Kinetics and mechanism of natural wolframite interactions with sodium carbonate

The kinetics and mechanism of natural wolframite interactions with sodium carbonate during air heating were studied. X-ray phase and X-ray microanalysis were used to establish that the initial monocrystalline wolframite consists of Fe_0.5Mn_0.5WO₄ and Fe_0.3Mn_0.7WO₄. Differential thermal analysis showed that the interaction of wolframite with sodium carbonate begins above 450℃ with the formation of tungstate, sodium ferrite, iron oxides, and manganese. Model experiments on sintering with the subsequent removal of water-soluble compounds (leaching) tracked the change in the structure of wolframite. The atomic ratio of Fe/Mn in wolframite does not change up to 600℃, and subsequently decreases to 0.2 during heating, which allows the mechanism of the process to be identified and indicates the greater reactivity of wolframites with an increased proportion of iron. Thermal analysis with data processing using non-isothermal kinetics established that the interaction of wolframite with sodium carbonate in an air stream proceeds via a two-stage mechanism, wherein the first stage is limited by diffusion (activation energy, E=243 kJ/mol) and the second stage is limited by autocatalysis (activation energy, E=212 kJ/mol) due to the formation of a Na₂WO₄-Na₂CO₃ eutectic.