# Removing Au/Pt/Pd from Diluted Solutions?



## NCLowe (May 20, 2011)

I'm hoping to find some help here. I do spectral analysis of precious metals, and constantly have solutions of varying concentrations of PGMs and Au, and am looking for how to precipitate the precious metals from it. I need to develop a method for 5 to 7 gallons a week worth of solution, and as cheaply as possible. The solution is mostly DI water, HNO3, and HCL. If anyone can offer some advice I'd greatly appreciate it! If I missed giving pertinent information let me know and I will fill in the gap. It needs to be a fairly passive system if possible, and I do have access to most standard lab supplies. What ever form they can be precipitated is fine- as long as they are out of the solution.


----------



## nickvc (May 20, 2011)

Most weak solutions take ages for the metals to precipitate, and PGMs are the worst, so I would be tempted to cement your values out on copper and save them until you have reasonable quantities to make refining worth while


----------



## NCLowe (May 20, 2011)

nickvc said:


> Most weak solutions take ages for the metals to precipitate, and PGMs are the worst, so I would be tempted to cement your values out on copper and save them until you have reasonable quantities to make refining worth while


 

That's what I'm hoping to do- I can wait to sell the precipitate, I just need to know how to remove it from the solutions. You mentioned cementing with copper- how does that work?


----------



## samuel-a (May 20, 2011)

I think you should use more reactive metal... I.e. - Iron/Aluminum/Zinc, IMHO

Seperate by filtration.


----------



## arthur kierski (May 20, 2011)

please,what is imho?


----------



## jimdoc (May 20, 2011)

In my humble opinion.


----------



## arthur kierski (May 20, 2011)

thanks jimdoc---as you know ,english is not my mother language.


----------



## nickvc (May 21, 2011)

Sam if the solutions contain only precious metals I'm in full agreement but if there are other metals in the mix why cement them out, using copper sheet will take out silver, gold and the PGMs and leave the rest in solution, less to complicate the refining after cementation.


----------



## samuel-a (May 21, 2011)

nickvc said:


> Sam if the solutions contain only precious metals I'm in full agreement but if there are other metals in the mix why cement them out, using copper sheet will take out silver, gold and the PGMs and leave the rest in solution, less to complicate the refining after cementation.



He said he is doing spectral analysis for the solutions, so i assumed he would have mentioned base metals if they ware there...

btw, cementing Pd with Cu, as i come to learn, isn't very practical idea.


----------



## Harold_V (May 22, 2011)

samuel-a said:


> btw, cementing Pd with Cu, as i come to learn, isn't very practical idea.


In my opinion, only because one rarely has much palladium to cement, so it isn't easily recovered after cementation. I used copper to recover platinum and palladium for years, although it was from my silver nitrate solutions. Works perfectly well, and the silver makes recovery easy. 

Harold


----------



## samuel-a (May 22, 2011)

I think that "nitrate" is the key word in this debate.



Harold_V said:


> In my opinion, only because one rarely has much palladium to cement, so it isn't easily recovered after cementation.



Yes, that was my intention. I don't think a 100% Pd recovry is possible using Cu with the exception of concentrated nitrate solution.
Dilute chloride solution, no way in my opinion that he could remove all Pd (and even Au) from solution. 
Dislute nitrate solution, maybe he could get away with that...

sorry, i should have been more specific.


----------



## lazersteve (May 22, 2011)

NCLowe said:


> I'm hoping to find some help here. I do spectral analysis of precious metals, and constantly have solutions of varying concentrations of PGMs and Au, and am looking for how to precipitate the precious metals from it. I need to develop a method for 5 to 7 gallons a week worth of solution, and as cheaply as possible. The solution is mostly DI water, HNO3, and HCL. If anyone can offer some advice I'd greatly appreciate it! If I missed giving pertinent information let me know and I will fill in the gap. It needs to be a fairly passive system if possible, and I do have access to most standard lab supplies. What ever form they can be precipitated is fine- as long as they are out of the solution.



If your goal is the analysis of the solution without precious metals, then I suggest you look into solvent extraction and/or resins to remove the precious metals from the sample to obtain your baseline solution for comparison. The solvent extraction methods may serve you better on larger scales, but for small sized test samples, the resins may be your best bet.

There are many posts here on the forum concerning both.

If your goal is an economical method of recovery of the precious metals from the solutions then copper would be my first choice.

Steve


----------



## 4metals (May 22, 2011)

Since you are trying to drop the values from the dilutions for spectroscopy, I assume you are talking about serial dilutions to the PPM range. If so the dilute nature may prevent you from getting complete recovery. You may do better with a resin column. 

