Certain types of organic material in Bayer liquors can cause problems during alumina production. A Parker Centre research team is working on identifying some of these compounds and determining the magnitude of their impacts and ways to get rid of them.

The methods currently used by alumina refineries to destroy Bayer liquor organics are relatively non-specific and have high energy costs. The researchers aim to develop more efficient and lower cost removal strategies by targeting only the detrimental organics.

Bayer liquor contains probably thousands of different organic compounds which largely result from degradation during the digestion stage of biological matter, including wood, root material and soil organism metabolites, in the bauxite ore.

Organics can slow the rate of gibbsite (alumina hydrate) precipitation. They can also reduce gibbsite product quality by increasing the incorporation of impurities such as sodium and promoting the production of finer crystals. The team is trying to pinpoint exactly which groups of organics are responsible for what effects. It is likely that more than 90% of Bayer organics cause no problems.

Ultrafiltration has been used to separate Bayer liquor organics into fractions on the basis of molecular weight. The fractions are being tested for their effects on gibbsite precipitation. It appears that the intermediate weight organics have the greatest effect on precipitation. The precipitation impact of organic extracts from bauxite, separated according to chemical reactivity, is also being determined, as is the effect of the oxidation that occurs during digestion.

Organic extracts from bauxite ore. Each differently coloured solution contains organics with differing chemical properties and different precipitation impacts.

The researchers will then work on characterising the subclasses of troublesome organics on the basis of reactivity and chemical composition, followed by possible identification of some representative compounds. Knowing the chemical characteristics of these organics should enable targeted removal or mitigation techniques to be developed.

The degradation process in refineries is being simulated by repetitive, sequential digestion of wood and bauxite extracts. Understanding how the organics in bauxite break down may assist with identifying the best point at which to intervene.

The research has a world focus. While bauxites around the world vary greatly in their organics, the types of organics identified as a problem in Australian liquors are likely to also cause trouble elsewhere and the degradative pathways will be the same.

A number of alumina companies are supporting the project by providing bauxite samples and refinery liquors as reference points. The project is a collaborative effort between CSIRO Minerals and Curtin University of Technology.

For further information, contact
Dr Chris Vernon, tel: (08) 9334 8043, e-mail: Chris.Vernon@minerals.csiro.au

By Ros Dilworth, Communications Officer, the Parker Centre.
First published in Process Magazine.