Effect Of Roasting Temperature And Alkalinity On Metallurgical Properties Of Hematite Pellets

Pellets are high-quality raw materials for blast furnace ironmaking.Because pellets,especially self-fluxing alkalinity pellets(R=0.33-1.0)are prone to volume expansion,strength reduction,and pulverization during the reduction process of the blast furnace,the permeability of the material column is reduced,which limits the self-fusion the alkalinity pellets are further improved in the blast furnace charge ratio.Therefore,studying the influence of roasting temperature and alkalinity on the metallurgical properties of hematite pellets is of great significance to solve the problem of reduction and expansion of pellets,improve the structure of blast furnace charge,and increase the blast furnace output of self-fluxing alkalinity pellets.In this paper,on the basis of mixing the analytical reagents of hematite(96%),SiO₂(3%),MgO(0.5%)and Al₂O₃(0.5%),the CaO analytical reagents are prepared with different alkalinity(0,0.33,0.67,0.83,1.0,1.33)briquetting pellets.Through Fact Sage thermodynamic analysis,the briquette pellets are instructed to be preheated at 950?and roasted at different temperatures(1230,1250,1280,1300?).The physical properties of the roasted briquette pellets were measured for porosity and compressive strength,and the phase composition,mineral phase microstructure and surface microstructure of the roasted briquette pellets were analyzed.The roasted briquette pellets were tested for reducibility and reduction and expansion metallurgical properties,and the surface microstructure of the reduced briquette pellets was analyzed by SEM-EDS.This reveals the effects of roasting temperature and alkalinity on the porosity,compressive strength,reducibility and reduction expansion of hematite pellets.The effects of different roasting temperatures and alkalinity on the porosity,compressive strength,reduction expansion and precipitation morphology of the reduced metallic iron of hematite pellets were investigated.The results of the study showed that:(1)As the roasting temperature increases,the porosity of pellets with different alkalinity decreases gradually.Increasing the roasting temperature and increasing the amount of liquid phase are conducive to the continuous crystallization of hematite,and the pellet structure is more compact.When the alkalinity is in the range of 0-0.83,the porosity of the briquettes after roasting increases rapidly;when the alkalinity of the pellets continues to increase(1.0-1.33),the porosity of the pellets rises slowly or slightly decreases.As the alkalinity increases,the solid solution formed during the roasting process hinders the recrystallization of the hematite grains;at the same time,the liquid phase between the particles increases,resulting in the gradual increase of large hole in the cooled pellets,thereby increasing the pellet Porosity.Under the condition of alkalinity of 1.33 and roasting temperature of 1230?,the porosity of the briquette pellets after roasted can reach up to 24.11%;under the conditions of alkalinity of 0 and roasting temperature of 1300?,The porosity of the briquette pellets after roasting can be as low as 5.12%.(2)With the increase of roasting temperature,the compressive strength of pellets with different alkalinity gradually increases.Increasing the roasting temperature,the increase in the amount of liquid phase in the pellets promotes the connection of the hematite grains and makes the pellets have higher strength.The compressive strength of pellets at different roasting temperatures increases first and then decreases with the increase of alkalinity,and when the alkalinity is 0.83,its compressive strength is the highest.When the alkalinity is in the range of 0-0.83,the liquid phase generated during the roasting process plays the role of binding the hematite grains,strengthening the consolidation of the pellets,thereby increasing the strength of the pellets;as the alkalinity increases continue to increase(1.0-1.33),the amount of liquid phase in the roasting process is limited,and the increase of high melting point solid solution is more,which limits the recrystallization of hematite and the binding of liquid phase,and the compressive strength of pellets decreases instead..Among them,the compressive strength of pellets reaches the maximum value at 3122N under the conditions of alkalinity of 0.83 and roasting temperature of 1300°C.(3)As the roasting temperature increases,the reducibility and reduction expansion rate of pellets with different alkalinity gradually decrease.As the roasting temperature increases,the porosity of the pellets gradually decreases,which is not conducive to the progress of the reduction reaction,resulting in the reduction of the reduction expansion rate and reducibility of the pellets.The reduction expansion rate of pellets at different roasting temperatures increases first and then decreases with the increase of alkalinity.When the alkalinity is 0.83,the reducibility and reduction expansion rate are the largest.When the alkalinity is in the range of 0-0.83,the porosity gradually increases,which is conducive to the progress of the reduction reaction,and the formation of a solid solution between Ca²⁺and the floatite promotes the growth of iron whiskers,which increases the reduction expansion rate;as the alkalinity continues increase(1.0-1.33),the porosity of the roasted pellets no longer increases,and the high melting point solid solution continues to increase,thereby inhibiting the growth of iron whiskers and reduction expansion.The reduction of pellets at a reduction temperature of 900?,reduction time of 60min,gas composition of CO=30%,N₂=70%,alkalinity of 0.83,and roasting temperature of 1230?.The expansion rate reached the maximum value,which were 56.68%and 21.48%respectively.(4)Different roasting temperature and alkalinity have different effects on the precipitation morphology of metallic iron in the reduced pellets.With the increase of alkalinity,the morphology of metallic iron in the reduced pellets gradually changed from a granular and reticulated structure to a slender whisker-like and stubby cone-like crystal structure,and it mostly grew near the hole.With the increase of the roasting temperature,the morphology of metallic iron in the reduced pellets gradually changed from dense whisker shape to massive and layered metallic iron.This shows that the whisker-like metallic iron can promote the reduction and expansion of the pellets;while the massive and layered metallic iron does not promote the reduction and expansion of the pellets.