Two 'Out in left field' questions...

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sctludwig

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I have two questions I would like to throw at the board, but first I need to say 'Hi', I'm Scott. A year-or-more lurker on the Gold Refining Forum. : )

- Does Aqua Regia adhere to the rules of reactivity? IE: Does Aqua Regia prefer to 'digest' aluminum and/or copper over gold and/or silver?

- Will a metal detector detect metal salts?

Thanks,
Scott
 
If the metals are physically touching you will get a phenomenon called contact passivation.

This means that when two metals are in contact in a liquid which is a solvent for both of them there will be no dissolution of one of them until all of the other metal has dissolved providing that there exists a great enough potential difference between the metals for this effect to occur.

This effect also occurs with metallic ores and is often the cause of poor metal recoveries in leaching circuits.

Despite long searching I have never found a complete series listing of the order of contact passivation for metals and ores.

If anyone has such I would appreciate a copy.

Deano
 
Normally, the dissolution will follow the electromotive series of metals where the most reactive metal will dissolve first. If the solution is highly corrosive with excess oxidizer, less reactive metals could dissolve alongside the more reactive metals but as soon as it dissolves, it will cement back out of solution on any base metal present. This is a classic way that beginners misplace their gold. Take AP (acid/peroxide) for example. It is possible to dissolve the gold foils if you add too much oxidizer (peroxide) to the solution. The gold then cements back out as the oxidizer is depleted as a black powder. The beginner is expecting to see gold foils and may mistake the black powder as something else and toss it out with the trash.
 
The first is a good question.

The most visible application of this I have seen is when dissolving Ceramic processor gold plated base lids as a batch in AR. The AR immediately goes yellow as it attacks the gold plating but a short while into the reaction it "finds" base metals and starts to attack those in preference to the gold. Within a short space of time the golden AR fills with base metals to the point at which it becomes saturated with base metals and a stannous test then shows no gold in solution. At this point one can literally pour off the top section of laden AR and add fresh HCl/Nitric. (I never let an AR solution like this run out of Nitric.) Over time, the plates dissolve leaving the gold flakes as the last metals to go into solution. It's a very visible answer to your question Scott, more so when you have a larger quantity of product, allowing you to see the steps as they occur. In small quantities it can be confusing as to exactly what is taking place.

Although I don't know the answer to your second question as I have never tried, I hope the first part of the post gives you a practical response to your query.

Jon
 
My metal detector can't even detect the gold powder I have collected so far. Detectors rely on electromagnetic eddy currents in the detected metal, which in an elemental metal powder, you may get if you have enough of it packed tightly. Metal salts, I think, would not have much luck.
 
Many years ago I used a Garrett Deepseeker metal detector to locate an asbestos cement pipe which had been in use for irrigation using salty water for several decades.

An unused piece of pipe gave no response but the used pipe gave a strong soft signal from the metre (3') deep pipe.

When dug up the pipe had a layer of scale inside which was about 1 to 2 mm thick, off white to very light tan in colour and very hard.

Looked like salty gypsum but certainly got a response from the detector.

Regarding the order of dissolution of contacting materials, the order is governed by the order of rest potentials, not the EMF series.

These are two entirely different series even though they overlap for some conditions.

Deano
 
Deano said:
Many years ago I used a Garrett Deepseeker metal detector to locate an asbestos cement pipe which had been in use for irrigation using salty water for several decades.

An unused piece of pipe gave no response but the used pipe gave a strong soft signal from the metre (3') deep pipe.

When dug up the pipe had a layer of scale inside which was about 1 to 2 mm thick, off white to very light tan in colour and very hard.

Looked like salty gypsum but certainly got a response from the detector.

Regarding the order of dissolution of contacting materials, the order is governed by the order of rest potentials, not the EMF series.

These are two entirely different series even though they overlap for some conditions.

Deano
Click to expand...

Deano did you by any chance try a set of devining rods also as a back up to find that pipe?
 
I do know that the metal detectors that refiners use to prevent metals from walking out of their facility will not detect gold digested in acid. Some brands will pick up salts in the form of plating salts, albeit a weak response but some miss it as well.
 

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