Fire assay questions

I have a problem that ocasionally happens when I run a fire assay, and I'm not sure of the cause.
Some times a thin layer of the lead button will stick to the slag at the top of the button. What causes this, and what can I do to avoid it?

Thanks for any help,

rookieminer

Not enough air. C W Ammens has a chapter on doing a fire assay and in it he describes a "stuck" cupple. the old assayers would stick a sliver of wood in the molten button to open the assay.

rookieminer said:

I have a problem that ocasionally happens when I run a fire assay, and I'm not sure of the cause.
Some times a thin layer of the lead button will stick to the slag at the top of the button. What causes this, and what can I do to avoid it?

Click to expand...

publius said:

Not enough air. C W Ammens has a chapter on doing a fire assay and in it he describes a "stuck" cupple. the old assayers would stick a sliver of wood in the molten button to open the assay.

Click to expand...

I don't think that's what he's talking about, publius. If it is, though, a "frozen" cupel would be caused by either TOO MUCH AIR or too low a temperature, or both. Usually, too low a temperature is the problem.

I think he's saying that, when he pours the fusion from the crucible into a cone mold, a thin layer of lead floats on top of the slag. Is that right, rookieminer? I've had that happen occasionally and usually didn't worry about it. It's extremely thin and probably doesn't represent even 1% of the total lead. If it were more, I would think it would sink. I really don't know the reason it happens. Maybe raising the temp at the end of the fusion would help. Maybe it happens mechanically when the fusion is poured. Can you see it floating on top of the melt right before it is poured?


Maybe 4metals and/or Westerngs can chime in on this.

Yes I am talking about the pouring after fusion, not cupeling. I have'nt noticed the lead float to the top, but a thin layer of lead sticks to the slag at the top of the button where it meets the slag.

Thanks for the replies,

rookieminer

GSP, that is what I get for trying to answer a question while sitting in Washington DC with my copy of Ammens' and Hoke's books on a bench in Northern Virginia...

rookieminer said:

Yes I am talking about the pouring after fusion, not cupeling. I have'nt noticed the lead float to the top, but a thin layer of lead sticks to the slag at the top of the button where it meets the slag.

Thanks for the replies,

rookieminer

Click to expand...

If you have a separate layer(s) on top of the lead button and underneath the slag, it's not lead, although it may look like it (sort of). It's either speiss or matte. Speiss consists of the arsenides and antimonides of Fe, Co, Ni, or Cu and it is quite brittle. Matte is a mixture of fused sulfides, usually Fe and Cu. It's possible to have both and, if so, from bottom to top, the order of layers (phases) would be lead, speiss, matte, slag. It's also possible to have more Ag and Au tied up in the speiss and matte than is in the lead. Therefore, if you get either of these, the assay results are low and you need to start over. You need to get a good fire assay book that discusses these speiss and matte phases and how to eliminate them. The best I've found on the subject is the Shepard and Dietrich book (pp 110-116) which is available for $20 at Legend. Much of what I've written in this paragraph came from that book.

http://www.lmine.com/mm5/merchant.mvc?Screen=PROD&Store_Code=LMS&Product_Code=17202A&Category_Code=assaying

Speiss and matte can be fairly common when assaying ores. There are quite a few assay books available as free downloads on Google Books. I'm sure most of them cover the subject. To download any of these, click on one of the books. Then click on the little arrow next to the "gear" at the upper right. Then, click on download pdf.

http://www.google.com/search?tbo=p&tbm=bks&q=fire+assay&tbs=,bkv:f&num=10

One more try. The more I think about this, it might be something else. That's why photos are important to define the exact problem.

Sometimes, when the slag breaks loose, easily and cleanly, from the lead button, there is an extremely thin layer of something that appears metallic in the depression on the slag where it made contact with the button. This is more obvious when the slag doesn't break up too much but comes off in a chunk. I've seen it a jillion times. I assume it's lead but don't know why it does it. In any case, it's super thin and I never worried much about it. If I find an answer in the books, I'll let you know.

Here are some excerpts discussing the formation.

http://www.coralsas.eu/DocsHtm/TContext05_Matte_Speiss.htm

I think I agree with GSP's initial thinking that it is speiss.

What exactly are you fusing that causes this? I occasionally come across this with arsenical ores, and it is difficult to avoid unless you roast the ore first.

If it is speiss, it will retain precious metals and the assay result will be low. I would suggest rerunning the assay and either roast the ore first (at a lower temperature than normal, say 1000 F instead of 1100) or else using a slight excess of potassium nitrate in the fusion to oxidize the arsenic.

Can you tell us what you are fusing and what flux you are using? Also fusion temperature will be useful.

Thanks for all of the replies.

I own the Shepard & Dietrich, and the fire assay book by Edward E Bugbee. They are both great books, and I have found them very usefull. I suspected speiss, or matte, but I was'nt quite sure how to identify it.

