Recovery of nickel and cobalt from Black Mass

The present invention lies in the field of pyrometallurgy and discloses a process and a slag suitable for the recovery of Ni and Co from Li-ion batteries or their waste, particularly from Black Mass. The slag composition is defined according to: 25% < MnO < 70%; AlO + 0.5 MnO < 45% SiO > 5%; LiO > 1%; MnO + LiO + AlO + CaO + SiO + FeO + MgO + PO > 90%; and, wherein (CaO + 2 LiO + 0.4 MnO) / SiO ≥ 2.0. This composition is particularly adapted to limit or avoid the wear or corrosion of furnaces lined with magnesia-bearing refractory bricks.