Oxidative dissolution of nickel metal in hydrogenated hydrothermal solutions

摘要

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 ℃. 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) = Ni~(2+)(aq) + H_2(g) and Ni~(2+)(aq) + nH_2O = Ni(OH)_(11)~(2-11) (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 (Δ_rG~0, Δ_rH~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 ℃ with 1 atm H_2 for temperatures below 309 ℃.