Control of Seepage Characteristics in Loose Sandstone Heap Leaching with Staged Particle Sieving-Out Method

Abstract

This paper studies the influence of the staged particle sieving-out method on the seepage characteristics in loose sandstone heap leaching. The staged sieving out of ore sample particles was conducted according to particle size, and ground pressure was applied to them. Subsequently, parameters such as the permeability, particle distribution, and pore distribution characteristics of the rock samples were obtained to investigate the influence of the staged particle sieving-out method on the seepage effect of loose sandstone heap leaching. The results indicate that sieving out particles smaller than 0.15 mm can significantly reduce the probability of hole blockage and increase the overall pore size, greatly enhancing permeability. Sieving out particles with sizes between 0.15 mm and 1.2 mm can result in the loss of skeleton particles, reducing the amount of flow channels and thereby decreasing permeability. Sieving out particles larger than 1.2 mm can reduce the overall particle size of rock samples, improve strength and pressure stability, and help maintain permeability. In the surface heap leaching of loose sandstone ore, by sieving out particles smaller than 0.15 mm during deep heap construction and sieving out particles larger than 1.2 mm during mid-level heap construction, and by using vat leaching for sieved-out particles, the seepage effect of the ore heap can be significantly optimized, and complete utilization of resources can be ensured.

1. Introduction

Surface heap leaching refers to the integrated production process of leaching useful components from ore by piling up the ore at specific sites after mining and spraying it with leaching solution [1,2,3,4]. This method is primarily used in industry for low-grade massive ores, as well as loose-sandstone-type ores that are difficult to leach in situ due to issues such as aquifer disruption or excessively low permeability. Currently, surface heap leaching is widely utilized in the metallurgy of metals such as copper, gold, and uranium due to its cost-effectiveness and simple process. Ore heap construction is a crucial step that influences heap leaching efficiency. During this step, it is essential to control the particle size of the ore to achieve an appropriate distribution within the ore heap [5,6]. This ensures that when the leaching solution is sprayed onto the surface of the ore heap, it can infiltrate through the heap at a moderate flow rate to reach the bottom of the heap. If the flow rate is too fast, adequate leaching reactions may not occur, while a slow flow rate can lead to leachate retention, affecting production efficiency. For large-volume blocky ores, crushing is typically required to reduce the particle size to less than 20 mm, increasing the surface area of the ore heap and enhancing the leaching efficiency. In contrast, loose sandstone ores exhibit the opposite trend, as they consist of granular deposits, with most particles having a diameter of less than 2 mm; while this ensures effective leaching, it results in low permeability of the ore heap. Additionally, loose sandstone has well-developed micropores, and under the influence of gravity from overlying layers, the ore heap gradually compacts from top to bottom, causing the seepage channels to shrink with increasing burial depth, leading to leachate retention issues in heap leaching. Using agglomeration methods to improve permeability by binding particles into blocks increases costs and significantly reduces the ore surface area, negatively influencing heap leaching efficiency [7,8]. Therefore, controlling the particle size of loose sandstone to improve its seepage characteristics is crucial for enhancing the efficiency of the heap leaching of such ores.

Existing research results indicate that the particle distribution of loose sandstone determines the pore distribution between particles, and the characteristics of the pore distribution directly determine the development of flow channels in the rock [9,10]. If particles of various sizes are distributed in loose sandstone, lower-level particles may exist between the pores, filling or blocking the flow channels. Therefore, controlling and sieving out particles of various sizes in loose sandstone to remove particles within specific size ranges can increase pore volume and improve ore heap permeability; moreover, using vat leaching for sieved-out particles can ensure the complete utilization of resources. This approach is feasible for addressing the poor leaching performance of loose sandstone ores and enhancing the overall production efficiency.

