| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === In integrated steel plants, dusts and sludges, commonly referred to as “residual materials”, are inevitably generated during steel production. Due to high iron oxides and carbon contents, these residual materials can be converted into direct reduced iron (DRI) through the carbothermic reduction. DRI can be recycled as the feedstock of blast furnace for liquid iron production. It is known that the permeability of blast furnace would be deteriorated in case of the severe breakage of burden materials. Therefore, it is strictly required that the crushing strength of DRI has to be higher than 0.60 kg/mm2 to avoid its breakage during storage, transportation, and charging into blast furnace.
The study focuses on the effect of the softening and melting temperature of slag composition based on residual materials in China Steel Company. The results compared with data of phase diagram can estimate how to adjust recipe of residual materials in order to meet the feed demand of blast furnace. The results show both basicity (B2) and FeO content can affect the softening and melting temperature of slag. When B2 is lower than 1.13, the deformation temperature is probably less than 1250℃. The solidified softened slag could intensify DRI crushing strength. At next stage, the study investigates the effect of additives to residual materials on the crushing strength of DRI. By adding proper agents, it can assure DRI crushing strength to meet the feed demand of blast furnace. The mixture (Case A) which was made of nine kinds of residual materials was composed of 28.82% oily dewater sludge, 19.15% blast furnace sludge, 17.05% basic oxygen furnace slurry, and 13.55% oily mill scale. The rest of the residual materials were basic oxygen furnace dust, blast furnace flue dust, wastes incinerator fly-ash, blast furnace high-zinc sludge, and cold-rolling sludge. Additives included powdery reagents of Fe2O3, SiO2, and graphite. Experimental conditions of the lab-scale carbothermic reaction included reaction time ranging from 10 to 20 minutes and reaction temperature between 1150 and 1250℃. Results shows adding proper amount of Fe2O3 will decrease the value of (C/Ored)mol and increase the iron content of DRI. It would raise the crushing strength of DRI. It also was found that adding a little SiO2 would induce the partially softening and melting slag phase under 1250℃ due to the lower B2 value (down to 0.89) of the residual materials specimens. The higher crushing strength of DRI was obtained due to sintering. Furthermore, the addition of graphite resulted in the value of (C/Ored)mol above 1.20. It declined the DRI strength. On the other hand, increasing reduction time or reaction temperature could enlarge effect of additives on the crushing strength of DRI. When reaction temperature decreased to 1200℃, adding 15% Fe2O3 made crushing strength of DRI higher than 0.60 kg/mm2.
In in-situ process of RHF, the intact green pellet of residual materials is essential. Blending water content with residual materials affects discharge.of extruder. The appropriate water content was between 17~21%. Besides, the condition of heat fragment was conducted at 950℃ for 5 minutes. The optimal water content was 15~19% for resistance of heat fragment. Synthesizing both results, 17~19% water content is the best condition for pilot scale. The mixture (Case B) had similar composition to Case A, but BOF slurry decreased from 17.05 to 10.10% and BF flue dust increased from 2.66 to 9.20%. This recipe expected to produce worse strength of DRI due to excess carbon content. The pellets via an extruder with water content (18%) were reduced at 1200℃ for 15 minutes. The result shows the crushing strength for Case B was 0.48 kg/mm2. Adding 2, 4, 6, 8% Fe2O3 from acid regenerate plant (ARP) to Case B was reduced at 1200℃ for 15 minutes. It shows 6 and 8% Fe2O3 can increase crushing strength of DRI to 0.61 and 0.71 kg/mm2.
According above results shows that high value (＞1.20) of (C/Ored)mol in residual materials would cause the crushing strength of DRI under 0.60 kg/mm2. So adding appropriate Fe2O3 which consumes carbon and introduces softened slag within DRI assures the crushing strength of DRI above 0.60 kg/mm2.
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