2.5. Sintering

Sintering is a thermal process (1300–1400�C) by which a mixture of iron ore, return fines, recycled products of the iron and steel industry (mill scale, blast furnace dusts, etc.), slagforming elements, fluxes and coke are agglomerated in a sinter plant with the purpose of manufacturing a sintered product of a suitable chemical composition, quality (physically) and 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 ensures an adequate sinter bed permeability and hence good productivity [2].

inorganic pellets are: bentonite, sodium carbonate, calcium carbonate or calcium hydroxide; while examples of organic pellets are: carboxymethylcellulose (CMC), other CMC-based poly-

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 tempera-

ture will depend on the kind of pellet. Pellets can be classified into three groups:

shipping, handling, and steelmaking processes [8].

• Blast Furnace grade. They are dried at 105–120�C, and then indurated at high temperature (1200–1400�C, depending on the kind of both ore and binder, the same for the time), allowing for a strong pellet that can resist shipping, handling, and processes in the blast furnace. This kind of pellets can have certain level of impurities since all constituents of the iron feed are melted during the process in the blast furnace, and they can float as slag above the reduced iron (being then removed). Typically contain 58–65% Fe and size range

• Direct Reduction grade. As opposed to blast furnace process, direct reduction of iron oxide happens in solid state (components are not melted). For that reason, impurities are necessary to be controlled. They are first dried at 105–120�C, and then indurated at high temperature (1200–1400�C, depending on the kind of both ore and binder, the same for the time, as in Blast Furnace grade pellets), allowing for a strong pellet that can resist

• Rotary Heating Furnace pellets, or iron ore-coal composite pellets. This kind of pellets does not require high strength as in the other two types, since in the same vessel that the green pellets are fed, they are transformed into direct reduction iron. The resistance is required only for feeding as they are dried in the first part of the furnace and then are reduced

Sintering is a thermal process (1300–1400�C) by which a mixture of iron ore, return fines, recycled products of the iron and steel industry (mill scale, blast furnace dusts, etc.), slagforming elements, fluxes and coke are agglomerated in a sinter plant with the purpose of manufacturing a sintered product of a suitable chemical composition, quality (physically) and

without requiring more handling than that to feed them into the furnace [8, 13].

mers, corn starch, wheat flour or molasses [8].

64 Iron Ores and Iron Oxide Materials

of 9–12 mm [8].

2.5. Sintering

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 read in [16]. The temperature at which the coke begins to burn depends on the size, oxygen partial pressure, volatile content, and component types in the coke. The ideal flame front thermal profile is characterized by short heating time (1.5 min) up to the high temperature zone (1100C) to avoid the formation of important amounts of FeO due to the low partial pressure of oxygen, and long cooling times (3–5 min) down to the room temperature to avoid a strong sinter structure by the formation of a gangue matrix [16]. Anyway, maximum temperature and heat distribution should be uniform to obtain a homogenous sinter with suitable quality, and with the maximum process efficiency. In this way, a double layer sintering is used in some plants to satisfy this question, the upper layer would have higher coke content than the lower one, and under that conditions the tendency of Tmax of being higher in the lower layer is avoided [16].

Softening melting test: this test was developed in Japan and UK to simulate the behavior of iron ore materials in the cohesive zone of the blast furnace (defined as the zone of the blast furnace where the load formed by coarse mineral grains, sinter and pellets begins to soft [20]).

Iron Ore Agglomeration Technologies http://dx.doi.org/10.5772/intechopen.72546 67

Tumbler Index (TI) (% > 6.3 mm) (min. 63, max. 79, typical > 74 [20]): this test was developed with the purpose of knowing the cold strength of the sinter, and in this way, evaluating the tendency to the formation of fines during the handling and transportation of the sinter from the sintering machine to the blast furnace throat (obviously informing about the losses of material as fines). The Tumbler index depends on the strength of each component, as well as on the strength of the bonding matrix components and ore composition [24]. Standards to calculate this index are IS 6495:2003 and ISO 3271:2015. There are other indices used with the same purpose as for instance Micum and Irsid indices. The generation of fines was studied by using a method different from TI in [25]. They simulated different steps of handling and transportation by using customized drop and vibration tests and considering sinter size, drop

