minerals
Article
Recycling Y and Eu from Waste Fluorescent Powder
and High Temperature Solid-State Synthesis of
Y2O3:Eu Phosphors
Xiaodong Chen 1 , Nian Liu 1 , Guangjun Mei 1,2, * and Mingming Yu 1
1
2
*
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070,
China; [email protected] (X.C.); [email protected] (N.L.); [email protected] (M.Y.)
School of Chemistry and Life Science, Hubei University of Education, Wuhan 430205, China
Correspondence: [email protected]; Tel.: +86-27-87882128
Academic Editor: Radostina G. Atanassova
Received: 11 January 2017; Accepted: 15 March 2017; Published: 20 March 2017
Abstract: Y2 O3 :Eu were prepared through precursors synthesized by leaching tests, removing
impurities, enrichment of Y and Eu from residual purified liquors, annealing treatment, and high
temperature solid-state reaction method, which is the most suitable for large-scale production.
The analysis of product shows that the purity is 99.42%. The resultant powders were characterized by
X-ray diffraction (XRD), differential thermal analysis (TG-DTA), scanning electron microscope (SEM),
and photoluminescence (PL). Compared with the commercial phosphors, the XRD spectrum of the
product samples revealed the synthesized particles to have a pure cubic Y2 O3 :Eu structure without
any impurities in the crystalline phase. On the morphology, the Y2 O3 :Eu particles synthesized by a
combustion and high temperature solid state process with sintering aids, were large and uniform.
For luminescence property, the emission intensity of Y2 O3 :Eu phosphors synthesized by combustion
process and high temperature solid state process with sintering aids were higher than those without
sintering aids, at 1400 ◦ C.
Keywords: waste fluorescent powder; leaching; impurities removal; synthesis; Y2 O3 :Eu particles
1. Introduction
Yttrium oxide doped with trivalent europium (Y2 O3 :Eu) phosphor—which have the
characteristics of optics, electricity, and magnetism—are unsurpassed and non-renewable strategic
resources, and have been extensively used in the field of electronic information, energy environment,
petrochemical, and metallurgical machinery, especially in rare earth luminescent materials. With the
popularization of fluorescent lamps, computers, mobile phones, and other related products, pollutive
waste fluorescent powder is sharply increasing. According to statistics, as much as eight thousand tons
of waste fluorescent powders were produced in China in 2010; additionally, according to the current
market value, rare earth resources in waste fluorescent powder are estimated to be about four billion
RMB in value [1]. If the waste fluorescent powders were recycled efficiently, not only could it reduce
the dwindling quantity of rare earth mineral, but it can also be fused to the industrial chain of rare
earth recovery, which could greatly improve the utilization of rare earth resources.
In recent years, recycling of rare earth elements from waste fluorescent phosphors has been
researched by many foreign and domestic researchers. Tsuyoshi Hirajima et al. [2] used collecting
agents and dispersants to separate rare earth trichromatic fluorescent phosphors by flotation method.
Hirajima et al. [3] used diazomethane as the layered medium and sodium oleate as a surfactant to
recycle rare earth fluorescent phosphors by centrifugal separation method. Akira Otsuki et al. [4]
adopted two-step liquid–liquid extraction methods to separate red, green, and blue rare earth
Minerals 2017, 7, 44; doi:10.3390/min7030044
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Minerals 2017, 7, 44
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fluorescent phosphors. Youming Yang et al. [5] used hydrochloric acid and hydrogen peroxide to leach
red powders, and adopted the sodium carbonate roasting method to leach green and blue phosphors.
Guangjun Mei et al. [6] recovered Y2 O3 from waste fluorescent powders by the process of leaching,
precipitation, and roasting. Hongmin Cui et al. [7] utilized Aliquat 336 functionalized chelating
adsorbent derived from chitosan to enrich and separate Y(III) from diluted solutions. However, rare
earth elements are not effectively retrieved by these methods. The above methods are used with
mixing power which has pure red, green, and blue phosphors, but waste fluorescent powder in China
is obtained by the crushing method.
For the synthesis of Y2 O3 :Eu nanoparticles, several processes such as the gas-phase condensation
technique, sol–gel route, polyol method, spray pyrolysis, and the hydrothermal method were studied
by researchers [8]. Jitao Chen et al. [9] reported the self-assembled Y2 O3 :Eu3+ 3D architectures
synthesized by a simple hydrothermal process, followed by calcination treatment. They found that
some of the nano lamellas wrinkled into the petals of flowerlike architectures, which may be potentially
applied in optoelectronics due to their strong red emission. Guowei Chen et al. [10] synthesized
Y2 O3 :Eu phosphor powder via a facile Pechini sol–gel method using pluronic F127 as a template agent.
Under the excitation of 245 nm UV radiation, the mesoporous Y2 O3 :Eu particles show stronger PL
intensity than that of particles without special structure, due to the advantage of surface area and
pores. Arunkumar Paulraj et al. [11] compared the hydrothermal method with the combustion method
and solid-state metathesis reaction.
In this paper, recycling and synthesis of high-purity rare earth oxides (Y2 O3 :Eu) from waste
fluorescent powder were studied. Compared to previous work, the innovation of this work is
that the purification step was added to the process, with ammonia, sodium sulfide, and Sodium
diethyldithiocarbamate trihydrate (DDTC), before precipitation with oxalic acid, which will reduce
the amount of oxalic acid. After annealing of the precursor at 1400 ◦ C, we obtained the materials of
Y2 O3 :Eu, which have excellent luminescent properties. The products were characterized by X-ray
diffraction (XRD), scanning electron microscope (SEM), and fluorescence spectrometer.
2. Experiment Procedure
2.1. Materials
The waste fluorescent powder was provided by a Chinese company which recycles and
reprocesses these types of wastes. All the reagents used in the present study were analytical-grade
materials. Hydrochloric acid and hydrogen peroxide (30%) were used as leaching reagents. Ammonia
was used to adjust pH of the leach liquors, and sodium sulfide and DDTC were used to remove heavy
metals such as Al, Fe, Pb, Zn, and Cu. Oxalic acid was used to precipitate the precursor. For the
synthesis of Y2 O3 :Eu nanoparticles, different proportions of NaF, boric acid, and sodium carbonate
were doped with the precursor.
