Iron ore pellet properties under simulated blast furnace conditions:investigation on reducibility, swelling and softening

Abstract A blast furnace is the dominant process for making iron in the world. Iron ore pellets are commonly used as iron burden materials in a blast furnace, in which iron oxides are reduced to metallic molten iron. While descending, the charge faces various stresses, which affect the gas flows in the shaft and the energy efficiency of the process. Charge material testing on a laboratory scale is of crucial importance in regard to the development of material quality. This doctoral thesis presents a couple of advanced novel experimental methods: a novel camera imaging method to determine the amount of swelling during reduction; a novel reducibility test to determine the reducibility in a solid state under simulated blast furnace conditions; and a novel experimental program for the ARUL reduction-softening test to more accurately simulate blast furnace conditions. Swelling tests under conditions of fixed temperature and gas composition showed that isothermal tests do not give a realistic insight into the material behaviour in a blast furnace. As a result, it is suggested that dynamic gas composition – temperature programmes simulating actual process conditions should be used. Additionally, the test results showed that circulating elements (sulphur and potassium) also affect the pellet volume change during reduction, however no abnormal swelling was observed in any of the swelling experiments. The factors affecting the high-temperature properties of iron burden materials for blast furnace use were evaluated by both the experimental methods and computational thermodynamics. It was shown that none of the studied pellet grades has as good reduction-softening properties as the fluxed sinter because of the differences in the chemistry and macro-porosity. FeO-SiO2-CaO-MgO-Al2O3 system examinations with FactSage was found to be a useful tool for predicting the softening of an iron burden material using the original chemical composition. FactSage computations suggest that the softening properties of an iron burden material can be improved either by decreasing the proportion of SiO2, increasing the proportion of MgO or introducing an appropriate amount of CaO in relation to the proportion of SiO2.Tiivistelmä Masuuni on merkittävin raakaraudan valmistusprosessi maailmassa. Masuunissa käytetään yleisesti rautamalmipellettejä rautapanosmateriaalina. Masuunissa raudanoksidit pelkistetään metalliseksi rautasulaksi. Vajotessaan panos kohtaa monenlaisia rasitteita, joilla on vaikutusta kuilun kaasuvirtauksiin ja masuuniprosessin energiatehokkuuteen. Panosmateriaalien testaus laboratoriomittakaavassa on merkittävässä roolissa, kun niiden laatua kehitetään. Väitöskirjassa esitetään useita kehittyneitä koemenetelmiä: uusi kamerakuvausmenetelmä, jolla voidaan määrittää turpoaminen pelkistyksen edetessä; uusi pelkistyvyystesti, jolla voidaan määrittää rautapanosmateriaalin pelkistyminen kiinteässä tilassa masuunia jäljittelevissä olosuhteissa; ja uusi koeohjelma, jolla voidaan jäljitellä aiempaa tarkemmin masuuniolosuhteita sulamis-pehmenemiskokeessa. Turpoamistestit vakioiduissa olosuhteissa osoittivat, että isotermiset testit eivät anna realistista kuvaa materiaalin käyttäytymisestä masuunissa. Tämän vuoksi dynaamisia kaasukoostumus–lämpötila-ohjelmia tulisi suosia. Lisäksi tutkimustulokset osoittavat, että myös masuunissa kiertävillä komponenteilla (rikillä ja kaliumilla) on vaikutusta pelletin tilavuuden muutokseen pelkistyksessä. Yhdessäkään turpoamiskokeessa ei kuitenkaan havaittu katastrofaalista turpoamista. Masuunin rautapanosmateriaalien korkealämpötilaominaisuuksiin vaikuttavia tekijöitä arvioitiin sekä kokeellisin menetelmin että termodynaamisin laskelmin. Yhdelläkään tutkitulla pellettilaadulla ei havaittu sintterin veroisia korkealämpötilaominaisuuksia, mikä johtuu eroista kemiallisessa koostumuksessa ja makrohuokoisuudessa. FeO-SiO2-CaO-MgO-Al2O3 systeemitarkastelut rautapanosmateriaalin lähtökoostumuksella todettiin toimivaksi menetelmäksi arvioida panosmateriaalin pehmenemiskäyttäytymistä. FactSage-laskennat antavat ymmärtää, että rautapanosmateriaalin pehmenemisominaisuuksia voidaan parantaa joko vähentämällä SiO2:n osuutta, lisäämällä MgO:n osuutta tai lisäämällä CaO:ta sopiva määrä SiO2:n osuuteen nähden

