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ACS Omega
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10.1021/acsomega.4c02088
Article
Decomposition Model of Bastnaesite and Its Fluorine Oxygen Coupling Escape Mechanism
Li Jianfei †‡§∥
Xu Yubo †‡§
Zhang Xiaowei †‡§
Wu Jinxiu †‡§
Dong Zhisheng †
https://orcid.org/0000-0003-4670-1403
Liu Zhaogang *†‡§#
Hao Xiaoyan ⊥
† School of Rare Earth Industry, Inner Mongolia University of Science and Technology, Baotou, Inner Mongolia 014010, China
‡ Key Laboratory of Green Extraction and Efficient Utilization of Light Rare Earth Resources, Ministry of Education, Baotou, Inner Mongolia 014010, China
§ Key Laboratory of Rare Earth Wet Metallurgy and Light Rare Earth Applications of Inner Mongolia Autonomous Region, Baotou, Inner Mongolia 014010, China
∥ School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
⊥ China North Rare Earth (Group) High Tech Co., Ltd., Baotou, Inner Mongolia 014010, China
# School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China
* Email: liuzg65@163.com.
06 09 2024
17 09 2024
9 37 3843738451
03 03 2024
26 07 2024
09 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Nearly 90% of rare earths worldwide currently originate from bastnaesite (REFCO3). Oxidative roasting, an effective method for the treatment of bastnaesite, has been extensively employed in industrial production practice. However, the roasting decomposition mechanism of bastnaesite at the molecular level remains controversial. In this study, two roasting atmospheres (i.e., Ar and O2) were adopted for the treatment of the mixed rare earth concentrate in Bayan Obo. Ar-Roasting ore was skillfully treated as an intermediate and then underwent oxidation roasting. The results achieved using research methods (e.g., XRD, FT-IR, TEM, and XPS) first revealed that the oxidation roasting of bastnaesite was performed in several stages. The first stage was the thermal decomposition of bastnaesite and the oxidation of cerium (REFCO3 → REOF + CO2). The second stage was the escape pathway of fluorine (REOF + O2 → RExOy + OxF and OxF → O2 + F2), describing the migration rules of F and O elements and the oxidation behavior of Ce. The results of this study can provide more theoretical support and guidance for the clean extraction of bastnaesite and mixed rare earth in Bayan Obo.

National Natural Science Foundation of China 10.13039/501100001809 51964040 Scientific Research Fund for the Introduction of High-level Talents to Institutions of the Inner Mongolia Autonomous Region NA NA Natural Science Foundation of Inner Mongolia Autonomous Region 10.13039/501100004763 2022QN05017 document-id-old-9ao4c02088
document-id-new-14ao4c02088
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pmc1 Introduction

Rare earth has been confirmed as a nonrenewable and vital strategic resource.1 Rare earth elements have been extensively employed in numerous highly sophisticated fields, especially in aerospace and military products that take on critical significance in a wide range of countries2−4 since these elements exhibit a special electron layer structure and numerous prominent physical and chemical properties from the macroscopic perspective.5 Currently, the global sources of rare earth are mainly concentrated in more than 10 kinds of rare earth minerals with industrial mining value (e.g., bastnaesite (REFCO3), monazite (REPO4), xenotime (YPO4), yttrium fluorite (Ca, Y)F2, [Al2Si2O5(OH)4]m-REO, and (Ce, La)2Ca2(CO3)F),6 and bastnaesite, monazite, and xenotime account for over 90% of the global supply of rare earth;7 bastnaesite has been confirmed as one of the critical raw materials in the global rare earth industrial production for its superior smelting performance, huge reserves, and simple process, and its rare earth recovery rate is capable of directly determining the global rare earth supply chain.8

In general, oxidative roasting is taken as the first step of the technical route in the recent industrial production of bastnaesite.9−11 After oxidation roasting, bastnaesite (with rare earth carbonate covered) can be decomposed into rare earth compounds that are more soluble in acid. Subsequently, most of the rare earths can be transferred into a solution through an acid leaching process, such that rare earths can be efficiently leached.12,13 The roasting process has been reported as a more mature method, and it has been extensively employed in rare earth production practices. Nevertheless, a considerable number of key issues remain about the transformation of the mineral phase of bastnaesite during roasting, comprising how O2 plays a certain role in the reaction, how the chemical bond of fluorine is broken, how to achieve controlled regulation of the roasting product, and the redox behavior of Ce3+/Ce4+ that have not been precisely described,14 coupled with the fact that most studies only use economic indicators (e.g., rare earth recovery and leaching rate) to examine the merits of the process, resulting in a lack of deeper understanding of the reaction mechanism in the roasting of bastnaesite, and the roasting mechanism has not been studied in depth. The roasting process of bastnaesite has not been capable of achieving a revolutionary change over the past few years.

