Steel has underpinned modern life for centuries, but it comes at an expensive cost. The industry is responsible for nearly 7 percent of the world’s total carbon dioxide emissions, driven by blast furnaces fueled by coke, a form of coal, to extract oxygen from iron ore.
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For decades, most attempts at cleaner steel-making, though, have been dashed on the problem of doing it large-scale. Scientists may now have an answer.
Scientists have long dreamed of using hydrogen as a cleaner alternative to coke. Theoretically, if hydrogen gas is used to smelt iron ore, the waste product would be nothing but water. But the method has languished behind thermodynamic hurdles. Smelting magnetite, the most common iron ore, with hydrogen alone demands extremely high temperatures above 900 Kelvin and then even hotter stages to complete the reaction. The result is substantial cost and substantial energy usage.
Researchers at the University of Minnesota Twin Cities, working in partnership with Hummingbird Scientific, have now shown how hydrogen plasmas that are not thermal can change the paradigm. As opposed to reactions from heat, these plasmas form fleeting but highly energetic hydrogen radicals—atoms so reactive that they can smelt iron ore at room temperature.
Until recently, no one had seen what these reactions looked like on the tiniest scales. Other experiments employed bulk samples that masked the fine details within disordered structures. To get around this problem, researchers developed a new device called operando plasma transmission electron microscopy, or TEM.
This instrument can image magnetite nanoparticles directly upon exposure to hydrogen plasma at a resolution of around one nanometer. That’s ten times better than earlier optical methods and allows researchers to watch the process in real-time.
"We developed a new technique that allows us to probe plasma-material interactions at the nanometer scale, which has not been achievable so far," said Jae Hyun Nam, lead author of the paper and graduate student in the mechanical engineering department at the University of Minnesota.
What the researchers saw was a drama. In ten seconds of exposure, magnetite particles began shrinking and developing cracks. At an atomic level, hydrogen radicals were stripping oxygen from the crystal structure, leaving metallic iron behind.
The particles followed a shrinking-core model, where the reaction initiated on the surface and propagated inward, relentlessly dissolving away the oxide. That meant that the reaction rate was governed by surface chemical steps rather than by transport across the particle. Scaling up is good news, as it suggests the focus should be put on controlling plasma radical density rather than worrying about diffusion through large chunks of ore.
"Plasma formation can be energetically much more efficient than heating the material," said Andre Mkhoyan, lead author on the paper and professor in chemical engineering and materials science at the University of Minnesota. "This technology has the potential to enable materials to be altered with lower energy consumption, ultimately making processes economically more efficient."
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