As a main charging form of BF (blast furnace), pellets play an important role in blast furnace process. However, comparing with sinters, pellets have many disadvantages, such as reduction swelling, low softening and melting temperature and so on. Therefore, the flux pellets have been applied in blast furnace widely, especially MgO containing pellets. The light burned magnesite is applied as MgO containing additive in pellet production. The characters of light burned magnesite are explored. Meanwhile, the effects of it on low-temperature metallurgical properties are investigated such as low-temperature reduction degradation index (RDI), compressive strength (CS) and the reduction swelling index (RSI). The light burned magnesite calcined at 850 ℃ manifests better grindability, larger specific surface area, and higher hydration activity. It is found that the addition of light burned magnesite can improve low-temperature metallurgical properties (RDI, RSI) of the pellets. With the increase of light burned magnesite in pellets, the RSI and RDI decrease gradually; when the proportion of light burned magnesite does not exceed 2.0% in pellets, the CS decreases slightly, but it still surpasses 2 689 N, which can still meet the demand of BF.
Calcined magnesite is a binding additive and an MgO-bearing flux for pellets production.The effects of calcination temperature and time on the characteristics of calcined magnesite were investigated.Experimental results indicated that the best calcination condition was 850℃and 1h.Under this condition,the hydration activity of the calcined magnesite was 80.56%,and the average diameter of crystal grain D,specific surface area Sand the medium particle size D50 were 25.4nm,45.40m2/g and 3.41μm,respectively.This kind of calcined magnesite was a good binding additive for pellets production.At the same proportion of calcined magnesite,the effects of activities of calcined magnesite on metallurgical properties of green pellet and indurated pellet showed that calcined magnesite with high activity could improve the dropping strength and compressive strength of green pellet and enhance the burst temperature of green pellet;however,the effects of activity on compressive strength,low-temperature reduction degradation index,reduction swelling index and reduction index of indurated pellet were not obvious.
The calcined magnesite was utilized as a kind of MgO bearing additive to produce MgO bearing pellets. The effects of MgO on densification and consolidation of pellets were investigated. The experimental results show that, at the same process parameters, the porosity and pore size distribution of green pellets have no evident relation with the MgO bearing additive, pore size of green pellets is between 15 μm and 35 μm and the porosity of green pellets is about 34%. There is a densification and consolidation phenomenon during the induration process; the pore size and porosity of product pellets decrease gradually; and the structure of product pellets becomes dense. MgO makes a negative effect on the densification and consolidation of product pellets, the densification ratio of pellets decreases from 46.3% to 28.6% with the addition of MgO bearing additive from 0 to 2.0 %. The porosity and the pore size of product pellets increase gradually with the increase of MgO content; When the mass fraction of MgO bearing additive increases from 0 to 2.0%, the pore size of product pellet increases and the pore size distributes in a large range. Also, the porosity increases from 18.61% to 24.06%.
The reaction of CO_2 gas with hot metal was investigated based on the thermodynamic analysis and experimental results.It shows that both silicon and carbon in hot metal can be oxidized by CO_2 gas in the temperature range of 1 300-1 500 ℃.When using graphite crucible,temperature has little influence on final mass percent of carbon w[C]because of the carburization effect.Decarburization degree rises significantly with increasing gas injection rate and w[C]can be reduced to 3.2% at most when using MgO crucible.Lower temperature or higher gas injection rate is propitious to promote desilication reaction,but only 5%-10% of desilication ratio could be obtained in 20 min.The final mass percent of silicon w[Si]when using MgO crucible is lower than that when using graphite crucible.Experimental results also demonstrate that CO_2 injection has no effect on the concentration of manganese,sulfur and phosphorus in hot metal.In view of the weak oxidation ability and temperature drop of hot metal,CO_2 gas is suggested to be used as carrier gas in desilication process rather than oxidizing agent.