Low temperature degradation tests: degradation that happens during reduction in the low temperature zone has harmful effects on burden strength in the blast furnace (loss of permeability to reducing gas and increase of coke consumption [26]). To have more information about the tests you can read [20]. Low Temperature Breakdown Test (LTB) was developed by the British Iron and Steel Research Association (BISRA) and Luossavaara-Kirunavaara AB (LKAB) with the purpose of evaluating the abrasion resistance of iron bearing materials under

Reducibility index (RI) (R60, %)) (min. 49, max. 78 [20]): this test informs about the ability of the sintered products for transferring oxygen during the indirect reduction process in the blast furnace stack. The porosity (non-occluded porosity is the surface available for gas–solid contact) and the mineralogical composition. RI is related with FeO content in the sinter (the higher the FeO content, the lower the reducibility) as FeO reacts with SiO2 to form calcium ferrites, which are difficultly reduced. There are eight tests to calculate the reducibility: Midrex-Linder, HYLSA Batch, VDE (Verein Deutcher Eisenhuttenleute), Japanese Industrial Standard, Gakushin, ISO Relative Reducibility Test, ISI test, and HYL-III test. They are described in [20].

Reduction Degradation Index (RDI) (% < 3 mm) (min. 27, max. 33, typical < 33 [20]): This test gives a measure of the sinter strength after the partial reduction of the material, and provides information about the degradation behavior in the lower part of the blast furnace stack [20]. Coke consumption (min. 39 kg t<sup>1</sup> sinter, max. 54 kg t<sup>1</sup> sinter [20]) is one important factor, as nowadays is the main combustible for the sintering process (around 88% [7], there are other traditional combustibles such as anthracite (with low volatile matter content), and other modern combustibles (biomass, biochar, etc.) developed in an attempt of reducing CO2, NOX and SOX as coke breeze (<5 mm) has 0.42–1% S and 1.06–1.23% N [7]) and for that reason it is necessary to guarantee the coal for coking production. The problem is that coking coal is only available in some regions, but also rises in coke price, unstable supply of anthracites and global warming by CO2 [2, 16, 27]. Size is an important factor in sinter productivity and reducibility,

reducing conditions, simulating those in the upper zone of a blast furnace stack [20].

See [20] to enlarge information.

height and conveying time [25].

There is a strong relationship between sinter structure and sinter characteristics. Sintering process is based on rising the temperature with the objective of producing a molten phase, which, during the cooling period, will crystalize or solidify into several mineral phases and will agglomerate the mixture. In this way, from a mixture whose main constituents are Fe2O3, CaO, SiO2 and Al2O3, it is possible to obtain a product having the following constituents: Fe2O3, Fe3O4, FeO, metallic iron, calcium ferrites and silicates. Calcium ferrites and silicates bond metallic oxides. It is, for that reason, possible to establish five categories of constituents that we can find in the sinter structure [20]: hematite (Fe2O3), primary (original mineral) and secondary or recrystallized (directly crystallized, formed by oxidation of magnetite crystals in liquid phase or formed by magnetite oxidation in solid phase); magnetite (Fe3O4), formed from the minerals or crystallized from the liquid phase; wüstite (FeO), formed by precipitation from the liquid in low potential of oxygen (typically associated to excesses of coke in the sinter, retained carbon grains or magnetite reduction in solid state); calcium ferrites (xSIO2 yFe2O3 5CaO zAl2O3 with x + y + z = 12), they are difficult to define as there is partial replacement of Ca2+ by Mg2+, Fe2+ by Mn2+, Fe3+ by Al3+ and Si2+, etc., so they include several chemical compounds formed by crystallization of the liquid phase according to the reaction above mentioned, and they are known as silicoferrites of calcium and aluminum (SFCA); and, silicates, which include olivine, fayalite, and calcium silicates. Reactions involved in the formation of these constituents are described in [16, 20]. It is obvious that each constituent has a certain influence in the quality of the sinter produced, in this way, for instance primary hematite is considered beneficial for the sintered product as improves reducibility index, and SFCA are also beneficial for the sinter structure as they have good reducibility, improve shatter index and tumbler index [20]. The optimum sinter structure for sinter reducibility in the blast furnace would be hematite nuclei (un-melted) surrounded by an acicular ferrite network [21, 22]. Considering the temperature of operation, the best sinter quality (maximum percentage of ferrites, high primary hematite, good porosity, and good quality indices) is reached when sintering at 1225–1275C [23].

