The nitric is an oxidizer, it is needed to dissolve (oxidize) the metal, but if used in excess, there will be HNO3 left in solution of the precious metal chlorides, when the precipitant is trying to reduce the metal chloride back to metal, but if free oxidizer in solution it cannot, the metal is again oxidized back to chlorides with free HNO3.
Oxidize to take electrons.
Reduce to gain electrons.
Using minimum nitric to begin with is one way to insure no excess nitric after metal is dissolved.
Adding more metal to consume nitric either while dissolving, or in an evaporation process is also a good method to use.
Sometimes for just a slight excess nitric a precipitant reagent will consume the small amount (by adding excess reagent).
Evaporating off the free nitric with three (or more) evaporation’s (below the boiling point of solution) adding HCl to keep wet or from crystallization or cooking salts (with gold a little H2SO4 can help with keeping crystal’s from forming, or from cooking them).
With gold sulfamic acid can be used to rid free nitric acid from solution (this forms sulfuric acid in solution which does no harm), but I am not sure how this would affect PGM's as they may form sulfates (I have not looked into this).
Some suggest Urea (it can cause complexes with PGM, create more salts), my study show’s it could also be dangerous, even with gold, and can add other impurity’s into the mix (I never recommend using it), except maybe in a test of free nitric, but even for this I find formic acid to be good test for nitric acid observing brown fumes formed when added.
You have a problem here.
I would consider several methods and choose which may work best with what I had.
evaporation, It could even form salts, but not cook salts, (these can be re-dissolved) Adding HCl to get salts back into solution (using minimum HCl as it is 68%water), and evaporate again repeating until excess nitric is removed, then dissolve salts with heat in HCl concentrate and to precipitate.
Or if you had more platinum, or palladium you could add it and heating to consume the free nitric acid,gold could also be used and recovered but it can add another metal contamination to deal with later.
Another option is to cement values with zinc, then you could refine the powders again with minimum nitric in your aqua regia, or just wash recovered values the way they are what ever you chose to do.
There are other options (but these are what I would consider).
