The mining industry has always needed a lot of water, but this is set to increase as the industry faces a growing demand for the minerals and metals needed to support the energy transition. One of those is copper.
“Copper even looks dry,” starts Simon Yacoub, Sector Leader, Mining, Minerals and Metals for the Americas at Worley. “The reddish-brown colour resembles the harsh and arid, high plain desert locations where it’s mined.”
Yacoub is based in Santiago, Chile, which is the world leader for copper production – accounting for just under 28 percent of global output. Around 1,600 km north lies the Atacama Desert, home to most of Chile’s copper reserves. Here, water is scarce. Existing natural groundwater sources are already insufficient to meet demands of agriculture and the local population.
The country produced around 5.4 million tonnes of copper in 2020. But copper demand may soon outpace supply. Citigroup estimates that the global copper market will reach a 521,000 tonne deficit in 2021 – a gap that only threatens to deepen as the green transition accelerates.
“With copper’s role in the energy transition, this is a supply gap we can’t afford,” says Yacoub. “But we can’t fill it with water-intensive mines. What’s more, the next decade of copper mine development, or any mine for that matter, must have strong environmental, social and government credentials.”
What can we do about water?
Copper is not an easy mineral to mine. Its scarcity and mineralogical complexity mean that extraction techniques need to improve continually to meet technical and community expectations. And responsible water usage ranks highly among those expectations.
Given fresh water needs to go to local communities first, copper miners must find alternative water supplies, while achieving more with water that’s already available. And with many mines designed decades ago when less consideration was given to water scarcity, retrospective changes are going to need to be made.
“There are several motivators to reduce water consumption,” says Yacoub. “The first is water stewardship, and the responsibility to minimize the impact on natural water resources that communities depend on. Another is cost. The drier we can make a mine, the lower the cost of production and the less the requirement to remediate and rehabilitate for closure.
Currently, to keep Chile’s existing copper mines operational and increase the volume they produce, millions of cubic meters of sea water is desalinated and pumped inland – in some cases up more than 4,000 meters of elevation. This consumes vast amounts of power and adds billions of dollars to a project’s capital and operating costs.
“The challenge is reducing a mine’s water use while simultaneously processing larger amounts of material. As the demand for copper increases, technology will be the single greatest enabler of a drier mine. But it’ll be a combination of steps that together form the water-saving story,” adds Yacoub.
It starts with extracting the right material
To scale up copper production without using more water, we need to process consistently higher-grade material. And technology can help to identify it.
“Copper processing plants have always been more efficient at producing waste than producing copper,” says Chris Beal, CEO of NextOre, our joint venture with Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) and with RCF Ambrian. NextOre uses magnetic resonance (MR) technology to provide near-instantaneous whole-of-ore grade estimates.
“If a plant’s feed grade is 0.50 percent copper, which is common, each tonne of copper will make at least 197 tonnes of waste by the time it ends up as pure metal. And the more material that’s processed, the more water is involved,” he says.
“A conventional copper processing plant uses between 0.45 and 0.6 m3 of water per dry tonne of ore. So, for a 50,000 t/d ore copper mine, this will consume around 30,000 cubic meters of fresh water a day. That’s enough to fill an 80,000-seat stadium, like Brazil’s Maracanã, full of water in about seven weeks.”
Bulk ore sorting technologies, such as NextOre, ensure only the higher grades of ore are processed. This means less waste is produced, and less water is needed. And as a result, even with increasing demand, processing plants can increase throughput whilst staying the same size, or perhaps become smaller and more efficient.
Saving water through a coarser grid
Once the higher ore grade is selected, it needs to be ground. Depending on the mineralogy, the average grind size of copper ranges from 50 microns – around the same size as a grain of talcum powder – to about 200 microns.
Could coarsening the grind size help achieve greater water savings?
“A coarser grind is achievable in the milling circuit, with a primary grind of ~300 microns (0.30 mm),” explains Yacoub. “That’s about the same coarseness that you’d grind a coffee bean to make your morning espresso – or coarser with only minor changes to the grinding circuit.”
After grinding, the material then undergoes flotation to separate the valuable minerals from the waste. However, maintaining the level of rougher recovery requires alternate coarse particle flotation, allowing metallurgists to process particles that were traditionally too coarse to float and were lost to tailings as misplaced material in the waste.
“This technology is ready for industry-wide adoption. The ability to float coarser material allows metallurgists to achieve lower comminution energy requirements, but it also gives the waste material better water saving credentials,” he explains. “Grinding coarser allows water to drain from the tailings faster and more completely, leading to better recovery.”
Drier techniques to deal with waste
Tailings dams represent the largest water loss at a mine site, so it’s no surprise that many operators want to move away from water retaining structures. And dry stacking is an appealing alternative.
As the name suggests, dry stacking involves taking the water out of tailings through filter presses. Despite retaining 10-20 percent moisture, the material acts as a solid and can be transported to a storage area, either above ground or back within the mine. Compared to wet tailings dams, this process can reduce net water loss by approximately 40 percent, allowing the water reclaimed from the filter operation to be recycled to the process plant.
While recent developments in large format filter presses have changed how mining waste is treated, it may not be until the widespread adoption of coarse particle flotation, or any comparable separation or dewatering technology, that filtration rates improve to the point where dry stack becomes more economic.
“Dry stacking may seem less economical in some parts of the world due to the cost of filtration, but less so in arid regions experiencing high water stress,” says Yacoub. “For example, a desalination option in the Chilean desert can result in water that costs approximately US$5 per cubic meter. That’s expensive, so in this scenario, dry stacking may become a much more sustainable option.”
Adapting today for the challenges of tomorrow
With each new mine the industry builds, and each additional tonne extracted from existing mines, the more pressing the need to save water becomes. This means miners need to prepare their future water saving strategies now.
“It’s all about aiming for a sustainable model,” says Yacoub. “And wherever there is water scarcity, we need to recover as much water as possible across every stage of the mining process.
“Water is a critical enabler for mining projects around the world, and it will continue to be even as weather patterns change and populations grow. But we need to be bold enough to evolve our existing techniques, or risk losing the social license to operate.”
With the copper supply-gap looming and water stewardship becoming more traceable, demonstrating the sustainable use of water is key to the future of mining.
“In most regions, water is now among the most pressing challenges miners are facing,” says Yacoub. “But if we get this right, the industry will have a lot to be proud of.”