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.     

Sintering characteristics and kinetics of acidic haematite ore pellets with and without mill scale addition

Haematite ore pellets require very high induration temperature (>1573?K) while, magnetite ore pellets require much lower temperature due to the oxidation of magnetite during induration. Mixing of some magnetite in haematite ore can improve the sintering property of pellets during induration. Mill scale is a waste material of steel plant which contains mainly FeO and Fe₃O₄. It can also be blended in haematite ore pellet mix which can enhance diffusion bonding and recrystallisation bonding and facilitate sintering at the lower temperature like magnetite ore. The extent of improvement in sintering property, sintering mechanism and its kinetics in the presence of mill scale is very imperative to study. In current study, the sintering characteristics of acidic iron ore pellet with 15% mill scale and without mill scale has been studied separately through microstructure observation, apparent porosity measurement and volume change. The volume changes due to heating at varying temperature and time has been measured by mercury displacement method and the data has been exploited for sintering kinetics study, wherein, extent of sintering α has a power relation with time. Several kinetics parameters such as time exponent (n), rate constant (k) and activation energies have been estimated for above two pellets and compared. While acidic pellet without mill scale requires 385?k?cal?mol^?1, acidic pellet with 15% mill scale requires only 310?k?cal?mol^?1 activation energy.     

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.     

Influence of magnesia on iron ore sinter properties and productivity

Abstract

Dolomite and other MgO bearing materials are being increasingly used as basic flux constituents for production of fluxed sinters. Addition of flux materials in sinter influences the resultant sinter microstructure and chemical properties. The physical and metallurgical properties of sinter mainly depend on mineralogy of the sinter. Dolomite is the source of double carbonate of calcium and magnesium. Recent studies reveal that, apart from the additional fuel needed, the addition of dolomite and MgO bearing material greatly influences the magnetite content and the properties of the sinter produced. The increasing use of MgO bearing fluxes in the blast furnace burden, and the trend to incorporate a major part of fluxes in the sinter mix led to an investigation of the influence of MgO on sinter properties and productivity. In this study, the systematic investigation has been made on the influence of MgO% (1·4 to 2·6) on sinter mineralogy and sinter properties with dolomite. Microstructural examination of dolomite sinter revealed that hematite and calcium ferrite phases decreased whereas magnetite phase increased with increase in MgO percentage in sinter. From the laboratory pot grate sintering results it was found that sinter reduction degradation index improved whereas tumbler index and reducibility decreased with increase in MgO%.     

Influence of raw material particle size on quality of pellets

Abstract

Pellet plant (4·2 MPta capacity) of JSW Steel Ltd imports iron ore fines from different mines to produce pellets for its Corex and Blast Furnace plants. The pelletisation process involves drying the ore fines to reduce the moisture content to less than 1%, grinding in open circuit ball mills to get required fineness. To produce good quality of pellets certain additives are important and limestone is employed for modifying the pellet basicity. Iron ore fines of ?10 mm size and limestone are ground together in a ball mill to get sufficient fineness for the balling process. However, as limestone is harder than iron ore fines the + 100 mesh size limestone particles is higher than required and not all the limestone is fully consumed in the reaction for melt formation. Microstructural studies were conducted under a Leica DMRX polarized microscope at different level fineness (?325# ? 56, 58 and 60%) to investigate its effect on the pellet quality. The cold crushing strength of the pellet improved from 203 to 220 kg p^?1 with increase in fineness. With increase in percentage of ?325# particle size in the ground product RDI of the pellet decreased from 13·8 to 11·9% with increased melt formation from 5 to 9%. With increase in fineness ?325# from 56 to 60% the 150 to 500 μm size pores decreased from 51·8 to 13·6%.     

Composite Pre-Reduced Pellet (CPR) — A New Way for Fines Utilisation

Composite pre-reduced pellet has a metallised core encased in a sintered shell of iron ore. It is produced under oxidizing atmopsphere and degree of reduction obstained is as high as 91%. Non-coking coal, charcoal, coke and char fines are used as reducing agent. The physical properties of this pellet make it suitable for use in blast furnace/steelmaking processes.

In this paper an attempt has been made to present a review of the work done in this area as well as ore: coal mixed pellet. The effect of process variables like, temperature, time, Fe₂O₃/C ratio, heating rate and heating conditions on degree of reduction has been discussed. Attempt has also been made to judge the suitability of the pellet for use in iron and steel industries.     

