Effects of Ore Particle Size Distribution on Rare Earth Leaching Process of Weathered Crust Elution-Deposited Rare Earth Ores

Abstract

This paper investigates the rare earth (RE) leaching process of weathered crust leached rare earth ores (WCE-DREOs) with different ore particle sizes. Ore particles were categorized into seven distinct size ranges: −0.106 mm, +0.106 mm to −0.15 mm, +0.15 mm to −0.25 mm, +0.25 mm to −0.85 mm and +0.85 mm. The study examined the correlation between the thickness of the electrical double layer (EDL) and the RE content, as well as the swelling ratio of clay minerals. The findings demonstrated that the order of RE content with respect to varying particle sizes was as follows: ${\mathrm{C}}_{-0.106}>{\mathrm{C}}_{+0.106~-0.15}>{\mathrm{C}}_{+0.15~-0.25}>{\mathrm{C}}_{+0.25~-0.85}>{\mathrm{C}}_{+0.85}$ (C represents RE content). The RE content of full particle size was found to be 7.93 mg/g, with the particle size −0.106 mm exhibiting the highest RE content of 2.02 mg/g. In the fitting of the relationship between the RE content and the thickness of the EDL, it was observed that the particle size −0.106 mm exhibited a consistent correlation with the Logistic function $\mathrm{y}=2.03-0.43/(1+{\left(1.49\mathrm{x}\right)}^{172.39})$. In the study of the swelling performance of the WCE-DREOs, the order of the swelling ratio of ore samples in different particle sizes was ${\delta }_{-0.106}>{\delta }_{+0.106~-0.15}>{\delta }_{+0.15~-0.25}>{\delta }_{+0.25~-0.85}>{\delta }_{+0.85}$($\mathsf{\delta }$ represents the swelling ratio), and the swelling ratio was 3.34%, 2.92%, 2.33%, 1.55% and 0.01%, respectively. In the fitting of the relationship between the swelling ratio of clay minerals and the thickness of the EDL, it was observed that a particle size of −0.106 mm corresponded to the exponential function $\mathrm{y}=3.65-99.89·\exp \left(-\mathrm{x}/0.12\right)$. The results of the study provide useful assistance in the mining of the WCE-DREOs.

1. Introduction

WCE-DREOs are rich in medium and heavy RE with high economic and strategic value, such as dysprosium, terbium and yttrium [1,2,3]. The first discoveries of WCE-DREOs were made in Jiangxi Province, China, with subsequent discoveries primarily occurring in the southern provinces of China, including Guangdong, Fujian, Guangxi and Yunnan [4,5,6]. WCE-DREOs have unique physical and chemical properties, which are difficult to be utilized by traditional mineral processing methods. The mining of WCE-DREOs has historically employed a range of techniques, including pool leaching, barrel leaching and heap leaching. However, the most recent decision has been made to adopt in situ leaching as the primary recovery method. This approach involves the injection of a liquid leaching agent, through a designated injection hole at the summit of the ore body. The cations of the leaching agent undergo an ion-exchange reaction with the RE ions in the ore body, and the leaching solution containing rare earths is collected in the collecting channel at the bottom of the ore body. Finally, the rare earths are recovered by a hydrometallurgical process [7,8,9]. WCE-DREOs are mainly composed of clay minerals, which are negatively charged and adsorb RE ions in the environment to form an EDL [10,11,12]. During the leaching process of rare earths from WCE-DREOs, the leach agent cations engage in ion-exchange reactions with the RE ions. The clay minerals adsorb the leach agent cations, thereby forming a more stable EDL [13]. A schematic diagram of the EDL model for the clay minerals of WCE-DREOs is depicted in Figure 1.

