近年研究发现，煤是重要的潜在稀土资源（REEs）来源，相比原煤，煤燃烧过程使稀土元素进一步富集至煤灰，从煤灰中提取稀土元素已成为国内外重要议题。针对煤灰中稀土元素的高效分离与提取涉及到的稀土元素赋存形态、稀土元素分布与富集规律、煤灰中稀土高效浸提回收3个关键科学问题，系统总结了煤、煤灰中稀土元素赋存形态特征。煤中稀土元素以矿物形式赋存为主，包括稀土磷酸盐、碳酸盐等稀土主矿物，以类质同象分布在硅酸盐、磷灰石等矿物晶格或以吸附态形式赋存于黏土矿物中，其具体种类和形式受成煤盆地物源类型、沉积环境、火山活动、后期岩浆/热水循环等地质活动影响；煤灰中稀土赋存形态不同于煤，一方面其化学态和晶体/配位结构发生一定程度变化，另一方面颗粒更加细小与分散，包裹、晶格替代等现象更常见。煤中不同形态稀土元素在炉膛内可能发生矿物破碎、分解、氧化、矿物间烧结、熔融以及矿物间化学反应等转化行为，燃烧后烟气输运过程中稀土可经历硫酸化、氯化等二次转化过程。总体而言，煤燃烧过程稀土主要向飞灰中迁移富集，不同级别飞灰中稀土总量差异不大，但轻稀土/重稀土质量比呈向细颗粒后排除尘器飞灰减小的趋势。基于燃煤过程煤中矿物-稀土高温转化行为与降温过程中灰成分形成过程，提出并描述了燃煤过程中铝硅酸盐捕虏稀土元素的2种富集机制。最后，以我国燃煤电厂为例，总结了不同煤种、炉型、灰类型对煤灰中稀土浸出率的影响，并以此为基础，介绍了基于物理分选和化学湿法提取的煤灰中稀土元素提取技术。未来研究应围绕煤灰中稀土元素超微结构识别与定量分析方法、燃煤工况条件-煤灰中稀土赋存形态-稀土浸出率关系、煤灰中稀土高效浸出方法与新型绿色药剂开发、煤灰中其他关键金属协同提取利用、煤灰提取残渣大规模综合利用5个方面开展系统与深入研究，为缓解我国未来稀土等关键金属资源危机、燃煤电厂煤灰固废资源化高值化利用开辟新路径。

Coal and coal ash have been regarded as important alternative sources for REEs （Rare earth elements）. Compared with coal, the combustion of coal can enrich REEs in coal ash and thus the recovery of REEs from coal ash is a hot issue in both domestic and foreign research institutes. Efficient extraction of REEs from coal ash must understand the speciation of REEs in coal ash, the distribution and enrichment trends in coal ash, and develop novel technologies for REEs extraction and separation from coal ash. Accordingly, to provide knowledge base and insights into these issues, this review summarized the speciation of REEs in coal and coal ash. REEs in coal mainly exists as inorganic mineral forms, including but not limited to REEs phosphates, REEs carbonates, silicates, adsorbed to clay minerals. Detailed REEs speciation in coal relied on the sediments sources of the coal basin, sedimentary environment, epigenetic igneous activities, and the circulating of hydrothermal solutions in the coal seams. Compared with coal, REEs in coal ash is different with regard to their different chemical states and structure and physical associations such as particle size and encapsulation in aluminosilicate glass. Then, the combustion behavior of REEs including thermal composition, fragmentation, oxidation, sintering, and mineral-mineral chemical reactions were discussed. After coal combustion, REEs in the flue gas may react with acid gas to form REEs sulfates or chlorides. The distribution and enrichment of REEs in coal-fired power plants were summarized. REEs mainly transported to coal fly ash during coal combustion, where no significant variations can be observed among different hopper ashes. Based on these results, two mechanisms were proposed to elucidate the capture processes of REEs in coal fly ash in the high-temperature combustion process and flue gas cooling process. Current REEs recovery methods from coal ash were reviewed and they can be classified as physical beneficiation and hydrometallurgical extraction. Future work should focus on developing new methods for measuring the micro-structure of REEs and quantifying the REEs species in coal ash, understanding the relationship of coal combustion conditions-REEs speciation in ash-REEs extractability from coal ash, designing high-efficient leaching methods and green reagents for extraction and separation of REEs from coal ash,co-extraction of other critical metals from coal ash, and comprehensive disposal and utilization of the extraction ash residue. 

0 引言

1 燃煤过程稀土元素迁移转化规律

   1.1 煤及煤灰中稀土赋存形态特征

   1.2 炉膛尾部烟气输运过程稀土元素形态转化规律

2 稀土元素在燃煤电厂分布与富集规律

   2.1 燃煤电厂稀土元素分布特征

   2.2 燃煤电厂稀土分布规律的影响因素

   2.3 煤灰中稀土元素富集机理

3 煤灰中稀土元素提取方法

   3.1 物理分选

   3.2 化学湿法提取

4 结语

5 展望