Lost Gold - Sodium Thiosulfate?

[snoman701]

Just did a stone removal. I am missing a couple grams of gold.

I am curious if there is a possibility it was picked up by in my sodium thiosulfate leach to break down silver chloride. I ran the AR leach until the reaction stalled, then decanted all liquid and rinsed everything. I then put a couple prills of sodium thiosulfate in the beaker and let it sit in the ultrasonic cleaner for a couple minutes. The silver chloride was dissolved, leaving karat scrap and gold powder. The thiosulfate + powder was filtered leaving a clear thiosulfate, and the gold powder was added back to AR.

I of course still have the thiosulfate, which is assumed to be only silver, but I know that thiosulfate can be used to leach gold as well.

I have no way to determine if there is gold in the thiosulfate. The last time I tried to acidify it with HCl, then leach the precipitant with AR. All tests were negative. Last time my numbers were dead on as well, and my procedure the same.

There is a very real possibility that I lost the gold as vapor, as a hard boil was required. I was always covered with a watchglass, but that doesn't contain everything. Given the size of the pieces, there is no way I could have done it in an erlenmeyer, which is my preference. My reactor/condenser won't be online until tomorrow.

I just want to make sure I'm giving my client my best possible work.

 

[orvi]

Just did a stone removal. I am missing a couple grams of gold.

I am curious if there is a possibility it was picked up by in my sodium thiosulfate leach to break down silver chloride. I ran the AR leach until the reaction stalled, then decanted all liquid and rinsed everything. I then put a couple prills of sodium thiosulfate in the beaker and let it sit in the ultrasonic cleaner for a couple minutes. The silver chloride was dissolved, leaving karat scrap and gold powder. The thiosulfate + powder was filtered leaving a clear thiosulfate, and the gold powder was added back to AR.

I of course still have the thiosulfate, which is assumed to be only silver, but I know that thiosulfate can be used to leach gold as well.

I have no way to determine if there is gold in the thiosulfate. The last time I tried to acidify it with HCl, then leach the precipitant with AR. All tests were negative. Last time my numbers were dead on as well, and my procedure the same.

There is a very real possibility that I lost the gold as vapor, as a hard boil was required. I was always covered with a watchglass, but that doesn't contain everything. Given the size of the pieces, there is no way I could have done it in an erlenmeyer, which is my preference. My reactor/condenser won't be online until tomorrow.

I just want to make sure I'm giving my client my best possible work.

If you are losing gold by boiling the solution too hard - always look to the drop that clinge to the center of the watchglass - and if it is coloured by the colour of the solution - aerosol reaches to the top of the beaker - which means it could easily carry values out. Also, it is good to see the colour of the condensing vapors in the "beak" of the beaker - that would give you very clear idea if values are lost, or not. I use to soak piece of filter paper in this condensate and test with stannous, if the rate of loosing values is just too high.

But I would say several grams are quite a high number - I will keep this as possibility, but several grams of gold would be seen... After wiping the surfaces in flow direction of the air in fumehood - from the beaker out to the vent, and testing with SnCl2 - you can make pretty sure if it is/isn´t there.

I would personally measure it with XRF... But that maybe isn´t quick enough option for you.
I usually tend to cement the metals out of solution (concentrating them), then getting rid of barren liquid and redissolving the residue in very small ammount of suitable acids - which enable good SnCl2 testing. For acidic enviroment, I would choose zinc or iron (for chloride leaches), or zinc/aluminium for basic solutions. For nitrate, copper. Thiosulfate can be reduced by zinc or aluminium to even H2S, so do it in the hood and in small quantity

But I may be off with this assumption it would work with thiosulfate. Hopefully somebody more experienced in this field chime in.

 

[snoman701]

Well, I believe there is vapor loss, but I don't believe it was that high. The watch glass drop is positive for gold (I tested that). I think the loss was actually solder loss as compared to XRF, as they were big bulky pieces that had a lot of solder (it's fun when you get to see the piece decompose in front of you).
There is also slight loss when I melt the non-digested pieces as there is silver chloride present. I want to make a small funnel scrubber for the furnace so I can capture that.

