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• Iron existed in the form of excluded minerals in Zhundong coal was found. • The spherical particles in the raw coal were the main source of the coal ash. • The slagging mechanism of high iron Zhundong coal on water-wall was proposed. Due to the wide region of Zhundong mining area, the coal properties of different mines are quite different. In this paper, a Zhundong coal containing red and cyan spherical particles (named as RP and CP) which can be observed by naked eye was studied. To better understand the characteristics of coal and spherical particles, the raw coal with spherical particles was directly ground for density classification, and then ashed. Besides, the raw coal was also separated according to the macroscopic appearance, and then the separated samples were ground and then burned at different temperature. The results showed that 91.57% of the coal mass was contained in classes with density less than 1.50 g/cm3, but the ash mass only occupied 46.74% of the total ash mass. On the contrary, the mass ratio of pulverized coal which density greater than 1.50 g/cm3 was occupied 8.43%, but its ash content was more than half of the total ash content. Iron was the main element in CP and RP ashes, but there was still a high iron content (reached 6.58%) in the ash of the coal without CP and RP. Finally, the slagging mechanism of high iron Zhundong coal on water-wall during combustion process was proposed. [ABSTRACT FROM AUTHOR]