Tuesday, July 15, 2008

Placer Mining Equipment: the Suction Dredge

Placer Mining Equipment: the Suction Dredge

The suction dredge is the single most efficient piece of mining equipment for recovering placer gold from a river or stream. Dredges can process large amounts of auriferous ("gold-bearing") material and are quite good at recovering everything from "pinhead" flour gold and small flakes on up to large nuggets. Essentially, dredges are just gasoline-powered "vacuum cleaners" that can be floated on pontoons or used from a stream bank (the latter being the usual province of highbankers or very small dredges).

Dredge Categories: Suction dredges are typically categorized according to the diameter of the unit's intake hose, which can be as small as 2 inches and as large as 8-10 inches or more. However, most recreational or small-scale placer miners in the United States use dredges with intakes of 2.5, 3, 4, or 6 inches, since these are the types most commonly sold by suppliers. The larger the diameter of the dredge's intake hose, the more material it can process (usually expressed in terms of cubic feet per hour).

Requirements and Limitations: One thing all dredges have in common (regardless of their intake hose diameter) is that they require large volumes of water to operate correctly and efficiently. Obviously, larger dredges with wider intake hoses require greater volumes of water while smaller dredges with corresponding smaller intake diameters can be successfully employed on smaller streams carrying much less water.

Dredge Components: The standard components for most dredges are:

1) Gasoline engine(s) providing power for the unit;

2) Water pump or "venturi" jet creating suction and expelling water;

3) Intake hose and suction nozzle acting as vacuum "attachments;"

4) Sluice box(es) for processing auriferous gravels and trapping the gold;

5) Pontoons, inner tube(s), or other flotation devices for supporting the dredge on the water's surface;
and

6) Breathing apparatus such as a "hookah" line for diving underwater.

Types of Dredging: There are essentially three main types of dredging activities:

1) Deep water where diving experience is a must, the dredger is fully immersed in water reaching depths of 15-20 feet, and he or she is using an air line ("hookah") to breathe;
2) Shallow water where diving experience is not mandatory and the dredger is (typically) only partially immersed in water depths of 2-5 feet; or
3) Surface dredging where overburden is shallow or bedrock is nearly exposed and cracks and crevices can be easily vacuumed.

Advantages: There are a number of advantages associated with using a suction dredge:

1) Large volumes of auriferous material can be processed rapidly;
2) Dredges are extremely efficient "gold grabbers;" and
3) They can be extremely cost-effective if operated properly in productive areas.

Disadvantages: Often, with every advantage comes a concurrent disadvantage. Dredges are no exception to this rule:

1) Fairly substantial initial financial outlay ($1,000-$6,000 depending on the dredge type and size);
2) Like all things mechanical dredges require a certain amount of maintenance and repair, and they can break down ;
3) They are often bulky and cumbersome to transport or carry and require additional tools, gasoline, and materials that must also be transported or carried.

Safety Concerns: Safety concerns should be first and foremost in any dredger's list of important items, since there are some inherent dangers involved with running the larger intake dredges at greater depths. I recommend the "buddy system" for those placer miners who will be working fully immersed underwater. Hookah breathing lines can twist, kink or foul and large rocks or boulders beneath the surface can suddenly shift or move, trapping the dredger. Be smart and be safe. No amount of gold in the world is worth your life, pure and simple.

Permits: I can't think of one state in the U.S. that does not require an annual permit for operating a suction dredge (and highbankers with a suction nozzle). Before you dredge any location, make sure you have researched any and all permit requirements for that area and that specific location. Save yourself from potential legal hassles and fines, OK?

(c)  Jim Rocha (J.R.)  2008

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