Breaking the calcium paradigm: Sodium-mediated thermochemical synergy for low-temperature melting and enhanced metal solidification

High-temperature melting is an important method for the solidification and resource recovery of hazardous waste, but suffers from high energy consumption and inefficient solidification of volatile metals. This study introduces a sodium-based fluxing strategy to supplant conventional calcium-based fluxing agents during the co-melting of hazardous waste incineration bottom slag and fly ash. Ellingham and phase equilibria diagrams synergistically identified sodium oxide as the optimal fluxing agent, demonstrating both the thermodynamic spontaneity of heavy metal chloride-to-oxide conversion and a 40 °C reduction in the eutectic temperature as sodium oxide content increased from 3 % to 8 %. A novel thermodynamic activity model further revealed the dual role of sodium oxide in solidifying heavy metals through forward shifts in solidification reactions and lowering melting points via formation of low-melting-point silicates. Lab-scale experiments showed that adding 10 % sodium carbonate reduced the melting point of bottom slag and fly ash by 100 °C (compared to 60 °C for calcium oxide) and improved Pb solidification efficiency by 9 % (vs. 0.5 % for calcium oxide). Pilot-scale experiments confirmed that 10 % sodium carbonate addition at 1300 °C produced vitrified slag with a density of 2.71 g/cm³, vitrification efficiency >85 % and leaching toxicity well below regulatory thresholds for construction sand. Despite higher raw material costs for sodium-based agents, operational expenses remained competitive with calcium-based systems due to reduced energy consumption and enhanced metal solidification. This study establishes a systematic theoretical framework for developing novel fluxing agents, wherein the design methodology demonstrates broad applicability to thermochemical conversion processes.