# Arc Smelting Precious Metal Ores Concentrates



## plasmasmelter (Sep 7, 2010)

Hi Folks,

This is my first post on GRF and I want to thank you for being a great resource.
I am building an electrical arc smelter. I am in the process of getting assay results of several prospective ores. 

Among other things I am researching the following:
1. smelting different PM ore types.
2. finding an honest refiner.
3. doing inhouse chemical intermediate refining before sending to big refiners
4. uses of fluxes to get lower melt temps
5. uses of fluxes to get separation of pm from non-pm metals and gangues

Thank you for any guidance/advice you may be able to send my way


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## dtectr (Sep 7, 2010)

plasmasmelter said:


> Hi Folks,
> Would any forum members help provide some guidance to me on;
> 1. smelting different PM ore types.
> 2. dealing with refiners.
> ...



First off - welcome to the forum!
2nd - you may get busy!
3rd - post this on Rockman rick's "rockman" or "Ore something-or-other" thread in "Prospecting, ..." Topic/Section.
There are plenty of new-and old-timer prospectors here that can guide you along if you are willing to do your homework.
I've done a (very) little prospecting, & know you usually need to process your ore way before smelting, but i lack the 
knowledge & expertise to advise you. 
Never fear! If its advice you want, its advice you'll get! There is a rather strict quality standard on info dispensed here, 
so you'll be sure to get "the best of the best". It may be a little more straight forward (blunt) than on other forums, but 
everyone here feels responsible for your safety first, then your profit, but they don't treat you like a preschooler or underclassman
in high school, either. (yeah, i've been to that _OTHER_ site.

good luck & as they say, paraphrased, "get in, strap down, & hang on"!
dtectr
p.s. I like that set of priorities


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## plasmasmelter (Sep 9, 2010)

Dtector,

Thanks for the advice. I am spending as much time as I can learning
the basics so I can pose more educated inquiries to the other members.
I took your advice and purchased online a copy of the prospector's handbook
and I expect I will learn a lot about what we are getting from the miners
as ore concentrate. While we do not prospect ourselves, we are dealing
with individuals that are involved in prospecting and concentrating PM ore.
We are still waiting on results from several buttons. 

I look forward to hearing from more of the folks here at the forum. Meanwhile
I'll continue perusing the forum topics as time permits.

Plasma


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## butcher (Sep 9, 2010)

Are you roasting the concentrates to rid the ore of the acid that formed the rock, prior to smelting, and also oxidizing the base metals in the ore?
A lot of ore was in a melt in a volcano, and the valuable metals that came out are still not separated from that melting process, still mixed as microscopic values in the rock that formed, so just by melting the same rock will accomplish nothing but a waste of manmade energy. 
Sulfide can lock up values in the melt, flux and ingredient added to melt can be very important to help separate metals from slag, like adding iron metal can help eliminate the sulfide from melt.
Sometimes we want to oxidize the mix adding an oxidizer like KNO3 to help oxidize the base metals to get them into the slag, other times we may want to remove oxygen from the melt adding carbon source.
Glass or sand can give a slag for the base metals and gangue to form in floating on top of metals in a good hot liquid melt.
Borax and sodium carbonate in flux can help with making the melt more liquid, so values in melt can join together and go to bottom of melt, and base metals to mix with slag, (I also thought they help to keep atmospheric oxygen out?).
Getting the melt hot enough and keeping the temperature up to liquid state in the melt long enough for the separation of values and slag and for the values to collect together is also important for success.
Metals added can also act as collectors of values, such as adding silver to collect platinum. I have read nickel will collect PGM's but never tried it, red lead oxide has been used as a collector for values in the melt.
Ore’s can be very different and one flux will not always work on different types of ores, knowing what the ore type came from and how nature formed it can help you choose how to make up a flux for that melt, also understanding how each ingredient you add to the flux reacts in the melt is also important to the chemistry of your melt and success of the button formed, it does no good to have a high grade ore if you cannot get the values out of it.
I only do this for fun as a hobby so I am not a good source for your success in this venture.
Can you give us some more details on your arc furnace, such as how furnace was made the refractory, what type of electrodes and what type of power source voltage amperage frequency, this sounds like something that would be fun to build and expieriment with.


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## Lou (Sep 14, 2010)

I'm surprised that clay-graphite is holding up so well.

Anyway, I suggest you invest in an optical pyrometer.

