6. Induration technologies

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

Iron ore Binder Characterization Reference

Substitution degree,

absorption test) Viscosity

CMC, TPP No Drop number, strength

CMC, TPP, organic binders No Drop number, strength

Molecular design, wettability, ionization potential, electron affinity,

binding energy

(Floform 1049 V) polyacrylamide No Physical, chemical and

Viscosity as a function of pH and temperature, adsorption

pH, adsorption kinetic

viscosity

Peridur® and bentonite No Drop number,

Magnetite Peridur® No Drop number, strength

CMC (Na2CO3, NaCl), Peridur®,

Organic binders (including Peridur®) and bentonite

S-1 (modified starch type) and Funa (substance rich in humic acids obtained from lignite by caustic

HS (Humic substances extracted

Source: elaborated by the author with data from [8, 18–27].

Magnetite Organic binders and bentonite PWA (plate water

bentonite

52 Iron Ores and Iron Oxide Materials

extraction)

from lignite)

Hematite (CVRD nowadays VALE)

Taconite (Minnesota, USA)

Magnetite (Carol Plant, Canada)

CVRD Hematite (nowadays VALE)

Hematite (Ferteco)

Pellet feed (China)

Pellet feed (Russia)

Iron ore concentrate

Pellet feed (Baiama Plant— China)

Binder Pellets

compressive strength (green, dry)

Drop number, strength (green, dry, fired) chemical and metallurgical properties

Drop number, strength (green, dry, fired)

(green, dry, fired), metallurgical properties

(green, dry)

(green, dry)

Drop number, strength (green, dry), thermal shock (820–780�C)

metallurgical properties

Drop number, strength (green, dry), thermal shock (550–600�C)

Drop number, strength (green, dry), firing

(green, dry)

No Drop number, strength

[19]

[20]

[21]

[18]

[22]

[8]

[23]

[24]

[25]

[26]

[27]

Floerger, KemPel™ from Kemira, FLOTICOR PA 8000 from Clariant among others.

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

MHA (fulvic acid and humic acid) Viscosity, zeta potential,

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 pellets are thermally treated in the induration process.

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 drying steps are approximately 300C.

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 lead to the formation of liquid phase below 1250C.

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

To finalize the induration process, pellets are cooled down by contact with flow of ambient air. The resulting heated air is used in the upstream steps of firing and drying.

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

Figure 6. Isotherm at 1200�C phase diagram of CaO-SiO2-Al2O3 system.

is the reason for using very fine solid fuels such as anthracite or coke breeze in the mixture to be agglomerated during balling.

Since the hot air flows from the top of the bed in the high temperature steps, firing of pellets is not homogeneous. Pellets close to the top are treated at higher temperatures for longer times, while pellets at the bottom reach lower peak temperatures for shorter residence times. It is been reported [30] that pellets at the top may reach 1300�C for 6 minutes while pellets at the bottom peak at 1200�C with no residence time. These values may vary, but the difference is large enough to generate pellets with different mechanical strengths and different metallurgical proprieties. This is a disadvantage for the straight grate in comparison to the grate-kiln

Iron Ore Pelletizing Process: An Overview http://dx.doi.org/10.5772/intechopen.73164 55

Figure 7. Schematic diagram of the straight grate induration process [29].

Figure 8. Schematic diagram of the grate-kiln induration process [29].

In the grate-kiln process, shown in Figure 8, there are three different reactors. The drying, preheating, and cooling steps are similar to that of the straight grate process. The general concept

process.

The most widely used industrial processes for pellet induration are the straight grate and the grate-kiln.

The straight grate process is composed of a single furnace where an endless line of pallet cars moves. A layer of indurated pellets is arranged at the bottom of each car to protect it against the heat. The green pellets are then charged on top of the hearth of indurated pellets. A schematic diagram of this process is shown in Figure 7.

The process is designed to enhance heat recovery. Therefore, two flows of ambient air are heated while cooling the hot indurated pellets. These flows are directed to other zones of the furnace. This is a way of recovering the latent heat present in the hot indurated pellets.

The drying of green pellets is performed in two stages by blowing warm air through the bed of pellets. In the first stage, the hot air from the cooling zone is blown from the bottom. In the second drying stage, hot air from the firing zone is blown on the top of the car. The use of both updraft and downdraft drying ensures a more uniform treatment along the height of the bed of pellets.

The high temperature phase is divided into three steps: pre-heating, firing, and after-firing. In all these phases, pre-heated air is fed into burners to produce flue gases that flow through the bed of pellets from the top. The burners are usually fired with gaseous fuels, such as natural gas or atomized liquid fuels, such as diesel.

