Non-Chemical tin and gold separation

[Anonymous]

Dear all,

we have a small alluvial gold mine and the final concentrate is a mixture of fine gold and cassiterite (tin oxide). When we smelt it, sometimes some part of the tin goes into the gold and makes it grey and brittle. Why that happens, I don´t know . We first thought, it might be the smelting temperature (1200°C). There is no reducing agent (carbon) present. We lowered the temperature to 1100°C, but still every third smelting or so the gold is contaminated with tin.

We then tried to clean it with lead and used MgO cupels, but the gold is not completely clean, it seems. It still is brittle and not good for jewelery making. Off course, we could use electrolysis and get it refined. But our idea is to leave it as "natural" as possible, and our clients (goldsmiths) like this kind of "eco-gold" thing, especially we don´t want to remove the natural content of silver and copper, to maintain the original color.

Does anybody know how we can clean the gold and remove the tin? Is there anything we can add to the smelting process which oxidizes the tin and gets it into the slag? In some old literature I found that they have used some copper oxide, but I guess then we would get copper into the gold.

Best regards
Hermann

 

[nickvc]

My suggestion is to read C.M.Hoke page71, as Harold V will tell you its full of common sense advice.if you type in the name in the search box it will be available to download off this site.Sorry but i dont think a simple addition to your melting fluxes will help, you need to treat it chemically to remove the tin before you melt, i could be wrong, and if i am one of our cleverer members will give you better advice.

 

[qst42know]

Although I did not find a direct answer to your problem I did find a reference to cassiterite being apt to follow the slag in alkaline fluxes forming stannates.

Have you tried some soda ash and some silica in your flux?

http://books.google.com/books?id=BsUJAAAAIAAJ&pg=PA193&lpg=PA193&dq=assay+ch+fulton&source=bl&ots=lHRzmUvGn7&sig=QpXdxsxhe4cS7DJIEXk80H3uPdw&hl=en&ei=icUNS_zVHoyMMqGb0ccC&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAgQ6AEwAA#v=onepage&q=carbonates%20tin&f=false

The reference is for performing base metal assays but what they are trying to avoid you might capitalize on.

One of the pros here may have a better solution for you.

 

[Harold_V]

We then tried to clean it with lead and used MgO cupels, but the gold is not completely clean, it seems. It still is brittle and not good for jewelery making. Off course, we could use electrolysis and get it refined. But our idea is to leave it as "natural" as possible, and our clients (goldsmiths) like this kind of "eco-gold" thing, especially we don´t want to remove the natural content of silver and copper, to maintain the original color.

I refined for over 20 years, with my primary customers the very people you mention-----manufacturing jewelers.

I bit of advice. Lose the idea of using gold as it comes from the ground. There are many elements that have an affinity for gold that destroy its properties, making it brittle. In order to insure that gold that is used in jewelry has the normally required ductility, refining is highly recommended. It need not be electrolytic---gold of excellent quality can be produced by the acid processing method.

By contrast, if they are making jewelry that does not require fabrication of any kind, nor any stone setting, it's entirely possible you can get away with questionable quality. That's not uncommon when working with strange colors of gold (blue, purple, etc.).

The color balance is often lost in melting, due to a surface patina that can't be readily reproduced. However, by proper alloying, pretty much any color that may be desired can be reproduced by starting with clean elements. Melted nuggets can be rather unattractive. I melted my share in my years in the lab and was never impressed, especially after comparing the color with properly refined gold.

Harold

 

[butcher]

try a test sample look into flux recipees for type of ore, such as a flux of silca sand or crushed glass and borax tiny bit of KNO3, to get oxidized metals in melt to go up into flux, this will also remove copper from button in melt.
I agree with Harold I would purify the gold and silver, I believe a jeweler can add any metal to get his desired color, or alloy, and your market for jewlers who will buy and price paid may be well worth the work.

 

[Anonymous]

Thanks a lot to everybody for your answers. I will go through all of them and do some testing. I keep you informed what comes out.
I still try to avoid a complete refining process (electrolytic or with acid), but if the other methods do not work, I might have to.
Best regards
Hermann

 

[Lino1406]

Melting with Caustic will form soluble stannates, but (I assume) also
will dissolve gold.
I used hot 60% sulphuric acid, for dissolving tin, then diluting (a bit)
with water, to filter. Needs patiance, not to precipitate tin again

 

[Irons]

http://www.carnmetl.demon.co.uk/tin.htm

-snip-

Smelting

Cassiterite is smelted to metal by reduction with carbon most commonly in a reverberatory furnace. Temperatures in excess of 1200°C are required. The difficulty is that cassiterite is hardly ever produced entirely free from other minerals many of these are reduced to metal at the same time forming alloys with the tin. It is therfore necessary to refine the tin to make it commercially useful. Fire refining involves various procedures on the molten metal. Iron is removed by passing steam through the molten metal, arsenic and antimony are removed by additions of aluminium alloy and copper is removed with sulphur. Very impure tin can be refined by electrolysis to very high purity.

-snip-

Did you know that an alloy of 80% gold 20% tin melts at around 280°C(536° F.) this would easily allow casting into rubber moulds! Has anyone tried this? I've got the tin if you've got the gold! apparently this alloy is used as a special solder and has a slight green colour.

-snip--


So, if you heat CPUs to around this temp, they should come apart rather easily. I bet an old toaster oven would be ideal. :mrgreen:

 

[PreciousMexpert]

How can we get some tin
What kind of scrap would have this metal

 

[Irons]

How can we get some tin
What kind of scrap would have this metal

A plumbing supply outfit would carry it. It's not cheap.

 

[jimdoc]

Try here;
http://www.rotometals.com/Tin-Ingot-s/27.htm

Jim

 

[PreciousMexpert]

Hi Irons
any way to get free tin
Do people use tin only for making parts or is it always mix with other metals

 

[PreciousMexpert]

Hi Jim
That is site I had not seen before
Thanks

 

[Irons]

Hi Irons
any way to get free tin
Do people use tin only for making parts or is it always mix with other metals

It's mostly alloyed with other metals. The best free source that I know is Shooting Ranges. The alloys used in bullets usually contain substantial amounts of Tin as well as Antimony.

Start an environmental cleanup service and get people to pay you clean up all of the 'dangerous' Lead, Tin and Antimony. :mrgreen:

 

[butcher]

pewter cups can be high tin,
solder's can have high tin.
http://en.wikipedia.org/wiki/Tin

 

[Sodbuster]

I found 1 and 4 oz hand poured tin bars on E-bay.
Search (1 ounce Tin bar .999 pure) and or seller (ryan9598)
I don't know what tin spot prices are but I thought thay were afordable.
I know thay have that nice crackel report when you bite um.

Ray

 

[leavemealone]

I know thay have that nice crackel report when you bite um.

I think we need to find ray some help...and quick! :shock:

I don't know what tin spot prices are

$8 a pound (london)
Johnny

 

[Sodbuster]

At $8 a pound mabey it's not so afordable. For E-bay I guess thats normal.
It is a nice pure sorce for some small quantity if a person ever needed tin.

So am I the only one here that does the hardness test with his teeth?
I don't do diamonds.
I mite need some rehab :lol:
Ray