Bismuth in scorification

P

pablohdez3

New Member
#1
Hi guys I'm training as an Assayer and I found this great forum; I'm trying to find information about how the presence of bismuth in the assay of gold and silver can affect the dore bead? using the scorification process; so far I couldn't find anything on my text books or other papers.

Thanks.
 
fireguy

fireguy

Supermoderator
#3
Hi guys I'm training as an Assayer and I found this great forum; I'm trying to find information about how the presence of bismuth in the assay of gold and silver can affect the dore bead? using the scorification process; so far I couldn't find anything on my text books or other papers.

Thanks.
Hi Pablo:
Bismuth has a reputation, in general, of causing a low bias in the assay process. However, there are new assay methods that use bismuth as a replacement collector for lead so if this bias is present it seems to be quite low. I think you can start out by assuming that the bismuth will act the same as lead, but you should do some tests to see if there are any additional losses. If you have significant amounts of bismuth they may cause your lead button to be brittle and some cupels do not work well with large amounts of bismuth.

The bismuth should not effect your dore' bead- all of the bismuth will be absorbed into the cupel (or volatilized) along with the lead. You may also want to step up to the next larger size cupel to make sure you have plenty of capacity:
www.lmine.com/category/mabor_cupel.html
 
P

Puerorum

New Member
#4
Hi Fireguy,

The absorbation of cupels are normally given for lead. Do you (or any other in here) know how one should treat these numbers when using bismuth instead of lead? Is it the same or similar? Are there any ratio to use for easy conversion. Thanks!
 
fireguy

fireguy

Supermoderator
#5
Hi guys I'm training as an Assayer and I found this great forum; I'm trying to find information about how the presence of bismuth in the assay of gold and silver can affect the dore bead? using the scorification process; so far I couldn't find anything on my text books or other papers.

Thanks.
Hi Pablo:
Are you sure you want to be doing a scorification? Do you have high sulfur or other elements that would be removed in the scorification process? You may want to start by just doing a regular crucible fusion and work on proper fluxing (the book by Shepard & Dietrich has great sections on fluxing: www.lmine.com/product/17202A.html) .

The "old timer" philosophy on bismuth is that bismuth in your button will cause a low bias. You can correct for this by running a proof assay, determining the losses, and correcting for it. To start I would suggest you not worry too much about the bismuth as an associated losses will probably be quite small.
 
fireguy

fireguy

Supermoderator
#6
Hi Fireguy,

The absorbation of cupels are normally given for lead. Do you (or any other in here) know how one should treat these numbers when using bismuth instead of lead? Is it the same or similar? Are there any ratio to use for easy conversion. Thanks!
You can assume that it is similar (perhaps a little less). Different furnaces will provide different results, so you may want to do a little testing. It is good to always have a little "white" unused cupel material left over. If you add too much lead (or bismuth), the metal will run out the bottom of the cupel- it is good to always use a layer of furnace bedding to absorb any excess metal that runs out.

Furnace bedding:
I prefer the Maborite, but bone ash power will work as well:
www.lmine.com/category/furnace_bedding.html
 
A

Auassayer

New Member
#7
I have been using bismuth here for almost 3 years. I wanted to give it a fair shot for the simple health and regulatory reasons. I have found the following;

1.) You can't cupel at the same temperature as for lead, you need to be about 1580-1600 degrees F.
2.) For standard ores, say less than 0.5 opt Au. As long as you are careful with temperature on cupellation AND the atmosphere in the furnace. It can be a direct replacement for the lead based flux. But be careful to run more frequent standards.
3.) For higher grade concentrates and bullion, I will not use bismuth, without perfect conditions that most labs cant achieve, you will have significant losses. We have been trying to be very careful when doing the SMA round robins(to check the bismuth assays against others using lead), but the trend is for the bismuth you will have a lower assay.
4.) I have found that the bismuth trioxide in the flux is VERY destructive to furnaces, and furnace elements. I have found a way to reduce the destruction by simply lowering the furnace temperature to about 1850 and fusing for 70 minutes, but the destruction does continue and I have not tried the lower fusing temperature on higher grade samples as yet.

Bizmuth is 5-6 times more expensive than its lead flux counterpart, but it is safer to use (so far) and waste can be thrown out in the regular trash.
However, for reasons mentioned above, in the future if I had the choice, I would not use the bismuth flux at all. (Sorry Dave)
 
Top