Reduction and Swelling of Fired Hematite Iron Ore Pellets by Non-coking Coal Fines for Application in Sponge Ironmaking

In the present investigation, the reduction and swelling behaviors (in low grade coal) of fired iron ore pellets, prepared by blending hematite iron ore fines of ?100, ?18 + 25, and ?10 + 16 mesh sizes in different proportions, have been studied in the temperature range of 850–1000°C with an objective to promote massive utilization of fines in sponge ironmaking. An increase in temperature up to the range studied (850–1000°C) substantially enhanced the reduction rate and the rate was found to be highest in the first 15–30 min at all these temperatures. All the fired pellets, made by mixing iron ore particles of ± 100 mesh size, have shown approximately the same reduction rates and slightly higher swelling indices than those made from fines of ?100 mesh size only. In all the fired pellets reduced at temperatures of 850°C and 900°C, the results indicated an increase in the extent of swelling with reduction time. Reduction of fired pellets at temperatures of 950°C and 1000°C exhibited shrinkage in their reduced products, and the extent of this shrinkage increased with increase in exposure time.     

Studies on the Reduction–Swelling Behaviors of Hematite Iron Ore Pellets with Noncoking Coal

Studies on the reduction and swelling behaviors of fired pellets, made by mixing hematite iron ore fines of ?100, ?18 + 25, and ?10 + 16 mesh sizes in different proportions, were carried out with low-grade coal in the temperature range of 850–1000°C with an aim to promote the massive utilization of fines in ironmaking. The rate of reduction in all the fired iron ore pellets increased markedly with an increase in temperature up to 1000°C and it was more intense in the first 15-min soak time. Relatively higher reduction rates and swellings/shrinkage were observed in the pellets made by the addition of larger size (+100 mesh) particles in the matrix of ?100 mesh size fines. In general, highest swelling was observed in the fired pellets at a reduction temperature of 850°C, followed by a decrease at 900°C. At both these temperatures, the percentage of swelling increased with reduction time up to the range studied (120 min). The fired pellets reduced at temperatures of 950°C and 1000°C, showed shrinkage, and the extent of this shrinkage increased with increase in exposure time at 950°C. The percentage swelling/shrinkage in the fired pellets was found to be related to their crushing strengths and porosities.     

STUDIES ON THE REDUCTION KINETICS OF HEMATITE IRON ORE PELLETS WITH NONCOKING COALS FOR SPONGE IRON PLANTS

In the present investigation, fired pellets were made by mixing hematite iron ore fines of ?100, ?16 + 18, and ?8 + 10 mesh size in different ratios and studies on their reduction kinetics in Lakhanpur, Orient OC-2 and Belpahar coals were carried out at temperatures ranging from 850°C to 1000°C with a view toward promoting the massive utilization of fines in ironmaking. The rate of reduction in all the fired iron ore pellets increased markedly with an increase in temperature up to 1000°C, and it was more intense in the first 30 min. The values of activation energy, calculated from integral and differential approaches, for the reduction of fired pellets (prepared from iron ore fines of ?100 mesh size) in coals were found to be in the range 131–148 and 130–181 kJ mol^?1 (for α = 0.2 to 0.8), indicating the process is controlled by a carbon gasification reaction. The addition of selected larger size particles in the matrix of ?100 mesh size fines up to the extent studied decreased the activation energy and slightly increased the reduction rates of resultant fired pellets. In comparison to coal, the reduction of fired pellets in char was characterized by significantly lower reduction rates and higher activation energy.     

Effect of alumina on slag–metal separation during iron nugget formation from high alumina Indian iron ore fines

Abstract

Iron nuggets can be obtained from ore–coal composite pellets by high temperature reduction. Alumina in the ore plays a vital role in slag–metal separation during nugget formation, as it increases the liquidus temperature of the slag. In this study, the effect of carbon content, reduction temperature and lime addition on slag–metal separation and nugget formation of varying alumina iron ore fines were studied by means of thermodynamic modelling. The results were validated by conducting experiments using iron ore fines with alumina levels ranging from 1·85 to 6·15%. Results showed that increase in reduction temperature enhances slag metal separation, whereas increasing alumina and carbon content beyond the optimum level adversely affects separation. Carbon below the required amount decreases the metal recovery, and carbon above the required amount reduces the silica and alters the slag chemistry. Optimum conditions were established to produce iron nuggets with complete slag–metal separation using iron ore–coal composite pellets made from high alumina iron ore fines. These were reduction temperature of 1400°C, reduction time minimum of 15 min, carbon input of 80% of theoretical requirement and CaO input of 2·3, 3·0 and 4·2 wt-% for 1·85, 4·0 and 6·15 wt-% alumina ores respectively.     

