CRESCENT VALLEY, Nev. (AP) - Whether it’s a surface pit or the underground, mines need explosives and people to handle them.
A blast crew completes this integral part of the mining process.
Before the dirt can be dug up by a shovel and processed, it has to be broken apart. The best way to accomplish this is to loosen the hard rock.
The first step begins with the geologists and engineers. The geologists identify what types of materials are in the ground and the engineers design the blast pattern over the areas to be mined.
Once the pattern is designed, the drillers dig the holes for the explosive materials. When they are done, the ground resembles the most aggressive gopher field anyone would ever see.
After these initial steps, the blast or powder crew takes over the area.
The blast crew arrives at Barrick Gold Corp.’s Cortez Hills at 5:45 each morning. The crew goes over safety issues and other responsibilities during its meeting at 6:15 a.m., said Kevin White, Cortez Hills drill and blast supervisor. The blast usually goes off at 2:45 p.m. and everyone is on their way home by about 4 p.m. after working a 10-hour day.
“We blast Monday through Friday,” blast technician Travis Cummins told the Elko Daily Free Press (https://bit.ly/1jMHgNM). “We usually blast once a day. Sometimes it’s twice a day. It depends on how fast the shovels operate.”
The patterns vary in size. They can be 30 to 200 holes, depending on what needs to be mined and what kind of material is in the ground, Cummins said.
“The first thing we do when we get on a pattern is cone it off and put up signs,” Cummins said during a tour at the end of January. “It lets people know this is our work area. The signs and cones have to be a minimum of 50 feet from the outermost zone of our work area. No one is allowed in the area unless they communicate with the crew first.”
Among the safety measures used are the colors on the blast crew’s equipment - green. No one else in Cortez is allowed to use the color green for safety equipment. The trucks have green reflective stripes and the cones and signs also are green.
Choreographed Chaos
Like other miners, the blast crew must follow U.S. Mine Safety and Health Administration regulations, but it also is regulated by the U.S. Bureau of Alcohol, Tobacco and Firearms. Only a handful of people on the mine site are allowed to remove the boosters and blasting caps from their storage areas.
“There are restrictions on how explosives are stored, transported and handled,” White said.
A Pentex booster with a blasting cap is tossed into the bottom of the hole, White said. The wire connected to the blasting cap is tied around the stake on the surface.
Then the explosive mixture, called Anfo, is pumped into each hole. A member of the crew monitors how much Anfo is used, White said.
“It looks kind of like chaos, but it is very choreographed,” White said.
The amount of explosive is determined by the depth of the hole and the type of rock, and it is critical the holes are not overloaded or underloaded.
From a distance the Anfo looks like a light pink liquid, but it actually comes out of the trucks in dry pellet shapes about the size of a BB.
“They put a pre-determined amount of explosive in each hole,” White said. “We want to contain the energy. We don’t want it just flying up in the air, so we fill the holes with dirt.”
Good Vibrations
After the holes are filled and covered, the wires connected to the boosters and blasting caps are wired together in a pre-determined order, White said.
“The engineers determine where the holes go, but the blast crew determines how the holes are tied together,” he said. “We put a seismograph on every blast. It helps us identify the vibrations coming off the blast to maintain fragmentation and vibration.
“We’re not blowing the rock up, we’re blowing the rock apart,” White said. “The material will move the rock about 15 feet.”
Blast technicians Brian Dunn and Bruce Krajewski explained the process while they loaded each hole’s information into a Logger. The Logger is a device that helps to run the computer program that sets off the blast.
The machines are then connected to the fire line and a “surb” - a remote blasting box - which connects to a remote detonator.
Blast Time
Once the patterns are prepared, certain areas must be evacuated. All employees are moved outside the perimeter of the blast, which is a 1,500-foot radius.
The siren sounds three minutes before the blast.
The blasts started at a corner and moved through the pattern like a wave. Each hole went off one at a time, but they all exploded within milliseconds of each other. The bench goes from a flat surface to rolling dirt and then settles as the dust cloud clears.
“Once the shot is done, we can check what happened electronically, but we also do a visible inspection,” Cummins said.
The blast crew looks for faults or breaks. Some people also may need to walk the “shot muck” after the ground settles. Shot muck is what miners call the material that has been blown apart.
“To walk the shot muck, you have to be trained,” Cummins said. “There could be voids and other things under the shot muck, so you have to be careful.”
The other advantage to the electronic monitoring of the holes, is that it allows the blast crew to determine if some of the explosives did not detonate. The crew will give the information to the shovel operators, so they can treat the area as a hot zone.
The area that was blown apart at the end of January was probably reached by the shovels for mining at the end of the first week in February.
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Information from: Elko Daily Free Press, https://www.elkodaily.com
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