Beware Falling Rocks: Acoustic Measurement Helps

Oct. 13, 2015
A Canadian copper mine faced considerable safety and monetary issues because of its inability to accurately measure an area of crushed ore storage with laser, ultrasonic or radar technologies. Low-frequency acoustic measurements helped change that.

Imagine working in a copper mine and hearing 2 tons of ore bounce off steel structures around you after a 184-ft drop. The noise is intense. Dust fills the air. Flying debris can shatter lights and damage nearby equipment. “You feel it in your chest,” says Daniel Kemp, technical sales specialist for Lakeside Process Controls, a local business partner for Emerson Process Management in Ontario, Canada.

Worker safety was a key concern faced around the ore passes at a mine outside Sudbury, Ontario. But another important issue was the money and time lost from press frames and liners that were continually damaged by falling rock—an issue that could be avoided if only they could find a way to keep track of minimum volume levels in their ore passes.

Ore passes are used to store the ore once it’s been crushed and before it’s transported up and out for off-site processing. At the Morrison mine, five different types of ore get crushed and fed into the same ore pass. Although leaving large remnants in the ore pass can cause too much of a mixture of ore types—diluting a batch, which has cost implications—leaving too little volume leads to considerable damage of the liners on the press frame as well as the press frame itself. So rather than let the ore pass run completely empty, leaving a small amount of ore in there helps keep the rock from falling through and damaging the frame and its liner.

But it turns out that actually measuring the volumes inside an ore pass is easier said than done. “From a measurement point of view, you can treat it as a silo, albeit an unusual one,” Kemp said, presenting the mine’s challenges at the Emerson Global Users Exchange in Denver this week. But it’s a silo that brings some particular challenges with it.

For starters, it’s some 4,500 ft underground. With the instrument shop up at the surface, the mine needs a low-maintenance solution. The technology not only needs to be able to penetrate through the extremely dusty environment, but should ideally be self-cleaning as well to deal with the inevitable dust buildup.

Add to those requirements the fact that this is no typical silo. Unlike a silo’s smooth walls, an ore pass is rocky, making it uneven and subject to wear. And rather than a nice, flat bottom, it’s cone-shaped. The meter needs to be able to measure down into the bottom of the cone, as much as 185 ft down, and ensure an accuracy level of at least ±5 percent to meet setpoints.

The mine tried laser, ultrasonic and radar technologies, but none of them worked or got them the information they needed, according to Kemp. Using an acoustic transmitter from Emerson Process Management, the mine has been able to measure levels and volumes at an accuracy of less than ±3 percent, mitigating the risk of damage to press frames and liners, reducing downtime and improving worker safety.

Emerson’s technology provides top-down, direct volume measurement with low-frequency acoustic waves that are transmitted from antennas. The device collects echoes from multiple points to measure the exact volume and create a 3D mapping visualization. For bigger spaces, several scanners can be connected together.

“The type of vessel is very flexible,” said Eran Perlstein, product director for Emerson. “We can configure multiple scanners for different shapes. We can calculate the surface material as long as we have good coverage.”

Getting good coverage is a matter of finding the optimal location of the scanner. “If there’s no optimal coverage, we can use multiple scanners,” Perlstein said. “There’s no limitation on the number of scanners we can use to be able to calculate the volume of material.” A silo in Russia, for example, uses nine scanners to measure a space 84 meters in diameter and 46 meters high, he added.

The low frequency of the acoustic signal enables the technology to cover large surfaces. Unlike with ultrasonic technologies, dust has very little effect on the performance of acoustic technologies, Perlstein said. “The lower the frequency is, the lower the impact of dust or humidity on the performance,” he said.

Although level measurements are enough for liquids, solids require volume measurements instead. Rather than inferring the volume from a single-point measurement, this technology maps the true volume, Perlstein pointed out.

The 3D solids scanner is also self-cleaning for minimal to no maintenance. “You just configure it, and that’s it,” Perlstein said.

The reduction of preventive maintenance costs adds to the mitigated risk of damage to the press frame and liners. Each replaced liner costs $50,000 and two days of downtime. The mine has not had to replace a single liner since installing the system nine months ago, Kemp said. “They’re very, very pleased with the amount of damage reduction they’ve had.”

And, most important, miners are no longer exposed to falling rock.

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