Lead or Lead oxide for collector metal?

[lanfear]

But cant you also make a cupel from Portland cement?
I am going to smelt my MLCC sometime in the future. I have desided to use bismut as collector metal. It's not that exspensive.
Doe's anyone know if it is possible to make "litharge" out of it?

 

[Ultrax]

Cupel shouldn't contain metallic lead, and it is useless action to grind it for this purpose (gravitational separation of lead drops). You should grind cupel to powder if you want to recover lead from litharge, in an additional process(usually it is not a profitable option). The precious metals mixed with unoxidized lead drops can be stuck in matte and slag on the cupel surface if the cupellation hasn't finished successfully. And yes, slag and matte can be ground and separated by gravity. Otherwise, you shouldn't see any lead beads (except PM bead) on the cupel surface - just the yellowish-orange-brown color of the cupel walls soaked litharge when you break it.

 

[stoneware]

Litharge wins the day when used as a collector of precious metals during smelting.

Litharge mixed intimately into the flux along with the material being smelted, with the addition of one of the following ingredients sugar, flour, charcoal or powdered coal.

As the crucible comes up in temperature the carbon sources want to ignite in order to do this it robs the oxygen atom from the litharge turning it into lead metal.

As the small beads of lead fall through the molten flux, precious metals will attach. At the end of the melt you'll have a lead button in your cone mold.

When the lead button recovered from the cone mold is cupelled, the reverse takes place you need an oxygen rich atmosphere during the heat to oxidize the lead so it will soak into the cupel.

Table from attached document.

 

[goldshark]

I also learned that Litharge starts reacting at a higher temperature than metallic Lead.Litharge starts at around 850 C. Metallic Pb at around 225 C.. My understanding is the Litharge doesn't flow through the flux/ore, until the contents are pretty close to the molten stage. This means the Pb has a better chance of attaching to a metal particle, instead of draining through the solid mass and collecting at the bottom. I don't know if a good swirl is enough to make intimate contact between Pb and all fine metals still in the melt, resulting in possibly lower reports. I am assuming that is why assayers use Litharge, in lieu of already metallic Pb.

 

[orvi]

I also learned that Litharge starts reacting at a higher temperature than metallic Lead.Litharge starts at around 850 C. Metallic Pb at around 225 C.. My understanding is the Litharge doesn't flow through the flux/ore, until the contents are pretty close to the molten stage. This means the Pb has a better chance of attaching to a metal particle, instead of draining through the solid mass and collecting at the bottom. I don't know if a good swirl is enough to make intimate contact between Pb and all fine metals still in the melt, resulting in possibly lower reports. I am assuming that is why assayers use Litharge, in lieu of already metallic Pb.

That feature is good in my opinion. Mixed charge firstly burn the flour while reducing lead oxide to lead microdroplets, locked in the matrix of flux and sample. As it all heat up to temperature, only then lead is liberated and rain through the charge, catching all globules to one bead.

 

[Southfork]

Jason's videos are mentioned in this thread a number of times. In one of his newer videos seems he's getting away from lead as a collector metal and using Bismuth instead nontoxic or less toxic.

 

[goldshark]

I sent Jason a note about using Lead vs. Litharge, to see if he do a video on the performance of Lead vs Litharge. I only got back a note to speak with him on tatatlk or Facebook, or some other network that I don't have time for. But I did notice his recent use of Litharge, which I had not seem him do in any previous videos.

 

[kurtak]

Jason's videos are mentioned in this thread a number of times. In one of his newer videos seems he's getting away from lead as a collector metal and using Bismuth instead nontoxic or less toxic.

I am sorry but (I believe) Jason is wrong about the chemistry that takes place using NaOH as a flux in smelting

That said - I don't personally know what the chemistry is (at smelt temps) but there is certainly a reason why you never see NaOH used as a flux ingredient in any of the books about smelting &/or fire assay

What I do know - is that a number of years ago (9-10 years ago) in a discussion about smelting CATs (for the PGMs) & using NaOH as a flux to dissolve the ceramic comb sub straight this is what Lou had to say about it

NO don't use NaOH for melting PGMs or doing a fusion on catalyst. More dangerous than aqua regia and less effective.

And (from that same discussion)

NaOH attacks Pt/Pd.

That was in a discussion about smelting CATs which only contains PGMs wherein the Pt/Pd is being attacked/reacted with NaOH at molten smelt temps --- how it would react with gold &/or silver at molten smelt temps I don't know

In other words (at least according to Lou) at smelt temps --- NaOH + heat --- does not simply go to Na2O + H2O --- something else is taking place in the chemistry at those temps - in order for the NaOH to attack (at least) Pt/Pd

Maybe if Lou sees this he can chime in with more details

Kurt

 

[Yggdrasil]

I am sorry but (I believe) Jason is wrong about the chemistry that takes place using NaOH as a flux in smelting

That said - I don't personally know what the chemistry is (at smelt temps) but there is certainly a reason why you never see NaOH used as a flux ingredient in any of the books about smelting &/or fire assay

What I do know - is that a number of years ago (9-10 years ago) in a discussion about smelting CATs (for the PGMs) & using NaOH as a flux to dissolve the ceramic comb sub straight this is what Lou had to say about it



And (from that same discussion)



That was in a discussion about smelting CATs which only contains PGMs wherein the Pt/Pd is being attacked/reacted with NaOH at molten smelt temps --- how it would react with gold &/or silver at molten smelt temps I don't know

In other words (at least according to Lou) at smelt temps --- NaOH + heat --- does not simply go to Na2O + H2O --- something else is taking place in the chemistry at those temps - in order for the NaOH to attack (at least) Pt/Pd

Maybe if Lou sees this he can chime in with more details

Kurt

That would indeed be interesting.
Is it the OH group that attacks and such dissolves Pt/Pd or something else all together?

