Effect of Leaching on Particle Migration and Pore Structure of Ionic Rare Earth Ores with Different Fine Particle Contents

Abstract

In in situ leaching, fine particles can be stripped and transported with the leach solution, significantly altering the particle size distribution and pore structure of each layer of the rare earth ore body. In this study, water and magnesium sulfate were used as leaching agents. Based on indoor column leaching experiments, particle gradation experiments, and pore structure tests, this study investigates and analyzes the patterns of particle migration and changes in pore structure in rare earth ores with varying fine particle contents under leaching conditions. The results indicate that during the leaching process, the degree of change in particle gradation follows the order of upper layer > middle layer > lower layer. As the depth increases, the soil becomes denser, leading to reduced permeability, a slower seepage rate of the leaching solution, and a higher fine particle content, making the effect more pronounced. During magnesium sulfate leaching, the overall trend of porosity in the rare earth ore structure initially increases and then decreases. Additionally, a higher fine particle content corresponds to higher porosity. In the early and late stages of leaching, pore size changes involve the transformation of larger pores into smaller ones, followed by the conversion of smaller pores into larger ones. Moreover, the higher the fine particle content, the greater the degree of transformation between the pore sizes.

1. Introduction

Ionic rare earth ore is the primary source of heavy rare earth metals globally. Due to their unique properties, rare earth elements are widely used in the manufacturing of new materials, medical science and technology, aerospace technology, and other fields, contributing to advancements in science, technology, and economic development [1,2,3]. Medium and heavy rare earth elements are primarily found in the Gannan region of China, where they occur in magmatic granite and volcanic rock. Under the region’s unique climatic conditions, these elements weather from unweathered bedrock into semi-weathered mineral particles. Rare earth elements are primarily adsorbed onto the surface of clay mineral particles in the form of hydroxyl-hydrated ions [4,5,6].

Ionic rare earth ore mining technologies have undergone the development of three generations of techniques, evolving from initial pool leaching and heap leaching to the current in situ leaching process. This evolution has resulted in significant improvements in mining costs, leaching efficiency, and environmental protection [7,8]. During in situ leaching, small particles are able to migrate through the ore body due to the seepage of the leaching solution, rare earth ion replacement, frictional effects, and gravitational effects. This migration leads to changes and reorganization in the pore structure of the ore body, resulting in variations in the particle size distribution at different depths. The accumulation of fine particles within the ore body creates a muddy layer, which negatively impacts the solution permeability during the leaching process, leading to lower leaching efficiency and inconsistent ore body stability. Many scholars have conducted research in an attempt to solve this problem [9,10,11]. Ahfir et al. [12] found the highest non-homogeneous dispersion coefficient of mixed sand through step injection and fluorescence tracer experiments, particle inlet retention at low flow rates, and deeper migration and increased sedimentary particle size at high flow rates; large-size sand enhanced retention due to narrow pore space, confirming that the homogeneity coefficient of the porous media is positively correlated with the filtration efficiency. Jiang et al. [13] investigated the migration and deposition of suspended fine particles in soil through laboratory experiments by introducing three different shapes of suspended fine particles into the leaching solution. Bennacer et al. [14] found that a high ionic strength (0–600 mM), low flow rate (0.15 cm/s), and large particle/sand ratio significantly enhanced deposition through pulse experiments, but the deviation from DLVO theory revealed the need for the introduction of surface heterogeneity, whereby a coupled ion–particle size–flow rate deposition model was established to provide a basis for the prediction of particle migration. Zhang et al. [15] revealed the evolution of pore structure triggered by a decrease in matrix suction in the leaching of ionic rare earth ores by the filter paper method, NMR, and simulated leaching experiments: the matrix suction decreased abruptly when the volumetric water content was <20%, and the distribution of the pore radius shifted to macro/medium pores (with the saturated porosity enlarged by 17.5-fold), which directly enhanced the permeability coefficient and elucidated the pore mechanism of “permeability inhomogeneity-preferential channeling”. The pore mechanism of “uneven permeability-preferential channeling” was elucidated by Wang et al. [16]. Based on the column leaching test, triaxial test, and NMR analysis, Wang et al. found that when ammonium sulfate leached the weathered crust rare earth ores, the migration of fine particles led to pore stratification (top loose/bottom dense), and a decrease then increase in the permeability coefficient. The increase in salts strengthened particle cementation, but the change in the pore structure weakened the strength of the soil body and triggered the upper part of the shear damage, which revealed the mechanism of the correlation between the evolution of the pore space and the engineering properties. Wang et al. [17] studied the effects of column leaching methods on particle migration in rare earth ore using two techniques: first, injecting deionized water followed by ammonium sulfate solution, and directly injecting ammonium sulfate solution. It has been shown that the synergistic effect of particle migration and ion exchange reaction in the leaching process has a significant effect on the permeability and pore structure of the rare earth ore body. The production requirements of mines should be combined with the actual situation of the project to select the appropriate leaching method. Based on nuclear magnetic resonance technology and indoor column leaching experiments, Zhang et al. [18] found that as the degree of weathering decreases (residual soil → strong weathering zone), the rate of change in the porosity and permeability coefficient of rare earth ores decreases, and that the strong weathering zone is characterized by weak decomposition of feldspars/mica, less disintegration, a stable pore structure, and optimal leaching, which provides a basis for the development of resources from a multi-weathering zone. Zhou et al. [19] and Liu et al. [20] investigated the changes in the pore size distribution and multi-dimensional geometry of rare earth ore bodies before and after leaching. Luo et al. [21] studied the structure, pore distribution, pore throat size, and pore network characteristics of rare earth ore bodies in the surface, weathering, and semi-weathering layers using CT scanning and three -dimensional imaging technology.

