Stumped

[jsargent]

I did a copper/ammonium thiosulfate leach last week on some magnetite ore with perplexing results. The ore is finely ground, highly magnetic iron ore which fire assays at 20+ ounces per ton gold. A 5 gallon bucket weighed a little over a hundred pounds so the ounce or so of gold in it was worth retrieving. In any case I used an ammonium thiosulfate leach, with a few ounces of added copper sulfate at pH 10.2, filtered the lixiviant and precipitated all the dissolved metal with sodium borohydride. After rinsing and drying I had just over 300 grams of dark metal powder. I mixed this powder with 200 grams of silver powder and melted in a flux of borax, soda ash, and cornmeal. Poured to shot (cornflakes) which were characteristically black from the copper I had added.

Things were proceeding normally at this point...

After digesting in 30% nitric acid and decanting off the copper/silver nitrate fraction, I ended up with slightly less than 300 grams of gray-black residue and cornflakes which had retained most of their original shape. This residue is not soluble in any strength of nitric or hydrochloric, tested individually. It is non-magnetic. Under 40X microscope much gold is visible intermixed with darker, shiny mystery metal. PGM's perhaps?

Are there any base metals that could have been dissolved by ammonium thiosulfate at a high pH but are acid insoluble? I'm annealing it now at 1500F to get rid of any residual nitrate. Suggestions?

 

[Harold_V]

I did a copper/ammonium thiosulfate leach last week on some magnetite ore with perplexing results. The ore is finely ground, highly magnetic iron ore which fire assays at 20+ ounces per ton gold. A 5 gallon bucket weighed a little over a hundred pounds so the ounce or so of gold in it was worth retrieving. In any case I used an ammonium thiosulfate leach, with a few ounces of added copper sulfate at pH 10.2, filtered the lixiviant and precipitated all the dissolved metal with sodium borohydride. After rinsing and drying I had just over 300 grams of dark metal powder. I mixed this powder with 200 grams of silver powder and melted in a flux of borax, soda ash, and cornmeal. Poured to shot (cornflakes) which were characteristically black from the copper I had added.

I'm curious why you melted the recovered powder.
The sole purpose for inquarting is to eliminate residual silver, so the values can be dissolved in AR. In this case, you already had a finely divided powder, which would have lent itself perfectly well to processing without melting. My personal choice would have been to incinerate the powder, then do a preliminary dissolve in dilute nitric, followed by a digest in AR. That would have served the purpose perfectly well.

I have no clue what is going on, but I expect something is not as you think----there's no obvious reason, at least to me, for the remaining cornflakes to not dissolve. The sole exception would be if there is sufficient silver remaining. preventing dissolution by a hard crust of silver chloride forming. If the corn flakes assumed a rather unusual gray/green color, that may well be the problem. I'd suggest you add more silver, re-melt and pour cornflakes once again. It's important that you achieve a 75% base metal/silver content in order for complete extraction to take place.

Harold

 

[jsargent]

Harold- Thanks for responding. I added silver to the leach residue as I suspected there was considerable silver in it already, along with copper, but not enough for a clean parting. I based the amount I added (200g) on the projected amount of gold in the residue, based on the fire assay (40g-50g). I'm suspecting (hoping) what may have happened is that there is actually far more gold than 50g in the residue and the 200g of silver I added was just not enough for a clean inquart. The more dismal answer is that the ore had a lot of something in it like titanium, tantalum, chromium or something else than would (in theory) alloy with silver, but not dissolve in nitric. The cornflakes are sort of an iron-gray color intermixed with flecks of gold under the microscope (prior to annealing). Now if I end up with 5 or 6 ounces of gold instead of just one, I'll be in tall cotton!
Thanks for your help.
Jeff

 

[Harold_V]

Harold- Thanks for responding. I added silver to the leach residue as I suspected there was considerable silver in it already, along with copper, but not enough for a clean parting.

That, of course, is bad logic. When you precipitate values, they may be combined, but they are finely divided, so they can be easily digested. Once you melt, the rules change. I strongly advise that you do not melt in the future, but incinerate, to kill anything that isn't wanted, then go directly to a dilute nitric dissolve. Any copper or silver that is present will be readily dissolved, eliminating the problems you are having.

The more dismal answer is that the ore had a lot of something in it like titanium, tantalum, chromium or something else than would (in theory) alloy with silver, but not dissolve in nitric.

Proper inquartation will solve that riddle, too. If the percentage of silver is high enough, it will prevent the unknown element from protecting the silver. The negative is that if you use too much silver, instead of ending up with pieces, the material will revert back to powder (that's where you started). It will process perfectly well that way, but it's much easier to lose some of the values when they are finely divided, for the often don't settle rapidly.

The cornflakes are sort of an iron-gray color intermixed with flecks of gold under the microscope (prior to annealing).

I'm inclined to think you're seeing silver chloride. It looks totally different than that which forms and sloughs off. If you find you can boil the material in AR for a prolonged period of time, with little to no effect, I'm of the opinion that is what is the cause. If you still have samples, it would be interesting to see if the flecks of gold you spoke of are pale yellow to light green in color, and if they appear to be in pits. That is a sign that silver chloride has chipped off the piece, exposing the alloy beneath.

Luck. Keep us posted.

Harold

 

[jsargent]

Good points Harold. I re-inquarted this time and instead of pouring directly to shot I poured to an ingot mold. I then broke away the slag and found the silver/gold cleanly deposited at the bottom and the mystery metal was adhered to the top of the silver, but not alloyed with it. It took a considerable amount of hammering to dislodge it but it finally broke off in pieces. It looks like iron but totally non-magnetic. Odd. In any case I re-melted the silver, poured it to shot and it's dissolving into a very gratifying pile of black gold residue.

Thanks again for your help and advice!

Jeff

 

[Harold_V]

Good points Harold. I re-inquarted this time and instead of pouring directly to shot I poured to an ingot mold. I then broke away the slag and found the silver/gold cleanly deposited at the bottom and the mystery metal was adhered to the top of the silver, but not alloyed with it. It took a considerable amount of hammering to dislodge it but it finally broke off in pieces. It looks like iron but totally non-magnetic. Odd. In any case I re-melted the silver, poured it to shot and it's dissolving into a very gratifying pile of black gold residue.

Ok, I think you just solved the mystery. I hope you haven't discarded that brittle material.

What you had was a sulfide layer. You should put it back in a crucible and heat it to melting, then insert some scrap steel (a long piece of rebar works well---and provides an end that can be grasped with tools, for removal). A little borax and soda ash should be included. After a soak of several minutes, remove the scrap.

The material, as recovered, may still contain some of the values as a sufide. By providing some scrap steel, it will be replaced, liberating silver and gold. The end result will be a much finer grained sulfide layer, and, if there's any values present, a small button. I suggest you pour such material to a cone mold, instead of an ingot mold. That presents a tight button with a small surface area, from which the sulfide layer is much easier to dislodge.

You can trust me on this one. I've been through that very thing on several occasions. You might be pleasantly surprised at the amount of value recovered. Or not! 8)

Harold

 

[jsargent]

Awesome! I'll give it a shot. There was several ounces of the sulfide material and it's definitely heavy so maybe more Au 8) Thanks again!