Abstract Fly ash represents a promising alternative source of rare earth elements (REE). However, information on REE containing mineral phases and their association with other fly ash components, vital for REE recovery from fly ash, is currently lacking. Herein, the mass fraction, distribution, crystallography and solid-state chemistry of REE, U and Th in Nigerian simulated fly ash samples were characterised using a range of laboratory and synchrotron x-ray based analytical techniques to underpin future extraction methodologies. Inductively coupled plasma mass spectrometry following full-acid digest of forty-five samples revealed recoverable average total REE content which ranged between 442 mgkg−1and 625 mgkg−1, comprising over 30 wt% of the critical REE Nd, Eu, Tb, Dy, Y and Er. These REE within the fly ash samples were found to be most frequently associated with discrete monazite, xenotime and Y-bearing zircon mineral particles, with the former the most detected, which could be beneficiated through gravity separation. Analysis of monazite particles isolated from the composite samples through a complimentary suite of analytical synchrotron radiation techniques revealed a core-shell pattern, with the shell rich in colocalised Ce, Nd and La, and the core enrich in both U and Th. Ce in monazite was found to exist in a mixed trivalent and tetravalent oxidation state, with the monazite structure amorphized due to the high temperature combustion process. Such results demonstrate the strong co-association and physical distribution of REE, U and Th within monazite in fly ash; knowledge of which can subsequently be used to optimise or develop a more selective, cost-effective and environmentally friendly solvent extraction methodology, by targeting the strongly colocalised and surface bound REE in fly ash monazite particles.

中文翻译：

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粉煤灰中稀土元素浓度、分布、形态、晶体学和固态化学的高级分析评估

摘要 飞灰是一种很有前景的稀土元素 (REE) 替代来源。然而，目前缺乏关于含有矿物相的 REE 及其与其他粉煤灰成分的关联的信息，这对于从粉煤灰中回收 REE 至关重要。在此，使用一系列实验室和同步加速器 X 射线分析技术对尼日利亚模拟飞灰样品中 REE、U 和 Th 的质量分数、分布、晶体学和固态化学进行了表征，以支持未来的提取方法。对 45 个样品进行全酸消化后的电感耦合等离子体质谱显示，可回收的平均总 REE 含量介于 442 mgkg-1 和 625 mgkg-1 之间，包括超过 30 wt% 的关键 REE Nd、Eu、Tb、Dy、 Y和Er。发现飞灰样品中的这些 REE 最常与离散的独居石、磷钇矿和含 Y 的锆石矿物颗粒相关，前者检测得最多，可以通过重力分离进行选矿。通过一套免费的分析同步辐射技术对从复合样品中分离出来的独居石颗粒进行分析，揭示了一种核壳模式，壳层富含共定位的 Ce、Nd 和 La，核富含 U 和 Th。发现独居石中的铈以混合三价和四价氧化态存在，由于高温燃烧过程，独居石结构非晶化。这样的结果证明了飞灰中独居石中稀土、铀和钍的强共缔合和物理分布；