There have been numerous studies on the internal particle migration and pore structure of the orebodies; however, few studies have focused on the impact of fine particle content on internal particle migration and pore structure during the leaching process of rare earth ore bodies. This study aimed to investigate the effect of fine particle content on particle migration and pore structure during the leaching process of rare earth ore under different leaching conditions. Rare earth ore samples with varying fine particle contents smaller than 0.075 mm were prepared for leaching. Additionally, considering the environment and green chemistry, indoor column leaching experiments were conducted using deionized water and magnesium sulfate solution as environmentally friendly leaching agents to observe the changes in the particle size distribution and pore structure. The mechanism by which fine particle content affects particle migration and pore structure in the rare earth ore body was analyzed. This study provides scientific guidance and reference for mining technology and slope stability in rare earth mines, promoting the green, efficient, and safe production of critical minerals.

2. Theoretical Basis and Experimental Methods

2.1. Theoretical Basis

The application of fractal theory and relaxation theory in rare earth ore research mainly focuses on the dynamic evolution of the ore body structure and the mechanism analysis of the leaching process. The fractal theory quantitatively characterizes the complex heterogeneity of the pore network and the particle distribution of the ore body through the fractal dimension, reveals the self-similarity of the particle migration path and the seepage channel in the leaching process, and provides a basis for optimizing the permeability and leaching efficiency. The relaxation theory involves the process of the system recovering from the non-equilibrium state to the equilibrium state, which may be related to the dynamics of the rare earth ore leaching process. For example, in the ion exchange reaction, the relaxation time may affect the reaction rate and efficiency, which in turn affects the penetration of the leaching solution and the change in the ore body structure.

