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Guidelines on flux calculation gold smelting (1 reply)

8 months ago
Collin 8 months ago

I am currently using Borax,  Silica flour, Soda ash and Potassium nitrate,  the sludge contains also copper, I need a guideline on flux calculation to use in order to move the copper or any other Base metal to the slag. 

I would like to know more about flux calculation for gold smelting. I use Borax, Silica flour, Soda ash and Potassium nitrate. Most of my sludge contains Copper, what is the best flux calculation to use say if one have a dried calcine of 95 kg.

Please advice on a formula to use on fluxes for dried gold calcine to smelt in an induction furnace. 

8 months ago
David 8 months ago

The flux is calculated to contain enough lead Oxide (litharge) so that if it is completely reduced, it will produce a prill weighing (usually) 25-28 grams. How do I know how much that is? I’m going to tell you. Ain’t gonna be no more igerunt prospectors out there. Litharge is composed of one atom of lead and one atom of oxygen. We represent it as PbO. Pb being lead and O being oxygen. Lead has a molecular weight of 207 and oxygen 16. Since they are combined the molecular weight of PbO is 223 (see link for Periodic chart of elements). In other words pbO Is 98.2% lead. If we reduce it to metallic lead then for every 100 gr of pbO we will be left with 98.2 gr of pure lead. Does that help? 

Well, it helps me because I know something that you don’t. I know that we want a prill of around 25-28 gr. So I am going to put 26 gr of PbO in my flux. When this PbO is all reduced by the flour/ore it will produce a lead prill of 25.79 gr. That is plenty close enough for government work. If we get a prill that only weighs 18 grams, we know that some of the flour must have been used up in reducing, not the litharge, but the sample. There wasn’t enough flour to reduce both the sample and the PbO. So, what can be done to fix this problem? Simple, we run the sample again and this time we put in a little more flour. How much more? I’m going to get to that but right now my ESP tells me that some brains out there are beginning to overheat and some folks are thinking bad thoughts about me and why they ever started reading this to begin with. I think there are two things that we must accomplish. We must cool down the brain cells and we must adjust the attitude of the reader. I feel this can best be accomplished by a single action. Take a trip to the fridge and recover another (or two) of those cool, soothing, foamy, unguents that I know you have squirreled away. Hey guys, we ain’t playing now. This is "Jet Airline" stuff.

Now that things are back to normal (do I hear muttering out there?), I’m going to give you a recipe for a flux that will, with minor adjustments, work most of the time. I know you aren’t going to set up an assay lab but this is what happens when you send in a sample to be assayed.

This is a good starting flux for quartz or "neutral" ore samples.

  • Ground sample ---------30 gr
  • Litharge (PbO) --------- 30 gr
  • Soda (Na2CO3) -------- 30 gr
  • Flour ------------------------ 2 gr
  • Silica Sand -------------- 10 gr

The amounts used are not correct for all ores. If your prill is too small, add more flour. I happen to know that 1 gr of flour will produce about 12 gr of lead from the litharge. So, if your prill is 10 gr too light you should add another gram of flour to the next assay. It’ll be close enough. Another thing that can go wrong is that sometimes the ore produces a melt that is too thick and viscous to pour properly. Could probably add more sand or, more effective, would be to add a couple of grams of Borax. Borax produces a thinner, more liquid melt. If borax is used you should be aware that it attacks the clay crucible so don’t use so much that you get a hole in it.

I should also mention that molten Litharge will dissolve the crucible and the firebricks that line the furnace. That is another reason for the quartz sand in the flux. It protects the crucible.


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