Research on Hydrocyclone Separation of Palygorskite Clay

Abstract

The separation methods of palygorskite can be divided into dry separation and wet separation. For wet separation, which is more efficient, the centrifuge is the commonly used method but is characterized as having a high cost. It remains a challenge to separate ultra-fine palygorskite with an economical method. This work tried to separate palygorskite using a hydrocyclone with two separation stages. Orthogonal experiments are designed to investigate the influence of feed concentration, feeding pressure, and underflow port diameter on separation performance. The element content, mineral composition, and particle size distribution of the separation products are analyzed, respectively, by XRD, XRF, and laser particle sizers. The apparent viscosity of palygorskite pulp is characterized by a rotational rheometer. The purity of the feed and palygorskite concentrate is measured using an internal standard method. The results show that the purity of palygorskite increased from 45.1% to 64.2%, with a recovery of 95.9%.

1. Introduction

Palygorskite is a magnesium aluminum silicate clay mineral with an ideal molecular formula of Mg₅Si₈O₂O₂₀(OH)₂(OH₂)₄∙4H₂O, belonging to the sepiolite family [1]. The unique layered chain structure of palygorskite makes it have special physical and chemical properties, such as catalytic, rheological, adsorbability, and carrier qualities [2,3,4,5,6]. At present, palygorskite has been widely used in many fields, including the petroleum, chemical, food, medicine, and agriculture industries [7,8,9,10,11,12,13]. Palygorskite is widely used and has high economic value, but the purity of palygorskite in natural ore is low and needs to be separated to increase its purity.

Traditional methods for separating palygorskite can be categorized into dry and wet methods. In the dry method, grinding and classification are the key steps. Grinding ensures that palygorskite is fully dissociated from gangue minerals, while classification facilitates the separation of palygorskite from gangue minerals. The classification process often involves the use of an air-medium classifier. However, the separation efficiency in this process is difficult to guarantee, resulting in low concentrate purity. Therefore, it is often considered as a tailing discarding process with low purity of the concentrate [14]. In the wet method, gravity separation and flotation are commonly employed. Centrifugation is frequently used in the gravity separation process. Zheng et al. [15] utilized two methods for palygorskite separation: centrifugal separation without grinding and centrifugal separation after ball milling. The purity of the separated palygorskite reached 82.4% and 95.3%, respectively. Li [16] employed a two-stage flotation method to purify palygorskite. In the first stage, dolomite was excluded, while in the second stage, palygorskite was separated from silica. The recovery of palygorskite was 97.2%, with a purity of 92.6%. Compared to dry separation, wet separation processes offer higher separation accuracy, making them more commonly used in current practices.

A hydrocyclone is a classification device that utilizes centrifugal force to accelerate the sedimentation of particles. In this process, heavy and large particles are directed downwards to the underflow region through the external swirl, while light and small particles move upwards to the overflow region through the internal swirl. Different scholars have conducted in-depth research on the structural optimization and separation efficiency of hydraulic cyclones [17,18,19]. Silva et al. [20] investigated the influence of underflow diameter and vortex finder on the performance of hydrocyclones. Vieira et al. [21] studied the impact of cyclone diameter on the separation efficiency of a novel filtering hydraulic cyclone and found that the optimal diameter varies under different circumstances. For fine particle separation, a smaller diameter was preferred, whereas for concentrating solid suspensions, a larger diameter was preferred. Some scholars have also conducted hydrocyclones from an application perspective [22,23]. Gama et al. [24] conducted a study on the optimal parameters for the purification of bentonite using a hydrocyclone and performed an industrial-scale experiment to validate their findings. Indeed, hydrocyclones have been widely used in various industries for the removal of contaminants from different types of fluids [25]. In addition to bentonite purification, hydrocyclones have been employed to remove dyes from aqueous solutions and catalyst particles from oil slurries [26,27]. Palygorskite, being a low-density and soft material, can be effectively separated from gangue minerals with higher density and hardness by employing a hydrocyclone after thorough dispersion under the influence of mechanical stirring. In this study, we employed the hydrocyclone as a novel method for the separation of palygorskite. The effects of feed concentration, feeding pressure, and underflow port diameter on separation performance were investigated. The performance of the separation was evaluated based on the analysis of element content, mineral composition, and particle size distribution of the separated products.

