Study On Crystalline Deposition Of Calcium Fluoride In High Fluoride Wastewater And Fluoride Removal By Fluidized Bed Crystallization

Our country is a large country of rare earth resources,Meanwhile,there are abundant rare metal mineral resources in southern Jiangxi Province.In the process of smelting rare earth minerals in the region,a large amount of fluorine-containing wastewater will be produced,and the discharge amount will far exceed the national discharge standard.Although there are many methods of defluorination,calcium precipitation is the most effective and economical method in practical waste water defluorination.Therefore,it is of great practical significance to further study the deposition characteristics of calcium fluoride reaction and understand the basic parameters of calcium fluoride in the treatment of fluorine-containing wastewater in a follow-up fluidized bed plant and to improve the utilization ratio of fluorine ions,which has important practical significance.In this paper,the following studies were conducted on the crystallization thermodynamics,kinetics,and fluidized bed crystallization techniques of calcium fluoride.First,a study on the crystallization thermodynamics of calcium fluoride.The related parameters of the crystallization thermodynamics of CaF₂ were measured by using laser measurement of calcium fluoride supersolubility experiment device and ion chromatograph device.The effect of the presence of quartz sand and calcite on the super solubility of calcium fluoride was measured and showes that in the presence of seed crystals,the metastable zone of calcium fluoride can be significantly reduced.When quartz sand is used as a crystal,it has a significant effect on the reduction of the metastable zone of calcium fluoride.By investigating the effects of calcite and quartz sand on the super solubility of calcium fluoride,both seed crystals can reduce the metastable zone of the seed crystals.The quartz sand was selected as the crystal seed,with0.5-1 mm,1-2 mm and 2-7 mm respectively.When the amount of seed added is the same,the smaller the crystal size,the greater the surface,which make it easier to contact with calcium fluoride crystals,cause its super solubility decreases,and at the same time cause the interface stability zone width is reduced.Effect of pH on calcium fluoride the metastable zone gradually decreases under acidic conditions,and the width of the metastable zone slightly increases under alkaline conditions;the presence of impurity ions CO₃^2-and SO₄^2-reduces fluorine The super-solubility of calcium.The existence of rare earth ions La and Y can also increase the metastable region of calcium fluoride.Study on the crystallization kinetics of calcium fluoride.For the influence of seed crystals on the induction period of calcium fluoride,the seed crystals of calcite and quartz sand,as well as seed crystals of different sizes,have different degrees of reduction in the calcium fluoride induction period,with particle sizes.The effect of supersaturation and impurities on the induction period was studied,the increase in the amount of addition can effectively reduce the induction period,the impact of supersaturation and impurities on the induction period,indicating that the higher the degree of supersaturation of the solution,the smaller the induction period.For impurities,MgCl₂ and CuSO₄ have similar effects on the induction period,which can shorten the induction period slightly.For NH₄Cl,although Cl^-can increase the activity of F^-,the hydrolysis of NH₄⁺can reduce its activity and increase its induction period.The influence of rare earth impurity ions on the supersolubility of calcium fluoride is studied as well.It is found that the existence of impurity ions increases the induction period of calcium fluoride,because the existence of rare earth ions can form stable compounds with fluorine ions.The fluorine ion is not easy to precipitate.Study on treatment of wastewater containing fluoride by fluidized Bed crystallizer.The seeds used are quartz sand and calcite.The experimental results show that the use of fluidized bed crystallization technology to remove fluoride ions in the solution can reduce the concentration of fluoride ions to 6 mg/L.For different seed crystals,different particle sizes,the addition amount will change to the fluoride ion in the aqueous solution.The concentration has changed.Characterization of the obtained crystals by SEM,XRD,particle size distribution,although affecting the crystal size distribution,does not change the internal structure of the calcium fluoride crystals.When quartz sand was selected as the seed crystal,obtaining to calcium fluoride crystal particles more uniform,the effect of fluoride removal will be better,calcium fluoride crystal particles were more uniform and the size was mainly 1.5?m to 2?m,and the removal rate and recovery rate of negative ions were 95.73%and 92.2%,respectively.