MECHANISTIC INVESTIGATIONS OF GIBBSITE GROWTH

Vernon, C.F, Brown, M.J., Lau, D. and Zieba, M.P.

Despite decades of research into hydrate growth in the Bayer process, there is no consensus on how the mechanism operates or what form of equation best describes growth kinetics. Indeed, there are half a dozen equations in popular use and many others in the open literature. All seem to work within limits. Knowledge of the growth mechanism is a powerful tool, not just to arrive at a better equation to describe growth rate, but in order to understand the whole precipitation process including soda incorporation, crystal morphology, effects of liquor composition and the effects of organics and process additives.

The present paper reviews the fundamental studies undertaken by the Parker Centre and other institutions, especially in the AMIRA P380 series of projects. It brings together sometimes apparently contradictory conclusions from various authors to propose a mechanism for hydrate growth that is consistent with laboratory and refinery observations. There is little doubt that the rate determining step is surface controlled. It appears that the liquor composition has a significant role in moderating the surface chemistry, and therefore moderates the mechanism. Additionally, it appears that the mechanism changes according to process conditions and this may help to explain why the mechanism has been so difficult to elucidate.