The study of the seepage characteristics of loose sandstone presents a typical multiscale problem, where microscopic mechanisms reveal macroscopic characteristics. Currently, there are various summary studies on field production trials related to loose sandstone [11,12,13] and attempts to use numerical simulation methods to establish ore layer models and alter characteristic parameters to observe changes in seepage characteristics [14,15,16], providing a macroscopic summary of fundamental principles. However, in order to approach the mechanism level, relevant research from the mesoscopic perspective of pores [17,18] and particles [19,20] is clearly more valuable for reference. Some of the research on the interaction between pore distribution and particle distribution [21,22,23] provides a reference for the study of the mechanism of the influence of the staged particle sieving-out method on the seepage characteristics of loose sandstone.

To study the influence of particle size control on the seepage effect in loose sandstone heap leaching, this study classifies and groups rock sample particles, sequentially sieves particles within specific size ranges to alter rock sample characteristics, prepares loose sandstone samples, and applies confining pressure to simulate the pressure conditions of rock samples at different depths in heap leaching scenarios. By utilizing permeability meters, particle size distribution analyzers, and nuclear magnetic resonance instruments, parameters such as the permeability, particle distribution, and pore distribution characteristics of rock samples can be obtained. This research aims to explore the laws and mechanisms of how the staged particle sieving-out method alters the particle distribution and pore distribution of loose sandstone in the heap leaching scenario, thereby influencing its seepage characteristics, providing a theoretical reference for engineering applications and supporting the improvement of the efficiency of surface heap leaching operations for loose sandstone.

2. Methodology

2.1. Introduction to Heap Leaching

The typical surface heap leaching development and production process is shown in Figure 1a. Generally, at the selected site, the first step is to carry out the anti-seepage layer operation, lay down the leachate collection pipeline, and pretreat the ore particles before stacking the ore heap. The height of the heap varies from tens of meters to more than a hundred meters, depending on the production scale. After the heap is built, the leaching solution spraying pipeline is laid down. The leaching solution infiltrates from the surface of the ore heap to the bottom layer, reacts with the ore to generate ore-containing leachate, and is finally recovered through the collection pipeline. Due to the influence of the overlying rock layers, the stress on the ore gradually increases, the pore volume between the ores decreases, and stress analyses are conducted on three rock mass regions at different buried depths, as shown in Figure 1b–d. The magnitude of the stress on the rock mass is related to the density and thickness of the overlying rock layers and can be calculated using Equation (1):

$$\mathrm{P}=\mathsf{\rho }\mathrm{g}\mathrm{H}$$

(1)

where the parameters are pressure P, MPa; gravitational acceleration g, 9.8 m/s²; burial depth H, m; and density ρ, kg/m³.

2.2. Experimental Materials and Plan

This experiment segmented and grouped the particle size range of loose sandstone from small to large. Each group of rock samples had particles from that segment sieved out and then resampled. The rock samples were pressurized at shallow, medium, and deep depths to induce changes in particle and pore structures. Subsequently, seepage characteristic tests were conducted to study the influence of particle size control on the seepage characteristics of loose sandstone at different burial depths during surface heap leaching.

The particle size distribution and pore distribution curve of loose sandstone ore obtained from a surface heap leaching project in a specific region of Northwest China are shown in Figure 2. The particle size of the ore particles ranges from 0–2.3 mm, with a dry density of 1.69 g/cm³ and a mass of 174.11 g for the standard cylindrical rock sample (diameter 50 mm, height 50 mm).

According to engineering conventions, loose sandstone particles are usually classified into silt, fine sand, medium sand, coarse sand, and fine gravel during research. Their particle size ranges are as follows: 0–0.15 mm, 0.15–0.3 mm, 0.3–0.6 mm, 0.6–1.2 mm, and above 1.2 mm. As shown in

Table 1. Experimental sample grouping design.