2.2. Procedure
2.2.1. Leaching Tests
Leaching tests were performed with hydrochloric acid and hydrogen peroxide. Different leaching
tests of reaction conditions on the hydrochloric acid concentrations (1.0 mol/L, 2.0 mol/L, 3.0 mol/L,
4.0 mol/L, 5.0 mol/L), the amount of hydrogen peroxide (0.0 mL/g, 0.1 mL/g, 0.2 mL/g, 0.3 mL/g,
0.4 mL/g), the leaching temperature (25 ◦ C, 40 ◦ C, 60 ◦ C, 75 ◦ C, 90 ◦ C) and the leaching time (1 h, 2 h,
3 h, 4 h, 5 h) were compared, and the optimum condition is determined as: 4.0 mol/L of hydrochloric
acid, 0.2 mL/g of hydrogen peroxide, 7.5 mL/g liquid–solid ratio, 60 ◦ C, and 4 h of reaction.
Minerals 2017, 7, 44
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2.2.2. Impurity Removals
Impurity removals were carried out by using ammonia (1:1), sodium sulfide (0.5 mol/L solution),
and DDTC. These experiments were operated to precipitate the majority of impurities dissolved,
especially heavy metals. After adding ammonia into the leaching solutions and adjusting pH at the
range of 2–6, sodium sulfide was added to remove heavy metals such as Al, Fe, Pb, and Zn.
2.2.3. Rare Earth Element (REE) Enrichment
Rare earth elements were precipitated with oxalic acid (100 g/L), and more oxalic acid was used
to react with other residual elements.
2.2.4. Annealing Process
The oxalates were annealed at the suitable temperature, and the characteristics of the rare earth
oxide were analyzed with XRD, differential thermal analysis (TG-DTA), and SEM.
2.2.5. High Temperature Solid-State
Different proportions of sintering aids (NaF, boric acid, and sodium carbonate) were doped into
the precursor, then placed in an agate mortar and grounded to a certain particle size. After that,
the mixed powder was calcined at 1400 ◦ C for 5 h in a vacuum. Lastly, the product was chartered with
XRD, SEM, and Fluorescence Spectrometer.
3. Results and Discussion
3.1. Leaching Tests
The component analysis of waste fluorescent powder is listed in Table 1. Table 1 shows that
SiO2 and Al2 O3 are found to be the predominant impurities (23.25% of SiO2 , 22.79% of Al2 O3 );
the proportion of rare earth oxides (Y2 O3 , Eu2 O3 , CeO2 , Tb4 O7 ) reached 25.1%; and they have high
recycling value. In order to improve the leaching rate of Y and Eu, the effect of HCl concentration,
the addition of hydrogen peroxide, the leaching temperature, and time have been discussed, and the
results are shown in Figures 1–4, respectively.
Table 1. Main chemical composition of the waste fluorescent powder (wt %).
Component
SiO2
Al2 O3
Y2 O3
CaO
P2 O5
BaO
Na2 O
MgO
Eu2 O3
CeO2
Tb4 O7
Content (%)
Component
Content (%)
23.25
PbO
0.92
22.79
SrO
0.70
20.43
Fe2 O3
0.60
9.06
Cl
0.15
8.11
MnO
0.17
2.67
SO3
0.09
2.79
CuO
0.08
1.86
Sb2 O3
0.09
2.06
ZnO
0.05
1.60
WO3
0.055
1.01
-
The HCl concentration plays an important role in the leaching test. It is clearly shown from
Figure 1 that by increasing the concentration of hydrochloric acid, the leaching rate of Y and Eu greatly
improved. During the leaching test, the HCl in the solid–liquid interface was first to be consumed,
resulting in a great increase of the concentration gradient of hydrochloric acid in the solution, and
speeding up the rate of diffusion of hydrochloric acid to solid interface. When the concentration of
HCl was 4 mol/L, the leaching rate of Y and Eu reached 99.48% and 88.02%, respectively, so there
was no need to increase the concentration of HCl continuously. Figure 2 shows that adding hydrogen
peroxide into the solution has a great influence on the leaching rate of rare earth, especially Eu.
Hydrogen peroxide can improve the activation energy of the reaction in the solution and accelerate
the solid–liquid chemical reaction rate. The leaching rate of Y and Eu achieved 99.48% and 88.02%,
respectively, at the hydrogen peroxide amount of 0.2 mL/g. At higher concentrations of hydrogen
peroxide, the leaching solution generated a dense oxide film on the surface of the reactants, hindering
the reaction; thus, the amount of hydrogen peroxide was ultimately determined to be 0.2 mL/g.
Minerals 2017, 7, 44
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enough2017,
time
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diffuse to the inside of REE particles. The HCl solution reacted with phosphor
4 of 13
completely under the condition of 4 h leaching time.
enough time to diffuse to the inside of REE particles. The HCl solution reacted with phosphor
completely under the condition of 4 h leaching time.
Figure 1. Effect of HCl concentration on the rare earth element (REE) leaching test (0.2 mL/g hydrogen
Figure 1. Effect of HCl concentration on the rare earth element (REE) leaching test (0.2 mL/g hydrogen
peroxide,60
60◦°C
leaching temperature,
temperature,44hhleaching
leachingtime).
time).
peroxide,
C of
leaching
Figure 1. Effect
HCl concentration on
the rare earth
element (REE) leaching test (0.2 mL/g hydrogen
peroxide, 60 °C leaching temperature, 4 h leaching time).
Figure 2. Effect of the addition of hydrogen peroxide on the REE leaching test (4 mol/L HCl
concentration,
60 of
°C the
leaching
temperature,
4 h leaching
time).
Figure
2. Effect
addition
of hydrogen
peroxide
on the REE leaching test (4 mol/L HCl
Figure 2. Effect of the addition of hydrogen peroxide on the REE leaching test (4 mol/L HCl
◦
concentration, 60 C leaching temperature, 4 h leaching time).
concentration, 60 °C leaching temperature, 4 h leaching time).
