Removal Of Fluoride From The Acid Digestion Liquor And Viscosity-reducing Of Neutralized Slurry In Production Process Of Nitrophosphate

Nitrophosphate is one of the efficient compound fertilizers containing nitrogen and phosphorus(potassium).With the grade declining of phosphate rock in recent years,the fluorine in phosphate rock would cause more serious pollution to ecosystems and series of problems for the subsequent process steps.The viscosity of neutralized slurry increased obviously if using middle-grade or low-grade phosphate rock,which would seriously restrict the normal production of nitrophosphate.In this paper,removal of fluoride and viscosity-reducing of neutralized slurry in the nitrophosphate process were investigated.KN03 was chosen as precipitate,and effects of acidic concentrations,temperatures and different nitrates on the solubility of K2SiF6 in solutions were firstly studied.The influences of reaction time,temperature and KN03 addition were studied both in pure fluoride solution(H2SiF6+HF+H2O)and acid digestion liquor,and the precipitates obtained were characterized by XRD,SEM and XRF.The simulated mother liquors were prepared and the effects of impurities on neutralized slurry viscosity were investigated.Based on the viscosity-reducing mechanism of neutralized slurry,proper viscosity reducers were chosen,and the influences of different viscosity reducers were evaluated.The aims of this paper are to solve the practical problems in the production process of nitrophosphate,and to provide enterprises with green and practical technical solutions.The main conclusions could be drawn as follows:1.In the nitrophosphate process,fluoride was present in two forms of H2SiF6(0.78 wt%F-)and HF(0.10 wt%F-)in the acid digestion liquor.2.The solubility values of K2SiF6 always increased with the raising temperature whether in phosphoric acid,nitric acid or phosphoric acid and nitric acid mixed solutions.The solubility values of K2SiF6 increased with the increasing HNO3 concentration,while decreased with H3PO4 concentration.The solubility values of K2SiF6 increased with the increasing Mg(NO3)2 concentration in phosphoric acid and nitric acid mixed solutions,while the concentrations of Al(NO3)3 and Fe(NO3)3 did not cause the obvious changes of K2SiF6 solubility.The presence of KNO3 and large amount of Ca(NO3)2 in mixed acid solutions made the solubility of K2SiF6 decrease.3.In the pure fluoride solution(H2SiF6+HF+H2O),the precipitation of fluoride was almost completed in 5 min.And the fluoride removal efficiency could reach the maximum of 88.52%when 400%excess of KNO3 over the stoichiometric requirement was added in 1 h time at 25℃.In the actual acid digestion liquor,the reaction time of 1 h was totally enough for the precipitation of fluoride.The fluoride removal efficiency could reach 70.22%when 400%excess of KNO3 was added in 1 h time at 25℃,while the maximum fluoride removal efficiency of 88.18%could reach only when 700%excess of KN03 was added.Whether in the pure fluoride solution or acid digestion liquor,the obtained precipitates were the high-purity(>99.50%)K2SiF6.4.The viscosity of neutralized slurry increased significantly with the raising pH,the viscosity of neutralized slurry was 4986.3 mPa-s at a pH of 2.63,resulting in the difficulties of ammonia neutralization reaction.However,the slurry viscositis changed unobviously and were about 10～30 mPa·s when mother liquor contained only HN03 and H3P04.5.Removing the fluoride or calcium in mother liquor could reduce the the viscosity of neutralized slurry.And all the viscosities of neutralized slurry without the fluoride or calcium were about 200 mPa·s at a higher pH of 6,making the slurry have a good rheology behavior.The effect of impurities on neutralized slurry viscosity could be ranked with the following order:SiF62-,Ca2+>Mg2+>Al3+>Fe3+>AI(SiO2).6.The chelating agents were chosen as viscosity reducers and exhibited excellent effects of viscosity-reducing.When the mother liquor contained large amount of calcium,sufficient viscosity reducers were necessary to be added in the slurry and the foaming phenomena were becoming more and more seriously.When the mother liquor contained no calcium,all the three viscosity reducers(EDTA,Citric acid,Oxalic acid)showed great effects of viscosity-reducing at the two peak-viscosity areas,resulting in the superior fluidity of neutralized slurry.