2. Materials and Methods

2.1. Mineralogical Analysis

The raw material used in this study was palygorskite ore sourced from Gansu Province, China. The initial purity of palygorskite in the ore was determined to be 45.1% (internal standard method analysis results). Prior to conducting mineralogy analysis, the ore was immersed in water for a duration of 10 min. Subsequently, mechanical agitation was employed at a speed of 1200 revolutions per minute (r/min) for a duration of 20 min. The ore was then subjected to filtration and drying processes. In this study, the internal standard method was employed to measure purity. This method requires a pure mineral to establish a standard curve. A pure palygorskite sample obtained from Xuyi, Jiangsu, was used as a reference material to determine the purity of the raw ore as well as the hydrocyclone underflow and overflow products. A pure silica sample, provided by Shanghai Keyan Industrial Co., Ltd. (Shanghai, China), was used as a mixed mineral for the laboratory analysis. The element content and mineral composition of the palygorskite ore and the pure palygorskite were analyzed using X-ray fluorescence spectrometer and X-ray diffractometer techniques. The obtained results are presented in

Table 1. Element content of palygorskite ore and pure palygorskite.

Element	Content/%
	Palygorskite Ore	Pure Palygorskite
TiO2	0.45	1.51
MgO	7.28	21.75
Al2O3	17.49	15.36
SiO2	56.13	47.84
K2O	3.91	1.84
CaO	5.21	0.58
Fe2O3	7.78	10.49
Other	1.74	0.64

and Figure 1.

The results from Figure 1 and Table 1 indicate that the main impurities in the raw ore are quartz and dolomite, with quartz having a relatively high content. The pure ore contains a small amount of quartz.

Table 2. Hardness and specific gravity of palygorskite, quartz, and dolomite.

Property	Palygorskite	Quartz	Dolomite
The Mohs hardness	2–3	7	3.5–4
Specific gravity	2.05–2.32	2.22–2.65	2.8–2.9

displays the Mohs hardness and specific gravity of palygorskite, quartz, and dolomite. It reveals that palygorskite has the lowest hardness and specific gravity, while quartz has the highest hardness and dolomite has the highest specific gravity. Hydrocyclones can effectively separate light and small particles from heavy and large ones. Therefore, it is feasible to separate palygorskite from quartz and dolomite using a hydrocyclone.

2.2. Screening Test

To provide additional validation regarding the applicability of a hydrocyclone for palygorskite separation, a wet screening test was conducted on the palygorskite ore. This test followed the same preparation procedure as the mineralogy analysis. The diameter of the screen pore was between 10 μm and 45 μm. The specific steps were as follows: 100 g of palygorskite were placed into a nested sieve set. The vibrating sieve machine was activated to commence the screening experiment. The quantity of material passing through the sieve was observed. The screening process was considered complete when the amount of material passing through the sieve was less than 1% of the total sample weight. The material retained on the sieve was transferred to the subsequent sieve size, and the aforementioned steps were repeated until all sieves had been utilized. Upon completion of the experiment, the screened materials were collected from each sieve size. The collected materials were weighed, and the yield for each sieve size was calculated. X-ray fluorescence (XRF) analysis was conducted to determine the chemical composition of each sieve size fraction.

2.3. Hydrocyclone Apparatus

The hydrocyclone system used in this study was primarily composed of three components: a mixing tank, a hydrocyclone, and two product collection tanks. The hydrocyclone was purchased from Haiwang Cyclone Co., Ltd. in Weihai City, while the remaining equipment was assembled in-house in the laboratory. Figure 2 illustrates the schematic diagram of the system.

Table 3. The geometric dimensions of the hydrocyclone.