It is generally believed that bastnaesite will decompose to form CeOF, CO2, and other phases (eq 1) in the roasting process,15−18 but it is difficult to explain the escape of small amounts of F and the oxidation of Ce in the process.12 Several scholars suggested that bastnaesite will decompose to form CeO2, CO2, and F2 (eq 2), whereas CeO2 will continuously decompose to CeO2–X and O2 (eq 3) when heated.19,20 As indicated by recent research, the generated CeOF will continue to react with CO2 to form CeO2, CeF3, and CO (eq 4). Subsequently, CeO2 and CO continuously undergo redox reactions to form Ce2O3, CO2, and so on (eq 5).21 As indicated by existing research, there is still no unified conclusion on the roasting products and the roasting reaction mechanism of bastnaesite, and some of the above-mentioned research processes are only at the conjectural stage, with no definite evidence of the generation of gases such as F2 and CO. For this reason, research on the roasting reaction mechanism of bastnaesite can provide a theoretical reference and take on critical significance in the research and development of the rare earth industry.

Bayan Obo mixed rare earth ore serves as a vital source of bastnaesite, and its high-temperature roasting reaction process is more sophisticated.22,23 For this reason, a Bayan Obo mixed rare earth concentrate was employed as a raw material to explore the roasting reaction process of its bastnaesite, gain insights into the mechanism of fluorine and oxygen coupling and the oxidation behavior of Ce, infer the reaction process and element migration law, and provide a certain reference for promoting the clean production of the Bayan Obo mixed rare earth concentrate in depth.1

2

3

4

5

2 Experiments

2.1 Raw Materials and Reagents

The rare earth concentrate used in this study was obtained from China Northern Rare Earth (Group) Hi-Tech Co., Ltd., and the chemical compositions and rare earth contents of the sample are listed in Table 1.

Table 1 Chemical Analysis of Mixed Rare Earth Concentrates

REO (%)	F (%)	P (%)	CaO (%)	
62.38	8.23	2.54	6.45	

2.2 Experimental Methods and Test Methods

The raw material employed in this study was a Bayan Obo mixed rare earth concentrate, which was primarily composed of bastnaesite and a small amount of monazite (with rare earth grade of 65%; Ce accounted for over 50% of the rare earth allocation). The mixed rare earth concentrate was employed after sieving and drying (80 °C, 2 h). Figure 1 presents a schematic diagram of the research scheme. In this study, the changes in a series of physical phases and product structures of the rare mixed earth concentrate were mainly investigated in different roasting atmospheres (O2/Ar), the reaction process was predicted, and the roasting reaction mechanism was speculated. There were three main roasting processes, which are elucidated below.1. In the research on the Ar-roasting process, a box was customized to avoid and reduce the effect of residual O2 in the furnace chamber on the roasted products in the roasting process. Six layers of the mixed rare earth concentrate of identical mass were placed. Next, the packet was placed in an atmospheric furnace. The sample was evacuated to a furnace pressure of −0.04 MPa. Afterward, Ar was introduced to a furnace pressure of the standard atmosphere, and then, the sample was evacuated again to a furnace pressure of −0.04 MPa. We cycled the process three times following the aforementioned steps. The final pressure gauge display value should be −0.04 MPa. Afterward, we ensured precise control of conditions such as roasting time and temperature. After the preset conditions were reached, the product was cooled to ambient temperature and removed, and the product was termed Ar-roasting ore. Figure 1 shows the Ar-roasting ore in an Ar-roasting atmosphere at 550 °C.

2. In the research on the O2-roasting process, the atmosphere was air, and the mixed rare earth concentrate was directly roasted and then decomposed in a high-temperature furnace to prepare the product termed O2-roasting ore.

3. In this study, Ar-roasting ore was cleverly considered as an intermediate product, and the Ar-roasting ore was then oxidized and roasted (the second roasting method) to prepare the product termed O2–Ar-roasting ore.

Figure 1 Schematic diagram of the research scheme (exploring the reaction mechanism of bastnaesite through the regulation of different roasting atmospheres).

The TG-DSC curve was generated using a thermogravimetric analyzer (STA-449C) from Netzsch, Germany. XRD phase analysis was conducted using an X-ray diffractometer (D8 ADVANCE) from Bruker, Germany. The group or chemical bond breakage of the sample was analyzed through Fourier transform infrared spectroscopy (Nicolet iS50) from Thermo Fisher, US. The chemical valence states of Ce, F, O, and other elements in the sample were explored through X-ray photoelectron spectroscopy (ESCALAB 250Xi) from Thermo Fisher, US. The adsorption of O2 on the sample was examined using a chemical adsorption instrument (PCA-1200) from Beijing Pioneer Company. The pyrolysis gas was analyzed with an online mass spectrometer (HPR-20) from Ingehead, UK. The diffraction ring of the sample was analyzed and then characterized under a Japanese transmission electron microscope (JEM-2100F).

