We can help our customers to determine which process is the best for their ore, fuel and pellet requirements. Our systems combine the best features of both technologies to provide the most modern plant and to produce pellets at the lowest cost and highest quality.
This process generates iron ore filter cake which needs to be pelletized to be used in the steel making process. Also during the processing of high grade iron ores which dont need beneficiated, fines which are generated can be pelletized and used instead of being disposed of.
Iron Ore Pellets are formed from beneficiated or run of mine iron fines. The iron is usually ground to a very fine level and mixed with limestone or dolomite as a fluxing agent and bentonite or organic binders as a binding agent. If the ore is a Hematite ore, coke or anthracite coal can be added to the mix to work as an internal fuel to help fire the pellets. This mixture is blended together in a mixer and fed to balling discs or drums to produce green pellets of size typically about 9-16mm. The green pellets are then fed to the induration machine. Both straight grates and grate kilns dry the pellets out in a drying section, then bring the pellets up to a temperature of about 800-900 Cin a preheat zone, then finish the induration process at roughly 1200-1350 C. The pellets are then cooled to a suitable temperature for transporting to a load out facility. Both processes recycle the heat from the pellet back through the process to aid in energy efficiency and decrease fuel usage.
Both processes can be used to generate almost any type of desired pellet chemistry, from direct reduction pellets (DR pellets) to blast furnace pellets. By adjusting the amount of fluxing agent or limestone added, pellets can be made that are anywhere from acid (or non-fluxed) pellets to heavily fluxed pellets.
Mixing is where the properly ground ore is combined with binding agents like Bentonite or organic binders, fluxing agents like limestone or dolomite, and if the ore is a Hematite with coke or anthracite coal as an internal fuel. The mixing is done usually in vertical or horizontal high intensity mixers to achieve a homogenous blend of ore and additives.
From mixing the filter cake is sent to the balling area where the ore is agglomerated on balling discs or balling drums into green(or unfired) pellets. Both drums and discs ball the ore to about 9-16mm size. Drums typically have very high recycle rates so have a screening circuit to screen out undersize and oversize pieces to be put back through the drum. Discs usually do not have a separate screening circuit at the disc.
Green pellets are then transported to the induration process. Pellets that are oversized or undersized and any fines generated during the balling or transporting process are screened right before entering the induration machine and sent back to the mixer or the balling area. The on-size pellets are then fed to the induration machine. Both straight grates and grate kilns dry the pellets out in a drying section, then bring the pellets up to a temperature of about 800-900 Cin a preheat zone, then finish the induration process at roughly 1200-1350 C. The pellets are then cooled to a suitable temperature for transporting to a load out facility. Both processes recycle the heat from the pellet back through the process to aid in energy efficiency and decrease fuel usage
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The iron ore production has significantly expanded in recent years, owing to increasing steel demands in developing countries. However, the content of iron in ore deposits has deteriorated and low-grade iron ore has been processed. The fines resulting from the concentration process must be agglomerated for use in iron and steelmaking. This chapter shows the status of the pelletizing process with a special focus on binders. Bentonite is the most used binder due to favorable mechanical and metallurgical pellet properties, but it contains impurities especially silica and alumina. The importance of many researches concerning alternative binders is also discussed in this chapter. Better quality wet, dry, preheated, and fired pellets can be produced with combined binders, such as organic and inorganic salts, when compared with bentonite-bonded pellets. While organic binders provide sufficient wet and dry pellet strengths, inorganic salts provide the required preheated and fired pellet strengths.
Iron ore and iron ore pellets are important sources of iron for manufacturing steel. The iron ore production has significantly expanded in recent years, owing to increasing steel demands in developing countries, such as China and India. However, the content of iron ore in deposits has deteriorated and low-grade iron ore has been processed. The fines resulting from the enrichment by separation after liberation by size reduction must be agglomerated in a pelletizing plant. Consequently, the number of pelletizing plants is expected to increase in the future.
