Tuning Your Gold Sluice
Part Two – The River (Your Slurry) Runs Through It
By Doc – ©GoldHog.com
Part 1 is HERE
In this part of our “tuning your sluice” series, we’ll take a look at that wet stuff flowing through your sluice. I guess you could call it water, most call it a slurry, but whatever you call it, it’s more complex than you think. Again, understanding what it is, and what is happening, will help make you more of a gold recovery expert.
Now… stick with me on this one as all this “stuff” leads to something important.
Before we begin it’s important to understand one point. While GOLD… is GOLD…. (it’s an element) it does change slightly in its size and shape based on geographical location. The same is true for the “dirt” which contains the gold. It’s one of the reasons we now offer NINE different sluice mats in our lineup. We work with miners, commercial ops, and prospectors in 35 countries and each area presents its unique make up and challenges. There is no “one shoe fits all” when it comes to recovery in a sluice.
As defined… a SLURRY is mixture of water and particles or other materials that create a thickened substance. We all use water, but we all add different “materials or particles” based on our geographical location. I stress this point because it GREATLY impacts what your slurry will look like and how it reacts and performs in your sluice. If you’re not careful, or don’t understand it, it can be a gold robbing thief. We’ll break that down a bit later.
The Different Loads of a Sluice
While they’re referred to as LOADS… it probably makes more sense to many to call them layers or levels. Your sluice, like any river or creek, has three basic loads. The DISSOLVED load , the SEDIMENT load, and the BED load.
For now we’re going to skip the “entry point” of your slurry. The feeder system, header box, etc, that creates the slurry and drops it into the sluice. Instead we’ll be looking at the slurry as it travels down your sluice. As it travels down your sluice you have 3 different loads and let’s focus on them. It’s “somewhat” similar to a stream’s loads.
In a sluice, the dissolved load (the upper portion) really can be called the VERY FINE load, not just the dissolved load. The term dissolved comes from streams and creeks where the water “appears” to be clear and most particles are invisible. In our sluice, like a stream after a bad storm, the water is “murky” from top to bottom. So you’ll have VERY fine particles in the dissolved load as well as “dissolved” material. Clays and silt will be present in all three loads.
The Suspended load contains fine sediment particles suspended and transported through the stream. (Sluice) These materials are too large to be dissolved, but too small to lie (fall) on the bed of the stream. Stream flow keeps these suspended materials from settling on the stream / sluice bed load. Given enough time and distance, without turbulence, many of these particles will try and settle into the bed load.
The bed load often travels slower than the upper two layers / loads. However, this variation really depends on the water depth in your sluice. It includes the largest and heaviest materials in the stream / slurry, ranging from sands, to gravels, to black sand, larger rocks, metals, and of course gold.
It’s important to understand that these loads DEVELOP in your sluice, as well as CHANGE during the run of your sluice. Remember, you’re “creating the river” and its loads. Larger, heavier, denser “particles” will drop out of the slurry quickly and enter the bed load. Large gold nuggets usually are found at the top for a reason. This goes back to our first article dealing with hydraulic equivalence and settling velocity. They enter the bed load instantly. As the water travels things such as hematite, pyrite, and other heavy metals or particles will also leave the sediment load and enter the bed load quickly. However fine flat gold takes a longer time to settle into the bed load. That’s the problem WE have to deal with.
Let’s look at the SHAPE of gold one more time to stress this point. A FLAT piece of gold reacts MUCH differently to FORCE than a round nugget. I always go back to my “tin foil” example to clear this up in peoples’ minds.
Imagine dropping both of these into a fast moving river or creek. The SPHERE… ball of foil, would sink to the bottom (bed load) quickly. The FLAT foil would travel a LONG way before finally settling. So by using this example we can see that small FLAT gold might not “settle” for some time in our sluice. Or at least take longer than big round gold.
Now comes the problem…….. TURBULENCE
So we now know that heavy things are trying to settle into the bed load. The problem is that we need to create a way to capture them. For the most part this comes from us creating TURBULENCE. Most sluices have some form of FLOW INTERRUPTION, such as riffles, which force the settling bed load to be thrown back up into the sediment load.
You can clearly see how this can be counterproductive, especially with fine gold. You’ve worked so hard to get it to settle and now you’re throwing it back up again into the wrong load. It’s kind of like trying to rake leaves on a windy day.
