Krasnoyarsk scientists characterized the magnetic fractions of dispersed particles less than 10 microns in size, which are part of the fly ash from the industrial combustion of coal at thermal power plants. This will make it possible to use them in creating new functional materials and to reduce environmental pollution by aerosol particles which are harmful to human health. The results of the study are published in the journal ACS Omega.

Coal power plants generate annually 38% of the world's electricity and about 1 billion tons of fly ash as industrial waste. Depending on the type and conditions of coal combustion, ash contains up to 40% of dispersed particles of submicron and micron size. Particles-aerosols of the PM2.5 class with a size of less than 2.5 micrometers pose a particular environmental hazard, as they are almost constantly suspended in the atmosphere and cannot be removed from human lungs. To assess the risks of anthropogenic impact on the environment which is associated with the industrial combustion of coal, as well as to minimize pollution with microdispersed components, including those with the high iron content, it is necessary to isolate and characterize magnetic solid particles of fly ash and find their application.

Scientists of the Federal Research Center "KSC SB RAS" under the supervision of Doctor of Chemical Sciences, Professor Alexander Anshits for the first time in the world isolated the magnetic fractions of microspheres PM2.5, PM2.5-10, PM10 from the fly ash of the Ekibastuz coal which one of the high-ash coals in Russia, and determined their characteristics. The data obtained will make it possible to transform dispersed ash waste into raw materials for high-tech applications, for example, for the development of new composite sorbents with the core-shell structure, magnetic carriers, affinity sorbents or biosensors.

Fly ash is composed of many components with different characteristics and composition. The Krasnoyarsk researchers isolated from it only magnetic particles which are environmentally hazardous and distributed them into fractions, depending on their size and density. As a result, fractions with a narrow particle size distribution were obtained, their average diameter being 1, 2, 3 and 7 microns.

The main macrocomponents of their chemical composition are iron, silicon and aluminum oxides, and the phase composition includes amorphous components, that is, substances in the glassy state, and crystalline phases, including ferrospinel, hematite, mullite and quartz. In addition, the scientists observed the formation of nanoscale particles of a rare metastable phase which is difficult to obtain, that is epsilon iron oxide. The researchers found out the dependence of the content of individual components of the narrow fractions on the particle size. For example, with an increase in the average particle diameter from 1 to 7 microns, the proportion of iron oxide, hematite and glass phase in its composition increases, while that of mullite and quartz decreases.

According to the scientists, the spherical shape of the dispersed ash particles, combination of magnetic properties with high thermal stability, as well as possibility of modifying the particle surface using functional groups in order to change their properties, provide advantages for the successful design of microspherical functional materials based on them for effective high-tech use.

“Fly ash is a difficult object to study and to apply, since it is heterogeneous in size, composition, morphology of globules and, therefore, cannot be used to obtain new materials with predictable properties without preliminary classification. But at the same time, fly ash contains a number of valuable components, including hollow aluminosilicate microspheres - cenospheres, magnetic microspheres - ferrospheres, unburned coal particles. These components have unique technological properties, and thus, they can be used in many modern technologies. To do this, it is necessary, first of all, to isolate them in the form of fractions with a narrow size distribution, for determining the composition, structure and properties. To isolate narrow fractions of the dispersed magnetic microspheres, we used the method of magnetic separation and aerodynamic classification, which was successfully implemented on the unique equipment of the Federal Research Center of KSC SB RAS, that is a centrifugal classifier. As a result, we obtained magnetic fractions of the dispersed particles, which belong to the environmentally hazardous classes PM2.5 and PM10, and characterized them. Due to their high iron content, these particles can induce lung apoptosis in humans, so it is very important to increase their recovery from fly ash and find areas for their further use. Due to the treatment of the dispersed components, the amount of waste and, accordingly, environmental pollution will be reduced. For example, these particles can be used to create highly efficient microspherical functional materials: sorbents, magnetic carriers, catalysts and biosensors, ” says Elena Fomenko, Candidate of Chemical Sciences, Senior Researcher at Institute of Chemistry and Chemical Technology FRC KSC SB RAS.