Homogeneous nucleation of CaSO ؒ 2H O gypsum based on an optical diagnostic 4 2 technique was studied within a supersaturation range of 1᎐ 4 at 25᎐ 90ЊC. The experiments were carried out using an experimental apparatus consisting of a batch crystallizer with the related measurement de®ices. Signals of scattered and transmitted light coming ( ) from a He ᎐ Ne laser source were analyzed to measure the induction period t , that i n d is, the time delay necessary for homogeneous nucleation to take place. As expected from theory, it was found that t decreases when either temperature or supersaturation i n d increase; from the dependence of t on supersaturation, it was possible to distinguish i n d between the mechanisms of homogeneous and heterogeneous nucleation. From the ex-( ) perimental data relati®e to homogeneous nucleation, the interfacial tension ␥ between s ( ) CaSO ؒ 2H O and the surrounding aqueous solution and the acti®ation energy E 4 2 at t for CaSO ؒ 2H O crystallization were e®aluated. In particular, the dependence of t 4 2 i n d on temperature made it possible to e®aluate E at 30 kJrmol and the dependence of at t t on supersaturation offered the ®alue of ␥ to be about 37 mJrm 2 , which does not i n d s
®ary with temperature in the inter®al explored.
the gypsum produced is an important aspect of wet limestone scrubbing, since it significantly affects the byproduct dewatering properties, and consequently the economics of the whole process.