Iron Ore Pellet Dustiness Part II: Effects of Firing Route and Abrasion Resistance on Fines and Dust Generation

Iron ore pellets abrade during their production and handling, which lowers product quality and leads to dustiness issues. Pellets were collected from a variety of plants (operating either Straight-Grate (SG) or Grate-Kiln (GK) furnaces) to understand whether furnace type affects fines and dust formation. Results showed that pellets fired in SG furnaces were less abrasion-resistant (3.5 × lower) than pellets fired in GK furnaces. Concurrently, laboratory pellets were prepared using various ores, binders, and firing temperatures. These were tested to understand the relationship between abrasion index and dustiness. AI was observed to range from 1 to 14%. Dustiness, determined via AI and size distributions of abrasion progeny, ranged from 0.2 to 1.6%. For AI greater than 5%, AI can be used to indicate potentially high levels of dust. For AI less than 5%, there was a poor correlation between AI and dustiness. This was explained by the observation that as AI decreased, the abrasion product fineness increased. The results from parts I and II of this investigation suggest that material loss and levels of pellet dustiness may be significantly affected by pellet quality up to a certain point. Poorly fired pellets will be dusty during handling and transportation, while well-fired pellets will generate less – but finer – material as their quality improves. This could lead to little observed changes in dust generation over a wide range of pellet quality. Dust generation at each site would then depend on the quantity of material produced and their extent of handling.     

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.     

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.     

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.     

Abrasion Characteristics of Ores

Effects of time and size on t₁₀, the abrasion parameter (fineness indicator) using a calibration model for a given ore type is presented. A model for predicting t₁₀ from a small diameter core material for any ore type is also presented. The ores studied are: Ok Tedi 1, Ok Tedi 2, Red Dome, Alcoa, and Broken Hill. Shape factor of the particles that varies with both the particle size and the ore type affects the abrasion characteristics. Abrasion process in a tumbling mill is described by two mechanisms namely (1) chipping, and (2) abrasion, occurring at different rates. The chipping phenomenon depends on the ore and the shape factor. The abrasion phenomenon is independent of t₁₀° (t₁₀° is the ore-specific t₁₀ at 7 × 10^?4 kWh/t tumbling test energy) and the shape factor. Abrasion process occurs at a much slower rate compared to the chipping process. Transition between chipping and abrasion occurs approximately after 2 min of tumbling. The long-term rate is controlled by the abrasion or attrition rather than by the chipping mechanism.     

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.     

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%.     

Effects of Binder on the Properties of Iron Ore-Coal Composite Pellets

Large amounts of fines and superfines are generated in Indian iron ore and coal mines due to mechanized mining and mineral dressing operations. Utilization of these fines for extracting metal is of vital concern for resource utilization and pollution control. For agglomeration of these fines, a suitable binder is required. Iron ore-coal composite pellets were prepared by cold bonding. Various binders such as lime, Ca(OH)₂, slaked lime, dextrose, molasses, and sodium polyacrylate (SPA), alone or in combination, were employed for making composite briquettes. The slaked lime–dextrose combination produced the highest strength among the various binders employed for producing composite briquettes and was therefore selected for producing composite pellets for the smelting reduction. In cold bonding, the composite pellets attain the requisite properties due to physico-chemical changes of the binder in ambient conditions. It was possible to obtain a dry strength of more than 300 N per pellet in some cases and more than 200 N per pellet in many trials. Drop strength and shatter index values of composite pellets were also measured. In the present paper an attempt has been made to evaluate the mechanical properties of cold-bonded composite pellets so as to throw some light on the capacity of these pellets to withstand stresses during handling and transportation.     

