Monday, February 14, 2011

Gold Rush Alaska - a gentle cough about shaker tables

I do not normally yell at my TV, nor usually want to throw things at it. However I almost reached that point of reaction on Sunday. I had been told about the show “Gold Rush – Alaska” by a couple of folks in my old Department, including a graduate advisee, but had not thought to go and look for it. But setting up to exercise I ran through the channels and an episode had just started. So I watched it as I worked out. (And it did get me more exercised than usual). In this particular episode they were having problems with their Wave table. (Which looks as though it works in a similar way to a table of similar appearance called a Wilfey table, and which I have recently used, as I will show in a photo added to the bottom of this post).

It became very clear, early in the episode, that the folks running it had no clue as to what it was supposed to do, and after a demonstration (which I am not sure wasn’t faked for the camera) where it failed to separate gold from the material run through it, the table was shut down. So I want to explain in very simple detail how the table is supposed to work, and show a photo, albeit with lead rather than gold ore – but that was what we were looking at when I last ran it). This is not rocket science.

The table starts out as a flat surface, onto which a number of thin strips of wood are attached. (Modern ones are of molded plastic ) The strips taper a little as they move along the table. The table is mounted so that it can be tilted in two dimensions, what I will call down the table, and along the table. And for the explanation I am going to use sketches initially.

Schematic of the basic components of a Wifley table.

The crushed ore feeds onto the table in a slurry and water sprays along the top edge of the table are set to give enough fluid to allow the vibration of the table (caused by some sort of eccentric cam resting on the underside) to provide a partial buoyancy to the particles, as well as helping with separation. The combined action of the water flow and the vibration help to move the crushed material both down and along the table, until it hits the top bar (or riffle).

The vibrating action helps to lift the lighter and smaller particles so that they float over this riffle, but the denser valuable particles are not lifted enough. (Remember Archimedes) Instead these then move along the feed edge surface of the riffle and table. If the riffles are of reducing height along the table this means that at some point intermediate weight ores can be separated from the lightest, (which run almost straight down the table) since although initially confined they can lift over a lower barrier. They are also separated from the heaviest ore (gold or lead), which remains confined by the riffles and thus runs down the far end of the table. Smaller particles of the heavier material that get over the top riffle do not have as much water on the lower riffles, and thus become trapped and fed over to the collection stream at the end of the riffles, but lower down the table.

The adjustments to the table are made so that the slope is enough, and the water flow enough, so that the mineral to be collected does not have enough buoyancy from the water and table action to get over all the riffles. It therefore collects at the far end, while the waste material is carried over the riffles then down and off the table. If the table is tilted too steeply, or the flow of water is too high, then even the heaviest particles will be swept over the riffles. (Which was one of the things they wre doing wrong). On the other hand, as is noted below, gold is not a rich ore and so there will be a lot of material swept over the table for very few ounces of recovery.

That is the basic principle, and by more careful adjustment it is possible to separate a mixture of different minerals into separate streams, as I just mentioned, as the particle move across and down the table, and these can be collected at different points along the bottom of the table (ours has holes in the table that feed to collection buckets).

There are different forms of table, based on this initial concept. An initial Google search showed this one at an on-line tutoring site

There is a video of a table working here, and one that, as with the second illustration uses groves that the heavier ore can’t escape from, here using a gold sample.

At the end of writing this rant I did find the Web site where the table makers respond to the Gold Rush Alaska video. They comment (in part)
Mike happened to be up there at John Schnabel’s . . . . so the two of them went over to the Hoffman’s site. Mike adjusted the table, ran a sample that Dorsey had, and got a gold line. It was filmed and will hopefully be shown on the next episode.

here’s our answers to what they did wrong…..

They destabilized the table by taking the slab out of the ground and loading it onto a floor jack.

All the raising and lowering of the table was wrong. Once the material is screened properly, you find the correct height adjustment and leave it there. Dorsey almost had it running, and then it was sabotaged. (Ed note that was the first thing I noticed).

No classification – large flakes should have never even been on the table (according to Dorsey’s blog, it probably was not even on the table).

The wave table does not make gold, it recovers gold. . . . .From the onset, their desperation (and script acting) caused mistake after mistake. No professional miner would work this way. There’s definitely gold on this property, but 30 buckets of concentrate and only 2½ oz of gold total! Wrong area to work……
Sadly it is often shows such as this that lead folk to believe that technology is some form of black art, whereas with just a little more accuracy and demonstration it could have been shown to be a very valuable tool.

The ore that we were processing the other month was a lead ore, and the table was set up just to show that we had liberated the galena, so that it was not tuned to give the separation right on the edge of the riffles (I had too much dip along the table) but you can clearly see, in the photo below how the heaviests parts of the ore had been carried to the edge of the riffles, and the galena (the silver stream) is clearly separated from the rest of the minerals.

It has sadly been my experience that folk often spend large amounts of money on equipment (in my field usually pumps etc) but fail to focus their investment and knowledge on the critical aspect of the entire operation that determines whether or not it works. In my case this is the small nozzle at the end of the delivery line that controls the jet that comes from the pump, (which because it wears out is usually of a poor quality, because they are cheap) - in this case the entire operation was centered around the use of the table to achieve the final separation of the gold. But without that running properly the entire investment was threatened. (But then, as a thought, if it all worked properly maybe there wouldn't have been enough drama to justify the series - tsk, tsk, what a cynic!)


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