The separation of Co(II)/Ni(II) from concentrated HCl solutions by solvent extraction was experimentally studied. The leach solution of spent lithium secondary batteries containing about 22.5 g L−1 Co and 1.5 g L−1 Ni in 3N HCl was modeled. Batch experiments were first performed to select optimal conditions for effective separation including the type of the extractant (tertiary and quaternary amines) and its composition, and the type of the stripping agent (H2SO4, HCl, NaCl, the mixture of NaCl and citrate) and its concentration. It was screened that organic phase contained Aliquat 336 and the phase modifier 1-dodecanol in kerosene and the strip phase was 1N HCl. The separation of Co(II) from Ni(II) could be achieved on the basis of preferential stripping for Co(II) (Ni/Co < 5 × 10−3 in the strip phase). Finally, mass transfer rates of solvent extraction operations in a rotating disc contactor and a newly developed hollow-fiber membrane contactor were compared.