Beneficiation of Fine-Grained Bayan Obo Niobium Ore Using a Slime Vibrating Table

Abstract

In order to enhance the separation efficiency of fine-grained Bayan Obo Niobium Ore, a novel gravity separation equipment named Slime Vibrating Table (SVT) was developed. The SVT employs an electromagnetic drive to generate a reciprocating motion for the table, with a lower stroke and higher frequency than a conventional Slime Shaking Table (SST). Key parameters of SVT, including table slope, wash-water flow rate, vibration voltage, and vibration frequency, were tested for a niobium ore assaying 0.19% Nb₂O₅ with a particle size below 74 um by 68.78%. Under the optimized condition, SVT was able to obtain a primary concentrate assaying 1.31% Nb₂O₅ with a recovery of 52.64%, which was 0.22% and 26.59% higher than that of SST, respectively. Size-by-size analysis indicated that the enhanced separation performance of SVT was mainly attributed to its superior recovery of Nb₂O₅ in the −38 μm fraction. The SVT introduced in this study shows great potential for efficient recovery of fine-grained strategic metals, including rare earths, tantalum, tungsten, tin, and antimony, etc.

1. Introduction

Niobium is a crucial strategic metal resource with excellent superconductivity, a high melting point, and reinforced corrosion resistance. It is widely employed in high-tech fields, including aerospace [1], superconducting materials [2,3,4], the new energy industry [5,6], piezoelectric materials [7,8,9], corrosion-resistant materials [10,11], and alloys [12,13,14].

Niobium-bearing minerals are the primary source for the extraction of metal niobium.The Bayan Obo deposit in northern China contains 16.92 million tons of niobium ore, which accounts for 63.40% of China’s total reserves. However, this niobium ore has not been exploited yet due to its low grade, fine-grained dissemination, and complex mineral composition [15]. According to former reports, the ore contains 0.1–0.2% Nb₂O₅ and is primarily contained in niobite ((Fe, Mn)Nb₂O₆), aeschynite ((Ce, Th)(Ti, Nb)₂O₆), and pyrochlore ((Ca, Na, Ce)₂(Nb, Ti, Ta)₂O₆(F, OH)). These niobium minerals typically have a grain size smaller than 20 µm [16], which makes them difficult to be recovered in the ore dressing process.

In the early 1990s, the Baotou Steel Mining Research Institute conducted industrial-scale tests on niobium ore separation using reverse flotation, direct flotation, and high-intensity magnetic separation. These initial efforts yielded a niobium concentrate assaying 2.5% Nb₂O₅ and a secondary product containing 0.8% Nb₂O₅. However, the Nb₂O₅ grade of concentrate was too low. By the late 1990s, process optimization succeeded in producing a niobium concentrate with a 4.90% Nb₂O₅ at a recovery of 28.25%. Despite this improvement, the product still failed to meet the requirements for pyrometallurgical processing [17]. A fundamental challenge is the low grade of Nb₂O₅ in the raw ore, which prompted attempts to pre-concentrate the ore by rejecting large volumes of gangue. Magnetic separation is one of the adopted methods, since niobite and aeschynite are weakly magnetic. But this method was proved ineffective when pyrochlore (a non-magnetic niobium-bearing mineral) was contained in the feed [18,19].

Gravity separation is considered an effective pre-concentration method for niobium ore, since most niobium minerals are much heavier than gangue minerals [20,21,22]. For instance, Wang et al. developed a process for recovering tantalum-niobium minerals from lithium-bearing polymetallic pegmatite ore, involving coarse grinding, gravity pre-concentration (with 100% of feed particles passing 0.35 mm), strong magnetic separation, and centrifugal separation. The pre-concentration stage was achieved using a combined spiral chute and shaking table [23]. Research further indicates that the selection of optimal gravity separation equipment is highly dependent on particle size. Specifically, heavy-medium cyclones are recommended for −2 + 0.5 mm material, shaking tables for −0.5 + 0.074 mm material, and Knelson centrifugal concentrators for −0.074 + 0.02 mm particles, with the latter achieving the highest efficiency in their respective size classes [24]. However, the recovery of niobium minerals from Bayan Obo presents unique challenges due to their extremely fine grain size (smaller than 20 µm), which typically requires enhanced gravity separators (EGS) such as shaking tables, Falcon, Knelson, Kelsey jigs, and Mozley multi-gravity concentrators [25]. Furthermore, the low grade (~0.2% Nb₂O₅) and high content of iron-bearing gangue in the ore necessitate a process with a higher upgrading ratio than conventional pre-concentration. Among various EGS, shaking tables provide a superior upgrade ratio. Nevertheless, it faces significant challenges in recovering particles finer than 20 microns [26,27,28].

