Discussion on Pretreatment Method for Extracting Rare Earth Elements from Weathered Crust Elution-deposited Rare Earth Ores

Weathered crust elution-deposited rare earth ores are the dominant mineral resources in China^1-5. The occurrence of rare earth elements (REEs) in weathering crust elution-deposited is very complicated. At present, the “ionic” rare earth extraction process can only use the “exchangeable adsorption state” rare earth elements, that is, the “ionic phase” rare earth elements, and other phase rare earth elements cannot be effectively recycled⁶. How to effectively dissolve various forms of rare earth elements in ion-adsorbed rare earth ore is very important to improve the utilization rate of rare earth resources.

To compare the extraction efficiency of REEs in weathered crust elution-deposited rare earth ores by different pretreatment methods and discuss the influencing factors.

Five pretreatment methods, being open mixed acid digestion (HCl+HNO₃+HF+HClO₄+H₂SO₄), nitric acid digestion with or without hydrogen peroxide, hydrochloric acid digestion with hydrogen peroxide and ammonium sulfate leaching, were used to extract REEs in six rare earth samples from the Nanling area. The content of REEs in samples was determined by ICP-MS.　　For ion-adsorbed rare earth minerals, the open digestion method of five acids can replace the complicated alkali melting method for the determination of REEs in samples under certain conditions. Nitric acid, hydrochloric acid digestion or ammonium sulfate leaching can only dissolve part of REEs in ion-adsorbed rare earth samples.

The amount of REE extracted by different pretreatment methods is quite different. The results of mixed acid digestion (five acids) are the highest; the results of nitric acid with or without hydrogen peroxide digestion, hydrochloric acid and hydrogen peroxide digestion are similar. The amount of REEs dissolved by nitric acid or hydrochloric acid digestion is slightly lower than mixed acid digestion. The amount of REEs dissolved by ammonium sulfate leaching is the lowest. The amount of REEs dissolved by 50% nitric acid, hydrochloric acid and other digestion methods accounts for 75.3%-91.7% of the total phase REEs, and the amount of REEs dissolved by ammonium sulfate leaching (ionic phase rare earth) only accounts for 9.1%-5.5% of the total phase REEs.　　The extraction rate of scandium (Sc) is much lower than that of total REEs in nitric acid and hydrochloric acid digestion, and Sc can not be dissolved by ammonium sulfate leaching. There is no correlation between the extraction rate of cerium (Ce) and other REE, the total extraction rate of light REE or the total extraction rate of REEs.　　The difference of the results by different pretreatments is closely related to the occurrence state of REEs in the samples. Mixed acid digestion can completely destroy the structure of the sample, and all the REEs in the sample can be dissolved. The REEs are differentiated and enriched in weathering crust layers. Some REEs still exist in the form of mineral facies after weathering. The content of REEs in ionic phase is closely related to weathering degree and mineral composition of each layer of weathering crust. Proportion of REEs content in different parts of weathering crust is not the same. Hydrochloric acid or nitric acid can dissolve REEs in the ionic state adsorbed in clay minerals or iron and manganese oxides, as well as REEs in the form of carbonates, phosphates, etc. Some REEs exist stably in silicate mineral lattices that cannot be completely dissolved by nitric or hydrochloric acid. Ammonium sulfate leaching can only dissolve the ionic REEs in the sample, so the digestion result of hydrochloric acid and nitric acid is lower than that of the whole phase REEs, and higher than that of ammonium sulfate leaching.

The differences and influencing factors of REEs extracted from ion-adsorbed rare earth samples by different pretreatment methods are discussed, which can provide reference for further research on the extraction methods of REEs from elution-deposited rare earth ores. Mixed acid digestion can extract the full phase REEs in the weathered crust eluviated rare earth ore sample, which can be used to evaluate the total amount of REEs in the weathered crust eluviated rare earth ore. Nitric acid digestion, nitric acid and hydrogen peroxide digestion, hydrochloric acid and hydrogen peroxide digestion cannot dissolve REEs in the silicate structure completely. Therefore, this method is suitable for evaluating the content of REEs in the form of ionic states, oxides, carbonates and phosphates in samples. Ammonium sulfate leaching can be used to evaluate the content of ionic phase REEs in weathering crust leaching type rare earth ores.　　The extraction rate of REEs is greatly affected by the chemical characteristics and occurrence state of REEs. Because the ionic radius of Sc³⁺ is obviously smaller than that of other REEs, Sc³⁺ can enter the crystal lattices of many rock-forming minerals in the form of homogeneity, which results in that ammonium sulfate leaching cannot dissolve Sc. Only a small amount of Sc can be dissolved by hydrochloric or nitric acid digestion. The different enrichment-differentiation characteristics of Ce and other REEs also make the extraction rate of Ce by ammonium sulfate leaching inconsistent with other REEs. The REEs with similar ionic radii often have similar extraction efficiency in the same pretreatment.