If you are having waste issues, consider running your dilute solutions in an evaporator to concentrate the liquids then cementing will work better. Plus you will greatly reduce your waste load. 5 to 7 gallons a week can be reduced to a few 100 CC's of concentrate a week and you will still be a conditionally exempt small quantity generator.


----------



## skippy (May 22, 2011)

I haven't had much luck getting pgms to cement totally on copper wire. Sometimes the cemented layer is porous enough and flakes off by itself exposing copper to keep the cementation occuring, but other times the layer is quite solid. It even has a shiny matte look and takes a great deal of effort to remove from the wire. 
When this sort of tough layer occurs the cementation stops and you need to scrape the wire or add more copper. 

I've actually had some good experiences though with steel wool and zinc-aluminum alloy turnings. My first time using steel wool, I was left with countless black bristles at the bottom of my container. I thought 'why the heck isn't the steel wool dissolving any more?' The steel was in fact almost entirely gone, but the cemented precious metals made a sort of cemented metal replica of the tiny wool bristles. Which was nice as it settled and filtered very easily. The alloy turnings behaved similarly, leaving a crusty black cementation, so long as the pH wasn't extremely low, as the aluminum and zinc reation gets too vigorous then and cements very fine black metals. Of course these metals also cement base metals that are in the solution too, and so isn't the greatest for dilute solutions that are heavy on the base metals.

This all sort of makes me think that fine copper wool or moss might be a not bad way to cement dillute solutions, if it would more or less completely react, like the steel wool and Zn-Al turnings do. Then you wouldn't have to scrape the copper or add fresh copper, and you would also get selectivity. Anyone out the who has tried it? 

As and aside I've also had pgms precipitate on copper as a mirror like film. These films somehow managed to form in sheets much larger than the diameter of the wire I was using.
Wish I took a picture, it was quite pretty, and surprising as well. The films had no integrity and turned to black powder upon attempting to strain them out.

Hope someone find this interesting or helpful.


----------



## butcher (May 22, 2011)

With a buss bar of copper you have more surface area, the small wire will not have, so with a buss bar more copper is exposed to react with metal salt you are cementing values from (direct contact with metal is needed to move electron), the fine shaving like wool will react fast as they also have the surface area, but the powders will also have more of this metal you are using to deal will later (center of these may not completely react,or it will be difficult to know how much to add). good stirring and the acidity of solution also come into play here, if too basic reaction will slow or stop, too acidic and you will put the metal you are using into solution from excess un-reacted acids, I feel heating also helps (no proof but I think so, so I heat it).


----------



## Harold_V (May 23, 2011)

samuel-a said:


> I think that "nitrate" is the key word in this debate.
> 
> 
> 
> ...


No problem, but you're wrong about not achieving full recovery. In fact, it may be the only way you can (including using more reactive metals, of course, but they are not selective). It takes time, but you can shorten that by using a large amount of surface area. There's no way you can leave values in solution when you present copper. It just doesn't happen. When I'd recover from my silver nitrate solutions, silver was always first to come down, with the platinum group last. They are slow, but down they come. The concentration of platinum/palladium often so sparse that it was barely detectable. An obvious sign it was down when the solution turned a brilliant sky blue color, from the previous green tint. I often allowed a full day afterwards, but never failed to accomplish a full recovery, at least by testing with stannous. (I practiced what I preach---I used stannous routinely, daily, for all solutions). 

Harold


----------



## Oz (May 23, 2011)

Here are a couple of pictures of what Harold is talking about as to the green solution color when cementing from an Ag-Pd-Pt from a nitric solution. These pictures are of nitric solutions from digesting karat gold inquartations with silver that I happen to have going.


This green solution is a little more pronounced than usual since there was a fair bit of white gold I have been processing, but in this picture almost all silver has been cemented.


The blue jar is representative of the actual color of solution in the lower half, and after all platinum group metals have been cemented.

I do not tie up nice lab glass for cementations as there is zero benefit. For larger volumes I cut the top radius off of 1 gallon wine bottles with a steady hand on a wet tile saw. They make very serviceable 3 liter beakers for non-heated reactions. I also just use whatever free clean copper available that is at least not stranded wire, thin sheet, or tube (copper buss bar is getting pricy at the scrap yards). The solid copper rod I have been using lately is close to a ¼ inch diameter (I got about 30 feet of it free) and it takes about 4 days to finish cementing everything from a liter of solution if agitated 2 times a day. Since the silver used is mine, and the platinum group is non-payable in these quantities, I do not worry as to time.

If you have accountability on your silver and PGM from karat gold, you would want thick bar or plate to speed things up. This also goes slower in winter temperatures, but a heating pad would fix that.


