Detoxification of thallium from lithium smelting slag by reductive acid leaching

Lithium smelting slag (LSS), a hazardous by-product from lithium carbonate production, present potential environmental risks due to its geochemically association with, thallium (Tl)-a highly toxic element activated during lithium extraction process. This study developed a novel NH₂OH·HCl-oxalic acid-citric acid reductive leaching system for efficient Tl detoxification from LSS. 91.7 % Tl leaching rate was achieved under optimized conditions: oxalic acid/citric acid molar ratio 1:1, the molar ratio of NH₂OH·HCl/oxalic acid-citric acid molar ratio 3:1, compound leaching agent concentration 0.48 mol L⁻¹ (NH₂OH·HCl/oxalic acid-citric acid concentration 0.36 mol L⁻¹/0.06 mol L⁻¹–0.06 mol L⁻¹), liquid–solid ratio 5.5:1, temperature 50 °C, and a NH₂OH·HCl concentration of 0.1 mol L⁻¹ during the enhanced leaching step. The toxic leaching concentration of Tl in the leached residue was less than the general industrial solid waste Class I standard threshold (5 μg L⁻¹). It is revealed that Tl in LSS mainly existed in a reducible fraction (bound to Fe-Mn oxides), and encapsulated within gypsum phase. The synergistic leaching mechanism of Tl from LSS by NH₂OH·HCl-oxalic acid-citric acid reductive leaching system involved gypsum phase dissolution by citric acid and oxalic acid synergistically, and reductive release of Tl bound to iron-manganese oxides mainly by NH₂OH·HCl. Leaching kinetics and thermodynamics studies confirmed that the reductive leaching process follows the shrinking core model under diffusion control, and the low activation energy for Tl leaching (18.934 kJ/mol) further confirms that Tl leaching kinetics are governed by reagent transport through boundary layers rather than chemical bond breaking. This work established a green, efficient, and economical approach for the Tl detoxification in LSS.