Mill scale as a potential additive to improve the quality of hematite ore pellet

Hematite pellet is required to be indurated at very high temperature to achieve its good strength as there is no exothermic heat of oxidation unlike magnetite. As mill scale contains mainly FeO and Fe₃O₄, any minor amount of its addition in pellet can provide in situ heat and enhance diffusion bonding and sintering. In this study, the mill scale generated in steel plant is added as magnetite input in hematite pellet both in acidic and in basic condition. It has been found that in fluxed pellet, mill scale can improve the properties of pellet. In acidic pellet, the induration temperature has been reduced to a great extent (1250–1275°C) and all properties have been found to be improved due to the addition of 15% mill scale. Mill scale shows enough potential to eliminate the flux addition in producing blast furnace quality pellet from hematite ore. Thus, the flux free acidic pellet has been developed even at very low temperature (1275°C) of induration.     

Oxidation induration process and kinetics of Hongge vanadium titanium-bearing magnetite pellets

The induration process and oxidation kinetics of Hongge vanadium titanium-bearing magnetite (HVTM) pellets have been investigated by employing X-ray diffraction, scanning electron microscope, energy-dispersive spectroscopy, thermogravimetric and differential thermal analysis and thermogravimetric and differential scanning calorimetry. The results indicated that HVTM was a high-chromium vanadium-bearing titanomagnetite containing 1.48?wt-% Cr₂O₃, and the crystal stock strength was 625?N. The compressive strength of HVTM pellets could be improved by increasing the roasting temperature and roasting time. Under the optimum conditions of oxidation roasting at 1200°C for 15?min, the compressive strength was found to be 2893?N. The phase transformations of the valuable elements could be described as follows: Fe₃O₄→Fe₂O₃; Fe₂VO₄→(Cr_0.15V_0.85)₂O₃; Fe_2.75Ti_0.25O₄→FeTiO₃→Fe₉TiO₁₅; FeCr₂O₄→(Fe_0.6Cr_0.4)₂O₄, Fe_0.7Cr_1.3O₃, (Cr_0.15V_0.85)₂O₃. Three stages were identified during the induration process: initial oxidation, later oxidation, and haematite re-crystallisation, poly-crystallisation and induration. The development of strength mainly occurred in the last stage. Kinetic parameters of the oxidation process were determined from heating experiments. The results showed that the average value of activation energy was calculated to be 69.33?kJ?mol^?1 by the Flynn–Wall–Ozawa methods. This study aims to provide theoretical and technical bases for the effective utilisation of HVTM ore for use in either blast furnaces or shaft furnaces.     

Isothermal reduction kinetics of self-reducing mixtures

Isothermal reduction of haematite carbon mixtures was investigated at temperatures 750–1100°C under inert atmosphere. Mass loss curves proved the stepwise reduction of haematite to metallic iron. The non-linear feature of haematite to magnetite reduction kinetics was observed and an activation energy of 209?kJ?mol^?1 was calculated. Irrespective of carbon-bearing material type, reduction rate of magnetite was linear. Activation energy values were calculated to be 293–418?kJ?mol^?1. Significant increase in the reduction kinetics in the last step (Wustite reduction) was observed and explained by the catalytic effect of freshly formed metallic iron. During the initial stages of wustite reduction, the activation energy values were calculated to be in the range of 251–335?kJ?mol^?1 for all carbon-bearing materials.     

A New On-Line Method for Predicting Iron Ore Pellet Quality

The Abrasion Index (AI) describes fines generation from iron ore pellets, and is one of the most common indicators of pellet quality. In a typical pellet plant, dust is generated during the process and then captured. Can the dust be measured and used to predict AI? In this paper, the feasibility of using airborne dust measurements as an indicator of AI is investigated through laboratory tests and using data from a pellet plant. Bentonite clay, polyacrylamide and pregelled cornstarch contents, and induration temperature were adjusted to control the abrasion resistance of laboratory iron ore pellets. AI were observed to range from approximately 1% to 12%. Size distributions of the abrasion progeny were measured and used to estimate quantities of PM₁₀ (particulate matter with aerodynamic diameter less than 10 µm) produced during abrasion. A very good correlation between AI and PM₁₀ (R² = 0.90) was observed using the laboratory pellets. Similarly, a correlation was observed between AI and PM measured in the screening chimney at a straight-grate pelletization plant in Brazil, with an R² value of 0.65. Thus, the laboratory and industry data suggest that measuring dust generation from fired pellets may be an effective on-line measurement of pellet quality. The data also showed that particulate emissions from pelletization plants may be directly affected by AI.     

