In the Mond process, nickel is purified by
◦ dissolving the crude ore in sulfuric acid, and precipitating the nickel(II) ion as the sulfate salt.
◦ dissolving the crude ore in a lye (NaOH) solution, and precipitating the nickel(II) ion as the hydroxide salt.
◦ heating the crude ore to its melting point and electrolytically reducing the nickel(II) ions to form the metal.
◦ passing sodium gas over the crude ore at high temperatures, chemically reducing the nickel(II) to nickel metal.
◦ passing carbon monoxide over the impure metal at high temperature, forming the volatile Ni(CO)4 which is removed as a gas.