# PROBLEM WITH A WHITE SALT FORMING IN AQUA REGIA REACTION



## crvest (Jun 18, 2008)

Hello sirs
I have almost ten years working in a gold refinery, charging from 4 to 16 kg of gold scrap daily in an aqua regia plant. Normally I charge the gold in shots or sometimes I charge rings and chains directly. This week I am facing the problem that a insoluble sticky white salt is forming during the reaction and is incrusting inside the glass condensers above the reactors. Its a very rare salt that is insoluble in the mayority of solvents I use.
At the begining I thought it was a contamination problem in HCL or in HNO3, but I discarded this posibility after some testings to the acids.

I think this salt could be a tin compound generation maybe from the soft solder in some pieces.

If anybody can help me I will thank
Best regards


Estuardo Cabrera
Precious Metal Refiner


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## lazersteve (Jun 18, 2008)

Sounds like it may a tin compound (tin hydroxide or metastannic acid?) to me also. I run into it when I process certain monolithic capacitors. When I see it it appears as a floating white gelatinous blobs. 

Could also be a lead compound.

When the temperature of the solution increases it tends to disperse. When the solutions are cold it will coagulate. It's near impossible to filter as it has a tendency to pass through any filter medium including packed columns.

Incineration of the scrap and precipitated gold prior to dissolving and filtering can convert it into more soluble forms. Proper removal of the base metals prior to dissolving in AR can virtually eliminate it.

Here's a link to some tin and lead chemistry that may also help:

Tin Chemistry

Lead Chemistry

and one more for good measure:

More about tin

Welcome to the forum and good luck.

Steve


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## crvest (Jun 19, 2008)

Thank you steve for your information
I will tell you what happen
Estuardo


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## Lino1406 (Jun 19, 2008)

Silver chloride dissolves in ammoniac
or thiosulphate in near neutral solution
Tin dioxide dissolves in 60% sulphuric
and in (better iodine-less) Hydriodic acid


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## The Refiner49er (Jun 22, 2008)

Hey Steve-

This may be off the subject thread, but I noticed the reference to condensers being used with the AR reactor vessel. I have never seen an industrial setup, but wouldn't this work in a small scale application as well? 

Conservation and reuse of the nitric component? Or mainly control of the emissions?

Just thought I would ask.

Thanks!

John
AKA The Refiner49er


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## lazersteve (Jun 22, 2008)

John,

It is feasible to set up a glass rig to condense the NOx vapor from an AR reaction and bubble it through distilled water to collect it as nitric acid. The problems arise when the water becomes saturated and will no longer hold the vapor. A second issue is any vapors that don't immediately dissolve in the water continue through the system and must be handled accordingly.

In my opinion it would be more logical to set up a scrubber tower on the output of your fume hood to collect the vapors in a liquid mist such as sodium bicarbonate. This liquid could them be converted back into nitric by known methods.

Steve


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## Platdigger (Jun 22, 2008)

Hi guys,

I think what John may be refering to here with mention to a condencer with AR, is this.

Using a specially designed reflux type distilling head for the conversion of Aqua Regia to chloride solution.

In other words....instead of repeated boiling down or evaporating to remove the nitrates, using a reflux colum, which is less labor intensive.
Randy


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## crvest (Jun 23, 2008)

Hello guys
There are several reasons to use condensers inmediatly after the aqua regia reactors and before the scrubbers. I want to mention you some of them:

1. To keep the liquid level in the reactors.
The solutions are kept in the boiling point during at least 4 hours. In this time the liquid level could be reduced to a dangerous level. Condensers help to keep the liquid level by returning all the condensable vapors tho the reactor.

2. To increase the acids efficency
Returning unreacted acids to the reactor

3. To reduce the acid vapors reaching the scrubber

About the white salts I told you, I have not solved the problem, but I am working in it. I assayed the salt and this is not a tin compound. I will tell you what happen.


Estuardo


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