Influence of Rare Earth Occurrence States on Extraction Rate in Weathered Crust Elution-Deposited Rare Earth Ores

BRIEF REPORTSignificance: The occurrence state of rare earth elements (REEs) in weathered crust elution-deposited rare earth ores is highly complex and can change under external forces such as mechanical grinding[18-20]. This alteration reduces the content of exchangeable adsorbed REEs and may affect the accuracy of laboratory-based rare earth resource evaluations. By analyzing the amount of exchangeable adsorbed REEs and the proportions of different occurrence states in this type of rare earth sample before and after grinding, significant changes were observed. During mechanical grinding, the contents of REEs in the ion-exchange state and water-soluble state decreased, while those in the carbonate-bound, iron–manganese oxide-bound, and humic acid-bound states increased.Introduction: Currently, ammonium sulfate in situ leaching is the primary process utilized in weathered crust elution-deposited rare earth mining areas. Rare earth ions that can be replaced by NH4+ are referred to as exchangeable adsorbed REEs, which are the only form that can be extracted by current rare earth processing technologies[17]. To accurately assess and fully utilize rare earth resources in such deposits, the study of the occurrence states of REEs has attracted increasing attention from researchers[21-22]. The continuous sequential extraction method has been applied to the study of the occurrence state of REEs in weathered crust elution-deposited rare earth ores, in order to explore the laws of rare earth element migration, transformation, and enrichment, which is of great significance in rare earth mineralization background and resource utilization[26-27].　　To investigate the effects of mechanical grinding on the occurrence states of rare earth elements, two samples from a weathered crust elution-deposited rare earth ore in southern China were selected. The total REEs content in the samples was analyzed by mixed acid digestion followed by inductively coupled plasma-mass spectrometry (ICP-MS). The occurrence states of REEs were examined through ammonium sulfate leaching and sequential extraction. Furthermore, the relationship between ammonium sulfate leaching rates and REEs occurrence states was preliminarily discussed. This study provides technical support for establishing evaluation standards for weathered crust elution-deposited rare earth ores and for improving the utilization efficiency of rare earth resources.Methods: The samples G1 and G2 used in the experiment were initially obtained as rare earth-enriched ores of different particle sizes, mainly composed of quartz, rock fragments, and clay minerals such as illite and kaolinite. A vibration ball mill was used to grind the samples to a particle size of less than 80 mesh. The ground samples were then analyzed for total REEs content. Ammonium sulfate leaching and sequential extraction studies were performed on the samples before and after grinding.　　The mixed acid (HCl + HNO3 + HF + HClO4 + H2SO4) digestion method is based on the classic four-acid (HCl + HNO3 + HF + HClO4) digestion method, with the addition of a small amount of H2SO4, which can effectively increase the temperature of the acid-driving process. At the same time, the solution does not evaporate completely during the acid-driving process, which is conducive to the decomposition of insoluble rare earth fluorides and can avoid the precipitation of rare earth fluorides as much as possible. For ion adsorption type rare earth ores, the five-acid digestion method replaced the complex alkali melting method under certain conditions for determining the total amount of REEs in samples[29]. Moreover, the results of national standard reference materials (GBW07160 and GBW07161) were within the limit of reference.　　The rare earth ions adsorbed on the surface or in the interlayer of weathered crust elution-deposited rare earth ores are in the form of ions or hydroxyl hydrated ions, which can be replaced by NH4+ during ammonium sulfate leaching and thereby enter the solution. The ammonium sulfate leaching method was used to analyze the content of exchangeable adsorbed REEs in the sample, before and after grinding. The Tessier seven step continuous extraction method was used to analyze the contents of REEs in the water-soluble state, ion-exchange state, carbonate-bound state, humic acid-bound state, iron–manganese oxide-bound state, strong organic-bound state, and residual state of the samples, before and after grinding.Data and Results: Compared with the sample before grinding, the total amount of REEs leached by ammonium sulfate decreased by 21.5% in G1 and by 51.6% in G2. The leaching rate of cerium (Ce) in these samples was significantly lower than that of the other light rare earth elements (LREEs).　　The occurrence state of REEs also changed after grinding. In G1, the enrichment of REEs (excluding Ce) in different occurrence states was in the following order: ion-exchange state > carbonate-bound state > residual state > humic acid-bound state ≈ iron–manganese oxide-bound state > strong organic-bound state > water-soluble state. After grinding, the content of ion-exchange state REEs in G1 significantly decreased, while the content of REEs in the carbonate-bound state, humic acid-bound state, and iron–manganese oxide-bound state increased to varying degrees. Regardless of before or after grinding, Ce in G1 mainly existed in the form of the residual state and the iron–manganese oxide-bound state. In G2, the enrichment sequence of REEs was very different. Among the LREEs other than Ce, the content of the carbonate-bound state was the highest, followed by the ion-exchange state and the residual state, with a small amount in the iron–manganese oxide-bound state and the humic acid-bound state. The content of the strong organic-bound state was even lower, and the content of the water-soluble state was extremely low. Heavy rare earth elements (HREEs) mainly existed in the form of the residual state and the carbonate-bound state, followed by the iron–manganese oxide-bound state, the humic acid-bound state, and the ion-exchange state, with lower levels in the strong organic-bound state and very small amounts in the water-soluble state. After grinding, the content of REEs in the ion-exchange state decreased, while the content of REEs in the carbonate-bound and humic acid-bound states increased, and there was no significant change in other states. Whether before or after grinding, Ce in G2 mainly existed in the form of the residual state and the iron–manganese oxide-bound state.　　Comparing the ammonium sulfate leaching rates of REEs and the proportion of REEs in different occurrence states, it can be observed that the ammonium sulfate leaching rate of REEs other than Ce is positively correlated with the sum content of the ion-exchange and iron–manganese oxide-bound states, and negatively correlated with the sum content of the carbonate-bound and residual states. The results indicate that the REEs in ion-exchange and iron–manganese oxide states are important sources for exchangeable adsorbed REEs. The higher the content of REEs in carbonate bound and residual states, the lower the leaching rate of ammonium sulfate.