Two types of powder after SMB drop.

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rjamjb

Active member
Joined
Sep 6, 2015
Messages
29
Hi,

It has been a while since I posted here I have a question relating to the picture which I've never come across as follows. Process first.

1. Collection of pins, gold plated parts primarily from old mobile phones and a few SIM cards over many months (there may have been an odd component in there although I tried to avoid including, being fairly meticulous about this).
2. Added HCL to cover (approx 0.5 litre) and then Nitric acid in small increments until all metals had dissolved.
3. Added Urea until any excess Nitric was gone.
4. Precipitated with SMB.

I got a normal drop from the SMB as I've done on a few occasions.

When I washed the powder in water it is clear that there are two powders, a light tan with about the amount of gold I'd expected (probably 1-1.5 grams) but also a 'dark powder'. When I swirl the powders they separate as you can see in the picture which is taken looking down into the beaker. One would appear to be more dense than the other.

20180609_124643.jpg


I thought that I must have put too much SMB in and dropped another metal as well as gold, but this is not dissolving in conc HCL - the picture in the photo shows the two powders in HCL.

I've wondered whether i may have precipitated out a higher metal (Pd for example) but there would appear to be far too much compared with the gold.

Does anyone have an idea what the additional powder is? I've not seen this before. (the tan powder, Au, is not acclomorating in the usual way either, remaining very fine and silty, I suspect because of it being mixed with the other powder. At least with some idea what it is then that might give me the basis to separate it out.

Thanks.
KR
Russell
 
SMB will precipitate a small amount of Pd but not that much. Normally, it's just trace amounts but it can throw your numbers off. Try dissolving the whole batch in HCl and bleach. Forego the AR this time. If you are expecting a couple of grams, less than 5 ml's of 8% sodium hypochlorite should dissolve it. Use a pipette and add the bleach directly into the powder and HCl a ml at a time instead of just dropping it in on top of the HCl. Give it a good swirl and let it set for a few minutes between additions. 30 ml's of HCl will be too much but that is what I would use. Heat it to steaming for an hour and let it cool completely before precipitating the gold again.
 
When precipitating gold from a mixed metal solution you will firstly get fairly clean gold dropping.

This is followed by a co-precipitate of gold and base metals which is fluffier and lighter in that the particles are less dense due to the presence of the base metals.

The entire drop is usually boiled in 50% HCl until the gold clumps, may take up to an hour.

After cooling the redrop will come down fairly clean as the base metal tenor in the HCl is much lower than it was in the original solution.

The process can be repeated if very pure gold is desired.

Substantial amounts of the gold will dissolve in the HCl boiling solutions, you will most likely need to cool the solutions and drop this gold as a separate sequence.

You will always have low levels of gold in solution in the solution being precipitated from no matter how much precipitant is added.

Depending on the base metal tenor and the type of base metals the residual gold will be from 0.1 to 2.0ppm in solution.

Keep in perspective that 1ppm gold in solution is a total of 1.0 milligrams in a litre of solution, usually not worth the effort to recover.

Deano
 
Thanks Geo, thanks Deano,

I had re-dissolved the mix in AR prior to reading either post. However this time very meticulous about adding SMB in increments, so just enough to cause a drop. The gold is much cleaner looking and I'll go ahead and wash/boil etc in the next couple of days.

I will try the bleach method on one of my next batches - I've never done that before and I know a lot of people use it as their primary choice. I'm running out of Nitric and bleach is definitely cheaper so if I like it I might adopt the method anyway.

Still not sure what the darker powder was but it has stayed in solution this time, gold only having dropped.

KR
Russell
 
rjamjb said:
Thanks Geo, thanks Deano,


Still not sure what the darker powder was but it has stayed in solution this time, gold only having dropped.

KR
Russell

Russell

That's answered in the first two paragraphs of Deano's post. 8)

Jon
 
Bigger gold particles gives a powder that drops fast. Smaller gold particles becomes darker and drops slower. Sometimes it even looks like and behaves as black ink.

The reason for differences in particle size are many but I guess that concentration and purity is two major factors.

If a batch doesn't behave when I precipitate gold I just let the gold collect on the bottom, decant the majority of the liquid and redissolve the gold again. The second drop is often very good and gives a powder that is a lot easier to handle than the first time.

You made the right decision when you redissolved your gold.

Göran
 
Thanks all for the input - great team here - as there was when I first posted a few years ago.

Keep up the good work.

KR
Russell
 
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