Iron Ore Pellets
Iron Ore Pellets
Iron ore pellet is a type of agglomerated iron ore fines which has better tumbler index when compared with that of the parent iron ore. Iron ore pellets are widely used as a substitute of lump ore for the production of direct reduced iron (DRI) and in the blast furnace (BF) for the production of hot metal. Iron ore pellets are used in large proportion, which is continue to rise because of the lack of supply of high-quality lump ores. The term iron ore pellet refers to he thermally agglomerated material formed by heating a variable mixture of iron ore, limestone, olivine, bentonite, dolomite, and miscellaneous iron bearing materials in the range of 1,250 deg C to 1,350 deg C.
Iron ore pellets can be made from beneficiated or run of mine iron ore fines. Lean iron ores are normally upgraded to higher iron ore content through beneficiation. This process generates iron ore filter cake which needs to be pelletized so that it can be used in an iron making process. Also, during the processing of high-grade iron ores which do not need beneficiation, generated fines can be pelletized and used instead of being disposed of. Tab1 shows identification details of iron ore pellets.
|Tab 1 Identification details of Iron ore pellets|
|Chemical name||Iron ores, agglomerates|
|Other names||Iron ore pellets, iron oxide pellets|
|Molecular weight (gram/mole)||159.7|
|Synonyms||Di iron trioxide|
|Mineral of identical or similar composition||Hematite|
|Other identity code: Related CAS No.||Hematite (Fe2O3) 1317-60-8|
|REACH registration No.||01- 2119474335-36-0013|
The typical specification for iron ore pellets is given in Table 2 below. Several pellet producers screen their pellets to remove fines prior to loading on board vessels or railway trains for delivery to the customers. The resultant screened off fines are known as pellet fines (also as pellet screenings or pellet chips). The specification for pellet fines is the same as that for pellets, except for nominal size, and pellet fines can be considered as pellets for the purposes of REACH (Registration, Evaluation, Authorization, and restriction of Chemicals). Tab 2 shows the general properties of iron ore pellets.
|Tab 2 General properties of pellets|
|Size||8 mm-20 mm|
|pH (40 gm/l, 20 deg C, slurry in water)||5.0 – 8.0|
|Melting point||1,500 deg C-1,600 deg C|
|Bulk density||2.0-2.2 t/cum|
|Tumbler index (+6.3 mm)||93 %-94 %|
|Abrasion index (-0.5 mm)||5 %-6 %|
|Compression strength (daN/p)||250 min|
|Size distribution (+8mm to -18 mm)||95 % min|
|Size distribution (-5 mm)||1.50 %|
Tab 3 gives typical range of chemical specification of iron ore pellets.
|Tab 3 General chemical specification of iron ore pellets|
|Fe2O3||%||More than 80|
|SiO2||%||Less than 10|
|Al2O3||%||Less than 3|
|CaO||%||Less than 8|
|MgO||%||Less than 5|
|P||%||Less than 0.2|
|S||%||Less than 0.1|
|Free moisture at 105 deg C||%||Less than 5|
|Under size||%||Less than 5|
|Nominal size – pellet fines||mm||Less than 10|
Iron ore pellets are normally produced in two types of grades namely DRI grade and BF grade. The requirements of DRI grade pellets are (i) low quantity of silica and alumina (less than 0.9 %), (ii) high basicity, (iii) low reduction disintegration, (iv) low sticking tendency, and (v) high reducibility. The requirements of BF grade pellets are (i) high and consistent quality, (ii) high productivity, (iii) low energy demand, (iii) additives to optimize BF process performance and (iv) pellets to match high basicity sinter.