Components	The Cylindrical Part		The Cylindrical Part		Overflow Outlet Diameter	Underflow Outlet Diameter
	Diameter	Height	Diameter	Height
Dimensions/mm	85	100	85	400	40	20

presents the geometric dimensions of the hydrocyclone. The sample and water were mixed thoroughly in a mixing tank to achieve homogeneity. The mixture was then pumped into a hydrocyclone for separation experiments. Underflow and overflow collection tanks were provided to collect the sorted materials, allowing for the option of either directly extracting them for subsequent experiments or returning them to the mixing tank for cyclic tests. In this research, the volume of the mixing tank was 80 L, and the motor power of the mixing tank was 2.2 kW with a frequency of 50 Hz. The power of the circulation pump was 1.5 kW, also with a frequency of 50 Hz. The volume of the sample collection tank was 50 L.

2.4. Hydrocyclone Separation Experiments

2.4.1. Design of Orthogonal Experiments

The orthogonal experiments were designed with three factors and three levels. The specific experimental parameters are shown in

Table 4. Orthogonal experiment.

One-Stage Exp. N.°	Factor 1	Factor 2	Factor 3
	Underflow Port Diameter/mm	Feeding Pressure/MPa	Feed Concentration/g/L
1	6.00	0.10	50
2	6.00	0.15	80
3	6.00	0.20	100
4	8.00	0.10	80
5	8.00	0.15	100
6	8.00	0.20	50
7	10.00	0.10	100
8	10.00	0.15	50
9	10.00	0.20	80

.

2.4.2. The Rough Separation

The palygorskite ore underwent crushing using a jaw crusher to achieve a size smaller than 25 mm. Subsequently, it was introduced into the hydrocyclone mixing tank. The predetermined weights of 5, 8, and 10 kg of ore were mixed with water in the tank, resulting in pulps with solid concentrations of 5%, 8%, and 10%, respectively. Prior to separation, the ore was immersed in water for 24 h to ensure complete dispersion.

Following the 24-h soaking period, the pulp was agitated in the mixing tank at a speed of 1200 r/min for 1 h, thereby facilitating further dispersion of the palygorskite. Based on the nine sets of experimental conditions outlined in Table 4, hydrocyclone separation experiments were conducted at varying levels of underflow port diameter, feeding pressure, and feed concentration. Throughout the separation process, samples of the feed, underflow, and overflow were collected twice. During the first sampling, the samples were subjected to particle size distribution analysis using a laser particle sizer. For the second sampling, the masses of the feed, overflow, and underflow collected simultaneously for a duration of 5 s were determined after filtration and drying. This allowed for the calculation of the yield for each batch.

2.4.3. Underflow Scavenging

The nine groups of underflows after the rough separation by hydrocyclone were collected, and they were used as the feed for underflow scavenging. The feed concentration was kept unchanged, and the underflow port diameter and the feeding pressure were the same as those shown in Table 4. The other experimental operations were consistent with the rough separation.

2.5. Particle Size Analysis

The GSL-1000 laser particle sizer (developed by China Dandong Liaoning Instrument Research Institute Co., Ltd., Dandong, China) was used to conduct the particle size distribution analysis of the test sample. The test sample was placed in 50 mL of distilled water. The sample was stirred for 5 min using a magnetic stirrer to ensure uniform dispersion. After the sample was evenly dispersed, it was transferred to the testing dish for analysis using a dropper. The experimental results were recorded when the optical density was between 25% and 35%.

2.6. Internal Standard Method Analysis

The internal standard method was utilized to characterize the purity of palygorskite in this study [28]. Synthetic corundum was employed as the internal standard material, with a fixed content of 50%. The remaining 50% of the mixture consisted of pure palygorskite and silica, with the proportion of palygorskite gradually increasing from 5% to 50% in increments of 5%. XRD analysis was performed on these 10 samples to qualitatively and quantitatively analyze their mineral composition. The strongest diffraction peaks of palygorskite (d(110) 1.04 nm) and corundum (d(113) 2.086 nm) were selected as the characteristic peaks for content analysis. The integral area ratios of the strongest palygorskite diffraction peak to that of corundum in the 10 samples were calculated. These ratios were plotted on the Y-axis, while the content of palygorskite in the samples was plotted on the X-axis. A linear curve was obtained, representing the relationship between the area ratio and the purity of palygorskite.

