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Visit our on-line presentation of the "Modern Gold Dredge" by Bruce Harris. This includes a great Gold Mining Dictionary.
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This booklet "The Modern Gold Dredge©" is provided for our web site's visitors & customers personal enjoyment. Copyright© 1992 by HMMC. All rights reserved worldwide.
Table Of Contents
High Lift DredgeSubSurface DredgeSurface Dredge
Modern Gold Dredge
Gas & Oil
(Suction or Vacuum Dredges > Jet Dredge)
NOTE: The term suction dredge is the same as vacuum dredge. Many dredgers are not aware of this. Many have gone on the assumption, that it was directly associated with where the power-jet intake; called a venturi; was placed.
Example; if the power-jet was placed in the power-jet tube up closer to the sluice, it was assumed to be a vacuum dredge or if it was placed in the nozzle head, such as with the suction nozzle, it was assumed to be a suction dredge. In some instances, it has been thought to be the other way around.
A modern dredge works materials, by means of a suction system, which draws gold-bearing materials up through the suction hose by a vacuum effect, from the use of a power-jet intake, which powers the dredge. This allows gold-bearing streambed materials, to be vacuumed up from underwater and processed through a sluice. The correct term for the modern dredge is jet dredge. The term is derived from the power-jet intake in the dredge suction system.
There are three types of jet dredges; high lift, subsurface, and surface. The difference is where the sluice or other primary recovery system is placed to process materials.
HIGH-LIFT DREDGE: A dredge generally processes all materials through a sluice recovery system aboard the floating platform, but this is not always the case. With a high lift dredge, one has the floating platform; dredge; and suction system to draw materials up from the bottom, but can direct the materials to the shoreline to process.
This is quite unique, because it can allow one to not only use other types of wet recovery systems for their primary recovery, but can be processed even by a sluice with better results. This is because of its stability on the shore. A sluice out on the water for example, will move around from the currents and because there is many changes in the amount of material build-up and water flow across the sluice itself, it will be less efficient than one stabilized.
This is very similar to that of high-banker sluices or hydromatic jig for example, that have the ability or available kits to convert them to dredge materials and process them on the shore. The main difference is where the motor and water pumps are located. The high-lift dredge places these on a floating platform, whereas the others are mounted on the shore next or attached to the recovery system.
High-lift dredges have a few advantages over these. For one, they are available in many sizes, allowing for various capacity ratings. Another is that you can work anywhere out in a waterway in the same manner as would any other dredge. This is especially helpful when working deep overburden, one has better control over the nozzle and suction hose.
The modern portable high-lift dredge generally range from 3 to 4 inch in size. Depending on the size or model they can transport slurries of rock, sand and gravel several hundred feet and lift material as high as 25 feet. The larger 5 to 8 inch dredge size may move materials up to 1,000 feet and as high as 100 feet or more.
Though the common jet-dredge is mainly used for the recovery of valuable materials, a design such as this provides other uses, inexpensively compared to the larger commercial reclamation dredges; such as, pollution control, reclamation, sand & silt removal, harvesting fish & clams, and underwater growth & plant life harvesting.
SUBSURFACE DREDGE: A dredge is classified as a subsurface dredge, when the sluice is placed below the water surface, yet firmly mounted aboard the dredge platform. When the sluice sets below the surface of the water, it reduces lift and increasing suction power on materials. This is somewhat an advantage over the standard surface sluice, yet both have their advantages and disadvantages, depending on preferences or requirements.
A subsurface dredge processes materials through a submersible sluice below the surface of a stream, river, etc. A submersible sluice is designed to process materials underwater in the same manner as submersible dredging tubes.
SURFACE DREDGE: Most modern dredges are designed with the sluice placed or mounted above water level; called a surface dredge.
The sluice design on these may vary according to the specific type; or size; of valuable, one is looking to recover. These follow into three categories; those designed especially for gem or diamond recovery, gold recovery, and coin recovery. Some sluices are designed to recover only one type, whereas some can recover both gem and precious metals with minimal adjustments. Those designed for gold recovery are called a gold dredge.
