Evaluation of the thermal and rheological characteristics of minerals in coal using SiO₂-Al₂O₃-CaO-FeO_x quaternary system

In the present work, the synergistic effect of components on the mineral behavior in the SiO₂-Al₂O₃-CaO-FeO_x quaternary system was tentatively evaluated. The mineral transformation and reaction were analyzed by thermo-mechanical analyzer (TMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermodynamic calculation (FactSage). In addition, the apparent viscosities of synthetic slags, expressed as a function of temperature and composition, were determined using a high temperature rotary viscometer with temperature ranging from 1 700℃ to re-solidifying temperature. The results demonstrated that the TMA and DSC approaches were applicable to characterize the mineral behavior under in-situ conditions. Silica and alumina preferred to be transformed to high temperature cristobalite and α-alumina with relatively stable structure and high viscosity, respectively. On the other hand, the reaction and transformation of silica and alumina could be accelerated by some valuable fluxing agents, particularly calcium oxide and iron oxides. The addition of ferrous oxide into synthetic slags could lower its viscosity compared with that of ferric oxide and ferroferric oxide. Furthermore, the reduction of iron oxides to metallic iron remarkably increased the viscosity. Ferric oxide may take part in the random glass network in a similar fashion with alumina. Besides, iron oxides with the oxidation state of Fe²⁺ may also act as a modifier under slightly reducing conditions and higher temperatures. The sensitivity of viscosity of mineral matters to temperature excursion decreased with increasing calcium oxide content as calcium oxide was able to enhance the solution ability of iron oxides in the SiO₂-Al₂O₃-CaO-FeO_x quaternary system.