Effect of a novel compound on leaching process of weathered crust elution-deposited rare earth ore

The leaching process of weathered crust elution-deposited rare earth ore is a typical noncatalytic heterogeneous reaction in liquid–solid systems. In order to intensify the leaching process of rare earth and reduce the impurity leaching accompanying with rare earth, ammonium sulfate and ammonium formate were mixed as a novel compound leaching agent, in which ammonium formate was used as an inhibitor for aluminum, as well as an aid agent to speed up rare earth leaching. Effects of ammonium formate concentration, liquid/solid ratio, leaching agent pH and leaching temperature on the leaching process of rare earth and aluminum were investigated, and then were systematically evaluated by the leaching kinetic theory. It is found that ammonium formate could effectively enhance the leaching rate of rare earth and significantly inhibit the leaching efficiency of aluminum. Leaching agent pH has a greater impact on the leaching efficiency of aluminum, and no effects on that of rare earth. A higher leaching temperature in the tested range could enhance the leaching rate of rare earth and aluminum, but it will largely increase the production cost. The optimum conditions of leaching rare earth and aluminum are 0.1 mol/L ammonium sulfate compounded with 0.032 mol/L ammonium formate, 1:1 liquid:solid (mL/g), pH 5.0 ∼ 8.0 and room temperature. At these conditions, the leaching efficiency of rare earth and aluminum are 92.97% and 37.79% respectively. Moreover, the leaching equilibrium time for rare earth was 320 min shorter than the traditional leaching process by the ammonium sulfate, confirming an acceleration from this novel compound on the leaching of rare earth, especially for the difficult-to-infiltrate weathered crust elution-deposited rare earth ore. The leaching results are well fitted with the shrinking core model with apparent activation energy of 15.83 kJ/mol for RE and 9.60 kJ/mol for Al in the temperature range of 283 ∼ 323 K, indicating a diffusion controlled reaction, and the reaction orders are 1.06 for RE and −1.77 for Al.