Placer Examination Techniques

Suction Dredge Operation

The active stream gravels of streams and rivers are logically mined using a floating suction dredge. Suction dredging using small dredges with an intake of 5 inches or less is a popular, relatively low cost mining method. The gravel within the active stream channel is suctioned from the bottom of the stream and processed over a sluice on a floating platform (pontoons). The gravel tailings are deposited just off the tail (end) of the sluice, in the stream. Fines, such as clay and silt size particles are carried downstream. A gasoline powered motor and pump are mounted on the floating platform for powering the suction apparatus and for driving the air pump which supplies air to the persons working underwater. Gasoline and motor oil are commonly carried to the dredge for servicing the engine(s). Other equipment commonly used while suction dredging includes: cables or ropes to hold the dredge in place against the stream current; winches, either hand or gasoline engine powered, to move or stabilize large boulders; wet suits for protection from lengthy exposures to fast, cold water; and various hand tools for loosening gravel and processing the concentrates in the sluice. Some “recreational” miners use a "high banker" which is a variation of the suction dredge used to process bench or bar gravel lying outside of the active stream channel. The high banker can be mounted on pontoons in the stream or on the bank. Dredges of this size are typically used by operations that are often more recreational in nature than for commercial full-time mining operations.

Suction dredges are rated by the size (i.e. 4-inch) of the inside diameter of the intake hose. The size of dredges used in California ranges from 2-inches up to 10-inches or more. Six-inch and larger dredges are rarely used for recreational purposes. Two to 2-1/2 inch dredges are generally “backpacking” models used in very small drainages with low water or difficult access. They can be carried by one person without completely disassembling. Air compressors are not used with dredges this small. Weights of the dredge range from about 50 to 80 pounds. Three-inch dredges are used in small, shallow drainages or for cleaning up or sniping along the banks of larger streams or rivers. They are powered by engines with about 5-horsepower (hp.) and weigh about 150 to 200 lbs. Four-inch dredges are powered by 5 to 8-hp. gasoline engines and weigh about 200 to 250 lbs. They are typically partially disassembled for transporting. One person can transport a 4-inch dredge by partially disassembling the dredge. Five-inch dredges are powered by single 8 to 11-hp. engines or dual 5-hp. engines and weigh 300 to 400 lbs. These dredges are normally disassembled to transport over dry ground. If only one person is working, complete disassembly is required to move the dredge when it is out of the water. Assembly times range from ½ hour to 1-hour for the various dredge sizes.

A gasoline engine powers a small centrifugal pump and possibly an air compressor. The pump drafts water out of the stream and pumps it into a pressure hose. Suction dredges are usually either a “suction nozzle” type or a “power jet” type, based on where the pressure hose connects to the intake hose. In the suction nozzle type, the pressure hose runs from the pump directly to the suction nozzle. The gravel is pushed up the intake hose to the sluice. In the power jet type, the pressure hose from the pump connects to a power jet tube which connects to the baffle box on the sluice at one end and the intake hose on the other end of the jet. The gravel is pulled through the intake hose from the intake nozzle. The high pressure water in the pressure hose is jetted through a small tube into a larger tube in the jet or suction nozzle creating a Venturi effect (increase in velocity of fluid flow and corresponding decrease in fluid pressure resulting in a suction), or a strong suction. The suction nozzle can be operated more efficiently in shallow water where the intake and pressure hoses may be above the water line, and there is no power jet that needs deeper water due to its length. The power jet creates a stronger suction and flow of water since the jet is located nearer to the sluice. In addition, the intake nozzle operator doesn’t have two hoses to contend with in the power jet set-up.

Figure 1 - Power Jet Dredge

Figure 2 - Suction Nozzle Dredge

Gravel enters the sluice at the baffle box, which breaks up the material and spreads it out across the width of the sluice, helps reduce the length of the sluice-box by dropping the material on the head of the riffles, and slows down the water velocity. The material drops onto a classifier, a perforated plate, where the larger material is washed over the classifier and part way down the sluice by a higher velocity of water. The heavier and finer material is forced through the classifier onto the head of the sluice with slower water which allows the gold to start to settle out. The riffles in the sluice create an eddy which allows the heavier material, such as gold to settle in the slower water behind the riffles. A “carpet” is placed under the riffles to catch and hold the gold. At the bottom end of the classifier, the slower water from under the classifier meets the faster water flowing above the classifier, and picks up speed to wash lighter and larger material off the tail end of the sluice as tailings.

Figure 3 Baffle Box

Clean-up of the sluice generally consists of picking coarse pieces of gold out the sluice with tweezers on a daily basis. The concentrates remaining in the sluice, under the riffles and in the carpet are washed out on a daily or weekly basis. The concentrates (aka: black sands for the heavy iron-bearing sands normally in them) are reduced down by panning to separate the gold or processing through another device such as a gold screw or gold wheel. Very fine gold usually requires amalgamation with mercury to separate from the concentrate.

Surface air is supplied underwater by a “hookah” system which operates by low-pressure air lines connected to an air compressor on the dredge or separate from the dredge. The compressor is normally powered by the dredge engine. A floating air line runs from the compressor to an air reserve tank, which acts as an air reservoir supplying a constant volume of air, cools the air and allows condensation, suppresses surges in air supply, and provides a couple of minutes of air supply should the compressor or engine stop operating. Floating air lines connect the air reserve tank to the regulators worn by the dredgers. Regulators use a valve to deliver low pressure air to the diver on demand.

Dredge manufacturers provide production capacities for suction dredges based on the volume (cubic yards) of material the dredge can process per hour. These capacities are based on optimal conditions that are rarely duplicated under actual dredging conditions. Our experience indicates that actual production capacities range from about 10% to about 50% of the maximum capacity cited by the manufacturers. We have estimated production rates ranging from 1.5 to 5 cubic yards per hour for 4-inch to 6-inch dredges. The difference between maximum and actual capacities is due to factors such as: time spent moving oversize material by hand; variable size and shape of gravel, which means you have to slow down or control the feed rate to prevent clogs; time spent opening fractures and breaking up bedrock; time spent breaking up compacted gravel; time spent maneuvering the intake hose and nozzle and dredge; time spent working around objects such as boulders and irregular bedrock; fatigue factor if the operator does not pace their activity; and discontinuous gravel deposits.