Figure 3. Effect of leaching temperature on the REE leaching test (4 mol/L HCl concentration, 0.2 mL/g
hydrogen peroxide, 4 h leaching time).
Figure 3. Effect of leaching temperature on the REE leaching test (4 mol/L HCl concentration, 0.2 mL/g
Figure 3. Effect of leaching temperature on the REE leaching test (4 mol/L HCl concentration, 0.2 mL/g
hydrogen peroxide, 4 h leaching time).
hydrogen peroxide, 4 h leaching time).
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Figure 4. Effect of leaching time on the REE leaching test (4 mol/L HCl concentration, 0.2 mL/g
Figure 4. Effect of leaching time on the REE leaching test (4 mol/L HCl concentration, 0.2 mL/g
hydrogen peroxide, 60 ◦°C leaching temperature).
hydrogen peroxide, 60 C leaching temperature).
The waste fluorescent phosphors were leached in the optimum condition (4.0 mol/L of
To obtainacid,
the best
leaching
rate of Y peroxide,
and Eu, it7.5
is very
to find
suitable
leaching
hydrochloric
0.2 mL/g
of hydrogen
mL/gimportant
liquid–solid
ratio,a 60
°C, and
4 h of
temperature
and
leaching
time.
As
shown
in
Figure
3,
raising
the
reaction
temperature
can
promote
reaction). Table 2 lists the dissolved yields of the elements by leaching 20 g waste fluorescent phosphors.
hydrochloric
acid that
to diffuse
and penetrate
to the phosphor
lattice,
andinaccelerate
It is clearly shown
the selected
leaching conditions
are very
efficient
dissolvingthe
redsolid–liquid
phosphors,
chemical
reaction
rate.
Simultaneously,
while
increasing
the
leaching
temperature,
the
thermal
motion
and that the leaching rates of Y and Eu were 99.54% and 88.61%, respectively, but the green and
blue
of
molecules
in
solution
speeded
up.
It
resulted
in
a
corresponding
reduction
in
solution
viscosity
and
phosphors were hardly dissolved. There were also some heavy metals that needed to be removed.
reduced
the
diffusion
resistance
of
soluble
substances.
The
optimal
leaching
temperature
was
kept
at
The following reaction may have occurred in the leaching process:
60 ◦ C. Figure 4 presents that when the leaching time was 1 h, the leaching rate of Y and Eu were only
Y2Oto
3:Eu + 6H+ → 2Y3+ + Eu3+ + 3H2O
60.52% and 23.11%, respectively, due
the leaching solution not having enough time to diffuse to (1)
the
+
2+
3+
2+
−
3−
inside of REECa
particles.
The
HCl
solution
reacted
with
phosphor
completely
under
the
condition
of
4h
10 (PO4)6(F,Cl):Sb,Mn + 42H → 10Ca + Sb + Mn + 6PO4 + F + 21H2O
(2)
leaching time.
Al2O3 + 6H+ → Al3+ + 3H2O
(3)
The waste fluorescent phosphors were leached in the optimum condition (4.0 mol/L of
◦
3+ + mL/g
hydrochloric acid, 0.2 mL/g of hydrogen
4h
Fe2O3 +peroxide,
6H+ → Fe7.5
3H2O liquid–solid ratio, 60 C, and (4)
of reaction). Table 2 lists the dissolved yields of
the elements by leaching 20 g waste fluorescent
PbO + 2H+ → Pb2+ + H2O
(5)
phosphors. It is clearly shown that the selected leaching conditions are very efficient in dissolving
red phosphors, and that the leaching rates of Y and Eu were 99.54% and 88.61%, respectively, but the
2. Inductively
coupled
emission
spectroscopy
of the metals
dissolved
solution
greenTable
and blue
phosphors
were plasma
hardly (ICP)
dissolved.
There
were alsoanalysis
some heavy
that
needed to
by
leaching
20
g
waste
fluorescent
powder.
be removed. The following reaction may have occurred in the leaching process:
Element
Y
Eu
Ce
Tb
Ca
Al
Fe
Sr
Sb
Ba
Y2 O3 :Eu + 6H+ → 2Y3+ + Eu3+ + 3H2 O
(1)
Yield (mg) 3919 281.45
0
0.288 1643 86.72 35.32 19.75 11.90 19.55
Element
Mn + 42H
W + → 10Ca
Zn 2+ +Pb
Ca10 (PO4 )6Mg
(F,Cl):Sb,Mn
Sb3+ +Cu
Mn2+ +Na
6PO4 3− K+ F− + Si
21H2 O (2)
Yield (mg) 14.52 11.18 1.446 4.217 6.311 6.22 34.45 14.15 22.31
Al2 O3 + 6H+ → Al3+ + 3H2 O
(3)
3.2. Purification of Leaching Solutions
Fe2 O3 + 6H+ → Fe3+ + 3H2 O
(4)
The purification process can be separated +into two
steps:
adjustment
of
the
leach
solutions
with
PbO + 2H → Pb2+ + H2 O
(5)
ammonia to remove aluminums and iron, and purification of heavy metals with sodium sulfide and
DDTC.
Table 2. Inductively coupled plasma (ICP) emission spectroscopy analysis of the dissolved solution by
20 g Removal
waste fluorescent
powder.
3.2.1.leaching
Impurities
with Ammonia
ItElement
is found that
containsTba lot of
(Al,FeFe, Pb,SrZn, etc.),
that the
Y the leachate
Eu
Ce
Caimpurities
Al
Sb and Ba
3+ and Fe3+. Figure 5 shows that Al3+ and Fe3+, which
neutralization
method
can
be
used
to
react
with
Al
Yield (mg)
3919
281.45
0
0.288
1643
86.72
35.32
19.75
11.90
19.55
Element
Mg when
Mnthe pHWwas 3.08
Zn and 1.62,
Pb respectively,
Cu
Nacan be Kremoved
Si effectively
started
to hydrolyze
by
Yield (mg)
11.18
1.446
6.22
34.45
14.15
22.31
adjusting
the pH 14.52
of leaching
solution.
