Smelting flux recipe notes from a large gold mine

I'm not sure if this subject material was discussed in other posts, but I could not find it recently, and certainly not a couple years ago when I started up an assay lab and refinery at a new gold mine. I talked to an old refiner and took notes as summarized below: This is apparently common knowledge in refining circles, but it was new to me.

I used recipe #3 as we had access to these ingredients. The slag was soft and chalky and assayed almost no gold.

The recommended flux recipes in refining are:

1) Mostly gold concentrates, heating at 1980F:
2 parts Borax
1 part Sodium Carbonate
1 part Manganese Dioxide
1 part Silica
1 part flourospar
1 part lime – Ca(OH)2

2) Table concentrates
3 parts Borax
1 part Sodium Carbonate
1 part silica
1 part Manganese Dioxide

3) Recommended for us as we are beginners, but with table concentrates composition unknown:
For 100 kg of clean table concentrates, add:

30 kg borax (liquefies the charge better)
10kg niter (acidic and tends to eat crucibles, and this is the ingredient you would vary depending… a given range is 10-20 kg - more niter allows the charge to heat up faster, but it promotes boiling and off gasses badly and lowers crucible life)
5 kg Silica

Blend the ingredients in buckets.

If there is a bit of mud in the concentrates, add a bit more niter, up to 20 kg maximum to clean up the dore. In general, use as little niter as possible as niter is the ingredient that eats up crucibles. At one mine, a crucible in the electric furnace lasted 18 pours, but crucible life is up to 40 pours now.

With a new crucible, condition the crucible with old slag. Fill the crucible with cold slag and heat it up slowly to 50% temperature. As the slag charge melts, add more and more slag until full. Pour off the old slag, turn off the fuel, and allow the crucible to cool somewhat. Pour the charge into the hot crucible and turn the flame back on. Slowly heat up the charge so the charge does not bubble and smoke does not erupt. Keep a metal rod handy to poke the charge to allow a path to prevent gasses blowing upwards through the charge.

As the charge melts, this full charge will melt to ~50% of its original volume. Keep pouring additional charge in as it melts, if desired. Be aware the charge volume will increase upon fusion. The rate of heating he was talking about was 10% of full temperature for half an hour, and an additional 25% temperature each half hour. Once the charge starts to liquify around the edges, bring up the temperature. More time is OK and is recommended until we get a bit of experience with our blends. The downside to more time is decreased crucible life. Keep the charge heated an hour after it liquifies and keep at ~2000F for 30 minutes. Look for a glow in the center of the charge before pouring - this is the niter burning off something.

Note: With a diesel fired crucible, measure the outside of the crucible for reference. When the crucible is down to 1/2" to 3/4" thick, replace it. After I left this mine, the "refiner" tried to use the crucible one too many times and spilled several hundred lbs of gold on the floor. One of the lab techs was tasked with cutting up this mess with a hacksaw.

Note: Niter tends to eat rings out of the crucible, so vary the charge volume to minimize eating out the crucible at one elevation.

Note: Make sure everything going into the crucible is dry. NO moisture allowed. Use a watch glass on cons to check for moisture before blending.

Note: Wear protective clothing. This should go without saying, but the refiner emphasized it. He does not want to see anyone get hurt.

Old slag goes not off gas. Off gassing is bad [due to blowing part of the charge out the crucible?] It is important to save old slag for this purpose of conditioning a new crucible.

Hello Blingmaker and welcome to the forum.

Thanks for this very informant first post. I don't think I'll ever need to use it as I've only worked in grams and never with concentrates from mines but I always enjoy reading new stuff.

I really liked the trick of using old slag for preparing a new crucible for the first melt. That could be useful even for hobby sized operations as mine.

Göran

Flourospar in the first recipe seems way too high, for normal flux blends, it usually only takes a little bit to dissolve the silica, or ceramic refractory type materials... and even that begins to eat up the ceramics of crucible (which can be drastic with overuse).
(This lead me to believe that ore from that mine had a lot of that type of that material in their ore).


fluorspar would be harder on most crucibles (silica, or refractory type materials) than the nitrate would be.
This may be the reason for the high lime content in that recipe.

Manganese Dioxide and nitrates are oxidizers to help oxidize the base metals so they report into the slag.
There are many different House blends of flux recipe's, these are normally used as starting points, and are then the ingredients of the house flux is adjusted, through experimentation, sometimes only minor adjustment may be needed. other times drastic adjustments or ingredients may be needed, for example the oxidizer (nitrates, Manganese Dioxide...) may need to be replaced with a reducing agent (charcoal carbon, flour, sugar...) or even a collector metal added with the reducing agent... each ore may need a completely different recipe.

In assay they will test the ore (using lead oxide) in a pre-assay melt to determine if the ore needs a reducing or oxidizing agent (indicated by how much lead is reduced, or kept oxidized...).
How well the flux "flows" or becomes liquid is an indicator of what may need to be adjusted in the recipe

A house blend recipe may be a good starting point, to begin experimenting on but normally will not be the recipe needed for best results (unless the same material (same ore or same sweeps...) as the flux recipe was made for.
Understanding what each ingredient of the flux does, its purpose, or how it reacts, is the way to be able to make up the flux recipe, or adjust it, for the material at hand, that and through experimentation.

The House blend flux recipe for one large gold mine, may be completely useless for another gold mine with a different ore.... although many basic starting flux blends may have the basic ingredients components, to get started making up your own recipe for your ore...
Like making bread most use some type of flour or grain meal, grease, animal or vegetable oil, and water as basic ingredients, but from there the recipe can change drastically for different types of breads or different cooks recipe's...

Thanks, Butcher, for expanding on this topic. Sorry, forgot where I had this topic saved.

Of course you would experiment in the assay lab to come up with a good working mixture, but you need to start from somewhere. I dont' recall seeing recipes like this in either the Bugbee or Furman texts on fire assaying.

I used recipe 3 as a starting point as we had pallets of these ingredients of recipe 3 in a seacan, and not much Manganese dioxide. I ended up using a fairly small amount of nitre as I was experimenting with really clean gravity gold. I loved the soft free flowing slag with a very low gold assay. Perfect for our needs.

One of the fluxes I was trying to make a blend of before was hard as granite and fairly viscous when pouring. I don't recall the blend, but I was barking up a wrong tree with this recipe.

I'll stay out of the lab and stay in the mill from here on out... I'd rather be milling than assaying.

Yes, these are severe, aggressive (and polluting*) fluxes for challenging materials.
I would use these only after trying others and milling it well with the troublesome feedstock.


*Using a flux with 20 kg of nitre like that will be very unpleasant and likely illegal in many jurisdictions. It decomposes to NOx when heated to high temperatures.