<Original Papers>Studies in the Ammonia Pressure Leaching of Metallic Nickel Powder

The dissolution reaction of nickel powder in aqueous ammonia solutions in the presence of oxygen under pressure is an electrochemical process involving separate anodic and cathodic steps as follows : Anodic process : [chemical formula] Cathodic process : [chemical formula] The step (3) is the rate-determining process. The velocity equation derived from the experiment is as follows : [chemical formula] where [Ni] is the amount of nickel (g/l) dissolved in time t(min), K a constant, A the surface area of nickel, PO_2 the partical pressure of oxygen (atm), and V the rev/min of stirrer (x is unknown). The role of NH_3 and NH_4^+ ion which is required for efficient extraction of nickel appears to lie in stabilizing nickel ion as the complex ion (step (2)) and preventing hydrolysis (step (4)). It was found that this leaching reaction could be used to extract nickel from a wide variety of Ni-Fe alloy and good separation of nickel from iron could be achieved more effectively with the solution containing ammonium sulphate than with that containing ammonium carbonate