Kinetics of column leaching of rare earth and aluminum from weathered crust elution-deposited rare earth ore with ammonium salt solutions

• NH4Cl and NH4NO3 were mixed as leaching agent to enhance the leaching of RE from weathered crust elution-deposited RE ore.
• Effects of column leaching conditions on leaching efficiency and leaching kinetic of RE and Al were discussed.
• The leaching process could be described by shrinking-core model and the leaching rate is controlled by inter-diffusion.

In this study, a mixture of ammonium chloride and ammonium nitrate was used as a leaching agent to improve the leaching efficiency and leaching rate of rare earth (RE) from weathered crust elution-deposited RE ore. By performing the column leaching experiments, effects on the leaching efficiency and leaching rate of RE, such as the molar ratio of the two components, concentration of ammonium (NH4+), liquid/solid ratio and pH of the leaching agent, were investigated. In addition, the effects of above parameters on the leaching performance of aluminum (Al), which is the main impurity in RE leachate, were also studied. It was found that the optimal conditions of column leaching were a 1:1 (molar ratio) of NH4Cl and NH4NO3, 0.2 mol/L of ammonium concentrate, 2:1 liquid:solid (mL/g) and a leaching agent with a pH between 4.0 and 8.0. The maximum leaching efficiency of RE could be 91%. Moreover, the leaching equilibrium time for RE was 25 min shorter than for Al, which is of benefit to reduce the leaching of Al. The leaching process was analyzed using the shrinking-core model to examine the kinetic parameters and rate controlling step. The leaching rate was controlled by the inter-diffusion of the reactants and leaching products through the mineral porous layer. An empirical equation based on the apparent rate constant and ammonium concentration was established for process optimization, and the reaction orders were 1.92 for RE and 1.48 for Al. The results of this study may play a vital role in developing a high efficient leaching technology for weathered crust elution-deposited RE ore.