2.7. Apparent Viscosity Test

A 3.5-g sample was taken and mixed with water to prepare a solution with a mass fraction of 7%. The mixture was then stirred using a high-shear disperser at a speed of 8000 r/min for 20 min. Subsequently, a 20-mL well-mixed sample was taken for apparent viscosity analysis using a DHR-2 rotational rheometer (TA Instruments, New Castle, DE, USA). The diameter of the flat rotor used during the testing process was 40 mm. The air purification device and temperature controller were turned on, and the temperature was set at 25 °C. Before testing, the equipment was calibrated for host inertia, internal resistance, and temperature. The distance between the two plates was set to 50 μm. The shear rate was varied from 1 to 400 s⁻¹ in flow scanning mode. The shear stress was measured at each shear rate, and the apparent viscosity of the slurry was calculated based on these measurements. The apparent viscosity can be calculated using the formula:

$$\eta =\frac{\tau }{\gamma }$$

(1)

where η is the apparent viscosity, τ is the shear stress, and γ is the shear rate.

3. Results and Discussion

3.1. Screening Test Results

The yield of different particle size fractions is shown in Figure 3. According to Figure 3, the majority of particles (93.38%) were found in the ≤10 μm fraction. The fractions of particles in the 45–10 μm and ≥45 μm size ranges were only 1.7% and 4.9%, respectively. Subsequently, an elemental content analysis was performed on the particles within the ≥45 μm, 45–10 μm, and ≤10 μm size ranges, as presented in

Table 5. Element content of different size range.

Element	Content/%
	≤10 μm	10–45 μm	≥45 μm
TiO2	0.51	0.40	0.26
MgO	5.65	2.19	1.88
Al2O3	18.80	12.79	11.89
SiO2	63.89	77.82	79.38
K2O	4.30	2.99	2.88
CaO	6.05	2.18	1.90
Other	0.80	1.63	1.81

.

According to Table 5, the content of Si in the ore gradually increased, and the content of Mg and Al gradually decreased with the increase in particle size. The pure minerals of palygorskite contain a large amount of Mg and Al elements, and the main gangue mineral quartz in the raw ore contains a large amount of Si elements. The Mg element in the fine-grained product may also originate from dolomite, but it can be observed that the content of quartz was significantly reduced in the fine-grained fraction. These results indicate that the content of palygorskite was higher in the fine-grained size range due to its low hardness, whereas quartz was mainly present in the coarse-grained component. After comparing the elemental content of the product within the particle size range of ≤10 μm with the pure minerals shown in Table 1, it was observed that the content of Mg, Al, and Si elements in the ≤10 μm product was 5.65%, 18.80%, and 63.89%, respectively, whereas the pure minerals had contents of 21.75%, 15.36%, and 47.84% (the corresponding elemental content for >10 μm products was approximately 2%, 12%, and 78%, respectively). Compared to the products in the other two particle size ranges the ≤10 μm product exhibited a relatively closer elemental content to that of the pure minerals. This further validated the feasibility of hydrocyclone separation.

3.2. Hydrocyclone Separation Results

3.2.1. The Rough Separation

Particle size analysis was conducted on the underflow and overflow of the hydrocyclone postseparation. The obtained results are depicted in Figure 4. It is evident from the data that the overflow particle size for One-stage 5 and 9 exhibited relatively fine characteristics, with D₈₀ (the volume of particles smaller than this diameter accounts for 80% of the volume of all particles) values of 4.88 μm and 5.45 μm, respectively. The underflow particle sizes for One-stage 2 and 6 appeared coarse, with D₈₀ particle sizes of 8.2 μm and 8.60 μm, respectively.

Table 6. Particle size property and yield of overflow and underflow of hydrocyclone.

