatures (such as about 75 °C) is an economic more valuable process to extract and recover terbium.

In summary, the present invention relates to a new recycling process for lamp phosphor waste based on the use of the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet] This innovative method allows the selective dissolution of the valuable red phosphor Y203: Eu³⁺ (YOX) without leaching any other constituents of the waste powder (such as other phosphors, glass particles and alumina). A selective dissolution of YOX is useful because this phosphor holds 80 wt% of the REEs although it only represents 20 wt% of the lamp phosphor waste. The proposed recycling process is a major improvement compared to currently used hydrometallurgical processes were the non-valuable halophosphate (HALO) phosphor (Sr,Ca)io(P04)6(CI,F)2:Sb³⁺'Mn²⁺ is inevitably leached when attempting to dissolve YOX. Since the HALO phosphor can make up as much as 50 wt% of the lamp phosphor waste powder, this consumes significant amounts of acid and complicates the further processing steps. The dissolved yttrium and europium can be recovered by a stripping step using a stoichiometric amount of solid oxalic acid or by contacting the ionic liquid with a hydrochloric acid solution. Both approaches regenerate the ionic liquid, but precipitation stripping with oxalic acid has the additional advantage that there is no loss of ionic liquid to the water phase and that the yttrium/europium oxalate can be calcined as such to reform the red Y203: Eu³⁺ phosphor, effectively closing the loop after only three process steps. The red phosphor prepared from the recycled yttrium and europium showed excellent luminescent properties. The resulting recycling process for lamp phosphor waste features a selective leaching, a fast stripping and an immediate revalorization step. Combined with the mild conditions, the reusability of the ionic liquid and the fact that no additional waste water is generated, this process represents a very green and efficient alternative to traditional mineral acid leaching.

The present invention will now further be illustrated by the following non-limiting examples.

EXAMPLES

Example 1: Synthesis of [Hbet] [Tf₂N]

HbetCI (0.390 mol, 59.91 g) and LiTf₂N (0.390 mol, 111.97 g) were dissolved in water (50 mL) and stirred for 2 h at room temperature. The mixture was then allowed to phase-separate and the water phase containing LiCI was removed. [Hbet]

was then washed several times with ice water (10 mL) to remove chloride impurities until the AgN03 test was negative in the water phase after the washing step. The remaining water was removed using a rotary evaporator under reduced pressure. The yield was 79 % (0.310 mol, 123.47 g) with a chloride content below detection limit (TXRF analysis). Example 2: Dissolution tests

Small glass vials (4 ml.) were filled with a fixed amount of [Hbet][Tf₂N] (2 g), a 10 mg sample of each of the four phosphors per g of ionic liquid (4 x 10 mg/g) and also a small amount of water during some experiments (1 to 5 wt%). A magnetic stirring bar was then added to each of the vials and they were closed using a plastic screw cap. The dissolution experiments were carried out using a heating plate with a silicone oil bath, an integrated magnetic stirrer and a temperature sensor. The vials were placed in the silicone oil bath at the appropriate temperature and stirred for a certain amount of time. The vials were then placed in a centrifuge (5300 rpm, 20 min) to precipitate the undissolved phosphor powders and to obtain a clear ionic liquid. The metal content dissolved in the ionic liquid was determined using total reflection X-ray fluorescence spectroscopy (TXRF).

Example 3: Influence of water

Water-free ionic liquid was used and increasing amounts of water were then added (0, 1, 2.5, 5, 7.5 and 10 wt%). Care was taken never to exceed the solubility of water in the ionic liquid (13 wt%), because it was important to maintain one homogeneous phase during the leaching process to avoid the loss of metal ions to the water phase. A mix of YOX, HALO, BAM and LAP was added to the ionic liquid (4 x 10 mg/g) and stirred (600 rpm) at 90 °C. Water-free ionic liquid was compared with water- containing ionic liquid (1, 2.5 and 5 wt% water added). Closed vessels with screw caps were used to contain the small amounts of water at elevated temperatures. The results show that the water-free ionic liquid is unsuitable for the dissolution of YOX. The leaching is incomplete and increases only slowly with increasing time and temperature. The addition of water has a positive influence on the leaching efficiency and speed. With a water content of 5 wt% (50 pL/g) in the ionic liquid, full dissolution of YOX was observed after 24 h (Fig 1). More importantly, in these conditions very little dissolution of HALO was observed (< 0.05 wt%) and no leaching of BAM and LAP could be detected (Fig. 2).

Example 4: Influence of temperature

The influence of temperature on the leaching of YOX was investigated for the optimized ionic liquid system ([Hbet] [Tf₂N] + 5 wt% H₂0) and compared with water- free ionic liquid. In both cases a noticeable increase in leaching speed was observed with increasing temperature. A temperature of 90 °C with 5 wt% (50 μί/g) of H₂0 in the ionic liquid was chosen as the optimal system because of its high leaching efficiency and the fact that the water pressure is still manageable. A higher temperature is favorable because it accelerates the dissolution reaction and lowers the viscosity of the ionic liquid which improves the diffusion. This is important because the leaching rate of this solid material is diffusion-controlled.

