Study on the Chemical Dissolution Behavior of Clay Minerals Under CO₂+O₂ In Situ Leaching Conditions for Uranium Recovery

Abstract

The accumulation of dissolved species during the in situ leaching (ISL) of uranium can lead to ore-layer blockage and reduced production capacity. Clay minerals, including montmorillonite, chlorite, kaolinite, and illite, are the primary cementing and filling materials in sandstone-hosted uranium deposits in China. However, previous studies have predominantly focused on the leaching behavior of uranium minerals, while systematic investigations into the dissolution mechanism of clay minerals under CO₂+O₂ conditions remain scarce. In this study, laboratory dissolution experiments, scanning electron microscopy–energy-dispersive spectroscopy (SEM-EDS), computed tomography (CT) scanning, and field verification were conducted to systematically investigate the dissolution behavior of major clay minerals and their contribution to ore-layer blockage under CO₂+O₂ leaching conditions. The results indicate the following: (1) Montmorillonite exhibited the most significant dissolution, with granular deposits rich in Ca and Si formed on its surface, which were inferred to be Ca-Si-rich precipitates. (2) Obvious changes in both microstructure and macroscopic physical properties of clay minerals were observed before and after leaching, with porosity decreasing by approximately 12.96% and permeability decreasing by approximately 10.16%. (3) Field verification revealed that the scale in the ore layer, filter cloth blockage, and resin surface caking were primarily composed of silica gel and hydroxide/carbonate precipitates of Al, Ca, and Fe, which are closely related to the dissolution and leaching of clay minerals. This study confirms that montmorillonite is the main source of clogging substances, and long-term closed circulation of injection and extraction leads to the accumulation of precipitates, exerting a significant impact on the uranium-leaching system. The findings provide a theoretical basis for anti-clogging and permeability enhancement in CO₂+O₂ ISL operations.

1. Introduction

The CO₂+O₂ in situ leaching (ISL) of uranium is the dominant mining technology for ensuring natural uranium production capacity in China [1,2]. As mining operations progress, CO₂ and O₂ are injected into underground ore layers, where they undergo a series of chemical reactions with reservoir minerals. The dissolved species produced during these reactions migrate through porous media. Due to changes in pressure, flow velocity, and chemical conditions, these species readily precipitate. Over time, ore-layer blockage has become increasingly prominent, not only affecting the migration efficiency of underground leaching solutions but also interfering with normal hydrometallurgical operations, severely constraining the stability of production, and similar situations also exist in other industries [3,4,5,6].

At a uranium mine in Xinjiang employing CO₂+O₂ leaching technology, with ore layers at approximately 600 m depth, severe borehole blockage occurred in 2019, reducing the total extraction/injection flow rate from the design value of 1170 m³/h to 645 m³/h. The blockage material primarily contained Ca, Mg, Si, Fe, Al, and organic matter. At Tongliao Uranium Industry in Inner Mongolia, which also uses CO₂+O₂ leaching with ore layers at 300–400 m depth, the leaching solution exhibited complex composition and markedly deteriorated permeability, requiring high-pressure injection (1.4 MPa). The bag filter experienced frequent clogging, and precipitates accumulated in the upper layer of the resin tower, severely affecting production operations. At the Nalingou uranium mine, also using CO₂+O₂ leaching with ore layers at approximately 400 m depth, borehole blockage material mainly consisted of Ca, Mg, Al, Fe, Si, and organic matter. Although air-compressor borehole washing was employed for maintenance, the extraction/injection flow rate continued to decline, and the increased external drainage volume led to insufficient evaporation pond capacity [7,8].

Since the application of ISL technology to uranium production, researchers have conducted extensive and fruitful studies on sandstone ore-layer permeability improvement and borehole washing techniques, mainly including mineralogical characteristics and permeability of sandstone uranium deposits [9], hydrogeochemistry and groundwater colloid research and modification of low-permeability sandstone uranium ore layers [10]. Additionally, research results from the petroleum industry on permeability enhancement of low-permeability oil-bearing sandstone have provided important references for ISL ore-layer modification. However, studies on the dissolution behavior of clay minerals, which are abundantly present in sandstone, remain scarce.

Previous studies have shown that uranium minerals in the Ordos Basin mainly exist as uraninite microparticles (UO₂), with sandstone being primarily composed of plagioclase, quartz, muscovite, chlorite, and epidote. The cementing materials consist of clay minerals, carbonates, and minor limonite [11]. The relatively high clay content results in numerous dead volumes in pores, smaller average pore throat diameters, and poor seepage capacity, with montmorillonite and chlorite having particularly significant effects on permeability [12]. Related studies have indicated that uranium deposits in northern China are severely cemented, with cementing materials blocking intergranular pores and resulting in low ore permeability. Currently, most research has focused on uranium mineral leaching performance and ore-layer permeability [13], while systematic studies on the dissolution behavior of clay minerals under CO₂+O₂ conditions are limited, lacking relevant laboratory experimental characterization and field verification.

In typical sandstone uranium deposits, framework grains are mainly quartz, K-feldspar, and albite, with cementing materials primarily consisting of calcite and clay minerals. Taking a sandstone uranium deposit in northern China as an example, X-ray diffraction analysis results show that the ore contains relatively high clay-mineral content, mainly montmorillonite (59%, relative content), followed by chlorite (20%) and kaolinite (17%), with minor illite (2%) (

Table 1. X-ray diffraction analysis results of ore samples (wt.%).

Mineral	SZ01-7	JC-17	JC-18	JC-19	JC-21	JC-22	Average
Quartz	36.9	32.8	34.6	41.5	33	27	34.3
K-feldspar	13.3	14.7	20.5	10.9	12.1	21.2	15.5
Plagioclase	14.4	14.4	13.6	14.9	16.2	10.3	14
Calcite	12.9	20.9	7.8	10.5	16.1	24.4	15.4
Dolomite	—	—	1	—	1.6	1.4	0.7
Pyrite	—	—	—	—	0.6	—	0.1
Total clay minerals	22.5	17.2	22.5	22.2	20.4	15.7	20.1
Montmorillonite	64	52	59	67	55	59	59.3
Chlorite	20	29	19	10	22	21	20.2
Kaolinite	14	16	18	21	19	16	17.3
Illite	2	3	4	2	4	4	3.2

). Clay minerals and calcite coexist with uranium minerals as cementing materials, directly affecting uranium-leaching operations. While calcite has been extensively reported in previous studies, the behavior of chlorite, montmorillonite, and kaolinite during leaching and their mechanisms of influence on ore-layer blockage remain insufficiently understood [14,15].

