Dropped gold - sulfurous smell upon drying

[orvi]

I have observed it several times, when drying washed gold in the beaker. I could certainly smell sulfur smell, and if the temperature was cranked past 300°C for few minutes, it nearly completely disappeared.
I always wondered why is that. I just noticed, that if the gold smelled like sulfur during the drying, it was of lower purity (averaged 995-997).
Strange observation is that today, I refined some gold (around 6g). It was inquarted gold mixed with gold dropped after dissolution of 18k alloy (75Au20Cu5Ag) - feed was 99% purity or above. It was dissolved in AR (in usual way, no extra nitric was present after dissolution). Diluted with water/ice to 3 times original volume, filtered and gold dropped with SMB (ACS reagent purity). After proper drop, liquid was decanted and gold washed with water, then boiling HCL (2x20 ml) - second wash was colourless, then 2x boiling water. After this treatment, sulfurous smell was clearly evident even without heating. After drying I obtained gold of 99,7% purity, with 0,25% Cu and 0,05% Ag impurity. Liquid after gold drop was practically colourless, and first HCL wash was coloured like white wine, no gold in solution afterwards (SnCl2 tested).
Not a big issue, it could be flux-washed free from copper anyway and I reach desired 999+, but I wonder why sometimes this happen. Could be the issue over-addition of SMB ? (I never measure the ammount, I just judge by the colour of the solution and SO2 smell). Or not enough washing ? Should I wash it with water, HCL, water, ammonia and finally water to better remove copper from the precipitate ? Does agitation play important role in washing (should it be stirred or boiling is enough) ? How long ?

I would be greatful for any suggestion
I was used to refine just to 99+% purity, and it was all fine for selling for "top" price. But now, I am in need of producing 999+ for getting top dollar for it. I was measuring the purity of gold drop from time to time, and I was hitting 995-999 on regular basis. But now it should be better quality everytime, if I want to save ton of time avoiding re-refining the gold.

Thank you
orvi

 

[4metals]

You can try melting it with flux containing a little Manganese Dioxide which is a strong oxidizing agent in the flux. It should put silver and copper oxides in the slag. Then you can de-slag the bar and remelt it without flux to make it pretty.

 

[orvi]

Thank you for advice. I will try this out next time I will melt the gold.

Does MnO2 also oxidize gold when molten in flux ? Or just silver and copper ? I do not have experience with MnO2 in smelting. Just in hydrometallurgy. I know that with chlorides, MnO2 in aqueous solutions generate chlorine, which can dissolve gold. But since I do not have chlorides in the melt, maybe it will be okay.

 

[FrugalRefiner]

After proper drop, liquid was decanted and gold washed with water, then boiling HCL (2x20 ml) - second wash was colourless, then 2x boiling water. After this treatment, sulfurous smell was clearly evident even without heating. After drying I obtained gold of 99,7% purity, with 0,25% Cu and 0,05% Ag impurity. Liquid after gold drop was practically colourless, and first HCL wash was coloured like white wine, no gold in solution afterwards (SnCl2 tested).

I know it's common to wash with water first, then HCl. Lou posted long ago that he recommends a first wash with HCl, and I have always followed his advice (on this as well as just about everything else). See Gold has been refined and melted, now has orange hue..

I should emphasize that washing with water first is often a bad idea, because the dilution of the dissolution acid (already partially neutralized by sulfite, which is a base) may cause the soluble impurity metals to precipitate as hydroxides.

I'm not sure if it's that folks never read this post, or that Harold's washing technique has been so often quoted, but I follow Lou's advice. Not sure if it will help with your problem.

Dave

 

[orvi]

I know it's common to wash with water first, then HCl. Lou posted long ago that he recommends a first wash with HCl, and I have always followed his advice (on this as well as just about everything else). See Gold has been refined and melted, now has orange hue..



I'm not sure if it's that folks never read this post, or that Harold's washing technique has been so often quoted, but I follow Lou's advice. Not sure if it will help with your problem.

Dave

Now it start to make perfect sense to me... If excess of SMB is added, copper is reduced from Cu(II) to Cu(I) and form a complex. If there isn´t enough HCL, it would have a tendency to precipitate together with gold and stay locked inside the particles (fairly concentrated solutions = bigger clumps of particles). It is then much harder to wash out of them, furthermore if the water precipitated some hydroxides out. I wonder if Cu(I)OH exist and what properties it has...

Thanks for advice. I have a second batch of 20g inquarted gold, which should be finished tomorrow. I will XRF the obtained sponge, then re-refine it with this advice incorporated, and XRF the result. And of course, I will write detailed procedure.
If it does not help somehow, I will follow 4metals advice on MnO2 pinch in the flux to clean CuAg to the flux.

 

[Shark]

I know it's common to wash with water first, then HCl. Lou posted long ago that he recommends a first wash with HCl, and I have always followed his advice (on this as well as just about everything else). See Gold has been refined and melted, now has orange hue..



