Preparation of core-shell iron ore-biochar composite pellets for microwave reduction

Abstract

Due to its high carbon and low impurities contents, as well as good reactivity, biochar can partly or totally replace coal or coke as a promising reducing agent in metallurgy. This study offers a new method to produce qualified metallized pellets for steelmaking after microwave-assisted self-reduction of iron ore-biochar composite pellets. The key point was to prepare green composite pellets with good quality indexes which also allowed sufficient self-reduction under microwave irradiation. For this purpose, core-shell iron ore-biochar composite pellets were prepared with inhomogeneous biochar distribution. It was found that high grade iron concentrates A and B could be used together for improving pelletizing. Controlling the distributions and proportions of iron concentrate B (different additions in the core and shell) and biochar (all located in the core of the pellets) would contribute to high-quality iron ore-biochar green composite pellets with the drop number of 3.6 times/0.5 m, compressive strength of 11.1 N/p, and decrepitation temperature of 410 °C under the pelletizing conditions of total proportion of iron concentrate B of 30%, pelletizing time of 16 min, rotation speed of 25 r/min, bentonite dosage of 2.3%, C/Fe mass ratio of 0.225, pellet moisture content of 13.0%, and proportions of iron concentrates A and B in the shell of 10% and 0%, respectively. The subsequent microwave reduction tests showed that the core-shell metallized pellets with the total iron content and metallization degree of 89.15% and 95.52%, respectively, could be obtained after microwave reduction at 1050 °C for 30 min under N2 atmosphere. They had better reduction indexes than those produced from the pellets with homogeneous structure, serving as a good burden for steelmaking directly.