Abstract The stability of Rare Earth Elements (REE), both sulfates and phosphates, was investigated using monazite ore, a residue from a synthetic rutile manufacturing process and a high-grade lanthanum sulfate sample. It was demonstrated that REE sulfates are converted into phosphates at temperatures within the range of 450–800 °C. This reaction depends on excess phosphorus present in compounds that are less stable than monazite. The presence of high partial pressures of P2O5 in the system will reconvert REE sulfates into insoluble REE polyphosphates – REE(PO3)3. Relatively high partial pressure may be attained by the decomposition of phosphorus compounds present in the sample, such as monocalcium phosphates and phosphoric acid generated by the addition of sulfuric acid. The direct contact with phosphoric acid leads, in turn, to the formation of monazite-like compounds. The gas–solid mechanism is believed to be the most suitable due to the easier access of the gas phase to the REE sulfate particles, thus making it possible to achieve the drastic decrease (conversion into phosphate) in REE extraction observed at 800 °C. This study’s results are expected to contribute to a better understanding of the factors favoring selectivity in REE extraction over iron from monazite ores.

中文翻译：

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镧在硫酸盐和磷酸盐体系中的稳定性以及对选择性稀土提取的影响

摘要 使用独居石矿石、合成金红石制造过程中的残余物和高品位硫酸镧样品研究了稀土元素 (REE)（硫酸盐和磷酸盐）的稳定性。结果表明，硫酸稀土在 450–800 °C 范围内的温度下会转化为磷酸盐。该反应取决于存在于比独居石不稳定的化合物中的过量磷。系统中存在高分压的 P2O5 会将 REE 硫酸盐重新转化为不溶性 REE 多磷酸盐 – REE(PO3)3。通过分解样品中存在的磷化合物，例如通过添加硫酸产生的磷酸一钙和磷酸，可以获得相对高的分压。与磷酸的直接接触反过来又会导致 形成独居石类化合物。气固机制被认为是最合适的，因为气相更容易接近硫酸稀土颗粒，因此可以实现在 800 °C 下观察到的稀土萃取的急剧减少（转化为磷酸盐）。预计本研究的结果将有助于更好地了解有利于从独居石矿石中提取 REE 的选择性而不是铁的因素。