One-Stage Exp. N.°	Underflow D80/μm	Overflow D80/μm	Underflow Yield/%	Overflow Yield/%
1	7.85	6.36	50.40	49.60
2	8.25	6.13	56.47	43.53
3	7.47	5.62	44.89	55.11
4	7.47	6.08	50.89	49.11
5	6.23	4.89	44.80	55.20
6	8.60	5.95	55.50	44.50
7	5.03	5.48	58.10	41.90
8	7.08	5.93	50.30	49.70
9	6.79	5.45	56.45	43.55

presents the particle size (D₈₀) property and yield data for the overflow and underflow of the hydrocyclone. It is evident from the table that One-stage 1, 3, 5, and 8 exhibited relatively high overflow yields. Among these four experimental groups, One-stage 5 demonstrated the highest overflow yield and the smallest overflow D₈₀, suggesting a higher content of palygorskite. From this perspective, One-stage 5 can be considered the optimal group for the rough separation.

It is important to highlight that the underflow D₈₀ of all the one-stage experiments did not show a significant deviation from the D₈₀ of the raw ore (Figure 4). Furthermore, the overflow yield was not high. These observations suggest that a considerable number of palygorskite particles were not effectively recovered but instead reported to the underflow. Hence, it is crucial to implement a scavenging process for the underflow in order to enhance the recovery of palygorskite particles.

The above results were analyzed based on the particle size and yield of the separated product. Next, further evaluation was required to assess the significance of the orthogonal experiment and the separation efficiency of the hydrocyclone.

The significance of the factors in the orthogonal tests was evaluated by analyzing the overflow D₈₀ as the criterion for analysis. The results indicate that the overflow D₈₀ decreased with an increase in the underflow port diameter. As the feeding pressure increased, the overflow D₈₀ initially decreased and then increased. Moreover, with an increase in feed concentration, the overflow D₈₀ decreased.

The analysis reveals that the underflow port diameter plays a significant role in determining the cone ratio of the hydrocyclone and the yield of the overflow [20]. Increasing the underflow port diameter leads to the discharge of more coarse and medium-sized particles through the underflow port, resulting in a decrease in both the yield and particle size of the overflow.

The feeding pressure is another crucial parameter that affects the classification performance of the hydrocyclone. Insufficient feeding pressure leads to a low centrifugal force within the hydrocyclone, making it difficult to achieve satisfactory classification. Conversely, excessively high feeding pressure or low feed concentration can create an unstable hydrodynamic environment within the hydrocyclone, thereby deteriorating the classification efficiency. Therefore, it is essential to carefully select an appropriate feeding pressure and feed concentration to optimize the hydrocyclone’s performance.

A significance analysis of the three factors (underflow port diameter, feeding pressure, and feed concentration) was conducted, and the results, as shown in

Table 7. Significance analysis of hydrocyclone experiments.

Factor	Deviation Sum of Squares	Degree of Freedom	F Ratio	F Critical Value
Underflow port diameter	0.33	2	0.688	5.14
Feeding pressure	0.196	2	0.408	5.14
Feed concentration	0.914	2	1.904	5.14
Deviation	1.44	6

, indicate that feed concentration has the highest significance, followed by the underflow port diameter, while feeding pressure has the lowest significance. These findings are consistent with Yu Jianfeng’s conclusions [29].

The classification performance of the hydrocyclone was evaluated via the particle size distribution rate according to nine sets of experimental results, as shown in Figure 5. According to Figure 5, the classification performance was evaluated by classification particle size, classification efficiency, and mismatched material, as shown in

Table 8. Classification performance of hydrocyclone experiments.

One-Stage Exp. N.°	Classification Particle Size/μm	Classification Efficiency/%	Upper Mismatched Material	Lower Mismatched Material	Average Mismatched Material
1	13.75	47.24	0.20	0.85	0.53
2	12.29	41.15	0.29	0.78	0.53
3	13.01	36.06	0.27	0.82	0.54
4	12.54	30.94	/	0.78	/
5	12.84	40.95	0.22	0.82	0.52
6	8.62	28.94	0.19	0.60	0.39
7	/	/	/	/	/
8	11.20	20.09	/	0.70	/
9	9.21	19.28	0.17	0.59	0.38

. The specific calculation formulas are as follows:

• Classification efficiency calculation formula:

$$\eta =E_c+E_f-100$$

(2)

$$E_c=\frac{{\gamma }_u\times u_c}{F_{c,r}}\times 100$$

(3)

$$E_f=\frac{100F_{f,r}-{\gamma }_u\times u_f}{F_{f,r}}$$

(4)

where:

• η—Classification efficiency, %
• E_c—The recovery efficiency of coarse particle, %
• E_f—The recovery efficiency of fine particle, %
• γ_u—Underflow yield, %
• u_c—Coarse particle content in underflow, %
• F_c,r—Coarse particle content in calculated feed, %
• F_f,r—Fine particle content in calculated feed, %
• u_f—Fine particle content in underflow, %

2.

