燃煤电厂稀土元素的迁移转化与提取技术

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.