Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning2017-04-21T02:33:06-04:00
  • To participate in the 911Metallurgist Forums, be sure to JOINLOGIN
  • Use Add New Topic to ask a New Question/Discussion about Pyrometallurgy and Electrometallurgy.
  • OR Select a Topic that Interests you.
  • Use Add Reply = to Reply/Participate in a Topic/Discussion (most frequent).
    Using Add Reply allows you to Attach Images or PDF files and provide a more complete input.
  • Use Add Comment = to comment on someone else’s Reply in an already active Topic/Discussion.

Melting gold concentrate without flux (2 replies and 6 comments)

3 years ago
Reidy 3 years ago

Hi, has anybody melted gold concentrate and poured it without using any flux's? Our goal isn't to refine the gold as the mint does that anyway and the cost will be the same. We have low iron and lead levels and I was just curious of anybody having done this before and could give  some feedback?

We were going to start with 60kg of concentrate.


3 years ago
(unknown) 3 years ago

You don't need flux to melt, you need flux to smelt. That is the difference between Melting and Smelting.

A good Flux lowers melting point and removes impurities. Otherwise melting without flux, you end up with a block of AgAuZn


There is a whole bunch of difference between melting and smelting. We are gonna try to put this subject to rest. It is simple but I seem to have a big problem trying to explain the difference. I think we all understand pretty well what it means to melt something. That is what happens when you put a glob of hog lard in a frying pan and stick heat to it. It is what happens when you touch a piece of solder to a hot soldering iron. It is the changing of a solid to a liquid by the application of heat. When the heat is removed the melted material returns to it's solid state. It's chemical content has not been changed. O.K., maybe if it was a powder when you stuck the heat to it, when it cools, it will probably be just a lump of something that is not finely divided. It is still the same stuff with the same composition. If it was 20% Zinc when in a powder form, it is still 20% Zinc after it cools to a solid lump. Are ya with me? Yeah, I think so. That ain't too hard to grasp, even for us miner types.

Now, there is a thing called "smelting". That is a metallurgical term. It really has nothing to do with "melting" other than the fact that both things have to be done at high temperatures where the material in question is probably a liquid. Just a coincidence.
Lets try explaining the difference with a "fer instance". Fer instance, if I had 10 grams of Gold. Hell, we all know it ain't all gold. So lets say it is 20% copper and 80% Gold. If it happens to be in a powder form and we melt it down to a nice lump of metal, it is still 20% Copper and 80% Gold. Right? Don't want to lose anyone this early.

O.K., so you are a purist and don't like the idea that your beautiful lump of metal has all that nasty Copper in it. Won't do no good at all to melt it again. You just end up with the same stuff. So, being a Basement Chemist, how are you gonna skin that cat? Well, one way is to smelt it. So how does that differ from melting? Physically, not much but chemically it is like being on a different planet. So what is this great difference that we are going to utilize? We are simply going to subject the molten metal to a highly "oxidizing" environment. Hey, you know about redox. You also know that it is very difficult to "oxidize" Gold but pretty easy to "oxidize" Copper. That is why Gold stays nice and shiny but Copper turns green pretty quick. Well, that is what we Are gonna use to clean up our gold a little. We only have to do two things. First we have to convert the Copper to some salt that will not form an amalgam with Gold. Second, we have to give the Copper salt some way to "escape" from the Gold. Hey, that ain't no big problem for a Basement Chemist.

O.K., now an easy, cheap, way to this is simply mix the Gold/Copper with an "oxidizing agent" such as Sodium Nitrate. That will get the job done. However, the Copper is sort of "locked up" inside the Gold so the Nitrate can't really get at it. Hey, we solve that problem by melting the whole mess. Now the nitrate can contact the Copper and oxidize it to Cupric Nitrate. Yeah, yeah, I know you all knew that but I had to say it.

Well, that solves the problem, right? Remember, we still have to get the Gold and Copper Nitrate separated. Otherwise, it will just be sort of a mixed mess. Hey, we can do that too. What we gotta do is provide a second liquid that will dissolve the Copper Nitrate but not the Gold. And it has to be a liquid at the temperature of molten Gold. Water don't last long at that temperature. So what we are gonna do is use a hi-temp liquid. Molten glass works great. So you just got to toss in some white, quartz construction sand, a smashed up disposable beer bottle, or a smashed piece of yr wife's best crystal (if you got the balls). Any of that stuff will work.

So now we got the nitrate, we got the silica (beer bottle). We heat that up with the Gold/Copper until it all melts. About 1,100 deg C or 1,800 deg F. Now all we have to do is pour it into some sort of mold and let it cool, (you can use that cast iron breadstick pan that yr wife has stashed away somewhere), bust off the glass containing the Copper and we got a button of pretty good Gold.

