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.     

A Kinetic Study on Carbothermic Reduction of Hematite with Graphite Employing Thermogravimetry and Quadruple Mass Spectrometry

The reduction of hematite–graphite pellet was investigated from a kinetic viewpoint in the temperature range of 1173–1473 K (900–1200°C). The experimental procedure included thermogravimetric analysis (TGA) for measuring the weight change of the pellet and quadruple mass spectrometry (QMS) for monitoring the compositional change of the product gases. By applying a uniform internal reduction model to the current system, the activation energies for the reduction of Fe₃O₄ to wustite and for that of wustite to Fe were evaluated to be 91.0 and 25.9 kJ mol^?1, respectively. Due to the product gas analyses by QMS, it was observed that the reduction rate of hematite–graphite pellet was accelerated up to the equilibrium concentration of CO determined by the carbon gasification curve. Changeover of reduction mechanism with increasing reduction degree was explained in terms of a schematic diagram based on the results obtained in the current study.     

Reduction-Swelling Behaviour of Pellets Bearing Iron Ore and Charcoal

Abstract

Self-reducing pellets containing iron ore and charcoal were reduced under argon at 1123, 1273, 1323, 1373, and 1423 K. The pellets were cold bonded with Portland cement. Pellets measuring 9.1 and 15.3mm diameter were tested. Thermogravimetry was used to follow the reduction process. X-ray diffraction was used to identify the present phases at the different steps of the reduction reaction. The change in volume was related to the reaction fraction, reaction temperature and the size of the pellet. Scanning electron microscopy images of the reduced pellet have shown that swelling is due to the formation of whiskers of iron during the wiistite to iron step of reduction. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Résumé

Des pelotes auto-reductrices faites de minerais de fer et de charbon ont été réduites sous atmosphère d'argon à temperatures de 1123, 1273, 1323, 1373,et de 1423K. Les pelotes furent liées à froid avec un ciment Portland. Des pelotes mesurant de 9,I a 15,3 mm furent étudiées. Afin d'étudier l'évolution de la réduction, des analises thermogravimetriques furent performées. La technique de diffraction de rayons X fut utilisée pour identifier les différentes phases présentes au cours des differentes étapes de la réaction de réduction. Le changement de volume fut relié à l'avancement de la réaction, la temperature de la reaction et à la taille de la pelote. Des observations avec microscope a balayage des pelotes ont révelé que le changement de volume était du a la formation de fibres de fer au cours de la wiistite de la reduction du minerais de fer.     

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.     

Electric arc furnace dust-coal composite pellet: effects of pellet size,dust composition,and additives on swelling and zinc removal

Abstract

Electric arc furnace (EAF) dust-coal composite pellets were heated from room temperature to 1423 and 1468 K under flowing argon by means of two heating patterns (non-isothermal tests). Apparent volume variation, compressive strength after heating, and zinc removal efficiency were evaluated, the last as a function of the additives used in the present work. A decrease of pellet size (from 14 to 7 mm in diameter) as well as the presence of Portland cement contributed towards avoiding abnormal swelling caused by growth of iron whiskers, and, as a consequence, there was no significant decrease of compressive strength at ~1323 K. At 1397 K, highest zinc removal was obtained for pellets with 12 wt-%KCl, and reasons for this result are proposed.     

Enrichment of phosphorus in reduced iron during coal based reduction of high phosphorus-containing oolitic hematite ore

With the objective of phosphorus enrichment in the metallic iron during coal based reduction, high phosphorus oolitic hematite ore was reduced in the presence of coal with the coal/ore molar ratio (C/O, the molar ratio of fixed carbon in coal to oxygen in iron oxides of ore) varying from 1·0 to 2·5 at temperatures ranging from 1473 to 1548 K. The metallic iron was beneficiated from reduction products by magnetic separation. The results showed that the enrichment of phosphorus in the metallic iron improved with increasing temperature and C/O molar ratio. The phosphorus content and the phosphorus enrichment could reach 2·5 and 77·5%, respectively, with a C/O molar ratio of 2·5 at 1548 K and after 60 min reduction. The high phosphorus-containing metallic iron so obtained could then be converted to steel and high phosphorus steelmaking slag that can be used as a phosphate fertiliser. Kinetic analysis demonstrated that the process of phosphorus enrichment in the metallic iron could be divided into two stages, early and late, described by phase boundary controlled reaction and diffusion controlled, respectively. At the early stage, the apparent activation energy and pre-exponential factor of phosphorus enrichment decreased from 182·12 kJ mol^?1 and 9509·06 min^?1 to 132·60 kJ mol^?1 and 395·44 min^?1, respectively, when the C/O molar ratio was increased from 1·0 to 2·5. At the later stage, the apparent activation energy and pre-exponential factor were 245·87 kJ mol^?1 and 172?818·99 min^?1 at a C/O molar ratio of 1·0, respectively, whilst those were reduced to 210·73 kJ mol^?1 and 13?930·28 min^?1 at a C/O molar ratio of 2·5.     

