Theoretical And Experimental Research On New Technology For Resource Utilization Of Bastnaesite

Rare earth elements play an important role in the transformation of traditional industries and the development of new industries.They are also an important non-renewable strategic resource of our country.Bastnaesite is one of the most important rare earth minerals currently known in the world;which is the main source of light rare earth,therefore,the study of the decomposition and smelting process of bastnaesite is very important for the rare earth industry.As an associated ore of fluorine and rare earth,the separation and purification of rare earth has been seriously affected by the strong electronegativity and coordination of fluorine,and the corresponding resource and environmental problems have been highlighted.The "oxidation roasting-sulfuric acid leaching-solvent extraction" process is considered to be the most advanced technology for smelting of bastnaesite.However,a series of problems such as the third phase,waste of resources and environmental pollution are produced due to the existence of fluorine,which hinder the development of the process.In this paper,based on the "oxidation roasting-sulfuric acid leaching-solvent extraction" process,aiming at efficient utilization of fluorine and rare earth,the fundamental research such as thermodynamic properties of fluorine-bearing rare earth sulfate solution,as well as the theory and practice of separation and recovery of fluorine and rare earth were conducted,so as to provide a theoretical basis for the development and application of new purification separation technology for fluorine and rare earth with high efficiency,low energy consumption and environmental protection.It is considered to have important theoretical value and practical significance in the new technology of resource utilization of bastnaesite.First of all,the interface properties of system of HDEHP/HEHEHP and(Ce4+-H2SO4)/(Ce4+-F-H2SO4)were studied.The interfacial tension data were fitted with Szyzkowski isotherm and Polynomial isotherm,and the interfacial adsorption parameters such as maximum adsorption excess ?max,minimum concentration of extractant in the bulk phase at the saturated interface Cmin and molecular boundary area of saturated adsorption Ai under different conditions were calculated,so as to provide a theoretical basis for the extraction of Ce4+ and F-and the formation of the third phase.The results showed that the interfacial activity of HDEHP was higher than that of HEHEHP,and the introduce of F-ions could increase the interfacial activity and the extraction capacity.The pysicochemical properties of Ce(SO4)2-H2SO4-H2O and Ce(SO4)2-HF-H2SO4-H2O systems were studied systematically.The solution density,refractive index and conductivity were measured,and the data were fitted to obtain the relation between density,refractive index and conductivity with electrolyte concentration and temperature.The apparent molar volumes of solutions were calculated.The activity coefficients of H2SO4 were determined with EMF method using a reversible cell assembled with a pH glass electrode and a Hg/Hg2SO4 electrode,and the experimental data were fitted with Pitzer interaction model.The Pizter ion interaction parameters were obtained,then,the activity coefficients of Ce(SO4)2+ and CeF22+,as well as the osmotic coefficients of solution were calculated.The results showed that the density and refractive index of Ce(SO4)2-H2SO4-H2O and Ce(SO4)2-HF-H2SO4-H2O systems were directly proportional to electrolyte concentration,and were inversely proportional to temperature.The changing law of conductivity of Ce(SO4)2-H2SO4-H2O system with electrolyte concentration and temperature was a nonlinear relationship with several variables.The complexation of F" and Ce4+could reduce the conductivity of solution.The activity coefficient of CeF22+ was higher than that of Ce(SO4)2+ in sulfuric acid solution,showing that the introduce of F-could promote the extraction of Ce4+.Using the ions such as Al3+,B3+ and Zr4+ which have strong coordination with fluorine as fluorine complexing agent,the coordination chemistry with F-in sulfuric acid solution was studied using fluorine ion selective electrode method,and the cumulative stability constants were determined.The complexation mechanism of fluorine complexing agent and fluorine in F-Ce-B,F-Ce-Al and F-Ce-Zr sulfuric acid solutions were researched,and the existence condition of free rare earth ions in the presence of fluoride complex ions were explored so as to provide a theoretical basis for the separation of fluoride and rare earth with fluorine complexing agent.The results showed that the Ce-F and B-F could form a four coordinated complex,and Al-F and Zr-F could form a six coordinated complex.The stability constants of B-F system were higher than that of Ce-F system,and the stability constants of high coordination