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Methods of Recovery–Suction Dredges

Historically, rivers have yielded great quantities of gold, along with lesser amounts of tin and gemstones. And in recent years it has been proven that with the application of modern mining concepts, techniques and technology, rivers remain a valuable source of minerals! Modern methods of extracting minerals from rivers use diver-controlled suction dredges and excavator-fed floating trammel plants for mining in smaller-scale situations, along with shaking tables for further refinement and extraction.  For larger operations, bulk mining in the lower reaches of rivers utilizes bucket dredges and suction-cutter dredges.

Suction dredges are preferable for both sampling and for mining selectively on a small-scale. These dredges may also be employed very effectively to work in conjunction with excavator-fed floating trammel plants. Additionally, suction dredges may be the only suitable means of mining where access is not possible for an excavator, or when regulations prohibit the use of heavy earth-moving equipment in environmentally sensitive areas. Virtually any river anywhere in the world can have access difficulties, and therefore the portability of suction dredges offer additional advantages (especially in Guatemala and particularly after major storms or hurricane events).

The basic concept of a suction dredge is to lift mineral bearing gravel from a river bed, pass it through a recovery system, and then return the waste gravel back to the river in as short a distance as possible. Each dredge component must form an integral part of the complete unit, with the pumps/s, suction velocity, and recovery plant designed as a whole, to form a complete and efficient unit suited to the deposit, operating conditions, and production level required to achieve viable economic operation.

How does a gold suction dredge work?

Suction dredges utilize an engine powered high pressure water pump to suck up water, gravel and gold and inject this material into a sluice through a header box. The header box dampens the rush of water and gravel so that it flows evenly into the sluice. The sluice box then expels the worthless gravel and retains the gold.

In addition to pumping water, the dredge’s engine can also produce compressed air for the dredge diver to use while underwater (though in cases where the water is shallow the diver may simply use a snorkel). This is often called “long arm dredging”.  Old suction dredges come in a wide variety of designs and sizes of intake. Newer dredges are more standard in design and commonly available “off-the-shelf”, ranging from a small backpack size with an intake hose diameter of 2 inches up to a very large commercial dredge with intake of 10 inches. The larger the intake size, of course, the more material can be processed through the dredge each hour of operation.


Gold suction dredge, 8 ¨, made by Keene Engineering

However, the larger the intake, the heavier the dredge will be, and with increased weight there is decreased portability. Further, dredges with intake sizes larger than 4 or 5 inches can be difficult for a single operator to manage. For safety reasons it is recommended that at least two people operate or tend the dredge if one person is operating underwater. Numerous improvements have been made in dredge sluice box efficiency, and many designs use further proprietary systems to increase fine gold recovery. The lightweight, portable suction dredge allows an individual prospector the ability to explore a world of gold deposits that the old timers had great difficulty reaching.

A power jet is attached directly to the header box and the pressurized water from the pump is injected into the power jet and that the water is blown directly into the header box. This causes a vacuum to form in the material hose which sucks up water and gravel from the other end of the hose, and they are likewise sucked up into the header box. The header box is simply a deflector attached to the end of the sluice which dampens the force of the water and gravel so that it flows evenly out of the sluice. The sluice captures the gold behind the riffles exactly like it would when you use it as a stream sluice.

A suction nozzle works in a similar fashion except instead of being attached directly to the header box it is attached at the far end of the material hose. A vacuum is created which sucks up water and gravel but the material is pushed through the hose instead of being sucked through the hose, as in the case of the power jet.

Which system is better? Power Jet or Suction Nozzle?

The answer is that they each have advantages and disadvantages. The advantage of the power jet is that the operator of the dredge has to only deal with one hose when he is dredging. The power jet also allows deeper dredging because the power jet will lift material higher than a suction nozzle can push it. The disadvantage of the power jet is that the power jet and supply hose must be kept under water at all times. Therefore they do not work very well when working in shallow water. The advantage of the suction nozzle system is that you can dredge in very shallow water and walk around with the suction nozzle completely out of the water without affecting the performance of the sluice. The disadvantage is that you have two hoses that the dredge operator must deal with. Generally speaking a dredge designed to work on flotation, with a compressor for underwater dredging, is almost always equipped as a power jet. A dredge that is designed to work with the sluice while standing in shallow water, is almost always equipped with a suction nozzle.