Reduction Behaviour of Olivine Iron Ore Pellets in the Experimental Blast Furnace

An experimental study was conducted to determine the reduction behaviour of olivine iron ore pellets and associated reduction mechanisms in the experimental blast furnace (EBF) located at Luleå. Two sets of EBF samples, namely slowly annealed excavated samples and rapidly quenched probe samples of olivine bearing iron ore pellets were examined in detail. Pellet samples were analysed using SEM, XRD and SIROQUANT analysis to quantitatively determine iron ore phase transformations during descent in the EBF. In the tested EBF campaign, up to 75% of reduction occurred at less than 1100°C, i.e. before the pellet reached the cohesive zone while rest of 25% reduction was completed when pellets reached a temperature of 1300°C and hence within the cohesive zone. The reduction degree of pellets was found to have a linear correlation with distance from the stock line of the EBF. This study showed that the presence of olivine did not have a significant effect on reduction degree for temperatures less than 1100°C in the upper zone of the EBF. However, olivine increased the reduction rate in the final stage of reduction for temperatures in excess of 1100°C in the cohesive zone, which was attributed to the formation of an increased amount of molten FeO containing slag within the pellet. This study is expected to make important contributions towards further improvements in the pellet design as well as the optimization of blast furnace operation and efficiency.     

Utilization of Coke Oven Gas and Converter Gas in the Direct Reduction of Lump Iron Ore

The application of off-gases from the integrated steel plant for the direct reduction of lump iron ore could decrease not only the total production cost but also the energy consumption and CO₂ emissions. The current study investigates the efficiency of reformed coke oven gas (RCOG), original coke oven gas (OCOG), and coke oven gas/basic oxygen furnace gas mixtures (RCOG/BOFG and OCOG/BOFG) in the direct reduction of lump iron ore. The results were compared to that of reformed natural gas (RNG), which is already applied in the commercial direct reduction processes. The reduction of lump ore was carried out at temperatures in the range of 1073 K to 1323 K (800 °C to 1050 °C) to simulate the reduction zone in direct reduction processes. Reflected light microscopy, scanning electron microscopy, and X-ray diffraction analysis were used to characterize the microstructure and the developed phases in the original and reduced lump iron ore. The rate-controlling mechanism of the reduced lump ore was predicted from the calculation of apparent activation energy and the examination of microstructure. At 1073 K to 1323 K (800 °C to 1050 °C), the reduction rate of lump ore was the highest in RCOG followed by OCOG. The reduction rate was found to decrease in the order RCOG > OCOG > RNG > OCOG-BOF > RCOG-BOFG at temperatures 1173 K to 1323 K (900 °C to 1050 °C). The developed fayalite (Fe₂SiO₄), which resulted from the reaction between wüstite and silica, had a significant effect on the reduction process. The reduction rate was increased as H₂ content in the applied gas mixtures increased. The rate-determining step was mainly interfacial chemical reaction with limitation by gaseous diffusion at both initial (20 pct reduction) and moderate (60 pct reduction) stages of reduction. The solid-state diffusion mechanism affected the reduction rate only at moderate stages of reduction.     

Novelty on reduction of raw and pre-oxidised titaniferous magnetite ore with boiler grade coal

Pre-oxidation of fines of magnetite containing materials is usually carried out to get better yield of metals. Titaniferous magnetite ore (TMO) is one kind of low-grade iron ore (around 45–50% of total Fe) with a significant amount of TiO₂ (23.23%) and V₂O₅ (0.403%). TMO fines have been pre-oxidised at 973?K (700°C) for 9?h under air atmosphere. The effect of reduction of raw TMO fines as well as the pre-oxidised TMO fines using boiler grade coal in the form of cylindrical briquettes has been studied in the temperature range of 1273?K (1000°C) to 1473?K (1200°C) for periods of 10, 20, 30, 40 and 60?min to estimate the relative yield of iron. The influence of temperature and time on reduction experiments has also been investigated with XRD, FESEM analyses along with chemical analysis of the reduced samples. The most novel result is that the yield of Fe by direct reduction of raw TMO (92.42%) is even marginally better than that of reduction of pre-oxidised TMO (90.89%) at 1473?K (1200°C) for 60?min. Thus the single-step reduction of raw TMO is techno-economically more viable than the pre-oxidation followed by reduction technique.     

