2024 Karenlampi Low temperature sulfuric acid leaching of allanite

Allanite is a common REE-bearing silicate mineral, which is found in potentially mineable abundances in many areas but has not been economically exploited for the REEs in an industrial scale. Conventional methods, such as sulfuric acid baking at high temperatures (200 to 650 °C) followed by extended water leaching near boiling temperature, have proven effective in decomposing allanite but come with substantial economic and environmental drawbacks. This study presents an alternative approach, investigating the dissolution behavior of allanite at low temperatures through direct treatment of an allanite concentrate with sulfuric acid. Through parametric optimization, the study reveals the practical and selective dissolution of allanite from a relatively coarse concentrate sample (particle size distribution D50 = 327 µm) after a 3-h leaching period with a mild dilute (2 M) H2SO4 solution at 22 °C, yielding overall recoveries of approximately 80% for light REEs and 60% for heavy REEs. The dissolved gangue elements, predominantly derived from allanite (e.g., Th, Fe, Al, Si, and Ca), exhibit contents in the leach solution comparable to or slightly higher than those reported in other studies utilizing acid bake–water leach processes. The notable efficiency of low-temperature sulfuric acid extraction is likely attributed to the heightened chemical reactivity of metamict allanite-(Ce) within the investigated concentrate. While these results are encouraging and demonstrate the potential of low-temperature leaching for allanite concentrates, additional research is necessary before testing the procedure on a larger scale.