Hai-yan Zheng

Effects of MgO Containing Additive on Low-Temperature Metallurgical Properties of Oxidized Pellet

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 °C 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 2689 N, which can still meet the demand of BF.

Characteristics of Calcined Magnesite and Its Application in Oxidized Pellet Production

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 °C and 1 h. Under this condition, the hydration activity of the calcined magnesite was 80.56%, and the average diameter of crystal grain D, specific surface area S and the medium particle size D50 were 25.4 nm, 45.40 m2/g and 3.41 μm, 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.

Gas-solid reduction kinetic model of MgO-fluxed pellets

The reduction process of MgO-fluxed pellets was investigated and compared with traditional acidic pellets in this paper. Based on the piston flow concept and experimental data, a kinetic model fitting for the gas-solid phase reduction of pellets in tubular reactors (blast furnace, BF) was built up, and the equations of reduction reaction rate were given for pellets. A series of reduction experiments of pellets were carried out to verify the model. As a result, the experimental data and calculated result were fitted well. Therefore, this model can well describe the gas-solid phase reduction process and calculate the reduction reaction rate of pellets. Besides, it can give a better explanation that the reduction reaction rate (reducibility) of MgO-fluxed pellets is better than that of traditional acidic pellets in BF.

Effect of magnesia on the compressive strength of pellets

The compressive strength of MgO-fluxed pellets was investigated before and after they were reduced. The porosity and pore size of green pellets, product pellets, and reduced pellets were analyzed to clarify how MgO affects the strength of the pellets. Experimental results show that when the MgO-bearing flux content in the pellets increases from 0.0wt% to 2.0wt%, the compressive strength of the pellets at ambient temperature decreases, but the compressive strength of the pellets after reduction increases. Therefore, the compressive strength of the pellets after reduction exhibits no certain positive correlation with that before reduction. The porosity and pore size of all the pellets (with different MgO contents) increase when the pellets are reduced. However, the increase in porosity of the MgO-fluxed pellets is relatively smaller than that of the traditional non-MgO-fluxed pellets, and the pore size range of the MgO-fluxed pellets is relatively narrower. The reduction swelling index (RSI) is a key factor for governing the compressive strength of the reduced pellets. An approximately reversed linear relation can be concluded that the lower the RSI, the greater the compressive strength of the reduced pellets is.

Effect of MgO on Oxidation Process of Fe3O4 in Pellets

Induration process of oxidized pellets involves the oxidation of Fe3O4 and re-crystallization of Fe2O3. The oxidation process of Fe3O4 is significant for pellets to obtain better ambient strength. Thus, the effect of MgO on oxidation process of Fe3O4 was investigated. The unreacted core model was applied to analyze the oxidizing induration process of pellets. The experimental results show that MgO plays a negative role in the oxidation process of Fe3O4. The oxidation rate of Fe3O4 in MgO-fluxed pellets (95.0% Fe3O4 + 5. 0% MgO) is slower than that in standard acid pellets (100% Fe3O4). The relation between oxidation ratio of Fe3O4 and time was calculated based on the unreacted core model for both MgO-fluxed pellets and standard acid pellets. According to verification experiments, the values calculated by model coincide well with the experimental values. Therefore, the unreacted core model could be applied to describe the oxidizing induration process of pellets.