8th International Congress on Science and Technology of Ironmaking - ICSTI 2018

In the blast furnace, the onset of softening and melting of ferrous burdens give origin to the region known as the cohesive zone, which is critical to process stability and performance. The softening and meltdown temperatures of ferrous burdens are usually accessed by high temperature softening melting (HTSM) apparatus, which can, to some extent, simulate the blast furnace reduction, temperature and mechanical conditions. However, such properties are highly dependent on the formation of eutectic points as well as on the *solidus* and *liquidus* temperatures of the slag phases, whose characteristics cannot be accurately measured during conventional tests. In this regard, the present work evaluates the suitability of computational thermodynamics as a tool for enhancing the comprehension of the ferrous burdens softening and melting phenomena. For that, composition, reduction profile and softening-meltdown properties of a lump ore, a pellet and a super-fluxed sinter (basicity from 0.02 to 2.07) were obtained from literature and used for carrying out thermodynamic calculations. An iterative strategy was adopted to realize equilibrium calculations from 800 to 1550°C, with an incremental step of 10°C, using Gibbs energy minimization principle along with thermodynamic databases from the commercial package FactSage 7.1. The simulation results were used to predict the whole curve of reduction degree as a function of temperature for different ferrous burdens; since reduction degree from thermodynamics is greater than the experimental one, a restriction factor in the available amount of reducing gas was introduced in the simulation procedure for taking into account kinetic and diffusional constraints in practical reduction process. In this way, the theoretical curve can accurately reproduce the experimental one, and the amount of liquid and solid oxide phases, as well as their compositions, can be known for the whole curve of reduction degree. The conceived methodology for thermodynamic simulation showed potential to improve understanding of the alterations originated from high-temperature reduction and the onset of softening and meltdown process. The chemical composition of liquid slag phase and its proportion to solid oxide phases were accessed in details during the course of reduction. A correlation was established between the calculated temperatures for first and maximum liquid phase formation and the experimental pressure drop profiles recorded for different samples, which could make it possible to estimate the temperature of formation and thickness of cohesive zone. Finally, the differences in behavior between sinter, pellet and lump ore were discussed from a thermodynamic viewpoint.