Citric acid-PESA synergistic system for pre-leaching permeability improvement and scale inhibition in sandstone uranium deposits

Serious carbonate mineral clogging easily occurs during CO 2 + O 2 in-situ leaching of low-permeability sandstone uranium deposits in Inner Mongolia, which severely reduces reservoir permeability and uranium leaching efficiency. To solve this engineering bottleneck and realize pre-mining permeability enhancement with low premature uranium leaching, an eco-friendly synergistic dissolution system composed of citric acid and polyepoxysuccinic acid (PESA) was proposed in this work. Integrated static dissolution batch tests and PHREEQC geochemical saturation index simulation were carried out to clarify the dissolution and secondary precipitation rules of scale-forming minerals in single citric acid and citric acid-PESA synergistic systems. The results show that single citric acid solution at 1.5 g L - 1 can efficiently dissolve calcite cement, but supersaturated calcium citrate, iron oxyhydroxide and aluminum hydroxide will precipitate secondarily and block rock pores. After adding 150-200 mg L - 1 PESA as green chelating scale inhibitor, the carboxyl groups of PESA form stable soluble complexes with Ca 2+ , Fe 3+ and Al 3+ simultaneously, which completely eliminates calcium citrate crystallization and significantly reduces the supersaturation risk of iron-aluminum hydroxide precipitates. Under the optimal combined formula, the maximum dissolved Ca 2+ concentration reaches 437 mg L - 1 , realizing thorough dissolution of carbonate scale minerals, and the premature uranium leaching is controlled below 5.04 %. Multi-scale microstructural characterization including scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) surface area analysis indicates that the synergistic system effectively dissolves carbonate minerals, removes flaky secondary precipitates from mineral surfaces, and leads to pore expansion and widened pore size distribution at the microscopic level. These micro-scale modifications are expected to contribute to the improvement of reservoir pore space in uranium-bearing sandstone cores. Collectively, these results provide solid theoretical support and technical reference for alleviating mineral clogging damage and improving uranium resource recovery in CO 2 + O 2 in-situ leaching of low-permeability sandstone uranium deposits.