Effect of Calcination on Hydrogen Reduction Kinetics, Porosity, and Microstructural Evolution of a High-Carbonate Manganese Ore

Abstract This study investigates the hydrogen reduction behavior of high carbonate manganese ore, focusing on how calcination influences reduction kinetics, microstructural evolution, and gas–solid reaction mechanisms. Isothermal reductions were performed at 700–900 °C on dried and calcined ore using a vertical tube thermogravimetric furnace, supported by x-ray fluorescence (XRF), x-ray diffraction (XRD), scanning electron microscopy (SEM)–energy-dispersive x-ray spectroscopy (EDS), porosity measurements, and JMAK kinetic modeling. Calcination of the ore at 900 °C effectively removed the carbonate and hydroxide phases, producing a more open-pore structure and significantly enhanced early-stage reducibility by hydrogen at 700–800 °C. Fractional-conversion and rate analyses confirmed higher initial reaction rates for calcined samples due to improved gas accessibility at these temperatures. At 900 °C, both raw and calcined ore types exhibited reduced porosity associated with temperature-induced densification, with the calcined ore showing a slower progression toward high conversion, consistent with increased diffusion resistance at advanced stages. Gas-evolution profiles revealed strong CO 2 release in the dried ore and highlighted the accelerating influence of the reverse water–gas shift (RWGSR) reaction during carbonate decomposition. JMAK modeling yielded activation energies of 81.46 kJ/mol (dried) and 24.62 kJ/mol (calcined), indicating a shift in the rate-controlling mechanism from surface-controlled behavior in the dried ore to diffusion-controlled behavior in the calcined ore. These findings provide mechanistic insight relevant to hydrogen-based low-carbon Mn processing. Graphical Abstract