THIS… was one of the driving inspirations in the development of our matting systems.
A CRITICAL POINT FOR FINE GOLD RECOVERY…… If we only need to treat the BED LOAD… let’s do it with something that will keep the fine gold settled, and NOT throw it back up into the sediment load. You can see from the design of our Scrubber mat as an example. This mat gently LIFTS the bed load and allows it to fall. It does NOT create that “cutting interruption” that throws bed loads back into the sediment load.
In the Talon mat below, you can see that we create an AGGRESSIVE exchange surface without using large tall riffles.
Again, the design is VERY active but the slurry in the sluice looks very calm.
This is also true with the new MotherLode mat.
What if we removed ALL the “BUMPS”?
So if throwing fine gold back into the upper layers is bad, why don’t we just remove ALL the turbulence? Don’t disturb it at all and our recovery should skyrocket, right? Not really. What ends up happening when you remove ALL turbulence is you end up with a PILE / LAYER of heavy junk riding along the bottom of your sluice. Everything rides along the bottom creating a mess. It’s all competing for the BOTTOM…. Things don’t get separated based on factors we want to use to separate them. (such as specific gravity ) It simply becomes a messy, unorganized, pile riding down the sluice.
So, the key is to have a SMOOTH slurry that still offers ways to work the bed load and keep it actively exchanging.
Slurry Density / Viscosity
A lot of us watch Gold Rush. Those that have remember the tragic season in the jungle. Todd and his crew’s efforts to capture gold went horribly wrong. It wasn’t their fault; they simply didn’t have the time or expertize to figure out the problem of huge losses. We don’t have that luxury. When a commercial op in Africa calls us and says they have the same problem, we need to fix it and we have. They’re problem and most ops problem in that region is slurry viscosity. The fine silt clay that turns the slurry a thick, crazy yellow orange and high sand content create a recovery nightmare.
Thick slurries have more scouring energy. It’s kind of like difference between running motor oil down your sluice vs. water. When you create vortexes, they have more energy / power. Add a bunch of fine sands into the mix and now you have a great power washer for cleaning your deck…. but not a good gold recovery system.
Thick slurries are often a problem in recirculation systems and in areas where there are high concentrations of powdery silts or clays. Also, areas with very high concentrations of fine sands can also create the same issue. To help lower the risk of losses try some of the following.
1- Reduce your feed rates / sediment percentage and make your slurry thinner. Use a lower ratio of pay to water.
If you want to keep your production up, try adding sluice width and length as well as GPH.
2- Eliminate as much large turbulence as possible while maintaining a good exchange. Use a smaller faced exchange surface. Large riffles and large expanded metal just keep fine gold suspended. Let it settle.
3- Decrease water depth. We have found in larger ops this was a great help. Going from a 3” depth to a 1.5” depth
really helped reduce bed load pile ups.
Variety is the key to life…. and gold recovery.
When you go fishing in a lake are the fish spread out evenly throughout the lake? No, you’ll usually find “pockets” or areas where fish LIKE to concentrate due to something being just right there. When you look at a river / creek, are the rocks and cobbles spread out evenly? No, they’re usually concentrated or piled up in certain areas like inside bends and varying size rocks deposit in varying areas. What about sand piles and piles of leaves. Starting to see a pattern?
Different things of different shapes, sizes, and density like to REST… in different areas. (Hydraulic Equivalence) What works well for some pieces of gold might not work well for others.
One of the best things you can do to a sluice based system is to add varying capture zones, water depths, and exchange energies. Very large, smart, commercial ops do this all the time. The slurry does not get treated in just one sluice with the same “capture system” for the whole run. They’ll vary the width, flaring out, use different capture surfaces, and even run totally different systems in stages such as jigs.
The same is true for the sluice of a prospector or small scale miner. Don’t just use one capture zone or type of riffles, matting etc. Vary it. Vary the width of your sluice. If it starts off at 10” wide, try adding a flared end section that is about 14 – 16” wide and change the matting / capture zones. See our Raptor highbanker below.
Again, some gold might like the top section, and some may like the bottom, and don’t think all the fine gold will be found in the lower section. 100’s of controlled tests runs shows differently. Again… the fish (gold) will concentrate in areas it likes.
Pay attention to your slurry, play, experiment and above all TEST.