Hydraulic Riffles ?

[golddigger7]

So rry to go back to this old post, but what was meant by...So Earlier on this year I have been doing some retesting with a side by side testing of three types of recovery systems to see which preformed the best; one of them was based on the angle iron riffles simular to yours. Getting the feed flow, angle, spacing and the height of the riffles including the slope of the ‘box were all very critical for good recovery and clearance (riffles not loading up). The riffles with a backward slope of aprox 15 degrees was about the best so far with a ‘box slope 12 to 15 degrees....my understanding from this is that the riffles were sloped back against the flow, so that with the normal slope of the sluice box they would then be straight up and down, is that correct.

[2dredge]

Yes that is correct….Thanks for bringing it up, I have been so busy I had forgotten about this……..The actual backward slope ended up at 10 deg. This enables the ‘box to be run flatter, at lesser angle,= slower water flow, which in turn should be better for fine/flaky gold.
What I did was to build three 2 ½” dredges to experiment with, Header box style for compactness.
The first with conventional Hungarian riffles.
The next had a flared outlet box, about 3 ½’ long with only 5 riffles over custom made raised expanded mesh to my specifications; based on an old design from 1980 as used by the original Jessie, it had a good reputation for fine gold recovery. This proved to be true, to true in fact it was grabbed by an old dredging mate as a sampling /crevis machine. and there is no way he will now part with it. ;D
The third was fitted with the backward sloping riffles, as above in your question. These style of riffles work well in a static, (on shore plant) but not so good in a dredge, the idea was to find out WHY, and could they be made to work? The popular method of making the backward sloping riffles is to use modified angle iron, 1” on the upward leg and ½” on the top: Far too big and heavy for the little 2 ½’ box so taking a que from the expanded mesh and that small riffles can and do work well for recovery I folded up my own, scaled down to about ½ size an less. One of the main problems turned out to be that the riffles were very water speed sensitive, as the flow at the top of the box was low and the outlet was fast. Not to give up get this sorted out.
Testing, running old concentrates it appears to work very well, but what counts will be in the field testing and side by side against the conventional Hungarian riffled ‘box. Until this is done, hopefully soon, I cannot make a report yet…..
Conclusions; - was it worth the trouble making the three boxes and doing the testing? The answer is yes, several niggling questions that I had have now been answered resulting in changes to my other ‘boxes, one was that extending the classifier over the first few riffles was Not a such a good idea and counter productive , showed no benefit at all. Expanded under the classifier proved far more effective. All ‘Boxes were fitted with deep ribbed matting, not ‘moss as it doesn’t self clean so well and holds too much crap, just a personal choice.
Backward sloping riffles were fiddley to manufacture and get ‘tuned’ in to work right, but they might have a lot of potential… dr.edger