# gold and silver strippers



## future gold refiner (Feb 12, 2021)

Hello Members,

I am GRF member since 2008 and still working on precious metals for all the years.
From time to time I read what´s going on in the forum... now I wrote a lot about the ECO-goldex and Iodine Leaching and wrote this very attentive.

After reading hundreds (!) of pages of research about ferro, ferri and glycine I was incited - I wanted to make some tests too. Because new stuff is always interesting to me.

(procedures like AR high grade material, low grade from different sources [different treatments], small/big batches done successful ...gold 99.99 and 99.99 silver is no problem at all - apart from the troubles every refiner have especially with new "not standard" material.

I have the possibility to get large quantities (really large ) of gold and silver plated material on different carriers....
Dissolving "everything" except PM is not an option (cost, waste...).....we are talking about large quantities 
Copper cell could be a option ....but melting everything will be a lot of energy and, two materials would not be processed this way .....beryllium copper and aluminium as carrier.

So thats the reason why I am interested in stripping of gold and silver (maybe palladium).
(I tried the iodine stuff, but it is nasty in large scale - especially the recovery of iodine from solution to prepare a new, fresh stripper)

I ordered ferro - , ferri - cyanide, glycine, and different oxidisers ... Peroxide, 3-Nitrobenzenesulfonic acid. 
For a first guess I always make some different concentrations and tried what is written in the literature....
different combinations of reagents.

to my surprise none of the combinations stripped the gold of the gold plated pins (on copper) 
can someone tell me why nothing of it worked at all ?

Every solution was heated to 50°C and stirred for 2 hours and than let stand for 80 hours at room temperature.

after two hours and after 80 hours was taken a sample and measured by ICP-OS
-> for the results look at the table

Gelb BLS -> means Potassium ferrocyanide
Rot BLS -> means Potassium ferricyanide
Glycin -> means glycine 
pH was adjusted with NaOH - measured with calibrated pH-meter
Peroxide was 30% ....3 drops from time to time 
test-Pins were gold plated (on copper with nickel as barrier) [pins have 7.9g Au / kg]

Any ideas, why nothing happened - ? (like described in literature) ... the few ppm Au I got into solution is not economical.


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## FrugalRefiner (Feb 12, 2021)

Did you expose the ferro and ferricyanide to UV light?

Dave


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## future gold refiner (Feb 12, 2021)

FrugalRefiner said:


> Did you expose the ferro and ferricyanide to UV light?



yes it stood close to window, but actually it is not super sunny here in germany

I wrote in GRF forum that it is needed to "gernerate" free CN and also that someone "played" with UV lamp to simulate/boost this reaction. But as I remeber right - not with large succes


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## FrugalRefiner (Feb 12, 2021)

You may not be getting enough UV.



Deano said:


> Most common glass is to some degree opaque to UV light waves.
> 
> How much is blocked depends on the type of glass.
> 
> ...



from the Eco-goldex thread.

Dave


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## future gold refiner (Feb 12, 2021)

it is standart lab borosilicate glass.

How do they do it in mining operations? is there engouth UV from the surface of the leaching tank?

second question, if there is enought UV is the ferro/ferri comperable quick in stripping fingers like classic cyanide?

I also read a lab instruction for dissolving gold (powder) in ferri and NaCl with oxigen as oxidiser.
but no word about UV ?


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## FrugalRefiner (Feb 12, 2021)

I wasn't talking about the beaker. You said "it stood close to window, but actually it is not super sunny here in germany".

I'll hope that deano, the person I quoted, will see this thread and respond to your question about how it's done in mining operations. He is our expert here on cyanide use in mining.

Dave


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## Lino1406 (Feb 12, 2021)

Mining works with cyanides, not ferro or ferri, and air bubbling or H2O2, is more important than UV


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## Deano (Feb 12, 2021)

A cyanide leach requires two things, enough cyanide in alkaline solution to dissolve all the gold and enough oxidant in solution for the leaching of this gold to occur.

In small batch leaches where the gold content of the "to be leached material" is known then the cyanide requirement can be calculated. It is usual for more than the theoretical cyanide requirement to be used, cyanide is cheap, usually much cheaper than the cost of your time spent waiting for the last of your now very dilute cyanide solution to dissolve the last of the gold.

What is not always appreciated is that cyanide is attacked by oxidisers, the addition of chemical oxidisers is always walking a fine line between adding enough oxidiser to speed up the leaching process and adding more than enough oxidiser so that the cyanide is destroyed before leaching has occurred.

