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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 1 329°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 1 160°C and only reduced to a reduction degree of 48.7%. The olivine pellet had intermediate reduction-softening properties with a final temperature of 1 252°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-SiO 2 -CaO-MgO-Al 2 O 3 systems with constant CaO, MgO and Al 2 O 3 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.

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.

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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.

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Large amounts of injectants are used in the blast furnace (BF) process to reduce coke consumption, but this changes the gas composition in the BF shaft where iron ore reduction takes place. H2 and H2O gases change markedly in the gas atmosphere at high injection levels, which makes it important to investigate their effects on the reduction of iron oxides in a CO-CO2 atmosphere. The gaseous H2 and H2O content in the BF shaft atmosphere is approximately 8%. In the present work olivine pellets were reduced in H2-H2O-CO-CO2 and CO-CO2 atmospheres with equal reducing potentials of H2 and CO by fixing the H2/H2O and CO/CO2 ratios. No significant differences in the reduction rates of iron oxides were found between the H2-H2O-CO-CO2 and CO-CO2 atmospheres at high temperatures but at lower temperatures H2-H2O-CO-CO2 had higher reduction rate. Activation energies determined for hematite to magnetite reduction for both gas mixtures indicated better initial stage and later stage reduction in the H2-H2O-CO-CO2 atmosphere. Field Emission Scanning Electron Microscope (FESEM) analysis was carried out on samples, and wüstite relics were found in the inner parts of pellets reduced to iron in the CO-CO2 gas but not in the samples reduced in the H2-H2O-CO-CO2 gas.