Nevertheless, the WCE-DREOs’ in situ leaching technology is not without its limitations. For instance, during the mining process of RE resources in thick weathered crusts, the leach solution containing rare earths cannot be collected in time due to the large thickness of the ore body. This results in anti-sorption of RE ions in the ore body, which consequently leads to low RE recovery [14,15]. Furthermore, the leaching agent solution seepage efficiency is poor in the presence of ore particles of varying sizes and other factors, resulting in prolonged rare earth recovery periods [16,17]. Additionally, the hydration and swelling of clay minerals during the RE leaching process can lead to landslides and other complications [18,19]. In order to further illustrate and optimize the aforementioned issues, scholars have made efforts to study the ore properties of WCE-DREOs, to improve the RE leaching efficiency, to increase the solution permeability and to reduce the swelling ratio of clay minerals. Studies have shown that the ore size of RE deposits of different mineralization types is different and that the type of host rock, degree of weathering, ambient temperature and vertical migration of particles can also have an impact [20,21]. Chen et al. [22] investigated the effects of magnesium ion concentration and pH conditions on the swelling process of WCE-DREOs. The results demonstrated that magnesium nitrate exhibited a significant ability to inhibit the hydration swelling and dispersion of clay minerals at room temperature. The anti-swelling rate of 0.20 mol/L magnesium nitrate was found to be 65%. Zhang et al. [23] proposed a methodology involving the injection of the leaching agent directly into the completely weathered layer without penetrating the humus layer. This method effectively lowers the content of impurity aluminum in the leach solution, while avoiding the influence of mineral swelling of the humus layer on the stability of the mine. He et al. [24] used a mixture of ammonium nitrate and ammonium chloride as an RE leaching agent. It increased the RE leaching efficiency of WCE-DREOs to 91%. Xiao et al. [25] utilized magnesium sulfate as a leaching agent to leach WCE-DREOs. Their findings indicated that magnesium sulfate, in comparison to ammonium sulfate, exhibited a capacity to leach over 95% of rare earths while concurrently impeding the leaching of aluminum impurities by 10%.

The majority of extant studies have focused on the selection of leaching agent or the improvement of leaching technology. However, there has been a paucity of research on the leaching mechanism in relation to different ore particle sizes. The leaching process of RE from WCE-DREOs with different particle sizes has been studied in this paper, as has the swelling process of clay minerals under different ore sizes. The study provides valuable insights into the relationship between the thickness of the EDL and the content of RE, as well as the relationship between the EDL and the swelling ratio of clay minerals. The findings of this study offer practical guidance for the optimal utilization of leaching agents and safe production in mines for WCE-DREOs.

2. Materials and Methods

2.1. Ore Sample Properties

The WCE-DREO samples used in the experiments were collected in Yunnan Province. The chemical compositions of the WCE-DREO samples are shown in

Table 1. The chemical composition of WCE-DREO samples (wt.%).

Component	REO	Al2O3	CaO	SiO2	MnO2	ZnO	MgO	P2O5	K2O
Content	0.177	37.212	0.683	53.056	0.053	0.031	0.194	0.172	1.591
Component	SO3	TiO2	Fe2O3	Rb2O	SrO	ZrO2	Na2O	CuO	Loss
Content	0.014	1.481	2.982	0.017	0.009	0.017	0.062	0.006	2.243

.

The WCE-DREO samples are primarily composed of Al₂O₃ and SiO₂, with contents of 37.212% and 53.056%, respectively. This is indicative of a typical aluminosilicate mineral. The REO proportion of the WCE-DREOs was found to be 0.177%, classifying them as high-RE-grade WCE-DREOs and thus possessing high utilization value. The WCE-DREO samples were sieved with a standard sieve, and the particle size distribution and mass accumulation curves of the WCE-DREO samples are depicted in Figure 2.

The WCE-DREO samples exhibited a broad spectrum of particle size distribution, with the largest proportion of the sample mass distributed in the particle size range of +0.85 mm, constituting 30.83% of the total. The proportion of the WCE-DREO samples in the −0.106 mm size category was 30.63%. The mass proportions of other ore samples in the particle size ranges of +0.106~−0.15 mm, +0.15~−0.25 mm and +0.25~−0.85 mm were found to be 7.30%, 7.91% and 23.33%, respectively. RE ions were found to be mainly adsorbed on small clay mineral particles. Therefore, it was imperative to divide the WCE-DREO samples into regions. The particle sizes were categorized into the following intervals: −0.106 mm, +0.106~−0.15 mm, +0.15~−0.25 mm, +0.25~−0.85 mm and +0.85 mm. The mineral composition of WCE-DREO samples with different grain sizes was characterized using X-ray diffraction, and the WCE-DREO samples’ XRD patterns are shown in Figure 3.

