Mineral transformation characteristic of Xinjiang iron-rich low-rank coal during steam gasification

Iron-rich coal is widely distributed in China, and it is necessary to study the mineral transformation of iron-rich coal under elevated temperature and pressure for expanding its utilization. Using a pressurized fixed bed reactor, the mineral transformation characteristics of the Xinjiang Zhundong iron-rich low-rank coal(ZC) were studied during steam gasification. The mineral composition, morphological characteristics and organic carbon microstructure of the raw coal, pyrolysis semi-coke and gasification residue were analyzed by XRD,SEM-EDS and Raman spectroscopy. The results show that the iron in the ZC coal mainly exists in the form of siderite and sphalerite. In the ashing process, the iron-bearing minerals in raw coal are converted into hematite at 500 ℃,and the iron-bearing low-temperature eutectic glass bodies such as iron spinel and fayalite are partially formed at 815 ℃. In the semi-coke obtained by pyrolysis under nitrogen atmosphere, the existence form of iron is mainly elemental iron. In the process of steam gasification of the ZC coal, the calcium and sodium-containing aluminosilicate low-melting mineral phase is first formed on the surface of carbon matrix. With the increase of gasification pressure, the content of low-melting mineral phase increases. It indicates that the pressure can promote the formation of calcium and sodium-containing low-melting aluminosilicates. At 3 MPa, the content of the plagioclase crystal phase increases, and the increased pressure can promote the significant participation of iron-rich minerals in the formation of the aluminosilicate glass phase, which will further reduce the ash melting temperature. The effect of gasification time on mineral transformation is more complex. With the increase of gasification time, the type and content of the iron-bearing mineral phase in the gasification residues increase. Under the gasification time of three hours, the low-temperature eutectic of iron-containing aluminosilicate and iron-containing silicates, such as ferric pyroxene and garnet, are mainly formed, which is mainly because that with the increase of gasification time, the carbon matrix that hinders the mutual contact of ash particles disappears, so that the mineral phase is fully contacted. The unreacted iron-containing phase is finally oxidized to form hematite in the presence of the gasifying agent. The existence of iron-bearing minerals also promotes the aggregation of low-temperature eutectic on the surface of carbon matrix, and then inhibits the transformation of carbon structure from ordered to amorphous to a certain extent.