Two coal ashes (one with high-melting temperature and one with low-melting temperature) obtained at 600 °C in air were pressed into pellets and further treated for 1 h at different temperatures from 700 to 1200 °C at the interval of 100 °C in reducing atmosphere (mole ratio CO:CO 2 = 60:40) to investigate the melting behavior of typical ash particles. The final structures were characterized by XRD and SEM-EDX. The results showed that most Na, K, Ca, and Fe took the form of aluminosilicates. Although the low-melting particles (Na−K-enriched aluminosilicate particles and Ca−Fe-enriched aluminosilicate particles) were small in number percentage, these particles were already melted at 1000 °C. It is possible that Fe was enriched more readily in particles than Ca. The extent of fragmentation of calcite particle was closely related to the calcite type. The calcite particles with layered textures fragmented more. Gaussian distribution successfully simulated the calcite particle size distribution after calcite particle fragmentation. The detailed mechanism analysis showed that the Fe−O particle dissolution was primarily controlled by diffusion. The Fe−O crystals from pyrite and siderite showed similar patterns of crystal growth.

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Two representative Chinese bituminous coals (one with rich Ca content, having a low melting temperature, and the other with rich Fe content, having a high melting temperature) were selected for this study to investigate the effect of quenching temperatures on melting characteristics of coal ash in a reducing atmosphere (6:4 CO/CO2). The final structures were characterized by X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray (SEM–EDX), and differential thermal analysis (DTA). The results show that, with the quenching temperature increasing, the crystalline volume percentage experiences a maximum value. However, the thermal properties (the glass transition temperature and the melting temperature) determine the maximum crystalline volume percentage temperature. There is a large variation in the phase compositions with an increase of the quenching temperatures, but the maximum diffraction peak intensities of most minerals simultaneously occur. A comparison of the coal ash samples after quench and direct heat treatment shows that there is a low crystalline volume percentage and simple phase compositions for the quenching process at corresponding temperatures, which are due to a preheat treatment at high temperatures prior to quench. Anorthite can form a better crystal shape in the coal ash sample with a low melting temperature and high Ca content for the quenching process. The diffusion controls the dissolution of the Fe–O particle, especially for the high melting temperature coal ash with a high Fe content.

Heavy rail steels are widely used in railway transportation owing to their high strength and great toughness properties nowadays. [1][2][3] The number, size, composition, morphology, and spatial distribution of nonmetallic inclusions in steel have a crucial impact on the cleanliness and performance of the steel product. [4][5][6][7][8] Particularly, large and long MnS inclusions precipitated during heat treatment after rolling of the steel and have a detrimental effect on the strength and corrosion resistance of heavy rail steels. [9][10][11][12][13] Many studies were reported to control large-sized MnS inclusions according to the characteristic and source of MnS in the steel, such as slag refining, [14,15] calcium/magnesium addition, [16][17][18][19] zirconium/cerium addition, [20][21][22] heat treatment process, [23,24] and heterogeneous nucleation improvement. [25][26][27] Oikawa et al. [28,29] studied the effect of Ti addition on the formation and distribution of MnS inclusions in Fe-0.1%C-1%Mn-0.02%S steels during solidification. The reason for the significant size reduction of MnS inclusions was the (Ti, Mn)O composite oxides generated at solid/liquid interface of the steel with Ti addition, which acted as heterogeneous nucleation cores for the formation of MnS. Zhang et al. [30] investigated the characteristics of MnS particles at three different cooling rates of 80.4 K s À1 (water cooling), 3.8 K s À1 (air cooling), and 1.8 K s À1 (furnace cooling). With the decreasing cooling rate, the 3D morphology of MnS changed from nearly spherical into rod like and the area fraction and average diameter of MnS increased. Li et al. [25] studied the mechanism of MnS precipitation on Al 2 O 3 -SiO 2 inclusions during the solidification of nonoriented silicon steel. MnS can precipitate on micrometer-sized oxides and its precipitation behavior was governed by the phase structure of the oxides. Nowadays, promoting the heterogeneous nucleation of MnS inclusions was acknowledged as an effective method to reduce flaw detection defects of steel heavy rails.The precipitation of MnS inclusions mostly occurred in steel during cooling and during heating processes. [31,32] It was reported that MnS inclusions near the grain boundary of the steel were usually large and long and well deformed during steel rolling while MnS that precipitated around oxide inclusions were nearly spherical and difficultly deformed during steel rolling.

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