Figure 7. Schematic diagram of the straight grate induration process [29].

is the reason for using very fine solid fuels such as anthracite or coke breeze in the mixture to

The most widely used industrial processes for pellet induration are the straight grate and the

The straight grate process is composed of a single furnace where an endless line of pallet cars moves. A layer of indurated pellets is arranged at the bottom of each car to protect it against the heat. The green pellets are then charged on top of the hearth of indurated pellets. A

The process is designed to enhance heat recovery. Therefore, two flows of ambient air are heated while cooling the hot indurated pellets. These flows are directed to other zones of the

The drying of green pellets is performed in two stages by blowing warm air through the bed of pellets. In the first stage, the hot air from the cooling zone is blown from the bottom. In the second drying stage, hot air from the firing zone is blown on the top of the car. The use of both updraft and downdraft drying ensures a more uniform treatment along the height of the bed of pellets. The high temperature phase is divided into three steps: pre-heating, firing, and after-firing. In all these phases, pre-heated air is fed into burners to produce flue gases that flow through the bed of pellets from the top. The burners are usually fired with gaseous fuels, such as natural

furnace. This is a way of recovering the latent heat present in the hot indurated pellets.

be agglomerated during balling.

54 Iron Ores and Iron Oxide Materials

schematic diagram of this process is shown in Figure 7.

Figure 6. Isotherm at 1200�C phase diagram of CaO-SiO2-Al2O3 system.

gas or atomized liquid fuels, such as diesel.

grate-kiln.

Since the hot air flows from the top of the bed in the high temperature steps, firing of pellets is not homogeneous. Pellets close to the top are treated at higher temperatures for longer times, while pellets at the bottom reach lower peak temperatures for shorter residence times. It is been reported [30] that pellets at the top may reach 1300�C for 6 minutes while pellets at the bottom peak at 1200�C with no residence time. These values may vary, but the difference is large enough to generate pellets with different mechanical strengths and different metallurgical proprieties. This is a disadvantage for the straight grate in comparison to the grate-kiln process.

In the grate-kiln process, shown in Figure 8, there are three different reactors. The drying, preheating, and cooling steps are similar to that of the straight grate process. The general concept

Figure 8. Schematic diagram of the grate-kiln induration process [29].

of heat recovery by using hot gases from downstream in the process for drying and for feeding the burners is also present. However, a rotary kiln is used for the firing step.

companies. Hence, the amount and quality of supplied iron ore products are important but

Iron Ore Pelletizing Process: An Overview http://dx.doi.org/10.5772/intechopen.73164 57

Therefore, the reasons why new investments in pelletizing capacity are likely to occur are the

• the pelletizing process is currently the most widely used option for producing suitable

• quality requirements for DR pellets are higher and since lump ore of the required quality

• pellets provide advantages to end users, such as improved productivity of blast furnaces, opportunity to increase the Fe content of the charge materials, and superior environmen-

• the main iron producing systems, blast furnace, and DR reactors, will not be replaced in

The authors would like to thank assistant researcher Dafne Pereira da Silva and interns (mining engineering students) Milton Candido Torres da Silva and Lucas Shin Takyia for

\*, José Renato Baptista de Lima2 and Tiago Ramos Ribeiro<sup>1</sup>

agglomerates for ironmaking applications from fines of iron ore concentrates;

is not available, pellets are the only viable feed for new DR plants;

tal performance of pellet plants as compared to sinter plants;

also to provide a diversity of types of pellets.

following [29]:

the near future.

Acknowledgements

Author details

References

May, 2014]

Sandra Lúcia de Moraes<sup>1</sup>

helping find and organize figures and references.

\*Address all correspondence to: sandralm@ipt.br

1 Institute for Technological Research, Sao Paulo, Brazil

2 School of Engineering of the University of Sao Paulo, Sao Paulo, Brazil

[1] Ball D. Agglomeration of Iron Ores. New York: American Elsevier Pub. Co.; 1973

[2] Tuck C, Virta R. 2011 minerals yearbook: Iron ore. USGS [Internet]. 2013. Available from: http://minerals.usgs.gov/minerals/pubs/commodity/iron\_ore/myb1-2011-feore.pdf [Accessed:

[3] Mourão J. Estudo Prospectivo do Setor Siderúrgico: NT Minério de Ferro e Pelotas Situação Atual e Tendências 2025. Brasília: Centro de Gestão e Estudos Estratégicos; 2008

The pre-heating zone is divided into two steps: tempered pre-heating zone and pre-heating zone, where maximum temperatures may reach between 1000 and 1100�C. Pellets need to gain some mechanical strength during pre-heating to withstand the tumbling inside the rotating kiln where firing is performed.

The firing in the rotating kiln generates pellets with more uniform properties. The movement of the kiln causes pellets to mix during the firing treatment and the temperature is more even among different pellets. The furnace is heated with a flame on the discharge side. The use of fuel is more flexible in this case in comparison to the straight grate. Besides gaseous and liquid fuels, solid fuels such as coal may also be used. This is of particular interest in regions with availability of cheap solid fuels.

After firing, pellets are discharged in a pallet car for the cooling stages.

Therefore, pellets undergo more charging and discharging operations during the grate-kiln process than in the straight grate. This causes a greater generation of fines during the process. However, the final pellet properties are more uniform, and fines generation during transport for final use is therefore expected to be lower for pellets produced in the grate-kiln process.

The worldwide pelletizing capacity is divided into 60% for the straight grate process and 40% for the grate-kiln. Both systems are used to produce quality blast furnace and direct reduction pellets.
