CARLIN – While other mines are holding steady or cutting back, Newmont Mining Corp.’s Leeville Underground is ramping up production and hiring more people.
The mine commissioned the Turf No. 3 Ventilation Shaft in November, said Veronica Tough, chief engineer for Leeville.
When the underground only had two shafts, the No. 1 shaft was exhaust -- hot and humid air -- and the No. 2 shaft was intake. The new system and No. 3 shaft allows both of those to be intake, she said. The air flow direction changed on the No. 1 shaft and the No. 2 shaft kept the same direction but the quantity of air through it changed, Tough said.
Prior to the new system, the mine had 1.2 million cubic feet per minute of air through the mine. Now Leeville is at 1.9 million CFM.
“So the volume increased,” Tough said. “I would say the big change is the ventilation system prior was set up to mine the areas that we had at the time. … The new vent shaft allows much fresher and more volume of air down to the area where we’re doing a lot of the work.”
Complaints from the miners used to be it was too hot to work and now they're complaining that it’s too cold or too windy, she said.
The new shaft will allow Newmont to explore more underground.
“Prior to putting in the new shaft, ventilation was our limiting factor for how much we could produce, how much drift we could drive, how far we could get away from our main infrastructure,” Tough said.
The new shaft allows Leeville to ventilate in areas it couldn’t before.
“The big justification to the shaft was being able to bring more equipment into the mine and mine at a higher pace,” Tough said. “Between the three groups -- engineering, maintenance and operations -- we’re adding over 100 people over the next year to the workforce, and that’s just our people. We’ve got contractors too, that we’ll be bringing in to help us out. It’s a huge benefit for the economy. You look at a lot of other sites are ramping down and we’re ramping up. So it’s a very unique position and a good place to be with the gold prices the way they are.”
Albert Keim, maintenance superintendent, said the new system, “in some aspects,” makes the maintenance more complicated.
“In order to make this change underground, we had to install, just for the commissioning portion, roughly 40 bulkheads, air doors and booster fans,” Keim said. “So that means just more maintenance on those bulkheads, or those structures. But for moving the fans from underground to surface, the conditions that they’re in, it’s more acceptable. You don’t have to deal with ground conditions, ground movement, mine sap getting in your blades and stuff like that. Everything is located on the surface now. They’re a lot easier to work on.”
The technical name for mine sap is “Copiapite” which is naturally occurring at Leeville, he said.
Keim said having redundancies for the ventilation system also helps. When there were only two fans, it limited production underground at times.
“Before we had two main fans that had to be running all the time in order for us to provide the mine with enough air,” he said. “Now we have four fans. At this time, two are off – one’s in maintenance repair at all times and one’s a backup at all times. So we have redundancy now, so that’s a definite benefit.”
Keim said the ventilation is operated by the hoist house at Leeville. The operator can start and stop fans.
Changing Air Flow
Once construction was completed, the fans had to be turned on and the direction of air flow was changed in a portion of the mine.
“We turned on the main fans on Nov. 9, so it was kind of an interesting process on how to do that,” Tough said. “The days leading up to Nov. 9, we had full crews underground, but a lot of the work was around making sure the air doors and things like that were in the correct setting and position.”
Newmont had a “very limited number of people on site” during the commissioning process, she said.
“So we had six teams of three to four people each that went underground and did that last minute work, so it consisted of mostly engineering and maintenance folks to take care of that last minute stuff that was affecting the ventilation underground, and then we finished all of that by noon on the 9th and turned on the main fans that afternoon,” Tough said. “That night we allowed the mine to gas out, to make sure that the ventilation stabilized.”
Crews were on site Nov. 10, but no one was allowed underground until the ventilation and geotechnical inspections were completed. Tough said 11 geotechnical inspection teams and six ventilation inspection teams started at various points throughout the mine.
“It was a long day,” she said. “I think it took us the full 10 hours. Crews were underground by 7 and the last ones came up by about 4.We were actually able to get back into production that night.”
She said the inspections were done as a safety precaution and to make sure nothing went wrong underground.
“It’s certainly not something that we do every day. It’s kind of a once in a lifetime thing,” Tough said. “I think in one of our pre-meetings, the question was asked, ‘Who’s ever reversed the ventilation in a mine before?’ I think only one or two people out of the 30 people that were in the room had actually ever done this in their career. So we wanted to make sure we took the proper steps to make sure it was done correctly.
“The ground conditions at Leeville are pretty unique. We wanted to make sure that areas at the top of the mine, that used to be really hot and moist, were now fresh air intakes, so very cool, dry air, and we just wanted to make sure that wasn’t negatively affecting the ground conditions in the area before we send people back to work.”
History of the Project
Designing for the project started in 2010 and construction began in 2012, said Craig Gammill, construction manager for the project group. At the height of construction it employed 160 people. The project was a $360 million investment for Newmont.
When the shaft was under construction, it was sunk through conventional means, but the ground was in an unusual state, because it had to be frozen. The shaft was sunk through an aquifer -- 400 feet down to 1,800 feet was saturated -- so the freezing allowed the construction crews to control the drilling.
As the shaft was drilled, construction crews poured concrete every 15 feet, Newmont stated previously.
The shaft also has a conveyance feature, Gammill said.
“The unique feature that’s associated with the hoisting set up over here is an airlock for the conveyance to enter and exit the shaft without interruption to the ventilation flows,” he said. “So that was also an opportunity that we took advantage of, by sealing the shaft, starting the fans and then completing the headframe and airlock construction afterwards.”
The airlock will allow access to the shaft for conveyance and inspections.
“We’re currently in the process of commissioning that hoisting system now,” Gammill said.