Precipitating PGMs from Aqua Regia

[Bentfunky]

I am quite comfortable testing small samples of concentrates for gold using aqua regia dissolution and stannous chloride test. This requires only a few mls of acid.

Today, I encountered a new twist, concentrates that appear to test positive for platinum with stannous chloride tests. Not sure how convinced I am by test results, so I decided to drop PGMs from solution to validate test. I'm far less familiar with PGMs, so just wanted to run a couple questions by the forum.

Not being terribly familiar with PGM processing, I consulted the great and wise Hoke. After filtering, neutralizing nitric, and evaporating liquid to a highly concentrated solution, I added ammonium chloride solution a little at a time. I immediately had dark granuals of material precipitate out of solution. This was followed by a yellow precipitate, which is ostensibly platinum. Solution will sit for a few hours to allow all precipitate to settle out. I'll go back later tonight to pick up where I left off.

Questions:
1. Concentrated solution likely contains a little gold, some silver, perhaps PGMs and some base metals. Will ammonium chloride precipitate base metals in addition to some of the PGMs?

I've read that ammonium chloride will likely not precipitate base metals. I've also read that it will and the chances of it doing so increases with amount of time it remains in solution.

2. Related to first question, what are possibilities for dark material that first fell out of solution? According to literature, Rh and Ir will precipitate out with platinum if present. Will precipitate from these other PGMs be similarly colored to Pt or will they be dark?

3. I am seeing an unexpectedly large amount of yellow precipitate given small sample size. Might this material be something else such as lead (not that I anticipate lead in prepared sample)?


Update:
I believe that it's possible that solution also contains palladium, but I need more information before attempting to precipitate it. Back-burning Pd for now. Hoke and others indicate that sodium chlorate can be used to precipitate it. Sodium chlorate is terribly dangerous and I am not inclined to mess about with it. DMG is also listed as an option, but this compound is rather expensive. I've seen articles and videos that use chlorine gas. Again, a bit too dicey for my taste.

I've seen references to using bleach as chlorine source and, in practice, I've seen HCL (incidentally) precipitate a bit of what appears to be a palladium compound. However, I'm looking for more details in this topic. Would appreciate it if someone cohld point me to additional literature on this topic.

 

[Geo]

Starting material and pics of what you have now may get better answers.

 

[snoman701]

Sodium chlorate is just the source for chlorine gas, which is what causes the ammonium hexachloropalladate to precipitate. You can also sparge chlorine via chlorine gas generator using pool shock granules and HCl in an equalizing funnel for a more controlled reaction. Either way, it's the chlorine gas and safety concerns regarding chlorine are warranted.

But this tends to not work as well for low grade palladium concentrations. You are probably better off using atomized copper (or a copper busbar and agitation) to first precipitate the PGM's so that you can re-dissolve at greater concentration. Plus you can redissolve using chlorine gas as the oxidant instead of nitrates, so you don't have to cook it down to a syrup.

 

[Bentfunky]

Sodium chlorate is just the source for chlorine gas, which is what causes the ammonium hexachloropalladate to precipitate. You can also sparge chlorine via chlorine gas generator using pool shock granules and HCl in an equalizing funnel for a more controlled reaction. Either way, it's the chlorine gas and safety concerns regarding chlorine are warranted.

But this tends to not work as well for low grade palladium concentrations. You are probably better off using atomized copper (or a copper busbar and agitation) to first precipitate the PGM's so that you can re-dissolve at greater concentration. Plus you can redissolve using chlorine gas as the oxidant instead of nitrates, so you don't have to cook it down to a syrup.

@snoman701, great info. I really Ike the copper suggestion. Hadn’t considered it before.

I’m less than thrilled that chlorine gas is a necessary evil. I’ll have to spend some time upping my safety game. Exhaust fan isn’t going to meet the need WRT chlorine gas ventilation, at least for me. Was exposed to a small amount as a kid. Left a lasting impression.

 

[Bentfunky]

Have everything set up for a redo using a flattened piece of copper to cement out PGMs if present. Sample was placed in warm AR for a couple hours. Filtered out liquid and concentrated by heating/evaporation. Used a little formic acid to verify solution had been denoxed. The liquid was cranberry red before concentrating. It was deep blood red afterward and crystals had started to form. Dissolved crystals by adding a bit more HCL.

Really having a tough time dissolving a couple tablespoons of sample material. I’m using warm AR, so I’m a bit surprised. Repeated above process with fresh AR on sample material that remained. Barely made a dent in remaining sample. Could have kept adding nitric but figured I probably had enough in solution for the purpose of this test.

Did stannous chloride test on some dilute AR solution that was collected when rinsing undissolved sediments. Got a yellow-pink result. Was a bit cloudy. Encouraging? Perhaps.

Everything is cooling down for the night and will pick up where I left off tomorrow.

The test material is dark-silvery color and ferromagnetic. Same even after a few hours in warm AR. Curious. Took a close up of post-AR sample material. Hard to focus on small shiny stuff, apparently. See below.

 

[Bentfunky]

@snoman701, Cementing out values with copper was definitely the way to go! Not the fastest process in the world, but best way ever to extract precious metals from solution containing base metals.

Didn't have atomized copper on hand, so used a flat piece of copper. Still waiting for the cementing process to finish. If the black material forming on the copper is one or more PGMs, there's more than I anticipated in my small test sample. This would validate my earlier stannous chloride tests.
Again, thank you for the great suggestion!

Update: the following pic shows the precipitate from cementing values with copper. Starting test material was approximately 2 tablespoons in volume. Whatever this material is, it dissolves very slowly in AR. Hence, the below pictured precipitate isn’t much more than originally dissolved. (Rest of test material is still sitting in AR. Have less than half of it in solution so far.)

You are looking as bottom of mason jar.

 

[jadedalex]

Bent, you write:

I've seen references to using bleach as chlorine source and, in practice, I've seen HCL (incidentally) precipitate a bit of what appears to be a palladium compound. However, I'm looking for more details in this topic. Would appreciate it if someone cohld point me to additional literature on this topic.

The Library on this site is second to none. I would suggest you stop and read before further attempts. Consider this adventure as lesson #1. If you want to call yourself a "Refiner" than stop mixing charges. You are trying to refine. Garbage in, Garbage out.

 

[nickvc]

Most PGMs are fairly resistant to dissolving in AR even hot AR so much of what you are hoping to dissolve may well be in your sample, if I remember correctly you will have better dissolution using either chlorine crystals or hydrogen peroxide with HCl, read more posts in dissolving PGMs here on the forum for confirmation.

 

[Geo]

What Nick said. They use catalyzing metals in catalytic converters to convert NO2 to nitrogen and oxygen. The catalyst has to be able to withstand the oxidizing effects of NO2. A catalyst initiates a chemical reaction without being effected by the reaction itself. In a AR solution, catalytic metals do the same thing. It breaks the bonds between the nitrogen and oxygen effectively destroying the NO2 in the process. Very little of the metal is dissolved using a huge (relatively) amount of acid. This is why chlorine is a better etchant for PGM's than AR. The catalytic metal does not effect chlorine but chlorine does dissolve all of the PGM's.

 

[cosmetal]

As Geo asked, and if you don't mind divulging, what is the source material?

Peace and health,
James