To enhance the recovery of fine-grained (<20 μm) niobium minerals from the Bayan Obo deposit during pre-concentration, a novel EGS named Slime Vibrating Table (SVT) was developed from Slime Shaking Tables (SST) in this study. The separation performance of the SVT was systematically tested and compared with SST. The results confirm the significant potential of the SVT for the efficient recovery of fine-grained strategic metals, including rare earths, tantalum, tungsten, tin, and antimony, etc.

2. Equipment Descriptions

The shaking table is one of the most widely used gravitational separation equipment. Figure 1 displays the structure of a shaking table suited for slime materials, termed the Slime Shaking Table (SST). It primarily comprises an inclined table and an eccentric mechanism. The table can shake back and forth at the drive of the eccentric mechanism. When slurry is fed and distributed along the feed side, particles contained in the slurry will shake in a longitudinal direction, while the fluid will bring them to move in a transversal direction. These particles are subjected to two orthogonal forces: shear from the table’s motion and drag from the flowing water film. This results in diagonal migration across the table, with trajectories that vary based on particle size and density. Consequently, heavier particles are delivered to the concentrate launder, while lighter particles are discharged into the tailing launder on the tailing side.

Shaking stroke and frequency are critical operational parameters that can be adjusted through the drive head; their typical operating ranges are 10–25 mm and 240–325 times per minute, respectively. The speed and acceleration of the shaking table deck are determined by the stroke (n) and frequency (s). The speed is proportional to “ns”, and the acceleration is proportional to “ns²”. During separation, a bed layer of pulp forms on the deck. For fine particles with slow settling velocity, a thin bed layer is used. This requires a relatively low deck speed coupled with high acceleration to ensure that the fine particles settle to the bottom and are then transported longitudinally to the concentrate launder. Therefore, a shorter stroke and a higher frequency create a regime of low deck speed coupled with high acceleration that enhances the recovery efficiency of fine particles [28,29]. However, the mechanical drive head of traditional shaking tables inherently limits the reduction in stroke and the increase in frequency through structural optimization.

To overcome this limitation, an electromagnetic drive head was applied to replace the mechanical drive head. Coupled with an optimized feeding device and table design, an enhanced shaking table called ‘Slime Vibrating Table’ (SVT) was developed in this study, as shown in Figure 2. The fundamental distinction between the SST and SVT lies in their drive mechanisms and resultant motion. As shown in Figure 2b, when the electromagnet strikes at the armature, the top seat of the table would bring the table to experience a reciprocating motion, and the pattern was quite different between SST and SVT. Figure 3 records the position offset along with time in the longitudinal direction of the table for SST and SVT by a high-speed camera. It can be found that the shaking stroke of SVT (1.24 mm) is much smaller than that of SST (8.23 mm), while the shaking frequency of SVT (2499 s⁻¹) is much higher than that of SVT (274 s⁻¹). Furthermore, the velocity in the transversal direction of SVT is non-zero, which is also different from SST. As a result, a more obvious boundary between heavy and light was observed, as shown in Figure 2d.

3. Material and Methods

3.1. Properties of the Tested Sample

The test sample was obtained from the middlings of a shaking table in Bayan Obo Niobium Ore, which assayed 0.19% Nb₂O₅ as determined by chemical element analysis. The mineral composition of the sample was tested using mineral liberation analysis (MLA). The MLA is an automated mineralogy system that integrates a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS) for the rapid, quantitative analysis of polished sections. It provides critical data on mineral identification, grain size, shape, and intergrowth relationships, which is essential for optimizing mineral processing workflows. In this study, MLA(FEl Company, Hillsboro, OR, USA)was performed using FEI MLA 650 and FEI Quanta 650 MLA instruments with a Bruker X-Flash dual EDS system.

As shown in

Table 1. Mineralogical composition of feed material.

Mineral	Niobite	Aeschynite	Pyrochlore	Biotite	Pyroxene	Calcite
%	0.16	0.03	0.01	55.39	17.38	10.72
Mineral	Dolomite-Ankerite	Amphibole	Siderite	Feldspar	Pyrite	Others
%	6.41	2.10	1.81	1.12	1.04	1.75

, the niobium-bearing minerals in the sample were niobite (0.16%), eschynite (0.03%), and pyrochlore (0.01%). The main gangue minerals were biotite (55.39%), pyroxene (17.36%), calcite (10.72%), and dolomite-ankerite (6.41%). The target mineral, niobite, had a specific gravity of 5.2, whereas the major gangue minerals (biotite, pyroxene, calcite, dolomite, and ankerite) exhibited specific gravities of 3.0, 3.3, 2.7, 2.8, and 3.1, respectively. The apparent disparity in density between the target mineral and the gangue minerals facilitated the enrichment of niobite through gravity concentration.

