Exploring rare earth mineral recovery through characterization of riebeckite type ore in Bayan Obo

Abstract

The aim of this study was to characterize the riebeckite type rare earth ore found in the Bayan Obo deposit, in order to identify the distribution and occurrence of both rare earths and gangue species within the ore. Several analytical techniques were utilized to accomplish this, such as chemical analysis, quantitative XRD, a single polarizing microscope, and a mineral automatic analysis system. The analysis revealed that the primary rare earth minerals (REMs) in the ore were bastnaesite and monazite, with huanghoite, parisite, aeschynite, and fergusonite identified as secondary rare earth minerals. The main gangue species was magnetite, accompanied by smaller quantities of riebeckite and dolomite. The ore was rich in rare earth oxides, with a 3.81 wt% grade. The screen analysis of the pulverized ore indicates that the 43–100 μm fraction is the dominant size, while the finer size fractions below 43 μm contain the bastnaesite and monazite, as well as huanghoite, parisite, aeschynite, and fergusonite. Microstructural characterization showed that the REMs were both coarse-grained and fine-grained, occurring as granular aggregates and fine disseminations within the gangue. Bastnaesite and monazite were the major REMs, with dominant amounts of cerium, lanthanum, praseodymium, and neodymium, while parasite was identified as an impurity. Huanghoite and parisite contained barium and calcium as impurities, respectively. Aeschynite and fergusonite were REMs that included niobium in their composition. Bastnaesite and monazite were found to contain a much higher rare earth content than huanghoite, parisite, aeschynite, and fergusonite. Potential methods for recovering rare earths from this ore, such as magnetic separation and froth flotation, have been identified and may be applicable to similar ferruginous rare earth-bearing ores.

1. Introduction

Rare earth elements (REEs) have unique and irreplaceable electrical, optical, magnetic, and thermal properties due to their large atomic magnetic moments, anisotropy, rich electronic energy level transitions, and unique lanthanide shrinkage [1,2]. Known as “vitamins of new materials,” REEs are a key element of strategic resources and technological development [3]. REEs are used widely in metallurgy, ceramics, the petrochemical industry, and military materials [4]. Over the past two decades, the demand for REEs has significantly increased, driven by their use in high-end technology, environmental, and economic sectors [5]. Unlike metallic elements, REEs are found in a diverse range of minerals, including oxides, carbonates, halides, phosphates, and silicates [6,7]. Although approximately 200 rare earth-bearing minerals have been identified, only a few of them are commercially significant. The primary minerals that contain 95% of the total rare earths resources are bastnaesite [(Ce,La)CO3F], monazite [(Ce,La)PO₄], and xenotime (YPO₄) [8]. The world's total rare earth reserves amount to approximately 130 million metric tonnes (in terms of oxide content), with 50% of the reserves situated in China. Currently, China dominates the global supply base for REEs [9,10]. China's REEs reserves are distributed mainly in Bayan Obo in Inner Mongolia, Gannan in Jiangxi, Weishan in Shandong, and Liangshan in Sichuan; the Bayan Obo rare earth reserves constitute over 90% of China's total [11]. The rare earth deposit was located at the border of Mongolia, about 100 km from Bayan Obo (Fig. 1). Large lenses define the Main Orebody and East Orebody, which are distributed along a west-east striking belt. Arranged from south to north (Fig. 1), they can be classified into four groups: the riebeckite type, aegirine type, massive type, and banded type. However, due to the variety of ore types and their complex embedding, the process mineralogy characteristics of different types of rare earth ores are very different. Therefore, a detailed study of process mineralogy for each type of ore is important for the development and utilization of ore deposits and the formulation of mineral processing strategies.

Fig. 1.

Structural map of the Main and East Orebody [12].

