Rare Earth Element Contents and Occurrence Forms in Weathering Crust Ion Adsorption Rare Earth Ore

The weathering crust ion adsorption type rare earth deposit has the characteristics of complete REE types, low radioactivity, high heavy rare earth contents, and easy mining, making it an extremely important type of rare earth deposit. At present, most of the research on the determination of REEs in this deposit only measures the total amount and ionic phase components of REEs, and some methods have complicated operating procedures, while there is relatively little research on the analysis of various REE forms. However, when studying the mineralization laws of weathering crust ion adsorption type rare earth deposits, it is not only necessary to analyze the total amount of REEs, but also to analyze the chemical forms that affect the mineralization background, migration, enrichment process, long-term differences, and availability of REEs in the deposit. Therefore, it is necessary to accurately determine the total amount and contents of various occurrence REE forms in weathering crust ion adsorption type rare earth deposits. In this article, a five acid mixed digestion was used as a pretreatment method for determining the total amount of REEs, and the results of rare earth elements speciation analysis determined by BCR method and Tessier method were compared. ICP-MS was used as the detection method to explore the distribution of the total amount and various REE forms in the samples. The results show that the five-acid mixed digestion can completely dissolve all REEs in the rare earth ore sample, and the operation is simple. The precision (RSD) of this method was between 0.82% to 5.19%, the detection limit was between 0.002g/g to 0.027g/g, and the relative error between the measured value and the recognized value of each element was between −4.70% to 6.65%. The ∑LREEs/∑HREEs was between 1.25 to 16.50, covering the enrichment of light rare earths and heavy rare earths. The relative deviation (RD) of rare earth forms extracted by the two methods of BCR and Tessier was 0.79% to 8.07%, and the extraction results correspond and match each other. The rate of recovery was between 84.75% to 107.13%. The RD of ∑REEs determination values was between 0.62% to 21.00%, and the relative error (RE) was less than 40%. In contrast, the BCR method has a simpler pre-processing flow, but the partitioned forms are not as intuitive and specific as the Tessier method, which cannot be used to obtain more detailed data on each form. Thus, under the conditions of this experiment, the Tessier method may have good adaptability for rare earth form analysis. REEs in the fully weathered layer samples of the weathering crust mainly exist on the surface of clay minerals in the form of ion adsorption, resulting in the highest content of ion exchange state. As the depth of the weathering crust profile increases and the pH rises, REEs are more likely to bind with carbonate or bicarbonate ions, leading to an increase in the rare earth contents in the carbonate bound state. Elemental Ce is easily oxidized from Ce³⁺to Ce⁴⁺ in this layer, and stays in situ in the form of precipitation, resulting in Ce anomaly; REEs in the parent rock are mainly enriched in the independent mineral lattice, with the highest residual content. The variation patterns of all REEs under different occurrence forms are basically consistent.