The4.217
Al(OH)6.311
3 and Fe(OH)3 generated were stable and not easy
to dissolve because of their large solubility product constant; however, the pH cannot be too high,
otherwise it may contribute to the loss of rare earth metals.
Minerals 2017, 7, 44
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3.2. Purification of Leaching Solutions
The purification process can be separated into two steps: adjustment of the leach solutions with
ammonia to remove aluminums and iron, and purification of heavy metals with sodium sulfide
and DDTC.
3.2.1. Impurities Removal with Ammonia
It is found that the leachate contains a lot of impurities (Al, Fe, Pb, Zn, etc.), and that the
neutralization method can be used to react with Al3+ and Fe3+ . Figure 5 shows that Al3+ and Fe3+ ,
which started to hydrolyze when the pH was 3.08 and 1.62, respectively, can be removed effectively
by adjusting the pH of leaching solution. The Al(OH)3 and Fe(OH)3 generated were stable and not
easy to dissolve because of their large solubility product constant; however, the pH cannot be too high,
otherwise
Minerals 2017,it7,may
44 contribute to the loss of rare earth metals.
6 of 13
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Figure 5.
5. Relationship between
equilibrium concentration
concentration of some
some metal ions
ions and pH.
pH.
Figure
Figure 5. Relationship
Relationship between
between equilibrium
equilibrium concentration of
of some metal
metal ions and
and pH.
100
100
100
100
80
80
80
80
Al
Al
Fe
Fe earth
rare
rare earth
60
60
60
60
40
40
40
40
20
20
20
20
0
0
2
2
3
3
4
4
pH
pH
5
5
6
6
loss
rare
earth
loss
of of
rare
earth
rate
and
removal
rate
of ofAlAl
and
FeFe
removal
Figure 6 reports the relationship between pH and the yields of impurities and rare earth metals.
Figure 66 reports
reports the
the relationship
Figure
between pH and the yields of impurities and rare earth metals.
3+ and Fe3+ were precipitated to some degree, and there existed the loss
With the adjustment of pH, Al3+
3+ were
Withthe
theadjustment
adjustmentof
ofpH,
pH,Al
Al3+ and Fe3+
existed the
the loss
loss
With
were precipitated
precipitated to some degree, and there existed
of rare earth elements (≥30%) at all conditions in these experiments. When pH was adjusted to 5.0,
of rare
rare earth
earth elements
elements ((≥30%)
of
≥30%) at
at all conditions in these experiments. When
When pH was adjusted to 5.0,
the impurities had been removed completely and the loss of rare earth metals was minimized. With
the impurities
impurities had
had been
been removed
removed completely
completely and
and the
the loss
loss of
of rare
rare earth
earth metals
metals was
was minimized.
minimized. With
With
the
the pH reaching 6.0, the rare earth elements almost completely precipitated.
the
pH
reaching
6.0,
the
rare
earth
elements
almost
completely
precipitated.
the pH reaching 6.0, the rare earth elements almost completely precipitated.
0
0
Figure 6. Effects of pH on the yields of impurities and rare earth metals.
Figure 6. Effects of pH on the yields of impurities and rare earth metals.
Figure 6. Effects of pH on the yields of impurities and rare earth metals.
3.2.2. Purification of Heavy Metals with Sodium Sulfide and DDTC
3.2.2. Purification of Heavy Metals with Sodium Sulfide and DDTC
and Fe3+, there was still a mass of
Although the neutralization method had precipitated Al3+
of
Although the neutralization method had precipitated Al3+ and Fe3+, there was still a mass 2+
heavy metal elements that needed to be removed in the leaching solutions. Figure 7 shows that Cu 2+,
heavy
metal
elements
that
needed
to
be
removed
in
the
leaching
solutions.
Figure
7
shows
that
Cu
,
Pb2+
, and Zn2+ could be precipitated with sodium sulfide. We can conclude that the final pH of the
Pb2+, and Zn2+ could be precipitated with sodium sulfide. We can conclude that the final pH of the
leaching solution could be controlled to 5.0, and that the addition of ammonia and sodium sulfide
leaching solution could be controlled to 5.0, and that the addition of ammonia and sodium sulfide
Minerals 2017, 7, 44
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3.2.2. Purification of Heavy Metals with Sodium Sulfide and DDTC
Although the neutralization method had precipitated Al3+ and Fe3+ , there was still a mass of
heavy metal elements that needed to be removed in the leaching solutions. Figure 7 shows that Cu2+ ,
Pb2+ , and Zn2+ could be precipitated with sodium sulfide. We can conclude that the final pH of the
leaching solution could be controlled to 5.0, and that the addition of ammonia and sodium sulfide
should be added in an appropriate proportion. After pH adjustment, sodium sulfide was directly
added
Minerals into
2017,the
7, 44solutions to control pH up to 5.0.
7 of 13
Figure 7.
7. Relationship
Relationship between
between the
the residual
residual concentration
concentration of
of metal
metal ions
ions and
and pH.
pH.
Figure
Table 3 reports that Cu2+, Pb2+, Zn2+, and Sb2+ reacted at a large extent. The best way to remove
DDTC is an excellent collector for heavy metal ions. The sulfur atom in the active group N-CS2
impurities was adjusting pH to 4.0 with ammonia, and adding sodium sulfide and DDTC to control
can capture the heavy metal ions from the solution to generate water-insoluble salt precipitates,
pH up to 5.0.
which is called DDTC. The generated DDTC has a strong flocculating effect. Thus, in the case of low
concentration Table
of heavy
metal
ions
in the solution, adding appropriate amounts of DDTC into the
3. Effect
of Na
2S after adding ammonia (R. = Rate of impurities removal).
sulfide precipitation should be taken into consideration.
pH that Cu2+ , Pb2+Pb
R.2+(%)
(%) extent. The best
Cu R.
(%)to remove
Table 3 reports
, Zn
, and Sb2+ reacted Zn
at aR.large
way
3.0
98.37
0.164
100
impurities was adjusting pH to 4.0 with ammonia, and adding sodium sulfide and DDTC to control
98.18
0.172
100
pH up to 5.0. 4.0
4.5
92.81
0.175
100
5.0
0 adding ammonia (R. = Rate
0 of impurities removal).0
Table 3. Effect of Na2 S after
1200
RE3+ + PO4 3− → REPO4 ↓
(6)
0
Y
Y
E
Y
E
Y
300
E
Y
600
Y
E
E
900
Y
E
Intensity(a.u.)