This paper presents a comprehensive characterization study of rare earth ore of riebeckite type. The ore was analyzed using XRF, ICP-OES, ICP-MS, and quantitative XRD to determine its composition. The textural and microstructural properties of the rare earth minerals (REMs) were examined using SEM/EDX, and the mineral automatic analysis system (AMICS) was used to determine the liberation characteristics of the REMs. The implications of these results for beneficiation processes aimed at recovering REMs from this ore are also discussed.

2. Materials and methods

2.1. Materials

The samples required for this study were collected using the riebeckite type rare earth ore from Main Orebody and East Orebody. According to the field survey, ore distribution, and ore belt location, a total of 20 sampling points (15 for the Main Orebody and 5 for the East Orebody) are designed, and the sampling points are evenly distributed within the entire stope boundary. The samples were collected with grid picking method within 10 m around the design sampling point.

To prepare the riebeckite-type rare earth ore for analysis, a composite sample of approximately 10 kg was crushed to ∼2 mm and then riffled to produce 5.0 kg batches. One batch was further riffled to yield representative sub-samples of approximately 100 g for chemical composition and mineral analysis. A 100 g sub-sample was then crushed into 4-mm powders and wet ground in a ball mill. After drying, 1.0 g of the ground material was mixed with epoxy resin and amino-terminated polyoxypropylene, placed in a plastic mold, and cured for 24 h at 100 °C to obtain AMICS resin samples. The samples were then polished with sandpaper particle sizes of P500, P1000, P1500, and P2000 (machine strength 30 n, rotating speed 200 rpm), followed by treatment with a polishing solution. After cleaning, drying, and coating with carbon, the polished samples were analyzed using AMICS.

2.2. Analytical methods

The chemical analysis of the ore samples was conducted using a combination of XRF (PANalytical Axios) and ICP-MS (PerkinElmer NexION 300X), while ICP-OES (PerkinElmer 8300) was used to analyze total sulfur (inorganic and organic). To perform

Quantitative phase analysis (QPA), the crushed ore sample was micronized in ethanol (EtOH) in a McCrone micronizer for 20 min, and then centrifuged to remove excess EtOH. The samples were then dried at 60 °C, and XRD data (Ultima IV) were collected in 2θ range of 5–140° using a PANalytical MPD instrument equipped with a cobalt tube (Co Kα radiation) that was operated at 40 kV and 40 mA. QPA was conducted using the Rietveld method [13,14] with TOPAS V5 software [15]. Crystal structure information was obtained for hematite [16], monazite [17], and dolomite [18].

To analyze the structural characteristics of the ore, polarizing microscope was used to identify the major mineral associations by mineral automatic analysis system (AMICS). The samples were imaged with a Zeiss Sigma 500 high-resolution FE-SEM (Carl Zeiss, Germany) equipped with back scattering SEM and a Bruker XFlash6/60 EDS detector. The imaging parameters were set to the acceleration voltage of 20 kV, a working distance of 9.6 mm, a frame resolution of 500 pixels, a probe current of 10 nA, and an overall electron beam acceleration voltage of 20 kV. Particles were selected as the measurement mode, the mineral standard generated by J was checked, and the minimum mineral particle area was set to 100 μm. X-ray acquisition time was 20 μs. The magnification was 200×, the upper limit of background gray was 40, and other parameters were set to suitable data. After the test, the mineral list was automatically generated by the AMICS tdmanager software, and then the minerals in the list were classified and dyed in the AMICS process software to obtain a series of process mineralogy related data [19].