The quality requirements of pellet, such as physical, chemical and metallurgical specifications, depend on each ironmaking furnace and those requirements influence the operation of the iron ore pelletizing plant.
The idea of rolling moist fine ore in a drum to form balls and then drying and firing it was first patented by A. G. Andersson in Sweden in 1912. Further development was performed to bring the idea to reality. In 1943, E. W. Davies and co-workers demonstrated the process using an experimental shaft furnace. Commercial operation started in the 1950s in Sweden using vertical-shaft-kilns for firing the pellets. Plant capacities were between 10,000 and 60,000 tons/year .
The world installed pelletizing capacity is currently estimated to be 480.7 million tons/year . As shown in Figure 1, China presents the largest production capacity, followed by the United States and Brazil.
The iron ore pelletizing process consists of three main steps:Pelletizing feed preparation and mixing: the raw material (iron ore concentrate, additivesanthracite, dolomiteand binders are prepared in terms of particle size and chemical specifications, dosed, and mixed together to feed the pelletizing process;Balling process: the green pellet is the rolled pellet without any thermal process. It is obtained under strict control of moisture and has a spherical shape and diameter of 816mm;Induration process: the green pellets are hardened in a high temperature processing at controlled heating rates, and aiming to achieve the physical and metallurgical requirements for handling, transportation, and final application.
Pelletizing feed preparation and mixing: the raw material (iron ore concentrate, additivesanthracite, dolomiteand binders are prepared in terms of particle size and chemical specifications, dosed, and mixed together to feed the pelletizing process;
Induration process: the green pellets are hardened in a high temperature processing at controlled heating rates, and aiming to achieve the physical and metallurgical requirements for handling, transportation, and final application.
This chapter aims to provide an overview and the evolution of iron ore pelletizing process including:Pelletizing process and raw materialsBalling technologiesBonding mechanismsEvolution of binders for iron ore pelletizingInduration technologiesChallenges and innovations in iron ore pelletizing
The iron ore is mined mostly from open pit deposits through mining operations and the raw product, run of mine, is subjected to mineral processing. Thus, the material is exposed to a series of operations of fragmentation, separation by size, concentration, dewatering, etc., aiming to adequate the chemical, physical, and metallurgical characteristics to meet the demands of ironmaking processes. The particle size distribution of iron ore is a very important requirement to be characterized after its mineral processing.
Materials containing a very fine particle size distribution are not adequate to be used directly in the reduction reactors, requiring to be agglomerated by different processes such as sintering or pelletizing.
The main used reduction reactors are the blast furnace (BF) and direct reduction reactors (DR). In the blast furnace, iron is reduced and melted and the most common product is liquid iron called hot metal. In direct reduction, iron remains in solid state and the product is the so-called direct reduced iron (DRI).
Pellets are balls formed by rolling moist concentrates and fines iron ores of different mineralogical and chemical composition, with the addition of additives and binder, in a horizontal drum or in an inclined disc [1, 4].
Pellets produced to be used in ironmaking processes must have characteristics that meet the list of quality specifications regarding physical, chemical, and metallurgical properties. Almost all of those properties are standardized as shown in Table 2.
Aiming to achieve those specifications, binders and additives are used in the pelletizing process. Additives such as limestone, dolomite, and hydrated lime are used to modify the chemical composition of the pellets, most often for correction of the basicity.1 Certain substances such as hydrated lime serve as both additive and binder. Fines of anthracite or coke are also added during the pelletizing process for reducing the consumption of fuel required for internally heating the ball .
Pellets are obtained by adding an appropriate amount of water to the iron ore concentrate; this is a fundamental factor in the formation and growth of pellets, which creates a surface tension that holds the mineral grains cohesive, thus allowing their handling [6, 8].
This cohesive tension of fine particles due to water is called neutral tension. Neutral tension, however, is not sufficient to keep cohesive grains as dense as iron minerals. Furthermore, when the pellet is heated, the vaporization of water occurs and the pellets tend to disintegrate.
