Migration And Distribution Of Associated Elements Of Phosphorus Ore In Nitrophosphate Process Using Freezing Method

Phosphorus rock is a non-renewable resource,which is mainly composed of fluorapatite and accompanied with some important elements such as fluorine,iodine,rare earth,etc.It is mainly used as a raw material for the producing phosphoric acid and phosphate fertilizer.At present,in the process of decomposed phosphate rock to produce phosphoric acid by sulfuric acid,the research about the distribution and extraction technology of associated resources of phosphorus rock has mature development.However,the process of using nitric acid to decomposing phosphate rock is different from using sulfuric acid,that has the characteristics of low reaction temperature,oxidability,and freezing crystallization for calcium removal,but there is less research about the distribution of associated resources of phosphorus rock.Therefore,in the range of nitrophosphate process control parameters,this paper explored the migration behavior and distribution of associated elements of fluorine,iodine,and rare earth elements in the operation unit of acidolysis,freezing crystallization,and ammonia neutralization processes.It provides technical support for the comprehensive utilization of associated element resources of phosphorus rock in the production of nitrophosphate.Firstly,HSC thermodynamic was used to calculate and ananlyze the reaction thermodynamic of nitric acid decomposing phosphate rock,The results showed that the reaction of Ca₅（PO₄）₃F in phosphate rock and nitric acid mainly generated calcium nitrate,phosphoric acid and hydrogen fluoride.The products of reaction of rare earth elements and nitric acid mainly existed in the solution in the form of REEH₂PO₄²⁺and REE³⁺.The HF produced by the reaction of phosphate rock and nitric acid reacted with SiO₂ of the phosphate rock easily,so the fluorine in the acidolysis solution mainly existed in the form of HF and SiF₆^2-.Iodine is oxidized to I₂ by nitric acid in the acidolysis process,so the main existence form of iodine during acidolysis was I₂.Secondly,the effects of acidolysis process parameters,including acidolysis temperature,nitric acid concentration,acidolysis ratio and acidolysis time,on the decomposition rate of P₂O₅ in phosphate rock and the migration and distribution of fluorine,iodine and rare earth during the acidolysis were studied.The results indicated:with the increasing of acidolysis temperature,the decomposition rate of P₂O₅ in phosphate rock,the leaching rate of rare earth,and the distribution rate of fluorine in liquid phase showed a trend of rising followed by decreasing,and the distribution rate of iodine in gas phase showed an increasing trend.With increasing nitric acid concentration,the decomposition rate of P₂O₅ in phosphate rock,leaching rate of rare earth,and the distribution rate of fluorine in the liquid phase in the phosphate rock decreased gradually,and the distribution rate of iodine in the gas phase gradually increased.With the increasing of acidolysis ratio,the decomposition rate of P₂O₅ in phosphate rock,the leaching rate of rare earth,and the distribution rate of fluorine in liquid phase increased gradually and the distribution rate of iodine in gas phase showed a trend of rising followed by decreasing.With the increase of the acidolysis time,the decomposition rate of P₂O₅ in phosphate rock,leaching rate of rare earth,the distribution rate of iodine in the gas phase increased gradually and the distribution rate of fluorine in the liquid phase showed a trend of increasing followed by decreasing.The acidolysis parameters were controlled at acidolysis temperature was 60°C,w（HNO₃）55%,acidolysis ratio was 1.25:1,acidolysis time was 120 min,the decomposition rate of phosphate rock was 96.76%,leaching rate of rare earth was 95.65%,the distribution rate of iodine in the gas phase was 65.21%,the distribution of fluorine in the liquid phase was92.89%.Thirdly,the effects of the freezing crystallization process parameters including endpoint temperature and crystallization time on the calcium removal rate and the migration and distribution of fluoride and rare earth during the acid solution were studied.The results showed that with the decreasing of the crystallization endpoint temperature and the prolongation of the crystallization time,the calcium removal rate increased and the distribution rate of rare earth and fluorine in the liquid phase decreased;The freezing crystallization process parameters were controlled at the endpoint temperature of crystallization was-2°C,and the crystallization time was 120 minutes,the calcium removal rate was 81.49%,the distribution of rare earth in the liquid phase was 95.47%,and the distribution rate of fluorine in the liquid phase was 60.13%.At last,the effects of ammonia neutralization process parameters including pH value,neutralization temperature and time on the precipitation yield of rare earth and the composition of neutralized sediment was studied.The results showed that the precipitation yield of rare earth increased with the increasing of pH,the prolongation of neutralization time and the rising of neutralization temperature.The ammonia neutralization parameters were controlled at pH=1.7,neutralization time was 180min,neutralization temperature was60℃,and rare earth recovery rate in the crystallization mother solution could reach 87.81%.The precipitate mainly consists of NH₄H₂PO₄,Ca₂（NH₄）H₇（PO₄）₄·2H₂O and Ca（H₂PO₄）₂,and REEs are present both in REEPO₄·nH₂O and coprecipitation with calcium phosphate as REEnCam（PO4）_{（3n+2m）/3}.