Extraction of Rare Earth Metals from NiMH Battery Scrap via Selective Sulfation Roasting

Controlled recycling of battery scrap to recover valuable metals has become essential nowadays not only to fulfill the demand for critical raw materials but also to mitigate the environmental impacts due to the possible release of toxic chemicals and heavy metals from this waste fraction. In the current research, selective sulfation roasting combined with a water leaching route is investigated for the extraction of rare earth metals (REMs) from waste NiMH batteries. The effect of the sulfation roasting kinetic parameters—partial pressure of SO₂ (pSO₂), and O₂ (pO₂), and temperature (T), time (t)—on REMs leaching efficiency was examined. The study demonstrated that it is possible to selectively extract REMs from spent NiMH batteries (< 125 µm) by this route. The results showed that, with an increase in temperature and pSO₂, the REM leaching efficiency increased linearly. Optimized sulfation roasting conditions are suggested to be 3% SO₂–3% O₂–Ar at 850 °C for 60 min, resulting in a leaching yield into water of 65% La, 58% Ce, 59% Pr, and 58% Nd along with a very low contamination of Ni (0.4%). These results suggest excellent separation of REMs from the other metallic elements of NiMH scrap.