You may also entertain the notion of adding copper, nickel, or silver to the feedstock to percolate through the melt and collect more values. I would say lead, but it'd mostly be boiling off and takes a hell of a toll on refractory. Try fluorospar, carbonate, and borax like Butcher expertly suggested.


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## Harold_V (Sep 14, 2010)

Lou said:


> Try fluorospar, carbonate, and borax like Butcher expertly suggested.


Do by conservative in the amount of fluorspar you use. I used to use it to thin my flux when I'd run my waste materials. My tilting furnace was lined with a refractory that was primarily alumina (94%), yet it was still heavily attacked. Doesn't take much to thin your flux, which is important in permitting the prills to agglomerate, so be certain to balance the mixture so you use just enough. 

Harold


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## plasmasmelter (Sep 16, 2010)

Butcher,

I wasn't ignoring you, but rather reading up before I responded. I really appreciate your input.

Yes and no. So far we have done no ore concentrate roasting ourselves, but we are receiving ores that have been. The pyrite material was not roasted but probably should have been first. We have the option of running the arc furnace with either a reduced or oxidizing environment. How best can we oxidize the large amount of iron that begins as FeS2 and get it into slag and separate from the values?

Yes, being ignorant, we have tried throwing heat at it and it doesn't work on the pyrite and some other materials. We reran the pryite ore and added some borax and got a second slag, but not a totally metallic button. Looks like we will need to try again with a better recipe. Any insights here would be very welcome since we are not sure what to do to get a better separation and good button.
Idea/Question: Should we rerun the "bad button" in a oxidizing environment first to oxide the base metals and then add silicate and limestone and borax and other suggested fluxes to slag the oxides?

Pyrite, a sulfide of Iron melts around 1100 C. We are way beyond that and have iron. With such a high % of iron can we oxidize the pyrite ore
and slag it in the initial smelt and get a clean button assuming values are there? The sulfur boils off and goes to scrubber . Or should we reduce in an o2 starved smelt first and then rerun in an oxidation environment?

For pyrite ore, we estimate (don't have pyrometer yet) we are reaching a melt temp of 2,100 C and holding between an estimated rising 1,800 C - 2,100 C for 30 minutes. Any insights into staging the fluxes and whether reduce/oxidize approach based on my responses?

We have not inquarted any smelts yet, but I can see that it makes sense. o you have a guide as to how much inquarting is required of the collector metal? We are working with PGM ores and the ones that have high silica seem to be self fluxing. The others are not.

Can you or anybody else recommend a good manual/book that documents how to apply the appropriate flux for a given ore type?


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## plasmasmelter (Sep 16, 2010)

Lou said:


> I'm surprised that clay-graphite is holding up so well.
> 
> Anyway, I suggest you invest in an optical pyrometer.
> 
> You may also entertain the notion of adding copper, nickel, or silver to the feedstock to percolate through the melt and collect more values. I would say lead, but it'd mostly be boiling off and takes a hell of a toll on refractory. Try fluorospar, carbonate, and borax like Butcher expertly suggested.



Hi Lou,

Thank you for your response. 
The clay/graphite crucibles work fine for one time uses even at the higher temps around 2,400 C. They appear no worse for the wear when we bust them up. And they are expensive. We use 120's, 150's and 200's. Does anybody have a good source for clay/graphite crucibles?

Yes, we need a pyrometer. Do you or anybody else know where I can source one from and an idea of cost for 2,300 C?

How do I get an idea of how much inquarting of collector metal is required for success?
How do I get an idea of how much flux is required for success? We have used borax and it has a dramatic effect on fluidity and it alters the melt color to a darker/richer color. We have also begun using pelletized limestone. The limestone has a plethora of different elements. Should I be using a more pure form of carbonate?

Any additional thoughts and recommendations are most welcome as I continue my studies?

Thanks Lou for your help.


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## jimdoc (Sep 16, 2010)

You can try here for graphite;

http://www.graphitestore.com/stores.asp/cat_id/12

Jim


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## plasmasmelter (Sep 16, 2010)

jimdoc said:


> You can try here for graphite;
> 
> http://www.graphitestore.com/stores.asp/cat_id/12
> 
> Jim



Jim,

We use the larger bilge crucibles #120 - #200. We have had problems sourcing significant stateside quantities at a reasonable price.
If I missed something on the website, please let me know. It appeared they had up to #40 size and they are high dollar. I'm waiting
on a small shipment from china (unfortunately) to see if they are sufficient so we can get a container load for production.

Thank you for your response.


