University of Arizona scientists Stephen...
Sustaining the Mining Supply Chain
The new Lowell Institute for Mineral Resources is charged with making the process of extracting copper more sustainable and energy efficient.
Mary Poulton, head of the University of Arizona's Department of Mining and Geological Engineering, was explaining how copper is extracted from rock. She looked around her office and produced a baseball-size specimen speckled with shiny deposits.
"You have to crush this rock down to something that's the size of powder," she said. "Literally, you start with blocks that are as big as this desk." Several stages of grinding reduce the blocks to a powder. That goes into vats of a chemical-and-water soup that is aerated to create bubbles. The chemicals bind the copper to the bubbles and it can be skimmed from the surface of the broth. The process requires a lot of water – potable water – and energy.
"What we're looking at," Poulton said, "is can we replace fresh water with effluent or brine – something that's non-potable – and still manage the chemistry without throwing away too much of the copper?"
Such a breakthrough would have a profound effect on the mining industry in the mineral-rich but parched Southwest. But concern for water and energy consumption and mining's impact on the land goes much farther.
"We think the UA is really unique in the world in terms of the depth and breadth of environmental people on campus," said Poulton. "It's let us put mining into a much broader context in terms of sustainable resource development."
The department's newly established Lowell Institute for Mineral Resources, funded by Science Foundation Arizona and industry partners, has 27 research projects under way, Poulton said.
"We've done things because they've worked, and we've never really understood why," she said. "And if you don't understand why, you really can't optimize the process. And now we finally are getting the tools and the science and the fundamental understanding that let us go down and see why this works so we can redesign the process."
The new tools include atomic force microscopes to examine the chemistry of processes at the molecular level. It's not only good business for the mining industry, it's crucial to keep the supply line running. Copper, long one of the "5 C's" along with cattle, climate, citrus and cotton that were the
bedrock of the Arizona economy, will become even more important because of the large volume needed for electronic devices – "even more so as we go to renewable energy," said Poulton, "because wind and solar are big consumers of copper, as are hybrid cars."
Jinhong Zhang, assistant professor of mining and geological engineering, is trying to unravel the mystery of the chemical changes involved in the flotation process of mineral separation.
The principle is to turn minerals like copper hydrophobic, or repellant to water, much like car wax makes water bead up. In his basement lab, Zhang demonstrates assorted mineral processing equipment. One device grinds the rock, and then the particles are put into a sifting machine. A series of stacked sieves can capture different size particles. Finally, the deposits are put into a metal flotation bucket with an aeration mixer, almost like an ice cream soda mixer. Aeration causes big bubbles to form. Zhang adds just a few drops of chemical with a hypodermic needle to create finer bubbles. This will float the particles and they can be pushed over the top of the bucket. Silicates, or clay, fall to the bottom of the bucket.
He uses an atomic force microscope to see what happens on the surface of the copper in nano scale. "We want to find out why this one makes recovery decrease," he said. "We have to know the reason, then we find the solutions. We suspect maybe there is one thing in effluent. In effluent there's a little bit of organic content inside."
He tested various chemicals used in processing and found one that floated the copper but did not bind to the silicates. He will test to see if droplets form the same patterns with different concentrations of reclaimed water. "Let's see what happens on the surface," he said. "If it becomes less adsorption on the surface we know, OK, we have to do something to treat this water to change it back."
Poulton said similar nano technology can be used to study if they can soften the bonds in minerals "so that we can use less energy to break the rock."
"We really have an opportunity, because Arizona is so well-endowed with mineral resources," Poulton said. "We have the ability to be leaders in basically new mine designs for the future that have a much smaller environmental footprint. Doesn't change the fact that somehow this has to come out of the ground in some way. But beyond that, the things that we can control if we understand fundamental science that goes with it and the engineering, then we can design a better, cleaner process."
These are important issues, Poulton said, because consumption of resources on the planet is going up exponentially. "Our ability to supply at the level the U.S. is used to consuming and wasting is not going to happen," she said. "The materials are so complex and the sourcing of the minerals for those materials becomes so complex and the time that it takes to bring a mine into production seems to get longer rather than shorter. If you tell me you're going to have a critical need for something today, I'm probably going to say, Sorry, it's going to be 10 years before we can help you. I hope you can wait that long."
The advance of technology – cell phones, GPS systems, plasma screens – requires new sources of increasingly hard-to-find, hard-to-extract minerals. "Better check to see if we have it," Poulton said with a laugh.