Leaching kinetics of lanthanum in sulfuric acid from rare earth element (REE) slag

• Leaching of lanthanum oxide from REE slag was studied in the sulfuric acid.
• A two stage shrinking core model was proposed as a leaching mechanism.
• Rate determining step was determined by chemical reaction and ash layer diffusion.
• Activation energy was found to be 10.0 kJ mol− 1 in the first chemical reaction step.
• Activation energy was found to be 24.8 kJ mol− 1 in the second ash layer diffusion step.

REE slag was obtained from reduction smelting of magnetite ore containing monazite. Using the REE slag, the leaching kinetics of lanthanum from the slag was studied under the conditions of slurry density of 1.5 g slag per L of 0.3 M H2SO4, agitation of 750 rpm and temperature ranging from 30 to 80 °C. After five hours of leaching, the lanthanum was completely leached from the slag. Leaching kinetics can be described by the shrinking core model for spherical particles of unchanging size and leaching was found to occur in two stages. The initial leaching rate was controlled by chemical reaction. Using the Arrhenius equation, the apparent activation energy of the chemical reaction rate controlling step was determined to be 10.0 kJ mol− 1. As leaching of the lanthanum increased, the reaction rate was controlled by diffusion through an ash layer. The apparent activation energy of the second stage was determined to be 24.8 kJ mol− 1. The leaching rate and extent of leaching of lanthanum from the slag increased with an increase in leaching temperature.