Fire Assay - Lead concerns - Valid?

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snoman701

Well-known member
Joined
Oct 8, 2016
Messages
2,108
Location
SE MI
I've read 4Metals description of assay/refining lab in assay forum, and being able to perform a fire assay is the direction I'd like to go in.

For someone performing maybe 1 fire assay per day on average, is exhaust scrubbed? Should it be? How concerning is the lead?

I'm a former potter, so lead is the devil. It contaminates your kiln, and it can be leached from utility ware by citric acid...worse though, the person firing the kiln is exposed to it as it fumes off. Thus, my biggest concern....am I just vaporizing the lead, then exhausting it outside, and having it condense and land on my garden and local land.

For the average small refining operation, are they scrubbing their fire assay exhaust?
 
That is a good question and that is the same reason I have not done any fire assays. I know that scrubbing is an absolute. Land downwind of historic smelters tends to be impacted with lead and since it settles out, it would seem that the lead vapor is really a particulate. Possibly some type of cyclonic system coupled with HEPA filtration?
 
Very true, but in smelting it's a bit...well, bigger process, and the lead is somewhat expected to fume off. In cupellation, it's absorbed by the magnesia cupel...and that's what I'm not sure, is if the amount of lead that is fumed off is actually of concern for an environmental impact, or just enough that you don't want to be standing over the furnace breathing it.

I mean, my whole yard used to be an apple orchard. Lead arsenate was used for quite some time as a pesticide...but at the same time, I'm not interested in loading up a broadcast spreader with lead oxide.
 
Found this...not too bad.

http://www.santafesymposium.org/2009-santa-fe-symposium-papers/2009-fire-assay-laboratory-design-1

Def within the realm of possible, to have a fully operational system.
 
I am quite sure you all ready know this, but, you could use bismuth instead.
It is more expensive, but if you only plan on doing one or so per day it just may be the way to go.
It has been talked about here on the forum before, and there is a patent that talks about it's use.
At least you wouldn't have to worry about lead contaminating you, others, and the enviro.
 
About 10% of the lead is volatilized and the balance goes into the cupels. The lead can be scrubbed out but it usually condenses out of the fume easily. The top of the kiln gets high concentrations of lead around the vent hole.

Bismuth is a better option, I remember in the late '80's there was a fear that the DEC in New York City would crack down on assayers but it never happened. I actually ran many assays in duplicate using bismuth for a few months to be certain if I had to I could switch and be confident with the results. It is still available from Legend. http://www.lmine.com

A 25 kg can of litharge costs about $190 the same size can of Bismuth oxide costs $1,200 Same is true for lead boats and bismuth boats. $180 for bismuth boats (100) and $50 for lead boats (100)

Used lead cupels are one of those items that slip through the cracks and are discarded with the trash. Not something I like to see.
 
Wowsa! Hydrometallurgical methods seem a lot more reasonable now!

I knew the bismuth was expensive, but didn't realize it was that expensive.

Have you ever checked with a lead recycler to see if they'd take the cupels? It seems like just crushing it then smelting in a reducing environment would bring the lead back to metal...which they are doing with the rest of their alloy anyway. Well, not the crushing part.
 
snoman701 said:
lead recycler to see if they'd take the cupels?
The value is too low to be bothered with.

In a bone-ash cupel there'd be negligible value versus gangue for a lead recycler.

A bit of lead pipe or a fishing weight would be much more valuable for them.

Better to thoroughly scrape the surface (so it can be re-used) leach out the lead from the dust with nitric, then react it with potassium iodide in the 'Golden Rain' experiment.
(assuming the cupel is bone-ash)

Beautiful experiment, and makes fake gold flakes that a noob would think was, er, not just lead iodide ;)
 
If I remember right, a bone ash cupel will hold a weight of lead oxide equal to the weight of the cupel.

I don't agree with hydrometallurgical being as good as fire assay. Of the some 10,000 fire assays I have done, I would say that about 80% were electronic components. These were mostly (probably 98-99%) done with no chemical preparation at all. Hydrometallurgical methods on much of these could be complex and quite subject to losses, I would suspect. Also, the time and labor of using hydrometallurgical methods would usually be much greater than with fire assay.
 
goldsilverpro said:
If I remember right, a bone ash cupel will hold a weight of lead oxide equal to the weight of the cupel.

I don't agree with hydrometallurgical being as good as fire assay. Of the some 10,000 fire assays I have done, I would say that about 80% were electronic components. These were mostly (probably 98-99%) done with no chemical preparation at all. Hydrometallurgical methods on much of these could be complex and quite subject to losses, I would suspect. Also, the time and labor of using hydrometallurgical methods would usually be much greater than with fire assay.

Given your experience with e-scrap, any issues using bismuth as a collector?

I'm still leaning more towards lead. I'll be able to pick up an air cleaner/scrubber at auction for a reasonable price.

My goal is something that's cheap and low maintenance to run.

I don't get any of my inventory free, and it would be a LOT easier to determine if I should buy if I could just mill a few chips down to determine yield. The lead will just cost more on the front end for the appropriate air handling.
 
snoman701 said:
goldsilverpro said:
If I remember right, a bone ash cupel will hold a weight of lead oxide equal to the weight of the cupel.

