The program called Gold Rush Alaska which is being aired on the Discovery Channel is well worth watching if you are curious about what it takes to do placer mining.   In addition to the strenuous task of digging down to the gold bearing layer of sediment, the miners are challenged by the short mining season in Alaska (~100 days or 2400 hours) and the remoteness of the location. 

There are several unit operations in play. The first operation using the trommel classifies or sorts the sediment by size.  This results in cobbles and pebbles being excluded from the sand and silt. This is a classification process that uses gravity to roll the large rocks to a separate location. 

The next step is more of a density driven process wherein the material stream is taken through a shaker station where sedimentation of the dense fines is accelerated by mechanical agitation and the resulting material flow is transferred to a sluice where the heavy gold particles and nuggets are agitated by the riffling action of the water and settle to the bottom. The less dense solids are washed out of the sluice and discharged to a waste pile.

All of the slurry flow is gravity driven, so the process train must begin uphill and work its way down. The sluice section is where the density separation occurs in earnest and this is where the gold will accumulate.

Periodically, the sluice section must be cleaned out and the resulting gold laden silt must be further processed to isolate the gold. The fellows in the program must use panning or a shaker table to isolate the dust and larger pieces of gold.  This a definite disadvantage compared with miners in the past.

The buckets of silt isolated from the sluice would have been treated to amalgamation in times past.  This selective dissolution of gold and silver could be used to accumulate the gold until the amalgam would begin to solidify. This process requires less skilled labor than panning or using a shaker table. The amalgam would eventually be placed in a retort and heated strongly to distill out the mercury leaving the non-volatiles behind.

The gold would then be sold and sent to a smelter for further refinement (i.e., parting) of the crude gold.

Without mercury, present day miners have a rather more complicated task in isolating the gold.

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