| Summary: | Weathered crust elution-deposited rare earth ores (WREOs) are significant strategic mineral resources. In industry, in situ leaching technology is usually applied with ammonium chloride and ammonium sulfate as the leaching solution. However, the slow seepage velocity of the leaching solution and low rare earth leaching efficiency still need to be improved. Ammonium formate can effectively improve the WREO leaching process. In order to further explore the effects of ammonium formate on the ammonium salt leaching process for WREOs, ammonium chloride and ammonium sulfate compounded with ammonium formate were used as leaching agents to determine their effects on leaching efficiency, seepage velocity and swelling. The results show that in the presence of ammonium formate, the rare earth leaching efficiencies with ammonium chloride and ammonium sulfate are both slightly increased, the seepage velocity of ammonium chloride and ammonium sulfate is increased by 1.67 × 10<sup>−4</sup> cm·s<sup>−1</sup> and 1.18 × 10<sup>−4</sup> cm·s<sup>−1</sup>, and the swelling percentage falls by 0.14% and 0.37%, respectively. The thickness of the adsorbed water layer and thermogravimetric and XRD results confirm that ammonium formate can inhibit surface hydration and thus improve the WREO leaching process.
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