You can go through the literature and find examples where large amounts of strong oxidisers are used, usually in leach reactors, as a way to rapidly solubilise gold.

What is not usually appreciated is that the gold rich feed for such a reactor is such that the monetary loss of cyanide to oxidation is minor compared to the increased leaching rate and thus gold throughput.

What you should always search for in the literature is what does the industry run as standard practice.

The standard cyanide conditions are 500 ppm cyanide ions in solution, the near equivalent of 1 gram per litre of sodium, calcium or potassium cyanide.

pH 11 is used so that there is minimal HCN gas generated and evolved from the solution, you can get less HCN generated at higher pH but you then trade this off against the slower leach rates caused by the higher alkalinity.

The standard oxidant of choice is oxygen from the air, this is cheap and easy to maintain in the leach solution and it will still provide a commercial leaching rate for gold while minimising the destruction of the cyanide in the leach solution.

If you are using ferro or ferri cyanides to generate the cyanide complexes then there are chemical rules you need to follow otherwise disappointment will result.

The reaction of uv with a ferro or ferri cyanide is reversible so only an equilibrium level of cyanide complexes is maintained in solution at pH 7.

You can drive the reaction to permanent completion by raising the pH to 11, which is actually where you want it to be to fit into industry practice.

At pH 11 the ferro and ferri components of the cyano complexes form hydroxyl complexes, these complexes are effectively insoluble in water and will precipitate out as fine orange particles. This effect prevents the uv reaction from reversing as there are now no soluble ferri or ferro groups to recombine with in solution.

The reaction with uv light is strongly dependant on the intensity of the uv rays at the surface of the water, uv does not penetrate water to any great degree. This means that you need to have a pH 11 water solution with dissolved ferro or ferri cyanide complexes, the water having large surface area to maximise uv contact area or to have slow agitation such that the cyano complexes are exposed at the surface of the water on a continuing basis.

You have the option of using a CN meter to see when the free cyanide level peaks or just running the reaction under uv for several days and hoping that you are near completion.

Different glass types act as blockers for different uv wavelengths, the easiest way to ensure full reaction is to have the water surface exposed directly to the sun.

Cyanide leaching rate is totally dependant on keeping a continuous supply of cyanide complexes at the surface of the gold. In other words, no agitation of the leach liquor means very slow dissolution of the gold.

A gold plated item will look the same gold colour before and during leaching, the only difference is in the surface texture which becomes roughened during leaching.

If you have a combination of low cyanide concentration, poor or no agitation and a thick coating of gold on an object then you have all of the necessary requirements for incredibly slow leaching which is difficult to actually see happening.

Deano


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## butcher (Feb 12, 2021)

Deano, thanks for sharing, I just wanted to say how much I have enjoyed reading and learning from what you have shared with us over the years. thanks for sharing in such detail, it is like we are getting an education from reading your posts.


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## nickvc (Feb 13, 2021)

Listen to Dean he certainly knows his leaching and the do,s and don’t,s.
Understanding the chemistry of leaching is fine but the real problems arise when you have gold over various base metals and with differences in plating thicknesses, in mining usually you have very low concentrations of cyanide but used in huge volume, with little amounts of base metals, and constantly moved by spraying exposing the gold to fresh solution, in laboratory recoveries you normally have a higher percentage of gold by volume so higher amounts of cyanide are needed to strip the gold in reasonable time but that comes with the danger of exposing the base metals on the lower thickness plated items and which in most cases the cyanide will leach those preferentially to gold so slowing the leaching of the gold by consuming the cyanide, you could say it’s a vicious circle and it can be frustrating to put it mildly.
To be totally honest cyanide leaching e scrap is an art more than a science requiring constant monitoring and knowing when to add either more cyanide, more oxidizer or agitation or all three, to add to the fun temperature also plays a key role, you also need to understand that you will not recover all the gold on mixed materials ever unless you run the leach at incredibly low levels of cyanide and for a very long time with constant agitation or the solution making the cyanide much more selective in what it will leach, this comes with the problem of safe waste disposal of large volumes of solutions.
So in a final statement I’d say that for most hobbyist refiners the cyanide process is not one I’d hurry to try and that’s without the risks of actually using a highly poisonous element that allows no mistakes or second chances, stick to the workarounds recommended here on the forum that are easier to control and fully understand.
There are other leaches discussed on the forum and they may be of use to the smaller refiner and hobbyist and perhaps deserve more research and development, who knows perhaps they will be the next move forward.


