Coordination–reduction leaching process of ion-adsorption type rare earth ore with ascorbic acid

The magnesium sulfate (MgSO4)–ascorbic acid (Vc) compound leaching technique can extract rare earth elements (REEs) existing in ion-exchangeable phase and colloidal phase from ion-adsorption type rare earth ore through the synergy effect of coordination and reduction, but its reaction process and mechanism remain unclear. In this paper, the coordination–reduction leaching mechanism was analyzed from the perspectives of leaching thermodynamics and kinetics, which provide theoretical guidance for the compound leaching process. In the case of neodymium, about 45% of dissolved neodymium will exist as the complex species of NdVc3(aq) in Nd–Vc–sulfate system. Based on this, it is deduced that the Gibbs free energy of the leaching reaction of ion-exchangeable phase REEs will change to a more negative value through the coordination of REEs cations and Vc anions in the MgSO4 leaching process. In addition, the Eh–pH diagrams of Ce–SO42––H2O and Fe–SO42––H2O together with the dissolution experiments confirm that the added Vc initiates the leaching process of colloidal phase REEs through reduction–dissolution reaction. Through the study of leaching kinetics, the leaching of REEs is controlled by diffusion and chemical reaction in the compound leaching system since colloidal phase REEs are leached. Therefore, the addition of Vc can shift the leaching equilibrium to a more favorable state and accelerate leaching process. The rare earth leaching efficiency of ion–exchangeable phase and colloidal phase can be effectively improved by increasing the reaction temperature, the concentrations of leaching agent and Vc, and the leaching agent acidity.