Preliminary evaluation on the occurrence of critical elements in Bulgarian coal fly ashes and their potential industrial significance

A preliminary evaluation on the occurrence of 31 critical elements (CE) in Bulgarian coal fly ashes (FA) and their potential industrial significance was conducted based on our own studies for eleven thermoelectric power plants. The chemical and phase-mineral composition of these FA is variable and depends mostly on the type of combusted coal. The qualitative presence of common minerals and phases in different FA is normally similar; however, their origin and quantitative occurrence is variable. It was found that 17 CE such as Ba, Be, Co, Ga, Mg, Ni, P, Sc, Sr, Tb, Ti, V, and especially As, Cu, Li, Mn, and Sb in definite FA are the most perspective for recovery. Some strong associations in FA were identified such as: Al, K, Si, Ti and Fe with rare earth elements; Fe with Co, Cu, and Ni; Ca with Mn and Sr; P with Ba, Ge, and Li; and Ti with Ni and V; which are important for multi-elements recovery. The modes of CE occurrence in FA are highly variable and complex; however, discrete minerals of Ba, Ce, Cu, La, Mg, Mn, Nb, P, Sb, Sr, Ti, W, and Y were identified, which is important from industrial and ecological points of view. FA are promising sources for extraction of some CE that can yield fractions meeting the cut-off grade for industrial recovery after some preliminary pre-treatment such as magnetic, size and density separations, as well as water leaching; however, much more detailed investigations are required for that purpose.