Effect of calcium-sodium composite flux on ash fusibility and mineral transformation of Pingshuo coal

In this paper, we investigated the effect of calcium-sodium composite flux on the ash fusibility of Pingshuo coal with high ash fusion temperatures. X-ray diffraction (XRD) and FactSage were used to analyze the mineral transformation behavior at high temperature. Combined with molecular dynamics simulations, the effect of calcium-sodium composite flux on the melting mechanism was revealed at the molecular level. The results showed that the silicon-aluminum ratio (Si/Al mass ratio) of Pingshuo coal ash with high ash fusion temperatures (AFTs) were approximately 1.0, with the total silicon-aluminum content (Si+Al, mass sum) about 90%. When the addition of calcium-sodium composite flux (CaO and Na2O) was 20%, the AFTs of coal ash with the addition of calcium-sodium composite flux were lower than that with the addition of CaO and Na2O, which demonstrates a synergistic effect of the calcium-sodium composite on the fusibility of Pingshuo coal ash. The FT of the two Pingshuo coal ash decreased to 1377 and 1279 ℃ when the calcium-sodium ratio of the composite flux was 3∶7. With the addition of calcium-sodium composite flux, quartz in coal ash reacted with Na2O and CaO to form low melting point minerals such as nepheline, melilite, gehlenite, inhibiting the formation of mullite. The formation of low eutectic minerals occurred among anorthite, albite, nepheline and other minerals. Besides, the ordered structural was disrupted by the high diffusivity of Na+, inducing the preferential coordination of Ca2+ with [AlO4]5− tetrahedra. As a result, the Si-O-Si network of coal ash at high temperatures was further broken. The addition of calcium-sodium composite flux significantly enhanced atomic diffusion capabilities within coal ash at high temperatures. The values of mean square displacement (MSD) were higher than those with the additions of Na2O and CaO.