Study On Mechanism And Technology Of Preparing Ultrafine Yttrium Oxide By Precipitation With Sodium Carbonate

Compared with ordinary yttrium oxide,ultrafine yttrium oxide has better properties of light,heat resistance and chemistry,and has higher price,so it is widely used in the field of high-tech materials.At present,most of the preparation processes of ultrafine yttrium oxide are restricted by the problems of poor product quality,environmental pollution and high equipment requirements,so it is difficult to achieve mass production.The sodium carbonate precipitation process has the advantages of low cost,pollution-free and simple operation,which is easy to realize industrialization.However,rare earth precipitated by sodium carbonate is not easy to crystallize,and it is difficult to obtain ultrafine yttrium oxide powder with good dispersion without the introduction of dispersant.In view of the above problems,this paper took yttrium chloride as the research object,used sodium carbonate as the precipitant,studied the precipitation crystallization process and roasting process of yttrium carbonate in detail,obtained the precipitation crystallization mechanism and roasting mechanism of yttrium carbonate,developed the deposition preparation technology of ultrafine yttrium oxide,and finally obtained ultrafine yttrium oxide with good dispersion.The mechanism of precipitation and crystallization can provide theoretical reference for the preparation of other crystalline rare earth carbonate.The development and implementation of this technology has an important role in reducing the backlog of common yttrium oxide,improving the performance of rare earth functional materials and product added value,and promoting the upgrading of China’s rare earth industry.The main research contents and results are as follows:FBRM-PVM can track the changes of particles and image information in situ,and can visualize the crystallization process.In this paper,the process and mechanism of precipitation and crystallization of yttrium carbonate were studied by FBRM-PVM.It was found that the crystallization process of yttrium carbonate was a dynamic equilibrium process from induction nucleation to crystal growth to Ostwald ripening at 80 ^oC and pH of 5.9-6.0.At this time,precipitation products with certain crystallinity could be obtained.In order to further increase the crystallinity of yttrium carbonate,the aging process and seed crystals circulation process of yttrium carbonate were studied.The aging process did not play a positive role in the crystallization of yttrium carbonate,but the existence of shear nucleation and contact nucleation phenomenon could reduce the average particle size in the system.In the process of seed crystals circulation,the nucleation of seed crystals could promote the crystal transformation and accelerated the Ostwald ripening.The crystal growth process was nucleation→needle like and flake like growth→needle like and flake like crystals aggregate into spherical like crystals.Finally,the crystal yttrium carbonate with 12.0μm particle size and uniform dispersion was obtained under the condition of one seed crystals cycle.On this basis,the effect of precipitation conditions such as the concentration of feed solution on the particle size of yttrium carbonate was investigated,and the best precipitation process was obtained.Finally,the high crystallinity Y₂（CO₃）₃·2H₂O with D₅₀=14.8μm,(D₉₀-D₁₀)/2D₅₀=0.64 was obtained under the conditions of parallel feeding,yttrium chloride concentration of 1.0 mol·L^-1,reaction temperature of 40 ^oC,pH of 5.5-6.5,feeding speed of 10ml·min^-1 and stirring speed of 600 r·min^-1.The obtained yttrium carbonate was gradually dissolved and dispersed by stirring aging,and the needle like and flake like spherical aggregate crystals were gradually transformed into small particles with uniform dispersion.Under the condition of 40 ^oC,after 48 h of stirring aging,the ultrafine yttrium carbonate with impurity content lower than 100 ppm,particle size D₅₀=3.77μm and particle size distribution(D₉₀-D₁₀)/2D₅₀=0.70<1 was obtained by natural filtration washing with 1500 mL of constant temperature water.In order to realize the heredity of particle size and other properties in the roasting process of ultrafine yttrium carbonate and prevent agglomeration,the thermal decomposition process and kinetics of the ultrafine yttrium carbonate（Y₂（CO₃）₃·2H₂O）obtained above were studied.It was found that the thermal decomposition process was divided into three stages.The first stage:when the roasting temperature was lower than 350 ^oC,the free water and crystal water were lost,and Y₂（CO₃）₃ was formed.The apparent activation energy E of thermal decomposition was between 45.07 and 103.19 kJ·mol^-1.The pre exponential factor A was 3.68s^-1,and the order of reaction was about 1.8.The second stage:when the roasting temperature was 350 ^oC⁵46 ^oC,Y₂（CO₃）₃ was lost two CO₂,and Y₂O₂CO₃ was generated.It was controlled by the equation of mechanism function Avrami Erofeev（n=4）,and the corresponding decomposition function was g（α）=[-ln（1-α）]⁵.The apparent activation energy E was 385.44kJ·mol^-1.The pre exponential factor A was 7.71×10²22 s^-1,and the order of was about 1.The third stage:when the roasting temperature was higher than 546 ^oC,Y₂O₂CO₃ continued to lose a CO₂,and Y₂O₃ was generated.It was controlled by the equation of mechanism function Avrami Erofeev（n=1/3）,and the corresponding decomposition function was g（α）=[-ln（1-α）]^1?3.The apparent activation energy E was 505.44 kJ·mol^-1.The pre exponential factor A was 3.95×10²⁸s^-1,and the order of was about 1.Finally,guided by the above thermal decomposition mechanism,through the adjustment of the roasting conditions,it was found that the ultrafine yttrium carbonate was dried at 50 ^oC for 24 h,and then held at 200 ^oC for 3 h—350 ^oC for 1h—550 ^oC for 2 h under 9 ^oC·min^-1,the ultrafine yttrium oxide powder with D₅₀=3.00μm,(D₉₀-D₁₀)/2D₅₀=0.89,and small particles of 40-50 nm in diameter aggregated on the crystal surface was obtained.It was consistent with the thermal decomposition mechanism of Y₂（CO₃）₃·2H₂O.