Method for separating cobalt and nickel

A method for separating cobalt and nickel, in which cobalt and nickel are separated from a lithium ion secondary battery, the method comprising:

• a crushing and sorting step of crushing and classifying the lithium ion secondary battery to obtain an electrode material containing at least cobalt, nickel, copper, and lithium;
• a leaching step of immersing the electrode material in a processing liquid containing sulfuric acid and hydrogen peroxide to obtain a leachate;
• a copper separation step of adding a hydrogen sulfide compound to the leachate with stirring and subsequently carrying out solid-liquid separation to obtain an eluate containing cobalt and nickel and a residue containing copper sulfide;
• and
• a cobalt/nickel separation step of adding an alkali metal hydroxide to the eluate to adjust a pH within a range of 3.0 to 4.0 and subsequently, adding a hydrogen sulfide compound with stirring while maintaining the pH of the eluate within a range of 2.0 to 3.5 from start to end of the addition of the hydrogen sulfide compound and carrying out solid-liquid separation to obtain a precipitate containing cobalt sulfide and nickel sulfide and a residual liquid containing lithium.