溫室效應為最近極受矚目的環保議題，各界皆致力於減少溫室氣體的排放。而鋼鐵工業的溫室氣體排放量佔總量的3%，故各家鋼鐵工業皆致力於發展節能減碳技術。

其中高料層碳熱還原煉鐵製程將煤礦與鐵礦混合造粒後堆料7~8層，並使用熱空氣由上方加熱球團進行碳熱還原(Carbothermic Reduction)反應，在還原過程中藉由球團的產氣保護球團免於再氧化，最後可得到高金屬化率之直接還原鐵(Direct Reduced Iron, DRI)。在高料層碳熱還原煉鐵製程中，若球團產生熔融崩塌，則整個料層亦會受到影響並坍塌，影響製程的產率以及金屬化率。

為探討煤鐵礦複合球團在碳熱還原過程中的熔融性質，本研究使用不同原料配比的球團進行碳熱還原實驗，利用還原過程影像紀錄、SEM微觀結構分析、XRD相分析、熱分析、化學分析等討論球團熔融崩塌的成因。並且挑選在碳熱還原實驗中產生熔融的球團，藉由添加劑調整球團中的渣相配比，觀察渣相配比對於球團熔融性質的影響。

實驗結果顯示，球團因低熔點的渣相熔化會在球團表面產生液泡，並且球團中心會因金屬鐵滲碳作用而使金屬鐵液化而流失，在特定配比下的球團會呈現中空的金屬鐵殼。而調整渣相配比亦使球團呈現不同形貌，添加氧化鋁可以使球團生成熔點1310 oC之FeAl2O4，減少低熔點1205 oC的Fe2SiO4的生成量，故可使球團表面觀察到液泡的時間點延後，並且還原後球團可維持完整形貌；添加氧化鈣的球團會使球團產生FeO-CaO的造渣反應，使球團的熔融現象加劇，並且球團會產生軟熔崩塌的現象；添加氧化鎂的球團可提高二元渣相熔點，但因金屬鐵滲碳造成的液相流失現象，故還原後球團僅餘中空的金屬鐵殼。而圓柱狀樣品在碳熱還原實驗下除添加氧化鈣及二氧化矽會使圓柱軟熔崩塌及形成不規則的形貌外，皆無表現出軟熔性質。

In order to reduce the CO2 emissions of the blast furnace process, inromaking industry are working on a low CO2 emission process. Tall-bed carbothermic reduction uses iron ore/coal composite pellets packed in several layers. It uses the gas generated by the carbothermic reduction to protect the pellets from re-oxidation, and finally we can get high-metallization-degree direct reduced iron(DRI). In tall-bed carbothermic reduction process, if the pellets melt or collapse, the layer will also collapse and re-oxidize.
In this study, we focus on the melting properties of the pellets during reduction. We use optical imaging, SEM and XRD, to analyze the mechanism for pellet melting. Next we use an additive to adjust the ratio of the slag system (FeO, Al2O3, CaO, MgO, and SiO2), and then we observe the property changes caused by the additive. The result shows that both low-melting-point slag melting and iron carburization are part of the reason for pellet melting. When these two mechanisms work together, the pellets will melt, and maybe even collapse. By adding the Al2O3, it will make the pellets form FeAl2O4, which has a melting point of 1310 oC, and the amount of Fe2SiO4, which has a lower melting point of 1205 oC, will become lower, so the temperature when the pellets form liquid bubbles will be delayed. Adding CaO will make the pellets severely melt and collapse, which is caused by FeO-CaO binary slag, as it has a melting point of only 1125 oC. MgO will form a 1400 oC melting point in FeO-MgO binary slag, but iron carburization still works, and the center of the pellet will be lost, with only the iron shell remaining. Adding SiO2 will promote the formation of Fe2SiO4, so the pellets will melt more severely.

