A carbon-free and energy-efficient pathway to direct reduced iron production: Hydrogen cooling reduction of iron ore pellets

Abstract

This study proposed a novel technology named hydrogen cooling reduction (HCR) for preparing direct reduced iron (DRI), i.e., metallized pellets, by hydrogen reduction of hot iron ore pellets during their cooling process, featured by simultaneous increase in H₂ concentration from 12.5% to 100% and decrease in temperature from 1150 °C to 450 °C. It was shown that the reduction degree (RD), iron metallization degree (MD), total iron content (TFe), and reduction swelling index (RSI) of the resulting metallized pellets increased with initial H₂ concentration, with the fractional concentration increase of 12.5% to 100% during the cooling process. Meanwhile, the compressive strength (CS) increased initially and then decreased. From the perspective of phase transformation, the content of Fe increased while those of Fe₂SiO₄, FeO, and SiO₂ decreased. The transmission electron microscopy (TEM) results revealed that Fe and Fe₂SiO₄ in the metallized pellets were connected by amorphous SiO₂, and they existed in a single-crystal form which was beneficial for improving the pellet strength. Higher initial H₂ concentrations promoted metallic iron particle growth and increased porosity and pore size. Furthermore, there was a linear relationship between the average pore size and RSI, given by the equation y = 1.7498x – 3.0732, where y and x denote RSI and average pore size, respectively. When the initial H₂ concentration was 75% with the fractional concentration increase of 12.5% to 100% in stages during cooling, the pellets had the optimal reduction performance, namely RD of 88.90%, MD of 81.05%, TFe of 80.63 wt%, RSI of 4.51%, and CS of 1975 N/p. The findings verified the viability of energy-efficient preparation of metallized pellets by HCR.