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Size of granule
Granule size (distribution) is the most obvious descriptor in the granulation and sintering process. It determines the packing structure of the sinter bed, which in turn influences the bed permeability and airflow rate through the bed. In addition, the granule size distribution can affect the rate of descent of the flame front as well as the sintering reactions that take place in the sinter bed (Ball et al., 1973; Ellis et al., 2007; Ennis and Litster, 1997; Formoso et al., 2003; Khosa and Manuel, 2007; Litster and Waters, 1988; Voice et al., 1953). For instance, if the granules have a wider size distribution range, a closer packed bed will be formed and airflow will be reduced due to obstruction of local voids by fine granules. Hence, the progress of the whole sintering process will be slow.
The structure of the packing is dependent on the size of particles. Previous studies reported that that fine particles tend to form close packing with low permeability whereas coarse particles likely form loose packing with high permeability (Haughey and Beveridge, 1969; Hausner, 1972; Hinkley et al., 1994a; Pahl, 1975; Rankin et al., 1985; White and Walton, 1937; Wooten, 1998; Yu and Standish, 1993). This justifies the need to increase the mean granule size and tighten the size distribution of granules through granulation prior to the sintering process.
Many researchers explored the possibility of replacing fine iron ore by ultrafine materials (concentrates, dust, iron ore wastes) in sinter making (Bartlett et al., 2009; Borges et al., 2004; Kasai et al., 1989; Pal et al., 2013; Rankin et al., 1985, 1983; Socalici et al., 2011). Unfortunately, a higher amount of ultrafine materials in the feedstock has an adverse influence on the sintering process. Concentrates drastically obstruct the passage of the flame front through the sinter bed, resulting in a lower permeability of the sinter bed and lower productivity of the sinter machine. To mitigate the negative effects of concentrates, these very fine particles can be transformed into micropellets so that they can be used as an alternative feed material in the granulation process (Bartlett et al., 2009; Borges et al., 2004; Pal et al., 2013; Rankin et al., 1985, 1983; Socalici et al., 2011).
Although the granule size plays an important role in determining the behaviour of the granular material (flow, packing and compaction), its definition is not exclusively expressed by one equation. For a large number of randomly orientated particles with different sizes, the particle size of a granular material is often defined in terms of weight – based size fractions obtained by sieving. Some typical definitions of particle size for granular material are given in Table 2.1 (Allen, 1990).
Shape of granules
Granulation of fine iron ore produces granules with irregular shape. This can significantly affect the structure and permeability of the green sinter bed. Hinkley et al. (1994a) predicted the effect of the particle shape on the structure of the packed bed.
Rounded particles create less particle-to-particle interlock than angular particles and thus provide easier compaction. Irregular particles often tend to impede compaction resulting in an increase in permeability of the packed bed. Wooten (1998) reported that bed permeability increases the more the shape of particles deviates from the spherical shape.
Pahl (1975) studied the effect of orientation of cylindrical particles with different aspect ratios on the flow through packed beds. Particles packed in different manners. It is evident that irregular particles can orientate themselves at various angles, reducing the reproducibility in pressure drops after repacking the same bed. However the orientation of spheres does not influence the structure of the bed.
The incorporation of concentrates and micropellets into the ore mixture can affect the overall particle shape distribution of granules. Shatokha et al. (2009) reported that granules constituted from high mass percentages of concentrate are much more regular. Their rounded-shapes are quite similar to those of pellets. Their structure consists of a coarse ore particle as core, which is surrounded by adhering fines (concentrates). Shatokha et al. (2009) also found that distinct spherical lumps of fines might be formed in the coalescence process, which form the nuclei of iron ore granules.
1. Introduction
1.1. Granulation for iron ore sinter
1.2. Objectives of this study
1.3. Outline of the Thesis
2. Literature Review
2.1. Granulation
2.2. Mechanisms of granulation for iron ore sinter
2.3. Parameters of granulation
2.4. Granulator devices
2.5. Permeability of the sinter bed
3. Experimental work
3.1. Experimental
3.2. Results and discussion
3.3. Conclusions
4. Study of structure and shape of granules produced during granulation of mixtures with addition of concentrate and micropellets
4.1. Introduction
4.2. X-ray microtomography
4.3. Analysis of three-dimensional particle shape
4.4. Experimental
4.5. Results and discussion
4.6. Conclusion
5. Prediction of the granule size distribution of iron ore sinter feeds that contain concentrate and micropellets
5.1. Introduction
5.2. Background to Litster’s model
5.3. Materials and methods
5.4. Results and discussion
5.5. Conclusions
6. Numerical validation of the pressure drop across glass bead beds using Rocky DEM – Fluent CFD coupling
6.1. Introduction
6.2. Background
6.3. Experimental procedure
6.4. Results and discussion
6.5. Conclusions
7. DEM – CFD coupling simulation of the pressure drop through a packed bed of iron ore granules
7.1. Introduction
7.2. Key parameters of DEM – CFD coupling method
7.3. Numerical simulation configuration
7.4. Results and discussion
7.5. Conclusions
8. Conclusions
8.1. Introduction
8.2. Conclusions
9. Recommendations for future work
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