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A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of α-Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp ≈ 340 kJ mol−1.