Revolution in treating contaminated mines

A by-product of bauxite refining is proving invaluable as a method of cleaning up metal-rich, acid mine water.

Acid mine drainage (AMD), failed tailings dams, sulphidic mine wastes and acidic soils blight the mining industry year after year.

There is no more obvious example of the impact the mining industry can have on the environment than the Baia Mare cyanide spill of February 2000. In one of Europe’s worst environmental tragedies, toxic wastewater poured from a failed tailings dam. Up to 100,000m³ of tailings water contaminated with a lethal cocktail of cyanide and heavy metals was discharged from a Romanian tailings pond when abnormally high winter temperatures and precipitation caused the dam to be breached.

Virotec International was called in to demonstrate how it could use BauxolTM technology to prevent such an event ever occurring again. The results surprised independent observers including former EU mining executive, Folmer BangHansen. ‘We found this [BauxolTM] to be one of most effective methods for cleaning up acid mine water and heavy metal pollution,’ he said.

Baia Mare is just one case of the devastating potential of AMD. The combination of heavy metals and high acidity can damage water resources, kill off eco-systems and trigger disease. The cost to the mining industry in political and financial terms is enormous. In the US around 2.7 million tonnes of acid rock drainage is generated each year polluting more than 26,000km of waterways. Globally over the past 50 years, there have been more than 250 recorded tailings dam failures.

The useful properties of BauxolTM technology were first recognised by scientists in Australia in the early 1990s. The technology has since been developed following eight years of environmental research and laboratory testing. BauxolTM is produced by chemically and physically modifying the residue from alumina refining enabling it to neutralise acid and reduce the concentration of heavy metals by up to 100,000 times.

The technology works by attacking the dangerous components of AMD. When Bauxol TM is added to metalladen mine water it triggers a chemical reaction with the heavy metal contaminants which are then bound on to the fine grains as insoluble minerals. Dr David McConchie, professor of engineering and environmental geochemistry at Southern Cross University and a Virotec director explains: ‘Bauxol TM adsorbs on in the first place but then the minerals present recrystallise to form totally new minerals with low solubilities.’

The new minerals are very stable. “ We have found that the longer you leave it after the metals have been bound on, the more tightly they get bound,’ says McConchie. ‘If you try and leach them off a week after the treatment, you get a small proportion off but if you wait six months, you get even less off.’

Bauxol TM grains settle within 48 hours to form a thin layer of sediment. No more than 2-3mm thick, the sediment continues to extract trace metals from the AMD water for up to 160 days.

McConchie, who leads Virotec’s research team, is confident his company has a product for remediating AMD, acid sulphate soils, sulphidic mine tailings, waste rock dumps and a range of other environmental prob lems. As well as a high trace metal trapping potential, Bauxol TM has a high acidic neutralising capacity: 2.57.5 moles of acid per kg.

The potential of Bauxol TM technology was first demonstrated on a large scale in July 2000. Virotec applied it to a 10m deep, 1,500Ml tailings dam at Mount Carrington in northern New South Wales. It was the largest body of contaminated AMD water ever treated by direct addition methods. Red Bauxol TM powder was sprayed directly onto the dam. It dispersed, settled within 48 hours and left the water completely clear.

‘The Mount Carrington dam was classified by the mines department of New South Wales as the third worst potential disaster in the state,’ says Virotec’s executive chairman Brian Sheeran. But after treatment with Bauxol TM, more than 99.99% of the heavy metals were bound up and converted into new and harmless crystalline substances which quickly settled out of the water. Also, the acidic water was neutralised.

In little over a few weeks, Virotec cleaned the toxic dam water to stringent standards allowing it to be released safely into the nearby Plumbago Creek catchment over the following 12 months at the rate of 1 to 2Ml per day. Discharge water has bettered background levels in the catchment over the period and aquatic life has returned, not only to the dam but also to the immediate discharge catchment.

The results achieved in the 12 months following treatment show that the water quality is maintained, but treatment still occurs long after active treatment ceases. Before treatment, pH levels in the dam were 5.2. Immediately after treatment, the pH of the water increased to 7.3 and 12 months later stood at 8.1. Zinc levels before treatment were in the region of 11,570µg/l. After treatment, the level dropped to 39µg/l and currently stands at less than 1µg/l. Similar reductions in other heavy metals have been noted.

During the treatment process, metal contaminants remain locked in the Bauxol TM sediment with no evidence of redissolution.

The fine residue remaining after treatment differentiates Bauxol TM from traditional processes which can leave a highly dispersed sludge. The residue is inert and can be used as a soil conditioner after it has captured and bound up heavy metals. Treated water can then be safely released and sold on for irrigation, process or drinking water purposes, or simply released back into local ecosystems.

Bauxol TM technology has been used to remediate AMD at other sites. Similar reductions in heavy metals have been observed at Mount Morgan, Queensland and Captains Flat, NSW. Projects are being planned for western Europe, which is supplied with Bauxol TM from an alumina refinery on Sardinia.

Bauxol TM is also being applied to environmental scalds such as acid sulphate soils and is helping to renew vital plant life. Other applications of the technology include treating chromium-rich acidic tannery effluent, treating leachates from domestic and industrial waste disposal sites, removing arsenic from drinking water, phosphate removal to prevent blue green algal blooms, improving plant growth in soils with poor water and nutrient holding capacity, and treating sulphidic marine sediments.

In July 2001, Virotec started trading on London’s Alternative Investment Market (AIM) after a $10.7M capital raising closed oversubscribed. Later this year, the company intends to apply for listing on New York’s NASDAQ exchange.


| mining



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