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.     

CHARACTERISTICS OF INDIAN NON-COKING COALS AND IRON ORE REDUCTION BY THEIR CHARS FOR DIRECTLY REDUCED IRON PRODUCTION

Studies on the chemical and physical properties (proximate analysis, sulphur content, reactivity, iron ore reduction potential, caking index, and ash fusion temperatures) of coals, procured from 16 different mines in Orissa, India, were undertaken for their judicial selection in Indian sponge iron plants. These coals were found to have low sulphur (range of 0.40–0.66%) and a moderate-to-high ash (range: 22–53%) contents. The results indicated that there were no caking characteristics in any of the coals except Basundhara. The majority of the studied coal ashes were found to have higher fusion temperatures (ST: 1349–1547°C; HT: 1500–1663°C; and FT: 1510–1701°C). An increase in the fixed carbon content in the coal char, in general, led to a decrease in its reactivity toward CO₂. The majority of the chars exhibited significantly higher reactivities (>4.0 cc of CO/g·sec). Further reduction studies in coal chars at 900°C indicated an increase in the degree of reduction of fired hematite iron ore pellets with an increase of char reactivity and reduction time. The authors recommend using the majority of the studied coals as such and some of them (Lakhanpur, Samleshwari, Orient OC–4, and Dhera coals) after blending or beneficiation.     

Compressive Strength of Fired Pellets Using Organic Binder: Response Surface Approach for Analyzing the Performance

In the present investigation, boric acid was used in the ball formation of iron ore fines to improve the compressive strength (CS) of fired pellet. Boric acid was used in combination with carboxymethyl cellulose (CMC) and saw dust and the pellets were fired at different firing temperatures from 1000 to 1300 °C. Box–Behnken statistical design was followed for analyzing the CS at different levels of boric acid, CMC and firing temperature. Results were discussed using 2D surface plots. Response function predictions determined by the regression analysis showed coefficient of correlation (R²) for CS as 0.96. Highest CS of 450 kg/pellet was obtained with addition of 1% boric acid, 0.1% CMC and a temperature of 1300 °C within the range of parameters under investigation.     

Effect of Microwave Treatment Upon Processing Oolitic High Phosphorus Iron Ore for Phosphorus Removal

Influence of microwave treatment on the previously proposed phosphorus removal process of oolitic high phosphorus iron ore (gaseous reduction followed by melting separation) has been studied. Microwave treatment was carried out using a high-temperature microwave reactor (Model: MS-WH). Untreated ore fines and microwaved ore fines were then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA). Thereafter, experiments on the proposed phosphorus removal process were conducted to examine the effect of microwave treatment. Results show that microwave treatment could change the microstructure of the ore fines and has an intensification effect on its gaseous reduction by reducing gas internal resistance, increasing chemical reaction rate and postponing the occurrence of sintering. Results of gaseous reduction tests using tubular furnace indicate both microwave treatment and high reduction temperature high as 1273 K (1000 °C) are needed to totally break down the dense oolite and metallization rate of the ore fines treated using microwave power of 450 W could reach 90 pct under 1273 K (1000 °C) and for 2 hours. Results of melting separation tests of the reduced ore fines with a metallization rate of 90 pct show that, in addition to the melting conditions in our previous studies, introducing 3 pct Na₂CO₃ to the highly reduced ore fines is necessary, and metal recovery rate and phosphorus content of metal could reach 83 pct and 0.31 mass pct, respectively.     

The sticking of iron ore during reduction by hydrogen in a fluidized bed

Abstract

Sticking of particles in the fluidized bed reduction of iron ore at temperatures above 600°C will occur whenever clean iron surfaces impinge. Some as-received ores are sufficiently dirty that iron-iron contact does not take place at low temperatures. However, as the temperature of reduction is raised, there is an increasing tendency for the iron nucleating on the surface of particles to grow into nodules which pierce the dirt film allowing iron-iron contact and hence sticking. Silica in the surface film stops iron nucleation near the surface of particles and thereby prevents iron nodule formation. Ores with silicacontaining coatings can be fully reduced in a fluidized bed at temperatures up to 840°C without sticking.