3. Results and discussion

3.1. Chemical composition of ore samples

Table 1 presents the chemical composition of the ore sample, which is rich in rare earths with a 3.81 wt% rare earth oxide (REO) grade. Light rare earth oxides (LREO) are the most abundant, accounting for 96.89% of the total REOs. High contents of Ce (1.85 wt% CeO₂), La (0.93 wt% La₂O₃), Nd (0.71 wt% Nd₂O₃), and Pr (0.20 wt% Pr₆O₁₁) were found in the ore. The ore showed slight enrichment in the medium rare earth oxides (MREO), comprising Sm (0.057 wt% Sm₂O₃), Gd (0.023 wt% Gd₂O₃), Eu (0.011 wt% Eu₂O₃), and Y 0.019 wt% Y₂O₃), which accounted for 2.89% of the total. Less than 1% of the total content was comprised of heavy rare earth oxides (HREO), with Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃ accounting for only 0.23% of the total REO. Despite this, due to the huge total rare earth reserves in Bayan Obo, it is important to consider the mineral processing of rare earth minerals and not underestimate the value of the HREO. The ore also contains iron, niobium, scandium, and titanium, with concentrations of 0.13 wt% Nb₂O₅, 0.016 wt% Sc₂O₃, and 0.25 wt%TiO₂, which are potential high-value by-products.

Table 1.

The elemental composition of the ore/wt%.

Element	Na2O	Ka2O	MgO	TiO2	CaO	BaO	SiO2	MnO2	F	S	P	TFe
1	1.87	0.54	3.76	0.18	11.26	1.09	15.79	1.50	3.58	1.68	0.64	32.17
2	1.46	0.47	2.96	0.18	9.83	0.36	17.14	1.52	1.46	1.21	0.58	33.65
3	1.99	0.38	2.52	0.43	10.73	1.59	12.96	1.65	2.47	0.73	0.71	36.85
4	2.01	0.42	3.14	0.29	13.65	0.97	14.86	1.38	3.71	0.88	0.58	32.72
5	1.74	0.63	4.08	0.19	11.94	0.73	16.11	1.46	2.94	1.39	0.66	26.32
Average	1.81	0.49	3.29	0.25	11.48	0.95	15.37	1.50	2.83	1.18	0.63	32.34

Element	ThO2	Sc2O3	REO	Nb2O5	La2O3	CeO2	Pr6O11	Nd2O3	Sm2O3	Eu2O3
1	0.042	0.014	4.23	0.14	0.98	2.06	0.23	0.82	0.069	0.011
2	0.034	0.016	3.24	0.12	0.80	1.59	0.17	0.60	0.04	0.007
3	0.039	0.026	4.58	0.11	1.08	2.18	0.25	0.93	0.066	0.014
4	0.037	0.013	3.08	0.16	0.60	1.51	0.18	0.68	0.068	0.012
5	0.031	0.010	3.93	0.13	1.21	1.92	0.18	0.54	0.04	0.0091
Average	0.037	0.016	3.81	0.13	0.93	1.85	0.20	0.71	0.057	0.011

Element	Gd2O3	Y2O3	Tb4O7	Dy2O3	Ho2O3	Er2O3	Tm2O3	Yb2O3	Lu2O3
1	0.025	0.032	0.0007	0.0022	0.0001	0.0016	0.00012	0.00040	0.00005
2	0.014	0.011	0.0010	0.0041	0.0002	0.0008	0.00007	0.00037	0.00004
3	0.03	0.023	0.0006	0.0023	0.0001	0.0009	0.00008	0.00039	0.00004
4	0.023	0.016	0.0047	0.0065	0.0004	0.0012	0.00013	0.00052	0.00006
5	0.021	0.013	0.0044	0.0064	0.0003	0.0011	0.00009	0.00038	0.00005
Average	0.023	0.019	0.0023	0.0043	0.0002	0.0011	0.0001	0.00041	0.00005

Iron was the most abundant element among the gangue elements, with a concentration of 32.34 wt% TFe. Calcium (11.48 wt% CaO), silicon (15.37 wt% SiO₂), and magnesium (3.29 wt% MgO) were also identified, albeit in lower concentrations. The concentration of harmful elements such as F (2.83 wt%), P (0.63 wt%), S (1.18 wt%), and ThO₂ (0.037 wt%) was low. However, these impurities are expected to have a significant influence on the grade and quality of rare earth concentrate and may affect subsequent enrichment and metallurgy processes.