----------



## Harold_V (May 23, 2011)

Oz said:


> I do not tie up nice lab glass for cementations as there is zero benefit.


As much as I speak against the use of plastic vessels in refining, I used 5 quart ice cream containers for cementing silver. There was reason to my madness. I often used pieces of copper that were large enough to easily break the container if it was bumped when stirring the silver. By using plastic, that wasn't a problem, and should a bucket outlive it's usefulness and form any cracks, it was incinerated for recovery of the metal that had fused to the sides of the bucket (the very reason I refused to use plastic for gold processing). 

The only other place I used plastic was in dealing with my solutions after values were extracted. Base metals were extracted in 5 gallon buckets filled with scrap steel. 

Harold


----------



## samuel-a (May 23, 2011)

Harold_V said:


> No problem, but you're wrong about not achieving full recovery. In fact, it may be the only way you can (including using more reactive metals, of course, but they are not selective). It takes time, but you can shorten that by using a large amount of surface area. There's no way you can leave values in solution when you present copper. It just doesn't happen.



Harold

I stand corrected, I'll take your expirience over my own at any time.
At least until i will perform an expiriment with palladium chloride solution and copper cementing with consequative DMG tests over few day period, once the findings will confirm your claim (which probably will) I'll become a true believer.

I'm a strong supporter of "seeing it with my own two eyes", even if it mean to disprove my own logic.

This experiment is defintly on my to do list. Thanks.


----------



## Harold_V (May 24, 2011)

samuel-a said:


> Harold_V said:
> 
> 
> > No problem, but you're wrong about not achieving full recovery. In fact, it may be the only way you can (including using more reactive metals, of course, but they are not selective). It takes time, but you can shorten that by using a large amount of surface area. There's no way you can leave values in solution when you present copper. It just doesn't happen.
> ...


Take note that I specifically mentioned the use of abundant copper. If you introduce a few strands of wire, I expect your results would be much as you described. I NEVER used wire---it was always pieces, and often quite large, so I had a lot of surface area. A small amount of copper may well fail to make a complete recovery. I wasn't considering that possibility. 

Harold


----------



## freechemist (May 24, 2011)

Hello NCLowe,

It depends mostly on what type of spectral analysis you are performing. If you analyze sample solutions of pure precious metals for impurities, the collected liquid after analysis may be fairly concentrated in precious metals (grams/liter), whereas base metals (impurities) are the traces in solution. If you are analyzing process-solutions for Au / Pd / Pt in course of precious metals recovery and refining, PM-concentrations are much weaker (milligrams/liter), due to high dilution necessary for spectroscopic measurement, eg by ICP. For treatment of highly diluted solutions I would recommend a strongly basic anion-exchange resin, which usually holds back Au, Pt and Pd as complex chloro-anions, especially if you take care, that Pd is in the +4-oxidation state. This can easily be done by dissolving small quantities of sodium-chlorate in the solutions to be treated, until a faint smell of gaseous chlorine is noteable. Most important condition is, that the solutions contain enough excessive HCl. A common anion exchange resin (Amberlite, Dowex) can be loaded with up to 10% of its weight with PMs, before the concentration in the effluent (resin-column) or filtrate (batch-procedure) becomes significantly higher than 1 mg/liter (1 ppm). The PM-values can be recovered from the loaded resin by first burning it carefully and slowly down in a oxygen rich atmosphere, and second, heating it for about two hours at 800oC.
For more concentrated solutions (grams/liter) there exists a chemical reduction procedure, using formic acid and urea in a HSO4(-)/SO4(2-)-buffered system at a pH-value not much higher than 2. It precipitates the PMs as metals in an easily filterable form quantitatively, leaving most base metals, except lead, in solution. This process is described in more details in the Proceedings of the 1999 Annual Conference of the International Precious Metals Institute (IPMI) in Acapulco (Mexico). 

freechemist


----------



## NCLowe (May 25, 2011)

I wanted to say thanks for all the advice! I should have been more specific about the solutions- they are all post analysis, after I've established values for Pt/Pd/Au. They are mostly nitric solutions with minor amounts of HCL. The silver has been dropped already, and base metals are non-existant. I am just trying to develop a method to drop the Pt/Pd and minor amounts of Au so as not to waste them. So far I've found that neutralizing the acid with soda ash is the most economical solution there- does anyone know if that would adversely affect the cementing process? Also, does anyone know ratios for how much zinc or copper should be used per L of neutralized solution? Right now the mix is running about 75 PPM Pt, and 110 PPm Pd., and my experiments so far have yielded only about 50% recovery using granular zinc. Thanks a lot!


----------

