Steel is the dominant metallic material. The production per year is 1.8 billion tons, of which 30% can be produced out of recycled melted scrap. The huge rest amount has to be newly produced from oxide minerals reduced by CO in blast furnaces, followed by partial removal of C by O2 in converters. The CO2 emission of these two processes is enormous, approx. 2.1 tons of CO2 per 1 ton of steel. Steel making thus becomes the largest single greenhouse gas emitter worldwide (~ 8% of all emissions). ROC is intensively involved in basic research needed to drastically cut down these CO2 emissions, by up tp 80% and beyond. This is the biggest single leverage we have to fight global warming.
Figure 1.(a) Schematic illustration of the arc melting furnace equipped with a tungsten electrode and charged with Ar-10%H2 gas mixture. The input hematite is placed on the water-cooled copper hearth inside the furnace before the reduction process. (b) Sequential steps for the reduction process. (c) Hematite pieces reduced with hydrogen plasma under different plasma exposure times: 1 min; 2 min; 5 min; 10 min; 15 min and 30 min. The red arrows indicate the presence of millimetric-scale transformed iron in the bottom of the samples. The upper part of the samples corresponds to the untransformed remaining oxide portion.
Figure 1.(a) Schematic illustration of the arc melting furnace equipped with a tungsten electrode and charged with Ar-10%H2 gas mixture. The input hematite is placed on the water-cooled copper hearth inside the furnace before the reduction process. (b) Sequential steps for the reduction process. (c) Hematite pieces reduced with hydrogen plasma under different plasma exposure times: 1 min; 2 min; 5 min; 10 min; 15 min and 30 min. The red arrows indicate the presence of millimetric-scale transformed iron in the bottom of the samples. The upper part of the samples corresponds to the untransformed remaining oxide portion.
The project ROC is based on two approaches (a) using H instead of C as reductant and (b) using electric arc furnaces operated with a H-containing reducing plasma. This will merge the multiple steps of traditional steel making into a single melting plus reduction process which can run with green electricity, an electric arc furnace operated with a H-containing reducing plasma. ROC’s approach is possible and can be upscaled by modifying existing furnace technology. The incentive is that solid Fe from other synthesis methods such as direct reduction must anyway be melted after reduction. Hybrid processes, where partially reduced oxides from direct reduction are fed into a reducing plasma, for high energy and H2 efficiency at fast kinetics and high metallization are also addressed. The physical and chemical foundations of these processes, down to atomic scales, are explored using instrumented laboratory furnaces as well as characterization, simulation and machine learning. Specific topics are the elementary nucleation, transport and transformation mechanisms, mixed scrap and ore charging, influence of contaminants from feedstock, plasma parameters, C-free electrodes, slag metallurgy and the role of nanostructure. Project ROC explores how steelmaking can contribute to the drastic reduction of CO2,which is the biggest challenge of our time,by cutting its emissions by 80% and more.
Figure 3. Phase transformation kinetics during hydrogen plasma-based reduction of the 15 g-hematite pieces. (a) Changes in the phase weight fractions of magnetite, wüstite, and ferrite (b) Iron conversion kinetics and corresponding transformation rates.
Figure 3. Phase transformation kinetics during hydrogen plasma-based reduction of the 15 g-hematite pieces. (a) Changes in the phase weight fractions of magnetite, wüstite, and ferrite (b) Iron conversion kinetics and corresponding transformation rates.
Figure 2. Microstructural characterization of the sample partially reduced for 2 min. (a) Overview of the sample. The red frame shows the chosen area for the correlative EBSD-ECCI probing approach. (b) ECCI-image of the area delimited by the red frame in (a). The white arrows evidence the presence of small domains of iron. (c) ECCI-image showing an enlarged view of the region delimitated by the yellow frame in (a). The corresponding EBSD maps of the area displayed in (c) are as follows: (d) image quality (IQ) map; (e) inverse pole figure (IPF) map in which the crystallographic orientations are shown perpendicular to the normal direction of the sample, i.e. parallel to the solidification direction; (f) phase map where wüstite and ferrite are represented by green and red respectively; (g) oxygen distribution and (h) silicon distribution maps. The corresponding captions are properly indicated in the bottom left-hand side of the figure.
Figure 2. Microstructural characterization of the sample partially reduced for 2 min. (a) Overview of the sample. The red frame shows the chosen area for the correlative EBSD-ECCI probing approach. (b) ECCI-image of the area delimited by the red frame in (a). The white arrows evidence the presence of small domains of iron. (c) ECCI-image showing an enlarged view of the region delimitated by the yellow frame in (a). The corresponding EBSD maps of the area displayed in (c) are as follows: (d) image quality (IQ) map; (e) inverse pole figure (IPF) map in which the crystallographic orientations are shown perpendicular to the normal direction of the sample, i.e. parallel to the solidification direction; (f) phase map where wüstite and ferrite are represented by green and red respectively; (g) oxygen distribution and (h) silicon distribution maps. The corresponding captions are properly indicated in the bottom left-hand side of the figure.
Souza Filho, I. R.; Springer, H.; Ma, Y.; Mahajan, A.; Corrêa da Silva, C.; Kulse, M.; Raabe, D.: Green steel at its crossroads: Hybrid hydrogen-based reduction of iron ores. Journal of Cleaner Production 340, 130805 (2022)
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