The main components of WCE-DREO samples are quartz (SiO₂), feldspar (KAl-Si₃O₈) and clay minerals (AlSi₂O₅(OH)₄), and the mineral composition of ores with different particle sizes is basically the same. The smaller the particle size of ores, the larger the peak strength of clay minerals, indicating a higher content of clay minerals. RE is mainly adsorbed on the clay minerals in a hydrated or hydroxyl hydrated form [26,27]. Consequently, the greater the proportion of clay minerals in the grain size interval of WCE-DREOs, the higher the concentration of RE ions.

2.2. Apparatus and Experimental Procedure

The main devices used in the experiment include a self-made controlled velocity leaching device and a PCY intelligent clay dilatometer (as shown in Figure 4), as well as an X-ray diffractometer (XRD, D8-ADVANCE, Bruker, Germany) and an X-ray fluorescence spectrometer (Axios advanced, Panalytical B.V., Malvern, UK), Fourier transform infrared spectrometer (FT-IR, NICOLET FTIR-7600, Lambda, Australia) and inductively coupled plasma mass spectrometer (ICP-MS, AGILENT-7700, Agilent, MA, USA).

2.3. Experimental Procedure

The raw ore was screened, and samples of 200 g of RE ore with different particle sizes were weighed. These samples were then loaded into a glass exchange column for a column leaching test [28,29]. The leaching agent solution was introduced into the column at a liquid-to-solid ratio of 4:1, with a pump flow rate of 0.5 mL/min. The experimental design involved the establishment of three groups of parallel samples, and the analysis of the RE content and leaching efficiency was conducted by averaging the results. The lixiviums were collected and the RE contents were detected by ICP-MS.

The swelling ratio of clay minerals was determined by PCY intelligent dilatometer, and the swelling ratio was calculated by the following formula [22]:

$$\delta ={\displaystyle \frac{∆H}{H_0}}\times 100\%$$

(1)

where $\delta$ is the swelling ratio. $∆H$ is the changed height of the WCE-DREO sample. $H_0$ is the original height of the WCE-DREO sample. When there was only one electrolyte solution, the thickness of the EDL could be calculated as follows [30]:

$${\displaystyle \frac{1}{K}}={\left({\displaystyle \frac{{\epsilon }_0{\epsilon }_r{k}_{B}T}{2000·e^2·c·N_A}}\right)}^{{\displaystyle \frac{1}{2}}}$$

(2)

where ${\epsilon }_0$ is the permittivity in vacuum/F/m. ${\epsilon }_r$ is the relative dielectric constant. $k_B$ is the Boltzmann constant/J/K. T is the thermodynamic temperature/K. e is the elementary charge/C. $N_A$ is Avogadro’s number. c is the ionic strength.

3. Results and Discussion

3.1. The Change in 1/K with Ammonium Chloride as the Leaching Agent

In order to facilitate a more profound comprehension of the influence that leaching agent concentration and temperature exert on 1/K, a visual description was presented. Ammonium chloride was selected as the leaching agent. The value of the constant in Equation (2) can be known in the ideal case, where ${\epsilon }_0=8.854\times {10}^{-12}\mathrm{F}/\mathrm{m}$, ${\epsilon }_r=78.36$, $k_B=1.3806\times {10}^{-23}\mathrm{J}/\mathrm{K}$, $e=1.6\times {10}^{-19}\mathrm{C}$ and $N_A=6.02\times {10}^{23}$. Under the different thermodynamic temperatures and the concentrations of ammonium chloride solution, the EDL thickness was calculated. The results are shown in Figure 5.

As demonstrated in Figure 5, 1/K was inversely proportional to the concentration of electrolyte solution, which was proportional to the thermodynamic temperature. The concentration of the electrolyte solution exerted a significant influence on 1/K. As the concentration of the electrolyte solution increased, 1/K underwent a gradual decrease. This phenomenon can be attributed to the fact that a decrease in 1/K results in a reduction in the potential of the diffusion layer, and the ${\mathrm{N}\mathrm{H}}_4^{+}$ of the completed ion exchange reaction was residual in the clay mineral structure, thereby gradually balancing the reaction. Furthermore, an increase in temperature was observed to result in a slight thickening of 1/K. This phenomenon can be attributed to the fact that an increase in temperature results in a forward shift of the end point of the ion exchange reaction. It is also the result of the Brownian molecular motion. In order to further clarify the relationship between the thickness of EDL of clay mineral particles and RE leaching, the column leaching process of RE mineral samples with different ore particle sizes was studied.

