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Increasing zinc metal concentration in acid zinc solutions

I am a chemist in a jobbing shop, we have more than 70k liters of rack acid zinc solutions.
We have a problem with zinc metal concentration increasing at the rate of 2-3 g/l a month.
Due to the type of work processed, drag out is minimal. Continual dilution is not an option.
Anodes are titanium cages with zinc domes. Solutions are KCL based, boric acid has been increased to approx. 30 g/l and pH maintained 5.2 - 5.4 .
Any suggestions how to prevent zinc from increasing.

John Mawby
- Birmingham, UK
2007

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First of three simultaneous responses --

Your complaint is a common problem with acid chloride zinc solutions. The primary cause is that anode efficiency is greater than cathode efficiency hence metal increases greater than the plate out. Viable options are few though I have heard of some people using platinized titanium to replace some anode area.

Gene Packman
- Great Neck, New York
2007

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Second of three simultaneous responses --

You might try reducing the anode surface area a bit; maybe not fill the baskets to the top or shield the bottoms with a rubber boot.

James Totter, CEF
- Tallahassee, Florida
2007

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Third of three simultaneous responses --

Take the anodes out and store them in the first rinse tank when not in use.
You might have more anode area than you need Anode:cathode ratio is 1:1.
You might get better answers if you provided complete plating data, as what you quote does not really compute.
You might need to use higher purity zinc or even go to an zinc-aluminum alloy, depending on what your operation really is.

James Watts
- Navarre, Florida
2007

Prior to changing to alkaline zinc solutions sometime ago we operated acid zinc solutions and found that increasing the total chloride concentration to about 200-210 grams per litre had the effect of preventing the zinc content rising. This was based on advice from a technical rep who had seen this practice work in other plants. We were skeptical but followed the advice anyway. The theory behind it being that the solution reaches a saturation point where it won't accept any more zinc into solution. That may or may not be the reason it works, but once we tried it we ran for several years without any problems with rising zinc concentrations.

Dougie Lightfoot
- Fife, Scotland
2007

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Following on from my query of rising zinc metal concentration in acid zinc, the plating solution operate 24/6 plus 12 hours Sunday. Down time only for general maintenance. Zinc anodes are 99.99% purity.
If platinised titanium inert anodes used to replace some of the zinc anodes will not chlorine gas be liberated from the anode ?

JOHN MAWBY
- Birmingham, UK
2007

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John,
when the Zn metal content in the bath is on the higher range and too high,the rate of acid attack on the anodes must be decreased by operating at lower temperature and increasing the pH to the higher range.Try not to make bulk addition of acid to adjust pH.
Try this
Regards,

T.K. Mohan
plating process supplier - Mumbai, India
2007