DuPont Pit has been reaching out to educate the community and to, hopefully, improve people’s opinions of the aggregate business. “Recently, we’ve been doing a lot of field trips with grade schools,” Tweedy says. “They have a required curriculum where they have to study rock and minerals. So, at the end of the terms, the classes come out. We talk about concrete and the different things we make out of our materials, and where they see and use it in their daily lives. It’s a lot of fun.”
“In 1997, when the plant was built, it was probably one of the most automated plants in the country,” Nicholson says. “Of course, it’s not that now, but it’s still pretty impressive.” Many improvements have been made since then.
The whole system is designed around 2,400-ton-per-hour production out of the mine, while shipping is typically about 2,000 tons per hour. The material is mined in a simple scoop and dump fashion. A dozer maintains a workable slope in the mine to ensure safe working conditions for the loader operators. Two wheel loaders scoop up the sand and gravel and dump it into a feeder over a conveyor system that carries the material up to a fractionated processing plant. No processing is done in the mine itself.
“We bring it in to the primary screening plant and sort it off into three raw surge piles, two different sizes for the crushers and one for the wash plant,” Tweedy says. “We wash everything 1½ inch and finer through the wash plant. Out of the wash plant, we make four different sizes of washed gravel and one size of coarse sand. All the fines are pumped over to our sand house where we have four classifiers that sort off three more sizes of fine sand. On the crushed side, we feed the crushers from the surge piles. The material comes through the crushed/screening house, where we make four sizes of crushed gravel. Out of those 12 basic sizes of materials, there are few limitations on how many different blends we can make.”
The plant is designed with two parallel 1,500-foot-long tunnels running under the stockpiles. The stockpiles on one side are all washed sand and gravel and the other side contains crushed gravel. The processing plant runs right between the two rows of stockpiled materials, which helps contain the noise inside the processing plant. Quality control (QC) monitors material gradations in the stockpiles continuously to ensure consistency.
Two feeders under each product stockpile enable the plant to produce different blends. “When a customer wants a certain product, our QC lab analyzes what the customer wants in their specifications and selects, proportionately, the blend he needs to pull to make those specifications,” Tweedy says. “Then he designates that blend a product number and it goes into the data base. When an order is entered in the system, the product number dictates which feeders are activated to make that blend and ensures they are the only feeders that will open. It’s timed so that, no matter where you’re pulling from, when the material hits the shipping belts, all of it is perfectly blended. We currently run about 350 different blends on our data base.”
While barging is the primary method of transport for DuPont’s material, trucks transport the remaining 20 percent. The plant simply flips a gate to change loading from the shipping dock to the truck lines, and vice versa.
A series of two conveyor belt systems carries material to the loadout dock on Puget Sound. The first conveyor in the journey is covered. “Once the material is on the loadout conveyor belt, the only place for it to go is on the barge,” Nicholson says. “It’s about a 3,100-foot conveyor and is covered for two reasons. The primary reason is that we can’t discharge a drop of anything into Puget Sound. The Sound is downhill, so if it rained on the conveyor, it would discharge material into the water. The other reason is that this is a long, flat conveyor. If the power went out during a storm and the belt sat in the rain with material on it, it would get pretty sloppy.”
The second conveyor goes down the bluff to the dock on Puget Sound at an extreme angle and is completely enclosed in a tube to prevent any spills. This conveyor has a very large drive motor and gear box with a hydraulically controlled clutch and large disk brake, so that if there’s a problem on the barge, the conveyor can stop quickly.
“A 350-horsepower drive keeps the conveyor from overrunning,” Nicholson says. “With all that weight and momentum going down the hill, that conveyor would just take off if you had a small motor. We use about 125 horsepower to start the conveyor, and then it settles back to about 75 horsepower in an empty condition. When the material hits the belt, it actually goes into negative horsepower and regenerates power back into the grid. It’s a pretty unique conveyor system.”
It takes one operator in the tower on the dock and one person handling barge lines to control the loading of the barge. The operator can move the barge wherever he wants it during the loading process with the winch system. The conveyor will luft, extend, retract, and swing to reach all areas on the barge. The tug boats that bring the barge to the dock simply tie up to the barge and wait until loading is complete.
“We do 3 million tons per year, which sounds like a lot, but it’s 50 percent of what we’re used to doing,” Nicholson says. “In 2005, we did 6.2 million. We typically load 24/7, but we’re down to two shifts now, five days a week.”
Both Nicholson and Tweedy agree that the best part of DuPont Pit is the employees. “We have really high-quality employees here,” Tweedy says. “The current average tenure is 10.4 years for plant employees. They really take a lot of pride in what they do. We do most fabrication in house. We built all our own conveyors, feed distributors, chutes, and structural supports. The employees come up with many of the ideas, and we have our in-house engineer do the design drawings.”