## 3 Permeability of the Rare Earth Pile under Lateral Liquid Injection
### Variation of the Permeability Coefficient with Hydraulic Head
This causes the leaching liquid to flow intensively to the lower part of the pile, and this results in a blind zone of leaching at the top of the pile, thereby reducing the leaching rate of the entire pile. With regard to the fine-medium particle soil sample S2, the permeability coefficients at different observation locations change relatively slowly with the increasing hydraulic head. This means that in the fine-medium particle soil sample, the degree of the fine particle migration  
Figure 6: Variations of the permeability coefficients with the hydraulic head of leaching liquid: (**a**) soil sample S1; (**b**) soil sample S2; (**c**) soil sample S3; (**d**) soil sample S4. is lower. During the heap leaching of rare earths, the stable permeation is conducive to the leaching of rare earth ions [27]. For the soil samples S1, S2, and S3, the permeability coefficient between the observation points B and C, namely \(K_{\text{BC}}\), grows the least with the increase of the hydraulic head \(H\). This is because the fine particles migrated from the liquid injection end to settle down in the BC section, thus hindering the formation of connected pores at this location. In addition, the adsorption of the leaching liquid ions on the surfaces of the deposited fine particles will form an electric double layer. The action of the electric double layer reduces the cross-sectional area of the leaching liquid flow between the pores, which further hinders the seepage of the leaching liquid [28]. With regard to the coarse-fine particle soil sample S4, the fine particles are also deposited during the leaching process. However, the deposition state is difficult to maintain in the pore structure that uses the coarse particles as the skeleton. Consequently, when the hydraulic head is increased from 24 to 26 cm, the permeability coefficient \(K_{\text{BC}}\) of the soil sample S4 increases suddenly. Compared with the permeability coefficient \(K_{\text{BC}}\), the permeability coefficient \(K_{\text{CD}}\) between the observation points C and D varies more significantly with the increasing hydraulic head \(H\). This is because the observation section at CD is close to the liquid outlet end, and the exudation of the fine particles from the liquid outlet end leads to an increase in the pore connectivity and the permeability coefficient. In comparison with the standard soil sample S1 and the fine-medium particle soil sample S2, the gap between the particles is larger and the pore connectivity is better for the medium-coarse particle soil sample S3. Therefore, the permeability coefficient \(K_{\text{CD}}\) of the soil sample S3 changes more significantly with the increase of the hydraulic head. Figure 7 presents the permeability coefficients at different locations under the hydraulic head \(H\) = 30 cm for the soil samples S1, S2, and S3. It is clearly seen that under the lateral liquid injection conditions, the permeability coefficient of the rare earth pile exhibits a non-uniform distribution in space. Specifically, the permeability coefficients at both ends are greater than those in the middle part of the pile. For the standard soil sample S1, the permeability coefficient at the liquid injection end is greater by 89.3% than that at the middle location. The relative differences are 74.6% and 89.2%, respectively, for the fine-medium particle soil sample S2 and the medium-coarse particle soil sample S3. The spatial distribution of the permeability is more inhomogeneous for the standard soil sample S1 and the medium-coarse particle soil sample S3.