Thermodynamic Analysis of Formation of Fluoride from Gangue in Bayan Obo Iron Concentrate Containing Fluorite

In order to fill up the deficiency of the theoretical basis about fluoride formation during Bayan Obo iron concentrate roasting process, the thermodynamic conditions of the interactivity between the components of the gangue and calcium fluorite were studied by means of thermodynamic calculation, DTA-TG thermal analysis and XRD characterization. The results revealed that KF, NaF and SiF4 (gaseous) could be formed during the roasting process, and the tendency of the generation of KF is greater than that of NaF or SiF4 in standard state. Besides, the results of roasting experiments showed that the products of KCaCO3 F and KCaF3 formed in the temperature range of 800–1250 °C and KF appears when the roasting temperature was higher than 1250 °C in K2 O-CaF2 system. For the Na2 O-CaF2 system, the product of NaF appears at temperature higher than 1050 °C. The formation reaction of gaseous SiF4 with solid phase CaO – SiO2 in SiO2-CaF2 system took place only at temperature higher than 1150 °C. In the natural potash feldspar-CaF2-CaO system, the fluorination reaction products involved KF at temperature higher than 1270 °C, while in the natural aegirine-CaF2-CaO system, NaF formed at temperature higher than 980 °C during roasting process.