minerals
ArticleInvestigation of Cerium Reduction Efficiency by Grinding with
Microwave Irradiation in Mechanochemical Processing
Tatsuya Kato 1,2 , Motonori Iwamoto 3 and Chiharu Tokoro 1,4, *
1

2

3

4

*

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Citation: Kato, T.; Iwamoto, M.;
Tokoro, C. Investigation of Cerium
Reduction Efficiency by Grinding
with Microwave Irradiation in
Mechanochemical Processing.
Minerals 2022, 12, 189. https://Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan;
kato.tatsuya@nitech.ac.jp
Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokisocho, Showa Ward,
Nagoya 466-8555, Japan
Powder Technology Center, Nippon Coke & Engineering Co., Ltd., 1 Koh-machi, Tochigi 328-8503, Japan;
motonori-iwamoto@n-coke.com
Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Correspondence: tokoro@waseda.jp; Tel.: +81-3-5286-3320

Abstract: This study evaluated the efficiency of cerium reduction by grinding with microwave
irradiation in mechanochemical processing. Grinding experiments with microwave irradiation wereconducted using an agitating mixer. Since the structure of the ground samples was amorphous andthe cerium concentration was much lower than those of other elements, the valence change andstructural change of cerium after grinding with microwave irradiation were investigated using X-ray
absorption fine structure (XAFS) analysis in the cerium K-edge. The X-ray absorption near-edge
structure (XANES) analysis revealed that a portion of tetravalent cerium was reduced to trivalentcerium by grinding with microwave irradiation. In addition, it was confirmed by extended X-ray
absorption fine structure (EXAFS) analysis that oxygen vacancies were produced as a result of thecerium reduction reaction. To evaluate the efficiency of cerium reduction efficiency, the percentage
reduction by grinding with microwave irradiation was compared to that by planetary ball millingand microwave irradiation. As a result, it was revealed that the efficiency of cerium reduction via
grinding with microwave irradiation was higher than that via microwave irradiation and the same asthat via planetary ball milling. Moreover, a larger amount of tetravalent cerium could be reduced totrivalent cerium by grinding with microwave irradiation than when using planetary ball milling andmicrowave irradiation.

doi.org/10.3390/min12020189
Academic Editor:
Konstantinos Komnitsas

Keywords: X-ray absorption fine structure; extended X-ray absorption fine structure; local structure;oxygen vacancy; mechanochemistry

Received: 7 December 2021
Accepted: 28 January 2022
Published: 31 January 2022
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1. Introduction
During the past two decades, the mechanochemical reactions caused by high-intensity
grinding have attracted increasing academic and commercial attention. This is due to thefact that mechanochemical reactions offer rapid, cleaner alternatives to the conventional
chemical reactions used in nanomaterials, waste recycling, and mineral processing [1–12].
The use of mechanochemical reactions is also known as mechanochemical processing or
mechanical activation [2]. The phenomenon of a mechanochemical reaction corresponding
to high-intensity grinding for many materials is widely recognized [1–6]. In addition,
the process of mechanochemical reaction has recently been investigated and revealedby X-ray diffraction analysis (XRD), kinetics, and X-ray absorption fine structure (XAFS)
analysis [13–18]. However, despite the wide recognition of mechanochemical processing
by high-intensity grinding and its benefits, the development and optimization of attractive
proof-of-principle laboratory experiments into viable large-scale processes have not cometo pass [1,12].

4.0/).

