To Inquart or Not to Inquart

[RickRag]

Hi all,

I'm new to the forum and have a question or three. First, a little background. I'm a dentist who has been practicing for over 40 years. I majored in Chemistry and Math in undergrad but it was mostly organic/biochem so my inorganic knowledge is rusty. I have read Hokes book three times now and perused the dental scrap topics available on this forum. A lot of good info here and I soaked it up like a sponge.

I have about 19 ounces of dental scrap that I have accumulated over the years, most of which is of unknown composition. My intent is to recover the precious metal present, Au, Pd and Pt. Ag, although present, isn't likely to be worth much, so its recovery is optional.

The entire lot likely is mostly yellow and white gold alloy used in major dental restorations. Also, there is likely to be some non-precious and semi-precious metal substrate as well, as these alloys were used commonly in the 80's and 90's. All obvious silver, amalgam, chrome-cobalt and other base metals have been physically removed. The scrap has been pickled and no ferrous metals or dental cements, tooth fragments, etc. are present. I have not attempted to separate the rigid non-precious/semi-precious metals from the more ductile pieces using physical pounding as there are literally hundreds of pieces and it likely would take days. I'm hoping there is an easier way.

My original plan was to melt down the scrap and create one large ingot free of all of the non-metallic contaminants, SiO2, B, feldspathic porcelain, etc using a borax flux. Then inquart the resulting ingot with Ag and create cornflakes for further refinement. Without knowing what the percentage of gold, silver, and other base metal content is, I wouldn't know how much silver to add, therefore I considered getting a XRF test on the ingot to help determine that prior to inquartation. This method has its disadvantages as Harold_V has pointed out that stratification can occur in solidification of the ingot and that any assay returned is likely to be less than accurate.

Question #1 - Is the xrf assay accurate enough to be able to use that data to assess the amount of Ag required? If so, then...

Question #2 - Is inquartation using the method outlined above likely to be the easiest way to isolate the precious metals and recover them using conventional methods, i.e., AR and SMB or oxalic acid, ammonium Chloride, DMG etc? If so, then...

Question #3 - In which order should the metals be recovered? using which method(s)?

If the answer to Question #2 is no, then...

Question #4 - What method(s) would be preferred, more efficient and/or desirable.

Thanks in advance for any help. I look forward to learning a lot here..

Regards,

Rick

 

[FrugalRefiner]

First, I have to yell CAUTION! Dissolved platinum is very hazardous! We have had a number of forum members who have either become highly allergic or have died from their exposure. Unless you're set up properly, and you're scrupulous with your lab hygiene, platinum is not the best thing for a beginning refiner.

Question #1 - Is the xrf assay accurate enough to be able to use that data to assess the amount of Ag required?

I've never worried about the exact gold content with dental crowns. I assume them to average around 14k to 16K. Inquartation doesn't need to be exact. While the target is to have the gold content around 25%, it will actually still work from lower than 25% up to around 30%. If the gold content is too high, you may not eliminate all the silver and palladium. If it is too low, the cornflakes will tend to crumble and break down in the nitric leach. Neither situation is ideal, but they're manageable problems.

Question #2 - Is inquartation using the method outlined above likely to be the easiest way to isolate the precious metals and recover them using conventional methods, i.e., AR and SMB or oxalic acid, ammonium Chloride, DMG etc? If so, then...

Yes, with a couple of caveats. First, you don't go directly to AR. After inquartation, you do a nitric acid leach. This will dissolve all the silver you've added, as well as the palladium, base metals, and some of the platinum. While platinum by itself will not dissolve in nitric alone, when it is alloyed with silver some of it will "follow" the silver into solution. Depending on how much platinum is present, you may find some in the nitric leach and some remaining undissolved with the gold. Potassium chloride is preferable to ammonium chloride. DMG produces a very voluminous precipitate with palladium. It's great for testing, but not necessarily the best choice for precipitation.

Question #3 - In which order should the metals be recovered? using which method(s)?

The nitric leach will have separated the palladium from the gold and the undissolved platinum. I would drop the silver as a chloride, then cement the palladium and any dissolved platinum with copper.

I would then dissolve the gold with AR. Drop it with SMB, ferrous sulfate etc. Then cement the platinum with copper again.