Solid state sintering in the induration of iron ore pellets

Data is developed to show the relationship between pellet strength and shrinkage which accompanies induration. Systematic measurements of shrinkage kinetics of a pure hematite reagent powder and of a commercial magnetite concentrate are presented, and rate constants and activation energies evaluated. The discussion links the results of pellet induration research with sintering theory, developed in support of powder metallurgy and ceramics. Transport mechanisms active during strength development of pellets are dealt with.     

Replacement of bentonite in hematite ore pelletisation using a combination of sodium lignosulphonate and copper smelting slag

Bentonite is the most common binder used in iron ore pelletisation owing to its good bonding properties in green and dry pellets at both ambient and elevated temperatures. However, due to its high alumina and silica content, it increases the slag volume and energy consumption in downstream processes. Organic binders may be used to replace bentonite; however, they fail to provide strength at a high temperature (700–900°C) due to poor thermal stability during pellet induration. In the present study, an organic binder Na lignosulphonate (NLS) has been used along with copper smelting slag (Cu-SS). FeO in Cu-SS provides diffusion bonding at high temperature and maintains the strength of pellets even after evaporation/burning of NLS. It also enhances recrystallisation bonding at relatively lower temperature to provide good strength. The study has been carried out with hematite ore and varying amounts of NLS and Cu-SS. Copper smelting slag (1.0%) addition with 0.5%NLS has been found to be optimum to provide very good green properties and ～300?kg/pellet cold crushing strength (CCS) at 1250°C induration temperature. However, hematite pellets of similar basicity with 0.5% bentonite requires higher induration temperature (1300°C) to achieve a similar CCS. The developed pellet also shows better reducibility (80%), similar reduction degradation index (18%) and swelling index (10%) to the usual bentonite pellet. Thus, the induration temperature of hematite pellet has been lowered by 50°C using a combination of NLS and Cu-SS eliminating bentonite completely, which can provide a considerable energy and cost saving.     

Characterisation studies on swelling behaviour of iron ore pellets

At JSW Steel Limited (JSWSL), pellets form the major part of the iron-bearing feed to corex and blast furnace. JSWSL produces low-basicity pellets ((CaO/SiO₂) – 0.40 to 0.50). The quality of the pellet is affected by the raw material chemistry (gangue content), flux proportion and their subsequent heat treatment to produce the fired pellets. The raw material silica, limestone addition, i.e. basicity – CaO/SiO₂ of pellet decides the mode, temperature and the amount of melt formed. The properties of the pellets are, therefore, largely governed by the form and degree of bonding achieved between ore particles and also by the stability of these bonding phases during the reduction of iron oxides. In the present study, laboratory pelletisation experiments have been carried out to know the effects of silica and basicity on the microstructure and swelling behaviour of pellets during reduction. Phase analysis was carried out using image analyser, and chemical analysis of oxide and slag phases was carried out using SEM–EDS. From the laboratory studies, it was observed that the swelling index of the pellets decreased with an increase in silica content due to the decrease in porosity. The presence of higher silica in pellet hinders the reduction step of haematite to magnetite at lower temperatures. Pellets with basicity range 0 to 0.1 exhibited lower swelling index due to the formation of high melting point fayalite phase and also at this basicity range the structure is held together by the seam-like compounds between Fe₂O₃ and SiO₂ primarily at high silica content. Higher swelling index was observed at the basicity range 0.3 to 0.7 due to the presence of low melting point calcium olivines (1115°C) between fayalite (FeSiO₄) and dicalcium silicate (Ca₂SiO₄). Low melting point slag phase enhances the swelling index of the pellets. Swelling index of the pellets considerably dropped between the basicity range 0.9 to 1.1 due to the formation of calcium ferrite phases with a close pore structure.     