Minerals 2022, 12, 189. https://doi.org/10.3390/min12020189

https://www.mdpi.com/journal/minerals

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In many of the previous studies [7–11], a planetary ball mill is commonly used to cause
the mechanochemical reaction for many kinds of grinding samples. However, a planetaryball mill is commonly used not on a large scale but a laboratory scale, due to the high energy
densities and low acquisition costs [19]. Several researchers reported a scaling-up method
for planetary ball mills, based on a discrete element method (DEM) simulation [20,21].
However, the development of large-scale planetary ball mills was problematic, and theycannot as yet be implemented. Thus, the development of alternative mills as a substitutefor planetary ball mills is desirable.
In this study, mechanochemical processing by grinding with microwave irradiation
using an agitating mixer has attracted attention as a substitute for planetary ball milling.
Microwave irradiation is known to be one of the factors that cause mechanochemical
reactions [22,23]. Comparing microwave irradiation and mechanochemical processing via
ball milling, Ribeiro et al. [23] reported that mechanochemical processing by ball milling
was a more effective procedure than microwave irradiation in terms of catalyst preparationand the catalytic reaction. However, it was reported that the most effective processes forthe catalytic oxidation of cyclohexane were a combination of microwave irradiation andmechanochemical processing by ball milling [23]. According to this study, it is possible forgrinding with microwave irradiation to become a more effective process than planetaryball milling.
Weathered residual rare-earth ore is considered to be an alternative source of rare-earth
minerals [13]. In our previous paper, we reported that cerium dissolution was increased
by a mechanochemical reaction corresponding to planetary ball milling [13]. In addition,
the cerium reduction reaction (Equation (1)) occurred as a mechanochemical reaction
during planetary ball milling [13,14]. The structural change and mechanism of the cerium
reduction reaction were revealed using XAFS analysis in our previous paper [14]. Since
the mechanochemical phenomenon and its mechanism have already been revealed forweathered residual rare-earth ore, this ore was selected as a target material in this study.
The cerium reduction reaction is as follows:x
CeO2 → CeO2−x + O2
2

(1)

The objective of this study is to evaluate cerium reduction efficiency via mechanochemical processing, by grinding with microwave irradiation using an agitating mixer. First,
the weathered residual rare-earth ore was ground by an agitating mixer with microwave
irradiation. Next, the valence change and structural change of ground samples after grinding with microwave irradiation were investigated, using XAFS analysis in cerium K-edge
since the structure of the ground samples was amorphous and the cerium concentration
was much lower than that of other elements. Finally, the cerium reduction efficiency bygrinding with microwave irradiation was compared with that by planetary ball milling andmicrowave irradiation to evaluate the cerium reduction efficiency.
2. Materials and Methods
2.1. Analysis
The detailed analysis methods for weathered residual rare-earth ore were described in
our previous paper [13]. Briefly, the major chemical composition of this ore was determinedby X-ray fluorescence (XRF; ZSX PrimusIII+, Rigaku Corporation, Tokyo, Japan) while theconcentrations of rare-earth elements were analyzed by laser ablation inductively coupledplasma mass spectrometry (LA-ICP-MS; 7500 Series, Agilent Technologies, Santa Clara,
CA, USA). The mineral composition in this ore was revealed by mineral liberation analysis(MLA; QuantaF Co., FEI, Hillsboro, OR, USA).
2.2. Heating Experiments by Microwave Irradiation
Heating experiments by microwave irradiation were performed using a microwave
irradiation device (µReactor EX, Shikoku Instrumentation Co. Ltd., Kagawa, Japan) underMinerals 2022, 12, 189

