Silver dore oxidizing go wrong.Help needed!

[Joko sulistyo]

Hi,all.
I have dirty dore that contain silver,tin,lead and may small amount of gold.My plan is remelt the dore,adding lead and then cupelling with portland cement.When the dore start to melt,I add lead to the molten dore and stir with graphit rod until all metal mix completely.Something wrong happen when I stop the burner.The molten metal start to oxidized and all became gray powder.Nothing metal left in the crucible.All is became gray powder.I try to remelt the gray powder with soda ash and borax as flux,and success.All gray powder became molten metal again.But when I stop the burner,the molten metal start oxidized became gray powder again.where my silver and gold gone?How to recover back my silver from that gray powders?Please help.

 

[butcher]

It is hard for me to follow what you are doing and what is happening, so I may be wrong here.

I see several possibilities.
The heat source is not hot enough to melt the metals, and you may be just fusing the powders (as they cling together in a lump they may appear to be molten, but are only fusing together (prior to melting).

Oxygen is an oxidizing agent in a melt, oxidizer or oxygen will convert the lead and many other base metals in a melt to oxides with the high heat involved in the melting of the metals.


Silver loves oxygen and will absorb it when molten.
you may have too much oxygen involved, either from the oxygen in the fuel source or from atmospheric air around your melt, oxidizing lead to lead oxide instead of melting it into lead...

Carbon added or carbonates in a flux melt can help to create a reducing environment.

Changing your flame source (torch or furnace burner from an oxidizing flame to a reducing flame, and or by changing the furnace environment from an oxidizing environment to a reducing environment which is high in CO2 gases (eliminate or limit the fresh air introduction).


As for how to proceed I see several options.

Try melting again with a reducing flux in a reducing environment.
This gives you lead metal which will then need to be returned to the oxide state that you already have it in now, for the bone ash, portland cement, or magnesium cupel to absorb the lead. if going that route you could just cupel the lead oxide as it is now (powders), but this does not help us to deal with tin, we still have silver and tin in metal form, nitric later here is a problem. dissolving the silver without nitric becomes a problem ...

Although I hate dealing with silver chloride sometimes it is the better way to go.
If you have silver tin and lead oxide in these powders (which have already been roasted red hot in the air).
I think I would go with HCl on the powders to put tin in solution as stannous chloride, converting the lead, and silver into an insoluble lead and silver chloride.

Lead chloride is fairly insoluble in cold water but becomes much more soluble in boiling hot water.
Silver chloride is insoluble in hot or cold water.

You can heat the lead and silver salts in (almost but not boiling) hot water, the silver chloride is light and fluffy so give it time to settle while keeping the solution hot, decanting the solution of lead salts to another vessel and letting that vessel cool the lead salts become insoluble, and we can reuse this water to recover more lead salts from our silver chloride powder, keeping the waste solutions contaminated with lead at a minimum as possible.

leaving you with silver chloride, which can be converted back to silver before meting using dilute H2SO4 and iron metal, or you could go with other methods such as the sodium hydroxide and Karo syrup method. or trying to flux melt the silver with carbonates...

 

[rickzeien]

"I think I would go with HCl on the powders to put tin in solution as stannous chloride"

what happens to the tin in this scenario? Does it stay in solution?

Sent from my SM-G950U using Tapatalk

 

[Owltech]

Hi,all.
I have dirty dore that contain silver,tin,lead and may small amount of gold.My plan is remelt the dore,adding lead and then cupelling with portland cement.When the dore start to melt,I add lead to the molten dore and stir with graphit rod until all metal mix completely.Something wrong happen when I stop the burner.The molten metal start to oxidized and all became gray powder.Nothing metal left in the crucible.All is became gray powder.I try to remelt the gray powder with soda ash and borax as flux,and success.All gray powder became molten metal again.But when I stop the burner,the molten metal start oxidized became gray powder again.where my silver and gold gone?How to recover back my silver from that gray powders?Please help.

Tin is the culprit. https://patentimages.storage.googleapis.com/63/66/ba/61a64e4d804772/US2235423.pdf
I use iron rod or iron shavings to remove it.

"When the gold is alloyed with tin, cupellation with lead alone will not succeed, because the tin, with part of the lead, forms a spongy and refractory oxide, and floats on the surface of the metal, and at the same time retains part of the gold. But as iron is found to combine with tin into an alloy that may be scorified by lead, the addition of iron filings during the process removes the difficulty." from Encyclopedia Britannica https://books.google.bg/books?id=inV6wuh6518C&pg=PA422&lpg=PA422&dq=iron+tin+cupellation&source=bl&ots=n1PVXRMZHH&sig=ACfU3U3ekmMGveBnq3chwgEAn8rCaLA-zA&hl=en&sa=X&ved=2ahUKEwiDx6yrvoXqAhWyunEKHc_lAhsQ6AEwDXoECAkQAQ#v=onepage&q=iron%20tin%20cupellation&f=false pg. 421-422

 

[Joko sulistyo]

Thank you,Butcher and Owltech.I will try your advice.I will recover my silver with acid.