To avoid such effects, binders are added to the material to be pelletized, aiming to:increase the strength of pellets before heating (green strength);prevent the collapse of pellets during the initial stages of heating, when a large volume of gas generated by water vaporization tends to crack the pellets.
Evenly distributed moisture and binder in the feeding process is decisive to improve the characteristics of pellets, especially to avoid the formation of undesirable agglomerates before the pellet formation.
However, bentonite incorporates silica and alumina, which are undesirable contaminants to pellets. Additionally, it is a natural material with variable composition depending on its origin. Obtaining a suitable binding effect requires a relatively large amount of material, around 0.5% by weight, which makes handling more difficult and increases logistics costs.
Figure 2 shows a flowchart of a typical pelletizing plant, highlighting the additive preparation, mixing and pelletizing feed preparation, the balling step, in this case using a disc pelletizer, and the induration step.
Figure 3 schematically shows a laboratory disc pelletizer. The balling disc basically consists of a pan with a peripheral wall, which rotates with a certain inclination to the horizontal . In general, in a balling disc, the pan angle can be adjusted between 40 and 60 .
When the disc rotates, the feed material with less moisture than that required for the pellets formation, is charged to the bottom of the disc, where it get in contact with the water from the sprayers, initiating the nucleation stage. At this stage, the nuclei begin to take the form of small pellets, which by rolling action, occur in the lower section, to the left of the disc, toward the top. As the added ore aggregates unto the surface of the pellets, they increase in size and the coefficient of friction is reduced causing the pellets to acquire a centrifugal force that carries them out of the nucleation zone. This movement takes the pellets to the top of the disc following a semicircular trajectory before returning to the base of the disc [1, 12].
The height and width of the trajectory increase as the ball size increases until the balls hit the scraper blades. After that, they move down and pass under the water sprayers so that they can find fresh feed.
The balling drum equipment fundamentally consists of an inclined rotating cylindrical shell with water sprays in its inlet end, where the feed material is introduced to make balls. All formed pellets are discharged, regardless of particle size, which is different from the disc where only balls larger than a certain size are discharged. Because of this, the product has to be screened by a roller screen, which has increasingly replaced vibrating screen to extract undersize and oversize .
The small pellets in the undersize will serve as seeds forming rightly sized pellets , the oversize is shredded, and both return to the feed addition point, forming a closed circuit. The water sprays promote nucleation and seed growth in the feed addition zone, while the assimilation process, responsible for the ball growth, occurs along the length of the drum .
The drums usually have a length-to-diameter ratio of 2.53.5 and very low slope with angles of inclination of the drum axis to the horizontal between 6 and 10. The optimum rotating speed is generally between 25 and 35% of the critical speed that is the speed in which balls will centrifuge causing their degradation . Speed control is necessary to develop a correct rolling and tumbling action to produce balls.
The rotation speed, the depth of the material in the drum (controlled by scrappers), and the time required for balling are constraints that need to be satisfied. The residence time in a drum is given by Eq. (1):
The main bonding mechanisms of size enlargement are described as being [11, 13]:solid bridges between agglomerating particles, which may occur by sintering, partial melting, chemical reaction, hardening binders, recrystallization of dissolved substances, and deposition of colloidal particles;interfacial forces and capillary pressure in movable liquid surfaces (liquid bridges);adhesional and cohesional forces in bonding bridges, which are not freely movable (highly viscous binder), adsorption layers <3nm thickness);attraction between solid particles: molecular forces (Van der Walls forces, free chemical bondsvalence forcesassociationsnonvalenceand hydrogen bridges), electric forces (electrostatic, electrical double layers, excess charges), and magnetic forces;interlocking, depending on the shape of particles, for example, fibers, threads, or lamellae.
solid bridges between agglomerating particles, which may occur by sintering, partial melting, chemical reaction, hardening binders, recrystallization of dissolved substances, and deposition of colloidal particles;
attraction between solid particles: molecular forces (Van der Walls forces, free chemical bondsvalence forcesassociationsnonvalenceand hydrogen bridges), electric forces (electrostatic, electrical double layers, excess charges), and magnetic forces;
The agglomerated growth mechanism was defined by Sastry and Fuerstenau , 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.