DRI grade pellets are also known as acid pellets while BF grade pellets are basic pellets. These pellets are fluxed pellets and have higher basicity than the DRI grade. DRI pellets do not contain CaO, while BF grade pellets are fluxing pellets containing CaO. For BF grade pellets, reducibility and swelling index are important properties while for DRI grade disintegration is an important property. The typical properties of BF and DRI grades pallets are given in Tab 4.
|Tab 4 Typical Chemical analysis of pellets|
|Parameters||Unit||BF grade||DRI grade|
|Fe||%||63 – 65.5||65 -67.8|
|SiO2 + Al2O3||%||Less than 5||Less than 5|
|CaO + MgO||%||Up to 3||Up to 0.10|
|Disintegration (-3.15 mm)||%||2|
The pellets are acid, partially fluxed, super fluxed according to their acid or basic oxide content. Acid pellets have basicity (CaO/SiO2) value of less than 0.1. In the case of highly acid pellets with a basicity of less than 0.1, the gangue is predominantly present as silica and alumina. The fired pellet strength is, to a certain degree, because of the hematite bridges of polycrystalline structure. These pellets contain large quantities of open pores. The reduction gas can quickly penetrate through these pores into the pellet core and simultaneously attack the structure in several places. The structural change begins very early at low temperatures over the whole pellet volume.
Partially fluxed basic pellets have basicity value in the range of 0.1 to 0.6. In case of pellets with low CaO content at a basicity level of 0.1 to 0.6, a glassy slag phase consisting of SiO2, CaO, and Fe2O3 of varying proportion is formed during the firing. Because of the increased flux addition, slag formation is to a certain extent and slag bonding with iron ore crystals is common.
Super fluxed basic pellets have basicity value higher than 0.7. If the pellet gangue contains more CaO than corresponds to a basicity level of around 0.7, not only glassy phase, consisting of SiO2, CaO, and Fe2O3, but also calcium ferrite (CaO·Fe2O3), is formed. During pellet firing, the availability of CaO considerably favours the crystal growth of hematite. It is to be noted that pellets with a high CaO content and a basicity which exceeds around 0.6 has a high mechanical strength after pellet firing.
Fluxed pellets can be produced as equivalent to the best sinter in terms of reducibility and softening meltdown properties and are superior in terms of strength and low temperature breakdown (LTD/RDI). Fluxed pellets show good strength, improved reducibility, swelling and softening melting characteristics. Because of these properties fluxed pellets give better performance in the blast furnace.
Quality of the pellets is influenced by the nature of the ore or concentrate, associated gangue, types and quantities of fluxes added, and their subsequent treatment to produce pellets. These factors, in turn, result in the variation of physico-chemical properties of the coexisting phases and their distribution during the pellet induration. Hence properties of the pellets are largely governed by the form and degree of bonding achieved between the ore particles and the stability of these bonding phases during reduction of iron oxides. Since the formation of phases and microstructure during induration depends on the type and quantity of fluxes added, there is an effect of fluxing agents in terms of CaO/SiO2 ratio and MgO content on the pellet quality.
Other factors which have effect on pellet quality are (i) cold crushing strength (CCS) of the pellets is found to increase with increasing temperature of firing, (ii) decrease of mean particle size (MPS) does not have a relationship with the pellet strength, but it reduces the porosity of the pellet, (iii) acid pellets (DRI pellets) and MgO free pellets show higher swelling at 0.6 basicity and decreased thereafter, (iv) fluxed pellets show good strength, improved reducibility, swelling and softening melting characteristics and because of these properties these pellets give better performance in the blast furnace, and (v) high swelling (swelling indicates volume change of pellets during reduction) reduces the strength of the pellets after their reduction.
The strength of iron ore pellets is important in minimizing degradation by breakage and abrasion during handling and shipping, and in the blast furnace. Strong bonding in pellets is believed to be due to grain growth from the accompanying oxidation of magnetite to hematite, or recrystallization of hematite. Although slag bonding may promote more rapid strengthening at slightly lower firing temperatures, pellet strength is normally decreased, especially resistance to thermal shock. Compressive strengths of individual pellets depend upon the mineralogical composition and physical properties of the concentrate, the additives used, the balling method, pellet size, firing technique and temperature, and testing procedure. The compressive strengths of commercially acceptable pellets are usually in the range of 200 kg to 350 kg for pellets in the size range of 8 mm to 18 mm.