 

[butcher]

My understanding is sodium carbonate flux Na2CO3 can act as a reducing flux, producing CO2 gas in the melt which can help to prevent oxidation of some metals or reductions.
Na2CO3 --> Na2O + CO2
Na2CO3 + SiO2 --> Na2SiO3 +CO2

At the same time the Na2O can help to form glass (like in the lowering melting point of glass to make soda glass) to more easily form slag glass out of silica or sands lowering the melting point of the silica, other ceramics will react similarly

Sodium Hydroxide lowers the melting points of glass silica or ceramic or refractory-type materials. KOH has a melting point of around 400 deg. C (752 deg.F), a mix of NaOH and KOH more or less 200 deg C. (392 deg F). melting point.

In a "dry" flux mix the sodium hydroxide NaOH flux reacts as a more basic flux in the chemical reactions of the fusion and melt forming more oxide anion crystals.

In a "wet" Flux mix the sodium hydroxide NaOH acts as a much more caustic flux reagent oxidizer (Kx Nax Bax BiO3)...

https://www.tf.uni-kiel.de/matwis/amat/iss/kap_a/illustr/sa_2_1.html

 

[Lou]

There's really no need for NaOH unless you're wholesale dissolving silica, tungsten oxides, molybdenum oxides, etc. and that requires some trials and tribulations to balance it sufficient that you're not making stuff you don't want to be making. But to act like PGMs don't get it on with base is not entirely true..gold doesn't really react, silver it seems like it diffuses into but doesn't really react, but Pt and Pd are severely attacked by lye.

Why did I say I don't like it?

Lot of spitting and misting from molten alkali melts, and I used to do some of these in a big nickel cauldron I could comfortably stand in.

I've found Pd to be mobilized into the slag and to stay somewhat soluble in hyper alkaline environment. It also filters like crap.
For gamma alumina, you can use it to dissolve it away from the PMs. Heraeus does this on the big scale and a video of it is seen on Youtube. I have an autoclave and have used that to do similar small scale things.

@butcher,
There's no redox happening with NaOH under any circumstance--there's dehydration to Na2O and that's what's doing the work with the glass formers and that's why it lowers melt temperatures of the flux. But sodium carbonate does the same thing and instead it makes CO2. This means that when you have oxygen get into a melt or have oxides already present your start getting sodium metallates, i.e. sodium bismuth-ate tungst-ate, silic-ate, zircon-ate. They're already as Bi2O3, WO3, SiO2, ZrO2.

 

[orvi]

I can relate to the difficulties using alkaline fluxes and also alkali metal compounds fluxes in general. Decomposition of say carbonates or lye isn´t instant, and it bubbles and spits for quite a time, even at temperatures far exceeding stated decomposition point of these compounds.
Sodium oxide, as well as all alkali metal chloride salts is somewhat volatile, and this isn´t very handy when performing larger scale melts.

And espetially one thing I hate about sodium in melts - it glows yellow and SO BRIGHT, even at 1000 °C, that you can hardly see anything going on in the crucible. Combine it with spitting nature, fumes... And you make mix of properties that make the smelt very nasty, impractical operation, accompanied by lots of swearing and cleaning all the involved stuff from corrosive white basic fumes settled on anything in proximity. Mainly your face-shield

There's really no need for NaOH unless you're wholesale dissolving silica, tungsten oxides, molybdenum oxides, etc. and that requires some trials and tribulations to balance it sufficient that you're not making stuff you don't want to be making. But to act like PGMs don't get it on with base is not entirely true..gold doesn't really react, silver it seems like it diffuses into but doesn't really react, but Pt and Pd are severely attacked by lye.

Why did I say I don't like it?

Lot of spitting and misting from molten alkali melts, and I used to do some of these in a big nickel cauldron I could comfortably stand in.

I've found Pd to be mobilized into the slag and to stay somewhat soluble in hyper alkaline environment. It also filters like crap.
For gamma alumina, you can use it to dissolve it away from the PMs. Heraeus does this on the big scale and a video of it is seen on Youtube. I have an autoclave and have used that to do similar small scale things.

I know about dissolution of Pd in hyper-alkaline solutions, but corrosion of Pt/Pd in very basic fluxes is new to me. I was not aware of this, but anyway, never done this or ever smelted PGMs in very basic flux mixtures. Good to know, thanks for very interesting info. What is the driving force of this... Formation of some metallate salts ? I suppose oxygen is acting as oxidant here. What about instances of using graphite as smelting crucible ?

