The bucket-line dredges that changed the character of gold mining in Alaska and the Yukon were invented in New Zealand. Many changes and additions were made to make them suitable for working on the frozen ground of Alaska, but the technology changed little for the 80 years they were in use. Although they seem complex, their concept is quite simple. The buckets scoop up the gravel and dump it into sluice boxes inside the floating dredge, water is then pumped in to separate the gold from the gravel, and the worthless gravel is then dumped out the back. You can see the piles of gravel from these early mining operations still today around Fairbanks and Dawson City.
Today most dredging is done with surface suction dredges either operated from shore or on a small boat. Suction dredging is a popular form of recovering gold from the numerous placer streams in Alaska. Various sizes of suction dredges are used, varying from those “recreational” models with a small 11⁄2 inch intake hose to large, heavy dredges with 8-inch and 10-inch intake hoses, driven by powerful engines, and capable of processing large amounts of material in a single day. The surface dredge floats on the surface of the water or sits on the nearby shore. The material is pumped to the surface through a suction hose into an efficient sluice box that is designed for recovering extremely fine particles of fine gold. The sluice box can either rest on the bank, or float on the surface of the water. Another advantage of the surface dredge is it can easily be operated with or without diving equipment. Normally, the smaller size dredges from two to three inches in size are equipped with single sluice boxes, as their primary function is portability and compactness.
Running a Gold Dredging Operation in Alaska
For a more complete understanding of this subject, we recommend you read any one of a variety of books and movies available through Amazon’s Gold Dredging Section. The dredge operates when a volume of water is pumped through a tapered orifice (jet), by a specially designed water pump. A high-velocity jet stream is created within the jet tube producing a powerful vacuum. A slurry of water and gravel enters the header box and is spread evenly over a classifier screen. The smaller and heavier particles drop below the classifier screen into an area of less velocity, allowing a slower and more selective classification of values. Often values are recovered and easily observed before they even enter the riffle section. The lighter nonbearing values and larger aggregate are returned back into the water. The riffles or gold traps in the sluice box are best described as “Hungarian Riffles”. This type of riffle has proven to be the most efficient gold recovery system. As material flows over the riffles an eddy current is formed between each riffle opening. This force allows the heavier material to settle out of suspension and the lighter, non-value-bearing material to be washed away. This continuous self-cleaning principle allows a dredge to be operated for prolonged periods of time. Normal conditions require a sluice box to be cleaned only once or twice a day.
First Prime the Pump
Before starting the engine of the gold dredge, the pump must be fully primed (filled with water). All jetting pumps provided with our dredges have a mechanical water pump seal. Without the presence of water in the pump, friction can cause a seal to overheat and require replacement. Priming the pump on some of the smaller models is accomplished by thrusting the foot valve back and forth under the surface of the water in a reciprocating motion. This will cause water to become pumped into the foot valve assembly into the pump. A pump is fully primed when water is observed flowing out of the discharge end of the pump. It sometimes may become necessary to hold the discharge hose above the level of the pump to complete the priming operation. The larger dredges that have a rigid foot valve, are easily primed by removing the cap provided on the foot valve and filling until water overflows. Caution must be exercised to prevent sand from entering the foot valve or intake portion of the pump. Excess amounts of sand could damage the water pump seal, or pump impeller. It is recommended that the intake portion of the foot valve be placed in a sand-free environment underwater, such as a small bucket or pan.
Next Prime the Suction Hose
Priming the suction hose need not be of concern in most dredging operations, but is important to understand the principle. When the tip of the suction hose is taken out of the water during operation air will to enter the suction system and cause the suction power to cease temporarily until submerged again. The suction will commence as soon as the air is passed through the system. It is important to ensure that no air leaks occur in the suction system.
If the Suction Home Becomes Plugged
The suction system can become jammed while dredging. This can be caused by dredging an excess of sand, causing the suction hose to load up, or a rock that has become stuck in the suction system. Rock jams generally occur in the jet, or just before entry into the jet. This can easily be cleared by removing the rubber plug located on the front of the header box and thrusting the probe rod through the header box and down through the jet in an effort to strike the obstructed area. It may occasionally be necessary to remove the suction hose to remove an obstruction. Sometimes obstructions can easily become dislodged by backflushing the system. Backflushing a suction system can be accomplished on some models by reversing the flow of the suction hose at the header box, by blocking the flow of the water as it enters the header box. If this is not successful. it may be necessary to locate the blockage in the transparent hose and dislodge it by striking the obstruction, taking care not to damage the hose.
Managing the Intake of Gold Bearing Gravel Into the Gold Dredge.
Care must be exercised to prevent dredging excess amounts of sand. A solid-to-water balance must be maintained. The solid content being dredged should never exceed 10%. If a suction tip is buried into the sand and not metered properly the solid content could cause the suction hose to become overloaded with solids and suction will cease, this will also cause the sluice box to become overloaded with solid content, resulting in a loss of values. Most models have a slight adjustment to raise or lower the sluice box. The proper sluice box adjustment can affect the recovery of values. If the sluice does not have enough angle, the sluice box will “load up” causing the riffle openings to fill with unwanted excess material. Too much angle will cause the material to flow too fast, resulting in a loss of values, evidenced by the riffles running too clean. The optimum adjustment of a properly working sluice box is evident by only a portion of the riffle visible while operating. A loss of values can also occur if the solid content of the suction discharge is too heavy in solid content. Remember, the solid content should not exceed 10 %. A normal sluice box tilt is approximately 1/2 inch to the running foot. A four-foot sluice box should have an approximate tilt of 2″
Getting the Gold Out at the End of The Day.
Before attempting to clean the sluice box, it should be allowed to run with only water for a few minutes in order to wash out any excess gravel that has accumulated. Either turn the engine off or let run with a slow idle, then remove the classifier screen and replace the wing nut to prevent losing it. Unsnap the riffle latches, fold the riffle tray up, and let rest against the header box, taking care not to let it drop back into place while cleaning. This could result in a potential injury! Place a wide tray, bucket or large gold pan at the end of the sluice, then carefully roll up the riffle matting and wash into the container at the end of the sluice. Rinse any excess gravel that remains in the sluice into a container. All material must be removed before replacing the riffle matting, riffle tray, and classifier screen. You’ll then want to pan any of the concentrates you have down into fine gold. Enjoy the cleanup.