The swelling index of pellets is an Important of metallurgical property. Swelling indicates volume change of pellets during reduction. The volume expansion of pellets during the reduction results in lower compressive strength of pellets. High swelling inside the furnace causes increase in volume of the pallet which in turn decreases voids in charge. This Impedes gas flow in the furnace and results into pressure drop. This in turn causes burden hanging and slipping inside the blast furnace. The addition of dolomite is favorable for the improvement in swelling property of pellets. Maximum allowable swelling of pellets for the blast furnace ranges from 16 % to 18 %. Acid pellets (DRI pellets) and MgO free pellets exhibit higher swelling. Other important properties of the pellets are reducibility, porosity, and bulk density. With some concentrates these can be varied within certain limits.
The wear rate due to the sliding is directly correlated to the total surface area of pellets whereas the wear rate due to the collisions is dependent on number of collisions and collision energy. It can be suggested that the dust particles generated by the sliding wear has small sizes in comparison to those produced by the collision wear. The size of dust particles generated by the collisions is directly related to the collision energy. It can be assumed that the sliding wear is dominant for large size pellets since the number and total surface area of large pellets is considerably lower. The higher collision energy of large sized pellets results in generation of larger sized dust particles. In a study, the characteristics of iron ore pellets and their influence on the wear of pellets has been observed. Based upon the presented results, the conclusions of the study are (i) large sized pellets (deq higher than 13.5 mm) have shown a 10 % to 20 % higher wear rate compared to the small sized pellets (deq less than 12.5 mm), (ii) the dust generated from the pellets of large size during the wear tests has contained a larger number of large sized particles (higher than 15 micrometers) than that of small size, and (iii) the sliding wear of pellets is dominant wear mechanism for small size pellets whereas the wear due to the pellet collision is considerably higher in case of large size pellets.
Fig 1 shows typical qualitative trends found in the fired pellets. It presents a qualitative influence of iron ore reducibility and carbon content in the green pellet on the microstructure and mechanical strength of the fired pellets. The figure shows that the lower the reducibility of iron ore and the lower the carbon content, the higher the mechanical strength of the fired pellets is, which is related to the microstructure formed. On the other hand, the increased carbon content in the pellets produced with high reducible iron ore in the green pellets lead to a negative scenario regarding the mechanical strength. The results show that for the firing conditions as per the industrial practice, the limitation related to the carbonaceous material addition in the pelletizing mixture varies according to the iron ore used and with the restrictions of objectified quality. To enable the use of larger quantities of solid fuel as well as introduce new ores in the mixture, while maintaining the quality in terms of mechanical properties of the pellets, it seems to be a crucial requirement to understand the behaviour of the raw materials and mixtures in the pilot plant aiming to adjust the characteristics of the induration process (time, temperature, heating rate, etc.) of the pellets.
Fig 1 Typical qualitative trends found in the fired pellets
The mechanical properties of iron ore pellets have a considerable influence on the performance of reduction reactors. Induration of green pellets occurs through the firing at high temperatures, and the thermal levels imposed in the process are related to the microstructure formed in the agglomerate which directly affects the mechanical properties of the fired pellets. During the firing process, the pellets undergo partial reduction due to the generation of reducing gases produced from reactions with the carbon contained in the mixture, and after consumption of the carbon, they undergo reoxidation due to the effect of the atmosphere in the furnace. The extent of the reduction and reoxidation reactions depend essentially on the characteristics of the pellet (size, porosity and such), the proportion and the characteristics of the raw materials used (such as ore porosity, reactivity, and quantity of carbonaceous material etc.), and on the processing conditions (temperature, heating rate, and atmosphere).
On the other hand, there is a consensus among specialists that the nature of the iron ore affects the quality of the pellets produced. As per one of the studies, the mechanical alteration caused by the thermal processing is very pronounced on goethitic ore while it is less expressive in hematite ore. In another study, it has been affirmed that the origin of the hematite (in natural or secondary) contained in the pellets interferes with the mechanism of the induration of the pellet. In a recent study, it has been observed that the ore composition exercises great influence on some quality parameters of the pellets such as its mechanical strength. In light of these studies, it is factual that the mechanical properties of the pellets are very sensitive to the solid fuel added to the mixture and this effect varies for different types of iron ore.
