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Author: Subject: Help please ! - Low Plating Efficiency
kernels
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[*] posted on 12-3-2018 at 15:27
Help please ! - Low Plating Efficiency


Hi there,

I was hoping to get some advice regarding an electrowinning experiment I am conducting.

I am a hobby E-waste Gold recovery type guy, part of the process of recovering Gold from plated parts is etching away Copper with a CuCl2 leach, and you can relatively quickly build up a few liters of saturated waste CuCl2.

In the past I have treated this waste by cementing the Copper out on Iron and raising the ph of the resultant solution to precipitate out the Iron Hydroxide, but, have always been quite interested in plating the Cu out to make the precipitation step easier.

I set up a 2000mL beaker with 1600mL of saturated CuCl2 solution. I am using two graphite electrodes that are 10cm x 15cm in size, running to a 5V power supply. The distance between the electrodes can be varied, so I move them apart until I get about 10A flowing through the circuit. I also have a 'cooling' container around the outside that I fill with cold water to maintain the solution temperature between 20 and 40 degrees celcius. I also have a small air bubbler in the solution to circulate the solution.

When running this setup, I seem to only be getting about 6g of Copper plating out after 3 hours, some quick math suggested that the maximum amount would be over 30g in 3 hours, so I am only getting about 20% efficiency ?

Does this sound correct for a real-world implementation ? Is there anything I can do to dramatically increase the speed at which the Copper plates out ?

Thanks!!!
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elementcollector1
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[*] posted on 12-3-2018 at 15:45


Some things you can try:

1) Increase current by moving electrodes closer together. 10A is already pretty good, though, so if you're sure you're already getting that much, best not to try your luck.

2) Don't use graphite. Switch to a scrap copper anode and a generic cathode. In my experience, graphite raises the resistance horrifically and doesn't have very good adhesion properties for the plate.

3) Add ammonium chloride. I've heard this improves plating dramatically by providing a more uniform (less crystalline) deposit that doesn't form any flaky 'trees'.




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OldNubbins
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[*] posted on 12-3-2018 at 18:15


10 amps seems like a lot. I usually plate at around 5 amps / sq ft. and none of my electrodes are anywhere near 1 sq. ft in size. Are you guessing on the amperage or do you have a meter?

At high amps, your anode efficiency can exceed 100% (low anode current density) and saturate the solution with copper ions and your cathode cannot keep up. Too high of a current density and your cathode develops an oxide layer and plating essentially stops.

Chloride is typically present in very small amounts in an acid sulfate bath. Although it is possible to plate out copper chloride, its not very practical. I would neutralize the solution and convert to copper sulfate.

Also iron, nickel, chromium, zinc and a whole host of other contaminants will interfere with plating efficiency. Not to mention you need to keep the solution agitated or else the area around your cathode will become depleted of ions and plating will stop.

in a nutshell:
convert to sulfate
agitate/stir/bubbler
filter electrolyte through activated carbon - probably dilute it as well as you don't want a saturated solution

*edit - found the electrode size

[Edited on 3-13-2018 by OldNubbins]
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kernels
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[*] posted on 13-3-2018 at 15:09


Thank you very much for your advice elementcollector1 and OndNubbins, I will study it and attempt some of the suggestions.
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[*] posted on 14-3-2018 at 01:12


Based on Wilipedia, CuCl2 can be reduced to insoluble CuCl,
cathodes reduce most compounds.

Was there an insoluble precipitate ?

I think that there must be side redox reactions because metal deposition usually follows Faraday's laws quite closely.
https://en.wikipedia.org/wiki/Faraday%27s_laws_of_electrolys...




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[*] posted on 14-3-2018 at 02:12


As Sulaiman already suggested, there probably are sidereactions taking place on the electrodes. The situation is made worse by high current density as mass transfer of the desired metal becomes the limiting factor (especially in dilute solutions). Basically what happens is that the copper ions will not be able to reach the electrode surface in sufficient quantity to supply the charge needed for the high current being forced through the cell by the power supply....this causes the electrodes to be polarized to a higher potential and sidereactions that were not energetically favored at lower electrode potentials, now become prevalent and supply a good deal of the required charge, resulting in an apparent low efficiency. To counter the situation try using electrodes with larger surface area, warm the solution and control the current density....should make a difference to plating efficiency.



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kernels
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[*] posted on 14-3-2018 at 11:15


Thanks for the suggestions Sulaiman and markx.

Sulaiman, there is no insoluble CuCl forming at the cathode, I'm very familiar with CuCl as it always forms when the HCl concentration in the CuCl2 leach gets low. The plating is very nice Cu colored metal crystals only. But I believe you are right, there must be some side reaction happening!

I'm surprised that there are so many suggestions to lower the current, at 10A I'm running at less than 0.1A/cm2, which seems on the low side compared to some of the experiments I've read about. I probably should also clarify that when I talk about the plating efficiency, I don't really care how electrically efficient it is, I just want to plate all the Cu out of the solution as quickly as possible.

I will give it a run at about 3A tonight and see how that goes, certainly still feels like I'm definitely doing something wrong!
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OldNubbins
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[*] posted on 14-3-2018 at 13:34


According to the metal finishing handbook, sulfate baths should be run from 15-30A/ft2. For an anode/cathode of 150cm2 that equals out to about 2.4-4.8 amps so 3 should put you in the ballpark.

I have attached an excerpt from the handbook - you might find a digital copy floating around the web. It is a valuable resource.

Attachment: COPPER SULFATE BATHS.pdf (239kB)
This file has been downloaded 64 times
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kernels
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[*] posted on 15-3-2018 at 13:09


Hi OldNubbins, great information, thanks! It looks like in practice electrowinning will not be the way to process waste, unless you can implement some very large surface area electrodes!
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[*] posted on 15-3-2018 at 13:21


Depending on your waste volume... maybe. But I process virtually all my waste through electrowinning or precipitation and storage of salts and my anodes are flattened copper plumbing pipes. I also use pieces of polished stainless steel to make copper sheets. Plate it on, peel it off. I just set it up and let it run, besides, lower current densities tend to give more consistent thickness and the resulting metal is less brittle and stays together/less likely to crumble.
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[*] posted on 15-3-2018 at 13:44


I guess as long as I run the current low enough that I don't have to monitor the progress it could just sit by the fume hood and run 24/7. Do you think my Chloride waste is similar enough to your Sulfate that the same principles apply ?
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[*] posted on 15-3-2018 at 14:03


I use a power supply similar to this one:



I can set the voltage and amperage and it will limit each as conditions change (cycling between constant current or constant voltage). As long as the electrodes are spaced appropriately and there is no chance of plating buildup bridging the gap, I have let my tank plate out over days just checking on it periodically. My only concern using copper chloride would be the evolution of chlorine gas. Since I use copper sulfate, I don't really need a hood, just a well ventilated garage with a fan blowing to prevent accumulation of hydrogen.
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[*] posted on 15-3-2018 at 14:28


Great, yes, I have a couple of CVCC power supplies, so no trouble setting the current limit. I will run it overnight @3A when I get home tonight and report back with the results. Thank you for your time.
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