Production Gold Dredging
Dredging technique in
gold-bearing streambeds mainly is focused upon removing the oversized rocks
(any rock which is too large to be sucked up the suction nozzle) from the work
area of the dredge-hole. In most hard-packed streambeds, this removal of the
oversized material is the bulk of the dredger’s work. From a production standpoint,
a large portion of this work has to do with freeing and removing the oversized
rocks from the streambed in an orderly fashion.
Hard-packed streambeds are laid down horizontally, in layers, during major flood storms. Generally, the streambed is put together like a puzzle, with different rocks locking other rocks into place. Usually, because of gravity, rocks higher up in the streambed (laid down later) are locking lower rocks (laid down earlier) into their position in the streambed. Because of this, we have found the best means of production is to dredge the hole down a layer at a time. We call this a “top cut.” If you take down a broad horizontal area of the streambed together, you uncover a whole strata of rocks which are interconnected like a puzzle. Then, you can see which rocks must be removed first in order to free the others more easily. This really is the key to production dredging! Generally, there are two things that drastically slow down the production of less-experienced gold dredgers: (1) plug-ups and (2) “nitpicking.” The time and energy spent freeing plug-ups from the hose and power-jet cuts directly into how much progress will be made in the dredge-hole. Plug-ups are caused by rocks jamming in the dredge’s suction hose or power-jet. Everyone gets a few of these. Inexperienced dredgers get many! This comes from not understanding which types of rocks, or combination of rocks, to avoid sucking up the nozzle. Basically, this knowledge comes from hard-won experience in knocking out hundreds of plug-ups to the point where you are more careful about what goes into the nozzle. I have covered this area quite thoroughly in my book, “Basics of Successful Gold Dredging.”
During the past several years, the various dredge manufacturers have started putting out dredges using oversized power-jets. This means that the inside-diameter of the jet-tube is no longer reduced in size from the inside diameter of the suction hose, helping to greatly reduce the number of plug-ups a dredger will get while in production. So, the combination of some practice in learning which types of rocks to avoid sucking up the nozzle, and the over-sized power-jet on modern dredges, will eliminate most of the plug-ups that would otherwise hinder production. The other main loss of production, which we are all guilty of to some extent, comes from a poor production technique which we call "nitpicking." Nitpicking is when you are trying to free rocks out of the streambed which are not yet ready to come out. Nitpicking is dredging around and around rocks which are locked in place by other rocks that need to be freed up first. When you find yourself making little progress, the key is almost always to widen your dredge-hole.
Production dredging means moving gravel through the nozzle at optimum speed. This is accomplished by making your hole wide enough to allow the oversized rocks to be easily removed. As soon as you find the rocks are too tight to come apart easily, it is usually time to widen the hole again. An underwater dredging helper can be a big help to remove oversized material from the dredge-hole. Basically, there are two different types of jobs in an underwater mining operation: (1) nozzle operator (N/O) and (2) rock person (R/P). The nozzle operator is responsible for getting as much material up the nozzle as possible. Therefore, it is his or her responsibility to direct how the dredge-hole is being taken apart. The rock person has the responsibility to help the nozzle operator by removing those rocks that are immediately in the way of production. Since there are plenty of oversized rocks that can be removed from a production dredging operation's work area, it is important that the R/P have some judgment as to which rocks ought to be moved first. Ideally, he will pay attention to what the N/O is doing, and focus his efforts primarily on those rocks which are immediately in the nozzle’s way. Silt is usually released when a rock is moved out of the streambed. So an R/P must be careful to not cloud-out the hole (loss of visibility). This can be avoided by concentrating on (a) the rocks that are near to where the N/O is working, or (b) the rocks that have already been freed from the streambed, but not yet removed from the hole. On occasion, an R/P unwittingly takes over the production operation by randomly moving whatever rocks he or she happens to see. This, then, causes the N/O to have to follow the R/P around, sucking up the silt which would otherwise cloud-out the hole. This generally results in slowing down production. We have found that is better if the R/P accepts the position of being the N/O’s assistant, which allows the latter to direct the progress of the dredge-hole. Since production is controlled by how efficiently rocks can be removed from the dredge-hole, it is important to understand that they must also be strategically discarded in a manner which, if possible, will not require them to be moved a second or third time. Generally, this means getting each rock well out of the hole--as far back as necessary. Good judgment is important on this point. You only have so much time and energy available. How far that oversized rocks will need to be removed from the hole is somewhat dependent upon how deep the excavation is going to go. Early on, we often must guess about this. Sometimes, the streambed turns out to be deeper than we thought it would be. Then, we find ourselves turning around and throwing all the cobbles further back away from the hole. This is not unusual.
We also must use our best judgment to not waste our limited time and energy resources. We don’t really want to move all those tons and tons of rocks any further than necessary to allow a safe, orderly progression in the dredge-hole. As we move our hole forward, and as we dredge layers (“top cuts”) off the front of the hole, we try to leave a taper to prevent rocks from rolling in on us. This is an important safety factor. Also, since the N/O’s attention is generally focused on looking for gold, the R/P should be extra vigilant in watching out for safety concerns. Any rocks or boulders that potentially could roll in and injure a team-mate should be removed before they have a chance to do so. The rock-person’s attention should always be on safety. If there is danger, he or she should point it out to the Nozzle-operator.
Some rocks will be too large and heavy to throw out of the hole. Therefore, it is good technique to leave a tapered path to the rear of your hole, so that boulders can be rolled up and out. If you can't remove boulders from your hole, the hole may become bound-up with oversized material. This can create a nitpicking situation. If you see boulders being uncovered which are too large to roll out of the hole, you can make room for them at the back of your hole, on the bedrock, by throwing or rolling other rocks further behind. This takes a little advanced planning.