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.     

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.     

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.     

CHARACTERIZATION OF PROPERTIES AND REDUCTION BEHAVIOR OF IRON ORES FOR APPLICATION IN SPONGE IRONMAKING

Studies on the chemical and physical properties, and the reduction behavior (in coal) of hematite iron ores procured from 10 different mines of Orissa, were undertaken to provide information for the iron and steel industries (sponge iron plants in particular). The majority of the iron ores were found to have high iron and low alumina and silica contents. All these iron ores were free from the deleterious elements (S, P, As, Pb, alkalies, etc.). The results indicated lower values of shatter and abrasion indices, and higher values of tumbler index in all the iron ore lumps except Serazuddin (previous) and Khanda Bandha OMC Ltd. For all the fired iron ore pellets, the degree of reduction in coal was more intense in the first 30 min, after which it became small. Slow heating led to higher degree of reduction in fired pellets than rapid heating. All the iron ores exhibited more than a 90% reduction in their fired pellets in 2-h time interval at a temperature of 900°C. Iron ore lumps showed a lower degree of reduction than the corresponding fired pellets.     

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.     

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%.     

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.     

LEACHING RATES OF ICEL-YAVCA DOLOMITE IN HYDROCHLORIC ACID SOLUTION

Additives can give rise to obvious, step-wise changes both in the oxidation process and in the sintering process. Therefore, the oxidation and sintering characteristics measured in dried pellets prepared from pure magnetite concentrates can not be representative for those characteristics in dried pellets containing additives. The oxidation and sintering characteristics of magnetite iron ore pellets balled with a novel complex binder (namely MHA) were mainly investigated by batch isothermal oxidation measurements in this research. Combined results reveal that the thermal decomposition of MHA binder influences the oxidation and sintering processes of dried pellets. Oxidation rate of pellets increases obviously with increasing the oxidation temperature in the range from 800°C to 1000°C. And the remaining FeO content declines gradually when separately heated for 10 min at low temperature (<1000°C). However, the oxidation rate of pellets decreases distinctly when oxidation temperature is higher than 1000°C. In addition, when oxidation temperature increases from 1000°C to 1250°C, the FeO content of pellets goes up obviously, particularly at 1250°C. The FeO content in the core of sintered pellets heated at 1250°C can even reach 29.68%. SEM spectrum analysis demonstrate that some iron appears in forms of wustite in sintered pellets, which indicates that the reduction reaction of iron oxide occurs during the high temperature sintering process. This is explained by the occurrence of reducing atmospheres because of the pyrogenic decomposition of MHA binder.     

Reduction Behaviour of Iron Ore–Coal Composite Pellets in Rotary Hearth Furnace (RHF): Effect of Pellet Shape,Size, and Bed Packing Material

Reduction of iron ore–coal composite pellets in multi-layers at rotary hearth furnace (RHF) is limited by heat and mass transfer. Effect of various parameters like pellet shape, size, and bed packing material that are supposed to influence the heat and mass transfer in the pellet bed, have been investigated, on the reduction behaviour of iron ore–coal composite pellets at 1250 °C for 20 min in a laboratory scale RHF. Reduced pellets have been characterised through weight loss measurement, estimation of shrinkage, porosity, and qualitative, quantitative phase analysis by XRD. A significant difference in the degree of reduction is observed layer-wise in the pellet bed with the variation in pellet shape and size. Pellet bed without any packing material or packed with coal have demonstrated higher degrees of reduction compared to the pellet bed packed with graphite and sand.     

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.     

Dissolution Kinetics of Iron from Low-Grade Mn Ore and Optimization Using Response Surface Methodology

In this study an attempt has been made to increase Mn/Fe ratio in dump Manganese ore fines so that it can be used for the production of ferromanganese. For this purpose non-coking coal was used as reductant and dilute hydrochloric acid as leaching medium for the roasted ore. The effects of acid strength, leaching time, leaching temperature, stirring speed, ore particle size and pulp density have been studied. The dissolution of iron follows the kinetic model 1 ? 2x/3 ? (1 ? x)^2/3 = k_dt. Thus product layer diffusion is the controlling mechanism and the activation energy has been determined to be 26.23 kJ/mol at 40–95 °C. Another set of experiments have been conducted according to 2³ full factorial design, and regression equation for iron dissolution has been developed.     

