Rare earth elements and yttrium (REY) have attracted considerable attention over the last decade because of their vital roles in clean energy, consumer product, national defense and security applications, among other uses. Due to the retention of REY during coal burning, coal combustion ash is considered as potential alternative sources for REY. Understanding the content, speciation, retention and/or transformation behavior of REY during coal combustion not only expands our knowledge of the combustion behavior of the trace elements in coal, but also provides basis for modeling REY partitioning during coal combustion and for developing economically viable REY recovery technologies. This review makes a critical summary of recent progress in the study of REY in coal ash. The contents and the extraction potentials of REY in coal ash derived from 15 major coal-producing countries worldwide were summarized and evaluated. Various analytical methods for determining REY bulk contents and speciation, together with the solid sample pretreatment, analytical accuracy and precision, advantages and disadvantages were summarized and compared. Modern analytical approaches combined indirect methods (e.g., sequential extraction) shed light on the physical distribution, mineralogy, and the chemical state of REY in coal ash. Three types of REY occurrences in coal ash, including Si-Al glassy association, discrete minerals or compounds, and organic association (bound with unburned carbon) were defined in the review. The glassy association can be further divided into REY minerals closely bound to glass phases and dispersed throughout the glassy structure. REY partitioning in various emission streams, the size distribution, and their enrichment behavior in coal ash were discussed. Thermal behavior and transformation of various REY forms in coal during combustion process, including organic-associated REY, REY phosphates, REY carbonates, clay-bound REY and among others were summarized. Two possible retention mechanisms of REY by aluminosilicate glass at boiler temperature were proposed: the incorporation of the individual REY phases into the glass as inclusions and the diffusion of REY phases throughout Si-Al glass structures in the melting process. Feed coal mineral types, mineral-mineral associations, boiler conditions, and other factors control the retention process. After coal combustion, the speciation of REY in fly ash may be modified by the reactions of REY phases with flue gas components. Further, an overview of REY transformation mechanisms during coal combustion was deeply discussed. Finally, current extraction techniques for REY recovery from coal combustion ash were introduced. Future outlooks and research problems were also identified.

稀土元素和钇 (REY) 在过去十年中引起了相当大的关注，因为它们在清洁能源、消费品、国防和安全应用等用途中发挥着重要作用。由于在燃煤过程中保留了 REY，燃煤灰被认为是 REY 的潜在替代来源。了解煤燃烧过程中 REY 的含量、形态、保留和/或转化行为不仅扩展了我们对煤中微量元素燃烧行为的认识，而且为模拟煤燃烧过程中 REY 的分配和开发经济可行的 REY 提供了基础恢复技术。本综述对煤灰中 REY 研究的最新进展进行了重要总结。对来自全球15个主要产煤国的煤灰中REY的含量和提取潜力进行了总结和评价。总结并比较了测定REY体积含量和形态的各种分析方法，以及固体样品的预处理、分析准确度和精密度、优缺点。现代分析方法结合间接方法（例如，顺序提取）揭示了煤灰中 REY 的物理分布、矿物学和化学状态。审查中定义了煤灰中 REY 的三种类型，包括 Si-Al 玻璃态组合、离散矿物或化合物和有机组合（与未燃烧的碳结合）。玻璃态结合体可以进一步分为与玻璃相紧密结合并分散在整个玻璃态结构中的 REY 矿物。讨论了 REY 在各种排放流中的分配、粒度分布及其在煤灰中的富集行为。总结了燃烧过程中煤中各种 REY 形式的热行为和转化，包括有机伴生 REY、REY 磷酸盐、REY 碳酸盐、粘土结合 REY 等。提出了在锅炉温度下铝硅酸盐玻璃对 REY 的两种可能保留机制：将单独的 REY 相作为夹杂物掺入玻璃中，以及在熔化过程中 REY 相在整个 Si-Al 玻璃结构中的扩散。饲料煤矿物类型、矿物-矿物组合、锅炉条件、和其他因素控制保留过程。煤燃烧后，REY 相与烟气组分的反应可能会改变飞灰中 REY 的形态。此外，深入讨论了煤燃烧过程中 REY 转化机制的概述。最后，介绍了当前从燃煤灰中回收 REY 的提取技术。还确定了未来的展望和研究问题。