Research On The Isomorphism Mechanism Of Rare Earths In Zhijin Phosphorite In Guizhou

The Zhijin phosphorus deposit in Guizhou Province is a marine sedimentary phosphorus deposit with light rare earth（LRE）and heavy rare earth（HRE）components,which is a reserve RE resource.The research on isomorphic substitution of RE in phosphorite can clarify the occupied site,occupied number,occupation tendency of LRE and HRE in corresponding minerals of phosphorite and the controlling factors affecting RE entering into the lattice of fluorapatite.It has important theoretical significance for the recovery and utilization of RE in Zhijin phosphorite and recognition of enrichment and mineralization of RE in marine sedimentary phosphorus deposit.This research takes Zhijin phosphorite and synthetic RE-doped fluorapatite as the research objects.Firstly based on process mineralogy and chemical phase analysis,using metallographic microscope,X-ray diffraction（XRD）,X-ray fluorescence spectrometer（XRF）,inductively coupled plasma mass spectrometer（ICP-MS）,scanning electron microscope and energy spectrometer（SEM-EDS）,time-of-flight secondary ion mass spectrometer（TOF-SIMS）,electron probe micro-analysis（EPMA）and other methods to preliminarily determine the occurrence of RE in phosphorite from a macroscopic perspective;then based on the X-ray absorption fine structure（XAFS）of synchrotron radiation,the occupation sites and coordination structures of La,Ce,Nd and Y in phosphorite and RE-doped fluorapatite are measured,and the occurrence of RE in phosphorite is confirmed to be isomorphic substitution from the atomic level;further the XRD refinement is used to determine the number of La,Ce,Nd and Y occupying the Ca1 and Ca2 sites of fluorapatite,and to clarify the distribution of RE in the Ca sites.Based on the density functional theory（DFT）,the substitutional energies,Mulliken charge distribution and bond distribution,charge density,and differential charge density of RE at the Ca1 and Ca2 sites in fluorapatite are calculated.Finally,the mechanism of isomorphic substitution of RE is revealed from the perspectives of electronegativity,spatial configuration and bond valence.The following main understandings and conclusions are obtained:（1）The phosphorite contains all the RE except Pm,and the content ofΣRE is 1011.49μg/g;the content of La,Ce,Nd,Y is 219μg/g,127.3μg/g,158.5μg/g and 327μg/g,respectively.The correlation coefficients ofΣRE with P₂O₅ and Cao are 0.9516 and 0.9498,respectively.According to SEM-EDS,TOF-SIMS and EPMA analysis,La,Ce,Nd,Y mainly occur in the phosphorite in the form of isomorphism.La,Ce,Y occasionally have independent minerals,and the corresponding independent mineral particles are very small（visible at the resolution of5μm）,and the distribution range is small.La and Ce independent minerals may be monazite,Y independent minerals may be yttrium phosphorite,and no independent minerals are found in Nd.There are trace La,Ce,Nd,Y in dolomite,which is lower than that in fluorapatite.The contents of La,Ce and Nd are similar,and the content of Y is low.RE is not enriched in quartz,organic matter,aluminosilicate,hematite and other minerals.A small amount of RE is distributed in dolomite occasionally.Most of RE occurs in Ca in fluorapatite.（2）XAFS shows that La,Ce,Nd have four coordination shells,and Y has five coordination shells;La,Ce,Nd coordination numbers at Ca1 site and Ca2 site are 9 and 7,respectively;Y coordination number at Ca1 and Ca2 site are both 6;the valence state of La,Ce,Nd,Y in phosphate rock is close to+3;La,Ce,Nd,Y all replace Ca in fluorapatite in the form of isomorphism,and there are substitutions at Ca1 and Ca2 site.（3）The results of XRD refinement show that La,Ce,Nd and Y occupy the Ca1 and Ca2 sites of fluorapatite,which is consistent with the XAFS test results.The occupation numbers of La,Ce,Nd and Y at Ca1 site are 0.005,0.003,0.004 and 0.024 respectively,and those of La,Ce,Nd and Y at Ca2 site are 0.021,0.011,0.012 and 0.032 respectively.The crystal structure formula of fluorapatite is deduced as Ca_4.7La_0.073Ce_0.039Nd_0.044Y_0.144(P_2.811Si_0.189)O₁₂F.The ratios of La,Ce,Nd,Y at Ca2 and Ca1 sites are 4.20,3.70,3.00 and 1.33,showing a decreasing trend.La,Ce,Nd tend to occupy Ca2 site,while Y tend to occupy Ca1 site.The RE occupation number and occupation tendency of La,Ce,Nd,Y at Ca2 and Ca1 sites in RE doped fluorapatite are similar to those in phosphorite.（4）The substitution energy of RE at Ca1 and Ca2 sites is generally"oblique W shape"and has obvious"gadolinium（Gd）disconnection".The change trend of RE substitution energy at Ca1 and Ca2 sites is consistent.With the increase of atomic number,the substitution ability of LRE at Ca1 and Ca2 sites increase gradually,and reach the maximum at Gd.The substitution law of HRE at Ca1 and Ca2 site does not change monotonically.It is necessary to judge the substitution rule by combining the coordination ability of atoms and spatial configuration.When Ca1 and Ca2 are substituted by RE,the corresponding coordination polyhedron is distorted.The distortion of HRE at Ca1 site is smaller and that of LRE at Ca2 site is smaller.Finally,HREs tend to replace Ca1 and LREs tends to replace Ca2,which is consistent with the results of XRD refinement.（5）The isomorphic substitution of RE at Ca site of fluorapatite is mainly controlled by spatial regulation.In order to form a stable coordination structure,the effective coordination number of HRE is changed to 6,and the volume of coordination polyhedron is reduced to 1/2 of Ca1coordination polyhedron.After adjustment,the HRE is closer to the stable structure of 6coordination,with smaller steric hindrance,and tends to replace at Ca1 site;the effective coordination number of LRE and HRE at Ca2 site are 5.8 and 6.3 respectively,but the volume difference between HRE and Ca2 is much larger than that between LRE and Ca2,so LRE is more likely to replace at Ca2 site.The substitution ability of LRE increases gradually with the increase of electronegativity at Ca site of fluorapatite,while the substitution ability of HRE does not change monotonically.The difference between the average bond valence of RE single bond and that of Ca1 single bond at Ca1 site is 0.111.In order to neutralize the crystal charge,the LRE tends to replace at the Ca2 site,while the HRE tends to replace at the Ca1 site.