Vast aluminium global resources that are currently uneconomic to mine could be opened up through a CSIRO-patented process that recovers caustic soda from red mud. Tim Thwaites reports that it may be environmental factors that drive adoption.

Article from resourceful: Issue 7, June 2015 

Researchers from CSIRO’s Mineral Resources Flagship have developed a process that lowers the cost of refining bauxite (aluminium ore) to alumina (aluminium oxide) by treating the waste product generated—red mud—to recover a costly ingredient, caustic soda.

While lowering the costs was the primary intent, a secondary benefit is that it makes the red mud less polluting and easier to handle. Ironically, tougher regulations on the disposal of red mud might well end up driving adoption of the process, according to a principal research scientist in CSIRO’s alumina production group, Dr Peter Smith.

"It is a bolt-on process," Dr Smith says. "We just take the residue, and remove the caustic soda. You wouldn’t have to change anything upstream in the current refining process at all."

The process can be applied to some or all of the residue —even independently to reprocess red mud already stored.

Working at the the electrodialysis unit in CSIRO’s Waterford laboratory

The vast majority of the world’s bauxite resources includes valuable alumina minerals and alumino-silicate clays which are intimately mixed. Insoluble components of the bauxite are removed by digesting the ore with very hot caustic soda (sodium hydroxide) in what is known as the Bayer process.

Initially the aluminium and the silicate clays dissolve in this solution. Sodium in the caustic soda solution combines with reactive silica to form a disilication product (DSP), which falls out of solution taking sodium with it.

The DSP becomes part of the residue, and the loss of sodium is at significant cost. So, the higher the level of reactive silica in the ore, the more expensive it is to refine. Bauxite with more than about eight per cent reactive silica is generally considered to be uneconomic to process, according to Dr Smith.

Australia has vast reserves of bauxite containing at least that level of silica—at Aurukun, south of Weipa on Cape York Peninsula, for instance, and on the Mitchell Plateau between Kununurra and Derby in the very northwest of Australia.

"Along with the rest of the world, we have cherry-picked the high quality ore, and are rapidly getting to the point where it is running out. But if we could make it economic to process bauxite with up to, say, 12% silica, we would increase our economic reserves significantly."

It is a bolt-on process. We just take the residue, and remove the caustic soda. You wouldn’t have to change anything upstream in the current refining process.

Between 2008 and 2011, financed by the Australian Government as part of the Asia-Pacific Partnership, CSIRO researchers identified three ways of approaching this problem:

  • pre-treating the ore in some way to remove reactive silica
  • changing the operating conditions of the Bayer process to minimise the loss of sodium or
  • reprocessing the residue from the process to recover the sodium.

The patented process falls into the last category and uses acid dissolution and electrodialysis (salt splitting) to recover the caustic soda.

The residue of the Bayer process is treated with sulphuric acid to neutralise the heavily alkaline mud and scavenge the retained sodium as a sodium sulphate solution. This solution is then subjected to electrodialysis reforming sulphuric acid, which can be recycled, at one electrode and sodium hydroxide, which can be fed back into the Bayer process, at the other electrode.

The treated red mud is left less alkaline and far more tractable for disposal. “We’ve proven the process, both technically and economically,” says Dr Smith.

The project has reached a point where further testing at an refining plant is needed. "You could set something up on a small scale, treating perhaps 10 per cent of the residue."

Bauxite mining and refining companies are aware of the work. For example, Rio Tinto, which mines the Weipa deposit on Cape York and owns refineries in Gladstone, central Queensland, has been advising the researchers on the sorts of industrial conditions their process would have to accommodate and how costs and benefits would be viewed. The team has also been approached by interested foreign companies.

"This technology was developed in Australia for Australian bauxite to maximise Australian resources," says Dr Smith. "We would like to see an Australian company take it up."

Recovery of Soda from DSP/BR by Acid Leaching and Electrochemical Processing.  ©Takuo Harato

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