3 Results and Discussion

3.1 XRD and TG-DSC Analysis

Figure 2 presents the XRD patterns of the mixed rare earth concentrate. As depicted in Figure 2, the mixed rare earth concentrate primarily comprised bastnaesite-Ce (●-CeFCO3), monazite-Ce (▼-CePO4), apatite (■-Ca5(PO4)3F), fluorite (▲-CaF2), thorianite (◆-Pb0.875Th0.125F2.25), cerianite-Ce (☆-Ce0.33Th0.33La0.33O1.83), and other phases. While rare earth minerals largely covered bastnaesite and monazite, bastnaesite served as the most abundant mineral in the mixed rare earth concentrate. The significant difference in the thermal decomposition behaviors of bastnaesite and monazite can lay a solid basis for the study of the roasting reaction mechanism of bastnaesite in the mixed rare earth concentrate from Bayan Obo. Accordingly, the above mixed rare earth concentrates were roasted in different roasting atmospheres, and their physical phase structure changes and evolution patterns were examined.

Figure 2 XRD patterns of the mixed rare earth concentrates (the test conditions: 2°/min, copper target).

Figure 3 illustrates the XRD patterns of Ar–O2-roasting ore, Ar-roasting ore, and O2-roasting ore. As depicted in Figure 3, the main phases of Ar–O2-roasting ore comprised Ce11O20, Ce(PO4), CaF2, Ca2F(PO4)3, and Ce0.33Th0.33La0.33O1.83. The valence state of Ce in the most abundant Ce11O20 ranged from +3 to +4, with an average value of +3.32, and no F element played a certain role in bonding. The main phases of Ar-roasting ore comprised Ce3O4F3, Ce(PO4), CaF2, Ca5F(PO4)3, and Ce0.33Th0.33La0.33O1.83. The average valence state of Ce in the most abundant Ce3O4F3 was +3.66, whereas elemental F played a significant role in bonding. The main phases of O2-roasting ore covered Ce7O12, Ce(PO4), CaF2, Ca(PO4)3, and Ce0.33Th0.33La0.33O1.83. The average valence state of Ce in the most abundant Ce7O12 reached +3.43, and no F element played a certain role in bonding. After the main peaks of the three calcined ores were magnified, the result indicated that the relative angular offsets of the four main peaks of Ce11O20, Ce7O12 and the four main peaks of Ce3O4F3 reached 0.92, 0.43° (b1-a1, b1-c1), 1.17, 0.56° (b2-a2, b2-c2), 1.22, 0.60° (b3-a2, b3-c2), and 1.71 and 0.88° (b4-a4, b4-c4), respectively. Accordingly, Ar-roasting ore may serve as one of the intermediate products in the roasting process of bastnaesite, and O2 may facilitate the oxidation of Ce3+ while causing the escape of F.24

Figure 3 XRD analysis of mixed rare earth concentrates under different roasting atmospheres: (a) Ar–O2-roasting ore, (b) Ar-roasting ore, and (c) O2-roasting ore (the test conditions: 2°/min, copper target).

The weight loss rate and TG-DSC data of the O2-roasting ore and the O2–Ar-roasting ore were identified (Figure 4) to verify the above conjecture. As depicted in Figure 4a, with the gradual increase of the roasting temperature, the O2-roasting ore had a significant weight at nearly 500 °C with a weight loss rate of 11.36%, consistent with the results presented in Figure 4c. The possible reason for this result is the release of CO2. In contrast, the argon roasting started extensive decomposition only at 550 °C, about 50 °C higher than the oxygen roasting temperature, consistent with the literature results.21 Accordingly, the involvement of O2 will be conducive to reducing the decomposition temperature of bastnaesite. Moreover, to try to avoid experimental errors arising from unreacted CO32–, the Ar-roasting ore at 850 °C was selected for the oxidation roasting test again (Figure 4b). As indicated by the result, with the gradual increase of the roasting temperature (100–400 °C), the weight of the O2–Ar-roasting ore tended to be increased. The possible reason for this result is that the free radical O· in air gradually filled the remaining oxygen vacancies and defects of Ce3O4F3 while causing the transformation of Ce3+ to Ce4+. During this transformation, the adsorption of free radical O· exceeded the escape. At the roasting temperature of ≥500 °C, although the weight gain phenomenon was still notable, the weight gain ratio tended to decline. This study suggested that the escape of free radical O· was increased at this time, and its ability to fill oxygen vacancies gradually decreases. The above finding was an important reason why CeO2 exhibits excellent oxygen storage and release capacity and has been extensively used in catalysis.25 When the roasting temperature was increased to >800 °C, a significant weight loss was identified, and the weight loss rate was about 0.33%, probably attributed to the escape of O or F from Ce3O4F3. Although this result differed from the decomposition temperature at which the weight loss began in Figure 4d by nearly 50 °C, this can be attributed to the reabsorption of some of the radical O· in the cooling process with the furnace in Figure 4b. It is noteworthy that the TG curve (blue) of Figure 4d displayed irregular fluctuations in the range of 100–800 °C with an exothermic tendency. The above-mentioned result may suggest that the adsorption and escape of free radical O· during low-temperature roasting may be in a dynamic equilibrium.

