Setting Up the Proper Recovery System
By Dave McCracken
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Rubies and sapphires from Cambodia. |
We have experimented a lot with secondary recovery systems to catch fine gold
and gemstones that get sucked up the suction nozzle of a floating dredge. There
are numerous issues involved, which must be carefully resolved. I strongly
advise you to study these issues for yourself as much as possible before
deciding how to acquire accurate samples, and what to do for a production dredge
if you decide to move forward.
If these problems are not carefully considered and resolved in the construction
of the equipment, the problems will definitely have to be dealt with in the
field, where it is much harder to fix them.
Since the purpose of
sampling is to
accurately determine the real value of gold (and/or gemstones) in the river
gravels, it is important how you acquire the samples. And if the samples turn
out well, it is vital that you design your production dredge and recovery system
from the beginning, to make certain they will actually do the job. The reason I
stress this point, is because I have been called in so many times to help with
projects that did not acquire equipment that would efficiently recover fine gold
and/or gemstones in the first place.
I also advise you to not take for granted that gemstone-dredges advertised by
various dredge-builders within the industry will recover diamonds or other
gemstones efficiently just because the builders advertise that they do. I
encourage you to review the points I follow with here, so you will have a
foundation with which to ask questions and make your own judgments at the time
when you will need to make pivotal decisions.
Gemstones are not heavy like gold, so they are much more difficult to recover.
Conditions must be set up to near-perfection to effectively recover gemstones
from the volume of sand and gravel which passes through a dredge nozzle.
Especially a
production dredge in the hands of experienced operators!
 Any enquiry into gemstone-recovery on suction dredges should certainly lead you
to the subject of jigs. A mineral-jig is a mechanical device that can be
adjusted to create a specific suspended medium inside. As raw material flows
into a jig which has been set up properly, different minerals are separated by
their specific gravity. Minerals that are lighter than a specific weight-range
are allowed to flow out of the jig as tailings. Minerals that are heavier are
allowed to settle to the bed, or to the bottom of the jig. The heaviest
finer-sized materials (mostly gold and iron) are allowed to bleed out the bottom
of the jig to keep it from filling up with material. Heavier materials are then
collected elsewhere, or are directed to more finely-tuned recovery systems.
Any enquiry into suction dredges and jigs should prompt a series of important
questions:
1) How are you going to classify the raw material that is sucked up through the
dredge’s suction nozzle?
2) What size-classification and volume of material will feed the jig?
3) How much volume of water will be included with the feed to the jig?
4) What will you do with the heavy material that flows from the bottom of the
jig?
Let’s take these important questions up one at a time:
1) Classification: You cannot direct large materials (rocks) into a mineral jig
and expect it to perform well. This is actually true of any recovery system
being set up to recover gemstones or fine-sized gold. Some method of screening
is necessary to “classify” the size-range of materials that you want to direct
into each type of recovery system. The more that different size-fractions of
material are separated from each other, the easier it is to separate gold or
gemstones from the other materials.
Since dredges have limited space to work with (usually on a floatation platform
or two), classification systems must be kept reasonably simple and portable.
Most suction dredges are set up with a fixed (not mechanical) classification
screen which material and water flow across inside the sluice box.
Riffles and various types
of traps are constructed below the screen to trap gold and other valuable
minerals out of the water-flow. All of the material that passes over top of the
screen, or that is not trapped by the riffles under the screen, is allowed to
flow out of the box and be discarded as tailings. For lack of a better term,
let’s call this a “hydraulic classification and recovery system,” because it
depends entirely upon water-flow to move raw material across the classification
screen and through the riffles. This is the type of system you can expect to
receive as standard dredging equipment on today’s market.
Hydraulic classification and recovery systems have evolved over the years to the
point where they generally recover gold and platinum with a reasonable degree of
efficiency down to size-fractions relatively small in size. How fine in size,
depends upon various factors, like the purity of the gold, its shape (round,
flat or crystalline), and the nature of the material (slurry) that is flowing
through the recovery system along with the gold or platinum.
It is reasonably safe to say that any recovery system is efficient down to a
certain size-fraction of gold or platinum in any given area. The size-fraction
might vary from one place to another. The reason for this is that the specific
gravity of gold and platinum is generally 5 to 6 times greater than the average
of other materials that exist in a streambed. This incredible difference in
weight will generally allow pieces of the heavier metal to penetrate the screen
and drop behind the riffles in a sluice box – even though there is a strong
force of water present to wash larger-sized material over top of the screen.