Particle size analysis (

Table 2. Nb₂O₅ analysis of individual size fractions.

Size, μm	Wt., %	Nb2O5, %	Distribution of Nb2O5, %	Negative Cum Wt., %	Negative Cum. Distribution of Nb2O5, %
+75	31.22	0.08	12.12	100.00	100.00
+38	33.83	0.11	18.06	68.78	87.88
+20	23.00	0.18	19.93	34.95	69.82
+10	8.72	0.60	25.40	11.95	49.89
−10	3.23	1.56	24.49	3.23	24.49

) revealed that 49.89% of the total niobium was distributed in the −20 μm fraction, confirming the extremely fine-grained nature of the sample and its associated processing challenges.

3.2. Methodology of the Tests

For each experiment, a 500 g sample was conditioned in a 5 L agitation tank at a pulp density of 10 wt.% solids. Subsequently, the slurry was fed into the SVT by a peristaltic pump at a regulated flow rate of 0.25 L per minute. To enhance the separation efficiency of fine particles, the impacts of crucial operational parameters, including table slope, wash-water flow rate, vibration voltage (positively correlated with the stroke length), and vibration frequency, were systematically assessed. The main parameters of the SVT used in the experiments are shown in

Table 3. Parameters of the SVT used for tests.

Particle size	68.78% passing 75 μm
Slurry density	10% (by Wt.)
Slurry flow rate	0.25 L/min
Table slope	2.0°, 2.4°, 2.8°, 3.2°
Wash-water flowrate	1.2 L/min, 1.5 L/min, 1.8 L/min, 2.1 L/min,
Vibration voltage	170 V, 180 V, 190 V, 200 V
Vibration frequency	35 Hz, 38 Hz, 41 Hz, 44 Hz

.

3.3. Evaluation on Separation Efficiency

A separation efficiency was defined to evaluate the performance of SVT and SST in niobium ore separation, as shown in Equation (1).

$$E={\displaystyle \frac{(\alpha -\theta )(\beta -\alpha )}{\alpha (\beta -\theta )(1-\alpha /{\beta }_m)}}$$

(1)

where E is separation efficiency, %, α is grade of raw ore, %, β is grade of concentrate, %, θ is grade of tailing, %, β_m is Nb₂O₅ content of pure Niobite (78.88), %.

4. Results and Discussions

4.1. Effect of Table Slope

In SVT, the table slope is a critical parameter governing particle stratification and transport dynamics. A small slope hinders the effective removal of gangue, thereby diminishing the concentrate’s grade. While a big slope cut down the residence time of particles on the table, leading to the loss of valuable minerals to the tailings.

Experiments were conducted to evaluate the effects of various table slopes (2.0°, 2.4°, 2.8°, 3.2°) under fixed operating conditions (vibration voltage 190 V, vibration frequency of 38 Hz, and wash-water flow rate of 1.5 L/min). Figure 4 illustrates the effect of table slope on the Nb₂O₅ grade, recovery of the concentrate, as well as the separation efficiency.

As shown in Figure 4, increasing the table slope from 2.0° to 2.4° significantly improved the separation performance. The Nb₂O₅ grade increased from 0.88% to 1.15%, and the recovery rose from 43.46% to 47.51%, and the separation efficiency reached a peak value at 50.41%. This enhancement can be attributed to the optimized stratification and scouring intensity, which facilitated the efficient rejection of gangue while retaining the valuable minerals.

However, a further increase in the slope to 2.8° and 3.2° resulted in a decline in recovery (to 41.55% and 34.90%, respectively) and separation efficiency (to 44.88% and 39.47%, respectively), despite a continued rise in concentrate grade (to 1.26% and 1.72%). The steeper table slope accelerates the particle’s transportation, reducing the residence time available for the heavy particles to settle into the concentrate. Although the increased moving kinetic energy favored the removal of gangues (resulting in a higher grade), it also entrapped valuable minerals into the tailings. These results confirm the existence of an optimal table slope (2.4° in this case), which maximizes separation efficiency by striking a balance between concentrate grade and recovery.

4.2. Effect of Wash-Water Flow Rate

The wash-water flow rate is another important factor that affects the hydrodynamic conditions and the efficiency of gangue rejection in the SVT separation process. A slow flow rate impedes the removal of low-density particles, thereby diminishing the concentrate grade. Meanwhile, a rapid flow rate results in the entrainment of valuable minerals into the tailings.

