To flare? Or not to Flare? That is the question…
Utilizing a sluice box is among the easiest methods to prospect for gold.
Discovering suitable water flow and speed for your situations is essential.
What are advantages/disadvantages of the flared sluice design?
How does the flared sluice compared to (input flow) over/under (recovery) sluice?
An over-under riffle flow system is commonly accomplished in the same basic sluice box, which is constructed with a removable false bottom, consisting of riffles and under catchment.
The larger the dredge, the faster and more powerful water-flow must be, to wash larger rocks and a larger volume of material through sluice.
A sluice input flare gathers in more water.
If you have plenty or a great deal of water then the input flare might make setting sluice up more difficult, as seen below.
Karl Von Mueller pioneered modern gold mining/treasure hunting methodology.
Karl Von Mueller was a big fan of tapered sluices and wrote a book, "Gold Dredgers Handbook" with the complete plans for his Spartan Dredge.
Karl Von Mueller introduced his flared dredge box design in the mid 70's as the Spartan Dredge, in his Examino Press publication. It contains much valuable information and detailed plans for construction.
Sluice construction material is wasted by utilizing odd shaped pieces.
Spartan dredge sluice box
Karl used the vibration from the power assembly to add better classification within the expanded metal riffle system.
The taper is 1" wider for every 5.5" of length. This taper twists lateral currents generated by sluice walls from crossing sluice to running parallel to center-line of sluice.
It does a good job on fine gold but needs a fair depth of water in upper section so as sluice widens and water level drops, there remains enough water to move slurry/rocks out.
Water column depth can be problem. Too large of material and not enough water velocity and depth will cause sanding in the lower end of sluice. Easiest way to prevent sanding is by classification.
Classification aspect is problematic. How to dredge material, as big as dredge will lift, and then discard it before it reaches sluice. One way is to use a high banker header.
Another if your stream-bed m Screen nozzle. It reduces the through-put and increases the hand pitching of rocks.
Another technique is shortening his taper section from 49" to 24" and using a straight section 24-36" at the head. This reduces box sanding problems.
Tapered sluice only caught on with a few dredgers due to problems outlined above.
Run correctly, it works great. Too large of material, not enough water or slope and it doesn't do well. Tapered sluice will do well with a much steeper slope.
On dredges with no flare, flow slows down to give it enough time to de-aerate.
Max recovery of fines when fines are under water surface tension. Use damping on incoming flow surface (See below.).
A flared sluice used in a dredge configuration must still maintain a minimum water velocity and depth to clear large material.
Advantage - Velocities are changing through the sluice allowing for different action of riffles, which capture different fractions of gold. If you deploy this methodology, take advantage of different velocities by using appropriate sluice riffles. Use expanded in lower velocity box end will reduce clogging of larger rocks.
Disadvantage- overall higher velocity of water flow must be maintained in areas designed for fine gold to clear larger material and scouring of riffles.
Over-under has many advantages by running smaller material at slower velocities over appropriate riffles and keeps larger material from scouring riffles.
Disadvantage - complexity of getting it right. You are dealing with a sub-surface environment at this point and water flow rates in under sections and to under-sections, including material flow, are now a function of pressure differentials. Getting a box "tuned" to operate properly can take time and work.
Combination of the two designs is interesting (flare only the under-sluice).
It's imperative to de-aerate and drop gold out of flow, not when it's built velocity moving down a fast moving dredge box. Inertia steps in and your specific gravity advantage is greatly reduced.
De-aeration and removing gold from high velocity flow is one of the key functions of a well-designed over/under sluice box. Do not dismiss use of Clarkson riffles in a dredge. They work best with fast flows and generate needed vortex energy to draw gold out of its desired path in fast moving water. To clarify further, specific gravity of gold helps in collection. If we want it to "drop out" in slow to medium flows. Specific gravity also works against high velocity water because of mass. It doesn't want to change direction of movement without strong forces being applied to change direction. That is why Clarkson style riffles can benefit a dredge when installed correctly at the right locations.
Clarkson style riffles
Flare over/under design one possible way would not require extensive fabrication might be to fabricate flared box as desired. Install punch plate as a 3 sided box maintaining same width of punch plate from entry all way down box.
As flare requires greater water flow more water/material will be drawn through punch plate while keeping larger material inside sluice box area. Using punch plate has low area ratio of openings.
A considerable amount of material will flow over openings in fast moving water flows. It makes it difficult to get enough flow to under sluice for proper operation.
Widening cross section is most reliable gold catch.
Water in sluice of constant pitch is constantly speeding up, due to gravity. Sluice length is wasted if gravity isn’t counteracted. If fine gold won't hold up in head of box where water is slower, it can never deposit in faster water down below.
Another important factor, especially in larger boxes with heavily built riffles, is the riffle tray or ladder, which is easier to install and remove from a tapered sluice. Sand can't pack in to lock it in place.
It doesn't take much flare, box expands 1 inch per foot in length.
It's especially useful to put higher, more aggressive riffles in the narrower section and finer riffles down below. Allow for a slick plate section between the two to maximize the efficiency of lower, wider riffles.
Any sluice system that doesn't widen towards tail is defective in design. This statement may not be true in all cases using advanced systems. The best sluices grade off the coarser material along the way.
Unconfined fluids, like water, have virtually no actual "pressure", rather, they have flow instead.
Water flow, or velocity is very affected by cumulative friction. As water/gravel slurry travels through riffles, expanded metal and drops, energy is used up, yielding progressively slower travel of slurry. A widening of sluice's raceway will slow slurry. It produces dramatic slowing on edges of flow than across full face.
Three slick plates for recovering small gold in its lower tray increase velocity using gravity, as well as providing stratification of flowing slurry which then presents that slurry to three separate drop fields. Those three drop fields catch a wide range of shaped and sized gold.
Any sluice can slow slurry travel by extension of catchment area. Added friction will slow slurry down. An additional sluice at end of primary sluice will also.
This produces recovery of different sizes of heavies, whirlpool vortexes and more concentrates.
Result is more gold for the same stream side effort, with additional concentrate clean-up time.
Also undercurrents after slick plate work well for slowing water and improving fine gold recovery.
Sluice may be set up with different riffles and under catchment materials, ribbed mat, miner’s moss, corduroy material, coconut matting, Astro Turf, carpet, etc., according to conditions.
Flared sluice can be set up as a high banker, one sluice over another.
At crash box, a baffle that breaks up water input stream and dirt/clay balls, exit, a short piece of miners moss can be removed while running to spot check for gold.