separating pgm powder from each other(pd,pt,rh)

[arthur kierski]

i use ar and heat the solution many times(3 times) and add hcl every time to eliminate the nitric----when this is done, i filter the solution and in the filter paper stays the rhodium----the rhodium left in the filter paper is fused with nahso4 at 500centigrates and then passed thru boiling water---here a problem appears:for example ; 8grams of rhodium powder dissolves and becomes (left)3,9grams-----the 3,9grams left is treated again with nahso4 and after the proceess ,still 3,5grams is left---i suspect that the powder left is pdo or silica dissolved in the extraction of the pgm from catalitic convertors-----can any one sugest what is the powder left?had anyone had the same problem?
thanks for any help
Arthur

 

[lazersteve]

If you are using powdered zinc in your process be sure it is free of Cab-o-Sil (SiO2). Test a small sample of your powdered zinc by dissolving in HCl. If you have a fine brown sediment remaining it contains Cab-O-Sil as a shipping stabilizer. I switched to zinc turnings for this very reason. I sell the turnings at my web store (link below).

If it is Cab-O-Sil it should slowly dissolve in a solution of boiling lye. Be very cautious with hot caustic solutions as they can be more dangerous than hot acids.

Steve

 

[arthur kierski]

Steve,i use powdered iron for all my precipitations and then clean the precipitates with 10%h2so4(hot) and then plenty of hot water (until the solution is clear from iron sulfate--which i test with nh4oh)---
Arthur
the powder left might be silica from the cat substrate-----i will try to dissolve it with a bit of hf(very carefully)--just to make sure

thanks again
Arthur