Minimizing the impact of passivation during allanite-(Ce) decomposition in sulfuric acid media for rare earth recovery

Abstract

Acid decomposition using concentrated H2SO4 at elevated temperature (>200 °C) is a common process to recover rare earth elements (REE) from refractory minerals where the reactions form REE sulfates, which are dissolved during a subsequent water leach. While the decomposition of REE orthophosphates and fluorocarbonates is well-documented, investigations focusing on REE silicates are more limited. The current study focuses on allanite-(Ce), since, in addition to the REEs, the presence of other sulfate-forming cations in the crystalline structure can help better define the decomposition of complex silicates. After treatment using concentrated H2SO4 at temperatures between 175 °C and 225 °C, important passivation effects from the formation of amorphous silica layers were observed. Decomposition occurs rather during the water leach if its temperature is maintained at near-boiling conditions for 20 h, suggesting that water helps attenuate passivation. Consequently, experiments were performed where water was premixed with H2SO4 during the initial acid treatment to lower concentration down to 55 wt%. The water addition drastically enhanced decomposition rate at acid treatment temperatures down to 100–125 °C, where REE recoveries >90 wt% were achieved after a room-temperature water leach. The extent of passivation may be linked to the solubility of produced sulfates. With concentrated acid, low sulfate solubility induces local saturation at the crystal dissolution front leading to diffusion-limited cation transfer across the growing silica layer. When the water content is increased, the sulfate-forming cations can enter in solution providing efficient transport, hence minimizing passivation. This decreases both acid treatment and water leach temperatures, significantly reducing energy consumption without additional reagent costs.