electrolytic refining of gold filled jewelry

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M

marethyu

Member
Joined
Jun 9, 2018
Messages
16
I was looking into refining some nonferrous gold filled jewelry and am looking for ways to get around the use of large amounts of nitric acid. I was researching the feasibility of building an electrolytic cell using anodes cast from gold filled jewelry, around 3% gold content, and I came across an old research paper about brass refining. It is my understanding that the base metal for most nonferrous gold filled jewelry is brass. In the paper it outlined an experiment of running brass anodes through an electrolysis cell of a copper sulfate solution that deposits copper at the cathode, pulls the zinc into solution and leaves less reactive metals, gold and lead, as anode slimes where I could recover the gold. I haven't seen anyone else try this before and was wondering if there are any potential issues with this process. Are there other base metals in gold filled jewelry that would interfere with the process? or are there any other variables that I'm not accounting for?

here is the link to the paper I found
https://digitalcommons.mtech.edu/cgi/viewcontent.cgi?article=1086&context=bach_theses
 
I'm not sure what you mean by "large amounts". A gallon of nitric acid will work quite a bit of gold filled jewelry. It is often able to produce well over $1000+ worth of gold per gallon. A copper sulfate cell under optimal conditions will produce a lot of copper, but gold filled is well below optimal material. If all the material was exactly the same, maybe, but the majority I have worked with the non precious metals varies by quite a lot, and so does the precious metals as well. I don't work at the rate the refineries do, so my way is slower, but I have often started a one or two pound lot of gold filled on Monday, and have it sold on Friday morning using nitric. And I like my gold very clean before I sell it as I get extra percentage for it that way. I have used Hydrochloric on a few test lots, and removing the base metals alone took almost a week in it's self, five days under optimal conditions. My small copper cells take about three days to move a pound of copper on to the cathode, then another two full days normally to process the slime's, which contain any precious metals that were involved. The thing that sold me on nitric the most though is that I only have to deal with the same waste materials. Don't get me wrong, I get a lot of enjoyment from a small copper cell, and still run one pretty often, I just haven't found it to beat nitric out for running gold filled. I mainly run low grade pins in my copper sulfate cell, and mostly because I like melting things in my furnace.

With all that said, the big boys may find it to be completely the opposite. The bigger guys do things the way they do for a reason and that alone lends reason to follow suit when ever possible. Which ever way you try, lets us know, it is always enjoyable to follow others progress and learning, maybe you can teach us some new things as well.
 
When I say large amounts of nitric acid, I just mean using it as a means of separating the base metals. I know its inevitable that I'll need to use some to process the anode slimes as purifying the gold with AR or inquartation is the best way to do it. I distill my own nitric a couple of liters at a time but I'd like to cut down using it if possible due to associated costs, emissions, and the time I put into a larger distillation. I agree it would absolutely be faster with a nitric digestion but I'd like to give electrolysis a go with the gold filled jewelry. I'd like to build a medium/small cell that uses a maximum of 20A and reading the paper it seems that a copper sulfate solution is the way to go, possibly with the addition of nitric acid if there is more than 1% lead. With further reading of some industrial processes, I've found that It is best to not allow the zinc content to rise above about 70g/L (still not completely sure of this) and since brass has a zinc content of around 30% ill have to replace or electowinn the electrolyte approximately every 140g of dissolved anode per liter. So I'm looking at making a cell with between 2-6L capacity to avoid too frequent processing of electrolyte and I'll be able to dissolve 280-840g before adjusting the electrolyte depending on cell volume. Looking at large scale processes, it appears like I will have to keep my current density between 10-20A per square foot of anode surface area. I was wondering if anyone has seen a small scale electrolytic brass cell or had any other knowledge as to why this would or wouldn't work? Or if anyone has better knowledge of the base metals in gold filled jewelry?

Also, Shark do your pins have a brass base metal and if so have you had any issues with zinc in solution? Any new information is much appreciated.
 
So much for that, I had a post written out and lost it by clicking the wrong button....

Gold filled is a mix of many different metals, just because it is all jewelry each maker will use a different combination of metals to make their own. It is almost impossible to determine the make up from a large lot of it. Besides a cell you will need a furnace to smelt the material with, which done right can help remove some of those base metals. That in it's self can be a whole other education, especially if you build your own. Besides zinc you will have to deal with nickle, which will foul your electrolyte quite fast as well. I am sure there others mixed into gold filled as well. Often each metal alone will give a sign that it is becoming in excess, trying to figure out what the problem is with a mixture can be more difficult. A hull cell could help identify the adjustments needed to keep it operating cleanly. If the cost of nitric is prohibitive there are options such as "dry poor mans nitric" that can be used.

My most used cell is in a plastic container that was made to hold a 5 pound bag of sugar, it holds around four liters of fluid but I operate it with around two to three liters. I pour anodes into a 4 3/4 inch cast iron skillet that was used to bake a big cookie in and came from a Christmas gift pack from Wal-Mart. Most of my Anodes come out around two pounds. My cathode is made from a piece of copper gutter that was gave me by a friend. With this I can use a 30 volt 10 amp power supply with out over stressing it to much. I have ran one as small as two cups but I have only tried one up to about three gallons which didn't work well with the other things I had going on at the time, and was hard on my small, Chinese power supply.

