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.