Dredge Diagram

The diagram left shows a basic suction dredge flowchart. The footvalve is a one way valve with a screen. Nearly all dredge pumps are not self priming because the mechanism that enables self priming results in a less efficient pump. Simple fire fighting type high pressure, high volume pumps are employed which need to be primed before starting the motor. An explanation from Keene Engineering follows:

“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.”

The intake screen prevents large items from entering the pump, but sand can still pass through and cause excessive wear over time. Place the footvalve on rocks, in a bucket underwater, or strap it under the floats to keep it off the bottom and out of sand. Once a system is properly primed the one way valve retains the prime even if the pump is shut off, allowing for easy restarts. If the footvalve is allowed to suck air and the prime is lost, shut off the pump as quickly as possible to prevent damage to the pump seal. It is a good idea to always have a spare pump seal as once one is damaged the dredging operation will be at a standstill until a new one is installed.

Water entering the footvalve passes through the pump intake hose and into the pump. It is pumped into the pressure hose and then into the venturi device that creates the suction in the main suction hose. The most common device is the power jet, which is installed in between the sluice box and the main suction hose. A power jet pulls material up and into the sluice box. An alternative is the suction nozzle which installs on the end of the suction hose in place of the suction tip. A suction nozzle pushes material up the suction hose and into the sluice box.

Power jets take less hose and are more efficient and therefore very common. However, care must be taken to not suck air into the main suction hose or the system will stop working until all the air is purged from the hose. A suction nozzle keeps water in the system at all times and so is preferred for shallow water use where sucking air into the nozzle is inevitable. Suction nozzles are rarely seen as a stock item except on 1.5″ or 2″ dredges and must be added as an accessory item on larger dredges.

The main suction hose is attached to the intake end of the power jet. It varies from 10 feet long on small dredges to 20 feet or more on larger dredges. A suction tip is fastened into the intake end of the hose which has a reduced opening to help prevent hose clogs.

The discharge end of the power jet plugs into a flare. The flare widens the flow of material out to the width of the sluice box, and in doing so slows the speed at which the material enters the sluice box.

When the material enters the sluice box it passes over a classifier screen and under a damper. The damper is usually a large sheet of rubber best seen in the photo at the top of this page, that rides on top of the flow of water. It smooths the water flow and forces any small gold that may be riding on top of the water back into the main flow. The classifier screen also acts to slow the flow of water and create a partial separation of the large rocks from the smaller gold bearing material, which passes through the screen and directly into the first couple riffles. The screen may be a punch plate or a heavy wire mesh with holes varying from 1/4″ to 1″ in size.

The main gold capturing section of the sluice box has riffles over a carpet. The riffles create gold traps which direct the gold into the carpet which captures and holds the gold. Many types of riffle designs exist, the most common being an inverted “L” design referred to as “Hungarian riffles”. Expanded metal screening of various types is also used in many sluice boxes as a riffle or in conjunction with other riffle types. Carpets employed included black ribbed rubber matting, riffled indoor/outdoor carpeting, and a type of carpeting referred to as “miners moss”. Miners moss is a nickname for a type of mat commonly used in building entryways that looks like pressed spaghetti. The open weave design that makes it good for trapping dirt from the bottom of shoes also makes it an excellent gold trapping carpet.

An optional component on a suction dredge is an air compressor which is run off the same motor as the pump via a belt and pulley arrangement. The air compressor is a special breathing air compressor than can deliver air to a diver underwater for as long as the motor is running. The air is delivered to the diver with an air line, reserve tank, and regulator. Surface air supply dive systems are commonly referred to as hookah diving systems.

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