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)+2H+(aq)=Ni2+(aq)+H2(g)Ni(s)+2H+(aq)=Ni2+(aq)+H2(g)andNi2+(aq)+nH2O=Ni(OH)n2-n(aq)+nH+(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 (ΔfG0, ΔfH0 and S0) for Ni2+(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 H2 for temperatures below 309 °C.