Kinetics and Mechanism of Low-Grade Manganese Ore Reduction by Natural Gas

The calcination and the reduction behaviors of a low-grade manganese ore by methane was studied at 973 K to 1273 K by several techniques. The onset temperatures of thermal decompositions of pyrolusite and calcite phases for this ore were found to be 868 K and 1056 K, and the rates of decompositions are obtained as 1.4 × 10⁻³ s⁻¹ and 1.53 × 10⁻³ s⁻¹, respectively. Because of the presence of manganese in the goethite structure of the ore, the goethite decomposition resulted in higher degrees of decomposition and magnetite formation at the studied temperature range. The exothermic reduction of manganese oxides to MnO started prior to the reduction of the coexisting iron oxides, and the apparent activation energy of reduction of Mn₃O₄ to MnO was determined. The reduction of manganese oxides resulted in CO₂ gas release due to the reaction of H₂O with the carbon supplied via methane cracking. Although manganese oxide minerals of the ore were not carburized at the experimental conditions, the manganese associated with iron oxides was carburized to cementite ((Fe,Mn)₃C), and it was accompanied with CO gas release. Higher rates and extents of manganese oxides reduction were obtained with increase in temperature; however, the simultaneous formation of manganese silicates was more favorable at higher temperatures and hindered complete reduction.