Martin Engineering installed the cannons in a chute carrying damp fines through the refining process at the mine in Michigan's Upper Peninsula.
By mitigating blockages and facilitating the movement of material, the cannons resulted in a calculated 1000% in savings to the cost of operation over existing solutions, as well as improved safety, reduced labour, greater production, and less downtime, said Martin Engineering.
"Safety is a top priority for us," said Ted Lakomowski, Lead Reliability Technician at Eagle Mine. "When we experienced clogging and downtime at the processing mill, our crew naturally swung into action to resolve it, but we immediately sought a safer long-term solution."
Eagle Mine is the only primary nickel mine in the United States, producing 1.5% of the world's total nickel production. The company extracts approximately 2,200 st/pd (2,000 mt/pd) from the underground nickel-copper mine using a bench-and-fill stoping process.
Ore from the mine is stored in a covered coarse stockpile facility prior to transport to the Humboldt Mill. A former iron ore processing plant, the Humboldt facility's three-stage crushing circuit reduces the material to 3/8 inch-minus, then a single stage ball mill grinds it further to sand, where it is mixed into a slurry.
To liberate the nickel and other minerals from the waste materials, a refining process of selective flotation is used. During the crushing process, a mesh screen separates the fines from the remaining aggregate, which are fed back through the process.
Fines that pass through a screen fall into a wide-mouthed hopper, leading to a chute that narrows to approximately 8 ft (2.5 m) wide by 2 ft (0.6 m) high and -- after a dead drop of several feet -- slopes abruptly in a ~45º angle of decline.
This slope slowed the descent of the fine material for a low impact and centred discharge onto a conveyor belt leading to the ore bins. Material buildup began at the hopper and at the discharge slope, but could also occur at virtually any point, blocking the chute.
Accumulation would stop the entire crushing process approximately 3-4 times per shift for as long as an hour, blocking input of material all the way back to the ore storage area. Workers attacked the clog with 15 foot (4.5 m) long air lances from the top of the hopper and bottom of the chute. The method used a tremendous amount of compressed air and diverted manpower from other essential duties. Moreover, air lances caused excessive splash-back of wet material, which was extremely messy and potentially hazardous.
Eagle first installed a polymer lining in the chute. Offering a low coefficient of friction, the lining was bolted to the chute wall and acted like a smooth slide for the material to ride down. Less effective against the adherent qualities of the material than hoped, Eagle next installed pneumatic vibrators onto the vessel wall, intended to agitate the adhered material and promote its descent down the chute slope. But the fact that the polymer lining was bolted to the vessel caused it to dampen the vibration of the units, limiting the force to only the impact zone and not much farther.
"We were forced to default back to air lances, but kept on looking for a better solution," Lakomowski explained. "Having worked with Martin Engineering in the past, we asked them to come in, examine the issue and offer a safe, effective and affordable solution."
Lakomowski advocated for the initial installation of five 35 Liter (9.25 gallon) Martin® Hurricane Air Cannons, followed by two more placed in essential spots in the chute. One unit was placed at the area where material discharged into the hopper, two others were positioned at the hopper slope where the most accumulation was observed and two more were placed along the drop chute. All of the tanks were accompanied by a 4-inch (101 mm) pipe assembly ending in fan jet nozzles.
Strategically positioned at a 30º angle, the cannons keep material flowing.
Offering more force output than designs double their size with considerably less air consumption, the compact air cannon tanks measure only 16 in. (406 mm) in diameter 24.92 in. (633 mm) long, weighing 78 lbs (35 kg) each. The units fire a shot of air at up to 120 PSI (8.27 BAR) through the pipe assembly to a fan jet nozzle. The nozzle spreads the air stream 12 inches (304 mm) at the exit point, distributing the blast pattern across the surface of the wall.
Safety cables secure the cannons to the vessel.
Operating on a regular firing schedule of every 1-10 minutes -- readjusted for production volume, time of year and moisture level -- revealed the seven-cannon configuration reduced clogging issues and downtime. This significantly lowered the risk to operators and reduced the cost of operation.
"When I did the cost assessment, I was surprised to discover that there was a 1000 percent compressed air savings in using the air cannons over the air lances," Lakomowski said. "It's a significantly lower effect on our system than initially predicted, and managers are very happy about that."
The project also improved safety, as workers spent less time diverted from other assignments to use air lances or create vibration by beating on the vessel walls. By being able to perform maintenance on wear parts like valves from the outside of the cannon without tank removal, upkeep can be safely performed by a single technician with no heavy lifting involved.
"Just from a safety aspect, this solution has paid for itself," Lakomowski concluded. "The Martin Engineering team was easy to work with, and they were cognizant of our budget restrictions. Overall, this was a successful project."
