In order to reveal the influence of ammonium salts on the rare earth leaching process of weathered crust elution-deposited rare earth ores, ammonium acetate, ammonium chloride, and ammonium sulfate were used as leaching agents. The effects of the leaching agent on the rare earth leaching efficiency and the expansion, dissolution, and transformation behavior of clay minerals in the rare earth leaching process were studied. The results showed that rare earth leaching efficiency followed the order ammonium acetate > ammonium chloride > ammonium sulfate, with values of 90.60%, 85.96%, and 84.12%, respectively. The swelling ratio of clay mineral followed the order ammonium acetate < ammonium chloride < ammonium sulfate; the clay mineral swelling ratio was 2.09% when ammonium acetate was the leaching agent. Thermogravimetric analysis showed that the interlayer water content was the lowest when ammonium acetate was used as the leaching agent. Under the conditions of different leaching agents, the clay mineral contents changed from illite and halloysite to smectite and kaolinite. When ammonium acetate was used as the leaching agent, the relative conversion of illite was 1.49%, and that of smectite was only 0.17%. SEM analysis showed that the clay minerals expanded and dissolved obviously when ammonium chloride and ammonium sulfate were used as the leaching agents. Meanwhile, the clay mineral layered structure was relatively complete when ammonium acetate was used as the leaching agent. Therefore, when ammonium acetate was used as the leaching agent, it had the least effect on the swelling, dissolution, and transformation of clay minerals. This can provide a theoretical basis for the safe production of weathered crust elution-deposited rare earth ore, and for the screening of green and efficient leaching agents.

To reveal the regularity of variation in the rare earth occurrence states of weathered crust elution-deposited rare earth ores, ore samples from different weathering crust layers were obtained by performing the sequential extraction procedure. The order of rare earth contents firmly obeyed the following sequence: the weathered layer > humic layer > partly weathered layer. The occurrence states of rare earth elements were mainly the ion exchange state, carbonate bound state, iron–manganese oxide state, organic binding state and residual state. The proportions of rare earth elements found in the rare earth ion exchange state of the weathered layer, humic layer and partly weathered layer were 78.55%, 73.53% and 53.88%, respectively. The light rare earth elements (LREEs) found in the rare earth ion exchange state were enriched in the upper part of the weathering crust, while the heavy rare earth elements (HREEs) were enriched in the lower part. There were also obvious negative anomalies in the content of cerium in the ion exchange state. The content of rare earth elements found in the carbonate bound state was small, and the rare earth partition pattern was basically consistent with that of the ion exchange state, which had little effect on the differentiation of the rare earth elements. The iron–manganese oxide state was mainly enriched with cerium, and the content of cerium increased with the depth of the weathering crust. The iron–manganese oxide state was the main factor causing the phenomenon of the anomaly in the cerium content. Meanwhile, the iron oxides in the iron–manganese oxide state were mainly hematite and goethite. The organic binding state mainly beneficiated yttrium and cerium by complexation and certain adsorption. The content of elements found in the rare earth residual state was related to the degree of weathering and reflected the release sequence of rare earth elements in the mineralization process. Clarifying the rare earth occurrence states is conducive to better revealing the metallogenic regularity of weathered crust elution-deposited rare earth ores. In addition, the results can provide a valuable reference for expanding the available rare earth resources and the efficient comprehensive utilization of rare earth ore.

Weathered crust elution-deposited rare earth ores (WCE-REOs) are the primary global source of medium and heavy rare earth elements (M/HREEs). The recent discovery of high-altitude (1500–2500 m) WCE-REOs in southern Yunnan Province, China, presents new opportunities for the development of M/HREE resources. This study investigates the enrichment and fractionation mechanisms of rare earth elements (REEs) in these deposits through a systematic analysis of three representative weathering profiles associated with the Lincang granite batholith. The analytical results indicate that the profiles consist mainly of clay minerals (kaolinite, halloysite, illite, minor montmorillonite) and iron oxides, with high SiO₂ (64.10–74.40 wt.%) and Al₂O₃ (15.50–20.20 wt.%) and low CaO/MgO—typical of weathered REE deposits. The total REE contents (238.12–1545.53 ppm) show distinct fractionation: LREE-enriched upper layers and HREE-enriched deeper zones. Sequential extraction revealed that the REEs in the Lincang granite weathering profiles predominantly occur in ion-exchangeable, residual, and iron-manganese oxide-bound states (>95% total REEs). Ion-exchangeable REEs showed depth-dependent enrichment (peaking at 819.96 ppm), while iron-manganese oxides exhibited a strong REE affinity (up to 47% total REEs), with amorphous phases that were preferentially enriched in Ce (partitioning >80%). Fissure systems exerted critical control over the redistribution of elements, particularly REEs.

This paper investigates the rare earth (RE) leaching process of weathered crust leached rare earth ores (WCE-DREOs) with different ore particle sizes. Ore particles were categorized into seven distinct size ranges: −0.106 mm, +0.106 mm to −0.15 mm, +0.15 mm to −0.25 mm, +0.25 mm to −0.85 mm and +0.85 mm. The study examined the correlation between the thickness of the electrical double layer (EDL) and the RE content, as well as the swelling ratio of clay minerals. The findings demonstrated that the order of RE content with respect to varying particle sizes was as follows: ${\mathrm{C}}_{-0.106}>{\mathrm{C}}_{+0.106~-0.15}>{\mathrm{C}}_{+0.15~-0.25}>{\mathrm{C}}_{+0.25~-0.85}>{\mathrm{C}}_{+0.85}$ (C represents RE content). The RE content of full particle size was found to be 7.93 mg/g, with the particle size −0.106 mm exhibiting the highest RE content of 2.02 mg/g. In the fitting of the relationship between the RE content and the thickness of the EDL, it was observed that the particle size −0.106 mm exhibited a consistent correlation with the Logistic function $\mathrm{y}=2.03-0.43/(1+{\left(1.49\mathrm{x}\right)}^{172.39})$. In the study of the swelling performance of the WCE-DREOs, the order of the swelling ratio of ore samples in different particle sizes was ${\delta }_{-0.106}>{\delta }_{+0.106~-0.15}>{\delta }_{+0.15~-0.25}>{\delta }_{+0.25~-0.85}>{\delta }_{+0.85}$($\mathsf{\delta }$ represents the swelling ratio), and the swelling ratio was 3.34%, 2.92%, 2.33%, 1.55% and 0.01%, respectively. In the fitting of the relationship between the swelling ratio of clay minerals and the thickness of the EDL, it was observed that a particle size of −0.106 mm corresponded to the exponential function $\mathrm{y}=3.65-99.89·\exp \left(-\mathrm{x}/0.12\right)$. The results of the study provide useful assistance in the mining of the WCE-DREOs.