Chapter 27 - Hydrogen Reduction of Nickel from Ammoniacal Sulfate Solutions

Abstract

An alternative process to electrowinning for the recovery of nickel from purified nickel solutions is hydrogen reduction. The objectives of this chapter are to describe the chemistry of hydrogen reduction of nickel from aqueous solutions; to discuss how this chemistry is applied safely and efficiently; and to compare hydrogen reduction and electrowinning. Hydrogen reduction is carried out by injecting hydrogen into aqueous ammoniacal nickel sulfate solutions in stirred high-pressure autoclaves. An alternative method of removing of the nickel that remains in solution is the precipitation of a mixed double salt consisting of Ni (NH4)2(SO4)2. The reduction of nickel by hydrogen solutions are produced by the leaching of sulfide mattes using air and ammonia and intermediate sulfide precipitates. The operation of the autoclave for hydrogen reduction occurs in the following steps: the preparation of the nickel ‘seed’ powder; the reduction of solution batches; the finishing of a 50 tonne “lot” of nickel powder; and, the preparation of the autoclave for a new cycle. The initial seed powder, with a particle size of about 0.01 mm, is made in the autoclave by the reduction of nickel by hydrogen from a dilute solution of nickel ammonium sulfate. The depleted solution is discharged through the flash tank, the autoclave is flushed with nitrogen, and the slurry containing the nickel powder is drained out through the bottom of the autoclave at ambient pressure. The nickel powder from hydrogen reduction is less pure than the nickel from electrowinning. The principal product of the process is nickel powder with a particle size of between 0.1 and 0.2 mm. The powder is washed, dried and usually briquetted before being sold to customers.