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.     

Kinetics and mechanism of smelting reduction of fluxed chromite Part 1 Carbon–chromite–flux composite pellets in Fe–Cr–C–Si melts

Abstract

Experimental studies on the smelting reduction of fluxed carbon–chromite composite pellets in Fe–Cr–C–Si alloys were carried out at 1520–1600°C. The reduction reaction was found to be favoured by high temperatures, a high lime addition in the pellets, a long pellet dissolution time, and a moderate melt Cr content. For a given CaO addition, however, the reduction rate initially slowed before increasing with an increasing silica addition to the pellets. A three stage reduction mechanism is proposed. The first stage is very likely to be controlled by solid state and/or gas diffusion with an apparent activation energy of 472 kJ mol^-1 for pellets fluxed with 15%CaO and 25%SiO₂ . The third stage proceeds via smelting mechanisms, with mass transfer in the slag phase possibly rate controlling.     

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.     

Kinetics and mechanism of reactions between iron oxides and iron‐carbon melts

The reaction between iron oxides and iron‐carbon melts was studied in the temperature range 1523–1973 K. Pellets made from three different sources of iron oxides were added onto the melt surface, and the reaction time was measured through the constant volume pressure increase (CVPI) method. The effects of reaction temperature, oxide and melt surface areas, oxide type and weight on reaction time were evaluated. Analysis of partially reduced pellets through SEM was also performed. It has been determined that the reaction time increases as the pellet weight increases, and decreases as the temperature, the contact area between the oxide and the melt and the surface area of the melt increase. Examination of partially reduced pellets had shown that the reduction occurs topochemically. Based on the results, it is proposed that the overall reaction rate is determined by the consecutive reactions of dissociation of FeO and formation of CO.     

Kinetics of smelting reduction of fluxed composite iron ore pellets

Kinetic studies on smelting reduction of unreduced fluxed composite pellets (FCP) and fluxed composite pre-reduced iron ore pellets (FCRIP) have been carried out in an induction furnace. The pellets are charged into the slag layer floating on the carbon saturated molten iron bath in a graphite crucible. The slag basicity was however varied such that it has the same value as that of the pellets charged. The temperature of the slag is varied within the range of 1623K to 1823K for the pellets of basicity 2.0. Kinetic studies show a mixed kinetic model of both diffusion and chemical reaction controlled. While the smelting of FCRIP follows the model expressed as G(or) = 1-(2/3)(α) -(1-α)^2/3, the unreduced FCP pellets initially follow the diffusion controlled model of G(α) = α² followed by a chemical reaction controlled first order model of -ln(1-α) at the latter stages of smelting reduction, where α denotes the degree of reduction. The basicity dependence on the kinetics is not very significant. Comparison of the activation energy values explains that the smelting reduction with pre-reduced pellets seems to be a rather less energy intensive process.     

Direct reduced iron production using cold bonded carbon bearing pellets Part 1 – Laboratory metallisation

Abstract

A new cold bonding technology for producing coal bearing composite pellets was developed. Alumina cement was used as binder, which gave high mechanical strength to the pellet even at elevated temperatures. Laboratory test results showed that the metallisation rate of the pellets was high owing to the intimate contact of the particulates of coal and the iron ore in the pellet. The developed cold bonding method can also be used to recycle electric arc furnace (EAF) dust, from which valuable zinc and lead can also be recovered.     

Cupric chloride leaching of a complex copper/zinc/lead ore

A complex Cu/Zn/Pb ore from Cayeli, Turkey, was reacted with cupric chloride solutions under different conditions. Energies of activation were calculated for dissolution of copper (37 kJ mol^?1), iron (33 kJ mol^?1, zinc (26 kJ mol^?1) and lead (7.5 kJ mol^?1, values which indicate diffusion control of the reaction, probably through the sulphur layer formed round each particle. Particle size/leaching relationships corroborated microscopic assessments and indicated that chalcopyrite dissolved at a very low rate. Calculation of Fe:Cu ratios of metal leached showed considerable dissolution of pyrite from finely-ground ($\text{d}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 3?5 μ}\text{m}$) ore. Examination of residues using SEM X-ray fluorescence line scan techniques revealed little attack of large pyrite crystals, suggesting that fine pyrite particles in complex relationship with the sphalerite and chalcopyrite were dissolving.     