#### 2.5.1. Sinter quality indices

As it was mentioned previously, blast furnace operators require a homogeneous product with the suitable thermal, chemical, physical, and mechanical properties to be fed into the blast furnace, ensuring the highest productivity and performance. The main quality indices are:

Softening melting test: this test was developed in Japan and UK to simulate the behavior of iron ore materials in the cohesive zone of the blast furnace (defined as the zone of the blast furnace where the load formed by coarse mineral grains, sinter and pellets begins to soft [20]). See [20] to enlarge information.

read in [16]. The temperature at which the coke begins to burn depends on the size, oxygen partial pressure, volatile content, and component types in the coke. The ideal flame front thermal profile is characterized by short heating time (1.5 min) up to the high temperature zone (1100C) to avoid the formation of important amounts of FeO due to the low partial pressure of oxygen, and long cooling times (3–5 min) down to the room temperature to avoid a strong sinter structure by the formation of a gangue matrix [16]. Anyway, maximum temperature and heat distribution should be uniform to obtain a homogenous sinter with suitable quality, and with the maximum process efficiency. In this way, a double layer sintering is used in some plants to satisfy this question, the upper layer would have higher coke content than the lower one, and under that conditions the tendency of Tmax of being higher in the lower

There is a strong relationship between sinter structure and sinter characteristics. Sintering process is based on rising the temperature with the objective of producing a molten phase, which, during the cooling period, will crystalize or solidify into several mineral phases and will agglomerate the mixture. In this way, from a mixture whose main constituents are Fe2O3, CaO, SiO2 and Al2O3, it is possible to obtain a product having the following constituents: Fe2O3, Fe3O4, FeO, metallic iron, calcium ferrites and silicates. Calcium ferrites and silicates bond metallic oxides. It is, for that reason, possible to establish five categories of constituents that we can find in the sinter structure [20]: hematite (Fe2O3), primary (original mineral) and secondary or recrystallized (directly crystallized, formed by oxidation of magnetite crystals in liquid phase or formed by magnetite oxidation in solid phase); magnetite (Fe3O4), formed from the minerals or crystallized from the liquid phase; wüstite (FeO), formed by precipitation from the liquid in low potential of oxygen (typically associated to excesses of coke in the sinter, retained carbon grains or magnetite reduction in solid state); calcium ferrites (xSIO2 yFe2O3 5CaO zAl2O3 with x + y + z = 12), they are difficult to define as there is partial replacement of Ca2+ by Mg2+, Fe2+ by Mn2+, Fe3+ by Al3+ and Si2+, etc., so they include several chemical compounds formed by crystallization of the liquid phase according to the reaction above mentioned, and they are known as silicoferrites of calcium and aluminum (SFCA); and, silicates, which include olivine, fayalite, and calcium silicates. Reactions involved in the formation of these constituents are described in [16, 20]. It is obvious that each constituent has a certain influence in the quality of the sinter produced, in this way, for instance primary hematite is considered beneficial for the sintered product as improves reducibility index, and SFCA are also beneficial for the sinter structure as they have good reducibility, improve shatter index and tumbler index [20]. The optimum sinter structure for sinter reducibility in the blast furnace would be hematite nuclei (un-melted) surrounded by an acicular ferrite network [21, 22]. Considering the temperature of operation, the best sinter quality (maximum percentage of ferrites, high primary hematite, good porosity, and good quality indices) is reached when

As it was mentioned previously, blast furnace operators require a homogeneous product with the suitable thermal, chemical, physical, and mechanical properties to be fed into the blast furnace, ensuring the highest productivity and performance. The main quality indices are:

layer is avoided [16].

66 Iron Ores and Iron Oxide Materials

sintering at 1225–1275C [23].

2.5.1. Sinter quality indices

Tumbler Index (TI) (% > 6.3 mm) (min. 63, max. 79, typical > 74 [20]): this test was developed with the purpose of knowing the cold strength of the sinter, and in this way, evaluating the tendency to the formation of fines during the handling and transportation of the sinter from the sintering machine to the blast furnace throat (obviously informing about the losses of material as fines). The Tumbler index depends on the strength of each component, as well as on the strength of the bonding matrix components and ore composition [24]. Standards to calculate this index are IS 6495:2003 and ISO 3271:2015. There are other indices used with the same purpose as for instance Micum and Irsid indices. The generation of fines was studied by using a method different from TI in [25]. They simulated different steps of handling and transportation by using customized drop and vibration tests and considering sinter size, drop height and conveying time [25].