Clay minerals readily form bridging cementation in pore throats or attach to clastic grain surfaces as thin films. Kaolinite can be dispersed by rapid leaching solution flow, with fragments easily blocking pore throats; montmorillonite exhibits high cation exchange capacity and strong hydrophilicity, showing significant water sensitivity; chlorite is iron-rich and can be dissolved during acid washing operations to release Fe³⁺, generating gel-like precipitates that affect ore-layer permeability; illite commonly occurs as pore linings, with filamentous structures easily displaced by water flow to block pore throats. Therefore, under CO₂+O₂ ISL conditions, it is urgently necessary to investigate the dissolution behavior mechanisms of clay minerals in sandstone uranium deposits, analyze the sources of ore-layer blockage, and provide fundamental support for blockage removal and permeability enhancement in CO₂+O₂ ISL sandstone ore layers [16].

In view of this, this study systematically investigates the dissolution behavior of major clay minerals (montmorillonite, chlorite, kaolinite, and illite) during the CO₂+O₂ ISL of uranium. Through laboratory dissolution experiments, micro-characterization, CT scanning, and field verification, the dissolution mechanisms of clay minerals and their contribution to ore-layer blockage are revealed, providing experimental data support for identifying the sources of ore-layer blockage and theoretical foundations for blockage removal and permeability enhancement in CO₂+O₂ ISL sandstone ore layers.

2. Materials and Methods

2.1. Research Design

This study employed an integrated experimental design combining laboratory simulation tests with field verification to systematically investigate the dissolution behavior of major clay minerals and their contribution to ore-layer blockage under CO₂+O₂ ISL conditions for uranium recovery. This research study was structured into three hierarchical levels: single-mineral dissolution tests, sandstone uranium ore sample leaching tests, and field validation.

The single-mineral dissolution tests were designed as single-factor time-series experiments to compare the dissolution characteristics of four primary clay minerals (montmorillonite, chlorite, kaolinite, and illite) under CO₂+O₂ leaching conditions. Five sampling time points were established at 12 h, 24 h, 48 h, 72 h, and 96 h. Original minerals before leaching served as blank controls, and the dissolution degree was evaluated by comparing elemental content changes before and after leaching.

The sandstone uranium ore sample leaching tests comprised two components: (1) agitated leaching tests to investigate the effects of leaching solution chemical conditions (HCO₃⁻ concentration, pH value, silicon content, and cation concentration) on scale formation behavior; (2) column leaching tests to simulate the actual ISL process and study the dynamic variation patterns of ion concentrations and structural changes in ore samples. Prior to column leaching, the leaching column was treated with deionized water for 16 h to obtain baseline data.

Field validation involved the collection of solid blockage materials from filter bags, paste-like blockage materials from the upper layer of resin, and scale deposits from the underground ore layer at a uranium deposit water metallurgy plant. These field samples were analyzed and compared with the laboratory test results to verify the consistency of the findings.

2.2. Materials

2.2.1. Test Samples

Natural single-mineral specimens (montmorillonite, chlorite, kaolinite, and illite) were purchased from Hangzhou Yuhang Education Geological Specimen Factory (Hangzhou, China). The specimens were crushed and sieved to obtain the 0.25–0.50 mm grain size fraction for laboratory dissolution experiments. Sandstone uranium ore samples were collected from a typical sandstone-hosted uranium deposit in northern China. The ore samples were also crushed and sieved to the 0.25–0.50 mm grain size for agitated leaching tests. Intact core samples (diameter: 25.4 mm; length: 50 mm) were obtained from the field for structural change analysis.

2.2.2. Chemical Reagents

The leaching agent consisted of CO₂ and O₂ gases (both at 1.5 MPa injection pressure) and HCO₃⁻ solution. To intensify the experimental leaching conditions and accelerate the dissolution process for pure single-mineral samples, the HCO₃⁻ concentration was set to 3.0 g/L. For ore-sample leaching tests, 1.15 g/L HCO₃⁻ was used to approximate the actual hydrochemical environment of field in situ leaching. H₂O₂ (388 mg/L) was used as an oxidant in the structural change tests. Sodium silicate was used to prepare solutions with varying silicon concentrations for scale formation studies. Deionized water was used for baseline treatment of the leaching column. The autoclave test was performed under ambient-temperature conditions.

2.2.3. Equipment

The main methods, instruments and equipment used in this study included: three-phase high-pressure autoclave (single-mineral dissolution tests), leaching column (loading height: 40 cm; fixed bed volume: 450 mL), scanning electron microscopy (SEM; surface morphology observation), energy-dispersive spectrometry (EDS; elemental composition analysis coupled with SEM), X-ray diffractometry (XRD; mineral phase analysis), computed tomography (CT) scanner (the micro-focus CT scanning system model was Phoenix v|tome|xL300; three-dimensional imaging of rock sample structure), triaxial servo testing machine (simulating formation pressure of 0.8 MPa), constant-temperature oscillator, X-ray fluorescence spectrometry (XRF; blockage material composition analysis), chemical analyzer (ion concentration determination), and pH meter.

2.3. Experimental Methods

2.3.1. Single-Mineral Dissolution Tests

The single-mineral dissolution tests were conducted in a three-phase high-pressure autoclave. The natural single-mineral specimens were crushed and sieved to obtain the 0.25–0.50 mm grain size fraction. The experimental conditions were as follows: HCO₃⁻ concentration of 3 g/L, CO₂ injection pressure of 1.5 MPa, O₂ injection pressure of 1.5 MPa, liquid-to-solid ratio of 3:1, and shaking duration of 90–100 h. Samples were collected at 12 h, 24 h, 48 h, 72 h, and 96 h for chemical analysis. The concentrations of SiO₂, Mg²⁺, Al³⁺, Ca²⁺, and Fe in the leaching solution were determined. Additionally, SEM-EDS analysis was performed on the mineral surfaces before and after dissolution to compare morphological changes and elemental composition variations.

