5. Evolution of binder for iron ore pelletizing

Binders are used in the pelletizing of iron ore aiming to improve the performance of the process in the following aspects [1, 4, 11, 16, 17]:


Bentonite, an inorganic binder, has been the main binder used in the iron ore pelletizing process since the beginning of pellet production in the 1950s. Bentonite promotes the formation of ceramic bridges between particles, which can minimize the number of pellets that collapse during firing. Despite its low cost, the inorganic compounds from bentonite are contaminants increasing the amount of acid gangue in the pellet. This increases the amount of slag formed in iron and steelmaking, which add to the energy needs of such processes [18].

not considered to belong to the population of agglomerates undergoing size change. In addition, the snowballing mechanism is considered to cause continuous change in pellet size, resulting in an increase in the total mass of the system and does not change the total number

Coalescence (Figure 5c) refers to the production of large-size species through the aggregation of two or more colliding granules. Binary coalescence is considered an elementary event. Thus, the collision coalescence of two agglomerated species leads to the formation of a larger sized pellet with mass. The coalescence mechanism causes discrete changes in the agglomerate mass and contributes to the decrease in the number of pellets, but does not change the total mass of

The breakage of pellets (Figure 5d) leads to the formation of a collection of fragments that are considered to belong to the class of well-formed species. These fragments are redistributed on the surviving pellets, causing the so-called layering according to the layering mechanism.

In the abrasion transfer mechanism (Figure 5e), a certain mass of material is transferred from one species to another due to the interaction and abrasion of the agglomerate during the pelletizing process. Mathematically, it is expected that on each encounter between species, an infinitesimal mass of material will be transferred from one to the other, with no preference of exchange in any direction. The abrasion transfer growth mode does not change the total

The optimum moisture content and particle size distribution are two decisive factors for green pellets formation. The moisture interferes with two important properties of green pellets: compressive strength and drop resistance. These two properties are complementary; to obtain a high compressive strength a lower water addition is necessary, whereas to achieve better

Urich and Han [15] studied the effect of grind on the quality of pellet of specular hematite and found that as the amount of particles smaller than 44 μm increases, the compressive strength (both green and indurated pellets), abrasion resistance, and other related properties improve

Binders are used in the pelletizing of iron ore aiming to improve the performance of the

• preventing the collapse of pellets in the initial stages of heating, when a large volume of

number or total mass of pellets in the system, causing only continuous changes in size.

resistance to drop the pellet should present higher moisture content [4].

5. Evolution of binder for iron ore pelletizing

• increasing the green and dry strength of the pellets;

gas generated by water vaporization tends to crack pellets;

process in the following aspects [1, 4, 11, 16, 17]:

• improving the properties of the fired pellets.

• promoting and facilitating the balling;

of pellets.

50 Iron Ores and Iron Oxide Materials

the system.

considerably.

Organic binders have been used as an attractive alternative to bentonite in iron ore pelletizing process, mainly because it burns without leaving any residue in the final pellet. There are two main types of organic binders, those based on cellulose compounds and other based on polyacrylamide polymers.

Table 3 shows some patents from chemical industries claiming the employment of organic binders in iron ore pelletizing aiming to replace bentonite in the process. The effectiveness of the binders is given in terms of compressive strength of pellets compared with the results from using bentonite.

Regarding research papers, Table 4 lists some publications, which report studies applying organic binders to iron ore pelletizing since the 1980s. All analyzed publications show results of compression strength (green and dry) and drop test from using organic binders. In some cases, the characterization of binders is also presented along with the discussion of their effects on the pellet properties. However, these studies do not explain how organic binders act to improve the properties of the pellets.


Table 3. Patents about organic binders in the iron ore pelletizing process.


6. Induration technologies

drying steps are approximately 300C.

pellets are thermally treated in the induration process.

lead to the formation of liquid phase below 1250C.

controlled by the rate of oxygen diffusion in the hematite crystal [28].

The resulting heated air is used in the upstream steps of firing and drying.

The final use of iron ore pellets in ironmaking reactors requires minimum mechanical properties. Pellets must withstand tumbling and falling during transport and mechanical loading inside the reactors due to the charge weight. In order to increase its mechanical strength, green

Iron Ore Pelletizing Process: An Overview http://dx.doi.org/10.5772/intechopen.73164 53

Pellets undergo drying, firing, and cooling steps. First, the water in the form of moisture is removed from the pellets in the drying steps. There is water in the pores and capillaries of pellets, that is, between different ore particles. In the case of porous ores, water may also be found in the pores of individual grains. Since this type of pores are normally smaller in size than the pellet pores, the temperature required to eliminate this water is expected to be higher. In the industrial process, the maximum temperatures reached in the solid phase during the

After drying, pellets undergo firing steps, at which temperatures may reach 1350C. In these steps, the roasting of all pellets components (ore, limestone, binders, etc.) occurs, liberating chemically bonded water and CO2. Additionally, the sintering of ore grains also happens, leading to the development of mechanical strength. This sintering may be caused by solid state interaction of particles, but also with the presence of liquid phase, which can act as transport media increasing the sintering rate. The liquid phase also acts as bonds among ore particles. The presence of liquid or semi-liquid phases is more pronounced in fluxed pellets where acid constituents normally from the ore (e.g., SiO2 and Al2O3) may react with basic ones added in the form of fluxes (e.g., CaO and MgO). This reaction may result in the formation of a slag phase. Figure 6 shows the phase diagram of the ternary system Al2O3-CaO-SiO2 at 1200C. At this temperature, liquid phase is present and indicated as "ASlag – liq." The reaction between iron oxide and fluxes or impurities is also possible. The interaction of CaO with Fe2O3 may

The major development regarding pellet strength occurs at temperatures above 1200C and is caused by the formation of necks between ore grains followed by pellet densification. These mechanisms are typical of solid state sintering. Pellet densification with increase in strength is

To finalize the induration process, pellets are cooled down by contact with flow of ambient air.

Induration processes were initially developed for ores composed of magnetite, since they are oxidized, producing hematite, and generating heat (482.4 kJ/mol of Fe3O4). In the case of ores composed of hematite, this heat liberation does not happen and needs to be compensated. For this reason, hematite is agglomerated with controlled amounts of carbon (1–2% wt.) that burns during induration, generating the required heat. For both cases, heat is induced inside the pellet by the diffusion of hot air through the pores of pellets and subsequent chemical reaction. In the case of magnetite, heat generation is more uniform over the pellet volume, while for hematite it will be concentrated around carbon particles that must be evenly distributed. This

Table 4. Scientific papers which investigated the effect of organic binders in iron ore concentrate pelletizing.

There are currently some organic binders available in the market for palletization of iron ore such as Peridur® from Akzo Nobel, Alcotac® from Basf Corporation, FLOFORM™ from SNF Floerger, KemPel™ from Kemira, FLOTICOR PA 8000 from Clariant among others.