I usually run all of my assays at 2000 F, and I only assay ores for gold and silver. I have been using the sulfide ore flux from action mining, and I add flour for a reducing agent in different amounts, depending on the ore, it's usually around 2.5 g. My previous attemps at using potassium nitrate always ended up making the fusion boil over, so I have'nt used it since. The main ore I've been working with lately does have some arseneopyrite, but in very small amounts. I will try a low temp roast on the ore and see if that helps. Tomorrow I will see if I can find a large piece of slag in my bucket that I can take a picture of to show you what I'm talking about, hopefully that will show you better than I can explain it.

What kind of losses can I expect if speiss forms in my assay fusion? I guess the only way to know for sure is to adjust my methods to eliminate it, and compare the results.

I hope this better explains my problem, and I will try to post some pictures tommorow.

Thanks again,

rookieminer

Shepard said:

In the presence of copper, speiss will form in the assay fusion unless the charge is proportioned with sufficient excess of litharge and soda to keep the copper as well as the arsenic and antimony oxidized.

Uncontrolled reduction methods should never be used for assaying materials containing large amounts of arsenic or antimony. The excess reduction used in uncontrolled reduction methods will reduce arsenic and antimony, as well as some of the speiss-forming metals, so that speiss is almost certain to form.

Click to expand...

From the text an excess of litharge and more soda ash.

Is it possible you are using too much flour and reducing the litharge too quickly before the oxygen can do it's job?

qst42know said:

Shepard said:

In the presence of copper, speiss will form in the assay fusion unless the charge is proportioned with sufficient excess of litharge and soda to keep the copper as well as the arsenic and antimony oxidized.

Uncontrolled reduction methods should never be used for assaying materials containing large amounts of arsenic or antimony. The excess reduction used in uncontrolled reduction methods will reduce arsenic and antimony, as well as some of the speiss-forming metals, so that speiss is almost certain to form.

Click to expand...

From the text an excess of litharge and more soda ash.

Is it possible you are using too much flour and reducing the litharge too quickly before the oxygen can do it's job?

Click to expand...

I will try using more litharge and soda ash, but if I decreas the flour to much I'm not left with much of a lead button. 2.5g of flour gives me a bout a 25g lead button.

Actually I just read the quote you posted last night, and I had another question about that. I'm not quite sure of the difference between a controled reduction, and an uncontrolled reduction.

And how do you slow down the reduction of litharge to lead?

Thanks for the reply,

rookieminer

The page I posted has links at the bottom right.

Shepard describes the controlled reduction starting on page 91, under the heading Use of Excess Litharge.

Also read Crucible Assay starting on page 97.

This is a picture of what I was talking about. This slag is a few weeks old, and when it is fresh the layer is shiney, and looks like lead.

Please post your flux mix, sample size and what you think is in the ore.

The slag should not remain in one piece that way, it should break into small pieces as it cools or when you break away the button. Seems you need to adjust the flux mix.

The flux I have been using is:
41g litharge
28g soda ash
18g borax
1.6g silica
about 2.5g flour, depending on the ore

The peice of slag in the picture did break up some around the edges, and sometimes it does shatter to peices. The main ore I have been working with contains very small amounts of arseneopyrite, iron pyrite, and some black mineral that I suspect is mostly iron. This is all in an oxidized quartz vein.

For a sample I pulverize about 10lbs of a good representative sample to about 1/8" minus. I then screen the 100 mesh material out of it. I usually get about enough to run 5-6 assays from this. I then mix this up before measuring out one assay ton (29.16g).

If this flux seems not right I would love to hear about it. Maybe I'll be lucky and the ore really has more values than I think.

Thanks for the reply,

rookieminer

I would use more PbO to provide more oxidizing power for the ore. I would first try 50 grams then 60 if that didn't work. I have gone as high as 120 grams of PbO on some ores that contained a lot of Telluride.

If by 60 grams you aren't getting better results, I would add a little potassium nitrate. Maybe a couple of grams.

I looked at some of my old notes. I sometimes had to use as much as 100g PbO and there were even a few instances where I cut my sample to 1/2 assay ton.

So I ran two assays side by side with the same sample, well mixed.

On one I ran it the same as before. On the other one I did a 30 minute roast at 1000F, added 15g litharge, and 5 grams soda ash to the original flux. The one with the roast had no speiss present (if that's what it was), and the slag shattered to pieces, and the button came off clean. The one ran as usual did have the speiss layer (again, if that's what it was), and the slag stayed more intact.

But after cupeling both, the result was almost identicle. One ran 6.1opt, and the other 6.2opt. The one with the roast was actually the lower one.

The assay did run cleaner, but with almost identicle results it's not worth the extra time and flux ingredients. Atleast not with this ore.

Thanks for all of the suggestions though, I'm sure they will come in handy in future assays.

rookieminer

Try adding a common nail (not a galvanized one) to the assay, it will help with the reduction of the sulfides which will give you a cleaner button.

P88