Effect of blast furnace sludge (BFS) characteristics on suitable recycling process determining

Abstract The present study aims to give a detailed characterization of blast furnace sludge (BFS) by using different techniques, in order to determine the most effective recycling method to recover valuable metals from this waste. BFS is composed mainly of hematite, as its iron-bearing phase, and carbon, in addition to fractions of silicate and carbonate materials. The studied BFS shows relatively high contents of iron (Fe) (390 g.kg-1), and carbon (C) (290 g.kg-1), due to abundance of hematite and coke, while the concentration of zinc (Zn) (2.5 g.kg-1) is low. The XRD analyses indicated that, hematite is more concentrated in the fine fraction (<20 μm), while the coarser fraction (90 - 250 μm) is dominated by calcite, quartz and X-ray amorphous coke. SEM-EDX analyses confirmed that particles rich in iron and zinc were detected in the fine fraction (<20 μm) of the sludge. Due to high Fe and C content in BFS, it can be utilized as self-reducing material and briquetting represent a potential method for recycling of blast furnace sludge

Identification of rate, extent, and mechanisms of hot metal resulfurization with CaO-SiO₂-Na₂O slag systems

Abstract The resulfurization of hot metal has not been comprehensively studied in literature. This study presents an experimental and mathematical modeling study of resulfurization in thermodynamic and kinetic point of view. The rate, extent, and mechanisms of resulfurization were evaluated by analyzing concurrently the physical properties and sulfur-extracting ability of the slag. Experiments were conducted in a chamber furnace in an argon atmosphere, and the hot metal was sampled with pre-defined basis. The experiments were continued until the metal–slag system reached an apparent thermodynamic equilibrium. To obtain a quantitative measure on the effect of system properties on the rate and extent of resulfurization, the results of this study were combined with previous studies handling the sulfide capacities of Na₂O-SiO₂ and CaO-SiO₂-Na₂O slag systems. The sulfide capacities of the slag and corresponding metal–slag sulfur partition ratios were mathematically modeled with data-driven techniques such as multiple linear and non-linear regression and artificial neural networks. Finally, with the help of these, to study the kinetics of resulfurization, a simple mechanistic reaction model was derived. The results suggest that resulfurization of hot metal follows 1st-order kinetics and that the rate and extent can be regulated through the control of the associated thermodynamic driving force and by modifying the physical properties of the slag. The rate-limiting factor was found to be determined by the morphology of the slag phase

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

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

Effect of circulating elements on the dynamic reduction swelling behaviour of olivine and acid Iron ore pellets under simulated blast furnace shaft conditions

Abstract Sulphur and alkalis in the blast furnace gas have been associated affecting the reduction swelling behaviour of iron ore pellets. A tube furnace was used in this study to examine the dynamic reduction swelling behaviour of olivine and acid pellets in CO–CO₂–N₂ atmosphere with sulphur and potassium in gaseous phases up to 1100°C simulating the conditions in the blast furnace shaft. No abnormal swelling was detected in sulphur or potassium containing CO–CO₂–N₂ atmospheres during dynamic reduction. Instead, sulphur in the reducing atmosphere was associated with pellet contraction and FeO–FeS melt formation which became more dominant with increasing sulphur partial pressures. In the extreme case, having a maximum of 1.0 vol-% S₂ gas in the reducing atmosphere, the reduction reaction of wüstite to metallic iron was hindered. The formation of FeO–FeS liquid phase extends the cohesive zone towards the blast furnace top and lower temperatures and decreases the gas permeability. Furthermore, large amounts of potassium in the reducing atmosphere (max. 0.03 vol-%) led to swelling and cracking in the olivine pellets still remaining in the range of normal swelling

Water-gas shift reaction in an olivine pellet layer in the upper part of blast furnace shaft

Abstract In order to reduce CO₂ emissions in the iron and steel industry, the utilization of H₂ gas as a reducing agent is a feasible option. The use of hydrogen bearing injectants in the lower blast furnace (BF) area increases H₂O concentration in the upper part of the BF shaft and the charging of moist burden has a similar effect as well. For efficient BF operation, it is important to investigate the effect of high H2 and therefore high H₂O concentrations in the reducing gas. This study focuses on the upper BF shaft area where hematite to magnetite reduction takes place and temperature is in the range of the forward water-gas shift reaction (WGSR). The effect of the WGSR on the composition of the reducing gas was estimated by experimental methods. A layer furnace (LF) was used to determine the temperature for the occurrence of the WGSR under simulated BF shaft conditions. The feed gas conversion was investigated in an olivine pellet layer. The WGSR was observed in an empty LF with CO–H₂O–N₂ gas at 500°C. With CO–CO₂–H₂O–N₂ gas the WGSR was observed in an olivine pellet layer at 400–450°C and in a pre-reduced magnetite pellet layer at 300–400°C indicating the catalyzing effect of magnetite on the WGSR. The results offer additional information about the effect of high H₂O concentration on the composition of the reducing gas through the WGSR. The occurrence of the WGSR in the actual BF and its effects were discussed

Reduction behavior of cold-bonded briquettes under simulated blast furnace conditions