3.2. Textural characteristics of ore samples

Riebeckite is idiomorphic and semi-idiomorphic with directivity, and the mineral particle size about 20 μm–100 μm (Fig. 2a). Fluorite is interspersed between riebeckite and mica mineral particles as irregular aggregates (Fig. 2a). Magnetite is distributed in riebeckite in disseminated form, with mineral particle size about 20 μm–300 μm (Fig. 2a). Hematite is mainly distributed between amphibole and phlogopite in an irregular granular aggregate with mineral particle size about 20 μm–150 μm (Fig. 2b). Magnetite is in allomorphic granular form and distributed in riebeckite in disseminated form, with mineral particle size of about 20 μm–300 μm (Fig. 2c). Some magnetites are distributed in granular form between the grains of riebeckite and dolomite, and the mineral particle size is uneven, distributed in 20 μm–600 μm (Fig. 2d). Dolomite and riebeckite are distributed alternately in veinlets and ribbons, most of them are in allomorphic granular structure, and the particle size is about 0.1 mm. Basnaesite and phlogopite can be seen between dolomite and riebeckite grains (Fig. 2d). Most rare earth minerals cannot be observed under the microscope due to their low content and fine particle size.

Fig. 2.

Textural characteristics of the ore

Fl: Fluorite; Mag: Magnetite; Rbk: Riebeckite; Dol: Dolomi; Hem: Hematite; Phl: Phlogopite; Bas: Basnaesite.

3.3. Distribution of REO and other valuable elements in the ore

The mineral composition and relative content of the ore were measured using AMICS, and the results are presented in Fig. 3. It can be seen that the iron minerals were mainly magnetite, hematite, pyrite, and pyrrhotite, with a content of 37.93 wt%, 0.69 wt%, 1.06 wt% and 2.13 wt%, respectively. Rare earth minerals (REM) included bastnaesite, monazite, huanghoite, parisite, aeschynite, and fergusonite with the content of 3.11 wt%, 2.33 wt%, 0.09 wt%, 0.34 wt%, 0.13 wt%, and 0.01 wt%, respectively. Niobium-containing minerals included niobite and pyrochlore in addition to aeschynite and fergusonite. The main gangue minerals were amphibole, pyroxene, mica, and dolomite. Bastnaesite, huanghoite, and parisite are carbonate minerals that can be effectively recovered together through flotation. In contrast, monazite is a phosphate mineral with a higher content of HREO and weaker floatability compared to carbonate REM. Separation of REM from silicate minerals such as sodium blende and feldspar is relatively easy. However, due to its close symbiosis with gangue minerals (dolomite, calcite, fluorite) and similar floatability, it brings great difficulties to flotation separation.

Fig. 3.

The analysis image of AMICS mineral/wt %. (a)SEM, (b) Mineral composition.

The distribution of REO in each mineral in the ore is shown in Table 2. The REO were mainly hosted in bastnaesite, parasite, and monazite, with a distribution of 84.10%. The distribution of REO in iron minerals was 12.43%, the distribution of REO in fluorite minerals was 2.38%, and this part of REO was easily lost with iron concentrate and fluorite concentrate in the process of iron separation. Other minerals had low REO content and were difficult to undergo beneficiation.

Table 2.

Distribution of REEs in minerals.

Mineral type		w(REO)/wt%	Distribution/%
Iron minerals	Magnetite	0.12	1.61
	Hematite	0.63	6.46
	Limonite	2.06	4.35
	Pyrite	0.11	0.01
Niobium minerals	Ilmenorutile	0.98	0.01
	Ferrocolumbite	1.35	0.04
	Aeschynite	36.60	0.26
	Fergusonite	38.81	0.01
Rare earth minerals	Bastnasite/Parisite	57.49	42.64
	Monazite	71.36	41.46
Other minerals	Fluorite	0.36	2.08
	Mica	0.05	0.03
	Apatite	3.19	0.38
	Quartz	0.07	0.03
	Aegirine	0.03	0.01
	Riebeckite	0.10	0.17

