Study On The Process And Mechanism Of Improving The Pelletization Of Brazilian Hematite By Adding Boron Bearing Compound Additives

In recent years,with the rapid development of steelmaking,the demand for imported iron ores has been greatly growing all over the country.In 2008,the consumption of imported iron ores was about 440 Million Tons.Particularly,with the development and large-scale of grate-kiln process,the growing shortage of the magnetite concentrate with high grade iron,fine size and good firing behavious become more serious.The utilization of imported hematite concentrate is an effective way for sustainable development of pellets industry in our country. Though Brazilian specularite is characterized by poor firing behaviours,it has gained world-wide recognition as an indispensable charge to replace magnetite for pelletizing plant due to its high grade iron and abundant. Therefore,the utilization of imported hematite concentrate as pellet feed by new technology,particularly,using additives to improve the firing behavious of pellets,will be of practical significance for Chinese iron and steel enterprises.In this paper,the process and mechanism of improving the pelletization of Brazilian PFCJ hematite by adding boron bearing compound additives have been studied.The effects of additives on balling, pellet preheating and firing,metallurgical properties have been studied systemically.It is shown that the optimum conditions are as follows: 1.8%bentonite in pellet feed,balling for 15min,11%moisture, preheating at 1000℃for 14min,and firing at 1280℃for 15min. Greenballs were manufactured with drop strength of 6.5 times from 0.5m hight,compression strength of 12.3 N/P and thermal shock temperature of 509℃,the compression strength of preheated and fired pellets are 773 and 2843 N/P,respectively.Good metallurgic properties of fired pellets are the following:RI 81.67%,RSI 14.42%,and ROI_+3.15mm 99.98%. Though the properties of green balls and metallyrgic properties of fired pellets are excellent without additives,the firing performance is poor,the compression strength of fired pellets is only 1792 N/P.Even if increasing the fire temperature and extending the fire time,the compression strength of the fired pellets without additives can not meet the product requirement. The results show that good quality fired pellets can be made from Brazilian hematite by adding boron bearing compound additives,which possess high mechanical strength and good metallyrgic properties at lower firing temperature for a short time.The consolidation mechanism of fired pellets with borax or boron bearing compound additives were studied.It is revealed that adding borax into the hematite pellet helpes to form more liquid phases at lower temperature,which enhance grain growth,leading to higher strength of fired pellets due to forming more coarse and dense hematite pellets. Results from the kinetics of grain growth are shown that borax or boron compound additive can fastens the grain growth rates when firing at 1150 to 1300℃for 20 to 50min.The effects of borax or boron bearing compound additives on the reduction of hematite pellets were studied.Resultes from isothermal reduction kinetics show that the reduction rate of pellets containing borax or boron bearing compound additives is controlled by chemical reactions when the reduction temperature ranges from 850 to 950℃,and reduction lasts for 180min in CO/N₂=30/70 atmospher.The apparent activation energy of boron bearing compound additives pellets is 32.192kJ/mol,less than that of borax pellets with 43.233kJ/mol.The lower the apparent activation energy,the easier the reduction reaction take place.So the reduction properties of pellets containing boron bearing compound additives are better than that containing borax.Adding borax into pellets worsens reduction swelling.It is probably that the reason is the sodium ion of borax leads to lattice distortion,resulting in anormal reductive swelling.The low melting point of liquid phases from borax bring about lower mechanical strength during reducting,leading to poorer resistance against swelling stress.The borax containing pellets are too coarse and dense,which impedes heat transfer and increases the thermal stress inside the pellets,leading to higher swelling.When adding boron bearing compound additives,magnesium from the additives not only can stabilize hematite to restrain the lattice distortion and increase the melting point to improve the mechanical strength during reducting,but also increase the porosity,which can offset part of thermal stress.Therefore,the RSI of boron bearing compound additives is lower than that containing borax.