2.3.2. Single-Mineral Agitated Leaching Tests

The agitated leaching tests were conducted under the following baseline conditions: ore sample grain size of 0.25–0.50 mm, liquid-to-solid ratio of 3:1, shaking duration of 72 h, and sampling time points of 12 h, 24 h, 48 h, and 72 h. Four sub-experiments were designed to investigate the effects of different chemical parameters on scale formation: (1) effect of bicarbonate concentration; (2) effect of pH on silica scale formation; (3) effect of silicon content on scale formation; and (4) effect of cations on scale formation.

2.3.3. Uranium Ore Column Leaching Tests

Uranium ore was compacted and loaded into a leaching column with a loading height of 40 cm and a fixed bed volume of 450 mL. The leaching agent entered from the bottom of the column and exited from the top (upflow configuration). Prior to the leaching test, the column was preconditioned with deionized water for 16 h, and the treated solution was chemically analyzed to obtain baseline data. The leaching test duration was 150 h. A sampler was used to collect samples at regular intervals, and the following parameters were analyzed: leachate volume, U concentration, pH value, and mass concentrations of Mg, Fe, Ca, Al, and Si ions.

2.3.4. Structural Change Tests of Sandstone Uranium Ore Samples

Intact core samples (diameter: 25.4 mm; length: 50 mm) were collected from the field. The leaching solution was prepared with a HCO₃⁻ concentration of 1.15 g/L and a H₂O₂ concentration of 388 mg/L. A simulated pressure of 0.8 MPa was applied using a triaxial servo testing machine to simulate formation pressure. Comparative structural analyses were conducted on the samples before and after the experiments. Representative elementary volumes were extracted to analyze structural changes, including matrix, mineral, and connectivity pore comparisons.

2.3.5. Field Blockage Material Collection and Analysis

Field blockage materials were collected from three sources at a uranium deposit water metallurgy plant: (1) solid blockage materials from raw solution filter bags; (2) paste-like blockage materials from the upper layer of resin; and (3) scale deposits from the near-ore-layer surface. X-ray fluorescence (XRF) spectroscopy was employed to determine the contents of SiO₂, Al₂O₃, TFe₂O₃, SO₃, CaO, MgO, TiO₂, and loss on ignition. Chemical analysis was performed to measure the contents of U, Ca, Mg, Si, Al, and Fe and water content.

2.4. Data Collection and Analysis

Data collection in this study encompassed multiple types of measurements obtained through various analytical techniques. Leaching solution chemical composition was obtained through timed sampling and chemical analysis; mineral surface morphology was observed by SEM; mineral elemental composition was analyzed by EDS spectroscopy; ore sample structural parameters were obtained via CT scanning; and field blockage composition was determined by XRF and chemical analysis.

Data analysis methods included: (1) dissolution degree assessment—evaluated by calculating the change in elemental content (wt.%) before and after leaching; (2) structural change rate calculation—change rate (%) = [(Value_after − Value_before)/Value_before] × 100%; (3) solubility product calculation—Ksp values of various precipitates were calculated based on actual ion concentrations and compared with standard values; and (4) scale formation trend analysis—precipitation behavior was observed by varying a single variable.

2.5. Statistical Methods

This study primarily employed descriptive statistical analysis, including mean calculation (average values of mineral contents from six samples in XRD analysis), change rate calculation (structural parameter comparison before and after), comparative analysis (elemental content before and after dissolution), and trend analysis (ion concentration variation over time).

2.6. Quality Control

To ensure the reliability and reproducibility of the experimental results, the following quality control measures were implemented: (1) standardized sample processing—all ore samples were uniformly crushed and sieved to the 0.25–0.50 mm grain size fraction; (2) baseline data acquisition—the leaching column was treated with deionized water for 16 h prior to column leaching tests; (3) replicate verification—XRD analysis was performed on six ore samples, and average values were calculated; and (4) multi-method cross-validation—laboratory test results were corroborated through CT scanning and field sampling.

Arithmetic mean values were calculated for instrumental replicate measurements of liquid-phase samples. Standard deviation (SD) was computed for datasets with valid replicates. Linear regression was performed using Origin 2024, and the coefficient of determination R² was applied to assess fitting quality. Owing to the lack of independent parallel leaching runs for time-series autoclave experiments, statistical significance tests were not carried out.

3. Results

3.1. Dissolution Characteristics of Single Clay Minerals

3.1.1. Variation in SiO₂ Content in Leaching Solution

The variation in SiO₂ content in the dissolution system with time is shown in Figure 1. After the start of leaching, the Si concentration in all leaching solutions showed an increasing trend. Montmorillonite released the most silicon ions, indicating its loose structure, large porosity, and large contact reaction area with the leaching agent, resulting in markedly higher dissolution degree than cementing materials such as kaolinite and illite. The Si concentration in the leaching solution reached a peak at 40–50 h and then gradually decreased; after 100 h, only the montmorillonite leaching solution maintained a relatively high Si concentration. The decrease in silicon concentration may be attributed to the reaction of Ca²⁺ in formation water with the added leaching agent to form calcium carbonate particles, which adsorb some silicon ions as crystal nuclei, leading to reduced silicon-ion concentration in the solution.

3.1.2. Variation in Cation Content in Leaching Solution

The variations in Mg²⁺, Al, Ca²⁺, and Fe contents in the dissolution system are shown in Figure 2, Figure 3, Figure 4 and Figure 5, respectively.

The concentrations of Mg²⁺, Al³⁺, and Ca²⁺ released by montmorillonite dissolution were all markedly higher than those of the other three minerals. Due to its inherent iron-rich composition, chlorite exhibited high Fe content in the leaching solution, with Fe concentration increasing rapidly after the start of leaching. Additionally, Fe content in the montmorillonite leaching solution began to increase from 50 h. These results indicate that montmorillonite is the most actively dissolved under CO₂+O₂ leaching conditions and is the primary source of dissolved species.