3.4. Characteristics of rare earth mineral

The AMICS analysis (Fig. 3) identified bastnaesite as the primary REM in the ore, making it the focus of mineral processing and recovery. Fig. 4 shows SEM and EDS images of the bastnaesite. Bastnaesite is embedded in riebeckite in irregular shape and has different particle size. REEs content of bastnaesite are presented in Table 3. The total amount of REEs in bastnaesite was 58.75 wt%, of which the content of Ce, La, Pr, Nd were 56.19 wt%, accounting for 95.64% of the total REEs. The sum of the content of MREEs and HREEs was only 2.56 wt%.

Fig. 4.

SEM image and EDS analysis of bastnaesite. (a)SEM, (b) EDS.

Table 3.

The REEs content of bastnaesite/wt%.

Elements	La	Ce	Pr	Nd	Sm	Tb	Dy
1	24.05	32.71	2.50	7.04	2.50	0.10	0.57
2	11.93	23.24	2.49	9.63	1.46	0.07	0.33
3	17.55	27.57	2.20	6.76	1.78	0.13	0.17
4	21.50	24.49	1.66	5.50	3.09	0.08	0.20
5	21.09	28.39	1.92	8.77	1.95	0.05	0.28
Average	19.22	27.28	2.15	7.54	2.16	0.09	0.31

Monazite is a widely distributed REM in the Bayan Obo deposit. Most of the monazite appears in extremely fine granular or ellipsoidal structures, and larger particles can be irregular or plate-shaped. Monazite minerals were dispersed in the riebeckite in an irregular fine-grained form, with a low degree of automorphism (Fig. 5). The REEs content of monazite are presented in Table 4. The total amount of REEs in monazite was 53.26 wt%, with Ce, La, Pr, and Nd as the richest, having content of 26.28 wt%, 13.29 wt%, 9.44 wt%, and 2.60 wt%, respectively and accounting for 96.90% of the total elements. The monazite minerals contained more types of MREEs (0.41 wt% Sm; 0.03 wt% Eu; 0.13 wt% Gd; 0.37 wt% Y) and HREEs (0.26 wt%Tb; 0.45 wt% Dy) than bastnaesite (see Table 5).

Fig. 5.

SEM image and EDS analysis of monazite. (a)SEM, (b) EDS.

Table 4.

The REEs content of monazite/wt%.

Elements	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y
1	14.82	30.72	3.29	12.35	0.78	–	0.11	–	0.5	0.67
2	12.95	25.39	2.76	9.44	0.23	0.02	–	–	0.35	0.42
3	9.03	23.08	2.59	11.1	0.58	–	0.13	0.13	0.25	0.38
4	12.45	24.59	2.09	6.8	–	0.04	0.09	0.65	0.94	0.24
5	17.19	27.63	2.27	7.49	0.04	0.02	0.18	0.01	0.22	0.42
Average	13.29	26.28	2.6	9.44	0.41	0.03	0.13	0.26	0.45	0.37

Table 5.

The REESs content of parisite/wt%.

Elements	La	Ce	Pr	Nd	Sm	Eu	Gd
1	10.79	29.34	4.35	12.09	1.11	0.15	0.26
2	11.73	23.78	4.83	14.41	2.25	2.37	0.02
3	9.77	27.96	2.56	9.75	1.96	0.24	0.19
4	11.81	25.77	4.52	13.15	2.04	0.42	0.22
5	9.68	31.62	6.9	11.86	1.41	0.37	0.46
Average	10.76	27.69	4.63	12.25	1.75	0.71	0.23

Parisite [Ca(Ce,La)CO₃F], the crystal structure of parisite is similar to that of bastnaesite, but the form of atomic combination arrangement along the c axis is different [20]. The SEM image and EDS analysis of monazite are presented in Fig. 6. Some REEs in the lattice of parisite are replaced by calcium ions, which causes the total amount of REEs in single minerals, i.e., 58.02 wt%, to be less than that of bastnaesite. The Nd content was 12.25 wt%, greater than the Nd content of bastnaesite.

