Abstract
Sulfuric acid baking followed by water leaching (ABWL) is commonly used to recover rare earth elements (REEs) and thorium (Th) from refractory REE-phosphate ores like monazite. This study assessed a low- to moderate-temperature ABWL process for a South African monazite ore containing 30.0% total REEs (TREE) and 2.0% Th. The effect of baking temperature (250–310 °C) on mineral transformation and extraction efficiency was examined through XRD, SEM-EDS, and Raman spectroscopy. Direct acid leaching was ineffective, extracting only 31% of REEs and 29% of Th, while dissolving notable impurities such as Fe (61%), P (79%), and Cu (64%). Residue analysis confirmed that the monazite largely remained intact, explaining the poor recovery. Baking at 250 °C increased extraction to 55% REEs and 74% Th. Raising the temperature to 280 °C significantly improved recovery to 92% REEs and 83% Th, with minimal residual monazite detected, indicating extensive breakdown of the phosphate structure. However, increasing the temperature to 310 °C decreased the recoveries to 75% REEs and 26% Th due to sulfur loss and the formation of a Th–anhydrite–silica network that impeded leaching. These findings show that moderate-temperature acid baking effectively breaks down monazite and maximizes REEs and Th extraction.