The results of preliminary studies of Bulgarian natural raw materials as possible sources for the synthesis of high-quality ceramics of the “yellow” pavers type are presented. For the purposes of comparative analysis, a short geological-mineralogical literature review of Bulgarian and Hungarian deposits of marl raw materials and of Bulgarian deposits of non-marl raw materials rich in magnesium is made. The capabilities of the X-ray fluorescence (XRF) analysis and the powder X-ray diffraction (PXRD) techniques for the rapid acquisition of reliable data on the phase-chemical characteristics of the investigated materials have been demonstrated. A relation is made between these characteristics and the physicochemical and mechanical properties of possibly synthesized products (color, hardness, etc.). Considerations for choosing methods for processing the primary data are presented, allowing for some simplification without affecting the reliability of the results. The latter have been analyzed and possibilities for improving the qualities of the starting raw material have been discussed in order to obtain products with desired characteristics.

Eight biomass ashes (BAs) generated from agricultural, herbaceous, woody and aquatic biomass types were studied for the occurrence of 14 nutrient elements (NEs) and 6 contaminant elements (CEs) to evaluate their significance for potential soil fertilization. For that purpose, a combination of different mineralogical and chemical analyses, and leaching procedures was used. The data show that the contents of NEs in BAs are highly variable; however, the concentrations of B, Ca, Cl, Cu, K, P, S, and Zn, and, to a lesser extent, Mg, Mn, Mo, Na, and Ni in BAs are perspective for potential soil fertilization or soil conditioners. The inorganic matter of BAs comprises amorphous matter and different minerals among carbonates, silicates, chlorides, sulphates, oxyhydroxides, and phosphates. BAs have high yields of water-soluble solutions with alkaline character, which are favourable for BA use. NEs such as Ca, Cl, K, Mg, Na, P, and S occur significantly in water-soluble and bioavailable salts. Various NEs and CEs in BAs fulfilled the available regulation limits for fertilizing products, excluding some problematic concentrations of Cd, Co, Cr, Fe, Mo, Ni, and P for specific BAs. Therefore, each BA needs to be studied on a case-by-case basis for that purpose. The establishment of advanced regulatory framework for BA utilization as fertilizing products is required to generate strict limit values of NEs and CEs in BA leachates. The water-extractable proportions of elements from BA and their release in time are very important to predict the supply of bioavailable NEs or CEs for crops.

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.