As for the pins, yes they contain a large amount brass or bronze. I melt them in a ratio of 95% copper to 5% pins, and have ran it as high as 15% pins. By evaporating the electrolyte some it can be filtered and reused to some degree, which will vary according to your mix of metals in your anode.

Here is where I started and offered the clarity I needed to give it a try.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=25596

As for smelting, there is another thread that helps, but covers only a fraction of what can be involved.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=25592

Both links are found in the Library Section and that section is well worth the read.
 
Thanks for those links, had some useful information about smelting I was not fully familiar with. I currently have a waste oil furnace for smelting and I am familiar with the process but still always learning. Also, I'm looking into building an electric foundry, I have all the materials but just need to put it together. I did not know nickel was present in significant amounts in gold filled base metals I'm going to have to look out for that. Due to the low cost of the electrolyte, I will probably just change it out frequently to avoid fouling problems. Maybe I'll attempt to electrowinn the zinc out of it once or twice before disposing of the electrolyte just to help minimize waste. I was planning on building a smaller scale refining cell to start off with similar to a hull cell to work out any kinks before sinking more money into a larger cell. I'm familiar with the process of copper refining like the process outlined in the link you shared, my only concern was that those typically use anodes of 95%+ copper while mine will probably be below 70% copper so ill be dealing with far more contaminates. I just haven't seen anyone else try a cell with similar anodes so I was unsure. I'm not too concerned with the frequency of changing out my electrolyte because it is not too difficult to treat and if I build a 6L cell it will probably cost less than $1 every time I change it out while recovering greater than 12 grams of gold in the slimes so it should be fairly cost efficient. Also, I can probably make a larger cell if I thought It was necessary as I got a really nice free 60A power supply from my job :D. Thanks for the information.
 
I do often run a filter system in my cells. As the fluids evaporate, some metals will fall out on their own. This can be caught with a very basic filtering system, but it must be kept under close watch or it will stop up and over flow the filtrate back into the the cell. I use an air powered lift type fish aquarium filter. They only come in the 1 to 3 gallon size but a person might be able to make one work on a larger scale, or with the use of multiply filters( I have run two together). The more brass/bronze you run the faster the filters will stop up. This led me to believe that something in the brass mix will fall out fairly easy as the amount of fluids lost to evaporation was minimal.

Hope that helps some.
 
Have you found the filters to be necessary? Personally, I would just prefer to process the dropped out impurities with the anode slimes if they aren't present in large amounts.
 
They don't get left in the slime's bag, so I never mixed them in. There should be no reason for any precious metals to carry over to the solution unless you add something that would carry it, such as nitric will do with silver. With out the filter these powders will drop to the bottom of the cell. If you add solution or disturb the cell they will become suspended in solution for a while. I found this out while running a gallon sized cell and needed to add to the electrolyte. It never quite settled out to suit me. It was just enough to make it hard to see through the solution and I had to lift the anode and cathode to check them, disturbing them all over again. It also caused some poor plating that I never quite figured out. Since I had the aquarium filter system, I gave it a try and it worked pretty good on a the small cell. Long term I was working towards a three cell system in which I could run the electrolyte until it became badly fouled and then pump it into one side while adding new electrolyte from the other side. It could then be cleaned or changed and be ready to go again in the opposite direction as soon as the existing solution fouled. Maybe sometime I can get back to playing with them and see what else I can come up with as well. I can't explain much of what is going on with these cells very well as I have no chemistry back ground and have learned about all I know on the subject from reading some books and this forum.
 
My guess is if there was only enough to make the solution cloudy then it was probably just lead/cadmium sulfate. Those are the only metals I can think of that would act like that because they have very low Ksp values so if the sulfate ion concentration of the cell increases, it would reduce the solubility of the lead and cadmium making them drop out of solution. Just a theory though. Do you keep free sulfuric acid in your electrolyte?
 
I do keep some extra sulfuric in the cell most of the time as it seemed to help with several things. It could be lead and that is my first thought as I remove a lot pins with a small torch. I also used a very high amount of pins with that batch as well. I am not positive but I am thinking about a 50-50 mix of copper and pins. Most of the pins are the type used on CD or DVD drives, but I often do pins from a large mix of items. I do keep the better pins separate and process them in a sulfuric cell most of the time. I used nothing that should have had cadmium but I guess it is possible that I had some.
 
I know of a company that sells small gold recovery systems that use an electrolytic process based on the paper you described. The system seems composed of two half-cells separated by a filter/membrane and uses a dilute nitric acid solution as the electrolyte. Copper forms the cathode and the material to be refined is cast as an alloy of gold 10 to 40 percent to be the anode. Copper is plated out on the cathode. Gold is left in the 1st half cell and other base metals are left in solution. Sorry, I don,t know if a link is permissable as this is not an advertisment but maybe what you are interested in process wise.
 

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