Enhancing calcium carbonate precipitation by heterogeneous nucleation during chemical softening

Abstract

Heterogeneous nucleation refers to precipitation on the surface of nucleating agents such as seed crystals, whereas homogeneous nucleation is precipitation in the absence of available nucleating surfaces. Unlike homogeneous nucleation, heterogeneous nucleation lowers the free energy of crystallization in precipitative water softening processes. To determine the importance of the amount of surface area provided by the nucleating agent, different‐sized calcium carbonate seed crystals were added to various hard waters during precipitative softening with caustic soda. Synthetic hard waters (with and without natural organic material) and naturally occurring hard waters were softened to remove dissolved calcium through homogeneous nucleation and heterogeneous nucleation using calcite seed crystals of two sizes. The naturally occurring hard waters were also softened in the presence of softening sludge from selected water softening treatment plants. Study results demonstrated that calcite seed crystals improved the removal of dissolved calcium during precipitative softening and that the optimal seed dose depended on the surface area available for nucleation.