In September 2021, an article entitled "A review of rare earth elements and yttrium in coal ash: content, modes of occurrences, combustion behavior, and extraction methods" by a research team led by Prof. Yao Hong was published in "Progress in Energy and Combustion Science" (IF = 29.394; Volume 88, 2022, 100954), a top international journal in the field of energy. The first author is Postdoc Fu Biao, with Prof. Yao Hong as the corresponding author, and other collaborators include Prof. James C. Hower from the University of Kentucky, Associate Prof. Wencai Zhang from Virginia Polytechnic Institute and State University, Prof. Luo Guangqian and Associate Prof. Hu Hongyun from the Huazhong University of Science and Technology (HUST).
Rare earth elements (REEs) are an indispensable strategic resource in the development of modern and high-tech industries. They are widely used in high-tech fields such as national defense and military industry, aerospace, new energy, new materials, and electronic information, etc.; besides, REEs have been incorporated in the national strategic plans of developed countries like Europe, Japan, and the U.S. for the development of key minerals. Coal and coal ash are important potential sources of REEs. The process of coal combustion can further enrich coal ash with REEs, with the rare earth content of REE-rich coal ash reaching up to 1% to 3%, close to the traditional rare earth deposits (0.1-1*n%), and the proportion of key rare earths exceeds that of most traditional rare earth deposits. Thereby the extraction of rare earth resources from coal ash has become a topic of widespread concern both at home and abroad.
The article focuses on the content and spatial distribution characteristics of China's REEs in coal ash for the first time based on the average value of rare earth content (435.45 ppm) of 15 major coal consuming countries across the world, while discussing the key scientific and technical issues such as the content distribution, occurrence, enrichment pattern and efficient recovery methods of REEs related to the efficient separation and extraction of REEs from coal ash. The REE-rich coal ash area in China can be divided into three zones, and the spatial distribution of rare earth content is estimated as: Zone I (623.5 ppm)> Zone II (430.9 ppm)> Zone III (318.6 ppm); the rare earth reserves in China's REE-rich coal ash are roughly estimated to be 11.3 million tons, accounting for up to 25%-30% of the existing traditional rare earth deposits (44,000,000 t), which is of strategic significance in exploration and development. Then, it summarizes the analytical methods of REE content and form in coal ash based on a variety of modern energy and mass spectrum and spectrum, and discusses the key steps, advantages and disadvantages, scope of application, and existing problems of the various methods. On the other hand, the article further defines the glass-bound state, discrete mineral state and organic-bound state as the main occurrence forms of REEs in coal ash based on the petrography and mineralogy of the REEs in coal ash, and its associated combination with other components. The thermal decomposition and transformation behaviors of different forms of REEs in the coal during combustion in the furnace chamber, and the transformation mechanism of REE forms in the process of flue gas transportation at the chamber tail are summarized and discussed. In-depth discussion of the migration behavior of REEs in coal-fired power plants and the distribution and enrichment pattern of different ash in the dust removal system are made. The high-temperature process and rapid cooling process are considered in discussing the formation mechanism of coal ash silicate melt-glass body-minerals and studying rare earth morphology, with two enrichment mechanisms of REEs as captured by fly ash aluminosilicate melt during coal combustion are proposed and described. The research progress of REE extraction technology in coal ash based on physical separation and wet chemical extraction is introduced, with the advantages and disadvantages of various methods, existing problems and possible solutions pointed out. In the end, it points out the existing problems in the research on the REE extraction and utilization in coal ash, and looks forward into the future research directions, in order to be of scientifically referential value to the high-value utilization of coal ash solid waste from coal-fired power plants in China.
Prof. Yao Hong-led research team has been committed to cutting-edge basic research in the field of solid fuel combustion, focusing on combustion reaction kinetics, pollutant emission and control, and thermal conversion of solid waste and biomass, etc. This research was supported by the National Natural Science Foundation of China (52006082), the China Postdoctoral Science Fund (2019M662586), the U.S. Department of Energy (DE-FE0027167 & DE-FE0029007), and the U.S. National Natural Science Foundation (CBET-1510861).
Paper Link: https://www.sciencedirect.com/science/article/pii/S0360128521000526
