Function And Environment Oriented Precipitation And Crystallization Technologies Of Rare Earth Compounds

Tuning the chemical property indexes of the rare earth compounds, reducingwastewater discharge are key research fields in the rare earth industrial manufacture.The present study focuses on the controlling of precipitation and crystallizationprocess for tuning the particle properties and chloride content of rare earth carbonatesor oxalates.The main results include:The phase transformation of lanthanide carbonates RE2(CO3)3·8H2O of La, Ce,PrNd to their subcarbonate REOHCO3are realized by the hydrolysis reactions in hotand alkaline solution. The results of scanning electron microscopy(SEM), X-raypowder diffraction(XRD) and chemical analysis of samples such as La2(CO3)3·8H2O,Ce2(CO3)3·8H2O,(PrNd)2(CO3)3·8H2O and their REOHCO3show that the phasetransformation occurs in neutral and alkaline solution at temperature higher than80℃,and the transfer percentages are increased with the increase of temperature, solutionpH and aging time. With the phase transformation conducting, the solution pH fallssignificantly at first and reaches a stable value finally. Correspondingly, the largeaggregates are dispersed into many single particles, which results in the decrease ofparticle size D50, particle size distribution D90-D10, the chloride content, and theincrease of bulk density of powders. Therefore, a novel method to prepare rare earthbasic carbonates with high bulk density, low chloride content, fine particlecharacterization is developed, with which the particle characterization of finalproducts can be easily tuned by changing the crystalline phase transformationconditions.The formation conditions of crystalline rare earth carbonates by the directreaction of solid RE2(SO4)3and NH4HCO3in aqueous solution containing a certainamount of free rare earth ions and (NH4)2SO4were investigated. The limitedconcentration of ammonium sulfate for the transfer of RE2(SO4)3to crystalline rareearth carbonates were determined. It is demonstrated that the limited concentration ofammonium sulfate depends on the solubility of their sulfates, and the addition of acertain amount of rare earth sulfate with higher solubility can break out the concentration limit of ammonium sulfate for the sulfates with lower solubility andcan obtain high concentration of ammonium sulfate solution after filtrating out thecrystalline rare earth carbonates, which provides possibility for recoveringammonium sulfate with low cost. The mother liquor with high concentration ofammonium sulfate can easily be treated by slaked lime, producing ammonia andcalcium sulfate. The clear solution after filtrating solid calcium sulfate can be re-usedin above transfer crystallization. The calcium content in the obtained rare earthcarbonate is only0.1%.Crystalline cerium carbonates were prepared by a gas-liquid-solid multiphasereaction with carbon dioxide, base and cerium chloride as materials. The results ofXRD, SEM and chemical analysis of the obtained samples show that assuming molarration of RE3+to OH-at1:3and reaction time1h, temperature at60℃or90℃, withthe increase of CO2partial pressure, the transfer percentage of cerium hydroxide tocerium carbonates as well as the cerium basic carbonate to cerium carbonate are allincreased. At the same time, the particle morphology changed from sphere to rod. Sothe final products are the mixture of cerium basic carbonate and cerium carbonate.However, assuming a low CO2partial pressure at0.04MPa, alkali amount90%of thetheoretical value, reaction temperature at60℃for1h, pure cerium basic carbonatewith sphere morphology and monodisperse particle size can be prepared.The variation of particle size and chloride content of rare earth carbonate andoxalates caused by the useless of ultrasonic or additives were investigated. It is foundthat ultrasonic in the feeding process of precipitation can improve the disperseproperty of precipitates or crystals and then lower the chloride content in finalproducts. Meanwhile, the addition of a certain amount of oxalate acid or ammoniumcitrate tribasic can also speed down the crystallization rate and reduce the chloridecontent in final products. Therefore, a novel method for preparing rare earthcompounds with fine particle size and low chloride content is suggested.