How to electrowin/recover copper (plating waste)

A. I assume by the term "rota cat" that you are referring generically to a rotating cathode system, and not a particular system commercially sold as a Rota-Cat^TM. They are two entirely different concepts.

The rotating cathode design (normally a rotating metal cylinder onto which metal is electrodeposited) is useful for a variety of electrowinning applications. The caveat here is that you need to have a proper design on the other side of the reaction -- the anode -- to allow electrowinning from these types of solutions that you mention. Sulfuric acid based electrolytes are amenable to most electrode designs, however caution must be used on ionic strengths. More is not necessarily better.

The use of any type of anode in a HF environment is severely limited due to the materials of construction and the cost. You would at a minimum require a niobium substrate and platinum catalyst coating. Normally, this has to be either a hot dipped platinum coating (i.e the patented "DeGussa Process", or the platinum could be clad onto niobium in small sheets and slit and expanded to get larger geometries. Depending on how much surface area that you need, you are likely to pay several tens of thousand dollars for anodes in a system such as these -- and they might not last 3-6 months time.

There has been some limited success in the use of ion specific membranes to envelope the anodes. This requires a separate electrolyte solution to promote conductivity between the membranes and the electrodes. Fluoride, being a very small ion, will permeate the membrane over time, and you will have to deal with the issues it causes.

I would stay away from any precious metal oxide based electrodes, as these are micro-cracked coatings and will allow free fluoride attack on the substrate, which will dissolve, and the coating will literally fall off in a process called exfoliation.

As far as the actual process goes (electrode issues aside) the strength of the acids and such will determine the efficiency of the process. You have to be your own gauge on whether the costs to operate the unit, including power, man-hours to maintain and remove deposits, and replacement parts have a positive value as compared to the recovered value of the metals. Zinc and tin are the least likely to be economical, copper fluctuates, and nickel is usually a positive to break-even consideration. I am not aware of anyone who has successfully electrowon iron from solutions on a commercial scale.

You need to be prepared to run an electrowinning unit much the same as a plating bath, which means additives, buffers, and other things will be required in the case of some metals, such as nickel. Also, remember that deposit integrity is a must - you're trying to get a thick deposit, and EVERY inconsistency in the deposit will be magnified, which usually means dendrites, and low quality plating.

Lastly, I would tend to stay away from any electrowinning cell that requires the use of raw media (other than reusable substrates like stainless steel or titanium) to plate on to operate. In some cases, they might tell you that you need copper shot as a starting media. It might have a value of US$3.00/kg, but as soon as you put in the cell and use it, its value drops to type 1 scrap or less - which means you lose money right from the start. Titanium makes for an excellent cathode, and if polished and used correctly, will give you almost immediate release of the reclaimed metals. I have seen too many cells where cathode prep was non-existent, and they use air chisels [adv: air chisels on eBay or Amazon] to try to remove the metal deposits.

Super informative posting, Tom. Thanks!

Regards,

A. Try adding some gelatin, coumarin or naphthalene trisulphonic acid. I have never done it with electrowinning, but try about 1 g/litre.

Q. Thanks, I will give a try to gelatin. Do you recommend a special one? from pig? or other?

The solution contains about 10 g/liter of copper. I reduced it first to get only Cu+ chloride complex, else the solution etches my cathode. pH is adjusted to 2. Chloride concentration is about 20 g/L.

Regards,

A. As the copper concentration decreases, you will need to do one or more of the following.

Reduce current density, and/or
Increase agitation, and/or
Use a high surface area cathode (which is the same effect as reducing current density).

A. Hi Yan

Many things in your question suggest that you are not an experienced plater or chemist and it would be interesting to know how you arrived at your starting conditions ... but I will try to point you in a more productive direction.

It is important to know what you mean by electrowinning. It usually applies to a method of extracting copper from ore or a crude copper anode e.g what you might get from melting electronic scrap. In that case, your objective is to obtain as pure a deposit as possible and the appearance does not matter because it will be melted down. Therefore you would plate onto a copper plate and avoid additives. The cathode plate is either stripped or, more usually, smelted with the deposit. Electrowinning is a crude large scale operation using impure materials with very little control of the plating conditions.

The term is sometimes used to describe extracting metals from rinse waters.

Firstly, if I have counted all those zeros correctly, you have about 21 g/l of copper, which is rather low. In a practical electrowinning situation, you would want it as high as possible to allow more current to pass and speed up the process.

You should aim for about 100 g/l of sulfuric acid. Unless you have very specialised equipment and knowledge, pH values under about 3 are meaningless.

I have no idea why you are adding ammonia. It is a very bad idea as it will complex with the copper and cause endless problems.

