Exploration of the role of SiO₂ in the carbothermic reduction of tripotassium phosphate

Carbothermic reduction of potassium phosphates contributes to phosphorus release during thermal conversion of P-rich biomass, but its temperature-resolved reduction pathways and interaction with SiO₂ remain poorly understood. This study investigates the carbothermic reduction of tripotassium phosphate (K₃PO₄) and the effect of SiO₂ over 600–1090 °C. Experiments were conducted in a fixed-bed reactor coupled with gas analysis and solid residue characterization. In the absence of SiO₂, K₃PO₄ follows parallel pathways: carbothermic reduction producing gaseous K and P, and char-assisted conversion to condensed phosphates (K₄P₂O₇ and KPO₃). The addition of SiO₂ alters this behavior by acting as a potassium getter. It promotes the formation of stable potassium silicates, which uncouple K from P, thereby lowering the onset temperature for phosphorus volatilization by 120 °C and increasing total P release by ∼10%. At temperatures above 940 °C, the formed potassium silicates undergo secondary reduction, releasing the trapped K. Thermodynamic equilibrium calculations support these experimental findings, confirming that the presence of K-silicate drives the redistribution of K–P speciation toward enhanced P volatilization. These results provide mechanistic insights into alkali phosphate transformation, offering a foundation for developing SiO₂-assisted strategies for phosphorus recovery from biomass. Novelty and significance statement This study investigates the carbothermic reduction of tripotassium phosphate (K₃PO₄) and the effect of SiO₂ in the temperature range of 600–1090°C. The results provide new insight into the coupled transformation of phosphorus and alkali metals during high-temperature processes relevant to biomass pyrolysis, combustion and gasification. By clarifying how SiO₂ influences potassium mobility and phosphorus release, this study connects fundamental reaction mechanisms with practical phosphorus recycling strategies. The mechanistic understanding on the role of SiO₂ offers new strategies for selective P–K recovery. The results contribute to broader efforts in circular utilization of P-rich residues in thermal conversion technologies.