Notes on the occurrence of phosphate mineral relics and spheres (phosphospheres) in coal and biomass fly ash

Abstract The occurrence of phosphorus minerals in coal is common, and its study is mainly related with the metallurgical coke making. Large amounts of phosphorus are emitted from coal and biomass combustion units and less direct methods, such as carbon black, are used for tracking stationary combustion sources of phosphorus. During the study of fly ash from coal burning power plants in the state of Jharkand, India, using XRD, SEM/EDS, XRF, ICP-MS and leaching tests, it was found that P-bearing spheres have unique visual characteristics. Therefore, this study is thought to represent the first time that P-spheres in coal fly ash are described and herein named as “phosphospheres”. To complement their characterization and how they form other fly ash types such as Quercus suber cork and chicken litter fly ashes were also studied using SEM. Under the SEM the phosphospheres are brighter than the associated aluminosilicate spheres, they may show a spongy (or “punctuated”) structure, have a high P-peak in the EDS profile, and their cross-sections typically show a “pomegranate” texture resulting from immiscible silicate and phosphate melts and the segregation of the P when the glassy matrix is still in a fluid stage. Therefore, with these unique characteristics, phosphospheres are easily recognized, and may be used as a direct method for phosphorus tracking.