Résumé

Les particules de minerai de fer au cours de la réduction en lit fluidisé aux températures supérieures à 600°C ont tendance à s'agglomerer aussitôt que des surfaces de fer sont en contact. Quelques minerais utilisés comme tels sont suffisamment impurs pour que le contact fer sur fer ne se produise pas aux basses températures. Cependant, quand la température est élevée, la nucléation du fer a la surface des particules se prolonge par la formation des modules qui percent la couche d'impuretés et favorisent l'agglomération. La présence de silice dans la couche superficielle empêche la nucléation du fer au voisinage de la surface donc aussi la formation de modules de fer. Les minerais contenant de la silice peuvent être réduits complètement dans un lit fluidise a des températures allant jusqu'à 840°C sans s'agglomérer     

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.     

Diffusion of H2-H2O through porous iron formed by the reduction of iron oxides

Measurements were made of the rate of equimolar counterdiffusion of hydrogen and water vapor through porous iron formed by the reduction of dense hematite, magnetite, and commercial iron ore pellets with hydrogen. The experiments were conducted at temperatures between 400° and 1000°C and at pressures between 0.1 and 40 atm. It is demonstrated that the structure of the porous iron is primarily a function of reduction temperature and that the diffusion process at the higher reduction temperatures is normal. The effect of gaseous diffusion on the rate of reduction of dense hematite with hydrogen is discussed. It is shown that gaseous diffusion limits the rate at the higher temperatures and pressures.     

Solid-phase prereduction of iron-vanadium concentrates and liquid-phase separation of the products of their reduction

The transformations that occur in ore grains during solid-phase carbon reduction of the metals from the iron-vanadium concentrates formed upon the beneficiation of the titanomagnetite ores from Southern Ural deposits are studied. Upon heating to 1000°C, the solid solution in titanomagnetite grains decomposes with the formation of magnetite and ilmenite; the reduction of iron begins in the temperature range 1080–1110°C, and the reduction of titanium begins at above 1215°C. The reaction mixture should be held at 1250°C for 45 min to ensure almost complete iron reduction and the minimum degree of titanium reduction. For rapid separation melting, this procedure results in vanadium-containing cast iron (0.43–0.5% V) with <0.15% Ti and a slag with 42–43% titanium oxides.     

Carbon deposition on iron surfaces in CO–CO2 atmosphere

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO₂ atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T?<?T_m) and a high carbon potential (α_c>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO₂ at 650, 706 and 750°C. Cementite (Fe₃C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe₃C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe₃C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.     

还原气氛对流态化还原铁矿粉黏结失流的影响

采用热态可视流化床装置研究了973~1173 K不同气氛条件对流态化还原铁矿粉黏结失流的影响.研究发现,一定表观气速条件下温度和还原气氛组成对失流时铁矿粉的金属化率影响不大,而失流时颗粒微观形态受还原气种类和温度的影响较显著,但还原气体积分数对形态的影响较小.此外,流化时间随着还原气体积分数的增大而逐渐缩矩,并通过线性拟合得到了不同温度时二者间的数学关系式.      

Reaction mechanism on the smelting reduction of iron ore by solid carbon

The kinetics of the smelting reduction of iron ore by a graphite crucible and carbon-saturated molten iron was investigated between 1400 °C and 1550 °C, and its reaction phenomena were continuously observed in situ by X-ray fluoroscopy. In the smelting reduction by graphite, it was shown from the observation results that the smelting reduction reaction proceeded by the following two stages: an initial quiet reduction without foaming (stage I) and a following highly active reduction with severe foaming (stage II). At 1500 °C, by the graphite crucible, the reduction rate of iron ore was found to be 8.88×10⁻⁵ mol/cm² · s, and by the molten iron, 8.25×10⁻⁵ mol/cm²·s. The activation energies for the reduction by the graphite crucible and the molten iron were 24.1 and 22.9 kcal/mol, respectively. Based on the results of kinetic research and X-ray fluoroscopic observations, it can be concluded that these two types of smelting reduction reactions of iron ore by the graphite crucible and by the molten iron are essentially the same.     

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%.     

Coating microstructure development during Zinquench galvanising of sheet steel

Abstract

The influence of an elevated strip entry temperature on the kinetics of the galvanising reaction has been investigated for a titanium stabilised, interstitial free steel. Elevated strip entry temperatures (>550°C) have been shown to accelerate greatly alloying between the substrate and the molten zinc, increasing the growth rate of the δ phase from 1 to ～7 μm s^-1 and preventing the formation of the ζ phase. The substantial Fe–Zn phase layer seen at the coating/substrate interface at high strip entry temperatures (>550°C) is attributed to the dissolution of iron from the substrate into the molten zinc in the first second of immersion, and the limited inhibition provided by the bath aluminium content (<0·15 wt-%effective).