Research On The Effect Of Hematite Ratio On Oxidation Pellets

At present, magnetite concentrate is the mostly used mineral for oxide pellet production in China. With the rapid development of steel industry, magnetite resources can hardly satisfy the needs of pellet production. With advances in mineral processing technology, hematite resource is gradually developed and utilized, many manufacturers attempt to increase hematite ratio in raw materials, even to100%. This paper focuses on change of raw material structure and the effects of hematite ratio on pelletizing, quality of preheated and roasted pellets are studied.Effect of hematite ratio on pelletizing was studied. The results show that with the same bentonite addition, increasing hematite ratio from0%to100%, drop strength and compressive strength of green ball show a decreasing trend. When adding0.75%composite bentonite, drop strength of green ball reduces from4.3times/0.5m to2.6times/0.5m, and compressive strength reduces from13.8N/P to11.1N/P. In order to meet the requirements of production for pellet quality, the amount of bentonite increases with hematite ratio. Micro-fine content, specific surface area, particle morphology and granulometric composition of magnetite and hematite are detected. The results show that with the increase of hematite ratio, micro-fine content of mixture reduces, the amount of smooth surface particles increases, and average particle size increases, the combination of which leads to a increase in pellet porosity. Moreover, larger pellet porosity and average particle size result in a lower bond strength between particles in pellet, and thus the quality of green pellet is worse. The impact of increasing hematite ratio on quality of green pellet can be modified by using suitable composite bentonites which have good adaptability to iron ore concentrates.Effect of hematite ratio on quality of preheated and roasted pellets was studied. The results show that under the same thermal regulation, increasing hematite ratio from0%to100%, compressive strength of preheated pellets falls down continuously and compressive strength of roasted pellets shows a downward trend, especially when the hematite ratio is above50%. With composite bentonite addition, compressive strength of preheated pellets reduces from488N/P to50N/P, while roasted pellets strength drops from3992N/P to1052N/P.Higher preheating temperature and roasting temperature, as well as longer preheating time and roasting time are required for up to grade pellet production. Microstructures of magnetite-based and hematite-based pellets were studied. The results show that oxidation takes place during preheating process in magnetite-based pellets and Fe2O3with higher activity is generated and microcrystalline connections are formed which improve the strength of preheated pellets; which in hematite-based pellets, during preheating process, although the particles get closer to each other, no connection can be formed, resulting in a lower strength. During roasting process, compared to Fe2O3generated by oxidation of magnetite, primary Fe2O3has slower particle diffusion velocity due to worse activity, which results in poorer crystallization ability of Fe2O3, especially in high hematite ratio pellets, consolidations among particles are not significant even at a temperature of1250℃, which leads to a reducing compressive strength of roasted pellets.Effect of hematite ratio on reduction swelling of pellet was studied. The results show that increasing hematite ratio from0%to100%, reduction swelling of pellet increases from12.13%to20.43%. It is undesirable swelling when hematite ratio is100%. Primary Fe2O3content in roasted pellets increases with hematite ratio, which makes the pellets more destructive. Pellets have weaker resistance to destruction due to poorer pellet strength after roasting and lower gangue mineral content such as SiO2, which can result in a larger reduction swelling index. Reduction swelling under high hematite ratio can be inhibited by addition of magnesium flux. When hematite ratio is100%, increasing MgO content to2.5%by adding dolomite, the reduction swelling index falls from20.43%to12.7%; increasing MgO content to2.5%by adding magnesite, the reduction swell index falls from20.43%to12.36%.