Aluminium could hold key to Ranger water purification

By Catherine Norwood
Source: CSIRO’s PROCESS magazine

Dr Tony Milnes, ERA’s general manager, environmental strategy, said unless treated, process water must be stored on-site and evaporated. After almost 30 years of operation at the company’s Ranger mine in the Northern Territory, the quantity of process water being stored in the mine’s tailings dam, and in a nearby mined-out pit, is building up and the need for a purification process is becoming more urgent so that water can be released.
ERA has developed an active treatment strategy that neutralises process water taken from the tailings dam to pH 10. The process involves lime addition, carbonation and microfiltration/reverse osmosis, and could purify up to 2,000 megalitres of water a year.
However, the process would also result in significant quantities of alkaline residues, precipitated from the water, which create their own handling and storage issues.
Scientist Ron Pleysier, who is working on the project through the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions, said the first step in evaluating potential residue storage options for ERA was to assay the residue to determine its chemical make-up. A number of techniques were used to identify the mineralogy and morphology of the residues including x-ray diffraction, x-ray fluorescence, thermogravimetric analysis and laser-sizing.
“We found it was 75 per cent gypsum (calcium sulfate), 20 per cent brucite (magnesium hydroxide) and almost five per cent manganese hydroxide, with small amounts of other metals such as aluminium,” Pleysier said.
If the residues, at about pH 10, were returned to the tailings dam, which is acidic (about pH 4.5) the magnesium in particular is likely to remobilise, which would defeat the purpose of the purification process – the solutes would dissolve back into the water ERA is trying to purify.
In assaying the residue, researchers realised that the chemical structure was a mixture of hydroxides and sulfates, which lent themselves to the formation of hydrotalcites; these would be more stable at lower pH levels, which might allow the residues to be returned to the tailings dam.
Pleysier said hydrotalcites are layered double hydroxides with a specific ratio of plus-two and plus-three cations in their structure, which can trap the less stable mineral ions between the layers.
The plus-two cations are already present in the process water and adding another reagent to the water - in this case aluminium - creates the necessary plusthree cations to stabilise the more soluble elements in the residue.
CSIRO has now patented this technology.
ERA is continuing to investigate this and other process water treatment options.
The company has now commissioned the Minerals Down Under Flagship to investigate other ways to neutralise the Ranger acidic tailings slurry stream without using lime, thus producing a different form of process water that may not require complex secondary treatment before it can be disposed of.
Dr Milnes said that, as with the hydrotalcite solution described by Pleysier, the new research is also looking at the role of aluminium in the neutralisation of Ranger’s tailings slurry.
It is possible such a step could make use of waste-water streams from the alumina industry, for example from Rio Tinto Alcan’s operation at Gove in the Northern Territory.

For more information contact Ron Pleysier at email: Ron.Pleysier@csiro.au or tel: +61 (0)8 9334 8923