 

[Lou]

Yes, it's some sort of metallates of the form nNa2O * MxOy.

I guess in this case I was wrong to say no redox has occurred when smart mouthing Butcher! Maybe some redox does happen with it, but not of the sodium instead you have OH- doing weird things with the precious metals.

I know if I take Ru sponge and fuse in KOH under inert atmosphere, almost no reaction happens. The melt colors but that is due to RuO2 on the surface uniting. Now if I add KNO3, then it starts to go down!

 

[snoman701]

Get some rose didymium glasses. It cuts down on the sodium flare.

Glass blowing trick.

 

[goldshark]

Other assayers I have spoken with, state the best reason for using Sodium Hydroxide, is that it produces less bubblling ( Production of CO2 ) than Sodium Carbonate, and a lot less than Sodium Bicarbonate. That is the only reason I have heard for it's use. Maybe less chance for loss of PM's through spitting is what I am gathering. I haven't used the Hydroxide, so cannot say how it affects the outcome personally.

 

[orvi]

Other assayers I have spoken with, state the best reason for using Sodium Hydroxide, is that it produces less bubblling ( Production of CO2 ) than Sodium Carbonate, and a lot less than Sodium Bicarbonate. That is the only reason I have heard for it's use. Maybe less chance for loss of PM's through spitting is what I am gathering. I haven't used the Hydroxide, so cannot say how it affects the outcome personally.

Assaying is clear, there is no debate about using standard charges with sodium containing components. Apart from that, I found advantageous in many many instances to use calcium reagents instead. Yes, of course, they have higher melting points, but to the roughly 1400°C, it does not matter much for me - and I must say the difference is striking.
Classic slags like calcium silicate are one example, but I rather prefer calcium borate. I am now speaking about general flux like - i am gonna melt this impure cemented metal with some junk in it, or some ceramic pieces stuck in the whole charge of metallic material. Easily mixed from quicklime and boric acid, fluidity is very very nice already around 1100-1200°C and best feature is good thermal contraction, practically no fumes. It shrinks usually enough to get the molten charge from inside graphite crucible out without need for pouring the melt or breaking the crucible. And last advantage, separation of metal and this slag is very nice, it is brittle, non-sticky and separates cleanly from gold as well as PGMs. Incompatible with silica dishes, of course but in graphite, it work wonders.

 

[goldshark]

It would be interesting to know what the losses would be, in comparison to a 2100 F, firing . Many of the assay books I have studied, do not advocate the use of the temperatures in the 1400 C range, particularly for Au/Ag. This is relative to losses by volatilization. PGMs would not have the losses as much as Au/Ag. I do like the concept of your idea very much. Use your flux recipe, induction furnace, special graphite mold with a conical bottom. Let it cool, turn it over ,boom, out pops everything. No molds, additional tongs, loss of temperature causing the flux to congeal prematurely, all in all great concept. I think you should apply for a design patent on the graphite mold, with a conical bottom, before someone else does.

 

[rusti2]

Lo siento pero (creo) Jason está equivocado acerca de la química que se lleva a cabo usando NaOH como fundente en la fundición.

Dicho esto, personalmente no sé cuál es la química (a temperaturas de fusión), pero ciertamente hay una razón por la que nunca se ve el uso de NaOH como ingrediente de fundente en ninguno de los libros sobre fundición y/o ensayo de fuego.

Lo que sí sé es que hace varios años (hace 9-10 años) en una discusión sobre la fundición de CAT (para los PGM) y el uso de NaOH como fundente para disolver el peine de cerámica recto, esto es lo que Lou dijo. sobre eso



Y (de esa misma discusión)



Eso fue en una discusión sobre la fundición de CAT que solo contiene PGM en la que el Pt / Pd está siendo atacado / reaccionado con NaOH a temperaturas de fusión fundidas --- cómo reaccionaría con oro y / o plata a temperaturas de fusión fundidas No sé

En otras palabras (al menos según Lou) a temperaturas de fusión --- NaOH + calor --- no va simplemente a Na2O + H2O --- algo más está ocurriendo en la química a esas temperaturas - para que el NaOH atacar (al menos) Pt/Pd

Tal vez si Lou ve esto, puede intervenir con más detalles.

kurt

Lo mismo sucede con el carbonato de sodio, por encima de los 500°C se descompone en Na2O(s) y CO2(g) que, a altas temperaturas, pueden oxidar Pt y Pd, pasando estos óxidos a la escoria.

 

[Yggdrasil]

Lo mismo sucede con el carbonato de sodio, por encima de los 500°C se descompone en Na2O(s) y CO2(g) que, a altas temperaturas, pueden oxidar Pt y Pd, pasando estos óxidos a la escoria.

English please

 

[Nountaineer]

Most modern cupels (I mean industrial) are made from active magnesium oxide mixed with magnesium chloride.
Bone meal is now used in animal husbandry, but of course, you can make some DIY cupels using bone ash. But making ash from bones it is a very stink process

A friend tried making his own bone ash crucibles then just bought some instead.