[en] Full text: Rare earth elements are used in a myriad of technological applications that underpin modern civilisation. These applications span industry, defence and private sectors, creating a strong dependence in developed nations on a consistent supply of this group of 16 elements. In the last decade, the supply risks associated with a Chinese domination of the global market has driven strong interest in developing commercially viable extraction processes for other rare earth deposits around the world. Processes for extraction of rare earth elements from rare earth ores are typically complex due to low concentrations of rare earths, the presence of radioactivity in the ore and the large number of co-occurring rare earth elements which must be separated before use. Processing can be broken down into three main stages: (a) physical separation of gangue (unwanted) minerals to produce a rare earth mineral concentrate, (b) chemical decomposition of the rare earth mineral and (c) purification and separation of individual rare earths to produce saleable products. The major industrial process for the chemical decomposition step, used by both China and Australia, is the sulfuric acid bake. This simply involves mixing the mineral concentrate with concentrated sulfuric acid and heating to over 200°C. The rare earths are converted into soluble sulfates and are dissolved in a subsequent water leach to produce a rare earth containing solution ready for further purification. Although the sulfuric acid bake process is well established in industry, a fundamental understanding of the chemistry involved is severely lacking. Improving this understanding could have great value in assisting development of economic extraction processes for new rare earth deposits. In this study, the effects of variables such as ore composition and bake temperature on reaction processes was examined using XRD, SEM-EDS and FT-IR. The impact of reaction processes on extractions of rare earths and impurities was also explored. The effect of ore composition was studied by addition of specific common gangue minerals to a rare earth mineral concentrate. In the absence of gangue minerals it was found that rare earth and impurity extractions were high for bake temperatures of <300°C. For bake temperatures >300°C the extraction of impurities decreased sharply, while the extraction of rare earths decreased slightly. The decreased extractions were found to be due to formation of insoluble phosphate phases, which partially incorporated rare earths. Addition of apatite, a calcium phosphate, exacerbated this effect by acting as a source of additional phosphate, and led to large decreases in rare earth extraction for bake temperatures of >300°C. Addition of iron oxide minerals had the opposite effect, acting to increase rare earth extractions. The mechanism was through incorporation of iron into the insoluble phosphate phase in preference to the rare earths, thereby allowing the rare earths to remain as soluble sulfates. These results highlight the strong effects of ore composition and bake temperature in determining rare earth and impurity extractability and are key to understanding and improving extraction processes for rare earth ores. (author)