Figure 4 Weight loss rate and TG-DSC. (a) Weight loss rate of O2-roasting ore (red lines) and Ar-roasting ore (blue line), (b) weight loss rate of O2–Ar-roasting ore, (c) TG-DSC of O2-roasting ore, and (d) TG-DSC of O2–Ar-roasting ore.

The structure of O2–Ar-roasting ore phases at different temperatures was analyzed in depth to further explain the weight gain and weight loss changes exhibited by O2–Ar-roasting ore during roasting. As depicted in Figure 5, with the increase of the roasting temperature, the rare earth physical phase then gradually shifted from Ce3O4F3 (●) to Ce11O20 (◆), while the main diffraction peaks of the (111), (200), (220), and (311) crystalline surfaces shifted to higher angles with the offset (Δ) of nearly 1.04, 0.9, 1.8, and 2.22°, respectively. The possible reason for this result is that the ionic radius of oxidation of Ce4+ (0.092 nm) is smaller than that of Ce3+ (0.1034 nm),26 resulting in a decrease in the cell parameters. Notably, the offset angle was the smallest (78°) when the roasting temperature was elevated from 700 to 800 °C, and a gradual broadening of the diffraction peak was identified, suggesting that the phase transition was most significant at this stage, consistent with the results in Figure 4b,d. As revealed by the in-depth analysis of O2 adsorption by Ar-roasting ore, Ar-roasting ore exerted a certain adsorption effect (Figure 6), thus confirming that Ar-roasting ore can have more oxygen vacancies, such that the adsorbed O2 is the root cause of Ce3+ oxidation.

Figure 5 XRD diagram of O2–Ar-roasting ore at different roasting temperatures.

Figure 6 O2 adsorption test (the blue line represents the adsorption of oxygen by Ar-roasted ore, while the black line represents the adsorption of oxygen by O2–Ar-roasted ore).

3.2 XPS Analysis

XPS analysis was conducted on the raw ore, O2-roasting ore, Ar-roasting ore, and O2–Ar-roasting ore (Figure 7) to further illustrate the redox mechanism of Ce3+/Ce4+ in the respective roasting product. As depicted in Figure 7a, the electron spin magnetic field was coupled with the orbital angular momentum, such that the Ce 3d spin–orbital energy level fell into two spin–orbital energy levels, i.e., Ce 3d3/2 and Ce 3d5/2,27 with a splitting difference of nearly 18.6 eV, while two distinct components controlled each spin–orbital where w’ = 905 ± 0.1 eV and w” = 900 ± 0.1 eV peaks pertain to Ce 3d3/2. r’ = 905 ± 0.1 eV and r” = 900 ± 0.1 eV peaks can be classified as Ce 3d5/2. Due to the coupling between O 2p and Ce 4f orbitals (O 2p → Ce 4f electron leap), the final state of 3d94f1 O 2p6 resulted in w’, r’, w”, and r” as the final state of Ce 3d94f2 O 2p5; w’, r’, w”, and r” express the characteristic peaks for the presence of Ce3+ ions,28 suggesting that the element Ce in the original ore was primarily present in the +3 valence form. Notably, Ce3+ ions did not exhibit the characteristic appearance of the 4f0 states.

Figure 7 Valence state analysis of the Ce element in different roasted ores: (a) mixed rare earth concentrate, (b) 500 °C O2-roasted ore, (c) 500 °C Ar-roasted ore, (d) 850 °C Ar-roasted ore, (e) 500 °C O2–Ar-roasted ore, and (f) 900 °C O2–Ar-roasted ore.

As depicted in Figure 7b, 10 Ce 3d characteristic peaks appeared in O2-roasting ore (u’ = 916.64 ± 0.1 eV, u” = 906.6 ± 0.1 eV, u”’ = 900.93 ± 0.1 eV, v’ = 898.17 ± 0.1 eV, v” = 887.12 ± 0.1 eV, v”’ = 882.63 ± 0.1 eV, w’ = 902.67 ± 0.1 eV, w” = 898.83 ± 0.1 eV, r” = 884.46 ± 0.1 eV, and r” = 881.5 ± 0.1 eV).29 The value of 889 ± 0.1 eV was the Osher peak of Fe (Fe LM6), and w’, w”, r’, and r”’ belonged to the characteristic peaks of Ce3+ ions (consistent with the results presented in Figure 5a), The possible reason for this result is from CePO4 (monazite) or attributed to insufficient oxidation. u’, u”, u”’, v’, v”, and v”’ belonged to Ce4+ ion characteristic peaks.30 At this time, Ce 3d3/2 and Ce 3d5/2 spin orbitals of Ce4+ ions were controlled by three characteristic components, where Ce 3d94f0 O 2p6, Ce 3d94f1 O 2p5, and Ce 3d94f2 O 2p4 express u’(v’), u”(v”), and u”’(v”’) final states and appeared as 4f0 final states.31

As depicted in Figure 7c, the characteristic peaks of Ce3+ and Ce4+ appeared in the Ar-roasting ore. Still, the intensity of the characteristic peak of Ce4+ was lower than that of Ce3+ (1:1.47, Table 2, c). The possible reason for this result is the reaction of Ce3+ on the surface with the residual trace O2 or the decomposition of part of Ce3O4F3 to generate CeO2 and CeF3 (eq 6). When the roasting temperature tended to be increased to 850 °C, the ratio of the sum of characteristic peak areas of Ce4+ to the sum of characteristic peak areas of Ce3+ increased (1:1.58, Table 2, d), suggesting a weaker reaction of eq 6.

