Nucleation characteristics of aqueous nickel ammonium sulphate solutions

Abstract

The nucleation of nickel ammonium sulphate hexahydrate from aqueous solution has been studied by two different methods which involved measuring (a) the maximum allowable undercooling (the metastable zone width) and (b) the induction period before the onset of visible nucleation.

Three different nucleation mechanisms were observed. For unseeded solutions (primary nucleation) the nucleation appeared to be homogeneous for supersaturations S > 1.8 and heterogeneous for S < 1.3. In the presence of a seed crystal secondary nucleation occurred.

Expressing the nucleation kinetics by the empirical relationship J=k_n(Δc)ⁿ, values of n(the “order” of the nucleation process)= 3.2, 2.0 and 1.1 were obtained for homogeneous, heterogeneous and secondary nucleation respectively. There was good agreement between the two different methods of measurement for primary heterogeneous nucleation (n= 2.0).

Trace quantities (10 ppm) of polyacrylamide suppressed nucleation considerably, but did not appear to change the “order” of the nucleation process.