As shown in Figure 5, experiments were conducted to evaluate the impact of various flow rates (1.2, 1.5, 1.8, and 2.1 L/min) under fixed conditions (vibration voltage 190 V, vibration frequency 38 Hz, and table slope 2.4°). Figure 5 illustrates the effect of the wash-water flow rate on the Nb₂O₅ concentrate grade, recovery rate, and separation efficiency. As the flow rate increased from 1.2 to 1.5 L/min, a significant increase in separation performance was observed. The Nb₂O₅ grade of concentrate improved from 0.83% to 0.92%, and the recovery rate increased from 43.46% to 47.51%. This improvement can be attributed to the optimization of fluidization and the effective scouring of gangue, which also helped in retaining valuable minerals.

Further increases in the flow rate proved detrimental to the overall performance. When the flow rate was 1.8 L/min, the Nb₂O₅ grade was only 0.78%, and the recovery dropped to 41.55%. At a flow rate of 2.1 L/min, the grade reached a peak value at 1.15%, but the recovery rate sharply declined to 34.9%.

Theoretically, an intensified hydraulic flow reduces the retention time of particles. This leads to the entrainment of heavy minerals into the tailings, thereby decreasing recovery. Meanwhile, it enhances the removal of gangue minerals, thereby increasing the grade. The maximum recovery (47.51%) and separation efficiency were achieved at a flow rate of 1.5 L/min. At this flow rate, an adequate fluid shear was maintained for gangue rejection without exceeding the critical entrainment velocity. Higher flow rates promote the suspension of particles beyond the separation zone, confirming the inverse relationship between grade and recovery dictated by hydrodynamic limitations.

4.3. Effect of Vibrating Voltage

In the SVT process, the vibrating voltage plays a pivotal role in influencing the kinetics of table stratification and separation efficiency through the regulation of particle dispersion and segregation energy. Insufficient vibration hinders the detachment of light gangue particles from the concentrate layer, consequently impacting the grade. Conversely, an excessive voltage promotes the misplacement of heavy minerals into the tailings stream, resulting in a decrease in recovery. The vibrating voltage and stroke length exhibit a positive correlation, where higher voltage results in bigger stroke length.

As shown in Figure 6, experiments were conducted to evaluate the impact of vibrating voltage (170, 180, 190, and 200 V) under fixed conditions (vibrating frequency of 38 Hz, table slope of 2.4°, and wash-water flow rate of 1.5 L/min). The results indicated that as the voltage increased from 170 V to 200 V, the grade of Nb₂O₅ concentrate decreased from 1.36% to 0.79%, while the recovery rate of Nb₂O₅ concentrate increased from 43.46% to 51.55%. At a voltage of 190 V, the separation efficiency reached its maximum value of 50.80%. This can be attributed to improved bed fluidization and increased forces of particle segregation, which subsequently enable the efficient removal of gangue minerals while preserving the valuable components.

4.4. Effect of Vibrating Frequency

The vibration frequency is a critical operational parameter in the separation process of the SVT, governing the dynamics of particle fluidization and the efficiency of stratification. Insufficient frequencies can disturb the equilibrium between gangue rejection and the retention of valuable minerals, thereby directly affecting the quality of concentrates and recovery rates. Experiments were conducted at a fixed table slope of 2.4°, a vibration voltage of 190 V, and a wash-water flow rate of 1.5 L/min to systematically evaluate the impact of vibration frequency (ranging from 35 to 44 Hz).

As shown in Figure 7, when the vibration frequency was elevated from 35 Hz to 38 Hz, the separation efficiency exhibited a notable improvement, rising from 47.13% to 50.80%. Simultaneously, the recovery of Nb₂O₅ increased by 5.71% (from 44.63% to 50.34%), whereas the concentrate grade experienced a moderate decline from 1.12% to 0.92%. This phenomenon implies that the heightened particle mobilization at 38 Hz contributed to more favorable stratification, which was conducive to the removal of gangue while ensuring the sufficient retention of valuable minerals. The maximum separation efficiency achieved at 38 Hz suggests an optimal equilibrium between particle suspension and settling kinetics.

An increase in frequency to 41 Hz and 44 Hz significantly impaired performance. Separation efficiency decreased to 45.20% and 37.52%, respectively, and recovery rates gradually declined to 49.06% and 44.41%. Notably, the concentrate grade continuously decreased to 0.69% (at 41 Hz) and 0.57% (at 44 Hz). These trends indicate that excessive vibration energy leads to turbulent hydraulic conditions, which shorten the effective residence time and disrupt the stratified layers. Consequently, finer valuable particles are discharged into the tailings due to intensified fluid drag, and insufficient settling opportunities impede the upgrading of the concentrate.