Strength and high temperature behaviour of carbon composite pellets containing BOF fine dust

Abstract

Steel plants produce significant amounts of dust and sludge during iron and steel production. These wastes contain valuable elements, such as Fe, Cr, Ni, C, K and Na and should be handled properly to prevent them from polluting the environment. In order to utilise the BOF fine dust, the effects of the dust on cold bonded pelletising, solid state reduction and reduction melting behaviours of composite pellets made from iron ore and anthracite with added BOF fine dust were investigated at laboratory scale. The BOF dust was found to improve the cold compressive strength of the wet green carbon composite pellet, and increased with increasing dust content. Almost four times the amount of dust was needed to get the same effect on the strength of the pellet when it was used to replace bentonite. The carbothermic reduction of the composite pellet proceeded effectively at temperatures above 1200°C. The BOF dust had a positive effect on the reduction rate of the pellet, and the rate increased with increased dust content. The reduction of iron oxide was topochemical and conformed to a shrinking core kinetic model. The dust was found to improve the iron and slag melting separation rate of reduced pellets at 1400°C when its content was less than 23·11 wt-%. The liquidus temperature of the slag would decrease with the content of BOF dust increasing from zero to ～30 wt-% and then increase if the content continued to become more in the experiment. Utilising the BOF dust as the binder and flux to adjust the composition of the slag system can potentially reduce the slag ratio and production cost compared with using bentonite and limestone. This work can help to find a new process for the effective utilisation of BOF dust in a more appropriate and environmentally friendly way.     

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.     

Design and Fabrication of Pellets for Magnesium Production by Carbothermal Reduction

For carbothermal reduction (CTR) to be an economic and clean process for magnesium metal production, operational challenges must be overcome. Strong and reactive precursor pellets are necessary to effectively and selectively produce Mg_(g) from any feedstock. In this study, the effects of ore (magnesia and dolime), carbon (petroleum coke, charcoal, algal char, and carbon black), and binder (organic and inorganic) on pellet strength and reactivity, product yield and purity, and reduction selectivity were analyzed. Theoretically and experimentally, the CTR of dolime (MgO·CaO) favored MgO reduction over CaO reduction; however, with enough carbon and heat, both oxides could be reduced. CaO carbothermal reduction produced CaC₂ and Ca_(g). The selectivity to CaC₂ remained constant (7 ± 4 pct) for all C/MgO·CaO ratios analyzed, while the selectivity to Ca_(g) increased (5 pct → 40 pct) when C/MgO·CaO was increased from 0.5 to 2.0. As the overall metal yield decreased (77.6 pct → 59.7 pct) with increasing CaO reduction (38.2 pct → 78.1 pct), Ca_(g) reverted faster than Mg_(g). Heavy metal impurities primarily remained in the residue (< 30 pct volatilized) and, when volatilized, condensed at high temperatures (700 °C to 1450 °C), relative to light metal impurities (350 °C to 1000 °C, > 78 pct volatilized). Organic binders added reducing power to the pellets but produced frail pellets (radial crush strength = 9.1 ± 0.7 N) after pyrolysis, relative to pellets with inorganic binders (15.1 ± 3.2 N). Kinetic parameters were determined for extruded pellets to predict the reaction rate as a continuous function of pressure and temperature.     

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

Abstract

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

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.     

Influence of CaO-SiO2-Al2O3 Ternary Oxide System on the Reduction Behavior of Carbon Composite Pellet: Part II. Morphological Properties

Carbon composite pellets composed of hematite, synthetic graphite, and oxide system were reduced at 1773 K (1500 °C) in a laboratory-scale horizontal tube furnace. The morphological properties of produced pellets after reduction were examined using light optical microscopy and a scanning electron microscope. Microscopic observation confirmed the correlation between morphological change of pellets and compositional difference of CaO-SiO₂-Al₂O₃ ternary oxide systems. Samples were also analyzed by X-ray diffraction to investigate phase change during the reduction process. The impact of oxide chemistry was established as each pellet illustrated different states of iron oxide as a function of time. In addition, the surface area was measured by the BET-N₂ absorption method to clarify the reaction mechanism which is significantly affected by physical contact.     

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.     

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.     

