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Rare earths are critical to numerous materials and applications underpinning modern civilisation. The majority of the world's rare earth reserves are hosted in the three minerals bastnasite, monazite and xenotime. A key step in the processing of rare earth mineral concentrates is the chemical decomposition of the mineral structure to release the constituent elements. The sulfuric acid bake has historically been, and is also currently, one of the major processes used for this step. Current sulfuric acid bake processes for the Bayan Obo deposit in China and the Mt. Weld deposit in Australia together account for more than half of the world's rare earth production. In the sulfuric acid bake, the rare earth elements are converted to rare earth sulfates which are dissolved in a subsequent water leach. The conditions required to achieve mineral decomposition vary widely for different rare earth minerals. Adjustment of process conditions may often be used to achieve some degree of impurity rejection which is beneficial to downstream processing. This paper reviews the application of the sulfuric acid bake process to ores/concentrates containing mainly monazite, xenotime and bastnasite, and other less common rare earth minerals including euxenite, samarskite, fergusonite, loparite, allanite, eudialyte and pyrochlore. The reported effects of feed mineralogy and process variables such as reaction temperature, bake duration, acid to concentrate ratio and particle size are presented along with a brief review of current understanding of the bake chemistry and water leach results.

Apatite is one of the important host minerals of the rare earth elements (REEs). Separation of REEs from apatite has not been studied extensively compared with other main minerals such as monazite, bastnasite and xenotime. Therefore, it is essential to develop an effective method for recovery of REEs from apatite-based ores. This paper presents the results of laboratory studies for the recovery of rare earth elements from an apatite ore sample by using sulphuric acid leaching, solvent extraction, and precipitation processes. Results showed that the most effective dissolutions of light REEs (>85%) and heavy REEs (>89%) from the apatite ore sample were achieved with a dilute sulphuric acid solution (1 M H2SO4) at a temperature of 20 °C in an hour. The heavy REEs present in the pregnant leach solution were almost entirely extracted using 1.2 M di-(2-ethylhexyl) phosphoric acid (D2EHPA) as a cation exchanger, while the most light REEs remained in the raffinate. The heavy REEs loaded into the organic phase were stripped completely by 3 M H2SO4 solution. REEs remained in raffinate and stripped into the aqueous solution were precipitated as RE oxalates by the addition of 0.08 M oxalic acid.

In this study, the ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HBetTf2N), has been applied directly as a leaching agent on a low-grade rare earth element (REE) ore called Rodberg, from the Fen carbonatite complex ore deposit located in Norway. REE in Rodberg are mainly present in the form of fluoro-carbonates of bastnasite group minerals. Factors studied during leaching were water ratio into ionic liquid, leaching temperature, pulp density (%w/v) and retention time. Leaching of Rodberg ore with HbetTf2N resulted to 65–100% recovery of light REE and 40–60% recovery of heavy REE. Additionally > 80–90% of Ca and Mg content dissolves, whereas the HbetTf2N is selective against iron and silicon resulting in a Fe/Si rich residue. The metals dissolved into HbetTf2N after leaching are stripped with an aqueous acidic solution, regenerating the ionic liquid for reuse. Through acid stripping, REE concentration is doubled in the aqueous acidic solution, for further purification. The carboxylic acid functional group attached to the cation side of HbetTf2N is capable of dissolving bastnasite thus enabling the development of new alternative methods for treating low-grade REE primary resources.

We investigated the calcination of a rare earth carbonatite ore from Niobec in Quebec, Canada, containing bastnasite and monazite. The bastnasite decomposed at a relatively low temperature of approximately 400 °C while the monazite decomposed through solid-state reactions with associated minerals at temperatures exceeding 700 °C. A global kinetic model was developed that represented the calcination kinetics of the whole ore based on the individual calcination kinetics of the minerals nested in the ore. The decomposition of bastnasite and the interaction of monazite with calcium oxide in the ore was used to estimate rare earth oxide production during calcination. Initially, the model was developed for small particles and was thus adapted to larger particle sizes by including a coefficient based on physical particle size cut parameters such as surface area, density, and particle diameter.

Rare earth metals are essential ingredients for the development of modern industry as well as designing and developing high technology products used in our daily lives. Consequently, the worldwide demand of rare earth metals is rising quickly and predicted to surpass the supply by 40,000 tons annually. However, their availability is declining, mainly due to the export quotas imposed by the Chinese government and actions taken against illegal mining operations. This has laid emphasis to exploit and expand technologies to meet the future necessities of rare earth metals. Bastnasite, monazite, and xenotime are their chief mercantile sources, which are generally beneficiated by flotation, gravity or magnetic separation processes to get concentrates that are processed using pyro/hydrometallurgical routes. To develop feasible and eco-friendly processes, R&D studies are being conducted for the extraction of rare earth metals from leached solutions (chloride, nitrate, sulfate, thiocyanate, etc.) using different cationic, anionic and solvating solvents or ions depending on material and media. Commercial extraction of rare earth metals has been carried out using different extractants viz. D2EHPA, Cyanex 272, PC 88A, Versatic 10, TBP, Aliquat 336, etc. The present paper reviews the methods used for the recovery of rare earth metals from primary as well as secondary resources, with special attention to the hydrometallurgical techniques, consisting of leaching with acids and alkalis followed by solvent extraction, ion exchange or precipitation. The piece of comparative and summarized review will be useful for the researchers to develop processes for rare earth recovery under various conditions.