Recovery of rare earths from weathered crust elution-deposited rare earth ore without ammonia-nitrogen pollution: I. leaching with magnesium sulfate

• Magnesium sulfate was proposed to be the leaching agent.
• The optimum conditions lead to 92% of total rare earth leaching efficiency.
• The leaching kinetics of rare earth could be a film diffusion control mechanism.
• The content of ion-exchangeable magnesium in washing tailings was 560 mg/kg soil.
• Synergic leaching was identified as an excellent alternative leaching agent.

The practice of in-situ leaching of weathered crust elution-deposited rare earth ore using (NH4)2SO4 solution has revealed serious environmental problems such as ammonia-nitrogen pollution in water body and plant growth difficulties. The present study investigated the use of magnesium sulfate instead of (NH4)2SO4 as the leaching agent in the recovery of rare earth elements to solve the problem of ammonia-nitrogen pollution. It was determined that the optimum leaching condition in 298 K, a flow rate of 0.60 mL·min− 1, and 0.20 mol·L− 1 magnesium sulfate solution, lead to 93% of total rare earth leaching efficiency and around 50% aluminum leaching efficiency. The results suggested that the magnesium sulfate was capable of producing identical recovery efficiency as (NH4)2SO4 and reducing aluminum leaching efficiency at the same time. In addition, the leaching mechanism was analyzed with different kinetic models, it was determined that the leaching kinetics of the weathered crust elution-deposited rare earth ore could be appropriately described by Pseudo-first-order model, which indicated that the film diffusion was rate-limiting for the leaching process. The apparent activation energy was calculated to be 8.90 kJ·mol− 1. However, the content of ion-exchangeable magnesium in rare earth washing tailings was 560 mg per kilogram soil. This would lead to an imbalance of ion-exchangeable calcium and ion-exchangeable magnesium, which would have a bad effect on the plant growth. Based on these findings, the synergic leaching with magnesium salt and calcium salt was identified as an excellent alternative leaching agent for further studies.