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## plasmasmelter (Sep 28, 2010)

plasmasmelter said:


> jimdoc said:
> 
> 
> > You can try here for graphite;
> ...



Edit: Sorry for double post. Accident. does anyone know of a crucible source in North America?


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## plasmasmelter (Sep 28, 2010)

A photo of a smelt.


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## HAuCl4 (Sep 28, 2010)

Nice photos!. I bet that torch can go through a vault door in no time!. 8) :lol: 

Keep the photos coming!.

You could make your own crucibles for a fraction of the cost of commercial ones... You said you are breaking them anyway after one melt. No?.


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## plasmasmelter (Sep 28, 2010)

photos removed


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## HAuCl4 (Sep 28, 2010)

A magnificent piece of equipment!. Hats off. 8) :shock: 8)


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## plasmasmelter (Sep 28, 2010)

HAuCl4 said:


> A magnificent piece of equipment!. Hats off. 8) :shock: 8)



Thank you for the compliments.... I admit we are proud of our hard work.

Please tell me more about home made crucibles..... :shock: 

PS


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## HAuCl4 (Sep 28, 2010)

plasmasmelter said:


> HAuCl4 said:
> 
> 
> > A magnificent piece of equipment!. Hats off. 8) :shock: 8)
> ...



http://goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=7004

Mostly with clay as the material, but I'm sure you can make some experiments adding graphite powder to the clay and experimenting, if you want clay-graphite. :shock:


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## HAuCl4 (Sep 28, 2010)

Geez. I decided that I like your torch better than an induction furnace!. It is a lot simpler. Simpler is always better in my book.

How much did it cost you to develop it and how much can you make a new one for?. :shock:

Does it blow a lot of air into the crucible?.

If you fail it won't be because of lack of heat!. 8)


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## HAuCl4 (Sep 28, 2010)

plasmasmelter said:


> Any additional thoughts and recommendations are most welcome as I continue my studies?



This is a KISS suggestion:

After you mix the proper flux according to the ore you want to smelt. (Don't invent here, check the books about that, basic, acid, sulfides, etc). 

Bring the melt to the highest temperature you normally use, then take the torch out, and carefully sprinkle dry Silver oxide in fine powder mixed with borax above the high temperature melt.

This creates a "rain" of silver droplets through your liquid melt that collects all metals it encounters on the way down to the bottom of the crucible. Then bring the temperature to the maximum you use again, and let it settle.

With a little trial and error you can establish the proper amount of silver oxide to add, so that the button you get will be ready for acid treatment and recovery. :shock: 

If none of the bullshit above works, you didn't waste much time trying it out either, but given the high value concentrates you have posted about it should work just fine and dandy!. 8)


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## plasmasmelter (Sep 28, 2010)

AuCl4

The plasma torch is more complicated than the carbon arc. There are limitations
for each system. 

You can make the furnace function as a reducing or oxidizing atmosphere.
Yes, heat is not a problem unless too much is a problem. And it sometimes is.

Thanks for the flux recipe idea. I will read up on that approach.

PS


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## rusty (Sep 28, 2010)

Appears that the Australians invented the Sirosmelt a submerged lance used in precious metals smelting.

http://www.aussiethings.biz/sirosmelt_isasmelt.html

The Company: http://www.isasmelt.com/

Looks like a Thermo Lance used in demolition work, you would know it from the movies as a safe cracking device.
http://www.thermolance.com/Thermolance_Oxygen_Lance/thermolance_oxygen_lance.htm


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## plasmasmelter (Sep 29, 2010)

Rusty,

That is pretty interesting. I can see how that would be useful
in smelting. I guess they run an oxidized bath to separate nobles
from bases. What are your thoughts about that? Any idea what temps
and types of ores they are working with?

PS


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## depperl001 (Sep 29, 2010)

rusty said:


> Appears that the Austrians invented the Sirosmelt a submerged lance used in precious metals smelting.
> 
> http://www.aussiethings.biz/sirosmelt_isasmelt.html
> 
> ...


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## rusty (Sep 29, 2010)

depperl001 said:


> Hi,
> 
> Not the Austrians but Australians.
> 
> ...



No offense Mate, at least I made a feeble attempt at using a spell check. I have made the correction to my post.

Now of you would do yours we would not perpetuate my error.


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## plasmasmelter (Sep 29, 2010)

I'm glad that is settled. :lol: 
Now can we get back to arc smelting? :mrgreen: 

PS


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## plasmasmelter (Sep 29, 2010)

Photos of a metal-slag separation.