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.
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 .
Urich and Han  studied the effect of grind on the quality of pellet of specular hematite and found that as the amount of particles smaller than 44m increases, the compressive strength (both green and indurated pellets), abrasion resistance, and other related properties improve considerably.
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]:promoting and facilitating the balling;increasing the green and dry strength of the pellets;preventing the collapse of pellets in the initial stages of heating, when a large volume of gas generated by water vaporization tends to crack pellets;improving the properties of the fired 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 .
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.
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.
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 .
Induration processes were initially developed for ores composed of magnetite, since they are oxidized, producing hematite, and generating heat (482.4kJ/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 (12% 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 is the reason for using very fine solid fuels such as anthracite or coke breeze in the mixture to be agglomerated during balling.
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.
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  that pellets at the top may reach 1300C for 6minutes while pellets at the bottom peak at 1200C 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, pre-heating, and cooling steps are similar to that of the straight grate process. The general concept 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.
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 1100C. 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.
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 properties of iron ore depend much on its genesis. Different ores have distinct characteristics and varying performances in the mining and in the metallurgical processes. Ore particle shape, size, texture and capability of water retention, capillarity and cohesive strength among different particles determine the velocity of pellet growth during balling and pellet porosity. Ore characteristics also influence on the required amount of binder to produce pellets with satisfactorily quality. The use of iron ores with smaller crystal size and less dense structure usually results in pellets with better reducibility, impacting the performance of ironmaking reactors. Therefore, different iron ores should be valued in function of the benefit from their use .
The influence of ore characteristics on beneficiation, concentration, and pelletizing are important for process optimization, improving product quality, and consequently, more efficient use of natural resources and energy. Hence, as described by Moraes and Ribeiro , the growing importance of this topic that is referred to as geometallurgy is evident.
Mining companies work toward obtaining pellets with customized properties for each type of ironmaking process and also to meet specific requirements of different iron- and steelmaking companies. Hence, the amount and quality of supplied iron ore products are important but also to provide a diversity of types of pellets.
Therefore, the reasons why new investments in pelletizing capacity are likely to occur are the following :the pelletizing process is currently the most widely used option for producing suitable agglomerates for ironmaking applications from fines of iron ore concentrates;quality requirements for DR pellets are higher and since lump ore of the required quality is not available, pellets are the only viable feed for new DR plants;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 environmental performance of pellet plants as compared to sinter plants;the main iron producing systems, blast furnace, and DR reactors, will not be replaced in the near future.
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 environmental performance of pellet plants as compared to sinter plants;
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 helping find and organize figures and references.
2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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specification that the DRI pellets are not oxidized nor reduced while descending through the top segment so that their mass and composition entering the bottom segment are the same as when they are top charged; and
replacement of [mass scrap steel descendinginto the bottom segment] column of Chapter 43, Top-Charged Scrap Steel, with [mass DRI pellets descendinginto the bottom segment] column as shown in Table 44.1.
In a DR process, iron ore pellets and/or lump iron ores are reduced by a reducing gas to produce DRI or hot briquetted iron (HBI). Depending on the generation of the reducing gas, two different DR processes are commercially available: gas-based and coal/oil-based. In the gas-based DR process, the reducing gas is produced by chemically reforming a mixture of natural gas and off-gas from the reducing furnace to produce a gas that is rich in hydrogen and carbon monoxide. Typical examples of the gas-based DR process include MIDREX and HYL, which are often the preferred technology in countries where natural gas is abundant. However in the coal/oil-based DR process, the reducing gas is generated from hydrocarbons (primarily coal, but sometimes oil and natural gas) in the reduction zone of the furnace, which is typically a rotary kiln. Typical examples of the coal-based process include the SL/RN and ACCAR processes. The coal-based DR process is more popular in India and China. Different types of reactors, such as shaft furnaces, fluidized beds, rotary kilns, and rotary hearth furnaces, have been used in different variations of the processe to achieve the metallization required.