Table 2 Ratio of the Ce4+ to Ce3+ 3d Characteristic Peak Area Suma

 	A, the sum of the areas of Ce4+ 3d peaks, eV	B, the sum of the areas of Ce3+ 3d peaks, eV		
a	 	22,242.42	 	
b	20,930.21	28,876.88	1:1.38	
c	17,808.60	26,206.06	1:1.47	
d	18,801.20	29,652.90	1:1.58	
e	20,684.95	14,378.75	1:0.69	
f	24,398.04	4711.32	1:0.19	
a (a) Mixed rare earth concentrate, (b) 500 °C O2-roasted ore, (c) 500 °C Ar-roasted ore, (d) 850 °C Ar-roasted ore, (e) 500 °C O2–Ar-roasted ore, and (f) 900 °C O2–Ar-roasted ore.

As depicted in Figure 7e,f, the characteristic peaks of Ce3+ and Ce4+ were also present in the O2–Ar-roasted ore, whereas the characteristic peaks of Ce3+ were primarily from monazite. It is noteworthy that the sum of the characteristic peak areas of Ce4+ in the O2–Ar-roasted ore was significantly higher than that of Ce3+ compared with those in the O2-roasted ore and Ar-roasted ore (1:0.69, Table 2, e), and the sum of the characteristic peak areas of Ce4+ was increased significantly with the gradual increase of the oxidation roasting temperature (1:0.19, Table 2, f). The above result suggested that compared with direct oxidation roasting of bastnaesite, the Ar-roasted ore displayed more defects and oxygen vacancies, such that it was also more susceptible to oxidation.6

The O 1s spectra of different samples in the range of 525–540 eV were analyzed (Figure 8) to further illustrate the role of O2 in the roasting process of bastnaesite. As depicted in Figure 8a, there were three types of oxygen structures in the mixed rare earth concentrate of Bayan Obo: nonbridging oxygen K’ (C–O and P–O) centered at 532.2 eV, bridging oxygen K” (CO–C and PO–P) centered at 531.3 eV, and 529.8 eV centered on a small amount of lattice oxygen K”’ (Ce3+–O),32−34 where the nonbridging and bridging oxygen primarily originated from the PO43– and CO32– groups of monazite and bastnaesite. As depicted in Figure 8b, after oxidative roasting at 500 °C, four oxygen structures appeared in the roasted products, comprising nonbridging oxygen K’ (P–O), bridging oxygen K” (PO–P), lattice oxygen K”’ (Ce4+–O) and adsorbed oxygen M. Simultaneously noting a significant increase in the proportion of bridging oxygen K” and lattice oxygen K”’.35,36 The probable reason for the above result is that REFCO3 decomposed when heated, and considerable rare earth phases were generated (e.g., CeOF and CeO2), resulting in the gradual appearance of the characteristic peak of the lattice oxygen (Ce4+–O). At the same time, REPO4 remains almost unchanged, and the stable presence of the PO43– group and the significant reduction of the CO32– group lead to the decrease of the bridging oxygen K” (PO–P) peaks that gradually appeared. Notably, as indicated by the appearance of the characteristic peak of adsorbed oxygen M, free radical O· started to fill the oxygen vacancies and defects on the mineral surface and caused a Ce3+ to Ce4+ transition, consistent with the results in Figure 7b.

Figure 8 Valence state analysis of the O element in different roasted ores. (a) Mixed rare earth concentrate, (b) 500 °C O2-roasted ore, (c) 500 °C Ar-roasted ore, (d) 850 °C Ar-roasted ore, (e) 500 °C O2–Ar-roasted ore, and (f) 900 °C O2–Ar-roasted ore.

As depicted in Figure 8c, when the mixed rare earth concentrate was roasted at 500 °C in the Ar atmosphere, the roasted products showed three oxygen structures comprising nonbridging oxygen K’ (P–O), bridging oxygen K” (PO–P), and lattice oxygen K”’ (Ce3+–O) with similar peak patterns as presented in Figure 7a, whereas no obvious characteristic peak of adsorbed oxygen was identified. When the roasting temperature was increased to 850 °C, a small amount of the characteristic peak of adsorbed oxygen (532.4 eV) appeared. The possible reason for this result is that the residual O2 or Ce3O4F3 decomposed and released part of O atoms. Furthermore, the central peak of lattice oxygen K”’ (Ce3+–O) shifted toward the low binding energy and the peak intensity increased (Figure 8d), suggesting that the decomposition of REFCO3 was more complete.

