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 the fact that mechanochemical reactions offer rapid, cleaner alternatives to the conventional chemical reactions used in nanomaterials, waste recycling, and mineral processing [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
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,
2,
3,
4,
5,
6]. In addition, the process of mechanochemical reaction has recently been investigated and revealed by X-ray diffraction analysis (XRD), kinetics, and X-ray absorption fine structure (XAFS) analysis [
13,
14,
15,
16,
17,
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 come to pass [
1,
12].
In many of the previous studies [
7,
8,
9,
10,
11], a planetary ball mill is commonly used to cause the mechanochemical reaction for many kinds of grinding samples. However, a planetary ball 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 they cannot as yet be implemented. Thus, the development of alternative mills as a substitute for 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 preparation and the catalytic reaction. However, it was reported that the most effective processes for the catalytic oxidation of cyclohexane were a combination of microwave irradiation and mechanochemical processing by ball milling [
23]. According to this study, it is possible for grinding with microwave irradiation to become a more effective process than planetary ball 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 for weathered residual rare-earth ore, this ore was selected as a target material in this study. The cerium reduction reaction is as follows:
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 by grinding with microwave irradiation was compared with that by planetary ball milling and microwave 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 determined by X-ray fluorescence (XRF; ZSX PrimusIII+, Rigaku Corporation, Tokyo, Japan) while the concentrations of rare-earth elements were analyzed by laser ablation inductively coupled plasma 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 atmospheric conditions. Each experiment heated 10 g ore. The increasing speed was fixed at 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]. Each grinding 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 dm
3. During grinding experiments with the planetary ball mill, the energy consumption of 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 experiment ground 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 volume of the mixer was 20 dm3. During grinding experiments with microwave irradiation, the energy 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 described in 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, with a 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 mode using a 19-element germanium (Ge) solid-state detector. Cerium phosphate (CePO4) and cerium oxide (CeO2) were used as reference materials.
The EXAFS analysis was conducted under the same conditions as described in our previous paper [
14]. Briefly, the EXAFS function was Fourier-transformed from a
k3-weighted EXAFS function to a radial distribution function (RDF) using a Hanning window function of within 1–12 × 10
10 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 both the
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 first to the fourth coordination shells. The theoretical phase and amplitude function for each shell, as used in curve fitting, were calculated by FEFF 6.0 [
24,
25]. All EXAFS analysis was performed using Athena and Artemis software [
26].
4. Conclusions
As a result of EXAFS analysis, it was confirmed that the atomic distances of both Ce(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 the same, based on the XAFS analysis results. Since the decrease in atomic distance achieved by 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.