3 of 13

atmospheric conditions. Each experiment heated 10 g ore. The increasing speed was fixedat 2 ◦ C/min and the ore was kept for 1 h at the target temperatures. After experiments,
the samples cooled naturally. The target temperature was set at 700, 900, 950, and 1100 ◦ C.
During heating experiments, the output of the microwave was monitored every second.
Based on this information, the energy consumption was calculated.
2.3. Grinding Experiments by Planetary Ball Mill
Grinding experiments using a planetary ball mill (PM100, Verder Scientific Co. Ltd.,Tokyo, Japan) were performed under the same conditions, as reported in our previous
paper [13]. The experimental results are quoted from our previous papers [13]. Eachgrinding experiment ground 100 g of ore. The rotation speed of the mill was fixed at
300 rpm and 25 chrome steel balls, with a diameter of 19 mm, were used as the grinding
medium. The grinding times were set at 10, 60, and 720 min. The volume of the mill was
0.5 dm3 . During grinding experiments with the planetary ball mill, the energy consumptionof the planetary ball mill was monitored.
2.4. Grinding Experiments with Microwave Irradiation
Grinding experiments with microwave irradiation were conducted using an agitating
mixer (FM20, Nippon Coke & Engineering Co. Ltd., Tokyo, Japan). Each experimentground 1000 g of ore. The rotation speed of the mill was fixed at 200 rpm. Grinding time
was set at 60, 180, and 420 min. The reaction temperature was fixed at 250 ◦ C. The volumeof the mixer was 20 dm3 . During grinding experiments with microwave irradiation, theenergy consumption was monitored using the same method as for the planetary ball mill.
2.5. X-ray Absorption Fine Structure Analysis of the Cerium LIII - and K-Edge
The conditions of XAFS analysis of the cerium LIII edge were as already describedin our previous papers [13,14]. Briefly, XAFS analysis of the cerium LIII edge was
performed using the BL5S1 beamline in the Aichi Synchrotron Radiation Center, Japan.
The X-ray absorption near-edge structure (XANES) analysis was performed using a
range of 5720–5740 eV to reveal the valence of the cerium.
The XAFS analysis at the cerium K-edge was performed using the BL01B1 beamline at
the SPring-8 Synchrotron Radiation Facility, Japan. All XAFS spectra at the cerium K-edge
were obtained at room temperature. The electron storage ring was operated at 8.0 GeV, witha stored current of 99.5 mA. The energy was from 40130 eV to 41920 eV. The continuous
X-ray synchrotron radiation was monochromatized using a silica (311) double-crystal
monochromator. The XAFS spectra for all samples were obtained via the fluorescence modeusing a 19-element germanium (Ge) solid-state detector. Cerium phosphate (CePO4 ) andcerium oxide (CeO2 ) were used as reference materials.
The EXAFS analysis was conducted under the same conditions as described in ourprevious paper [14]. Briefly, the EXAFS function was Fourier-transformed from a k3 weighted EXAFS function to a radial distribution function (RDF) using a Hanning windowfunction of within 1–12 × 1010 m−1 . Structural parameters for different coordination shells
surrounding both the tri- and tetravalent cerium atom, namely, coordination number,
atomic distance, and Debye-Waller factor, were obtained by a curve fitting using boththe k3 -weighted EXAFS function and RDF. The structural parameters were obtained after
fitting the RDF at an interval of 1.5–4.0 × 10−10 m, consisting of contributions from the firstto the fourth coordination shells. The theoretical phase and amplitude function for eachshell, as used in curve fitting, were calculated by FEFF 6.0 [24,25]. All EXAFS analysis was
performed using Athena and Artemis software [26].
3. Results and Discussion
3.1. Characterization of Weathered Residual Rare-Earth Ore
The detailed characteristics of weathered residual rare-earth ore have already been
described in our previous paper [13]. The major chemical composition and concentrations

Minerals 2022, 12, 189

4 of 13of rare-earth elements in this ore are shown in Tables 1 and 2. MLA analysis revealed
that this ore contained four kinds of cerium minerals; britholite-(Ce) ((Ce, Ca, Th, La,Nd)5 (SiO4 , PO4 )3 (OH, F)), cerianite ((Ce, Th)O2 ), cerite (Ce9 Fe(SiO4 )6 [(SiO3 ), (OH)](OH)3 )and monazite-(Nd) ((Nd, Ce, La)(P, Si)O4 ). Among these cerium minerals, britholite-(Ce),cerite and monazite-(Nd) are trivalent cerium minerals, while only cerianite is a tetravalentcerium mineral.
Table 1. Chemical composition of weathered residual rare-earth ore, as analyzed by X-ray fluorescence(mass %) [13].
SiO2

Fe2 O3Al2 O3

Na2 O

MgO

P2 O5

K2 O

CaO

TiO2

MnO

Others

68.6

16.2

7.3

0.3

0.4

0.3

2.1

0.2

0.3

0.2

4.1

Table 2. Concentrations of rare-earth elements in weathered residual rare-earth ore, as analyzed bylaser ablation inductively coupled plasma mass spectrometry (mg/dm3 ) [13].
Sc

