Richard Shell has been manufacturing very large suction dredges for over forty years. Figure 1 shows the largest suction dredge in Alaska. This Shell 15-inch dredge has an overall length of 123 feet from the nozzle tip to the tail race. It is 23 feet wide. The pump motor is 300 horsepower and can pump 4000 gallons per minute. The dredge can process up 400 cubic yards an hour. It generates 500 kW which runs the pump, concentrators and other electrical controls. This dredge has a high tech undercurrent concentrator-classifier recovery system (figure 4) that is excellent at fine gold recovery. The cleanup equipment is even onboard.
Floating suction dredges are the most efficient way to mine unconsolidated sediments ever developed. Suction dredges can continuously move material from the earth to the processing equipment and dump onto the tailings pile in one non-stop motion using a minimum of moving parts and machinery. (Compare to a bucketline dredge and typical heavy equipment placer mines.) There is no repetitive feeding by excavators endlessly cycling bucketfuls into a hopper, then pushing the tailings away from the plant and finally reclaimed. Suction dredges’ continuous feed rate is many times more than of the largest excavator and the feed is much more consistent. No more of the surging and purging associated with conventional placer mining. The feed gravel is only picked up once, the gold and other heavy minerals are stripped out in the concentrator-classifiers, and down the tail chute to the pond floor in only seconds. Handling the gravel only once substantially lowers the operating cost of placer mining. In fact the operating costs are so low; it might make it feasible to re-mine tailings of past generations of miners.
Figure 2 shows the major components of a 15-inch dredge. This also shows the shovel-scoop cutter head on the end of the suction pipe. The shovel-scoop cutter head operates as a shovel which pivots to open as the dredge yaws from side to side. The spuds hold the dredge in one place while it yaws back and forth.
Figure 3 is a close up of the shovel scoop cutter head that pivots to open in the direction of the yaw.
The intense suction at the cutter-head is developed by 200 pounds of pressure in the venturi just behind the shovel scoop cutter head. The dredge can generate enough horizontal torque when yawing to rip several inches into most types of bedrock. In the unlikely and rare event, a rock does manage to get jammed in the intake pipe, there is a rifle port shown in figure 3 at the end of the venturi. It is possible to shoot a high velocity projectile and break up the clog.
The feed gravel is screened down to half inch minus, then distributed to the concentrator-classifier trays. The concentrator-classifiers have slits of various widths transverse to the current direction. The heavier particles fall into slits when the trays vibrate at a specified frequency. This system is extremely efficient at fine gold recovery and has been proven to be over 90% efficient at recovering flour gold. The coarse gold is caught in a nugget trap behind the half inch screen.
Figure 4 shows some of the concentrator-classifier trays not in operation. The slit widths vary from one tenth of an inch to 6/1000th of an inch.
These dredges are impressive and complicated with their high tech components, but they actually are simpler and with far fewer moving parts than the old bucketline dredges. Much of the controls are done with joysticks. Shell dredges can be produced with all electrical control systems, or with hydraulic control systems.
Let’s compare large suction dredges to modern bucketline dredges, such as the Bima which plied the water off of Nome 25 years ago. The Bima had a total horsepower of almost 7000 horses pulling 5667 kW and was capable of mining 1,300 to 2,600 cubic yards per hour. It weighed 12,000 metric tons. It was capable of mining three to five times as much as a Shell 15-inch which is capable of mining 400 cubic yards per hour, but the Bima used more than 10 times the electricity, 12 times the horsepower, and at least 10 times as many employees. Needless to say, it would have had at least 10 times the operating costs too. Even with the huge economy of scale help, the Bima would have had; it was far more expensive to operate per cubic yard than a Shell suction dredge.
Suction dredges can dig much deeper water than bucketlines too. Bucketlines could only dig as deep as their ladders would permit. Lengthening the bucketline ladder was a very major undertaking. Lengthening the suction pipe on a suction dredge takes only a few hours. For really deep ground such as 80 to 120 feet, air injection at the intake nozzle can economically bring gravel up. Compare this to adding many more thousand pound buckets to the bucketline. Plus suction dredges are more flexible for cleaning off irregular bedrock surfaces. The operating cost saving are obvious.
Another feature that deserves pointing out is environmental remediation by recovering mercury from past placer mining. Since of operating costs are so low with a suction dredge, it may be feasible to strip mercury on a low bid contract. The low operating cost often makes it feasible to mine ground that has been mined several times. Of course, it all depends on the amount of precious metal in the old tailings. These dredges also can make it feasible to mine ground with too much heavy gangue minerals for conventional mining equipment. Shell engineers can outfit a dredge to suit the property with nugget detectors, magnetic separators, sonar, even underwater televisions, as well as onboard cameras for watching the concentrators and clean-up operations. In fact our dredges are often custom designed to best suit the client and to the ground conditions of the property.
Our suction dredges dump the fines and the coarse tailings close enough to together that plant roots can more easily reach shallow moisture in the fines.
Using a large floating dredge with fine gold recovery concentrators can make it possible to go after the gold that was too difficult and expensive to mine before. Shell dredges can be outfitted for marine mining too.
Figure 5 is a simplified schematic of a dredge in operation. Mining with a large suction dredge enables much more cost-effective and simpler mining by only picking up the ground once. Suck it up, strip the gold out, and drop the tailings back on the pond bottom. None of this digging it with one machine, carrying it to the wash plant with another, picking it up again to run it into the wash plant, then picking it up again to carry away from the wash plant. Dozers are needed for stripping overburden and smoothing the tailing and re-spreading topsoil, but that is all. A dredge and a dozer complete the list of the costly heavy machinery.
Shell dredges come in three sizes: 10-inch, 12-inch and 15-inch. The 10-inch can produce 200 yards per hour. They weigh 8 tons and powered by 700 horsepower diesel. A 10-inch suction dredge cost $2.5 million. The 12-inch cost $3 million. The 12-inch can mine 300 yards per hour. They weigh 25 tons and powered by 733 horsepower diesel. For more details, go to: https://www.shelldredging.com/
The cross sections in figure 6 is similar that of other highly efficient mobile wash plant cross sections.
Figure 7 is a view of the tail end of the dredge. The coarse tailings splitter and the spuds are the most obvious features. The splitter dumps the tailings close enough to the spuds to help secure their footing. When the dredge yaws left and right (port and starboard to be more fitting), one of the spuds is pulled so it can pivot on the other spud.
Looking at the economics of this 15-inch dredge: one costs $5 million and it can produce 400 cubic yards an hour. If you mine 20 hours per day and 30 days a month, it computes to 240,000 cubic yards a month. If the average grade of the ground is $10 per yard, then your gross revenue is $2.4 million per month. The operating cost should be in the vicinity of $5 per yard. Two to three men per shift can operate a dredge. In one summer, a dredge can be paid off. However suction dredges can operate longer than the three months of an Alaskan summer as the F.E. Company found out in Fairbanks by working for five months some years. If the dredge is enclosed and insulated, you can mine until the dredge pond is too thick with ice. Good news! There will be no irrigation pipes to thaw out each morning making placer mining a real pleasure…hopefully.
One of our 15-inch dredges can be shipped on six flatbed 40 foot long trailers and can be assembled at the mine site by four men in less than seven days. Our technicians will be onsite to re-assemble it and instruct how to use it.
Take advantage of the ground water rather than fight it. Lengthen your mining season. Increase your fine gold recovery. Re-mine that ground to clean bedrock with a giant vacuum cleaner and get all the gold out of those tailings. Use a really big suction dredge. Buy or lease a Shell suction dredge.
For more information contact Jim Halloran at 907-250-3726