Coupled effect of SiO2 content and hydrogen-enriched atmospheres on the reduction behavior and microstructural evolution of fired hematite pellets

Abstract

As demand grows for low-carbon ironmaking, it is essential to understand how hydrogen reduces iron ore pellets under varying gangue compositions and gas atmospheres. In this work, fired hematite pellets with a basicity (mass ratio of CaO to SiO2) of 0.3 and SiO2 contents ranging from 1wt% to 4wt% were systematically investigated under three typical shaft furnace atmospheres (Midrex, HYL, and coke oven gas (COG)) as well as under 100% H2, to clarify the reduction kinetics, reaction mechanism, and microstructural evolution of the fired pellets. The results indicate that a higher hydrogen proportion significantly accelerates the reduction rate of the fired pellets, while an increase in SiO2 content generally leads to a decrease in the overall reaction rate. However, the effect of hydrogen concentration on the reduction behavior of the fired pellets varied markedly with their silicon content. For the fired pellets containing 1wt% and 2wt% SiO2, an increase in hydrogen concentration causes deterioration in reduced pellet characteristics, as evidenced by the increase in reduction swelling index from 26.14% to 34.26% and the decrease in cold compressive strength from 110 to 78 N. In contrast, fired pellets with 3wt% and 4wt% SiO2 exhibit the opposite trend, with the reduction swelling index decreasing from 15.26% to 9.23% and cold compressive strength improving from 179 to 271 N. Kinetics analysis indicates that under 100% H2, the reduction of fired pellets with 1wt% SiO2 is governed by a mixed gas-diffusion and uniform reaction model, whereas fired pellets with 4wt% SiO2 follow an unreacted core model. These differences in reduction kinetics, reduction behavior, and post-reduction properties are closely associated with the formation of more Al-bearing calcium silicate slag phases in high-SiO2 reduced pellets, which strengthen intergranular bonding, buffer phase-transformation-induced stress, and promote the evolution of metallic iron from whisker-like to granular or layered morphologies.

1. Introduction

With the intensification of global climate change, the steel industry—accounting for approximately 7% to 9% of global CO2 emissions—is under growing pressure to transition toward low-carbon production [1–2]. Low-carbon technologies have become essential for steel enterprises pursuing sustainable development [2–3]. The direct reduction iron–electric arc furnace (DRI–EAF) short process, which uses iron ore pellets as the primary feedstock, is regarded as a key decarbonization route due to its low emissions and high efficiency [4–5]. However, the depletion of high-grade iron ore and rising magnetite costs have driven increased use of hematite in pellet production, introducing new challenges to the reduction process [6]. Gas-based technologies such as Midrex and HYL require strict control over pellet chemistry and reducibility, as poor-quality pellets can impair equipment stability and DRI quality [7]. Compared to magnetite, hematite-based pellets show greater swelling and lower strength during reduction, which may negatively impact process performance in the furnaces [8].

Numerous studies [9–11] have shown that variations in SiO2 content within iron ore fired pellets influence their consolidation mechanisms, microstructure, and phase composition, thereby affecting the reduction process, swelling behavior, compressive strength after reduction, and ultimately the quality of the DRI product. Zhang et al. [12] reported that high SiO2 content (5.6wt%) combined with CaO addition suppresses iron whisker growth and promotes the formation of high-melting-point liquid phases, which facilitates hematite intergrowth structures. These phases possess strong bonding capacity, resisting internal stress during reduction and effectively suppressing pellet swelling. Gan et al. [13] found that adding 4wt% silica powder to chromite fired pellets increased their strength to 1298 N, aided by 1.5wt% liquid phase formation that promoted iron oxide grain growth and pore filling, enhancing pellet densification. El-Geassy et al. [14] investigated Fe2O3–SiO2 reagent compacts and revealed the mechanisms by which SiO2 affects swelling behavior. Their results showed that adding 2.5wt% SiO2 significantly reduced the reduction swelling index compared to pure Fe2O3, and led to the formation of relatively compact iron grains.

Moreover, the formation of fayalite (2FeO·SiO2) during reduction improved the compact strength and inhibited the growth of iron whiskers and plate-like metallic iron, thereby mitigating catastrophic swelling. In addition, the SiO2 content is closely correlated with the reduction performance of pellets. For every 1wt% increase in SiO2, the reduction rate of iron oxides decreases by approximately 5wt%–10wt% [15–16].