lazersteve said:
Harold would be the best member to answer this one in my opinon.
Real simple. Platinum and palladium refuse to precipitate from dilute solutions. Keep them as highly concentrated as is possible for the best results. Someplace in Hoke's book you should read a statement to that effect. It's been years, so my memory has dimmed.
This is something you can prove for yourself. Dissolve a small amount of platinum, then split it in halves after filtering. Leave one half well concentrated, then dilute the other by several volumes. Introduce ammonium chloride and tell me which one works best.
As for gold precipitations, if the solution is concentrated the gold will literally plate out onto the vessel when the SMB is added. For this reason I always dilute my auric chloride 2-3 times over with water before adding SMB.
Steve
That doesn't often occur if the vessel is very clean, although I have seen the entire surface covered as if it was plated. After precipitation, a sheet would peel off the surface, aided by a plastic policeman.
My solutions were heavily concentrated when I re-refined---I started with about a liter of gold chloride that contained roughly 17-19 ounces, then added enough ice to the 4,000 ml cylinder to bring it near the top. SO2 was then percolated through the solution, with the gold precipitating progressively. When the operation was finished, the 4 liters of solution were quite warm, and the gold was down.
I think if you use this bit of logic, you'll know which metal should be precipitated first. It all revolves around drag down, and your ability, or lack thereof, to wash the precipitant.
Gold powder can be washed endlessly, so you can, for all practical purpose, eliminate drag-down of other elements. That isn't true for the platinum group, which is precipitated as a salt-------which is very soluble in water. Washing the salts is not easy without dissolving them. I strongly advise that gold be precipitated before platinum------and considering that the process for palladium begins with ammonium chloride, platinum will come next, with palladium being last.
Harold
