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Tuesday, December 28, 2010

Sluice box efficiency...

We have in our hands, “Use of Nuclear Tracers to Evaluate Gold Recovery Efficiency,” a bulletin dated April 1994. This bulletin addresses feed rates and water flows for recovery of different types, size and weight of placer gold.

The study was based on gold tracers (irradiated particles) tracked through a large sluicing operation to determine the most efficient means of gold recovery.  A large amount of the information does not apply to the small placer miner, but some interesting facts apply to both large and small scale operations.

In a comparison of expanded metal versus angle iron, the report states, “Flat bar riffles are not recommended for fine gold recovery.” An example of this would be homemade angle iron riffles. “The vortex is launched up to the top of a turbulent slurry column instead of onto the next riffle,” meaning the force of the water (vortex) carries the gold into turbulence rather than onto the next riffle, floating it right out of the sluice box due to bubbling and irregularity of gold particles.

“Regular or modified angle iron riffles should have a 40-60mm. gap and be tilted at 15 degrees upstream of the sluice box‘s vertical in a sluice run, with a slope of 12-15 degrees. Packing and extreme gold loss were often observed when any of the following conditions occurred: Shallow slopes, narrow gaps between riffles, excessive feed rates, insufficient water flow, and riffles larger than 25mm.

“Expanded metal riffles are shallow riffles which are sensitive to scouring and the resulting (which results in) coarse gold losses when they are subjected to surging or excessive water flows and/or steep sluice box gradients.”

We’ve run into this situation many times with our guests… more is better! A high water flow will float finer gold right out of the sluice box. They are amazed that we even watch the ebb and flow of the stream during the day, adjusting for increased water flow.  We try to remove our sluice box if an afternoon rain occurs... the increased flow of the stream might clean it out for us!

Feed rates, as mentioned above, are also critical. Many times a tired prospector starts feeding too much material to the sluice. This will not only dislodge accumulated gold but also float gold across overloaded riffles right back into the stream! We’ve joked about re-running the tailings of someone we know is overloading the sluice box or has too much water flow… but content ourselves with testing our own tailings periodically by pan to adjust for possible loss of gold.

Use of expanded metal or angle iron is stated to, “severely reduces the opportunity for gravels and anything but very coarse gold and nuggets to enter the riffles.”

For those of you wanting to build your own sluice box, a 15 degree upstream angle is needed on riffles, and a slope of 12-15 degrees of your sluice box in the creek.

The article concludes with, “The fine screened sluice boxes had the lowest loss of all… one of the triple run boxes and a single run box lost more gold than they recovered. The fine screen systems with recommended sluice box designs recovered 99% of their placer gold.”

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