Problem holding Zinc concentration in alkaline Zinc plating bath

A. Hi Pankaj. There are several terrific articles available, like "E-Brite Ultra Alk Alkaline Non-Cyanide Zinc Plating Process" from EPI [a finishing.com supporting advertiser] and https://www.columbiachemical.com/wp-content/uploads/Article-ZincGeneratorTanksForAlkalineZincPlaters.pdf -->
which detail what the zinc and starter concentration should be, how to easily verify the ratio with a Hull Cell, how to calculate the required size and zinc capacity of the generation tank, and several alternate ways to design it.

You apparently recognize that good galvanic contact between the steel and the zinc is required in order to generate the galvanic current necessary to dissolve the zinc fast enough. So I would suggest first comparing your numbers to what those two articles or others suggest in order to determine whether what you have should be adequate to maintain concentration. If it should be, but it isn't, then I would suspect that perhaps something is amiss with this "fine MS sheet" arrangement you speak of, but which I am not picturing. A sketch or photo to mooney@finishing.com for posting here might help.

Luck & Regards,

A. Hi Everyone!

You are using more zinc and less produce zinc to galvanize reaction.
Make sure potassium hydroxide concentration and temperature within limit and enough zinc slabs into regeneration tank.

Your generation tank produces less zinc than your process tank.
Need mixer in generation tank; make sure potassium hydroxide concentration is within limit; check that temperature in generation tank within spec. Descale your steel baskets where you add zinc slab.

A. Hi, Pankaj Chaudhary.

I think your system started should be fine, but suddenly there is a problem with the concentration of zinc.
I want you to try to check what you can visually first.
1.Chemical flow from the overflow of the plating tank. Is it convenient flow to the solution tank?
The pipe size too small? It should be large, at least three hundred percent of the return pipe from the filter. (The design should not save this pipe).
2.Placing the steel basket in the solution tank, does it block the flow of chemical? If the basket arrangement is not good, ingot zinc or balls will not dissolve well, resulting in the zinc flow back to the plating tank is not concentrated enough.
3. Chemical return pipe From filters to plating tanks --
At the outlet pipe, is the solution evenly distributed?
If you spread only one point the concentration in the plating tank at each point is not the same.
Therefore, you need to split the end of the return pipe at more than one point. (The more the better)
4.Easier. Do it now --
When was the last time you checked for blockages in your filter?

A. You should have more than enough zinc dissolving to maintain the concentration. Your problem should be in keeping the zinc concentration down to a manageable level with that setup.

What concentration are you running the NAOH at and what temperature is the plating solution running at? Are you dissolving zinc and then adding the solution to the plating bath or is the plating solution being continually replenished by 'enriched' solution?

A. I agree with the other reply that you should have sufficient anode weight to maintain the zinc concentration in solution if you are using the correct level of sodium hydroxide (caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] ). Generally, depending upon individual supplier chemistry the range is from 12 to 20 ounces per gallon (sorry, my clients and I have been referring to things this way not in metrics for 30 years) maintaining a bath zinc metal of 1.6 to 2.4. You should maintain some type of agitation in the zinc generating tank and keep the temperature at 70-85° F. Lower temperatures may cause some precipitates to form.

The sand could be impurities from your anode material. The preferred zinc anode material for alkaline non cyanide baths is 99.99% pure.

I hope this is somewhat helpful

A. Dimitrij,

For how long time has you used your alkali zinc solution?

It could be carbonate from the NaOH you see. What types of anode sheet did you use? please tell me more so I can solve your problems.

Regards

A. I obviously don't know the characteristics of your particular solution but your NaOH seems far too low. It should be up near 140 g/l. Doesn't your solution come with a technical data sheet and supplier back up?

Haven't experienced the 'white sand' problem before but I agree with Gene that contaminants from the anodes is a likely source. Na2CO3 shouldn't be a problem unless the concentration rises above 100 g/l (although newer chemistry can cope with this figure). It wouldn't come out of solution anyway unless the temperature fell below 5 °C (41 °F).

A. Hello Dimitrij

I came in just now and do agree with most of what has been said so far.

Factors that affect Zinc Generator performance are listed and you could check these out.

- Caustic should be around 125 gpl and above to ensure effective dissolution of zinc. 140 is really good if your supplier has the right system that works at this level..

- The iron used for the baskets should have been treated as follows before first use.. .. Clean. Light pickle to remove rust if any. Rinse. dry. Then use a blowlamp to heat the basket till it develops a bluish film all over, It has been seen that this film helps dissolution of zinc in caustic solutions and this is how we do it . Baskets treated this way work very well..

- The purity of the zinc is definitely important. 99.99

- Lack of caustic will tend to polarise the dissolution and cause a white coating over the anodes.

- Light air agitation helps dissolve zinc.

-Temperature above 20 centigrade is recommended.

- A low energy Mild steel heater can be used in winters to control temperature around 25 Centigrade.

- Check the white powder for zinc content. If it is zinc that is oxidised and won't react with caustic then it is a sign your caustic was low.

I hope this helps.

Regards

A. Dimitrij,

All answers here are good enough to solve your problem I think. You should analyze your zinc anodes to see if they are 99,99 zinc.

Regards

A. Hello,

It was interesting to read information on solution to the problems with Zn bath. I read at many places that purity of Zn anodes is of high significance in Zn plating process.

Mr. Dimitrij, I would appreciate if you could let me know what kind of Zn solution do you use..is it sulphate/ chloride.. When you say you are running a alkaline bath, what is the pH of your solution?

Many Thanks.

With Regards,

A. Dear Dimitrji, Dear Asif,

With interest I've followed the suggestions to eliminate the problems Dimitrji had in his production. In fact I do absolutely agree to the suggestions of Asif.