Y
pH of rare earth, the
Pbdark
R. (%)
Znwere
R. (%)
R.and
(%) XRD. Table 4
Due to the loss
solid precipitates
analyzed with Cu
XRF
and Figure 8 show
process of removing impurities.
The rare
3.0 that rare earth element
98.37 loss existed in the0.164
100
4.0
98.18
0.172
100
earth precipitation during impurities removal may be attributed to two aspects. One reason is that
92.81
100when they pass.
the precipitation4.5of impurities can also
entrap Y3+ and Eu3+0.175
, and bring them down
5.0
0
0
0
The other reason may be that the rare earth ions can react with phosphate in the case of low pH, and
these precipitations have strong adsorptions with metal ions; this result also explains why that in the
Due toofthe
of rare earth,
dark solid
precipitates
were
analyzed
and
condition
pHloss
adjustment
withthe
ammonia,
there
existed 30%
loss
of rare with
earthXRF
all of
theXRD.
way.Table 4
and Figure 8 show that rare earth element loss existed in the process of removing impurities. The rare
RE3+ + PO43− → REPO4↓
(6)
earth precipitation during impurities removal may be attributed to two aspects. One reason is that
the precipitation of impurities can also entrap Y3+ and Eu3+ , and bring them down when they pass.
1800
The other reason may be that the rare earth ions can react with phosphate in the case of low pH, and
Y---YPO4·xH2O
these precipitations have
strong adsorptions with metal ions; this
result·xH
also
O explains why that in the
E---EuPO
1500
4
2
condition of pH adjustment with ammonia, there existed 30% loss of rare earth all of the way.
4.0
4.5
5.0
98.18
92.81
0
0.172
0.175
0
100
100
0
Due to the loss of rare earth, the dark solid precipitates were analyzed with XRF and XRD. Table 4
8 of 13
and Figure 8 show that rare earth element loss existed in the process of removing impurities. The rare
earth precipitation during impurities removal may be attributed to two aspects. One reason is that
3+, and bring them down when they pass.
the precipitation of impurities
canchemical
also entrap
Y3+ andofEu
Table 4. Main
composition
the
solid precipitation.
The other reason may be that the rare earth ions can react with phosphate in the case of low pH, and
Eu2 Oadsorptions
SiO2 withAlmetal
P2 O
BaO
CaO why
Fethat
2 O3
3
2 O3
5
2 O3 in the
theseCompound
precipitationsYhave
strong
ions;
this
result
also explains
Contentof
(%)
44.83
5.40 ammonia,
1.60 there 5.48
22.93
0.85 earth all
0.32of the way.
3.29
condition
pH adjustment
with
existed 30%
loss of rare
Minerals 2017, 7, 44
Compound
Content (%)
Na2 O
0.04
SrO
0.25
ZnO
Sb2 O3
PbO
SO3
13.55
0.353+ + PO43−
0.56
RE
→ REPO0.38
4↓
Cl
0.09
-
(6)
1800
Y
Y---YPO4·xH2O
E---EuPO4·xH2O
1200
0
20
30
40
Y
50
Y
E
Y
10
Y
E
300
E
Y
600
Y
E
E
900
Y
E
Intensity(a.u.)
1500
60
70
80
2-Theta(Deg.)
Figure 8. The XRD pattern of the RE solid precipitation.
Figure 8. The XRD pattern of the RE solid precipitation.
3.3. Recycling of Rare Earths with Oxalic Acid
The residual colorless liquid was then used to carry out the rare earth elements of Y and Eu with
oxalic acid. As RE3+ and Ca2+ can generate oxalates, residuary Mg2+ and Al3+ can generate complexes
with oxalic acid in the residual solutions. Hence, the amount of oxalic acid could be divided into three
parts: one part used to precipitate rare earth and calcium; one part to maintain the rare earth ions
precipitate completely; and the remaining oxalic acid should react with impurities [12].
2Y3+ + 3H2 C2 O4 ·Y2 → Y2 (C2 O4 )3 + 6H+
(7)
2Eu3+ + 3H2 C2 O4 → Eu2 (C2 O4 )3 + 6H+
(8)
Ca2+ + H2 C2 O4 → CaC2 O4 + 2H+
(9)
Oxalic acid (100 g/L) was added to the residual solutions, and the temperature of the solutions
were kept at 60 ◦ C for 0.5 h. The precipitations were then separated, thoroughly washed with deionized
water, dried, and used for annealing studies.
Rare earth oxalates Y2 (C2 O4 )3 and Eu2 (C2 O4 )3 would decompose under high temperature
conditions. Figure 9 shows the TG-DSC curves of rare earth oxalates in air atmosphere. The thermal
decomposition of these materials may be as depicted below [13]: dehydration, decomposed into rare
earth carbonate, decomposed into rare earth oxides, and crystal transition ordering. DSC curve reports
that there are four absorption peaks from 130 ◦ C to 750 ◦ C. Table 5 reports the weight loss of rare
earth oxalates at different temperatures, and it can be found that the weight loss of the four stages are
13.73%, 7.84%, 29.97%, and 5.87%. When the temperature was up to 800 ◦ C, the weight is no longer
reduced from the TG curve. Therefore, the final annealing temperature is set at 850 ◦ C.
Figure 9 shows the TG-DSC curves of rare earth oxalates in air atmosphere. The thermal decomposition
of these materials may be as depicted below [13]: dehydration, decomposed into rare earth carbonate,
decomposed into rare earth oxides, and crystal transition ordering. DSC curve reports that there are
four absorption peaks from 130 °C to 750 °C. Table 5 reports the weight loss of rare earth oxalates at
different temperatures, and it can be found that the weight loss of the four stages are 13.73%, 7.84%,
Minerals
2017, 7,and
44 5.87%. When the temperature was up to 800 °C, the weight is no longer reduced from9 of 13
29.97%,
the TG curve. Therefore, the final annealing temperature is set at 850 °C.