Y

La

Ce

Pr

Nd

Sm

Eu

5.3
Gd
224.2

1384.1
Tb
37.4

926.5
Dy
236.2

2495.5
Ho
50.4

236.8
Er
153.7

903.3
Tm
23.1

200.0
Yb
137.9

11.2
Lu
19.6

3.2. Cerium Reduction by Heating Experiments
The Ellingham diagram of cerium reduction in the range from 0 to 1200 ◦ C is shownin Figure 1 [27]. The standard free energy (∆G0 , 25 ◦ C, 1 bar) is 330.6 kJ/mol [27]. Since
the free energy charge (∆G) in this area is always a positive value, the cerium reduction
reaction did not occur in the range from 0 to 1200 ◦ C.
The XAFS spectra for heating samples corresponding to the XANES region at thecerium LIII -edge are shown in Figure 2. In addition, the valence of cerium in each heating
sample is shown in Table 3. Above 700 ◦ C, the concentration of trivalent cerium wassignificantly increased (Table 3). Thus, it was confirmed that the cerium reduction reaction
occurred during heating by microwave irradiation from XANES analysis. However, thecerium reduction reaction does not occur as shown in the Ellingham diagram in the rangeof 0 °C to 1200 °C (Figure 1). In a previous study [28], it was reported that the free energy
charge decreased when the lattice defects were present in the structure of cerianite. The
weathered residual rare earth ore is very fine-grained because of strong weathering over along period. Thus, it was suggested that lattice defects occurred naturally in the structureof cerianite in this ore, and cerium reduction reaction occurred more easily.
The structural change of cerianite, corresponding to cerium reduction by heating,
was reported in a previous paper [29–32]. The crystal structure of cerianite and cerium
trioxide(III) (Ce2 O3 ), which is produced after the cerium reduction reaction, corresponds toa fluorite structure (Fm3 m) and a C-type structure (la3), respectively [29–32]. When thecerianite is reduced to Ce2 O3 , eight unit cells of cerianite are synthesized and produce 25%of oxygen vacancy in the structure. This structural change from cerianite to the C-type of
Ce2 O3 involves a mineral reorganization of the skeleton arrangement of the cerium atoms.
Table 3. Concentration of tri- and tetravalent cerium in the heating samples, based on the X-ray
absorption near-edge structure analysis of the cerium LIII edge (%).
Temperature (◦ C)

Concentration of
Trivalent Cerium

Concentration of
Tetravalent Cerium

25
700
900
950
1100

50.7
50.8
51.4
61.0
75.9

49.3
49.2
48.6
39.0
24.1

Temperature (°C)
25
700
900
Minerals 2022, 12, 189
950
1100

Minerals 2022, 12, x

Concentration of Trivalent Cerium
50.7
50.8
51.4
61.0
75.9

Concentration of Tetravalent Cerium
49.3
49.2
48.6
5 of 1339.0
24.1

Figure 1. Ellingham diagram of cerium reduction
reaction [27].
6 of 16
Figure 1. Ellingham diagram of cerium reduction reaction [27].

Figure 2. Cerium L edge X-ray absorption fine structure spectra, corresponding to X-ray
Figure2. Cerium
LIII edge
X-ray
absorption
finestructure
spectra, corresponding to X-ray absorption
absorption near-edge
structure analysisof the region,
both without
and with grinding,showingthenear-edge structure analysis of the region, both without and with grinding, showing the resolved
components of tri- and tetravalent cerium based on cerium fluoride(CeF3 ) and tetravalent cerium
oxide(CeO2 ) as reference spectra, respectively.
III

3.3. Cerium Reduction by Mechanochemical Processing via Planetary Ball Milling
Details of the mechanochemical phenomenon, structural change analysis, and themechanism of cerium reduction by mechanochemical processing via planetary ball milling
have already been given in our previous papers [13,14]. Briefly, the valence of cerium
without and with grinding samples is shown in Table 4. From Table 4, the concentrationof trivalent cerium was significantly increased according to grinding time. After finishingMinerals 2022, 12, 189

6 of 13planetary ball milling, the temperature of the grinding sample was at a maximum of about
50 ◦ C. Thus, it was confirmed that the cerium reduction reaction occurred during planetaryball milling as a mechanochemical reaction.
The results of structural change analysis corresponding to cerium reduction reaction
are shown in Table 5 [14]. As shown in Table 5, the atomic distances of both Ce(IV)-O
and Ce(III)-O decreased as the grinding time increased. This trend shows that the oxygen
vacancy is generated in the structure of cerianite by planetary ball milling [33]. When an
oxygen vacancy is generated in the structure of cerianite, it has been reported that the
local structure around the oxygen vacancy becomes an A-type or B-type structure [14]. In
addition, it was reported that when both A-type and B-type structures were generated in
the structure of cerianite, the atomic distances of Ce(IV)-O and Ce(III)-O was decreased.
Table 4. Concentration of tri- and tetravalent cerium samples without and with grinding, based on
the X-ray absorption near-edge structure analysis in cerium K-edge (%) [14].
Grinding Time (min)Concentration of
Trivalent Cerium