As depicted in Figure 8e, when the Ar-roasting ore was oxidatively roasted again, its products also comprised four oxygen structures, namely, nonbridging oxygen K’ (P–O), bridged oxygen K” (PO–P), lattice oxygen K”’ (Ce4+–O), and adsorbed oxygen M with similar peak patterns to Figure 7b, suggesting that the mineral surface information on Ar-roasted ore after reoxidation roasting was similar to that of O2-roasted ore. Thus, a theoretical basis was laid for viewing Ar-roasted ore as an intermediate. The peak intensity of the bridging oxygen K” (PO–P) was reduced when the roasting temperature was increased to 900 °C, probably because PO43– also started to react locally.

3.3 FT-IR Analysis

To gain insights into the change of chemical bonds better during roasting, FT-IR(ATR) analysis was performed on the respective roasting product, as presented in Figure 9 (where υ represents the asymmetric vibration, δ represents the symmetric vibration, γ represents the bending vibration, and π represents the contraction vibration of the lattice).

Figure 9 FT-IR(ATR) analysis of (a) pure substance, (b) mixed rare earth concentrate, Ar-roasted ore, and O2-roasted ore, (c) mixed rare earth ores at different oxidation roasting temperatures, and (d) O2–Ar-roasted ore at different roasting temperatures.

The infrared spectra of four chemicals, CePO4, Ce2(CO3)3, CeF3, and CeO2, were first studied to reasonably determine the breakage of the chemical bonds among the atoms within the rare earth minerals during the roasting process. As depicted in Figure 9a, the characteristic infrared spectra of PO43– were mainly asymmetric vibrations υ (1090, 1040, and 990 cm–1), symmetric vibrations δ (950 cm–1), and bending vibrations γ (867, 840, and 729 cm–1). The characteristic infrared spectra of CO32– were mainly asymmetric vibrations υ (1435 cm–1) with bending vibrations γ (867, 840, and 729 cm–1). The chemical bonding of CeF3 and CeO2 was mainly ionic, and the wavenumbers of their IR characteristic peaks were below 400 cm–1, mainly the contraction vibrations of the lattice π (Ce–F, 342 cm–1) and π (Ce–O, 266 cm–1). Notably, CePO4, Ce2(CO3)3, and CeF3 had a small amount of lattice contraction vibrations π (Ce–O) in the wavenumber range of 287–266 cm–1, suggesting that CePO4 and Ce2(CO3)3 were also subjected to some interactions between Ce3+ and O2– during molecular conformation, while CeF3 was probably attributed to air oxidation or contamination from the preparation process.

As depicted in Figure 9b, in the wavenumber range of 1800–400 cm–1, no matter what type of atmosphere the mixed rare earth concentrate was roasted in, the peak strength of the asymmetric vibration υ and the bending vibration γ of CO32– was reduced to varying degrees, whereas the asymmetric vibration υ, symmetric vibration δ, and bending vibration γ peak of PO43– did not vary notably. In the wavenumber range of 400–200 cm–1, trace amounts of lattice contraction vibrations π (Ce–F and Ce–O) were still visible in the 850 °C Ar-roasted ore, whereas the lattice contraction vibrations π (Ce–O, 257 cm–1) became more significant in the O2-roasted ore. The possible reason for the above result is that during the thermal decomposition of the mixed rare earth concentrate, the CO32– group in REFCO3 first decomposed (eq 1) to produce the REOF phase and CO2, and the contraction vibration of π(Ce–F) was more vigorous. Afterward, with the continuous intervention of O2, the free radical O· was adsorbed to the remaining oxygen vacancies and lattice defect sites on the surface of REOF minerals such that the contraction vibration of π(Ce–O) was strengthened, while the contraction vibration of π(Ce–F) was masked. This trend became more significant with the gradual increase of the roasting temperature (Figure 9c). It is worth noting that the intensity of the π(Ce–O) characteristic peak in the low-temperature stage (100–400 °C) is stronger than that in the high-temperature stage (500–900 °C), which can be attributed to the fact that most of the REFCO3 is not yet decomposed during the low-temperature stage. Indeed, a small amount of fluorine escape may be one of the reasons for the reduction of the π(Ce–F) lattice shrinkage vibration. In this process, REPO4 was relatively stable and hardly varied, such that the asymmetric vibration υ, symmetric vibration δ, and bending vibration γ peaks of PO43– were always present.

As depicted in Figure 9d, the characteristic peak pattern was essentially the same in the wavenumber range of 1800–400 cm–1 with the gradual increase of the roasting temperature. In the wavenumber range of 400–200 cm–1, the contraction vibration π(Ce–F) of the lattice was almost undetectable, suggesting that the adsorption of radical O· can be achieved at low temperatures for Ar-roasted ores. In contrast, the contraction vibration of π(Ce–O) was slightly enhanced with increasing temperature, consistent with the previous findings in Figure 9b,c. In brief, in addition to the calcination temperature, O2 may be one of the inducing factors that promote the escape of a small amount of F atoms.

