Swelling and softening behavior of iron ore-spent mushroom substrate composite pellets during carbothermal reduction

In the present day, we have reached close to an optimum condition of CO2 emission when using coal or coke for iron making process. Carbon neutrality is achieved during carbothermal reduction by employing biomass as a reducing agent in composite pellets. The swelling behavior of composite pellets during reduction has detrimental effects, and catastrophic swelling leads to subsequent disintegration of pellets, which causes low gas permeability through packed beds in the industry. In this study, we used coal, spent mushroom substrate, torrefied mushroom substrate (300 °C), and de-ashed torrefied mushroom substrate as reducing agents in our iron-ore composite pellets. Volume change of the composite was investigated with heating profile (15 °C/min until 1350 °C), reduction time is 70min, the maximum swelling observed in torrefied mushroom pellets is +158%, the pellets experienced swelling in between (700°C-1000 °C) and started to soften from 825 °C. However, no swelling is observed in the coal and de-ashed torrefied pellet. On the other hand, de-ashed torrefied pellets with the addition of alkali metal oxides and soda lime glass had significant swelling and decrease in softening temperature. With the presence of Na2O and K2O in pellets ash, during reduction in some localized areas, they form low melting slags like Soda-lime (~1000 °C) and Potash-lime (~1200 °C) compositions. These slags start to soften at a low temperature ~700 °C for soda-lime composition. As the temperature increases, viscosity decreases, which causes pellets to soften more, and parallelly gas evolving from the pellet during reduction (mainly CO product of the Boudouard reaction) causes these slags to expand, like a glass blowing mechanism.