A review on the kinetics of iron ore reduction by hydrogen

Abstract A clean energy revolution is occurring across the world. As iron and steelmaking have a tremendous impact on the amount of CO₂ emissions, there is an increasing attraction towards improving the green footprint of iron and steel production. Among reducing agents, hydrogen has shown a great potential to be replaced with fossil fuels and to decarbonize the steelmaking processes. Although hydrogen is in great supply on earth, extracting pure H₂ from its compound is costly. Therefore, it is crucial to calculate the partial pressure of H₂ with the aid of reduction reaction kinetics to limit the costs. This review summarizes the studies of critical parameters to determine the kinetics of reduction. The variables considered were temperature, iron ore type (magnetite, hematite, goethite), H₂/CO ratio, porosity, flow rate, the concentration of diluent (He, Ar, N₂), gas utility, annealing before reduction, and pressure. In fact, increasing temperature, H₂/CO ratio, hydrogen flow rate and hematite percentage in feed leads to a higher reduction rate. In addition, the controlling kinetics models and the impact of the mentioned parameters on them investigated and compared, concluding chemical reaction at the interfaces and diffusion of hydrogen through the iron oxide particle are the most common kinetics controlling models

Review on the phase equilibria in iron ore sinters

Abstract Sintering process is a commonly used pre-treatment process for iron containing burden materials with an aim to produce porous, agglomerated sinter material with suitable properties to be charged into the blast furnace. During the sintering process the material undergoes a series of reactions, during which the conditions vary considerably. These changes in temperature and state of oxidation cause changes in the mineralogical composition of the material and although the sintering process does not completely reach the chemical equilibrium, it is important to understand the phase equilibria of the sinter system in order to analyse and control the effect of various factors on the sintering process. The purpose of this paper is to give a review on the research related to phase equilibria in iron ore sinters. The main components of the sinter are FeO, Fe₂O₃, SiO₂, CaO, Al₂O₃ and MgO and by studying the phase equilibria of this system, the behaviour of sinters can be evaluated. Based on the experimental data, oxide databases have been created to provide thermochemical data of all the necessary compounds within this system. Concerning the solutions, more research is required related to SFCA phases. These databases are commercially available with thermochemistry software and can be used to compute phase diagrams illustrating the effect of different factors on the phase equilibria within the FeO–Fe₂O₃–SiO₂–CaO–Al₂O₃–MgO system. Phase diagrams provide a useful tool to study the behaviour of the material in both sintering process itself as well as in the following reduction processes such as the blast furnace

Reduction of iron ore pellets, sinter, and lump ore under simulated blast furnace conditions

Abstract A blast furnace (BF) is the dominant process for making iron in the world. The BF is charged with metallurgical coke and iron burden materials including iron ore pellets, sinter, and lump ore. While descending in the BF the charge materials reduce. The iron‐bearing materials should reduce fast and remain in the solid form until as high a temperature as possible to ensure reaction contact with reducing gas and iron oxides. Herein, the reducibility of the iron ore pellet, sinter, and lump ore in the BF shaft are focused on. The experiments are conducted isothermally with a blast furnace simulator (BFS) high‐temperature furnace at four different temperatures (700, 800, 900, and 1000 °C) for 300 min. The experimental atmosphere consists of CO, CO₂, H₂, H₂O, and N₂ simulating the conditions in the BF shaft. It is found that lump ore has lowest reduction rate in all test conditions, and at lower temperatures iron ore pellets reduce faster than sinter, and this is reversed at higher temperatures. Furthermore, the reduction rate of sinter and iron ore pellets begins to resemble each other at higher temperatures

Estimation of iron ore pellet softening in a blast furnace with computational thermodynamics

Abstract In blast furnaces it is desirable for the burden to hold a lumpy packed structure at as high a temperature as possible. The computational thermodynamic software FactSage (version 7.2, Thermfact/CRCT, Montreal, Canada and GTT-Technologies, Aachen, Germany) was used here to study the softening behavior of blast furnace pellets. The effects of the main slag-forming components (SiO₂, MgO, CaO and Al₂O₃) on liquid formation were estimated by altering the chemical composition of a commercial acid pellet. The phase equilibria for five-component FeO-SiO₂-CaO-MgO-Al₂O₃ systems with constant contents for three slag-forming components were computed case by case and the results were used to estimate the formation of liquid phases. The main findings of this work suggested several practical means for the postponement of liquid formation at higher temperatures: (1) reducing the SiO₂ content; (2) increasing the MgO content; (3) reducing the Al₂O₃ content; and (4) choosing suitable CaO contents for the pellets. Additionally, the olivine phase (mainly the fayalitic type) and its dissolution into the slag determined the amount of the first-formed slag, which formed quickly after the onset of softening. This had an important effect on the acid pellets, in which the amount of the first-formed slag varied between 10 and 40 wt.%, depending on the pellets’ SiO₂ content

Reduction of acid iron ore pellets under simulated wall and center conditions in a blast furnace shaft

