Oxygen-pressurized Leaching of Metallic Nickel in Ammoniacal Ammonium Carbonate Solution

Abstract

The oxygen-pressurized leaching of metallic nickel powder was studied kinetically in ammoniacal ammonium carbonate solution. The dissolution of nickel required the presence of ammonium ions as well as oxygen; the degree of dissolution was determined by the amount of ammonium ions. When the sample amount was less than 6 g-Ni/200 ml, the rate-determining step was assumed to be diffusion of dissolved oxygen through a liquid boundary film. In this case, the initial velocity of dissolution of nickel powder was expressed by the following equation, an apparent activation energy being L 5 kcal/mol (30∼410°C):
1-(1-x)^1/3=k't,
where x was the degree of nickel dissolution, k' was constant, and t was reaction time. The dissolution of nickel powder was significantly accelerated by increasing the partial pressure of oxygen and the stirring velocity. These phenomena also supported that the rate-determining step was diffusion of oxygen. When the sample amount was more than 6 g-Ni/200 ml, the rate-determining step was assumed to be dissolution of oxygen into the solution.
The nickel in coal ashes obtained by steam-gasification was easily leached as long as the nickel was present in a state of metal. Thus, more than 90% of the nickel could be leached within 10 min from a coal ash sample, in which the gasification degree was about 70%, under the conditions: (NH₄)₂CO₃0.5 mol/l, free NH₃ 3 mol/l, reaction temperature 50°C, partial pressure of oxygen 4 atm and stirring velocity 1000 rpm. When the gasification degree. was nearly 100%, 80% of the nickel was converted to oxide and the degree of leaching of nickel was as low as about 20% under the same conditions.