Kinetic Studies of Iron Ore–Coal Composite Pellet Reduction by TG–DTA

An attempt has been made to study the effect of coal quality on the reduction kinetics of iron ore–coal composite pellets under non-isothermal condition in inert atmosphere. During non-isothermal reduction of composite pellets, it is observed that (i) reduction rate of iron oxide increases with increasing temperature, (ii) reduction rate increases with increase in porosity of pellets and (iii) the computed values of activation energy (E) are lower during the initial stage of reduction (0.86–8.82 kJ mol⁻¹) than those in the later stages of reduction (12.37–38.32 kJ mol⁻¹). These values indicate that the initial stage reduction is controlled by gaseous diffusion mechanism and at final stage, mixed control reaction mechanism (i.e., both gaseous-diffusion and chemical reaction) is the rate controlling step. The present investigation aims at to assess the effect of Fe_tot/C_fix ratio in pellet, volatile matter in coal, and temperature on the reduction kinetics of iron ore–coal composite pellets using simultaneous thermogravimetric and differential thermal analyser (TG–DTA).     

Reduction swelling behaviour of haematite/magnetite agglomerates with addition of MgO and CaO

Abstract

The influence of three kinds of CaO and MgO additives (dolomite, burnt lime and serpentine) on the reduction swelling behaviour of haematite–magnetite (H–M) concentrates pellets was studied. Burnt lime and dolomite increased the reduction swelling index of H–M oxidised pellets, while the reduction swelling index was able to be reduced when serpentine was added. CaO accelerated the formation and growth of metallic iron whiskers and led to abnormal swelling of the magnetite briquettes, while MgO was able to be dissolved in wüstite and reduced the migration rate of Fe²⁺; therefore, there was no catastrophic swelling in either the haematite or magnetite briquettes. As far as H–M concentrate pellets were concerned, because the solubility of CaO in magnetite was greater than that in the primary haematite and the secondary haematite generated from magnetite during the oxidation was easy to be reduced to wüstite, there was abnormal swelling in the reduced H–M pellets with CaO addition.     

Reaction rate and product morphology in carbon composite iron ore pellets with and without Portland cement

Abstract

The aim of this work is to study the reaction rate and the morphology of intermediate reaction products during iron ore reduction when iron ore and carbonaceous materials are agglomerated together with or without Portland cement. The reaction was performed at high temperatures, and used small size samples in order to minimise heat transfer constraints. Coke breeze and pure graphite were the carbonaceous materials employed. Portland cement was applied as a binder, and pellet diameters were in the range 5·6–6·5 mm. The experimental technique involved the measurement of the pellet weight loss, as well as the interruption of the reaction at different stages, in order to submit the partially reduced pellet to scanning electron microscopy. The experimental temperature was in the range 1423–1623 K, and the total reaction time varied from 240 to 1200 s. It was observed that above 1523 K the formation of liquid slag occurred inside the pellets, which partially dissolved iron oxides. The apparent activation energies obtained were 255 kJ mol^–1 for coke breeze containing pellets, and 230 kJ mol^–1 for those pellets containing graphite. It was possible to avoid heat transfer control of the reaction rate up to 1523 K by employing small composite pellets.     

Effect of fluxing agents on reduction degradation behaviour of hematite pellets

During induration at a high temperature, a considerable amount of slag/melt phase forms inside the iron ore pellets, comprising SiO₂, Al₂O₃, CaO, MgO and FeO. After cooling, the slag phase solidifies and acts as an important bonding phase in the finished pellets and influences their room temperature as well as high temperature properties, especially reduction degradation. Fluxing agents play an important role in forming these bonding phases depending on the type and amount of flux. In the present study, the effect of different fluxing agents, namely, limestone, dolomite, magnesite and pyroxenite, on melt formation and microstructure during induration and on reduction degradation behaviour during reduction was examined. From the results, it was understood that to reduce the disintegration during reduction it is essential to increase the amount and distribution of bonding phases like silicates, which are more stable as compared to oxide phases like hematite. Acid pellets exhibited highest reduction degradation due to the presence of more hematite bonds and less silicate bonds. In limestone fluxed pellets, reduction degradation index dropped considerably with increasing CaO content due to the formation of more amount of bonding phase. Dolomite–pyroxenite pellets, on the other hand, showed lower reduction degradation index up to 0.4 basicity, and beyond that, higher degradation was observed due to the increased pore size, which resulted in poor strength of the reduced pellet matrix and hence more degradation. Low reduction degradation observed in pyroxenite and magnesite fluxed pellets could be due to the formation of magnesioferrite and silicate melt, which are more stable phases compared to hematite.     