2.1.1. Fractal Theory

The application of fractal theory began in the early 1980s and has since been applied to various research fields, including mathematics, physics, and chemistry [22,23,24]. In this study, due to the irregular particle size and distribution of ionic rare earth ore, fractal theory is introduced to describe the particle size distribution of the ore. The fractal dimension F value of rare earth ore can be determined by analyzing the curve slope of ln (m/M) and ln (r/R) of the rare earth ore. The equation is shown in Equation (1):

$$\mathrm{l}\mathrm{n}(\mathrm{m}/\mathrm{M})=(3-\mathrm{F})\mathrm{l}\mathrm{n}(\mathrm{r}/\mathrm{R})$$

(1)

In the formula, m is the cumulative mass of rare earth particles, g; M is the total mass of the rare earth sample, g; r is the aperture of any sieve, mm; R is the maximum particle size of the rare earth sample, mm; and F is the fractal dimension.

2.1.2. Relaxation Theory

According to the relaxation principle of nuclear magnetic resonance, with the stop of the radio frequency pulse, the hydrogen nuclei gradually return to equilibrium, and the absorbed energy is released as a current signal. The instrument can count the number and position of hydrogen in the sample. This process is called a relaxation process. The required time is the relaxation time [25,26,27,28]. Since $T_{2B}$≫$T_2$, and the $T_E$ value is small enough, the functional relationship between the T2 value and the pore radius R can be derived from the relationship between S and V. The equation for calculating the pore radius is shown in Equation (4):

$${\displaystyle \frac{1}{T_2}}={\displaystyle \frac{1}{T_{2B}}}+{\rho }_2\left({\displaystyle \frac{S}{V}}\right)+{\displaystyle \frac{D{\left(\gamma G{T}_E\right)}^2}{12}}$$

(2)

Since $T_{2B}$≫$T_2$, and the value of $T_E$ is sufficiently small, Equation (2) can be simplified to

$${\displaystyle \frac{1}{T_2}}={\rho }_2{\displaystyle \frac{S}{V}}$$

(3)

Through the relationship between S and V, Equation (3) can again be simplified as

$${\displaystyle \frac{1}{T_2}}={\rho }_2{\displaystyle \frac{2}{R}}$$

(4)

In the formula, R is the radius of the aperture, μm. $T_2$ is the free relaxation time of the fluid in the tested sample, ms; $T_{2B}$ is the free relaxation time, ms; ${\rho }_2$ is the transverse surface relaxation strength, μm/ms; S is the surface area of the pores, μm²; V is the volume of pores, μm³; D is the diffusion coefficient, μm²/ms; γ is the magnetogyric ratio, ${(\mathrm{T} \mathrm{m}\mathrm{s})}^{-1}$; G is the magnetic field gradient, ${10}^{-4}$ T/μm; and $T_E$ is the echo interval, ms.

2.2. Experimental Methods

2.2.1. Experimental Materials

The rare earth samples used in this study were collected from a rare earth mining area in Dingnan County, Jiangxi Province, in the People’s Republic of China. The basic physical parameters of the mine are presented in

Table 1. Basic physical parameters.

Parameter	Natural Density (g/cm3)	Dry Density (g/cm3)	Moisture Content (%)	Specific Gravity of Soil Particles	Porosity (%)
Value	1.63	1.35	20.47	2.70	49.87

. In this experiment, three different ore blocks without in situ leaching were selected, with a sampling depth ranging from 2 to 3 m. The field appearance is shown in Figure 1. The soil particle size distribution for this experiment was determined by sieving the undisturbed soil and by drying the collected undisturbed rare earth mineral samples in a desiccator at 105° for 8 h, followed by dry sieving. The soil particles obtained after screening were dried and weighed, and the proportion of each particle size was calculated and is presented in

Table 2. Particle gradation of undisturbed rare earth.

Particle Diameter (mm)	<0.075	0.075–0.25	0.25–0.5	0.5–1	1–2	2–5	>5
Percentage of interval (%)	13	10	15	18	15	24	5
Cumulative percentage (%)	13	23	38	56	71	95	100

. The particle size distribution of the rare earth samples from the mine is favorable, with the proportion of particles smaller than 0.075 mm accounting for approximately 13%, classifying the material as fine-grained sand. All fine particles referred to in this paper are smaller than 0.075 mm, and this fine particle size content of ionic rare earth ore is the primary focus of this study. After screening the ore particles smaller than 0.075 mm from the original graded ore soil using a sieve, the cumulative particle size distribution of the ore soil was obtained, as shown in Figure 2.

