Over 90% of Bayan Obo rare-earth ores (Bayan Obo, Inner Mongolia, China) are decomposed in rotary kilns using concentrated sulfuric acid, typically under a reducing atmosphere generated by internal heating. However, externally heated rotary kilns, offering precise temperature control and automation, are emerging as a promising alternative. This study investigates the decomposition behavior and reaction mechanisms of Bayan Obo mixed rare-earth concentrate under an oxidizing atmosphere with concentrated sulfuric acid. Experiments were conducted at temperatures ranging from 300 to 1000°C, with Fe 3 O 4 added to control the Fe/P molar ratio between 1 and 6. The roasted products and leach residues were analyzed using inductively coupled plasma–optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). Results showed that Fe 3 O 4 significantly enhanced rare-earth element leaching efficiency and markedly promoted phosphorus removal, whereas its effects on the removal of Th and Ca were relatively limited compared to the control experiments. The improved REE dissolution was attributed to the formation of iron phosphate phases, which prevented the formation of insoluble rare-earth–calcium–thorium polyphosphates. Additionally, at high temperatures, iron oxide formation hindered rare-earth oxidation, favoring the conversion of rare earths into soluble sulfate forms, thereby facilitating leaching. This study offers valuable insights into Fe 3 O 4 ’s role in sulfuric acid oxidation roasting and its potential for advancing rare earth extraction technologies.
Zhang et al. (Wed,) studied this question.