Figure 9. TG-DSC curves of RE oxalates in air atmosphere (negative value is exothermal).
Figure 9. TG-DSC curves of RE oxalates in air atmosphere (negative value is exothermal).
Table 5. Loss-weight rate of RE oxalates at different temperature.
Table 5. Loss-weight rate of RE oxalates at different temperature.
Temperature Range (°C)
Thermal Decomposition Steps
Loss-Weight Rate
130–200 ◦
RE2(C2O4)3·nH2O → RE2(C2O4)3 + nH2O
13.73%
Temperature Range ( C)
Thermal Decomposition Steps
Loss-Weight Rate
200–430
RE2(C2O4)3 → RE2(CO3)3 + 3CO2↑
7.84%
130–200
RE2 (C2 ORE
·nH23)O3 →
→RE
RE22O(C
13.73%
4 )32(CO
2O
4 )3 2+↑nH2 O
430–641
3 +
3CO
29.97%
200–430
RE
(C
O
)
→
RE
(CO
)
+
3CO
↑
7.84%
2 Crystal
2 4 3 transition
2
3ordering
3
2
641–750
5.87%
430–641
RE2 (CO3 )3 → RE2 O3 + 3CO2 ↑
29.97%
641–750
Crystal transition ordering
5.87%
According to the result of Figure 9, the rare earth oxalates were annealed at 850 ◦ C for 2 h, and
Minerals 2017, 7, 44
9 of 13
turned
into rare earth oxides Y2 O3 and Eu2 O3 . Table 6 shows the chemical composition of rare
earth
products. It
is
clear
from
the
data
that
Y
O
and
Eu
O
account
for
99.42%,
and
the
impurities
take
2 3 oxalates were annealed at 850 °C for 2 h, and up
According to the result of Figure2 9,3the rare earth
0.58%.turned
Compared
with
waste
fluorescent
powder,
the purity of rare earth had been improved greatly.
into rare
earth
oxides
Y2O3 and Eu
2O3. Table 6 shows the chemical composition of rare earth
products. It is clear from the data that Y2O3 and Eu2O3 account for 99.42%, and the impurities take up
Tablefluorescent
6. Main chemical
composition
RE earth
oxides.
0.58%. Compared with waste
powder,
the purity ofofrare
had been improved greatly.
Compound
Y2 O3
SiO
AlRE
TableEu
6. Main
composition
of
2 O3 chemical
2
2 Ooxides.
3
Content
(%)
96.39 Y2O3
Compound
Content (%)
96.39
3.03Eu2O3
3.03
0.097
SiO2
0.097
0.02
Al2O3
0.02
CaO
SO3
0.29
CaO
0.29
0.078
SO
3
0.078
The synthesized precursor material was annealed at 800 ◦ C for 3 h; its powder XRD pattern is
The synthesized precursor material was annealed at 800 °C for 3 h; its powder XRD pattern is
depicted in Figure 10. The diffraction pattern clearly shows that the main diffraction peaks of the
depicted in Figure 10. The diffraction pattern clearly shows that the main diffraction peaks of the
product correspond well to the body centered cubic structure of Y2 O3 . As the peak type of the product
product correspond well to the body centered cubic structure of Y2O3. As the peak type of the product
is sharp
and its
height
is small,
it can
thecrystalline
crystallinegrain
grain
product
is uniform
is sharp
andhalf
its half
height
is small,
it canbebededuced
deduced that
that the
of of
product
is uniform
and the
crystallinity
is
well.
and the crystallinity is well.
Figure 10. XRD pattern of the precursor.
Figure 10. XRD pattern of the precursor.
The morphologies of precursor samples were investigated by SEM images. It is shown from
Figure 11 that the precursor is composed of lamellar crystals which were densely packed and
irregular.
Minerals 2017, 7, 44
Figure 10. XRD pattern of the precursor.
10 of 13
The morphologies of precursor samples were investigated by SEM images. It is shown from
The
of precursor
samplesofwere
investigated
SEMwere
images.
It is shown
Figure
11morphologies
that the precursor
is composed
lamellar
crystals by
which
densely
packedfrom
and
Figure
11
that
the
precursor
is
composed
of
lamellar
crystals
which
were
densely
packed
and
irregular.
irregular.
Figure
Figure 11.
11. SEM
SEM image
image of
of the
the RE
RE precursor.
precursor.
3.4. High Temperature Solid-State
3.4. High Temperature Solid-State
Currently, solid phase method is commonly used in the industry to synthesize Y2O3:Eu because
Currently, solid phase method is commonly used in the industry to synthesize Y2 O3 :Eu because of
of its simple operation. Phosphors synthesized with the solid phase method (reaction, process,
its simple operation. Phosphors synthesized with the solid phase method (reaction, process, synthesis)
synthesis) have high brightness and stable performance, and can be easily extended in the industry.
have high brightness and stable performance, and can be easily extended in the industry. Thus, in this
Thus, in this study, the solid phase method was used to synthesize Y2O3:Eu. This nano crystalline
study, the solid phase method was used to synthesize Y2 O3 :Eu. This nano crystalline precursor was
precursor was processed further by annealing
at 1400 °C with or without sintering aids to yield
processed
by annealing at 1400 ◦ C with or without sintering aids to yield phosphor
Mineralsfurther
2017, 7, 44
10 material
of 13
with varying crystallite sizes and morphologies. In the present work, we used NaF, boric acid, and
phosphor
material
with
varyingaids
crystallite
sizes and
morphologies.
the presentatwork,
we used
sodium
carbonate
as the
sintering
in different
proportions.
AfterInannealing
high temperature,
NaF,
boric
acid,
and
sodium
carbonate
as
the
sintering
aids
in
different
proportions.
After
annealing
the sintering aids were removed by washing with hot water repeatedly, and the powder XRD patterns
at high temperature, the sintering aids were removed by washing with hot water repeatedly, and the
of the materials were recorded [9]. The selected XRD patterns of the processed materials are depicted
powder XRD patterns of the materials were recorded [9]. The selected XRD patterns of the processed
in Figure
12. From Figure 12, we can see that all of the XRD peaks could be indexed to the cubic phase
materials are depicted in Figure 12. From Figure 12, we can see that all of the XRD peaks could be
of Y2 O
(No.
