# Cementing PGM's from AR using Copper Bus bar



## RickRag (Dec 12, 2018)

Hi Guys,

Per Frugal Refiners suggestion I droopped all of the gold from an AR solution and transferred the solution to a larger vesssel to cement all the PGMs remaining with a copper bus bar using an inexpensive air pump intended to aerate aquariums. The copper plate/bus bar is 1/4" thick by six inches by 8 nches. It's been bubbling now for about a week and I still get a positive reaction with SnCl. See picture enclosed. Could the bus bar too encrusted with precipitated metals to continue to dissolve? I've tried to scrape the thin coating of black off to wxpose fresh Cu but but it is tenacious and won't wipe or rub off, at least not without some mechanical abrasion which I haven't tried. Is the test in the picture still positive for values or could it be a false positive due to the SMB used to drop the gold? Suggestions please!!


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## anachronism (Dec 12, 2018)

It's not a false positive from SMB, certainly not after a week anyway. 

How much base material did you have, and what exactly was it? So how much and what types of PGMS do you believe you have? 

Also let's see a pic of the copper bar. 

Jon


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## RickRag (Dec 12, 2018)

This was all from dental scrap that wasn't inquarted, just directly dissolved in AR. There is expected to be some copper, zinc, nickel and a fair amount of palladium and possibly some platinum, although there is not much Pt used in dental gold. There is no silver in the filtrate as it was precipitated and filtered from the AR after all the gold dissolved. All nitric was killed with either the dissolution process itself or what little remained was neutralized with sulfamic acid. It did not require much as I was careful with the nitric additions. After filtration the solution was diluted three times in preparation for dropping the gold. The filtrate from the gold drop was crystal clear but had a greeish blue tint to it and it tested positive for values, but not any dark purple or black indicating the presence of gold, just blue-green and reds mixed. After dropping gold, it was decanted off, allowed to settle overnight again and just to be sure I had dropped all the gold, I added another teaspoonful of SMB to the depleted AR just to see if any more gold dropped. Tested again and got nothing. I got the slightest trace of precipitant and concluded that the gold had indeed been dropped. The filtrate was transfered to a larger vessel to accommodate the large bus bar and cementation was begun with the aerator. A pic of the bus bar is included. The greenish stuff was black when the plate was removed from solution. This pic was taken approximatey an hour after it had been exposed to air. The entire solution containd appproximatley 3-5 oz. of gold although I haven't weighed it. Because it dissolved directly in AR it likely had 15-30% base metals as most quality dental gold is 14-18 karat. I got maybe 10 - 15-grams AgCl. Total solution volume after dilution and dropping was approximatley 8 liters. I'm guessing that this is likely Pd remaining in solution but I can't seem to figure out why it hasn't been cemented out as yet. Should I try concentrating the solution and abrading the bus bar to expose fresh Cu? Maybe switch to Cu powder as Dave recommends? Is the solution acidic enough after the dilution process to enable good Cu dissolution for cementation?


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## Lino1406 (Dec 13, 2018)

Stannous test is sensitive to traces of pm's. But so high copper concentration seems to interfere somehow, as we see copper color in your test


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## RickRag (Dec 13, 2018)

I found this link to a previous discussion on this topic. Does anybody have anything to add to maybe clarify the solution? No pun intended. 

https://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=14482

Another search revealed the following:



> _Cementation of metals from acid solutions
> United States Patent 3902896
> 
> Abstract:
> ...




The discussion above encompasses methods to recover copper and some of the difficulties in the process. These appear to be analagous to the problems encountered here. Although a thiosulfate solution is not going to be introduced here, the physical method of keeping the cemented metals from occluding the base metal, in this case copper, may be incorporated by other means. Per 4 metals post previously, powdered Cu is likely going to be a good answer. Or Kurt's suggestion of adding nitric so the copper plate is constantly be attacked to "freshen" it's surface area.

Thoughts anyone??


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## Platdigger (Dec 13, 2018)

Well, at this point you could use iron, zinc or a strong reducer, and basically just start over.


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## RickRag (Dec 14, 2018)

I appreciate your suggestion Platdigger, but that isn't an option. Iron or zinc would drop other base metals, including a lot of copper that is now in solution, that I am attempting to exclude. I do intend to re-refine both the gold and the PGM's a second time for greater purity. I just want to eliminate as much trash as possible the first time around.

This is a solvable problem and others on this forum have success cementing out values from gold free chloride solutions. I am having difficulty and don't know why and would like some help. I suspect the solutions are too dilute, thus depositing a very fine tenaciously adherant particulate on the copper plate and thus occluding access to fresh metal. It does not flake off and settle as it did in a previous attempt that used a more concentrated solution and a much thinner copper plate. I would like some input from others that either have had this problem and what they did to solve it or from those that dont seem to have this problem and what they do differently. 

Based on my research, there seem to be three easy alternatives that make sense. I'm sure there are others, I just don't know where to look or haven't enough expreience yet.

1. Concentrate the solution - Someone, I think it was Frugal Refiner or 4 Metals, posted that dilute solutions drop a finer particulate than concentrated solutions and from a physical perspective this makes sense as a means to reduce or eliminate the aggregation of occluding layers on the substrate.

2. Use a finer particle size for cementation, such as 300 mesh copper powder, also suggested elsewhere by either Frugal Refiner or 4 Metals, as a way of maximizing the surface area for ionization/electron transfer. Thinner/smaller substrates would react both faster and more completely. The potential problem here is the introduction of too much copper powder, which may not be a problem if I re-refine the metals dropped. Keep in mind that I'm trying to eliminate base metals, not reintroduce them. 

