Thermal analysis and mechanism of α-Zn₂SiO ₄:Mn²⁺ formation from zinc oxalate dihydrate under hydrothermal conditions

Reaction of zinc oxalate dihydrate in high-temperature water at 200-400°C was investigated to understand the mechanism of α-Zn ₂SiO₄:Mn²⁺ formation and specifically why zinc oxalate dihydrate forms α-Zn₂SiO₄ crystalline phase at a higher rate compared with other water soluble and insoluble zinc sources studied in our previous works. From experiments under hydrothermal conditions, it was determined that the dehydration of zinc oxalate dihydrate became suppressed as the temperature and pressure increased. From thermal analyses, zinc oxalate dihydrate, which dehydrates at 160°C in air, was stable at conditions up to 300°C in water and this resulted in the lack of formation of the intermediate phase, hemimorphite, which forms at 150-175°C from other zinc compounds (e.g. ZnO). The stability of the zinc oxalate dihydrate under hydrothermal conditions can possibly be attributed to changes in the chemical equilibrium with temperature and pressure which leads to the compound having less reactivity with silica so that reaction paths to form hemimorphite and β-Zn₂SiO₄ become unfavorable. Zinc oxalate dihydrate as starting material provided α-Zn₂SiO₄ at a higher formation rate than other starting materials.