(a) Sieved-out particle size segmentation and grouping (mm)
Group	A	B	C	D	E	F
Sieved-out particle size	-	0–0.15	0.15–0.3	0.3–0.6	0.6–1.2	1.2–2.4
(b) Sample particle mass of each particle size segment (g)
0–0.15 mm	28.75	Sieved out	35.36	44.93	38.12	30.01
0.15–0.3 mm	32.55	38.99	Sieved out	50.87	43.16	33.98
0.3–0.6 mm	62.70	75.10	77.12	Sieved out	83.13	65.45
0.6–1.2 mm	42.79	51.25	52.63	66.87	Sieved out	44.66
1.2–2.4 mm	7.32	8.77	9.00	11.44	9.70	Sieved out
Total mass	174.11	174.11	174.11	174.11	174.11	174.11
(c) Sample numbers and the ground pressure they experienced
Pressure-free (0 m)	A0	B0	C0	D0	E0	F0
0.17 MPa (10 m)	A1	B1	C1	D1	E1	F1
0.83 MPa (50 m)	A2	B2	C2	D2	E2	F2
1.66 MPa (100 m)	A3	B3	C3	D3	E3	F3

a, using the passable particle sizes of the 0–2.4 mm standard sandstone sieve set as a reference, the rock samples were divided into 5 segments based on the particle distribution curve in Figure 2a. The particle mass corresponding to each group of rock samples is shown in Table 1b (after sieving out particles from a certain segment, the particle mass of other segments is increased in the original proportion to maintain a consistent mass for each group of rock samples). Based on this, assuming a heap leaching site with a height of 100 m, the rock samples were pressurized at depths corresponding to the surface (0 m), shallow (10 m), medium (50 m), and deep (100 m) depths, resulting in 5 experimental groups, B–F, and the original rock sample group, A, as a control, as shown in Table 1c. In the calculation of stress, for ease of comparison, the density ρ of the overlying rock layer was uniformly set to 1690 kg/m³.

Using wrap, filter, and fixer to create rock samples with particles grouped by particle size, as shown in Figure 3a,b, confining pressure was applied to the rock sample using a high-confining-pressure gripper, as shown in the Figure 4, to simulate ground pressure. Subsequently, the permeability of the rock samples was obtained using a permeameter.

3. Seepage Characteristics Analysis

3.1. Permeability Characteristics Analysis

The depth–permeability relationship points for each set of loose sandstone rock samples were calculated and plotted, as shown in Figure 5.

For sample Group A, the permeability gradually decreases with increasing depth, which is consistent with the trend of pore shrinkage due to increased ground pressure. For the other three sample groups (C, D, and E), the permeability is generally slightly lower than that of Group A, showing a similar decreasing trend. Notably, the initial permeability of sample Group B is extremely high, more than six times that of Group A, but it rapidly decreases with increasing ground pressure and reaches the same level as that of Group A at a depth of 100 m. Additionally, sample Group F shows stable permeability at shallow and medium depths, and it is generally greater than that of Group A, but as the depth increases to 100 m, its permeability rapidly decreases below that of Group A.

To conclude, sieving out mid-sized particles (0.15–1.2 mm) from rock samples will reduce the permeability, with a similar decreasing trend. Removing particles from either end of the rock samples (0–0.15 mm or 1.2–2.4 mm) significantly altered the permeability of the rock samples and their variation trends with depth, indicating that controlling the particle size distribution of loose sandstone can alter its seepage characteristics. Explaining this pattern at the mechanistic level will require subsequent research, starting from the microscopic particle distribution and pore distribution characteristics.

3.2. Analysis of the Particle and Pore Distribution Characteristics

After testing, the rock samples disintegrated while retaining their particle distribution characteristics. The particle distribution parameters of the rock samples were obtained using a particle size analyzer [24]. The particle distribution and cumulative curves of the rock samples are plotted in Figure 6.

Compared to the surface unpressurized rock sample’s particle distribution, particle fracturing occurred in each group of rock samples with increasing depth. Significant particle fracturing around the 600-micron particle size range was observed in all six rock sample groups with increasing depth. As shown in Figure 6b–e, new particles were generated within the sieved-out particle size segments as the depth increased, with the generation rate significantly increasing with depth. In particular, as shown in Figure 6f, at shallow and medium depths, the quantity of the particles in the 0.6–1.2 mm segment did not decrease until a depth of 100 m, at which point the particles in this segment began to fracture, aligning with the permeability variation pattern of sample Group F, shown in Figure 5f.