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## future gold refiner (Feb 14, 2021)

FrugalRefiner said:


> I wasn't talking about the beaker. You said "it stood close to window, but actually it is not super sunny here in germany".
> 
> I'll hope that deano, the person I quoted, will see this thread and respond to your question about how it's done in mining operations. He is our expert here on cyanide use in mining.
> 
> Dave



Now I got it - you mean the Window glass - thats true, could be a problem


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## future gold refiner (Feb 14, 2021)

Lino1406 said:


> Mining works with cyanides, not ferro or ferri, and air bubbling or H2O2, is more important than UV



Thats true, they work with "classical" cyanide - but:

1. there are papers which indicates that ferrro / ferri could enhance the preformance
2. Because of the eco-Goldex .... it could be that there is a other cyanide source used than classical cyanide


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## future gold refiner (Feb 14, 2021)

Deano said:


> A cyanide leach requires two things, enough cyanide in alkaline solution to dissolve all the gold and enough oxidant in solution for the leaching of this gold to occur.
> 
> In small batch leaches where the gold content of the "to be leached material" is known then the cyanide requirement can be calculated. It is usual for more than the theoretical cyanide requirement to be used, cyanide is cheap,* usually much cheaper than the cost of your time spent waiting for the last of your now very dilute cyanide solution to dissolve the last of the gold.* right!
> 
> ...



Dear Deano, 
overall I understand what you said ....because I read almost everthing in other threads what you wrote about cyanide.Thanks a lot for your advices!


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## future gold refiner (Feb 14, 2021)

nickvc said:


> Listen to Dean he certainly knows his leaching and the do,s and don’t,s.
> Understanding the chemistry of leaching is fine but the real problems arise when you have gold over various base metals and with differences in plating thicknesses, in mining usually you have very low concentrations of cyanide but used in huge volume, with little amounts of base metals, and constantly moved by spraying exposing the gold to fresh solution,* in laboratory recoveries you normally have a higher percentage of gold by volume so higher amounts of cyanide are needed to strip the gold in reasonable time but that comes with the danger of exposing the base metals on the lower thickness plated items and which in most cases the cyanide will leach those preferentially to gold so slowing the leaching of the gold by consuming the cyanide, you could say it’s a vicious circle and it can be frustrating to put it mildly.*
> I could live with a little more cunsumption of CN and base metal in solution - I would use a dissolving tank - after stripping - filter - add ion exchange resin -> ash and leach the ash / or elute ....differs on the used resin
> 
> ...


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## future gold refiner (Feb 14, 2021)

Dear Deano,

I would like to run the material in a basket in a glass lined vessel.
A pump on bottom will pump the liquid in circle.
Maybe a extra stirrer.

I could install a UV lamp inside the vessel or I have a UV desinfection pipe-unit.
If i would run it with ferri/ ferro

could you imagine what I wanna do Deano?


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## nickvc (Feb 14, 2021)

No insult meant but that reactor isn’t really useful in leaching especially e scrap unless you are running identical items, you need to be able to see what is happening especially if you are running cyanide, as I said it’s as much an art as a science.
I don’t think any large scale operation is using ferri or ferro and I’m guessing for a good reason, they have limited capabilities compared to actual cyanide in my opinion, they are safer but not as effective at stripping the values at reasonable speed, if you have the necessary safety in place why even consider using them, using cyanide isn’t always the easy choice it’s not as simple as dissolving all the metals and then selectively precipitating the values, there are many many variables in selective stripping and visibility is essential to see exactly where you are and what’s happening, it’s very much a hands on process and doing the process while fully understanding the dangers and reactions is needed to become successful, it’s almost impossible to learn from books or reading, the solution can cover the product but if the plating is shielded it won’t strip so the product needs moving as much if not more than the solution.
I’m not saying you can’t do this I’m sure you can but there is no simple way to learn this process and be able to do it successfully with decent yields.
To give you an idea of what I’m talking about take a couple of kilos of mixed fingers and strip them, they are probably the easiest e scrap to work with, some the modern ones will almost instantly strip the older heavier plated ones can take 1/2 hour to fully strip, now think mixed products different base metal substrates, usual e scrap lots, and the stripping becomes harder and harder and if you have any aluminum in the mix then bye bye stripping.


