1

Recovery of an yttrium europium oxide phosphor from waste fluorescent tubes using a

Brønsted acidic ionic liquid, 1-methylimidazolium hydrogen sulfate

Nicolas Schaeffer, Xiaofan Feng, Sue Grimes*, Christopher Cheeseman

Department of Civil and Environmental Engineering, Imperial College, London SW7 2AZ

United Kingdom,

*Corresponding Author: s.grimes@imperial.ac.uk

Telephone: +44-0207-594-5966

Abstract

Background: Spent fluorescent lamps, classified as hazardous waste in the EU, are segregated

at source. Processes for the recovery of critical rare-earth (RE) elements from the phosphor

powder waste, however, often involve use of aggressive acid or alkali digestion, multi-stage

separation procedures, and production of large aqueous waste streams which require further

treatment.

Results: To overcome these difficulties phosphor powder pre-treated with dilute HCl was

leached with a 1:1 wt. [Hmim][HSO4]:H2O solution at a solid:liquid ratio of 5 %, at 80 oC for

4 h with stirring at 300rpm to recover 91.6 wt.% of the Y and 97.7 wt.% of the Eu present.

The yttrium-europium oxide (YOX), (Y0.95Eu0.05)2O3, recovered by precipitating the dissolved

RE elements from the leach solution with oxalic acid and converting the oxalate to an oxide

phase by heating, was characterised by FTIR, XRD and luminescence analysis. The analyses

suggest the recovered oxide has the potential to be directly reused as YOX phosphor.

Regeneration and reuse of the ionic liquid is achieved with only minor leaching efficiency

losses found over four leaching/recovery cycles.

Conclusion: The recovery of yttrium europium oxide from waste fluorescent tube phosphor

by a simple efficient low cost ionic liquid process has been developed.

Keywords: rare earth elements, strategic material recovery, hydrometallurgy, fluorescent

lighting phosphors, spent fluorescent lamps.

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1. Introduction

The rare earth (RE) elements (lanthanides, Y and Sc) have distinct metallurgical,

chemical, catalytic, electrical, magnetic and optical properties that are exploited in electrical

and electronic applications such as permanent magnets, rechargeable NiMH batteries, fibre

optics, light-emitting diodes (LEDs), catalysts and phosphors. Alternatives to rare earth

elements are limited and demand for these metals has increased dramatically in recent years1

and is projected to continue increasing at a rate of 3.7-8.6% per annum.2,3 The European Union

classify RE elements as critical materials for the future development of key technologies that

are subject to a high supply risk.3

Waste Electrical and Electronic Equipment (WEEE) is the fastest growing global waste

stream. It contains a variety of RE elements of which only about 1% are currently recovered

for re-use.4 Phosphor powders used in fluorescent lamps are chemically complex with some

compositional variation between manufacturers, but typically consist of a calcium

halophosphate matrix doped with RE elements (La, Ce, Eu, Gd, Tb and Y). They also contain

Al, Si, P, Ca and Ba at relatively high concentrations with Sr, Mg, Mn, Sb, Cl, F, Hg, Pb and

Cd present at trace levels.5 It is estimated that waste fluorescent lamps will contain up to 25,000

tonnes of RE elements by 2020 making waste phosphor a potential secondary source of these

critical metals.1

Spent fluorescent lamps, classified as hazardous waste in the EU, are segregated at

source through a well defined collection and treatment chain. Processing the waste typically

involves separating the Al end caps from the phosphor-coated glass, with the Hg present being

recovered by thermal desorption and distillation.6 The surface layer of the fluorescent tubes

containing the RE elements is separated from the bulk glass substrate, powdered and seived to

< 50µ to concentrate the RE elements in this size fraction. Conventional processes to solubilise

the RE elements in the phosphor powder and to recover them from solution generally involve

the use of aggressive acid or alkali digestion, multi-stage separation procedures, and the

production of large aqueous waste streams which require further treatment.7-11 Selective

recovery of RE elements from the acid or alkaline digestates can be achieved by a number of

processes12 including: precipitation, solvent extraction, ion exchange, electro-winning, and

3

chromatography13 with solvent extraction and oxalate precipitation being the two most

commonly reported methods. An acid digestion process has been used, for example, to recover

yttrium oxide from a waste phosphor powder in 20% H2SO4 at 90oC using a solid:acid w/v

ratio of 1:5 followed by precipitation of the yttrium as yttrium oxalate.