3.4 TEM Analysis

As depicted in Figure 10b, the diffraction ring spacing of each crystal plane of Ar-roasted ore was similar to that of the (La, Ce)OF (PDF no. 00-057-0608). The above result suggested that while the F element played a significant role in Ce3O4F3 bonding (Figure 10a,c), the O2-roasted ore and O2–Ar-roasted ore diffraction ring of each crystal plane spacing was closer to that of CeO2 (PDF no. 03-065-2975), whereas O2-roasted ore had (200) crystal planes protruded out in the O2-roasted ore. The possible reason for this result is the different degree of oxidation of Ce3+ to Ce4+, consistent with the results in Figure 3. Thus, it was assumed that the Ar atmosphere-roasted ore (CeOxF) was an intermediate product of REFCO3 in the oxidative roasting process, and the main physical phase change that occurred was expressed as CeFCO3 → CeOxF → CeOx.

Figure 10 TEM analysis ((a) O2–Ar-roasted ore, (b) Ar-roasted ore, and (c) O2-roasted ore).

3.5 Fluorine Loss during Thermal Decomposition

The gases of the Ar-roasting ore oxidation process were analyzed (Figure 11) to further investigate the possible reaction processes during the roasting of bastnaesite. As indicated by the result, with the gradual increase of the roasting temperature, the content of O2 tended to decline, arising from the adsorption of O2 on the mineral surface (consistent with the results in Figure 6). Moreover, trace amounts of F2 were produced. The fluorine escape behavior of the mixed rare earth concentrate at different roasting temperatures was examined to investigate the cause of F2 production. As depicted in Figure 12, with the gradual increase of the roasting temperature (100–700 °C), the fluorine loss rate was between 5 and 6%, while most of the fluorine was still present in the mineral. As depicted in Figure 13, the difference in chemical bonds shifted between the F 1s central peak of the mixed rare earth concentrate, and different roasted ores were within 0.5 eV, suggesting that a higher energy was required for the chemical bond breakage of fluorine. To verify this conjecture, the morphology and elemental distribution states of the mixed rare earth concentrate during high-temperature roasting were analyzed, as presented in Figures 14 and 15 and Table 3.

Figure 11 Thermal decomposition gas analysis.

Figure 12 Loss rate of fluorine (loss rate of F in mixed rare earth concentrate at different roasting temperatures).

Figure 13 F 1s XPS. (a) Mixed rare earth concentrate, (b) 500 °C O2-roasted ore, (c) 500 °C Ar-roasted ore, (d) 850 °C Ar-roasted ore, (e) 500 °C O2–Ar-roasted ore, (f) 900 °C O2–Ar-roasted ore, and (h) 1400 °C O2-roasted ore.

Figure 14 Face scan (surface scanning of mixed rare earth concentrates at different roasting temperatures) ((a) 1100, (b) 1200, (c) 1300, (d) 1400, and (e) 1500 °C).

Figure 15 SEM-EDS (morphology and energy spectrum of mixed rare earth concentrates at high-temperature roasting temperatures).

Table 3 EDS Pattern of Roasted Ore

 	temperature	La	Ce	Ca	P	F	O	
①	1100 °C	20.03%	36.66%	6.06%	0.78%	12.18%	4.96%	
②	1200 °C	5.99%	45.68%	8.94%	0.71%	10.44%	10.32%	
③	1300 °C	3.71%	65.73%	0.71%	0.45%	0.82%	8.36%	
④	1400 °C	3.65%	69.14%	0.43%	0.47%	0.37%	10.64%	
⑤	1500 °C	3.17%	72.45%	0.66%	0.78%	0.89%	11.00%	

Figure 14 illustrates that as the roasting temperature increases, the La, P, and O elements exhibit a strip-like structure and gradually accumulate together, while the Ce element gradually accumulates with Ca, F, and O elements. At temperatures exceeding 1200 °C, spherical ceria particles with a diameter of approximately 1.0 μm begin to form (Figure 15). The size of the spherical particles increases with the calcination temperature. Table 3 illustrates that as the roasting temperature increases (1100–1500 °C), the F content in ceria spherical particles declines from 12.18 to 0.89%, while the O content rises from 4.96 to 11.00%. In comparison to the low-temperature calcination process (100–700 °C) (Figure 12), the high-temperature calcination method is more conducive to the escape of fluorine.

In brief, F2 may originate from the mineral surface due to the presence of unsaturated bonds in a small amount of F· on the mineral surface (Figure 16) and the relatively low energy required for the reaction between O· and F·, which can easily react to form OF· and OF2 (eqs 7 and 8). In addition, OF2 can decompose to O2 and F2 at 200 °C (eq 9).377

8

9

Figure 16 Mechanism of fluorine escape.

