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

NaCl-induced crystallization recovered Na 2 SiF 6 from real glass-thinning wastewater. • H 2 SiF 6 -equivalent removal was 95.50%; recrystallization mass recovery was 54.96%. • Recrystallization increased Na 2 SiF 6 content to 99.52 wt%, meeting GB/T 23936-2018. • Apparent whole-process recoveries were 24.54% for F and 51.19% for Si. 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 Na 2 SiF 6 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 H 2 SiF 6 -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 (R 2 = 0.99991). Recrystallization at 1:75 (g:mL) provided approximately 55% stage dry-mass recovery and increased Na 2 SiF 6 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 Na 2 SiF 6 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, SO 4 2- , Cl - , and acidity loads, the process should be regarded as a front-end resource-recovery step requiring downstream mother-liquor treatment.