Low temperature degradation tests: degradation that happens during reduction in the low temperature zone has harmful effects on burden strength in the blast furnace (loss of permeability to reducing gas and increase of coke consumption [26]). To have more information about the tests you can read [20]. Low Temperature Breakdown Test (LTB) was developed by the British Iron and Steel Research Association (BISRA) and Luossavaara-Kirunavaara AB (LKAB) with the purpose of evaluating the abrasion resistance of iron bearing materials under reducing conditions, simulating those in the upper zone of a blast furnace stack [20].

Reducibility index (RI) (R60, %)) (min. 49, max. 78 [20]): this test informs about the ability of the sintered products for transferring oxygen during the indirect reduction process in the blast furnace stack. The porosity (non-occluded porosity is the surface available for gas–solid contact) and the mineralogical composition. RI is related with FeO content in the sinter (the higher the FeO content, the lower the reducibility) as FeO reacts with SiO2 to form calcium ferrites, which are difficultly reduced. There are eight tests to calculate the reducibility: Midrex-Linder, HYLSA Batch, VDE (Verein Deutcher Eisenhuttenleute), Japanese Industrial Standard, Gakushin, ISO Relative Reducibility Test, ISI test, and HYL-III test. They are described in [20].

Reduction Degradation Index (RDI) (% < 3 mm) (min. 27, max. 33, typical < 33 [20]): This test gives a measure of the sinter strength after the partial reduction of the material, and provides information about the degradation behavior in the lower part of the blast furnace stack [20].

Coke consumption (min. 39 kg t<sup>1</sup> sinter, max. 54 kg t<sup>1</sup> sinter [20]) is one important factor, as nowadays is the main combustible for the sintering process (around 88% [7], there are other traditional combustibles such as anthracite (with low volatile matter content), and other modern combustibles (biomass, biochar, etc.) developed in an attempt of reducing CO2, NOX and SOX as coke breeze (<5 mm) has 0.42–1% S and 1.06–1.23% N [7]) and for that reason it is necessary to guarantee the coal for coking production. The problem is that coking coal is only available in some regions, but also rises in coke price, unstable supply of anthracites and global warming by CO2 [2, 16, 27]. Size is an important factor in sinter productivity and reducibility, being the best results found when coke size is 0.25–3 mm [28]. The fraction 1–3.15 mm is more economical in terms of consumption [28–30]. The reduction in coke breeze consumption is being carried out mainly by replacing coke breeze by new renewable solid fuels as biochar, biomass, or charcoal [31–33].

MgO content was increased, but [42] observed that MgO did not affect RDI, and [44] observed

Iron Ore Agglomeration Technologies http://dx.doi.org/10.5772/intechopen.72546 69

Alkalis content (%) (min. 0.11 [20]): they are mainly K and Na compounds and enter in the blast furnace with the charge materials (sinter, 32–58%; coke ash, 21–33%; pellets, 16–37% [46]). They cause several problems in the blast furnace performance: decrease coke hot strength, deteriorate sinter RDI, weak refractory lining and facilitate the formation of slag crusts, scaf-

Data analysis is nowadays widely used with the purpose of correlating variables and, in this way, evaluating the effect of them in a certain process, whose have a higher influence, and how changing them will affect the sintering process. We have studied what variables are and what weight have them on three sinter quality indices as Tumbler Index, Reduction Degradation Index, and Reducibility Index. The objective of our work was to estimate them by using multivariate statistics, and thus avoiding long and expensive laboratory tests. Once each index was defined by a mathematical equation whose parameters are easily measured, we used fuzzy inference to develop a fuzzy control system that could warn in case of deviations from the

For our work, we used real data from a sintering machine. The variables with a higher weight for each quality index are: RDI (% < 3.15 mm): fines (%), fluxes (%), Fe2O3 (%), FeO (%), CaO (%), S (%), 10–20 mm (%), average particle size (mm), productivity (t/h), basicity index, and abrasion (% < 0.6 mm); Tumbler Index (% > 6.35 mm): Fe2O3 (%), CaO (%), MgO (%), MnO (%), P2O5 (%), 0–10 mm (%), 10–20 mm (%), basicity index, and maximum temperature in box prior to sinter discharge (�C); and, RI: Recoveries from other stages of the ironmaking and steelmak-

We elaborated a mathematical equation that correlated the RDI, the TI and the RI with those variables mentioned above, and in this way, it is possible to predict the indices by using these

