At first it sounded a little like popcorn popping, then a lot like a washing machine struggling out of balance.
Considering there were nearly 200,000 volts being pulsed into a baseball-size piece of rock, one might expect an ensuing explosion. But the experiment ended quietly after all.
University of Minnesota Duluth geology professor Christian Schardt and UMD graduate student Matt Matko opened the door to the Swiss-made High Voltage Pulse Power Fragmentation device and got a peek at what remained of the rock.
Coffee grounds, it looked like. In reality, fine fragments, almost like grains of black sand, had burst apart when the high-voltage sonic beams hit the water bath the rock was encased in.
Schardt used his finger to sort across the black grounds and expose flecks of gold. Real gold. The stuff that's $1,100 per ounce.
"What it does, in one very fast, easy step, is separate the (valuable) minerals from the black stuff that you don't want,'' Schardt said.
That particular rock had come from a gold mining region. But Schardt and Matko had also blasted apart a piece of copper-and-nickel bearing rock taken from the Eagle Copper Mine in Michigan's Upper Peninsula. While most rock mined at Eagle averages about 6 percent copper, this particular rock held a much smaller percentage of valuable metals - so little, in fact, that the mining company doesn't bother to process it.
"But it's right down there with the stuff they are going after. So if we can develop a way to get at that lower-grade ore that's cost-effective, it would give them another option ... more bang for their buck,'' Schardt said. "Right now, with traditional crushing and grinding, it's not worth the effort."
Schardt also hopes to see how well the blaster separates different minerals within the rock, or whether copper and nickel will remain clinging together after the blast.
The sonic blaster comes from Switzerland-based Selfrag AG and is housed at UMD's Natural Resources Research Institute through January, on special loan for a test drive.
"If we sent these samples out it would cost between $500 and $900 apiece, so it's nice to have this option right here,'' Schardt said.
Schardt and other geologists (Twin Metals and PolyMet have expressed interest) would like to keep the device permanently. But University of Minnesota officials might have sticker shock about buying their own pulse fragmentator at a cool $500,000 each.
"There are only two of these in the U.S.,'' Schardt said, noting the other is at Columbia University (in New York City). There's also one in Kingston, Ont.
But mining companies in South Africa, Canada and Australia already are adapting the technology to their mineral-processing efforts. In South Africa, a commercial-scale prototype can blast 10 tons of rock per hour.
Schardt, who is part of UMD's Swenson College of Science and Engineering, said the sonic blaster's most intriguing and potentially valuable characteristic is its ability to separate the valueless rock, or waste product, from valuable sulfide minerals like copper and nickel.
That leaves the stuff you want, the valuable stuff that's high in sulfide and a waste product with almost no sulfide, Schardt said.
It's that high-sulfide rock that's causing the most concern over proposed Minnesota copper mining projects because of the potential runoff when that sulfide meets air and water. Critics of copper mining say that waste product could lead to polluted downstream waters.
But the end result of the blaster "separates out the elements that are potentially environmentally dangerous and leaves you with a waste stream that is, for the most part, environmentally neutral,'' Schardt said.
The sonic fragmentation does not destroy any part of the rock but causes it to "disaggregate'' along weak lines, essentially breaking the rock down into its original particles.
That can help geologic historians like UMD Professor Latisha Brengman separate out the various types of mineral that make up a rock and study their basic chemistry, which helps explain how and when the rocks formed in the first place, billions of years ago. (Iron from the Mesabi Range, for example, originated thanks to volcano-like thermal activity under an ancient ocean.)
The blaster "eliminates three or four steps. It saves a lot of time and effort,'' Brengman said. "It's also a lot cleaner process. You have less chance for contamination of the samples."
Schardt has gone beyond geologic applications in the few weeks he has to test the blaster. He recently threw in a defunct circuit board and blasted it apart. What came out were bits of plastic separated from bits of metal.
"You get metals and plastics and you could recycle both. They wouldn't have to throw this in a landfill'' where the heavy metals could eventually pollute groundwater, Schardt said.
He has other stuff lined up, too, like battery chargers and television remotes.
Civil engineers are hoping the blaster will disaggregate old concrete so it could be reused and recycled rather than landfilled.
But, deep down, as a geologist, Schardt sees High Voltage Pulse Power Fragmentation as a potential boon to Northland's mining industry and the region's environment.
"I think, if they can scale this up to commercial level at a reasonable cost, this could save a lot of time and energy and a lot of waste product problems,'' Schardt said. "I think this might be able to help solve some problems here in Minnesota."