## 3 Permeability of the Rare Earth Pile under Lateral Liquid Injection
### Variation of the Leachate with Hydraulic Head  
During the tests, when the hydraulic head of leaching liquid reaches a certain level, the leachate that is exuded from the outlet end becomes turbid, especially for the coarse-fine particle soil sample S4, as shown in Figure 8. After the leaching test of each soil sample was completed, the leachate was left for the sediment. Then, the mineral particles were  
Figure 7: Permeability coefficients at different locations under the hydraulic head \(H\) = 30 cm. separated from the mixed leachate and dried for a particle gradation analysis. Figure 9 gives the gradation curves of the exuded rare earth particles for the four testing soil samples. For the standard soil sample S1, the coarse particles that were larger than 2.35 mm do not exude during the leaching. The exuded mineral particles were mainly the fine particles that were smaller than 0.15 mm, whose weight accounts for 64.7% of the total weight of the exuded particles. With regard to the fine-medium particle soil sample S2, the exuded particles were all of the fine particles that were smaller than 0.15 mm. When the medium-coarse particle soil sample S3 was tested, the exudates were mainly the medium particles with sizes of 0.25-0.60 mm, and their mass accounted for 57.1% of the total. Actually, these are also the finer particle components in this soil sample. Among the exuded particles of the coarse-fine particle soil sample S4, the fine particles that were smaller than 0.15 mm accounted for 78.8% of the weight. The proportion of the coarse particles is larger than 2.35 mm, they account for only 8.8% of the total. Clearly, for any of the four soil samples, the exuded rare earth particles during the leaching tests are mainly the finer particle components of the soil samples. The rare earth particle exudation from the outlet end is clearly adverse to the pile stability, and furthermore, the faster the particles exude, then the worse the pile stability is. Figure 10 shows the variations of the particle exudation rate with the increasing hydraulic head for the four soil samples. The hydraulic head at which the rare earth particles begin to exude is defined as the critical hydraulic head of particle exudation \(H_{\text{c}}\). It is seen from Figure 10 that the relationship of the critical hydraulic head between the four soil samples is \(H_{\text{c}}\)(S4) \(<H_{\text{c}}\)(S2) \(=H_{\text{c}}\)(S1) \(<H_{\text{c}}\)(S3). For the soil sample S4, because the sizes of the fine  
Figure 8: Collection of the leachate exuded from the outlet end. Figure 9: Gradation curves of the exuded rare earth particles. and coarse particles vary widely, the fine particles easily migrate within the pores between the coarse particles and exude from the rare earth pile. Therefore, it is critical that the hydraulic head of particle exudation is the smallest among the four soil samples. In the medium-coarse particle soil sample S3, driving the medium particles to exude from the outlet end requires a greater seepage force. Hence, the critical hydraulic head of particle exudation in this soil sample is the highest. In the soil sample S2 that lacks coarse particles, its critical hydraulic head of particle exudation is 32 cm, and it is the same as that in the standard soil sample S1. This means that the coarse particles have little constraint on the migration of the fine particles, and the medium particles play a major role in the constraint of this. In comparison with the soil samples S1 and S4, the soil samples S2 and S3 have a relatively gentle change in the particle exudation rate as the hydraulic head increases. It is consistent with the gentle variation of the permeability coefficient \(K_{\text{CD}}\) in the soil samples S2 and S3. This shows in the fine-medium particle soil sample S2 and the medium-coarse particle soil sample S3, the particle migration degree near the outlet end is lower.