Based on statistics (Anon 3, 2014), India is the world leader in DRI production producing about 17.8Mt of DRI in 2013, approximately one-forth of world DRI production. The gas-based DR processes are producing almost 80% of the world's DRI. MIDREX is the key variant of the gas-based DR processes accounting for about 63.2% of world DRI production in 2013, followed by HYL (15.4%). Therefore, the following discussion focuses mainly on the MIDREX process.
The free swelling test determines the volume increase of iron ore pellets during reduction. When pellets were first introduced, a swelling tendency led to damage to the BF stack, poor permeability to gas flow, and irregular burden descent. The test does not apply to lump ore or sinter.
An electrically heated furnace with a vertical reduction tube that contains a wire basket with room for 18 individual pellets is used. The pellets with sizes ranging from 10 to 12.5mm are placed in three levels of six pellets each. The tube is 75mm in diameter and is preheated by hot reduction gas flowing in the space between the walls. The pellets are dried at 105C, and their volume is measured. Afterward, they are placed in the wire basket and lowered into the test furnace. The pellets are first preheated with hot inert gas to the test temperature of 900C in a N2 atmosphere, after which reduction gas with composition 30%/70% (CO/N2) is introduced at a flow rate of 15L/min. The pellets are subjected to isothermal reduction at 900C for 60minutes. The reduction gas is then substituted with N2 gas and the pellets are cooled to room temperature. The post test volume of the pellets is measured, and the free swelling index is expressed as the percent volume increase.
There are two main types of pelletizer that are used to produce iron ore pellets at industrial scale, the rotary drum and the disc. Besides iron ore agglomeration, these pelletizers can also be used for other materials such as copper ore, gold ore, coal, and fertilizer .
The rotary drum pelletizer was first used for taconite pellets in the early 1940s [14, 18]. A large drum-shaped cylinder is slightly elevated at one end, approximately 34. The iron ore and binder mixture enters the high end and finished pellets exit the low end. A roller screen is usually attached to the exit to separate pellets within the desired range from undersize and oversize, the latter two streams being recirculated (oversize after being crushed). The recirculating load tends to be approximately 150250% by weight of feed. Although a rotary drum pelletizer requires a roller screen it provides a more complete control of size. For a drum pelletizer flow sheet, see Figure 1.2.6.
Disc pelletizers are also used extensively worldwide. The advantage of the disc pelletizer is that there is no recirculation. The desired blend is fed to the pelletizer, which is a large disc inclined at 4060 to the horizontal (Figure 1.2.7). The rotation of the disc causes the formation of seeds, which grow into full-sized pellets. Factors affecting the final pellet size include the disc angle, feed rate, water addition, and rotation speed. As the diameter of the pelletizer increases, the speed should be decreased, otherwise due to the high impact pellets will start breaking. Disc pelletizers are very simple to design and have excellent performance .
Available sources of iron oxide include high-grade lump ore, beneficiated iron ore fines, iron ore pellets, and agglomerates from dusts produced by the BF, basic oxygen furnace, and the EAF. Most DRI is produced in shaft furnaces, which require a uniform-sized coarse feed. Due to the high gas velocities and abrasive conditions in shaft furnaces, fine particles are not suitable as charge materials. They tend to be carried out with the gas stream, from which they must be collected and recirculated. Fluidized bed DR processes are exceptions. Shaft furnaces use pellets (produced in the same way as pellets for the BF), or lump ore. Raw material for pellets is produced by crushing and grinding low-grade iron orestypically of the taconite class and finer than 325 mesh (0.044mm)and magnetically separating the iron oxide (magnetite, Fe3O4) from the siliceous gangue. The fine particles are reconstituted into moist pellets about 1cm in diameter, and then indurated by heating to temperatures approaching 1300 C. This is sufficient to bring about complete oxidation to recrystallized hematite (Fe2O3).
