Fundamental And Technological Investigation On Production Of Oxide Pellets From Refractory Hematite Concentrates

Oxide pellets are quality burden for blast furnace. At present, grate-kiln is the main process for oxide pellet production in China. Acid pellet production by grate-kiln process with magnetite as the main raw material is becoming mature. However, diversified oxide pellet is required by the complex raw material structure in China, so that impoverishment and diversification problem of iron ores can be solved and higher demand of pellet quality by high coal injection and high-intensity smelting can be met. Therefore, it is an important subject of pellet technology to broadening the range of raw material and improving pellet quality.Compared with magnetite, though with resource reserves and price advantage, utilization of hematite is limited by poor induration performance, higher preheating and roasting temperature demand, as well as higher reduction swell index of hematite pellet. In this study, a certain hematite from Brazil is taken as the main raw material. Based on the systematic study of hematite oxide pellet, mineral composition and microstructure of hematite pellet are reconstructed by improvement of surface properties, control of solid diffusion conditions, activation of crystal lattice, control of liquid phase quantity, etc. to improve the quality of hematite pellet according to the poor strength of green pellet, high induration temperature, and high reduction swell index.The research on basic characteristics of hematite oxide pellet indicates that the iron ore concentrate used in this test has high iron grade, but little fine particles and small specific surface area (644cm2/g), thus it has bad balling capability. It requires a thermal regulation of preheating temperature 1100?, preheating time 8-48 min, roasting temperature 1300-1320?, and roasting time 10-18 min to obtain the preheated strength of 400 N/P and roasted strength of 2500 N/P. The thermal regulation above is hard to realize with regular grate-kiln process.Both high pressure roller grinding and ball milling are able to micronize iron ore concentrate particles, increase its specific surface area. The main effect of high pressure roller grinding is shape change while the main effect of ball milling is ore grinding. The balling capability of hematite concentrate can be remarkably changed by pretreatments of ball-milling and high pressure roller grinding. After the pretreatment, when the specific surface area is over 1600cm2/g, the dosage of bentonite can reduce 0.75%, the feasible preheating temperature is 100?lower, the roasting temperature is analogous while the pellet strength increases to higher than 500N/P.Compared with carbon-free pellets, hematite pellets with proper amount of carbon burden require a 20??50?lower roasting temperature,5min?10min shorter roasting time, and the feasible range is 50?larger. The main effects of anthracite on induration process of hematite pellet include:?heat supply:anthracite catches on fire during preheating, and a certain amount of heat is released, therefore, the preheating temperature can be lower;?reduction:the carbon in anthracite, as well as the gas product of combustion, CO, can reduce the hematite. The hematite is partially transformed into magnetite which is reoxidized with heat releasing out during induration process. These magnetite are then transformed into secondary hematite with higher activity. Therefore, roasting temperature of hematite is lower, and indurated strength of pellet is better.The research on flux adding indicates that limestone and dolomite are mainly composed with carbonate minerals. A certain amount of heat is absorbed by decomposition during preheating, therefore, pellet temperature falls as well as the strength of preheated pellet. Solid state reactions take place between CaO and Fe2O3 during preheating. The low melting point reaction products, such as calcium ferrite, are produced. These low melting point materials form the liquid phase during roasting which promotes microcrystalline connection of iron oxides and pellet strength improvement. Moreover, reduction swell index can be decreased by addition of dolomite.Optimizing technological parameters are obtained by pilot experiment. The research on process of carbon burden fluxed pellet indicates that fluxed pellet with high quality can be produced by grate-kiln or straight grate process. And grate-kiln pellet is more uniform in quality, has higher tumbler strength and better abrasion resistance comparing with the latter process.