Avoiding or minimizing pollutants is of large importance in nowadays environmental industrial policies, but also the objective is reducing the ecological footprint of the sintering process. In a second plane, environmental improvements will enhance the profitability of the sintering

Sintering process is also used in the ironmaking and steelmaking as recycling system as apart from using iron ore fines and additives (such as fluxes and coke breeze), mill scale, collected dusts (and to a much lower extent sludges) from gas cleaning, other recycled by-products of steel manufacture and recycled sinter particles from sinter screening are treated in the sintering

machine. Air pollutants are the most significant and are presented here:

ing process (%), FeO (%), P2O5 (%), MgO (%), S (%), Al2O3 (%) and >50 mm (%).

that increasing MgO increased RDI.

folding and burning of the tuyeres [47].

2.5.2. Multivariate statistics in the sintering process

conditions that ensured the best quality indices.

easily measured variables.

2.5.3. Environmental issues

process from the economical point.

Fe total (%) (min. 51, max. 61, typical > 56 [20]): iron ore market is mainly composed by hematite (Fe2O3), goethite (α-FeOOH) and magnetite (Fe3O4), with low impurities content (alkalis, sulfur and phosphorus) and iron average content of 60–65% in 2016 (around 40% in 1940) [2]. However, the depletion of rich iron ore mines will lead to exploitation of complex chemistry ores and low-grade iron ore mines, considering in some cases the bioprocessing [5].

Basicity index (typical 1.70 [20]): reduction degradation index (RDI) decreases when basicity increases [20]. Obviously, basicity index has importance in the hardness and reducibility of the sinter as expresses the relation CaO/SiO2. See [20] for a deeper explanation on sinter basicity index and sinter structure.

FeO content (%) (min. 4.0, max. 11 [20]): this parameter informs about the thermal state of the sintering process [20] and about coke rate. RDI is reduced when FeO content is increased [20, 34]. The problem is that increasing FeO content affects coke rate, deteriorates the sinter reducibility [35, 36], and can also affect blast furnace productivity [35].

Al2O3 content (%) (min. 0.6, max. 1.8, typical 1.35 [20]): sinter quality is related with Al2O3 content as when Al2O3 content is increased, RDI is higher. This point is under discussion as Kumar et al. observed that with 10–10.5% CaO an increase of 0.1% in Al2O3 increases RDI 2 points [34], while Hiesch observed no relation between RDI and Al2O3 [37]. Alumina is also related with reducibility, but the same as for RDI, there are different opinions, Yu et al. observed that maximum reducibility is reached when Al2O3 content is 2.5% in the sinter ore [38], while Umadevi et al. observed that reducibility increased when alumina content is raised from 2 to 5.5% [39].

MgO content (%) (min. 0.7, max. 2.2, typical 1.65 [20]): MgO is related with the obtaining of an optimum blast furnace slag (flowability and desulphurization). For that reason, MgO is added as dolomite or dunite, directly or through the sinter. It was observed an increase in the utilization of dolomite (containing CaO and MgO) or other MgO-bearing materials with low SiO2 (as dunite) due to the use of iron ores with high SiO2 content. There are mixed opinions regarding how this constituent affects reducibility, RDI, Tumbler and productivity. Umadevi et al. observed that reducibility was decreased with increasing MgO addition (in low and high silica iron ore fines) [40]. Kalenga and Garbers-Craig observed an increase in Tumbler index and coke rate with increasing MgO content [41]. Bhagat observed that varying MgO content within certain limits (1.5–2.5%) did not affect productivity and Tumbler, while coke breeze consumption was reduced [42]. On their behalf, Umadevi et al. observed that increasing MgO content from 1.4 to 2.6% increased heat consumption, and productivity was reduced [43]. And Yadav et al. observed that raising MgO content from 1.75 to 3.25% caused a deterioration in productivity and an increase in Tumbler index [44]. With regard to how MgO content affects RDI, there are several opinions, and in this way, [41, 43, 45] observed that RDI improved when MgO content was increased, but [42] observed that MgO did not affect RDI, and [44] observed that increasing MgO increased RDI.

Alkalis content (%) (min. 0.11 [20]): they are mainly K and Na compounds and enter in the blast furnace with the charge materials (sinter, 32–58%; coke ash, 21–33%; pellets, 16–37% [46]). They cause several problems in the blast furnace performance: decrease coke hot strength, deteriorate sinter RDI, weak refractory lining and facilitate the formation of slag crusts, scaffolding and burning of the tuyeres [47].
