Stanislav V. Vassilev

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.

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.

This study explores the synergistic potential of ladle slag (LS) and sunflower shell fly ash (SSFA) in alkali-activated binder systems, focusing on their chemical and mineralogical characteristics and the influence of SSFA addition on the mechanical performance of LS-based pastes. X-ray fluorescence and XRD analysis revealed that LS is rich in CaO and latent hydraulic phases such as γ-belite and mayenite, while SSFA is dominated by K₂O, SO₃, and KCl/K₂SO₄ phases, reflecting its biomass origin. Infrared spectroscopy and thermal analysis confirmed the presence of carbonate, hydroxide, and hydrate phases, with SSFA exhibiting more complex thermal behavior due to volatile-rich composition. When used alone, LS produced weak binders; however, a 10 wt% SSFA addition tripled compressive strength to nearly 30 MPa, indicating a significant activation effect. Further increases in SSFA content led to strength reduction, likely due to increased porosity and excess salts. Microstructural analysis showed that SSFA promotes the formation of AFm phases such as Friedel’s salt and hydrocalumite, altering hydration pathways and enhancing early strength through chemical activation and carbonation processes. The findings highlight the potential of combining LS and SSFA as a sustainable binder system, offering a waste-derived alternative for low-carbon construction materials.

This study explores the synergistic potential of ladle slag (LS) and sunflower shell fly ash (SSFA) in alkali-activated binder systems, focusing on their chemical and mineralogical characteristics and the influence of SSFA addition on the mechanical performance of LS-based pastes. X-ray fluorescence and XRD analysis revealed that LS is rich in CaO and latent hydraulic phases such as γ-belite and mayenite, while SSFA is dominated by K₂O, SO₃, and KCl/K₂SO₄ phases, reflecting its biomass origin. Infrared spectroscopy and thermal analysis confirmed the presence of carbonate, hydroxide, and hydrate phases, with SSFA exhibiting more complex thermal behavior due to volatile-rich composition. When used alone, LS produced weak binders; however, a 10 wt% SSFA addition tripled compressive strength to nearly 30 MPa, indicating a significant activation effect. Further increases in SSFA content led to strength reduction, likely due to increased porosity and excess salts. Microstructural analysis showed that SSFA promotes the formation of AFm phases such as Friedel’s salt and hydrocalumite, altering hydration pathways and enhancing early strength through chemical activation and carbonation processes. The findings highlight the potential of combining LS and SSFA as a sustainable binder system, offering a waste-derived alternative for low-carbon construction materials.

Toxic Hg species pose a global ecological threat due to the significant anthropogenic emissions of Hg to the atmosphere, particularly during coal combustion. An investigation about the content, association and modes of occurrence of Hg in diverse biomass ash (BA) varieties and its behaviour during biomass combustion was performed. These BAs belonging to woody, herbaceous, agricultural and aquatic biomass groups were examined with different chemical and mineralogical methods, as well as thermal and leaching procedures. The Hg contents in BAs are low and vary between 0.0032 and 0.0452 ppm (mean 0.0225 ppm). About 60–99% (mean 87%) and 63–100% (mean 91%) of Hg in biomass were volatilized at 500 °C and 700 °C, respectively, and only limited amounts of Hg were captured by BAs enriched mostly in salts such as carbonates, oxyhydroxides, phosphates, sulphates, and chlorides. The affinity of residual Hg in the BA system is towards relatively stable Fe-, P-, Ti- Al-, K-, and Si-bearing minerals (especially K aluminosilicates and Fe oxides), as well as less stable chlorides (particularly sylvite). The potential mode of Hg occurrence in BA is likely in the form of impurities in the above minerals. Alternative renewable fuels poor in Hg such as appropriate sustainable biomasses (0.001–0.043 ppm Hg) are suggested to partially or completely substitute the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants and thus minimize the ecological problems related to this element.

Toxic Hg species pose a global ecological issue due to the significant anthropogenic emissions of Hg to the atmosphere. То asses the behavior of Hg during combustion of diverse biomasses, here we discuss published mineralogical and geochemical data on different biomass types and present preliminary Hg content results on their ashes. About 63–100% of Hg in these biomasses were volatilized at 700 °C and the Hg contents in their biomass ashes are low (0.0032–0.0452 ppm). Only limited parts of Hg were captured by specific biomass ashes enriched in oxyhydroxides, sulphates, chlorides, carbonates, and phosphates. Alternative renewable and sustainable biomasses poor in Hg (0.001–0.043 ppm) are suggested to substitute partially or completely the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants to minimize the ecological problems related to this element.

Toxic Hg species pose a global ecological problem due to the significant anthropogenic emissions of Hg into the atmosphere, especially during coal combustion. A study about the content, association and potential modes of occurrence of mercury and their ecological significance in eight biomass varieties belonging to woody, herbaceous, agricultural and aquatic biomass groups was conducted based on a combination of different chemical and mineralogical analyses and leaching procedures. The Hg contents in these biomasses vary between 0.003 ppm and 0.043 ppm (mean 0.013 ppm), which are about an order of magnitude lower than the Clarke value of coal (0.10 ppm). It seems that Hg occurs in both inorganic and organic matter of biomass, as the preferable association and potential modes of occurrence of this element include inorganic matter, hemicellulose, and water-soluble Cl-, S-, N-, and Na-bearing chlorides, sulphates, and nitrates such as halite, sylvite, arcanite, Ca nitrate, and nitrocalcite in the biomass system studied. Alternative renewable and C-neutral solid fuels poor in Hg such as appropriate sustainable biomasses are suggested to substitute partially or completely the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants to minimize the ecological problems related to this element.

The contents of 49 trace elements in sub-bituminous Pernik coals and their waste products from preparation and combustion processes were investigated. The studied coals have trace element contents higher than the respective Clarke values for brown coals and some of them may pose environmental concerns. The elements Li, Rb, Cs, Ba, Sc, Y, La, Ce, Nd, Sm, Eu, Er, Ga, Zr, Sn, V, Nb, Ta, W, F, Cu, Zn, In, Pb, Cr, Co, Ni, and Th in the feed coals have concentrations that exceed twice the Clarke values. Most element contents in bottom ash are enriched compared with those in feed coal. Some of the volatile elements are equal or significantly depleted including Sn, Mo, Sb, F, Bi, Cd, Ge, and Pb. Fly ash has higher contents of Ga, Zr, Hf, Sn, V, Nb, Mo, and F in comparison with bottom ash. Most elements have a significant positive correlation with ash yield, indicating their inorganic association. The mixed wastes (coal slurry, bottom ash, and fly ash) in the disposal pond are slightly depleted of most of the elements studied with the exclusion of Cl, Ba, and Br. The Pernik coals and their waste products are unpromising for the extraction of REY due to their low element contents.