# Oxidizing and slagging base metals in smelt



## plasmasmelter (Sep 29, 2010)

We are getting good metals separation in our arc furnace running a reduced environment.
I need to determine how to oxidize the base metals and put them into slag during
the same smelting operation. My perception at this time is that we should convert the furnace
to an oxidizing state and convert the base metals to oxides and put them into the silica carbonate
"sponge" slag. We can use forced air and or chemical oxidizers. Does anyone have any insight into
this? 

It appears that we want to keep out any chlorine atoms so not to cause volatization of nobles.
Should we be concerned about other elements that will volatize nobles when oxidizing?

Also, can we substitute ammonium nitrate for KNO3 or NaNO3 as an oxidizer?

Thanks in advance for your advice,

PS


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## HAuCl4 (Sep 29, 2010)

There are several processes, separate from each other. 

One is the recovery of all precious metals (along with some base metals) into a slab/button/etc using a reducing environment and a "collector". This collector is usually litharge mixed with charcoal, and other stuff like borax, etc for the "flux".

This lead button separates from the "slag" on cooling.

Usually this lead button (or ingot) contains too much base metals, including lead, and it is oxidized in a cupellation furnace, where the litharge is recovered and re-cycled to use in the previous process.

At some point the cupellation is complete (or it can be stopped at a certain point), and the metal is shot and further refined with nitric or sulfuric acids, and then aqua regia, or electrolysis, or, or, or...etc.

Please note that this is a very quick summary of how it has been done, and not a definitive guide.

Basically the process parallels what is done in a fire assay of ores.

Further notice that lead cupellation generates very toxic fumes, and requires equipment to deal with such.

For your hi-temp application, and since your ores/concentrates are so rich I suggested silver oxide instead of lead as the collector, since lead VAPORIZES at around 1,750 C and your arc torch is simply too hot.

If you read one of the fire assay books that are around, you will get many ideas of fluxes, procedures, etc. that may help optimize your proposed operation. Also in Hoke's book there is a description on how they process the precious metals from a nickel refinery, including the furnaces, cupellation, and more.

Cheers and good luck!. 8)

Edit: In your high temperature case, I imagine that iron is playing a large role in your metal button, and I suspect this is why you want to oxidize. But you can't have both oxidation and reduction at the same time in the same flux and of the same metals!.(Stating the obvious). In smelting, the reductive environment is used for RECOVERY, and the oxidative environment is used for PURIFICATION. Two different concepts/processes that allow CONCENTRATION of precious metals. :shock:


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## plasmasmelter (Sep 29, 2010)

Thanks HAuCl4 for your response,

Doesn't it stand to reason that after we have separated the base and noble metals
into a button (i.e. reduced furnace) that we can convert the furnace to an oxidizing
environment and eliminate the base metals from the button via oxidation of the base
metals? The conversion from reducing to oxidizing would take place "on the fly" and
would involve adding oxidizer agent and or forced air to stir the melt to facilitate
a faster oxidizing reaction between the base metals and the oxygen. And then possibly
using granulated silver as a collector to round up any rogue nobels that have been suspended
into the melt.

Ideas anybody?

PS


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## HAuCl4 (Sep 29, 2010)

That sounds like a good plan plasmasmelter. I guess its success would depend on how much base metals % you have in the crucible and wish to oxidize, and you would have to stop and pour out the non metal slag first, so really it would technically still be 2 melts and not "on the fly", but sequential. :idea:


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## plasmasmelter (Sep 29, 2010)

I may be offbase here, but my idea is that the volume in the crucible will not change
significantly when we convert from reducing to oxidizing while adding the oxidizer
and or forced air. However, I think there will be some measurable increase in volume as
the metals join with oxygen to form the oxides. That would need to be accounted for in
the charge and we would leave some post-melt freeboard in the crucible during the reducing
stage. As far as pouring off the slag, I think we need to keep that in the melt to act as a
sponge for fusing the oxides and getting a clean slag separation.

does this make sense?