I don't believe I'm losing any to my AgCl rinse, which is usually where the loss is.

 

[orvi]

Well, I believe there is vapor loss, but I don't believe it was that high. The watch glass drop is positive for gold (I tested that). I think the loss was actually solder loss as compared to XRF, as they were big bulky pieces that had a lot of solder (it's fun when you get to see the piece decompose in front of you).
There is also slight loss when I melt the non-digested pieces as there is silver chloride present. I want to make a small funnel scrubber for the furnace so I can capture that.

I don't believe I'm losing any to my AgCl rinse, which is usually where the loss is.

I would say exactly that. It is there, it can be non-negligible, but usually not very high. Some times, when working in less efficient fumehood, I used to do inquarts in ground glass erlenmeyer with reduction on the joint - attached PVC tube submerged to the water. I intentionally bent the tubing to "U" shape, to catch any aerosols. And it catched some of course. Small bubbles are the biggest culprit - ones that form upon dissoluton of the metal (NOx-es etc). When they pop, they create very fine aerosol, which need more time to properly "settle".

Solder is tricky to characterize, you don´t know how much of it is there usually, unless you melt the bulk.
If you can, add flux to the melt, that can avoid losses of AgCl in smoke to the minimum. I usually melt in induction, graphite crucible. Put material of concern in, and to the top, I add layer of either soda ash or calcium oxide (depending on what flux composition I intend to use next). I heat the pot very slowly, taking at least dozen minutes to reach like 600-700°C (basic flux component does not melt truly). Then I slow down and take another dozen minutes to get to like 900. Slow vaporization of AgCl can make "scrubbng" it into the slag and subsequently reducing it much more tame and complete. Then, I add the second part of the flux, melt the charge properly and pour.

Funnel scrubber is excellent additional thing to either prevent losses and help the main scrubber unit to cope with fumes effectively. One old dedicated vacuum cleaner for suction, with power regulator either integrated or added (cheap DIY triac power regulators can be bought on Aliexpress for few bucks) does the "dirty job". Hose is changed to stainless steel flexible hose, which is packed on accessible end by mineral wool. You only need to adjust suction that way you do not overheat the machine insides. Vacuums for ashes are good as they have fiberglass lined "reusable filter bags", which are somewhat fireproof.

This concept can be used for cadmium or zinc scrubbing during smelting - as oxides tend to de-sublimate in cooler parts and deposit there. Mineral wool plug catch like 90% (if cold enough till it reach it), baggie inside like 9%, and the rest is unfortunately blown out - work for the main hood scrubber.

 

[Lou]

It's almost assuredly solder. It is extremely difficult to lose grams of gold from a covered beaker, even on a full boil. Not saying that misting doesn't carry out values entrained in the steam, because it does (this can be seen when you move to a fully condensed system with heads).

Many refiners and many customers expect their stone removal jobs to run plumb but in my experience, they never do.

Thiosulfate can and does dissolve small quantities of gold, but that is in the context of treatment by swirling the silver chloride. I never used thiosulfate or ammonia for stone removal jobs.

I did the platinum stuff in a pressure vessel and did the other stuff in a sonicator after a good, hard boil in aqua regia. I sometimes used straight nitric for 10K material.

 

[EMRE]

Well, I believe there is vapor loss, but I don't believe it was that high. The watch glass drop is positive for gold (I tested that). I think the loss was actually solder loss as compared to XRF, as they were big bulky pieces that had a lot of solder (it's fun when you get to see the piece decompose in front of you).
There is also slight loss when I melt the non-digested pieces as there is silver chloride present. I want to make a small funnel scrubber for the furnace so I can capture that.

I don't believe I'm losing any to my AgCl rinse, which is usually where the loss is.

About 8 to 10% of your Thiosulfate filtered residue will contain gold rest silver. May I ask how do you plan on converting it to metallic silver? What is your go to process?