Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC₂O₄·2H₂O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

中文翻译：

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从粉煤灰中回收稀土元素 (REE) 的绿色方法

由于对稀土元素 (REE) 的需求不断增长以及 REE 易受潜在供应中断的影响，人们越来越关注从粉煤灰 (CFA) 等废物流中回收 REE。同时，CFA 作为美国 (US) 的大型工业废物流，造成了巨大的环境和经济负担。从 CFA 中回收 REE 是解决 REE 稀缺问题的有希望的解决方案，也为 CFA 管理带来了机遇。本研究展示了一个用于从 F 类和 C 类 CFA 中回收 REE 的绿色系统，该系统由三个模块组成：使用柠檬酸盐的 REE 浸出、使用草酸盐的 REE 分离和浓缩，以及使用来自模块 I 和 II 的二次废物的沸石合成。在模块 I 中，分别使用 pH 4 的柠檬酸盐从 F 类和 C 类 CFA 样品中浸出约 10% 和 60% 的稀土元素。₂ O ₄ ·2H ₂ O)，而其他微量金属则留在溶液中。在模块 III 中，沸石是使用模块 I 和 II 中的废物合成的。这项研究的特点是成功回收 REE 和二次废物的升级再循环，解决了 REE 回收和 CFA 管理方面的挑战。