Similarly, boric acid has no use here.

If you really must have a fine grain deposit, the simplest additive is probably gelatin about 1-2 g/l but some will be in the deposit. It is rarely used in electrowinning

Current density is about 3-5 A/dmsq at room temperature (neither is critical).

A. Hi Apurva. Your nomenclature is reversed. The negative pole where you are accumulating your plated-out material is the cathode, and your carbon positive pole is the anode. I am assuming that you succeeded in dissolving the powder into the acids before you started trying to plate it out?

You haven't given us any details on pH, current density, etc., but from your description of bubbles and "suddenly turns black", it sounds like the current is far too high. What you probably need to do as a first step is to maintain a current density so low that copper plates out preferentially to iron, nickel, and hydrogen.

Good luck.

Regards,

Q. Thank you so much for your reply. It helped a lot. I have used hydrogen peroxide to increase leaching but as it is strong oxidant; I suppose that's why I get black copper. I am using 20% sulfuric acid for leaching and electroplating in same cell. I am using 5V 0.4A DC source for plating. My raw material is smelter slag containing copper, zinc, and iron in major part. So I am directly mixing 25 grams of sample in 20% sulfuric acid and apply electroplating in same cell. My main target is to recover copper and zinc from the raw material. Please guide me in right direction. Am I doing it right?

My questions are:

1. At what voltage I can only get copper from the produced mixture of copper sulphate, zinc sulphate and iron sulphate.
2. Should I use different solvent to increase electroplating efficiency?
3. I have heard about some organic chemicals that can separate copper sulphate and make it float over the solution. Is it true? If it is then suggest some names.
4. Can I recycle electrolyte?
5. Can I recover zinc and copper both from the same solution? What changes should I apply.
6. Can I use iridium oxide coated titanium anodes for this application to eliminate anode corrosion? Or to provide better anode.

Thank you so much for helping.
I guess you are my only hope.

Thank you.

A. Hi Apurva. I'm sorry to hear we're your only hope because my experience is in electroplating, and while there are certain similarities, I can't help you very much in the details of differential electrowinning ...

Yes, a strong oxidizing agent like peroxide can wreak havoc on electrodeposition -- hopefully there is little of it remaining by the time you do the plating. Bubbling the solution should help it escape.

1. Copper is more noble than iron or zinc and will selectively plate out if you keep the voltage below the voltage needed to plate those other metals out. But it's not quite as easy as looking up the potentials in the galvanic series because it depends on concentration. 5V sounds Way too high unless the anode is quite far from the cathode so there is over 4V of solution resistance.
2 & 3. I have not heard of such in my limited experience.
4. Recycling may be possible to some extent, but the acidity is consumed whenever hydrogen bubbles are released from the process. 2H20 ⇨ H2^ + 2OH^-1, leaving the solution more alkaline.
5. In practice I doubt that you will be able to recover the zinc. Zinc electroplaters generate tons of zinc-rich sludges, and even in their more favorable position of having solutions richer in zinc and lower in other metals, practical zinc recovery escapes us.
6. Yes, I think iridium oxide coated titanium anodes would be fine. Good luck.

Regards,

A. Hi Khagani. We appended your inquiry to a similar thread to give you some food for thought. Please get back to us with additional detail or your comments on the solutions that were proposed. Thanks.

Regards,

A. Nodulation is normal on thick deposits, especially in high current density areas. Are the nodules in just one area of the part or all over? Is it possible that your current density is too high?

A. Hi. I have only ever seen one shop do actual production copper electroplating with scrap copper as the source of the copper. If you are serious about your project try to reach out to Laurens Willard of Galvan Industries, Harrisburg, NC and see what he can share with you. I went there on a consulting assignment many years ago and learned at least 10X as much from him as I taught him, and I don't feel at liberty to reveal even what little I now know about it. So you'll have to wait for another reader to talk about the chemistry because I can't.

But I think it will be terribly difficult to use scrap as the source of a reliable plating solution without analysis and instrumentation, so I think you'll need to up your game quite a bit before you'll succeed :-)

You already made a math error because 31 inch2 is not anywhere even close to 81 cm2 :-(
Although it's true that you multiply inches by 2.54 to convert to cm, to convert inches2 to cm2 you need to multiply by 2.54 twice, whereas to convert in3 to cm3 you have to multiply by 2.54 three times.

Anode to cathode ratio is not critical, you can just eye in making the surface area of the flattened pipe twice as big as the part you are plating. But I suspect another error: the width of a flattened cocktail straw might be 0.3-0.5 cm; surely the width of even a small flattened copper pipe is way bigger than that.

Luck & Regards,