Novel insights into the problem of enthalpy and entropy convergence in thermal decomposition of coal slag using the data from non-isothermal kinetic measurements

This study provides insight into benefits of thermo-chemical conversion of coal slag as recovery process into value-added products. This research involves kinetic analysis of process conducted through non-isothermal thermal analysis measurements, with additional raw material characterization. Kinetic results showed that decomposition proceeds through two consecutive reactions steps (first one, including anorthite P1̅ → I1̅ phase transition, and then production of incongruent melting product (ternary system: CaO·Al₂O₃·2SiO₂ (CAS₂), where viscosity of slag changes), and second one including dehydration and formation of meta-muscovite, and subsequently, thermal disruption of muscovite de-hydroxylated phase, which proceeds with breaking of octahedral Al–O bonds), and one single-step reaction (attributed to CO-reduction of hematite to magnetite). Thermodynamic results showed an existence of physically meaningful isokinetic temperature (T_iso), which corresponds to active vibrational frequency of surroundings of SiO₂ reaction site, manifested through Si‒O bond weakening by catalytic reaction of freed hydroxide ion (OH⁻). It was concluded that at temperature T = T_iso, the course of process loses its dependence on temperature and pressure, regulating changes between thermodynamic parameters, through enthalpy-entropy compensation (EEC) effect.