Study On Preparation Technology Of Silica And Ammonium Bifluoride From Fluosilicic Acid

Fluosilicic acid is important a byproduct of phosphorus chemical industry, it has certain toxicity and corrosion, which will bring about severe environmental pollution if discharged directly. According to statistics, for every 1 million tons of phosphate fertilizer (equivalent to P2O5), there are more than 50,000 tons of fluorosilicic acid byproducts.As domestic phosphate fertilizer production in 2013 is expected to reach about 14.5 million tons, then the fluosilicic acid byproducts will exceed 700,000 tons, plus the amount of fluorine chemical industry,the annual output of fluosilicic acid will be millions of tons. If such huge numbers of fluosilicic acid is not being fully exploited but being discarded directly, it will not only cause serious waste of resources, but also bring tremendous pressure on the environment. Therefore, taking fluosilicic acid and ammonia as raw material, ammonium bifluoride and silica were prepared via liquid phase-pyrolysis crystallization technology. This is of vital significance for reducing pollution and improving the environment,as well as winning great economic benefits.Reaction factors such as volume ratio of added ammonia and total ammonia, the volume ratio of base solution and ammonia, the dropwise add volume ratio of fluosilicic acid and ammonia, reaction temperature and aging time that influencing the DBP value of silica were investigated, and the effect of the concentration temperature, evaporated amount and crystallization temperature to the yield and purity of ammonium bifluoride were studied by response surface methodology design and orthogonal experiment,respectively. The optimum silica preparation conditions were confirmed as follows:the volume ratio of primary reaction added ammonia and total ammonia was 0.31, the volume ratio of base solution and ammonia was 0.6, the dropwise add volume ratio of fluosilicic acid and ammonia was 1:5.75, reaction temperature was 74.66℃, aging time was 6h.Under the optimized conditions, the DBP value of silica was 3.12mL·g-1.The optimum condition of ammonium bifluoride were:the concentration temperature75℃, the evaporated amount of water70%, the crystallization temperature25℃. Under these conditions, the purity and yield of ammonium bifluoride were 98.07% and 90.43%,respectively.The final samples were characterized by infrared spectrum, XRD,TEM, TG, specific surface area measurement and particle size detection. Results showed that the sample was amorphous silica in structure, average particle size was 218nm, specific surface area was 125.99 m2·g-1, silica content was 99.05%, heating loss was 7.13%, roasting loss was 5.84% and pH was 6.7; XRD and SEM detection indicated that the ammonium bifluoride sample has complete crystal shape and high crystallinity in monoclinic phase system, the HG/T3586-1999 detection also showed the product was medium superior quality.The experiment results proved that the established liquid phase-pyrolysis crystallization technology can make full use of fluorine and silicon in fluosilicic acid,and converted into ammonium bifluoride and silica with excellent properties. It demonstrates that the process is reasonable and feasible.