2.2.2. Column Leaching Experiment

The column leaching experiment consists of a liquid injection device, a sample, and a support. Particle migration and pore structure change measurements of rare earth ores were conducted during the experiment. The experimental setup is shown in Figure 3a,b. To facilitate the measurement and collection of the experimental parameters during the column leaching process, transparent PVC cylinders of different sizes (height × diameter: 230 × 110 mm and 110 × 45 mm) were selected for the rare earth ore particle migration and pore structure column leaching experiments. When preparing the rare earth ore samples with varying fine-grained content, a pre-screened sample of fine rare earth particles (<0.075 mm) was slowly and thoroughly mixed in specific proportions to achieve a fine-grained content (as a percentage of the total sample mass) of 5%, 10%, 15%, 20%, 25%, and 30%. The rare earth samples, with a coarse-grained distribution similar to the original ore soil gradation, were used as the experimental controls. The specific size distributions accounting for the variation in the fine fraction are shown in Figure 4. In the rare earth ore particle migration experiment and the pore structure column leaching experiment, the sample heights were set to 180 mm and 60 mm, respectively. The prepared samples, weighing 2566 g and 155.46 g, were loaded into PVC pipes for 6 cycles and 3 cycles, respectively. Each sample weighed 427.7 g and 51.82 g, and each layer was gently compacted with a wooden hammer to a thickness of 3 cm and 2 cm, respectively. To prevent obvious stratification, the surface of the sample was “roughened” before loading each layer of the soil sample. After loading, a layer of qualitative filter paper was placed on top as a buffer layer, and then the column immersion experiment was conducted. Deionized water and 3% magnesium sulfate solutions were used as the leaching agents. The water acted as a control, and magnesium sulfate is commonly used as an effective leaching solution for rare earths [29]. The entire column leaching process was carried out at indoor room temperature for 14 days.

2.2.3. Particle Gradation Experiment

In the particle grading experiments, samples of raw soil and the rare earth ore columns that had undergone continuous leaching for 14 days were dried in an oven in a natural air atmosphere at 105° for 8 h. Once the rare earth sample had dried enough to be easily separated from the PVC mold, it was slowly removed. Each sample was then divided into 60 mm thick, equidistant ore layers corresponding to the upper, middle, and lower sections. The different layers of the separated rare earth samples were loosened with sticks to prevent the agglomeration of fine particles, which could affect the subsequent screening experiment. The prepared ore samples were placed in the screening machine for screening, and the weight ratio of each particle size greater than 0.075 mm was measured and recorded; the sieve mesh particle sizes were 10, 2, 1, 0.5, 0.25, and 0.075 mm in that order. A Winner 2000 (Beijing Haixinrui Technology Co., Ltd., Jinan, China) laser particle size analyzer was used to analyze the particle size of ore samples with particle sizes smaller than 0.075 mm, and the particle size distribution changes of the full particle size and fine particle size before and after leaching with 3% magnesium sulfate and clear water were obtained.

2.2.4. Pore Structure Experiment

In the pore structure experiment, the rare earth columns, prepared with different fine particle contents, were saturated for 72 h [30]. Once the saturation treatment of the rare earth columns reached more than 95% saturation, the NM-60 nuclear magnetic resonance instrument Suzhou Newmai Co., Ltd., Suzhou, China) from Newman Technology was used to test the pore structure before leaching. After the completion of the test, the column leaching experiment was conducted using 3% magnesium sulfate and water (control group). During the experiment, the 60 mm sample was subjected to nuclear magnetic resonance testing every two hours. The porosity, T2 spectrum curve, and pore size distribution of the sample at each time interval were recorded. Each test was repeated three times, and the average value was calculated.

