The Workings and Operation of a Portable Dredge
There are many books and videos dedicated to dredge operation, and to anyone considering purchasing a dredge, we recommend this type of research, along with procuring a site or club membership that allows dredging before buying any equipment.
Dredges work by creating a powerful vacuum from water forced at a high velocity, through a tapered jet. Water and gravel from the bottom of streams or lakes are vacuumed into a suction hose and delivered onto the sluice jet flare and are distributed along the classifier screen. Smaller and heavier particles drop into the riffles, or become trapped in the miners moss, carpet, or other material designed for this purpose. Lighter and larger aggregate are returned back to the water. The force of water flowing over the riffles allows heavier material to settle, while the lighter material is washed away. This continuous self-cleaning allows for prolonged dredge activity. The sluice would need to be cleaned only once or twice in the course of a normal dredging day.
Before starting the engine, the pump must be fully primed, meaning it must be full of water, and air must be removed. On smaller models, thrust the foot valve back and forth, under water, in a reciprocating motion until you can see water flowing out of the discharge end of the pump. On larger models with a rigid foot valve, remove the cap and fill to overflowing, taking care to keep out any sand or debris. You can put the intake portion of the foot valve in a bucket placed under water to keep sand out during operation.
Priming the suction hose is not necessary, but you should know that when the tip of the suction hose is taken out of the water during operation, air will enter the suction system and cause suction to cease until resubmerged. Suction will commence as soon as all the air has passed through the system. Make sure to patch any air leaks.
To avoid jamming the vacuum, see that mostly water is being vacuumed and solid material being dredged never exceeds 10% of the total volume. Most jams can be cleared by flipping the rubber damper back and using a probe to loosen solid material. If the suction hose becomes jammed, the hose may need to be removed in order to get it dislodged.
If the adjustable sluice box isn’t at the right angle, it could overload and riffles will fill with excess material, or material will flow too fast, causing material to be lost back into the water. A normal tilt angle is about ¾” to the running foot. A 4-foot sluice box should have an approximate tilt of 3”.
Check-list for Portable Dredging
Buying a Dredge? Here’s a list of equipment, supplies, and accessories for you to consider in order to make the most of your dredging experience. Pick and choose according to what you’ll be moving, time spent in one location, and the remoteness of the location.
Dredging Equipment: Pry-bar, winch and rock net, floating rope, fuel containers, tools for servicing gear, fuel and oil.
Spare Parts: tune-up replacement parts for motors, pump seals and bearings, compressor and alternator belts, re-build kits or repair kits for compressors, regulators and air lines, extra pressure and intake hoses, extra water hose and air line fittings, extra rubber hose seals, extra face masks or strap repair kits, duct tape, nylon line.
Diving Equipment: Face mask, air tank and fittings, air lines, harness, and regulators, dive skin and wetsuit, weight belt and weights, footwear and gloves, dive hood, ear drops.
Clean-up Gear: Gold pans, buckets, classifiers, tubs, magnet, trowels or scoops, snuffer, vials, scale, tweezers, magnifier, fine gold recovery equipment.
Camping Gear: Tent, stove and fuel, cooking and eating utensils, wash tub, toilet facilities, chairs & table, first-aid kit, map or GPS, camera, lamp, flashlight & batteries, knife, paper & pens, drinking water, food and containers, dish/hand soap, trash bags, zip-lock bags.