Concentration of
Tetravalent Cerium

0
10
60
720

30.5
31.9
50.0
74.4

69.5
68.1
50.0
25.6

Table 5. Cerium fitting results of the radial distribution function for samples without and withgrinding, assuming four kinds of cerium shells for tetravalent cerium oxide [14].
∆E0SampleShell

CN

R [× m−10 ]σ2 [× m−20 ]

7.7

2.35

0.009

CeO2

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

12.2

3.84

0.005

Without grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

8.7
0.2
0.3
12.1

2.35
2.46
3.81
3.83

0.02
0.04
0.002
0.008

−9.3

10 min grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

8.0
0.06
0.2
11.4

2.36
2.47
3.81
3.84

0.02
−0.01
0.001
0.008

−10.6

60 min grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

4.0
3.9
7.4
5.4

2.31
2.42
3.77
3.79

0.006
0.02
0.008
0.009

2.4

720 min grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

0.1
8.4
10.0
2.6

2.30
2.42
3.76
3.79

0.009
0.03
0.04
0.0009

−3.4

-

−10.3

CN: coordination number; R (× m−10 ): atomic distance; ∆E0 : threshold E0 shift; σ2 (× m−20 ): Debye–Waller factor.

3.4. Cerium Reduction by Mechanochemical Processing via Grinding with Microwave Irradiation
The XAFS spectra at the cerium K-edge and the valence of cerium in samples without
grinding, and then with grinding and microwave irradiation, are shown in Figure 3 andTable 6, respectively. When the valence of cerium is evaluated using XAFS analysis, the
XAFS analysis at the cerium LIII edge has more advantages than at the cerium K-edge.
However, we confirmed that the XAFS spectra at the cerium K-edge were sufficient toevaluate the valence of cerium in our previous paper [14]. Thus, we evaluated the valenceof cerium using the XAFS spectra at the cerium K-edge.

Minerals 2022, 12, 189

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The concentration of trivalent cerium significantly increased with grinding time, inthe same way as planetary ball milling (Tables 4 and 6). Since the reaction temperature
was fixed at 250 ◦ C during grinding with microwave irradiation, it was confirmed that thecerium reduction reaction occurred as a mechanochemical reaction.
The k3 -weighted EXAFS spectra and RDF are shown in Figures 4 and 5, respectively.
The curve fitting results are listed in Table 7 and are plotted in Figures 4 and 5. In RDF(Figure 5), the peak position of the four shells is shown. Since the contribution of a third
shell (Ce(III)-Ce(III)) is quite minor, relative to that of a fourth shell (Ce(IV)-Ce(IV)) [14],the peak positions of both the third and fourth shells overlap. As shown in Table 7, theatomic distance of both Ce(IV)-O and Ce(III)-O decreased as the grinding time increased,in the same way as planetary ball milling. This trend is the same as that in planetary ball
milling (Section 3.3). Thus, the structural change of cerianite by grinding with microwave
irradiation showed that oxygen vacancies occurred in the structure. In addition, twotetravalent cerium structures surrounding the oxygen vacancy took an electron and werereduced to trivalent cerium when the oxygen vacancy occurred [14]. The cerium reduction
mechanism caused by grinding with microwave irradiation was the same as that achievedby planetary ball milling [14].
Table 6. Concentration of tri- and tetravalent cerium in the samples by grinding with microwave
irradiation, based on X-ray absorption near-edge structure analysis in the cerium K-edge (%).
Grinding Time (min)Concentration of
Trivalent Cerium

