Effect of magnesium oxide and its occurrence on the roasting and metallurgical performance of magnetite pellets

Increased Al₂O₃ content in blast furnace slag in China presents adverse effects in blast furnace smelting. In response to this problem and to improve fluidity of blast furnace slag, MgO pellets are added. Blast furnace can be operated smoothly, but on the other hand, higher MgO content is unfavorable to the pelletizing property and pellet roasting performance of raw materials. Five kinds of pellets containing high-magnesium magnetite, forsterite, dolomite, magnesite, and magnesia powder, respectively, were made to investigate the effect of MgO content and its occurrence on induration behavior and metallurgical performance of pellets. Results show that various magnesium-containing fluxes have different influences on the quality of green balls. Both magnesia powder and high-magnesium magnetite can improve the drop numbers of green balls, which is due to their chemical properties and specific surface areas. With fixed firing temperature and time, increasing MgO content leads to lowered compressive strength of the preheated and fired pellets, with the lowest impact from dolomite observed. With the oxidation degree elevation of preheated pellets, compressive strength of roasted pellets improves, which indicates that we can increase the preheating time in actual production to improve the roasting performance of magnesium magnetite pellets. Under the same source of MgO, increasing MgO content will lead to an increase in porosity of pellets, which presents a negative effect on the strength of pellets. For five kinds of fired pellets, increased MgO content improves the reduction swelling index, low temperature degradation indices, and reduction degree. The reduction swelling index and low temperature degradation indices of pellets with high-magnesium magnetite are observed to be better than those of other magnesium-containing pellets.