Suction Dredging for Gold
Covering the Basics of a Gold Dredging Operation

By Dave McCracken
 

Most gold mining today is done in small operations -- one or two men working at a time -- often with the use of suction dredges. A suction dredge is a powerful underwater-type of vacuum cleaner. It sucks up streambed material (rocks, sand, gravel, silt, gold and other minerals), passes it up through a suction hose, and runs it across a recovery system floating at the surface. Pieces of gold, which are very heavy, are separated from the other streambed materials and trapped, as the gravel and other material wash through the recovery system and are then washed back into the stream.

Most intermediate and larger-sized gold dredges come with built-in hookah-air systems. These attach to the same engine that powers the water pump. Air for breathing underwater is generated by an air compressor, passes down through an air line, and provides air to a diver through a regulator, similar to what is used by SCUBA divers. Dredging is usually done in ten feet of water or less, but some work is done at greater depths. 

Using a dredge, an (experienced) operator is able to process a much larger volume of streambed material than with any other small-scale mining apparatus. Most of the gold-bearing river-bottom streambed material is sucked up as quickly as the operator is able to feed it into the suction nozzle.  Rocks too large to pass through the suction nozzle are moved out of the way by hand.

The early miners who came to California (and elsewhere) during the 1849 gold rush (and later) did find and recover many of the easy-to-find gold nuggets and rich deposits. During those early days, the deposits had to be easy to find and recover; because recovery methods and processing capabilities were very limited. 
Suction dredge technology allows modern-day gold and gemstone miners to prospect and mine for mineral deposits in places where earlier miners were not able to go. This is true in the deeper rivers (3-meters or more of water depth) all over the world. It is especially true in remote locations and/or developing countries where modern technology is generally not available to village-miners.

Because a modern (experienced) dredger is able to process substantially more volume of streambed material, with better gold recovery, the gravel deposits of today do not need to be as rich in gold as was necessary during the past.

One of the main advantages of having the capability to process more streambed material is that an area can be more-effectively sampled. Therefore, the success-rate in modern underwater mining is much greater than it’s ever been using other technologies. This has caused a lot of interest in suction dredging equipment, which has resulted in a competitive market. At present, very good equipment for suction dredging can be obtained at relatively low cost.  Just to give you an idea, a top-of-the-line five-inch gold dredge, and the miscellaneous gear needed to run a small dredging operation, can be obtained for under $5,000. 

The size of a gold dredge is determined by the inside-diameter of its suction hose--usually anywhere from two to ten inches. A single person customarily can operate a four, five or six-inch dredge. Two men commonly operate six, eight or ten-inch units. Sometimes, when streambed material is deep, and there is a lot of oversized material (large rocks and boulders) that needs to be moved out of the way, as many as four or five persons can be utilized underwater to operate a production gold dredge.

A single, experienced operator, sampling with a four-inch dredge, can process many times more than could be processed at the surface using conventional pick & shovel methods. A six-inch dredge, in experienced hands, can process around twice as much material as can be accomplished with a 4-inch dredge -- and can also dredge several feet deeper into the streambed material, while remaining efficiently-productive. An 8-inch dredge can about double the production over a 6-inch dredge and dredge even deeper into the streambed material. And a 10-inch dredge can double production over an 8-inch dredge and excavate even deeper holes.

The other side of this equation is that each larger dredge-size about doubles the bulk and weight of the equipment that must be moved around and managed. Because of this, some locations may be too remote to support a larger-sized dredge.
The limiting-factor on a suction dredge is not the horsepower or the size of the suction hose.  It is the size of the nozzle opening. Please trust me on this one:  It is all about the size of rock that will go up the nozzle.  Once again, I invite you to closely watch the underwater video segments on my videos and see what is happening underwater.  It is almost all concerned with moving the oversized material out of the way.  The size of the nozzle-opening determines what can be sucked up, and what must be otherwise moved out of the way by hand.

A cutter-head will just get bogged down (and damaged) in a normal hard-packed streambed.

Some dredges are available that are operated from the surface with hydraulic-powered cutter-heads at the nozzle. Cutter-heads are mechanical devices that help feed material evenly into the nozzle. They are most-productive in doing channel-work in harbors or making navigation-channels (where the material mostly consists of sand or silt) deeper or wider. Cutter-heads cannot replace the need for divers when mining in hard-packed streambeds which are made up mainly of oversized rocks and boulders, which must be moved out of the excavation by hand.


If you want to do serious excavations with a suction dredge, you must leave the opening of the suction-nozzle as large in diameter as possible, while still reducing it enough to eliminate un-necessary plug-ups in the suction hose.
Streams, rivers and creeks in gold-bearing areas are constantly being replenished with the valued metal. In the last 150 years, natural erosion has caused a substantial amount of new gold to become deposited in today’s waterways. Some rivers and streams that were once thoroughly mined out are presently paying gold dredgers in very handsome deposits. Rivers that ran too deep for local miners to gain access to the bottom during the past are giving up rich, virgin gold deposits to suction dredgers.

Gold found in streambeds is called “placer gold.” Placer gold is most commonly found in flake form, usually about the size of flattened grains of rice and smaller. Some deposits carry a larger amount of such flakes and fine-gold. Other deposits carry substantial amounts of larger pieces and nuggets. Gold nuggets can be worth more than actual weight-value, because of their uniqueness as jewelry or specimens.

Gold is one of the heaviest metals. It has a specific gravity of 19.6, meaning that it weighs 19.6 times an equal volume of pure water. It is about six times heavier than the average sand, gravel, rocks and other materials normally found in a streambed. So it takes a substantially-greater force to move gold, than it does to move the other streambed materials. This principle is used in gold recovery systems. The same principle is also used to predict where high-grade gold deposits are most likely to be found in a riverbed.
 

Because of its enormous weight, gold tends to follow a certain path of its own when being washed down a waterway, and will get hung up in various common locations where the water force lets up enough to drop gold. One example is the inside of a bend where a stream makes a turn. Another example is at the lower-end of a section of white water. Gold will form “pay-streaks” in areas such as this--where the water slows down on a large scale during large flood storms.

A gold-dredger has an advantage, in that he/she is able to float equipment where he or she wants it to go, sucking up gravel from various strategic areas. This is much easier than having to carry equipment around and set it up in each new area, as in conventional mining.

There is some amount of gold to be found just about anywhere in a gold-bearing waterway. The important key is to find it in paying quantities. Most commonly, experienced dredgers locate rich pay-streaks by systematically sampling various locations where it seems that gold should have been deposited. Sometimes it takes numerous sample holes to locate a pay-streak, and sometimes it only takes a few. This often depends on an individual’s understanding of where gold gets hung up in a stream, and on his or her familiarity with the area being sampled. 

Dredging can be an exciting and remunerative activity if you are willing to work hard at it. It takes a bit of study and persistence in the beginning--just like any other activity.
Anyone contemplating suction dredging as a commercial activity should be aware that there is a learning curve involved, and they should plan on it.

Logistical Planning

The Preliminary Evaluation

Prospecting for Gold in Hard-packed Streambeds

Outfitting an Underwater Mining Project

Production Gold Dredging

Hard Work