But, wouldn't ya know it. Sumpin went to hell. The molten goo was so thick it just wouldn't pour out of the crucible. Maybe you should use a different brand of beer bottle. Well, before we go to that extreme, lets see if the wife has some ol 20 Mule Team Borax lurking around the kitchen/utility room somewhere. Now if you just toss a teaspoon of that stuff right in the mix with her smashed crystal, you will find that when it melts, it will be much thinner and you can pour it much easier.

Now you just bash off the glass and you will find that your Gold is much cleaner than when you started. Now it is probably 95% Gold and 5% Copper. I can't guarantee the numbers, but something like that.
So, fellow scientists, that is the difference between Melting and Smelting.

3 years ago
GeoffC 3 years ago
2 likes by Marshal Meru and David

You probably can melt gold concentrate if it is reasonably pure. Typically a flux is added to even re-melting gold bullion to reduce volatilisation and if it is in a finely divided form the lower melting point flux as a liquid provides better heat transfer to the solids. In its most basic form a borax cover is often all that is used. Other flux reagents are added to prevent oxidised compounds fuming off by combining with these in a liquid from. You mentioned the concentrate had low lead levels but by not using a flux there is potential for most of what lead is there to fume off causing possible safety issues. The other thing fluxes do is remove troublesome elements that can interfere with producing a "clean" bullion bar and  they also reduce gold losses.  An example of this can be production a matte or speiss that forms a separate layer on top of bullion bars and contain significant amounts of gold. The aim of smelting on site with fluxes is to produce a bullion product with minimum precious metal losses that is in a form that can be readily sold to the refiner. The flux use is to ensure this can be done with minimal risk and produces an acceptable produce and is not to refine the gold as such. It may do this by default by removing deleterious components in the concentrate.

3 years ago

We completed the trial with 60kg of concentrate but it failed as the molten metal was too viscous to pour. We added some borax and soda ash and within 30 minutes it was good to pour.. The bars were 5% copper so I believe the copper oxidised which caused the glugginess/ viscosity of the molten material.

3 years ago

You might want to take a small sample and try a rock salt and borax mix, It is one of my sampling tactics that rides my assay heat for the same stock. I putter with flux's and it is an old tried and true method, I do not know if you roast your materials or crushing techniques, This is a very oxidizing reducing mix that will eat almost anything, do not let it's simplicity fool you and do not use a production pot to try it, a small scale 30-45 gram assay or sample crucible will do. If properly crushed , you should have no problem doing a 60 gram sample with 10 grams borax and 20 grams rock salt, not table salt. Good Luck

2 years ago

Auexplorer. I am trying to find out best way for me. I have gold with alot of pyrite/sulfides. I want to get the gold from the pyrite/sulfides? Was told crush fine 400 mesh, roast with charcoal and salt, dump in water to shock, clean and smelt. I use chapman flux. Was told Im losing the gold from sulfides. Best cheapest way please. Cant get the acids here. Was told hydrofloric acid then smelt. Cant find it. Any and all help is appreciated.

2 months ago

If you effectively roàsted off the sulfides, i.e. S-- to SO2, then you should be able to smelt Au concentrates with a small amount of niter KNO3 t9 oxidize any residual sulfide and liberate any Au in the aulfide mineral.

2 months ago

Continuing above, if it is a pyrite, Au concentrate, then I would use a neutral flux, àpproximately equal amounts of Silica and some CaCO3 or Ca(OH)2a little borax glass cover on top, and use a very little flours0ar for xtra fluidity to flux and molten.mix.
Slag should on top at say 1800°F and gold/silver at bottom of crucible last and pour out clean. If not, try this and see what you get and.I'll look for further posts if I can help.

2 months ago

Forgot to mention,if you skip the roasting step, use your basic KNO3, niter recipe to oxidize sulfide, S-- in smelting step. This will get rid of, oxidize S-- and add silica, BG on top and maybe flourspar if needed for fluidity.

Please join and login to participate and leave a comment.

BUY Laboratory & Small Plant Process Equipment

We have all the laboratory and plant equipment you need to test or build/operate your plant.

ENTER our Mining Equipment' Store

We Sell EQUIPMENT for all types of Mineral Treatment PROCESSES and Laboratory Testing needs


View the Services we Provide

911Metallurgist Mineral Processing & Process Development Laboratory

We have a metallurgical test for every possible mineral type and treatment.


We can IMPROVE ALL PLANTS / Mineral Processing Engineering & LABORATORY Ore Testing

911Metallurgy Engineering

Contact us for process engineering, metallurgical investigations, plant optimization, plant troubleshooting, needs. WE “FIX” METALLURGY.

I Need Consulting Engineering Help
I Need Ore Laboratory Testing