Mechanism and kinetic study of rare earth extraction from allanite by direct acid leaching

Abstract

Direct acid leaching was conducted to recover rare earth elements (REEs) from an allanite ore assaying 9,861.4 ppm of REEs. The effects of operational variables, such as acid type, sulfuric acid (H2SO4) concentration, temperature, solid/liquid (S/L) ratio, and particle size, on the total REE (TREE) recovery were systematically examined. The REE mineralogy and the involved leaching mechanisms were investigated by electron microscopy characterization and residue re-leaching tests. It was found that approximately 80% of REEs was extracted using 1 M H2SO4 at 75 °C for 2 h. Increasing temperature significantly improved the REE extraction, while changing acid type, H2SO4 concentration, and particle size did not cause noticeable influences on the extraction efficiency. The low fraction of Si leached (<4.0%) suggested that the extraction did not significantly alter the crystal structure of the dominant minerals. The leaching kinetics of REEs was fast within the first 10 min, followed by a much slower stage (i.e., 10–120 min). The kinetic modeling results showed that both stages were likely controlled by both chemical reaction and diffusion, implying a mixed control mechanism. The activation energies for REE leaching using 1 M H2SO4 were determined to be 20.3 kJ/mol for the 0-10 min stage and 10.8 kJ/mol for the 10-120 min stage. The residue re-leaching results suggested that the initial fast leaching kinetics was caused by the preferential dissolution of metamict allanite, while the following slower leaching stage was due to the dissolution of difficult-to-leach REE-bearing minerals, such as well-crystallized allanite.