Reduction kinetics of carbon containing pellets made from metallurgical dust

Abstract

Reduction experiments of carbon containing pellets made from metallurgical dust were conducted under a weak oxidising atmosphere in the temperature range of 1348–1573 K. Analysis of kinetics and the reduction mechanism revealed that the rate determining step of the reduction of the pellets is the interfacial or local reaction with the activation energy 111·66 kJ mol^?1. The reduction rate can be expressed by the McKewan equation 1?(1?R)^1/3?=?kt. In addition, temperature is an important factor influencing the reaction rate as dezincification and metallisation increase with the increased temperature. The amount of dezincification and metallisation could be up to 97·8 and 79·9% respectively at 1573 K compared to a minimum of 75·3 and 60·2% at 1348 K.     

Reduction of Nickel Oxide Powder and Pellet by Hydrogen

Reduction of nickel oxide (NiO) powder and pellet by hydrogen was studied in a thermogravimetric apparatus. The variables studied were temperature (573, 673, 773, 873, 973?K) and hydrogen flow rate (100, 150, 200?cc?min^?1). With NiO powder 70?C80?% reduction and with NiO pellets about 95?% reduction were achieved. With both NiO powder and pellets, the rate of reduction increased with increasing temperature and hydrogen flow rate and the linear nature of the fractional reduction versus time (F vs. t) plot, for the most part of the reduction at all temperatures, supported a rate control by gas film diffusion. The activation energies for the reduction of NiO powder and NiO pellet were found to be 20.14 and 19.21?kJ?mol^?1, respectively. The SEM images showed that the grain size of nickel (Ni) produced was 1?C2???, and the XRD analysis established the presence of Ni (and the absence of NiO) in the reduced sample.     

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).     

Influence of CaO-SiO2-Al2O3 Ternary Oxide System on the Reduction Behavior of Carbon Composite Pellet: Part I. Reaction Kinetics

The reduction behavior of composite pellets comprising of hematite, synthetic graphite, and several oxide binder systems was investigated in a laboratory-scale horizontal tube furnace. Three oxide binder systems using silica-rich, alumina-rich, and conventional blast furnace slag compositions were selected to examine the effect of oxide chemistry on the reduction behavior of pellets. Compositional differences in the CaO-SiO₂-Al₂O₃ ternary system were confirmed to influence the reactions occurring in composite pellets during the reduction of iron oxide. An in situ visualization approach was used to observe the oxide/iron/carbon interactions at high temperatures from 1623 K to 1773 K (1350 °C to 1500 °C). The off-gas composition was measured by means of an infrared analyzer to determine the pellet reaction rates. Changes in physical appearance during the in situ reaction experiments demonstrated a strong correlation between the oxide composition and internal reactions. Moreover, the mechanical properties of pellets were investigated by measuring compressive strength to understand the relationship between physical properties of pellets and the associated oxide binder systems selected for this study.     

含碳球团低温熔分过程中硫的行为

 根据低配碳比含碳球团还原低温熔分制备粒铁的技术思想,对含碳球团还原熔分过程中硫的分配及硫的行为进行了试验研究。结果表明,影响产品铁粒中硫质量分数的主要因素为碳质垫料、球团原料中硫量以及熔分后铁粒在炉内的停留时间。为了降低铁粒中硫质量分数,应尽量限制煤粉、铁矿带入的硫量。要选择硫量较低的碳质垫料,尽量缩短熔分后铁粒在炉内停留时间。垫料中混和适量的固硫剂有利于降低铁粒中硫质量分数。球团中添加CaO未见有脱硫作用,铁粒中硫质量分数反而稍有增加。球团中配碳比的增加会导致含碳球团中煤粉带入的硫量的增加,因此配碳比的增加会稍微提高铁粒中硫质量分数。     

Cold bonded ore–coal composite pellets for sponge ironmaking Part 1 Laboratory scale development

Abstract

There is a need for the development of efficient industrial processes to use iron ore fines of high grade. Attention is particularly drawn to rotary kiln sponge ironmaking technology using lump iron ore, where productivity is low and energy consumption high compared with gas based processes. Fundamental studies carried out elsewhere indicate that the reduction of lump iron is accelerated if a limited amount of carbonaceous material is incorporated in the agglomerate of iron ore fines. Based on these considerations, cold bonded ore–coal composite pellets have been developed for sponge ironmaking in a rotary kiln. These composite pellets were tested in the laboratory and found to reduce very quickly, compared with lump iron ore. Composite pellets were also tested in an 8 t/day rotary kiln sponge iron plant giving enhanced productivity and lower coal consumption, and these results will be presented in Part 2 of this paper (next issue).     

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.