It is also reasonably safe to say that the smaller a piece of heavy metal is
(gold or platinum), the more it will be influenced by the fast, turbulent flow
of water required to wash larger-sized material through a sluice box (for
example, the force of water to wash 9-inch rocks over top of a screen in the
recovery system of a 10-inch dredge). So the smaller it is, the less likely that
a piece of gold will drop through a hydraulic screen and get trapped behind a
riffle. Therefore, hydraulic classification systems lose efficiency as the
particle-size of a heavy metal becomes smaller.
 It is important to understand, that because gemstones are only slightly heavier
than quartz, and are within the same weight-range as the other materials
generally found in a streambed, hydraulic classification systems on dredges are
not an efficient method of sizing raw material, meaning that probably more
gemstones wash across the top of a screen (into tailings), than drop through it.
Furthermore, hydraulic recovery systems (riffles and baffles) are actually
designed to discard gemstones.
In other words, because gemstones are light, you cannot expect them to drop
through a classification screen that has a torrent of water passing over top.
And, because riffles will quickly accumulate a concentrate of material behind
them that is heavier than the average specific gravity of a gemstone, you should
not expect to recover gemstones efficiently using riffles.
As far as I know,
Pro-Mack is the only dredge-builder that has accomplished mechanical
classification on a suction dredge. We do it by placing a vibrating screen
(powered by a hydraulic pump) in place of the sluice box. Raw material from the
suction nozzle is directed onto the vibrating screen. Minus-size materials drop
into a hopper under the screen and are then pumped to a recovery system –
usually on a second platform.
Summary: On suction dredges, there are basically 2 kinds of
classification systems:
(A) A fixed screen which a flow of raw material is washed across by the force of
water, with some portion of minus-sized gravels dropping through. This system
works relatively well on heavy metals down to a certain size-fraction.
Efficiency is lost below that size, and there is no efficiency on gemstones
(they are too light).
(B) Mechanical classification, when set up properly, can be depended-upon to
provide nearly 100% of the size-fraction you want to separate out from the raw
material, then to be directed into a recovery system. Please take a look at the following free video segment to see how we recently worked this out on a Pro-Mack commercial dredge system that is being used on a diamond recovery project in India:
 India Shaker Screen
2) Size and volume: Jigs are available in different types and sizes. Generally,
a certain size of jig is designed to process a given volume of material. Each
manufacturer will have their own set of guidelines.
I say “guidelines,” because we are back to variables that will change from one
location to the next. The main consideration is the difference in weight between
the mineral you are trying to save, and the medium that it is mixed with.
 For example, because the weight-difference is so great, it is relatively easy to
drop a particle of gold (19.6 times heavier than water) through a suspended
medium of pre-sized quartz crystals (3 times heavier than water), because the
difference in weight is more than 6 times. Therefore, with heavy metals, there
is greater margin to introduce a larger variation of size-fraction into the jig,
or a larger volume of raw material, without forfeiting recovery.
If you are trying to drop gold particles through a raw material made up of iron
(8 times heavier than water), you will be required to tighten-up the sizing and
slow down the feed; because the weight-difference is only around 2 ½ times.
Sizing and volume are critical in the recovery of gemstones (usually only around
3.5 times heavier than water), because there will be only the smallest
weight-difference between the stones and the other streambed materials.
Summary: Sizing and volume requirements for jigs are largely affected by the
difference in weight between the particle you are trying to recover, and the raw
material you want to discard as tailings. This becomes critical as you try and
recover gemstones with efficiency.
3) Water feed: Suction dredges operate on a venturi system, where high-pressure
water is pumped into a power-jet at an angle, to create a vacuum through the
suction hose and nozzle. In this way, gravel and rocks can be sucked up from the
bottom of the river and directed to a screening or recovery system floating at
the surface, without having to pass them through a water pump. This allows a
suction dredge to be manufactured at a small fraction of the cost to produce a
dredge of the same size which must pass rocks and material through a pump.
But venturi-dredges are limited, in that they cannot lift streambed material and
water more than around a foot (or less) above the water’s surface. Therefore,
anything that is going to initially be done to raw material excavated by a
suction dredge must be accomplished right at the water’s surface. This is the
reason why all standard suction dredges are equipped with hydraulic
classification and recovery systems; because there is very little room at the
water’s surface to do much else.