3.2. Morphology and Elemental Changes of Clay Minerals Before and After Dissolution

3.2.1. Kaolinite

Kaolinite mostly exists as pore-filling material in sandstone intergranular pores, exhibiting vermicular or book-like morphology with relatively loose intercrystalline structure, easily dispersed by fluid flushing and subsequently blocking or dividing pore throats, particularly affecting fine throats. The elemental content changes of kaolinite before and after leaching are shown in

Table 2. Changes in elemental content of kaolinite before and after leaching (wt.%).

Item	SiO2	Al2O3	MgO	K2O	CaO	Na2O	Fe2O3
Before leaching	50.8	39.5	0.1	0.13	0.03	0.22	0.68
After leaching	50	39	0.04	0.5	0.1	0.14	0.35
Change	−0.8	−0.5	−0.06	0.37	0.07	−0.08	−0.33

. The contents of Mg, Al, and Si in kaolinite all decreased, indicating that these elements entered the dissolution system during reaction with the leaching agent, with relatively large silicon-ion dissolution. The damage to kaolinite by the leaching agent was mainly manifested as reduced silicon content on the mineral surface, with the surface changing from rough to relatively smooth and flat. Microcrystalline materials were found on some grain surfaces, inferred to be silicate scale deposits, which were relatively loose and small in particle size and existed in disordered accumulation. The EDS component analysis of kaolinite before and after leaching is shown in Figure 6, and the SEM analysis of the leached core is shown in Figure 7.

3.2.2. Montmorillonite

Montmorillonite exhibits cotton-like or honeycomb morphology, with strong water sensitivity and abundant intergranular cementing materials on the mineral surface. The elemental content changes of montmorillonite before and after leaching are shown in

Table 3. Changes in elemental content of montmorillonite before and after leaching (wt.%).

Item	SiO2	Al2O3	MgO	K2O	CaO	Na2O	Fe2O3
Before leaching	73	13.2	0.62	1.29	2.96	2.39	1.31
After leaching	71.9	13.2	1.05	1.36	2.46	2.22	2.69
Change	−1.1	0	0.43	0.07	−0.5	−0.17	1.38

. SiO₂ content decreased by 1.1%, CaO content decreased by 0.5%, while Fe₂O₃ content increased by 1.38%, and MgO content increased by 0.43%. The increase in Fe and Mg contents may be related to surface adsorption or deposition during the leaching process. Of particular importance, granular deposits were found in some core voids, with high Ca and Si elemental contents determined by EDS, inferred to be calcium silicate precipitates. The leaching solution could markedly dissolve the montmorillonite cementing materials between sand grains, indicating that montmorillonite is an important source of dissolved species. Fe and Mg show a net increase; this phenomenon is likely due to adsorption or precipitation processes.

The EDS component analysis of montmorillonite before and after leaching is shown in Figure 8, and the morphological characteristics are shown in Figure 9.

3.2.3. Chlorite

Chlorite is an aluminosilicate mineral, occurring as needle rose- or leaf-like forms filling sandstone pores, generally rich in high-valence iron ions. Under acid washing conditions at ISL sites, chlorite is easily dissolved to release Fe³⁺, subsequently precipitating and causing reservoir blockage, classifying it as an acid-sensitive mineral. The elemental content changes of chlorite before and after leaching are shown in

Table 4. Changes in elemental content of chlorite before and after leaching (wt.%).

Item	SiO2	Al2O3	MgO	K2O	CaO	Na2O	Fe2O3
Before leaching	38.7	15.8	9.5	0.29	0.7	0.18	26.5
After leaching	38.5	14.8	7.42	0.27	0.76	0.22	23.6
Change	−0.2	−1	−2.08	−0.02	0.06	0.04	−2.9

. MgO content decreased by 2.08%, and Fe₂O₃ content decreased by 2.9%, indicating that the iron–magnesium components in chlorite were dissolved into the solution system during leaching. The EDS component analysis of chlorite before and after leaching is shown in Figure 10, and the morphological characteristics are shown in Figure 11.

3.2.4. Illite

Illite is an aluminosilicate mineral, occurring as leaf-like or filamentous forms attached to grain surfaces or filling intergranular pores. The filamentous structure is easily displaced by water flow to block pore throats; flaky microcrystals divide pores into numerous small pores, increasing the tortuosity of the leaching solution. The elemental content changes of illite before and after leaching are shown in

Table 5. Changes in elemental content of illite before and after leaching (wt.%).

Item	SiO2	Al2O3	MgO	K2O	CaO	Na2O	Fe2O3
Before leaching	51.1	34.6	0.07	0.22	0.36	0.17	0.37
After leaching	39.7	27.8	0.05	0.14	0.16	0.07	0.13
Change	−11.4	−6.8	−0.02	−0.08	−0.2	−0.1	−0.24

. SiO₂ content decreased by 11.4%, and Al₂O₃ content decreased by 6.8%, indicating significant dissolution reactions occurred during the leaching process. The EDS component analysis of illite before and after leaching is shown in Figure 12, and the morphological characteristics are shown in Figure 13.

3.3. Leaching Test Results of Sandstone Uranium Ore Samples

3.3.1. Agitated Leaching Tests

The basic chemical ion composition of the leaching solution is shown in

Table 6. Chemical ion composition concentrations of leaching solution.

U (mg·L−1)	HCO3− (mg·L−1)	pH	Ca2+ (mg·L−1)	Mg2+ (mg·L−1)	SO42− (mg·L−1)	Cl− (mg·L−1)
27.49	1150.51	6.4	472.35	99	651	216.34

.

First, Effect of bicarbonate concentration: With the increase in HCO₃⁻ concentration in solution, soluble Ca²⁺ gradually decreased. At low HCO₃⁻ content (approximately 100 mg/L), precipitates were mainly flake-like silicic acid and silicates; at high HCO₃⁻ content (approximately 1100 mg/L), precipitates transformed into granular carbonates. Since the solubility product of carbonates is generally lower than that of silicates and the solubility product of magnesium carbonate is higher than that of calcium carbonate, flake-like carbonate precipitates mainly appeared in the system as HCO₃⁻ concentration gradually increased, with relatively increased magnesium-ion concentration.