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## kurtak (Feb 14, 2021)

future gold refiner

I believe what Nick & Deano are trying to tell you is that in order for the ferri or ferro cyanide complexes to work the iron (ferri or ferro) part of the complex needs to be disassociated (separated) from the CN part of the complex so that the CN becomes free CN in order to dissolve the gold --- in other words - once you are able to disassociate the iron in the complex from the CN in the complex - you in fact then have CN - which then allows the gold to dissolve

The iron can be disassociated from the CN with UV --- the problem is in getting "proper" exposure of the UV to the iron/CN complex to get "effective" disassociation of the iron from the CN - so that the CN can be effective in doing its work to dissolve the gold

As much as doing this in a small batch beaker can (or not) work (if you get everything right) it becomes "much" more complicated to get everything right on a LARGE scale in order to free the CN in order to get "effective" use of the CN to dissolve the gold

Though I may be wrong - I believe trying to use the ferri/ferro/CN complexes as a stripper on a "large" scale will do nothing but cause you frustration from one batch to the next

To put it another way --- what you are "in fact" trying to do is strip/leach gold/silver plating by way of CN stripping/leaching --- but you are also complicating the process because you first need to disassociate the iron from the CN for the CN to work --- so why not just go with actual CN in the first place ?

I understand that you don't need permits to get the ferri/ferro/ CN complexes - whereas yes you will need to be properly permitted to set up for actual CN processing --- I would think that on the scale you are talking about the cost of getting properly permitted would "more" then pay for its self - then the frustrations you are likely to run into with a "work around" in order to avoid permits

Also -keep in mind that once you disassociate the iron from the CN - you in fact have CN - so your waste is going to be CN waste - so you are going to need to deal with that waste the "same as" if you had started with actual CN in the first place

Also - as Nick pointed out in order to get FULL stripping - its not a matter of just putting the solution in motion - the material its self needs to be put in motion - other wise the solution will not get to the gold on parts that are touch each other

That's why the "big boys" do this in open tanks (with proper ventilation on &/or over the tanks) the parts are then put in a barrel (that has holes & screens over the holes) the barrel is then submerged in the tank & turned so that the material moves in the barrel allowing the solution to come in contact with ALL surfaces of the plated parts - other wise part that are touching each other will not be completely stripped

Also you can not - under "normal" CN leaching conditions strip gold/silver plating if the plating is on aluminum - that's because CN leaches are normally run at (about) Ph 11 --- at that PH you will dissolve the aluminum instead of the plating --- so stripping gold or silver on aluminum is another story in & of it's self 

Kurt


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## kurtak (Feb 14, 2021)

Example of how the big boys do it - watch the second video of the video's on this page

:arrow: https://advchem.com/index.php/services/chemical-refining

Kurt


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## Deano (Feb 14, 2021)

Using ferro or ferri cyanide complexes to generate cyanide is done only when you cannot access sodium, potassium or calcium cyanide as leaching material. As mentioned above in this thread the ferro or ferri complexes will give you exactly the same cyano complexes in solution as the straight commercial cyanide products.

You will have to work under the same safety and waste disposal protocols for both starting chemistries because you will have free cyanide complexes in solution no matter which method you use to generate these complexes.

I have seen the ferri and ferro cyanides used in small commercial setups and at the time I wondered why they were used when permitting for straight cyanides was possible. I assumed that they were being used because the operators presumed, incorrectly, that they were safer than straight cyanide, at least that was the explanation I was given.

These operations used a separate tank for the dissolution step so that they could get the CN concentration to the right level before leaching started.

Nick is totally correct when he says that CN leaching of e-waste is a learned art, the variations in substrate types and plating thickness mean that substantial experience is needed to even get your product sorted properly before leaching.

Assuming that you can get an aluminium free waste source you have two options.

Firstly you can use high cyanide levels with strong oxidisers to quickly strip the gold but, as pointed out by Nick and Kurt, you will solubilise a lot of base metals as well as the gold. This is not the cheap option you might have been expecting for processing.

Secondly you can use a lower cyanide level with oxygen from the air as the oxidiser. This will lessen the base metal dissolution but will require a longer gold leaching time.

Everything is a trade-off, if you want a rapid leaching system you will have much higher reagent costs, if you want cleaner leaching you will have a slower processing system.