Both room temperature ionic liquids (ILs), that usually consist of organic cations and

inorganic anions with melting points below 100 oC, and deep eutectic liquids can, if properly

selected14-17, offer environmentally beneficial alternatives to both organic and inorganic

solvents in selective leaching and recovery processes. The interactions between ionic liquids

and lanthanide and actinide elements have, for example, been reviewed by Binnemans.18

Exploitation of these interactions to RE element separation and recovery processes have been

described in a number of studies including: the homogeneous liquid–liquid extraction of

Nd(III) by choline hexafluoroacetylacetonate in the ionic liquid choline

bis(trifluoromethylsulfonyl)imide19; the use of functionalized ionic liquids in the solvent

extraction of RE elements20; the selective extraction and recovery of rare earth metals from

phosphor powders from waste fluorescent lamps using an ionic liquid system21; the solid–liquid

extraction of yttrium22 and of RE elements from waste fluorescent tube phosphors on ionic

liquid impregnated resins13; use of a functionalized ionic liquid in the solvent extraction of

trivalent RE metal ions23; the use of [Hbet][Tf2N]:H2O systems in the development of a one-

step process for the separation of light from heavy rare earth elements24; the recovery of rare

earth elements from simulated fluorescent powder using bifunctional ionic liquid extractants25

and the use of the mixed IL system ([DMAH][NTf2]):[Bmim][Tf2N] to dissolve the RE-

containing mineral, bastnaesite.26

The recovery efficiency of phosphors from their thin film oxide layers present in waste

fluorescent tubes is limited7 by a number of factors including, the grade of the rare earth oxide

product, reagent costs, energy consumption, and the environmental impacts of the process used.

Conventional acid- or alkali-based methods such as that used in the sulfuric acid leaching

processes (Figure 1) are limited by their use of aggressive leaching agents, the need to use

higher temperatures and the large volumes of aqueous waste streams produced. The use of

ionic liquids in recovering rare earth oxides from phosphors has been limited by factors such

as low metal loading capacity, long leaching times and high reagent costs. The fluorinated ionic

liquids such as [Hbet][Tf2N] can, for example, be 5-20 more expensive than alternative leach

media27. We now report on the use of a low-temperature recyclable low-cost Brønsted acidic

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ionic liquid leach step to replace sulfuric acid in a typical process (Figure 1) for the recovery

of RE element phosphors from waste fluorescent tubes.

Figure 1. Typical process for recovery of Y and Eu oxides.

2. Materials and methods

2.1 Materials

Waste phosphor was obtained from a leading UK fluorescent tube recycler (Balcan

Engineering Limited, UK). The X-ray data for the as-received waste phosphor are shown in

Figure 2. The Brønsted acid ionic liquid [Hmim][HSO4] (Figure 3) was synthesised following

the method described by Chen et al.27 All chemicals used were of reagent grade purchased

from either Sigma-Aldrich or Fisher Scientific and used without further purification.

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Figure 2. XRD analysis of the as-received waste phosphor with the main crystalline phases

identified

Figure 3. Schematic of 1-methyl imidazolium hydrogen sulfate IL - [Hmim][HSO4]

2.2 Experimental Techniques

The as-received phosphor fractions were characterised by particle size analysis using a

Beckman Coulter LS-100 Series with a particle size detection range between 0.4 and 900μm,

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by X-ray fluorescence (XRF) analysis using a Bruker S4 Explorer spectrometer and by high

resolution scanning electron microscopy using a field emission gun scanning electron

microscope (FEGSEM) fitted with an Oxford Instruments INCA energy dispersive (EDS) X-

ray spectrometer (Gemini 1525, USA). The main crystalline phases in both the as-received

phosphor and in the final product of the recovery process were identified by X-ray diffraction

(XRD) using a Philips X’pert PRO PANalytical with an automatic divergence slit, a graphite

monochromator and Cu-Kα radiation and PANalytical X'Pert HighScore Plus software. Metal

analyses on phosphor leach solutions and the final recovered product were obtained, in

triplicate, using inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin

Elmer Optima 7000 DV). All sample solutions were diluted to avoid potential interference in

the analyses by the ionic liquid. The recycled ionic liquid and the recovered oxide were

characterised, by comparison with pure samples, using Fourier transform infrared (FTIR)

spectra on a Thermo Scientific Nicolet 6700 spectrometer with samples examined directly

using a Quest single reflection diamond attenuated total reflection (ATR) accessory.

