butcher said:
Copperas (ferrous sulfate) (FeSO4), easy to make works good if PGM in solution, will precipitate gold and not platinum, (SO2 or SMB can carry down PGM), I have not heard of trouble with Iron being dragged down as stated above, I believe the iron should stay in solution, (I would like to learn more about how the iron would be dragged down).
Refining twice using a different precipitant than was used in the first refining will greatly improve the quality of gold.
From above we see copperas would make a good choice for the first refine if PGM was in solution, and SO2 the second refining.
Butcher, I have recently started using ferrous sulfate for the first refining, and SMB for the second refining.
The gold powder from the second refining comes out looking real good, that nice light colored tan look, and the gold always tests three nines via XRF.
I did notice that the barren solution (tested with stannous chloride) from the second (SMB) refining was tinted yellow so I did a third refining using SMB and the barren solution was then colorless. But before this third refining I put a few drops of the yellow barren solution in a spot plate and added a crystal of ammonium thiocyanate.
Sure enough it turned red indicating the presence of iron. If the pure gold powder was not properly washed free of this yellow solution then wouldn't this cause a slight bit of iron contamination in the gold? Of course it would not be "drag-down" of the iron, but if ANY of the yellow tinted iron solution remained in the gold powder, due to poor washing, and the gold powder melted, then the iron would report in the gold, correct?
The reason I bring this up is because I turned in some gold once, precipitated with ferrous sulfate, and the tech said that the gold had some iron in it.
Also, I use copperas from the hardware store. I dissolve a couple of pounds in a little hot water to make a concentrated solution, and filter. THERE ARE BITS OF IRON IN THE COPPERAS that filter out as well as other solids. If one was to try and add the dry copperas, right out of the bag, then these bits of iron would contaminate the gold powder for sure.
I then pour the filtered concentrated ferrous sulfate solution into a large PYREX dish and let it evaporate to a hard green solid.
I keep the solid in a covered dish until I need it. To use, I put some of the green solids in a beaker, add distilled water and a little hydrochloric acid, then heat it on a stirrer until it dissolves.
Thank you,
kadriver