A perspective on the growth-only zone, the secondary nucleation threshold and crystal size distribution in solution crystallisation

Abstract

The metastable zone between the solubility curve and the crystallisation curve can be divided into two regions, separated by the secondary nucleation threshold. At a supersaturation less than that of the secondary nucleation threshold seeds will grow but not proliferate. At greater supersaturations, they will proliferate. Although this concept has been known in connection with the commercial crystallisation of sucrose and of lactose for almost a century it is virtually unrecognised beyond the sugar and dairy industries. The consequences are explored of the existence of such a boundary on the crystallisation process and the influence on crystal size distribution. It is recognised that the secondary nucleation threshold is the ‘seeded metastable limit’, but there are important consequences of avoiding the latter term. Mullin's ‘latent period’ is shown to be closely related to the ‘growth only’ zone within the metastable zone.