3. Results and Discussion

3.1. Rare Earth Ore Column Leaching Full Particle Size Gradation Analysis

3.1.1. The Change Rule of the Full Particle Size Gradation of Deionized Water Leaching Ore Soil

The full particle size distribution curves of the upper, middle, and lower layers of the rare earth ore with different fine particle contents, before and after clear water leaching, are shown in Figure 5a–f. The particle size is categorized into four ranges: (1~75 μm), (75~500 μm), (500~2000 μm), and (2000~10,000 μm). From the figure, it can be seen that compared with before leaching, the overall changes in the coarse and fine particle contents of each fine-grained content of the rare earth ores after leaching are upper layer > middle layer > lower layer. For the coarse particle content, the trend is lower layer > middle layer > upper layer. Since no significant chemical reaction occurs during the water leaching process and the particles of rare earth ore in each particle size range are evenly distributed before leaching, the fine particles within the sample will migrate to some extent during the in situ column leaching process. This migration results in an uneven distribution of the coarse and fine particles. As fine particles accumulate during migration, the coarse and fine particles are more thoroughly separated during the post-leaching screening process. In other words, the content of fine particles attached to the surface of the coarse particles in the rare earth samples screened after clear water leaching decreases, while the content of fine particles that are screened out increases. This ultimately leads to a decrease in the content of coarse particles in the upper layer and an increase in the content of separable fine particles.

From the analysis of the full particle size distribution curve for each layer in the diagram, only a qualitative assessment can be made regarding the water leaching of rare earth ore with varying fine particle contents. To further investigate the particle size distribution characteristics of each layer after water leaching, “Fractal theory” was introduced, with the fractal dimension used to characterize the impact of different fine-grained rare earth ores on the particle size distribution of each layer before and after leaching, reflecting the uniformity of the layers. The larger the fractal dimension of the rare earth ore, the more compact the internal structure of the ore body, resulting in poorer permeability. Conversely, the smaller the fractal dimension, the looser the internal structure of the ore body, leading to better permeability.

Table 3. Fractal dimensions of upper, middle, and lower layers of rare earth ore with different fine particle contents before and after deionized water leaching.

Sample	Before Ore Leaching	Upper Layer	Middle Layer	Lower Layer
L05	2.21	2.17	2.16	2.21
L10	2.32	2.28	2.29	2.31
L15	2.39	2.34	2.35	2.36
L20	2.42	2.35	2.38	2.41
L25	2.45	2.37	2.41	2.44
L30	2.50	2.40	2.42	2.44

shows the calculated fractal dimensions of the upper, middle, and lower layers of rare earth ore with different fine grain contents before and after water leaching. The fractal dimension range for L₀₅ to L₃₀ is between 2.21 and 2.50. The comparison reveals that the higher the fine particle content of the rare earth ore, the greater the fractal dimension of the particles before leaching [31]. Due to the high content of fine particles, the internal ore body is more complex, with fewer coarse particle skeletons, a more compact structure, and poorer permeability. As water leaching progresses, more connected pores will form in the upper layer of the ore body under the influence of a larger hydraulic gradient. These connected pores facilitate preferential flow during the water infiltration process. At the same time, the downward seepage of water will transport some fine particles to the lower part of the ore body, thereby altering the particle size distribution in each layer of the ore body. The hydraulic gradient varies across different layers. The smaller the hydraulic gradient, the less likely the hydrodynamic force is to facilitate fine particle migration. Therefore, the further down the layer, the greater the fractal dimension of the ore body, indicating that the lower down the layer of the rare earth ore body, the more compact and less permeable it is.