Example 5: Stripping tests

Two different stripping methods were investigated. Method 1 involved the use of an aqueous HCI phase to extract the rare-earth ions from the ionic liquid to the water phase, method 2 consisted of adding pure (solid) oxalic acid directly to the ionic liquid to precipitate the rare-earth ions as rare-earth oxalates. For every stripping experiment, Y₂03: Eu³⁺ (10 mg/g) was first fully dissolved in a large batch of ionic liquid (5 wt% H2O) . Small glass vials (5 mL) were then filled with a fixed amount of this rare-earth-containing ionic liquid (1 g). For method 1, an HCI solution (1 g) was then added to the rare-earth-containing ionic liquid. This lowly viscous biphasic mixture was shaken (1500 rpm) using a mechanical shaker at a set temperature (25, 70 and 80 °C). The samples were then centrifuged (1 min, 5300 rpm) to speed up phase separation, and analyzed by TXRF to determine the remaining metal content in the ionic liquid phase. For method 2, pure (solid) oxalic acid was added directly to the ionic liquid phase. These samples were more viscous and consisted of only one phase, so they were stirred using a magnetic stirring bar and a stirring plate/oil bath set-up to control the temperature (25, 50 and 70 °C). The samples were then centrifuged (5300 rpm, 20 min) to remove the oxalate precipitate and the ionic liquid phase was analyzed with TXRF to determine the remaining metal content (Fig. 7). Example 6: Stripping with HCI

The different stripping parameters were investigated for stripping with HCI, starting with the influence of acid concentration. First, 10 mg/g of YOX was dissolved in [Hbet] [Tf2N] (5 wt% H2O) . Then, the loaded ionic liquid was contacted with aqueous solutions containing varying concentrations of HCI (1 : 1 phase ratio by mass) and shaken (1700 rpm) for 1 h at 25 °C. The remaining metal content in the ionic liquid was then analyzed using TXRF. The results showed that contacting the ionic liquid with 1 M HCI resulted in 99.6% and 99.7% stripping of Y³⁺ and Eu³⁺ respectively (Fig. 5). A sufficient amount of HCI is needed to protonate the betaine and to form the water-soluble YCI3 and EuC compounds. In this set-up, the stoichiometric HCI concentration is 0.3 M in the water phase, but in reality a slight excess of HCI is required to obtain full stripping (0.4 M). In diluted acidic conditions with an excess of rare-earth ions compared to HCI, Y³⁺ is easier to strip than Eu³⁺. However, for HCI concentrations > 0.4 M no difference was observed and the stripping process became very efficient and reproducible. An HCI concentration of 1 M was retained as the optimal stripping concentration because of the fast and reliable stripping. Example 7: Stripping with oxalic acid

The different parameters of this stripping method were investigated. First 10 mg/g of YOX was dissolved in a large batch of ionic liquid containing 5 wt% of H2O. Once the YOX was fully dissolved in the ionic liquid, separate portions were taken (1 g) and increasing amounts of pure (solid) oxalic acid were added. The samples were then stirred (600 rpm) at 70 °C for 1 h. The precipitated rare-earth oxalates were removed by centrifugation (5300 rpm, 20 min) and a sample of the ionic liquid was taken to determine the remaining metal content with TXRF analysis. The results show that a stoichiometric amount of oxalic acid compared to the amount of rare-earth ions (3 : 2) is sufficient to obtain a stripping efficiency of 100% (Fig.6). Oxalic acid does not show individual selectivity for yttrium or europium in these conditions.

Example 8: Regenerating the Υ2θ3:Ειι³⁺ phosphor

The yttrium/europium oxalate or precipitate was separated from the ionic liquid by filtration with a Buchner funnel and washed with water. The mixed oxalate was then dried in a vacuum oven at 50 °C for 12 h. The dry yttrium/europium oxalate was then placed in an oven at 950 °C for 5 h to obtain the Y₂03: Eu³⁺ phosphor. The purity, the luminescence properties, and crystal structure were verified using ICP-MS, luminescence spectroscopy and XRD respectively. The ICP-MS and XRD results confirm the successful synthesis of Y₂03: Eu³⁺ with a high purity directly from the stripping product. The luminescence properties of this material were compared with the commercially obtained Y₂03: Eu³⁺ phosphor. When the spectra are scaled to have the same intensity of the ⁵Do→ ⁷Fi transition, it is interesting to compare the intensity of the ⁵Do→ ⁷F₂ transition since this is a so-called hypersensitive transition, meaning its intensity is very dependent on its environment. It can be seen that the maximum intensity peak (⁵Do→ ⁷F∑) has the same intensity for the recycled phosphor. The luminescence lifetime of the recycled phosphor (0.989 ms) and the commercial phosphor (1.003 ms) are very similar. Analysis by scanning electron microscope (SEM) shows that the average particle size and distribution of the recycled phosphor (4.11 ± 1.41 pm) is in the same range as those of the commercial phosphor (3.6 ± 1.5 μιτι). Example 9: Recycling process

