A new dual powered mobile crushing plant has more than doubled aggregate production while improving product quality and creating unprecedented versatility for one of the only government-owned quarries in the United States.
Six mornings a week, dozens of dump trucks wind up the aptly named Rock Crusher Road to the Oconee County Quarry.
Located outside the town of Walhalla in northwestern South Carolina, near the Georgia and North Carolina borders, the county-owned quarry produces blue granite, South Carolina’s state rock.
Not only does the quarry make aggregate for county government needs that include road building, storm drainage and slope protection, it also serves more than 600 local customers, who range from paving and grading contractors to rural residents who just need a little gravel for a driveway.
“We’re able to support local government needs as well as the needs of the local citizens,” says assistant manager Thom Moxley. “We’re very proud of that.”
An aging crushing plant left the quarry struggling to satisfy demand in recent years. The 30-year-old stationary plant could only produce 300 tons of crushed stone per hour running at full capacity, and it often wasn’t running at all. Unscheduled downtime took a toll on production.
“We had got to the position where we were running our plant to try to meet production needs and not having enough time to do the preventative maintenance that we needed to do,” Moxley says.
Moxley, colleagues at the quarry and the county administrator began discussing and researching replacement options in early 2017, comparing both stationary and mobile plants. Moxley and crew leader Billy Buchanan, who started working at the quarry within a week of each other six years ago, traveled to Sweden to inspect a dual power mobile plant in 2017.
“It’s a real shift in the way of thinking of instead of taking the rock to the plant, you take the plant to the rock,” Buchanan says. “The efficiency of that plant was impressive.”
In February 2018, the Oconee County Council green-lighted $7.5 million USD in improvements to the quarry. Quarry staff recommended the purchase of a mobile crushing plant, and the county initiated a request for proposal (RFP) process.
The quarry needed a plant that would produce at least 650 tons per hour of finished product, including at least 200 tons per hour of 1-inch #57 aggregate.
“Those were the stringent requirements that we had to meet for Oconee County to feel like we could make this investment,” Moxley says.
The quarry also required the plant to be able to produce several other products, including #789, class A riprap and asphalt sand. The crusher run needed to meet South Carolina Department of Transportation specifications for gradation in order to be used on state roads, and both the #57 and #789 also had to meet state specifications for flat or elongated particles.
Moxley and his colleagues were also intrigued by the flexibility of a plant that could enable the quarry to freely alternate power supply between onboard diesel generators and the main electric network.
“Dual power machines were very attractive to us because we can be most efficient if we are on the grid,” he says. “But certain times it’s costly to be on the grid, so we could manage the different operations using diesel over electric so that we can stay off of the grid during peak times. The electric is quieter and so there’s a lot of things that helped us ultimately be able to justify the dual power system.”
The Oconee County Council awarded the RFP to Sandvik, whose $5.8 million USD solution was $750,000 under budget. It included a five-year extended warranty, local dealer service support, scheduled equipment inspections and an on-site parts consignment container, as well as comprehensive operator training.
“We just felt like that through the whole process that Sandvik’s technology was on the cutting edge and they make a very durable, strong product,” Buchanan says. “All the companies in the RFP sent us offers of their solutions, and Sandvik actually sent us four options. The one we went for has provided some solutions that we never asked for, and the potential to grow. In the future the quarry might need more capacity. Sandvik was looking out for us.”
The quarry invested in a mobile plant primarily because it provides pit design flexibility. The new plant can be relocated indefinitely, enabling the quarry to size, crush, screen and stockpile as close to the face as possible.
“We’re not ever going to be in a situation again that we have a stationary plant sitting on rock that we need to mine,” Buchanan says. Moxley expects the plant to eventually reduce the operation’s diesel budget, too.
“Ultimately, when we get our quarry in a position to accept the equipment that we have, we can accommodate the customers there and it’s going to decrease the amount of trucking and equipment necessary to give the customer what they need,” Moxley says. “When the time comes, we can just move our plant and we won’t have to redesign a plant or start all over again. We’ll just go to a different footprint.”
The new diesel-electric mobile plant was commissioned in November 2019 after Sandvik reconfigured it from a 400 V voltage and 50 Hz frequency to North American mains electric power, 480 V and 60 Hz.
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