Magnetite Particle Size Distribution and Pellet Oxidation

Oxidation of magnetite pellets is commonly performed to prepare strong pellets for ironmaking. This article presents a contribution to quantitative understanding of fundamental pellet oxidation kinetics, based on measured oxidation kinetics of magnetite particles and pellets. The commonly observed “plateau” oxidation behavior is confirmed to be consistent with the effect of very large differences in magnetite particle sizes in the concentrate from which pellets are produced. The magnetite particles range in size from less than a micron to several tens of a microns; changing the size distribution by inert sintering of pellets decreases both the plateau level of oxidation and the specific surface area, in ways that are compatible with an assumed Rosin-Rammler magnetite particle size distribution.     

X-ray microtomography for characterisation of cracks in iron ore pellets after reduction

Abstract

This work presents a method, based on X-ray microtomography and three-dimensional (3D) image analysis, of characterising and quantifying crack distribution in iron ore pellets. The aims have been to verify the method and to determine to what extent crack propagation contributes to the decrease in compressive strength that occurs during reduction at 500°C as haematite transforms into magnetite. Raw materials known to cause disintegration problems were selected in order to promote crack propagation. Pellets displayed crack lengths of sizes roughly corresponding to half the pellet diameter already before reduction and, during reduction, a further crack propagation of ～50% occurred. Through estimations by finite element analysis of the crack size and the pellet geometry, it has been possible to determine that this crack growth most likely is a mechanism that contributes to the decrease in compressive strength. The decrease of ～90% that was experimentally determined to occur after 30 min of reduction is, however, too large to be explained by crack propagation alone. The study shows that the proposed techniques allow 3D imaging of iron ore pellets and characterisation of cracks. The scans are non-destructive and can be carried out repeatedly, which allows a specific sample to be studied at different stages during a process. Through future use of the proposed method, our aim is to reach a deeper understanding of the mechanisms behind low temperature disintegration of iron ore pellets and the performance of LKAB olivine pellets inside the blast furnace.     

Recycling of steel plant mill scale via iron ore pelletisation process

Abstract

Mill scale is an iron oxide waste generated during steelmaking, casting and rolling. Total generation of mill scale at JSWSL is around 150 t/day and contains 60–70%FeO and 30–35%Fe₂O₃. To recover the iron, the mill scale must be smelted in a blast furnace or other reduction furnace; however, it is usually too fine to use without previous agglomeration such as via pellet or sinter mix. JSWSL operates a 4·2 Mtpa pellet plant to produce pellets for Corex and BF ironmaking units. The aim of this study is to determine the effect of mill scale on pellet properties. Detailed laboratory basket trials were conducted using up to 40% of mill scale in the pellet mix. The addition of mill scale up to 10% is considered to provide the optimum balance of chemical, physical and metallurgical properties of the pellet.     

Sintering behaviours of iron ore with flue gas circulation

Flue gas circulation is an important method for energy conservation and pollutant emission reduction in iron ore sintering. In this paper the effects of flue gas recirculation ratio on sintering of different iron ores including haematite, magnetite and limonite were studied by illustrating the variation of sinter bed temperature, atmosphere and mineralisation characteristics of different types of iron ores induced by the circulation. It shows that the proper flue gas circulation ratios for haematite, magnetite and limonite are 37, 30 and 25%, respectively. For magnetite ore, preheating and high consumption of oxygen in combustion zone caused more silicate minerals and less acicular calcium ferrite, thereby lowering sinter tumbler strength. As for haematite ore, the rapid change of temperatures of combustion, melting and solidification zones leads to elevated combustion efficiency and increased formation of acicular calcium ferrite, which enhances the sinter strength. When using limonite ore as the main raw material, high oxygen consumption, lower maximum temperature of sintering bed, higher cooling rate and larger porosity of sinter are observed.     

