Effects Of Gangues On Reduction Swelling Character Of Iron Ore Pellets

Reduction swelling is one of the most important metallurgical properties for iron ore pellets. In this paper, an attempt has been made to study the reduction swelling characters and mechanism of pellets made mainly of Carajas hematite concentrate.In order to discover the effects of MgO, A12O3, SiO2 and CaO content on reduction swelling characters of pellets, three kinds of relatively pure iron bearing raw materials (ferric oxide powder, magnetite concentrate from China and hematite concentrate from India) have been attained and compacting-firing-reducing technology has been used. Results show that MgO addition can decrease reduction swelling index (RSI) for all of the three kinds of compacts. A12O3 addition increases the RSI of compacts of ferric oxide powder and magnetite concentrate, while decrease the RSI of hematite compacts. With the increase of SiO2 addition, the RSI falls firstly, then rises up and drops again in the sequel for ferric oxide compacts and magnetite compacts, but the RSI decreases for hematite compacts. CaO addition can lead to abnormal and even disastrous swelling for ferric oxide compacts and magnetite compacts, but greatly alleviate the swelling of hematite compacts.On the basis of research above, the effects of additives on reduction swelling characters of the iron concentrate pellets have been studied. The results show that addition of MgO or serpentine can ease pellet swelling and with the increase of addition, RSI drops quickly. On the other hand, addition of CaO, dolomite and partly calcined dolomite can aggravate pellet swelling, even lead to abnormal swelling.The investigations of reduction swelling mechanism of the iron concentrate pellets show that the addition of MgO or serpentine can improve reducibility of pellets. At the beginning stage of reduction, spongy iron appears and forms a dense layer on the periphery of pellets, so that the following reduction can go smoothly and therefore pellets maintain part of strength and structure. Furthermore, stable Mg-Fe olivine in the pellets has good strength, which helps to maintain the wholestructure of pellet. Otherwise, addition of calcium bearing additives can lead to dissociative SiO2 coming out, and the SiO2 particles can breed cracks in the pellets. It is a probable reason that a crystal transformation to a- SiO2 takes place during reduction and the transformation brings volume expansion, then cracks appear and become bigger and bigger, finally lead to fierce swelling.