A RATE EQUATION FOR CRYSTALLIZATION OF SODIUM OXALATE UNDER BAYER CONDITIONS

During the digestion of bauxite in caustic, and subsequent operations in a Bayer plant, entrained organic matter is degraded, much of it to the relatively stable oxalate anion. There is consequently a continuous build up of sodium oxalate in the Bayer liquor, and this must be removed in order to prevent unwanted crystallization during hydrate precipitation.

In other industries, rate equations have been employed to predict the extent of crystal growth. Although rate equations have been developed for the crystallization of sodium oxalate in aqueous systems, no work has been published on crystallization under Bayer conditions. This work will describe the development of a rate equation to predict sodium oxalate crystallization in a synthetic liquor system. The effect of supersaturation, seed charge, stirring rate and temperature on rate of crystal growth will be discussed. From activation energy calculations it has been discovered that the crystallization process is predominantly controlled by surface-integration and not bulk diffusion.

A comparison of growth on blocky and acicular oxalate seed has shown that the rate of crystallization is not a simple function of available surface area, which suggests that some surfaces are more active than others. This will be demonstrated with aid of SEM pictures.