Luminescence measurements on the recovered RE oxide phase in the range 350-750 nm were

performed on a Jasco FP-6000 Series spectrofluorometer equipped with a 150 W xenon lamp

with an excitation of 254 nm.

The ionic liquid-based methodology developed for the recovery of a YOX phosphor

from waste fluorescent tubes in this work involved five stages – sample characterisation and

pre-treatment; optimisation of leaching with an ionic liquid; rare-earth element concentration

as oxalates; YOX product characterisation; and ionic liquid recovery for recycle, with the

characterisation techniques used in each stage being shown in the following flow diagram:

2.3 Phosphor Pre-Treatment

XRF analysis of the as-received phosphor from waste fluorescent tubes (Table 1) has

the particle size distribution that shows that RE elements account for about 18.3 wt.% of the

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as-received waste phosphor powder with Ca, P, Si, Al and Ba arising mainly from the glass

matrix elements being the other major elements present. SEM micrographs of the as-received

phosphors indicate that the larger (≥25 µm) are composed mainly of the glass matrix (identified

by the presence of strong Si peaks in EDS analysis) whilst the smaller diameter particles

contain most of the phosphor powder (identified by the presence of strong yttrium EDS peaks)

(Figure 4). The RE elements are concentrated in the waste phosphor powder by two pre-

treatment stages: (i) removal of the > 25 µm particles from the as-received material by sieving

to leave the ≤ 25 µm fraction in which the phosphor is concentrated. The ≤ 25 µm fraction

represented only 10% of the total waste sample but contained 63% of the rare earth elements

present and for this reason the ≤ 25 µm fraction was used to demonstrate the methodology

developed in this work. The largest size fractions (>50 µm) contained only 15% of the rare

earth elements but could be further crushed to increase the mass of the ≤25 µm fraction; and

(ii) leaching the sieved powder (≤ 25 µm fraction) with 0.5M HCl. The data in Table 1 show

that (a) the ≤ 25 µm fraction of the waste phosphor has a greater Y content (17.6 %), and an

increased total RE content (26.5%) than the as-received powder, and (b) contacting the ≤ 25

µm fraction of the waste phosphor for 1 hour at room temperature with 0.5 M HCl at a

solid:liquid ratio of 1:10 with mixing at 500 rpm. The HCl treatment results in the removal of

relatively soluble glass matrix elements to give a final RE content in the treated waste phosphor

of >22%.

Table 1. Chemical analysis of the phosphor powder and percentage leached during the HCl

pre-treatment step

Concentration (wt.%)

As-received†

≤ 25 µm fraction†

≤ 25 µm fraction after

HCl pre-treatment‡

Y 11.7 17.6 26.5

Eu 1.1 1.3 1.9

La 2.6 3.5 5.4

Ce 1.2 2.4 3.7

Tb 1.1 1.2 1.9

Gd 0.6 0.5 0.8

Ca 21.2 23.8 20.9

Ba 4.5 4.4 2.5

Al 4.5 7.4 11.1

Na 1.0 1.9 2.4

Fe 1.0 0.9 0.7

Others 49.5 35.1 22.2

† XRF analysis, ‡ ICP-OES analysis

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Figure 4. SEM images of the as-received phosphor powder at a) 500× magnification and b)

2500× magnification. c) EDS spectrum of three regions in Figure 4b highlighted in white

2.4 Optimisation of waste phosphor leaching using [Hmim][HSO4]

All leaching tests were performed on the HCl pre-treated ≤ 25 µm fraction of the waste

phosphor to limit the amount of residual glass particles present. Initial leaching conditions in

50 wt.% aqueous [Hmim][HSO4] with stirring at a mixing velocity of 300 rpm were: leaching

time, 2 h, leaching temperature, 80 oC, and a solid:liquid ratio, 10%. Leaching parameters were

varied individually whilst maintaining all others at the initial test conditions to obtain the

optimum leaching conditions with respect to [Hmim][HSO4] concentration (0-100 wt.%),

temperature (20-100 oC), leach time (0.5-24 hr), and solid:liquid ratio (1:20-1:5). The leach

solutions obtained under different conditions were filtered through a 0.22 µm cellulose nitrate

filter prior to analysis of RE element content by ICP-OES. The variations in leaching

efficiencies (wt %) of Y, Eu, Tb, Ca and Al with, [Hmim][HSO4] concentration, temperature,

contact time and solid:liquid leach ratio, are in Figures 5a-d respectively.

