Issues Magazine

Putting the Squeeze on Mining Water

By Jason Du

Mining research is finding ways to reduce the water use of a very thirsty industry.

Australia needs to manage its limited freshwater resource more wisely to maintain a sustainable population and economic growth. The Australian Bureau of Statistics reports that in 2004–05 Australia’s total water use was nearly 80,000 GL (1 GL is approximately the amount of water in 500 Olympic swimming pools) in agricultural irrigation, household, mining and other industries. More than half of Australian water use occurs in the Murray–Darling Basin, even though this area has only 6% of Australian total surface water runoff (www.nwc.gov.au/www/

html/236-water-use-in-australia.asp). Largely due to drought, water consumption in Australia decreased by nearly 3000 gigalitres, or 14%, between 2000–01 and 2004–05. Over the same period, agricultural consumption was down 23% and household consumption decreased by 8%. However, the mining industry used 29% more, particularly in Western Australia, which used 49% of national mining water consumption due to the mining boom.

So does Australian mining industry use too much water or use water wisely enough?

The Australian mining industry disturbed less than 0.01% of Australian land and accounted for 2% of national freshwater consumption during 2000–01, but increased its share of GDP from 5% throughout the 1990s to 8% in the 2008–09 financial year, according to the Australian Bureau of Statistics. Compared with 67% water consumption by agricultural industry and only 3% contribution to GDP, the mining industry is actually very good at converting water consumption to GDP.

However, while the mining industry is a small user of water on a national scale, it can be the largest user at a local level, especially considering that most of the Australian mine sites are in water-limited outback or inland areas. The 2% water consumption means that 401 GL of precious freshwater were taken away from underground or surface water natural resources (see http://www.environment.gov.au/soe/2006/publications/report/inland-waters...).

The mining industry needs water for purposes such as exploration, mining, minerals processing, refining, smelting and staff amenities. Generally, the minerals processing tends to use the most water within a mining operation. It is estimated that on average 0.5 tonnes of fresh make-up water consumption is used for each tonne of ore ground.

When water is continually added to ore grinding and processing operations, these processes generate mining waste or tailings. It is estimated that more than 10 billion tonnes of tailings are produced globally every year. The tailing stream, normally significantly larger than the mineral concentration stream, contains 90% or more of recyclable water.

A typical process to recycle and reuse the water in mineral tailings uses gravity thickener units to separate the water from mineral solid particles. Due to limited solid–liquid separation efficiency of gravity thickeners, the thickener underflow tailings normally still trap 45–60% of water. Most of this trapped water in the tailings is later lost through evaporation during long-term sedimentation in huge tailings storage dams, which could easily occupy hundreds of hectares of land.

As a result, most of the make-up water is lost in this tailing thickening and water recovery process, which results in a constant demand for fresh make-up water. For example, according to a 2006 Rio Tinto sustainable development report, Rio Tinto Comalco Weipa in north Queensland used as much as 21,050 ML of fresh make-up water in 2005.

As one of the largest mining companies in Australia, Rio Tinto is always investing money in scientific research to investigate methods to reduce water consumption during mineral tailings processing. To reduce the water consumption without cutting the productivity, the key is to squeeze more water out from the mineral tailings during the water recovery process. By releasing more water from mineral processing, recycling and reusing the processed water, the pressure on the fresh make-up water would be eased.

A research project at the University of South Australia, funded by Rio Tinto, the Australian Research Council and the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, found that the application of ultrasound could squeeze more water out from the mining waste. During my PhD research I found that the mineral tailings have a similar structure to a honeycomb . The current mechanical raking process only breaks the honeycomb into a smaller closed structure, which is why the thickener underflow still traps a significant amount of water.

After identifying this problem I introduced ultrasonic vibration to collapse this closed, self-supporting structure and achieve a much denser sediment so that water can escape. The principle is similar to shaking a jar of flour, which causes the flour to be compacted.

At just one of Rio Tinto’s mines in the Murray–Darling Basin, the technology could save 436 ML of water per year. These research findings were presented to the public for the first time by the national program Fresh Science in June 2010.

It is hoped that when this technology is used in the field, more freshwater could be allowed to flow back into natural waterways such as the Murray–Darling Basin system.