Enrichment And Fractionation Of Rare Earth Elements In Iron Oxid-clay Mineral Composite

Recently,a super-sized weathered crust elution-deposited rare earth ore with the features of high elevation and thick weathered crust development was found in the southern Yunnan Province,China.Several researchers have done a lot of study surrounding these types of mineral deposits in response to this finding,however there are still diverse viewpoints and insufficient research on the occurrence status,migration,and enrichment behavior of rare earth elements in this mining area.There have been fewer studies on rare earth resources with low grade and difficult extraction in various regolith parts.Based on the shortcomings,it is proposed in this paper to investigate the physical and chemical properties of weathered profiles,the fractionation mechanism of clay mineral-iron oxide complexes on the occurrence status of rare earths,and the adsorption mechanism of iron oxides and their complexes on rare earths in weathered profiles,using the typical weathered profiles of three typical granite weathered crust elution-deposited rare ore in Lincang,China.To comprehend rare earth migration and metallogenic regularity,as well as the role of iron-oxide-clay mineral composite in the enrichment and fractionation of rare earth elements.The following are the primary research findings and contents:The granites in the research region,such as porphyritic medium-grained and coarse-grained biotite monzogranite,weathered and leached under favorable environmental circumstances,and minerals such as potassium feldspar and biotite experienced secondary alteration,generating clay minerals such as kaolinite.The 2:1 clay minerals illite and montmorillonite progressively converted into 1:1 kaolinite and eloxite in the lower section of the whole regolith,eventually undergoing desilication and aluminum enrichment by oxidative leaching.The distribution of major elements in weathered profile minerals in the study area is characterized by high Silicon（64.10-74.40 wt%）,high Aluminum（15.50-20.20wt%）,low Calcium（0.04-0.05 wt%）and low Magnesium（0.45-0.90 wt%）distribution.The overall quantity of rare earth ranges between 238.12 and 1545.53 ppm,with the maximum LREEs enrichment degree in the middle and upper regolith.The content of HREEs increases with depth.The distribution of rare earth elements in the weathered crust profile is high in the middle and low in the upper and lower parts.The enrichment degree is the highest in the whole regolith,and the upper part is enriched with LREEs and the lower part is enriched with HREEs.Except for the independent rare earth minerals（residual state）,most of the rare earth elements released during the weathered process are in the ion exchange state and the two types of iron mangane oxide states,with just a trace in the carbonate and organic bound states.The content of ion-exchanged rare earth increases progressively with depth,reaching up to 327.73 ppm and 819.96 ppm,respectively,accounting for 54.25%and 56.40%of the total rock content.They are more abundant in the middle and lower parts of the weathered profile and show a declining trend with iron and manganese oxidation state.Amorphous iron manganese oxides play a key role in the fractionation and enrichment of rare earths in weathered profiles,and their proportion in the total rock is even higher than of ion-exchange state rare earths.The LREEs are substantially richer in amorphous iron manganese oxides,and Ce element is a positive anomaly with more than 80.00%Ce component.The proportion of amorphous iron manganese oxides state rare earth in weathered profiles is greater than that of crystalline iron manganese oxides state rare earth,owing to amorphous iron manganese oxides larger specific surface area and increased adsorption ability for rare earth elements.However,compared with amorphous iron mangane oxides state rare earth,the crystalline iron mangane oxides state rare earth is significantly enriched in HREEs,indicating that crystalline iron mangane oxides have stronger selective adsorption for HREEs.The LREEs tend to be adsorbed by clay minerals,which results in the formation of ion-exchange state with obvious enrichment of LREEs and weathered crust elution-deposited rare earth ore with significant rare earth fractionation effect.The adsorption process of rare earth ions on iron oxides and their complosite follows the Langmuir model（R²>0.96）,indicating that the adsorption of rare earth ions on iron oxides and their complosite belongs to monolayer uniform adsorption.The higher the degree of crystallization of iron oxide,the more stable it is.The adsorption difference of three REEs in crystalline Fe₂O₃-kaolin complosite is greater than that in amorphous Fe OOH-kaolin complosite.These results indicate that amorphous iron-oxide-clay mineral complosite has better complexation adsorption and enrichment ability for rare earth ions,but crystalline iron-oxide-clay mineral complosite has a significant effect on the fractionation and enrichment of rare earth.The increase of reaction temperature and p H is conducive to the adsorption of iron oxide and its complosite to rare earth ions,and Ce element is more sensitive to ionic strength and p H conditions in solution.The complexation of rare-earth elements by crystalline iron-oxides is selective to HREEs.The rare earth ions are mainly adsorbed on the surface of iron oxides by inner ring complexation.During the adsorption process,the Zeta potential on the surface of iron oxide and its complosite increases with the increase of solution p H.The higher the Zeta potential,the more cations adsorbed in the diffusion layer.Fe OOH-kaolin complosite has the highest concentration of oxygen vacancy and has higher chemical reactivity,which further confirms that amorphous iron-oxide-clay mineral complosite has better complexation adsorption effect and stronger enrichment ability for rare earth ions.