In southeastern Missouri lies a metal-rich region called the Viburnum Trend. The ore — which consists primarily of lead, with small amounts of zinc and copper mixed in — is buried 1,000 ft. (304 m) underground.
To recover the metals, the ore must be drilled, blasted, hauled to a shaft, lifted to the surface, and processed in mills. The annual production of six mines in the area, completed by the SEMO (Southeast Missouri) Mining and Milling Division of The Doe Run Company, totals approximately 5.5 million tons of ore.
The first mine in the Viburnum Trend opened in 1962. Other mines followed; at one time there were nine mines operated by four different companies. After a series of buy-outs and mergers, those nine mines became six, all of which are now owned and operated by The Doe Run Company.
Today, four of the six mines are connected underground. One can climb into a mining truck at the north end of the Casteel Mine and drive south for 22 mi., traveling through the Buick, Brushy Creek and Fletcher mines.
Some 850 workers are employed at The Doe Run Company mines, and safety is a major part of their workplace culture. “We have a huge safety commitment,” said Greg Sutton, general mine manager at The Doe Run Company SEMO Mining and Milling Division. “We want people to go home every day in the same shape they came in. We pride ourselves on being able to do that.”
Here’s proof. Every year, the U.S. Mine Safety and Health Administration presents the Sentinels of Safety Award, recognizing the safest mines across the country in several categories. In the fall of 2012, Doe Run Company Sweetwater Mine and Mill earned the Outstanding Safety Performance in the Large Underground Metal Mine category, marking the seventh time Sweetwater received the recognition. Doe Run and its predecessor companies have earned the Sentinels of Safety award 25 times since 1971.
Volvo Plays Its Part
Volvo operators and the machinery that supports their work play a big role in achieving that safety record. The SEMO Division began running Volvo A30 trucks to move ore underground in the mid 1990’s. “Within a year or two, we moved into Volvo A35 trucks at some of the other mines,” said Sutton. “In about 1999, we bought our first A40 haulers. Today, 23 of the 45 haulers in the four large mines are from Volvo Construction Equipment.”
Of the 23 Volvo machines, 19 are A40D units. The four remaining are A40E FS full suspension haulers. Full suspension — an exclusive Volvo feature — uses electronics and nitrogen gas to help cushion the ride and level the load. “Full suspension absolutely enhances our hauling production,” said Sutton. “It is easier on the truck and the operator.”
Sutton said Volvo suspension trucks also help increase efficiencies in their operation. “Our speed limit underground for the non-full suspension trucks is 27 miles per hour, but we can bump that up to 29 with the full suspension units,” he said.
“The full suspension units are smoother, and the operators can handle them better. Because of that, I would estimate that we get between 5 and 10 percent more production with the full suspension trucks,” he added.
Underground to the Grizzly
The average underground haul length is 2 mi. (3.2 km), one way, with a maximum haul distance of 5 mi. (8 km). The 2-mi. cycles — including loading and dumping — can take a truck about 20 to 25 minutes to complete. The hauler travels from a mining face, where the blasted ore is loaded, to the grizzly, where it dumps its contents, back to the face.
The grizzly is a hole guarded by spaced steel bars, which feeds the ore to a lower level, containing an ore crusher, or skip pocket, or both, according to Sutton. A skip pocket is an area where rock is loaded into the skip, or steel box, to be hoisted to the surface.
The SEMO Division uses the room-and-pillar method of mining. The actual mined-out openings — widths of a drift —measure about 32 ft. (9.8 m) wide. The pillars which support the roof are approximately 28 ft. (8.5 m) wide, with distances between the pillars measuring about 60 ft. (18 m), center to center. Drift heights measure anywhere from 10 to 120 ft. (3 to 36.6 m) in one room. The average width of an ore body is about 400 ft. (122 m), according to Sutton. In plain view, the pillars are checker-boarded over the ore body.
Drilling and Blasting
To drill holes for explosives at the mining faces, the SEMO Division runs one- or two-boom jumbos. The booms are mounted on a carrier, resembling a center-articulated truck with an engine, hydraulic system and electrical system. During drilling, the jumbo carrier remains stationary and the booms move to drill holes averaging 14 ft. (4.3 m) deep.
“In the four mines, we have about 200 faces with approximately 20 drills working at any given time,” said Sutton. “We usually shoot twice a day, at the end of each 10-hour shift. Then, we do what is called mucking, which consists of loading the ore into haul trucks with front-end loaders.
“Somewhere in the middle of the mucking process, we have to scale the blasted faces because anytime you blast you make loose rocks,” he said. “We don’t want loose rocks to fall down unexpectedly, so we have to knock the loose rocks down, or scale them, before somebody gets under them.” With hand scaling, a worker knocks down loose rocks with a pry bar. Mechanical scalers use a truck with a boom-and-pick system.
“Every day, in multiple faces, we are drilling, blasting, mucking, scaling, and more,” continued Sutton. “And we keep up with our utilities such as power and ventilation. At any given time, we are completing all of these tasks at one of our mines.”
Each of The Doe Run Company mines contain ore hoists with skips in the shaft. The ore hoist sits on the surface and raises and lowers the skip. The loaded skip travels up the shaft to the surface, where a tripping mechanism opens the skip door to dump the load.
When primary mining is complete, the SEMO Division uses specialized drills to remove pillars that are left behind. “We come back and evaluate the area for what we call pillar recovery,” said Sutton. “Pillar recovery is about 30 percent of our production. We use remote control equipment to take out what pillars we can after we’re finished with primary mining.”
Sutton said the Volvo articulated haulers perform well for his operation. “We have been working with Volvo trucks for nearly 15 years, and they have made a number of modifications over the years to make them better for our application,” he said.
The SEMO Division puts up to 15,000 hours on a Volvo articulated hauler before sending it to the dealer for rebuilding. “We take the hauler apart underground and pull the parts up to the surface,” said Sutton. “Our dealer, Rudd Equipment, takes the truck to the shop and tears it down to the frame. They fix problems with the frame, if any exist, and rebuild the engine and entire drivetrain. We get excellent service from Rudd Equipment, our Volvo dealer,” said Sutton. “They do a good job of keeping our trucks running.”