Average mismatched material calculation formula:

$${PE}_m=\frac{{PE}_u+P{E}_L}{2}$$

(5)

$$P{E}_u=\frac{S_{75}}{S_p}$$

(6)

$$P{E}_L=\frac{S_p}{S_{25}}$$

(7)

where:

• PE_m—Average mismatched material
• PE_u—Upper mismatched material
• PE_L—Lower mismatched material
• S₇₅—Particle size corresponding to a classification efficiency of 75% on the particle size distribution rate curve
• S_p—Partition size corresponding to a classification efficiency of 50% on the particle size distribution rate curve
• S₂₅—Particle size corresponding to a classification efficiency of 25% on the particle size distribution rate curve.

It was found that the overall actual classification particle size of the nine groups of experiments was quite different. The classification particle sizes of One-stage 6 and 9 were small, and One-stage 1 had the largest actual classification particle size. The classification efficiency of One-stage 1, 2, and 5 was large, and for One-stage 8 and 9 was small, while One-stage 7 had no classification effect. It could be seen from Figure 5 that only the distribution rate curves of One-stage 1, 2, 3, 5, 6, and 9 were complete, and the upper and lower mismatched material under these experimental conditions could be completely obtained. For the other experiments, the classification effect was weak, and the mismatched material could not be obtained. It could be seen from Figure 5 that the curve of One-stage 7 did not cross the horizontal line on which the distribution rate was 50%, so it had no classification effect. The curves of One-stage 4 and 6 did not cross the horizontal line on which the distribution rate was 75%, so the upper mismatched material could not be obtained. The results showed that among the six groups of experiments that had an effective classification effect, One-stage 9 had the least mismatched material, but their classification efficiency was low.

Based on the analysis above, it can be concluded that One-stage 5 had the largest overflow yield and the smallest classification particle size. Although its actual classification particle size was large, its classification efficiency was high. Therefore, One-stage 5 was the most effective group for the rough separation. However, in order to improve the recovery of palygorskite, underflow scavenging is required.

3.2.2. Underflow Scavenging

Particle size analysis was conducted for the underflow and overflow of the hydrocyclone after scavenging. The results are presented in Figure 6. It can be observed that the overflow particle size of Two-stage 9 was fine, with a D₈₀ of 3.40 μm. The difference in overflow particle size among the other groups was relatively small. The underflow particle sizes of Two-stage 2 and Two-stage 6 were coarse, with D₈₀ particle sizes of 9.93 μm and 11.38 μm, respectively.

Table 9. Particle size property, yield of overflow, and the hydrocyclone total.

Two-Stage Exp. N.°	Experimental Result
	Underflow D80/μm	Overflow D80/μm	Overflow Yield in Scavenging/%	Total Overflow Yield/%
1	7.53	6.10	56.22	77.93
2	9.93	6.08	48.81	71.09
3	9.18	5.46	49.38	77.28
4	8.28	6.08	50.54	74.83
5	7.66	5.69	50.98	78.04
6	11.38	5.64	41.31	67.43
7	5.39	5.39	50.02	70.96
8	9.22	5.60	43.77	71.72
9	9.10	3.40	46.55	69.83

displays the particle size (D₈₀) property and yields of the total overflow and underflow of the hydrocyclone after two-stage separation. It can be observed that the overflow yields of Two-stage 1, 4, and 5 were relatively high, similar to those observed in the rough separation. The overflow yield for all experimental conditions was approximately 70%, with the highest being 78.04% in Two-stage 5. In terms of palygorskite recovery, Two-stage 5 exhibited the optimal separation performance. However, it is worth noting that the overflow D₈₀ particle size for Two-stage 5 was coarse, while the underflow D₈₀ particle size was fine.

