The effect of reaction conditions on the precipitation of sodium hexafluorosilicate produced from waste hexafluorosilicic acid

Abstract

The purpose of the study is to investigate the optimum conditions for the precipitation of sodium hexafluorosilicate (Na 2 SiF 6) from waste hexafluorosilicic acid, an effluent from the phosphoric acid industry. Sodium chloride and sodium hydroxide were used as reactants to produce Na 2 SiF 6. The effect of various parameters on the precipitation was investigated and includes; the molar ratio of the reactants, contact time, the temperature and the effect of seeding. The optimum reaction conditions were found to be as follows; excess sodium chloride or sodium hydroxide to hexafluorosilicic acid of 25%, contact time 40 minutes and a reaction temperature of 40°C. The reaction of hexafluorosilicic acid with an aqueous solution of sodium chloride at optimum conditions gave a maximum yield of 94.26% Na 2 SiF 6 while the reaction of hexafluorosilicic acid with the aqueous solution of sodium hydroxide at optimum conditions gave a maximum yield of 97.3% Na 2 SiF 6. The X-Ray diffraction (XRD) analysis reveals that the only crystals present in the precipitate are Na 2 SiF 6. Also, Scanning Electron Microscope (SEM) analysis shows that the different morphology of these crystals depend on the precipitation conditions.

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration–Bipolar Membrane Electrodialysis-Based Closed-Loop Process

“…When the pH was increased to 8, the residual fluoride concentration increased to 767.3 mg L –1 , and the Na 2 SiF 6 yield decreased to 2.1%. This could be ascribed to the Na 2 SiF 6 dissolution in alkaline solution (Na 2 SiF 6 + 4OH – → 2Na + + 6F – + SiO 2 ·2H 2 O) . Overall, the Na 2 SiF 6 yields at pH 6 and 7 showed higher values than those at pH 5 and 8.…”

“…This could be ascribed to the Na 2 SiF 6 dissolution in alkaline solution (Na 2 SiF 6 + 4OH − → 2Na + + 6F − + SiO 2 •2H 2 O). 20 Overall, the Na 2 SiF 6 yields at pH 6 and 7 showed higher values than those at pH 5 and 8. To avoid Na 2 SiF 6 decomposition due to the impracticality of controlling the pH value exactly at 7 in the large-scale experiment, pH 6 was selected for the following experiments.…”

“…Previous studies have reported that the chemical reaction of hydrofluoric acid (HF) and silica generates silicofluoride (SiO 2 + 6HF → SiF 6 2– + 2H 2 O + 2H + ) . Moreover, the precipitation reaction of silicofluoride with sodium ions can be used to produce Na 2 SiF 6 (SiF 6 2– + 2Na + → Na 2 SiF 6 ↓) . Due to the fluoride ions being transformed into HF at lower pH (pH < 3.16), it is possible to acidify the fluoride-rich wastewater to form HF using a strong acid.…”

“…19 Moreover, the precipitation reaction of silicofluoride with sodium ions can be used to produce Na 2 SiF 6 (SiF 6 2− + 2Na + → Na 2 SiF 6 ↓). 20 Due to the fluoride ions being transformed into HF at lower pH (pH < 3.16), 21 it is possible to acidify the fluoride-rich wastewater to form HF using a strong acid. Then, the HF in fluoride-rich wastewater can react with SiO 2 to generate Na 2 SiF 6 using silica-rich wastewater.…”

Zero-Liquid Discharge Treatment of Wastewater from a Fertilizer Factory

“…It is used as a fluoridation agent for drinking water, insecticide, production of high purity silicon tetrafluoride and the production of enamels for china and porcelain products [9].…”