Metso caustic leach process: alkaline extraction of lithium from petalite and micaceous aluminosilicates

Reducing environmental impact while maintaining cost competitiveness is crucial for operators in the production of battery-grade lithium. While spodumene is well-known as the primary source of lithium from hard rocks, the projected increase in demand for lithium has highlighted the importance of other lithium-bearing minerals. This paper introduces a hydrometallurgical approach for processing non-spodumene lithium feedstocks, expanding upon previous Metso alkaline leaching developments by adapting the existing soda-based process to utilize a stronger alkali: caustic soda (NaOH). Under pressure caustic leaching conditions at 220°C, the conversion of lithium from uncalcined petalite to Li2SiO3 exceeded 99% within 5 min. Similarly, the conversion of lithium from calcined zinnwaldite reached >99% within 30 min at 200°C. Subsequent bicarbonation yielded ≈94% lithium extraction for petalite and just under 90% for zinnwaldite. The alkaline caustic leaching process provides a sulfate-free route for lithium extraction and enables the processing of a wider range of lithium mineralogies while maintaining a simplified process flowsheet and potential cost advantages.