August 1, 2009
Eagle Peak Rock & Paving is “mining” aggregate by crushing rocks left over from 1930s mechanized gold-panning operations in mountainous northern California.
by Carl Emigh
During the famous 1849 Gold Rush, miners panned for gold in California streams. Many years later, some enterprising companies took it to a new level by panning a number of river valleys in northern California with floating dredges.
One such dredging operation started circa 1936 and continued into the early days of World War II. A 100-yard-long, barge-mounted dredge was used to pan the Scott River Valley just south of the Oregon border near Yreka, Calif., in Siskiyou County. On the front end of the dredge was a 76-unit rotating bucket line that dug up dirt and rock. Gold was removed inside the hull, using a trommel screen apparatus with 1/4-inch and 1/2-inch holes. A 120-foot-long stacker at the back of the barge deposited the excavated rocks in giant windrows.
For Alturas, Calif.-based Eagle Peak Rock & Paving, the “gold” is in those windrows of large, smooth, very hard rocks still remaining from the dredging operation. “Our mining consists of crushing the rocks for use in our two asphalt plants,” says Eagle Peak President Tony Cruse. He says the company’s equipment includes three Terex Pegson crushers and two Powerscreen units, all in closed circuit. The current configuration has allowed the company to nearly double its production. “We produce what we call asphalt rock. This includes 3/8-inch clean, 1/2-inch clean, and #4- to 0-inch crusher dust,” Cruse says. “We blend all three in the asphalt production process. We reject all natural sand.”
The closed-circuit process consists of several steps. Material from the rock windrows is fed into a 26- by 44-inch jaw plant and crushed to 5-inch size. The 5-inch goes to a double-deck dry screen that rejects 1-inch minus stone. The 1-inch plus goes to a cone crusher that produces 2-inch minus which goes to a triple-deck producing 3/8-inch, 1/2-inch, and crusher dust. The oversize goes to a second cone, which also produces 3/8-inch, 1/2-inch, and crusher dust in closed circuit with the triple-deck screen.
The equipment features quick setup and tear down, as well as quality service from the dealer, Powerscreen of California. “They’re here anytime we need them, and sometimes they just show up on their own to check everything over on site wherever we are,” Cruse says.
A Paktronic control panel on the cone crushers helps ensure both product consistency and equipment durability. It sets the closed-side setting and shows the current setting. Further, the control senses anything uncrushable entering the crushing chamber and lifts up (dumps) to allow it through before automatically reverting to the current setting. The control also records and displays wear so the operator can gauge how much life is left in the liners.
The cone crushers include a feed hopper, product conveyor, crushing chamber, and power pack on a single chassis. The plants are engineered for high specification, flexibility, mobility, and low operating costs. The smaller cone can crush up to 220 tons per hour, while the larger unit can process up to 420 tons per hour, depending on the application. Typically, pre-screening is not required. The hydraulic system can be adjusted quickly, even while crushing. The entire feeder-hopper assembly can be hydraulically lowered into the feed ring for transport or raised for re-metaling.
The jaw crusher is designed for high-production quarrying, demolition, and mining operations. An aggressive crushing action with a single-toggle, high-swing jaw facilitates greater material entry into the crushing chamber. The design incorporates a hydraulic setting adjustment system that changes the jaw size at the press of a button for subsequent quick, product-sizing changes, thus reducing downtime. Throughput capacities range up to 400 tons per hour, depending on material and setting.
The two-deck 20- by 5-foot dry screen is designed and built for large-scale operations. The unit provides uniform sizing and can process up to 500 tons per hour, depending on mesh size and material type. The force in the screen provides optimum performance in wet, sticky applications. The plant is highly mobile. It sets up and is ready to run in 15 minutes.
Operators keep close tabs on the quality of the processed material to ensure it meets the needs of its asphalt plants. “We do gradation testing on samples every 500 tons. That’s six samples a day, and the product is very, very consistent,” Cruse says. “I couldn’t be happier with that.”
Working in tandem, the plant setup allows Eagle Peak to effectively turn a challenging by-product into a valuable revenue stream. “The rock feed material is very, very hard,” Cruse notes. “But our crushing and screening equipment handles it very well.”
Mining it old school
This artist’s concept of the dredge used in mining gold in the Scott River Valley is based on a 1930s postcard illustration. The hull and 120-foot stacker conveyor totaled about 100 yards in length. The hull had a 10-foot draft. At the front of the barge, a 76-unit rotating bucket line dug up the river bottom and as much of the shoreline as possible, sometimes re-routing the river in the process. Electric power was provided by a hydroelectric plant located 60 miles away. The dredge handled up to 9,000 cubic yards per day in three eight-hour shifts. Richard S. Moore of Callahan, Calif.-based Moores Gravel provided information about the gold dredging operations and equipment.
Carl Emigh is a freelance writer specializing in the construction materials market.