This is an good informative thread so I wanted to explain some of what I said and what Lou added in a little more detail so there is no confusion. And, as always, discussion is good.
Years ago, like 30, I had an excellent relationship with a gentleman who was the head chemist for a major dental alloy refiner. He told me that since most refiners were a bit under equipped on the analytical end, he had to come up with a methodology that would work with dilute as well as concentrated solutions. Ironically small refiners have the same issues today and this would apply to our members as well. He called his method bulletproof because it would yield the highest yields regardless of concentrations in solution.
Some of his methods are because of what Lou mentioned in his post above.
He did not affect a more complete precipitation because his Pt was not all Pt(IV) but some was Pt(II) from the gold reduction process.
First and foremost this method recommends dropping the gold from the aqua regia dissolve first, and any reducing agent the refiner was familiar with could be used, because all they were dropping from this acid was gold.
Once the gold was dropped, the solution was cemented on slabs of copper. This was done for a few reasons, one was to allow the refiner to work with a smaller quantity of acid, and the second was to drop any remaining traces of gold with ferrous sulfate.
Back when I was told this method, the nitric was removed by the classic 3 times evaporation procedure. Urea was never used because platinum and palladium have a definite ability to form nitrogen compounds; which will affect recovery. Since then, the use of sulfamic acid for de noxing solutions has replaced the need for the lengthy classic evaporation method.
Another benefit of cementing the values allowed the small refiner to get some idea as to how much metal they were looking at. The cemented values were roasted (which also serves to eliminate any PdO which is insoluble in aqua regia) and weighed.
As Lou said, the quantity of Ammonium chloride Geo added is un-necessary but in the bullet proof recipe that is what is called for. If a refiner is more analytical (my kind of refinery) the ammonium chloride (or potassium chloride) can be added at the rate of 0.6 grams per gram of expected platinum. However, an excess doesn’t hurt.
The roasted powders are digested in aqua regia taking care to use as little nitric acid as possible. The acid is filtered and de noxed, and the volume of the solution is noted. Next the gold is dropped with ferrous sulfate. Since this powder should have very little gold, it is dropped from the solution while it is as concentrated as possible.
Now the gold is filtered out and the solution is diluted with 3 times the original volume of water. To this solution, a volume of saturated ammonium chloride (or potassium) is added equal to the starting volume before dilution. Now the platinum drop will be more complete. This was never done hot or even warm for that matter but heat shouldn’t hurt and may speed things up.
The platinum salts are filtered out of the solution and the remaining solution is treated with powdered sodium chlorate by slowly sprinkling it over the surface. The red palladium salt will drop.
I have always done this at ambient room temperature, as heat was not suggested in the instructions I received. In light of what Lou said about trichloroamine formation, I would say heat would benefit by allowing the gasses to escape the surface of the reaction much easier.
The above method has proven to be effective for refiners without the ability to perform assays before processing so they could add closer to the stoichiometric quantities needed in these reactions. And while I have never experienced what Geo did, I would add some heat to the drops. It can’t hurt and especially for some refiner in an unheated barn up north it is better to be safe.
Finally, Lou said;
Please no weighing of dried platinum salts if at all humanly possible to avoid it
I can understand this and completely agree that handling platinum group metal salts can have terrible consequences and exposure of exposed skin or inhalation of the dust can result in Platinosis.
https://en.wikipedia.org/wiki/Platinosis
But small refiners often pay on PGM's by out-turn, meaning they have to collect the salts and weigh them to quantify the yield for settlement purposes. For most small jobs, from jewelry lots or sweeps, or small dental lots, this is rarely in excess of a few ounces.
I do prefer to weigh the dried salts on tared filters and collect them until there is sufficient quantity to reduce the salts to metal. I do this in a good hood (with good air flow in excess of 100 CFM/sq ft of hood opening) and I wear gloves, long rubber sleeves, an apron, and a respirator. I encourage those who do this to keep dedicated protective equipment for this work and wash it down after each use.
Fortunately, most of our members are not needing to pay a customer for the PGM's as they are recovering stock pot values. These values can be reduced to metal by whatever method you choose without having to dry them out and can even be stored damp to accumulate until you have enough to process further.
