Some features of the composition and properties of ash slag and fly ash from the burning of Ekibastuz coals

Introduction. According to the Russian Ministry of Natural Resources, the country currently generates 22-25 million tons of fly ash and slag waste per year, and up to 10% is disposed. By the Government Order of the Russian Federation No. 1557-p dated June 15, 2022, the “Comprehensive Plan for increasing the volume of ash and slag waste disposal of hazard class V” was approved. Ekibastuz coals are the highest ash fuel coals used at thermal power plants in the Russian Federation. The properties of fly ash and ash slags depend on the type of fuel being burned - its mineral part (overburden), as well as other technological features of the processes of preparation, combustion, capture and disposal of materials. That is why the properties of these materials should be considered in relation to a specific fuel.

Methods and materials. The object of the study was fly ash and ash slag obtained by the combustion of Ekibastuz coals at Russian thermal power plants. With the use of physical and chemical analysis methods, the chemical and mineralogical composition was studied, the physical properties and environmental parameters of these materials were determined.

Results. As a result of the conducted research, the chemical, mineralogical and phase composition of fly ash and ash slag of Ekibastuz coals have been established. The specific features of physical properties have been identified and the causes of their formation have been identified. Specified data on the environmental characteristics of fly ash and ash slag have been obtained.

Conclusion. Ekibastuz coals are characterized by the highest ash content among all fuel coals, reaching 45%, which necessitates the active involvement of ash and slag wastes in the country’s construction industry. A number of specific properties must be taken into account for the effective use of fly ash and ash slag formed during the combustion of these coals. The fly ash of Ekibastuz coals, characterized by a superacid chemical composition, does not demostrate the properties inherent in mineral binders. However, the fly ash obtained by dry capture has the ability to interact with hydrolysis products of mineral binders and be involved in hydration processes, which indicates its potential activity in building composites. Kaolinite, which dominates the mineralogical composition of the Ekibastuz coal rock with a limited presence of fluxes, necessitates high temperatures for modification and thermal activation of the mineral component. This feature has a direct effect on the phase composition of the formed fly ash and ash slag, determining their characteristics and properties. The dissociation of minerals in finely ground coal stimulates the formation of both closed and open microporosity in ash particles. This process leads to a significant increase in their specific surface area and chemical activity, determining the physical properties of fly ash and ash slag. The slag component from the burning of Ekibastuz coals is heated only to the sintering stage, but not melting. This determines the reduced density and mechanical properties of the ash slag. Ekibastuz coals are characterized by a high content of fusain in the organic part. The petrographic characteristics of this organic mass determine the increased resistance of carbonaceous residues to aggressive influences. The radioactivity of the Ekibustuz fly ash and ash slag does not exceed the permissible limit for classifying them to the 1-st hazard class according to the specific effective activity of the latter. Ekibastuz ash and slag wastes have been assigned the V-th hazard class for the environment, defining it as practically non-hazardous waste.

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