Fate Of Phosphorus In Phosphate Rock Under Na₂-EDTA And Heat Treatment And Recovery Of Fluorine And Iodine In Wet Phosphoric Acid Process

China is rich in phosphate rock reserves,but most of them are low-grade phosphate rock,and most of the phosphate rock needs to go through a complex beneficiation process before it can be utilized.The surge in demand for phosphorus in agriculture and the scarcity of high-grade phosphate rock in nature have led to serious challenges for the fertilizer industry.Therefore,the improvement of phosphate rock grade and the release of inert phosphorus are imminent.Phosphate rock often contains a certain amount of iodine.China’s iodine resources are scarce,and a large amount of iodine needs to be imported from abroad every year.Therefore,the recovery of iodine in the phosphorus chemical production process is of great significance.Phosphate rock has more fluorine content.The wet phosphoric acid process often produces a large number of fluorine-containing substances such as Si F₄ and HF,and the recovery of these fluorides can not only reduce environmental pollution,but also obtain high value-added material resources.The research content of this thesis consists of the following three parts:1.Determination of phosphorus and iodine in phosphate rock by acid digestion,distillation and ion chromatography.A new method was developed for determination of phosphorus（P）and iodine（I）in phosphate rock.Firstly,the phosphate rock was digested with HCl solution to generate H₃PO₄.Then the remaining HCl was evaporated by distillation,the generated H₃PO₄ reacted with Na Ac to generate HAc,and HAc was evaporated by distillation finally.The escaping gases including HI,I₂,HCl and HAc were absorbed by a diluted solution of Na OH and Na₂SO₃.Ac^-and I^-in the absorption solution were determined by ion chromatography（IC）and the content of P in phosphate rock was confirmed according to the Ac^-determination.The results showed that the optimal operating parameters were respectively 4.0 mol L^-1 HCl,50℃and 35 min for the acid digestion of phosphate rock,160℃and 40 min for HCl evaporation,and 140℃and 40 min for H₃PO₄ reaction and HAc evaporation.The correlation coefficients of P and I in this method were good（R²>0.999）,the relative standard deviations（RSDs）of the measured values and the recoveries of P and I ranged from 1.28%to 7.21%and from 91.90%to 98.62%,respectively.This method exhibits suitable accuracy and precision,with simple and mild operating conditions and is good for determining P and I in phosphate rock.2.Na₂-EDTA combined with heat treatment promotes the release of phosphorus and the improvement of grade in phosphate rock.In this study,low-grade phosphate rock（PR）was first heat treated with tubular furnace,and then PR and its calcination residue（CPR）and pyrolysis residue（PPR）were treated with Na₂-EDTA solution.The results show that the CPR and PPR grades obtained by heat treatment alone do not increase much.Then the Na₂-EDTA solution was added to CPR and PPR,and the results showed that more phosphorus was released and the phosphate rock grade was also greatly improved.Compared with the extraction of PR with Na₂-EDTA solution,calcining combined with Na₂-EDTA can effectively promote the release of phosphorus and improve the grade of PR.Pyrolysis combined with Na₂-EDTA is beneficial to improve PR grade but not to phosphorus release.Therefore,the extraction of Na₂-EDTA solution after PR heat treatment can effectively release phosphorus and improve grade,which can provide new ideas for alleviating the demand for phosphorus in agriculture.3.The release and recovery of fluorine and iodine in the production and concentration of wet-process phosphoric acid（WPA）from phosphate rock.In this work,the phosphate rock was digested with H₂SO₄,and the remaining H₂SO₄ in the WPA was removed with Ca（H₂PO₄）₂,then the WPA was concentrated under vacuum and different temperatures.Fluorine and iodine in the absorption solution were recovered in the forms of Ca F₂ and I₂,respectively.The results showed,under the optimal digestion condition of phosphate rock,the release rate of phosphorus was 97.21%in WPA.In gas and liquid phase,the mass percentages were 1.35%and 94.87%for fluorine,and 65.21%and 29.89%for iodine,respectively.In the concentration of WPA under 15-17 k Pa,the release rates were 89.79%for fluorine at 80-130^oC and 62.44%for iodine at 80^oC.Recovery rates and purity were 93.26%and 97.24%for fluorine,85.63%and 99.47%for iodine,respectively.Therefore,this method was simple,low consumption,high recovery and purity for fluorine and iodine.