EFFECT OF ALKALIS AND FLUORINE ON SWELLING PROPERTIES OF PELLET DURING REDUCTION

The effect of alkalis, fluorine or combination of both, either in vapour state or in contaminated state on the swelling properties of the pellets during reduction were studied. It was proved that the abnormal swelling of pellets contained alkalis and fluorine during reduction mainly occur in the stage of Fe_2O_3-Fe_3O_4, which was accompanied by abnormal rupture of iron oxide crystal, whereas the pellets contained high K occur obvious swelling in the stage of Fe_xO-Fe which was appeared with serious iron whisker growth. By means of step reduction of oxidized high pure Fe sample permeated with K or Na, it has been proved that the abnormal rupture of the iron oxide crystal and iron whisker growth are resulted from the high alkalis content. On the other hand, the bonding phase contained alkalisfluorine in the pellet became liquid during reduction and lost their bonding force to resist the stress caused by the volume expansion due to rupture of iron oxide crystal or iron whisker growth, thus caused more serious abnormal swelling of the pellets. From the analysis of the reduction sample by X-ray diffraction and Mossbauer spectra and study on the reduction rate at the interface between iron oxides, the causes of crystal rupture and iron whisker growth were clarified, the mechanism mode of the rupture of iron oxide crystal and iron whisker growth has also been proposed.