The fluxing agents also affect the pellet quality. To meet the blast furnace slag chemistry, different fluxes, like limestone, dolomite, magnesite or pyroxenite need to be added to the burden. But it is uneconomical to add fluxes directly to the blast furnace, as their dissociation demands more energy leading to higher coke rate. They are normally added in the agglomeration process (sintering or pelletizing) so as to increase the content of either CaO or MgO in the agglomerates. Effect of these basic oxides on the quality of sinter is well known, but their effect on pellet quality is scanty due to limited studies have been carried out.
CaO added in the form of limestone affects the pellet quality. During induration, green pellets are subjected to high temperature to attain strength which also results in the physical and chemical changes. High temperature results in thermally activated process like solid state diffusion, compound formation, melting, dissociation and precipitation, and grain growth etc. When limestone is added as flux, after its calcination, it forms calcium ferrite (CaO.Fe2O3) at contact points between lime and hematite for pellets with CaO/SiO2 ratio above 0.5. At around 1,150 deg C to 1,200 deg C, calcium ferrite reacts with hematite to form calcium di-ferrite (CaO.2Fe2O3). Calcium ferrite melts at 1,216 deg C whereas calcium di-ferrite melts at 1,185 deg C. With further increase in temperature, liquid ferrite reacts with silica particles to form complex iron calcium silicates which form the bonding phase. Calcium di-ferrite is thermodynamically unstable and it decomposes to calcium ferrite and hematite below 1,155 deg C. Calcium di-ferrite formation is undesirable as it precipitates secondary hematite in the slag phase and its reduction generates stress which breaks the bonding in the pellets.
Addition of limestone to the pellets decreases their strength after preheating since it hinders the micro-crystallization of hematite grains. Strength of fired pellets with limestone addition increases up to the basicity (CaO/SiO2) value of 0.4 to 0.6 and decreases thereafter. At lower basicity, small quantities of liquid phases formed help in recrystallization of hematite grains hence increasing the strength, whereas at high basicity, increased quantity of liquid phase destroys the structure of pellets.
In the limestone fluxed pellets, maximum swelling occurs in the range of the basicity value of 0.2 to 0.8, because of the formation of low melting point olivines with 80 % Fe2SiO4 and 20 % Ca2SiO4 with melting point as low as 1,115 deg C. One of the studies on self-fluxed pellets also confirmed that highest swelling occurs in the CaO fluxed pellets in the basicity range of 0.5 to 0.7 because of the solid solution of slag components in iron oxide. CaO in the pellet, during reduction, forms solid solution with wustite to form calcio-wustite. During metallization, CaO promotes nucleation and growth of iron whiskers, especially at high CaO sites. CaO also diffuses into the wustite phase in solid state and lowers its melting point. In the CaO fluxed pellets, decomposition of ferrite and formation of CaO-FeO-SiO2 ternary liquid slag increases the swelling tendency. Among the pellets with varying basicity ranging from 0 to 1.6, very good reducibility is achieved at a basicity value of 1.3. This can be due to the fact that CaO forms a solid solution with wustite, forming porous iron morphologies as final product as compared to dense iron layer formed from pure wustite at lower basicity levels.
MgO added in the form of dolomite also has effect on the pellet quality. When dolomite is added to the pellets as fluxing agent, its calcination to CaO and MgO, is followed by the formation of magnesio-ferrite (MgO.Fe2O3) by solid state reaction at temperature which is higher than 700 deg C. In the dolomite fluxed pellets, after calcination, first reaction takes place, in solid state, between CaO-Fe2O3 and MgO-Fe2O3. As the MgO has less solubility in calcium ferrites, the former enters the slag phase only after the formation of calcium-iron-silicates. MgO after calcination enters and stabilizes spinal structure by forming magnesio-ferrites. CaO combines with silicates to form calcium silicates and also forms calcium ferrites at high basicity. Dolomite fluxed pellets with 1.5 % MgO at a basicity value of 0.8 has shown superior reducibility at high temperature, because of the porous structure and presence of calcium ferrites.
