4. Bonding mechanisms

The main bonding mechanisms of size enlargement are described as being [11, 13]:


Nucleation (Figure 5a) is defined as any formation of new pellets in an agglomeration system from an extra feed of moist material. The nucleation of new species results from the capillary attraction between a collection of individual moist feed particles. Thus, the occurrence of nucleation promotes changes in the mass and number of well-formed species in the system.

Figure 5. Details of the mechanisms proposed by Sastry and Fuerstenau [14]. (a) Nucleation, (b) snowballing/layering, (c)

Iron Ore Pelletizing Process: An Overview http://dx.doi.org/10.5772/intechopen.73164 49

coalescence, (d) breakage, and (e) abrasion transfer.

Whenever a new feed is supplied to a pelletizing system, the pellets act as seeds and tend to accumulate the newly added moist material. This mechanism is called snowballing or layering (Figure 5b). In this case, it is considered that all new moist feed nuclei are of unit mass and are

e. interlocking, depending on the shape of particles, for example, fibers, threads, or lamellae.

The agglomerated growth mechanism was defined by Sastry and Fuerstenau [14], considering that in industrial balling systems, the balling devices are continuously operated and the new feed is constantly supplied to previously seed pellets. The authors describe five agglomerate growth mechanisms as shown in Figure 5.

4. Bonding mechanisms

48 Iron Ores and Iron Oxide Materials

and deposition of colloidal particles;

growth mechanisms as shown in Figure 5.

viscous binder), adsorption layers <3 nm thickness);

The main bonding mechanisms of size enlargement are described as being [11, 13]:

Figure 4. Typical balling drum arrangement. Source: Modified by the authors with data from Ball [1].

b. interfacial forces and capillary pressure in movable liquid surfaces (liquid bridges);

a. solid bridges between agglomerating particles, which may occur by sintering, partial melting, chemical reaction, hardening binders, recrystallization of dissolved substances,

c. adhesional and cohesional forces in bonding bridges, which are not freely movable (highly

d. attraction between solid particles: molecular forces (Van der Walls forces, free chemical bonds—valence forces—associations—nonvalence—and hydrogen bridges), electric

e. interlocking, depending on the shape of particles, for example, fibers, threads, or lamellae. The agglomerated growth mechanism was defined by Sastry and Fuerstenau [14], considering that in industrial balling systems, the balling devices are continuously operated and the new feed is constantly supplied to previously seed pellets. The authors describe five agglomerate

forces (electrostatic, electrical double layers, excess charges), and magnetic forces;

Figure 5. Details of the mechanisms proposed by Sastry and Fuerstenau [14]. (a) Nucleation, (b) snowballing/layering, (c) coalescence, (d) breakage, and (e) abrasion transfer.

Nucleation (Figure 5a) is defined as any formation of new pellets in an agglomeration system from an extra feed of moist material. The nucleation of new species results from the capillary attraction between a collection of individual moist feed particles. Thus, the occurrence of nucleation promotes changes in the mass and number of well-formed species in the system.

Whenever a new feed is supplied to a pelletizing system, the pellets act as seeds and tend to accumulate the newly added moist material. This mechanism is called snowballing or layering (Figure 5b). In this case, it is considered that all new moist feed nuclei are of unit mass and are 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 of pellets.

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].

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

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

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

(Patent N�) Binder Others compounds Compressive strength (kg)

Copolymer of acrylamide Sodium acrylate

Carboxymethyl cellulose Sodium

Carboxymethyl cellulose (Peridur®) Source of hydroxide

Copolymer of acrylamide/ Carboxymethyl cellulose

Carboxymethyl cellulose (Peridur®), polyacrylate, polyacrylamide, cellulose derivatives, guar gum, starches,

acrylamide, maleic acid

Homopolymer of methacrylic acid,

dextrins, pectins

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

(Na3Citrate) Na2CO3

Sodium acrylate (Na3Citrate) Na2CO3

tripolyphosphate

Source of hydroxide ions (NaOH)

Limestone/esmectite 1.24–1.61/

1.44–2.61

ions (NaOH)

Green Dry

Iron Ore Pelletizing Process: An Overview http://dx.doi.org/10.5772/intechopen.73164 51

0.4–0.71 2.07–6.02

0.4–0.71 2.07–6.02

0.77–1.54 4.76–7.85

— 0.64–5.4

— 0.68–3.22

2.9–6.59

polyacrylamide polymers.

improve the properties of the pellets.

using bentonite.

Allied Colloids Limited (EU 0225171—1987)

Allied Colloids Limited (US 4.684.549—1987)

Aqualon Company (US 4.863.512—1989)

Peridur® Nobel (US 5.698.007—1997)

Peridur® Nobel (US 6.071.325—2000)

Clariant S.A. Brazil (EP 2548978 A1—2013)

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 system.

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

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

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 considerably.