Fig. 6.

SEM image and EDS analysis of parisite. (a)SEM, (b) EDS.

Aeschynite [(Ce,Th)(Ti,Nb)₂O₆] is a rare earth mineral that contains niobium. The mineral crystal morphology is mostly in granular or plate shape, and the aggregate is radial or lumpy, with uneven particle size. The soluble stone in the riebeckite type ore is embedded in the edge of pyrite in an irregular lump-like aggregate and is associated with the riebeckite (Fig. 7). In addition, riebeckite is embedded in the soluble stone in a fine-grained form. The content of Nb was 23.68 wt %, the content of Ti was 14.70 wt%, and the total amount of REEs was 32.98 wt% (Table 6). It was obvious that there were more types and content of MREEs and HREEs than other rare earth minerals, and the total amount of Sm, Eu, Gd, Tb, and Y was 5.92 wt%, which accounted for 17.01% of the total amount of REEs in the mineral.

Fig. 7.

SEM image and EDS analysis of aeschynite. (a)SEM, (b) EDS.

Table 6.

The REEs content of aeschynite/wt%.

Elements	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Nb	Ti	Y
1	1.42	13.05	3.03	10.95	1.20	0.37	1.23	0.21	22.14	18.55	0.62
2	2.19	5.04	1.10	15.37	3.78	2.12	0.37	0.39	27.89	12.78	0.86
3	1.90	8.86	1.99	18.25	2.41	1.96	0.85	0.24	22.56	13.36	0.91
4	1.10	12.48	5.90	13.66	1.96	2.10	0.94	0.5	21.36	13.52	0.77
5	1.09	10.27	4.36	12.39	2.44	0.97	1.37	0.36	24.47	15.31	0.68
Average	1.54	9.94	3.28	14.12	2.36	1.50	0.95	0.34	23.68	14.70	0.77

Huanghoite [Ba(Ce,La)CO₃F], is widely distributed in the main and eastern orebodies, and is associated with albolite, dolomite, fluorite and other minerals, often in granular aggregate (Fig. 8). Its chemical composition and physical properties are different from the properties of cordylite. The REEs content of monazite are presented in Table 7. The total amount of REEs in huanghoite was 27.61 wt%, with LREEs (Ce, La, Pr, and Nd) having content of 13.47 wt%, 9.66 wt%, 0.80 wt%, and 2.66 wt%, respectively and accounting for 96.92% of the total elements.

Fig. 8.

SEM image and EDS analysis of huanghoite. (a)SEM, (b) EDS.

Table 7.

The REEs content of huanghoite/wt%.

Elements	La	Ce	Pr	Nd	Sm	Eu	Tb	Dy
1	8.8	13.77	0.72	3.11	0.11	0.07	0.09	0.52
2	12.06	11.28	0.86	1.98	0.14	0.05	0.11	0.45
3	9.85	14.06	0.67	2.45	0.09	0.12	0.13	0.39
4	7.59	15.33	0.74	2.97	0.23	0.09	0.11	0.68
5	10.02	12.91	0.99	3.62	0.17	0.06	0.08	0.57
Average	9.66	13.47	0.80	2.83	0.15	0.08	0.10	0.52

Fergusonite (YNbO₄) is a type of normal niobate tantalate mineral that contains niobium and tantalum and REEs (Fig. 9). The content of Nb in fergusonite was 41.08 wt%, the content of Ti was 1.72 wt%, and the total amount of REEs was 28.08 wt%. The total amount of Y was 18.10 wt%, accounting for 64.46% of the total REEs (Table 8).

Fig. 9.

SEM image and EDS analysis of fergusonite. (a)SEM, (b) EDS.

Table 8.

The REEs content of fergusonite/wt%.

