There are three basic types of dredges on the market today. They include surface dredge, submersible dredging tube and the underwater submersible dredge. The surface dredge is the most popular, efficient and versatile gold recovery machine.
They are also far more efficient than the machines of old. It is not uncommon to see a dredge profitably working the tailings of some of the old mines and tailing dumps.
A small portable backpack dredge of today can weigh as little as forty pounds and cost around eight hundred dollars. It can process as much gravel as a larger three hundred-pound dredge, some twenty years ago. One of the most exciting features of this type of dredge is that it allows the prospector to penetrate areas that were otherwise impossible to reach with heavier and more cumbersome equipment.
In the early 1900s, several crudely built steam powered dredges were active on some of the northern rivers of California. The divers worked futility on the bottom of the rivers with heavy diving helmets and cumbersome diving suits.
In the beginning of the Gold Rush, the miners were limited because they could only work the areas that were accessible to hand tools along the banks of the streams and rivers. Their equipment was limited to gold pans, sluice boxes and rockers. As time progressed and as they became more experienced, they realized that the deeper gavels in the riverbeds were often richer than the surface gravel along the banks.
The submersible dredge of today is mainly used for sampling and when a good streak is found, the surface dredge is employed to do a more efficient job of recovery.
The submersible dredge must be held relatively level while in operation and cannot reach around corners and hard to get at places. It also is not practical to use in shallow water, as it must be completely submergedin order to operate properly. It is physically described as a flared metal or plastic tube with an attached metal elbow at a forty five to sixty degree bend. High pressure water is pumped into the bend, creating a vacuum at the end of the bend. It is powered by a high pressure water pump which is normally located on a float that sets at the surface of the water and is pumped down to the dredge via a high pressure hose. At the end of the flared tube a riffle tray is attached containing a series of gold traps. As the gold bearing gravel is sucked into the dredge the heavier particles, including gold, becomes entrapped into the riffle tray. The lighter non gold bearing particles flow back into the river.
The underwater dredge is the less popular of the dredges available, because it lacks somewhat in its ability to recover as fine of gold as the surface type. It is designed mainly for compactness and portability, but is limited also in its application, as it is cumbersome to handle underwater.
The top edges of the sluice box are rolled for Greater safety and strength. They are equipped with heavy duty latches and a longer rubber damper that is used for more even distribution of material over the recovery area to assist in settling fine gold out of suspension. Currently available only with 4, 5, and 6 inch dredges.
2. The material passes over two different classifier screens allowing more complete separation. Experience finer gold recovery, in a lighter and more compact dredge that will out perform any double or triple sluice on the market! This system is extremely easy to operate for quick and easy clean up. It provides quick identification of values in the primary recovery riffle section.
1. The material is classified 1/3 of the way down the box allowing the fine gold to fall and settle out of suspension
The latest technological design is the new 3 stage sluice box. The new box works similar to the double or triple sluice, classifying the fine material into separate compartments or sluices for processing. The differences are:
The double or triple sluice also separates and classifies the dredged material at the entrance of the sluice. The smaller heavy material falls into separate sluices for a more selective recovery. When the dredged material is separated by size, it ensures better recovery. Higher velocity water is required to move the larger cobbles through the sluice. Lower speed or velocity is required to recover fine gold in the lower or separate sluices. When the speed is high enough to carry off the larger non-value cobbles through a single sluice box, a loss of fine gold can occur if the material and flow is not separated.
The single sluice box processes all dredged material through a single recovery box. A single box includes a short classifier screen at the entrance, to separate the larger cobbles from the smaller, which are most likely to contain small gold particles. This design is still used in most small dredges, due to their lightweight compact design.
The following information should provide you with a basic understanding of operating a portable dredge. For more complete understanding on this subject, we recommend you read any one of a variety of books available through the Keene Library of Books, such as The Gold Miners Handbook, Dredging for Gold or Advanced Dredging TechniquesThe vacuum on a portable dredge is created by a "venturi principal". A volume of water is pumped through a tapered orifice (jet), by a special designed water pump. A high velocity jet stream is created within the jet tube producing a powerful vacuum. As indicated in the diagram gravel is dredged into the suction hose and is delivered to the sluice box header. As 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 non bearing 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 a 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 principal 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.
Most small engines are throttle controlled. The speed of the engine can be controlled with the use of a lever. Although the rated horsepower is achieved on most small engines at 3600 R.P.M., it may not be necessary to operate the dredge at full speed. Lower speeds conserve engine life and fuel economy. Be sure to read all instructions and especially the engine instructions that are provided with each unit.
NOTE: ENGINES ARE NOT SHIPPED FROM THE FACTORY CONTAINING OIL. OIL MUST ADDED PRIOR TO USE! ENGINES OPERATED WITHOUT SUFFICIENT OIL SUPPLY WILL INVALIDATE ENGINE WARRANTEE!
IF SUCTION DECLINES:
1. Check the suction device for an obstruction. An obstruction can be removed by probing the obstructed area with the provided probe rod. I may be necessary to check the suction hose for a visible obstruction. This can be remedied by either back flushing the system or dislodging the obstruction with a gentle blow.
2. Check the pump for loss of prime or blockage. The foot valve may be too close to the surface of the water and air may enter the intake of the pump via a small whirlpool. The pump intake or foot valve screen may be plugged with leaves or moss, restricting flow into the intake of the pump. Check and tighten all clamps to prevent an air leak.
1. Check all clamps for an air leak.
2. It may be necessary to check the foot valve for a small leak. This is accomplished by removing the foot valve assembly from the pump and blowing air into the hose portion of the assembly and listening for an air escape. It may be necessary to remove the hose and check the rubber valve for an occurrence of a leak, or for a small obstruction preventing the valve from sealing.
3. If a water pump seal is either defective or damaged, a leak will be evident on the inside portion of the pump around the drive shaft. Often a new pump will leak slightly, until the seal and gasket has become fully seated. This is a common occurrence in most new pumps.
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