The pore distribution testing of the rock samples was conducted using a nuclear magnetic resonance imaging analyzer. The pore distribution and cumulative curves of the rock samples are plotted in Figure 7.

From the overall pore distribution perspective, the loose sandstone rock samples used in this study consist of small pores (0–0.1 microns), medium pores (0.1–5 microns), and large pores (above 5 microns), with medium pores being the most dominant. This pore distribution characteristic is similar across all six sample groups, with the main differences lying in the proportions of small, medium, and large pores. The pore distribution curves of the samples within the same group are highly similar, but shift to the left with increasing depth, indicating overall pore contraction in the rock samples. Regarding total porosity, for sample Group A (Figure 7a), the decrease in porosity becomes more significant with increasing depth, while sample Groups C–E (Figure 7c–e) show smaller decreases in total porosity. In particular, sample Groups B and F (Figure 7b,f) exhibit smaller decreases in porosity at shallow and moderate depths, but a significant increase in the rate of decrease is observed when the depth reaches 100 m.

3.3. Analysis of the Influence Patterns

In Figure 6, the particle size distribution curves of each rock sample can only be qualitatively assessed, making it challenging to conduct a quantitative analysis. Therefore, referring to previous studies and international standards [25], we divided the sandstone particles into free particles and skeleton particles. Skeleton particles are relatively stable and immobile in loose sandstone, while free particles refer to particles smaller than 63 microns, which are the smallest in the international sandstone particle size classification and are relatively mobile, leading to potential pore blockages and significantly influencing rock permeability.

Building upon Figure 6, we focused on particles smaller than 63 microns as the research target and plotted the distribution and cumulative curves of the free particles, as shown in Figure 8. Additionally, we plotted the depth–free particle proportion relationship points in Figure 9.

Figure 8 clearly shows that with increasing depth, the quantity of free particles increases, and the particle size further decreases. In particular, in sample Group B, even after removing all the free particles, new free particles were gradually generated with increasing ground pressure, with the quantity of free particles approaching the level of Group A at a depth of 100 m (see Figure 9b). This phenomenon explains the significant decrease in permeability with depth in Group B.

Figure 7 shows that both sample Groups D and E exhibit higher porosity levels than sample Group A at the same depth. However, as shown in Figure 5, sample Group A consistently has higher permeability than Groups D and E under the same conditions. This contradicts the logic that higher porosity facilitates greater permeability. This discrepancy arises because porosity does not directly equate to permeability. Figure 7 shows that all the sample groups contain a certain proportion of pores smaller than 0.1 microns, where molecular forces come into play [26]. These pores typically require significant pressure differentials for liquid to pass through, necessitating the exclusion of pores less than 0.1 microns in size when evaluating the flow channel volume. In this study, pores larger than 0.1 microns were defined as effective seepage pores. Based on Figure 7, the effective seepage pore distribution curves of each sample group are plotted in Figure 10, and the depth–effective seepage pore proportion relationship points are depicted in Figure 11.

As shown in Figure 10, at the same depth, the effective seepage pore distribution curve of sample Group A is notably shifted to the right compared to that of Groups C–E, indicating larger overall effective seepage pore sizes. This suggests that sieving out particles between 0.15 and 1.2 mm may lead to an overall reduction in pore size, resulting in a lower proportion of effective flow channels. For sample Group B, although its effective seepage pore distribution peak is not high, there is a significant increase in the number of pores between 2 and 20 microns in size, leading to greater effective seepage pore proportions at all depths compared to those of Group A. In particular, at a depth of 50 m, the effective seepage pore proportion of sample Group F is close to that of Group A. Figure 9f shows that at a depth of 50 m, the proportion of free particles in sample Group F is slightly lower than that in Group A, and as shown in Figure 5f, at a depth of 50 m, the permeability of sample Group F is higher than that of Group A. This indicates that sieving out particles between 1.2 and 2.4 mm in size can reduce the generation of free particles, slow the collapse of effective seepage pores, and maintain the permeability of rock samples.

