Effect of magnesium oxide and its occurrence on the roasting and metallurgical performance of magnetite pellets
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摘要: 分別以5種不同的含鎂添加劑（高鎂磁鐵礦、鎂橄欖石、白云石、菱鎂石和氧化鎂粉）制備鎂質球團，闡述MgO含量和來源對磁鐵礦球團焙燒特性及冶金性能的影響。研究結果表明：不同的含鎂添加劑對于生球的落下強度有著一定影響，其中氧化鎂粉與高鎂磁鐵礦均能夠提高球團的落下強度。相同的預熱焙燒制度下，提高MgO含量會增加球團孔隙率，降低預熱和焙燒球團的抗壓強度，其中白云石對焙燒球團強度的不利影響最小。增加預熱球團的氧化度有利于促進鎂質焙燒球團固結，提高其抗壓強度。在MgO來源相同的情況下，MgO含量的增加會導致球團孔隙的增減，降低了球團強度，而配加不同種類的含鎂添加劑，均能不同程度改善球團的還原膨脹性、低溫還原粉化性和還原性，其中配加高鎂磁鐵礦的球團的還原膨脹性和低溫還原粉化性均優于于其他含鎂球團。Abstract: Increased Al2O3 content in blast furnace slag in China presents adverse effects in blast furnace smelting. In response to this problem and to improve fluidity of blast furnace slag, MgO pellets are added. Blast furnace can be operated smoothly, but on the other hand, higher MgO content is unfavorable to the pelletizing property and pellet roasting performance of raw materials. Five kinds of pellets containing high-magnesium magnetite, forsterite, dolomite, magnesite, and magnesia powder, respectively, were made to investigate the effect of MgO content and its occurrence on induration behavior and metallurgical performance of pellets. Results show that various magnesium-containing fluxes have different influences on the quality of green balls. Both magnesia powder and high-magnesium magnetite can improve the drop numbers of green balls, which is due to their chemical properties and specific surface areas. With fixed firing temperature and time, increasing MgO content leads to lowered compressive strength of the preheated and fired pellets, with the lowest impact from dolomite observed. With the oxidation degree elevation of preheated pellets, compressive strength of roasted pellets improves, which indicates that we can increase the preheating time in actual production to improve the roasting performance of magnesium magnetite pellets. Under the same source of MgO, increasing MgO content will lead to an increase in porosity of pellets, which presents a negative effect on the strength of pellets. For five kinds of fired pellets, increased MgO content improves the reduction swelling index, low temperature degradation indices, and reduction degree. The reduction swelling index and low temperature degradation indices of pellets with high-magnesium magnetite are observed to be better than those of other magnesium-containing pellets.
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Key words:
- magnetite pellets /
- MgO content /
- MgO sources /
- roasting behaviour /
- metallurgical performance
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圖 1 MgO質量分數及來源對高品位磁鐵礦球團生球落下強度的影響
Figure 1. Effect of MgO mass fraction and source on the drop numbers of green balls
圖 2 MgO質量分數及來源對磁鐵礦預熱球團抗壓強度的影響（預熱溫度950 ℃，預熱時間12 min）
Figure 2. MgO mass fraction and source on compressive strength of preheated pellets (preheating at 950 ℃ for 12 min)
圖 3 MgO質量分數及來源對磁鐵礦焙燒球團抗壓強度的影響（預熱溫度950 ℃，預熱時間12 min，焙燒溫度1240 ℃，焙燒時間15 min）
Figure 3. MgO mass fraction and source on compressive strength of fired pellets (preheating at 950 ℃ for 12 min, roasting at 1240 ℃ for 15 min)
圖 4 配加不同含鎂熔劑的焙燒球團微觀結構圖（預熱溫度950 ℃，預熱時間12 min，焙燒溫度1240 ℃，焙燒時間15 min，w(MgO)=2.45%）。（a）氧化鎂粉；（b）菱鎂石；（c）白云石；（d）高鎂磁鐵礦；（e）鎂橄欖石（H—赤鐵礦；S—(Fe，Mg)Fe2O4；P—孔洞；MgO—氧化鎂；T—鐵酸鈣；F—鎂橄欖石）