Development of fluxed micropellets for sintering utilising iron oxide waste fines

Abstract

Ultrafine iron oxide wastes such as slime, blue dust and Linz–Donawitz (LD) converter sludge have very limited use in sintering of iron ore due to their excessive fineness (?50 μm). Pelletisation of these ultrafine materials for use in blast furnace involves high temperature curing, which is a highly energy intensive process. Briquetting of LD sludge requires costly binders and contains high moisture, which creates problem at high temperature of the downstream process. In order to alleviate these problems, the current study has developed a process for preparing micropellets of waste iron oxide fines (2–6 mm size) without using any binder. The strength of the micropellet has been increased by a novel CO₂ treatment process at room temperature. Developed micropellets exhibit very suitable drop strength (125 Nos), tumbler properties and cold compressive strength (～9 kg/pellet) to withstand cold handling. Low lime containing micropellets have the possibility of being used as a mixed material in usual sinter making, and high lime containing micropellets may be exploited for making super fluxed sinter that can be used as synthetic flux in the basic oxygen furnace process towards the formation of low melting oxidising slag at the early stage of blow.     

Tuyere failure analysis in Corex process

Abstract

Corex is an alternative smelting reduction ironmaking process where non-coking coal and pure oxygen is used instead of coke and air. The temperature of the tuyere region is much higher than in a blast furnace, and sustainability of tuyeres is a major challenge. At JSW Steel Ltd, almost 15% of the total shutdown is due to burnt tuyere replacement. A detailed analysis of tuyere failure and process parameters effecting tuyere burning was conducted to understand the failure mechanism and the root causes. A common reason does not exist for all types of tuyere failure; however, the collective reasons for failure are excessive coal fines (–6·3 mm) and small mean particle size, low back pressure, blocking of tuyeres, scab formation and slip.     

Dissolution Characteristics of CO2-Treated Fluxed Pellets in Hot Metal Bath

Lump lime and iron ore are generally used in the basic oxygen furnace as flux and cooling material, respectively. Owing to high melting point, poor dissolution property, fines generation tendency, and hygroscopic nature of lump lime, delay in process and operational complexities are generally encountered. On the other hand, iron ore charging creates slag foaming. In order to alleviate the above problems and to utilize waste materials, fluxed lime–iron oxide pellets (FLIP) containing waste iron oxides and lime fines (10%–40%) were prepared and subsequently strengthened with CO₂ gas treatment. FLIP may have the potential to partially replace scrap and lump lime in the conventional basic oxygen furnace charge. In order to assess the applicability of FLIP in steelmaking, the dissolution characteristics of these pellets were studied in a high-temperature pot furnace equipped with a charge-coupled device (CCD) camera under varying experimental conditions. It was found that the dissolution time decreased with increasing hot metal temperature, increasing specific surface area of the pellet, and decreasing lime content of the pellet. The melting of the pellet in the absence of hot metal took much higher time than its presence.     

Preparation of Iron Ore Micro-pellets and Their Effect on Sinter Bed Permeability

At present around 6–7% of iron ore slimes, out of total production, are being generated and accumulated at iron ore mine sites of National Mineral Development Corporation Limited, India. The accumulated slimes of finer size and relatively inferior grade should be utilized in an economical way for sustainable mining. These slimes can be agglomerated into micro-pellets for subsequent use in sinter making through hybrid pellet sintering method. However, the micro-pellets of sufficient size and strength are required for hybrid sinter making. The green properties of the micro-pellets depend upon various parameters such as surface area, moisture, binder, etc. In this study, iron ore slimes were beneficiated through gravity, and magnetic separation and concentrate of grade 65% Fe (Total) was obtained. Since the concentrate obtained had low surface area (700–900 cm²/grams) rendering it unsuitable for micro-pellet making, it was further subjected to grinding in a ball mill. The requirement of surface area for producing an optimum quality of green micro-pellets was established. The resultant micro-pellets were further used in studying sinter bed properties. The effect of moisture and size of micro-pellets on permeability of sinter bed were examined. The results confirmed that the addition of micro-pellets to the sinter mix improved the permeability of the sinter bed. The sinter bed with highest permeability of JPU 25.25 and void fraction of 36.27% was achieved with micro-pellets of size 3–6 mm at 7% moisture level. Mean granule size of sinter mix was also studied with respect to moisture content and size of the micro-pellets.     

Transfer of iron to Cu–Ni alloys from the lining of the vacuum induction furnace

In Ni–Cu alloys, iron must be excluded in many cases. Iron may enter the alloy from the batch or the furnace lining. Since the Fe₂O₃ content in refractories may be as much as 2.5%, it is important to assess the increase in iron content in alloys on account of interaction with the furnace lining. In the present work, the influence of the Fe₂O₃ content in the crucible and the volume of the crucible on the iron content in the final alloy is studied. Thermodynamic analysis and experimental data indicate that the nickel and copper in Ni–Cu alloys may reduce iron that is present in the lining. When using low-iron batch, iron from the crucible is transferred almost completely to the melt. The increase in iron content in Ni–Cu alloys is investigated as a function of the capacity of the vacuum induction furnace and the Fe₂O₃ content in the periclase crucibles, with complete transfer of the iron from the lining to the melt. With increase in furnace capacity, less iron enters the melt from the crucible. With more than 200 kg of metal, the increase in iron concentration mainly depends not on the furnace capacity but on the Fe₂O₃ content in the refractory. In order to produce Ni–Cu alloys with <0.01% Fe, refractories with Fe₂O₃ content no higher than 0.5% must be used. To produce Ni?Cu alloys with <0.05% Fe, the use of lining refractories with Fe₂O₃ content no higher than 2.5% is recommended.     

