Reduction Behaviour and Microstructural Evolution Mechanism of Pellets Based on Blast Furnace Dissection

Given the increasing proportion of pellets used in Chinese blast furnaces, elucidation of the reduction process of pellets and their microstructural changes within the furnaces is important for their efficient utilization. In this work, pellet samples from different areas within a blast furnace were analysed through dissection and sampling, and the microstructure evolution mechanism during pellet reduction was explained. The results show that the pellet reduction process can be divided into three stages from top to bottom in the blast furnace and that the porosity increases gradually as reduction proceeds. The first stage involves the rapid reduction of haematite, during which coarse-grained haematite minerals within the pellets are gradually reduced from the edges of the crystals to the interior to form magnetite. The second stage involves the slow reduction of magnetite and w & uuml;stite, during which the magnetite is reduced to fibrous or fine-grained w & uuml;stite, the metallic iron reduced from the w & uuml;stite gradually grows into worm-like shapes, and the two phases are reduced simultaneously. The third stage involves the high-rate reduction of w & uuml;stite, during which w & uuml;stite is reduced to metallic iron through indirect and direct reduction, and the generated iron aggregates and grows into a sintered or network structure, eventually forming an iron shell. The local reduction conditions differ in the radial direction of the blast furnace, leading to differences in the microstructure and reduction behaviour of the pellets. From the centre to the wall of the blast furnace, the density of the iron shell in the pellet periphery decreases, and the proportion of the iron phase in the pellet core increases. The porosity increases from 25.31 to 27.84 pct, and the degree of reduction decreases from 61.29 to 56.58 pct.