NaCl-induced crystallization for Na₂SiF₆ recovery from real glass-thinning etching wastewater

Recovering resources from real glass-thinning etching wastewater is challenging because of its high acidity and complex ionic composition. This study developed a NaCl-induced crystallization-recrystallization process for selective Na₂SiF₆ recovery and evaluated NaCl dosage, stirring time, temperature, and recrystallization solid-to-liquid ratio. The wastewater contained 29.439 g L^-1 total F, 3.269 g L^-1 dissolved Si, and 4.33 mol L^-1 total acidity. At an NaCl dose of 1.2 times the stoichiometric requirement, 40 °C, and 45 min, primary crystallization achieved a dissolved-Si-based H₂SiF₆-equivalent removal efficiency of 95.50 ± 0.08% and crude-product purity of 91.30 ± 1.26%. The Avrami model empirically described the depletion kinetics (R² = 0.99991). Recrystallization at 1:75 (g:mL) provided approximately 55% stage dry-mass recovery and increased Na₂SiF₆ purity to 99.52 ± 0.08 wt%, meeting GB/T 23936-2018 requirements. An apparent F-Si mass balance showed that the final product retained 24.54 ± 0.19% of wastewater-input F and 51.19 ± 1.91% of wastewater-input Si, distinguishing elemental recovery from liquid-phase removal and product purity. XRD, FTIR, and Raman identified Na₂SiF₆ as the predominant phase. Thermal decomposition occurred mainly at 500-620 °C and yielded NaF as the predominant detectable solid product. Because the primary mother liquor retained high total-F, SO₄^2-, Cl^-, and acidity loads, the process should be regarded as a front-end resource-recovery step requiring downstream mother-liquor treatment.