3.2. WCE-DREOs’ Column Leaching with Different Ore Particle Sizes

3.2.1. Effects of Leaching Agent Concentrations on RE Leaching Process

To investigate the influence of leaching agent concentration on the leaching process of WCE-DREO samples with different particle sizes, ammonium chloride with concentrations of 0.05, 0.10, 0.15, 0.20 and 0.25 mol/L was selected as the leaching agent. The RE leaching solution of WCE-DREOs with various particle sizes was collected, and the RE leaching efficiency and RE contents were detected. The experimental results are shown in Figure 6.

As demonstrated in Figure 6a–e, the RE leaching efficiency of ores of different particle sizes exhibited an initial rapid increase before 15,000 s, followed by a subsequent gradual rise. Ultimately, the RE leaching efficiency approached a state of equilibrium. Furthermore, Figure 6a–e demonstrate that the leaching efficiency of RE under different particle sizes exhibited optimal performance at a concentration of ammonium chloride of 0.2 mol/L. It can be seen from Figure 6f that the concentration of the ammonium chloride reached 0.20 mol/L, and the RE contents of particle sizes −0.106 mm, +0.106~−0.15 mm, +0.15~−0.25 mm, +0.25~−0.85 mm and +0.85 mm were 2.02, 1.98, 1.87, 1.11 and 0.88 mg/g, respectively. When the concentration of the leaching agent was greater than 0.20 mol/L, the content of RE in the leaching solution increased slowly and tended to balance. As illustrated in Figure 6g, the RE content with full particle size was 7.86 mg/g. The RE content was ranked according to different particle sizes: ${\mathrm{C}}_{-0.106}>{\mathrm{C}}_{+0.106~-0.15}>{\mathrm{C}}_{+0.15~-0.25}>{\mathrm{C}}_{+0.25~-0.85}>{\mathrm{C}}_{+0.85}$. The maximum RE content observed in the size range −0.106 mm of the WCE-DREO samples was 2.02 mg/g. This phenomenon can be attributed to the correlation between particle size and the larger specific surface area, with smaller particles exhibiting larger surface areas and more active adsorption centers. Consequently, as particle size decreases, the capacity for RE ions to be adsorbed on the surface of clay minerals increases [31]. Therefore, particle size −0.106 mm had the highest industrial utilization value.

In order to explore the relationship between the RE content and 1/K under different leaching agent concentrations, the RE content with different particle sizes was fitted with 1/K under normal temperature. The fitting results are shown in Figure 7. The fitting result conforms to Logistic function $\mathrm{y}={\mathrm{A}}_2+({\mathrm{A}}_1-{\mathrm{A}}_2)/(1+{\left(\mathrm{x}/{\mathrm{x}}_0\right)}^{\mathrm{p}})$, where x is 1/K and y is the RE content. The fitting coefficients are shown in

Table 2. The fitting coefficient of RE content and 1/K under different leaching agent concentrations.

Particle Size/mm	A1	A2	X0	p	R2
−0.106	2.04	1.53	0.85	12.93	0.9991
+0.106~−0.15	2.03	1.45	0.85	10.27	0.9951
+0.15~−0.25	1.99	1.25	0.82	8.06	0.9879
+0.25~−0.85	1.15	0.55	0.85	11.11	0.9954
+0.85	1.04	0.39	0.81	5.78	0.9632

.

As demonstrated in Figure 7, a negative correlation was observed between 1/K and the RE content. In accordance with formula (2), a negative correlation was also identified between 1/K and the concentration of the leaching agent. Given the positive correlation between the RE content in the leaching solution and the concentration of the leaching agent, it can be concluded that the RE content and 1/K in the leaching solution are negatively correlated. As demonstrated in Table 2, the fitting coefficients R² were all greater than 0.96. The model demonstrated a high degree of fit, with R² for the particle size interval −0.106 mm exceeding 0.999.

