Low Concentrations Of Rare Earth Precipitation And Crystallization Of Ammonium Bicarbonate Solution Studies

For the advantages of simple operation and low cost and so on, NH4HCO3precipitationprocess has become a main method of extracting rare earth from low-concentration solution,especially ion-adsorption rare earth leaching solution. But it resulted to some problems thatprecipitation volume is large, difficult to filter and preparation because rare earth carbonatesolubility is very small and so easy to precipitate amorphous flocks, Aimed at the above practicalproblems and difficulties, based on the relationship between rare earth carbonate precipitationand crystallization process and pH change of equilibrium solution, and combining with moderntest methods such as SEM, laser particle size analyzer and XRD, rare earth carbonateprecipitation and crystallization process was systemically studied, and also studied the optimumforming conditions and influence factors. The crystallization reaction mechanism was discussedand the specific method of crystal rare earth carbonate production was also proposed.Taking low-concentration rare earth chloride solution as raw materials, the precipitation andcrystallization process of rare earth carbonate was systemically studied. The results indicated thatthe process had correspondence with equilibrium solution acidity. Along with pH value declining,amorphous flocks gradually transformed to crystal forms. The greater the declining degree, thebetter crystallization, and the faster the declining degree, the faster the crystallization speed.Theresults of crystallization reaction mechanism of rare earth carbonate showed that there was acrystallization reaction of consuming HCO—3and releasing H+in the process, involving crystalnucleating, crystal growth, coagulation and aging and so on. It was a dynamic equilibrationprocess of crystal nucleating then dissolve and recrystallization. So pH declining was proposed toevaluate rare earth carbonate characteristics.The major factors influencing crystal process were found out through study on crystalforming conditions and influence factors. The results indicated that the main factors influencingcrystal process were molar ratio of NH4HCO3and RE3+, rare earth concentration, aging time andimpurities content, etc. Light rare earth carbonate was easy to crystal in low molar ratio ofNH4HCO3and RE3+, too high and low concentration could cause negative impact on crystalprecipitates. Over aging time, precipitate volume might decrease and particle might becomelarger and amorphous flock precipitates gradually transformed to crystal forms. At the same time,the impurities content not only had an effect on production quality, but also could reduce crystalactivity of rare earth carbonate, especially aluminum and iron ions. With crystal activity of rareearth carbonate was decreased with the increase of ions content, and the crystal speed was sloweddown until difficult to form crystal precipitates. The key to production of crystal precipitates from practical rare earth leaching solution wasrare earth concentration in leaching solution and impurities content. Speed-control trickleleaching process was adopted for rare earth ore in Xunwu and the major parameters wereoptimized, a better index with extraction rate of83.21%, rare earth concentration in leachingsolution of2.26g/L was obtained in the conditions that water content0%,(NH4)2SO4concentration5%, leaching speed0.42mL/min. Rare earth leaching solution was conductedfurther purification treatment. On the premise that rare earth loss of4.43%, it could obtainimpurities-removed solution with rare earth concentration of2.15g/L, Al3+and Fe3+content of3.48mg/L,0.26mg/L respectively.On the basis of study on precipitation and crystal process, crystallization reactionmechanism, preparation of crystal rare earth carbonate was studied with practicalimpurities-removed solution as raw materials. As was shown from the results, conventional agingcrystal-transformed method could generally form non-crystal rare earth carbonate, but add aamount of fresh seed crystal or sesbania gum surfactant in the crystal process, could promotecrystal transformation to obtain crystal rare earth carbonate with the advantages of easy-filtration,big particle and good crystal characteristics.