Theoretical Investigation Of The Difference For Light And Heavy Rare Earth Adsorption/Desorption On Clay Surface

Ion-adsorption rare earth ores is an important strategic resource for China,in which the rare earth is mainly adsorbed onto clay minerals such as kaolinite,halloysite,illite and montmorillonite in the form of ions.The mining technology of ion-adsorption rare earth ores in China is leading in the world,and a lot of research has been carried out in the optimization of exploite,leaching agent,recovery rare earth,etc.,and rich research results have been achieved.However,there are few attention on the influence of clay gene minerals on the leaching process at present,and similar leaching processes are used for all type of ion-adsorption rare earth minerals,resulting in the low leaching rate and the underutilization of rare earth resources and some of rare earth resource can not be exploited.Whereas,"genetic mineral processing engineering"has been widely recognized in the field of mineral processing.In this article,the adsorption/desorption characteristic of light/heavy rare earth on kaolinite,halloysite,illite and montmorillonite were studied,the molecular dynamic and density functional theory were applied to investigate the mechanism of light/heavy rare earth adsorbed on kaolinite,which is helep for the leaching of ion-adsorption rare earth with different clay gene.The main results of this paper are as follows:（1）The adsorption/desorption behavior of RE ions from kaolinite,halloysite-7?,illite and montmorillonite surface were investigated,and the mechanism was clarified from the perspectives of crystal structure and surface properties of clay minerals.Due to the strongest deprotonation ability of kaolinite,when it is cleaved along the most common（001）surface,RE ions can undergo stable inner-adsorption on the deprotonated surface,which need to be completely desorbed by the synergistic action of H⁺and NH₄⁺ions.Hence,the desorption rate of RE ions in kaolinite decreases significantly with the increase of pH value of the leaching agent.Halloysite-7?curls inward in the way of aluminum hydroxyl group to form a tubular structure,which prevent rare earth ions to be adsorbed with inner-layer adsorption,so that the desorption rate of Nd³⁺,Eu³⁺and Lu³⁺ions is almost not affected by leaching pH.On the other hand,the both of（001）surface of illite and montmorillonite are silicon-oxygen layer,while only a small amount of aluminum hydroxyl groups are exposed on the broken edges can adsorb rare earth ions by inner-layer adsorption.Hence,the change of rare earth desorption rate with leaching pH from illite and montmorillonite is not as large as that of kaolinite.（2）Molecular dynamics and first principles methods were used to build the model of competitive adsorption of Eu³⁺（light rare earth）and Lu³⁺（heavy rare earth）ions on kaolinite（001）surface,and the microscopic mechanism of competitive adsorption of Eu³⁺and Lu³⁺ions was elucidated from thermodynamic and kinetic perspectives.Because the first hydrate number（6）and first hydrate peak（2.23?）of Eu³⁺ions are smaller than those of Lu³⁺ions（7,2.45?）,so that they preferentially occupy the inner-layer adsorption sites,while Lu³⁺ions can only exist in the outer-layer adsorption form,that is,the adsorption of Eu³⁺is kinetically superior.Whereas,those Eu³⁺ions need to be completely desorbed by the synergistic action of H⁺and NH⁴⁺ions.First-principles simulation results show that the adsorption energy of Eu³⁺and Lu³⁺ions is comparable,that is,there is no obvious difference for the adsorption of Eu³⁺and Lu³⁺ions in thermodynamics.Therefore,when a single Eu³⁺and Lu³⁺ion is adsorbed on the deprotonated kaolinite surface,the desorption rates of both decrease sharply with the increase in the pH value of the leaching agent.（3）The desorption behavior of RE ions on kaolinite,halloysite-7?,illite and montmorillonite under different leaching systems was investigated.The desorption effect of NH₄⁺ions on rare earths from those four clays was generally better than that of Mg²⁺ions,but when the concentration of leaching agent was lower,Mg²⁺ions was slightly better than that of NH₄⁺ions.The desorption efficiency of anion is:SO₄^2->Cl^->NO₃^-.（4）A prototype of classification enhanced leaching technology of rare earth based on ionic rare earth clay mineral genes was constructed.When the main occurrence mineral of rare earth ions is kaolinite,low leaching agent pH can increase the leaching rate of rare earth by 6.8%compared with the traditional process,while the leaching rate of aluminum ions is only increased by 3%.If the main occurrence mineral of rare earth ions is halloysite,reduce the leaching agent pH,the rare earth leaching rate is only increased by 3.9%,but the leaching of aluminum ions is increased by 24.2%,so this type of ion-adsorption rare earth ore should be leached under low acidity.