Nucleation and Growth of Scheelite in a Diffusing- Reacting System

Abstract

In this paper a study of the nucleation and growth behaviour of scheelite (CaWO4) in a porous medium at 25 º C is presented. The experimental set-up involves the counter diffusion of calcium and tungsten through a silica gel column. This results in the continuous evolution of pH and Ca²+ and WO4²- concentrations and therefore in the generation of scheelite supersaturation profiles along the gel column. The limited ionic mobility of the system determines that nucleation occurs at high supersaturation. The characteristics of scheelite nucleation (waiting time, precipitate position and metastability levels) are explained by attending to both the way in which supersaturation is created and the [Ca²+] and [WO4²-] distribution along the diffusion column. Crystal morphologies developed after nucleation are consistent with the high supersaturations observed. The results presented here are in agreement with the general crystallisation behaviour reported by PRIETO et al. (1989, 1991,1997) for other sparingly soluble salts.