Furthermore, Effect of pH on silica scale formation: With the increase in pH, the equilibrium concentrations of bicarbonate and silicon ions increased; when pH > 9, bicarbonate gradually completely transformed into carbonate, and silicon- and magnesium-ion concentrations gradually decreased. Therefore, decreased pH is an important factor promoting silicate scale formation.

In addition, Effect of silicon content on scale formation: When silicon-ion content was 80 mg/L, powdery precipitates appeared in the solution; when silicon-ion content was 180 mg/L, partial granular precipitate aggregation occurred. In the 50–400 mg/L range of silicon ions, Ca²⁺ content in the solution decreased markedly, with most Ca²⁺ reacting with the leaching agent, while silicate reaction with Ca²⁺ promoted calcium-salt deposition; when silicon-ion content was 250–400 mg/L, increased silicon content further promoted calcium-salt scale deposition, with increased scaling rate.

Finally, Effect of cations on scale formation: With the increase in cation dosage, calcium- and magnesium-ion contents in the solution also increased, but the difference between actual ion content and theoretical added amount also increased, indicating that calcium and magnesium aggregated and deposited in the scaling system, leading to reduced ion content, with calcium deposition magnitude being much greater than that of magnesium. In actual production, calcium-salt content in scale samples is also much higher than content of magnesium salt.

3.3.2. Column Leaching Tests

The concentrations of major components in the leachate of sandstone uranium ore samples vary with leaching time. The concentrations of dissolved species reflect the net balance between mineral lattice destruction (release) and secondary phase formation (consumption). The leached uranium concentration and pH value leaching curves are shown in Figure 14.

The variation pattern of calcium-ion concentration is similar to that of uranium mass concentration. During mineral dissolution, uranium, calcium, and magnesium in the uranium ore enter the leaching system through mineral dissolution, leading to increased ion concentrations. As the leaching reaction proceeds further and the liquid-to-solid ratio exceeds 1.5, the uranium concentration begins to decrease, manifesting as reduced effluent volume and partial blockage of some ore layers; subsequently, the calcium-ion concentration in the leachate also declines. The leaching curves of Ca, Mg, and uranium concentrations are shown in Figure 15.

The leaching curve of Si in the leaching system follows a similar pattern to that of uranium (Figure 16), indicating that silicon and uranium have similar leaching behaviors and a certain linear relationship.

Throughout the experiment, the aluminum concentration remained below 0.5 mg/L, reflecting the low solubility of Al³⁺ under near-neutral-pH conditions. Within this pH range, hydrolysis of Al and precipitation of Al(OH)₃ effectively buffer dissolved Al to trace levels. Although EDS measurements revealed significant Al₂O₃ loss (up to 6.8% in illite), the dissolved Al did not persist in solution but rapidly precipitated near the mineral surface as amorphous Al-hydroxide phases. The iron-ion concentration stayed below 0.3 mg/L, and its impact on ore leaching blockage was also minor. The leaching curves of Fe, Al, and uranium concentrations are shown in Figure 17.

3.4. Structural Changes of Sandstone Uranium Ore Samples

Leaching experiments were conducted on rock samples obtained from an in situ sandstone uranium deposit. A simulated in situ leaching solution was prepared with NaHCO₃ at 1.5 g/L and H₂O₂ at 388 mg/L. The leaching experiments were performed over a duration of 7 days. Micro-focus CT scanning was employed to characterize the samples before and after leaching, enabling a comparative analysis of the matrix, minerals, and connected pore structures. The structural variations observed before and after leaching are summarized in Table 6. The pore-scale flow velocity and streamlines obtained from micro-CT scanning are shown in Figure 18, where (a) and (b) correspond to pre-leaching and post-leaching status, re-spectively. Significant changes in seepage paths occur after leaching.

A comparative analysis of the characteristic images—Image a (pre-leaching) and Image b (post-leaching)—reveals distinct differences in the CT scan visualizations. In Image a (initial stage), high-velocity regions (red/orange) are predominantly concentrated in the central primary flow channels, with intermediate-velocity zones (yellow/green) serving as transitional areas, while low-velocity regions (blue/purple) are prevalent along the periphery and corners. In Image b (later stage), high-velocity streamlines are markedly reduced and more dispersed; intermediate-velocity zones exhibit a substantial decrease, and low-velocity/dead zones undergo extensive expansion (represented in gray/white).

Figure 18. Micro-CT scan of uranium-bearing sandstone pore-scale flow velocity and streamlines: (a) pre-leaching; (b) post-leaching.

Figure 18. Micro-CT scan of uranium-bearing sandstone pore-scale flow velocity and streamlines: (a) pre-leaching; (b) post-leaching.

As listed in Table 7, the structural parameters of intact ore-core samples are com-pared for pre-reaction and post-reaction states. The table quantitatively records the differences in ore-core structure caused by chemical reaction. Such structural alteration provides direct evidence for the reconstruction of pore-throat networks within ore matrix during the reaction, which further affects the seepage conditions of in-situ leaching. Prior to leaching, the sample exhibited favorable connectivity with a porosity of approximately 20%. Following the leaching reaction, the generation of chemical precipitates resulted in the blockage of connected pores, leading to a deterioration in pore connectivity and a reduction in porosity to approximately 18%. Regarding mineralogical changes, the leaching reaction consumed a portion of uranium-bearing minerals, accompanied by a corresponding decrease in volume. The flow velocity of the pre-leaching sample exceeded that of the post-leaching sample. The proportion of larger pore throats and apertures decreased, whereas the proportion of smaller pore throats and apertures increased; the proportion of intermediate pore throats and apertures remained relatively stable. After leaching, the tortuosity of connected pores increased from the original value of 2.09 to 2.123, and the permeability declined from approximately 0.131 mD to 0.117 mD, which is primarily attributable to the blockage caused by chemical precipitates.

