The Gold Dredge and Dredging for Gold
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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.
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.
Although history reveals that dredging has been in existence throughout the world for many years, it is just recently that it has reached such a high degree of popularity due to advanced technology in dredging equipment. Dredges of today are lighter, more portable and more efficient than ever.
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.
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.
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.
Picture: Two Divers operating a New Keene 4 inch Dredge in shallow water.
The Surface Dredge
The surface dredge floats on the surface of the water. The material is pumped to the surface through a suction hose into an efficient sluice box that is capable of 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. Marlex plastic floats are normally preferred as they are rugged and stable in rough water and are extremely lightweight.
Modern dredges are provided with a Single or “multi stage” recovery systems, such as a the new 3 stage sluice. The multi stage sluice boxes are preferred, because they have a greater capacity to recover a finer grade of gold and black sand concentrates that often hold values. 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.
The Underwater Dredge
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 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 submerged when operated 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 is located normally 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 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.
WHAT IS A SINGLE SLUICE BOX, DOUBLE SLUICE BOX, TRIPLE SLUICE BOX AND THE LATEST TECHNOLOGY OF THE 3 STAGE SLUICE BOX?
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 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 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:
1. The material is classified 1/3 of the way down the box allowing the fine gold to fall and settle out of suspension.
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. 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.
GENERAL OPERATING INSTRUCTIONS
THE FOLLOWING INFORMATION SHOULD ENABLE YOU TO UNDERSTAND THE BASIC THEORY OF OPERATION OF 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 Techniques. The 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.
PRIMING THE PUMP
Before starting the engine, the pump must be fully primed. This means the pump must be full of water and all air removed. All jetting pumps provided with our dredges have a mechanical water pump seal. Without the presence of water in the pump, friction could 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 dam age 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.
PRIMING THE SUCTION HOSE
Priming the suction hose need not be of concern in most dredging operations, but is important to understand the principal. 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.
SUCTION SYSTEM OBSTRUCTIONS
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 back flushing the system. Back flushing 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 a striking the obstruction, taking care not to damage the hose.
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.
SLUICE BOX ADJUSTMENT
Most models have a slight adjustment to raise or lower the sluice box. The proper sluice box adjustment can effect 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 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"
CLEANING THE SLUICE BOX
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 have accumulated. Either turn 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 container. All material must be removed before replacing the riffle matting, riffle tray and classifier screen.
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. 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!
[A] 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.
IF PRIMING THE PUMP BECOMES DIFFICULT
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.
Principal of dredging operations courtesy of Keene Engineering. Keene is our preferred dredge mfg. Visit keene at: http://www.keeneengineering.com/