How to remove carbonates from alkaline zinc plating bath

A. You will need to lower the temperature to around freezing at which point the carbonate will drop out of solution. From there I'm afraid you'll need to pump out the zinc solution and then get in the tank with a spade. There are crystallization units available but they are quite expensive. I did hear of a plating company who bought an old milk tanker and used that to cool the solution.

A. Sir

The best solution is:

Chill the plating solution to 5 °C. Carbonates will precipitate out. The carbonates are removed mechanically.

This can be done in batches, or applied to the whole tank , depending on the volume you wish to treat.

Prevent carbonates forming . Stop air agitation. Use moving cathode in rack plating. Barrels should be fully submerged below solution to prevent churning. It is Carbon dioxide in air that's responsible for Carbonates, so best to avoid excessive air and solution vigorously mixing / contacting each other.

Regards

A. Hi Tim,
The easy alternative is to cut the bath.
Or try adding calcium hydroxide and precipitate in the form of calcium carbonate and remove.
But this has to be done first in the lab and then proceed with the bath.
Do not forget to inform us the result thru this forum!
Regards

A. Tim,

The above answer is absolutely correct. Now about chilling (note I did not say freezing). I am presuming you are building carbonates because you are running a barrel installation. If you are not, stop right now and figure out where you are aerating the bath to form extra carbonates. Guessing that your bath is too large to chill, but that you do have chilling capacity, I would suggest the installation of a side tank to chill a portion of the solution on a batch basis regularly or run it as an ongoing process utilizing a very low flow rate. I have been successful doing this previously.

A. You can also use calcium oxide / calcium hydroxide to precipitate carbonates, but see that temperature does not rise too much which will lead to cyanide breakdown and carbonate buildup.
Good luck
Mukesh

A. Hello Robert,

As you are able to completely remove carbonates at one go, I suggest you analyze the pre treatment and post treatment content.

In my experience a minimum of 20-30 grams is required in every bath like a kind of buffer, a total removal to under 10 grams per liter is not advisable.

Please let me know again over here what the analytical values are.

After a complete carbonate removal it would help restoring additives by a Hull Cell empirical analysis.

A. Generally when we see adhesion problems that points to improper pre-treatment and cleaning. I agree that we do want a certain level of carbonates in the bath as having none can cause a whole other mess of issues. If I was you I would check all parameters of the pre-treatment processes that happen before the strike. That includes: process times, bath components levels, pH, temp, proper racking/barrel configuration, most recent dumps of any acid or cleaners, good rinsing and what ever else may be involved in these pre-plate processes. If you can rule out pre-treatment then you can look towards the nickel plating tank as the issue.

That is my 2 cents on what is the best troubleshooting for this type of issue.

! We have two identical lines so we checked all pre-treatment on both lines with the trouble shooting method on what bath caused it. The parts came out as pictured in only the one zinc bath no matter what pre-treatment alternatives we used.

We even moved the bath content from one line to the other, to see if the bath equipment like anodes, rectifier and so on was at fault, but still the same problem with the one bath fluid mix.

Once we dumped the bath fluid and mixed a new, everything was fine again.

With regards to the carbonates, we did measure them just after and it never gets below 20, even on new baths we start at about 25.

The only thing we have not yet tried is treatment with active carbon. It looks like this happens each 1-2 years.

The supplier is also is clueless on why this happens.