number complexes of Al-F and Zr-F were much higher than that of Ce-F system.The complexation mechanism of[CeFx]4-x and[BFy]3-y were studied by potentiometric titration.The results showed that the F-and Ce4+ existed as CeF22+ complex ion,and the B3+ could capture F-in CeF22+ to form BF4-.The complexation mechanism of F-Ce-Al system was studied by conductivity titration method.When n(F-)/n(A13+)was lower than 2,the complex ion with low coordination number was not stable.A variety of aluminum fluoride complex with different coordination number could form in sulfuric acid medium,and the coordination number was determined by the concentration of F-and n(F-)/n(Al3+)in solution.The exploratory experiment of separation of fluorine/cerium in fluorine bearing rare earth sulfate solution by selective adsorption using hydrous zirconium oxide was carried out,and the Raman,FT-IR and XPS spectra suggested that an ion-exchange reaction between the hydroxyl ion on hydrous zirconium oxide and fluoride was involved.The adsorption capacities of fluoride and cerium both increased with an increase in solution pH,and the optimum pH was suggested to be 0.3-0.6.The loss of cerium was higher at a low n(F-)/n(Ce4+)ratio.High initial fluoride concentration was unfavorable for the separation.The accompanied rare earth ions had no significant influence on the adsorption of fluoride.The bastnaesite was roasted with aluminum compound as anti fluorine agent,andthe decomposition kinetics was studied by thermal analysis method,then the fluorine and rare earth were leached by sulfuric acid,and the parameters affecting the leaching rates of fluorine and rare earth such as roasting temperature and time,n(Al3+)/n(F-),sulfuric acid concentration,leaching temperature and leaching time were investigated.The fluorine could be separated from rare earth by extraction,and was recovered through preparation of cryolite.The results showed that using Al(OH)3 as additive could effectively promote the decomposition of bastnaesite.The thermal decomposition reaction order was 0.82,and the decomposition activation energy was 265.463kJ/mol.The optimum conditions of "complexation roasting-sulfuric acid leaching" process were determined as n(Al3+)/n(F-)1/2,roasting temperature 500 ?,roasting time 1h,sulfuric acid concentration 3mol/L,leaching temperature 90 ? and leaching time 1h,at which the leaching rates of F,Ce and total RE reached 92.71%,98.92%and 98.57%.The fluorine and aluminum was recovered as cryolite with the recovery rates of fluoride and aluminum as 96.33%and 97.38%.The stripping kinetics Ce4+ with HEHEHP was studied with laminar constantinterfacial cell method,and the effects of stripping acid,stirring speed,temperature,specific surface area,Ce4+ concentration in loaded organic phase,free extractant concentration in organic phase,HCl concentration and H2O2 concentration on the stripping rate were investigated.The control model and the control step of stripping process were deduced.The stripping kinetic equation was derived,and the mechanism of stripping process was also dicussed.The results showed that the stripping process was controlled by diffusion and interfacial chemical reaction,and the apparent activation energy Ea is 22.40kJ/mol.The reductive stripping kinetic equation of Ce4+ was R=10-3.66[Ce](o)1.09[HEH/EHP](o)-1.04[H+](a)0.99[H2O2](a)0.53,and the apparent rate constant k was 10-3.66(mol-0.57·L0.57)/min.The extraction of Ce4+ from sulfuric acid solution by emulsion liquid membrane using HDEHP as carrier was investigated.The influences of HDEHP,Span80 and liquid paraffin concentrations,HCl and H2O2 concentrations in inner aqueous solution,volume ratio of membrane phase to internal phase,volume ratio of external phase to membrane phase,external phase acidity,extraction time and stirring speed on extraction rate were investigated in order to select the ideal extraction system and condition factors.The separation of Ce4+ and RE3+ in external phase and the effect of fluorine on extraction process were also studied.The results showed that the optimum conditions for Ce4+extraction could be summarized as:HDEHP concentration 12%(v/v),Span80 concentration 2?3%(v/v),liquid paraffin concentration 2?4%(v/v),HCl concentration 4?5mol/L,H2O2 concentration 0.02mol/L,external phase acidity 0.4?0.5mol/L,volume ratio of membrane phase to internal phase 1.5,volume ratio of external phase to membrane phase 2,extraction time 15min and stirring speed 250rpm,at which the extraction rate of Ce4+ could reach 98%.The emulsion liquid membrane system had obvious selectivity to Ce4+ in Ce4+-RE3+ mixed solution.The introduce of A13+ in internal phase could promote the extraction of Ce4+ and F-,and the introduce of Al3+ in external phase could realize the separation of Ce4+ and F-as F-could not be extracted.The demulsification was conducted by ultrasonic treatment,and the recovery of Ce4+ was higher than 97%.