Common dredge sizes for non-commercial dredges are 2, 2.5 , 3, 4, 5, 6, and 8 inches. Dredge size is based on diameter of intake/suction hose nozzle. The Inch Rating pertains to the size of the intake hose measurement. When you see a dredge being advertised, as a 2-inch dredge, this means that the inside diameter of the suction hose that goes up to the back of the sluice box is 2 inches. It is not the diameter of the suction nozzle, as many people think. The diameter of the suction nozzle is anywhere from 1/4 inch to a full inch less than the inside diameter of the suction hose. This is so rocks of the same size wont pass through the nozzle to continually clog up the hose.
Recreational dredge size(s) are considered to be 4 inches or less. Some areas of public lands available for mining, have limitations on dredge size. In some cases, dredging is restricted to recreational dredging only. The standard dredge permit issued from the State Fish & Game Department for use within a given States waters, is for use of 8 inch dredge size or smaller.
Recreational dredges are mainly used by those considering backpacking into remote areas for a short period or for weekenders who have little time to set up equipment as with the larger dredges. These are also used by the more experienced dredgers, to sample remote areas ahead of the larger dredge size.
The 4 and 5 inch dredge, are the most common sizes used by recreationalists and small mining operations. These dredges are designed to be easily dismantled and have the same air capability for hooka diving as the larger dredges. They also process a greater volume of materials than the prospecting type small dredges. Each inch in diameter of the intake greatly increases the material flow into the sluice concentrator allowing more values to be recovered in the same amount of time. The inside diameter of the suction hose, also has a direct relation to the amount of gravel that the dredge will move per hour. The general rule for this is; if you double the inside diameter of the suction hose, you increase the capacity of the dredge by a factor of four.
Depth capability depends greatly on many factors; size of motor & pump combination, additional equipment (such as compressor size) and accessories, whether or not the pumps are belt driven or shaft mounted, whether the pumps are free standing or centrifugal jetting pumps, manufactures intended design, height above sea level, temperature of the area, etc.
Dredge capacity depends also on the above, but includes type & size of materials worked and the depth one is working. Yardage per hour specified by a particular manufacturer, is based upon the amount of loose gravels; all the the same basic size; that a dredges intake is capable of picking up and processing in an hours time. These figures relate to the maximum amount of materials the dredge can process under ideal conditions. Also, one has to take into account downtime, due to problems; mainly clogs in the suction system.
A recovery system is the method used to process gold-bearing materials for its gold or other heavy mineral content. This is done by means of separators and/or concentrators. The term separator as used in connection with dredging, is a device employing mechanical means to screen or divide streambed materials into sizes and/or allows tight packed materials or clay to be broken up before entering the concentrator; such as a hydraulic separator. On the other hand, a concentrator is a device employing mechanical means to separate gold or heavy minerals from streambed materials to recover a heavy concentrate.
A primary recovery system, is the main method of processing materials and may employ the subdividing of materials into sizes to be processed by different concentrators specifically designed for the size of materials passing through. This is seen with common dredge designs employing double or triple classifiers.
A secondary recovery system is a separate concentrator(s), which reprocesses materials a second time for any lost values. The primary recovery system aboard a dredge is usually a combination hydraulic separator and sluice concentrator; on the larger dredge sizes of 8 inch or larger, there may be two separate kinds of concentrators used for the primary recovery systems, such as a sluice for the larger gravels and a hydraulic jig for the smaller.
In most dredge designs or methods, where there is a secondary recovery system, the sluice is also the most common; such as with an undercurrent sluice (A large, flat, broad, branch sluice, placed beside or at the end of the main sluice recovery system).
Any recovery system design, has a manufacturers capacity rating. This is the amount of materials, which can be processed in a given amount of time. Dredge recovery systems use cubic yards per hour to specify its production rate. It is important to understand, this information is based on ideal conditions. This does not measure what should be expected in a normal operation; taking into consideration human and mechanical factors. In order for a dredge to operate at its maximum capacity, all the gravel would have to be of the correct size to enter the suction portion of the nozzle, and run a continuous flow of gravel, which is extremely unlikely. One should consider about one half to one third the rated capacity as a "probable capacity" under normal conditions.
Copyright 1992© Modern Gold Dredging / Published by Heavy Metal Mining Company, Springfield, Missouri, USA.