Oxidative dissolution of nickel metal in hydrogenated hydrothermal solutions

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of metallic nickel in hydrogenated ammonia and sodium hydroxide solutions between 175 and 315 °C. The solubility measurements were interpreted by means of an oxidative dissolution reaction followed by a sequence of Ni(II) ion hydrolysis reactions: Ni ( s ) + 2 H + ( aq ) = Ni 2 + ( aq ) + H 2 ( g ) and Ni 2 + ( aq ) + n H 2 O = Ni ( OH ) n 2 - n ( aq ) + n H + ( aq ) where n = 1 and 2. Gibbs energies associated with these reaction equilibria were determined from a least-squares analysis of the data. The extracted thermochemical properties (Δ f G 0, Δ f H 0 and S 0) for Ni 2+(aq), Ni(OH) +(aq) and Ni(OH) 2(aq) were found to be consistent with those determined in a previous solubility study of NiO/Ni(OH) 2 conducted in our laboratory. The thermodynamic basis of the Ni/NiO phase boundary in aqueous solutions is examined to show that Ni(s) is stable relative to NiO(s) in solutions saturated at 25 °C with 1 atm H 2 for temperatures below 309 °C.