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Episode 2 - Shakers

Auto Panning System
by Mike Leahy

Alluvial Gold
A Simple Sluice
Automated panning
How to make an automated sluice
Pumping water up to the sluice

The challenge
To make an automated panning system in order to obtain alluvial gold from the river.

Alluvial Gold
Alluvial gold Alluvial gold is made up of small flakes of gold washed down from high in the mountains by flood water. 'Alluvial' means 'containing alluvium,' which are the deposits of sand, earth, and so on left by water flowing over land that is not permanently submerged, especially deposits left in river valleys and deltas. Alluvial gold is difficult to see because the particles are extremely small and mixed with lots of sand, mud and pebbles. However, the gold can be separated from the stuff we don't want because gold is much denser than sand — in fact, gold is about seventeen times heavier than most rocks or sand.

A Simple Sluice
SluiceTo separate the gold from the other materials, we used a simple sluice. A sluice is a wooden channel along which the gold-bearing gravel, sand and water runs. For gold prospecting a slat, bar, or cleat is placed across the bottom of the sluice to break the current and detain the gold. The water passes over these slats, called riffles (riffles obstruct a rapid to form a piece of broken water). Light material like sand flows over the riffles with the water, but heavier material like gold is trapped between the riffles.

In addition to the riffles, gold may be caught in burlap sacking, in gold pans, or with some punga wood (the wood of a New Zealand tree fern, which has a surface textured like shredded wheat). But in order to increase our yield we want to automate the system, mainly because it's such hard work.

Automated panning
There are several steps in gold panning that can be automated:

Digging the gold-bearing sand — this is difficult because much of the gold is hidden behind and underneath huge boulders.

Taking water to the sluice — this is easier to do and an important step to automate, because gold prospecting uses water in large volumes.

Agitating the sand and stones on the sluice in order to allow the heavy gold particles to fall out of the sand to the bottom of the sluice — this should also be easy to do, and will save our fingers from the cold.

To operate a water pump or an automated sluice we need to harvest energy from the environment. We had several choices:

Wind power — it isn't that windy here on New Zealand's west coast.

Solar power — although the sun ultimately supplies all the energy needed to sustain life on earth, man has found it very difficult to harvest without the help of plants. It is totally inappropriate to try solar power here in rural New Zealand.

Water power — this is the obvious option because alluvial gold is usually found close to running water.

We chose to make a water wheel because we were working next to a rapidly flowing river. As water runs down from the mountain it converts potential energy (energy stored in the water) to kinetic energy (movement). This can be harnessed by a water wheel, which uses the energy in the river to make it turn.

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How to make an automated sluice
We used a rocking motion to agitate the sand and pebbles as they moved down the channel. We first built a compact sluice, rather than a long thin one, and then put rockers on it, like a baby's cradle. In fact, we called the device our 'cradle'. As water, sand and pebbles ran down the various wooden boards, gold flakes were caught in either a ribbed rubber mat, between wooden riffles, or between and within punga wood before the water flowed out back into the river.

Sketch of rod and drainpipe
Sketch of water flow
Sketch of inside drainpipe
Sketch of inside drainpipe

Using the simple technology available, how did we pump water up to the sluice?
We needed to take water from the river and move it vertically for about one yard. There were plenty of pieces of drainpipe around - but how could we defy gravity and lift water up the pipe?

We took a lesson from nature and made a type of petal valve (similar to the valves found in the heart). We made a wooden disc that fit exactly inside the drainpipe and secured it to the bottom of a rod with a broomstick. The disc fit tightly enough within the drainpipe to prevent water from leaking past it too quickly. So when the rod was pulled up the pipe, it brought water with it.

Then we drilled holes through the disc. Now water would leak out of the holes, so they needed to be sealed with a circular piece of rubber. If the rubber was placed on top of the holes, but only secured in the middle by the broomstick rod, water could pass through the disc from below when the rod is forced down into the water, because the flexible seal would move freely out of the way, but when the rod is drawn back up the tube, the weight of the water would force the flexible seal down onto the fixed wooden disc, closing the holes. Therefore, water that passed the seals on the down-stroke to get to the top chamber of the pump is caught on the up-stroke and pours out of the top of the tube.

CradleA water wheel harvests energy in the river and provides a turning or 'rotational' motion, which can power machines. We needed to change this 'rotational' motion into a 'rocking' motion for the cradle sluice, and a 'reciprocating' (back and forth, or up and down) motion for the water pump.

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How do we do this?

Much like a bicycle gear or a car engine, the circular motion of a crankshaft is converted to a reciprocating linear action, or vice versa, by using connecting rods. You can see this on the wheels of an old steam railway engine, for example, where crankshafts convert a reciprocating 'in and out' movement of the steam engine into a circular motion at the wheels. We used the same technology in the form of more broomsticks.

Wheel and cradleSo, now that we can rock the cradle and pump water, what do we do next? In an ideal world we would have found a fast-running current next to gold deposits. What's the problem with this scenario? Well, gold tends to be deposited on the inside of bends in the river, so it is more likely to be found near fairly slow-moving water.

So if the water is moving slowly near the gold, what can we do?

Use a long rod or rope to connect the water wheel to the cradle and water pump — not really suitable.

Try to make the water current faster near the gold deposits — probably the best idea. We can make the water flow through a channel, which will make it speed up and turn the water wheel faster.

So, will all this paraphernalia get us more gold? Well, it should save us a lot of back-breaking work during the search!

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Photo: Rough Scientists at work
Metal Detector Interactive