Abstract The operational conditions, including temperature and gas composition, vary along the radial position in a blast furnace. Nevertheless, very few studies can be found in the literature that discuss how the reduction behavior of the ferrous burden varies along the radial position. In this study, the effect of the radial charging position on the reducibility of acid iron ore pellets was investigated using a laboratory-scale, high-temperature furnace in CO-CO₂-N₂ and CO-CO₂-H₂-H₂O-N₂ atmospheres up to 1100 °C. The experimental conditions were accumulated based on earlier measurements from a multi-point vertical probing campaign that was performed for a center-working European blast furnace. The main finding of this study is that the pellet reduction proceeded faster under simulated blast furnace conditions resembling those in the center area, compared to the wall area, because of a higher share of CO and H₂ in the gas. Therefore, the pellet charging position affects its reduction path in a blast furnace. Additionally, it was shown that the presence of H₂ and H₂O in the reducing gas enhanced the progress of reduction reactions significantly and enhanced the formation of cracks slightly, both of which are desirable in blast furnace operation. The reducibility data attained in this study are important in understanding how temperature and gas composition is connected to the reduction degree under realistic process conditions

Compression strength of coke after gasification

Summary In this work gasification of different types of coke (higher quality and lower quality) were subjected with two different gas-atmosphere profiles (H₂-H₂O-CO-CO₂-N₂ and CO-CO2-N2) simulating Blast Furnace circumstances. It was found out that coke was more reactive towards H₂ and H₂O containing gas atmosphere. After gasification, coke samples were subjected to compression strength tests. It was found out that although higher quality coke reacted more intensively in H₂ and H₂O containing atmosphere, the compression strength properties were not harmed by the reaction of a higher extent. The compression strength properties were improved after gasification in H₂ and H₂O containing atmosphere for the higher quality coke. This could be due to increase in small-scale pores which make the porous structure more uniform and therefore decrease the heterogeneity of elasticity throughout the coke structure. Image analysis showed a linear correlation between strength, strain and area percentage of large pores in the coke structure

Effect of iron ore pellet size on metallurgical properties

Abstract Iron ore pellets are small and hard spherical particles agglomerated from a fine iron ore concentrate. They are used in the blast furnace process to produce hot metal. The diameter of blast furnace pellets is usually between 8 and 16 mm. In this study, a batch of magnesia iron ore pellets was first sieved into particle sizes of 8–10 mm, 10–12.7 mm, 12.7–16 mm and 16–20 mm, and the four different size fractions were used to study the effect of pellet size on metallurgical properties. The metallurgical experiments showed a decrease both in reducibility under unconstrained conditions and in low-temperature reduction-disintegration but showed an increase in cold crushing strength as the pellet size increased. In the reduction-softening test, pellets sized 10–12.7 mm reached the highest final temperature and the highest reduction degree among the pellet samples of different sizes. Based on the implications drawn from this study, the amount of 10–12.7 mm pellets should be maximized in a blast furnace operation

Gas composition change in a single sinter, pellet and coke layer in simulated blast furnace conditions

Abstract In spite of the vast amount of research regarding the operation of blast furnace, the gas composition change in a single charge material layer in a blast furnace is not an extensively studied research area. Iron-bearing material and coke are charged in turn as layers into the blast furnace as raw material. With no percolation of layers taken into account, the composition of gas varies in turn in the blast furnace shaft, losing its reducing potential in an iron-bearing material layer and being reformed in a coke layer. In this paper, the effect of reactions in sinter, pellet and coke layers on the gas composition between the blast furnace top and the cohesive zone has been discussed. The gas was analysed on-line on multiple heights of a tube furnace loaded with a material bed of 1.0 m in height and uniformly heated at a rate of 2°C/min or 3°C/min up to 1100°C or 1200°C. As a result, the H₂–H₂O gas composition change occurred in a higher temperature than the CO–CO₂ change in sinter, pellet and coke beds. This led to a conclusion that the reduction of iron oxides by hydrogen and gasification of coke by water vapour started in somewhat higher temperatures than the reactions with carbonaceous gas components. Additionally, olivine pellets were more reducible in moderate temperatures compared with sinter and the utilisation rates of CO and H₂ gases rose higher in a pellet bed than in a sinter bed, mainly due to the higher hematite percentage in pellets

Influence of H₂–H₂O content on the reduction of acid iron ore pellets in a CO–CO₂–N₂ reducing atmosphere

Abstract Using hydrogen as a reducing agent for iron production has been the focus of several studies due to its environmental potential. The aim of this work is to study the influence of H₂–H₂O content in the gas phase on the reduction of acid iron ore pellets under simulated blast furnace conditions. Temperature and gas compositions for the experiments were determined with multi-point vertical probes in an industrial blast furnace. The results of the reduction tests show that higher temperatures and H₂ content increase the rate and extent of reduction. For all the gas and temperature combinations, morphological, mineralogical, and microstructure changes were observed using different characterization techniques. Microscopy images reveal that H₂–H₂O, in the gas phase, has a positive influence on reduction, with metallic iron forming at the pellet’s periphery and core at lower temperatures compared to CO–CO₂–N₂ reducing gas. Porosity and surface area changes were determined using a gas pycnometer and the BET method. The results indicate that increasing the reduction temperatures and H₂ content results in greater porosity and a larger surface area. Moreover, carbon deposition did not take place, even at lower temperatures. A rate minimum was detected for pellets reduced at 800°C, probably due to metallic iron formation, hindering the diffusion of reducing gases through the product iron layer