Table 7. Comparison of structural changes of intact core ore samples before and after reaction.

Parameter	Before Reaction	After Reaction	Magnitude of Change
Permeability, μm2	0.00013075	0.00011746	−10.16%
Porosity, %	0.202707	0.17643	−12.96%
Tortuosity	2.09	2.123	1.58%

Table 7. Comparison of structural changes of intact core ore samples before and after reaction.

Parameter	Before Reaction	After Reaction	Magnitude of Change
Permeability, μm2	0.00013075	0.00011746	−10.16%
Porosity, %	0.202707	0.17643	−12.96%
Tortuosity	2.09	2.123	1.58%

The significant reduction in effective permeability is dominated by micropore blockage. The pronounced deterioration in pore connectivity is likely attributable to the isolation of pore throats by precipitates. The increased tortuosity compels the fluid to follow more circuitous flow paths. The enhanced heterogeneity in seepage uniformity leads to the disappearance of preferential flow channels, while the reduction in movable fluid saturation corresponds to an increased proportion of irreducible fluid. The seepage capacity of the core underwent a reversal from “dissolution-enhanced permeability” to “precipitation-induced blockage,” resulting in a substantial deterioration in overall seepage performance. This constitutes the microscopic mechanism responsible for the decline in efficiency during the later stages of the alkaline in situ leaching of uranium.

Similar dual-evolution patterns of reservoir properties (dissolution-driven permeability gain offset by precipitate-induced clogging) have also been reported for sandstone-hosted uranium ISL systems [16].

3.5. Solubility Product Theoretical Calculation

The solubility product constants and solubility calculation results for various ion concentrations in column leaching are shown in

Table 8. Solubility product constants and solubility calculation results of various ion concentrations in column leaching (20 °C).

Precipitate	Ion-Activity Product (IAP)	Calculated Solubility (g/L)	Standard Ksp (25 °C)	Standard Solubility (g/L)	Note
CaCO3	3.34 × 10−7	0.0578	3.36 × 10−9	0.0069	Sparingly soluble
MgCO3	7.28 × 10−8	0.0226	6.82 × 10−6	0.0158	Slightly soluble
CaSO4	1.61 × 10−4	1.728	4.93 × 10−5	2.03	Slightly soluble
MgSO4	—	—	—	350	Soluble

.

In this work, the parameter calculated from measured solution-phase ion concentrations is the ion-activity product (IAP). Activity coefficients were neglected during calculation, so the IAP is approximated directly by the measured ion concentrations. Minor ion-pair and trace complexation effects are ignored. The standard thermodynamic solubility product constant Ksp of calcite is sourced from public geochemical databases. The saturation state is judged by comparing the calculated IAP against the standard Ksp.

The calculated actual solubility product of CaCO₃ ion concentration in this experiment (3.34 × 10⁻⁷) was markedly higher than the standard Ksp (3.36 × 10⁻⁹), indicating that CaCO₃ in the system was in a supersaturated state and highly prone to precipitation, making it the main cause of ore-layer blockage. The IAP of MgCO₃ (7.28 × 10⁻⁸) was lower than the standard Ksp (6.82 × 10⁻⁶), indicating weaker precipitation tendency than CaCO₃. The IAP of CaSO₄ was close to the standard value, and its solubility was relatively high, contributing little to blockage. MgSO₄, as a soluble salt, mainly existed in ionic form in the leaching system and was not prone to precipitation [17].

3.6. Field Blockage Material Composition Analysis

3.6.1. XRF Analysis of Filter Solid Blockage Materials

The elemental composition of solid blockage materials collected from the filter was charac-terized by X-ray fluorescence, and the corresponding mass fraction data are listed in Table 9. The major elemental constituents of filter clogging solids can be quantitatively obtained from these results, which helps to identify the primary chemical components responsible for filter blockage during in situ leaching.

Table 9. X-ray fluorescence (XRF) analysis results of filter solid blockage materials (wt.%).

Sample No.	SiO2	Al2O3	TFe2O3	SO3	CaO	MgO	TiO2	LOI
1	10.77	28.29	12.01	1.98	4.85	0.66	0.09	37.46
2	10.76	29.42	12.14	1.93	4.73	0.98	0.09	36.92

Table 9. X-ray fluorescence (XRF) analysis results of filter solid blockage materials (wt.%).

Sample No.	SiO2	Al2O3	TFe2O3	SO3	CaO	MgO	TiO2	LOI
1	10.77	28.29	12.01	1.98	4.85	0.66	0.09	37.46
2	10.76	29.42	12.14	1.93	4.73	0.98	0.09	36.92

3.6.2. Analysis of Paste-like Blockage Materials from Upper Resin Layer

Table 10 presents the chemical composition of paste-like blockage materials sampled from the upper layer of resin, with all values given in weight percent. The compositional features of this pasty substance reflect the accumulation of secondary precipitates on the resin surface, which is one of the key factors leading to resin fouling and performance degradation in the ura-nium recovery system.

Table 10. Analysis results of paste-like blockage materials from upper resin layer (wt.%).

Sample	U	Ca	Mg	Si	Al	Fe	Water Content
Filter bag scale	0.114	0.370	0.193	0.411	5.028	1.643	91.13
Backwash tower flocculent precipitate	0.007	0.602	0.638	0.279	2.760	0.875	75.37

Table 10. Analysis results of paste-like blockage materials from upper resin layer (wt.%).

Sample	U	Ca	Mg	Si	Al	Fe	Water Content
Filter bag scale	0.114	0.370	0.193	0.411	5.028	1.643	91.13
Backwash tower flocculent precipitate	0.007	0.602	0.638	0.279	2.760	0.875	75.37

The viscous materials found in the surface treatment process were mainly silica gel, aluminum, calcium, iron precipitates, and organic matter. These materials accumulated over long-term closed operation in the leaching and leaching solution treatment system, with the leaching solution from filter cloths and the resin layer surface for uranium recovery by ion exchange producing caking, indicating chemical blockage. Underground ore-layer scale composition analysis showed that the elements with relatively high contents were C, Fe, Na, and Al. The combination of these elements most likely originated from clay minerals. The ore-layer blockage materials in this deposit were mainly CaCO₃, SiO₂, Fe(OH)₃, etc. After viscous impurities appeared on site, the extraction/injection flow rate decreased.