Figure 4. Mineral phases of roasted pellets with different MgO source under optical microscope (preheating at 950 ℃ for 12 min, roasting at 1240 ℃ for 15 min, w(MgO)=2.45%): (a) magnesia powder; (b) magnesite; (c) dolomite; d) high magnesium magnetite; (e) forsterite (H—Hematite; S—(Fe2O4) spinel; P—hole; T—calcium ferrite; F—forsterite)
圖 5 具有不同MgO來源的預熱球團氧化度對焙燒球團抗壓強度的影響（預熱溫度950 ℃，焙燒溫度1240 ℃，焙燒時間15 min，w(MgO)=2.45%）
Figure 5. Relationship between compressive strength of fired pellets and different degree of preheated pellets (preheating at 950 ℃, roasting at 1240 ℃ for 15 min, w(MgO)=2.45%)
圖 6 MgO質量分數（a）與不同含鎂添加劑（b）對成品球團孔隙率的影響
Figure 6. Effect of MgO mass fraction (a) and different magnesium-containing flux (b) on porosity of pellets
圖 7 MgO質量分數（a）與不同含鎂添加劑（b）對球團低溫還原粉化率RDI?3.15 mm的影響
Figure 7. Effect of MgO mass fraction (a) and different magnesium-containing flux (b) on RDI?3.15 mm of pellets
圖 8 MgO質量分數（a）與不同含鎂添加劑（b）對球團還原膨脹率的影響
Figure 8. Effect of MgO mass fraction (a) and different magnesium-containing flux (b) on swelling index of pellets
圖 9 MgO質量分數(a)與不同含鎂添加劑(b)對球團還原度的影響
Figure 9. Effect of MgO mass fraction (a) and different magnesium-containing flux (b) on reducing degree index of pellets
表 1 原料的主要化學成分（質量分數）
Table 1. Chemical composition of raw materials
% Raw materials TFe FeO MgO Al2O3 SiO2 CaO K2O Na2O P S LOI* Ordinary magnetite 69.660 15.710 0.340 0.010 1.660 0.180 0.010 0.010 <0.010 0.020 ?1.010 High magnesium magnetite 67.730 24.660 2.480 1.250 0.970 0.270 <0.010 0.010 0.020 0.520 ?1.240 Forsterite — — 35.670 — 52.290 2.350 0.049 0.130 <0.010 0.054 9.560 Dolomite — — 20.930 — 1.230 30.520 0.022 0.040 <0.010 0.018 45.990 Magnesite — — 45.780 — 1.480 0.820 0.020 0.029 0.027 <0.010 50.640 Bentonite 2.820 — — 13.430 69.990 2.090 0.440 2.310 0.030 0.050 9.240 Note：* is burning loss. 
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表 2 原料的物理性能
Table 2. Physical properties of raw materials
Raw materials Specific surface area/ (cm2·g?1) Ratio for different particle sizes/% ?0.074 mm ?0.043 mm Ordinary magnetite 1061 89.94 69.13 High magnesium magnetite 669 69.92 64.29 Forsterite 2836 88 74.9 Dolomite 2282 94.8 81.8 Magnesite 3068 86.5 67.8 Magnesia powder 4826 99.5 97.2
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表 3 膨潤土物理性能
Table 3. Physical properties of bentonite
Colloid index/
(mL·g?1)Swelling coefficient/
(mL·g?1)Water absorption
(2 h)/%Ethylene blue
adsorbed/gMontmorillonite
content/%Size
(<0.074 mm)/
%10.45 25.25 605.33 39.09 80.42 99.73
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表 4 成品球團礦化學成分（質量分數）
Table 4. Chemical analysis of fired pellets
% MgO source TFe FeO SiO2 CaO MgO K2O Na2O P S — 68.35 — 1.82 0.18 0.35 0.010 0.010 <0.010 <0.010 Magnesia powder 68.18 0.25 1.62 0.18 0.60 0.010 0.010 <0.010 <0.010 Magnesia powder 66.33 0.33 1.58 0.17 1.19 0.010 0.010 <0.010 <0.010 Magnesia powder 64.13 0.38 1.53 0.17 1.78 0.010 0.010 <0.010 <0.010 Magnesia powder 61.59 0.48 1.47 0.16 2.45 0.010 0.010 <0.010 <0.010 Magnesite 67.39 0.41 1.66 0.21 2.42 0.010 0.010 <0.010 <0.010 Dolomite 65.84 0.42 2.32 3.27 2.43 0.020 0.030 <0.010 <0.010 Forsterite 64.49 0.39 4.64 0.31 2.45 0.010 0.020 <0.010 <0.010 High magnesium magnetite 66.70 2.33 1.37 0.31 2.44 <0.010 <0.010 <0.010 <0.010
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