Effect of MgO bearing flux material of different sources in iron ore pellets

Abstract

MgO is an important flux material in high alumina hematite ore pellet which can reduce RDI of pellet and improve other properties too. Pure MgO is not available in nature. It is available in form of either Magnesio silicate (pyroxenite, olivine, dunite, etc) or magnesite, or dolomite. At present various naturally available minerals containing MgO viz. olivine, pyroxenite, dunite, dolomite etc. are widely used in different steel plants. However, properties of each of the above fluxes are physically and chemically different and they contain different types and quantities of gangue materials like silica, alumina, CaO etc. While, pyroxenite, olivine and dunite contain a significant percentage of silica (41-53%), dolomite contains a significant percentage of CaCO3 (52-55%). On the other hand, magnesite and olivine require endothermic heat due to the calcinations of carbonates in it but dolomite, pyroxenite and dunite do not involve with calcinations of carbonates. Further, olivine is rich with magnesia (47%) but pyroxenite is comparatively lean (29.5%). Though, MgO has specific roles, other gangue materials carried by MgO bearing fluxes may also change the phases or slag bonding in pellet that changes the pellet properties. Thus, different MgO bearing flux shows different behavior in pellets. It is very imperative to study the actual role of MgO and effect of different gangue materials in it. Aim of this study is to examine the actual role of MgO and its optimum requirement as well as the effect of other gangue materials in MgO bearing fluxes. I have carried out the work at National Metallurgical Laboratory, Jamshedpur with a high alumina Noamundi hematite ore of Tata Steel Ltd. As different MgO source, pure MgO, olivine and waste magnesite brick powder have been used separately in pellet making. The properties of green pellets like drop numbers, green compressive strength, dry strength have been measured. The pellets were indurated at varying temperature in a chamber furnace and subsequently characterized by measuring cold crushing strength, apparent porosity, reducibility indices, reduction degradation indices and swelling indices as per standard. The phase identification and morphology has also been studied through XRD analysis and optical microstructure. The pellet characteristics have been compared. It has been found that olivine fluxed pellet shows highest CCS and pure MgO fluxed pellet shows lowest CCS i.e. CCS increases with increase in gangue content of MgO bearing fluxes because of more amount of slag bond formation in higher gangue materials. However, RDI is an important factor for a pellet to consider it as good quality. While, in pure MgO fluxed pellet, RDI is as low as 7.5% for 0.9% MgO level in pellet, the waste magnesite brick fluxed and olivine fluxed pellets shows 28.8% and 17%, respectively. RI for olivine pellet decreases drastically due to the high amount of slag bond formation by silica in olivine and decrease of porosity resulting hindrance of gas permeability. Swelling index of olivine pellet is slightly higher (14.5%) than other two pellets (11-13%). MgO containing fluxes have high gangue content which are helpful from strength point of view but pure MgO provide good RDI, RI and swelling properties than other two. However, it is not used in pelletization due to its non-availability.