Leaching behavior and clogging mechanisms of the Hailijin sandstone-type uranium deposit, Songliao Basin, China

Mineralogy, uranium occurrence, and permeability evolution are key controls on the performance of in situ uranium leaching. However, the dissolution behavior of ore and gangue minerals, as well as the origin of permeability decline during oxygen-enhanced bicarbonate leaching, remain insufficiently constrained. Here, the Hailijin sandstone-type uranium deposit in the Songliao Basin was investigated using SEM-EDS, XRD, XRF, ICP-MS, particle-size analysis, thin-section and powder leaching experiments, and PHREEQC simulations. The results show that uranium mainly occurs as pitchblende, with only minor uranium hosted in Ti oxides. Pitchblende is effectively dissolved under COv + Ov leaching conditions and in Ov-enriched bicarbonate groundwater, whereas uranium-bearing Ti oxides show no observable dissolution. Dolomite undergoes incongruent dissolution, releasing Ca into solution while Mg is retained as magnesite. Siderite dissolution is kinetically inhibited under near-neutral oxic conditions. Pyrite is oxidized to goethite, but goethite rims form passivation layers that slow further oxidation. PHREEQC simulations and precipitate observations indicate that gypsum does not precipitate at the initial stage of leaching and that its total amount remains limited unless local enrichment occurs. The decrease in permeability is mainly caused by the detachment, migration, and retention of mineral particles and by the aggregation of ferruginous colloids with mineral particles, rather than by widespread carbonate or sulfate precipitation. Because the Hailijin groundwater contains abundant bicarbonate ligands, direct oxygen oxidative leaching is suitable for this deposit. An oxygen dosage of approximately 240 mg/L and quartz-sand filtration of the produced solution are recommended to improve uranium recovery and mitigate permeability damage.