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Men installing the Catch-hopper which mounts under the shaker
screen on a 10-inch commercial Pro-Mack dredge.
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Classification systems used on a suction dredge will end up dropping the
minus-gravels into a sump or hopper that is located below the water’s surface.
Therefore, to get the classified material up into a recovery system, it will
need to be pumped.
Any jig is designed to allow only so much water-flow with the feed. The reason
is that too much water-flow will wash sized-material across the top of the jig
before the suspended medium has an opportunity to place particles where they
should go.
 Water-flow is even more critical in the recovery of gemstones because they are
so light.
Therefore, important consideration must be given to how classified materials
will be directed into the feed of a jig. We use hydraulic-powered gravel pumps
on the Pro-Mack design, because we have found that venturi-elevators (using a
high-pressure water flow) deliver too much water volume with the feed.
4) Bleeding off the heavy material: One of the reasons why jigs work so well, is
that they are allowed to keep bleeding off the heaviest materials from the
bottom, which otherwise would build up and alter the suspended medium that
creates the desired separation of your target-mineral from the other streambed
materials.
What you should do with the heavy materials from the bottom of a jig entirely
depends upon what they contain.
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On the production dredges we build at Pro-Mack, it is common to have a series of
three jigs. The first (called a “rougher”) accepts the classified raw material
from the sump under the dredge’s screen. The rougher accepts a larger
classification of material at volume speed. Its purpose is just to make an
initial classification and trap the largest gemstones and heavy metals on top of
a bed (smaller classification screen) inside the jig. Large materials and the
lightest small materials flow off the top of the rougher-jig as tailings.
Heavier, classified materials are bled off the bottom into a “secondary-jig.”
The secondary-jig can be more finely-tuned to further separate a
finer-classified, slightly-heavier material at a slower speed. Then the
finer-classified, heavier material from the bottom of the secondary jig is fed
into a “finishing-jig” – which can be tuned to complete a final separation.
Most or all of the gemstones will become trapped on top of the jig-beds
(classification screens) inside of the jigs. If there are fine-sized heavy
metals present, the bleed from the bottom of the finishing-jig usually is
directed into a final concentrating device – commonly a centrifugal bowl. The
final concentrate is then separated in camp, often with the use of a shaker
table.
2 Images: Shaker table Final fine gold separation over a shaker table in a
production operation.
Shaker
As all of these mechanical recovery systems are very sensitive to sudden jerking
movements, changes in water pressure and other factors, we have found that it is
much better to set up the recovery system for a production dredging operation on
its own independent platform. This includes the water pumping system that
supports the recovery system(s). Here are a few reasons why we have found this
works better:
1) Dredge platforms jerk around a lot as the suction nozzle is managed
underwater. The suction hose is flexible, so there is an accordion-affect when
varying amounts of suction are used at the nozzle. This causes the dredge to
bounce around. The bouncing can throw off critical settings on mechanical
recovery equipment.
2) Demands made upon the dredge’s main water pump fluctuate widely, depending
upon how much suction is being used at the nozzle. If the same water pump is
being used to supply water to mechanical recovery systems, the
pressure-fluctuations can throw off the suspended mediums that make the systems
work.
3) A production gold dredging platform has a constant parade of divers, managers
and tenders moving about while doing their various jobs. Most recovery systems
are designed to be fastened down to a level, stable platform. The movement of
numerous people around a dredge platform can throw off important settings.
4) Security: The final product(s) on a commercial mining operation should
accumulate in a safe location where traffic can be carefully controlled. In
addition, these mechanical recovery systems have a lot of moving parts. It’s
better to keep wet, slippery divers and all their gear clear of the machinery.
In my view, the best way to do it is to set up two platforms:
A)
 A dredge platform that you can move around, put divers and tenders on, pump
raw material to; and pump classified materials from.
B) A recovery platform that receives the classified materials and processes
them. This system needs to be carefully engineered, and large enough to manage
the volume of raw, classified material that is directed to it from the dredge.
You only need to have one or two operators on this platform, so as to not upset
the delicate balances that can be easily offset by people walking around
changing the way the platform is sitting in the water.
I have found that when you try and put it all on one platform, you are forced to
ignore several vital factors which add up ultimately to a dredge-package that
does not do the job very well.
Different Kinds of
Sampling
Outfitting an Underwater
Mining Project
The Preliminary
Evaluation
Logistical
Planning
The Size of Riffles
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