Abstract Recycling of fine sized iron-rich by-products back to blast furnace (BF) process in the form of cement-bonded briquettes has become a common procedure in steel plants. Replacing part of the cement by Ground Granulated Blast Furnace Slag (GGBFS) is also a common method to reduce cement consumption. When the briquettes are subjected to high temperature and reducing atmosphere in the BF, the cement phases decompose and the iron oxides undergo a series of phase transformations. To avoid early disintegration and to improve the performance of the briquettes, it is necessary to study these reactions during the reduction. In the present study the reduction behavior of the BF briquette samples was studied by experimental methods in a laboratory scale furnace, which simulates the conditions of the BF shaft in a CO–CO₂–N₂ atmosphere. With interrupted experiments the composition of the briquette was studied in different reduction stages of the BF shaft. The effect of GGBFS as a binder material on the reduction was studied with GGBFS containing briquette samples. The reduction of briquettes was compared to an olivine pellet which was used as a reference sample. Considerably higher reduction rate was detected with the briquettes compared to the pellet at 1100°C when reduced to metallic iron. 25–50 vol-% swelling in the briquette samples was detected during the wüstite-iron reduction step at 900–1000°C. X-ray diffraction (XRD) was used to observe the phase transformations in the Fe–Fe₂O₃–CaO system of the briquette and the results are in agreement with the theory

Evaluating the reduction-softening behaviour of blast furnace burden with an advanced test

Abstract A ferrous burden loses its permeability in the cohesive zone of a Blast Furnace (BF), where the iron burden materials soften and melt. A tailor-made, high-temperature furnace named ARUL (Advanced Reduction under Load) was used here to study the reduction-softening behaviour of acid and olivine pellets and basic sinter under simulated BF gas, temperature and pressure conditions. The ARUL test showed the best reduction-softening properties for the basic sinter. The sinter sample resisted up to 1329°C and achieved a reduction degree of 90.2% until a gas-impermeable structure was formed in a packed bed, whereas the acid pellet lost its permeability at 1160°C and only reduced to a reduction degree of 48.7%. The olivine pellet had intermediate reduction-softening properties with a final temperature of 1252°C and a final reduction degree of 68.7%. The differences between the test materials were assessed as being caused mainly by different chemistry, but it was also revealed that the sinter sample remained its macro-porosity markedly better in relation to the pellets, providing routes for reducing gases. The experimental results were compared to the phase diagrams calculated with the computational thermodynamic software FactSage. Phase diagrams for the 5-component FeO–SiO₂–CaO–MgO–Al2O₃ systems with constant CaO, MgO and Al₂O₃ contents were used to estimate the formation of liquid phases in the test materials. The computed phase diagrams gave an estimate of the liquid formation; however, some limitations were also found in the utilization of the computations because of the need to define the system in certain simplicity

Dynamic and isothermal reduction swelling behaviour of olivine and acid Iron ore pellets under simulated blast furnace shaft conditions

Abstract Pellet swelling has been widely studied, being simultaneous with reduction reactions and common in the operation of blast furnaces. A tube furnace equipped with a camera recording system was used here to study the dynamic and isothermal reduction swelling behaviour of olivine and acid pellets under simulated BF shaft conditions. The olivine pellets were magnetically separated into three fractions, containing low, medium and high amounts of magnetite in the core. The divalent iron (FeO) content of these fractions was 0.1 wt-%, 0.2 wt-% and 2.9 wt-%, respectively. Pellets with a large magnetite nucleus were observed to encompass numerous cracks, which was reflected in a poor LTD test value, while SiO₂-rich reference pellets with a different slag chemistry had more restrained swelling and cracking behaviour in dynamic reduction. Swelling in the olivine pellets was associated with cracking at the boundary between the original magnetite nucleus and the hematite shell. The dynamic reduction swelling test results showed lower reduction swelling indices (max 17% in volume) than under isothermal conditions (max 51% in volume), in which case the pellets were suddenly exposed to a strongly reducing atmosphere. It is thus suggested that the reduction swelling behaviour of iron ore pellets should preferably be studied dynamically under simulated blast furnace conditions in order to achieve a realistic understanding of their swelling behaviour in a blast furnace

Effect of adding limestone on the metallurgical properties of iron ore pellets

Abstract In order to produce high-quality pellets with good reducibility and superior softening and melting properties, certain additives are important. One of the most common fluxing materials for iron ore pellet production is limestone, which is mainly calcium oxide (CaO). In this study, the effect of adding limestone on the metallurgical properties (reducibility, swelling, cracking, softening temperature, Low-Temperature Disintegration, Cold Crushing Strength) of acid iron ore pellets was investigated using a comprehensive set of metallurgical laboratory tests. The dynamic reducibility test under unconstrained conditions showed a higher final degree of reduction in limestone-fluxed pellets compared to non-fluxed ones. Also in the reduction–softening test under load, the fluxed pellets reduced to a higher final degree of reduction, although they started to soften at a somewhat lower temperature. Swelling and cracking of the pellets during dynamic reduction were slightly increased by the addition of limestone, but not remarkably. Adding limestone slightly decreased the Cold Crushing Strength and increased the formation of fines in the hematite to magnetite reduction stage in the LTD test. However, all four parameters (CCS, LTD, swelling, and cracking) are within the acceptable range for blast furnace use