Improved understanding of Sodium hydroxide concentration role and kinetic model of cryolite reactive extraction in cathode Spent Pot Linings

Spent Pot Lining (SPL) cathode pot, waste from the aluminium smelting process needs detoxification from cyanides, washing out water-soluble salts and extraction of the cryolite (Na₃AlF₆) decomposition products to be recycled. Revealed cryolite decomposition mechanism with NaOH opens possibilities to explore its critical role in the reactive extraction process. Common Na⁺ ion from NaOH hinders the solubility of the product but also drives mass transfer to the reaction site. Reaction mass balance provides adequate liquid to solid ratio (L/S) and NaOH concentration range. A newly developed kinetic model based on Whitman film theory and NaOH mass flow enables prediction of the reaction time to decompose cryolite to a low enough level. Results show that the internal particle resistance to transport (1/k_s) is 19 times larger than the external (1/k_l) one and governs the whole process.