There are some key differences in the pellet chemistry for DRI versus BF use. In DRI production, the primary chemical change is the removal of oxygen and the addition of some carbon; the other constituents remain with the DRI. In smelting, the formation of a slag allows substantial removal of the ore contaminants. For this reason, the iron content of DRI pellets should be as high as possible and preferably >67%. Pellet reducibility, strength, and swelling specifications are similar to those of BF pellets. Coal-based processes have the potential disadvantage of contributing coal ash oxides to the product.
The term induration describes the hardening of a powdery substance. For example, in steel production, iron ore pellets are fed into melt furnaces. To avoid dusting and loss of ore, small oxide particles are agglomerated by sintering. Although initially applied to iron ore, soon the briquette agglomeration concept spread to a variety of materials .
Percy  describes iron ore agglomeration in 1864 and notes how oxide inclusions are detrimental. By the early 1900s large scale sintering agglomeration systems were in use . Figure 2.13 shows one such plant that helps demonstrate the large scale application of sintering by 1912. In the 1930s and 1940s, ore sintering included zinc, lead, lead sulfide, carbonates, chlorides, and precious metals.
Today iron ore agglomeration is the largest tonnage application for sintering, with plants operating at up 20,000 metric tonnes per day. For example, Figure 2.14 is a picture of a modern agglomeration facility which incorporates off-gas capture to reduce environmental damage. In such a facility, the iron ore fines are mixed with fluxes, carbon fuel, and water. The mixture is continuously fed onto trays or belts. As the conveyor moves through the sintering furnace, the mixture is heated to ignite the fuel and sinter the powder. Reaction waste consists of carbon dioxide, carbon monoxide, as well as nitrous oxides and sulfur oxides.
The so-called tumbler tests are usually used for testing material like coke, coal, iron ore pellets or tablets. They can be divided into drum tests and ball mill type tests. The latter type is used to derive both the Hardgrove Index and the Bond's Work Index, which are often used to classify the material friability as described in Sec. 3. They are generally more suited to coarse material. The Hardgrove Grindability test requires an initial size range form 595 to 1190 microns.
The Grace-Davison jet-cup attrition test is often used to test the friability of catalysts (e.g., Weeks and Dumbill, 1990; Dessalces et al., 1994). The respective jet-cup apparatus is sketched in Fig. 5. The catalyst sample is confined to a small cup, into which air is tangentially added at a high velocity (about 150 m/s). Some authors (e.g., Dessalces et al., 1994) assume that the stress in the jet-cup is similar to that prevailing in gas cyclones. With respect to fine catalysts, this type of test works as good as the impact test described above, but its applicability is limited to smaller sizes because larger particles tend to slug in the small cylinder. However, in the catalyst development, where at first only a little batch of catalyst is produced, this apparatus is an important friability test because it requires only a small amount of material (approximately 5 to 10 g).
The Fastmet is a continuous process which basically consists of a rotary hearth furnace where one or two layers of self-reducing iron ore pellets are placed. These self-reducing pellets are made from a mixture of iron ore concentrate, reductor (coal or coke), and binder. Unlike the other processes previously described, the Fastmet process uses a solid instead of a gas to reduce the iron oxide. The pellets travel through the rotary hearth furnace and are heated to 12501350C by burners placed throughout the length of the furnace. The rapid reduction rate of 12minutes is attributed to the high reduction temperatures and the close contact of the reductor and the iron oxide particles. DRI produced in this process is also unstable in air and must be briquetted to avoid reoxidation.
JKMRC has developed a slightly different method of estimating abrasion. Their method is similar to the standard laboratory Trommel Test applied for testing the abrasion of iron ore pellets and coke. In this test, 3kg of dry ore, size 55mm+38mm is charged into a horizontal cylindrical steel drum ID 0.30m 0.30m with lifter bars 2.54cm in height. The drum is rotated for 10 min at 53rpm (70% of the critical speed). The sample is then removed and screened to 38m. The cumulative mass percent passing each screen size is plotted. The mass percent passing 1/10th (T10) of the original size is taken as the abrasion parameter, Ta.
