We have an 80-acre placer operation in an ancient Tertiary channel with cemented gravels. Since I completed major rework of our old trommel, we needed a far more efficient sluice arrangement than the 20+ feet of 12" wide metal conduit trays and old carpeting that was used by the prior owners of our claim. This new reciprocating sluice is made of 14 gage steel 18" wide, 10 feet in length, and is mounted in an angle steel frame with adjustable sluice box angle provided by the chain supports at the discharge end. It is currently (no pun intneded!) powered with a 120-volt two-speed 1/2 HP washing machine motor, and the throw of the box is 3/4" by means of an offset shaft supported in pillow blocks at the feed end. Speed of motion can be selected with one of two switches mounted in a water-proof box (the other switch is ON/OFF). We can fine tune the optimal speed(s) additionally with pulley diameter changes, but I believe I have an optimum range figured out. We'll see in the next couple of weeks.
My design came from inspiration and efficiencies gained from our portable trommel which has a similar setup, but shorter length. Riffle material is #3 raised steel grating laid on ribbed outdoor carpeting (like corduroy), and cinched tight to the carpeting with 8 foot lengths of 2 x 4 pressure-treated lumber wedged tight with wood wedges against angle steel bolsters welded to the sides of the box. Length of each carpet and grating section is 4 feet to make cleanup easier and more convenient. The carpeting is waterfalled in an overlapping fashion so that no concentrates are lost.
Our portable trommel was recovering over 80% of the gold from gravels washed earlier this year in the first four feet of the box. The remainder was found primarily in first 1 1/2 feet of the lower section with very little black sands. What was especially interesting was the large amount of minus 500 mesh gold found in the first section. I believe the present choice of riffle and carpeting arrangement is the best I've ever seen. Our recovery is at least as good, if not better than our neighbor's claim where they're using a trommel, jig, and table (no sluice box). We should be able to handle around 50-60 yards per day with the new box; probably more if we screen the infeed with a grizzly and minus 2" shaker deck (this will be my next design project!).
I'm looking at building a smaller version for sampling and portable use. Stay tuned!
Here are some photos: