Dynamic and Isothermal Reduction Swelling Behaviour of Olivine and Acid Iron Ore Pellets under Simulated Blast Furnace Shaft Conditions

Abstract

Pellet swelling has been widely studied, being simultaneous with reduction reactions and common in the operation of blast furnaces. A tube furnace equipped with a camera recording system was used here to study the dynamic and isothermal reduction swelling behaviour of olivine and acid pellets under simulated BF shaft conditions. The olivine pellets were magnetically separated into three fractions, containing low, medium and high amounts of magnetite in the core. The divalent iron (FeO) content of these fractions was 0.1 wt-%, 0.2 wt-% and 2.9 wt-%, respectively. Pellets with a large magnetite nucleus were observed to encompass numerous cracks, which was reflected in a poor LTD test value, while SiO2-rich reference pellets with a different slag chemistry had more restrained swelling and cracking behaviour in dynamic reduction. Swelling in the olivine pellets was associated with cracking at the boundary between the original magnetite nucleus and the hematite shell.The dynamic reduction swelling test results showed lower reduction swelling indices (max 17% in volume) than under isothermal conditions (max 51% in volume), in which case the pellets were suddenly exposed to a strongly reducing atmosphere. It is thus suggested that the reduction swelling behaviour of iron ore pellets should preferably be studied dynamically under simulated blast furnace conditions in order to achieve a realistic understanding of their swelling behaviour in a blast furnace.

Effect of Circulating Elements on the Dynamic Reduction Swelling Behaviour of Olivine and Acid Iron Ore Pellets under Simulated Blast Furnace Shaft Conditions

“…Each swelling experiment followed the same operation line in the Fe-O-CO-CO2 phase diagram presented in the earlier paper. 18) The operation line simulates the temperature and the CO/CO2 ratio in the blast furnace shaft and is consistent with the general understanding of how CO-CO2 gas composition changes in the blast furnace shaft. [20][21][22][23] The basic gas composition and temperature conditions in the reduction swelling tests are shown in Fig.…”

“…The swelling mechanism of olivine and acid pellets is more precisely discussed in the earlier paper by authors. 18) In the "High-S" experiment -representing the extreme case of the sulphur concentration in the blast furnace atmosphere -the pellets began to melt and partially penetrated through the sample basket leading also to the partial melting of the basket and the furnace tube and to totally different internal structure of pellets compared to the experiments with lower sulphur partial pressures. Light optical microscope (LOM, Olympus BX51) images from the periphery of a low-magnetite olivine pellet after (a) the "Medium-S" experiment and (b) the "High-S" experiment are shown in Fig.…”

“…The divalent iron (FeO) content of the olivine pellet fractions was 0.1 wt-%, 0.2 wt-% and 2.9 wt-%, respectively. Detailed descriptions of the sample preparation and the experimental procedure are given in the earlier paper 18) as well as the chemical analysis of the pellets. The equations for determining the degree of reduction and reduction swelling index (RSI) are also presented there.…”

“…In addition to the operational description of the Blast Furnace gas phase Simulator (BFS) presented in the earlier paper, 18) adjusting the partial pressure of sulphur and potassium is introduced here. The partial pressure of potassium was adjusted by controlling the temperature of the reactor vessel beneath the main furnace leading to controlled reaction rates (with carrier gas) between sieved graphite chippings and potassium carbonate.…”

Softening Behaviors of Acid and Olivine Fluxed Iron Ore Pellets in the Cohesive Zone of a Blast Furnace

“…The pellets of the present work were reduced by the so-called "Iron 1" program. 15,16) The pre-reduction conditions are shown as a function of time in Fig. 1 and illustrated in the Fe-O-CO-CO 2 phase diagram in Fig.…”