Nearly all professional gold placer mining operations classify their gravel to maximize the amount of gold that they recover. Doing this step prior to running material through a sluice box allows for maximum retention of gold, especially the finer gold dust.
Classifying gravel is the process of separating the aggregate material into consistent sizes by running it through a screen or sieve. The larger rocks are separated out and the smaller gravels and sands are left behind, along with the gold nuggets and fine gold dust of course.
There are a few benefits to doing this, and although it does add an extra step to the mining process, the extra effort and time will pay off in extra gold recovered.
Consistent Separation in the Sluice
Simply put, by running similar sized material through the sluice box, you are allowing for the gold to settle out as efficiently as possible. The whole process works more smoothly and you don’t have to deal will the problems that arise from taking material directly from the shovel to the sluice.
Organic matter such as sticks and wood can cause added turbulence that may dislodge a gold nugget from the riffles of the sluice box, and this is especially true for fine gold dust. It is very easy to lose fine gold from a sluice box, but keeping gravel sizes consistent will help to prevent the loss of fine gold.
Adding material that is of consistent size will allow it to be added more quickly, which increases productivity of a mining operation.
No Rocks to Dislodge Gold
Similar problems arise when large rocks are run through a sluice box. For one thing, there is no need to run a rock through a sluice box. You already know that it doesn’t contain gold, so there is no value in doing it. A large rock bouncing overtop of a sluice boxes riffles does cause turbulence within the sluice that is unnecessary and can easily disrupt the consistent flow of water over top of the riffles.
In gold country that produces large gold nuggets and specimens, it may be worthwhile to check your tailing piles with a metal detector to make sure that you didn’t screen out a piece of gold. While uncommon, this can happen, especially if you are screening down to a small size. You might also want to check the waste piles after you have run material through the sluice. Sometimes despite your best efforts, you will still lose some gold. This is particularly common with drywashing in the desert.
Breaks Up Clays
Running your gold-bearing material through a classifier will also break up clays, which are perhaps the biggest enemy in fine gold retention. Clays will trap small gold particles, causing them to be lost out of your sluice box. Running it through a screen will help to break clays apart and substantially increase your recovery rates.
Fine Gold Adds Up
You always want to think about how to capture as much gold as you possibly can from your mining operations. Very small gold dust can accumulate into considerable amounts over time depending on the size of your placer mining operation, so it is critical that you set everything up properly to catch as much of it as possible.
Large and Small, Classify it All!
Classifying gold-bearing gravels is important not only for large mining operations, but for general prospecting as well. Even a weekend gold panner will find that they have much more success if they use a simple screen to classify material before they add it to their gold pan.
If classifying gravel to be run through a small sluice box, it may work best to shake the material into a 5-gallon bucket. Once you have filled a bucket you can slowly scoop the material into the sluice from the bucket.