A relatively recent fad uses sluice boxes that don’t use riffles of any type and rely instead on capture mats covered with a screen of expanded metal mesh. The makers of these boxes intend them to be used in the recovery of ‘fine’ gold and they do work as promised, up to a point. Unfortunately even the best of these boxes are extremely prone to losing materials due to scouring every time a new load of material is introduced into the sluice, especially if the material contains larger stones and pebbles.
The materials they capture are collected in the upper-most layer of the capture fabrics and held there primarily by friction, so almost any disruption dislodges them back up into the laminar flow. These are good sluices but without riffles there is no centrifugal force that drives (or holds) the concentrated materials deeper into the capture fabric.
In field conditions, these sluices lose about as much material as they collect but this isn’t necessarily a bad thing as they do a very good job of collecting extremely fine material. The drawback is that you have to work a huge amount of raw materials to make their use cost-effective as compared to more conventional sluices where you can run more material in less time.
I’m sure these boxes have a use at certain sites but they are extremely slow if operated properly to maximize the collection of fine material. They are most effective, actually extremely effective, if you do a clean up after running only a few scoops of material. Unfortunately this slows down production to a snails pace.
I’m sure other people will use these devices at their sites with good results so the comments are not intended to be a put-down but rather a casual observation.
It is very easy to overload any type of riffled or non-riffled sluice. The box will only process so much material at any given point in time based up the flow velocity. It is extremely tempting to just keep feeding the box hoping for the best but overloading the riffles just guarantees that you’ll loose at least 50% of the gold you’re trying to recover.
Recovery rates will go up dramatically almost in direct proportion to how the feed rate to the sluice is slowed down. Maximum recovery occurs when feeding the box at an agonizingly, almost unbearably slow rate regardless of the riffle type being used. Almost 100% recovery rate can be achieved by feeding the sluice at a very slow rate.
Unfortunately, when you slow down the operation to a certain extent you will eventually get to the point where you can actually pan faster and more efficiently than you’re sluicing.
Besides overloading a box by feeding it to fast it is also very easy to overload a box by trying to run to much water through it. This usually isn’t a problem with sluices of small electrical-pump high-bankers but it can become a serious problem when running a larger gas-pump power sluice since water is fed so fast that it creates a huge amount of turbulence and interjects a tremendous amount of air into the slurry. Since gold is hydrophobic any small, or even large flat particles, will simple ride down the chute attached to the air bubbles rising to the surface.
Most power-sluices use dredge pumps that in actuality are far larger than necessary and pump at flow velocities of between 26 and 32 feet per second at the header box while even relatively large sluices perform best with maximum velocities of 15 feet per second. As you can easily see when the water enters the box it is moving about twice as fast as it should be so in the entire length of the sluice we typically see massive amounts of turbulence, foaming, frothing and air entrapment which is deadly to fine particle recovery. In fact it is almost as detrimental to the recovery of even large particles.