4. Discussion

4.1. Dissolution Mechanism Analysis of Clay Minerals

After contact between the leaching agent and reservoir minerals, the dissolution reaction follows a progressive process from surface to interior. First, M-O bonds (M = Ca, Mg, Al, Fe, etc.) and Si-O bonds on the mineral surface break, causing surface dissolution of the rock; subsequently, O-Si-O, O-Ca-O, and O-Mg-O bonds inside the rock are gradually destroyed, leading to rock structure disintegration; simultaneously, divalent metal-ion exchange reactions with Na⁺ and K⁺ occur on mineral microparticle surfaces, with divalent metal ions entering the solution system [18].

The following reactions are schematic representations for qualitative illustration of mineral dissolution processes; only selected reactions are strictly mass- and charge-balanced for thermodynamic calculation.

The specific dissolution reactions of each clay mineral are as follows:

Clay-mineral dissolution plays an important role in pore-throat evolution during leaching. First, regarding the dissolution reaction of kaolinite:

Al₄[SiO₄](OH)₈·nH₂O + H⁺ → Al³⁺ + SiO₃²⁻ + H₂O

After kaolinite dissolution, silicon enters the aqueous solution and exists in multiple dissolved silicate species, such as Si(OH)₄, HSiO₃⁻ and SiO₃²⁻, during migration, according to aqueous-silicon chemistry theory. Next comes the dissolution reaction of illite:

(K,H₃O⁺)(Al,Mg,Fe)₂[(Si,Al)₄O₁₀](OH)₂ + H⁺ → K⁺ + Al³⁺ + Fe³⁺ + Mg²⁺ + SiO₃²⁻

For montmorillonite, its dissolution reaction is expressed as:

(Na,Ca)_m(Al,Mg,Fe)₂[(Si,Al)₄O₁₀](OH)₂·nH₂O + H⁺→Al³⁺ + SiO₃²⁻ + H₂O + Na⁺ + K⁺ + Ca²⁺ + Fe³⁺ + Mg²⁺

Finally, the dissolution reaction of chlorite can be written as:

(Mg,Al,Fe)₃(Si,Al)₄O₁₀(OH)₂ + H⁺ → Al(OH)₃ + Fe²⁺ + Fe³⁺ + Mg²⁺ + SiO₃²⁻ + K⁺

From the above reactions, after contact between the leaching agent and reservoir minerals, M-O bonds and O-Si-O bonds on the mineral surface first break, followed by destruction of internal M-O bonds and O-Si-O bonds, accompanied by divalent metal-ion exchange reactions with K⁺ and Na⁺, ultimately transforming minerals into soluble species or microparticles entering the formation fluid. Mineral soluble species mainly exist in the formation fluid as HSiO₃⁻, Si(OH)₄, Na⁺, K⁺, Ca²⁺, Fe²⁺, Mg²⁺, Al(OH)₃, and AlO₃³⁻ ions or migrate in suspension as hydroxides and mineral microparticles. Therefore, with extended injection time and increased leaching agent injection volume, silicon in the reservoir is continuously dissolved, with silicon concentration showing an increasing trend.

4.2. Comparison of Dissolution Degree Among Four Clay Minerals

From the total dissolved ion release and elemental content changes, the dissolution activity of the four clay minerals shows significant differences. Montmorillonite exhibits the most significant dissolution, with granular deposits rich in Ca and Si formed on its surface, inferred to be Ca-Si-rich precipitates, potentially associated with calcium silicate phases. Montmorillonite is a 2:1-type layered silicate mineral with lattice substitution phenomena; interlayer attraction is mainly represented by van der Waals forces, which are relatively weak, allowing water molecules to easily enter the interlayers, with numerous exchangeable cations. These structural characteristics give it the highest dissolution activity under CO₂+O₂ leaching conditions.

Although illite showed the largest decrease in SiO₂ content after dissolution (−11.4%), its absolute content change was still smaller than that of montmorillonite. Illite is a 2:1-type layered silicate mineral with electrostatic forces between crystal layers, relatively strong attraction, and water molecules not easily entering the interlayers, which to some extent limits its dissolution degree. Kaolinite is a 1:1-type layered silicate mineral with strong hydrogen bonding between layers, tightly connected crystal layers, no lattice substitution, and extremely few exchangeable cations, resulting in the lowest dissolution degree. Chlorite is rich in iron–magnesium components, with Fe₂O₃ and MgO contents decreasing by 2.9% and 2.08% respectively during dissolution. Its acid-sensitive characteristics make it prone to releasing Fe³⁺ under acidic leaching conditions, subsequently forming iron hydroxide precipitates.

4.3. Analysis of Ore-Layer Blockage Mechanism

This study confirms that montmorillonite is the main source of clogging substances. Long-term closed circulation of injection and extraction leads to precipitate accumulation, exerting a significant impact on the uranium-leaching system. The formation mechanism of ore-layer blockage can be understood from three levels:

First, at the underground ore-layer level, Si, Al, Ca, Fe, Mg, and other ions released by clay-mineral dissolution migrate through porous media. Due to changes in pressure, flow velocity, and chemical conditions, these species precipitate. The calculated solubility product of CaCO₃ is markedly higher than the standard value, indicating that it is in a supersaturated state and highly prone to precipitation at pore throats, causing pore blockage.

Second, at the surface treatment system level, silica gel and metal hydroxide/carbonate precipitates carried in the leaching solution accumulate on filter cloths and resin surfaces, forming caked deposits. Field verification showed that filter bag scale and backwash tower flocculent precipitates contained relatively high contents of Si, Al, Ca, and Fe, with water contents of 91.13% and 75.37% respectively, indicating that these materials have colloidal characteristics and are prone to adhesion and accumulation.