3.6 Roasting Reaction Mechanism

The possible reaction pathways of bastnaesite during roasting decomposition were deduced based on the above data analysis (Figure 17). As indicated by the result, the bond energy of ⑥C=O (σ bond and π bond) in CO32– exceeded that of ① or ②C–O (σ bond). On that basis, with the gradual increase of the roasting temperature, the polarization of C and O elements in CO32– intensified. Subsequently, Ce3+ preferentially seized an O2– in CO32– bonded by an σ bond, leading to the breakage of the ① or ②C–O bond. For convenience, it was assumed that the ①C–O bond occurred first, and the Ce-O·– (a p orbital electron) and C·– (an sp2 orbital) had an unpaired electron after the break, which tend to form Ce=O and C=O. Moreover, the breakage of ①C–O caused electron rearrangement and site resistance effects, resulting in further elongation and weakening of the ④Ce–O bond, and the Ce-O·– bond length was shortened, such that ④Ce–O started to display a trend of breaking. Afterward, two reaction pathways were identified next. (1) When O2 was present, the high-temperature thermal radiation caused the chemical bond of O2 to break evenly, producing radical O· (O2 + hυ (180–240 nm) → 2O·), which competed with O2– in the ④Ce–O bond for the 6s5d orbital electrons of Ce3+ and eventually attracted the p orbital electrons of O2– in the ④Ce–O bond to form a π bond. As a result, a lower reaction potential and decomposition temperature were achieved, such that the breakage of the ④Ce–O bond was facilitated to produce CO2 gas. Moreover, due to the change of the charge field, the 4f electron shielding effect was reduced, and 4f electrons started to participate in bonding, such that the valence state of Ce in the main phase Ce11O20 of O2-roasting ore had an average state of +3.32. (2) When Ar was present, the fracture of ④Ce–O should give higher energy to generate the Ce3O4F3 phase due to the lack of active oxidant radical O· attraction. Thus, the temperature of complete decomposition of CO32– in Ar-roasting ore was higher than that of the O2-roasting ore, consistent with the results in Figure 4. (3) When the obtained Ar-roasting ore was roasted and oxidized again or in the presence of residual O2, the radical O· generated by the high-temperature thermal radiation first attracted 4f orbital electrons to generate a structure similar to Ce3O4F3. The above-mentioned process was shown as a weight gain phenomenon, accompanied by the oxidation behavior of Ce, consistent with the results in Figure 4. Afterward, the molecular vibration intensified with the gradual increase of the roasting temperature (≥800 °C). The ⑤Ce–F bond started to be elongated and weakened, whereas the mutual gravitational force between the valence level electrons of the radical O· and the F element was strengthened. In addition, the attraction and competition with the O atom for the 6s5d orbital electrons of Ce were enhanced, such that the ⑤Ce–F bond was broken. Subsequently, protons and neutrons were transferred to generate the Ce7O12 phase and intermediate OF·–, such that a weight loss phenomenon was generated. Due to the lively chemical nature of intermediate OF·–, it continued to combine with F·– to generate O2F, whereas the molecular mass of the O2F was larger than that of Ar molecules and easy to decompose. On that basis, it continued to decompose at the bottom to generate F2 and O2 (eq 9), and the generation of O2 was the cause of partial oxidation of the minerals at the bottom during the Ar-roasting process, which is the reason why the oxidation phenomenon at the bottom in Figure 1 was generally significant. Nevertheless, intermediates, O2F and OF·–, and F2 were adsorbed on the mineral surface due to their lively chemical properties, which is also the reason why the weight loss rate fluctuated up and down.

Figure 17 Mechanism of the roasting reaction of bastnaesite.

4 Conclusions

Regardless of the roasting atmosphere, monazite (REPO4) almost did not vary notably, whereas the model of thermal decomposition of bastnaesite primarily fell into two stages. At the first stage, bastnaesite was subjected to first decomposition to generate REOF and CO2, and considerable oxygen vacancies and lattice defects were generated, while the free radical O· began to adsorb and escape on its mineral surface, thus facilitating the transformation of Ce3+ to Ce4+. Since the sum of CO2 loss and free radical O· escape at this stage was much larger than the free radical O· adsorption, this stage finally displayed the trend of weight loss. At the second stage, with the gradual increase of the roasting temperature (800–1000 °C), the vibration of the REOF lattice was increased, which led to the decrease of oxygen storage and the release capacity of REOF minerals while strengthening the gravitational force between radical O· and F· atoms. Lastly, the atoms escaped in the form of OF2, which continuously decomposed to generate F2 and O2. Due to the weak reaction of OF2 generation and the small amount of free radical O· escape from REOF minerals themselves, the weight loss at this stage reached only 0.74%. The roasting model is capable of successfully explaining some macroscopic phenomena in the roasting decomposition of mixed rare earth minerals in Bayan Obo, revealing its essential laws from a microscopic perspective. This model also explains part of the reaction mechanism of other models and has been more extensively applied with some theoretical innovation.

The authors declare no competing financial interest.

Acknowledgments

This work was financially supported by the Inner Mongolia Autonomous Region Natural Science Foundation(2022QN05017) and the National Natural Science Foundation of China (51964040). Thanks to the support of the Scientific Research Fund for the Introduction of High-Level Talents (Jianfei Li) to Institutions of the Inner Mongolia Autonomous Region (research on the oxidation/sodium leaching strategy of mixed rare earth ore and key technologies for synergistic low-carbon disposal of “three wastes”).
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