3.4. Correlation Analysis

The analysis indicates that the variation characteristics of the permeability in each group of rock samples are correlated with the proportion of free particles and the proportion of effective seepage pores. However, the correlation patterns between the different sample groups are not the same. To further investigate the correlation patterns among these factors, scatter plots of free particle proportion–permeability and effective seepage pore proportion–permeability were established for different groups of rock samples, as shown in Figure 12.

In general, the permeability shows a significant negative correlation with the proportion of free particles and a significant positive correlation with the proportion of effective seepage pores. However, due to the influence of particle size control on rock characteristics, significant differences exist among the characteristics of each group of rock samples, making it impossible to establish a unified expression for these relationships.

Regarding the correlation between permeability and the proportion of free particles, there are notable differences in the slopes and intercepts of the fitted lines for each group. This is attributed to variations in the pore distribution characteristics of each group of rock samples, leading to differences in the ease of particle plugging, reflected in the varying slopes of the fitted lines. The differences in the proportion of free particles among the groups are manifested in the differing intercepts of the fitted lines. In particular, since the effective seepage pore proportions and free particle proportions of sample Groups A and F are close (see Figure 9f and Figure 11f), the fitted lines for A and F are similar (see Figure 9a).

In terms of the correlation between the permeability and the effective seepage pore proportions, the slopes of Group B differ significantly from those of the other groups, while the correlation parameters of the fitted lines for the remaining groups are relatively close. This is because the proportion of free particles in Group B is notably lower than that in the other groups (see Figure 9), which fundamentally alters the seepage characteristics of the rock samples in Group B.

4. Analysis of the Influence Mechanisms and Optimal Piling Methods

4.1. Analysis of the Particle and Pore Variation Characteristics

Based on the particle distribution characteristic curves in Figure 6, the differences in the particle distribution curves between the rock samples at different burial depths and the surface rock samples were calculated for each group. The resulting rock sample particle distribution difference curves are shown in Figure 13a–f. Additionally, by cumulatively summing this difference curve from right to left, the cumulative pore distribution difference curves were obtained, as shown in Figure 13g–l.

Similarly, using the pore distribution characteristic curves in Figure 10 as a basis, the differences in pore distribution curve between the rock samples at different burial depths and the surface rock samples were calculated for each group. The resulting rock sample pore distribution difference curves are depicted in Figure 14a–f. By cumulatively summing these interpolated curves from right to left, the cumulative pore distribution difference curves were obtained, as shown in Figure 14g–l.

In Figure 13a–f and Figure 14a–f, the curves fluctuate around the 0 value, and positive values indicate an increase in the diameter of the particles or pores relative to the surface rock sample. In Figure 13g–l and Figure 14g–l, as the curve develops from right to left, and a downward trend indicates particle disintegration or pore contraction in that diameter range.

Figure 13a,g shows that the natural rock sample’s degree of particle disintegration is strongly correlated with the particle proportion. Specifically, if the particle proportion of a certain diameter is high, the amount of particle disintegration under ground pressure will be greater for that diameter. As shown in Figure 13g,k,l, particles with a diameter of approximately 400–600 microns exhibit both particle disintegration on the right side and particle generation on the left side. This phenomenon occurs because the most common particle size in the rock sample is 600 microns, making changes in particles around this size more apparent. However, the situation changes after sieving out particles in a certain segment; larger particles on the right side of that segment disintegrate to fill the missing particles in that segment, and this becomes more significant with increasing burial depth. These observations suggest that the particle distribution of natural rock samples can maintain the structural stability of the rock sample, while controlling the particle size can disrupt this characteristic, leading to the emergence of new particle distribution features under ground pressure.

