7. Hydrate Precipitation, Classification and Filtration

The precipitation area is an area of a Bayer plant that is mostly responsible for the production and product quality. The control of the circuit is seen by many as art rather than a science because of its complexity and the many aspect of the process that are difficult to understand easily at first. Another factor that is particular to this area is the long inertia and slow response of the circuit, partly due to the large inventory of seed, equivalent to more than a week of production for most circuit. This chapter covers the science to help understand this area in order to control and optimise it better. Classification of the precipitated hydrate is an integral part of the precipitation circuit operation and the presentation is based on hydrocyclones as primary and secondary classification devices which have replaced conventional thickeners in modern refineries. Fine seed thickening is performed in a conventional thickener. 3-stage classification flowsheets are discussed versus 2- and 1-stage and the influence on final strength of alumina is discussed as a consequence of the number of classification stages. Disc filters with large diameter discs are the standard equipment for coarse and fine seed hydrate filtration. These filters have replaced other vacuum filters, mainly disc filters with smaller disc diameter, due to their significantly higher filtration capacity which is a result of a significantly increased hydraulic capacity of the filter components in combination with a short cycle time. For product hydrate filtration horizontal pan filters are the state of the art technology, allowing for effective cake washing with up to three counter current wash stages, in combination with the application of steam for reducing the cake moisture.