3.1.2. The Change Rule of the Full Particle Size Gradation of Magnesium Sulfate Leaching Ore Soil

The full particle size distribution curves for the upper, middle, and lower layers of rare earth ore with varying fine particle contents, before and after leaching with 3% magnesium sulfate solution, are shown in Figure 6a–f. The relationship between the fine particle content in each layer is as follows: lower layer > middle layer > upper layer. However, the differences in the fine particle content between the layers of the same sample are not significant. During the chemical reaction between the magnesium sulfate solution and the rare earth ions, the low-valence magnesium ions (Mg²⁺) in the solution gradually replace the high-valence rare earth ions (RE³⁺). At this point, the concentration of high-valence cations in the solution increases, which reduces the Zeta potential on the surface of the rare earth particles and the thickness of the electric double layer surrounding the particles. As a result, the electrostatic repulsion between the particles decreases. At this point, the van der Waals attraction becomes the dominant force, leading to an increase in the number of small pores and a decrease in the number of large pores. This restricts the development of continuous pore channels, which in turn hinders the downward migration of fine particles. When magnesium sulfate solution is used for leaching, the migration of fine particles in each layer is less pronounced compared to leaching with clear water.

The calculated fractal dimensions of the upper, middle, and lower layers of rare earth ore with varying fine particle content, before and after magnesium sulfate solution leaching, are presented in

Table 4. Fractal dimensions of the upper, middle, and lower layers before and after leaching with magnesium sulfate solution.

Sample	Before Ore Leaching	Upper Layer	Middle Layer	Lower Layer
L05	2.21	2.34	2.37	2.39
L10	2.32	2.43	2.44	2.45
L15	2.39	2.47	2.50	2.52
L20	2.42	2.50	2.49	2.50
L25	2.45	2.48	2.51	2.52
L30	2.50	2.49	2.49	2.50

. Compared to clear water leaching, the fractal dimension of the lower layer after leaching is larger [32]; however, the key difference is that the fractal dimension of the ore body before leaching is lower than that of each layer after leaching. This is due to the chemical replacement reaction that occurs during the leaching process. During this reaction, more fine particles are detached from the surface of the coarse particles. As the solution flows downward, the pores connected by the seepage action gradually become blocked by the fine particles. This causes the pores to shrink, preventing further migration of fine particles to the lower part of the ore body, which impacts the upper layer of the rare earth ore.

3.2. The Change in Behavior of Fine Ionic Rare Earth Ore Particles During In Situ Column Leaching

3.2.1. Fine Particle Size Distribution of Rare Earth Ore After Water Leaching

In order to further study and analyze the transport of fine particles during the column leaching process, the fine particle size fractions of the upper, middle, and lower layers of the rare earth ores before and after deionized water leaching are shown in Figure 7a–f. From the figure, it can be seen that after deionized water leaching, the particle size distribution of small fines has shifted to become bimodal for each, and the entire particle size range of rare earth fines has become more narrow. The full-size rare earth particles are classified into coarse particles (>75 μm), fine particles (15~75 μm), and ultrafine particles (1~15 μm). During the clear water leaching process, fine particles are peeled off from the surface of the coarse particles, and a small number of ultrafine particles adhere to the surface of the fine particles before leaching. After leaching, due to the stripping and migration processes, fine particles accumulate on the surface of coarse particles and ultrafine particles accumulate on the surface of fine particles. As a result, the particles are more thoroughly separated during the screening process that follows leaching. Finally, the overall fine particle size range screened after leaching was reduced. Specifically, the percentage of ultrafine particles in the range of 1–15 μm increased, while fine particles in the range of 15–75 μm decreased. This indicates that ultrafine particles with a size of approximately 1–15 μm are more likely to adhere to the surface of coarse or fine particles.