Figure 14 shows that the pore change characteristics of the rock samples in each group are consistent and can be roughly divided into three pore change segments based on pore size: 0.005–0.05 microns, 0.05 microns–1 micron, and 1–10 microns. In these three segments, there is a trend of larger pores on the right shrinking toward smaller pores on the left. Comparing Figure 7 and Figure 14a,g, it is evident that these three segments correspond perfectly to the three peaks on the pore distribution graph, indicating that under the influence of ground pressure, the pores in the rock samples also shrink as a whole.

It is worth noting that the waveform similarity of the difference curves of the six rock sample groups in Figure 14 is extremely high. However, there is a significant difference in their cumulative difference curves. Compared to rock sample Group A, the contraction of effective seepage pores is more pronounced in Groups C–E. By adding a reference line at a pore size of 0.1 microns, as shown in Figure 14, the intersection of the cumulative difference curve with this reference line represents the loss of effective seepage pores. The final value of the cumulative curve accumulated from right to left represents the total porosity loss. Comparing the A3–A0 curve (Figure 14g) with the C3–C0 curve (Figure 14i), the effective seepage pore loss for A3 is 8.84%, and the total porosity loss is 7.55%, resulting in a difference of 1.29%. On the other hand, C3 shows an effective seepage pore loss of 7.71% and a total porosity loss of 5.19%, with a difference of 2.52%. This indicates that under the influence of particle size control, the contraction magnitude of the effective seepage pores and the total porosity vary among the rock sample groups.

The effective seepage pore proportion relative to the total porosity (P_e/t) for each rock sample group at the different burial depths was plotted, as shown in Figure 15.

For the rock samples in Groups C–E (see Figure 15c–e), as the burial depth increases, the proportion of effective seepage pores to total porosity significantly decreases. Combined with the information in Figure 13 and Figure 14, it can be deduced that after excluding particles in the 0.15–1.2 mm range, the rock samples undergo significant particle fragmentation under ground pressure. This is evident in Figure 13i–k as a significant downward trend in the particle difference cumulative curve, leading to the collapse of a large number of pores above 0.1 microns in size without the support of aggregate particles, collapsing into smaller pores below 0.1 microns in size. Figure 14i–k shows a significant decrease in the number of effective seepage pores compared to the total porosity loss.

In particular, as shown in Figure 15b, the P_e/t values of the rock samples in Group B are similar to those of Group A, and when the burial depth reaches 100 m, the P_e/t value of Group B even exceeds that of Group A. According to the B3–B0 curves in Figure 13h and Figure 14h, the rock samples in Group B also experience significant particle disintegration under ground pressure, leading to a significant collapse of the effective seepage pores. However, the difference lies in the absence of particles smaller than 0.15 mm, causing the pore distribution curve of Group B to shift rightward compared to that of Group A, with larger pore sizes. This results in collapsed pores larger than 0.1 microns in Group B still maintaining higher pore sizes above 0.1 microns, thus sustaining the P_e/t value of Group B.

Furthermore, the P_e/t value fitting curve for the Group F rock samples is completely different from that for Group A. According to Figure 13l and Figure 14l, the particle changes at shallow and medium burial depths for the Group F rock samples are minimal, especially with no significant particle changes below 0.15 mm. Additionally, their pore distribution difference cumulative curves are smoother than those of the other groups, indicating that excluding particles in the 1.2–2.4 mm range can enhance the bearing capacity of the rock sample at burial depths from shallow to medium, reducing particle disintegration and pore collapse. As shown in Figure 9f, at a burial depth of 50 m, the proportion of free particles in the Group F rock samples is lower than that in Group A, and as shown in Figure 11f, at a 50 m burial depth, the proportion of effective seepage pores in the Group F rock samples is similar to that in Group A. However, at a burial depth of 100 m, the P_e/t value for the Group F rock samples significantly decreases. In Figure 13l, the F3–F0 curve shows significant particle changes in the 400–600 micron range, and in Figure 14l, the cumulative difference curve significantly decreases, indicating that the ground pressure generated at a 100 m burial depth exceeds the stable bearing capacity of the Group F rock samples, leading to particle disintegration and a significant loss of effective seepage pores.

