Development of suitable extraction techniques for rare earth elements from different geological sources

Abstract

Rare earth elements (REEs) have become indispensable metals in global technological expansion toward a green economy. Without REEs, several modern technologies would not exist in their current form. Due to the global supply uncertainty of REEs, rare earth (RE) extraction techniques need to be developed with the combination of conventional and novel engineering chemical practices up to the industrial level. However, current extraction techniques often suffer from low efficiency and high environmental impact, highlighting the need for innovative approaches. Therefore, this study aims to develop suitable extraction techniques for REEs from primary apatite crystals of the Eppawala Phosphate Deposit (EPD), lake sediments around the EPD, and gem mining waste from the Wagawatta area in Sri Lanka. The methodologies employed include mineralogical characterization, acid leaching, and the development of a kinetic model to optimize the leaching process for subsequent recovery. The average concentration of total REEs (TREEs) in primary apatite crystals was 4072 mg/kg with enrichment of light REEs (LREEs). A developed process in this study including leaching, solvent extraction, stripping, precipitation and calcination produced a purity of 92% of rare earth oxides (REOs). Downstream lake sediments also contained 804 mg/kg of TREE with an LREE enrichment. The leaching experiments revealed that about 34% of ΣREEs were present as ion-exchangeable REEs, whereas about 74% of ΣREEs were present as the acid-leachable fraction. Acid leaching was proven to be the best approach to leach out REEs from lake sediments in the studied lake. In the Wagawatta gem pit, the alluvial layer contained an REO percentage of 0.3. A simple physical separation method consisting of wet sieving and subsequent density separation via a shaking table was employed to upgrade the REO percentage up to 2.8 with an LREE percentage of 94. The organic-rich clay layer found at the Wagawatta gem pit contained 5348 mg/kg TREE. Leaching experiments showed that about 94% of TREEs from the clay layer could be leached out using H2SO4 under the optimum conditions. Furthermore, the developed leaching kinetic model revealed that the internal diffusion through the solid layer around the unreacted core was the rate-controlling factor of the leaching process (calculated Ea=11.219 kJ/mol). In conclusion, the study successfully developed and validated a novel leaching kinetic model, achieving significant REE recovery rates from the investigated materials. These results contribute to addressing the global supply challenges of REEs by proposing efficient and environmentally friendly extraction methods. These findings open up opportunities to initiate progressive steps in the development of a novel extraction technique to recover REEs from many other sources throughout Sri Lanka. Despite the economic benefit to the country, it will encourage the sustainable management of REE sources in Sri Lanka, reducing their environmental footprint.