Third, temporal asynchrony exists between the rapid release of Si, Ca, and Mg in the early stage and their slow release in the later stage, suggesting that dissolution follows two-stage kinetics. A two-stage dissolution kinetic model for clay minerals under CO₂+O₂ conditions is proposed: Stage 1 (before liquid-to-solid ratio reaches 1.5): surface-controlled rapid dissolution, where Si-O-M bonds (M = Si, Al, Fe, Mg) break rapidly and the dissolution rate is controlled by surface reaction kinetics; Stage 2 (after liquid-to-solid ratio has surpassed 1.5): diffusion-controlled slow dissolution, where reaction products (e.g., silica gel and Al/Fe hydroxides) accumulate on the mineral surface, forming a diffusion boundary layer, and solute transport shifts to diffusion control, markedly reducing the dissolution rate.

This model provides a theoretical framework for understanding the spatiotemporal evolution of ore-layer blockage during long-term operation.

The proportion of larger pore throats and pore diameters decreased, while the proportion of small pore throats and pore diameters increased, indicating that chemical precipitates preferentially blocked well-connected large pores, making fluid migration paths more tortuous.

4.4. Comparison with Existing Studies

Mineral alteration and secondary mineral precipitation are widely accepted as primary triggers for ore-bearing-formation permeability impairment during uranium in situ leaching, threatening production efficiency of sandstone-type uranium deposits. A large number of experimental and field observations have demonstrated that water–rock interactions can generate multiple pore-clogging products under different leaching media. Under acid-leaching conditions, feldspar dissolution–alteration into clay-rich assemblages and gypsum crystallization are two well-documented clogging pathways, which can significantly reduce reservoir porosity and seepage capacity. Secondary gypsum precipitates are regarded as some of the most intractable chemical clogging products for acid-ISL operations worldwide [19].

For neutral CO₂+O₂ in situ leaching systems, hydrochemical variables including bicarbonate concentration, pH, and Eh jointly govern uranium dissolution, migration, and secondary-mineral-precipitation risks. Existing experimental and case investigations mainly focus on pyrite geochemical responses and coffinite-uranium mineral dissolution, as well as permeability damage induced by carbonate precipitation in sandstone uranium aquifers. Most published interpretations attribute pore clogging predominantly to carbonate precipitates, gypsum, or feldspar-derived alteration assemblages.

Clay-mineral-related reservoir-damage mechanisms have been emphasized in petroleum and hydrogeology research, where particle migration, swelling and re-deposition deteriorate porous-media conductivity. Nevertheless, within the field of neutral-condition uranium CO₂+O₂ in situ leaching, far less attention has been paid to the contribution of clay-mineral dissolution–migration to permeability deterioration. Few published works systematically distinguish the clogging risks originating from different clay phases, especially smectite-group montmorillonite, in neutral CO₂-rich groundwater environments.

Through systematic laboratory experiments combined with field-scale verification evidence, the present study identifies montmorillonite as the dominant dissolved and clogging-causing substance under CO₂+O₂ neutral-leaching conditions. This finding supplements current understandings of reservoir-blockage mechanisms for sandstone-hosted uranium deposits and fills the specific research gap regarding clay-driven permeability evolution during neutral in situ leaching exploitation.

4.5. Research Limitations

This study is mainly based on laboratory simulation experiments. Although corroborated through field verification, the complexity of actual field geological conditions (such as temperature gradients, pressure fluctuations, and microbial effects) has not been fully simulated. Furthermore, monitoring data for long-term dynamic dissolution processes still need to be accumulated. Future research could consider conducting long-term field monitoring experiments to obtain more comprehensive dynamic dissolution data.

In terms of leaching experiments, the time-series autoclave leaching tests (Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5) were performed as single experimental runs without independent parallel leaching replicates. Although each liquid sample was measured two–three times for elemental analysis to ensure instrumental analytical precision, error bars representing overall experimental variability cannot be provided. The silicon-peak-and-subsequent-decline trend is supported by auxiliary SEM-EDS observations of solid leaching residues. Future work will implement multiple independent parallel leaching experiments to further verify this temporal variation trend.

5. Conclusions

(1) Sandstone uranium deposits in China belong to high clay-content deposits, with four main clay minerals: montmorillonite, chlorite, kaolinite, and illite. Under CO₂+O₂ leaching conditions, all cementing clay minerals can undergo dissolution reactions, with montmorillonite producing the most clogging substances after dissolution.

(2) During leaching, montmorillonite dissolved the cementing materials between sand grains. Granular deposits were found on the mineral surface, with high Ca and Si elemental contents determined by EDS, inferred to be Ca-Si-rich precipitates, potentially associated with calcium silicate phases. The concentrations of Si, Mg, Al, and Ca ions released by montmorillonite dissolution were all markedly higher than those of the other three minerals, making it the primary source of dissolved species.

(3) When the liquid-to-solid ratio exceeded 1 during leaching experiments, some components exceeded the solubility product constant, and calcium carbonate precipitates readily formed in the solution. The calculated solubility product of CaCO₃ (3.34 × 10⁻⁷) was markedly higher than the standard Ksp (3.36 × 10⁻⁹), indicating that it was in a supersaturated state and was the main cause of ore-layer blockage. The CT scanning of ore samples showed that the structure and physical properties of clay minerals in the ore changed markedly before and after leaching, with porosity decreasing by approximately 12.96%, permeability decreasing by approximately 10.16%, and tortuosity increasing by approximately 1.58%.

(4) Field verification showed that scale on the near-ore-layer surface, filter cloth blockage materials, and caked deposits on the upper resin layer surface had relatively high contents of Ca, Si, and Fe. After long-term closed operation and accumulation, these materials have negatively affected ISL site operations. The blockage materials were mainly CaCO₃, H₄SiO₄ and Fe(OH)₃ precipitates, and the test results showed that their sources were closely related to clay dissolution and leaching.

(5) The research findings provide a theoretical basis for anti-clogging and permeability enhancement in CO₂+O₂ ISL operations. It is recommended that in actual production, the monitoring of ore layers with high montmorillonite content be strengthened and leaching agent formulation and injection–extraction circulation systems be optimized to reduce the accumulation of clogging substances and ensure the stability of production operations.