4.2. Comprehensive Analysis of Influence Mechanisms

An integrated analysis of the influence mechanism of particle size control on the particle and pore distribution characteristics of loose sandstone at different burial depths under surface heap leaching is presented below.

The particle distribution of natural rock samples forms a stable pore structure. Under ground pressure, all the particles in a rock sample experience slight uniform fragmentation, all the pores are compressed, and some of the pores collapse due to particle fragmentation. Particle fragmentation leads to an increase in the proportion of free particles, while pore compression results in a decrease in the proportion of effective seepage pores. An increase in the particle plugging frequency and a decrease in the permeable pore volume lead to a slight decrease in the permeability.

After excluding particles in the 0–0.15 mm range, the free particles completely disappear, resolving the particle plugging issue in the rock sample and increasing the proportion of effective seepage pores. However, free particles also play a role in supporting pores. As the burial depth increases, the lack of lower-level particle support between skeleton particles leads to an excessive stress concentration, causing the skeleton particles to disintegrate, pore collapse, and the generation of new free particles. The rapid increase in the particle plugging frequency and the sharp decrease in the permeable pore volume cause the permeability of the rock sample to rapidly decrease from a high value.

Excluding particles in the 0.15–0.3 mm, 0.3–0.6 mm, and 0.6–1.2 mm ranges has the same influence on the rock sample. After the particles are excluded, the proportion of free particles slightly increases, leading to a decrease in the proportion of effective seepage pores. The particle plugging issue slightly worsens, resulting in a lower initial permeability compared to that of natural rock samples. With increasing burial depth, due to the lack of support from the sieved out particles, the higher-level particles undergo a certain degree of fragmentation, leading to more collapsing of the effective seepage pores. Consequently, the proportion of effective seepage pores in this type of rock sample at different burial depths is significantly lower than that in natural rock samples, with a notably greater amount of free particles. This results in the permeability of this type of rock sample being consistently lower than that of natural rock samples, with the gap widening with increasing depth.

After excluding particles in the 1.2–2.4 mm range, the rock sample loses some effective seepage pores due to the absence of large particles, resulting in a slightly lower initial permeability than that of natural rock samples. However, the overall particle size of the rock sample decreases, making the rock sample structure more stable than that of natural rock samples. When the burial depth is greater than 50 m, the rock sample experiences minimal particle fragmentation, maintaining a high proportion of effective seepage pores. However, at a burial depth of 100 m, the rock sample undergoes particle fragmentation due to its inability to withstand high ground pressure, leading to a significant decrease in the proportion of effective seepage pores. This allows the permeability of the rock sample to remain at a high level at shallow burial depths but significantly decreases as the burial depth increases.

4.3. Analysis of the Optimal Piling Methods

To balance the efficiency of heap leaching operations and the dissolution reaction effect, the on-site pilot production phase requires the overall permeability of the ore heap to be close to k = 1 mD. Since the permeability of natural rock samples continuously decreases as burial depth increases, directly using natural ore for heap leaching is clearly not in line with production requirements. Based on the burial depth, selectively sieving out certain particles to maintain permeability while using vat leaching for the sieved-out particles to ensure complete resource utilization is a better choice. Figure 16 shows a permeability comparison chart of different rock sample groups at various burial depths, allowing for the selection of the heap construction method for each burial depth.

When constructing a heap at the surface (0 m), natural ore can be used without controlling the particle size, which can maintain the permeability of the surface of the ore heap at 0.95 mD. When constructing a heap at a shallow depth (10 m), as the permeability of the Group F rock samples is only slightly higher than that of the Group A samples, it is still preferable to use natural ore for shallow heap construction, maintaining the permeability of the shallow layer of the ore heap at 0.84 mD.

