Distribution and occurrence of uranium in coal and its migration behavior during the coal utilization

Coal and uranium resources are closely related in spatial disposition as well as mineralization, and the reasonable development, utilization and pollution control of them play an essential role in the sustainable development of our national economy and society. Based on a large number of literature research and the previous results of our research group, some conclusions were drawn with regard to the distribution and occurrence of uranium in coal `and its migration in the process of coal preparation, combustion and leaching:(1) The enrichment and mineralization of uranium in coal has reflected the evolution of coal-forming tectonics:The U-rich coal in Southwest China is mainly related to Emei Mountain basalt and fault structure; the U-rich coal in Northwest China is generally located in depression and open area of fault-depression basin, and is connected with overlying sandstone-type uranium deposit. (2) Uranium is mainly combined with organic matter(mainly humic acid) in coal. It can occur in the form of micro-fine uranium bearing minerals, and is closely associated with organic sulfur and sulfide in U-rich coal. Therefore, in the process of coal preparation, whether gravity separation or flotation, the removal rate of uranium is not high(the highest is 68.3%), and uranium of some coal even enriched in clean coal during flotation; uranium will volatilize to the atmosphere more or less in the form of gas phase during coal combustion. (3) Generally, U-rich coal is also enriched with V, Mo, Se, Re, Cr and other variable valence elements in high valence, which is related to the strong reduction environment caused by the burial depth and decomposition of organisms; the precipitation enrichment of those constant-valence elements, such as Sc, Y, La, is mainly connected with the acidification conditions of humic acid formation. These paragenetic assemblage elements show consistent migration behavior during U-rich coal separation and the leaching process of coal gangue. (4) Uranium is mainly enriched(increase in the form of magnitude) in fly ash and bottom ash during coal combustion in power plant. Generally, with the increase of pH value of leaching solution, the leaching concentration of uranium in fly ash decreases; the extraction rate of it decreases with the increase of ashing temperature. The results provide reference and basis for uranium resource utilization and environmental pollution control.