Study On The New Preparation Technology Of Silica And Sodium Fluoride From Industrial Grade Sodium Hexafluorosilicate

Sodium hexafluorosilicate, which is toxicity and corrosive to some extent, is one of the important by-product of phosphorus chemical industry. Discharging indiscriminately of them will cause severe environmental pollution. According to statistics, the production of every 1 million tons of phosphate fertilizer(equivalent to P2O5) will bring more than 60,000 tons of sodium hexafluorosilicate. It is estimated that the phosphate fertilizer production in 2016 is about 20.6 million tons, then the sodium hexafluorosilicate by-products will exceed 1 million tons. The number will be doubled if we take other fluorine chemical industry into consideration. Not only will it cause serious waste of resources, but also will put tremendous pressure on the environment if such huge numbers of sodium hexafluorosilicate is being discarded directly instead of being fully exploited. Here, we develop two different chemical technologies to convert the fluorine source in sodium hexafluorosilicate to sodium fluoride, and convert the silicon source to silica. Hence, we obtain two additional higher economic value chemical products, meanwhile, it is also environmental friendly.We employed the method of adding crystal seed and preparing sodium fluoride and silica in fractional steps(Method I) and the method of adding surfactant and sodium carbonate(Method II) to acquire sodium fluoride and silica. In the experiment, we discussed the influence of several singer factors, optimized the preparation of sodium fluoride and silica through orthogonal experiments of the two methods.The process route of the method of adding crystal seed and preparing sodium fluoride and silica in fractional steps was carried out as follows: We used the sodium fluoride and ammonia as the raw materials, seed crystals was prepared at first, then added the seed crystals to the glass reactor, following the precipitating of the raw material, stirring and aging, filtering to obtain the filtrate the filtrate and the filtration residue. We added the sodium hydroxide solution to the filtrate then concentrating in vacuum, crystallization, filtrating, drying to afford the sodium fluoride products. sulfuric acid residues by adjusting pH, filtering and washing, drying silica product. The pH of filtration residue was adjusted by H2SO4, followed by filtering, washing and drying to afford the silica product.The optimum technological conditions of Method I to prepare silica: the volume concentration of dilute ammonia is 4.7%, the concentration of surfactant is 0.5%, the volume concentration of crystal seed is 5%, the proportion of quick addictive ammonia to total amount of ammonia is 0.2,the reaction temperature is 70℃, the molar ratio of the total ammonia to sodium fluoride is 1:8, the end reaction pH is 8.5, the aging time is six hours. The optimum technological conditions of Method I to prepare sodium fluoride: the concentrating temperature is 60℃, the amount of evaporation is 80%, the concentration of NaOH is 20 mol/L, the aging temperature is 20℃, the aging time is 20 min. The characterization results indicate that the particle size of silica shows a normal distribution, the median diameter is 20 nm, the specific surface area is 178.19 m2/g. The average particle diameter of the sodium fluoride crystal obtained is 110μm, the XRD test shows a significant crystalline diffraction peaks of sodium fluoride.The process route of the method of adding surfactant and sodium carbonate was carried out as follows: We used the sodium fluosilicate, one of byproduct of phosphorus industry and Na2CO3 as raw material, then adding the surfactant, following by adding Na2CO3 step by step, then the crude products of sodium fluoride and silica were obtained after the separation of liquid-solid fluidization separating, and then silica and sodium fluoride with higher purity were afforded by the subsequent process.The optimum technological conditions of Method II to prepare silica and sodium fluoride: the volume ratio of quick addictive sodium carbonate solution to the total amount of sodium carbonate solution is 0.3, the molar ratio of sodium carbonate and sodium fluoride is 1:3, the concentration of surfactants is 0.3%, the reaction temperature is 85℃, the reaction time is 3h, the flow rate of mother liquor in separation unit is 24 L/h. Sodium fluoride and silica can be effective separated through the liquid-solid fluidizating separation equipment designed by ourselves. The purity of silica and sodium fluoride is 92.18% and 98.92%, respectively. The characterization results indicate that the particle size of silica shows a normal distribution. The median diameter is 208 nm, the DBP value is 3.16ml/g and the surface area is 112.24m2/g. The average particle diameter of the sodium fluoride crystal obtained is 170μm, the XRD test shows a significant crystalline diffraction peaks of sodium fluoride.Taken together, both fluorine and silicon elements can be utilization effectively in sodium hexafluorosilicate through the two methods developed by this research, which enhances the economic value. Besides, our experiments verify the feasibility of the process routes.