Study of properties of hydrogen-reduced DRI related to steelmaking

Abstract

The present thesis investigates physical, mechanical and melting properties of hydrogen direct reduced iron (H-DRI) and how these relate to both reduction conditions and steelmaking practice. A combination of industrial pilot trials and controlled laboratory work is used in this endeavor. Carbonaceous reduction and resulting direct reduced iron are studied as references. Properties of H-DRI are of interest as hydrogen-based reduction is a proposed alternative to the blast furnace process, which represented 7-8% of global CO2 emissions in 2025.  As a first step, DRI reduced by natural gas or hydrogen in an industrial pilot shaft was investigated in terms of carbon and cementite contents, apparent porosity, fractures, tumbling strength and compression strength. It was found that tumbling and compression strength decreased with an increasing carbon content. The carbon-free H-DRI sample produced 90% less fines after the longest tumbling test and had nearly twice the average compression strength in comparison to the NG-DRI sample having the highest carbon content of 3 wt%. Cementite was found as concentrated near the pellets’ surfaces for the carbon-containing samples.  To gain deeper understanding, iron ore pellets were fully reduced in lab with hydrogen gas under varying temperature and time conditions. All reduction reached a maximum temperature in the range 700 to 1000 °C. The applied heating rates were between 1.5 and 10 °C/min. Holding times at the final reduction temperature were between 0 and 360 min. Volume change, apparent porosity, surface fractures, tumbling strength, compression strength, and microstructures at the pellet cross-sections were studied. The maximum reduction temperature had the most significant impact on the samples’ physical and mechanical properties. Samples reduced for a total time of five hours to 1000 °C in comparison to 700 °C had 9% lower fines generation after a 4-hour tumbling test, 3% lower apparent porosity and 13% fewer pellets with surface fractures. Increasing heating rate and holding time also had positive effect on apparent porosity, surface fractures, tumbling and compression strength. Further investigation is needed to understand the influence of conditions in the industrial reactor such as water content and pressure. Isothermal reduction experiments at 700 or 900 °C were conducted over an experimental time between 1 and 5 hours. CO+H2 gas mixture supplied into 

the bottom and exited through the top of the iron ore pellet bed with a gas flow rate of either 2 or 4 nL/min. Carbon and oxygen content analysis as well as cementite quantification over the pellets’ cross-sections was made for pellets placed along the bed’s vertical axis. Metallization, total carbon content and cementite content was greatest at the bottom and lowest at the top of the pellet bed for all experiments. Cementite content varied further within individual pellets, mostly found as concentrated near the pellets’ surfaces.  Melting of H-DRI and reference NG-DRI was investigated in a pilot-scale Electric Arc Furnace (EAF). Selected results from a large testing plan are included in this thesis work with focus on the melting behavior of highly metallized H-DRI (99%), low-metallized H-DRI (95%) and reference NG-DRI. Extensive sampling of slag composition, steel composition, temperature, ingoing and outgoing material weights was made. Slag foaming performance and identification of agglomerates of unmelted material termed ferrobergs were noted by visual observation. Maintaining an FeO content in the top slag above ~30 wt% seemed to have a greater influence on melting behavior than did slag basicity, foaming index or type of DRI. This may be because higher FeO content lowers the slag viscosity, which in turn enhances convective heat transfer within the bulk slag. Deeper understanding of melting behavior of H-DRI in EAF-type slags was sought through lab experimental work. A single H-DRI pellet was submerged in molten slags having varying FeO, Al2O3 and MgO. The slag and submerged pellet were quenched after melting times of 5, 10 or 15 s. The cross-section was studied and the surrounding slag was crushed and magnetically separated to see amount of melting. After 15 s melting time, between 50 and 80% apparent melting was achieved. The latter was achieved by submersion in the slag having theoretically highest thermal conductivity, i.e lowest FeO content and highest Al2O3 content. This stands in contrast to the findings in pilot-scale trials, showing that convection seems to be the dominant heat transfer mechanism in industrial-like conditions. 

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