2.4. Pelletization

Pelletization is an agglomeration process of iron ore concentrates with a granulometry lower than 150 μm and low concentration of impurities [10, 11]. This iron ore is mixed with water, bentonite (or other organic binders less expensive and contaminant [12]), and lime and treated in a furnace at temperatures of around 1200�C, with the purpose of obtaining a product of 10– 20 mm in diameter with suitable physical, chemical, and mechanical properties to be fed into the blast furnace or to the production of DRI (Direct Reduction Iron) [10]. Sintering raw mixes with a high proportion of fines (<150 μm) deteriorates the operation of the Dwight-Lloyd sinter machine, and for that reason pelletizing is the best method for treating this kind of material. The formation of liquid phases, which agglomerate the iron ore, in pellets is achieved by an external source of energy (fuel, natural gas or pulverized coal), as opposed to sintering, where liquid phases are consequence of the combustion of coke breeze [2].

Chemically, pellets are characterized by (approximately): 94% Fe2O3, 3.3% SiO2, 1.0% CaO, 0.20% MnO, 0.50% MgO and 1.0% Al2O3, and as we mentioned previously, a granulometry of 10–20 mm [2]. According to this question, it is possible to say that pellets are characterized by [10]: great uniformity regarding the size (10–20 mm in diameter); high mechanical strength; almost inert behavior facing to water as a consequence of the low CaO content that makes possible storing and transporting pellets outside; good reducibility; and, high iron content.

As in the case of the sintering, mixing is an important step to obtain homogenous pellets. This mixing is easy when the materials to be mixed have the same particle size and physical properties, however, binder additions (around 1%) are necessaries and they are less dense than the ore. For that reason, segregation between particles must be considered when mixing the materials in the production of pellets.

Bentonite clay is the most common binder agent in iron ore pelletization as we mentioned previously, and is added at levels of 0.5–1.5% by weight [8]. There are others, that can be classified into two categories: organics (do not add impurities to the pellet and can be added in amounts of 1/10 of the equivalent in bentonite, although sometimes do not provide sufficient strength to the indurated pellets), and inorganics (usually result in strong pellets, which is good for shipping and handlings, but have the problem of impurities) [8]. Examples of inorganic pellets are: bentonite, sodium carbonate, calcium carbonate or calcium hydroxide; while examples of organic pellets are: carboxymethylcellulose (CMC), other CMC-based polymers, corn starch, wheat flour or molasses [8].

granulometry to be fed into the blast furnace, ensuring a homogenous and stable operation in the blast furnace [2]. This definition proposed in [2], describes the sintering process, but prior to sintering there is an important process called granulation that is deeply reviewed also in [2]. Granulation is the homogenization of the iron ore mixture in a rotatory drum with 7–8% water having as objective the obtaining of a pre-agglomerated product, which is then delivered as a layer over a continuously moving grate or "strand" (Dwight-Lloyd machine) to obtain the sintered product. This process that takes between 30 and 60 min. (including the addition of moisture, granulation and feeding to the sintering machine) has a fundamental role as it

Iron Ore Agglomeration Technologies http://dx.doi.org/10.5772/intechopen.72546 65

Dwight-Lloyd technology is the main iron ore sintering technology. Basically, this equipment consists in a sintering grate that is a continuous chain of large length and width, which is formed by the union of a series of pallet cars making the sintering strand. The process in the Dwight-Lloyd machine begins with each pallet car passing below the charging hopper. Feeding is carried out in two layers: the hearth layer that protects steel grates from over-heating during the sintering process, which is formed by sinter of coarse granulometry (10–20 mm, with a granulometry not enough to be sent to the blast furnace) in a layer of 3–6 cm [14, 15]; and the layer of fine material (0–8 mm) granulated to be sintered (fine material, return fines, fluxes, and coke). Once charged, the pallet car passes below an initializing furnace, causing the combustible ignition on the surface of the mixture, and being at the same time the mixture subjected to downdraught suction through the load. The combustion of the coke breeze (or alternative combustible in the corresponding case) does not take place in the whole thickness of the bed instantaneously, on the contrary, combustion happens as a horizontal layer that moves vertically through the bed. In this way, permeability is important, and for that reason granulation is a fundamental step [16]. This movement of the combustion zone defines several zones in the sinter bed (from the lower to the upper zone) [16]: cold and wet zone, is the zone of the sinter mix to be sintered at a temperature lower than 100�C; drying zone, zone at temperatures between 100 and 500�C where the vaporization of moisture and dehydration of hydroxides take place; reaction zone, where the maximum temperatures are reached during the combustion of coke (exothermic), decomposition of carbonates (endothermic), solid phase reactions, reduction and re-oxidation of iron oxides and reactions of formation of the sintered mass take place; and finally, cooling zone, is the zone where the cooling and re-crystallization of the sintered product take place. The high thermal efficiency is consequence of the heat accumulation in a partial layer called sintering zone or flame front (which progresses at 10–30 mm/min) toward the sintering grate, and in this way a 500–600 mm bed height would be sintered in around 25 min [16, 17]. Finally, the flame front reaches the bottom of the layer at the end of the strand, where the sintered product is discharged and subjected to cooling, crushing, and screening, being three kinds of product finally obtained: 0–5 mm (or 0–10 mm depending on the author, see [16]), called return fines and sent back to the beginning of the sintering process; 5–20 mm (or 10–20 mm or 10–15 mm depending on the author, see [16]), used as hearth layer in the sinter strand with the functions previously mentioned; and, > 20 mm (or >15 mm depending on the

author, see [16]), sent to the blast furnace directly. Maximum sizes around 50 mm [18].

As we mentioned, flame front is the region limited by the moment where the coke begins to combust and the moment where the coke burned [19]. There are other definitions that can be

ensures an adequate sinter bed permeability and hence good productivity [2].

Regarding pelletizing technologies, it is possible to mention two that are used at industrial scale: rotary drum and rotary disc. The rotary drum is a large drum-shaped cylinder that is elevated at one of its ends (3–4�). The mixture (iron ore-binder) is fed at the elevated zone of the drum and lefts the drum at the lower zone, where it is classified into three groups, the undersize pellets (which are recirculated), the oversize pellets (which are recirculated after crushing), and the final product [8]. The pelletizer discs are the second technology, having as advantage if compared with the rotary drum that there is no recirculation. The mixture is fed to an inclined large disc (40–60� to the horizontal), and the rotation of the disc causes the formation of balls, which remain in the disc until they reached the adequate pellet size. The variables that should be controlled are the disc angle, feed rate, water addition, and rotation speed [8].

Once the green pellet is obtained, it must be subjected to induration as it is too weak for its application. Pellets are for that reason processed at high temperatures to strengthen them. Three types of furnaces are used: shaft furnace, traveling grate, and grate-kiln, and temperature will depend on the kind of pellet. Pellets can be classified into three groups:

