Iron ore pellets
Iron ore pellets
Iron ore pellet is a type of agglomerated iron ore fines which has better tumbler index when compared with that of parent iron ore and can be used as a substitute of lump ore for the production of direct reduced iron (DRI) and in blast furnaces for the production of hot metal. The term iron ore pellets refers to the thermally agglomerated substance formed by heating a variable mixture of iron ore, limestone, olivine, bentonite, dolomite and miscellaneous iron bearing materials in the range of 1250 deg C to 1350 deg C. Iron ore pellets are normally produced in two types of grades namely DRI grade and BF grade. BF grade pellets have higher basicity than the DRI grade. The general identification details of iron ore pellets are given Tab 1.
Tab 1 Identification details of Iron ore pellets
|Chemical name||Iron ores, agglomerates|
|Other names||Iron ore pellets, iron oxide pellets|
|Molecular weight (gram/mole)||159.7|
|Synonyms||Di iron trioxide|
|Mineral of identical or similar composition||Hematite|
|Other identity code: Related CAS No.||Hematite (Fe2O3) 1317-60-8|
|REACH (Registration, Evaluation, Authorization, and restriction of Chemicals) registration No.||01- 2119474335-36-0013|
DRI pellets donot contain CaO while BF grade pellets are fluxing pellets containing CaO. For BF grade pellets reducibility and swelling index are important properties while for DRI grade disintegration is an important property. The properties of pallets are given in Tab 2.
Tab 2 Properties of pellets
|pH (40 gm/L,20Deg C; slurry in water)||5.0 – 8.0|
|Melting point||1500-1600 deg C|
|Bulk density||2.0 -2.2 t/Cum|
|Tumbler index (+6.3 mm)||93-94 %|
|Abrasion index (-0.5 mm)||5-6 %|
|Compression strength (daN/p)||250 min|
|Size distribution (+8mm -18 mm)||95 % min|
|Size distribution (-5 mm)||1.50%|
The chemical analysis of pellets are given in Tab 3
Tab 3 Chemical analysis of pellets
|BF grade||DRI grade|
|Fe||%||63 – 65.5||65 -67.8|
|SiO2 + Al2O3||%||< 5||<5|
|CaO + MgO||%||Up to 3||Up to 0.10|
|Disintegration (-3.15 mm)||%||2|
Quality of the pellets is influenced by the nature of the ore or concentrate, associated gangue, type and amount of fluxes added. These factors in turn result in the variation of physicochemical properties of the coexisting phases and their distribution during the pellet induration. Hence properties of the pellets are largely governed by the form and degree of bonding achieved between the ore particles and the stability of these bonding phases during reduction of iron oxides. Since the formation of phases and microstructure during induration depends on the type and amount of fluxes added, there is an effect of fluxing agents in terms of CaO/SiO2 ratio and MgO content on the pellet quality. Other factors which have effect on pellet quality are given below.
- Cold crushing strength of pellets found to increase with increasing firing temperature.
- Decrease of mean particle size (MPS) does not have a relationship with the pellet strength, but it reduces the porosity of the pellet.
- Fluxed pellets exhibits good strength, improved reducibility, swelling and softening melting characteristics. Because of these properties these pellets give better performance in the blast furnace.
- Swelling indicates volume change of pellets during reduction. Higher swelling reduces the strength of the pellets after their reduction thereby resulting in high resistance to gas flow, burden hanging and slipping indiside the blast furnace. Maximum allowable swelling of pellets for the blast furnace ranges from 16 -18 %.
- Acid pellets (DRI pellets) and MgO free pellets exhibit higher swelling at 0.6 basicity and decreased thereafter.
Mineralogically iron ore pellets comprise essentially hematite (original surviving) particles of iron ore, crystalline silica (quartz, cristobalite and tridymite) and forsterite (Mg2SiO4). The principle variation in pellet mineralogy is in the proportion of gangue phases present in the product. These will vary depending upon the pellet feed material and the type and the amount of any additives to feed such as limestone, dolomite, olivine and bentonite etc.
The mosaic image of cross section of a pellet is shown in Fig 1. The radial variation of phases, especially pores, is readily visible in the image.
Fig. 1 Mosaic image of pellet cross section
Advantages of Pellets
Iron ore pellet has the following advantages.
- Pellets have good reducibility since they have high porosity (25-30%). Normally pellets are reduced considerably faster than sinter as well as iron ore lumps. High porosity also helps in better metallization in DRI production.
- Pellets have a uniform size range generally within a range of 8 -18 mm.
- Pellets have spherical shape and open pores which give them good bed permeability.
- Pellets have low angle of repose which is a drawback for pellet since it creates uneven binder distribution.
- The chemical analysis is uniform since it gets controlled during the beneficiation process. Fe content varying from 63% to 68% depending on the Fe content of Ore fines. Absence of LOI is another advantage of the pellets.
- Pellets have high and uniform mechanical strength and can be transported to long distances without generation of fines. Further it has got resistance to disintegration. High mechanical and uniform strength of pellets is even under thermal stress in reducing atmosphere.