To construct a heap at a moderate depth (50 m), since the permeability of the Group F rock samples is significantly greater than that of the Group A samples, it is possible to sieve particles larger than 1.2 mm in the ore and construct a medium-depth ore heap. This can maintain the permeability of the medium layer of the ore heap at 0.81 mD, resulting in a 16% increase in seepage efficiency compared to that of natural rock samples.

When constructing a heap at a deep depth (100 m), as the permeability of all the rock sample groups drastically decreases under high pressure, and the Group B rock samples maintain a relatively high level of permeability due to overall large pore sizes, it is preferable to sieve particles smaller than 0.15 mm in the ore and construct the heap. This can maintain the permeability of the deep layer of the ore heap at 0.89 mD, resulting in a 51% increase in seepage efficiency compared to that of natural rock samples.

Taking B3 (Group B, 100 m burial depth) and F2 (Group F, 50 m burial depth) as the research objects, their particle size distribution and pore distribution curves are subtracted from the corresponding curves of A3 and A2, and the cumulative difference values are obtained for B3 and F2 relative to A3 and A2 in terms of the resulting particle and pore distribution curves, as shown in Figure 17.

For the B3 rock sample, its skeletal particle quantity is equivalent to that of A3, but it contains more particles larger than 200 microns (see Figure 17a), which results in B3 having more effective seepage pores than A3 (see Figure 17d), revealing the reason why the permeability of B3 is significantly greater than that of A3.

For the F2 rock sample (see Figure 17b), the F2–A2 curve intersects the 63-micron line above the 0 axis, indicating that although F3 lacks particles in the 1.2–2.4 mm size range, F2 still has more skeletal particles; this leads to F2 having a lower pore size for effective seepage than A2, but the proportions of effective seepage pores between the two are still similar. Additionally, having more skeletal particles in F2 implies fewer free particles, making A2 more prone to particle migration and pore clogging issues, which explains why the permeability of F2 is slightly greater than that of A2.

5. Conclusions

• Segmenting the sieving of particles out of loose sandstone can alter its particle size distribution, adjust its pore distribution, and, consequently, influence its seepage characteristics.
• With the increase in burial depth, large particles are fractured, and particles in the sieved-out particle size segment will be generated, with large particles fractured. Especially after particles in the 0–0.15 mm segment are sieved out, a large number of new particles are generated, which makes the permeability rapidly decrease.
• The quantity of particles smaller than 63 microns determines the frequency of particle clogging, while the quantity of pores larger than 0.1 microns determines the proportion of effective seepage pores in the rock sample, both of which are highly correlated with the permeability.
• The mechanism by which particle size controls the seepage characteristics of loose sandstone is as follows: Sieving out particles in the 0–0.15 mm range resolves the issue of free particle clogging and increases the proportion of effective seepage pores, resulting in higher permeability. However, as the burial depth increases, pores collapse rapidly, particles disintegrate, and permeability decreases sharply. Sieving out particles in the 0.15–1.2 mm range reduces the proportion of effective seepage pores. As the burial depth increases, the higher-level sieved-out particles further disintegrate due to a lack of support, leading to pore collapse, causing the permeability of this type of rock sample to always be lower than that of natural rock samples, with the gap gradually widening. Sieving out particles in the 1.2–2.4 mm range reduces the overall particle size of the rock sample, making its structure more stable than that of natural rock samples. When the burial depth is not deep, the rock sample hardly undergoes particle disintegration. However, with increasing burial depth, rock sample particles cannot withstand high ground pressure and disintegrate, resulting in a relatively high permeability at shallow to moderate depths, although the permeability significantly decreases with increasing burial depth.
• Based on the permeability requirements for field production in this study, the following approach can be adopted: For the surface and shallow layers of the heap, use the natural ore rock as it is. For the middle layers of the heap, mineral rocks with 1.2–2.4 mm particles should be sieved out. For the deep layers of the heap, mineral rocks with 0–0.15 mm particles should be sieved out.

Conducting continuous seepage experiments to observe the influence of particle migration on the seepage characteristics of loose sandstone over extended periods is a feasible path for further in-depth study.