3.2.2. Fine Particle Size Gradation of Magnesium Sulfate Leaching Ore Soil

The particle size distribution of fine particles in the upper, middle, and lower layers of rare earth ore with different fine particle size contents, before and after leaching with magnesium sulfate solution, is shown in Figure 8a–f. It can be observed from the figure that magnesium sulfate leaching yields results similar to those of clear water leaching. After leaching, each particle size distribution curve exhibits a bimodal distribution centered around 3 μm and 20 μm; however, this is less pronounced compared to clear water leaching. As the fine particle content increases, the peak near 3 μm becomes less distinct, and the peak near 20 μm shifts closer to the peak observed before leaching. Overall, after leaching with magnesium sulfate solution, the particle size distribution curve of the rare earth ore with a higher fine particle content was closer to the particle size distribution curve observed before leaching. Under the influence of the flow-induced electric field, the net charge in the electric double layer on the surface of the rare earth particles moves in the opposite direction of the solution seepage. As a result, part of the solution is affected by the net charge’s viscous force and moves in the opposite direction, thereby hindering the migration of fine particles. At the same time, during the column leaching of ionic rare earth with magnesium sulfate, an ion exchange reaction occurs, leading to the replacement of a large number of rare earth cations. This promotes a decrease in the Zeta potential of the rare earth particles, ultimately reducing the electrostatic repulsion between them. At this point, the van der Waals attraction becomes the dominant force. The higher the fine particle content of the rare earth ore, the greater the specific surface area of the soil particles interacting with the magnesium sulfate solution. As a result, the van der Waals attraction becomes more pronounced, hindering the development of pore channels and limiting the migration of the fine particle content.

3.3. Porosity Variation of Rare Earth Ore During the Column Leaching Process

The porosity changes of the rare earth ore bodies with varying fine particle contents during water and magnesium sulfate column leaching are shown in Figure 9a and Figure 9b, respectively. It can be observed that, under the same conditions for ionic rare earth ore, the higher the fine particle content in the column, the greater the initial porosity before leaching. During the column leaching process with water, the overall porosity of the rare earth ore with different fine particle size contents did not change significantly, and the trend of change was roughly the same. Water has a minimal effect on the porosity of the ore body during the leaching process, which is consistent with corresponding research in this field [15]. The magnesium sulfate solution, however, exhibits a distinct difference from clear water in its leaching process. Figure 9b shows that the change in porosity in ionic rare earth ores initially increases and then decreases. By comparing and analyzing the porosity changes in the ore body during water leaching, the increase in porosity observed in the magnesium sulfate leaching process is attributed to the ion exchange reaction with rare earth particles during the seepage of the solution, which alters the internal pore structure. During the leaching process of rare earth ore, a significant amount of magnesium sulfate is injected to increase the concentration of high-valence cations in the solution, leading to a gradual decrease in the Zeta potential of the particles and a corresponding reduction in the thickness of the electric double layer around the rare earth particles, thereby reducing the electrostatic repulsion between them [33,34]. At this point, the primary force between the particles is the van der Waals attraction. This attraction causes the fine particles to move and migrate with the solution, resulting in their adsorption onto the pore surfaces.

When leaching with magnesium sulfate, the turning point of the overall change in porosity of rare earth ore bodies with different fine-grained contents occurs at around 5 h. Many fine particles are adsorbed onto the surface of the coarse particles or the pore structure, causing the overall rare earth ore body to become more compact compared to its initial state, with the preserved pores enlarging, thereby increasing the overall porosity. As the magnesium sulfate solution reacts with the rare earth ions, a large number of displaced rare earth ions are released, leading to an increase in the Zeta potential of the particle surface and the expansion of the double layer thickness, which results in greater electrostatic repulsion between the rare earth particles. The adsorbed fine particles undergo reaction and release from the filling pores, resulting in a decrease in the overall porosity of the ore body, which becomes lower than before.

3.3.1. Change in the Pore Size Distribution of Rare Earth Ores in Deionized Water Column Leaching

The relaxation time in the T2 mapping curve was converted to the corresponding pore radius, allowing for the determination of the pore aperture distribution for each column at various leaching periods. The pore sizes were classified into four groups: small pores (0–0.05 μm), medium pores (0.05–1 μm), large pores (1–10 μm), and super-large pores (10–30 μm). The changes in pore apertures of ionic rare earth ores with different fine-grained contents during water leaching over various periods are shown in Figure 10a–f. It can be observed that this rare earth ore has the highest occurrence of medium-sized pores, with fewer small and large pores, very few super-large pores, and a lower proportion of super-large pores. During the water leaching process, small and super-large pores are gradually converted into medium and large pores, increasing the proportion of medium and large pore sizes. However, for columns with higher fine grain content, the conversion of small and oversized pores to medium and large pores was less pronounced compared to columns with lower fine grain content.