Application of Surface Chemical Fundamentals to Improving Industrial Filtration Rates

The throughput of any process is limited by the step with the lowest throughput. In iron ore processing, filtration is used to dewater the iron ore concentrate created during reverse flotation. The conditions of the reverse flotation of iron ore and the efficient filtration of iron ore concentrate are found to be at odds with each other. We show that optimizing the filtration conditions should reduce cation retention in the filter cake, which also improves pellet quality in laboratory scale work. Experimental work was performed to investigate potential implementations at plant scale. Based on data collected at operating magnetite concentrators, it has been found that adding CO₂ into the filter slurry can increase filtration rates dramatically. In plant scale work, filtration rates were increased by up to 23.7% with the addition of 0.54kg of CO₂/ton of feed. We compare these results with laboratory scale results on the same process and elaborate on the theory leading to this discovery, while considering its potential impact on final pellet and iron product quality.     

Application of artificial neural network model to predict reduction degradation index of iron oxide pellets

Abstract

The reduction degradation index (RDI) is an important metallurgical property of iron ore pellets used for the production of RDI from shaft furnace or for use in blast furnaces. In order to develop a control strategy, a neural network model has been developed to predict the RDI of pellets from 13 input variables, namely feedrate of green pellets, bed height, burn through temperature, firing temperature, specific corex gas consumption, bentonite, moisture and carbon content in green pellets and Al₂O₃, SiO₂, CaO, MgO and FeO in fired pellets. The RDI of pellets was more sensitive to variation in MgO, CaO, bentonite and green pellet carbon content. The predicted results were in good agreement with the actual data.     

Influence of coke breeze particle size on quality of sinter

Abstract

Iron ore sintering is an extremely complex process involving fuel combustion to generate heat and reducing gases like CO. This heat allows physicochemical, solid and solid–liquid reactions to form liquids of complex components as fuel particles are consumed and cooling processes allow the formation of solid mineral phases. At JSW coke breeze from coke ovens is used as solid fuel in sinter. The properties (size) of the solid fuel play a very important role in determining the sinter microstructural properties and sinter quality. The microstructure of the sinter is a basic necessity and also the first step towards establishing the structural property relationship. Microstructural studies have been carried out to understand the effect of coke breeze particle size on sinter microstructure and sinter properties. The present paper is an attempt to understand and correlate the physical and metallurgical properties of sinter with varying size of the coke breeze particle in sinter mix. It was observed that as the proportion of coke breeze below 3 mm in the sinter mix increased from 53·0 to 90·0% the calcium ferrite phase increased, the number of bigger size pores decreased, and thereby decreased the reduction degradation index (?3·15 mm) of sinter from 39·7 to 23·5%. Superior sinter properties were obtained with the ?3 mm coke breeze size ～90% in the sinter mix.     

Iron ore reduction/cementation: experimental results and kinetic modelling

Abstract

Iron ore reduction and iron cementation by H₂-CH₄-Ar gas mixtures were investigated in a laboratory isothermal fixed bed reactor in the temperature range 600-925°C. Iron ore was first reduced to metallic iron by hydrogen, then metallic iron was carburised to cementite by methane. Increasing temperature and hydrogen content accelerated the reduction process. However, for >55 vol.-%, the effect of H₂ content was not significant. Methane had almost no effect on the reduction process. Increasing temperature increased the rate of iron cementation and also the rate of free carbon deposition. Optimum conditions for cementite formation were: temperature 750°C and reducing/carburising gas contents of 40-55 vol.-%H₂ and 35 vol.-%CH₄. Under these conditions, reduction of iron ore to cementite was completed in ~15 min. A two interface grain model and a volume reaction model were used to simulate the process of iron ore reduction and iron cementation. The simulated results for both reduction and cementation were consistent with the experimental data.     