Concentration of
Tetravalent Cerium

0
60
180
420

30.5
40.7
46.9
53.4

69.5
59.3
53.1
46.6

Table 7. Cerium fitting results assuming four kinds of cerium shell for tetravalent cerium oxide,
samples by grinding with microwave irradiation in the radial distribution function.
∆E0

SampleShell

CN

R [× m−10 ]σ2 [× m−20 ]7.7 ± 2.0

12.2 ± 12.0

2.35
3.84

0.009

CeO2

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)Without grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)8.7 ± 4.8
0.2 ± 1.8
0.3 ± 3.8
12.1 ± 11.9

2.35
2.46
3.81
3.83

0.02
0.04
0.002
0.008

−9.3

60 min grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)

7.5 ± 1.8
0.6 ± 0.6
1.0 ± 1.2
11.4 ± 6.6

2.35
2.47
3.81
3.84

0.003
0.016
0.005
0.006

2.6

180 min grinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)6.9 ± 4.8
1.1 ± 0.7
2.5 ± 1.0
10.2 ± 3.8

2.29
2.41
3.75
3.78

0.025
−0.002
0.008
0.005

−4.0

420 mingrinding

Ce(IV)-O
Ce(III)-O
Ce(III)-Ce(III)
Ce(IV)-Ce(IV)6.7 ± 13.1
1.8 ± 12.0
9.0 ± 4.9
3.4 ± 2.2

2.21
2.32
3.66
3.69

0.063
0.00006
0.004
0.014

−14.7

−10.3
0.005

Note that CN: coordination number; R (× m−10 ): atomic distance; ∆E0 : threshold E0 shift; σ2 (× m−20 ): Debye–
Waller factor.

Minerals 2022, 12, 189
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10 of 16

Figure 3. Cerium K-edge X-ray absorption fine structure spectra, corresponding
11 of 16 to X-ray absorption
near-edge structure analysis of the samples by grinding with microwave irradiation, showing resolved
components of tri- and tetravalent cerium, based on cerium phosphate(CePO4 ) and tetravalent cerium
resolved components of tri- and tetravalent cerium, based on cerium phosphate(CePO4) andoxide(CeO
2 ) as reference spectra, respectively.

Figure 3. Cerium K-edge X-ray absorption fine structure spectra, corresponding to X-ray absorption
near-edge structure analysis of the samples by grinding with microwave irradiation, showing

tetravalent cerium oxide(CeO2) as reference spectra, respectively.

Figure 4. The k3 -weighted cerium K-edge extended X-ray absorption fine structure spectra oftetravalent cerium
oxide, samples by grinding with microwave irradiation.
3
Figure 4. The k -weighted cerium K-edge extended X-ray absorption fine structure spectra oftetravalent cerium oxide, samples by grinding with microwave irradiation.

Minerals 2022, 12, 189

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12 of 16

Figure 5. The radial distribution function of tetravalent cerium oxide, samples by grinding with
Figure 5. The radial
distribution function of tetravalent cerium oxide, samples by grinding withmicrowave
irradiation.
microwave irradiation.