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Figure 5. Leaching efficiency of Y, Eu, Tb, Ca and Al as a function of (a) [Hmim][HSO4]

concentration, (b) temperature, (c) contact time and (d) solid:lquid ratio.

2.5 RE oxide recovery from aqueous [Hmim][HSO4] leachate by oxalic acid precipitation

followed by conversion to oxide

The recovery of REEs from the optimised aqueous [Hmim][HSO4] leach solution by

oxalic acid precipitation was optimised (Figures 6a-d) with respect to temperature (20-60 oC),

contact time (15-45 min), pH (0.9-1.4) and oxalic acid:REE molar ratio (0.8-1.75). The leachate

pH was adjusted using a 2 M NH3 solution. The final product from the oxalate precipitation

was filtered off, washed with deionised water and dried at 105 oC before conversion to a RE

oxide phase by raising the temperature of the solid to 650 oC for 1 hour. The final oxide product

was washed with deionized water and dried at 105 oC, and characterised using ICP-OES, FTIR,

XRD and luminescence measurements.

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Figure 6. The recovery efficiency of the main leached element as a function of (a) temperature,

(b) contact time, (c) solution pH, and d) OA:REE molar ratio.

2.6 [Hmim][HSO4] recycling and re-use over four leaching and recovery cycles

Following the optimised leaching and recovery process, the aqueous IL solution was

diluted with 5 times the quantity of distilled water and shaken with an excess of XAD-DEHPA

solvent impregnated resin for one hour to extract the non-precipitated metal components onto

the resin28. After filtering off the solvent impregnated resin, [Hmim][HSO4] was recovered by

evaporating the water present under vacuum at 60 oC for 4 hr. The recovered [Hmim][HSO4]

was then re-used in a further three cycles of recovering RE oxides from the phosphor in the

optimised leaching process.

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3. Results and Discussion

3.1. Pre-treatment

The main crystalline components identified by XRD (Figure 2) in the as-received

phosphors in addition to the yttrium europium oxide ((Y0.95Eu0.05)2O3) known to be present are

fluorapatite (Ca5(PO4)3F), cerium phosphate (CePO4) and barium silicate (BaSi2O5), and

consistent 8-9,21 with Y and Eu being present in the phosphors as oxide phases and Ce, Gd and

Tb being present as phosphates.

Removal of the large particles (>25µm) from the as-received phosphor followed by

leaching with dilute hydrochloric acid results in the concentration of the Y present in the waste

phosphor from 11.7 to 17.6 and finally to 26.5%,. and of the other REEs from 6.6 to 8.9 and

finally to 13.7% (Table 1). Leaching of the ≤ 25µm fraction with 0.5M HCl further removes

35.5 wt.% of the non-RE element content including over 40% of the Ca (the main component

of the <25µm fraction) present while leaching out only 2.8 wt.% of the RE elements (Table 1).

3.2. Optimisation of the [Hmim][HSO4]:H2O phosphor leaching process

The data in Figures 5 (a-d) showing the variations in the solubility of Y, Eu, Tb, Ca,

and Al in the pre-treated phosphor waste with [Hmim][HSO4] concentration, temperature,

phosphor:leach solution ratio, and contact time, have been used to determine the optimum leach

conditions for the recovery of RE elements from the phosphors.