I don't agree with hydrometallurgical being as good as fire assay. Of the some 10,000 fire assays I have done, I would say that about 80% were electronic components. These were mostly (probably 98-99%) done with no chemical preparation at all. Hydrometallurgical methods on much of these could be complex and quite subject to losses, I would suspect. Also, the time and labor of using hydrometallurgical methods would usually be much greater than with fire assay.

Given your experience with e-scrap, any issues using bismuth as a collector?

I'm still leaning more towards lead. I'll be able to pick up an air cleaner/scrubber at auction for a reasonable price.

My goal is something that's cheap and low maintenance to run.

I don't get any of my inventory free, and it would be a LOT easier to determine if I should buy if I could just mill a few chips down to determine yield. The lead will just cost more on the front end for the appropriate air handling.
Haven't used bismuth. Always thought it was to expensive. Litharge (PbO=92.8% Pb)) is yellow and minium (Pb3O4=90.7%) is bright red-orange. I have heard the minium called red litharge, but the yellow PbO is the only true litharge. A couple of times I was out of PbO, so I used Pb3O4. As far as the results were concerned, they worked the same. I could tell no difference. Litharge itself forms a basic flux, the opposite of an acid flux. It also deoxidizes, desulphurizes, and, when reduced to lead metal by the carbon formed by a small amount of flour or sugar, it collects the Au and Ag and some of the PGMs. In the old days, they used lees, the solids that form in a wine cask, to reduce the PbO. Litharge vaporizes at 1472F and lead at 2012F. Therefore, the source of the 10% lead loss must be the litharge that forms on the inside surface of the cupel, where most is absorbed into the cupel. Like I said, though, black evil smoky fumes form over the cupels during the cupellation. At the very least, you must evacuate the fumes from the building.

I used a lot of litharge. Too much is better than not enough. I never pre-mixed fluxes and probably 90% of my assays required a fusion step before cupellation. Fire assay, as far as fluxing is concerned is quite forgiving. For a 30 gram crucible, my most common mix was 2 heaping tsp of litharge, 2 of anhydrous borax, 1 of anhydrous soda ash, 1 of silica sand, and maybe 1/2g of sugar. This worked for about 90-95% of the electronic components. I kept these fluxing chemicals near the furnace in wooden bins I made. If the slag was too thick, I would run the sample again with a little fluorspar. If there was a lot of copper, I put it into the slag by first heating the sample with some yellow sulfur for awhile to form copper sulfide.

Fire assaying is extremely simple, once you know what you're doing and have everything you need. I could easily train a person in an 8 hr day. In my book, I list 25 pieces of equipment (like the assay furnace or small 6 or 2 place cone molds or a Stainless kitchen knife or a small ball peen hammer, etc., etc.) and 8 different chemicals that you need. I just read over the list and I don't see how you could do without any of them. The scale and the furnace are the biggest ticket items. Most of the others are cheap.
 
Is there a reasonably priced 0.1mg scale out there? I've got a mettler moisture meter, but it only goes to 2 places. I can usually find 1 mg scales reasonably priced, but that fourth decimal is expensive!

I've made so many different furnace variants, I don't know how this could be too much different. I worked with potters and glassblowers in my artisian days. Always making up some kind of frame.

Just sent you a paypal gift for the book.
 
snoman701 said:
Is there a reasonably priced 0.1mg scale out there? I've got a mettler moisture meter, but it only goes to 2 places. I can usually find 1 mg scales reasonably priced, but that fourth decimal is expensive!

I've made so many different furnace variants, I don't know how this could be too much different. I worked with potters and glassblowers in my artisian days. Always making up some kind of frame.

Just sent you a paypal gift for the book.
Just sent book. Thanks.

Bottom price on a new 4 place scale is about $1000. I would look for a used one for half or less but I would sure ask a lot of questions before I bought it. I would also make sure the seller knows how to pack it.

http://www.ebay.com/sch/i.html?_odkw=analytical+balance&_osacat=20094&_from=R40&_trksid=p2045573.m570.l1311.R4.TR9.TRC2.A0.H3.TRS0&_nkw=analytical+balance+0.1mg&_sacat=20094
 
goldsilverpro said:
Bottom price on a new 4 place scale is about $1000. I would look for a used one for half or less but I would sure ask a lot of questions before I bought it. I would also make sure the seller knows how to pack it.

I bought a used one from an on line auction something like 4 or 5 years ago - being an auction it was an "as is" deal with no return - they did not pack it right when they sent it - when it arrived it was "burnt toast" :cry:

Luckily I didn't pay to much for it so it wasn't a big loss - but a loss non the less

Kurt
 
I bought one a few years back from a GovDeals auction as well, I only bid on it because it was within driving distance and when I went to get it I brought along a 50 mg calibration weight just to check it out. Then I packed it and transported it myself. Some things you just don't leave up to the UPS driver!
 
I'm in the same boat you are, did plenty of reading before going anywhere.

The way I figured it, a exhaust is the bare minimum in terms of safety.
I let build a large fumehood all around the furnace, still in construction.

I was thinking of using a coverall besides a respirator. (got the latter free)
I don't need lead oxide powders on my everyday clothing.

Perhaps I'm being paranoid, but I'm also playing around with bismuth.
Got 1 Kg for approx 40$ of a fireworks store.
 
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