The fitting function of particle size −0.106 mm was $\mathrm{y}=1.53+0.51/(1+8.18·{\mathrm{x}}^{12.93})$, where x represents EDL thickness/nm and y represents rare earth content/mg/g. It can be seen from the fitting equation that the greater the RE content, the smaller the thickness of the EDL, that is, the larger the y value, the smaller the x value. Therefore, the selection of the appropriate leaching agent concentration is pivotal in ensuring a minimal change in the thickness of the EDL. This is instrumental in mitigating the risk of landslides during the production process.

3.2.2. Effects of Temperature on RE Leaching Process

To investigate the influence of temperature on the leaching process of WCE-DREO samples with different particle sizes, the experimental reaction temperature was adjusted to 283 K, 293 K, 303 K, 313 K and 323 K. The concentration of ammonium chloride was 0.20 mol/L. The experimental results of WCE-DREO samples with different particle sizes are shown in Figure 8.

Figure 8a–e demonstrate the RE leaching process for each particle size of the WCE-DREO samples. As the RE leaching reaction proceeded, the RE leaching efficiency increased rapidly in the first 10,000 s. Thereafter, the RE leaching efficiency tended to equilibrate. It was also found that with an increase in reaction temperature, the RE leaching efficiency increased in each particle size interval. However, when the reaction temperature reaches 303 K, the leaching efficiency of RE increases marginally. Therefore, it can be leached at room temperature. The higher the temperature, the higher the RE leaching efficiency, the more priority can be given to summer operation. It is evident from Figure 8f that the RE content in the leaching solution of each grain-size WCE-DREO sample increases with rising temperature. The RE content with different particle sizes increases with the increased temperature. This is attributed to the acceleration of thermal movement of ions in the leaching reaction, which in turn increases the rate of diffusion of ammonium ions. Consequently, a greater number of ammonium ions are available to participate in the reaction within a constant volume, thereby enhancing the leaching efficiency of rare earths [32]. As demonstrated in Figure 8g, at a temperature of 303 K, the ion-exchange reaction approaches equilibrium, with the total RE content recorded as 7.93 mg/g for each particle size. The RE contents of particles −0.106 mm to +0.85 mm are 2.02 mg/g, 2.00 mg/g, 1.90 mg/g, 1.12 mg/g and 0.90 mg/g, respectively.

The subsequent investigation sought to establish a correlation between RE content and 1/K at different thermodynamic temperatures. The relationship between RE content and 1/K was fitted for a NH4Cl solution concentration of 0.20 mol/L. The fitting results are shown in Figure 9. The fitting results conform to Logistic function $\mathrm{y}={\mathrm{A}}_2+({\mathrm{A}}_1-{\mathrm{A}}_2)/(1+{\left(\mathrm{x}/{\mathrm{x}}_0\right)}^{\mathrm{p}})$, where x is 1/K and y is the RE content. And the fitting coefficients are shown in

Table 3. The fitting coefficient of RE content and 1/K at different temperatures.

Particle Size/mm	A1	A2	X0	p	R2
−0.106	1.60	2.03	0.67	172.39	0.9989
+0.106~−0.15	1.57	2.00	0.67	219.99	0.9954
+0.15~−0.25	1.19	1.92	0.67	144.65	0.9997
+0.25~−0.85	0.71	1.13	0.67	168.50	0.9986
+0.85	0.38	0.92	0.67	124.13	0.9978

.

As demonstrated in Figure 9, a positive correlation was observed between 1/K and the RE content. The rate of the ion exchange reaction accelerates with increasing temperature. The content of rare earths in the solution increases, and the rate of formation of stable EDL structures in the WCE-DREO sample particles by the leaching agent cations increases [33]. It can be seen from Table 3 that the fitting coefficients R² were all greater than 0.99, indicating a good fit.

From Table 3, the fitting function of particle size interval −0.106 mm of the WCE-DREO samples was $\mathrm{y}=2.03-0.43/(1+{\left(1.49\mathrm{x}\right)}^{172.39})$, where y was the content of RE/mg/g and x was the thickness of the EDL/nm. The equation indicates that, under conditions of temperature fluctuation, an increase in the RE content in the leaching solution corresponds to a greater thickness of the EDL of the ore particles. Therefore, in the process of RE leaching, it is imperative to regulate the temperature to ensure the effective leaching of RE while concurrently averting the potential for the thickness of EDL from causing mine landslide.

