Nickel Powder Precipitation By High-Pressure Hydrogen Reduction

ABSTRACT

The effect of impurities on the precipitation behaviour of nickel powder produced by

high-pressure hydrogen reduction was investigated in order to determine the factors responsible

for the formation of powder with undesirable morphology. In nickel precipitation by hydrogen

reduction, two product morphologies have been observed: the spherical, open powder (desirable)

and the spherical, closing/closed powder (undesirable). Two major impurities were studied

namely; a morphology modifier (a polyacrylic acid derivative) used as an additive and iron

which is an inherent impurity. Reduction experiments to investigate the effects of the

morphology modifier were conducted on a pilot-plant scale using a 75 L autoclave with modifier

dosages in the range of 0.25-5 vol %. Experiments to investigate the effects of iron were

conducted on a laboratory scale using a 0.5 L autoclave fitted with a Teflon reaction beaker.

Both autoclaves were fitted with a double impeller configuration consisting of an upper axial

impeller and lower Rushton turbine. Iron was added to the reduction solution as ferrous sulphate

solution (acidified to pH 2.5 to prevent oxidation) to give reduction solutions with Fe2+

concentrations of 6, 20 and 200 mg/L. Reduction was conducted at a temperature between

180-190 oC and 2800 kPa pressure using a nickel ammine sulphate solution (free NH3:Ni2+ and

(NH4)2SO4:Ni2+ molar ratios of 2.0-2.1 and 2.2 respectively). Nickel powder samples were

collected from the autoclaves after each successive batch reduction (densification) within a

cycle. The powder was then separated from the mother liquor before being washed and dried for

subsequent analysis. The concentration of the nickel before and after reduction was also

measured to establish the nickel depletion rate.

The effects of the selected impurities were investigated by analysing the nickel depletion rates,

SEM micrographs, powder purity and transforming the particle size distribution (PSD) data of

the powder samples into moments. The evolution of the moments, volume or mass moment mean

size (D(4.3)), number based mean size ( L1.0 ) and BET surface area were used to generate

information on the particle rate processes responsible for powder formation. These findings were

validated by means of mathematical models based on the moment form of the population balance

equation.

The morphology modifier was found to act as a growth inhibitor, thus, decreasing the

aggregation rate and making the powder more prone to breakage. Iron was found to induce

surface nucleation, thus, creating more growth sites on the particle surface leading to an increase

in the growth rate. Based on mathematical modelling results and evidence from SEM

micrographs, the spherically shaped nickel powder particles were proposed to be formed through

the formation of a pre-cursor by secondary aggregation followed by spherulitic growth. The

degree of compactness of the spherulites (open or closed formation) was proposed to be

determined by the number of active growth sites on the nickel particle surface. The morphology

modifier was found to decrease the number of growth sites as a result of growth inhibition

leading to the production of more open spherulites which are more prone to shear-induced

breakage. Iron was found to increase the number of growth sites as a result of surface nucleation

leading to more compact spherulites which are more resistant to shear-induced breakage. Based

on these findings a modifier dosage of less than 1 vol % and Fe levels of less than 6 mg/L are

recommended if spherical, open particles are desired. Thus, by characterising the effect of any

impurity on growth it is possible to predict its impact on particle morphology.