Ferrospheres from fly ashes of Chelyabinsk coals: chemical composition, morphology and formation conditions

Ferrospheres originating during the pulverised fuel firing of brown coals from the Chelyabinsk basin (South Urals, Russia) have been examined for determination of their chemical and phase compositions, morphology and formation conditions. Most of the ferrospheres are close to ideal spheres with dendritic or skeletal structure. The appearance of microsphere inner anatomy is determined by morphology of ferrispinel aggregates, which compose more than 85 vol% of these globules. The analysed ferrispinels are complex solid solutions based on FeFe2O4 with impurities of MgFe2O4, MnFe2O4 and nCaO×mFe2O3. The glasses coexisted with ferrispinel crystallites are basic-ultrabasic in composition. Ferrospheres are the quenching products of high-ferrous melts originated from melting of iron-bearing carbonate admixtures in coals. The mass crystallisation of ferrispinels in ferrospheres was a result of iron changes from Fe2+ to Fe3+ and following ferritisation of high-ferrous melts during molten drops cooling. The residual melt is quenched to form a low-silicon, high-calcium, high-ferrous glass. The skeletal and dendritic forms of ferrispinel are due to their crystallisation under drastic supercooling conditions.