Thermodynamically Guided Three-Stage Separation for Deep Aluminum Removal from Delithiated LiFePO4 Slag Toward Battery-Grade FePO4 Regeneration

The recovery of high-purity iron phosphate (FePO 4 ) from lithium extraction slag (LES) is crucial for achieving closed-loop recycling of spent LiFePO 4 /C. However, this process is significantly challenged by the difficulty in removing aluminum impurities due to the similar chemical behavior of Fe 3+ and Al 3+ . Guided by thermodynamic principles, we developed a three-stage purification process for deep aluminum removal, consisting of “selective leaching-precipitation-dissolution/recrystallization.” In the first stage, highly selective aluminum leaching (96.3% Al vs. 21.6% Fe) is achieved under mild acidic conditions, leveraging the thermodynamic preference for aluminum dissolution over FePO 4 . During the second stage, by utilizing the tendency of Fe 3+ to precipitate more readily as FePO 4 ·xH 2 O than Al 3+ , precise pH control enables 95.5% iron precipitation, with only 4.02% aluminum coprecipitation. The third stage involves an in situ dissolution/recrystallization, which promotes efficient iron recrystallization (95%) while further removing residual aluminum, reducing its content by over 50% from an already low level. This enhancement is attributed to the higher crystallization tendency of FePO 4 ·2H 2 O compared to AlPO 4 ·2H 2 O. Ultimately, battery-grade FePO 4 with an aluminum content below 300 ppm is obtained. This work provides an industrially viable strategy for the deep removal of aluminum and the high-value utilization of LES derived from spent LiFePO 4 /C.