3.3.2. Change in Pore Size Distribution of Rare Earth Ores in Magnesium Sulfate Column Leaching

During the magnesium sulfate leaching of this rare earth ore, the changes in pore apertures over time differ significantly from those observed during clear water leaching, as shown in Figure 11a–f. From the figure, it can be observed that during the magnesium sulfate column leaching process, after approximately 4 h of leaching, large pores initially convert into small pores; however, later in the process, small pores revert back to large and super-large pores. Furthermore, the degree of interconversion between large and small pore sizes is higher in rare earth ores with greater fine-grained content. This phenomenon may be attributed to changes in the cation valence in the leaching solution, the pH value, and the strong ion exchange reactions, all of which reduce the electrostatic repulsion between rare earth mineral particles [35]. As a result, van der Waals attraction becomes the dominant force, causing some fine particles to migrate and be adsorbed onto the surface of the pore space due to gravitational forces, leading to the shrinkage of large pores into small ones prior to leaching. In the later stages of leaching, as rare earth ions are exchanged and seeped out, the cation valence in the solution decreases, which increases the Zeta potential on the particle surface and the thickness of the double layer surrounding the particles. This increase in electrostatic repulsion releases the fine particles previously adsorbed onto the pore surface, leading to an increase in the proportion of large pores and a decrease in the proportion of small pores.

4. Conclusions

This study investigated the effect of leaching ionic rare earth ore from China with different fine particle contents using deionized water (control group) and 3% magnesium sulfate solution. Changes in the particle size distribution, pore structure and pore size distribution were observed in the post-leaching column between the top, middle, and bottom layers, as well as at 1 h intervals during the early leaching time. The conclusions are as follows:

In the leaching process of ionic rare earth ores, the degree of influence and tightness of solution percolation on the particle gradation of each layer of rare earth ores is as follows: upper layer > middle layer > lower layer. The analysis of the particle size distribution and fractal dimension data of the content of fine particles revealed the effect that fine particles (<75 µm) have on the overall leaching of the ore body. The lower the rare earth ore layer, the denser the soil body, and the poorer the permeability, and the higher the fine particle content of the rare earth ore, the more pronounced the phenomenon becomes.

After leaching with deionized water, the forces attracting fine particles to the surface of the coarse particles are reduced, resulting in greater particle transport to lower layers of the ore body. More obvious increases are seen in ores with higher contents of fine particles. After the leaching of the magnesium sulfate solution, the van der Waals force and electrostatic repulsion caused the leaching activity to have less influence on the particle migration of rare earth ores with higher fine-grained contents, due to the seepage of the solution and the chemical reaction between the solution and the rare earth particles. After leaching in both water and magnesium sulfate, the particle size distribution of the fines became bimodal, and the overall size range was narrower than that of the raw ore.

The overall change in porosity and pore size distribution is not large when using deionized water leaching, although there will be some small pores (radii < 0.05 µm) and oversized pores (radii > 30 µm) transforming into medium and large pores (0.05–30 µm). In the leaching of magnesium sulfate solution, the overall porosity of the rare earth ore initially increased and then decreased after around 4 h of leaching. This change is attributed to the transformation of small- and medium-sized pores (<1 µm) into large and oversized pores (>1 µm) over time due to the ion exchange reactions occurring between the rare earth cations and the solution. Leaching in the late stage of the smaller pore aperture gradually transformed to the larger pore aperture. The higher the fine-grained content of rare earth ores, the greater the degree of transformation between larger and smaller pore sizes.