With the gradual depletion of high-quality iron-ore resources, ironmaking blast furnaces have gradually shifted to using high-content Al2O3 iron ore. Under this condition, the addition of MgO in the slag is adopted to alleviate operation problems caused by the high content of Al2O3. However, the proper value of the MgO/Al2O3 ratio for blast-furnace slag systems (Al2O3 = 8–25%) is not systematically studied. In this paper, we discuss the proper MgO/Al2O3 ratio on the basis of blast-furnace slag systems under different Al2O3 contents. On the basis of thermodynamics and phase-diagram analysis, it could be concluded that: (1) the MgO/Al2O3 ratio is not limited when Al2O3 in slag is less than 14%, (2) the MgO/Al2O3 ratio is required to be in the range of 0.40–0.50 when Al2O3 in the slag is 15–17%, and (3) the MgO/Al2O3 ratio should be 0.45–0.55 when Al2O3 in the slag is larger than 18%. The proper MgO/Al2O3 ratio value has been established in various industries, and it has achieved significant economic and social benefits.
The desulphurisation of blast furnace slag is an important metallurgical function of blast furnace smelting. To elucidate the desulphurisation behaviour of blast furnace slag with high Al 2 O 3 content, the slag-metal equilibrium method was employed to determine the sulfide capacity (C S ) of CaO-SiO 2-Al 2 O 3-MgO melts for blast furnace smelting at 1823 K. When basicity (R w =w(Basic oxide)/w(Acid oxide)) and w(MgO)/w(Al 2 O 3 ) were increased, the intensities of the characteristic peaks of [SiO 4 ] and [AlO 4 ] in Raman spectrum decreased, indicating that the degree of polymerisation had reduced. The cluster structures of [SiO 4 ] and [AlO 4 ] depolymerised and lgC S (i.e. the desulphurisation ability) increased, owing to which the number of free O 2− ions increased. However, when w(Al 2 O 3 ) was increased, the intensities of the characteristic peaks of [SiO 4 ] and [AlO 4 ] in Raman spectrum increased gradually, indicating that the degree of polymerisation had increased, and the simple structures of [SiO 4 ] and [AlO 4 ] were polymerised. As a result, both the number of free O 2− ions and lgC S decreased.
With the increase in using high Al2O3 iron ore, the content of Al2O3 in blast furnace slag has increased sharply in recent years. Thus, the activity of high Al2O3 slag in the blast furnace slag system needs further study. Herein, the activity of Al2O3 (a(Al2normalO3)) in CaO–SiO2–Al2O3–MgO slag equilibrated with molten Cu at 1873 K is determined by applying the reference slag method. Isoactivity contours for Al2O3 in the (CaO + MgO)–SiO2–Al2O3 pseudoternary system are achieved for blast furnace slag, and the effects of R (mass% CaO/mass% SiO2) and MgO/Al2O3 on the activity of Al2O3 are discussed. When Al2O3 = 20 wt% and MgO/Al2O3 = 0.4, with R increasing from 1.10 to 1.30, the activity of Al2O3 decreases from 0.0423 to 0.0244. When Al2O3 = 20 wt% and R = 1.25, with MgO/Al2O3 increasing from 0.35 to 0.55, the activity of Al2O3 decreases from 0.0470 to 0.0366. The reference slag method, herein, is feasible based on the experimental results and the reference slag method is simple, reasonable, and of high accuracy.
With the shortage of high-quality iron ore resources, more and more attention has been paid to the blast furnace (BF) smelting of Al2O3-rich iron ore in recent years. Thus, investigation on the thermodynamics property of Al2O3-rich blast furnace slag was inevitable in order to produce high quality hot metal with low cost. In this paper, based on the determination of Al2O3 activity by reference slag method at 1873 K in previous study, the effect of the mechanism of slag structure property on Al2O3 activity in CaO-SiO2-MgO-Al2O3 system was discussed by Raman spectra analysis. It was found that the [AlO4] structure was polymerized and [SiO4] structure was depolymerized with the Al2O3 content increasing from 10 wt.% to 18 wt.%, which caused the Al2O3 activity increased.
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