The behavior of pyrite during in-situ leaching of uranium by CO2 + O2: A case study of the Qianjiadian uranium deposit in the Songliao Basin, northeastern China

Oxidative weathering of pyrite, a widespread iron sulfide mineral, is crucial for the uranium mineralization in sandstone-hosted uranium deposits and the ore utilization leaching under manual intervention. To understand the behavior of pyrite under CO2 + O2 leaching conditions in sandstone-hosted uranium deposits, samples of uranium ore from the mineralized zone of the Qianjiadian IV uranium deposit located in the lower section of the Yaojia Formation were collected for this study. The stirring leaching experiment was carried out through the processes of the pyrite-bearing uranium ore samples thinned before and after the reaction, followed by an optical microscopic study and the TESCAN Integrated Mineral Analyzer analysis, and finally, the thermodynamic simulation by making use of PHREEQC3.0. The results revealed the following: (1) The dissolution particles of pyrite become smaller, which may increase the porosity of the ore layer and the exposure ratio of uranium minerals. And more soluble pyrite with a high free surface will be more competitive with uranium minerals for the consumption of O2. (2) The ion (Fe2+) produced by the dissolution of pyrite will be oxidized to Fe3+ by excess O2 (4 mol), which provides an oxidant for the oxidation of uranium minerals, and the generated SO42- will combine with the Ca2+ released from calcite to form a permanent precipitate of gypsum, which may block pores and cover the uranium ore. (3) Pyrite and calcite will consume O2 and CO2 in the injection system, thus jointly inhibiting the leaching of uranium minerals. After the injection of excessive CO2 and O2, the concentration of Ca2+, SO42-, Fe3+, UO22+, and HCO3– in the system will continue to accumulate, resulting in the secondary precipitation of calcite, co-precipitation of gypsum, iron minerals, colloids, and hexavalent uranium minerals, resulting in plugging pores and seriously affecting the effective leaching of uranium. This study contributes to the understanding of the behavior of pyrite during uranium recovery through CO2 + O2 leaching. It also contributes to the understanding of quantitative uranium mineralization in sandstone-hosted uranium deposits and process parameters in in-situ leaching (ISL) of uranium.