The solubilities of both of Y and Eu increase with increasing [Hmim][HSO4]

concentration in the aqueous leach solution to reach a maximum at 1:1 [Hmim][HSO4]:H2O

wt% (Figure 5a). In addition to the lower solubilities of the target elements Y and Eu in

solutions containing higher percentages of [Hmim][HSO4], the viscosities of the solutions

increase making them less suitable leach media - the viscosity of the 1:1 [Hmim][HSO4]:H2O

mixture at 20 oC (4.2 cP), for example, increases to 6.9 cP for a 98 wt.% [Hmim][HSO4]

solution. The efficiency of the leaching of Y and Eu increases by 50.6 wt.% and 73.7 wt.%

respectively as the temperature is increased from 22 oC to a maximum at about 80 oC (Figure

5b); and the leaching of both Y and Eu from phosphor wastes increases with time up to about

4 hours (Figure 5c). Higher temperatures are required to overcome the low solubility of RE

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phosphates present in the phosphor and at 100 oC, 36.0 wt% of Gd, 3.7 wt.% of Tb, 2.1 wt.%

of Ce and 0.14 wt.% of La are leached into the aqueous [Hmim][HSO4] solution. The recovery

of Eu (92.4%) and Y (81.4%) is high when phosphor:liquid ratios of 5% are used but decreases

with increasing solid: liquid ratio (Figure 5d) to 46.5 and 32% respectively with a solid:liquid

ratio of 1:5.

The results show that the optimum conditions for the use of [Hmim][HSO4]:H2O mixtures to

leach the rare earth elements Y and Eu from treated phosphor samples are: use of a 1:1 wt%.

[Hmim][HSO4]:H2O solution at 80 oC for 4 hours with a solid: liquid ratio of 5 %, and a mixing

speed of 300 rpm. Under the optimised leaching conditions, high recoveries of Y of 92% and

of Eu of 98% are achieved (Table 2).

Table 2. Percentage leached of elements present in the phosphor powder under the optimised

[Hmim][HSO4] leach conditions

Element wt.% leached

Y 91.6 ± 2.06

Eu 97.7 ± 2.23

Gd 39.5 ± 0.81

Tb 4.05 ± 0.13

Ce 2.40 ± 0.07

La 0.20 ± 0.01

3.3. Optimisation of Y and Eu recovery by oxalic acid precipitation followed by conversion

to oxide.

RE elements can be selectively recovered from acid or ionic liquid solutions by

precipitation with oxalic acid. Use of solid oxalic acid allows for (i) an efficient one-step in-

situ recovery process, (ii) recovery of [Hmim][HSO4] for re-use, and, (iii) easy conversion of

the RE oxalate product to an oxide phase by calcination. The data in Figure 6 show that the

optimum conditions for the precipitation of rare earth elements from the leach solutions with

oxalic acid are: use of a slightly greater than stoichiometric oxalic acid: RE ratio of 1.6:1; at

40oC and pH 0.9 with a contact time of 0.5h. The product from the oxalate precipitation was

filtered off, washed with deionised water and dried at 105 oC before conversion to an RE oxide

13

phase by calcination at 650 oC to give an oxide product with an average particle diameter of

1.4 ± 0.14 µm that was characterised by, XRD and photoluminescence analysis.

3.4. Characterisation of the recovered oxide product

The recovered oxide product has been characterised using ICP-OES, FTIR, XRD and

luminescence measurements.

The XRD of the recovered product (Figure 7) is consistent with (Y0.95Eu0.05)2O3. This

is chemically similar to the most widely used YOX phosphor for the emission of red light

(Y2O3:Eu3+). The FTIR spectrum shows the recovered oxide product to be identical to a

commercially YOX phosphor, with the characteristic peaks at 555, 461 and 415 cm-1.

Figure 7. XRD analysis of the recovered oxide product after calcination

14

The luminescence spectrum of the recovered powder under 254 nm wavelength

excitation (Figure 8) exhibits the optical properties of Eu3+ ions in a cubic Y2O3 host structure29-

31. The emission spectrum displays the characteristic 5D0 7Fj (j=0, 1, 2, 3) and 5D1 7F0 of

Eu3+ transitions. The high intensity red hypersensitive 5D0 7F2 transition at 612 nm is the

dominant emission with the 5D0 7F3, 5D0 7F1,

5D0 7F0, and 5D1 7F0 emission peaks

located at 631 nm, 587 – 600 nm, 582 nm and 533-539nm respectively The SEM image in

Figure 8 shows the final oxide product of the process under excitation at 254 nm. The

luminescent life-time of the sample was measured by following the decay of the 612 nm

emission for 25 ms after excitation at 254 nm. The resulting decay curves were fitted with a

monoexponential decay equation (R2=0.99)31 to give: It = 8.97et/0.72, where It is the intensity at

time t (ms), 8.97 the intensity at t = 0 (I0) and 0.72 ms is the luminescence lifetime (τ). The

quality of the luminescence spectrum and luminescence lifetime (0.72 ms) of the recovered

oxide product confirms that it has potential for direct re-use as YOX phosphor.

