Dredging

The modern day dredge floats on top of the water. The material is pumped to the surface through a suction hose into a sluice box (similar to but usually larger than the stream sluice) that will recover larger and fine pieces gold. An advantage to the surface dredge is it can easily be operated with or without diving equipment. The floatation of a dredge makes it very stable and extremely portable in the water simply because it floats like a boat.
     Normally, the smaller size dredges have single sluice boxes, and the larger one can have single, double, and triple stage sluices, depending on the brand name or style that you prefer.

OPERATION OF A DREDGE
    Special designed water pump pumps a volume of water through a tapered orifice (jet) and high velocity jet stream is created within the jet tube producing a powerful vacuum on a larger suction hose. Material is dredged or pulled into the suction hose and is deposited in the sluice box flare or header which slows down and spreads out the water gravel mix over a classifying screen. The smaller and heavier particles drop below the classifier screen into an area of slower water flow allowing the gold to drop out of suspension. A lot of gold and heavy material will be recovered before they even make it to the riffles. The lighter materials and smaller gold is washed further down the sluice to be separated by the design of the riffles. Many types or style of riffles are used and most do the job efficiently enough to recover gold. As material flows over the riffles, a swirl or back flow is created behind the riffle, which allows the heavies to drop out of water suspension and the lighter materials to be washed out of the box. This is basically a self-cleaning operation that allows a dredge to be used for long periods of time. A dredge can usually be run all day before a clean up of the box is necessary.

SLUICE ANGLE
General thoughts of the proper sluice angles are between ˝ inch to ľ inch per foot of length of the box. Most dredges have adjustments to raise or lower the sluice box. The sluice angle can affect the recovery of GOLD. If the sluice is too close to being level, the box will "load up" causing the riffles to fill up with overburden. If the sluice has too much angle, the material will flow too fast, causing a loss of GOLD, which you can see by the riffles will not have very much material behind them. Loss of gold can also be affected by engine speeds. It is recommended to follow the manufacturer instructions.

CLEANING THE SLUICE

You should allow the dredge to “run clean” or without any more material being pulled into the suction hose for a few minutes before cleaning. With the engine off, or running at idle speed, Carefully start disassembling the sluice classifiers and riffles with the sluice box material being dumped into a tub or a clean up bucket kit. Rinse out the sluice and all carpets and mats into the bucket. Remember that larger nuggets and small pickers may be visible on your black ribbed mat. Always have a large mouth specimen jar such as a baby food jar handy for just that purpose. Once all of the contents have been flushed into the bucket, you are ready for panning!