Elements	Nd	Dy	Y	Nb	Ti
1	3.28	6.68	18.01	41.06	1.16
2	3.44	7.23	20.14	39.65	2.31
3	2.97	7.41	15.96	42.32	1.98
4	4.01	5.96	18.95	40.07	2.01
5	2.66	6.25	17.44	42.31	1.14
Average	3.27	6.71	18.10	41.08	1.72

3.5. Grain size distribution of REMs

The grain size distribution of the principal REMs in the pulverized ore is presented in Fig. 10. Bastnaesite and monazite had an uneven particle size, +74 μm. Their distribution was 34.01 wt% and 30.57 wt%, respectively. The distribution of −10 μm grain size was 3.69 wt% and 6.34 wt%, respectively. It is difficult to recover this part of fine-grained REMs. The particle size distribution of bastnaesite and soluble stone is relatively similar, mainly 20–74 μm, which comprised 88.96 wt% and 80.50 wt%, respectively. Huanghoite and fergusonite had extremely fine particle sizes; the distributions of +74 μm fractions were only 8.94 wt% and 13.74 wt%., which made it difficult to be efficiently recovered by flotation. However, fergusonite is rich in niobium and HREEs, so further enrichment and recovery should be considered in rare earth tailings.

Fig. 10.

The grain sizes of the principal REMs in the ore.

3.6. Liberation characteristics of REMs

Liberation characteristics of REMs under different grinding fineness was investigated by AMICS analysis and this is presented in Fig. 11. With grinding fineness of −74 μm accounts for 80%, the dissociation degree of bastnaesite, monazite, parisite, aeschynite, huanghoite and fergusonite were 63.31%, 69.62%, 59.42%, 57.04%, 55.36%, 43.36%. With an increase in grinding fineness, the dissociation degree of REEs minerals increased significantly of −74 μm from 80% to 90%. When the grinding fineness was −74 μm 98%, the degree of dissociation of bastnasite, monazite, and parisite increased significantly, but huanghoite and fergusonite increased to a lesser extent. In particular, under the existing production grinding fineness (−74 μm 90%), the iron concentrate had a greater content of rare earth, a greater content of impurity calcium in rare earth concentrate, and a lower ratio of MREEs and HREEs than that of raw ore. As shown in Fig. 11, when the grinding fineness was −74 μm 90%, the conjoint with iron minerals entered the iron concentrate, and the carbonate and fluorite minerals that were conjoined with the REMs were enriched in the rare earth concentrate, which affected the grade and quality of the rare earth concentrate. At the same time, the aeschynite and fergusonite with high content of MREEs and HREEs were enriched in the rare earth tailings due to insufficient dissociation.

Fig. 11.

Liberation characteristics of REMs. (a)Bastnaesite, (b)Monazite, (c)Parasite, (d) Aeschynite, (e) Huanghoite, (f)Fergusonite.

4. Discussion for beneficiation processes

Rare earth ores beneficiation processes rely on physical or physico-chemical properties to separate REMs from the gangue. These include magnetic separation, gravity separation, and froth flotation [21]. The degree of beneficiation and final concentrate grade primarily depend on the hydrometallurgical process used to leach and purify the concentrate. Caustic leaching demands high-quality concentrates with 70–80% of REMs, whereas sulfuric acid baking-water leaching can use lower quality feeds having below 70% of REMs, making it a lower-cost alternative [22].

The ore mainly comprises magnetite and pyrrhotite, which are the major iron minerals and account for more than 40% of the ore by weight (Fig. 3). Therefore, any REM enrichment process must either target the separation of rare earths from magnetite and pyrrhotite or removal of magnetite and pyrrhotite. The removal of these minerals would significantly increase the rare earth grade in the concentrate, with the REO grade expected to increase from 3.81 wt% in the feed to over 6 wt% in the concentrate. Dolomite, calcite, and fluorite are other significant gangue minerals that account for 9.95%, 4.00%, and 2.93%, respectively (Fig. 3). While removing these minerals would not significantly boost the REO grade as much as removing magnetite and pyrrhotite, it is still desirable since calcium minerals can have negative impacts on flotation and hydrometallurgical processing.

