| Based on the investigation of cryolite technology and fluoride removal technology of CaF2, we had a research on the complex system of F", Al3+ and CaF2 systematically, and used two technologies to the process of cryolite in the low-temperature decomposition of silicates integrated, solved the problem of low output value of aluminum hydroxide generated in the reaction and excessive fluoride liquid in waste issues, further improved the process. The main contents are as follows.First, through the theory calculation and experiments of F-Al-Ca complex system, we obtained the follows:NH4+ was pulled in to reduce the reaction of K+ and F-, while separated the precipitate formed by F- and Al3+. Content of unreacted fluoride in the liquid waste would be further reduced by adding an appropriate amount of calcium; in acidic cryolite solution system, when pH≤5, aluminum hydroxyl ions are hardly found, when pH=6.0, the aluminum fluoride ions are over 60%, aluminum hydroxyl ions are more than 35%, when pH≥8.0, its almost all aluminum ions; in CaF2-Ca(OH)2 system, the total concentration of fluoride in solution depends on the remaining concentration of Ca2+ and pH in the solution; in CaF2-Ca(OH)2-H2SiF6 system, when adding Ca(OH)2 to the fluorine acid solution, just to ensure the pH<8.7 of solution, [SiF62-] will be ignored; in fluoride acid-aluminum fluoride acid system, when pH> 3.03 of the solution, AlF63- will become the main form of F; in the integrated system, the concentrations of SiF62- and AlF63- in solution are influenced by the concentration of Ca2+ and pH of the solution, and concentration of SiF62- is further influenced by pH; through comparative study of various ions in different pH conditions, we obtained that impurity ions will be removed gradually by adding ammonia to control conditions of the solution.Second, during the actual process of fluoride removal reaction, the final concentration of fluoride will not be reduced gradually because of the excess calcium. When F'and Ca2+ for their equivalent reaction, the lower concentration of F-, the slower reaction rate. Compare to fluoride wastewater treatment with lime, its reaction rate is much slower than the treatment with CaCl2; in the same molar ratio of Ca/F, the higher concentration of initial concentration of F", the more concentration of F- decreases. When the molar ratio of Ca/F is bigger than or equal to 4, it's no need to increase molar ratio of Ca/F to reduce the concentration of F-; in water, the presence of electrolytes dissolved will hinder the formation of CaF2, the balance will be broken to CaF2(S)' further dissolution; Influence on the solubility of CaF2 by pH will grow as the amount of CaF2 increase rapidly; in the process of adding Ca2+ to remove fluorine, excess Ca2+ and the SO42- of the solution will generate CaSO4 precipitation, affect the SO42-content in the potash, thus affecting the quality of the KNH4SO4, excess Ca2+ should be avoided during in the experiment to improve the quality of potash.Finally, the optimum conditions for the production of cryolite are:reaction temperature is 25℃, molar ratio of ammonia and aluminum ingredients is 3:1, reaction time is 30 min, ratio of adding water and waste is 1.5:1, pH of the reaction should be controlled to 5-6; the optimum conditions of fluorine removal in waste water are as follows:adding lime powder, pH=13 should be controlled as reaction end point, reaction time is about 60min; stirring intensity is 200r·min-1; pH≈6～7 should be controlled by anti-adjusting of (1+1)H2SO4, the concentration of remained F-in the solution would be finally lowed down to 5 mg-L-1. |
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