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## Harold_V (Sep 30, 2010)

Indications are you have a serious sulfide layer. It should be assayed for values. Should you find any, they can be recovered by melting and inserting scrap steel. 

Harold


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## plasmasmelter (Sep 30, 2010)

Harold,

We are supposed to get the assay back this friday. I'm hearing you say
that you think the button has sulfides in it and we should
reduce the sulfides with iron to get elemental metals.
What gave away the sulfides? The button's rainbow sheen?
How do we know how much scrap steel to use?

PS


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## Harold_V (Sep 30, 2010)

plasmasmelter said:


> What gave away the sulfides? The button's rainbow sheen?


That's what made me wonder, although my experience with sulfides dictates that the material will look like gray iron, with no colored sheen. Doesn't mean it can't have one, though. 



> How do we know how much scrap steel to use?


It tends to be self regulating. Insert an abundance, so reaction time is short. The lot will reach the point where there is no further dissolution of the inserted scrap. It's important that temperature is held below the melting point of steel. I did this process in a natural gas fired furnace, inserting lengths of rebar through the exhaust port. The inserted pieces can be easily removed to see your progress. 

I hope I'm not forgetting anything. It's been many years since I performed this operation. 

Harold


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## Oz (Sep 30, 2010)

Harold_V said:


> It's important that temperature is held below the melting point of steel



That is a key point, and I would think imposible in an arc furnace.


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## Harold_V (Oct 1, 2010)

Oz said:


> Harold_V said:
> 
> 
> > It's important that temperature is held below the melting point of steel
> ...


Oh, yeah! If memory serves, an electric arc yields something in the 10,000°F range. That's why I made mention of using a natural gas fired furnace. There are times when too much heat isn't in your best interest. 

Harold


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## qst42know (Oct 1, 2010)

I wonder if it would be prudent to have an assay done on the black crust that's building up on the face of your refractory.

You may be vaporising or simply blowing away elements of interest.


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## plasmasmelter (Oct 3, 2010)

Bad news. The assay came back with no significant pm's. Small amounts of Ag and PGM's
showed up, but no Au. There was 85% Mn and 15% Fe leaving only traces of other metals.  
I don't know if there were sulfides, since the ICP assay only reports back metals.

On friday I ordered an omega OS524E IR pyrometer



The temperature of the furnace can be controlled several ways in order to stay in a needed range.
Any PM volatization will be captured in the exhaust system or furnace wall. 

The material shown in the earlier posted slag/button photos was supposed to be an orecons and we did
not get it assayed before processing it so we flew blind into it. Is it possible that there are pm's help
up in the slag? Or volatized off? There were after all traces of Ag in it. The boiling temp of Mn is 2061 C
and it seems that we would be boiling off that base metal instead of the PM's if any metals were in fact boiling off.
We were running a reduced furnace. Any ideas on where the PM's may have gone?

I ordered the book "Chemistry of Gold Extraction" by John O.Marsden
http://www.smenet.org/store/mining-books.cfm/Chemistry-of-Gold-Extraction,-Second-Edition/240-8

At this time we are faced with needing to be able to reduce the base metal content of our buttons in order
for the refiner to accept them. We are getting a 2.6% PM button with an orecon with a reduced furnace
extraction. We are considering approaching better concentration by running smelts in an oxidized
furnace to fuse as much base metals into the flux slag as possible and then use wet chemistry acid to
dissolve the base metals and collect the PM particles. I'm wondering what the technical issues will be
with getting the PM's to settle to the bottom of the crucible while we are oxidizing the base metals.
Is this where a getter metal like silver would be employed to wash the PM's out of suspension? 

PS


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## HAuCl4 (Oct 4, 2010)

I still wonder what exact "edge" there is in having such high arc temperatures in smelting. The only angle I see is that the warmup period must be very short. Maybe someone can point the advantages of ultra high temperatures to me?. Many metal compounds just vaporize at those temperatures, including PM compounds.


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## plasmasmelter (Oct 4, 2010)

HAuCL4,

The high heat that can volatize PM's is confined to the vicinity of the arc itself
and the temp quickly falls off a short distance from the arc. 
The melt is theoretically not allowed to get beyond a certain temperature
to avoid pm losses to volatization. I think the flux material floating on top
shields the precious metals from volatization to a degree as well. I also think that
there is some volatization of pm's to a small degree resulting from direct impingement
of PM particles into the arc column.

So by controlling the temperature we can minimize PM losses. 


PS


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