Bauxite to alumina: a cleaner pathway

Through their understanding of fundamental chemical mechanisms in the Bayer process, Flagship researchers have been testing new approaches to the problem of organics removal.

Australian bauxite generally contains high levels of organic carbon – ranging from intact plant material through to decayed plant remnants.

These organic carbon compounds can have a detrimental effect on the alumina refinery process – reducing the productivity of solvent (or liquor) used to dissolve alumina-bearing minerals, affecting product quality, and disrupting plant operation. Organics entering the process can also break down and cause volatile organic carbon emissions.

Costly problem 
Australian refineries produce 18 million tonnes of alumina a year, contributing about $4 billion to our export trade. The evidence suggests that organics contamination could be costing these refineries several hundred million dollars a year. 

Earlier attempts to reduce the impact of organics have proved either too costly or chemically incompatible with the Bayer process – the process by which alumina is extracted from bauxite.

More recently, researchers from the Parker Centre for Integrated Hydrometallurgy Solutions have begun working on a Light Metals Flagship project to find new ways of removing organics from the Bayer process.  

Buildup in Bayer liquor
In the Bayer process, bauxite is mixed with hot caustic soda solution in an operation known as digestion. This causes the organic substances present to degrade further and become more soluble. 

After alumina removal, the caustic liquor is continually recycled to the digester, until dissolved organic impurities build up to a level at which additional caustic – and therefore more alumina – cannot be dissolved into solution without impairing the process.

Consequently, to recover a given amount of alumina, more Bayer liquor must be processed, decreasing refinery productivity and increasing energy use. 

Organics are blamed for many other problems with the Bayer process, from foaming and scale formation, to impaired product quality.

Assessing alternatives
The Light Metals Flagship project team, led by Dr Chris Vernon from CSIRO Minerals, has been testing new approaches to the problem based on fundamental chemical mechanisms in the Bayer process.

One approach involves novel oxidation mechanisms. With this approach, the team had some success in suppressing the buildup of sodium oxalate – a damaging organic by-product that can interfere with Bayer chemistry – but noted some limitations.

However, this oxidation mechanism may prove useful elsewhere as a liquor pre-treatment step for other organics-removal processes, such as salting out, wet oxidation, liquor burning or adsorption.

Another approach being assessed by the team is adsorption of organic impurities.

While adsorption is routinely used to treat water, the high pH and ionic strength of Bayer process streams inhibit its effectiveness in organics removal.

Through its investigations of improved adsorbent materials, however, the project team has found that some process streams respond favourably to adsorption. 

Beneficiation another option
The team is also looking at the option of removing organics before bauxite enters the refinery.

Its research has shown that careful manipulation of washing conditions changes the profile of organics removed. An economically viable beneficiation process to remove organics might also upgrade components within the ore at the same time.

Dr Vernon’s team has been collaborating with CSIRO Industrial Physics in Sydney, RMIT University in Melbourne, and Curtin University of Technology in Perth on the organics removal research, as well as liaising with industry to keep companies informed of progress.

Contact:
Dr Chris Vernon
Ph: 61 3 9334 8043
Chris.Vernon@csiro.au

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In brief...

The Light Metals Flagship is a CSIRO initiative and part of the National Research Flagships program that aims to deliver scientific solutions to advance Australia's most important national objectives. One of the largest scientific initiatives ever mounted in Australia, it aligns closely with the Federal Government's National Research Priorities. The initiative brings together our national research resources to deliver breakthroughs in fields ranging from healthcare to light metals and the environment.

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