Un-oxidized iron (elemental iron) in a roasting process (or smelt), of pyrites can be very beneficial at removing the sulfide that traps your values, the elemental iron has all its electrons, and readily chemically gathers the sulfides from the ore, on the other hand, the oxide of iron, or Ironsulfides, will not help in this conversion.
When smelting ore the chemical make up of ore is important to know if possible, and how flux and additives react chemically.
When I roast or incinerate ore, ore a recovered salt from one of my leaching or recovery processes, I actually go through a fusing phase,
say I have a damp salt or an ore, putting it on low heat till it dries out keeping heat low and a cover over it, it can create gasses that bubble up and pop splashing my values all over the place, during this drying process a chemical reaction can be occurring, more volatile gasses and may vapor off converting some of the salts in solution, once it dries, it will get hard and can form like a rock, I usually break it apart and crush it before completely dry,
now I can raise my heat without the gasses splattering my values all over the place, although I will still have metal salts with gasses that will evolve, now under this higher heat, and uncovering to expose to air I will still drive off gasses some of the less volatile ones, these gasses are from the acids that formed my rock salts, or metal salts, as these gasses leave the metal salt the metals become oxidized by the oxygen in the air,
Also any carbonous materials will break down and start to give off carbon dioxide gas, the ore, or recovered salts are still making chemical changes, and the oxides usually darken or go towards a blackish color, depending on their makeup they can take on several colors during these processes.
then these fusing salts (remember METAL + ACID = a salt of that metal), they will seem to start to melt again, the more volatile gasses escaping and forming acids or bases in this fusing stage (looks like melting to me but is not), here again allot of chemistry is going on, it can get liquid again and start bubbling and splattering so a cover may again be needed.
with ore or even recovered salts, making additions of chemicals (like flux or metals), can help you change your ore or recovered salt chemically, at this fusing stage and even before the smelting stage of recovery, or the leaching stage of your recovery process, example I may be wanting to leach this ore with a chlorine leach (and know it is low on silver content) so I may cover this ore with table salt bring up heat and stir and keep heat for awhile, this can convert some of my fusing metals into chlorides, helping me save chemicals in my later leaching stage and also help to convert some of the harder to convert metals, heat can be a big part of the chemistry equation.
a note here during this fusing stage I can also be volatilizing some of my metals, and they can be vaporizing off with these acidic gasses, if I see a yellow gas or a yellow acid like syrup on the cooler portion of my fusing dish, I may be vaporizing off my gold, here I would lower my heat, also many of these acids and metal vapors coming are very deadly to breath, and we do not want to change the chemistry of our lungs and end up on a respirator in the hospital, or buried 6 foot under in a pine box, cannot get much use out of gold that way.
Now my fusing salts will give off most of their gasses, and these salts will start to dry out again, still chemically changing, and more metals exposed to air have a chance to oxidize.
roasting or incineration or fusing can convert our product chemically, and by adding and controlling conditions we can get metal values that would otherwise be lost or not recovered in our later recovery and refining processes, Why else would one of the master refiners on this forum spend his time preaching this to us?
I am not saying James is wrong about the black sand being oxidized already, and the sulfides converted by exposed back sand, but I would always roast any ore or black sands. Sulfides can lock up metals chemically, and these crystals can be very hard to convert or oxidize even if exposed to the elements in the environment for millions of years, I believe the sulfides can still be locked up in their crystal structure.
fluxing and these pre processing (with heat) measure are almost a chemistry in themselves, as a hobbyist just learning I do not completely understand them, and may or may not be correct in how I see what little I do in this, but for the mining communities around the world for since man has made things from rocks they have also tried to learn it, and have known that it can mean the success or failure of getting metal out of that rock, it does not matter how much gold is in that rock if you cannot get it out cost effectively it will still only be worth rock.
and even today most any professional miner will tell someone who asks what flux do I use he will tell him to experiment their is no magic formula, I think learning as much as we can about the type of ore we have, and how metals and flux react chemically in these heated stages, can help us make better judgments on how we experiment. But pre-roasting, incineration or fusing is almost always practices in professional mining.
looking into assaying can also help us learn from the proceedures they use, and improve our ore or metal recovered salt by pre-treating them before leaching or smelting processes.
Maybe someone else with more experience will comment and give us more clues on which flux and what chemical effects they create in these melts, who knows maybe someone has the secret to a magic flux recipe hidden in their back pocket?
James, I find your process of generating a chloride leach in a five gallon bucket an ingenious process, and would like to try it, but I still believe if you pretreated the black sand with a roasting or fusing process you may recover more value's, but then again I can be wrong it would not be a first for me, as most of the time I get things wrong before I learn, and even then I have alot more to learn.
Traveller, iron being higher in series will still have to be leached from your ore before gold will go into solution, and can also cause problems with your leaching of the gold, untill it is removed from the leach, it is always best to eliminate the base metals as much as possible before extracting the gold, whether in leaching or refining.
silver can always create problems in chloride base leaches, also keeping your gold locked up out of your reach, whether 10% will give you grief here I cannot say, in mining sometime's they preleach to remove silver before they attempt to get the gold, or they would get neither.
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