3.5.
Comparison
CeriumEfficiency
among These Methods
3.5. Comparison
CeriumReductionReduction
Efficiency among
These Methods
The
relationshipbetweenbetween
energy consumption
the amount of ceriumThe
relationshipenergyandconsumption
and reduction
the amount of cerium reductionin each process is shown in Figure 6. When the energy consumption was the same in each
inprocess,each theprocess
is shown in Figure 6. When the energy consumption was the same inmechanochemical processing by grinding with microwave irradiation
each
process,themechanochemical
processing
by grinding
with microwave irradiation
showedthe highest
amountof cerium reduction among
these processes(Figure 6). Thisfinding shows
mechanochemical
by grinding
with microwave
showedthethathighestamountprocessingof cerium
reduction
amongirradiationthese processes (Figure 6). This
using the agitator mixer could be conducted as a large-scale process more easily than thatfindingshows
thatandmechanochemical
byIntroduction,
grinding thewith microwave irradiationby planetaryball milling
microwave irradiation. processing
As discussed in thedevelopment
and optimization
attractive
experiments
using
the agitator
mixer ofcould
be proof-of-principle
conducted aslaboratory
a large-scale
process more easily than that
into viable large-scale processes have not been achieved since a planetary ball mill isbyplanetary ball milling and microwave irradiation. As discussed in the Introduction,commonly used not on a large scale but only on a laboratory scale. However, based on
theandof attractive
laboratory experiments
this development
result, it is possiblethat optimization
the development andoptimization proof-of-principleof attractive proof-ofprinciple
laboratory
experiments
intolarge-scale
processes
couldbeconducted
in
into viable large-scale processes have not been achieved since a planetary ball mill ismechanochemical processing by grinding with microwave irradiation using the agitator
commonlyused ofnot
on aandlarge
scale butonly onirradiationa laboratory
However, based on thismixer. The method
heating
processing
by microwave
is the scale.
leasteffectiveitsince
the amountthatof ceriumis the lowest.
et al. [23] reported
result,is possible
the reduction
development
andRibeiro
optimizationof attractive proof-of-principlethat microwave irradiation is not always advantageous, due to its rather complicatedlaboratory
experiments into large-scale processes could be conducted in mechanochemical
relationship with the composition of the reaction mixture, the reaction mechanism, and aprocessing by grinding with microwave irradiation using the agitator mixer. The methodof heating and processing by microwave irradiation is the least effective since the amountof cerium reduction is the lowest. Ribeiro et al. [23] reported that microwave irradiation is
not always advantageous, due to its rather complicated relationship with the compositionof the reaction mixture, the reaction mechanism, and a number of other factors. Thus, it
was confirmed that heat processing by microwave irradiation is not effective for this ore.
When the value of x is 1 in Equation (1), the equation of cerium reduction reactionchanges to Equation (2). As described in Sections 3.3 and 3.4, the oxygen vacancy plays animportant role in the cerium reduction reaction during planetary ball milling and grinding
with microwave irradiation. Equation (2) can be substituted for the half-reaction equation(Equation (3)) involving oxygen vacancy (VO ) [34–36]:
2CeO2 → Ce2 O3 +1
O2
2

4Ce4+ + O2− → 2Ce4+ + 2Ce3+ + VO +

(2)
1
O2
2

(3)

Minerals 2022, 12, 189

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According to Equation (3), the amount of oxygen vacancy could be estimated by theamount of cerium reduction. As shown in Tables 4 and 6, we are able to evaluate theamount of cerium reduction from the XAFS analysis. The relationship between grindingtime and the oxygen vacancy ratio in mechanochemical processing by both planetary ball
milling and grinding with microwave irradiation is shown in Figure 7. In Figure 7, no significant difference between the mechanochemical processing by grinding with microwave
irradiation and that by planetary ball milling was found in the oxygen vacancy ratio. Thisfinding showed that the efficiency of cerium reduction by grinding with microwave irradiation was the same as that achieved by planetary ball milling. As shown in Table 7, theatomic distance of both Ce(IV)-O and Ce(III)-O by grinding with microwave irradiation
decreased more than that by planetary ball milling. When the cerium reduction reaction
occurs, oxygen vacancies are produced in the cerianite structure [14]. As a result of thisstructural change, the atomic distance of both Ce(IV)-O and Ce(III)-O decreases. Thus, the
EXAFS results shown in Section 3.4 revealed that it was easier to change the structure ofcerianite in weathered residual rare-earth ore by grinding it with microwave irradiation
13 of 16
than by planetary ball milling. In addition, it was reported that the Ce(IV)-O distanceof nanocrystalline CeO2 was slightly shorter, with small CeO2 particles, in the previous
study [37]. Based on the above results, it is suggested that the structural change of cerianite
numberof withothermicrowave
factors. Thus,
it was confirmed
by microwave
bygrinding
irradiation
contributedthat
not heat
only processing
to the cerium
reduction
irradiationisnoteffective
for
this
ore.
reaction but also to nanocrystallization.