The PGMs are very difficult to fully separate. Send them to a professional like Lou.

Dave

 

[RickRag]

Thanks for the reply Dave.

I was able to get theAR to act on the scrap without inquarting, however it took a long time and inquartation likely would have saved some time. Be that as it may, after quie a few days of mixing and heating AR and scrap, I finally have about 2 liters of solution of dissolved gold, platinum, palladium, base metals and possibly some iridium (although dental scrap isn't supposed to contain much). I still had some of the fine gold granulated scrap to process so I threw that into the pot to use up the remaining Nitric, hopefully. The solution was decanted off of the remaining undissolved scrap and sediment(AgCl, etc.) and a little Sulfuric acid was added to precipitate out any lead. Accorcing to Hoke, the next step is to recover the dissolved platinum with Ammonium Chloride however Frugal Refiner (Dave) posted that Potassium Chloride is preferred. Since this is not a nitric leach solution from which I wish to recover dissolved palladium and platinum, I have all three metals, gold, palladium and platinum in a solution of diluted AR that likely has little NOxx left in it. Assuming the NOxx is used up...

Questions...

What is the prefered method and order of recovery?

Accorcing to Hoke, recovery of dissolved platinum should be carried out first with Ammonium Chloride, but Dave suggests Potassium Chloride. Where can I read more about this?

After dropping the platinum, is gold next with SMB or is ferrous sulfate preferred? Will it pull down some of the palladium along with it?

Thanks for all you help.

Rick

 

[nickvc]

Rick Dave pointed out already that PGMs in solution are not the best thing to mess with as the salts are highly toxic, if this was my solution I’d precipitate the gold carefully pour off the solution and rinse the gold powder well several times adding the rinses to the original solution and then cement the PGMs using copper sheet, cementing PGMs can take time and adding an air bubbbler will help speed up the process.
Most of the value you have there will be the gold but the PGMs could provide a bonus but again if it was me I’d take Dave’s advice and send the cemented powders to Lou or another professional who knows how to handle them, if you don’t want the money ask if you could exchange the value for gold.

 

[Yggdrasil]

Hi Dave!
I'm just curious here.
Why is it preferable to use KCl over NH₄CL?
Since NH₄CL is composed of only various gases, wont it bring less impurities to the solution?
Or only impurities that can easy be converted to gas through calcining?

 

[g_axelsson]

As the ammonium ion could under some circumstances form explosive compounds, some people prefer to take the safe route and use alternative methods.

Some threads I found by searching...
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=20658
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=17713&p=179369#p179369

And a classic...
[youtube]https://www.youtube.com/watch?v=kABusu4Qcr0[/youtube]

Göran

 

[FrugalRefiner]

I learned from Lou that potassium chloride is preferable to ammonium chloride. In addition to the possible formation of explosive compounds that Göran mentioned, I believe the ammonium ion can also form some difficult complexes with the PGMs. Given that Lou is an expert at dealing with PGMs, I never question his sage advice on the subject.

Dave

 

[RickRag]

I learned from Lou that potassium chloride is preferable to ammonium chloride. I never question his sage advice on the subject.
Dave

Thanks Dave,

Do you have a reference or post for Lou's advice? At what point would one utilize KCl/NH3Cl in the process? After one drops the gold or before as Hoke suggests? Is using KCl a similar process to using NH4Cl?

Rick Dave pointed out already that PGMs in solution are not the best thing to mess with as the salts are highly toxic, if this was my solution I’d precipitate the gold carefully pour off the solution and rinse the gold powder well several times adding the rinses to the original solution and then cement the PGMs using copper sheet, cementing PGMs can take time and adding an air bubbbler will help speed up the process.
Most of the value you have there will be the gold but the PGMs could provide a bonus but again if it was me I’d take Dave’s advice and send the cemented powders to Lou or another professional who knows how to handle them, if you don’t want the money ask if you could exchange the value for gold.

I dont really need to separate the PGM's from each other as I'm only interested in recovering the values. Sending the cemented PGM's off to a professional such as Lou seems like a very attractive alternative, given the potential risks. If that were the plan, would Nick's suggestion above be the best way to proceed or Is there a chemical method for precipitating the values left in solution? Is cementation the best way to recover them? Are there other methods that might be more efficient or less costly/complicated/time consuming?

