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 NHOH·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 NHOH·HCl/oxalic acid-citric acid molar ratio 3:1, compound leaching agent concentration 0.48 mol L (NHOH·HCl/oxalic acid-citric acid concentration 0.36 mol L/0.06 mol L-0.06 mol L), liquid-solid ratio 5.5:1, temperature 50 °C, and a NHOH·HCl concentration of 0.1 mol L 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). 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 NHOH·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 NHOH·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.