Recovering lead and sulfur from spent lead paste by molten salt electrolysis: a clean and sustainable lead and sulfur loop†

The sulfur transfer is key to obtaining greenness of recycling spent lead paste (SLP) since conventional recycling methods always involve generating secondary wastes such as sulfur oxides (SO_x), sulfates, and sulfides. To address these challenges, we propose a combined process in which the SLP is first converted into lead sulfide (PbS) by carbothermic reduction, and then the resultant PbS is split into liquid lead (Pb) and sulfur vapor (S_x) by molten salt electrolysis (MSE). The faradaic efficiency of MSE reaches 92.29% with a Pb recovery rate of 97.85%, and the liquid Pb and gaseous S_x are respectively discharged from the bottom and top of the cell to allow the reaction to happen continuously. Compared with traditional methods, over 98.85% of SO₂ emissions are reduced, and no sulfides and sulfates are generated. Furthermore, the energy consumption is only 0.32 kW h (kg-Pb)⁻¹, which demonstrates a 67% reduction compared to mainstream recycling processes using a 50 g-scale electrolyzer. Overall, we offer a clean sulfur transfer route to reduce secondary wastes and energy consumption, which will benefit the recovery and utilization of various sulfate wastes and raw materials with low environmental footprints.