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## HAuCl4 (Sep 29, 2010)

plasmasmelter said:


> I may be offbase here, but my idea is that the volume in the crucible will not change
> significantly when we convert from reducing to oxidizing while adding the oxidizer
> and or forced air. However, I think there will be some measurable increase in volume as
> the metals join with oxygen to form the oxides. That would need to be accounted for in
> ...



Why continue to heat a silica/whatever slag barren of metals?. Better pour it out, keep the metal button, add your oxidizing-scavenger flux and start with the oxidation from there. That's what I'd do anyway. Most of the initial melt, after reduction, should be non-metallic slag.


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## plasmasmelter (Sep 29, 2010)

HAuCl4 said:


> plasmasmelter said:
> 
> 
> > I may be offbase here, but my idea is that the volume in the crucible will not change
> ...



My take on it is that the "silica/whatever slag barren of metals" is used to fuse (i.e. dissolve or capture) the metal oxides during the oxidation
stage of smelting and thus leaving behind the nobles. So the slag is viewed as necessary to fuse the oxides and get a cleaner separation.

Might it be possible to look at this another way and instead of reducing and then oxidizing to just oxidize and keep the heat up
to where the nobles will precipitate out of the slag. Maybe use a getter or collector metal to encourage the nobles to sink to bottom.

PS


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## HAuCl4 (Sep 29, 2010)

At this point I would suggest that you perform a complete fire-assay yourself of 30 grams of ore concentrates, following the step-by-step instructions given in the fire-assay books or threads in this forum. Then after that, you will have a better view of what I'm trying to convey.

One smelts to concentrate values without losing them and progressively elliminate slag and non-values. 

The first smelting of concentrates should leave you with maybe a 15% metal button and 85%+ barren slag. So you have removed 85% of non-values. From there, you can oxidize the metal button to remove base metals, etc.

Good luck!. This is supposed to be fun!. 8) 

minute 3:12 has the "smelting" you have to do something similar. Not that hard.
http://www.youtube.com/watch?v=apD5cPVsDjc&feature=related


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## plasmasmelter (Sep 29, 2010)

Thanks Au for your advice. I hear you saying that it may be easier
to oxidize the base metals from the button in the absence of the barren
slag that was formed in the reducing stage of smelting. Are you also saying
that once I remove the barren slag from the button containing both noble and
base metals that I should proceed with the oxidizing smelt without any flux for
fusing and only use oxidizing agent? I will continue researching the topic and 
hopefully come up with an approach that works. 

We are preparing to run ANOTHER (not the one mentioned in the above posts) material
for the third time and it appears we are starting with a large blob of reduced metals. The raw
orecon was over half metallic with O's and S's to start with. It is supposed to be a goody bag (+++1%),
so we are eager to crack this one.

Thanks again for your thoughtful response.

PS


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## HAuCl4 (Sep 29, 2010)

You say 50% of your concentrates are metal?.

Do you have % of the metals that compose the button after the reduction?.

This is very usunual, in my opinion, and may require more special treatments. Getting 30-40 Oz of values out of 500 Kg of iron would be definitely a "new challenge" for me.


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## plasmasmelter (Sep 29, 2010)

Yes,

We reduced the charge mass from approx. 50 kg to 22.5 kg. The original charge
assayed at approx. 50% MnO and 10% FeS2 and traces of other metals 
and oxides. So yes it is a very high base metal content that resulted from other
concentration process(s) and no, we don't have an assay of the base/noble button. 
We used only a small amount of borax in both earlier smelts. I'm working on the new recipe
now and will be smelting it a third time by this weekend. Hopefully the last time.
I'll post it when I get it formulated......meanwhile if anybody has any ideas....

PS


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## HAuCl4 (Sep 29, 2010)

plasmasmelter said:


> Yes,
> 
> We reduced the charge mass from approx. 50 kg to 22.5 kg. The original charge
> assayed at approx. 50% MnO and 10% FeS2 and traces of other metals
> ...



Interesting. This is way over my head. Good luck with it!.


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