Study On Theory And Process Of Recovery REES From Phosphoric Concentrate By Phosphoric Acid Leaching

The REES(Rare Earth Elements)in phosphate ore is an important resource after the ionic REES is found in southern china.The REES in phosphate ore occurs in an isomorphous way,which is completely different from the northern China and the ionic REES in southern China.The existing extraction and separation techniques for the conventional rare earth ore cannot economically handle the phosphorus associated rare earth resources.Due to the low grade of REES in phosphate ore,the extraction of REES has no economical advantage;it has to be combined with the phosphoric acid production.More than 80%of phosphoric acid by wet process was produced through the sulphuric acid method.With the sulphuric acid method,the REES scatters and the process is complex with high costs.Therefore,a highly efficient,low energy consuming,environmental friendly extraction technique for phosphorus associated rare earth resources has great significant for development and utilization of this kind of resources.In order to solve the problems existing in the process of the extraction of rare earth from phosphate ore,this paper has firstly proposed a new method to handle the REES containing phosphate ore by phosphoric acid process.Based on the mineralogy studies,a detailed study on the thermodynamics,kinetics,separation of rare earth in high-phosphorus complex solution and the phase evolution of rare earth in the leaching process was carried out to obtain the basic theory and technology of phosphoric acid process.The mineralogy studies show that the major phase in phosphate concentrate is collophane,calcite,dolomite and quartz followed by a small amount of limonite,clay minerals and fluorite.Almost all of collophane occurs as single elements,a small amount collophane is combined with calcite,dolomite,and quartz.Rare earth in phosphate rock mainly exists in the similar form of isomorphy(collophane)with about 95%of total rare earth and independent minerals(about 5%),and trace rare earth is in the form of ion adsorption(less than 0.1%).The thermodynamic data is theoretically summarized such as the standard Gibbs free energy,standard enthalpy,entropy and heat capacity of salts which are composed by rare earth cations and HPO42-,H2PO4-,F,Cl-and SO42-anions at 25 ? and 0.1 MPa in solution.The equilibrium constants of REES ions and inorganic ions are calculated and the equilibrium constant of REES and F-is more than the constant of REES and H2PO4-;in phosphoric acid leaching process,the leaching ratio of REES is benefit for increasing the concentration of F-;The Eh-pH graphs,such as RE-H2O system,RE-PO43--H2O system,RE-SO42--H2O system and RE-SO42--PO43--H2O at 298K,are drawn.Comparing with RE-H2O system,after adding the phosphoric acid and sulfuric acid,the rare earth phosphate stable region becomes narrow while a large new stable region is formed in which a new complex rare earth,phosphor and sulfur containing compound is the major phase.The experimental results show that,under the leaching conditions:retention time 8h,temperature 65?,phosphoric acid concentration 23%,82%of particles under granularity 400 mesh,ratio of liquid to solid 10,stirring speed 300rpm,initial Ca2+concentration 0g/L and initial SO42-concentration 0g/L,the extraction rates of REES and P2O5 have reached 85.4%and 96.4%respectively from the phosphoric concentrate which has contents of 31.92%P2O5,50.48%CaO,0.13%REES;At those experimental conditions:retention time 8h,temperature 65 ?,acid concentration 25%,82%particles under granularity 400 mesh,ratio of liquid to solid 10 and stirring speed 300 rpm,the extraction rates of REES and P2O5 have reached 90.5%and 96.2%respectively;The pilot tests were carried out with phosphoric acid(AR)and cycled acid from WPA factory,the leaching results are consistent with results in laboratory.By means of thermodynamic calculations,XRD,SEM,EDS,TEM and mineralogical microscopes,phase evolution is investigated in the leaching process of REES concentrate in phosphoric acid;the reaction mechanism of phosphoric acid process is determined,in which the phosphorus,calcium and rare earth are extracted into the solution with the form of dihydrogen phosphate salt and the fluorine stays in the slag in the form of crystal-like calcium fluoride.The research on kinetics of leaching with phosphoric acid has shown that the reaction process is controlled by surface chemical reactions and follows the "unreacted core shrinking model".The reaction activation energy is 44.6kJ/mol.The apparent reaction order is 0.993.By the experimental data,the kinetic equations about the single factors are set up such as granularity,temperature,acid concentration and initial calcium concentration.By SPSS software,the kinetic mathematical model for REES leaching process is 1-(1x)1/3=te8.596-5045.215/T+0.109ln[C]+28.687ln[r0]+0.082ln[CCa]Research shows that the absorption rate of REES in the high-phosphorus complex solution is 82.9%at a temperature of 30?,adsorption time 240min,liquid to solid ratio 100;the static resin adsorption capacity of 16.8mg/g,the dynamic adsorption capacity of 18.7mg/g;after desorption of the loaded resin with hydrochloric acid,oxalic acid precipitation and calcination,the obtained REO(rare earth oxide)has a content of 46.72%of the mixed rare earth oxide.In summary,this dissertation has proposed the phosphoric acid process for the extraction of rare earth and provided a new way for the development and utilization of the phosphorus associated rare earth;it has important academic significance and practical application value.