Leaching Behavior of Coffinite-Rich, Calcite-Cemented Sandstone Uranium Ore Under CO2 + O2 Conditions: Implications for In Situ Recovery at the Bayinqinggeli Deposit, Ordos Basin

Uranium occurrence and gangue mineralogy govern the choice of leaching process for sandstone-hosted uranium deposits. This study evaluates CO2 + O2 leaching of calcite-rich uranium ore from the Bayinqinggeli deposit in the northern Ordos Basin using mineralogical characterization, thin-section and agitated leaching experiments, and PHREEQC modeling. The dominant U–Si-bearing phase is interpreted as coffinite, although its mineralogical identification remains provisional in the absence of phase-specific structural and oxidation-state analyses. Thin-section observations revealed pronounced dissolution of exposed calcite and coffinite grains, but only minor surface corrosion of pyrite. Agitated leaching tests showed that increasing bicarbonate concentration improved uranium recovery, with diminishing gains above approximately 2 g/L. Calcite dissolution supplied bicarbonate, but equilibrium calculations indicated that the amount dissolved within a single pore-water volume was small relative to the calcite inventory. Thus, bicarbonate generation does not necessarily imply extensive liberation of encapsulated uranium minerals or a substantial increase in aquifer permeability. Pyrite oxidation may promote carbonate dissolution through acid generation, while also consuming oxygen and potentially producing pore-clogging precipitates. The results support further evaluation of CO2 + O2 recovery for this deposit. A modeled initial CO2 dosage of 5 g/L produces approximately 2 g/L bicarbonate, and maintaining pressure in the surface processing circuit may limit CO2 loss and calcite precipitation. These operating suggestions require validation under field flow and recirculation conditions.