Thanks everyone for your input. I'm learning a lot. And it's rewarding as well as fun.

 

[FrugalRefiner]

I learned from Lou that potassium chloride is preferable to ammonium chloride. I never question his sage advice on the subject.
Dave

Thanks Dave,

Do you have a reference or post for Lou's advice? At what point would one utilize KCl/NH3Cl in the process? After one drops the gold or before as Hoke suggests? Is using KCl a similar process to using NH4Cl?

I don't recall the post, but I recall the circumstance. Someone else had mentioned using KCl, and I posted saying that Hoke recommended NH₄Cl. Lou, a man of few words and kind spirit, gently pointed out that KCl was preferable. The rest I've picked up along the way.

One of the disadvantages of the forum is that it's a forum. People ask questions and get answers. Threads get activity and then they fade away. The result is that the information is scattered all over the forum, so only the dedicated few who devote the time to read it all get the full benefit.

While most of Hoke's advice is sound, she suggested some practices that we no longer consider safe. I tried to address them in the introduction I added to the versions of her book in my signature line. There are other cases where we've learned better ways, and that advice can only be gleaned by thorough study. Hoke is an excellent introduction to refining, but as with a college education, reading a single book does not provide a complete education.

Most people would drop the gold first, partly because it represents the highest percentage of the values, and partly because it can be dropped pretty selectively. In different circumstances, you might choose a different order.

I dont really need to separate the PGM's from each other as I'm only interested in recovering the values. Sending the cemented PGM's off to a professional such as Lou seems like a very attractive alternative, given the potential risks. If that were the plan, would Nick's suggestion above be the best way to proceed or Is there a chemical method for precipitating the values left in solution? Is cementation the best way to recover them? Are there other methods that might be more efficient or less costly/complicated/time consuming?

The platinum sisters are difficult to completely separate, even for experienced refiners. Copper is a chemical method for precipitating the values. There are others, but why complicate matters. Copper is a pretty selective reagent, cementing gold, silver, platinum, palladium, and mercury. Practice the tried and true methods until you're familiar with how they work. Then you can branch out into other methods as your experience grows.

It is important to note that as soon as you dissolve platinum, you are dealing with a platinum salt. It is in solution, but that just means it's a salt that is dissolved. Many people make the mistake of thinking that platinum "salts" are only the precipitated, dried powders. Be very careful when working with platinum.

Dave

 

[Yggdrasil]

Thanks.
That makes sense, Potassium is an easy element to clean anyway.
I had aquired an idea that the ammoniumchloride path, was safe with regard of detonation.
But better avoid it all together.

 

[nickvc]

No one has given you a clear answer to your question about cementing your PGMs so let me say it’s easy simple cheap and effective if done correctly but it does need agitation hence the suggestion of a cheap air bubbler to keep the solution moving so it all gets exposed to the copper.

 

[g_axelsson]

I don't recall the post, but I recall the circumstance. Someone else had mentioned using KCl, and I posted saying that Hoke recommended NH₄Cl. Lou, a man of few words and kind spirit, gently pointed out that KCl was preferable. The rest I've picked up along the way.

Can this be the post?
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=17026&p=174494#p174494

Seems like NCl₃ is the risk in this reaction. Quite a lot of famous people have been injured by that substance.
https://en.wikipedia.org/wiki/Nitrogen_trichloride

Göran

 

[FrugalRefiner]

I don't recall the post, but I recall the circumstance. Someone else had mentioned using KCl, and I posted saying that Hoke recommended NH₄Cl. Lou, a man of few words and kind spirit, gently pointed out that KCl was preferable. The rest I've picked up along the way.

Can this be the post?
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=17026&p=174494#p174494

That's a good thread, but I was thinking about Refining catalytic converters.

Dave

 

[RickRag]

I decided to take your advice Dave and Nick and recover the gold and then cement out the Pt and Pd using copper. I filtered out the AgCl and PbSO4 and was left with a fairly dilute solution of Au, Pt, Pd and other base metals. I wasn’t sure if I had sufficiently deNOxx’ed the solution so I took a small sample (50ml) and added dissolved SMB. Apparently I had little or no NO3 left in solution as gold dropped quite readily. I’m left with a pale green-blue solution that contains Pt, Pd and base metals and possibly some gold???