One of the studies has indicated that dolomite addition increases the pellet strength after induration and low temperature reduction (at 550 deg C) but increases the swelling. Dolomite fluxed pellet at a basicity value of 0.8 results in the formation of magnesio-ferrite and slag phase of two types, one rich in Fe and MgO and the other rich in CaO and SiO2. Dolomite addition also results in higher porosity because of the formation of less slag as MgO raises its liquidus temperature. Reducibility also found to increase at a basicity value of 1.3 with dolomite addition, because of the high porosity of the pellets.
The degree of reduction of pellets increases with increase in the MgO/SiO2 ratio. When MgO is added in the form of magnesite, the pellet porosity is expected to increase due to the decomposition of magnesite thereby improving the reducibility. The swelling tendency of pellets also decreases with increasing MgO/SiO2 ratio of pellets. Low swelling can be because of the fact that magnesio-ferrite, during reduction, does not cause volume increase as it has a spinal structure with a cubic system which is same as wustite. Hematite is the primary cause for swelling and it has been calculated that 1 % of hematite in the pellets leads to 0.15 % increase in the volume. The volume is including the volume caused by transformation of hematite, cracks caused by it, sintering among iron oxide particles, and solid solution of gangue minerals into iron oxide. From phase diagrams (FeO-SiO2-MgO and SiO2-CaO-MgO), it is understood that slag phase with high MgO/SiO2 ratio has high melting point irrespective of its FeO content. CaO is to be as low as possible as it lowers the slag formation temperature.
The reduction degree of CaO-free MgO pellets, at high temperature (1,250 deg C), increases considerably when the MgO/SiO2 ratio is higher than 0.4. In CaO fluxed pellets, reduction degree drops steeply up to CaO/SiO2 ratio of 0.3 and increases thereafter. Liquid slag formation which causes volume contraction during the reduction, influences the reducibility to a great extent. Acid pellets, during reduction, form higher quantity of low melting point (around 1,200 deg C) fayalite slag leading to closing of pores and hence low reducibility. With increasing MgO/SiO2 ratio, quantity of liquid slag decreases due to the formation of high melting point (around 1,500 deg C) olivine. In CaO fluxed pellets, up to CaO/SiO2 ratio of 0.3, formation of low melting point (around 1,250 deg C) anorthite increases leading to closure of pores. Pellet softening temperature increases with increasing MgO/SiO2 ratio. In CaO fluxed pellets, lowest softening temperature occurs at CaO/SiO2 ratio of 0.45. Softening degree of pellets below 1,100 deg C is more affected by the strength of iron oxides than the slag phase, whereas above 1,100 deg C, it is more affected by the pellet chemistry and slag phase. Softening properties of pellets can be improved by increasing the quantity of solid slag.
Addition of MgO in the form of olivine to magnetite concentrate has been found to stabilize the magnetite structure by forming magnesio-ferrite. Magnetic susceptibility of the pellets, in the form of high Fe2+ content, increases with increasing MgO/SiO2 ratio. Porosity of pellets also increases with increasing MgO content in magnetite pellets. This is because of the fact that magnetite is stabilized in view of the diffusion of Mg2+ ions into the iron oxide lattice, thereby minimizing the heat available from magnetite oxidation, recrystallization, and grain growth. Cold strength of the pellets also decreases because of the same reason.
MgO addition in the form of serpentine reduces the swelling of pellets made from both hematite as well as magnetite concentrates. This can be because of the fact that MgO is dissolved in FeO to form [(1-x)MgO.FexO]O.Fe2O3 thereby reducing the migration of Fe2+ during reduction which leads to no change in volume hence low swelling. One of the studies has shown that with increasing MgO/SiO2 ratio, the high temperature reducibility of pellet increases in olivine fluxed pellets. Another study has reported that addition of MgO addition to pellets causes more low temperature breakdown because of the formation of cracks between the reduced magnetite phase and magnesio-ferrite spinel. However, MgO improved the high temperature reducibility of pellets because of less liquid slag formation.