Minerals 2022, 12, x

14 of 16

Figure6.6.Relationship
Relationshipbetween
betweenenergy
energyconsumption
consumptionand
andthetheamountamountofofcerium
ceriumreduction.
reduction.
FigureWhen the value of x is 1 in Equation (1), the equation of cerium reduction reactionchanges to Equation (2). As described in Sections 3.3 and 3.4, the oxygen vacancy plays animportant role in the cerium reduction reaction during planetary ball milling and grinding
with microwave irradiation. Equation (2) can be substituted for the half-reaction equation(Equation (3)) involving oxygen vacancy (VO) [34–36]:
2CeO → Ce O +
4Ce

+O

→ 2Ce

+ 2Ce

1
O
2
+ V +

(2)
1
O
2

(3)

According to Equation (3), the amount of oxygen vacancy could be estimated by theamount of cerium reduction. As shown in Tables 4 and 6, we are able to evaluate theamount of cerium reduction from the XAFS analysis. The relationship between grindingtime and the oxygen vacancy ratio in mechanochemical processing by both planetary ball
milling and grinding with microwave irradiation is shown in Figure 7. In Figure 7, noFigureRelationshipbetweengrinding
time
and
the
oxygenvacancyratio. by grinding withsignificant
difference
betweenthetimemechanochemical
processing
Figure7. 7.
Relationshipbetweengrinding
andtheoxygenvacancyratio.
microwave irradiation and that by planetary ball milling was found in the oxygen vacancy
on the above
results,
the efficiency
efficiency ofof cerium
cerium reduction
reduction by
by grinding
grinding with
withratio.Based
This finding
showedthat themicrowave
irradiation
was
the
same
as
that
achievedbyplanetaryball
milling.
However,microwave irradiation was the same as that achieved by planetary ball milling. As shown
theTableamounttreatment
involved
inCe(IV)-O
grinding andwithCe(III)-O
microwave
irradiation
is microwave
larger thanin
7, theofatomic
distanceof bothby grindingwiththat involvedin planetary
the volumeof theWhen
agitatingmixer is40 times
irradiation
decreased
more ballthanmillingthat by sinceplanetaryball milling.
the cerium
reduction

Minerals 2022, 12, 189

11 of 13

Based on the above results, the efficiency of cerium reduction by grinding with microwave irradiation was the same as that achieved by planetary ball milling. However, theamount of treatment involved in grinding with microwave irradiation is larger than thatinvolved in planetary ball milling since the volume of the agitating mixer is 40 times thatof the planetary ball mill. Thus, it was revealed that a large amount of tetravalent cerium
could be reduced by grinding with microwave irradiation, with the same efficiency as that
achieved by planetary ball milling.
4. Conclusions
As a result of EXAFS analysis, it was confirmed that the atomic distances of bothCe(III)–O and Ce(IV)–O in the structure of samples ground down by grinding with microwave irradiation decreased, which indicated that the oxygen vacancy reaction occurred.
Since this trend is the same as that achieved by planetary ball milling, it was suggested
that the cerium reduction mechanism by grinding with microwave irradiation was thesame, based on the XAFS analysis results. Since the decrease in atomic distance achievedby grinding with microwave irradiation was bigger than that achieved by planetary ball
milling, it was revealed that it was easier to change the structure of cerianite in weathered
residual rare-earth ore by grinding with microwave irradiation than by planetary ball
milling. In addition, it is suggested that the structural change of cerianite by grinding
with microwave irradiation contributed not only to cerium reduction reaction but also
to nanocrystallization. When evaluating the cerium reduction efficiency, it was revealed
that a large amount of tetravalent cerium could be reduced by grinding with microwave
irradiation, with the same efficiency as that achieved by planetary ball milling.
Author Contributions: Conceptualization, M.I.; C.T.; formal analysis, T.K.; investigation, T.K.; M.I.;
resources, M.I.; writing—original draft preparation, T.K.; writing—review and editing, C.T.; supervision, C.T.; project administration, T.K.; C.T.; funding acquisition, C.T. All authors have read andagreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data available on request due to restrictions eg privacy or ethical.
Acknowledgments: The synchrotron radiation experiments were performed using a BL5S1 beamline
courtesy of the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi,
Japan (Proposal No. 201801021), and a BL01B1 beamline of SPring-8, with the approval of the Japan
Synchrotron Radiation Research Institute (Proposal No. 2021B1031). Part of this work was performed
as a component of the activities of the Research Institute of the Sustainable Future Society, Waseda
Research Institute for Science and Engineering, Waseda University.
Conflicts of Interest: The authors declare no conflict of interest.

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