Column Bioleaching of a Low-Grade Silicate Ore of Uranium

The bioleaching of a low-grade Indian uraninite ore (triuranium octoxide, U₃O₈: 0.024%), containing ferro-silicate and magnetite as the major phases, and hematite and pyrite in minor amounts, has been reported. Experiments were carried out in laboratory scale column reactors inoculated with enriched culture of Acidithiobacillus ferrooxidans isolated from the source mine water. The pH effect on uranium recovery was examined with the same amounts of ores in different columns. With the presence of 10.64% Fe in the ore as ferro-silicate, the higher uranium biorecovery of 58.9% was observed with increase in cell count from 6.4 × 10⁷ to 9.7 × 10⁸ cells/mL at pH 1.7 in 40 days as compared to the uranium recovery of 56.8% at pH 1.9 with a corresponding value of 9.4 × 10⁸ cells/mL for 2.5-kg ore in the column. The dissolution of uranium under chemical leaching conditions, however, recorded a lower value of 47.9% in 40 days at room temperature. Recoveries were similar with 6-kg ore when column leaching was carried out at pH 1.7. The bioleaching of uranium from the low-grade ore of Turamdih may be correlated with the iron(II) and iron(III) concentrations, and redox potential values.     

Reduction of Sn-Bearing Iron Concentrate with Mixed H<Subscript>2</Subscript>/CO Gas for Preparation of Sn-Enriched Direct Reduced Iron

The development of manufacturing technology of Sn-bearing stainless steel inspires a novel concept for using Sn-bearing complex iron ore via reduction with mixed H₂/CO gas to prepare Sn-enriched direct reduced iron (DRI). The thermodynamic analysis of the reduction process confirms the easy reduction of stannic oxide to metallic tin and the rigorous conditions for volatilizing SnO. Although the removal of tin is feasible by reduction of the pellet at 1223 K (950 °C) with mixed gas of 5 vol pct H₂, 28.5 vol pct CO, and 66.5 vol pct CO₂ (CO/(CO + CO₂) = 30 pct), it is necessary that the pellet be further reduced for preparing DRI. In contrast, maintaining Sn in the metallic pellet is demonstrated to be a promising way to effectively use the ore. It is indicated that only 5.5 pct of Sn is volatilized when the pellet is reduced at 1223 K (950 °C) for 30 minutes with the mixed gas of 50 vol pct H₂, 50 vol pct CO (CO/(CO + CO₂) = 100 pct). A metallic pellet (Sn-bearing DRI) with Sn content of 0.293 pct, Fe metallization of 93.5 pct, and total iron content of 88.2 pct is prepared as a raw material for producing Sn-bearing stainless steel. The reduced tin in the Sn-bearing DRI either combines with metallic iron to form Sn-Fe alloy or it remains intact.     

Recycling of steel plant mill scale via iron ore pelletisation process

Abstract

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

A case study on large-scale production for iron oxide pellets: Ezhou pelletization plant of the BAOWU

The Ezhou pelletization plant of the BAOWU with production capacity of 5,000,000 ton oxide pellets per year has the biggest single production line of Grate–Kiln–Ring cooler, which was provided by Metso Company. The iron concentrate was mainly imported from Brazil and self-provided by Jinshandian. The iron concentrates were dried and ground by the high pressure roller. The ground iron concentrates were pelleted by drum and disc pelletizers. The qualified green pellets were subjected to grate, rotary kiln, and ring cooler successively. The innovation and creation of process and equipment of Ezhou pelletization plant of BAOWU plays an exemplary role for the application of large-scale grate-kiln-cooler in iron and steel industrial. The use of the high pressure roller grinding significantly improved the pelletization ability of the iron concentrate and the quality of the finished pellets. Compared with other same scale grinding equipment, it is more energy-saving. The combined technique of high pressure roller with wet ball mill grinding was used to treat some hard specularite with large particle size. The first successful application and design of electrostatic precipitator on the large-scale grate-kiln in China provides a good foundation for other large-scale pellets plants.