THE USE OF NUCLEAR TRACERS TO EVALUATE THE GOLD RECOVERY EFFICIENCY OF SLUICEBOXES

Sluiceboxes can provide a much higher concentration ratio than most ot her gravity concentrators (up to 50 000:1) at very high overall placer gold recoveries (greater than 99%). They are also very reliable, inex pensive and sample to operate This combination is very difficult to be at and explains why the sluicebox is still the most important placer g old concentrator in Canada's Yukon Territory. A sluicebox is a rectang ular flume containing riffles on matting, through which a dilute slurr y of water and alluvial gravel flows. Sluiceboxes operating under deal conditions are actually centrifugal concentrators whose riffles overt urn ribbons of slurry to form vortices. At the bottom of these vortice s, centrifugal and gravitational forces combine to drive placer gold p articles into matting. Testing sluiceboxes with conventional sampling and evaluation techniques is very costly, time consuming and problemat ic. The effect of a single gold particle can cause large unpredictable errors (nugget effect) even when large sample volumes are processed w ith care. Nuclear tracer tests are more accurate, faster, cheaper and safer than conventional sampling. In 1989 through 1991, the recovery e fficiency of several sluicing systems was determined by mixing radioac tive gold particles (tracers) into the feed streams of 27 placer mines in the Yukon Territory. Four distinct sizes of nuclear tracers were u sed and their recovery was related to the design and operational chara cteristics of the individual sluiceboxes and their pay gravels. Over-a ll gold losses ranged between 71% and 0, or from $2.5 million to less than $1000 per 1200 hour season. One of the triple-run sluiceboxes and one of the single-run boxes lost more gold than they recovered. The s luiceboxes which were fed with pre-screened gravels (minus 25 mm) had the lowest losses of all, averaging only $47,000 per season. Three of these sluiceboxes recovered 99% of their gold. Many of the mine recove ry plants tested in 1990 had already implemented recommendations from the 1989 test program including the use of unbacked Nomad matting, coa rse expanded metal and 25 mm angle iron riffles. None were using doubl ed riffles and few were using cocoa matting or Monsanto matting. Four operators installed screening equipment which increased their gold rec overy by 10% to 20%. Another four operators modified their sluicing sy stems and increased their gold recovery by 3% to 44%. This paper prese nts a summary of the existing and potential gold recoveries, and recom mends sluicebox designs and operating parameter, based on the results of nuclear tracer testwork from 1989 through 1991, conventional sampli ng in 1988 and laboratory investigation in 1989-1990.