Influence of Chemical Additions and Specific Surface Area on Micro-explosion of Burning Iron Particles

Micro-explosions (ME) during combustion of iron strongly affect combustion time, morphology, and oxidation of the final products. These explosions are caused by a sudden release of gas, but its origin (if at all unique) remains unclear. This gas could come from impurity within the particles (e.g., carbon), hydrogen adsorbed on the surface, or a combination thereof. This study investigates how the carbon and hydrogen content, originating from passive hydroxide layer and thus varying with the powders specific surface area, influences the occurrence and intensity of ME in four iron-based powders: pure Fe, Fe-7Si, DRI (Direct Reduced Iron), and Fe-5.2Al2O3. The particles were ignited in an electrodynamic levitator and observed through high-speed imaging and luminosity measurements. This enabled statistical identification of ME through luminosity evolutions and quantitative assessment of the particle inflation level across powders. The results show that DRI and Fe-5.2Al2O3 particles exhibit the highest ME probability (>90%), correlating with their larger specific surface areas (1.26 and 0.61 m2/g) compared to the other powders (<0.1 m2/g). Fe-5.2Al2O3 particles also display the most intense inflation events, which are not attributable to higher initial carbon content. Instead, these findings support a new mechanism: hydrogen release from native hydroxide layers, confirmed present via X-ray Photoelectron Spectroscopy (XPS). These layers form during storage through reactions with ambient humidity. During combustion, hydrogen dissolves into molten iron and is expelled near peak temperature due to solubility limit or supersaturation. Fe-7Si results further indicate that a thicker hydroxide layer must exist to account for their inflation intensity, attributed to Si promoting formation of an additional independent hydroxide layer. Finally, impurity composition in the oxide liquid phase influences burst resistance after gas formation, demonstrated by the absence of ejection in inflated Fe-7Si particles, linked to their lower density, higher dynamic viscosity, and possibly higher surface tension due to Si presence.