Investigation of factors affecting pellet strength in straight grate induration machine

Abstract

During induration in a straight grate machine, the green pellets pass through four different thermal treatments, namely drying, preheating, heating and cooling. The pellet bed is fired with downdraught firing leading to thermal gradients through the bed. Corex sludge, which is used as fuel in the pellet mix, supplies the necessary energy for uniform heating of the pellet. The physicochemical conditions, e.g. the temperature and oxygen partial pressure mainly depend on the amount of fuel incorporated in the pellet mix. As a result the percentage and the distribution of various phases in the pellets vary, leading to deviation in quality. To study the distribution of phases and their impact on cold crushing strength at different carbon levels (1·20 and 1·35%), pellets from different layers of the induration bed in an industrial straight grate were characterised. It was observed that the strength of the pellets varied from 142 to 268 kg/pellet and 128 to 245 kg/pellet across bed, with carbon 1·20 and 1·35% respectively. It was found that middle layer pellets had higher strength compared to top and bottom layers. It was observed that amount of hematite, magnetite, porosity and the pore size plays a significant role on the pellet strength. Pellets with 1·20% carbon showed better physical and microstructural properties across the pellet bed compared to pellets with 1·35% carbon.     

ASSESSMENT OF REDUCTION BEHAVIOR OF HEMATITE IRON ORE PELLETS IN COAL FINES FOR APPLICATION IN SPONGE IRONMAKING

Studies on isothermal reduction kinetics (with F grade coal) in fired pellets of hematite iron ores, procured from four different mines of Orissa, were carried out in the temperature range of 850–1000°C to provide information for the Indian sponge iron plants. The rate of reduction in all the fired iron ore pellets increased markedly with a rise of temperature up to 950°C, and thereafter it decreased at 1000°C. The rate was more intense in the first 30 minutes. All iron ores exhibited almost complete reduction in their pellets at temperatures of 900 and 950°C in < 2 hours' heating time duration, and the final product morphologies consisted of prominent cracks. The kinetic model equation 1 ? (1 ? α)^1/3 = kt was found to fit best to the experimental data, and the values of apparent activation energy were evaluated. Reductions of D. R. Pattnaik and M. G. Mohanty iron ore pellets were characterized by higher activation energies (183 and 150 kJ mol^?1), indicating carbon gasification reaction to be the rate-controlling step. The results established lower values of activation energy (83 and 84 kJ mol^?1) for the reduction of G. M. OMC Ltd. and Sakaruddin iron ore pellets, proposing their overall rates to be controlled by indirect reduction reactions.     

Effect of TiO2‐content on Reduction of Iron Ore Agglomerates

The effect of titanium oxide on iron ore agglomerates is studied by the use of test sinter, test pellets and synthetic briquettes under laboratory conditions. Titanium favours secondary hematite rather than magnetite, which is the main phase in the sinter of Rautaruukki's Raahe plant. Additionally, the effects of sinter RDI and pellet LTD on the blast furnace process are evaluated using the test results of basket trials in LKAB's Experimental Blast Furnace. The effect of titanium in synthetic hematite is studied as hematite is reduced to magnetite in the RDI test. This occurrence causes deterioration in burden permeability. Synthetic titanium‐bearing iron oxides under controlled conditions are investigated at the University of Oulu. The effect of TiO₂, in solid solution in magnetite, on the magnetite to hematite oxidation is studied separately in order to simulate the final stage of the sintering process. In other experiments, hematite samples doped with various contents of TiO₂ are studied using thermogravimetry under a controlled gas atmosphere (CO/CO₂/H₂/N₂). The TiO₂ content of hematite has a clear effect on reduction degradation. Also increasing content of TiO₂ in solid solution in magnetite radically accelerates the oxidation rate. In the pilot tests, TiO₂ content has a similar negative effect on the reduction strength of both sinter and pellets     

Effect of Iron Ore Pellet Size on its Properties and Microstructure

The properties of the pellets and their microstructure mainly depend on the raw material mix proportion, raw material chemical composition and the physicochemical conditions like the temperature and oxygen partial pressure within the induration machine. The pelletising plant products are in the size range of 8 to 16 mm. With increasing pellet size, the sintering intensity, thermo‐chemical conditions and formation of different phases vary across its cross section. The time required for varies reactions within the pellet is directly proportional to the pellet size. Because of differences in pellet size, the reduction and oxidation process takes place under different conditions resulting in different phases and microstructures. In this work, detailed studies were carried out on pellets of different size (8 to 16 mm) produced from a 4.2 Mtpa pellet plant for their physical, metallurgical and microstructural properties. It was observed that the pellets in the size range of +8 to ‐12 mm showed good strength and lower RDI. It was observed that the amount of hematite, magnetite, porosity, pore density, pore size and slag phase play a significant role on pellet strength and RDI.