Reducing silica in zirconium leachate from washed and dried frit powder: Role of cold and concentrated nitric acid and avoidance of crud formation

Traditionally zirconium is leached from washed dried frit (WDF) powder using commercial-grade nitric acid at 80 °C, which results in a leachate containing significant colloidal silica, leading to emulsion and solvent loss. This work used red fuming nitric acid (RFNA) which consists of 84 % nitric acid, 13 % dinitrogen tetroxide and 1–2 % water, as a novel leaching agent. The reduced water content in RFNA enhances the acid strength and prevents colloidal silica formation. This improves zirconium recovery and reduces co-leaching of silica. Optimization efforts focused on operating parameters (impeller speed, temperature, time, acid concentration, liquid-to-solid ratio during reaction and washing steps) to maximize zirconium recovery while minimizing silica in the pregnant leach solution (PLS). Avrami-Erofeev model was used to identify the reaction regime and estimate the apparent rate of leaching. The leaching process was found to follow a complex mechanism. Fourier-transform infrared (FT-IR) spectra of leach residue were analysed to characterize the silica. Zirconium concentration in the PLS from WDF powder dissolved in concentrated nitric acid reached approximately 87 g/L compared to 67 g/L using hot acid leaching (29.8 % increase). Silica concentration decreased from 6500 mg/L to 400 mg/L (93.8 % decrease) using this novel approach, achieving nearly 30 % higher zirconium concentration in the PLS compared to traditional methods.