41-1105).
Samples
(b),
and (c) show sharp diffraction peaks due to good crystallinity
3
indexed to
the cubic
phase of(a),
Y2O
3 (No. 41-1105). Samples (a), (b), and (c) show sharp diffraction
◦ C for 5 h. The peaks of sample (a)—in which the sintering aids are smoother,
after calcination
at
1400
peaks due to good crystallinity after calcination at 1400 °C for 5 h. The peaks of sample (a)—in which
sharper,
where
noare
additional
peaks
of other
phasesnowere
observed
after
the and
sintering
aids
smoother,
sharper,
and where
additional
peaks
of calcinations—indicate
other phases were
3+ hasinto
observed
afterhas
calcinations—indicate
that
the
precursor
has
decomposed
Y
2
O
3 completely
and into
that the
precursor
decomposed into Y
O
completely
and
the
Eu
been
effectively
doped
2 3
the lattice
Eu3+ has[9].
been
effectively
doped
into the host of
lattice
[9]. It is clear
that the
crystallinity
of the aids
the host
It is
clear that
the crystallinity
the product
increased
when
the sintering
were product
added. increased when the sintering aids were added.
Figure
12. XRD
pattern
Y2O
3:Eu
3+ annealed at 1400 °C
◦ with sintering aids (a); without sintering aids
Figure
12. XRD
pattern
of Yof2 O
3 :Eu annealed at 1400 C with sintering aids (a); without sintering aids
(b); and commercial red phosphor (c).
(b); and commercial red phosphor (c).
3+
The well-known Debye–Scherrer’s equation as follows was used to calculate the crystallite sizes
of the material.
D = K λ / β cos θ
(10)
where D is the crystallite size, K is the dimensionless shape factor (0.9), λ is the X-ray wavelength, β
is the FWHM, and θ is the Bragg’s angle [11]. The crystallite size of these materials was calculated
Minerals 2017, 7, 44
11 of 13
The well-known Debye–Scherrer’s equation as follows was used to calculate the crystallite sizes
of the material.
D = Kλ/β cos θ
(10)
where D is the crystallite size, K is the dimensionless shape factor (0.9), λ is the X-ray wavelength, β is
the FWHM, and θ is the Bragg’s angle [11]. The crystallite size of these materials was calculated using
Scherer’s formula, and the values are tabulated in Table 7. It can be seen that the crystallite size of
these samples, which were annealed at 1400 ◦ C, increases when compared with the precursor. From
Table 7 we can also see that the material obtained by annealing at 1400 ◦ C in the presence of sintering
aids had given the material 90% of photoluminescence (PL) intensity.
Table 7. Physical Characteristics and PL Efficiency of Y2 O3 :Eu particles.
Temperature (◦ C)
2θ
FWHM (◦ )
Crystallite
Size (nm)
Shape of
Particle
Particle Size
PL (%)
800
1400 with sintering aids
1400 without sintering aids
Commercial red phosphor
29.02
29.11
29.18
29.05
0.116
0.155
0.161
0.153
70.61
53.03
51.05
53.71
Bulk
Ellipsoid
Bulk
Spherical
3–5 µm
0.89–5.26 µm
4.65–8.93 µm
3–5 µm
65
90
77
100
The morphologies of the samples after calcinations at 1400 ◦ C with and without sintering aids
were characterized by SEM. The SEM images of Y2 O3 :Eu obtained at 1400 ◦ C and 1400 ◦ C with
sintering aids are shown in Figure 13. It is found that the material presented ellipsoid morphology
with a particle size of 0.89–5.26 µm after annealing at 1400 ◦ C with the sintering aids, and its surface
Minerals 2017, 7, 44
11 of 13
is smoother; while those samples which were annealed at 1400 ◦ C without sintering aids showed
irregular shape with the particle size of 4.65–8.93 µm. Devaraju et al. [14] prepared nanorod and
irregular shape with the particle size of 4.65–8.93 μm. Devaraju et al. [14] prepared nanorod and
spherical shaped Y O :Eu particles by the solvothermal method, and observed that material with
spherical shaped Y22O33:Eu particles by the solvothermal method, and observed that material with
spherical morphology has higher PL efficiency than the rod-shaped one. Thus, it can be concluded
spherical morphology has higher PL efficiency
than the rod-shaped one. Thus, it can be concluded
that the samples which were annealed at 1400 ◦ C with sintering aids had higher PL efficiency because
that the samples which were annealed at 1400 °C with sintering aids had higher PL efficiency because
of their ellipsoid morphology.
of their ellipsoid morphology.
Figure 13. SEM image of (a) Y2O3:Eu sintered at 1100 °C;
(b) Y2O3:Eu sintered at 1100 °C with sintering
Figure 13. SEM image of (a) Y2 O3 :Eu sintered at 1100 ◦ C; (b) Y2 O3 :Eu sintered at 1100 ◦ C with sintering
aids; and (c) the commercial red phosphor.
aids; and (c) the commercial red phosphor.
Apart from these, photoluminescence properties were also investigated in this study. The PL
Apartoffrom
these,
photoluminescence properties were also investigated in this study. The PL
spectrum
the Y
2O3:Eu material sintered at 1400 °C with sintering aids, along with the material
◦ with sintering aids, along with the material
spectrumatof
the°C
Y2without
O3 :Eu material
at 1400
sintered
1400
sinteringsintered
aids, which
wereCexcited
at a wavelength of 254 nm, is presented
◦
sintered
1400
C without
sintering
which
werethat
excited
at a wavelength
254nm
nm,
is presented
in
Figureat14.
Figure
14 depicts
that allaids,
of the
samples
exhibited
emission atof610
corresponded
in
Figure
14.
