Investigation on the Mechanism of Rare Earth Oxide Prepared by Ultrasonic Spray Pyrolysis of Variable Valence Rare Earth Chloride Solution
Author:Xue Shou Feng
Supervisor:wu wen yuan
Green and clean metallurgical technology is an effective ensure for the sustainable development of metallurgical industry.To date,the preparation of rare earth oxides is mainly completed by precipitation and roasting method in industry scale,which it is known as simple principle and low technical operation.However,this method usually need to pay a higher price for the huge energy consumption,environment pollution and destruction of ecosystem.Hence,it is urgent to develop a clean and efficient technology for the production of rare earth oxide.On the basis of previous research,a foundment research was carried out to development a short process technology that the rare earth oxides was prepared by ultrasonic atomization pyrolysis of rare earth chloride solution,particularly,the thermal deomposition of variable valence rare earth element chloride such as cerium,praseodymium and terbium were investigated carefully.Some important conclusions were described in concise as following.The static thermal decomposition mechanisms of CeCl3·7H2O,PrCl3·6H2O and TbCl3·6H2O were investigated respectively by using the analysis technology of XRD、FESEM、EDS and TG-DTA/DSC-MS.The results showed that dehydration reaction completed at 224,234 and 284℃,the hydrolysis product were Ce(OH)3,Pr(OH)2Cl and Tb(OH)2Cl,Ce(OH)3 would be oxided slowly and give CeO2 in air atmosphere,the dehydration of Pr(OH)2Cl and Tb(OH)2Cl would lead to the formation of PrOCl and TbOCl respectively.And the oxidation reaction were observed that the oxidation of CeCl3 leads to the formation of CeO2 above 460℃,the oxidation of PrOCl leads to a few amount of Pr12O22 formed above 1000℃ and the oxidation of TbOCl also lead to the formation of Tb7O12 partially above 800℃.The thermal decomposition behavior of rare earth chloride was investigated by using the rare earth chloride aerosol as precursor.The resutls showed that CeCl3 aerosol could completely convert to CeO2 at 600℃,the formation of PrO2 were observed by pyrolysis of PrCl3 aerosol above 700℃ that different with the static thermal decomposition behavior,the formation of TbO2 also observed by pyrolysis of TbCl3 aerosol at 600℃.More amount of rare earth oxide was formed at the higher pyrolysis temperature of rare earth chloride aerosol(Pr,Tb).The preparation of rare earth oxide by ultrasonic spray pyrolysis:the amount of residual chlorine in cerium oxide increases with the increase of solution concentration,and decreases with the increase of carrier gas velocity,and decreases with the increase of pyrolysis temperature,the solution temperature diplayed little effect on the amount of residual chlorine in the product.The particle size of the prepared cerium oxide increases with the increase of the solution concentration,and decreases with the increase of the solution temperature,and slightly effect of the pyrolysis temperature and no significant effect of carrier gas velocity on the particle size were observed.Furtherly,the residual chlorine in cerium oxide could be ascribed to the incomplete dissociation of residual chlorine in the pyrolysis process and the absorption of Cl2/HCl.The residual chlorine content in the sample can be minimized by roasting in air atmosphere.The preparation of rare earth oxide by enhancement ultrasonic spray pyrolysis of rare earths(Pr,Tb)chloride:(1)The H2O2 assisted ultrasonic spray pyrolysis of PrCl3 solution for the preparation of praseodymium oxide.PrO2 could be prpepared when the concentration of PrCl3 solution is 0.05mol/L and with addition of 30%H2O2 at 1000℃.The prepared praseodymium was PrO2.below 1100℃,but it was Pr12O22 and PrO1.99 above 1100℃.And the formation of Pr(OH)2Cl was unavoidable during the enhancement ultrasonic spray pyrolysis of praseodymium chloride,more amount of Pr(OH)2Cl would found at lower pyrolysis temperature in the product.(2)The H2O2 assisted ultrasonic spray pyrolysis of TbCl3 solution for the preparation of terbium oxide.The results shown that the pure terbium oxide could be prepared under the condition of TbCl3 solution concentration is 0.05～0.1 mol/L and the concentration of H2O2 is 5%.The TbO1.81 phase could be obtained at 800 and 900℃,Tb7O12 could be obtained at 1000 and 1100℃.The morphology and structure formation mechanism of rare earth oxide particle:the formation of solid spherical particle is due to the longer residence of atomized droplet in low temperature zone and high temperature zone respectively.On the other hand the formation of hollow structure particle is ascribed to the short residence of atomized droplet in the tube furnace.The collisons with supersaturated droplets and unsaturated droplets or molten droplets with hollow structures and other molten droplets can result in the formation of porous cerium oxide particles.