Question: Since the filtrate still contains values, how can I be certain that I have precipitated all the gold? Is SnCl distinctive enough in its color palette to determine that with all three metals in solution?

If there was any NO3 left in solution, although not in sufficient quantity to prohibit the gold from dropping, would all the gold drop given an excess of SMB? Or is it possible that that there is still some gold left that would come down with the Pd and Pt when cementing with copper, which I obviously want to avoid.

I don’t want to proceed with the bulk of the solution if i still need to deNOxx it further.

 

[RickRag]

After posting that last reply, I realized my main reference, Hoke’s book, was noticeably deficient on precipitating gold with SMB and deNOxxing solutions using urea, formic acid and sulfamic acid. I also found the answer to my question about excess SMB in the presence of nitrates. I also discovered that my solutions did in fact have an excess of HNO3, thus requiring elimination to avoid using unnecessary amounts of SMB. Because I’m doing this outside I’m reluctant to use evaporation due to the nearby structures exposed to acid fumes. A chemical deNOxxing is thus preferred. I’ve seen some info on formic and sulfamic acid use but nothing quantitative. Urea use seems to have mixed opinions but the forum members with experience seem to avoid its use. Does anyone have a reference for the use of sulfamic or formic acids? Or urea if my specific method warrants its use? thanks again for all the help.

Rick

 

[Yggdrasil]

Hi Rick.
I think the concensus on the forum now, is to use sulfamic acid.
First and foremost because it actually "destroys" the nitric, turning it to sulfuric acid, which has the added benefit to drop any dissolved lead.
If I have understood it correctly: Add until there are no more fizzing.

I haven't used it myself yet, I mainly use HCl/H2O2 or HCL/Cl.
If needed, I use AR with nitric by the drop, so excess has not bee an issue yet.

 

[RickRag]

I successfully dropped the the gold from the sample solution and now have the remaining aqua regia solution containing palladium and maybe some platinum. Testing with SnCl reveals a blue green reaction as shown in the picture. I now have a copper bus bar suspended in the solution along with an air bubbler to create agitation. Should this solution be heated to increase the rate of reaction? Since the palladiun and platinum salts are to be precipitated out as the metal elements, is the preciptiate toxic? Will all of the values precipitate eventually? Are any precautions necessary before handling this material?

PS> How do I get the pictures to display right side up. I tried saving them rotated 90 degrees but that didn't seem to make any difference.

 

[nickvc]

The PGMs will cement out of the solution as black powders which are metallic so not really any worries handling them and yes they will all cement eventually especially if you have an air bubbler, heating will speed up the reaction but isn’t essential so long as the solution is moving.

 

[RickRag]

Thanks Nick,

Will I get a faster more complete precipitation if the solution is more concentrated? Accorcing to Hoke and others, unlike gold, Pt (and, I assume Pd, but correct me if I'm wrong)) precipitates better from concentrated solutions. Would a magnetic stirrer be as efficacious as a bubbler? Can you comment on the SnCl test I posted a picture of. Does the solution look fairly dilute? I recovered about 18 gms. of gold from that solution, which was about 1.5L., Also, should I scrape the PGM's off of the copper bar to expose more Cu? Or let them fall off under their own weight

 

[FrugalRefiner]

Will I get a faster more complete precipitation if the solution is more concentrated? Accorcing to Hoke and others, unlike gold, Pt (and, I assume Pd, but correct me if I'm wrong)) precipitates better from concentrated solutions.

When we precipitate gold, we're reducing the dissolved gold ions to solid metal, so if we use sufficient reducing agent, the gold will drop completely.

When we "precipitate" PGMs as their salts with ammonium or potassium chloride, we're precipitating a salt that is slightly soluble. That's why there is always still some PGMs in solutions precipitated in this way. By precipitating from concentrated solutions, we leave a smaller percentage of our values in solution to be collected in the stock pot. The stock pot recovers these values by using a more reactive metal to reduce these final tiny amounts.

That's what you'll be doing by putting a copper bar in your solution. It reduces the values to metals instead of precipitating them as salts.

Having said that, given enough time, you will recover all the values whether the solution is concentrated or dilute. The problem with cementing from dilute solutions is the precipitate is usually finer and more difficult to wash, so I prefer to cement from reasonably concentrated solutions.

Dave