The retained recycling process consisted of three steps. First an equimolar synthetic mixture of HALO, YOX, BAM and LAP phosphors was added to the ionic liquid [Hbet][Tf2N] containing 5 wt% of water. The system was optimized for 10 mg of YOX per g of ionic liquid, which corresponds to approximately 40 to 50 mg of real lamp phosphor waste. A stirring bar was added and the samples were stirred for 24 to 48 h at 90 °C to obtain a selective and 100 % leaching of YOX. The remaining solid lamp phosphor waste was separated from the ionic liquid by centrifugation (5300 rpm, 20 min). A stoichiometric amount of pure oxalic acid was then added to the ionic liquid loaded with europium and yttrium to precipitate the rare-earth ions as oxalates and regenerate the ionic liquid. The samples were stirred for 10 min at 70 °C to obtain 100% stripping. The oxalate precipitate was separated from the ionic liquid by filtration and washed with water. The mixed (yttrium, europium) oxalate was dried in a vacuum oven at 50 °C and then calcined at 950 °C (5 h) in an oven to obtain new Y₂0₃: Eu³⁺ (YOX).

Example 10: Microwave heating

A programmable microwave oven with temperature and pressure control was used to test the influence of microwave heating on the ionic liquid

At 100 W, the ionic liquid [Hbet]

was heated to a temperature of 100 °C in less than 15 s (Fig. 8).

Claims

A method for recovering rare-earth elements from a solid mixture comprising halophosphate and at least one compound comprising one or more rare-earth element the method comprising the steps of:

(a) mixing said solid mixture in an ionic liquid,

(b) separating from the mixture of a) the ionic liquid phase from the solid phase, wherein the solid mixture comprises the halophosphate.

The method according to claim 1, wherein the compound comprising one or more rare-earth elements is red phosphor or Y203:Eu³⁺ (YOX).

The method according to claim 1 or 2, comprising additional steps: (c) wherein the ionic liquid phase resulting from step b) is contacted with oxalic acid and mixed, thereby forming a rare-earth oxalate; and (d) wherein the rare-earth oxalate containing solid phase resulting from step c) is separated from the ionic liquid phase.

The method according to claim 3, wherein the oxalic acid in step c) is added as a solid or powder form.

The method according to claim 3 or 4, wherein the solid phase separated in step

(d) comprises the rear-earth oxalate (Y_n,Eui-_n)2(C204)3, wherein n is between 0 and 1, including 0 and 1.

The method according to any of claims 3 to 5, comprising additional step (e) of calcining the solid phase resulting from step (d).

The method according to claim 6, wherein the calcined product obtained in step

(e) is YOX.

The method according to claim 1 or 2, wherein after step a) the ionic liquid phase is contacted with an aqueous acid solution, and mixed; and wherein in step b) an aqueous phase comprising a rare-earth metal is separated from the ionic liquid phase.

9. The method according to claim 8 wherein the aqueous acid solution is an HCI solution.

10. The method according to claim 8 or 9, wherein the rare-earth metal in the aqueous phase comprises the water soluble YC and/or EuC salts.

11. The method according to any of the previous claims, wherein the ionic liquid is according to the general formula :

R^XR²R³R⁴A⁺ X^",

wherein independently from each :

A is selected from the group consisting of N, P, As, and Sb;

R¹, R² and R³ are each independently selected from the group consisting of Ci-12 alkyl and C3-12 cycloalkyl; or each of R¹ and R², or R¹ and R³, or R² and R³ form a substituted or unsubstituted cyclic structure;

- R⁴ is selected from the group consisting of Ci-12 alkyl-COOH and C3-12 cycloalkyl-COOH; and

- X^" is selected from the group consisting of chloride, bromide, iodide, thiocyanate, nitrate, perchlorate, sulfate, hydrogensulfate, hydrogenphosphate, di(Ci-i2 alkyl)phosphate, organic sulfonate, organic sulfate, organic sulfonylimide and organic carboxylate.

12. The method according to claim 11, wherein each of R¹, R² and R³ are methyl

13. The method according to claim 11 or 12, wherein X^" is selected from the group consisting of fluorinated organic sulfonates, fluorinated organic sulfates, fluorinated organic sulfonylimide anions and fluorinated organic carboxylates.

14. The method according to any of claims 11 to 13, wherein X^" is a perfluoro-Ci-12 alkyl-sulfonylimide. The method according to any of claims 1 to 14, wherein the ionic liquid beta ulfonyl)imide with the structure of formula I:

(I)

16. The method according to any one of claims 1 to 15 wherein the solid mixture is a mixture comprising a red lamp phosphor and other lamp phosphors. 17. Use of an ionic liquid for separating red lamp phosphor or europium-doped yttrium oxide Y₂03: Eu³⁺ (YOX) by selective dissolution from other lamp phosphors.

18. The use according to claim 17, wherein the ionic liquid is betainium bis(trifluoromethylsulfonyl)imide.

19. The use according to claim 17 or 18 for the recovery of europium-doped yttrium oxide Y2O3 : Eu³⁺ (YOX) from the lamp phosphor powder waste fraction from end- of-life fluorescent lamps.

The use according to any of claims 17 to 19, wherein oxalic acid is used for the stripping of dissolved yttrium and/or europium from the ionic liquid comprising said rare-earth elements.