Iron Ore Pellet Dustiness Part I: Factors Affecting Dust Generation

Iron ore pellets abrade during handling and produce dust. This study was conducted to determine what factors affect pellet dustiness, and whether dustiness can be related to the abrasion index. Factors studied included bed depth within a straight grate furnace; pellet chemistry; firing temperature; coke breeze addition; and tumble index. Abrasion indices for all pellet samples ranged from 1.9–5.0% (20 samples) and from 7.1–27.5% (5 samples). Pellets were dropped in an enclosed tower, which enabled the collection of airborne particles generated during pellet breakdown. The quantity of airborne particles generated by each pellet type was 10–100 mg/kg-drop, or 50–500 mg/kg over five drops through the tower. Pellet dustiness was predominantly affected by pellet chemistry and by pellet firing temperature. Results showed a nearly 21% increase in dustiness for every percent decrease in firing temperature – this was based on a typical firing temperature of 1280°C. Pellet dustiness was regressed to the pellet abrasion index (for AI < 5%), which yielded a correlation coefficient of 0.22. These results show that, although AI is one of the best indicators of fired pellet quality and can indicate high levels of dust, it could not explain the dustiness of good quality pellets.

The second paper (Iron Ore Pellet Dustiness Part II) explains the relationship between AI and dust for good-quality pellets; and compares fines generation between pellets fired in Straight-Grate (Traveling Grate) and Grate-Kiln furnaces.     

Pellets Prepared with Mechanically Activated Iron Ore

This work analyses pellets prepared with iron ore that has been mechanically activated by high energy ball milling. Pellet feed iron ore was submitted to high‐energy ball milling for 60 minutes, and the resulting material was analysed through measurements of particle size and specific surface area, as well as X‐ray diffraction. Pellets were prepared from this material. The pellets were heated at temperatures ranging from 1000 to 1250°C in a muffle furnace, and submitted to the maximum temperature during 10‐12 minutes. The samples were then tested regarding crushing strength, densification and porosity, and were examined in a scanning electronic microscope. The results were compared to those obtained with similar samples made from non‐milled pellet feed. It has been shown that through high‐energy ball milling of iron ore it is possible to achieve pellets presenting high densification and compressive strength at firing temperatures lower than the usual ones.     

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.     

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.     

Use of Iron Ore Fines in Cold-Bonded Self-Reducing Composite Pellets

The feasibility of producing direct reduced iron from cold-bonded, self-reducing composite pellets, constituted from beneficiated iron ore slime, coke, and different binders (dextrin, bentonite, calcium lignosulfonate, and carboxymethyl-cellulose [CMC]) was studied. This was done using a design of experiments approach. It was found that as-received beneficiated iron ore slime is suitable as a raw material for the production of self-reducing composite pellets with carboxymethylcellulose as the most suitable binder. Dry strengths in excess of 300 N/pellet were attained by curing the pellets under ambient conditions. The composite pellets reduced within 20 min to degrees of metallization in excess of 90% at 1100°C, with decrepitation indices significantly below 5%. The degree of metallization of composite pellets increased with an increase in reduction temperature (from 1000 to 1100°C), reduction time (20 min. vs. 40 min), and coke quantity (15% vs. 20%). CMC was identified as the most economical and suitable binder for the Sishen concentrate.     

Disintegration of Northern Cape iron ores under reducing conditions

Abstract

Cracking occurs in the first step of gaseous reduction of hematite iron ore, to magnetite, and can lead to the formation of fine material, with deleterious effects on operation of shaft furnaces. To study this, samples of three ore types from the Northern Cape iron ore field in South Africa, and one blended ore from this region, were studied. The methods were high temperature microscopy (during reduction) and quantification of fines formation following reduction disintegration tests. The ore types do differ significantly with regards to their propensity to form fines. Although disintegration is clearly triggered by reduction, no direct correlation could be established between the degree of reduction and the amount of fines generated. Reduction disintegration increased with higher hydrogen percentages (>5%) in the reduction gas, and at higher temperatures (in the 500–700°C range). Disintegration of the samples decreased at temperatures >750°C. There was no correlation between the presence of gangue minerals and fines formation.     

Development in smelting reduction processes

Corex process is a smelting reduction process to produce hot metal of blast-furnace quality. Coal is used instead of coke, and this replacement makes production costs of hot metal decrease. Iron ore reduction and melting is separated into two steps: in a melter gasifier reducing gas is generated and melting energy is produced by coal gasification; iron ore is reduced in a shaft furnace. Due to this separation, a great variety of untreated coals can be used. The Corex process is designed to operate under elevated pressure, up to 5 bar. Reducing gas is generated in a fluidized bed by partial oxidation of coal. After leaving the melter gasifier, the gas is mixed with cooling gas to obtain a temperature suitable for direct reduction, i.e. approximately 850–900°C. The fines captured in a hot cyclone are re-injected into the gasifier. Reducing gas is fed into the reduction furnace and ascends through the iron burden according to the counterflow principle. The hot DRI having a temperature of 800–900°C is continuously charged into the melter gasifier, where further reduction is effected and melting occurs. Hot metal and slag drop to the bottom of the melter-gasifier. Analogous to blast-furnace practice hot metal and slag are discharged by conventional tapping.