Figure 7 and

Table 10. Classification performance of hydrocyclone experiments.

Two-Stage Exp. N.°	Classification Particle Size/μm	Classification Efficiency/%	Upper Mismatched Material/%	Lower Mismatched Material/%	Average Mismatched Material/%
1	14.98	76.07	0.42	0.92	0.67
2	11.89	25.42	0.49	0.78	0.64
3	9.07	41.24	0.37	0.65	0.51
4	11.27	34.55	0.24	0.72	0.48
5	11.74	31.74	0.21	0.75	0.48
6	8.86	45.91	/	0.71	/
7	/	/	/	/	/
8	7.27	31.79	0.24	0.57	0.40
9	6.04	61.85	0.61	0.71	0.66

present the particle size distribution rate curve and classification performance of the hydrocyclone experiments for underflow scavenging. It was observed that Two-stage 1 had a larger actual classification particle size, while Two-stage 9 had a smaller classification particle size. The classification efficiency was high for Two-stage 1, 6, and 9, while Two-stage 7 showed no classification effect. From Figure 7, it can be seen that only the distribution rate curves of Two-stage 1, 2, 3, 4, 5, 8, and 9 are complete, indicating that the upper and lower mismatched materials could be completely obtained under these experimental conditions. However, for the other experiments, the classification effect was weak, and the mismatched material could not be obtained. The explanation for this phenomenon has been previously elucidated in the rough separation of the report, thus there is no need to reiterate it. The results demonstrate that Two-stage 8 had the least amount of mismatched material, but its classification efficiency was low.

Based on the above analysis, it can be found that Two-stage 2 had a relatively coarse underflow particle size, and the difference between underflow particle size and overflow particle size was large. Two-stage 6 displayed a coarse underflow particle size, along with higher classification efficiency and a lower average of mismatched material. Two-stage 9 demonstrated a finer overflow particle size, a finer classification particle size, and a higher classification efficiency. Consequently, Two-stage 2, Two-stage 6, and Two-stage 9 emerged as the optimal groups for underflow scavenging.

3.3. Internal Standard Method Analysis Results

The mineralogical composition analysis was conducted on the overflow and underflow of One-stage 5 and Two-stage 2, 6, and 9. Figure 8 illustrates the results of this analysis. It is observed that the peak of palygorskite in the overflow is significantly higher compared to that in the underflow. However, the peaks of quartz and dolomite still exist in the overflow. This suggests that after the hydrocyclone separation, the palygorskite was concentrated in the overflow with a high recovery rate, as evidenced by the weak peak of palygorskite in the underflow. Nevertheless, the purity of palygorskite in the overflow was not very high due to the presence of quartz and dolomite, which led to contamination.

In addition to the mineralogical composition analysis, an element content analysis was conducted, and the results are presented in

Table 11. Element content table after separation.

Exp. N.°	TiO2/%	MgO/%	Al2O3/%	SiO2/%	K2O/%	CaO/%	Fe2O3/%	Other/%
One stage-5 underflow	0.39	6.13	15.91	62.18	3.57	4.93	5.89	1.00
One stage-5 overflow	0.42	7.44	17.68	56.07	4.03	5.28	8.15	0.93
Two stage-2 underflow	0.34	5.39	13.86	65.73	3.20	5.10	5.19	1.19
Two stage-2 overflow	0.42	7.42	17.61	56.18	3.99	5.43	8.06	0.89
Two stage-6 underflow	0.33	5.22	13.29	66.59	3.10	5.19	5.05	1.23
Two stage-6 overflow	0.42	7.40	17.72	56.02	4.01	5.17	8.36	0.90
Two stage-9 underflow	0.36	6.12	14.53	62.89	3.29	5.93	5.75	1.13
Two stage-9 overflow	0.42	7.44	17.57	56.06	3.99	5.48	8.16	0.88

. It is observed that, after the hydrocyclone separation, the Si element content in the underflow of all four experimental groups was higher than that in the overflow. The highest difference was observed in Two-stage 6, with a difference of approximately 10.6%. The Mg and Al elements in the overflow of all four experimental groups were higher than those in the underflow, with the largest difference again seen in Two-stage 6. This finding is consistent with the results of the XRD analysis. Then, the internal standard method was used to assess the purity of the overflow after the hydrocyclone separation.