Figure 8. Luminescence spectra of the recovered powder under 254 nm wavelength excitation.

Inset photograph: Recovered oxide product under 254 nm light.

3.5. [Hmim][HSO4] recycling and reuse and a process flow

To be considered as a viable economic and environmental alternative to inorganic acids

such as H2SO4, [Hmim][HSO4] must possess a high selectivity and extraction capacity for Y

15

and Eu, incur minimal losses and possess excellent chemical stability. To assess the stability

of [Hmim][HSO4], the IL was recycled and re-used over four leaching and recovery cycles

under optimised conditions and the results are presented in Figure 9.

Figure 9. FTIR spectra of virgin and four time recycled [Hmim][HSO4]

Leaching of the REEs decreased slightly over the four leaching/stripping cycles with

decreases of 1.3 wt.% for Y, 5.6 wt.% for Eu and 11.3 wt.% for Gd. Complete removal of all

added oxalic acid is ,however, essential prior to re-use as it has a detrimental effect on the

leaching efficiency of RE oxides. The small decrease in Eu and Gd leaching efficiency is most

likely due to (1) a lower proton content in [Hmim][HSO4] and/or (2) partial saturation of the

IL. This is easily remediated by addition of H2SO4 in [Hmim][HSO4] to compensate for the

loss of H+ ions.

The recovery of yttrium europium oxide (Y0.95Eu0.05)2O3 from waste fluorescent tube

phosphor by a simple 4-step efficient low cost ionic liquid process, that includes recycle and

reuse of the ionic liquid [Hmim][HSO4] is presented in Figure 10.

Recently, Chen and co-workers27 demonstrated that the Brønsted acidic IL, 1-

methylimidazolium hydrogen sulfate ([Hmim][HSO4]) can be produced on an industrial scale

at prices between $2.96 and $5.88 kg−1 and it therefore provides a less expensive material for

16

use as a solvent particularly if it can be used in closed cycle applications like the recovery of

the YOX phosphor.

Figure 10. Schematic diagram of a process for the recovery of (Y0.95Eu0.05)2O3 from waste

fluorescent tubes using [Hmim][HSO4]

4. Conclusions

The recovery of yttrium europium oxide [(Y0.95Eu0.05)2O3] from waste fluorescent tube

phosphor by a simple efficient low cost IL process is described; a process which replaces a

sulfuric acid leach in the standard process. The optimum conditions for solubilising RE oxides

from acid pre-treated phosphor powder involves leaching with a 1: 1 wt. [Hmim][HSO4]:H2O

solution at a solid: liquid ratio of 5 %, 300 rpm, at 80 oC for 4 h. The percentage Y, Eu and Ca

leached under these conditions is 91.6 wt. %, 97.7 wt.% and 24.9 wt.% respectively. The REEs

in the leachate were precipitated by addition of oxalic acid at a molar ratio (OA: REE) of 1.5,

a stripping temperature of 60 oC, mixing time of 15 minutes and a solution pH of 0.9. Calcining

the oxalates at 650 oC for 1 h gives an 86.0 wt.% mixed rare earth oxide (Y0.95Eu0.05)2O3

recovered product with Ca being the major residual impurity. Luminescence analysis indicates

that the recovered yttrium europium oxide has the potential to be directly reused as YOX

phosphor. The replacement of the sulfuric acid leach stage with the ionic liquid [Hmim][HSO4]

leach has the following process advantages; it avoids both the use of aggressive acid leaching

that may also involve high temperatures and the production of large volumes of aqueous waste

water. The economics of the ionic liquid process are also favourable because [Hmim][HSO4]

17

can now be synthesised in bulk at less than $6/kg27 and can be recovered for reuse in the process

over at least four recycle steps.

Acknowledgement

We wish to acknowledge an EPSRC DTA Scholarship for N.S. and the EPSRC UK National

Mass Spectrometry Facility at Swansea University, United Kingdom, for mass spectrometry

analysis. The authors would like to thank the Chemistry Department at Warwick University for

performing the luminescence analysis.

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