Deano


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## future gold refiner (Feb 16, 2021)

nickvc said:


> No insult meant but that reactor isn’t really useful in leaching especially e scrap unless you are running identical items, you need to be able to see what is happening especially if you are running cyanide, as I said it’s as much an art as a science.
> I don’t think any large scale operation is using ferri or ferro and I’m guessing for a good reason, they have limited capabilities compared to actual cyanide in my opinion, they are safer but not as effective at stripping the values at reasonable speed, if you have the necessary safety in place why even consider using them, using cyanide isn’t always the easy choice* it’s not as simple as dissolving all the metals and then selectively precipitating the values*,
> I thnink dissolve everthing is not a option for large scale of plated stuff
> 
> ...


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## future gold refiner (Feb 16, 2021)

kurtak said:


> future gold refiner
> 
> *I believe what Nick & Deano are trying to tell you is that in order for the ferri or ferro cyanide complexes to work the iron (ferri or ferro) part of the complex needs to be disassociated (separated) from the CN part of the complex so that the CN becomes free CN in order to dissolve the gold --- in other words - once you are able to disassociate the iron in the complex from the CN in the complex - you in fact then have CN - which then allows the gold to dissolve
> *
> ...


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## future gold refiner (Feb 16, 2021)

kurtak said:


> Example of how the big boys do it - watch the second video of the video's on this page
> 
> :arrow: https://advchem.com/index.php/services/chemical-refining
> 
> ...


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## future gold refiner (Feb 16, 2021)

Deano said:


> *Using ferro or ferri cyanide complexes to generate cyanide is done only when you cannot access sodium, potassium or calcium cyanide as leaching material. As mentioned above in this thread the ferro or ferri complexes will give you exactly the same cyano complexes in solution as the straight commercial cyanide products.
> 
> You will have to work under the same safety and waste disposal protocols for both starting chemistries because you will have free cyanide complexes in solution no matter which method you use to generate these complexes.
> 
> ...


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## future gold refiner (Feb 16, 2021)

Is there any other way to strip with thiourea (I know carcinogen) or thiosulfate or something else to leach gold / silver? + oxidizer to speed up.
(except of iodine, bromine or chlorine)

Reagent costs are no big problem because I have acsess to big chemical companys - so chemicals are "cheap" to me.

Adsorbtion on Resin is preferred.


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## nickvc (Feb 16, 2021)

If you are looking to strip e scrap you need decent volumes even though it has a higher gold content than many mine products, if time isn’t a problem you are then looking at huge plant with very large volumes of solution at a very low level of cyanide and still have to move that product to effectively strip it, not just the solutions as that will leave gold on product.
To run at low levels, I think Dean has specified what level, you need to monitor your solutions to maintain those levels adding more reagents to keep the stripping going and at low levels it could be days if not weeks before your product is stripped to an acceptable degree.
I know nothing of the other stripping chemicals as I have always found cyanide a good way to strip reasonably cost effectively, efficiently and within acceptable time scales so have never bothered to research those processes, if that is the way you want to go you may well have to develop your own processes through trial and error, all those clever chemists working for the large mines and refiners haven’t bothered or abandoned the idea so unless you are very clever or lucky I guess you may well find the same result.


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## Deano (Feb 16, 2021)

Always look at what the mining industry uses in their processing, despite many myths about secret process methods the industry will always use the most cost effective method and there are a lot of papers available regarding these methods.

In the case of e-waste any acid system will cause attack on the substrates preferentially to the gold plate. This rules out thiourea and thiocyanide leaches.

Thiosulfate leaches are run alkaline but the balance of copper in the leach solution is easily upset by the copper solubilised from the substrates.

The biggest problem with thiourea and thiosulfate leaches is that, despite the claims of ion exchange representatives, gold complexes from these leaches load very poorly both on resins and carbon.

This means that gold recovery from these liquors has to be done by zinc or electrowinning, neither of these is an attractive option when you have low tenor solutions.

Typical cyanide concentrations w/w for sodium, potassium or calcium cyanide are;
High cyanide concentration. 1-2 % per litre
Mining industry usual level. 0.1% per litre
High base metal leaching. 0.01% per litre

Keep in mind that hydrogen peroxide in the presence of oxidisable materials such as cyanide and many base metals will very rapidly disassociate and evolve oxygen unless kept under pressure. This is difficult to do if you are trying to keep visual control of the processing and makes glycine leaching not terribly cost effective. It is also the reason this processing is only used in the mining industry for very high grade concentrates.

Deano


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