Spatiotemporal Dynamics of Hydrogen Plasma Smelting Reduction of iron ore: A Multi-Species Diagnostic Approach
Plasma-based mineral-processing routes, such as hydrogen plasma smelting reduction (HPSR), which converts iron-ore fines directly to liquid metal in a single scalable step are commonly modeled by treating the arc as a spatially uniform heat source. Yet the reduction chemistry is governed by the strongly non-uniform conditions at the plasma-melt interface, which spatially averaged diagnostics cannot resolve. Here we spatially and temporally resolve the arc of a transferred arc HPSR reactor using multi-species optical emission spectroscopy (OES), in which neutral and ionic argon (Ar I, Ar II), hydrogen Balmer, and neutral iron (Fe I) emissions serve as intrinsic spatial filters set by their differing ionization thresholds. Combined with infrared thermography of the melt surface and an LTE thermal-plasma model validated against the benchmark free-burning argon arc, the measurements reveal a strongly stratified, non-isothermal discharge: an argon-defined core (>10,000 K), a partially recombined Balmer envelope (7,000-10,000 K), and an Fe I-traced interfacial boundary layer (3,000-4,000 K) directly above a melt surface at ~1,900-2,300 K. Across this steep thermal drop, positive hydrogen ions recombine before reaching the surface, so the reductant flux delivered to the oxide is overwhelmingly neutral; atomic hydrogen (H) and vibrationally excited molecular hydrogen H2(v), rather than the energetic ions often assumed. The measured electron density and excitation temperature bound the interfacial ionization. These findings redefine the boundary conditions for kinetic modeling of plasma-based ore reduction and establish a spatially resolved multi-species diagnostic framework transferable across plasma mineral-processing systems.