Iron ore is a critical raw material in modern society; it is the basis of the steel industry, which provides us with everything from infrastructure to appliances. Courtesy of iron ore, steel is all around us.
The production of steel from iron ore has increasingly been employing the pelletizing or balling technique as a result of the many benefits it can offer, combined with changes in the market that have made pellets more favorable.
This process varies depending on the ore source, but typically involves various stages of crushing and grinding to reduce the size of the iron ore. Separation techniques such as magnetic separation or froth flotation are then used to separate the gangue (unwanted) materials from the iron content.
In addition to mined iron ore, other sources of iron, such as flue dust collected from blast furnaces, or the dust produced at mine sites, is also frequently pelletized so it can be utilized, as opposed to disposed of.
The production of iron ore pellets from fines to finished product can differ based on a variety of factors. As such, its important to note that the process described here is a generalized approach subject to many variations.
In order to pelletize iron ore fines and/or concentrate, a binder is needed. Binder selection can vary from process to process depending on the unique goals of the project at hand, but bentonite clay is a common choice. Various additives may also be included with the feedstock to improve performance in the blast furnace.
Material exiting the mixer is continuously fed into the pelletizing device, along with additional liquid binder. The binder causes the particles to become tacky, picking up more fines as they roll in the drum or disc a layering phenomenon known as accretion.
Pellets must meet a myriad of criteria in terms of characteristics, many of which can be controlled during the pelletizing process. This might include using additives to adjust the chemical makeup or metallurgical properties of the pellets, or adjusting variables in the process to control physical characteristics such as crush strength or particle size distribution.
Once pellets reach the desired size, they exit the pelletizing device and are carried on to the next step (a screening step is often necessary). At this stage, pellets are referred to as green pellets.
Disc pelletizers, or balling discs, consist of an inclined, rotating disc mounted on a stationary structure. Disc pelletizers are often chosen for their ability to fine-tune particle size control, producing a tight window of particle size distribution. Pelletizers also result in far less recycle.
As material enters the disc, it is taken up by the rotation. With the continuous addition of binder and feedstock, particles continue to pick up more fines, rolling and growing in similar fashion to a snowball.
Balling drums consist of a large, cylindrical drum, through which the material tumbles to promote agglomeration. Much like the disc pelletizer, binder and feedstock are continuously added and particles pick up additional fines as they tumble through the drum, which is set on a slight angle to allow gravity to assist in moving pellets through the process.
Upon balling, green pellets must be fired in order to cure into their final, hardened form. This is carried out through induration a thermal treatment that heats the pellets to just before their melting point, causing them to become extremely hard. This is typically preceded by a drying step, which may be carried out in the induration unit, or in a separate device. After induration, pellets may be cooled.
As there can be significant variation in process requirements and sources of iron ore fines, testing is often an essential part of the development of a successful iron ore pelletizing operation. Different sources of iron ore will respond differently to agglomeration, and process requirements will subsequently vary.
The FEECO Innovation Center, where FEECO process engineers conduct batch- and pilot-scale testing, has been working with iron ore pellet producers for decades; process engineers regularly test iron ore sources to work out process variables such as feed rates, additive inclusion, binder selection, required equipment specifications, and more be it for flue dust, concentrate, run of mine fines, electric arc furnace (EAF) dust, or other source of fines.
Iron ore is essential to meeting the demands of the steel industry that continues to build the world around us. Pelletizing, or balling, carried out through either a disc pelletizer or rotary drum, is a key part of efficiently and sustainably producing steel from iron ore fines of varying sources.
FEECO is a leader in feasibility testing, custom disc pelletizers and balling drums, and parts and service support for the iron ore balling/pelletizing industry. For more information, contact us today!