The hydrometallurgical separation of nickel from spent Raney Ni catalyst is based on the dissolution of separated nickel slurry in hot diluted sulphuric acid (H2SO4) with the addition of an appropriate oxidation agent. Consecutive diffusion dialysis, which was performed in a two-compartment countercurrent dialyzer with an anion-exchange membrane Neosepta-AFN at steady state, enables the removal of excess of H2SO4 from a solution of aluminium and nickel sulphate in H2SO4 and simple recycling of the isolated H2SO4. Next, the obtained dialysate was alkalized using excess of NaOH solution. Using this procedure, the Ni(OH)(2) contaminated with only 8.9 wt.% Al was obtained.