It is conventional to represent the bulk composition of complex oxide materials, such as iron ore pellets, iron sinter, minerals, ores, and refractory products, in terms of the simple oxides of the constituent elements. However, this does not indicate that the product is composed of a mixture of such simple compounds. It is simply a convenient means of representing the overall elemental composition of the material with each element concentration expressed in the form of its stable oxide.
Quality of the product pellets broadly depends on green pellet quality, type and quantity of binders, fluxes and additives used, and firing conditions. Quality of green pellets, in turn, depends on input parameters like mineralogy, chemistry and granulometry of ore fines, balling parameters like initial feed size, moisture content, and porosity etc. Physical and metallurgical properties of product pellets depend on the quantity and type of binder and flux additions, induration parameters like firing temperature and time etc. Ingredients of the green pellets react together, during firing, to form different phases and microstructure. The type and quantity of these phases, their chemistry and distribution play an important role in deciding the metallurgical properties of the pellets during reduction in the subsequent iron making process.
Primary phases present in iron ore pellets are hematite, magnetite, slag phase, calcium ferrite, and pores. The microstructures of pellets are presented in Fig 2. Hematite is the predominant phase in pellets with basicity from 0.08 to 1.15, other phases vary depending on the basicity of the pellet. In the fluxed pellets, bonding is achieved through silicate melt formation during induration. The quantity of gangue in the concentrate, CaO and MgO in the fluxes and binder influence the quantity and chemistry of silicate melt. CaO fluxes as well as silicate melt reacts with iron oxide to form different calcium ferrites. MgO either enters the magnetite lattice to form magnesio-ferrite or dissolves in the slag phase. These melting phases interact with each other and dissolve a variable quantity of iron oxides. As the formation of phases and microstructure during the induration depends on the type and quantities of fluxes added, there is a need to study the effect of these fluxing agents, in terms of CaO/SiO2 ratio and MgO content, on pellet quality by using limestone and dolomite.
Fig 2 Microstructural phase analysis of pellets
Mineralogically iron ore pellets comprise essentially hematite (original surviving) particles of iron ore, crystalline silica (quartz, cristobalite and tridymite) and forsterite (Mg2SiO4). The principal variation in pellet mineralogy is in the proportion of gangue phases present in the product. These vary depending upon the pellet feed material and the type and the quantity of any additives to feed such as limestone, dolomite, olivine and bentonite etc.
Pellets present an inhomogeneous microstructure, mainly due to porosity, and radial variations are typical. Hence, traditional sample scanning is required to be done carefully to avoid producing average results which actually hide the spatial variations that are to be identified and measured.
Microstructural studies are normally carried out by dividing the pellet into four segments. These segments are (i) shell which extends around 2 mm below from outer surface (peripheral), (ii) outer mantle which is just below the shell (2 mm-3 mm thick), (iii) inner mantle which is just below outer mantle (4 mm-5 mm thick) and (iv) core which is the innermost part of the pellet (4 mm-6 mm in diameter). Fig 3 shows segments of a pellet.
Fig 3 Segments of a pellet
The microstructural characterization of iron ore pellets contributes to the understanding of their properties and behaviour in the agglomeration processes, and their use in the production of DRI and hot metal. The mineralogical composition constitutes an important parameter which can be specified and controlled, as well the porosity and the spatial arrangement of pores. Reflected light microscopy (RLM) is typically applied to characterize iron ore pellets since it is capable of distinguishing the most common mineral phases through their distinctive reflectivities. In fact, RLM and digital image analysis are suitable tools for characterizing these materials. They can provide fundamental information like mineralogy, porosity, and texture.
A leading producer of iron ore pellets performs a qualitative evaluation of the microstructure of iron ore pellets, by visual examination under a reflected light microscope, regarding its correlation with conventional quality parameters. Nowadays, efforts on RLM and digital image analysis development are in progress in order to establish qualitative and quantitative characterization routines for iron ore pellets. Digital Microscopy techniques were developed and employed to allow this kind of assessment.