3. Introduce nitric acid to aid in the digestion of the copper. I'm not sure this is a viable or suitable alternative but it does make some sense. I have not come accross but one post where this was used as a remedy. I'm reluctant to ry it without further input from others that have more experience.

If you have a suggestion on solving the current problem, other than going higher on the electromotive scale, I would be very interested in learning more as I am a problem solver and don't like to give in prematurely.


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## Platdigger (Dec 14, 2018)

Ok, but you are missing something here. If you were to reduce all the metals out of solute with say (and I hesitate a bit to suggest) hydrazine, it would be a simple way to concentrate your values. I mean rather than trying to evaporate down the whole batch.
And then at that point a simple digest in dilute nitric will leave behind your pt. Any pd could be recovered from the solution a number of ways.
And this time don't use so much acid.
Only enough to get things in solution.


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## Geo (Dec 14, 2018)

Aluminum is much cheaper and easier to source. Add small amounts slowly because it cause an exothermic reaction but it will cement any noble metals quickly. Since the majority of the metal cemented will be copper, you can melt and part the cemented metal or simply redissolve it in the smallest amount of acid needed for a concentrated solution. From this solution, it will be much easier to remove the less reactive metal salts using copper.


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## kurtak (Dec 15, 2018)

Here is a discussion about cementing PGMs

It started out with a "problem" I was having

4metals got me on the right track

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=21218&hilit=cementing#p219231

Kurt


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## Lino1406 (Dec 15, 2018)

Wants to answer stannous chloride revealing copper question?
use the link in signature


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## Platdigger (Dec 15, 2018)

Could not read it Lino.
I suppose it's because I do not have facebook, and don't want it.


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## Geo (Dec 16, 2018)

Sulfur dioxide is a reducer of metal salts. The brown, false positive from stannous chloride is the reduction of copper by tin(II) chloride using the sulfur dioxide as a form of catalyst. It is not truly a catalyst because the sulfur dioxide is consumed in the reaction. Normally, stannous chloride will not react with copper chloride but with the excess sulfur dioxide present in the solution after adding SMB, the tin reduces the copper back to copper metal creating the brown stain. If you heat the solution after precipitating gold long enough to expel the excess sulfur dioxide gas, the false positive will not occur.


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## Lino1406 (Dec 16, 2018)

Former post edited in favor of those who don't use Facebook. 
N.B. Reduction by SO2. We know, from SMB dropping of gold that excess SO2 will reduce Cu++ in the presence of chlorine ions to CuCl
H2SO3 + 2Cu++ + 2Cl- + H2O = H2SO4 + 2CuCl + 2H+
although thermodynamically the reduction to Cu is quite differentially more favorable:
H2SO3 + Cu++ + H2O = H2SO4 + Cu +2H+


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## anachronism (Dec 16, 2018)

Geo said:


> Sulfur dioxide is a reducer of metal salts. The brown, false positive from stannous chloride is the reduction of copper by tin(II) chloride using the sulfur dioxide as a form of catalyst. It is not truly a catalyst because the sulfur dioxide is consumed in the reaction. Normally, stannous chloride will not react with copper chloride but with the excess sulfur dioxide present in the solution after adding SMB, the tin reduces the copper back to copper metal creating the brown stain. If you heat the solution after precipitating gold long enough to expel the excess sulfur dioxide gas, the false positive will not occur.




Honest question. If the result is brown and the result for gold is deep purple/black then how is it defined as a false positive? The colours are completely different. Surely a true false positive would by definition be a false purple/black that mimics the result for gold.

This is a different result. 

On a side note thanks for showing the workings behind this Jeff, I never understood it and when I've maintained that I have never had a false positive I've been using the logic/reasoning that my stain is brown not the colour for gold if that makes sense?

Jon


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## Geo (Dec 16, 2018)

The term "false positive" is for beginners to understand that not all dark reactions with stannous is gold. Sometimes there is a little gold in solution but the brown covers it up. It's just a darker brown. It's an easy fix but it takes someone to explain it sometimes.


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## Shark (Dec 17, 2018)

Thanks Geo, I had always been curious about that also.


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## Lino1406 (Dec 19, 2018)

Given the information above, we may see that obtaining Cu metal by stannous chloride reduction, though unfavorable, is a borderline case. Only 2.16 kcal/mol separating it from being favorable, a gap that normally can be bridged by slight heating.
We then render to potentials:
Anode: Sn++ + 2e- = Sn++++ ΔGa = 11.17 –(-6.55) = 17.72 kcal/mol
Ea = 17.72x4.18/96.5x2 = 0.38v on hydrogen scale
Cathode: Cu++ + 2e- = Cu ΔGc = 15.56 kcal/mol
Ec = 0.34v
That is Ec – Ea = - 0.04v only needed to turn the process favorable, namely to turn Ec – Ea positive
One way of doing it is dealing with the additional item in Ec – Ea, namely:
- 0.059/2 log□((⁡(Sn++++))/((Sn++)(Cu++))) 
1. By augmenting ( extremely) Sn++, Cu++ concentrations, or reducing Sn++++ such that the logarithmic item passes – 0.04
2. By driving the process towards SnO2, e.g. by higher temperature. This makes the process more favorable by 22 kcal/mol
Notice that SnO2 appearance also minimizes Sn++++


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## anachronism (Dec 19, 2018)

And your meaning? 8) 8)


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## Lino1406 (Dec 19, 2018)

The answer to: Under which conditions stannous chloride reduces copper ions to copper metal


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## anachronism (Dec 20, 2018)

Lino1406 said:


> The answer to: Under which conditions stannous chloride reduces copper ions to copper metal



Ahh and there I was thinking it was some random chemistry quote. :lol:


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