Scientists Map the Hidden Chemistry That Could Cut Steel’s Sintering Energy Bill

Steel is the backbone of modern civilization, but it is also one of the planet’s heaviest carbon emitters, responsible for roughly ten percent of global CO2 output. Now, a team of metallurgists from RWTH Aachen University, Brunel University of London, and NIT Andhra Pradesh has used computational thermodynamics to retrace, step by step, the molecular choreography that turns loose iron ore powder into the strong, porous chunks that feed blast furnaces. Their target is a crystal called silico-ferrite of calcium and aluminum, or SFCA, the glue that holds industrial sinter together, and their findings suggest that a simple flux tweak could theoretically slash the temperature at which this crucial phase forms by hundreds of degrees.

The study, published in the Journal of Materials Science: Metallurgy by Shubham Sanjay Joshi, Prabhakar Kumar, and Dieter Senk, tackles a deceptively simple question with a powerful tool: if we know exactly which chemical reactions build SFCA, and at what temperatures each one becomes spontaneous, can we find a cheaper, cooler route to the same product? To answer it, the researchers turned to CALPHAD-based modeling in FactSage, a Gibbs free-energy minimization framework drawing on the FToxid and FactPS databases to simulate the CaO-Fe2O3-Al2O3-SiO2-MgO system that dominates real sinter beds.

Sintering itself is a brutal, energy-hungry affair. Fine iron ore particles, too small to charge directly into a blast furnace, are mixed with flux, coke breeze, return fines, and moisture, then ignited on a moving grate where temperatures reach between 1300 and 1480 degrees Celsius. The blend partially melts, consolidates into a cake, and freezes into a complex mineral assemblage as it cools. The quality of that cake, its strength, reducibility, and resistance to degradation, hinges almost entirely on the bonding phases that precipitate from the melt, and SFCA, with the general formula Ca2(Fe,Ca)6(Fe,Al,Si)6O20, is the undisputed king among them.

What makes the new work distinctive is its insistence on mapping the full reaction network rather than a single endpoint. The authors trace two competing routes, which they label path ABC and path DEF, from raw sinter mix to the monocalcium ferrite CF phase. Along the way, limestone first calcines into lime, lime reacts with iron oxide to build dicalcium ferrite C2F and its aluminum-substituted variant C2(F1-xAx), and these intermediates branch toward brownmillerite CFA before silicon oxide enters the picture to unlock SFCA itself. Every arrow in this flowchart was assigned a computed temperature, and several of those predictions land remarkably close to experimental values reported in the literature.

The numbers tell an interesting story about efficiency. Calcination of limestone is predicted to become spontaneous between 476.85 and 526.85 degrees Celsius under idealized open-system conditions with minimal CO2 partial pressure, though the authors are careful to note that under real industrial conditions, where the gas phase approaches one atmosphere of CO2, the decomposition shifts up to roughly 790 to 800 degrees. The aluminum-bearing dicalcium ferrite C2(F1-xAx) forms at a computed 876.64 degrees, matching prior experimental work. Most striking is the fork in the road to CF: path ABC requires a scorching 1292.32 degrees, while path DEF, running through the direct combination of calcined lime and iron oxide, gets there at just 752.5 degrees, consistent with experiments showing the reaction proceeds between 650 and 780 degrees.

The brownmillerite CFA phase, which the simulations reveal as a combination of CA (CaAl2O4) and CF (CaFe2O4) components rather than a simple solid predecessor of binary CF, forms at 855.79 degrees. Here the team uncovered a subtle thermodynamic stabilizing effect: aluminum ions lower the Gibbs free energy of the complex ferrite matrix so substantially that the ternary phase becomes stable long before the binary one, suggesting CFA forms directly from solid-state interactions among CaO, Fe2O3, and Al2O3. Oxygen partial pressure, the analysis shows, plays no role at this stage, since the total gas fugacity only reaches one atmosphere at 1388.35 degrees, far above the formation temperature.

The headline result concerns magnesium. Dolomite is already added industrially at around 1.1 percent MgO to improve reduction-degradation resistance and protect furnace refractories, but the simulations reveal a far more dramatic consequence: the SFCA formation temperature plummets from 1156.75 degrees without MgO to just 788.92 degrees with it, a drop of roughly 367 degrees. The computed activity of the participating phases rises across the board when MgO is included, promoting phase stability at lower temperatures. Since the coke breeze burned in a sinter plant exists precisely to reach those temperatures, a bonding phase that forms hundreds of degrees cooler translates directly into less fuel, lower energy demand, and reduced CO2 emissions.

The model also pins down the melting behavior of SFCA, calculating a solidus of 1259.02 degrees and a liquidus of 1326.47 degrees, and tracks the iron-aluminum substitution reaction 2(Fe3+, Al3+) reversible with (Ca2+, Fe2+) through two distinct enthalpy transitions, at 1259.02 and 1326.47 degrees, where equilibrium shifts from an iron-rich phase toward an aluminum-rich one. At the top of the thermal profile, molten SFCA reacts with magnetite to form an iron-rich SFCA phase, predicted at 1420.33 degrees, though X-ray diffraction experiments place that event nearer 1200 degrees, a discrepancy of more than 220 degrees that the authors attribute squarely to the non-equilibrium reality of industrial sinter beds, where rapid heating, chemical heterogeneity, and particle size distributions defy the infinite-time assumptions of equilibrium thermodynamics.

That gap between calculation and furnace is where the authors plant their most important caveat. Equilibrium calculations say nothing about heating rates, diffusion, gas-solid heat transfer, melt viscosity, or bed permeability, all of which govern whether a lower formation temperature actually yields fuel savings in practice. The predicted MgO effect may partly reflect database extrapolations, and the narrower, more complex SFCA-I and SFCA-II phases remain beyond the model’s reach. The team calls for kinetic and continuous-cooling studies, experimental validation by XRD and SEM-EDS, and a full accounting of the coupled transport phenomena before the 367-degree promise can be banked.

Even with those limits, the work sketches a compelling vision for greener steelmaking. If the bonding phase that determines sinter quality can be coaxed into existence at lower temperatures, the coke breeze bill shrinks, emissions fall, and the industry’s most energy-intensive agglomeration step becomes measurably more sustainable. Combined with dolomite’s existing benefits, greater flexibility in flux selection and better use of recycled materials, the study positions computational thermodynamics as a genuine design tool for low-carbon metallurgy: a way to explore millions of reaction pathways on a computer before committing a single kilogram of ore, flux, or fuel to the sinter strand.

Subject of Research: Thermodynamic (CALPHAD) analysis of SFCA bonding-phase formation pathways in iron ore sintering and the effect of MgO on formation temperature

Article Title: Thermodynamic analysis of Silico‑Ferrite of Calcium and Aluminum (SFCA) formation via reaction pathways during iron ore sintering and its limitations: Considerations for energy efficiency in sustainable metallurgy

Article References: Joshi, S. S., Kumar, P., & Senk, D. (2026). Thermodynamic analysis of Silico‑Ferrite of Calcium and Aluminum (SFCA) formation via reaction pathways during iron ore sintering and its limitations: Considerations for energy efficiency in sustainable metallurgy. Journal of Materials Science: Metallurgy, 1(1), Article 23. https://doi.org/10.1007/s44492-026-00024-y

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00024-y

Keywords: SFCA, iron ore sintering, CALPHAD, FactSage, thermodynamics, sustainable metallurgy, energy efficiency, MgO addition, calcium ferrites, CO2 emissions, steelmaking, phase equilibria