1. Solomon, S. et al., 政府間氣候變遷小組第一工作組第四次評估報告, 2007
2. Midrex Technology Inc., The Midrex Process – The World’s Most Reliable and Productive Direct Reduction Technology.2014
3. Garza, C., HyL Direct Reduction. Millennium Steel.2006
4. Lu, W.K. et al., The Evolution of Ironmaking Process based on Coal-Containing Iron Ore Agglomerates. ISIJ international. 41(8): p. 807-812.2001
5. Wiberg, E. et al., Inorganic Chemistry. Academic Press. p. 810.2001
6. 黃希祜, 鋼鐵冶金原理. 冶金工業出版社. 1990
7. Fruehan, R.J. et al., Rate of Reduction of Iron-Oxides by Carbon. Metallurgical and Materials Transactions B. 8(2): p. 279-286.1977
8. Moon, J. et al., Investigation into the Role of the Boudouard Reaction in Self-Reducing Iron Oxide and Carbon Briquettes. Metallurgical and Materials Transactions B. 37(2): p. 215-221.2006
9. Chatterjee, A. et al., Sponge Iron Producction By Direct Reduction of Iron Oxide. p. 144.2010
10. Oda, H. et al., Dust Recycling System by the Rotary Hearth Hearth. Nippon Steel Tecchnical Report. 94: p. 147-152.2006
11. Ishikawa, H. et al., Rotary Hearth Furnace Technologies for Iron Ore and Recycling Application. Archives of Metallurgy and Materials. 53(2): p. 541-545.2008
12. Camci, L. et al., Reduction of Iron Oxides in Solid Wastes Generated by Steelworks, Turkish Journal of Engineering and Environmental Sciences. 26: p.37-44.2002
13. Borra, C.R. et al., Effect of Alumina on Slag-Metal Separation during Iron Nugget Formation from High Alumina Indian Iron Ore Fines. Ironmaking and Steelmaking. 40(6): p. 443-451.2013
14. Wang, G. et al., Effect of Carbon Species on the Reduction and Melting Behavior of Boron-Bearing Iron Concentrate/Carbon Composite Pellets. International Journal of Minerals, Metallurgy and Materials. 20(6): p. 522-528.2013
15. Birol, B. et al., The Effect of Reduction Parameters on Iron Nugget Production from Composite Pellets. Mineral Processing & Extractive Metallurgy Review. 34: p. 195-201.2013
16. Kong, L. et al., The Internal and External Factor on Coal Ash Slag Viscosity at High Temperatures, Part 2: Effect of Residual Carbon on Slag Viscosity. Fuel. 158: p. 976-982.2015
17. Koichiro, O. et al., Effect of Slag Melting Behavior on Metal-Slag Separation Temperature in Powdery Iron, Slag and Carbon Mixture. ISIJ International. 51(8): p. 1279-1284.2011
18. Koichiro, O. et al., Effect of Ash Amount and Molten Ash’s Behavior on Initial Fe-C Liquid Formation Temperature due to Iron Carburization Reaction. ISIJ International. 55(6): p. 1245-1251.2015
19. Nougueria, A.E.A. et al., Effect of Slag Composition on Iron Nuggets Formation from Carbon Composite Pellets. Materials Research. 13(2): p. 191-195.2010
20. Liu, S.H. et al., Effects of Basicity and FeO Content on the Softening and Melting Temperatures of the CaO-SiO2-MgO-Al2O3 Slag System. Materials Transactions. 56(6): p. 1448-1456.2009
21. Park, H. et al., Influence of CaO-SiO2-Al2O3 Ternary Oxide System on the Reduction Behavior of Carbon Composite Pellet: Part I. Reaction Kinetics. Metallurgical and Materials Transaction B. 44(B): p. 1379-1389.2013
22. Park, H. et al., Influence of CaO-SiO2-Al2O3 Ternary Oxide System on the Reduction Behavior of Carbon Composite Pellet: Part II. Morphological Properties. Metallurgical and Materials Transaction B. 44(B): p. 1390-1397.2013
23. Park, H. et al., Effect of Slag Composition on Reaction Kinetics of Carbon Composite Agglomerate in the Temperature Range of 1273 K to 1573 K (1000 oC to 1300 oC). Metallurgical and Materials Transaction B. 46(B): p. 1207-1217.2015
24. Park, H. et al., Reduction Behavior of Carbon Composite Pellets Including Alumina and Silica at 1273 K and 1373 K. ISIJ International. 54(6): p. 1256-1265.2014
25. Park, H. et al., Effect of Alumina and Silica on the Reaction Kinetics of Carbon Composite Pellets at 1473 K. ISIJ International. 54(1): p. 49-55.2014
26. Zhu, D. et al., Influence of Basicity and MgO Content on Metallurgical Performances of Brazilian Specularite Pellets. International Journal of Mineral Processing. 125: p. 51-60.2013
27. 張皓荀, 煤鐵礦複合球團於不同溫度歷程之碳熱還原和形貌變化研究. 國立成功大學材料科學及工程學系碩士論文. 2015
28. Chipman, J. et al., Thermodynamics and Phase Diagram of the Fe-C System. Metallurgical Transcations. 3: p. 56-64. 1972
29. Levin, E.M. et al., Phase Diagrams for Ceramists. Reser. The American Ceramic Society, p. 696.1964
30. Moore, J.J. et al., Chemical Metallurgy. Butterworth-Heinemann. p. 164.1990
31. Decterov, S.A. et al., Thermodynamic Modeling of the FeO–Fe2O3–MgO–SiO2 System. Journal of the American Ceramic Society. 85(12): p. 2903-2910.2002
32. Maitre. A, Plasma-Jet Coating of Preoxidized XC38 Steel: Influence of the Nature of the Oxide Layer. Physical Chemistry Chemical Physics. 4: p. 3887-3893.2002
33. Wu, P. et al., Critical Evaluation and Optimization of the Thermodynamic Properties and Phase Diagrams of the CaO-FeO, CaO-MgO, CaO-MnO, FeO-MgO, FeO-MnO, and MgO-MnO Systems. Journal of the American Ceramic Society. 76(8): p. 2065-75.1993
34. Prestes, E. et al., Post Mortem Analysis of Burned Magneisa-Chromite Brick Used in Short Rotary Furnace of Secondary Lead Smelting. Cerâmica. 55: p. 61-66.2009
35. Wu, P. et al., Prediction of the Thermodynamic Properties and Phase Diagrams of Silicate Systems – Evaluation of the FeO-MgO-SiO2 System. ISIJ International. 33(1): p. 26-35.1993
36. Jung, S.M. et al., A Kinetic Study on Carbothermic Reduction of Hematite with Graphite Employing Thermogravimetry and Quadruple Mass Spectrometry. Steel Research International. 84(9): p. 908-916.2013
37. Jung, S.M. et al., A Kinetic Study on Reduction in Carbon Composite Magnetite pellet Employing Thermogravimetry and Quadruple Mass Spectrometry. Steel Research International. 41(1): p. 38-46.2014