Tensile and fatigue properties of cold and warm compacted Alumix 431 alloy

Abstract

In this experimental study, tensile and fatigue properties of the Alumix 431 alloy (Al, Zn, Mg and Cu alloys) produced using the conventional press and sinter processes in different pressures and temperatures are investigated. The results clearly showed that the warm compacted specimens can reach the mechanical properties of the cold compacted ones under less pressure. In the fatigue tests it was observed that fracture started from large pores as shown in all scanning electron microscope (SEM) examinations and ductile fracture occurred. 85% of the 180 MPa/80°C and 77% of the 230 MPa/RT specimens fractured at the machined surface. Tensile and fatigue properties of warm compacted (180 MPa/80°C) and cold (230 MPa/RT) compacted specimens are almost equal at these same densities. This result indicates the economic benefit of warm compaction by the much lower applied compaction pressure.     

Industrial processing,microstructures and mechanical properties of Fe–(2–4)Mn (–0.85Mo)–(0.3–0.7)C sintered steels

Abstract

The potential of PM Mn steels has been established in laboratory experiments. This paper deals with sintering of Fe–(2–4)Mn–(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of ?55 °C. Tensile, bend (including fatigue) and miniature Charpy specimens were sintered in flowing gases and in semiclosed containers with a getter of ferromanganese, carbon and alumina. The quenched and tem- pered state was investigated, as was sinter hardening (cooling rate of 55 K min ?1), simulated for comparison with slow cooling at 10 K min ?1. As there was no forma tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Ellingham–Richardson diagram for Mn oxidation/reduction, the use of semiclosed containers was superfluous. The quenched and tempered specimens were brittle. Sinter hardening lead to an improvement in mechanical properties. The reproducibility of tensile and TRS data was high for the sintered materials, characterised by Weibull moduli m of 12–41. All the alloy microstructures were complex and heterogeneous, consisting of, depending on the local manganese and carbon contents, the diffusive and non-diffusive transformation products (pearlite, bainite, martensite) and additionally ferrite and retained austenite. The highest mechanical properties in the entire range of compositions investigated in the furnace cooled state: yield, tensile and bend strengths of 499, 637 and 1280 MPa, respectively, with impact energy of 18 J, and tensile and bend strains of 1˙17 and 1.57%, were achieved for the Fe–2Mn–0.85Mo–0.5C alloy, marginally superior to Fe–2Mn–0.7C. For the sinter hardened Fe–4Mn–0.3C alloy yield, tensile and bend strengths were 570, 664 and 1263 MPa, respectively, at an acceptable impact energy of 14 J, with tensile and bend strains of 0.52% and 1.8%. Many of the results compare favourably with the requirements of MPIF standard 35. Mn is a more effective strengthening agent than either Ni or Cu, or their combination, though generally at reduced plasticity.     

The effect of hydrogen addition on the carbon-deposition behaviour during the reduction of pellets for the blast furnace process

With the application of large amount of pulverised coal injection into the blast furnace, the hydrogen content in the gas will increase, which accelerates the reduction of iron ore in lump zone of the blast furnace as well as carbon-deposition reaction. This study has investigated the effect of hydrogen addition on carbon-deposition reaction during the reduction of pellets through thermodynamic calculation and experiment. The results show that H₂ can promote the carbon-deposition reaction, while the increase of temperature and CO₂ can significantly inhibit it. The preference region of temperature for C formation is about 600°C. Moreover, the promotion effect of H₂ on the carbon-deposition reaction at 700°C is better than that at 600°C. The SEM observation results show that the generated carbon is mainly distributed on the surface of the pellet, and only a little carbon is located inside the pellet. The agglomerated carbon could be more easily formed due to the dramatic carbon-deposition reaction caused by the lower temperature or higher H₂ content. But, most of the carbon just exists as an individual particle at the lower carbon-deposition reaction rate. The results of SEM–EDS reveal that carbon deposited is primarily in the form of elemental carbon rather than in the form of cementite. The study also shows that with increasing reduction time, the rate of carbon-deposition increases, mainly due to the promotion effect of reduced iron during the reduction process of pellets.     

Preparation of Mo–Si–B nanocomposite powders by mechanical alloying and heat treating

Abstract

In this study, an attempt has been made to synthesis Mo–Si–B nanocomposite alloys using a combination of mechanical alloying (MA) and heat treatment in various primary elemental compositions. For this purpose, Mo–14Si–10B, Mo–57Si–10B and Mo–47Si–23B (at.‐%) elemental powders were separately milled using an attritor mill. Mechanically alloyed (MAed) powders were annealed in an atmosphere controlled furnace under constant temperature for 10?h. Metallurgical characteristics of MAed and/or annealed powders were evaluated by atomic absorption spectrometry, SEM, TEM and X‐ray diffraction. The results did not show any formation of related intermetallics after MA. However, MoSi₂, Mo₅Si₃, Mo₅SiB₂, MoB and Mo were successfully formed, when the MAed Mo–57Si–10B powders were subjected to annealing at a high temperature.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.