A. Dear Dominik

Just saw your posting . Here are the reasons for what was suggested.

Mild steel basket with a bluish oxide layer on them , in contact with zinc, immersed in a caustic solution set up an excellent electrochemical couple that speedily dissolves zinc.

This light bluish film of oxide is required to be created once, when new baskets are about to be installed. A gas blowtorch with the light blue flame stationed on steel moved lightly and uniformly will form the film.

Light agitation be it mechanical, solution recirculation or light air only in the generator helps prevent solution stagnation and anode polarization.

We now design alkaline zinc tanks with the zinc generator as part of the working bath. One end of the bath contains static anode baskets. With zinc in them. The filter draw a solution from the non working end which is the generator end and discharges the filtered solution into the other far end, setting up a nice circulation.

We use mild steel anodes to pass current. Maintain caustic around 125 to 135 and the zinc stays above 12 gpl most of the year. North Indian winters call for low energy heater to maintain 22 °C when ambient falls to 5 °C.

There is no advantage with Nickel plating anode baskets. Its an older technique that has been discontinued with the newer brightener systems. The mild steel baskets get a little too active in summer and we have to remove some anode to keep the zinc from climbing past the 20 gpl mark. Yes the system works well.

I hope I was able to provide all the answers ?

Regards,

A. Do not use air agitation as it will add to carbonates, I ran a 6800 gal alkaline Zinc bath, I used a plate filter with a 5 micron poly pro cloth on top of plate, this should filter out the sand you refer to,the cloths can be rinsed off to clean and reused for a long time. Mechanical agitation in the generation tank should help dissolution rate. Run the filter intake from this tank to discharge in the process tank.The filter should be changed 3 to 5 times per week. A turnover rate of 1 to 2 times per hour worked for me. A bit of filter aid will help to put a pack on the plates and do a better job of filtration. Maintain your caustic levels and analyze the bath twice per shift.

Hope this helps.

Good luck.

A. I would like to add one more comment to this interesting discussion:
When analysing for NaOH, add BaCl2 to make sure that the Na2CO3 precipitates. Otherwise you can get a high incorrect result for NaOH.

Ed. note: The years have been kind to your command of the language, Dimitrij; your English is better than a lot of Americans.

Thanks for a compliment.
I hope, that your compliment will not scare all the others, to give an answer :)
I need some answers regarding to my problem badly....

A. Dimitrij,

It should work as long as there is no electrical circuit. I think this is the theoretical part of it which I will try to verify next week by doing some lab tests. I also will try to measure the potentials which are running through and will post the results asap here at this page.

Asif, I have tried several tests in the lab and in the field at customers your recommendation of bluish oxide film on the anode baskets. The initial dissolving reaction was very very good and very fast (1-1,5g/l/hour), but after a certain time and especially after work breaks (weekends and holidays)the dissolving reaction slowed down, it even stopped completely. After a fresh "oxidizing" of the mild steel anodes with a acetylene/oxygen burner the dissolving reaction was as fast as before. For our application (rotogravure industry)is this kind of dissolving technique not suitably. We run almost the double of zinc metal content in the bath as in conventional zinc plating baths.

As I said above, I make some lab tests with grounded iron sheets and some tests out in the field and will post the results here.

So long,

A. What temperature does the solution fall to over the weekend and what temperature is on Monday, Tuesday ......etc.?

A. Is your temperature definitely held at a constant 25? I've never seen an installation yet where that was achieved. Obviously if the temperature was lower during the early part of the week then the dissolution rate would be slower. Removing the baskets from the bath may also be part of your problem. Restoring the dissolution process to its previous rate can often take some time after it has been stopped for a while by removing the baskets from the solution.
Also, solution dragout from barrels can be horrendous. Is the NaOH concentration constant? You may want to consider raising that to help to dissolve more zinc.

A. Dimitrij,

I've read ALL that has been said about your problem and it's very interesting. You already have all the basics to set up a proper zinc generator tank.

What maybe has not been addressed is drag-out. With caustic level at 130 g/l you'll have a lot of dragout in a Barrel operation !

Maybe you are drag-in out too much solution that your tank generator can't keep up with it. You compared your operation with another plant running a bigger zinc plating tank with a smaller generator tank, remember they are running a Rack line, not a Barrel line.

There could be huge differences in dragout according to part configuration, dwell times or drip times, barrel hole's sizes, etc.

You are constantly adding adding back NaOH, maybe because of dragout.

We ran a 16,000 liter line with 2,000 zinc generator tank, and we ONLY pump the solution from this tank for 15-20 minutes a day ! This is a rack line.
Increase your drip times, rotate the barrel to drain more solution, use a drag-out tank and return this solution, etc.

Have you estimated how much zinc you are actually plating everyday? (Zinc deposit on parts)
Maybe if you compare this figure with the zinc actually being used (grams plated + dragout) you might see a big difference.

I'm only trying to give you a different approach, since I can see that everything else has been attempted. Maybe this won't eliminate the problem but it might help you work better.

Good luck,

A. I believe that steel can be used, Rajesh, but these plating baths can be very difficult to control unless you use a separate zinc generation tank. Does your supplier have any comment?

A. Dear Rajesh,
Definitely MS (mild steel) as Ted rightly said. The supplier of additives should have advised you on this. If you do not have a Zinc Generator tanks of a sophisticated type, just have a 200 ltrs drum withs lots of (300-400 grms/ltr) of caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] and lots of (up to 100 kgs of Zinc metal) added in it. Leave it overnight (Do not add the mix while hot). Start adding the mix to the bath in required qnty. as per analysis.
I have seen the working of up to 13000 ltrs tanks working in this style round the clock.
Please try this method and let me know!
Regards

Simple and interesting, TK, thanks!

Regards,

A. It is not required.