3.3. WCE-DREO Samples’ Clay Mineral Swelling Processes

3.3.1. WCE-DREO Samples’ Clay Mineral Swelling Mechanism

The RE leaching process of WCE-DREOs is commonly accompanied by the hydration and swelling of clay minerals, which poses a significant threat to the safe production of WCE-DREO mines. Therefore, it is very necessary to study the swelling mechanism of clay minerals in the RE leaching process of WCE-DREOs. A schematic diagram of the clay mineral swelling process during RE leaching of WCE-DREOs is depicted in Figure 10.

Clay minerals account for 40%–70% of the WCE-DREOs, mainly composed of kaolinite, halloysite, illite and montmorillonite [31]. These clay minerals possess a substantial number of active adsorption centers. During the mineralization process of WCE-DREOs, a large number of cations, including RE ions, are adsorbed between the layers of clay minerals. During the in situ leaching of WCE-DREOs, the clay minerals undergo hydration and swelling. The process of NH4+ ion exchange of RE ions with clay minerals to re-form the structure of EDL similarly affects the swelling and dissolution of clay minerals [34]. This series of problems would affect the safety and stability of WCE-DREO mines.

3.3.2. Swelling Ratios of Ore Samples with Different Grain Sizes of WCE-DREOs

The swelling experiments were carried out on different grain size intervals of WCE-DREO samples. Under ambient temperature conditions, different concentrations of ammonium chloride were selected as the liquid phase for the experiment. The swelling ratios of WCE-DREO samples in various grain size intervals are depicted in Figure 11.

As demonstrated in Figure 11, the swelling ratio of WCE-DREO samples decreased with the increase in the concentration of ammonium chloride solution. The swelling ratio of the ore sample of particle size D5 was essentially negligible, as the ore of this particle size was predominantly composed of quartz and feldspar, a rock-forming mineral. The order of swelling ratio of different particle sizes was ${\delta }_{-0.106}>{\delta }_{+0.106~-0.15}>{\delta }_{+0.15~-0.25}>{\delta }_{+0.25~-0.85}>{\delta }_{+0.85}$. This phenomenon can be attributed to the fact that the smaller the particle size of the WCE-DREOs, the greater the proportion of clay minerals [31], which exhibit enhanced swelling behavior upon interaction with the leaching solution. Using 0.20 mol/L ammonium chloride as the leaching agent, the swelling ratios of the ore samples with particle sizes −0.106, +0.106~−0.15, +0.15~−0.25, +0.25~−0.85 and +0.85 mm were 3.34%, 2.92%, 2.33%, 1.55% and 0.01%, respectively. The relationship between the thickness of the EDL and the swelling ratio at different ammonium chloride concentrations was investigated, and the fitting results are shown in Figure 12. The fitting results for ore samples in the −0.106 mm to +0.25~−0.85 mm grain size interval are consistent with equation $\mathrm{y}={\mathrm{A}}_1·\exp \left(-\mathrm{x}/{\mathrm{t}}_1\right)+{\mathrm{y}}_0$, where x is 1/K and y is the swelling ratio. The fitting coefficients are listed in

Table 4. Fitting coefficients of relationship between swelling ratio and EDL thickness.

Particle Size/mm	y0	A1	t1	R2
−0.106	3.65	−99.89	0.12	0.9660
+0.106~−0.15	3.28	−19.27	0.17	0.9859
+0.15~−0.25	2.69	−15.23	0.18	0.9847
+0.25~−0.85	1.93	−54.68	0.14	0.9943

.

The hydration swelling reaction of particle size +0.85 mm was not clearly evident, and thus the relationship between the EDL thickness and the swelling ratio was not fitted. As illustrated in Figure 12, the swelling ratio of WCE-DREO samples increases with the increase in the thickness of the EDL thickness of ore particles. The variation in swelling ratios of WCE-DREO samples in different grain size intervals is primarily attributable to the varying contents of swellable clay minerals. From Equation (2), it can be seen that the electrolyte solution concentration decreases with the increase in the thickness of the EDL thickness. Combined with the results in Figure 11, it is evident that the lower the electrolyte solution concentration, the higher the swelling rate of WCE-DREO samples. As can be seen from Table 4, the fitting coefficients R2 of the fitting equations for each ore particle size interval are all greater than 0.96. This indicates that the fitting effect is good and consistent with the experimental results.