The primary REMs in the ore are bastnaesite and monazite; however, it is important to recover secondary minerals such as huanghoite, parisite, aeschynite, and fergusonite to enhance overall resource utilization. The fine grain size and complex textural characteristics of the REMs in the ore (see Section 3.4) necessitate the need for fine grinding to achieve complete liberation of REMs from the gangue. Therefore, gravity separation cannot be employed as a primary REM recovery method. Alternative processes such as magnetic separation or froth flotation must be used instead [23].

Paramagnetic properties of REMs make them amenable to magnetic separation techniques which have been employed for the removal of highly magnetic gangue or recovery of REMs [21,23]. Promising results have been reported for magnetic separation studies carried out on the Bayan Obo ore of Inner Mongolia (China) [24]. Although magnetic separation cannot achieve high-grade rare earth concentrate, it can be used as a pre-enrichment technology.

Due to ore's fine-grained nature, froth flotation is the most appropriate conventional method for recovering REM [25]. There are two possible REM flotation routes: direct flotation of rare earth phosphates or reverse flotation, where rare earth phosphates are depressed and gangue minerals are floated [25]. Direct flotation is the preferred option. Despite its effectiveness in recovering rare earths, flotation poses several challenges in this particular case due to the high proportion of ultra-fines present in the ore. These ultra-fines can cause issues such as froth instability, high reagent consumption, low concentrate grades, and recoveries [[26], [27], [28]]. Although desliming can manage the ultra-fines, approximately 30% of the rare earths are still present in the 20 μm fraction of the ore. Disposing of the slimes would result in a considerable loss of rare earths and other valuable elements such as Nb.

Moreover, many of the collectors used for direct anionic flotation of REMs (fatty acids, hydroxamic acids) [23,29], are strongly floating towards iron oxides [30] and calcium minerals [31]. These challenges, however, can be overcome by selecting appropriate REM collectors and gangue depressants. Removal of coarse gangue minerals containing calcium could also be beneficial and simplify flotation.

5. Conclusion

In summary, the characterization of a riebeckite-type rare earth ore has provided insights into its mode of occurrence, distribution of rare earths and gangue, and their impacts on mineral processing. The ore is rich in rare earths with a grade of 3.81 wt% REO, predominantly consisting of LREEs (accounting for 96.89% of the total REEs). The presence of Fe, Si, Ca, and Nb, and the low concentration of impurities such as P, S and ThO₂, indicate that the concentrate quality is likely to be favorable. The primary rare earth-bearing minerals are bastnaesite and monazite and account for 90.52% of the total REMs, while the primary gangue minerals are magnetite, riebeckite, and dolomite accounting for over 50% of the ore by weight. Although desliming the ore can remove ultra-fines, it would result in a significant loss of rare earths to the tails. This is because around 10% of the rare earths are present in the 20 μm fraction. Additionally, the minerals huanghoite, parisite, aeschynite, and fergusonite have a very fine grain size and are disseminated in gangue. This suggests that fine grinding may be necessary to achieve sufficient liberation for processing.

Author contribution statement

Weiwei Wang: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Zhangkuang Peng, Shaochun Hou: Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Chunlei Guo，Qiang Li，Yanjiang Liu: Performed the experiments; Analyzed and interpreted the data. Hailong Jin: Analyzed and interpreted the data.

Funding statement

This work was financially supported by the Major projects of Natural Science Foundation of Inner Mongolia of China (2020MS05001), the National Key Research and Development Program of China (2022YFC2905301), the Program of State Key Laboratory of Bayan Obo Rare Earth Resource Researches and Comprehensive Utilization (2021Z2356, 2022Z2405).

Data availability statement

The authors do not have permission to share data.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.