Pellets present an inhomogeneous microstructure, mainly due to porosity, and radial variations are typical. Hence, traditional sample scanning is to be done carefully to avoid producing average results that actually hide the spatial variations which are to be identified and measured. Normally digital microscopy is employed to acquire mosaic images of pellet samples. These mosaics either scanned the sample along several diameters or covered the full sample surface in a single image. Image analysis is then employed to discriminate and measure the phase and porosity fractions and establish their systematic spatial variations.
Mosaic images are very useful both for qualitative and quantitative analysis as they provide a complete view of the sample, and the obtained measurements do not suffer from statistical limitations and edge problems associated with individual fields. On the other hand, at higher resolutions (and hence smaller field sizes) mosaic image files can quickly become so large that image processing becomes impractical. Hence, it is important to combine both low resolution mosaics covering the whole sample and higher resolution mosaics covering sample diameters only, which reveal the radial distribution of phases. Fig 4 shows the mosaic images of cross section of a pellet. The radial variation of phases, especially pores, is readily visible in the image.
Fig 4 Mosaic images of pellet cross section
The mosaic image of the complete sample section obtained with the lower resolution lens as shown in Fig 4 is very useful for qualitative analysis by a human operator. However, the accurate measurement of phase fractions is hindered by the limited resolution.
The bulk chemical compositional analysis of iron ore pellets is normally carried out using X‐ray fluorescence (XRF) spectrometry by the fused bead technique or with the original substance. For the fused bead method, typically, a 0.5 gram to 1 gram portion of finely ground and ignited iron ore pellets is mixed with alkali borate (e.g., lithium metaborate) and in the sample the sample to borate ratio is 1:10. The mixture is fused and cast into a circular glass bead. When the original substance is used, it is finely ground and mixed with a binding agent. In either case, the resultant test sample is subsequently subjected to multi‐element analysis by XRF spectrometry using well established calibration.
There are no specific EN standards for the multi‐element analysis of iron ore pellets by XRF spectrometry. However, ISO standard 9516‐1:2003 for the analysis of iron ore [Iron ores – determination of various elements by X‐ray fluorescence spectrometry – Part 1: Comprehensive procedure] is applicable to iron ore pellets. Tab 5 gives different ISO standard tests for pellets.
|Tab 5 ISO standard tests used for pellets|
|ISO standard test||Measurement values||Purpose|
|ISO 4700 /Crushing strength||daN||Pellet cold strength|
|ISO 3271 /Tumble strength||Fractions +6.3 mm and -0.5 mm||Tendency for abrasion|
|ISO 13930 / Low temperature reduction disintegration||Fractions +6.3, -3.15 and -0.5 mm||Tendency for low temperature degradation|
|ISO 4698 /Free swelling||Volume %||Increase Tendency for swelling|
|ISO 4695 / Reducibility||Reduction rate at 40 % reduced||Reducibility|
|ISO 7992 / Reduction under load||1. Reduction rate at 40 % reduced||Reducibility, Softening / melting behaviour|
|2. Pressure drop at 80 % reduced|
|3. Bed shrinkage at 80 % reduced|
Laboratories undertaking such analyses are required to have accreditation to ISO 17025 or ISO 9001. Multi‐element analysis of iron ore pellets provides the overall concentrations of the main constituents of the product, but does not give any indication of the identity of the individual compounds or chemical phases present, i.e., the pellet mineralogy.
Quantitative phase analysis of all the major chemical phases present in the iron ore pellets can be achieved only by means of X‐ray diffraction (XRD) analysis combined with Rietveld data analysis. Mineralogically iron ore pellets comprise, essentially, relict (original surviving) particles of iron ore, crystalline silica, and forsterite (Mg2SiO4), bound together by oxide bridging formed during the process. The identified mineralogical phases present in iron ore pellets are (i) hematite, Fe2O3 (ii) magnetite, Fe3O4, (iii) quartz, SiO2, (iv) cristobalite, SiO2 (v) tridymite, SiO2, and (vi) forsterite, Mg2SiO4. The principal variation in pellet mineralogy is in the proportion of gangue phases, such as crystalline silica and magnesium silicate (forsterite) present in the product. These vary depending upon the pellet feed material and the type and quantity of any additives to the feed, such as limestone, dolomite, olivine, bentonite etc.
