Mechanism-driven Regeneration of Ca2+-contaminated Tempering Baths: Effects of Oxide Additives and Waste-Derived Silica

Calcium contamination is a critical limitation in the chemical tempering of glass, as even trace levels of Ca2+ can inhibit potassium–sodium ion exchange and degrade mechanical strengthening. This study systematically investigates the effects of Ca2+ contamination and regeneration strategies for KNO3 baths using oxide-based additives and waste-derived silica. Controlled experiments identify ≈5 ppm Ca2+ as a critical threshold at which ion exchange is effectively suppressed, yielding no measurable surface compressive stress (CS) or depth of layer (DOL), while lower concentrations cause progressive degradation. These findings are validated using industrial salt bath samples, confirming the high sensitivity of chemical tempering to ppm-level contamination. Regeneration experiments with alumina, silicic acid, and silica powders show that efficiency is governed by both chemical affinity toward Ca2+ and particle-size effects. Ca2+-induced blocking originates from interfacial accumulation, whereas regeneration proceeds via immobilization in the molten salt. Alumina exhibits the most stable regeneration behavior, while silicic acid is effective at low additions (0.1–0.5 wt%) but less efficient at higher levels. Silica-based regeneration strongly depends on particle size, with fine silica achieving performance comparable to alumina. Notably, waste-derived silica synthesized via alkali fusion matches or exceeds commercial materials, demonstrating its potential as a sustainable regeneration agent.