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Sintering is the most economic and widely used agglomeration process to prepare iron ore fines for blast furnace use. Owing to the depleting reserves of traditional high grade iron ore, there have been considerable changes in iron ore resources available throughout the world, especially in steel mills in East Asia. Corresponding to the changes in the availability of iron ore resources, the amount of impurities in iron ore has been slowly increasing. Some of these impurities have been found to have deleterious impacts on sinter quality and sintering performance. In the meantime, an increasing number of large blast furnaces with inner volumes of more than 5000 m3 have been built in East Asia which require more sinter and are often more demanding in terms of the quality requirements of the ferrous materials. Finally, sinter plants are facing increasing pressure due to more stringent regulations regarding their environmental impact. This paper gives an overall review of a variety of technologies developed worldwide to tackle the changing raw material characteristics and mitigate emissions from sintering operations. Given the high sinter usage and volume of their blast furnaces, most of the recent sintering technologies have originated from East Asia, particularly Japan.

The formation and the growth of void (crack) in the sintering bed have not been explored. In this study the motions of particles and the air in the nearly actual scale sintering bed were simulated to elucidate the void formation and the growth mechanisms to large scale crack by the simultaneous calculation of NavierStokes equations and the Lagrangian DEM equations based on the simple sintering model in which the phase change of particles, the cohesion force due to the liquid film between particles and the fixation process above the melting zone were considered. The air flow among particles facilitated to grow the crack and finally to produce the large scale crack. The cohesion force by the liquid film caused the agglomeration among particles and grew the voids in the melting zone. In the fixation zone the large cohesive force which was 10 or 30 times larger than that in the melting zone in this study advanced the agglomeration and grew the void. Therefore the cohesion force between particles mainly affects the occurrence of the large scale crack. The decrease of mobility of particle motion by the fixation process in the fixation zone generated the locally large contact force which was about 250 times larger than the usual cohesion force between particles in the agglomerate and the large velocity difference between agglomerates. They broke down the agglomerate particle. Through the fixation zone the cracks (voids) further grew and merged to a large scale crack.

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In general, Fe content in iron ore is gradually decreasing. This fact affects worse performance of BF operation, for example, increase of RAR and Slag ratio. Depletion of high grade iron ore deposits is moving us to use concentrates in sintering process. Through magnetite concentration deteriorates reducibility because of high FeO content in sinter product. Such situation makes it to promote oxidation of magnetite iron ore during sintering process for improving sinter reducibility. In addition, promoting oxidation of magnetite possibly increases sinter strength with using oxidation heat. ISIJ sinter research group for utilization of magnetite concentration suggests that restricting melt formation is critical for promoting oxidation of magnetite concentration. In this paper, It is confirmed that "Separate Granulation" has been examined to apply their suggestion by sinter pot test. The main results obtained are described as follows: (1) "Separate Granulation" in case that magnetite is pre-granulated with high Al 2 O 3 iron ore without limestone and coke breeze resulted in decrease of FeO in sinter and improvement of both sinter reducibility and sinter strength. (2) Sinter micro structure featured restriction of pore, low circle factor and small mineral texture, which supported that melting restriction worked during sintering. (3) Magnetite decreased and hematite increased as sinter mineral, which corresponded with decrease of FeO content. (4) These facts shown (1) to (3) concluded that "Separate Granulation" is effective to improve both sinter reducibility and sinter strength due to restriction of melting during sinter reaction.