As mentioned above, the principal methods of analysis used for characterization of iron ore pellets are XRF spectrometry, for bulk analysis, and XRD for phase analysis. Since iron ore pellets are inorganic materials, analytical techniques such as nuclear magnetic resonance (NMR) spectroscopy, infra‐red (IR) spectrometry, and ultra‐violet (UV) absorption spectro‐photometry are not suitable, since these techniques are used to investigate the molecular bonding states of organic compounds which contain essentially covalent bonds. They are not appropriate methods for the identification of inorganic structures where the bonds are principally ionic or metallic in character.
Gas chromatography (GC) is also an inappropriate analytical technique for inorganic solids since it can only be applied to organic (covalent) substances which are vapourized at temperatures below around 320 deg C. Similarly, high‐performance liquid chromatography (HPLC), which is applicable principally to organic compounds, is not a suitable method for identification of inorganic substances. Mass spectrometry can only be applied if high energy excitation techniques, such as spark discharge or laser ablation, are used to vapourize the sample for introduction into the mass spectrometer. However, these techniques essentially provide the same information as X‐ray fluorescence spectrometry, which is highly developed and widely applied for product and process control purposes.
Thermo‐chemical methods of analysis, such differential thermal analysis (DTA), differential scanning calorimetry (DSC) or thermo‐gravimetric analysis (TGA) can be applied for specific studies on chemical phase changes and chemical reactions which occur when iron or iron oxide materials are heated, but the data provided by these techniques are not normally sufficient for identification of these materials.
ICP‐AES (inductively coupled plasma ‐ atomic emission spectrometry), ICP‐MS (inductively coupled plasma‐mass spectrometry), or AAS (atomic absorption spectrometry) methods can be used for the analysis of iron ore pellets, but these techniques are normally more time‐consuming and laborious than XRF spectrometry. Mossbauer spectroscopy is a useful technique for the identification and quantification of iron‐bearing phases (Fe3O4 and Fe2O3, etc.) in iron ore pellets, however, the technique is not commonly applied in industry since the instrumentation needed is specialized and normally only available in research institutions. The technique is less useful than XRD since it does not provide information on non‐ferrous phases such as silica and silicates. As far as particle size distribution is concerned, this is to be verified by means of a sieve test in compliance with an internationally accepted standard (ISO, ASTM, etc.).
Pellet strength is most commonly determined by compression and tumble tests. In the tumbler test 11.4 kg of +6 mm pellets are tumbled for 200 revolutions at 25 rpm in a drum tumbler (ASTM E279-65T) and then screened. A satisfactory commercial pellet should contain no more than about 5 % of minus 0.6 mm (minus 28 mesh) fines, and 94 % or more of plus 6 mm size, after tumbler testing. A minimum of broken pellets between 6 mm and 0.6 mm in size is also desirable.
Advantages of Pellets
Iron ore pellets are superior to iron ore lumps in the subsequent iron making processes such as direct reduction furnace, and blast furnace etc. The pellets have the following advantages.
- Pellets have good reducibility since they have uniform and high porosity (25 % to 30 %). Normally pellets are reduced considerably faster than sinter as well as iron ore lumps. High porosity also helps in better metallization in DRI production.
- Pellets have a uniform size range normally within a range of 8 mm to 18 mm.
- Pellets have spherical shape and open pores which give them good bed permeability.
- Pellets have low angle of repose which is a drawback for pellet since it creates uneven binder distribution.
- The chemical analysis is uniform since it gets controlled during the beneficiation process. Fe content varies from 63 % to 68 % depending on the Fe content of ore fines. Absence of LOI (loss on ignition) is another advantage of the pellets.
- Pellets have high and uniform mechanical strength and can be transported to long distances without generation of fines. Further, it has got resistance to disintegration. High mechanical and uniform strength of pellets is even under thermal stress in reducing atmosphere.
- The inherent higher mechanical strength and abrasion resistance of pellets improve the production rate of DRI in direct reduction furnace.