Figure 9 presents a linear curve illustrating the relationship between the area ratio and the purity of palygorskite. Based on the XRD results shown in Figure 1, the purity of the raw palygorskite ore used in this study was calculated to be 45.1%.

Table 12. Palygorskite purity and recovery in the overflow after hydrocyclone separation.

Exp. N.°	Purity of Palygorskite in Overflow/%			Recovery/%
	One-Stage	Two-Stage	Total	One-Stage	Total
One-stage 5	63.2	/	63.2	77.5	77.5
Two-stage 2	61.2	58.9	60.4	59.1	95.2
Two-stage 6	65.6	60.5	64.2	64.8	95.9
Two-stage 9	62.5	60.1	61.7	60.3	95.5

displays the purity of palygorskite in the overflow after hydrocyclone separation. The results indicate that, among the four experimental groups, the purity ranged from 60.4% to 64.2%, with corresponding recovery rates of 77.5% to 95.9%. Notably, Two-stage 6 exhibited the highest purity of 64.2% and a recovery rate of 95.9%. The results demonstrate a significant increase in the overall recovery of palygorskite in the overflow after underflow scavenging.

The findings of Kim et al. [30] align with the results obtained in this study. They utilized a hydrocyclone for the purification of sericite, resulting in a decrease in the SiO₂ content in the overflow from 60.9% to 51.4%. Similarly, in this study, the SiO₂ content in the underflow increased from 56.13% (Table 1) to 66.59% (Table 11) after hydrocyclone separation. However, the SiO₂ content in the overflow did not exhibit significant changes. This phenomenon can be attributed to the unique silicon-oxygen tetrahedral structure of palygorskite, which contains a substantial amount of Si. After separation, the purity of palygorskite increased by 19 percentage points, indicating a notable separation effect.

3.4. Apparent Viscosity Test Results

The apparent viscosity of the raw palygorskite ore and the underflow and overflow of One-stage 5, Two-stage 2, Two-stage 6, and Two-stage 9 were measured using a rotational rheometer, and the results are presented in Figure 10. It can be observed that the apparent viscosity decreased as the shear rate increased, indicating that the pulp exhibited pseudoplastic behavior. The difference in overflow apparent viscosity among the four experiments was minimal, but all of them were higher than that of the raw ore. On the other hand, the overall underflow apparent viscosity was significantly lower than that of the raw ore, with Two-stage 6 exhibiting the lowest apparent viscosity. At high shear rates, the underflow apparent viscosity showed a slight recovery. This may be attributed to the presence of excessive eddies in the fluid at high shear rates, which disrupt the original laminar flow state and lead to an increase in apparent viscosity. Due to the excellent colloidal properties (rheological properties) of palygorskite, the overflow contained a high concentration of fine palygorskite particles, resulting in a high apparent viscosity. The underflow was primarily composed of coarse quartz and other gangue minerals, leading to a decrease in the apparent viscosity of the pulp.

4. Conclusions

The hydrocyclone apparatus was used to separate the palygorskite from the gangue minerals with high density and hardness, such as quartz and dolomite. By changing the feed concentration, underflow port diameter, and feeding pressure, the conditional experiments of rough separation and underflow scavenging were carried out. The classification effect of hydrocyclone and the purity of palygorskite were analyzed so that the optimum experimental condition was found, under which the palygorskite recovery was 95.9%, with purity increasing from 45.1% to 64.2%. The optimum experimental condition was as follows: the underflow port diameter was 8 mm, the feed concentration was 50 g/L, and the feeding pressure was 0.2 MPa. As a physical separation method, hydrocyclone separation did not add any agent in the separation process, so the crystal structure of palygorskite separated by hydrocyclone was not damaged, and the gangue minerals with large particle sizes were discharged so that the purity of palygorskite was improved. Compared with the traditional palygorskite separating method, hydrocyclone separation with higher separating accuracy and yield, which also has a simple process, can be used on a large scale.