I'm not sure if it's that folks never read this post, or that Harold's washing technique has been so often quoted, but I follow Lou's advice. Not sure if it will help with your problem.

Dave

I have used HCl first for a long time. I am also pretty sure it goes back to that same post by Lou.

 

[FrugalRefiner]

I stand on the shoulders of giants.

Dave

 

[Palladium]

Cold hcl! I was using a warm hcl wash when he recommended cold hcl.
Cold does work better from my experience.

 

[orvi]

Cold hcl! I was using a warm hcl wash when he recommended cold hcl.
Cold does work better from my experience.

Now it all makes perfect sense to me. And I was doing it wrong the whole time... Practically never needed to do this exactly and precisely to obtain 999+gold. Now, I need to, and it isn´t always like I wanted

 

[Palladium]

If i'm going for .995-.999 it a double refine. If i'm looking for sure enough .999 + i'll do a triple refine. Dissolve, drop. 2 water washes. Dissolve, drop, two water washes. Dissolve, drop, two water washes, hcl wash, three water washes. It only takes a small amount of chemicals and a little more time. BUT ONLY if it comes at a premium on the price. Other than that it's .995 all day long.

 

[Lou]

I should probably sticky that post.

I never was a fan of Harold's washing procedure at all. Cold HCl is objectionable as it is, but boiling HCl!

Ralph, there should be no way you need to do 3 refines to get that high of purity. You are selling yourself short my man! First drop with any sulfite on any batch of decent size usually averages 999X for us on ICP.

The pictures I posted are from a batch I just happened to take a photo of a close up a few years back. Most of my gold averages at 99.995-7 wt % with us assaying following the ASTM B562 Specification for Refined Gold. Biggest impurity is iron. Sometimes it gets over gassed and Pd ends up in it. Note that the stainless pan, which got pulled out of an oven, is not rusty. Rusty stainless pan means entrained acids.

Funny enough, there's a few refiners producing metric tons of gold a week that do it exactly the way I recommend. Why would that be?

 

[orvi]

So I ended inqartation, washing and analytics. Washings passed negative brine test (Ag absence), gold is nice and crunchy. XRF of well crushed inqarted gold (to expose inside of chunks) shown 99,3% Au, rest CuAg, nearly equally.

So I think starting material for the standard AR/sulfite rafination is OK. Should be done tomorrow, if I make it, I will post the results.

 

[orvi]

Gold is dissolved and dropped.
Weight of inquarted alloy was 21,0 g. But it could contain some locked salts or another impurities, which did not read on XRF. So we will see how it end up.
Dissolved in 200 mL of HCL and nitric dosed till nearly all gold dissolved (I left around 0,1g undissolved). I used around 14 mL of nitric to do the job. Then cooled with ice to 0°C, diluted with water to triple original volume, filtered cold till no opalescence and rinsed the Buchner filter with around 40 mL of water.
Then the gold was dropped with SMB till white foam/colour loss - and then negative stannous + SO2 smell evident.
Precipitate is quite dark coloured to my surprise, but really heavy and very quickly settling.
I left before it properly settled, so I didn´t seen the colour of the solution.

I will process it and weigh it tomorrow, if I make it to the end. If not, tuesday will be the day lots of other work around

 

[Shark]

What is the piece under your beaker in the first photo?

 

[Lou]

Cork ring

 

[Shark]

I looked at it wrong and thought it was sitting on a hot plate.

 

[orvi]

I looked at it wrong and thought it was sitting on a hot plate.

Scales were tared also with cork ring (to easily keep up "standing" also round-bottomed flasks), so I used it as it was not waiting the scales to stabilize and allow me to tare them.

 

[orvi]

So I finished the batch. I decanted and washed the gold with room temp. HCL - not boiling. First wash of the powder was quite yellow, but stannous shown no gold, so I continued with washings till they were colourless (total of 3 washes x 100 mL of 31% HCL). Then I washed with water to neutral pH and dryied on a hotplate. And again - I was able to smell sulfurous smell. But this time only very subtle. Powder weighed 20,70 g. XRF shown 997Au rest copper... But we figured out that this could be error reading of XRF. It was sent for re-callibration to the manufacturer, but we think hardly anything was done with the machine. Complaint written and sent, we will see how it will go.
Anyway, I melted the powder and result was 20,59 g button. Crystallized from the bottom, vent on the top - but with slightly matt surface - dont know why... XRF show 999,3Au rest silver. From both sides.

So I finally made it, and thank everybody for the help I recieved. Now, I am confident that I can make it on regular basis, which could really help me sell the stuff for top dollar.

 

[Palladium]

Was it melted with any flux or borax?

 

[orvi]

Was it melted with any flux or borax?

Technically, no. I used new melting dish, but I previously glazed it with borax.