Effect of alumina occurrence form on metallurgical properties of hematite and magnetite pellets

The effect of alumina occurrence form on the metallurgical properties of both hematite and magnetite pellets was investigated at the same Al₂O₃ level of 2 wt.%, including reduction index (RI), low-temperature reduction disintegration index (RDI), reduction swelling index (RSI), and high-temperature softening–dripping performance. The mineralogy of fired pellets was also studied to reveal the influence of alumina occurrence form on the phase composition and microstructure. From the results, the alumina occurrence form presents tremendous impacts on the metallurgical performance of both magnetite and hematite pellets. Addition of all alumina occurrence forms contributes to inferior reducibility of pellets, especially in the case of gibbsite for magnetite pellets with a RI of 58.4% and kaolinite for hematite pellets with a RI of 56.8%. However, addition of all alumina occurrence forms improves the RDI of magnetite pellets, while there is no significant difference among various alumina occurrence forms. In contrast, alumina occurrence forms have little influence on the RDI of hematite pellets. The presence of free alumina, gibbsite, and kaolinite tends to improve the RSI of hematite and magnetite pellets, whereas hercynite gives the opposite trend with a RSI of 25.6%. For softening–dripping performance of magnetite pellets, all alumina occurrence forms contribute to narrower softening–melting interval. Meanwhile, alumina, gibbsite, and kaolinite give narrower softening–dripping interval, at 229, 217, and 88 °C, respectively, whereas addition of hercynite results in the largest melting range at 276 °C due to its high melting point. Regarding hematite pellets, free alumina, gibbsite, and hercynite tend to enlarge melting range, whereas kaolinite contributes to lower dripping temperature of 1148 °C and narrow softening–dripping interval of 88 °C due to the formation of a greater amount of slag phase at high temperatures.