Multistage Counterflow Leaching of Weathered Crust Elution-deposited Rare Earth Ore
The leaching of weathered crust elution-deposited rare earth ore was mainly achieved by ion exchange reactions between leaching agent ions and rare earth ions adsorbed on the surface of clay.Using(NH4)2SO4 as leaching agent,the leaching process of weathered crust elution-deposited rare earth ore was studied by a multi-stage counterflow leaching experiment.The leaching behaviors of rare earth ions(RE3+)and aluminum ions(Al3+,the major impurity)were investigated based on the composition variations of the leachates from each stage,and the reaction mechanisms of ammonium ions(NH4+)with both RE3+and Al3+as well as amount of net adsorption of the ore were discussed according to consumption of leaching agent.The results show that the con-centration of both RE3+and Al3+in the leachates gradually increases in the stage-by-stage leaching of the fresh ore along with the consumption of NH4+,however the amount of them exchanged into the leachates in a single leaching gradually decreases with stage.Meanwhile,sulfate ions(SO42-)in the leachates decrease stage-by-stage,indicating that besides NH4+consumption through ion exchange reactions there could also be net adsorp-tion of NH4+to the fresh ore.The ratio of adsorption sites in the fresh ore for rare earth,aluminum and net ad-sorption was calculated to be about 20∶41∶39.During the two series of counterflow leaching experiments,leach-ing of Al3+always behaves similar to that of RE3+and does not show correlation with pH variation of the leach-ates(from 4.5 to 4.0).In addition,the analysis shows that under pH of the leaching reactions RE3+is the domi-nant specie of rare earth while Al3+,AlOH2+,and Al(OH)2+are the main species of aluminum,thus it is in-ferred that the ion-exchange stoichiometric ratios of NH4+to rare earth ions and to aluminum ions should be close or equal to 3∶1 and 1.8∶1 respectively according to mass balance during the leaching process.
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