Direct synthesis of nickel powders from NiO slurry by hydrothermal hydrogen reduction process

High purity nickel powder (99.5% Ni) was prepared by direct hydrogen reduction of commercial NiO powder in an acidic slurry in an autoclave with a yield of approximately 100% at 250 °C, initial pH 5.4 and 300 psi partial hydrogen pressure. It was found that the initial pH of the solution and the addition of NiSO4 play critical roles in the NiO/Ni powder conversion process. They provide an adequate acidic media and a source of SO42− counter ions for dissolving NiO into Ni2+ ions, while the subsequent reduction of Ni2+ ions to metallic nickel proceeds.