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 carbo-thermal 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 degrees 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 degrees C/min until 1350 degrees C), reduction time is 70min, the maximum swelling observed in torrefied mushroom pellets is +158%, the pellets experienced swelling in between (700 degrees C-1000 degrees C) and started to soften from 825 degrees 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 degrees C) and Potash-lime (-1200 degrees C) compositions. These slags start to soften at a low temperature-700 degrees 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 Bou-douard reaction) causes these slags to expand, like a glass blowing mechanism.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This study investigated the use of different reducing agents in iron-ore composite pellets, including coal, spent mushroom substrate, torrefied mushroom substrate, and de-ashed torrefied mushroom substrate. The torrefied mushroom pellets exhibited significant swelling and softening behavior, while coal and de-ashed torrefied pellets did not swell. Additionally, the addition of alkali metal oxides and soda lime glass to de-ashed torrefied pellets resulted in increased swelling and decreased softening temperature.