Day 6, Eluvial, Fluvial and Alluvial, Part 2

Methods of recovery and processing of gold ores

The method of open pit is easier and quicker to exploit a deposit, but only applies to the shallow gold deposits covered by land. This method can be performed in different ways depending on the terrain, the shape and size reservoir.
Exploitation of small placers

The method is based on the principle of using gravity as a means to separate concentration of mineral grains of different specific weights, which exist in the record to be treated, when these grains are suspended in a liquid medium in turbulence. In this action the heavier mineral grains, tend to sink to the bottom of the vessel containing them, the light is washed away in the liquid turbulence. In the exploitation of small pleasures alluvial deposits or use the following methods: punt, the screen and the sluice.



Panning for gold, detail

The simplest in the operation of a gold deposit is made ​​by hitting the instrument to be that simple, as it consists of a single piece and is handled by a single operator. The tray is a plate of steel or wood 25 to 55 centimeters in diameter and 5 to 7.5 centimeters deep with sloping edges 30 to 40 degrees to the horizontal. In operation the tray is filled with sand, gravel or dirt gold is immersed in tap water and spins rapidly as possible until the fine sand and clay, are separated from the combined action of centrifugal force and water also to cause the heavy particles separated from the mass to go to the bottom, the light fall off the edge of the tray with the circulatory motion of the suspended mass. At the end of the operation the gold grains are at the bottom of the pan, heavy mineral associated with another.

The screen:

Screening or sieving is a distribution operation in which a mixture of minerals of different sizes and of different specific gravities into fractions separated by the action of a stream of water onto a gold gravel placed on a sieve or screen in a hopper. The screen consists of a small box, square, elongated without lid open on top of which there is a screen formed by a sheet of iron plate with holes and below which is a second box divided into compartments, made ​​of wooden beams or steel, also called rifles, In the first case a jet of water drops on gold or gravel material, the fragments larger that support the sieve are rejected and separated mechanically at the beginning of the operation. Fragments of smaller sizes for the sieve, are entrained by the water stream to a conduit and thence to the inclined floor of the box, with rifles or tarps where trapped small gold grains which are at the bottom of the water stream, the remaining material in the upper continuous and is expelled with the water stream. The material retained on the rifles is subjected to washing in a special cuvette to recover the gold. During operation the screen apparatus or can receive movements, manual or mechanical pulse which complement the action of the water stream.

The Sluice:


The sluice working

In the auriferous alluvial plains or placers, using sluices or channels is the simplest and cheapest to use, easy to build, easy to transport from one place to another, good performance, simple to operate and not require skilled workers in their operation. The sluice is a long box, typically of wood, inclined in a proportion of 1 to 20 degrees, where water runs dragging the gravel quickly gold. Gold and other heavy mineral, is retained by a series of rifles conventionally arranged along the floor of sluice. Sluice length must be such that allows the gravel disintegration and release of gold particles. In small gravels pleasure operations are emptied directly into the headwaters of the sluices. The gravels are transported pleasure taken from the site of the sluices in dollies. In larger operations, transport of gravel to the town of sluices made ​​more complex by mechanized means.


Sluice Box, How to use it

sluice box 3

Sluice box

Getting the flow right is the key to running a sluice. Too much water, moving too quickly will carry gold higher in suspension. The science might be tricky to explain, but basically the upper layer of water is not affected by the riffles as much as the lower layer. While the lower layer is rolling behind the riffle, the upper is flowing right over the eddy, and while the eddy itself can slow the upper layer some, it’s not as effective as the riffle itself.

On the flip side, too little flow, while allowing the gold to drop out quickly, also allows lighter material to drop out, filling the space between your riffles, eliminating the slow spots entirely, and allowing the rest of your gold to flow right out of the box.

The perfect flow allows the gold and other heavies to drop out of suspension and the lighter material to flow out of the box.

The general rule of thumb is 1″ drop per foot of length of your box. So if your sluice box is 24″ long, you’d start out with about 2″ overall drop from upstream to downstream ends of your sluice and adjust from there.

Now finding a spot in the stream that allows the flow and drop required to get your box running just right isn’t always easy, but it’s better to set up in a good spot to get the right flow and carry your paying material to your sluice than it is to set up in a bad spot close to your diggings.

In some areas it may be impossible to find a safe spot with the right flow and drop, and nobody wants to stumble and drown while lugging buckets to a sluice perched over the swirling abyss. In these areas you may be able to increase flow of a slower, shallower and safer section of stream by setting up a wing dam to hold your sluice and manage the flow. A wing dam is essentially a rock barrier, often extending in a V upstream and outward from the mouth of your sluice box, that channels water into the box increasing flow.

Depth of water flowing through the box is often dependant on circumstances, but you do want the water flowing through the channel of the box, not over the top of it. I usually shoot for a water line about midway up the side of the sluice, but don’t agonize over depth. It’s flow that matters most.

Once you get what you hope to be the right flow, it’s always a good idea to plonk a large rock across your box to hold it in place. The risk of losing your box or even just having it shift and dump your day’s work is enough to warrant spending a minute to find a rock for the job.

Now you dig. Hopefully you’ve sample panned the area and found some decent ground before you set up. If not, you probably want to do that before you start digging. Gold does not settle out evenly and is not distributed evenly throughout the gravels of a stream, so it’s always a good idea to pan material from a variety of likely spots and pick the best one before you start doing the heavy lifting.

At this point, you can classify the material to pull out the larger rocks, or run as is and pick them out by hand or sweep them with the current out the end of your box. The choice comes down to a matter of preference and the size of gold you’re after. Classification adds time, and means running less material overall, but it’s always easier to separate gold from material of a similar size, and larger rocks in a sluice can create unpredictable currents that may dislodge your gold and send it out the end of the sluice box. Nuggets aren’t as likely to wash out, but if you’re digging an area rich in fines, large rocks can be a problem. Personally, I like to classify my material before I run it.

Feed the material into the top of your sluice slowly, and give it plenty of time to wash through the box before adding more. Overloading will only clog your riffles and send gold downstream. I use a large garden trowel and run one scoop at a time.

When you’re ready to clean up, and when is another judgment call you’ll have to make based on the material you’re running and experience, let the water flow for a bit after your last scoop. Then carefully remove the rock and lift the sluice box out of the stream. Be careful not to slosh water and material out of the box. Place the downstream end in your clean-up tub or bucket and inspect for visible nuggets. Grab any nuggets or pickers and put in your snuffer or other container for safekeeping. Then release the riffle tray catches. Rinse the riffles before removing or folding out of the way. Slide out the carpet, miner’s moss and/or matting and rinse well before setting aside. Rinse everything into your tub or bucket.

Now it’s entirely up to you if you’d like to pan down your concentrates or get your sluice back in the water and do some more digging. I’ll usually pan a quick sample into my tub just to get an idea of how I’m doing, but I can always pan out my concentrates at home, so digging and sluicing more material is really the best use of my time on the stream.

There you have it. Pack out your trash, fill in your holes and best of luck sluicing for gold! and if you want to try sluicing gold in Guatemala, you are welcome!!! Motagua river is waiting for you.

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