| More than 90% of the phosphate rock contains fluoride. The recovery of fluorine in phosphate rock is mostly absorbed by water in the terms of the fluorine gas during the concentration of wet-process phosphoric acid, which can be used to prepare hydrofluoric acid and fluosilicic acid.Experiments of defluorination for wet process phosphoric acid are studied with the sodium sulfate as defluorination agent, silica as auxiliary. The optimum conditions of these experiments are as follow:1.45 times amount sodium sulfate of the theoretical, 45℃ for the reaction temperature,2h for the reaction time, 300r/min for the stirring speed,0.3% of the quality of wet process phosphoric acid for the auxiliary. Under the optimum conditions of this process, the rate of removing fluorine can get 42.68%. During the defluorination process of wet-process phosphoric acid, the author investigated the relationship of sodium sulfate to the content of liquid phase fluorine with the reaction time in the wet process phosphoric acid, so the dynamic model is established by it. The relationship between the rate constant and temperature of sulfur acid sodium for the defluorination reaction is k=0.5908exp(- 799.86/T).Ammonium fluoride solution was obtained by the ammonolysis reaction of defluorination product of sodium fluorosilicate and ammonia. And also the by-product precipitate of silicon with sodium fluoride are attained. Under the condition of 30℃and stirring thoroughly, the optimum conditions of the reaction of Sodium fluoride and ammonia are as fllow: The sodium fluosilicate and water mixing with quality ratio of 1:5, 100ml/h for the ammonia dropping speed, and 60 min for the reaction time. Make the first time by the ammonium fluoride solution generated by the first time reaction instead of water to mix with sodium fluosilicate and react with ammonia, do such cycle 2 times. It means 3 times of such aminolysis reactions is a unit operation. The filter cake from the 3 times of the aminolysis reactions was washed two times with water by the total amount of 1.5 times of with water for the aminolysis reaction of the first time. Under these conditions as above, the sodium fluosilicate can be reacted by 98.56%, the total yield of ammonium fluoride can reach above 95%, the concentration of ammonium fluoride solution increased from 9.91% of the first time reaction to 21.92% of the unit operation that the energy consumption of subsequent vacuum distillation of ammonium fluoride solution was reduced greatly.After the vacuum distillation of the ammonium fluoride solution, the purification to remove the sulfate ion, sodium ion and fluosilicaten from itwas studied. The optimum conditions of purification for the ammonium fluoride solution are as flow: the barium hydroxide dosage is 1.05 times of the theoretical amount to react withsulfate,the fluosilicic acid is 1.3 times amount of theory to sodium fluoride, pH value regulated by ammonia water is during 9.0 ~ 9.5 andthe stewing time is 4h after fluosilicic acid is added to the ammonium fluoride solution. Under these conditions,the purity of obtained ammonium fluoride is 95.12% andother indicators have reached the industry standard.Then the ammonium fluoride solution was concentrated to appearcrystal in vacuum and went on purification of cooling crystallization. The optimum conditions for crystallization are as flow:vacuum concentration at 70℃, cooling crystallization at 10℃, 200 ~ 250r/min for stirring the speed, 2 ~ 2.5h for the crystallization time.The separation of sodium fluoride and silica precipitation obtained by the aminolysis reaction were dissolved with sodium hydroxide. The optimum conditions of these experiments are as follow: The sodium fluoride and silica were mixed wettability, slowly adding a certain amount of sodium hydroxide and a small amount of water, stirring constantly for the reaction of 100 min, cooled with standing to filter to attain the sodium fluoride, and washed it with sodium hydroxide solution. The purity of the sodium fluoride can reach 97.13%. |
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