Mechanochemically activating lepidolite to allow easier lithium extraction and safer thallium handling through alunite formation

Abstract

The mineral processing and traditional metallurgical industry remains a high risk to the environment due to the release of toxic pollutants. Lithium resource extraction from lepidolite through high-temperature roasting of lepidolite with concentrated acid causes a large environmental burden due to the disposal of the concentrated wastewater. However, the issue of thallium contamination due to its volatilization during the roasting of lithium minerals has frequently come into the spotlight. In this study, as a pretreatment, mechanochemical activation of lepidolite was introduced to disrupt the mineral crystallinity to allow efficient lithium extraction under quite mild metallurgical conditions and to avoid the widespread Tl contamination resulting from the volatilization associated with current pyrometallurgical processes. After ball milling the lepidolite mineral for 2 h at 500 rpm, leaching efficiencies of over 99% for both Li⁺ and K⁺ were achieved even at room temperature with only 4% acid. Subsequently, Al(OH)₃ was ground and added to the acidic solution containing K⁺ and Al³⁺ cations (co-leached with lithium from lepidolite) to form an alunite mineral phase, which provides a geometrical framework for immobilizing both K⁺ and Tl⁺. Adjusting the molar ratio of Al(OH)₃, Al₂(SO₄)₃, and K₂SO₄ enabled the incorporation of both K⁺ and Tl⁺ by forming an alunite precipitate. Removal efficiencies exceeding 98% for both K⁺ and Tl⁺ were ultimately achieved at an Al(OH)₃/Al₂(SO₄)₃/K₂SO₄ molar ratio of 8 : 2 : 1, yielding a clean solution for subsequent lithium recovery. A set of analytical methods were applied to understand the mechanism of Tl incorporation inside the alunite structure. Development of a novel mineral processing technology and the disposal of the released toxic species based on suitable mineral phase regulation offer great potential for reducing environmental pollution in this field.