[Ken Wood (OP)]

Hello,  When sound waves are passed through a metal plate and sand it sprinkled on that plate, it creates different patterns depending on the frequency. My question is , If sands of different density were placed on the plate, would they form different patterns? I.e. if you had iron, glass , sapphire and sand of same size powder, would they separate into different patterns at different frequencies? I hope this is a clear question, I don't have the equipment to test this myself. Please can someone advise me. Many thanks 19kenwood61@gmail.com

[chiralSPO]

I don't think that the density (or size of the grains) of the "sand" should change the pattern (it might change how quickly a stable pattern is established)

However, changing the material that the plate is made of might have an effect. The spacing of the pattern depends on the frequency of the vibration, the speed of sound in the plate, and the shape of the plate (including where the plate is connected to the vibrator).

[Ken Wood (OP)]

Thank you for your kind reply. Does this mean that if you had a bag of mixed density particles of sand i.e lead (Pb) and aluminium (Al) and they were poured onto the middle of a large vibrating plate, that they would separate at different speeds so the (Al) would arrive into it's sound pattern first, and would then be caught up by the (Pb)? Or could it be possible that a frequency be found that would only be strong enough to move the Aluminium, but leave the Lead motionless?

[Bored chemist]

It's the plate that's doing the real moving, and it will respond to different frequencies. The sand grains just get kicked off the bits of the plate that move most and pile up where it moves least.
It would barely matter what the sand was made from.
You could tr it with flour and sand.

[evan_au]

Overlap with Bored Chemist...

If sands of different density were placed on the plate, would they form different patterns?

I think what you are describing is Chladni Figures.

This relies on the powder having an insignificant mass compared to the plate, and the energy absorbed by the bouncing particles to be small compared to the vibrational energy pumped into the plate.

The particles bounce around, driven by the force of vibration and brought back by the force of gravity, until they finally land on a spot which has minimal vibration (a vibrational node). This technique has been used by musical instrument builders to optimise their designs.

The requirement of minimal mass means that the throughput is very low, and the nodal lines are a property of the plate, so both ores will end in the same place. The velocity of vibration and the force of gravity is the same, so both ores will migrate at roughly the same rate. However, the density of the ores will be different, so they will be slightly separated by falling through the air.

I think better ways of separating aluminium and lead would be:
- Look for good ores. Lead and Aluminium ores have very different density and melting points, so they tend to be found in very different deposits.
- Rely on the physical and chemical properties of the ore.
- Lead typically uses separation by density, and bubble flotation of the ore.
- A large part of lead refining is separating out the small fraction of gold and silver, which is worth more than the lead itself. This uses the fact that gold and solver are more soluble in molten zinc than in molten lead.
- Aluminium is dissolved in a strong alkali, and then separated by electrolysis

See: https://en.wikipedia.org/wiki/Ernst_Chladni#Chladni_figures
https://en.wikipedia.org/wiki/Lead#Primary
https://en.wikipedia.org/wiki/Bauxite#Processing

[Ken Wood (OP)]

Thank you again, Chladni Figures was what I was referring to. And as a small time gold panner, I'm looking for an easy way to separate gold from black sand without using expensive chemicals, refiners or dangerous substances. So my idea would be to get riffled matting on a large 12ft trampoline then blast it with high frequency from a sub-woofer underneath, dump a bucket of paydirt in the middle and watch it separate. What do you reckon? Try it on a small scale  first? There must be a point where the lighter material moves before the heavier, even if very fine tuning required.

[Ken Wood (OP)]

Or perhaps with a plastic swimming pool on top of the trampoline half filled with water, the gold has an affinity to the water.

[Colin2B]

..... from a sub-woofer underneath, dump a bucket of paydirt in the middle and watch it separate.

You would need a lot more power than a subwoofer could provide. A bucket of dirt will provide too much damping to allow the nodes to form. Even on something that will resonate easily eg guitar soundboard, i use lightweight material eg tea leaves or glitter.

[chiralSPO]

You may be interested in using dense liquids, like sodium polytungstate disslved in water (https://en.wikipedia.org/wiki/Sodium_polytungstate), which can get up to about 3 g/mL. Obviously this won't separate everything out, but most dirt and sand will float on top, while denser materials (like iron, lead and gold) will sink. (there are more expensive and highly toxic alternatives that will allow even higher densities--closer to 5 g/mL--but it's not worth the added cost or risk).

Overall, unless you happen to have a particularly gold-rich area to pan in, it's probably not worth the investment to get the dense liquid, as it won't increase the speed of finding the gold that much, but it could be a fun experiment.

[Colin2B]

Overall, unless you happen to have a particularly gold-rich area to pan in, .......

Or you could wash for it as done in many cultures eg using a sheep fleece - as in golden fleece.

[alancalverd]

You can indeed use vibration to separate particles of different densities. It works best if they are all pretty much the same size, so it will separate gold dust from dross if you grind and sieve the whole lot. A Chladni plate is a fine demonstrator but less efficient than a single vertical column shaker.

[Ken Wood (OP)]

I thank you all for your various opinions and I will be trying my subwoofer bass speaker under a glass plate. It's loud enough to make the walls of my house shake so can't see why it won't separate a bucket of sand, I will try this link
szynalski.com/tone-generator/  the very high and low frequencies are unable to be heard by the human ear, but should get the sand jigging! I look forward to your future comments. I will write my results soon. Ken