Preparation Of MgO Bearing Additive For Pellet And Mechanism Investigation Of It On The Effect To Quality Of Pellets

Nowdays, Fluxed pellets, particularly MgO fluxed pellets, are developed as an alternative candidate. And the MgO bearing additives mainly include magnesite, dolomite, and forsterite and so on. However, there are many gangue minerals in these additives such as CaO, SiO2 and Al2O3 etc; therefore, it will increase the gangue compositon in pellets passively. Moreover, on the production of MgO fluxed pellets, it still needs some bentonite in order to improving the pelletizing, which may lead the grade of total Fe decreased and slag ratio of blast furnace (BF) increased gradually. Basing on above, a new MgO bearing additive is developed in present work; and the effects of MgO bearing additive on green pellets and product pellets are investigated; also, the consolidation mechanism of MgO bearing pellets is explored; and the feasibility of that the MgO bearing additive substitute bentonite and the universality of principle for effects of MgO bearing additive on the qulalitiy of pellets is investigated at the same time. The main findings are shown as follows:1. The magnesite was utilized as a raw material to develop the MgO bearing additive; after the calcinated experiment, it can obtain that the calcined magnesite have larger specific surface area, smaller of crystal grain size and smaller size distribution when the calcinated temperature is between the 800? and 850?. Among all the experimental conditions, the best calcinated condition was calcinated at 850? for 1h.2. The MgO bearing additive with higher hydration activity could improve the metallurgical properties of green pellets, such as compressive strength (CSGP/N), dropped strength (DS/Times) and burst temperature (BT/?), especially the burst temperature (BT/?) increased by 125?, when the hydration activity increased from 71.23% to 80.56%. However, hydration activity have no evident effects on the metallurgical properties of product pellets, such as compressive strength (CS/N), reduction disintegration index (RDI/%), reduction swelling index (RSI/%), and reducibility index (RI/%).3. The MgO bearing additive can improve the quality of pellets. With the addition of MgO bearing additive from 0% to 2.0%, the compressive strength of green pellets (CSGP/N), dropped strength (DS/Times) and burst temperature (BT/?) are all increased gradually; at the same time, the reduction disintegration index (RDI/%), the reduction swelling index (RSI/%), the reduction compressive strength (RCS/N) and reducibility index (RI/%) are all improved gradually, especially the reduction disintegration and reduction swelling are inhibited effectively; however, the compressive strength (GS/N) decreases gradually.4. The effects of MgO and SiO2 on Fe3O4 oxidation process are carried out in 1050?. The MgO play a negative role in oxidation process of Fe3O4; while the effect of SiO2 on oxidation process of Fe3O4 is not evident. Base on unreacted core model, the relationships between oxidation ratio of Fe2O4(X) and time (t) for Standard-reagent pellets (w(Fe3O4)=100%) and MgO-reagent pellets(w(Fe3O4)/w(MgO)=95.0%/5.0%) are proposed. The other experiments are carried out to verify the accuracy of model. The experimental results have conformed to the calculated value of the model. Therefore, the unreacted core model is suitable for the oxidized and roasted process of pellets.5. The MgO play a negative role on the re-crystallization of Fe2O3. Comparing with the acid pellets, with the addition of MgO bearing additive from 0% to 2.0%, the re-crystallization of Fe2O3 was frangible; the crystallite was mal-distributed and connected incompactly, and Fe3O4 was intermingled in the Fe2O3.6. The MgO counts against on the densification and consolidation of pellets. During induration process, there is a phenomenon of densification and consolidation, the pore size and porosity of green pellets decrease gradually; the oxidized consolidation index is proposed and there is a positive correlation relationship between the oxidized consolidation index (?2) and compressive strength (CS/N), The oxidized consolidation index (?2) could be used to denote the consolidation degree of pellets precisely; When the proportion of MgO bearing additive increased from 0% to 2.0%, the oxidized consolidation index (?2) decrease gruadlly, and the physical apparent of that is the porosity and pore size increased gradually, while the pore distributes of product pellets is in a large range.7. MgO make a distribution in iron phase dispersedly and which can mutual solute and produce MF phase-{(Fex·Mg1-x)O ·Fe2O3}. The MgO content of mutual solute in MF phase is different between the inner-layer and outer-layer of pellets. According to the anaysis of SEM-EDX, the mutual solute content of MgO in MF phase can be caculated, in outer-layer of pellets is about 8-16%; in inner-layer of pellets is about 3?10%, the rough composition of MF in present work:Outer-layer MF phase:(Mgo.35?0.77·Fe0.65?0.23) O ·Fe2O3Inner-layer MF phase:(Mg0.13?0.45·Fe0.87?0.55) O ·Fe2O38. The reduction process of acid pellets and MgO bearing pellets are compared and discussed. Based on the fundamentals of piston flow concept, a kinetic model fitting for the gas-solid phase reduction of pellets in tubular reactor (BF) is built up. A series of reduction experiments are carried out to verify this model. The values of experiments and calculation are coincident well. Therefore, this model could well describe the gas-solid phase reduction process of pellets.9. As an additive, MgO bearing additive can substitute bentonite to play a role as binding additive, and the substitution rate is about 0.75?0.80%; moreover, it can be utilized as MgO flux to produce MgO bearing pellets. And the effect of MgO bearing additive on the qulalitiy of pellets is universal after changing the iorn ore for pelletizing.