The fitting function of particle size −0.106 mm was $\mathrm{y}=-99.89·\exp \left(-\mathrm{x}/0.12\right)+3.65$, where y was the swelling ratio of WCE-DREOs/% and x was the thickness of the EDL, 1/K/nm. It can be seen from the equation that the thickness of the EDL is proportional to the swelling ratio of clay minerals, thereby indicating that the thickness of the EDL can exert an influence on mine safety.

3.3.3. Swelling Dissolution Mechanism of Clay Minerals in WCE-DREO Samples

In the process of in situ leaching of WCE-DREOs, the hydration expansion of clay minerals has been shown to have a significant impact on the safety of mine production. During the hydration expansion of clay minerals, water molecules first enter between the crystal layers to form a water interlayer, which enlarges the distance between the crystal layers and causes the soil to absorb water and expand. This process relies on the energy produced by cationic hydration in the soil so that the water molecules are regularly arranged with the hydration cations of the specific adsorption. Subsequently, the leaching agent cations exchange RE ions into the clay mineral layers, and the clay minerals adsorb the leaching agent cations to form a new EDL structure, which is accompanied by the change in the thickness of the EDL. When the concentration of the leaching agent is excessively high, the thickness of the mineral particle EDL increases rapidly, resulting in the instability of the ore body and eventually geological disasters such as landslides. The hydration expansion of clay minerals is a complex process involving many factors, in which the change in the thickness of the EDL exerts a significant effect on the expansion behavior. It is therefore vital to be able to control and improve the hydration expansion behavior of expansive soil by adjusting the characteristics of EDL layers. The greater the swelling ratio of clay minerals, the greater the risk of geological disasters such as landslides. Therefore, it is of great practical significance to find out the expansion law of clay minerals with different particle sizes. The risk of geological disasters such as landslide can be reduced by knowing the grain composition of ore and controlling and optimizing the injection amount of the leaching agent.

4. Conclusions

The WCE-DREOs exhibited a wide particle size distribution, with fine minerals measuring −0.106 mm accounting for 30.63% of the total. Optimum leaching outcomes were obtained at a leaching temperature of 303 K and an ammonium chloride dosage of 0.20 mol/L. Under these conditions, the order of RE content with various particle sizes was as follows: ${\mathrm{C}}_{-0.106}>{\mathrm{C}}_{+0.106~-0.15}>{\mathrm{C}}_{+0.15~-0.25}>{\mathrm{C}}_{+0.25~-0.85}>{\mathrm{C}}_{+0.85}$. The total content of RE in the leaching solution was found to be 7.93 mg/g, with the maximum content of RE observed in the leaching solution for different particle sizes being 2.02 mg/g of grain size −0.106 mm. The relationship between RE content and 1/K of the particle size −0.106 mm fitting relation was found to be a curve with a Logistic function $\mathrm{y}=2.03-0.43/(1+{\left(1.49\mathrm{x}\right)}^{172.39})$. In the study of the swelling process of the WCE-DREOs, it was found that the order of the WCE-DREO samples’ swelling ratios of different grain sizes was ${\delta }_{-0.106}>{\delta }_{+0.106~-0.15}>{\delta }_{+0.15~-0.25}>{\delta }_{+0.25~-0.85}>{\delta }_{+0.85}$, with swelling ratios of 3.34%, 2.92%, 2.33%, 1.55% and 0.01%, respectively. The relationship between the swelling ratio of RE ore and 1/K under different leaching agent concentrations was fitted, and the particle size −0.106 mm fitting relation was in a curve with the exponential function $\mathrm{y}=3.65-99.89·\exp \left(-\mathrm{x}/0.12\right)$. The leaching process of RE was accompanied by the obvious change in the thickness of EDL of ore particles. It is therefore concluded that effective control of leaching conditions is beneficial in reducing the thickness of the EDL of ore particles and has a positive effect on the safety of mine production. At the same time, before mine operation, it is of great significance to find out ore particle size characteristics and formulate suitable leaching conditions for mine safety production. In the future, research on mine hydrogeological conditions can also be carried out, and suitable mining conditions can be formulated for specific rare earth mines to ensure efficient, safe and green production of mines.