Occurrence state, occupying sites and coordination structure of rare earth elements in phosphorite

Rare earth elements (REEs) are associated with phosphorite, which is an important strategic reserve resource. During sorting process of phosphorite, REEs may move with specific host minerals, however, occurrence state and moving pattern of REEs from rock to products are still unclear, which limits separation and enrichment of REEs from phosphorite. Mappings of scanning electron microscope (SEM) and electron probe X-ray micro-analyzer (EPMA) of REEs are highly consistent with those of calcium and phosphorus, and complementary with that of magnesium, which indicates that fluorapatite (Fap) is the main host mineral of REEs. The results of flotation and leaching experiments further indicate that REEs are enriched along with Fap from phosphorite to products. Occupied sites and occupation number of REEs were obtained by X-ray diffraction (XRD) refinement based on the Rietveld method. La, Ce, Nd, and Y can occupy both Ca1 and Ca2 sites. The ratios of La, Ce, Nd, and Y at Ca2 and Ca1 sites are 4.20, 3.70, 3.00, and 1.33, showing a decreasing trend, indicating that La, Ce, and Nd tend to occupy Ca2 sites, while Y tends to occupy Ca1 sites. X-ray absorption fine structure (XAFS) shows that REEs mainly form coordinate structures with oxygen and fluorine, which is a direct evidence that REEs replace calcium(II) in phosphorite in an isomorphism form. Coordination structure and polyhedral configuration analysis indicate that substitution degree of La, Ce, Nd, and Y is Y > La > Ce ≈ Nd from easy to difficult at Ca1 and Ca2 sites. The research enriches the mineralization theory of REEs-bearing phosphorite and provides certain theoretical guidance for selective enrichment of REEs from phosphorite.