Occurrence and vertical distribution of aluminum and rare earths in weathered crust elution-deposited rare earth ore: Effect of soil solution pH and clay minerals

Weathered crust elution-deposited rare earth ore is crucial source of medium and heavy rare earths, with in-situ leaching being the most common mining method. The high contents of impurity of aluminum in the leach solution are a significant challenge for the subsequent enrichment process of rare earths. A comprehensive understanding of the occurrences and vertical distribution of aluminum and rare earths within typical vertical profiles can provide valuable insights into entire design of the in-situ leaching. This paper improves a five-step sequential extraction method to analyze the occurrences and vertical distribution of rare earths and aluminum in vertical profiles from Chongzuo and Longyan. Experimental results demonstrate that soil solution pH is the main factor affecting the vertical distribution of ion-exchangeable rare earths. Both samples have distinct areas of enrichment for ion-exchangeable rare earths or aluminum. Ion-exchangeable rare earths are primary concentrated in the middle and lower parts of the ore layer (4–13 m in Chongzuo, 14–22 m in Longyan), while the ion-exchangeable aluminum is mainly enriched in the upper part of the ore layer (1–5 m in Chongzuo, and 2–14 m in Longyan). The vertical distribution of inorganic hydroxy aluminum is likely influenced by the micromorphology and particle size of the clay minerals. The inorganic hydroxy aluminum concentration in Chongzuo samples decreases continuously from 415.65 to 120.95 mg/kg with increasing sampling depth, whereas the concentration in Longyan samples (110.55–171.27 mg/kg) is almost independence with sampling depth. These results provide direct guidance for the entire design of the injection well depth and the leaching parameters, thereby inhibiting the leaching of impurity of aluminum and lower the consumption of leaching agent.