The inverse correlation between concentrate grade and recovery across the frequency spectrum highlights a fundamental trade-off in separation dynamics. Higher frequencies enhance gangue scouring; however, they concurrently cause the entrainment of Nb₂O₅ particles into tailings streams. In contrast, lower frequencies enhance grade selectivity while sacrificing recovery. This research determines that 38 Hz is the optimal operational frequency under the tested conditions, achieving maximal separation efficiency (50.80%) through balanced hydrodynamic control.

4.5. Comparison Between SST and SVT

To evaluate the fine particle separation efficiency of the SVT, a comparative experiment was conducted with the widely used SST in industrial fine particle gravity separation. The table area and raw ore feed rate for the SST and SVT are 0.2 m² and 2.1 m², and 2 kg/h and 20 kg/h, respectively. The slurry density was fixed at 10%. In the comparative experiment, both devices operated under their respective optimal parameters. For the SVT, the optimal test was conducted at a vibration voltage of 190 V, a vibrating frequency of 38 Hz, a table slope of 2.4°, and a wash-water flow rate of 1.5 L/min. For the SST, the optimal test used a stroke length of 8.2 mm, a stroke rate of 274 r/min, a table slope of 2.8°, and a wash-water flow rate of 15 L/min. The separation indices of both devices are presented in

Table 4. Comparison of separation indices between SST and SVT.

Equipment	Product	Yield (%)	Nb2O5 Grade (%)	Nb2O5 Recovery (%)	Separation Efficiency (%)
SST	Concentrate	4.55	1.09	26.05	27.5
	Tailing	95.65	0.15	73.95
	Raw Ore	100	0.19	100
SVT	Concentrate	7.82	1.31	52.64	56.96
	Tailing	92.18	0.1	47.36
	Raw Ore	100	0.19	100

. The SST achieved an Nb₂O₅ concentrate grade of 1.09% and a recovery of 26.05%, whereas the SVT reached 1.31% and 52.64%, respectively. The separation efficiencies were 27.50% for the SST and 56.96% for the SVT. These results indicate that the SVT demonstrates significantly better performance in the separation of fine-grained niobium ore.

Figure 8 demonstrates the size-by-size analysis results of the experiments in Table 4. It can be seen that the SVT exhibited performance similar to that of the SST for the +38 μm fractions but was significantly superior to the SST for the −38 μm fractions. This advantage became more pronounced as the particle size decreased. For example, for the 20–30 μm fraction, the Nb₂O₅ recovery of SST is 37.95%, while that of SVT reached 68.89%; for the −10 μm fraction, the Nb₂O₅ recovery of SST is merely 6.01%, while that of SVT reached 68.24%.

It can be concluded that SST is only effective for particles larger than 30 μm, whereas the SVT can effectively recover particles as fine as 10 μm. The enhanced overall separation performance of SVT is mainly attributed to its superior recovery of Nb₂O₅ in the −38 μm fraction.

In the case of SVT, the recoveries of Nb₂O₅ in the −74 + 38 μm and +74 μm fractions were only 35.29% and 9.18%, respectively. This low recovery is attributed to the loss of niobium to the middlings, where it primarily exists as incompletely liberated intergrowths concentrated in the +38 μm fraction. As shown in Table 2, the +38 μm fraction accounted for 30.18% of the total niobium. Therefore, regrinding the middlings is essential to enhance the liberation of niobium and further improve the overall recovery.

5. Conclusions

This study introduced a novel gravitational separation equipment named Slime Vibrating Table (SVT), with its structure presented and separation performance tested for a niobium ore. The main conclusions were as follows:

(1)

The SVT was developed from the Slime Shaking Table (SST), with its drive head replaced from an eccentric mechanism to electromagnetic mechanism, and the driving mode changing from shaking to vibrating.

(2)

The effect of key parameters, including table slope, wash-water flow rate, vibration voltage, and vibration frequency, was tested. The optimized operating condition was table slope at 2.4°, wash-water flow rate at 1.5 L/min, vibration voltage at 190 V, and vibration frequency at 38 Hz.

(3)

Under the optimized operating condition, SVT produced a primary concentrate assaying 1.31% Nb₂O₅ with a recovery of 52.64%, which was 0.22% and 26.59% higher than that of SST, respectively. Size-by-size analysis indicated that the enhanced separation performance of SVT was mainly attributed to its superior recovery of Nb₂O₅ in the −38 μm fraction.