Figure
14
depicts
that
all
of
the
samples
that
exhibited
emission
at
610
nm
corresponded
to
to red emission, due to 5D0–7F2 transition [15–17]. The emission bands around 591 nm could be
5
7
red emission,
due
The emission
be ascribed
5D0to
0 – F2 transition
ascribed
to the
–7FD
1 magnetic
dipole[15–17].
transitions,
while thebands
weak around
peak at591
633nm
nmcould
and 711
nm are
5 D –7 F magnetic dipole transitions, while the weak peak at 633 nm and 711 nm are related to
to
the
5
7
5
7
related to0 the1transition from D0 to F3 and from D0 to F4, respectively [18]. Figure 14 also reveals
5 D to 7 F and from 5 D to 7 F , respectively [18]. Figure 14 also reveals that the
the transition
from
0 materials
3
0 14004 °C with sintering aids was higher than that of the
that
the intensity
of the
annealed at
◦ C with sintering aids was higher than that of the materials
intensity
of
the
materials
annealed
at
1400
materials annealed at 1400 °C without sintering aids. The emission luminance of Y2O3:Eu particles
◦ C without sintering aids. The emission luminance of Y O :Eu particles synthesized
annealed at 1400
2 390.12%, compared to the
synthesized
by high
temperature solid state with sintering aids reached
commercial phosphor. It can be seen that the PL intensity of nano-sized phosphor with sphere shape
is obviously higher than the phosphor with irregular shape.
sintered at 1400 °C without sintering aids, which were excited at a wavelength of 254 nm, is presented
in Figure 14. Figure 14 depicts that all of the samples that exhibited emission at 610 nm corresponded
to red emission, due to 5D0–7F2 transition [15–17]. The emission bands around 591 nm could be
ascribed to the 5D0–7F1 magnetic dipole transitions, while the weak peak at 633 nm and 711 nm are
related to the transition from 5D0 to 7F3 and from 5D0 to 7F4, respectively [18]. Figure 14 also reveals
Minerals 2017, 7, 44
13
that the intensity of the materials annealed at 1400 °C with sintering aids was higher than that 12
ofofthe
materials annealed at 1400 °C without sintering aids. The emission luminance of Y2O3:Eu particles
synthesized
by high temperature
solidsintering
state with
sintering
reached
90.12%,tocompared
to the
by high temperature
solid state with
aids
reachedaids
90.12%,
compared
the commercial
commercialIt phosphor.
can the
be seen
that the PL
intensity ofphosphor
nano-sized
phosphor
with sphere
shape
phosphor.
can be seenItthat
PL intensity
of nano-sized
with
sphere shape
is obviously
is
obviously
higher
than
the
phosphor
with
irregular
shape.
higher than the phosphor with irregular shape.
◦ C without
14. Photoluminescence
Figure 14.
Photoluminescence spectra
spectra of (c) Y22O33:Eu
:Eu annealed
annealed at
at 1400
1400 °C
without sintering
sintering aids;
◦
(b) Y22O33:Eu
:Eu annealed
annealed at
at 1400
1400 °CCwith
with sintering
sintering aids;
aids; and
and (a)
(a) the
the commercial
commercial red
red phosphor.
phosphor.
4. Conclusions
4.
Conclusions
fluorescent
In conclusion,
research
describes
the recycling
and synthesis
of Y2O3:Eu
In
conclusion,this
this
research
describes
the recycling
and synthesis
of from
Y2 O3waste
:Eu from
waste
lamps.
The
process
includes
leaching
tests;
impurities
removal
from
leaching
solution
using
fluorescent lamps. The process includes leaching tests; impurities removal from leachingammonia,
solution
sodium
sulfide, and
DDTC;
precipitations
rare earth using
oxalic
acid;
andoxalic
synthesis
Y2synthesis
O3:Eu by
using
ammonia,
sodium
sulfide,
and DDTC;ofprecipitations
of rare
earth
using
acid; of
and
high
temperature
solid-state
synthesis
method.
The
leaching
tests
showed
that
the
optimum
condition
of Y2 O3 :Eu by high temperature solid-state synthesis method. The leaching tests showed that the
to leach rare
earth was:
mol/L
hydrochloric
0.2of
mL/g
of hydrogen
peroxide,
7.5 mL/g
liquid–
optimum
condition
to 4.0
leach
rareofearth
was: 4.0 acid,
mol/L
hydrochloric
acid,
0.2 mL/g
of hydrogen
peroxide, 7.5 mL/g liquid–solid ratio, 60 ◦ C, and 4 h of reaction. This study is mainly focused on the
purification process aimed at optimization of experimental conditions for removal of impurities, as
well as reduction of rare earth loss during this procedure. Tests showed that the best way to remove
impurities is by adjusting pH to 4.0 using ammonia, and adding sodium sulfide and DDTC to control
pH up to 5.0. In addition, Yongsong Liu et al. [17] found that the morphologies have a great effect on
photoluminescence property, and that the microsphere Y2 O3 :Eu3+ exhibit the strongest red emission
intensity among structures obtained in the experiments. M.K. Devaraju et al. [8] put forward that for
successful applications, the phosphor particles should have a spherical shape and high luminescence.
Phosphor particles with a spherical shape are capable of minimizing light scattering on their surface.
Thus, in this paper, because of the smooth surface and ellipsoid morphology, the materials of Y2 O3 :Eu
synthesized by high temperature solid-state synthesis method at 1400 ◦ C with sintering aids exhibited
higher PL intensity at 610 nm under the excitation of 254 nm, when compared to those synthesized
without sintering aids. This result may provide helpful guidance for the application of phosphors.
Acknowledgments: This work was financially supported by “Key Projects in the National Science and Technology
Pillar Program during the 12th Five-year Plan Period” (No. 2014BAC03B07), “Key Laboratory for Solid Waste
Management and Environment Safety, Ministry of Education of China, Tsinghua University” (No. swmes 2011-03),
and “The Fundamental Research Funds for the Central University” (No. WUH165208005).
Author Contributions: Xiaodong Chen and Nian Liu conceived and designed the experiments, Xiaodong Chen
performed the experiments; Nian Liu analyzed the data; Guangjun Mei contributed reagents, materials, and
analysis tools; Xiaodong Chen and Mingming Yu wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Minerals 2017, 7, 44
13 of 13
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